Identification of putative neuroblasts at the base of adult neurogenesis in the olfactory midbrain of the spiny lobster, Panulirus argus

Continuous neurogenesis persists during adulthood in the olfactory midbrain of decapod crustaceans, including spiny lobsters, Panulirus argus. This encompasses generation of projection and local interneurons, whose somata are in the lateral soma cluster (LC) and medial soma cluster (MC), respectively. Both neuronal types originate from immediate precursors labeled by a single injection of BrdU and located in a small proliferation zone within each cluster. The aim of this study was to identify neuroblasts as a source of the dividing cells by multiple injections of BrdU over 2 days. All animals receiving multiple injections had one or a few 'extra' BrdU-positive nuclei near the proliferation zones, and these nuclei were significantly larger than nuclei of neurons or BrdU-positive cells in the proliferation zones. Since the defining morphological feature of neuroblasts in preadult neurogenesis in arthropods is being larger than their progeny, these large extra BrdU-positive nuclei represent "putative adult neuroblasts." Multiple BrdU-injections revealed a clump of small cells enclosing the putative adult neuroblasts in LC and MC, and these cells shared morphological characteristics with newly identified putative glial cells in the soma clusters and perivascular cells in the walls of arterioles. These results on P. argus suggest that adult neurogenesis is based on one adult neuroblast per soma cluster, adult neurogenesis appears to be a continuation of embryonic and larval neurogenesis, and the newly identified clumps of cells surrounding the putative adult neuroblasts might provide them with specific microenvironments necessary for their unusual lifelong proliferative and self-renewal capacity.     

Neurogenesis in neonatal rat brain is regulated by peripheral injection of basic fibroblast growth factor (bFGF)

Many major diseases of human brain involve deficiencies of select neuronal populations. As one approach to repair, we examined regulation of neurogenesis directly in vivo, employing postnatal day 1 (P1) cerebellar cortex, which is composed primarily of granule neurons and dividing precursors. We focused on basic fibroblast growth factor (bFGF), which stimulates precursor mitosis in culture and which is highly expressed in cerebellum during neurogenesis. Subcutaneous injection of bFGF increased [³H]thymidine ([³H]dT) incorporation, a marker for DNA synthesis, by 50% in whole cerebellar homogenates, suggesting that peripherally administered factor altered ongoing neural proliferation. Further, assay of isolated granule precursors revealed a 4-fold increase in [³H]dT incorporation following in vivo bFGF treatment, indicating that granule neuroblasts were the major bFGF-responsive population. Morphologic analysis indicated that twice as many granule precursors were in S-phase of the mitotic cycle after peripheral bFGF. To determine whether other neurogenetic populations respond to peripheral bFGF, we examined additional brain regions in vivo. bFGF stimulated DNA synthesis by 68% in hippocampus, and by > 250% in pontine subventricular zone (SVZ). In contrast, incorporation was not altered in basal pons or cerebral cortex, regions in which neurogensis has already ceased. To define potential direct actions of peripherally administered factor, ¹²⁵I-bFGF was used to study distribution. Intact 18 kDa ¹²⁵I-bFGF was recovered from brain following peripheral injection, suggesting that the factor acted directly to stimulate mitosis in dividing neuroblasts. The stimulation of neuronal proliferation by exogenous bFGF suggests that the factor normally regulates neurogenesis, and provides new therapeutic approaches to promote functional recovery from nervous system diseases. © 1996 Wiley-Liss, Inc.     

