The relationships between neuroglial and neuronal changes in Alzheimer’s disease,and the related controversies II: gliotherapies and multimodal therapy

Abstract

Since the original description of Alzheimer´s disease (AD), research into this condition has mainly focused on assessing the alterations to neurons associated with dementia, and those to the circuits in which they are involved. In most of the studies on human brains and in many models of AD, the glial cells accompanying these neurons undergo concomitant alterations that aggravate the course of neurodegeneration. As a result, these changes to neuroglial cells are now included in all the “pathogenic cascades” described in AD. Accordingly, astrogliosis and microgliosis, the main components of neuroinflammation, have been integrated into all the pathogenic theories of this disease, as discussed in this part of the two-part monograph that follows an accompanying article on gliopathogenesis and glioprotection. This initial reflection verified the implication of alterations to the neuroglia in AD, suggesting that these cells may also represent therapeutic targets to prevent neurodegeneration. In this second part of the monograph, we will analyze the possibilities of acting on glial cells to prevent or treat the neurodegeneration that is the hallmark of AD and other pathologies. Evidence of the potential of different pharmacological, non-pharmacological, cell and gene therapies (widely treated) to prevent or treat this disease is now forthcoming, in most cases as adjuncts to other therapies. A comprehensive AD multimodal therapy is proposed in which neuronal and neuroglial pharmacological treatments are jointly considered, as well as the use of new cell and gene therapies and non-pharmacological therapies that tend to slow down the progress of dementia.

Introduction

Since the original description of Alzheimer´s Disease (AD), ¹ research into this condition has mainly focused on assessing the alterations to neurons associated with dementia, and those to the circuits in which they are involved. In most of the studies on human brains and in many models of AD, the neuroglial cells accompanying these neurons (astroglia, oligodendroglia, microglia) undergo concomitant alterations that initiate and/or aggravate the course of neurodegeneration.^2-8 As a result, these changes to neuroglial cells are now included in all the “pathogenic cascades” described in AD. ⁹ The reactive changes of these neuroglial cells (generically grouped under the term “gliosis”: astrogliosis, oligodendrogliosis, microgliosis), both manifested by hypertrophy or hyperplasia, as well as by hyper-reactivity or hypo-reactivity of differential markers and by the expression of new genes that give rise to new phenotypes with new functionalities ¹⁰ ), Astrogliosis and microgliosis, the main components of neuroinflammation, have been integrated into all the pathogenic theories of this disease,^4-9 as discussed in the first part of this two-part review that focused on gliopathogenesis and glioprotection. ¹⁰

To understand the change produced in the neuronal conception of AD to the neuron-neuroglial or mainly neuroglial conception of AD, it is necessary to take into account the development of research in different fields on the CNS (both normal and pathological). Research into glial cells has advanced significantly in the four last decades. Hence, these cells are no longer considered as a mere support network for neurons and neuronal circuits but rather, they are seen to be active in maintaining the morphology and correct functioning of the nervous system, and as drivers of the adaptive responses that underlie cognitive and behavioral processes.^10-18 Neuroglia maintains homeostasis, that is, the physical-chemical conditions of the internal environment of the CNS so that the neurons have the optimal environment to fulfill their normal functions and carry out the adaptive changes necessary for the proper functioning of the neuronal circuits. However, neuroglial cells not only maintain the correct levels of ions and metabolites, but collaborate in neuronal neurotransmission, produce neurotrophic substances, collaborate in neuronal plasticity and synaptic remodeling. Astroglia also have a gliotransmitter character and microglia is the immune representative in the CNS. All these aspects have been analyzed in the first part of this monograph. ¹⁰ For all these reasons, alterations in normal neuroglial functions can theoretically lead to pathological changes, and many studies have shown that alterations to glia underlie pathological processes. Logically, therapeutic interventions aimed at maintaining or improving their activity should therefore help to prevent or treat neurodegeneration.

In the first part of this monograph, the different types, and subtypes of neuroglial cells (astroglia, oligodendroglia -including pro-oligodendrocytes or NG2+ cells- and microglia) as well as the reactive phenotypes that each subtype can originate have been presented. In a general sense, the glial responses configure diverse subsets of neuroglial cells that have two general different and opposing activities on the neurons: some neuroglial subtypes produce neuroinflammatory substances that induce neurotoxicity, and other subtypes produce anti-inflammatory substances that induce neuroprotection/neurorepair. This first part of the monograph also analyzes and discuss the presence of different subtypes of normal and pathological neuroglial cells in the brains of patients that suffered Alzheimer’s, or models of experimental Alzheimer’s, their relationships with altered neurons and pathological amyloid deposits, and the potential implications for the presentation and progression of AD. (See the reference 10 of this second part).

In this introduction, we wish highlight recent studies using new technologies in molecular biochemistry/genetics (genome-wide association studies -GWAS-, whole genome sequencing -WGS- and whole exome sequencing -WES- studies; transcriptomics and RNAseq data studies from isolated cells or selected cell groups, etc.) that show indubitatively the involvement of neuroglial cells in AD, both in its origin and in the development of the underlying neurodegeneration leading to dementia. Possible pathogenic mechanisms as well as definition of new therapeutic strategies can be inferred from these studies. Among the results obtained, we can highlight the demonstration of: (a) genetic risk factors in neuroglial cells in AD; (b) neuroglial cells with characteristic phenotypes closely related to AD that classify with much greater precision the phenotypes of different subtypes of neuroglial cells; (c) senescent neuroglial cells closely related to the development of AD.

(a) Common polymorphisms in the apolipoprotein E (APOE) gene are the major genetic determinants for AD: APOE4 is the strongest genetic risk factor of Late-onset Alzheimer’s disease risk. ¹⁹ Brain apoE is mainly produced by astrocytes and to a lesser extent by microglia, oligodendrocytes, vascular mural cells, and neurons.^20-22 ApoE, is a protein involved in a wide variety of functions, including lipid transport, neuromodulation, neuronal plasticity, neuronal repair, neurite outgrowth and regulation of Aβ formation and clearance.^10,20,23 ApoE also modulates the inflammatory response of microglia and astrocytes and is of special importance in neurodegeneration ¹⁰ , Three major isoforms, apoE2, apoE3, and apoE4, encoded by the ε2, ε3, and ε4 alleles, exits in humans with different abilities to carry out the assigned functions in the CNS. Apoe4 increases an individual’s risk for AD,^24-27 induces endoplasmic reticulum stress and impairs astrocyte and microglia functions; ²⁸ but apoe2 and 3 seem to be protective factors, ²⁶ increasing anti-inflammation. ²⁹ APOE4 genotype was associated with lower levels of secreted apoE from astrocytes and proinflammatory effects.^10,30,31 Genetic variants in brain-derived neurotrophic factor (BDNF) associated with Alzheimer’s disease have been described.^32-35 Sexually dimorphic effect of the Val66Met polymorphism of BDNF on susceptibility to Alzheimer’s disease has been observed. ³⁵ GWAS, WGS, WES and integrative analyses with transcriptomics, epigenomics and proteomics have nominated many microglial genes closely related with increasing risk for AD (more than 75 genes: TREM2, CR1, CD33, CLU, BIN1, CD2AP, PILRA, SCIMP, PICALM, SORL1, SPI1, RIN3, and so on.^19,31,36-38 These genes are involved in phagocytic recognition, focal adhesion, actin cytoskeletal rearrangement, endo-lysosomal network, phago-lysosomal network, auto-lysosomal network, and transcriptional responses in microglia. Microglial cells maintain CNS homeostasis by sensing, endocytosis, and phagocytosing apoptotic cells, debris, synapses, and pathological Aβ deposits. ³⁸

