The synthetic NCAM mimetic peptide FGL mobilizes neural stem cells in vitro and in vivo

Transcranial-Direct-Current-Stimulation Accelerates Motor Recovery After Cortical Infarction in Mice: The Interplay of Structural Cellular Responses and Functional Recovery

BACKGROUND

Transcranial direct current stimulation (tDCS) promotes recovery after stroke in humans. The underlying mechanisms, however, remain to be elucidated. Animal models suggest tDCS effects on neuroinflammation, stem cell proliferation, neurogenesis, and neural plasticity.

OBJECTIVE

In a longitudinal study, we employed tDCS in the subacute and chronic phase after experimental focal cerebral ischemia in mice to explore the relationship between functional recovery and cellular processes.

METHODS

Mice received photothrombosis in the right motor cortex, verified by Magnetic Resonance Imaging. A composite neuroscore quantified subsequent functional deficits. Mice received tDCS daily: either 5 sessions from day 5 to 9, or 10 sessions with days 12 to 16 in addition. TDCS with anodal or cathodal polarity was compared to sham stimulation. Further imaging to assess proliferation and neuroinflammation was performed by immunohistochemistry at different time points and Positron Emission Tomography at the end of the observation time of 3 weeks.

RESULTS

Cathodal tDCS at 198 kC/m² (220 A/m²) between days 5 and 9 accelerated functional recovery, increased neurogenesis, decreased microglial activation, and mitigated CD16/32-expression associated with M1-phenotype. Anodal tDCS exerted similar effects on neurogenesis and microglial polarization but not on recovery of function or microglial activation. TDCS on days 12 to 16 after stroke did not induce any further effects, suggesting that the therapeutic time window was closed by then.

CONCLUSION

Overall, data suggest that non-invasive neuromodulation by tDCS impacts neurogenesis and microglial activation as critical cellular processes influencing functional recovery during the early phase of regeneration from focal cerebral ischemia.

Novel Balance Mechanism Participates in Stem Cell Therapy to Alleviate Neuropathology and Cognitive Impairment in Animal Models with Alzheimer's Disease

Stem cell therapy improves memory loss and cognitive deficits in animal models with Alzheimer's disease. The underlying mechanism remains to be determined, but it may involve the interaction of stem cells with hippocampal cells. The transplantation of stem cells alters the pathological state and establishes a novel balance based on multiple signaling pathways. The new balance mechanism is regulated by various autocrine and paracrine cytokines, including signal molecules that target (a) cell growth and death. Stem cell treatment stimulates neurogenesis and inhibits apoptosis, which is regulated by the crosstalk between apoptosis and autophagy-(b) Aβ and tau pathology. Aberrant Aβ plaques and neurofibrillary tau tangles are mitigated subsequent to stem cell intervention-(c) inflammation. Neuroinflammation in the lesion is relieved, which may be related to the microglial M1/M2 polarization-(d) immunoregulation. The transplanted stem cells modulate immune cells and shape the pathophysiological roles of immune-related genes such as TREM2, CR1, and CD33-(e) synaptogenesis. The functional reconstruction of synaptic connections can be promoted by stem cell therapy through multi-level signaling, such as autophagy, microglial activity, and remyelination. The regulation of new balance mechanism provides perspective and challenge for the treatment of Alzheimer's disease.

NCAM regulates temporal specification of neural progenitor cells via profilin2 during corticogenesis

The development of cerebral cortex requires spatially and temporally orchestrated proliferation, migration, and differentiation of neural progenitor cells (NPCs). The molecular mechanisms underlying cortical development are, however, not fully understood. The neural cell adhesion molecule (NCAM) has been suggested to play a role in corticogenesis. Here we show that NCAM is dynamically expressed in the developing cortex. NCAM expression in NPCs is highest in the neurogenic period and declines during the gliogenic period. In mice bearing an NPC-specific NCAM deletion, proliferation of NPCs is reduced, and production of cortical neurons is delayed, while formation of cortical glia is advanced. Mechanistically, NCAM enhances actin polymerization in NPCs by interacting with actin-associated protein profilin2. NCAM-dependent regulation of NPCs is blocked by mutations in the profilin2 binding site. Thus, NCAM plays an essential role in NPC proliferation and fate decision during cortical development by regulating profilin2-dependent actin polymerization.

