Adult Neurogenesis Following Ischemic Stroke and Implications for Cell-Based Therapeutic Approaches

The advantages of multi-level omics research on stem cell-based therapies for ischemic stroke

Stem cell transplantation is a potential therapeutic strategy for ischemic stroke. However, despite many years of preclinical research, the application of stem cells is still limited to the clinical trial stage. Although stem cell therapy can be highly beneficial in promoting functional recovery, the precise mechanisms of action that are responsible for this effect have yet to be fully elucidated. Omics analysis provides us with a new perspective to investigate the physiological mechanisms and multiple functions of stem cells in ischemic stroke. Transcriptomic, proteomic, and metabolomic analyses have become important tools for discovering biomarkers and analyzing molecular changes under pathological conditions. Omics analysis could help us to identify new pathways mediated by stem cells for the treatment of ischemic stroke via stem cell therapy, thereby facilitating the translation of stem cell therapies into clinical use. In this review, we summarize the pathophysiology of ischemic stroke and discuss recent progress in the development of stem cell therapies for the treatment of ischemic stroke by applying multi-level omics. We also discuss changes in RNAs, proteins, and metabolites in the cerebral tissues and body fluids under stroke conditions and following stem cell treatment, and summarize the regulatory factors that play a key role in stem cell therapy. The exploration of stem cell therapy at the molecular level will facilitate the clinical application of stem cells and provide new treatment possibilities for the complete recovery of neurological function in patients with ischemic stroke.

Anti-LINGO-1 treatment restores myelination of corticospinal tract neurons and improves functional recovery after stroke

Demyelination of corticospinal tract neurons contributes to long-term disability after cortical stroke. Nonetheless, poststroke myelin loss has not been addressed as a therapeutic target, so far. We hypothesized that an antibody-mediated inhibition of the Nogo receptor-interacting protein (LINGO-1, leucine-rich repeat and immunoglobulin domain-containing Nogo receptor-interacting protein) may counteract myelin loss, enhance remyelination and axonal growth, and thus promote functional recovery following stroke. To verify this hypothesis, mice were subjected to photothrombotic stroke and received either an antibody against LINGO-1 (n = 19) or a control treatment (n = 18). Behavioral tests were performed to assess the effects of anti-LINGO-1 treatment on the functional recovery. Seven weeks after stroke, immunohistochemical analyses were performed to analyze the effect of anti-LINGO-1 treatment on myelination and axonal loss of corticospinal tract neurons, proliferation of oligodendrocytes and neurogenesis. Anti-LINGO-1 treatment resulted in significantly improved functional recovery (p < 0.0001, repeated measures analysis of variance), and increased neurogenesis in the hippocampus and subventricular zone of the ipsilateral hemisphere (p = 0.0094 and p = 0.032, t-test). Notably, we observed a significant increase in myelin (p = 0.0295, t-test), platelet-derived growth factor receptor α-positive oligodendrocyte precursor cells (p = 0.0356, t-test) and myelinating adenomatous polyposis coli-positive cells within the ipsilateral internal capsule of anti-LINGO-1-treated mice (p = 0.0021, t-test). In conclusion, we identified anti-LINGO-1 as the first neuroregenerative treatment that counteracts poststroke demyelination of corticospinal tract neurons, presumably by increased proliferation of myelin precursor cells, and thereby improves functional recovery. Most importantly, our study presents myelin loss as a novel therapeutic target following stroke.

