The G1 restriction point as critical regulator of neocortical neuronogenesis

Evolving features of human cortical development and the emerging roles of non-coding RNAs in neural progenitor cell diversity and function

The human cerebral cortex is a uniquely complex structure encompassing an unparalleled diversity of neuronal types and subtypes. These arise during development through a series of evolutionary conserved processes, such as progenitor cell proliferation, migration and differentiation, incorporating human-associated adaptations including a protracted neurogenesis and the emergence of novel highly heterogeneous progenitor populations. Disentangling the unique features of human cortical development involves elucidation of the intricate developmental cell transitions orchestrated by progressive molecular events. Crucially, developmental timing controls the fine balance between cell cycle progression/exit and the neurogenic competence of precursor cells, which undergo morphological transitions coupled to transcriptome-defined temporal states. Recent advances in bulk and single-cell transcriptomic technologies suggest that alongside protein-coding genes, non-coding RNAs exert important regulatory roles in these processes. Interestingly, a considerable number of novel long non-coding RNAs (lncRNAs) and microRNAs (miRNAs) have appeared in human and non-human primates suggesting an evolutionary role in shaping cortical development. Here, we present an overview of human cortical development and highlight the marked diversification and complexity of human neuronal progenitors. We further discuss how lncRNAs and miRNAs constitute critical components of the extended epigenetic regulatory network defining intermediate states of progenitors and controlling cell cycle dynamics and fate choices with spatiotemporal precision, during human neurodevelopment.

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

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

Ranbp1, Deleted in DiGeorge/22q11.2 Deletion Syndrome, is a Microcephaly Gene That Selectively Disrupts Layer 2/3 Cortical Projection Neuron Generation

Ranbp1, a Ran GTPase-binding protein implicated in nuclear/cytoplasmic trafficking, is included within the DiGeorge/22q11.2 Deletion Syndrome (22q11.2 DS) critical region associated with behavioral impairments including autism and schizophrenia. Ranbp1 is highly expressed in the developing forebrain ventricular/subventricular zone but has no known obligate function during brain development. We assessed the role of Ranbp1 in a targeted mouse mutant. Ranbp1(-/-) mice are not recovered live at birth, and over 60% of Ranbp1(-/-) embryos are exencephalic. Non-exencephalic Ranbp1(-/-) embryos are microcephalic, and proliferation of cortical progenitors is altered. At E10.5, radial progenitors divide more slowly in the Ranpb1(-/-) dorsal pallium. At E14.5, basal, but not apical/radial glial progenitors, are compromised in the cortex. In both E10.5 apical and E14.5 basal progenitors, M phase of the cell cycle appears selectively retarded by loss of Ranpb1 function. Ranbp1(-/-)-dependent proliferative deficits substantially diminish the frequency of layer 2/3, but not layer 5/6 cortical projection neurons. Ranbp1(-/-) cortical phenotypes parallel less severe alterations in LgDel mice that carry a deletion parallel to many (but not all) 22q11.2 DS patients. Thus, Ranbp1 emerges as a microcephaly gene within the 22q11.2 deleted region that may contribute to altered cortical precursor proliferation and neurogenesis associated with broader 22q11.2 deletion.

Histone deacetylase inhibition mediates urocortin-induced antiproliferation and neuronal differentiation in neural stem cells

During cortical development, cell proliferation and cell cycle exit are carefully regulated to ensure that the appropriate numbers of cells are produced. Urocortin (UCN) is a member of the corticotrophin releasing hormone (CRH) family of neuropeptides that regulates stress responses. UCN is widely distributed in adult rat brain. However, the expression and function of UCN in embryonic brain is, as yet, unclear. Here, we show that UCN is endogenously expressed in proliferative zones of the developing cerebral cortex and its receptors are exhibited in neural stem cells (NSCs), thus implicating the neuropeptide in cell cycle regulation. Treatment of cultured NSCs or organotypic slice cultures with UCN markedly reduced cell proliferation. Furthermore, blocking of endogenous UCN/CRHRs system either by treatment with CRHRs antagonists or by neutralization of secreted UCN with anti-UCN antibody increased NSCs proliferation. Cell cycle kinetics analysis demonstrated that UCN lengthened the total cell cycle duration via increasing the G1 phase and accelerated cell cycle exit. UCN directly inhibited the histone deacetylase (HDAC) activity and induced a robust increase in histone H3 acetylation levels. Using pharmacological and RNA interference approaches, we further demonstrated that antiproliferative action of UCN appeared to be mediated through a HDAC inhibition-induced p21 upregulation. Moreover, UCN treatment in vitro and in vivo led to an increase in neuronal differentiation of NSCs. These findings suggest that UCN might contribute to regulate NSCs proliferation and differentiation during cortical neurogenesis.