(b) Recent studies using the above-mentioned new technologies are highlighting the broad heterogeneity in phenotypes of neuroglial cells. These studies show that there is a much greater diversity of reactive neuroglial cells (especially astrocytes and microglial cells) that can no longer be generically considered as, in the case of astrocytes, phenotypes A1 (neurotoxic/proinflammatory) or A2 (neuroprotective/anti-inflammatory),^10,39,40 or as in the case of microglia, M1 (neurotoxic/proinflammatory) or M2 (neuroprotective/anti-inflammatory).^10,41-43 We have dealt with this topic in the first part of this monograph ¹⁰ but it should be remembered again in this second part of the monograph when trying to analyze the possible new neuroglial therapeutic strategies for AD. The role of each of these newly described neuroglial phenotypes in the neurodegenerative lesions and synaptic and neuronal damage, in the tissue injury or repair responses and trajectory of cognitive impairment, are incompletely understood.^10,44 Some studies have suggested that increased reactive glia in early mild cognitive impairment (MCI) stages may exert a beneficial role on preserving brain structure and function, ⁴⁵ while identical phenotypes in subsequent phases of the dementing disorder are associated with increased severity and faster rates of cognitive dysfunction. ⁴⁶ Recent data in human AD brains and animal models of AD have pointed to the existence of different profiles of normal (“homeostatic”), “Aging” and” AD” microglial genes with diverse transcriptomic expression (see the review of Boche and Gordon, 2022 ⁴⁴ regarding human specific, human/mouse dual expressed and mouse specific gene expression in microglia). This study present phenotypes with increasing (CD68, PICALM, SORL1, TREM2, TGB1, …) or decreasing transcripts as well as a relevant loss of homeostatic glial markers early in the disease ⁴⁴

(c) Senescent cells in CNS have been described in different studies on AD and related diseases. These cell subtypes have been linked to the neurodegeneration underlying AD.^47,48 Although senescence has been demonstrated in some neurons, the majority of senescent cells in brain are of neuroglial linages.^47,48 Senescent astrocytes,^49-53 oligodendrocytes ⁵⁴ and microglial cells ⁵⁵ have been observed and studied. These cells are characterized by growth arrest (in tissue or in culture after isolation), increased expression of senescence-associated genes p53 and p21WAF1 and increased senescence-associated β-galactosidase (SA-β-Gal) activity,^50,56,57 as well as a characteristic development of a multicomponent senescence-associated secretory phenotype (SASP).^49-53,58 The SASP consists of a myriad of cytokines, chemokines (CXCLs), growth factors, and proteases that initiate neuroinflammation or, in the contrary, growth responses in nearby cells. In young healthy tissues, the SASP is typically transient and tends to contribute to the preservation or restoration of tissue homeostasis. However, senescent cells increase with age, and a chronic SASP is known or suspected to be a key driver (though a chronic inflammation) of many pathological hallmarks of aging and neurodegenerative processes such as in AD (for example, tau and amyloid neuropathology, microglia proinflammatory reactivity.^49,53,55,59 All the studies above-mentioned suggest that the elimination or control of these senescent cells may be an effective strategy to combat neurodegeneration (ie the clearance of senescent cells prevents Tau-dependent pathology in AD mouse models ⁴⁹ or the removal of senescent NG2+ cells improves the AD pathology. ⁵⁴ Senescent astroglial cells appear to be able to be physiologically cleared from the brain via para-vascular pathways (glymphatic pathways^60-62 to meningeal lymphatic pathways, ⁶³ but senescent microglial or oligodendrocyte cell clearance systems are not well known. However, it seems that these senescent cells can be eliminated by “senolytic cocktails.”^49,54

We can conclude this introduction considering that there is already significant evidence that the study of glial alterations in AD will provide a basis to not only understand but also to treat this condition^10,18,64-70 Unfortunately, we remain far from deciphering the precise role of the different glial cells in each phase of this neuropathological processes as we have considered in the first part of this review. ¹⁰ However, we do have enough information to establish that knowledge of the alterations to glia will shed light on the mechanisms driving different neuropathological processes, and that this will help identify therapeutic targets to combat this disease. We are now in a new era to face the AD and many clinical trials have been approved supported by consistent preclinical studies and in-depth analyzes of human brains.

In this second part of the review, we will consider the possibilities of maintaining normal astroglial, oligodendroglial and microglial activity, and/or regulating astrogliosis and microgliosis, and neuroglial senescence in AD, to offer preventive or palliative treatment for this condition. Most of the studies considered here represent preclinical research, or they involve data from studies in humans prior to designing regulated clinical trials. Nevertheless, all of them offer reliable evidence that correcting the abnormal behavior of glial cells can produce therapeutic benefits in the prevention or treatment of AD.

Potential strategies to combat AD based on modifying the activity of normal and reactive glial cells

The data obtained from studies into AD and from experimental models of this disease suggest that there are therapeutic benefits to be gained by targeting glial cells. Here we will highlight some of the evidence supporting the use of neuroglial therapy in AD, most of which has come from preclinical studies or introductory studies on humans for regulated clinical trials. In general, these approaches aim to either stimulate the neuroprotective effects of neuroglia or they arrest and/or counteract the neurotoxic effects of these cells.

