Deviating from the well travelled path: Precursor cell migration in the pathological adult mammalian brain

Endogenous Brain Repair: Overriding intrinsic lineage determinates through injury-induced micro-environmental signals

Adult human neurogenesis has generated excitement over the last 2 decades with the idea that endogenous adult stem cells could act as a potential cell source for brain repair after injury. Indeed, many forms of experimentally induced brain injury including stroke and excitotoxic lesioning can promote proliferation from the subventricular zone and mobilise neuroblasts and oligodendrocyte progenitor cells to migrate through brain parenchyma to damaged regions. However the failure of neuroblasts to mature into appropriate neuronal subtypes for cell replacement has been an issue. Recent work by our group and others has indicated that micro-environmental signals released from areas of cell loss may be able to override intrinsic gene expression lineages and covert neuroblasts into oligodendrocyte progenitor cells. This commentary will discuss the enhanced fate plasticity of both adult neural progenitors and parenchymal NG2 cells after injury, and the importance of understanding brain-injury induced micro-environmental signals in the quest toward promoting endogenous regeneration after injury.

Suppression of MicroRNA let-7a Expression by Agmatine Regulates Neural Stem Cell Differentiation

PURPOSE

Neural stem cells (NSCs) effectively reverse some severe central nervous system (CNS) disorders, due to their ability to differentiate into neurons. Agmatine, a biogenic amine, has cellular protective effects and contributes to cellular proliferation and differentiation in the CNS. Recent studies have elucidated the function of microRNA let-7a (let-7a) as a regulator of cell differentiation with roles in regulating genes associated with CNS neurogenesis.

MATERIALS AND METHODS

This study aimed to investigate whether agmatine modulates the expression of crucial regulators of NSC differentiation including DCX, TLX, c-Myc, and ERK by controlling let-7a expression.

RESULTS

Our data suggest that high levels of let-7a promoted the expression of TLX and c-Myc, as well as repressed DCX and ERK expression. In addition, agmatine attenuated expression of TLX and increased expression of ERK by negatively regulating let-7a.

CONCLUSION

Our study therefore enhances the present understanding of the therapeutic potential of NSCs in CNS disorders.

The Effect of Pro-Neurogenic Gene Expression on Adult Subventricular Zone Precursor Cell Recruitment and Fate Determination After Excitotoxic Brain Injury

Despite the presence of on-going neurogenesis in the adult mammalian brain, neurons are generally not replaced after injury. Using a rodent model of excitotoxic cell loss and retroviral (RV) lineage tracing, we previously demonstrated transient recruitment of precursor cells from the subventricular zone (SVZ) into the lesioned striatum. In the current study we determined that these cells included migratory neuroblasts and oligodendrocyte precursor cells (OPC), with the predominant response from glial cells. We attempted to override this glial response by ectopic expression of the pro-neurogenic genes Pax6 or Dlx2 in the adult rat SVZ following quinolinic acid lesioning. RV-Dlx2 over-expression stimulated repair at a previously non-neurogenic time point by enhancing neuroblast recruitment and the percentage of cells that retained a neuronal fate within the lesioned area, compared to RV-GFP controls. RV-Pax6 expression was unsuccessful at inhibiting glial fate and intriguingly, increased OPC cell numbers with no change in neuronal recruitment. These findings suggest that gene choice is important when attempting to augment endogenous repair as the lesioned environment can overcome pro-neurogenic gene expression. Dlx2 over-expression however was able to partially overcome an anti-neuronal environment and therefore is a promising candidate for further study of striatal regeneration.

Inhibition of MAPK/ERK signaling blocks hippocampal neurogenesis and impairs cognitive performance in prenatally infected neonatal rats

Hippocampus endogenous neurogenesis has been postulated to play a favorable role in brain restoration after injury. However, the underlying molecular mechanisms have been insufficiently deciphered. Here we investigated the potential regulatory capacity of MAPK/ERK signaling on neurogenesis and the associated cognitive performance in prenatally infected neonatal rats. From our data, intrauterine infection could induce hippocampal neuronal apoptosis and promote endogenous repair by evoking neural stem cell proliferation and survival. We also found intrauterine infection could induce increased levels of p-ERK, p-CREB and BDNF, which might be responsible for the potential endogenous rescue system. Furthermore, inhibition of MAPK/ERK signaling could aggravate hippocampal neuronal apoptosis, decrease neurogenesis, and impair the offspring's cognitive performances and could also down-regulate the levels of p-ERK, p-CREB and BDNF. Our data strongly suggest that the activation of MAPK/ERK signaling may play a significant role in promoting survival of newly generated neural stem cells via an anti-apoptotic mechanism, which may be particularly important in endogenous neuroprotection associated with cognitive performance development in prenatally infected rats.

Transplantation dose alters the dynamics of human neural stem cell engraftment, proliferation and migration after spinal cord injury

The effect of transplantation dose on the spatiotemporal dynamics of human neural stem cell (hNSC) engraftment has not been quantitatively evaluated in the central nervous system. We investigated changes over time in engraftment/survival, proliferation, and migration of multipotent human central nervous system-derived neural stem cells (hCNS-SCns) transplanted at doses ranging from 10,000 to 500,000 cells in spinal cord injured immunodeficient mice. Transplant dose was inversely correlated with measures of donor cell proliferation at 2 weeks post-transplant (WPT) and dose-normalized engraftment at 16 WPT. Critically, mice receiving the highest cell dose exhibited an engraftment plateau, in which the total number of engrafted human cells never exceeded the initial dose. These data suggest that donor cell expansion was inversely regulated by target niche parameters and/or transplantation density. Investigation of the response of donor cells to the host microenvironment should be a key variable in defining target cell dose in pre-clinical models of CNS disease and injury.

