Abstract
Li Z, Chen L, Fei F, Wang W, Yang L, Wang Y, Cheng H, Xu Y, Xu C, Wang S, Gu Y, Han F, Chen Z, Wang Y. Adv. Sci. 2024; 11: 2410927. doi: 10.1002/advs.202410927. Enriched environment (EE), characterized by multi-sensory stimulation, represents a non-invasive alternative for alleviating epileptic seizures. However, the mechanism by which EE exerts its therapeutic impact remains incompletely understood. Here, it is elucidated that EE mitigates seizure susceptibility through the augmentation of adult neurogenesis within the EC circuit. A substantial upregulation of adult hippocampal neurogenesis concomitant with a notable reduction in seizure susceptibility has been found following exposure to EE. EE-enhanced adult-born dentate granule cells (abDGCs) are functionally activated during seizure events. Importantly, the selective activation of abDGCs mimics the anti-seizure effects observed with EE, while their inhibition negates these effects. Further, whole-brain c-Fos mapping demonstrates increased activity in DG-projecting EC CaMKIIα+ neurons in response to EE. Crucially, EC CaMKIIα+ neurons exert bidirectional modulation over the proliferation and maturation of abDGCs that can activate local GABAergic interneurons; thus, they are essential components for the anti-seizure effects mediated by EE. Collectively, this study provides compelling evidence regarding the circuit mechanisms underlying the effects of EE treatment on epileptic seizures, shedding light on the involvement of the EC-DG circuit in augmenting the functionality of abDGCs. This may help for the translational application of EE for epilepsy management.
Commentary
Optimal seizure control with minimal side effects remains the primary goal of epilepsy treatment. Despite a rapid increase in the number of new antiseizure medications (ASMs) approved by the FDA over the past two decades, the proportion of patients with drug-refractory epilepsy has remained stubbornly high, at approximately 30%. 1 Even among individuals whose seizures are effectively managed, adverse effects related to ASMs—such as cognitive impairment, mood disturbances, and systemic toxicity—are frequently reported and can significantly impact quality of life. Thus, there remains an urgent need for more effective and better-tolerated therapeutic strategies to improve outcomes for people living with epilepsy. Emerging noninvasive approaches, such as environmental enrichment (EE), have shown considerable promise in reducing seizure susceptibility while producing few, if any, adverse effects. However, the precise mechanisms by which EE exerts its antiseizure effects remain poorly understood. Further research is needed to elucidate the underlying biological pathways and to determine how these findings can be translated into more effective clinical interventions.
This new animal study 2 focused on elucidating how EE reduces seizure susceptibility and severity by regulating hippocampal adult neurogenesis and the circuits involved in this process. The authors first demonstrated that EE upregulates adult hippocampal neurogenesis, which may contribute to its profound antiseizure effects, as measured using a fast pentylenetetrazole (PTZ) kindling paradigm. Leveraging adult-born dentate granule cells (abDGC) specific modulation tools, the authors further showed that both chemogenetic (hM3Dq-DREADD) and optogenetic (ChR2) activation of abDGC is sufficient to recapitulate the antiseizure effects observed with EE. Conversely, chemogenetic inhibition of abDGC using hM4Di-DREADD abolished the protective effects of EE. The authors also conducted a comprehensive study on the potential circuit mechanisms underlying the antiseizure effects of EE, employing a combination of techniques including immediate early gene expression mapping, retrograde tracing, bulk calcium imaging, chemogenetics, and optogenetics. They identified and validated both afferent [entorhinal cortex (EC) CaMKIIα+ neurons → abDGC] and efferent (abDGC → hilar GABAergic interneurons) pathways to and from the abDGCs that regulate their maturation and the antiseizure effects of EE. Altogether, this study provides a framework for understanding the circuit mechanisms underlying the antiseizure effects of EE, paving the way for advancing EE and other strategies targeting adult-born neuron modulation for seizure control.