In gliotherapeutical studies aimed at regulating the behavior of each type/subtype of glial cell, it should not be forgotten that not only are the normal and reactive forms of these cells the therapeutic targets, but also that the activation or inhibition of the different types of neuroreparative/neuroprotective or neurotoxic substances (respectively) cold be the main targets. In addition to this, it must also be taken into account that a correct balance of the substances produced by astrocytes, oligodendrocytes and microglia cells must be maintained. The regulatory interrelationships of neuroglial cells are very close, with each type continuing control the others, both locally and generally. This interrelation, called glial crosstalk, is essential both to obtain optimal function of neurons and neuronal circuits in normal situations and to modify or correct alterations which cause or make progress the neurodegenerative processes such AD. ⁷¹ Furthermore, we have to take into account the close relationships of systemic immune cells (and their secreted substances) with microglial cells^71,72

Pharmacological gliotherapies

Astroglial therapies

Different strategies can be used to diminish the influence of astrocytes on the development of AD. First, we must consider the possibility of maintaining the functionality of astrocytes (including their adaptive responses) in order to maintain the neurons and neuronal circuits functional, and especially their synaptic connections. Among the many activities undertaken by astrocytes, four are of particularly importance: the production of neurotrophins; their participation in neuronal glutamatergic neurotransmission; gliotransmission (glutamate, GABA, adenosine, etc.); and the diverse activities to maintain homeostasis in the nervous system (regulating Ca²⁺, K⁺, etc.). Functional changes in astrocytes may affect these activities, inducing alterations to neurons that are likely to represent the true cause of AD. Dysfunctions of the astroglia surrounding neurons are likely to enhance AD progression when amyloidogenesis or a Tau pathology develops.^6,10,17,18 Indeed, the significant changes implicated in the development of AD in humans and in models of AD affect metabolic markers, as well as glutamatergic and GABAergic neurotransmission via the Glutamate Transporter 1 (GLT1), the glutamate/aspartate transporter (GLAST), glutamine synthetase (GS) and GABA synthesis.^70,73-75 This information is beginning to be translated to pharmacological research to design useful astrogliotherapies for AD. Here, we present what we consider to be some of the most interesting of these:

(A) One possible therapeutic strategy is to compensate the deficits in neurotrophins (mainly Nerve Growth Factor –NGF- or Brain Derived Neurotrophic Factor - BDNF) caused by the dysfunction or involution of astroglia. Studies on changes in neurotrophic factors have given rise to controversial results for years, both in the study of human brains and in the study of experimental models of AD. In humans, divergences between different brain areas have been shown.^76-78 The decrease in BDNF, both protein and mRNA, has been confirmed in numerous studies^76,77,79,80 but the putative changes in NGF were not being confirmed although the cholinergic theory of AD presupposed a decrease in NGF. Given the strong dependency of the cholinergic basalo-cortical forebrain neurons on NGF, it was hypothesized that their atrophy was caused by NGF deficits. This was confirmed by the cognitive disturbances in an AD NGF−/− model.^81,82 Many studies showed normal levels of NGF/pro-NGF proteins and mRNA in human brain ⁷⁷ However, in-depth studies on the metabolism of NGF and pro-NGF have shown that there is a functional deficit of NGF in AD. ⁸³ The alterations compromise the availability of NGF to basal forebrain cholinergic neurons. ⁸³ Moreover, AD models have shown evident deficits in the NGF metabolic pathway with advanced pathology. ⁸⁴

A large number of preclinical studies^85-90 and clinical trials have addressed this issue (Table 1; Supplemental Data 4), although the many attempts to administer NGF have failed due to the side-effects produced,^91-94 both when administered by injection or through other more sophisticated routes (eg, adeno-associated virus-based delivery ⁹² ; Table 1). However, new approaches are still being tested that may have potential therapeutic benefits (eg, the intranasal route investigated by Capsoni, 2012 ⁹³ ).

• (B) A second strategy to be considered is the possibility of controlling the reactive astrogliosis that enhances the progression of AD. Although there are many laboratories working on this approach, there is as yet no effective way to control toxic astroglial activation. However, there are data indicating that a reduction in oxidative stress significantly affects both the neuroprotective and modulatory aspects of the astroglial reaction.^94-102 Curcumin, quercetin, vitamin E and other substances can modulate the levels of glutathione in neurons and astrocytes.^103,104 Polyphenolic nutraceticals are potential preventive and therapeutical substances against AD ¹⁰⁵ (Table 1). Nicotinamide-ribose ameliorates cognitive impairment of aged and Alzheimer’s models. ¹⁰⁶ Although these approaches don’t seem to be fully effective in humans, ¹⁰⁷ they do suggest it may be possible to correct undesirable changes to the activity of astroglial cells. Many of these antioxidants that can help regulate astroglia function, also have anti-inflammatory effects by regulating microglia, as shown below. Although these substances have not yielded conclusive results in previous clinical trials, they have been shown to have potentially beneficial effects that are currently being used in multimodal therapies (Table 1).

• (C) Amyloid peptides bind specifically to Ca2+ sensing receptors (CaSRs) on normal astrocytes, leading to the production and secretion of large amounts of amyloid and other neurotoxic products (nitric oxide -NO, vesicular endothelial growth factor -VEGF, etc.). Interestingly, Ca2+ antagonists (“calcilytics”) have been identified^108-114 that decrease the production of neurotoxins.

• (D) Polyamines (putrescine, spermidine, etc.) are substances that interact with negatively charged molecules and that play critical roles in many cellular processes, including astroglial cells. Moreover, they have been shown to fulfil an important role in neuron-glia cell communication and in the modulation of glutamate receptors. As a result, biomolecule/polyamine conjugation is considered a promising strategy in AD.^115,116

• (E) The prevention of astroglial senescence could be a potential AD therapy as it has been considered in other neuroglial cells.^{49,53,59,117,118} The clearance of senescent cells prevents Tau-dependent pathology in AD mouse models. ⁴⁹ Senescent astroglial cells appear to be able to be physiologically cleared from the brain via para-vascular pathways (glymphatic pathways) to meningeal lymphatic pathways. ⁶³ The meningeal lymphatic vessels and the glymphatic system are being now studied as therapeutic targets to remove macromolecules, debris, and senescent or damaged cells.^61,62 Pharmacologically, these senescent cells can be eliminated by “senolytic cocktails.”^49,54

Table 1.

Resume of clinical studies (observational and clinical trials in different phases of development) recorded from the Clinicaltrials.gov website related to gliotherapy and multimodal therapy in AD.