Dynamic interactions between intermediate neurogenic progenitors and radial glia in embryonic mouse neocortex: potential role in Dll1-Notch signaling

The mammalian neocortical progenitor cell niche is composed of a diverse repertoire of neuroepithelial cells, radial glia (RG), and intermediate neurogenic progenitors (INPs). Previously, live-cell imaging experiments have proved crucial in identifying these distinct progenitor populations, especially INPs, which amplify neural output by undergoing additional rounds of proliferation before differentiating into new neurons. INPs also provide feedback to the RG pool by serving as a source of Delta-like 1 (Dll1), a key ligand for activating Notch signaling in neighboring cells, a well-known mechanism for maintaining RG identity. While much is known about Dll1-Notch signaling at the molecular level, little is known about how this cell-cell contact dependent feedback is transmitted at the cellular level. To investigate how RG and INPs might interact to convey Notch signals, we used high-resolution live-cell multiphoton microscopy (MPM) to directly observe cellular interactions and dynamics, in conjunction with Notch-pathway specific reporters in the neocortical neural stem cell niche in organotypic brain slices from embryonic mice. We found that INPs and RG interact via dynamic and transient elongate processes, some apparently long-range (extending from the subventricular zone to the ventricular zone), and some short-range (filopodia-like). Gene expression profiling of RG and INPs revealed further progenitor cell diversification, including different subpopulations of Hes1+ and/or Hes5+ RG, and Dll1+ and/or Dll3+ INPs. Thus, the embryonic progenitor niche includes a network of dynamic cell-cell interactions, using different combinations of Notch signaling molecules to maintain and likely diversify progenitor pools.

Primary cilia and Gli3 activity regulate cerebral cortical size

During neural development patterning, neurogenesis, and overall growth are highly regulated and coordinated between different brain regions. Here, we show that primary cilia and the regulation of Gli activity are necessary for the normal expansion of the cerebral cortex. We show that loss of Kif3a, an important functional component of primary cilia, leads to the degeneration of primary cilia, marked overgrowth of the cortex, and altered cell cycle kinetics within cortical progenitors. The G1 phase of the cell cycle is shortened through a mechanism likely involving reduced Gli3 activity and a resulting increase in expression of cyclin D1 and Fgf15. The defects in Gli3 activity alone are sufficient to accelerate cell cycle kinetics and cause the molecular changes seen in brains that lack cilia. Finally, we show that levels of full-length and repressor Gli3 proteins are tightly regulated during normal development and correlate with changes in expression of two known Shh-target genes, CyclinD1 and Fgf15, and with the normal lengthening of the cell cycle during corticogenesis. These data suggest that Gli3 activity is regulated through the primary cilium to control cell cycle length in the cortex and thus determine cortical size.

Cytokinetics of adult rat SVZ after EAE

Cytokinetics regulating cell cycle division can be modulated by several endogenous factors. EAE (experimental autoimmune encephalomyelitis) increases proliferation of progenitor cells in the subventricular zone (SVZ). Using cumulative and single S phase labeling with 5-bromo-2-deoxyuridine, we examined cell cycle kinetics of neural progenitor cells in the SVZ after EAE. 20% of the SVZ cell population was proliferating in adjuvant control rats. However, EAE significantly increased them up to 27% and these cells had a cell cycle length (TC) of 15.6h, significantly (P<0.05) shorter than the 19 h TC in non EAE SVZ cells. Few TUNEL (+) cells were detected in the SVZ cells of adjuvant controls. EAE increased (P<0.05) TUNEL (+) nuclei in SVZ suggesting early stage progenitor cell death. Cell cycle phase analysis revealed that EAE substantially shortened the length of the G1 phase (9.6h) compared with the G1 phase of 12.25 h in adjuvant control SVZ cells (P<0.05). This reduction in G1 contributes to EAE-induced reduction of TC because no significant changes were detected on the length of S, G2 and M phases between the two groups. Our results show a surge in proliferating progenitor cells in the SVZ with concomitant increase in apoptotic cell death after EAE. Furthermore, increase in the SVZ proliferation contributes to EAE-induced neurogenesis and this increase is regulated by shortening the G1 phase. Our investigation suggests the activation of quiescent cells in SVZ to generate actively proliferating progenitors. Moreover, the increase in the cell death in proliferating population may contribute towards negative regulation of proliferative cell number and hence diminished regenerative capacity of CNS following EAE.

