Cyclin-dependent kinase 5 is required for control of neuroblast migration in the postnatal subventricular zone

The Celsr3-Kif2a axis directs neuronal migration in the postnatal brain

The tangential migration of immature neurons in the postnatal brain involves consecutive migration cycles and depends on constant remodeling of the cell cytoskeleton, particularly in the leading process (LP). Despite the identification of several proteins with permissive and empowering functions, the mechanisms that specify the direction of migration remain largely unknown. Here, we report that planar cell polarity protein Celsr3 orients neuroblasts migration from the subventricular zone (SVZ) to olfactory bulb (OB). In Celsr3-forebrain conditional knockout mice, neuroblasts loose directionality and few can reach the OB. Celsr3-deficient neuroblasts exhibit aberrant branching of LP, de novo LP formation, and decreased growth rate of microtubules (MT). Mechanistically, we show that Celsr3 interacts physically with Kif2a, a MT depolymerizing protein and that conditional inactivation of Kif2a in the forebrain recapitulates the Celsr3 knockout phenotype. Our findings provide evidence that Celsr3 and Kif2a cooperatively specify the directionality of neuroblasts tangential migration in the postnatal brain.

In vitro Time-lapse Imaging of Primary Cilium in Migrating Neuroblasts

Neuronal migration is a critical step for the development of neuronal circuits in the brain. Immature new neurons (neuroblasts) generated in the postnatal ventricular-subventricular zone (V-SVZ) show a remarkable potential to migrate for a long distance at a high speed in the postnatal mammalian brain, and are thus a powerful model to analyze the molecular and cellular mechanisms of neuronal migration. Here we describe a methodology for in vitro time-lapse imaging of the primary cilium and its related structures in migrating V-SVZ-derived neuroblasts using confocal or superresolution laser-scanning microscopy. The V-SVZ tissues are dissected from postnatal day 0-1 (P0-1) mouse brains and dissociated into single cells by trypsinization and gentle pipetting. These cells are then transduced with a plasmid(s) encoding a gene(s) of interest, aggregated by centrifugation, and cultured for 2 days in Matrigel. Time-lapse images of migratory behaviors of cultured neuroblasts and their ciliary structures, including the ciliary membrane and basal body, are acquired by confocal or superresolution laser-scanning microscopy. This method provides information about the spatiotemporal dynamics of neuroblasts' morphology and ciliary structures, and is widely applicable to various types of migrating neuronal and nonneuronal cells in various species.

A Semi-automated and Scalable 3D Spheroid Assay to Study Neuroblast Migration

The subventricular zone of the mammalian brain is the major source of adult born neurons. These neuroblasts normally migrate long distances to the olfactory bulbs but can be re-routed to locations of injury and promote neuroregeneration. Mechanistic understanding and pharmacological targets regulating neuroblast migration is sparse. Furthermore, lack of migration assays limits development of pharmaceutical interventions targeting neuroblast recruitment. We therefore developed a physiologically relevant 3D neuroblast spheroid migration assay that permits the investigation of large numbers of interventions. To verify the assay, 1,012 kinase inhibitors were screened for their effects on migration. Several induced significant increases or decreases in migration. MuSK and PIK3CB were selected as putative targets and their knockdown validated increased neuroblast migration. Thus, compounds identified through this assay system could be explored for their potential in augmenting neuroblast recruitment to sites of injury for neuroregeneration, or for decreasing malignant invasion.

In vivo serial MRI of age-dependent neural progenitor cell migration in the rat brain

The subventricular zone (SVZ) is a neurogenic niche in the mammalian brain, giving rise to migratory neural progenitor cells (NPC). In rodents, it is well-established that neurogenesis decreases with aging. MRI-based cell tracking has been used to measure various aspects of neurogenesis and NPC migration in rodents, yet it has not yet been validated in the context of age-related decrease in neurogenesis. This validation is critical to using these MRI techniques to study changes in neurogenesis that arise in diseases prevalent in aging populations and their combination with advanced cellular therapeutic approaches aiming to combat neurodegeneration. As such, in this work we used MRI-based cell tracking to measure endogenous neurogenesis and cell migration from the SVZ along the rostral migratory stream to the olfactory bulb, for 12 days duration, in rats aged 9 weeks to 2 years old. To enable the specific detection of NPCs by MRI, we injected micron sized particles of iron oxide (MPIOs) into the lateral ventricle to endogenously label cells within the SVZ, which then appeared as hypo-intensive spots within MR images. In vivo MRI data showed that the rate of NPC migration was significantly different between all ages examined, with decreases in the distance traveled and migration rate as age progressed. The total number of MPIO-labeled cells within the olfactory bulb on day 12, was significantly decreased when compared across ages in ex vivo high-resolution scans. We also demonstrate for the first-time, provocative preliminary data suggesting age-dependent MPIO uptake within the dentate gyrus (DG) as well. Histology to identify doublecortin-positive NPCs, verified the decrease in cell labeling as a function of aging, for both regions. The dramatic reduction of NPC labeling within the SVZ observed with MRI, validates the sensitivity of MRI-based cell tracking to neurogenic potential and demonstrates the importance of understanding the impact of age on the relationship of NPC and disease.