Increased subventricular zone-derived cortical neurogenesis after ischemic lesion

In adult rodents stroke enhances neurogenesis resulting in the addition of neurons to forebrain regions such as striatum or cortex where postnatal neurogenesis under normal conditions plays a negligible role. In the cortex, new neurons are generated either from local cortical precursors that are activated by stroke or from precursors residing in the subventricular zone (SVZ) of lateral ventricles that under normal conditions supply neuroblasts by and large only for the olfactory bulb. In this study we used 5HT3A-EGFP transgenic mice in which all neuroblasts originating in the SVZ are EGFP-labeled. We induced stroke in these mice and by combination of EGFP detection with BrdU injections we labeled all post-stroke-generated SVZ-derived neuroblasts. We showed an increase in SVZ-derived neuroblasts 14 and 35 days after stroke in the ipsilateral hemisphere. Post-stroke-generated SVZ-derived neuroblasts migrated to the cortex and survived for at least 35 days representing 2% of BrdU-positive cells in peri-infarct area where they differentiate into mature neurons. Thus, stroke enhances SVZ neurogenesis and attracts newborn neurons to the injury zone.     

Rat forebrain neurogenesis and striatal neuron replacement after focal stroke

The persistence of neurogenesis in the forebrain subventricular zone (SVZ) of adult mammals suggests that the mature brain maintains the potential for neuronal replacement after injury. We examined whether focal ischemic injury in adult rat would increase SVZ neurogenesis and direct migration and neuronal differentiation of endogenous precursors in damaged regions. Focal stroke was induced in adult rats by 90-minute right middle cerebral artery occlusion (tMCAO). Cell proliferation and neurogenesis were assessed with bromodeoxyuridine (BrdU) labeling and immunostaining for cell type-specific markers. Brains examined 10-21 days after stroke showed markedly increased SVZ neurogenesis and chains of neuroblasts extending from the SVZ to the peri-infarct striatum. Many BrdU-labeled cells persisted in the striatum and cortex adjacent to infarcts, but at 35 days after tMCAO only BrdU-labeled cells in the neostriatum expressed neuronal markers. Newly generated cells in the injured neostriatum expressed markers of medium spiny neurons, which characterize most neostriatal neurons lost after tMCAO. These findings indicate that focal ischemic injury increases SVZ neurogenesis and directs neuroblast migration to sites of damage. Moreover, neuroblasts in the injured neostriatum appear to differentiate into a region-appropriate phenotype, which suggests that the mature brain is capable of replacing some neurons lost after ischemic injury.     

Distinct stages of adult hippocampal neurogenesis are regulated by running and the running environment

Hippocampal neurogenesis continues into adulthood in mammalian vertebrates, and in experimental rodent models it is powerfully stimulated by exposure to a voluntary running wheel. In this study, we demonstrate that exposure to a running wheel environment, in the absence of running, is sufficient to regulate specific aspects of hippocampal neurogenesis. Adult mice were provided with standard housing, housing enriched with a running wheel or housing enriched with a locked wheel (i.e., an environment comparable to that of running animals, without the possibility of engaging in running). We found that mice in the running wheel and locked wheel groups exhibited equivalent increases in proliferation within the neurogenic niche of the dentate gyrus; this included comparable increases in the proliferation of radial glia‐like stem cells and the number of proliferating neuroblasts. However, only running animals displayed increased numbers of postmitotic neuroblasts and mature neurons. These results demonstrate that the running wheel environment itself is sufficient for promoting proliferation of early lineage hippocampal precursors, while running per se enables newly generated neuroblasts to survive and mature into functional hippocampal neurons. Thus, both running‐independent and running‐dependent stimuli are integral to running wheel‐induced hippocampal neurogenesis. © 2010 Wiley Periodicals, Inc.     