Transcranial Current Stimulation Alters the Expression of Immune-Mediating Genes

Despite its extensive use in clinical studies, the molecular mechanisms underlying the effects of transcranial direct current stimulation (tDCS) remain to be elucidated. We previously described subacute effects of tDCS on immune- and stem cells in the rat brain. To investigate the more immediate effects of tDCS regulating those cellular responses, we treated rats with a single session of either anodal or cathodal tDCS, and analyzed the gene expression by microarray; sham-stimulated rats served as control. Anodal tDCS increased expression of several genes coding for the major histocompatibility complex I (MHC I), while cathodal tDCS increased the expression of the immunoregulatory protein osteopontin (OPN). We confirmed the effects of gene upregulation by immunohistochemistry at the protein level. Thus, our data show a novel mechanism for the actions of tDCS on immune- and inflammatory processes, providing a target for future therapeutic studies.

Substrate elasticity induces quiescence and promotes neurogenesis of primary neural stem cells-A biophysical in vitro model of the physiological cerebral milieu

In the brain, neural stem cells (NSC) are tightly regulated by external signals and biophysical cues mediated by the local microenvironment or "niche." In particular, the influence of tissue elasticity, known to fundamentally affect the function of various cell types in the body, on NSC remains poorly understood. We, accordingly, aimed to characterize the effects of elastic substrates on critical NSC functions. Primary rat NSC were grown as monolayers on polydimethylsiloxane- (PDMS-) based gels. PDMS-coated cell culture plates, simulating the physiological microenvironment of the living brain, were generated in various degrees of elasticity, ranging from 1 to 50 kPa; additionally, results were compared with regular glass plates as usually used in cell culture work. Survival of NSC on the PDMS-based substrates was unimpaired. The proliferation rate on 1 kPa PDMS decreased by 45% compared with stiffer PMDS substrates of 50 kPa (p < 0.05) whereas expression of cyclin-dependent kinase inhibitor 1B/p27Kip1 increased more than two fold (p < 0.01), suggesting NSC quiescence. NSC differentiation was accelerated on softer substrates and favored the generation of neurons (42% neurons on 1 kPa PDMS vs. 25% on 50 kPa PDMS; p < 0.05). Neurons generated on 1 kPa PDMS showed 29% longer neurites compared with those on stiffer PDMS substrates (p < 0.05), suggesting optimized neuronal maturation and an accelerated generation of neuronal networks. Data show that primary NSC are significantly affected by the mechanical properties of their microenvironment. Culturing NSC on a substrate of brain-like elasticity keeps them in their physiological, quiescent state and increases their neurogenic potential.

Neural cell adhesion molecule peptide mimetics modulate emotionality: pharmacokinetic and behavioral studies in rats and non-human primates

Recent evidence highlights the fibroblast growth factor (FGF) family in emotion modulation. Although ligands that activate FGF receptors have antidepressant and anxiolytic effects in animal models, FGF ligands have a broad range of actions both in the brain and the periphery. Therefore, identifying molecular partners that may function as allosteric modulators could offer new avenues for drug development. Since neural cell adhesion molecule (NCAM) activates FGF receptors, we asked whether peripherally administered NCAM peptide mimetics penetrate the brain and alter the behavior of standardized tests that have predictive validity for drug treatments of anxiety or depression. The NCAM peptide mimetic, plannexin, acutely increased and chronically decreased anxiety, but did not have antidepressant effects in rats. Another NCAM peptide mimetic, FGL_L, had acute anxiogenic effects and chronic antidepressant effects in rats. A related NCAM peptide mimetic, FGL_S, had antidepressant effects without modulating anxiety-like behavior, and these antidepressant effects were blocked by an AMPA receptor antagonist. Cisternal cerebrospinal fluid (CSF) levels of FGLs correlated with blood plasma levels in rats and non-human primates, and CSF-to-blood ratios of FGL_S were comparable in both species. Results indicate that NCAM peptide mimetics penetrate the brain and support the suggestion that FGL_S may be a candidate for further development as a novel treatment for major depressive disorder in humans.