Overlapping migratory mechanisms between neural progenitor cells and brain tumor stem cells

Neural stem cells present in the subventricular zone (SVZ), the largest neurogenic niche of the mammalian brain, are able to self-renew as well as generate neural progenitor cells (NPCs). NPCs are highly migratory and traverse the rostral migratory stream (RMS) to the olfactory bulb, where they terminally differentiate into mature interneurons. NPCs from the SVZ are some of the few cells in the CNS that migrate long distances during adulthood. The migratory process of NPCs is highly regulated by intracellular pathway activation and signaling from the surrounding microenvironment. It involves modulation of cell volume, cytoskeletal rearrangement, and isolation from compact extracellular matrix. In malignant brain tumors including high-grade gliomas, there are cells called brain tumor stem cells (BTSCs) with similar stem cell characteristics to NPCs but with uncontrolled cell proliferation and contribute to tumor initiation capacity, tumor progression, invasion, and tumor maintenance. These BTSCs are resistant to chemotherapy and radiotherapy, and their presence is believed to lead to tumor recurrence at distal sites from the original tumor location, principally due to their high migratory capacity. BTSCs are able to invade the brain parenchyma by utilizing many of the migratory mechanisms used by NPCs. However, they have an increased ability to infiltrate the tight brain parenchyma and utilize brain structures such as myelin tracts and blood vessels as migratory paths. In this article, we summarize recent findings on the mechanisms of cellular migration that overlap between NPCs and BTSCs. A better understanding of the intersection between NPCs and BTSCs will to provide a better comprehension of the BTSCs' invasive capacity and the molecular mechanisms that govern their migration and eventually lead to the development of new therapies to improve the prognosis of patients with malignant gliomas.

"Till Death Do Us Part": A Potential Irreversible Link Between Aberrant Cell Cycle Control and Neurodegeneration in the Adult Olfactory Bulb

Adult neurogenesis (AN) is an ongoing developmental process that generates newborn neurons in the olfactory bulb (OB) and the hippocampus (Hi) throughout life and significantly contributes to brain plasticity. Adult neural stem and progenitor cells (aNSPCs) are relatively limited in number and fate and are spatially restricted to the subventricular zone (SVZ) and the subgranular zone (SGZ). During AN, the distinct roles played by cell cycle proteins extend beyond cell cycle control and constitute key regulatory mechanisms involved in neuronal maturation and survival. Importantly, aberrant cell cycle re-entry (CCE) in post-mitotic neurons has been strongly linked to the abnormal pathophysiology in rodent models of neurodegenerative diseases with potential implications on the etiology and progression of such diseases in humans. Here, we present an overview of AN in the SVZ-OB and olfactory epithelium (OE) in mice and humans followed by a comprehensive update of the distinct roles played by cell cycle proteins including major tumors suppressor genes in various steps during neurogenesis. We also discuss accumulating evidence underlining a strong link between abnormal cell cycle control, olfactory dysfunction and neurodegeneration in the adult and aging brain. We emphasize that: (1) CCE in post-mitotic neurons due to loss of cell cycle suppression and/or age-related insults as well as DNA damage can anticipate the development of neurodegenerative lesions and protein aggregates, (2) the age-related decline in SVZ and OE neurogenesis is associated with compensatory pro-survival mechanisms in the aging OB which are interestingly similar to those detected in Alzheimer's disease and Parkinson's disease in humans, and (3) the OB represents a well suitable model to study the early manifestation of age-related defects that may eventually progress into the formation of neurodegenerative lesions and, possibly, spread to the rest of the brain. Such findings may provide a novel approach to the modeling of neurodegenerative diseases in humans from early detection to progression and treatment as well.

Dual role for DOCK7 in tangential migration of interneuron precursors in the postnatal forebrain

Throughout life, stem cells in the ventricular-subventricular zone generate neuroblasts that migrate via the rostral migratory stream (RMS) to the olfactory bulb, where they differentiate into local interneurons. Although progress has been made toward identifying extracellular factors that guide the migration of these cells, little is known about the intracellular mechanisms that govern the dynamic reshaping of the neuroblasts' morphology required for their migration along the RMS. In this study, we identify DOCK7, a member of the DOCK180-family, as a molecule essential for tangential neuroblast migration in the postnatal mouse forebrain. DOCK7 regulates the migration of these cells by controlling both leading process (LP) extension and somal translocation via distinct pathways. It controls LP stability/growth via a Rac-dependent pathway, likely by modulating microtubule networks while also regulating F-actin remodeling at the cell rear to promote somal translocation via a previously unrecognized myosin phosphatase-RhoA-interacting protein-dependent pathway. The coordinated action of both pathways is required to ensure efficient neuroblast migration along the RMS.

Disruption of neuronal nitric oxide synthase dimerization contributes to the development of Alzheimer's disease: Involvement of cyclin-dependent kinase 5-mediated phosphorylation of neuronal nitric oxide synthase at Ser(293)

Although previous studies have suggested that neuronal nitric oxide synthase (nNOS)-derived NO has neuroprotective effects on the development of Alzheimer's disease (AD), the underlying molecular mechanisms are not fully elucidated. Here, we investigated whether and how disruption of nNOS dimerization contributes to the development of AD. No differences in synaptic number or expression of synaptic markers, including synaptophysin and postsynaptic density 95, were found in the cortex of 5 × FAD mice, which possess 5 familial AD mutations, at 6 months of age compared with control littermates. nNOS dimerization was disrupted in the 5 × FAD cortex, accompanied by an increase in reactive oxygen species (ROS) production. The subcellular distribution of cyclin-dependent kinase 5 (CDK5) shifted more diffusely toward a cytosolic compartment, but there was no change in total expression. Furthermore, the levels of p25, a CDK5 activator, increased significantly and it colocalized with nNOS in the 5 × FAD cortex. In silico analysis revealed that a new nNOS-specific GSP (glycine-serine-proline) motif was well-conserved across species at nNOS-Ser(293), which is located ahead of the N-terminal hook. This motif was not present in the closely related isoform, endothelial NOS. Motif scan analysis also predicted that CDK5 can phosphorylate nNOS-Ser(293) with a high likelihood. An in vitro phosphorylation assay clearly showed that CDK5/p25 does indeed phosphorylate nNOS-Ser(293). Finally, nNOS-S293D mutant, a phosphomimetic form of nNOS-Ser(293), and nNOS-S293A mutant, a neutral form of nNOS-Ser(293), significantly decreased nNOS dimerization and NO production. Taken together, our results demonstrate that nNOS dimers are disrupted in the 5 × FAD cortex, and nNOS-Ser(293), a potential site of CDK5 phosphorylation, may be involved in the decrease in nNOS dimerization and NO production, and the development of AD.