Mitotic neuroblasts determine neuritic patterning of progeny

Neuronal precursor proliferation and axodendritic outgrowth have been regarded as strictly sequential, with process formation presumably beginning after mitotic activity ceases. We now report that sympathetic precursors in vitro often elaborate long neurites before dividing. Of 437 neuroblasts observed in 48 time-lapse recordings, 42 neuroblasts divided. Thirty (71%) of these mitotic neuroblasts had neurites prior to cytokinesis. “Paramitotic” neurites were found to contain microtubules (MTs), indicating that precursors elaborate neuritic cytoskeleton during proliferation. Remarkably, the precise neuritic pattern exhibited by parental neuroblasts was consistently reproduced by daughter cell pairs. Preservation of neuritic morphology occurred through asymmetric division, with individual neurites allocated to specific daughter cells. Paramitotic neurites either remained intact throughout mitosis (12 of 65), or “retracted” into the soma during prophase and then “regrew” within minutes after cytokinesis (53 of 65). “Retraction” and “regrowth” involved resorption of cytoplasm into the soma, then refilling of residual cell membrane, resulting in recapitulation of the parental neurite pattern. Paramitotic neuritogenesis appears to be intrinsically driven, but is responsive to environmental signals. The culture substrate influenced neurite length, but not the response of paramitotic neurites during mitosis or the preservation of neuritic morphology. However, the incidence of neurite-bearing neuroblasts increased from 38 ± 1.3% to 94 ± 1.1% with growth factor treatment. The surprisingly high incidence of paramitotic neurites and the fidelity with which patterning was conserved across cell generations raise the possibility that mitotic precursors engage in pathfinding. Our studies suggest a novel link between neurogenesis and cytoarchitectonic patterning. © 1996 Wiley-Liss, Inc.     

Effects of lesions of the optic nerve, optic tectum and nervus terminalis on rod precursor proliferation in the goldfish retina.

Teleost retinas grow throughout life by proliferation of neuroblasts at the retinal margin and dedicated rod precursors in the outer nuclear layer. Mechanisms regulating this proliferation are largely unknown. Previous investigators observed that rod precursor replication, as detected by incorporation of radioactive thymidine into cells of the outer nuclear layer, is enhanced after optic nerve crush. We attempted to determine whether this was due to severing of the retinopetal (nervus terminalis, n.t.) or retinofugal (retinal ganglion cell) axons in the optic nerve of the goldfish, Carassius auratus. In the first series of experiments, we ablated unilaterally the optic nerve, olfactory bulb (containing n.t. ganglia), or optic tectum (containing retinal ganglion cell axons and n.t. collaterals). Rod precursor proliferation increased dramatically in both retinas as soon as 5 days after surgery; in addition, the numbers of dividing cells were greater in the ipsilateral retina 10-15 days after optic nerve crush or tectal ablation and in the contralateral retina 20-25 days after olfactory bulb ablation. These observations are not accounted for by the known projections of retinal ganglion cells, but are consistent with the projections of the n.t. In the second series of experiments, n.t. projections to the brain and retina were severed bilaterally 7-8 weeks before the unilateral optic nerve crush or hemitectal ablation. Rod precursor proliferation increased as before, but the quantities of dividing cells were always equal in both retinas. We conclude that the n.t. may modulate rod proliferation locally and that injury to (some) brain regions may cause release of mitogens that affect rod precursors in both retinas.     

Differential vascular permeability along the forebrain ventricular neurogenic niche in the adult murine brain

Adult neurogenesis is influenced by blood‐borne factors. In this context, greater or lesser vascular permeability along neurogenic niches would expose differentially neural stem cells (NSCs), transit amplifying cells (TACs), and neuroblasts to such factors. Here we evaluate endothelial cell morphology and vascular permeability along the forebrain neurogenic niche in the adult brain. Our results confirm that the subventricular zone (SVZ) contains highly permeable, discontinuous blood vessels, some of which allow the extravasation of molecules larger than those previously reported. In contrast, the rostral migratory stream (RMS) and the olfactory bulb core (OBc) display mostly impermeable, continuous blood vessels. These results imply that NSCs, TACs, and neuroblasts located within the SVZ are exposed more readily to blood‐borne molecules, including those with very high molecular weights, than those positioned along the RMS and the OBc, subregions in which every stage of neurogenesis also takes place. These observations suggest that the existence of specialized vascular niches is not a precondition for neurogenesis to occur; specialized vascular beds might be essential for keeping high rates of proliferation and/or differential differentiation of neural precursors located at distinct domains. © 2015 Wiley Periodicals, Inc.     