Clinical studies recorded from the Clinicaltrials.gov website (July 2022)
ANTIOXIDANTS/ALZHEIMER DISEASE	51 studies
ANTI-INFLAMMATORY/ALZHEIMER DISEASE	35 studies
Vitamins (E, C, folate….) (some of them in multimodal therapy with neuronal therapy)	6 studies
Polyphenols (resveratrol, epigallocatechin-3-gallate. …) (Some of them in multimodal therapy with neuronal therapy)	17 studies
Quercetin + Desatinib (senolytic cocktail)	4 studies
Isoflavones	2 studies
Nicotinamide riboside	2 studies
Melatonin	2 studies
Sulforaphane	1 study
Exercise program	5 studies
Diet (Mediterranean diet, ketogenic diet, …)	5 studies
Non-steroidal anti-inflammatory drugs (NSAIDs)	17 studies
Anti-IL-1beta (Canakinumab)	1 study
Interpheron alpha (hrIFN-a2a)	2 studies
Cromoglycate (preventing the release of inflammatory substances)	1 study
CELL THERAPY/ALZHEIMER DISEASE	20 selected studies
Stem cells of different origin	20 studies
(Adipose cells)	(4 studies)
CERE-10 (adeno-associated virus injection)	(2 studies)
GENE THERAPY/ALZHEIMER DISEASE	7 selected studies
APOE 2 gene LCR injection	2 studies
NGF encapsulated cells	1 study
Fibroblast producing NGF, injected into basal forebrain	1 study
AAV (adenovirus) libering NGF gene	2 studies
AAV-hTERT (gene telomerase activator)	1 study

Four searches have been made with the web system for search. The antioxidant-AD and anti-inflammatory-AD searches are jointly considered as the studies in these fields do not differentiate well. The total records in these two searches were 123 and we have referenced the studies in Supplemental Data 1 (antioxidants in AD therapy) and Supplemental Data 2. In the cases of cell therapy-AD and gene therapy-AD searches, those studies that truly respond to the search interest have been selected (for example, most of the studies classified as gene therapy are studies dedicated to checking whether certain drugs are more or less appropriate for patients bearing certain genetic isoforms) and references have been collected in Supplementary Data 3 (cell therapy in AD) and 4 (gene therapy). Anti TNF-alpha therapy and TREM2 therapy were referenced in Supplementary Data 5 and 6.

Oligodendroglial therapies

Oligodendroglial cells are associated with different alterations in AD, yet their capacity to prevent or treat AD has been little studied.^119-123 Several therapeutic strategies for oligodendrocyte mediated remyelination are now in study (vitamins-B5; growth factors; activation and differentiation of oligodendrocytes; etc.), but no clear beneficial effects have been published.^119-125

At present, the prevention of cell senescence and/or the elimination of senescent NG2+ cells can be considered potential therapeutic strategies to prevent AD and other neurodegenerative diseases. A senolytic combination (“senolytic cocktail ”) of dasatinib (anti-leukemic) and quercetin (antioxidant: D + Q ) has been reported to cause the death of senescent oligodendrocyte precursors (OPCs) induced by amyloid in vitro ⁵⁴ (Table 1).

Microglial therapies

Inhibition of the neuroinflammatory response is considered one of the most promising gliotherapies to prevent the presentation of AD and other neurodegenerative diseases, or to halt their progression.^126-134 During the course of these pathologies, there are changes to the morphology and functional phenotypes of the different sets of microglial cells, both resident and invasive microglia. Microglial heterogeneity can drive phenotypic diversity of AD. ¹³⁵ In a generic way (see the introduction and in the first part of this monograph ¹⁰ the discussion on the nomenclature of microglia, M1 -neurotoxic/proinflammatory- or M2 -neuroprotective/anti-inflammatory-,^10,41-43 we can consider the (old) M1 phenotype to be responsible for neuroinflammation because these microglial cells secreting pro-inflammatory cytokines/chemokines (TNF-α, IL12, etc.), while a second phenotype (old M2) of reactive microglial cells secrete anti-inflammatory cytokines/chemokines (IL-10, TGF-β, etc.).^130-137 In different pathologies there is an imbalance between these two phenotypes, not only in the different phases of the disease but also, in different regions of the brain and in different areas of each of the regions affected. Many of these variations are still poorly understood but in general, it is thought that the shift from the (old)M1 to (old)M2 phenotype has beneficial effects in most processes, regardless of the phase of the disease or the areas that are affected.^{129,131-134,138} Although early activation of microglia is beneficial for clearing toxic Aβ from the brain, over time the chronic stimulation of microglia by Aβ may also be deleterious and lead to prolonged inflammation, excessive Aβ deposition and acceleration of the neurodegenerative process. During AD pathogenesis, the phagocytic activity of microglia seems to decline, while the production of proinflammatory cytokines and neurotoxic molecules increases^10,139,140

Various therapeutic strategies that focus on microglia have been tested. The microglial response is controlled by extracellular factors in the nervous tissue and by alterations to neurons and/or astroglial cells, which must be detected and corrected as quickly as possible.^{10,127-132,138} Cytokine signaling convergence regulates the microglial state transition in AD ¹⁴¹ and this opens up new therapeutic possibilities if we are able to know the specific effects of the different levels of cytokines and their local and general interrelationships. External environmental factors, diet, metabolic alterations (eg, obesity, insulin resistance, oxidative stress), cardiovascular processes, toxic drugs, etc., can trigger or accentuate the inflammatory phenomena that ultimately lead to AD.^{8,127-131,142,143} Thus, addressing AD risk and inflammatory phenomena should be among the initial measures to be taken in AD. For example, it is worth noting that hypercaloric diets appear to induce microgliosis in experimental AD models. ¹⁴⁴

On the other hand, it should not be forgotten that phagocytosis of damaged (neurotoxic) neurons and of amyloid deposits is of great importance in the control of AD and the possible stimulation of this microglial (and astroglial) function may be of great importance for fighting AD.^145,146 Many attempts are being made but without success up to now, but many research studies are continually being put in place to obtain the best benefits in AD treatment (Table 1; Supplemental Data 1-4).

After reviewing the bibliography as well clinical studies (Table 1), we consider antioxidants and anti-inflammatory agents the best studied substances to treat AD, both in humans and in experimental models of AD. ¹⁴⁷ In this regard, different lines of research have been followed:

(A) Cellular pathways that activate cyclo-oxygenase 2 (COX-2) produce inflammation in many tissues, including the CNS, suggesting that COX-2 inhibition might represent an effective treatment for AD. NSAIDs (non-steroidal anti-inflammatory drugs) have been tested in humans and in experimental models of AD (celecoxib, flurbiprofen, etc.) (Table 1; Supplemental Data 1 and 2), albeit with conflicting results.^148-151 Decreased expression levels of inflammatory cytokines (IL-1β and TNF-α) have been observed in models of AD but not a clear prevention or improvement of cognitive deficits in humans.^148-151 In addition, systemic problems have been reported with long-term use of NSAIDs. ¹⁵² To avoid gastro-intestinal disorders, the intranasal route has been tried in some cases.^153,154 While no NSAID have as yet been approved by drug agencies to treat AD, trials continue to search for an effective approach, especially based on the effects observed on the reduction of amyloid plaques in animal models of AD. ¹⁵⁵ NSAIDs may have other therapeutic targets (such as the regulation of heme-oxygenase-1) with neuroprotective and/or anti-amyloidogenic effects. ¹⁵⁶ To note that COX-2 is also activated by cholinergic receptors on neurons and astrocytes, acting through signaling pathways that are not involved in inflammatory phenomena. ¹⁵⁷ Thus, COX-2 activation cannot be exclusively considered as a neurodegenerative microglial response but rather, as a component of a global adaptive response in certain neuroprotective/reparative phases of disease when CNS alterations occur. ¹⁵⁷ In the search for novel and effective treatment strategies, the multi-target-directed ligand paradigm was applied to the rational design of series of new hybrid compounds endowed with anti-inflammatory and anticholinesterase activity via triple targeting properties, namely able to simultaneously hit cholinesterases, cyclooxygenase-2 (COX-2) and 15-lipoxygenase (15-LOX) enzymes ¹⁵⁸ as well as flurbiprofen-clioquinol hybrids as multitarget-directed ligands against oxidative stress and amyloid plaque formation.^159,160