Cell-permeable p38 MAP kinase promotes migration of adult neural stem/progenitor cells

Endogenous neural stem/progenitor cells (NPCs) can migrate toward sites of injury, but the migration activity of NPCs is insufficient to regenerate damaged brain tissue. In this study, we showed that p38 MAP kinase (p38) is expressed in doublecortin-positive adult NPCs. Experiments using the p38 inhibitor SB203580 revealed that endogenous p38 participates in NPC migration. To enhance NPC migration, we generated a cell-permeable wild-type p38 protein (PTD-p38WT) in which the HIV protein transduction domain (PTD) was fused to the N-terminus of p38. Treatment with PTD-p38WT significantly promoted the random migration of adult NPCs without affecting cell survival or differentiation; this effect depended on the cell permeability and kinase activity of the fusion protein. These findings indicate that PTD-p38WT is a novel and useful tool for unraveling the roles of p38, and that this protein provides a reasonable approach for regenerating the injured brain by enhancing NPC migration.

Control of the Cell Cycle in Adult Neurogenesis and its Relation with Physical Exercise

In the adult brain the neurogenesis is mainly restricted to two neurogenic regions: newly generated neurons arise at the subventricular zone (SVZ) of the lateral ventricle and at the subgranular zone of the hippocampal subregion named the dentate gyrus. The hippocampus is involved in learning and memory paradigms and the generation of new hippocampal neurons has been hypothesized to be a pivotal form of plasticity involved in the process. Moreover the dysregulation of hippocampal adult neurogenesis has been recognized and could anticipate several varieties of brain disease such as Alzheimer disease, epilepsy and depression. Over the last few decades numerous intrinsic, epigenetic and environmental factors have been revealed to deeply influence the process of adult neurogenesis, although the underlying mechanisms remain largely unknown. Growing evidence indicates that physical exercise represents one of the main extrinsic factor able to profoundly increase hippocampal adult neurogenesis, by altering neurochemistry and function of newly generated neurons. The present review surveys how neurogenesis can be modulated by cell cycle kinetics and highlights the putative role of the cell cycle length as a key component of the beneficial effect of running for hippocampal adult neurogenesis, both in physiological conditions and in the presence of defective neurogenesis.

The aged brain: genesis and fate of residual progenitor cells in the subventricular zone

Neural stem cells (NSCs) persist in the adult mammalian brain through life. The subventricular zone (SVZ) is the largest source of stem cells in the nervous system, and continuously generates new neuronal and glial cells involved in brain regeneration. During aging, the germinal potential of the SVZ suffers a widespread decline, but the causes of this turn down are not fully understood. This review provides a compilation of the current knowledge about the age-related changes in the NSC population, as well as the fate of the newly generated cells in the aged brain. It is known that the neurogenic capacity is clearly disrupted during aging, while the production of oligodendroglial cells is not compromised. Interestingly, the human brain seems to primarily preserve the ability to produce new oligodendrocytes instead of neurons, which could be related to the development of neurological disorders. Further studies in this matter are required to improve our understanding and the current strategies for fighting neurological diseases associated with senescence.

Drebrin regulates neuroblast migration in the postnatal mammalian brain

After birth, stem cells in the subventricular zone (SVZ) generate neuroblasts that migrate along the rostral migratory stream (RMS) to become interneurons in the olfactory bulb (OB). This migration is crucial for the proper integration of newborn neurons in a pre-existing synaptic network and is believed to play a key role in infant human brain development. Many regulators of neuroblast migration have been identified; however, still very little is known about the intracellular molecular mechanisms controlling this process. Here, we have investigated the function of drebrin, an actin-binding protein highly expressed in the RMS of the postnatal mammalian brain. Neuroblast migration was monitored both in culture and in brain slices obtained from electroporated mice by time-lapse spinning disk confocal microscopy. Depletion of drebrin using distinct RNAi approaches in early postnatal mice affects neuroblast morphology and impairs neuroblast migration and orientation in vitro and in vivo. Overexpression of drebrin also impairs migration along the RMS and affects the distribution of neuroblasts at their final destination, the OB. Drebrin phosphorylation on Ser142 by Cyclin-dependent kinase 5 (Cdk5) has been recently shown to regulate F-actin-microtubule coupling in neuronal growth cones. We also investigated the functional significance of this phosphorylation in RMS neuroblasts using in vivo postnatal electroporation of phosphomimetic (S142D) or non-phosphorylatable (S142A) drebrin in the SVZ of mouse pups. Preventing or mimicking phosphorylation of S142 in vivo caused similar effects on neuroblast dynamics, leading to aberrant neuroblast branching. We conclude that drebrin is necessary for efficient migration of SVZ-derived neuroblasts and propose that regulated phosphorylation of drebrin on S142 maintains leading process stability for polarized migration along the RMS, thus ensuring proper neurogenesis.

Functional synergy between cholecystokinin receptors CCKAR and CCKBR in mammalian brain development

Cholecystokinin (CCK), a peptide hormone and one of the most abundant neuropeptides in vertebrate brain, mediates its actions via two G-protein coupled receptors, CCKAR and CCKBR, respectively active in peripheral organs and the central nervous system. Here, we demonstrate that the CCK receptors have a dynamic and largely reciprocal expression in embryonic and postnatal brain. Using compound homozygous mutant mice lacking the activity of both CCK receptors, we uncover their additive, functionally synergistic effects in brain development and demonstrate that CCK receptor loss leads to abnormalities of cortical development, including defects in the formation of the midline and corpus callosum, and cortical interneuron migration. Using comparative transcriptome analysis of embryonic neocortex, we define the molecular mechanisms underlying these defects. Thus we demonstrate a developmental, hitherto unappreciated, role of the two CCK receptors in mammalian neocortical development.