Avoidance and escape conditioning adjust adult neurogenesis to conserve a fit hippocampus in adult male rodents

In this study, the connection between cognitive behaviors and the adult rodent hippocampus was investigated. Recording field potentials at performant pathway (PP)-hippocampal dentate gyrus (DG) synapses in transverse slices from the dorsal (d), intermediate (i), and ventral (v) hippocampus showed differences in paired-pulse responses and long-term potentiation in rats. The Barnes maze (BM) and passive avoidance (PA) tests indicated a decrease in escape latency and step-through latency in both rats and mice over training days. A decrease in the use of random or sequential strategy while an increase in the use of direct strategy to search for an escape box occurred in both groups. Evaluation of the levels of neurogenesis markers (Ki67 and BrdU/NeuN) by immunofluorescence assay in the dDG, iDG, and vDG revealed a long-axis disparity in the hippocampal dentate baseline cell proliferation and exposure to the BM and PA task changed the profile of baseline cell proliferation along the DG in both rats and mice. Also, these learning experiences changed the profile of BrdU+ /NeuN+ cells along the DG of rats. Quantitation of hippocampal BDNF protein levels using ELISA exhibited no changes in BDNF levels due to learning experiences in rats. We demonstrate that PP-DG synaptic efficacy and neurogenesis are organized along a gradient. Avoidance and escape conditioning themselves are sufficient to change and calibrate adult neurogenesis along the hippocampal long axis in rodents. Further research will be required to determine the precise mechanisms underlying the role of experience-derived neuroplasticity in cognitive function and decline.

Neuroprotective effect of Potentilla reptans L. root in the rat brain global ischemia/reperfusion model

Stroke is the most common cause of death among neurological diseases. The protective effects of Potentilla reptans L. include antioxidative, anti-inflammatory, and antiapoptotic effects. In this study, the brain protection and beta-amyloid effects of P. reptans root extract were investigated in the rat brain ischemia/reperfusion (IR) model. Forty male Wistar rats were randomly divided into five groups (n = 8), including IR, sham, and three groups receiving P. reptans with concentrations of 0.025, 0.05, and 0.1 (g/kg/b.w.), which were injected daily for 7 days. For the IR model, the common carotid artery was occluded bilaterally for 8 min. All injections were intraperitoneal (IP). The shuttle box test was used to measure passive avoidance memory. Then the brain tissue was extracted for the histological examination of neuron counts and β-amyloid plaques using a morphometric technique, and finally, Statistical Package for the Social Sciences software was used for statistical analysis of the data. Pretreatment with P. reptans improved memory impairment. Also, by examining the tissues of the CA1, CA3, and dentate gyrus areas of the hippocampus, it was observed that the number of plaques in the groups receiving P. reptans extract was reduced compared to the IR group, especially at the concentration of 0.05 g/kg/b.w. Also, P. reptans improved the number of neurons at all concentrations, in which the concentration of 0.05 g/kg/b.w. showed more effective therapeutic results. Taken together, we found that P. reptans root extract has beneficial effects on memory impairment, neuronal loss, and β-amyloid accumulation.

Pre-stroke exercise does not reduce atrophy in healthy young adult mice

Stroke is the main cause of acquired disability in adults. Exercise reduces the risk for stroke and protects against functional loss after stroke. An exercise-induced reduction in key risk factors probably contributes to the protective effect, but direct effects on the brain may also contribute to stroke protection. We previously reported that exercise increases angiogenesis and neurogenesis through activation of the lactate receptor HCA1. Here we exposed young adult wild-type mice and HCA1 knockout mice to interval exercise at high or medium intensity, or to intraperitoneal injections of L-lactate or saline for seven weeks before we induced experimental stroke by permanent occlusion of the distal medial cerebral artery (dMCA). The resulting cortical atrophy measured three weeks after stroke was unaffected by exercise or L-lactate pre-treatments, and independent of HCA1 activation. Our results suggest that the beneficial effect of exercise prior to stroke where no reperfusion occurs is limited in individuals who do not carry risk factors.

