Adipose Tissue-Derived Stem Cells in Neural Regenerative Medicine

Abstract

Adipose tissue-derived stem cells (ADSCs) have two essential characteristics with regard to regenerative medicine: the convenient and efficient generation of large numbers of multipotent cells and in vitro proliferation without a loss of stemness. The implementation of clinical trials has prompted widespread concern regarding safety issues and has shifted research toward the therapeutic efficacy of stem cells in dealing with neural degeneration in cases such as stroke, amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease, Huntington's disease, cavernous nerve injury, and traumatic brain injury. Most existing studies have reported that cell therapies may be able to replenish lost cells and promote neuronal regeneration, protect neuronal survival, and play a role in overcoming permanent paralysis and loss of sensation and the recovery of neurological function. The mechanisms involved in determining therapeutic capacity remain largely unknown; however, this concept can still be classified in a methodical manner by citing current evidence. Possible mechanisms include the following: 1) the promotion of angiogenesis, 2) the induction of neuronal differentiation and neurogenesis, 3) reductions in reactive gliosis, 4) the inhibition of apoptosis, 5) the expression of neurotrophic factors, 6) immunomodulatory function, and 7) facilitating neuronal integration. In this study, several human clinical trials using ADSCs for neuronal disorders were investigated. It is suggested that ADSCs are one of the choices among various stem cells for translating into clinical application in the near future.

Adipose Tissue-Derived Stem Cells (ADSCs)

ADSCs are mesenchymal stem cells (MSCs) that can be isolated from the abdominal subcutaneous, intra-articular and visceral stations, and ectopic fat tissue using minimally invasive methods (4,14). Notable markers of ADSCs that have previously been associated with differentiation include cluster of differentiation 29 (CD29), CD44, CD73, CD90, and CD105 as well as human leukocyte antigen class I. Other distinguishing surface marker expression profiles of ADSCs are likely to include CD9⁺, CD10⁺, CD13⁺, CD49d⁺, CD49e⁺, CD54⁺, CD55⁺, CD71⁺, CD106⁺, CD146⁺, CD166⁺, and STRO-1⁺ (5,61). The differentiation of ADSCs suggests that they could be induced into a wide variety of tissue-specific lineages, including adipocytes, osteoblasts, chondrocytes, hepatocytes, myocytes, and epithelial and neuronal cells (4). A growing body of evidence has indicated the therapeutic capacity of ADSCs in dealing with a variety of issues associated with stroke, amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), autoimmune diseases, multiple sclerosis, polymyositis, dermatomyositis, and rheumatoid arthritis (15). In addition, ADSCs have undergone intensive scrutiny with regard to their safety, including karyotype analysis, telomerase activity, and cell proliferation (38,44). However, Yu et al. (58) indicated that MSCs could favor tumor growth in vivo; thus, it is necessary to test the long-term safety of ADSCs. Ra et al. (39) tested the toxicity of intravenous infusion of ADSCs in animals and humans, and the results showed no side effects and no evidence of tumor development. Thus, the safety, therapeutic potential, and clinical applicability of ADSCs place them among the most promising resources in the field of regenerative therapy.

Chronic Stroke

The syndromes of stroke include the loss of nerve function with sudden hemiplegia, aphasia, and numbness in hands and feet, most of which are caused by the blockage of blood vessels in parts of the brain (46,47). Stroke can be classified as either ischemic involving the loss of normal blood supply or hemorrhagic (commonly known as hyperemia), which is determined by the type of blood vessel injury in brain (22,41). Acute stroke is defined as neurological deficits of vascular origin occurring within a period of a few hours. In contrast, chronic stroke is normally associated with attacks lasting for a prolonged duration (51). Most current regimens used in the treatment of acute stroke provide only a partial restoration of the injured region. The results showed that the administration of ADSCs intracerebroventricularly (ICV) and intravenously (IV) into the injured region have potential therapeutic effects and tissue regeneration ability in rodent models (4,18,19). The application of ADSCs to the problem of chronic stroke involves finding a workable strategy with which to promote functional neuron regeneration.