Neuronal migration and protein kinases

The formation of the six-layered structure of the mammalian cortex via the inside-out pattern of neuronal migration is fundamental to neocortical functions. Extracellular cues such as Reelin induce intracellular signaling cascades through the protein phosphorylation. Migrating neurons also have intrinsic machineries to regulate cytoskeletal proteins and adhesion properties. Protein phosphorylation regulates these processes. Moreover, the balance between phosphorylation and dephosphorylation is modified by extracellular cues. Multipolar-bipolar transition, radial glia-guided locomotion and terminal translocation are critical steps of radial migration of cortical pyramidal neurons. Protein kinases such as Cyclin-dependent kinase 5 (Cdk5) and c-Jun N-terminal kinases (JNKs) involve these steps. In this review, I shall give an overview the roles of protein kinases in neuronal migration.

Mechanisms of HIV-1 Tat neurotoxicity via CDK5 translocation and hyper-activation: role in HIV-associated neurocognitive disorders

The advent of more effective antiretroviral therapies has reduced the frequency of HIV dementia, however the prevalence of milder HIV associated neurocognitive disorders [HAND] is actually rising. Neurodegenerative mechanisms in HAND might include toxicity by secreted HIV-1 proteins such as Tat, gp120 and Nef that could activate neuro-inflammatory pathways, block autophagy, promote excitotoxicity, oxidative stress, mitochondrial dysfunction and dysregulation of signaling pathways. Recent studies have shown that Tat could interfere with several signal transduction mechanisms involved in cytoskeletal regulation, cell survival and cell cycle re-entry. Among them, Tat has been shown to hyper-activate cyclin-dependent kinase [CDK] 5, a member of the Ser/Thr CDKs involved in cell migration, angiogenesis, neurogenesis and synaptic plasticity. CDK5 is activated by binding to its regulatory subunit, p35 or p39. For this manuscript we review evidence showing that Tat, via calcium dysregulation, promotes calpain-1 cleavage of p35 to p25, which in turn hyper-activates CDK5 resulting in abnormal phosphorylation of downstream targets such as Tau, collapsin response mediator protein-2 [CRMP2], doublecortin [DCX] and MEF2. We also present new data showing that Tat interferes with the trafficking of CDK5 between the nucleus and cytoplasm. This results in prolonged presence of CDK5 in the cytoplasm leading to accumulation of aberrantly phosphorylated cytoplasmic targets [e.g.: Tau, CRMP2, DCX] that impair neuronal function and eventually lead to cell death. Novel therapeutic approaches with compounds that block Tat mediated hyper-activation of CDK5 might be of value in the management of HAND.

NFIX Regulates Proliferation and Migration Within the Murine SVZ Neurogenic Niche

Transcription factors of the nuclear factor one (NFI) family play a pivotal role in the development of the nervous system. One member, NFIX, regulates the development of the neocortex, hippocampus, and cerebellum. Postnatal Nfix(-/-) mice also display abnormalities within the subventricular zone (SVZ) lining the lateral ventricles, a region of the brain comprising a neurogenic niche that provides ongoing neurogenesis throughout life. Specifically, Nfix(-/-) mice exhibit more PAX6-expressing progenitor cells within the SVZ. However, the mechanism underlying the development of this phenotype remains undefined. Here, we reveal that NFIX contributes to multiple facets of SVZ development. Postnatal Nfix(-/-) mice exhibit increased levels of proliferation within the SVZ, both in vivo and in vitro as assessed by a neurosphere assay. Furthermore, we show that the migration of SVZ-derived neuroblasts to the olfactory bulb is impaired, and that the olfactory bulbs of postnatal Nfix(-/-) mice are smaller. We also demonstrate that gliogenesis within the rostral migratory stream is delayed in the absence of Nfix, and reveal that Gdnf (glial-derived neurotrophic factor), a known attractant for SVZ-derived neuroblasts, is a target for transcriptional activation by NFIX. Collectively, these findings suggest that NFIX regulates both proliferation and migration during the development of the SVZ neurogenic niche.

Early postnatal in vivo gliogenesis from nestin-lineage progenitors requires cdk5

The early postnatal period is a unique time of brain development, as diminishing amounts of neurogenesis coexist with waves of gliogenesis. Understanding the molecular regulation of early postnatal gliogenesis may provide clues to normal and pathological embryonic brain ontogeny, particularly in regards to the development of astrocytes and oligodendrocytes. Cyclin dependent kinase 5 (Cdk5) contributes to neuronal migration and cell cycle control during embryogenesis, and to the differentiation of neurons and oligodendrocytes during adulthood. However, Cdk5's function in the postnatal period and within discrete progenitor lineages is unknown. Therefore, we selectively removed Cdk5 from nestin-expressing cells and their progeny by giving transgenic mice (nestin-CreERT2/R26R-YFP/CDK5(flox/flox) [iCdk5] and nestin-CreERT2/R26R-YFP/CDK5(wt/wt) [WT]) tamoxifen during postnatal (P) days P2-P 4 or P7-P 9, and quantified and phenotyped recombined (YFP+) cells at P14 and P21. When Cdk5 gene deletion was induced in nestin-expressing cells and their progeny during the wave of cortical and hippocampal gliogenesis (P2-P4), significantly fewer YFP+ cells were evident in the cortex, corpus callosum, and hippocampus. Phenotypic analysis revealed the cortical decrease was due to fewer YFP+ astrocytes and oligodendrocytes, with a slightly earlier influence seen in oligodendrocytes vs. astrocytes. This effect on cortical gliogenesis was accompanied by a decrease in YFP+ proliferative cells, but not increased cell death. The role of Cdk5 in gliogenesis appeared specific to the early postnatal period, as induction of recombination at a later postnatal period (P7-P9) resulted in no change YFP+ cell number in the cortex or hippocampus. Thus, glial cells that originate from nestin-expressing cells and their progeny require Cdk5 for proper development during the early postnatal period.