Adiponectin Promotes Neurogenesis After Transient Cerebral Ischemia Through STAT3 Mediated BDNF Upregulation in Astrocytes

Newborn neurons from the subventricular zone (SVZ) are essential to functional recovery following ischemic stroke. However, the number of newly generated neurons after stroke is far from enough to support a potent recovery. Adiponectin could increase neurogenesis in the dentate gyrus of hippocampus in neurodegenerative diseases. However, the effect of adiponectin on the neurogenesis from SVZ and the functional recovery after ischemic stroke was unknown, and the underlying mechanism was not specified either. The middle cerebral artery occlusion model of mice was adopted and adiponectin was administrated once a day from day 3 to 7 of reperfusion. The levels of BDNF and p-STAT3 were detected by western blotting on day 7 of reperfusion. The virus-encoded BDNF shRNA with GFAP promoter and a STAT3 inhibitor Stattic were used, respectively. Neurogenesis was evidenced by the expression of doublecortin and 5-bromo-2'-deoxyuridine (BrdU) labelling and brain atrophy was revealed by Nissl staining on day 28 of reperfusion. Neurological functional recovery was assessed by the adhesive removal test and the forepaw grip strength. We found that adiponectin increased both the doublecortin-positive cells and NeuN/BrdU double-positive cells around the injured area on day 28 of reperfusion, along with the improved long-term neurological recovery. Mechanistically, adiponectin increased the protein levels of p-STAT3 and BDNF in astrocytes on day 7 of reperfusion, while silencing BDNF diminished the adiponectin-induced neurogenesis and functional recovery. Moreover, inhibition of STAT3 not only prevented the increase of BDNF but also the improved neurogenesis and functional recovery after stroke. In conclusion, adiponectin enhances neurogenesis and functional recovery after ischemic stroke via STAT3/BDNF pathway in astrocytes.

Minor Changes for a Major Impact: A Review of Epigenetic Modifications in Cell-Based Therapies for Stroke

Epigenetic changes in stroke may revolutionize cell-based therapies aimed at reducing ischemic stroke risk and damage. Epigenetic changes are a novel therapeutic target due to their specificity and potential for reversal. Possible targets for epigenetic modification include DNA methylation and demethylation, post-translational histone modification, and the actions of non-coding RNAs such as microRNAs. Many of these epigenetic modifications have been reported to modulate atherosclerosis development and progression, ultimately contributing to stroke pathogenesis. Furthermore, epigenetics may play a major role in inflammatory responses following stroke. Stem cells for stroke have demonstrated safety in clinical trials for stroke and show therapeutic benefit in pre-clinical studies. The efficacy of these cell-based interventions may be amplified with adjunctive epigenetic modifications. This review advances the role of epigenetics in atherosclerosis and inflammation in the context of stroke, followed by a discussion on current stem cell studies modulating epigenetics to ameliorate stroke damage.

Neural Stem Cells Therapy for Ischemic Stroke: Progress and Challenges

Ischemic stroke, with its high morbidity and mortality, is the most common cerebrovascular accident and results in severe neurological deficits. Despite advances in medical and surgical intervention, post-stroke therapies remain scarce, which seriously affects the quality of life of patients. Over the past decades, stem cell transplantation has been recognized as very promising therapy for neurological diseases. Neural stem cell (NSC) transplantation is the optimal choice for ischemic stroke as NSCs inherently reside in the brain and can potentially differentiate into a variety of cell types within the central nervous system. Recent research has demonstrated that NSC transplantation can facilitate neural recovery after ischemic stroke, but the mechanisms still remain unclear, and basic/clinical studies of NSC transplantation for ischemic stroke have not yet been thoroughly elucidated. We thus, in this review, provide a futher understanding of the therapeutic role of NSCs for ischemic stroke, and evaluate their prospects for future application in clinical patients of ischemic stroke.