Distinct functions of human numb isoforms revealed by misexpression in the neural stem cell lineage in the Drosophila larval brain

Mammalian Numb (mNumb) has multiple functions and plays important roles in the regulation of neural development, including maintenance of neural progenitor cells and promotion of neuronal differentiation in the central nervous system (CNS). However, the molecular bases underlying the distinct functions of Numb have not yet been elucidated. mNumb, which has four splicing isoforms, can be divided into two types based on the presence or absence of an amino acid insert in the proline-rich region (PRR) in the C-terminus. It has been proposed that the distinct functions of mNumb may be attributable to these two different types of isoforms. In this study, we used the outer optic anlage (OOA) of the Drosophila larval brain as an assay system to analyze the functions of these two types of isoforms in the neural stem cells, since the proliferation pattern of neuroepithelial (NE) stem cells in the OOA closely resembles that of the vertebrate neural stem/progenitor cells. They divide to expand the progenitor cell pool during early neurogenesis and to produce neural precursors/neurons during late neurogenesis. Clonal analysis in the OOA allows one to discriminate between the NE stem cells, which divide symmetrically to expand the progenitor pool, and the postembryonic neuroblasts (pNBs), which divide asymmetrically to produce neural precursors (ganglion mother cells), each of which divides once to produce two neurons. We found that in the OOA, the human Numb isoform with a long PRR domain (hNumb-PRRL), which is mainly expressed during early neurogenesis in the mouse CNS, promotes proliferation of both NE cells and pNBs without affecting neuronal differentiation, while the other type of hNumb isoform with a short PRR domain (hNumb-PRRS), which is expressed throughout neurogenesis in the mouse embryonic CNS, inhibits proliferation of the stem cells and promotes neuronal differentiation. We also found that hNumb-PRRS, a functional homologue of Drosophila Numb, more strongly decreases the amount of nuclear Notch than hNumb-PRRL, and could antagonize Notch functions probably through endocytic degradation, suggesting that the two distinct types of hNumb isoforms could contribute to different phases of neurogenesis in the mouse embryonic CNS.     

Cellular proliferation and migration following a controlled cortical impact in the mouse

Neurogenesis following neural degeneration has been demonstrated in many models of disease and injury. The present study further examines the early proliferative and migratory response of the brain to a controlled cortical impact (CCI) model of traumatic brain injury. The CCI was centered over the forelimb sensorimotor cortex, unilaterally, in the adult mouse. To examine proliferation, bromo-deoxyuridine (BrdU) was injected i.p. immediately post-injury and on post-injury days 1, 2, and 3. To assess migration, we labeled SVZ cells with inert latex microspheres immediately post-injury. By combining microsphere labeling with BrdU, we determined if migrating cells had gone through the S-phase of the cell cycle after the lesion. In addition, we used a marker of neurogenesis and migration, doublecortin, to further characterize the response of the SVZ to the injury. Lastly, we determined whether subregions of the SVZ respond differentially to injury. The current study demonstrates that 3 days following CCI cellular proliferation is seen around the cortex, in the SVZ, corpus callosum, and subcortical areas anatomically connected to, but not directly damaged by the impact. It delineates that an increase in proliferation occurs in the dorsal-most aspect of the ipsilateral SVZ following impact. Lastly, it demonstrates that proliferating cells migrate from the SVZ to cortical and subcortical structures affected by the injury and that some of these cells are migrating neuroblasts.     

Proliferation and programmed cell death of neuronal precursors in the mushroom bodies of the honeybee

We have studied proliferation and programmed cell death in the brain of the honeybee during metamorphosis. DNA fragmentation detection using the TUNEL method combined with 5-bromodeoxyuridine incorporation experiments reveal that in the mushroom bodies neurogenesis is terminated by extensive apoptosis. Proliferation of mushroom body neuroblasts is active until the fourth day of pupal development, ceasing abruptly within 1 day after the onset of apoptosis in the mushroom body proliferative clusters. Inside the mushroom bodies, apoptosis spreads from the apical ends of proliferative clusters, beneath the brain's surface, toward the basal ones. The distributions of apoptotic cells and those in the S phase of the cell cycle overlap significantly. Electron microscopic analysis gives further evidence that mushroom body neuroblasts themselves undergo programmed cell death. We suggest that programmed cell death may be the main factor controlling the final number of Kenyon cells produced during metamorphosis. The overlap in time and space between proliferation and apoptosis raises the question of whether the neuronal precursors switch to programmed cell death during the progression of the cell cycle, or afterwards.     