(B) Other anti-inflammatory and antioxidant compounds have also been tested for the treatment of AD and continue to be studied in clinical trials (Table 1; Supplemental Data 1 and 2), many of them in multimodal AD therapies. Most of these compounds are of vegetable origin like propentofylline (a xanthine-derived inhibitor of adenosine reuptake), phosphodiesterases, tannic acid, panpheniacol, polyphenols, etc.^105,161-163 The mechanism of action of these compounds are diverse, including the inhibition of NF-κB activation. Flavonoid antioxidants like Farrerol dampen the production of IL-6, IL-1β and TNFα, and these substances have been seen to diminish the pro-inflammatory reaction of microglia in some studies. ¹⁶⁴ Polyphenols are also inhibitors of NF-kB. ¹⁶⁵ Animal experiments and clinical studies have shown that consumption of diets rich in plant polyphenols have beneficial effects on various diseases including neurodegenerative diseases such as AD. The most representative polyphenols are epigallocatechin-3-O-gallate in tea, chlorogenic acids in coffee, resveratrol in wine, and curcumin in curry. ¹⁶³ Jujuboside A, a triterpene saponin reported to have antioxidant, anti-inflammatory, anti-apoptosis, and neuroprotective functions ¹⁶⁶ has been shown to inhibit abnormal activation of microglia and enhance microglial uptake of Aβ42, thereby ameliorating cognitive deficiency in APP/PS1 mice evaluated by Morris water maze (MWM) test and object recognition tes. ¹⁶⁶ Mechanisms involved may include up-regulation of heat shock protein 90β expression and restoration of peroxisome proliferator activated receptor γ levels dependent on the Axl/ERK pathway in microglia. Likewise, another anti-inflammatory drug, metformin, has been reported to effectively reduce activation of microglia and astrocytes and production of pro-inflammatory mediators in APP/PS1 mice, while promoting production of the anti-inflammatory cytokine IL-4. ¹⁶⁷ These beneficial effects suggest that these compounds have some therapeutic potential to halt the progression of both AD and other neurodegenerative diseases. However, none of these substances have a proven total efficacy and as yet there are still no protocols for their administration. Currently ongoing clinical trials (Table 1) may establish more precise guidelines for greater therapeutic benefits.

• (C) Urolithin B, a metabolite of ellagic acid that is produced in the gut of mammals, seems to decrease the pro-inflammatory reaction of microglia (involving NO and pro-inflammatory cytokines). While the use of this compound has been studied, no protocol has as yet been approved for its use. ¹⁶⁸

• (D) Nicotinamide adenine dinucleotide and panthenine-vitamin B5 (also active in myelin homeostasis, ¹²⁰ normalize astroglial and microglial functions.^169,170

• (E) Protein kinases ¹⁷¹ and phosphodiesterase 4B^172,173 are enzymes closely related to the production of pro-inflammatory cytokines by reactive microglia. Amyloid peptides and other toxic substances can enhance the activity of these enzymes, increasing the pro-inflammatory cytokines produced. Thus, these enzymes possibly represent interesting targets for AD therapies and such approaches are being studied by several laboratories.

• (F) Several studies have been conducted against oxidative stress via the activation of nuclear factor erythroid 2-related factor 2 (Nrf2-a transcription factor that induce the expression of detoxification, anti-oxidation, immune modulation and NLRP3 inflammasome).^174-176 Sulforaphane is the most studied of this type of compound. ¹⁷⁷ There are many preclinical studies on this substance and its mechanisms of action (antioxidant, anti-inflammatory), both in tumors and in neurodegenerative diseases. There are many clinical trials in the field of oncology, but only one in Alzheimer’s (Clinical Trial NCT04213391 Effects of Sulforaphane in Patients with Prodromal to Mild Alzheimer’s Disease. Recruiting phase) (Table 1). However, it is widely used as a supplement in the diet of Alzheimer patients. Chronic inflammasome formation and progression propagates cycles of inflammation mediated by microgliosis. Inflammasomes are protein complexes that can be triggered by pathogens or by alterations to the proteins associated with neurons in neurodegenerative diseases^178,179 being the NLRP3 inflammasome the most characterised in AD. The activation of the NLRP3 inflammasome causes the generation of caspase-1-mediated interleukin (IL)-1β and IL-18 in microglia cells and is considered a crucial therapeutic molecular target for regulating AD neuroinflammation. ¹⁸⁰

Control of neurotoxic microglia reactivity will be absolutely necessary in specific phases of neurodegeneration. ¹⁸¹ Nevertheless, more research is required to determine how these pathological subsets of microglia and/or neuroactive mechanisms can be controlled and used for therapeutic purposes.

• (G) Research aimed at finding drugs to shift microglia from a pro-inflammatory (‘M1-like’) to an alternatively activated (‘M2-like’) phenotype are being investigated in many laboratories around the world. Many problems, including the inability of the substances investigated to cross the BBB, have prevented a beneficial effect on AD. Recently, PPARα and PPARγ (Peroxisome proliferator-activated receptors are nuclear receptors that function as ligand-activated transcription factors) agonists have both been shown to shift microglia from a pro-inflammatory (‘M1-like’) to an alternatively activated (‘M2-like’) phenotype. Such strategies have been explored in clinical trials for neurological diseases, such as Alzheimer’s disease. Compounds as pioglitazone or D (dendrimer)-tesaglitazar (a dendrimer-PPARα/γ dual agonist conjugate to pass BBB), as well as extracts of a variety of rice seem to significantly decrease the expression of NF-κβ and the pro-inflammatory microglial marker (CD45) in parallel with increasing the expression of the anti-inflammatory microglial and phagocytic markers (arginase1, CD163, and CD36)^137,182

• (H) In recent years, many studies have addressed the possibility of blocking receptors that trigger the microglial pro-inflammatory response^146,147 using antibody therapies or specific drugs. ¹⁸³ In this regard, substances like BLZ945 inhibit the colony-stimulating factor 1 receptor ¹⁸³ and it may also be possible to use other novel antibodies that block pro-inflammatory substances. ¹³⁹