Osteopontin Attenuates Secondary Neurodegeneration in the Thalamus after Experimental Stroke

Cortical cerebral ischemia elicits neuroinflammation as well as secondary neuronal degeneration in remote areas. Locally distinct and specific secondary neurodegeneration affecting thalamic nuclei connected to cortical areas highlights such processes. Osteopontin (OPN) is a cytokine-like glycoprotein that is excreted in high amounts after cerebral ischemia and exerts various immunomodulatory functions. We here examined putative protective effects of OPN in secondary thalamic degeneration. We subjected male Wistar rats to photothrombosis and subsequently injected OPN or placebo intracerebroventricularly. Immunohistochemical and fluorescence staining was used to detect the extent of neuronal degeneration and microglia activation. Ex vivo autoradiography with radiotracers available for human in vivo PET studies, i.e., CIS-4-[¹⁸F]Fluor-D-Proline (D-cis-[¹⁸F]FPRO), and [6-³H]thymidine ([³H]thymidine), confirmed degeneration and proliferation, respectively. We found secondary neurodegeneration in the thalamus characterized by microglial activation and neuronal loss. Neuronal loss was restricted to areas of microglial infiltration. Treatment with OPN significantly decreased neurodegeneration, inflammation and microglial proliferation. Microglia displayed morphological signs of activation without expressing markers of M1 or M2 polarization. D-CIS-[¹⁸F]FPRO-uptake mirrored attenuated degeneration in OPN-treated animals. Notably, [³H]thymidine and BrdU-staining revealed increased stem cell proliferation after treatment with OPN. The data suggest that OPN is able to ameliorate secondary neurodegeneration in thalamic nuclei. These effects can be visualized by radiotracers D-CIS-[¹⁸F]FPRO and [³H]thymidine, opening new vistas for translational studies. Graphical Abstract Intracerebroventricular injection of osteopontin attenuates thalamic degeneration after cortical ischemia (pink area). Disruption of thalamocortical connections (blue) and degeneration of thalamic nuclei (encircled) leads to microglia activation. Osteopontin protects from both neurodegeneration and microglia activation as assessed by histological analysis and autoradiograpic studies.

Bioluminescence imaging visualizes osteopontin-induced neurogenesis and neuroblast migration in the mouse brain after stroke

Background

Osteopontin (OPN), an acidic phosphoglycoprotein, is upregulated in the brain after cerebral ischemia. We previously reported that OPN supports migration, survival, and proliferation of neural stem cells (NSC) in primary cell culture, as well as their differentiation into neurons. We here analyzed the effects of OPN on neuroblasts in vivo in the context of cerebral ischemia.

Methods

Transgenic mice expressing luciferase under the control of the neuroblast-specific doublecortin (DCX)-promoter, allowing visualization of neuroblasts in vivo using bioluminescence imaging (BLI), were injected with OPN intracerebroventricularly while control mice were injected with vehicle buffer. To assess the effects of OPN after ischemia, additional mice were subjected to photothrombosis and injected with either OPN or vehicle.

Results

OPN enhanced the migration of neuroblasts both in the healthy brain and after ischemia, as quantified by BLI in vivo. Moreover, the number of neural progenitors was increased following OPN treatment, with the maximum effect on the second day after OPN injection into the healthy brain, and 14 days after OPN injection following ischemia. After ischemia, OPN quantitatively promoted the endogenous, ischemia-induced neuroblast expansion, and additionally recruited progenitors from the contralateral hemisphere.

Conclusions

Our results strongly suggest that OPN constitutes a promising substance for the targeted activation of neurogenesis in ischemic stroke.