Failure to express GAP-43 during neurogenesis affects cell cycle regulation and differentiation of neural precursors and stimulates apoptosis of neurons

GAP-43 is first expressed in proliferating neuroblasts and is required for maturation of neurons. When GAP-43 is not expressed in differentiating embryonal carcinoma P19 cells, reduced numbers of neurons were generated. Here we show that neuronal differentiation is initially disrupted at the onset of cell-cycle arrest in aggregated, proliferating neuronal precursors. The ratio of nestin:beta-tubulin-labeled progeny generated at this stage suggests that the differentiation is asymmetric. Apoptosis of immature neurons subsequently produced was also significantly induced. In vivo, too, proliferation of neuroblasts was significantly reduced in cortex of GAP-43(-/-) mice at E14.5. These data demonstrate that when GAP-43 is not expressed in proliferating neuroblasts, neural differentiation is not initiated appropriately, inducing apoptosis. Moreover, the concurrent inhibition of Ca2+-dependent adhesion between differentiating P19 cells in aggregates implicates GAP-43 in CAM-mediated signaling during neurogenesis, as has been previously shown in growth cones.     

Drebrin E regulates neuroblast proliferation and chain migration in the adult brain

F‐actin‐binding protein drebrin has two major isoforms: drebrin A and drebrin E. Drebrin A is the major isoform in the adult brain and is highly concentrated in dendritic spines, regulating spine morphology and synaptic plasticity. Conversely, drebrin E is the major isoform in the embryonic brain and regulates neuronal morphological differentiation, but it is also expressed in neurogenic regions of the adult brain. The subventricular zone (SVZ) is one of the brain regions where adult neurogenesis occurs. Neuroblasts migrate to the olfactory bulb (OB) and integrate into existing neuronal networks, after which drebrin expression changes from E to A, suggesting that drebrin E plays a specific role in neuroblasts in the adult brain. Therefore, to understand the role of drebrin E in the adult brain, we immunohistochemically analyzed adult neurogenesis using drebrin‐null‐mutant (DXKO) mice. In DXKO mice, the number of neuroblasts and cell proliferation decreased, although cell death remained unchanged. These results suggest that drebrin E regulates cell proliferation in the adult SVZ. Surprisingly, the decreased number of neuroblasts in the SVZ did not result in less neurons in the OB. This was because the survival rate of newly generated neurons in the OB increased in DXKO mice. Additionally, when neuroblasts reached the OB, the change in the migratory pathway from tangential to radial was partly disturbed in DXKO mice. These results suggest that drebrin E is involved in a chain migration of neuroblasts.     

Absence of striatal newborn neurons with mature phenotype following defined striatal and cortical excitotoxic brain injuries

Experimental stroke and excitotoxic brain lesion to the striatum increase the proliferation of cells residing within the ventricular wall and cause subsequent migration of newborn neuroblasts into the lesioned brain parenchyma. In this study, we clarify the different events of neurogenesis following striatal or cortical excitotoxic brain lesions in adult rats. Newborn cells were labeled by intraperitoneal injection of bromo–deoxy–uridine (BrdU), or by green fluorescent protein (GFP)-expressing lentiviral vectors injected into the subventricular zone (SVZ). We show that only neural progenitors born the first 5 days in the SVZ reside and expand within this neurogenic niche over time, and that these early labeled cells are more prone to migrate towards the striatum as neuroblasts. However, these neuroblasts could not mature into NeuN⁺ neurons in the striatum. Furthermore, we found that cortical lesions, close or distant from the SVZ, could not upregulate SVZ cell proliferation nor promote neurogenesis. Our study demonstrates that both the time window for labeling proliferating cells and the site of lesion are crucial when assessing neurogenesis following brain injury.     