• (I) Closely related to the therapeutic focus of the previous section of blocking receptors for microglial neurotoxic activators, is the search for substances that directly block these neurotoxic activators. Different highly active in vitro and in vivo antibodies that block proinflammatory cytokines are known (for example, canakinumad, an anti-interleukin 1 beta ¹⁸⁴ but do not penetrate the brain parenchyma. Food and Drug Administration (USA) approved biologic (monoclonal antibodies) TNF-α inhibitors (anti-TNF includes infliximab, golimumab, etanercept, certolizumab, and adalimumab) for clinical studies on potential treatment for AD (Supplementary Data 5). These antibodies do not cross the blood-brain barrier, but they have shown (after peri spinal or Intrathecal administration) cognitive improvement and change in blood and CSF Aβ and tau in humans,^185-187 (Supplementary Data 5) as well as (after Intracerebroventricular injection) reduction in Aβ pathology, tau phosphorylation, and cognitive deficits in AD mouse models.^188-191 Studies to achieve a transfer of antibodies through the BBB are being carried out in many laboratories. ¹⁸⁷

• (J) Perhaps one of the most important tools in the fight against AD is the activation of microglial cells to phagocytose amyloid and produce an anti-inflammatory reaction via correction of genetically abnormal TREM2 (triggering receptor expressed on myeloid cells 2) or recovery of the loss-of-function that occurs in this receptor in aging or in the development of degenerative diseases such as AD. TREM2 is an innate immune receptor expressed in different populations of myeloid cells including microglia in CNS.^140,192,193 It consists of an extracellular immunoglobin domain, a transmembrane domain, an adaptor protein TYRO protein tyrosine kinase-binding protein (TYROBP, also known as DAP12) and a cytoplasmic tail.^193,194 TREM2 binds in vitro to anionic carbohydrates, bacterial products lipopolysaccharides, phospholipids,^192,195 and ApoE. ¹⁹⁶ TREM2 is recognized to have anti-inflammatory properties on macrophages activated in vitro.^140,192,197 Activation of TREM2 is essential for the transition of homeostatic microglia to a disease-associated microglial state fighting against involutive changes (although in the long term they can be neurotoxic).^140,192 Using an anti-TREM2-specific agonistic antibody, Hyb87, has been identified 300 upregulated and 251 downregulated expressions of microglial genes. ¹⁹⁸ In cells activated via TREM2, phagocytosis and lipid metabolism are increased. TREM2 variants (or in the signaling adapter DAP12) as well as loss-of-function of this receptor, have been linked to different types of neurodegenerative disorders such as AD.^199,200 Moreover, it has been observed that TREM2 activation decreases with AD progression. ¹⁹⁸ TREM2 activation was lower in AD microglia than in microglia from healthy subjects or patients with mild cognitive impairment. ¹⁹⁸ TREM2 activation in AD may lead to anti-apoptotic signaling, immune response, and cytoskeletal changes in the microglia, supporting a protective role of TREM2 activation in AD. ¹⁹⁸ Anti-TREM2-specific agonistic antibodies, such Hyb87, 4D9, ²⁰¹ AL002 ²⁰² (the most advanced from Alector now in phase II clinical trial Supplemental Data 6) may become the most effective therapeutic agents against AD.

Cell and molecular gliotherapies

New techniques that take advantage of different cell and molecular approaches have opened avenues to prevent or combat many diseases in different clinical areas. Preclinical studies have indicated that these approaches may offer promise in terms of future AD treatments. However, to date no therapeutic protocols have been approved to treat AD or other such neurodegenerative diseases, even though data on potentially useful therapies have emerged from seven Clinical Trials to date, and the body of encouraging preclinical data continues to increase.

Glial exosome therapy

In recent years, a novel system of intercellular communication mediated by substances like microvesicles (“exosomes”) has been shown to produce responses in the cells that recognize and capture these structures. In the case of microglia, exosomes released by stem cells have already been seen to attenuate CNS inflammation and demyelination in some models of neurodegenerative diseases.^203-205 Indeed, exosome treatment can produce a significant increase in anti-inflammatory cytokines (IL-10, TGF-β) and a decrease in pro-inflammatory mediators (TNF-α and IL-12). Increased microglial exosomal miR-124-3p alleviates neurodegeneration and Improves Cognitive Outcome. ²⁰⁶

Glial cell implants/grafts

Many of the possible pharmacological therapies for AD (aging or other neurodegenerative diseases) are based on the use of mediators/molecules that activate or inhibit neuroglia, inducing neuroprotective or neuroadaptive changes to compensate for the neuronal damage/involution that occurs. However, the failure of these therapies may be associated with the adequate application of these “neuropharmaceuticals,” either because they do not cross the blood-brain barrier or because they produce neurotoxic responses. The alternative of implanting progenitor or embryonic stem cells that can produce the desired effects/substances is an approach that has tremendous potential and that has been under study for several years. Although the main Drug Agencies (FDA, EMA, etc.) have yet to approve any such protocols, the results of preclinical and some initial clinical studies indicate that there is considerable therapeutic promise in these strategies. ²⁰⁷

The greatest success with cell therapy in the CNS has been achieved through the implantation of neuroglial cells (astrocytes, oligodendrocytes or microglia cells) or their precursors (astroblasts, pro-oligodendrocytes or cells of the mesodermal lineage that can mimic the effects of the microglia).^69,208-215 All these cells can produce a wide range of neuroprotective or neuroreparative substances that recover damaged neurons, and that can maintain the functionality of neuronal circuits by promoting new synapse formation, combating oxidative stress or normalizing the cellular environment that has become neurotoxic. ⁶⁹ The main problem in their use is that these implanted cells do not undergo a neuroinflammatory transformation (toxic gliosis) and as such, instead of repairing neurons and neuronal pathways they could favor the deterioration of neuronal centers and circuits. This possibility has been well documented, as has the genesis of glioblastomas. Systems biology approaches should identify molecular signatures and cell subtypes of interest to treat AD, while additional steps and precision medicine applications will need to be tested in cohort studies and Clinical Trials ²¹⁶ (Table 1; Supplemental Data 3 and 4).

Many of the implant studies with neuroglial cells or their precursors provide beneficial results that when later analyzed in more detail, indicate that they are not actually direct effect of the implants but rather, the benefits derived from the reaction to the exogenous tissue (neurons and/or neuroglial cells) or to the presence of new cells that do not form part of their functional structure, whether physiological or pathological. That is, the implanted cells are “bystanders” and they do not seem to take an active part in, or at least they are not primarily responsible for, the cellular or molecular changes manifested after implantation. ²¹⁷ This presents serious problems when interpreting the results of such manipulations. To avoid pathogenic phenomena, many of these cell lines must be reprogrammed to prevent them from inducing pro-inflammatory gliosis, often abolishing both the expression of receptors for substances inducing toxic/pro-inflammatory reactions and the expression of pro-inflammatory substances. Once these cells are implanted and they produce neuroreparative/neuroprotective substances continually, they offer the advantage of a long-lasting “drug therapy,” avoiding the drawbacks of classical drug therapy. Implantation of these cells may also require additional pharmacological therapy to maintain their normal function, either by activating neuroprotective secretory products or inhibitors that impede the production of pro-inflammatory toxins.^218-220

Preclinical studies have shown that cell therapy of this type could be used to dampen neurodegenerative astrogliosis and microgliosis, and to restore normal neuroglial functions/effects.^209,221-224 Indeed, both mesenchymal stem cells^211,212,217 and adipose-derived stem cells^223,224 can control or prevent microglial pro-inflammatory reactions.