Fascin regulates the migration of subventricular zone-derived neuroblasts in the postnatal brain

After birth, stem cells in the subventricular zone (SVZ) generate neuroblasts that migrate along the rostral migratory stream (RMS) to become interneurons in the olfactory bulb (OB). This migration is a fundamental event controlling the proper integration of new neurons in a pre-existing synaptic network. Many regulators of neuroblast migration have been identified; however, still very little is known about the intracellular molecular mechanisms controlling this process. Here, we show that the actin-bundling protein fascin is highly upregulated in mouse SVZ-derived migratory neuroblasts. Fascin-1ko mice display an abnormal RMS and a smaller OB. Bromodeoxyuridine labeling experiments show that lack of fascin significantly impairs neuroblast migration, but does not appear to affect cell proliferation. Moreover, fascin depletion substantially alters the polarized morphology of rat neuroblasts. Protein kinase C (PKC)-dependent phosphorylation of fascin on Ser39 regulates its actin-bundling activity. In vivo postnatal electroporation of phosphomimetic (S39D) or nonphosphorylatable (S39A) fascin variants followed by time-lapse imaging of brain slices demonstrates that the phospho-dependent modulation of fascin activity ensures efficient neuroblast migration. Finally, fluorescence lifetime imaging microscopy studies in rat neuroblasts reveal that the interaction between fascin and PKC can be modulated by cannabinoid signaling, which controls neuroblast migration in vivo. We conclude that fascin, whose upregulation appears to mark the transition to the migratory neuroblast stage, is a crucial regulator of neuroblast motility. We propose that a tightly regulated phospho/dephospho-fascin cycle modulated by extracellular signals is required for the polarized morphology and migration in neuroblasts, thus contributing to efficient neurogenesis.

Plexin-B2 regulates the proliferation and migration of neuroblasts in the postnatal and adult subventricular zone

In the postnatal forebrain, the subventricular zone (SVZ) contains a pool of undifferentiated cells, which proliferate and migrate along the rostral migratory stream (RMS) to the olfactory bulb and differentiate into granule cells and periglomerular cells. Plexin-B2 is a semaphorin receptor previously known to act on neuronal proliferation in the embryonic brain and neuronal migration in the cerebellum. We show here that, in the postnatal and adult CNS, Plexin-B2 is expressed in the subventricular zone lining the telencephalic ventricles and in the rostral migratory stream. We analyzed Plxnb2(-/-) mice and found that there is a marked reduction in the proliferation of SVZ cells in the mutant. Plexin-B2 expression is downregulated in the olfactory bulb as interneurons initiate radial migration. BrdU labeling and GFP electroporation into postnatal SVZ, in addition to time-lapse videomicroscopy, revealed that neuroblasts deficient for Plexin-B2 migrate faster than control ones and leave the RMS more rapidly. Overall, these results show that Plexin-B2 plays a role in postnatal neurogenesis and in the migration of SVZ-derived neuroblasts.

Cycling or not cycling: cell cycle regulatory molecules and adult neurogenesis

The adult brain most probably reaches its highest degree of plasticity with the lifelong generation and integration of new neurons in the hippocampus and olfactory system. Neural precursor cells (NPCs) residing both in the subgranular zone of the dentate gyrus and in the subventricular zone of the lateral ventricles continuously generate neurons that populate the dentate gyrus and the olfactory bulb, respectively. The regulation of NPC proliferation in the adult brain has been widely investigated in the past few years. Yet, the intrinsic cell cycle machinery underlying NPC proliferation remains largely unexplored. In this review, we discuss the cell cycle components that are involved in the regulation of NPC proliferation in both neurogenic areas of the adult brain.

Abnormal neuronal migration changes the fate of developing neurons in the postnatal olfactory bulb

Neuronal precursors are continuously integrated into the adult olfactory bulb (OB). The vast majority of these precursor cells originates from the subventricular zone and migrates along the rostral migratory stream (RMS) en route to the OB. This process, called postnatal neurogenesis, results from intricate pathways depending both on cell-autonomous factors and extrinsic regulation provided by the local environment. Using electroporation in postnatal mice to label neuronal precursors with green fluorescent protein (GFP) and to reduce the expression levels of doublecortin (DCX) with short-hairpin (Sh) RNA, we investigated the consequences of impairing migration on the fate of postnatal-formed neurons. First, we showed that electroporation of Dcx ShRNA plasmid efficiently knocks down the expression of DCX and disrupts cells migration along the RMS. Second, we found misplaced anomalous migrating cells that displayed defects in polarity and directionality. Third, patch-clamp recordings performed at 5-7 days post-electroporation (dpe) revealed increased density of voltage-dependent Na(+) channels and enhanced responsiveness to GABA(A) receptor agonist. At later time points (i.e., 12 and 30 dpe), most of the Dcx ShRNA(+) cells developed in the core of the OB and displayed aberrant dendritic length and branching. Additional analysis revealed the formation of GABAergic and glutamatergic synaptic inputs on the mispositioned neurons. Finally, quantifying fate determination by numbering the proportion of GFP(+)/calretinin(+) newborn neurons revealed that Dcx ShRNA(+) cells acquire mature phenotype despite their immature location. We conclude that altering the pace of migration at early stages of postnatal neurogenesis profoundly modifies the tightly orchestrated steps of neuronal maturation, and unveils the influence of microenvironment on controlling neuronal development in the postnatal forebrain.

Strategies for regenerating striatal neurons in the adult brain by using endogenous neural stem cells

Currently, there is no effective treatment for the marked neuronal loss caused by neurodegenerative diseases, such as Huntington's disease (HD) or ischemic stroke. However, recent studies have shown that new neurons are continuously generated by endogenous neural stem cells in the subventricular zone (SVZ) of the adult mammalian brain, including the human brain. Because some of these new neurons migrate to the injured striatum and differentiate into mature neurons, such new neurons may be able to replace degenerated neurons and improve or repair neurological deficits. To establish a neuroregenerative therapy using this endogenous system, endogenous regulatory mechanisms that can be co-opted for efficient regenerative interventions must be understood, along with any potential drawbacks. Here, we review current knowledge on the generation of new neurons in the adult brain and discuss their potential for use in replacing striatal neurons lost to neurodegenerative diseases, including HD, and to ischemic stroke.