Migrating neuroblasts of the rostral migratory stream are putative targets for the action of nitric oxide

It has been demonstrated that the gaseous messenger nitric oxide influences cell proliferation and cell migration, and therefore affects adult neurogenesis in mammals. Here, we investigated the putative targets for this action in the rostral migratory stream of the rat. We used immunocytochemical detection of the beta1 subunit of the enzyme soluble guanylyl cyclase, which can be activated by nitric oxide. Our results under light and electron microscopy demonstrated that the migrating neuroblasts (type A cells) were beta1-immunopositive. The astrocytes (type B cells), immature precursors (type C cells) and ependymal cells (type E cells) were beta1-immunonegative. The neurochemical characterization of the soluble guanylyl cyclase-containing cells confirmed these results. In this regard, the beta1-containing cells expressed doublecortin, a protein expressed by type A cells, and did not express glial fibrillary acidic protein, which is a marker for type B cells. Injection of 5-bromo-2'-deoxyuridine 2 h before killing demonstrated that proliferating cells did not contain soluble guanylyl cyclase. Finally, we found that beta1-containing type A cells also expressed the A3 subunit of the cyclic nucleotide-gated ion channels. Altogether, the present results indicate that nitric oxide may influence adult neurogenesis acting on the migrating neuroblasts of the rostral migratory stream. In these cells, nitric oxide may activate the enzyme soluble guanylyl cyclase, triggering the production of the second messenger cGMP. In turn, cGMP might induce the opening of cyclic nucleotide-gated ion channels, which are present in these cells.     

Naturally occurring cell death and migration of microglial precursors in the quail retina during normal development.

We compared chronotopographical patterns of distribution of naturally occurring neuronal death in the ganglion cell layer (GCL) and the inner nuclear layer (INL) with patterns of tangential and radial migration of microglial precursors during quail retinal development. Apoptotic cells were identified by the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling technique, and microglial precursors were identified by immunocytochemistry with an antibody recognizing quail microglial cells (QH1 antibody). Apoptotic cells were first detectable in the GCL at the seventh day of incubation (E7), were most abundant at E10, and were absent after E13. In the INL, apoptotic cells first appeared at E7, were most abundant at E12, and disappeared entirely after the third posthatching day (P3). In both retinal layers, cell death first appeared in a small central area of the retina and subsequently spread along three gradients: central-to-peripheral, temporal-to-nasal, and dorsal-to-ventral. The chronology of tangential (between E7 and E16) and radial migration (between E8 and P3) of microglial precursors was highly coincident with that of cell death in the GCL and INL. Comparison of the chronotopographical pattern of distribution of apoptotic nuclei in the GCL with the patterns of tangential and radial migration of microglial precursors neither supported nor refuted the hypothesis that ganglion cell death is the stimulus that triggers the entry and migration of microglial precursors in the developing retina. However, microglial cells in most of the retina traversed the INL only after cell death had ceased in this layer, suggesting that cell death in the INL does not attract microglial precursors migrating radially. Dead cell debris in this layer was phagocytosed by Müller cells, whereas migrating microglial cells were seen phagocytosing apoptotic bodies in the nerve fiber layer and GCL but not in the INL.     