Injury to the spinal cord niche alters the engraftment dynamics of human neural stem cells

The microenvironment is a critical mediator of stem cell survival, proliferation, migration, and differentiation. The majority of preclinical studies involving transplantation of neural stem cells (NSCs) into the CNS have focused on injured or degenerating microenvironments, leaving a dearth of information as to how NSCs differentially respond to intact versus damaged CNS. Furthermore, single, terminal histological endpoints predominate, providing limited insight into the spatiotemporal dynamics of NSC engraftment and migration. We investigated the early and long-term engraftment dynamics of human CNS stem cells propagated as neurospheres (hCNS-SCns) following transplantation into uninjured versus subacutely injured spinal cords of immunodeficient NOD-scid mice. We stereologically quantified engraftment, survival, proliferation, migration, and differentiation at 1, 7, 14, 28, and 98 days posttransplantation, and identified injury-dependent alterations. Notably, the injured microenvironment decreased hCNS-SCns survival, delayed and altered the location of proliferation, influenced both total and fate-specific migration, and promoted oligodendrocyte maturation.

Subventricular zone localized irradiation affects the generation of proliferating neural precursor cells and the migration of neuroblasts

Radiation therapy is a part of the standard treatment for brain tumor patients, often resulting in irreversible neuropsychological deficits. These deficits may be due to permanent damage to the neural stem cell (NSC) niche, damage to local neural progenitors, or neurotoxicity. Using a computed tomography-guided localized radiation technique, we studied the effects of radiation on NSC proliferation and neuroblast migration in the mouse brain. Localized irradiation of the subventricular zone (SVZ) eliminated the proliferating neural precursor cells and migrating neuroblasts. After irradiation, type B cells in the SVZ lacked the ability to generate migrating neuroblasts. Neuroblasts from the unirradiated posterior SVZ did not follow their normal migratory path through the irradiated anterior SVZ. Our results indicate that the migrating neuroblasts were not replenished, despite the presence of type B cells in the SVZ post-irradiation. This study provides novel insights into the effects of localized SVZ radiation on neurogenesis and cell migration that may potentially lead to the development of new radiotherapy strategies to minimize damage to NSCs and neuroblast migration.

Ciliary neurotrophic factor controls progenitor migration during remyelination in the adult rodent brain

Ciliary neurotrophic factor (CNTF) has been shown to be expressed after brain lesions and in particular after demyelination. Here, we addressed the role of this cytokine in the regulation of neural progenitor migration in the adult rodent brain. Using an acute model of demyelination, we show that CNTF is strongly re-expressed after lesion and is involved in the postlesional mobilization of endogenous progenitors that participate in the myelin regenerative process. We show that CNTF controls the migration of subventricular zone (SVZ)-derived neural progenitors toward the demyelinated corpus callosum. Furthermore, an ectopic source of CNTF in adult healthy brains changes SVZ-derived neural progenitors' migratory behavior that migrate toward the source by activation of the Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3) pathway. Using various in vitro assays (Boyden chambers, explants, and video time-lapse imaging), we demonstrate that CNTF controls the directed migration of SVZ-derived progenitors and oligodendrocyte precursors. Altogether, these results demonstrate that in addition to its neuroprotective activity and its role in progenitor survival and maturation, CNTF acts as a chemoattractant and participates in the recruitment of endogenous progenitors during myelin repair.

Neural stem cells: therapeutic potential for neurodegenerative diseases

INTRODUCTION

Neural stem cells (NSCs) from specific brain areas or developed from progenitors of different sources are of therapeutic potential for neurodegenerative diseases.

SOURCES OF DATA

Treatment strategies involve the (i) transplantation of exogenous NSCs; (ii) pharmacological modulations of endogenous NSCs and (iii) modulation of endogenous NSCs via the transplantation of exogenous NSCs.

AREAS OF AGREEMENT

There is a consensus about the therapeutic potential of transplanted NSCs. The ability of NSCs to home into areas of central nervous system injury allows their delivery by intravenous injection. There is also a general agreement about the neuroprotective mechanisms of NSCs involving a 'bystander effect'.

AREAS OF CONTROVERSY

Individual laboratories may be using phenotypically diverse NSCs, since these cells have been differentiated by a variety of neurotrophins and/or cultured on different ECM proteins, therefore differing in the expression of neuronal markers.

GROWING POINTS

Optimization of the dose, delivery route, timing of administration of NSCs, their interactions with the immune system and combination therapies in conjunction with tissue engineered neural prostheses are under investigation.

AREAS TIMELY FOR DEVELOPING RESEARCH

In-depth understanding of the biological properties of NSCs, including mechanisms of therapy, safety, efficacy and elimination from the organism. These areas are central for further use in cell therapy. CAUTIONARY NOTE: As long as critical safety and efficacy issues are not resolved, we need to be careful in translating NSC therapy from animal models to patients.