Despite this advance, several limitations remain. Firstly, to study the critical circuit projections to and from abDGC in regulating its antiseizure effect, a more rigorous investigation is required through targeted clozapine N-oxide (CNO) microinfusion and light stimulation at the axon terminals. Though systemic CNO injection offers ease and flexibility, circuit-specific manipulation can be achieved through intracranial microinjections of CNO, for instance, directly into DG, where axon-selective chemogenetic receptors are expressed at the projected axon terminals of the EC, thereby isolating chemogenetic manipulation to particular circuit projections. Similarly, positioning and illuminating via the optic fiber at the axon terminals of a specific pathway from a complex circuit would offer direct evidence of its functional involvement. Secondly, precise temporal control of abDGC activity is essential for the EE-mediated antiseizure effects. The authors demonstrated that short-term inhibition of EC CaMKIIα+ neurons→abDGC projections abolished the effect of EE treatment on both adult neurogenesis and seizure resistance. However, the long-term consequences of disrupting the activities of these EC CaMKIIα+ neurons→abDGC projections remain unknown—an important gap, given the need for sustained seizure management. To chronically manipulate the activity of EC CaMKIIα+ neurons→abDGC projections while avoiding concerns regarding chemogenetic or optogenetic manipulation efficiencies, genetic ablation strategies could be employed. Specifically, Cre-dependent adeno-associated viruses expressing Caspase3 or diphtheria toxin A could be targeted to the EC, enabling selective ablation of CaMKIIα+ neurons and their projections to the DG.
An important next outlook is whether EE can exert antiepileptogenic or disease-modifying effects. The fast PTZ kindling paradigm employed in this study primarily evaluates seizure susceptibility or ictogenesis. However, in contrast to ictogenesis, the mechanisms underlying epileptogenesis are likely distinct and evolve dynamically over time. For example, in animal models of temporal lobe epilepsy (TLE), an initial brain insult triggers a transient surge of aberrant adult neurogenesis, followed by a gradual depletion of beneficial new neurons over several months. 3 Consistently, the characteristics of the abDGC, including proliferation, morphology, and functional integration, are also highly heterogeneous during different stages of epileptogenesis.4,5 At the acute phase, neural progenitor proliferation increases, with many of these newborn neurons migrating abnormally within the dentate gyrus and forming inappropriate synaptic connections. Growing evidence implicates these ectopic neurons in the formation of epileptic circuits. At the chronic phase, the prolonged over-proliferation ultimately depletes the neural stem cell (NSC) pool, leading to long-term reductions in neurogenesis. Animal studies showed that bilateral grafting of hippocampal NSCs into hippocampi after status epilepticus can reduce the frequency and severity of spontaneous recurrent seizures and preserve cognitive and mood function for prolonged periods after status epilepticus insult. 6 Notably, similar patterns—including aberrant neuronal migration and loss of neurogenesis—have been observed in the hippocampi of patients with TLE. 7 These dynamic shifts in adult neurogenesis are now recognized as key contributors to hippocampal dysfunction in epilepsy. Although EE holds promise for modulating neurogenesis and promoting network stability, careful consideration of the timing and direction of adult neurogenesis modulation will be crucial for harnessing its potential antiepileptogenic effects.
Another significant challenge for the clinical translation of EE is that it remains primarily a laboratory experimental approach, with many barriers hindering its application in clinical practice. There is a lack of knowledge of which specific components of EE (e.g., cognitive stimulation, exercise, and sensory novelty) or secondary consequences (e.g., sleep alteration) are responsible for its potential antiseizure effects. 8 For example, introducing EE during the light cycle inevitably disrupts normal sleep patterns in mice, which could, in turn, influence adult neurogenesis and, consequently, impact seizure susceptibility. Despite these concerns and challenges with EE, therapeutic approaches directly targeting hippocampal adult neurogenesis hold great translational potential, given that this process is a highly conserved function across mammalian species, including humans. Within the adult human brain, the DG is one of the few brain regions that harbor resident NSCs capable of sustaining neurogenesis until the 10th decade of human life, 9 rendering the therapies targeting adult neurogenesis a viable strategy for epilepsy patients across a wide age range. A particularly intriguing implication of this study is that seizure control could be achieved by regulating adult neurogenesis via multiple strategies beyond EE. 10 For example, adult neurogenesis can be enhanced by overexpressing human homologs of proneurogenic factors, such as NeuroD1 and Prox1, or by deleting antineurogenic factors, such as p21 and Dkk1, possibly through emerging gene therapies. Pharmacological agents offer another promising route. Antidepressants such as selective serotonin reuptake inhibitors, particularly fluoxetine, are already FDA-approved and known to enhance hippocampal neurogenesis. Moreover, noninvasive neuromodulation techniques, such as transcranial magnetic stimulation and transcranial direct current stimulation, which are commonly employed in epilepsy treatment, have been shown to activate pathways associated with neurogenesis. Lifestyle interventions also hold promise; exercise, continuous learning, and good sleep hygiene are increasingly recognized as modifiable factors that can promote adult neurogenesis and, in turn, potentially confer antiseizure benefits. Taken together, these diverse strategies highlight a growing recognition that enhancing hippocampal neurogenesis could represent a novel and multifaceted therapeutic approach to epilepsy management.