Astrocyte or astroblast implants

Induced pluripotent stem cell derived astroglia can be very important to normalize CNS homeostasis. ²⁰⁹ Reactive astrocytes can restore normal functions of neuronal and glial cells, although it is necessary to establish close control of these cells.^222,225 Studies published in animal models of AD have provided evidence of neuronal repair following the implantation of astrocytes/astroblasts that release neurotrophic factors like NGF, BDNF and IGF (Insulin-like growth factor). ²²⁶ In some cases, the results point to a positive regulation of oxidative stress and the normalization of glutamatergic neurotransmission (remember that the maintenance of homeostasis and collaboration in glutamatergic neurotransmission is a primary function of astrocytes).

Astrocyte “senescence” may be key to the development of neuronal involution in AD, in the same way as pro-inflammatory reactive astrogliosis or astroglial atrophy. A challenge for astroglial cell therapy would be to identify areas of astroglial pathology to try to normalize the elements that have become dysfunctional via astroglial implants, or to alter neuronal populations. Indeed, recent studies have pointed to the possibility of obtaining direct reprogramming of reactive glial cells into functional neurons in AD “in vivo.” ²²⁷

Oligodendrocyte implants or pro-oligodendroglial cells

In recent years, the study of oligodendroglial cells (and pro-oligodendrocytes) in the genesis and progression of AD has intensified.^121,228 Demyelination is a well-known phenomenon in AD that could well be the basis for cell therapy to protect long communication pathways in AD, although such studies are in the early stages of development. According to recent studies, pro-oligodendrocytes (NG2+ cells) seem to be of great importance in AD, yet their physiological and pathological functions are not well understood. ⁵⁴ Pro-oligodendrocytes can give rise to homeostatic changes or neuronal or glial alterations in different scenarios, or they can generate new olidodendroglial cells and astrocytes. It has been documented that their proliferation increases with the loss of mature oligodendrocytes, aiding the remyelination of axons with a depleted myelin sheath. However, they also proliferate with the appearance of amyloid plaques and dystrophic neurons. ²²⁹ These new proliferating pro-oligodendroglial cells can create glial cells or even new neurons in neuroreparative processes, as well as an attempt to eliminate amyloid plaques.^230-234 This phenomenon has been observed in humans and AD models, and it could form the basis for cell therapies. However, the new elements created may undergo a process of “senescence” that would contribute to the progression of AD. The pharmacological elimination of these senescent cells has recently been raised as an anti-AD therapy, and several laboratories are working on the “safe” use of pro-oligodendrocytes in such AD cell therapies.

Microglial cells and mesodermal precursor cell implants/grafts

Most of the cell therapy studies carried out in the CNS have been dedicated to the use of microglial cells or their mesodermal precursors (“stem cells”)^{208,209,211,213,235-238} (Table 1; Supplemental Data 3). These studies are based on the fact that resident microglial cells serve to eliminate cellular debris or toxic dysfunctional cells, and to renew synaptic connections in the normal CNS. However, the pro-inflammatory microgliosis of these cells cannot be forgotten, which may be the basis of neuronal repair in a scenario of small alterations, as well as the basis for the progression of neurodegenerative disorders like AD. ²³⁹

There is considerable evidence from animal models of AD that brain implants of mesenchymal cells improve cognition/behavior. Likewise, the activation of mesenchymal cells outside the brain seems to be of therapeutic interest given the constant “cross-talk” between extracerebral mesenchymal cells and brain microglia. ⁷² In all cases, a cerebral anti-inflammatory reaction is sought, simultaneously avoiding the pro-inflammatory response in the brain that can provoke different toxic microglial reactions. Important benefits have been noted with the use of these cells, although microglial/mesenchymal cell therapy protocols to treat AD have not yet been approved. At present, 23 Clinical Trials are ongoing to test such approaches (Supplementary Data).

Controlling the number and function of the different subtypes of microglia cells need be considered a priority objective in the fight against AD and other neurodegenerative diseases. ¹²⁸ To achieve this goal, all possible therapeutic approaches must be used together. We can conclude that microglial cells are keystones in AD. These cells have a high capacity for rapid renewal and for reactive neuroprotective and neurotoxic changes; microgliosis is very relevant in neurodegeneration and inflammasome related microglia is a cell element for neuroinflammation progression.^240,241

It should also be noted that radiotherapy (as used in individuals suffering from different cancers) can alter cognitive circuits and microglial reactions, with normal microglial function recovered upon tissue recovery. Could this represent a possible therapeutic strategy for AD? ²⁴² Would this recolonization by self-renewing microglia reestablish microglial homeostasis? ²⁴³

New gene neuroglial therapies

Various lines of research have focused on new gene therapies in order to revert the dysfunctions and altered gene expression in neurons and glial cells associated with the degenerative processes observed in AD^244-249 (Table 1; Supplemental Data 4). Many of the alterations observed are produced by unknown causes, genomic alterations or changes that modify the expression of genes in neurons or glial cells that provoke either insufficient or aberrant production of proteins and thereby alter the normal behavior of cells. Gene therapies attempt to recover the normal expression of these components by CNS cells, neurons or neuroglial cells, to restore their physiological activity (APP, APOE-2, NGF, etc.…).^250-252 Different approaches to gene therapy have been studied, the most effective being the use of neuronal or glial viral vectors carrying the genes that are deficient or abnormal in these cells. Some studies have shown beneficial effects after “infection” with neurotropic viruses that carry genes driving the production of neurotrophic factors by neurons or glial cells, inducing the activity of “normal” receptors in microglial cells to avoid the pro-inflammatory reaction. ²⁵³ All these studies are still in their initial phases but may represent interesting therapeutic avenues, in the near future. Other gene therapy approaches attempt to produce antibodies against β-amyloid. ²⁵⁴ Focal transgene expression (ie, in the proximity of amyloid plaques), has been investigated in mouse models of AD ²⁵⁵

Multimodal AD therapy includes gliotherapy

Many different approaches have been considered to fight against AD (neuronal defense/repair, gliotherapies, AD preventive approaches) (Figure 1). Pharmacological and no-pharmacological therapies have been proposed. In recent years, new cell and gene therapies have been introduced (Table 1; Supplemental Data 1 and 2). The best results in the fight against Alzheimer’s have to be achieved using all resources and all strategies in a joint and coordinated manner (multimodal therapy for AD). From the preventive measures that they advise to lead a healthy life, to the most modern cell and gene therapies that are approved by supranational drug regulatory agencies (FDA, EMA).