Interleukine-17 Modulates Neurogenesis and Behavior Following Exposure to Trauma in Mice

Post-traumatic stress disorder (PTSD) is a psychiatric disorder accompanied by deficits in cognitive and social skills. Adult hippocampal neurogenesis is a lifelong phenomenon, with new neurons being formed in the granular cell layer of the dentate gyrus. Impaired neurogenesis is associated with multiple behavioral disorders including Alzheimer's disease and schizophrenia. PTSD patients often present hippocampal atrophy and animal models clearly present impaired neurogenesis. Previous studies on PTSD patients demonstrated elevated levels of Th17 cells and plasma levels of the pro-inflammatory cytokine interleukin-17A (IL-17A). Since IL-17A can impair neurogenesis in mice, we thus hypothesized that decreasing the serum levels of IL-17A will increase hippocampal neurogenesis and alleviate symptoms in a murine model of PTSD. Surprisingly, our results showed that attempting to neutralize IL-17A with an antibody resulted in increased serum levels of IL-17A, while targeting IL-23, the upstream regulator of IL-17, did lower the levels of IL-17A in trauma-exposed mice. As expected, increased levels of serum IL-17A (in anti-IL-17A treated mice) resulted in impaired neurogenesis, reflected by reduced number of proliferating Ki67⁺ neural progenitors and newly formed DCX⁺ neurons, which was correlated with increased expression of Hes1. Nevertheless, increased maturation was noted by the expression of Slit2 and Ache. In contrast, treatment with anti-IL-23 indeed resulted in increased neurogenesis. Behaviorally, both treatments did not affect trauma-related freezing behavior but did affect trauma-related social deficits. Unexpectedly, increased levels of serum IL-17A (in anti-IL-17A treated mice) prevented social deficits in trauma-exposed mice while anti-IL-23 exacerbated these deficits. We thus conclude that IL-17 is involved in regulating neurogenesis following exposure to stress but may be important in maintaining social behavior.

New Insights Into the Role of Aberrant Hippocampal Neurogenesis in Epilepsy

Data accumulated over the past four decades have confirmed that adult hippocampal neurogenesis (HN) plays a key role in the wide spectrum of hippocampal pathology. Epilepsy is a disorder of the central nervous system characterized by spontaneous recurrent seizures. Although neurogenesis in persistent germinative zones is altered in the adult rodent models of epilepsy, the effects of seizure-induced neurogenesis in the epileptic brain, in terms of either a pathological or reparative role, are only beginning to be explored. In this review, we described the most recent advances in neurogenesis in epilepsy and outlooked future directions for neural stem cells (NSCs) and epilepsy-in-a-dish models. We proposed that it may help in refining the underlying molecular mechanisms of epilepsy and improving the therapies and precision medicine for patients with epilepsy.

Research advances in the role of endogenous neurogenesis on neonatal hypoxic-ischemic brain damage

Hypoxic-ischemic brain damage (HIBD) is the main cause of perinatal mortality and neurologic complications in neonates, but it remains difficult to cure due to scarce treatments and complex molecular mechanisms remaining incompletely explained. Recent, mounting evidence shows that endogenous neurogenesis can improve neonatal neurological dysfunction post-HIBD. However, the capacity for spontaneous endogenous neurogenesis is limited and insufficient for replacing neurons lost to brain damage. Therefore, it is of great clinical value and social significance to seek therapeutic techniques that promote endogenous neurogenesis, to reduce neonatal neurological dysfunction from HIBD. This review summarizes the known neuroprotective effects of, and treatments targeting, endogenous neurogenesis following neonatal HIBD, to provide available targets and directions and a theoretical basis for the treatment of neonatal neurological dysfunction from HIBD.