In the chronic stroke region, the scattered blood vessels usually are not able to support nutrients for maintaining cell survival; thus, vascular endothelial growth factor (VEGF) might be a possible mechanism that protects neurons from cell death (11). The administration of ADSCs could induce neuronal differentiation and stimulate brain repair markers associated with neurogenesis to promote functional recovery from chronic stroke (6,18,43). Nonetheless, this could cause glial scarring at the infarct boundary in cases involving the excess proliferation or hypertrophy of glial cells, such as astrocytes, microglia, and oligodendrocytes (3,9). The application of ADSCs for the treatment of stroke is meant to reduce the generation of glial fibrillary acidic protein-positive cells, thereby preventing nerve fibrosis in the brain (56). In addition, neuronal recovery is usually accompanied by a reduction in brain injury-derived apoptosis as well as natural repair responses that are activated after brain injury (18). ADSCs also enhance the survival and/or differentiation of neuronal cells via neurotrophic factors (25,36). Another possible mechanism underlying the protective role of ADSCs may be through an immunomodulatory mechanism that displayed increased interleukin-10 and reduced tumor necrosis factor-α in middle cerebral artery occlusion rats (10). In summary, these results indicate that the administration of ADSCs in rodent stroke models can have significantly positive effects on neuronal function.

Als

ALS is a neuromuscular degenerative disease involving the death of lower and upper motor neurons (12). The primary clinical manifestation is a gradual weakening and atrophy of muscle tissue, resulting in progressive failure of the neuromuscular system and eventual death (7). The treatment modality remains an unsolved mystery, and no effective therapy is currently available for this disease (24). ADSC transplantation has been used in various diseases associated with neurological deficits; therefore, it may be a therapeutic agent for ALS. It has been reported that ADSCs can differentiate into neuron-like cells, and delivery of ADSCs through ICV or IV prior to the appearance of clinical symptoms has been shown to delay the onset of the disease and extend life spans in a superoxide dismutase 1 G93A-mutated (SOD1^G93A) ALS mouse model (25). Following transplantation, increased levels of neurotrophic factors, such as nerve growth factor, brain-derived neurotrophic factor, insulin-like growth factor, and VEGF, as well as decreased apoptotic cell death were detected in the spinal cord of the ALS mice (25). Another report showed similar outcomes with significant upregulation of glial cell-derived neurotrophic factor and basic fibroblast growth factor, indicating that ADSCs may play a role in neuroprotection as well as the mitigation of symptoms (32). These results from animal models provide valuable data to support future trials in ALS patients.

Parkinson's Disease

The motor symptoms of PD are due to programmed cell death of dopaminergic neurons in the substantia nigra, as well as interruptions in nerve conduction paths (16). The main cause of PD is mutations in the α-synuclein gene resulting in protein aggregation and the formation of Lewy bodies, Lewy neuritis, and cytotoxicity, which may induce spontaneous cell death (23). Previous studies have reported on the use of in vitro cell culture systems to test the ability of proteins secreted from ADSCs to recover damage caused by 6-hydroxydopamine (6-OHDA) (17). The results demonstrate that ADSC-conditioned media is able to recover 6-OHDA-induced reactive oxygen species and neurotoxicity in rat mesencephalic neurons and cerebellar granule neurons, and directly attenuate H₂O₂-induced neuronal death (17). In addition, ADSC-conditioned media has been shown to promote neurite regeneration in PC12 cells by increasing bone morphogenetic protein 2 and FGF2 (34). Moreover, ADSC implantation into hemiparkinsonian rhesus monkeys, generated by methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP), has been shown to have in vivo neuroprotective effects (60). Improved treatment protocols, such as an in vitro strategy for neuronal differentiation treated with LIM homeobox transcription factor 1a (LMX1A) and neurturin, have shown improvements in behavior and symptom amelioration in monkey models (60). In the future, ADSCs may provide a valuable resource in the transplantation of autologous cells for the treatment of PD.

Alzheimer's Disease

Although Alzheimer's disease is the most common type of dementia, little is known with regard to the symptoms of late onset Alzheimer's (50). Several opposing hypotheses have been forwarded to explain the cause of the dementia, including cholinergic (13), amyloid (28), tau (8), age-related myelin breakdown (2), and oxidative stress (48). Recent studies have reported that ADSCs are a promising new cell source for regenerative cures, capable of ameliorating the neuropathological deficits associated with Alzheimer's disease (31). Ma and his colleagues demonstrated the efficacy of intracerebral administration of ADSCs in the recovery of spatial learning/memory ability in amyloid precursor protein/presenilin 1 (APP/PS1) double transgenic mice (31). In addition, the IV administration of ADSCs has also been shown to have palliative effects on the symptoms of dementia and therapeutic effects in Alzheimer's disease mouse models (26). Perez-Gonzalez et al. investigated the issue of neurogenesis in the adult hippocampus (37), the results of which indicated an increase in the proliferation of neuronal precursors and survival of neurons in APP/PS1 double transgenic mice. In summary, the transplantation of ADSCs potentially has beneficial effects in the prevention of pathological deterioration associated with Alzheimer's disease.