Cdk5: multitasking between physiological and pathological conditions

Cyclin-dependent kinase 5 (Cdk5) is a peculiar proline-directed serine/threonine kinase. Unlike the other members of the Cdk family, Cdk5 is not directly involved in cell cycle regulation, being normally associated with neuronal processes such as migration, cortical layering and synaptic plasticity. This kinase is present mainly in post-mitotic neurons and its activity is tightly regulated by the interaction with the specific activators, p35 and p39. Despite its pivotal role in CNS development, Cdk5 dysregulation has been implicated in different pathologies, such as Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and, most recently, prion-related encephalopathies (PRE). In these neurodegenerative conditions, Cdk5 overactivation and relocalization occurs upon association with p25, a truncated form of the normal activator p35. This activator switching will cause a shift in the phosphorylative pattern of Cdk5, with an alteration both in targets and activity, ultimately leading to neuronal demise. In AD and PRE, two disorders that share clinical and neuropathological features, Cdk5 dysregulation is a linking event between the major neuropathological markers: amyloid plaques, tau hyperphosphorylation and synaptic and neuronal loss. Moreover, this kinase was shown to be involved in abortive cell cycle re-entry, a feature recently proposed as a possible step in the neuronal apoptosis mechanism of several neurological diseases. This review focuses on the role of Cdk5 in neurons, namely in the regulation of cytoskeletal dynamics, synaptic function and cell survival, both in physiological and in pathological conditions, highlighting the relevance of Cdk5 in the main mechanisms of neurodegeneration in Alzheimer's disease and other brain pathologies.

Neurogenesis in Alzheimer's disease

It is widely acknowledged that neural stem cells generate new neurons through the process of neurogenesis in the adult brain. In mammals, adult neurogenesis occurs in two areas of the CNS: the subventricular zone and the subgranular zone of the dentate gyrus of the hippocampus. The newly generated cells display neuronal morphology, generate action potentials and receive functional synaptic inputs, their properties being equivalent to those of mature neurons. Alzheimer's disease (AD) is the widespread cause of dementia, and is an age-related, progressive and irreversible neurodegenerative disease that results in massive neuronal death and deterioration of cognitive functions. Here, we overview the relations between adult neurogenesis and AD, and try to analyse the controversies in the field. We also summarise recent data obtained in the triple transgenic model of AD that show time- and region-specific impairment of neurogenesis, which may account for the early changes in synaptic plasticity and cognitive impairments that develop prior to gross neurodegenerative alterations and that could underlie new rescue therapies.

Modulation of aberrant CDK5 signaling rescues impaired neurogenesis in models of Alzheimer's disease

Recent studies show that in Alzheimer's disease (AD), alterations in neurogenesis contribute to the neurodegenerative process. Neurodegeneration in AD has been associated with aberrant signaling through the cyclin-dependent kinase-5 (CDK5) pathway via its activators p35/p25; however, the role of CDK5 in the mechanisms of defective adult neurogenesis in AD is unknown. First, to study AD-like abnormal activation of CDK5 signaling in an in vitro model of neurogenesis, neuronal progenitor cells (NPCs) were infected with a viral vector expressing p35, and exposed to amyloid-β protein (Aβ(1-42)). These conditions resulted in impaired maturation and neurite outgrowth in vitro, and these effects were reversed by pharmacological or genetic inhibition of CDK5. Similarly, neurogenesis was impaired in a transgenic mouse model of AD that expresses high levels of amyloid precursor protein (APP), and this effect was reversed in transgenic mice crossed with a CDK5 heterozygous-deficient mouse line. A similar rescue effect was observed in APP transgenic mice treated with Roscovitine, a pharmacological inhibitor of CDK5. Taken together, these data suggest that the CDK5 signaling pathway has a critical role in maintaining the integrity of NPCs and neuronal maturation in the adult hippocampus. Moreover, potential therapeutic approaches could focus on modulating the aberrant activity of CDK5 to target the neurogenic and neurodegenerative alterations in AD.

Prospects and limitations of using endogenous neural stem cells for brain regeneration

Neural stem cells (NSCs) are capable of producing a variety of neural cell types, and are indispensable for the development of the mammalian brain. NSCs can be induced in vitro from pluripotent stem cells, including embryonic stem cells and induced-pluripotent stem cells. Although the transplantation of these exogenous NSCs is a potential strategy for improving presently untreatable neurological conditions, there are several obstacles to its implementation, including tumorigenic, immunological, and ethical problems. Recent studies have revealed that NSCs also reside in the adult brain. The endogenous NSCs are activated in response to disease or trauma, and produce new neurons and glia, suggesting they have the potential to regenerate damaged brain tissue while avoiding the above-mentioned problems. Here we present an overview of the possibility and limitations of using endogenous NSCs in regenerative medicine.

Ectopic expression of α-synuclein affects the migration of neural stem cells in mouse subventricular zone

α-Synuclein (α-syn) is a key protein in Parkinson's disease (PD), and its abnormal accumulation is implicated only not in the loss of dopaminergic neurons in the substantia nigra but also in impairment of olfactory bulb (OB) in PD. Olfactory dysfunction could arise from these OB changes as an early symptom in PD. We reported previously the impairment of neuronal stem cell (NSC) proliferation in the subventricular zone, which is upstream of OB in PD models. Reduction of NSC generation could potentially lead to olfactory dysfunction, which is commonly associated with and precedes the motor symptoms by several years in PD. Here, we investigated neurosphere formation in vitro and migration of NSCs in vivo after transduction of α-syn-encoding retroviral vector to characterize the function of α-syn in NSC. Over-expression of α-syn caused less effective formation of neurospheres and induced morphological changes. Fluorescence-activated cell sorting showed diminished NSC cell cycle progression induced by over-expression of α-syn. Intriguingly, suppression of NSC migration along the rostral migratory stream was observed when the α-syn-encoding vector was directly injected into the subventricular zone of mice in vivo. These results indicate that α-syn affects the generation of NSC and suggest that this protein could serve as a tool for the design of potentially useful therapy for PD patients.