Figure 1.

Scheme on the various therapeutic possibilities in AD and the role of glyotherapy. The upper left image shows a representation of the adaptive interplay of normal neurons––neuroglial cells to maintain optimal function of neural circuits in normal scenario of the nervous tissue (adapted from Toledano et al ²⁵⁶ ). The upper right image shows a representation of the nervous tissue in AD after undergoing neuroinflammation and neurodegeneration processes. Multimodal AD therapies, against neuroinflammation and neurodegeneration, include: 1. Neuronal defense/repair; 2. Gliotherapy; and 3. AD prevention (pharmacological and non-pharmacological). There are several important lines of gliotherapy, some of them currently under study (pharmacological interventions, cell and gene therapies) that may be of paramount importance in the coming years. A (Astrocyte); Dn (Dystrophic neurites); HA (Hypertrophic astrocyte); HM (Hypertrophic microglial cell); HMf (Hypertrophic microglial cell, phagocytic subtype); M (Microglial cell); Npre (Presynaptic Neuron); Npost (postsynaptic neuron); NT (Neurofibrillary tangles); P (Amyloid plaques); R (Receptors for specific.

Different diets (mainly Mediterranean diet) are advised as prevention for AD as well healthy lifestyle of life ²⁵⁷ (Table 1; Supplemental Data 1 and 2). Lifelong supplementation of choline has been also proposed, but no effective results has been demonstrated.^258,259

Oxidative stress is considered to be one of the most important phenomena to combat neurodegenerative diseases.^{107,210-212,260-263} Oxidative stress not only induces and spreads neuroinflammatory astrogliosis/microgliosis, but it also damages neuronal activity. Different approaches to dampen oxidative stress are being developed, and they focus principally on diet, healthy lifestyles, antioxidants, etc., often used concurrently.^{68,143,264-267} However, some substances or new therapeutic approaches are considered more important elements in combatting AD progression. N-acetyl cysteine is a widely used clinically antioxidant/anti-inflammatory agent, although it produces limited benefits in human therapy. In recent years, different strategies have been evaluated to obtain greater control of microglia activation in order to diminish pro-inflammatory responses. For example, some studies into neurodegenerative diseases have focused on carriers of antioxidants that act on the mitochondria of activated microglia. ²⁶⁸ Accordingly, neutral hydroxyl-terminated polyadoamine (PAMAM) dendrimers appear to exert some control on pro-inflammatory microglial reactions, acting selectively on the mitochondria of these glial cells. ²⁶⁰ Mitochondrial regulation has been also indicated as a key element to prevent neurodegeneration in neurons and glial cells. ²⁷⁰ Microbiota regulation is also important to prevent or slow the progress of AD. ²⁶⁹

Conclusions

The demonstration that neuroglial cells are required for neurons to function correctly has given these cells a more prominent profile in clinical neurosciences. All glial cells are involved in global neuroprotection/neurorepair but similarly, they are implicated in neurodegeneration, particularly in pathological conditions and in association with CNS aging. Profound changes to glia occur during physiological aging of the brain, in AD and in neurodegenerative disorders. However, the biggest problem is to now differentiate which glial changes are beneficial and which are harmful to the brain.

Increasing our knowledge about the roles of neuroglial cells (astroglia, oligodendroglia and microglia), as well as their neuroprotective and neurodegenerative actions in AD and other neurodegenerative diseases, will enhance our possibilities of designing new therapeutic strategies. At present, it is not clear how to develop effective therapies that prevent or regulate disease progression, although many preclinical studies are underway. Different sets of neuroglial cells are likely to represent effective therapeutic targets. However, the therapeutic strategies adopted should simultaneously act on neurodegeneration and neuroprotection to achieve the desired goal. There are several important lines of gliotherapy currently under study and being tested in various laboratories and promising results in terms of combating AD have been generated through these. Experimental neurology produces great advances in the understanding and treatment of neurodegenerative diseases, especially in AD.

Oxidative stress is considered one of the most important phenomena to shut down neurodegenerative astrogliosis/microgliosis and neuroinflammation seems to be the main phenomenon to be controlled (as repeated in this review). Different approaches to dampen oxidative stress and neuroinflammation are being developed (focusing on diet, healthy lifestyles, antioxidants, regulators of the mitochondrial function, etc.) (Figure 1). Many preclinical studies are underway to develop effective therapies against AD by acting on neuroglial cells, often based on new immunological, cell or gene therapies. Thus, it is possible that new antioxidant drugs or regulators of operative signaling pathways specific to neuroglial cells can be incorporated, in the near future, into therapies against AD and other neurodegenerative diseases.

The fight against Alzheimer´s must be developed holistically, using all possible approaches. Multimodal AD therapies, against neuroinflammation and neurodegeneration, need to include different lines of action: (1) neuronal defense/repair; (2) Gliotherapy; and (3) AD prevention (pharmacological and non-pharmacological).

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Supplemental Material - The relationships between neuroglial and neuronal changes in Alzheimer’s disease, and the related controversies II: gliotherapies and multimodal therapy

Supplemental Material for The relationships between neuroglial and neuronal changes in Alzheimer’s disease, and the related controversies II: gliotherapies and multimodal therapy by Adolfo Toledano-Díaz, M Isabel Álvarez and Adolfo Toledano in Journal of Central Nervous System Disease

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Supplemental Material - The relationships between neuroglial and neuronal changes in Alzheimer’s disease, and related controversies

Supplemental Material for The relationships between neuroglial and neuronal changes in Alzheimer’s disease, and related controversies by Adolfo Toledano-Díaz, M Isabel Álvarez and Adolfo Toledano in Journal of Central Nervous System Disease

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Supplemental Material for The relationships between neuroglial and neuronal changes in Alzheimer’s disease, and related controversies by Adolfo Toledano-Díaz, M Isabel Álvarez and Adolfo Toledano in Journal of Central Nervous System Disease

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Supplemental Material - The relationships between neuroglial and neuronal changes in Alzheimer’s disease, and related controversies

Supplemental Material for The relationships between neuroglial and neuronal changes in Alzheimer’s disease, and related controversies by Adolfo Toledano-Díaz, M Isabel Álvarez and Adolfo Toledano in Journal of Central Nervous System Disease

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Supplemental Material - The relationships between neuroglial and neuronal changes in Alzheimer’s disease, and related controversies

Supplemental Material for The relationships between neuroglial and neuronal changes in Alzheimer’s disease, and related controversies by Adolfo Toledano-Díaz, M Isabel Álvarez and Adolfo Toledano in Journal of Central Nervous System Disease