The Evolving Role of Induced Pluripotent Stem Cells and Cerebral Organoids in Treating and Modeling Neurosurgical Diseases

Over the past decade, the use of induced pluripotent stem cells (IPSCs), as both direct therapeutics and building blocks for 3D in vitro models, has exhibited exciting potential in both helping to elucidate pathogenic mechanisms and treating diseases relevant to neurosurgery. Transplantation of IPSCs is being studied in neurological injuries and diseases, such as spinal cord injury and Parkinson's disease, whose clinical manifestations stem from underlying neuronal and/or axonal degeneration. Both animal models and clinical trials have shown that IPSCs have the ability to regenerate damaged neural tissue. Such evidence makes IPSCs a potentially promising therapeutic modality for patients who suffer from these neurological injuries/diseases. In addition, the cerebral organoid, a 3D assembly of IPSC aggregates that develops heterogeneous brain regions, has become the first in vitro model to closely recapitulate the complexity of the brain extracellular matrix, a 3-dimensional network of molecules that structurally and biochemically support neighboring cells. Cerebral organoids have become an exciting prospect for modeling and testing drug susceptibility of brain tumors, such as glioblastoma and metastatic brain cancer. As patient-derived organoid models are becoming more faithful to the brain, they are becoming an increasingly accurate substitute for patient clinical trials; such patient-less trials would protect the patient from potentially ineffective drugs, and speed up trial results and optimize cost. In this review, we aim to describe the role of IPSCs and cerebral organoids in treating and modeling diseases that are relevant to neurosurgery.

Gastrodin promotes hippocampal neurogenesis via PDE9-cGMP-PKG pathway in mice following cerebral ischemia

Gastrodin, which is extracted from the Chinese herbal medicine Gastrodia elata Blume, can ameliorate neurogenesis after cerebral ischemia. However, it's possible underlying mechanisms remain still elusive. PDE9-cGMP-PKG signaling pathway is involved in the proliferation of neural stem cells (NSCs) after cerebral ischemia. In this study, we investigated whether the beneficial effect of gastrodin on hippocampal neurogenesis after cerebral ischemia is correlated with the PDE9-cGMP-PKG signaling pathway. Bilateral common carotid artery occlusion (BCCAO) in mice and oxygen-glucose deprivation/reoxygenation (OGD/R) in primary cultured hippocampal NSCs were used to mimic brain ischemic injury. The Morris water maze (MWM) test was executed to detect spatial learning and memory. Proliferation, differentiation, and mature neurons were examined using immunofluorescence. The survival and proliferation of NSCs were assessed by CCK-8 assay and BrdU immunofluorescence staining, respectively. ELISA and western blot were used to detect the level of the PDE9-cGMP-PKG signaling pathway. In BCCAO mice, administering gastrodin (50 and 100 mg/kg) for 14 d restored cognitive behaviors; meanwhile, neurogenesis in hippocampus was stimulated, and PDE9 was inhibited and cGMP-PKG was activated by gastrodin. Consistent with the results, administering gastrodin (from 0.01-1 μmol/L) for 48 h dose-dependently ameliorated the cell viability and promoted greatly the proliferation in primary hippocampal NSCs exposed to OGD/R. Gastrodin further decreased PDE9 activity and up-regulated cGMP-PKG level. KT5823, a PKG inhibitor, markedly abrogated the protective effects of gastrodin on OGD/R-injured NSCs, accompanied by the down-regulation of PKG protein expression, but had no effects on PDE9 activity and cGMP level. Gastrodin could accelerate hippocampal neurogenesis after cerebral ischemia, which is mediated, at least partly, by PDE9-cGMP-PKG signaling pathway.

Baicalin Inhibits NLRP3 Inflammasome Activity Via the AMPK Signaling Pathway to Alleviate Cerebral Ischemia-Reperfusion Injury