Planar polarity of multiciliated ependymal cells involves the anterior migration of basal bodies regulated by non-muscle myosin II

Motile cilia generate constant fluid flow over epithelial tissue, and thereby influence diverse physiological processes. Such functions of ciliated cells depend on the planar polarity of the cilia and on their basal bodies being oriented in the downstream direction of fluid flow. Recently, another type of basal body planar polarity, characterized by the anterior localization of the basal bodies in individual cells, was reported in the multiciliated ependymal cells that line the surface of brain ventricles. However, little is known about the cellular and molecular mechanisms by which this polarity is established. Here, we report in mice that basal bodies move in the apical cell membrane during differentiation to accumulate in the anterior region of ependymal cells. The planar cell polarity signaling pathway influences basal body orientation, but not their anterior migration, in the neonatal brain. Moreover, we show by pharmacological and genetic studies that non-muscle myosin II is a key regulator of this distribution of basal bodies. This study demonstrates that the orientation and distribution of basal bodies occur by distinct mechanisms.

New neurons clear the path of astrocytic processes for their rapid migration in the adult brain

In the long-range neuronal migration of adult mammals, young neurons travel from the subventricular zone to the olfactory bulb, a long journey (millimeters to centimeters, depending on the species). How can these neurons migrate through the dense meshwork of neuronal and glial processes of the adult brain parenchyma? Previous studies indicate that young neurons achieve this by migrating in chains through astrocytic tunnels. Here, we report that young migrating neurons actively control the formation and maintenance of their own migration route. New neurons secrete the diffusible protein Slit1, whose receptor, Robo, is expressed on astrocytes. We show that the Slit-Robo pathway is required for morphologic and organizational changes in astrocytes that result in the formation and maintenance of the astrocytic tunnels. Through this neuron-glia interaction, the new neurons regulate the formation of the astrocytic meshwork that is needed to enable their rapid and directional migration in adult brain.

Subventricular zone-derived neural progenitor cells migrate along a blood vessel scaffold toward the post-stroke striatum

The subventricular zone (SVZ) of the adult brain contains neural stem cells that have the capacity to regenerate new neurons after various insults. Brain ischemia causes damage to brain tissue and induces neural regeneration together with angiogenesis. We previously reported that, after ischemic injury in mice, SVZ-derived neural progenitor cells (NPCs) migrate into the striatum, and these NPCs are frequently associated with blood vessels in the regenerating brain tissue. Here we studied the role of blood vessels during the neural regeneration in more detail. BrdU administration experiments revealed that newly generated NPCs were associated with both newly formed and pre-existing blood vessels in the ischemic striatum, suggesting that the angiogenic environment is not essential for the neuron-blood vessel interaction. To observe migrating NPCs and blood vessels simultaneously in damaged brain tissue, we performed live imaging of cultured brain slices after ischemic injury. In this system, we virally labeled SVZ-derived NPCs in Flk1-EGFP knock-in mice in which the blood vessels are labeled with EGFP. Our results provide direct evidence that SVZ-derived NPCs migrate along blood vessels from the SVZ toward the ischemic region of the striatum. The leading process of the migrating NPCs was closely associated with blood vessels, suggesting that this interaction provides directional guidance to the NPCs. These findings suggest that blood vessels play an important role as a scaffold for NPCs migration toward the damaged brain region.

Molecular mechanisms of neurodegeneration in Alzheimer's disease

Alzheimer's disease (AD) is characterized by cognitive impairment, progressive neurodegeneration and formation of amyloid-beta (Abeta)-containing plaques and neurofibrillary tangles composed of hyperphosphorylated tau. The neurodegenerative process in AD is initially characterized by synaptic damage accompanied by neuronal loss. In addition, recent evidence suggests that alterations in adult neurogenesis in the hippocampus might play a role. Synaptic loss is one of the strongest correlates to the cognitive impairment in patients with AD. Several lines of investigation support the notion that the synaptic pathology and defective neurogenesis in AD are related to progressive accumulation of Abeta oligomers rather than fibrils. Abnormal accumulation of Abeta resulting in the formation of toxic oligomers is the result of an imbalance between the levels of Abeta production, aggregation and clearance. Abeta oligomers might lead to synaptic damage by forming pore-like structures with channel activity; alterations in glutamate receptors; circuitry hyper-excitability; mitochondrial dysfunction; lysosomal failure and alterations in signaling pathways related to synaptic plasticity, neuronal cell and neurogenesis. A number of signaling proteins, including fyn kinase; glycogen synthase kinase-3beta (GSK3beta) and cyclin-dependent kinase-5 (CDK5), are involved in the neurodegenerative progression of AD. Therapies for AD might require the development of anti-aggregation compounds, pro-clearance pathways and blockers of hyperactive signaling pathways.

In vivo MRI of neural cell migration dynamics in the mouse brain

Multipotent neuroblasts (NBs) are produced throughout life by neural stem cells in the forebrain subventricular zone (SVZ), and are able to travel long distances to the olfactory bulb. On arrival in the bulb, migrating NBs normally replace olfactory neurons, raising interest in their potential for novel cell replacement therapies in various disease conditions. An understanding of the migratory capabilities of NBs is therefore important, but as yet quantitative in vivo measurement of cell migration has not been possible. In this study, targeted intracerebral injections of iron-oxide particles to the mouse SVZ were used to label resident NBs in situ, and their migration was tracked noninvasively over time with magnetic resonance imaging (MRI). Quantitative intensity metrics were employed to identify labeled cells and to show that cells are able to travel at speeds up to 100 microm/h en route to the olfactory bulb, but that distribution through the olfactory bulb occurs at a much slower rate. In addition, comparison of histological and MRI measures of iron-oxide particle distribution were in excellent agreement. Immunohistochemistry analysis 1-3 weeks after labeling revealed that the majority of labeled cells in the olfactory bulb were immature neurons, although iron-oxide particles were also found in astrocytes and microglia. This work indicates that dynamic measurements of endogenous cell migration can be made with MRI and represents the first in vivo measurement of NB migration rates. The use of MRI in future studies tracking endogenous NB cells will permit a more complete evaluation of their role during homeostasis at various developmental stages and during disease progression.