Forebrain neurogenesis after focal Ischemic and traumatic brain injury

Neural stem cells persist in the adult mammalian forebrain and are a potential source of neurons for repair after brain injury. The two main areas of persistent neurogenesis, the subventricular zone (SVZ)-olfactory bulb pathway and hippocampal dentate gyrus, are stimulated by brain insults such as stroke or trauma. Here we focus on the effects of focal cerebral ischemia on SVZ neural progenitor cells in experimental stroke, and the influence of mechanical injury on adult hippocampal neurogenesis in models of traumatic brain injury (TBI). Stroke potently stimulates forebrain SVZ cell proliferation and neurogenesis. SVZ neuroblasts are induced to migrate to the injured striatum, and to a lesser extent to the peri-infarct cortex. Controversy exists as to the types of neurons that are generated in the injured striatum, and whether adult-born neurons contribute to functional restoration remains uncertain. Advances in understanding the regulation of SVZ neurogenesis in general, and stroke-induced neurogenesis in particular, may lead to improved integration and survival of adult-born neurons at sites of injury. Dentate gyrus cell proliferation and neurogenesis similarly increase after experimental TBI. However, pre-existing neuroblasts in the dentate gyrus are vulnerable to traumatic insults, which appear to stimulate neural stem cells in the SGZ to proliferate and replace them, leading to increased numbers of new granule cells. Interventions that stimulate hippocampal neurogenesis appear to improve cognitive recovery after experimental TBI. Transgenic methods to conditionally label or ablate neural stem cells are beginning to further address critical questions regarding underlying mechanisms and functional significance of neurogenesis after stroke or TBI. Future therapies should be aimed at directing appropriate neuronal replacement after ischemic or traumatic injury while suppressing aberrant integration that may contribute to co-morbidities such as epilepsy or cognitive impairment.     

Meteorin is a chemokinetic factor in neuroblast migration and promotes stroke-induced striatal neurogenesis

Ischemic stroke affecting the adult brain causes increased progenitor proliferation in the subventricular zone (SVZ) and generation of neuroblasts, which migrate into the damaged striatum and differentiate to mature neurons. Meteorin (METRN), a newly discovered neurotrophic factor, is highly expressed in neural progenitor cells and immature neurons during development, suggesting that it may be involved in neurogenesis. Here, we show that METRN promotes migration of neuroblasts from SVZ explants of postnatal rats and stroke-subjected adult rats via a chemokinetic mechanism, and reduces N-methyl-D-asparate-induced apoptotic cell death in SVZ cells in vitro. Stroke induced by middle cerebral artery occlusion upregulates the expression of endogenous METRN in cells with neuronal phenotype in striatum. Recombinant METRN infused into the stroke-damaged brain stimulates cell proliferation in SVZ, promotes neuroblast migration, and increases the number of immature and mature neurons in the ischemic striatum. Our findings identify METRN as a new factor promoting neurogenesis both in vitro and in vivo by multiple mechanisms. Further work will be needed to translate METRN's actions on endogenous neurogenesis into improved recovery after stroke.     

Rapid axonal transport of phosphatidylinositol in the rabbit optic pathway

At different time intervals (4 hr up to 24 days) after intravitreal injection of myo-(2-³H)-inositol, the specific activity (sp act) of phosphatidylinositol (PtdIns) and water-soluble compounds extracted from the retina, optic nerve (ON), optic tract (OT), lateral geniculate body (LGB), and superior colliculus (SC) was measured. Furthermore, 2 days after biocular injection of the labeled precursor, the specific activity of PtdIns and other major phospholipids in the various subcellular fractions (myelin, axolemma, synaptosomes, mitochondria, and microsomes) purified from ON + OT, from LGB, and from SC was evaluated. In the ON and OT, the phospholipids were labeled 8 hr after the precursor injection; in the LGB and SC after 8–12 hr. In all the optic components examined, the lipid specific activity reached a maximum at day 6 and decreased slowly until day 24 after injection. In the ON + OT, the specific activity of PtdIns, isolated from various subcellular fractions, decreased as follows: myelin > axolemma; in the LGB and SC as follows: microsomes > synaptosomes > myelin > mitochondria. The radioactivity present in the retina and in the optic pathway was exclusively localized in the PtdIns during the whole time interval studied. Ten days after the precursor injection, a high portion of the radioactivity transported was detected in the water-soluble pool, separated from the ON and OT, whereas no significant counts in the corresponding LGB and SC pool were found. The kinetics of PtdIns labeling, observed in the different optic components, indicated a rapid axonal transport of the phospholipid; besides, the presence of labeled PtdIns in the purified myelin suggested a transaxonal migration of the phospholipid and its free precursor.