Huntington's Disease

Huntington's is a rare, inherited, late onset neurodegenerative genetic disorder, characterized by defects in muscular coordination and cognitive decline (42). The pathology mechanisms include activated apoptotic signaling, abnormal glutamine function, the impairment of energy production, and altered gene expression, due to cellular toxicity caused by the misfolded Huntingtin mutant protein (35). The transplantation of ADSCs into R6/2 transgenic mice with Huntington's disease was shown to promote the secretion of multiple paracrine growth factors (21). The effect of the protective factors is to prevent apoptotic phenomena and recover behavioral deficiencies in animal models. An alternative method has been proposed from the same research group in which cell extracts were used, instead of the direct transplantation of ADSCs, for intraperitoneal injection (20). Those results demonstrated significant improvements in behavior and the rotarod test as well as reductions in striatal atrophy and abnormal Huntington aggregation, which may be regulated via the cyclic adenosine monophosphate-responsive element-binding proteinperoxisome proliferator-activated receptor g, coactivator 1 a pathway. Other animal models have been used to develop strategies to deal with Huntington diseases (33). According to the results of rodent models, ADSCs may provide a curative autologous cell-based therapy for Huntington's disease.

Cavernous Nerve Injury

The cavernous nerve is in line with the posterolateral aspects of the prostate, which in turn provides the majority of autonomic input to erectile tissue. Injuries resulting from prostatectomy complications typically result in erectile dysfunction (52,57). Recently, much research has sought to develop 1) neurotrophic and neuroprotective agents to restore erectile function and 2) intracavernosal or IV ADSC injections to regenerate cavernous nerves (1,30,54). Previous studies demonstrated that ADSCs secrete neurotrophic factors (which promote cavernous nerve regeneration) and differentiate into Schwann cells (which play pivotal roles in peripheral nerves). Furthermore, other researchers have shown that ADSCs have therapeutic potential to treat postprostatectomy erectile dysfunction (1,27,59). For example, You et al. found that intracavernosal injections of human ADSCs effectively restored penile erectile function in a rat model of cavernous nerve injury (57), and Xu et al. found that using ADSC-based microtissues to treat postprostatectomy erectile dysfunction yielded significant benefits (54). Nonetheless, human clinical trials are necessary to further investigate therapeutic effects and safety issues associated with ADSC treatments.

Traumatic Brain Injury

Traumatic brain injury (TBI), also known as intracranial injury, occurs when external trauma causes the loss of motor and cognitive functions in the brain (49). In recent studies, stem cell treatments have been shown to effectively restore physiological functions in animal models of TBI (45,53). Furthermore, findings suggest that ADSCs may present an attractive cell-based therapy for regenerative medicine and wound healing in TBI (55). Specifically, the authors showed that adult ADSCs can be induced to differentiate into functional endothelial progenitor cells, which have beneficial effects on cell therapy. Moreover, Tajiri et al. treated F344 rats with an IV injection of hADSCs, conditioned media, or a vehicle (unconditioned media) 3 h following TBI (49). In that study, rats showed significant amelioration of motor and cognitive functions as well as significant reduction in cortical damage and hippocampal cell loss. Findings from that study and others have advanced scientific understanding related to potential therapeutic effects of ADSCs in TBI and have provided pivotal guidance for future human clinical trials.

Future Research Expectations

ADSCs are applicable to the treatment of human neurodegenerative disorders, due in part to their availability and safety (15). Future development will require improvements in cell manipulation technology. Besides, the development of stem cell treatments will require a safe method of visual tracking in order to avoid the need for toxic imaging and invasive diagnostic procedures (29,40). A study with the tracking and visualization of cells or the pharmacokinetics of small molecule drugs, verifying the safety and therapeutic benefits of proposed experiments is crucial to gaining approval for human clinical trials. In addition, developing a sterile system that requires minimal upkeep for the generation of nucleated cells could have considerable clinical applicability. Furthermore, auxiliary methods could enhance neurogenesis from endogenous progenitors, while drugs or chemical compounds could be included in combinational cures (12,13). In summary, the success of these treatments in animal experiments suggests their efficacy should be further explored in future human clinical trials.