[Endogenous repair mechanisms in the brain]

Most of neurons are generated by neural stem cells in the developing brain at the embryonic or neonatal stages. However, recent studies indicate that adult brain also contains neural stem cells that continuously generate new neurons. Neurogenesis can be observed in the adult subventricular zone (SVZ) at the lateral wall of the lateral ventricles of various animal species including primates. Young neurons generated in the SVZ migrate over long distances and mature after they reach their final destinations where they function. In this talk, I will present our recent studies using animal models on the mechanisms of neuronal production, migration and maturation in the adult brain under physiological and pathological conditions, and discuss the possibility of their application into regeneration therapies for ischemic brain diseases.

Activation of cyclin-dependent kinase 5 is a consequence of cell death

Cyclin-dependent kinase 5 (Cdk5) is similar to other Cdks but is activated during cell differentiation and cell death rather than cell division. Since activation of Cdk5 has been reported in many situations leading to cell death, we attempted to determine if it was required for any form of cell death. We found that Cdk5 is activated during apoptotic deaths and that the activation can be detected even when the cells continue to secondary necrosis. This activation can occur in the absence of Bim, calpain, or neutral cathepsins. The kinase is typically activated by p25, derived from p35 by calpain-mediated cleavage, but inhibition of calpain does not affect cell death or the activation of Cdk5. Likewise, RNAi-forced suppression of the synthesis of Cdk5 does not affect the incidence or kinetics of cell death. We conclude that Cdk5 is activated as a consequence of metabolic changes that are common to many forms of cell death. Thus its activation suggests processes during cell death that will be interesting or important to understand, but activation of Cdk5 is not necessary for cells to die.

Making a neuron: Cdk5 in embryonic and adult neurogenesis

Cyclin-dependent kinase 5 (Cdk5) has been implicated in the migration, maturation and survival of neurons born during embryonic development. New evidence suggests that Cdk5 has comparable but also distinct functions in adult neurogenesis. Here we summarize accumulating evidence on the role of Cdk5 in regulation of the cell cycle, migration, survival, maturation and neuronal integration. We specifically highlight the many similarities and few tantalizing differences in the roles of Cdk5 in the embryonic and adult brain. We discuss the signaling pathways that might contribute to Cdk5 action in regulating embryonic and adult neurogenesis, highlighting future research directions that will help to clarify the mechanisms underlying lifelong neurogenesis in the mammalian brain.

Transcriptional regulation of tangential neuronal migration in the developing forebrain

In the developing brain, the tangential mode of migration appears as an efficient strategy for newly generated neurons to reach destinations that are far away from their site of origin, as opposed to local migration along radial glia process. The ganglionic eminence, in the vertebrate subpallium, is the main source of tangentially migrating neurons in the forebrain. However, little is known about the transcriptional control of such long-distance tangential migrations. Here, we review recent findings showing that homeodomain (HD) transcription factors (TFs) regulate the tangential migration of telencephalic neurons through the expression of several downstream targets including other TFs, axon guidance molecules, and cytoskeletal components. This molecular mechanism also seems to apply to tangentially migrating neurons in other parts of the brain.

Adult neurogenesis and its alteration under pathological conditions

Even in the adult brain, neural stem cells in the dentate gyrus and subventricular zone continue to produce neuronal precursors, which migrate and differentiate into functional mature neurons. This physiological neurogenesis is thought to be involved in neuronal plasticity. Moreover, recent studies indicate that adult neurogenesis can change in response to various brain insults, including psychiatric diseases, stroke, and neurodegenerative disorders. Although increased neurogenesis in these pathological conditions could contribute to the restoration and regeneration of the damaged brain, an inadequate and/or excessive supply of new neurons, or suppressed neurogenesis, could contribute to their pathophysiology. To develop successful regenerative treatments for the injured brain, we need to understand more precisely and comprehensively the mechanisms regulating adult neurogenesis under both physiological and pathological conditions.

Cdk5 is essential for adult hippocampal neurogenesis

The molecular factors regulating adult neurogenesis must be understood to harness the therapeutic potential of neuronal stem cells. Although cyclin-dependent kinase 5 (Cdk5) plays a critical role in embryonic corticogenesis, its function in adult neurogenesis is unknown. Here, we assessed the role of Cdk5 in the generation of dentate gyrus (DG) granule cell neurons in adult mice. Cre recombinase-mediated conditional knockout (KO) of Cdk5 from stem cells and their progeny in the DG subgranular zone (SGZ) prevented maturation of new neurons. In addition, selective KO of Cdk5 from mature neurons throughout the hippocampus reduced the number of immature neurons. Furthermore, Cdk5 gene deletion specifically from DG granule neurons via viral-mediated gene transfer also resulted in fewer immature neurons. In each case, the total number of proliferating cells was unaffected, indicating that Cdk5 is necessary for progression of adult-generated neurons to maturity. This role for Cdk5 in neurogenesis was activating-cofactor specific, as p35 KO but not p39 KO mice also had fewer immature neurons. Thus, Cdk5 has an essential role in the survival, but not proliferation, of adult-generated hippocampal neurons through both cell-intrinsic and cell-extrinsic mechanisms.

Unravelling the complex role of Cdk5 in the developing cerebral cortex

The normal development of the mammalian CNS is entirely dependent on the coordinated behavior of its cellular components. Particular importance is attributed to the correct morphology, migration and communication of neurons. Recent years have seen the identification of many extracellular, cell surface and intracellular signaling molecules that are important for normal CNS development, consequently triggering huge progress in our understanding of the complex processes involved. A key molecule to emerge is Cdk5. To date, Cdk5 has been functionally linked with controlled neuronal morphology, migration, synaptic function, cognition, drug addiction, neuronal death and neurodegeneration. The complexity of its function has been confirmed by the ever increasing number of diverse upstream regulators, protein substrates and biological consequences of altered catalytic function. The aim of this review is to consolidate recent findings concerning the role of Cdk5 in the developing nervous system, particularly the cerebral cortex, where its importance is most clearly evidenced.