Baicalin has been reported to have ameliorative effects on nerve-induced hypoxic ischemia injury; however, its role in the NLRP3 inflammasome-dependent inflammatory response during cerebral ischemia-reperfusion remains unclear. To investigate the molecular mechanisms involved in baicalin alleviating cerebral ischemia-reperfusion injury, we investigated the AMPK signaling pathway which regulates NLRP3 inflammasome activity. SD rats were treated with baicalin at doses of 100 mg/kg and 200 mg/kg, respectively, after middle cerebral artery occlusion at 2 h and reperfusion for 24 h (MCAO/R). MCAO/R treatment significantly increased cerebral infarct volume, changed the ultrastructure of nerve cells, and activated the NLRP3 inflammasome, manifesting as significantly increased expression of NLRP3, ASC, cleaved caspase-1, IL-1β, and IL-18. Our results demonstrated that baicalin treatment effectively reversed these phenomena in a dose-dependent manner. Additionally, inhibition of NLRP3 expression was found to promote the neuroprotective effects of baicalin on cortical neurons. Furthermore, baicalin remarkably increased the expression of p-AMPK following oxygen glucose deprivation/reperfusion (OGD/R). The expression of the NLRP3 inflammasome was also increased when the AMPK pathway was blocked by compound C. Taken together, our findings reveal that baicalin reduces the activity of the NLRP3 inflammasome and consequently inhibits cerebral ischemia-reperfusion injury through activation of the AMPK signaling pathway.

The Role of NADPH Oxidase in Neuronal Death and Neurogenesis after Acute Neurological Disorders

Oxidative stress is a well-known common pathological process involved in mediating acute neurological injuries, such as stroke, traumatic brain injury, epilepsy, and hypoglycemia-related neuronal injury. However, effective therapeutic measures aimed at scavenging free reactive oxygen species have shown little success in clinical trials. Recent studies have revealed that NADPH oxidase, a membrane-bound enzyme complex that catalyzes the production of a superoxide free radical, is one of the major sources of cellular reactive oxygen species in acute neurological disorders. Furthermore, several studies, including our previous ones, have shown that the inhibition of NADPH oxidase can reduce subsequent neuronal injury in neurological disease. Moreover, maintaining appropriate levels of NADPH oxidase has also been shown to be associated with proper neurogenesis after neuronal injury. This review aims to present a comprehensive overview of the role of NADPH oxidase in neuronal death and neurogenesis in multiple acute neurological disorders and to explore potential pharmacological strategies targeting the NADPH-related oxidative stress pathways.

Prospects of Therapeutic Target and Directions for Ischemic Stroke

Stroke is a serious, adverse neurological event and the third leading cause of death and disability worldwide. Most strokes are caused by a block in cerebral blood flow, resulting in neurological deficits through the death of brain tissue. Recombinant tissue plasminogen activator (rt-PA) is currently the only immediate treatment medication for stroke. The goal of rt-PA administration is to reduce the thrombus and/or embolism via thrombolysis; however, the administration of rt-PA must occur within a very short therapeutic timeframe (3 h to 6 h) after symptom onset. Components of the pathological mechanisms involved in ischemic stroke can be used as potential biomarkers in current treatment. However, none are currently under investigation in clinical trials; thus, further studies investigating biomarkers are needed. After ischemic stroke, microglial cells can be activated and release inflammatory cytokines. These cytokines lead to severe neurotoxicity via the overactivation of microglia in prolonged and lasting insults such as stroke. Thus, the balanced regulation of microglial activation may be necessary for therapy. Stem cell therapy is a promising clinical treatment strategy for ischemic stroke. Stem cells can increase the functional recovery of damaged tissue after post-ischemic stroke through various mechanisms including the secretion of neurotrophic factors, immunomodulation, the stimulation of endogenous neurogenesis, and neovascularization. To investigate the use of stem cell therapy for neurological diseases in preclinical studies, however, it is important to develop imaging technologies that are able to evaluate disease progression and to “chase” (i.e., track or monitor) transplanted stem cells in recipients. Imaging technology development is rapidly advancing, and more sensitive techniques, such as the invasive and non-invasive multimodal techniques, are under development. Here, we summarize the potential risk factors and biomarker treatment strategies, stem cell-based therapy and emerging multimodal imaging techniques in the context of stroke. This current review provides a conceptual framework for considering the therapeutic targets and directions for the treatment of brain dysfunctions, with a particular focus on ischemic stroke.