The macrosphere model-an embolic stroke model for studying the pathophysiology of focal cerebral ischemia in a translational approach

The main challenge of stroke research is to translate promising experimental findings from the bench to the bedside. Many suggestions have been made how to achieve this goal, identifying the need for appropriate experimental animal models as one key issue. We here discuss the macrosphere model of focal cerebral ischemia in the rat, which closely resembles the pathophysiology of human stroke both in its acute and chronic phase. Key pathophysiological processes such as brain edema, cortical spreading depolarizations (CSD), neuroinflammation, and stem cell-mediated regeneration are observed in this stroke model, following characteristic temporo-spatial patterns. Non-invasive in vivo imaging allows studying the macrosphere model from the very onset of ischemia up to late remodeling processes in an intraindividual and longitudinal fashion. Such a design of pre-clinical stroke studies provides the basis for a successful translation into the clinic.

Osteopontin mediates survival, proliferation and migration of neural stem cells through the chemokine receptor CXCR4

Introduction

Osteopontin (OPN) is a phosphoglycoprotein with important roles in tissue homeostasis, wound healing, immune regulation, and stress responses. It is expressed constitutively in the brain and upregulated during neuroinflammatory responses; for example, after focal cerebral ischemia. To date, its effects on neural stem cells (NSC) remain to be elucidated and are, accordingly, the subject of this study.

Method

Primary fetal rat NSC were cultured as homogenous monolayers and treated with different concentrations of OPN. Fundamental properties of NSC were assessed following OPN exposure, including proliferative activity, survival under oxidative stress, migration, and differentiation potential. To elucidate a putative action of OPN via the CXC chemokine receptor type 4 (CXCR4), the latter was blocked with AMD3100. To investigate effects of OPN on endogenous NSC in vivo, recombinant OPN was injected into the brain of healthy adult rats as well as rats subjected to focal cerebral ischemia. Effects of OPN on NSC proliferation and neurogenesis in the subventricular zone were studied immunohistochemically.

Results

OPN dose-dependently increased the number of NSC in vitro. As hypothesized, this effect was mediated through CXCR4. The increase in NSC number was due to both enhanced cell proliferation and increased survival, and was confirmed in vivo. Additionally, OPN dose-dependently stimulated the migration of NSC via CXCR4. Moreover, in the presence of OPN, differentiation of NSC led to a significant increase in neurogenesis both in vitro as well as in vivo after cerebral ischemia.

Conclusion

Data show positive effects of OPN on survival, proliferation, migration, and neuronal differentiation of NSC. At least in part these effects were mediated via CXCR4. Results suggest that OPN is a promising substance for the targeted activation of NSC in future experimental therapies for neurological disorders such as stroke.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0098-x) contains supplementary material, which is available to authorized users.

In vivo imaging of endogenous neural stem cells in the adult brain

The discovery of endogenous neural stem cells (eNSCs) in the adult mammalian brain with their ability to self-renew and differentiate into functional neurons, astrocytes and oligodendrocytes has raised the hope for novel therapies of neurological diseases. Experimentally, those eNSCs can be mobilized in vivo, enhancing regeneration and accelerating functional recovery after, e.g., focal cerebral ischemia, thus constituting a most promising approach in stem cell research. In order to translate those current experimental approaches into a clinical setting in the future, non-invasive imaging methods are required to monitor eNSC activation in a longitudinal and intra-individual manner. As yet, imaging protocols to assess eNSC mobilization non-invasively in the live brain remain scarce, but considerable progress has been made in this field in recent years. This review summarizes and discusses the current imaging modalities suitable to monitor eNSCs in individual experimental animals over time, including optical imaging, magnetic resonance tomography and-spectroscopy, as well as positron emission tomography (PET). Special emphasis is put on the potential of each imaging method for a possible clinical translation, and on the specificity of the signal obtained. PET-imaging with the radiotracer 3’-deoxy-3’-[¹⁸F]fluoro-L-thymidine in particular constitutes a modality with excellent potential for clinical translation but low specificity; however, concomitant imaging of neuroinflammation is feasible and increases its specificity. The non-invasive imaging strategies presented here allow for the exploitation of novel treatment strategies based upon the regenerative potential of eNSCs, and will help to facilitate a translation into the clinical setting.