FGF15 promotes neurogenesis and opposes FGF8 function during neocortical development

BACKGROUND

Growth, differentiation and regional specification of telencephalic domains, such as the cerebral cortex, are regulated by the interplay of secreted proteins produced by patterning centers and signal transduction systems deployed in the surrounding neuroepithelium. Among other signaling molecules, members of the fibroblast growth factor (FGF) family have a prominent role in regulating growth, differentiation and regional specification. In the mouse telencephalon the rostral patterning center expresses members of the Fgf family (Fgf8, Fgf15, Fgf17, Fgf18). FGF8 and FGF17 signaling have major roles in specification and morphogenesis of the rostroventral telencephalon, whereas the functions of FGF15 and FGF18 in the rostral patterning center have not been established.

RESULTS

Using Fgf15-/- mutant mice, we provide evidence that FGF15 suppresses proliferation, and that it promotes differentiation, expression of CoupTF1 and caudoventral fate; thus, reducing Fgf15 and Fgf8 dosage have opposite effects. Furthermore, we show that FGF15 and FGF8 differentially phosphorylate ERK (p42/44), AKT and S6 in cultures of embryonic cortex. Finally, we show that FGF15 inhibits proliferation in cortical cultures.

CONCLUSION

FGF15 and FGF8 have distinct signaling properties, and opposite effects on neocortical patterning and differentiation; FGF15 promotes CoupTF1 expression, represses proliferation and promotes neural differentiation.

Japanese encephalitis virus infects neural progenitor cells and decreases their proliferation

Japanese encephalitis virus (JEV), a common cause of encephalitis in humans, especially in children, leads to substantial neuronal injury. The survivors of JEV infection have severe cognitive impairment, motor and behavioral disorders. We hypothesize that depletion of neural progenitor cells (NPCs) by the virus culminates in neurological sequelae in survivors of Japanese encephalitis (JE). We utilized both in vivo model of JEV infection and in vitro neurosphere cultures to study progressive JEV infection. Cellular infection and cell death was determined by flow cytometry. BrdU administration in animals and in neurospheres was used to determine the proliferative ability of NPCs. JEV leads to massive loss of actively proliferating NPC population from the subventricular zone (SVZ). The ability of JEV infected subventricular zone cells to form neurospheres is severely compromised. This can be attributed to JEV infection in NPCs, which however do not result in robust death of the resilient NPC cells. Instead, JEV suppresses the cycling ability of these cells, preventing their proliferation. JEV primarily targets at a critical postnatal age and severely diminishes the NPC pool in SVZ, thus impairing the process of recovery after the insult. This arrested growth and proliferation of NPCs might have an effect on the neurological consequences in JE survivors.

Lengthening the G(1) phase of neural progenitor cells is concurrent with an increase of symmetric neuron generating division after stroke

The proportion of neural progenitors that remain in (P fraction) and exit from (Q fraction) the cell cycle determines the degree of neurogenesis. Using S-phase labeling with 5-bromo-2'-deoxyuridine and a double nucleoside analog-labeling scheme, we measured the cell-cycle kinetics of neural progenitors and estimated the proportion of P and Q fractions in the subventricular zone (SVZ) of adult rats subjected to stroke. Stroke increased SVZ cell proliferation, starting 2 days, reaching a maximum 4 and 7 days after stroke. The cell-cycle length (T(C)) of SVZ cells changed dynamically over a period of 2 to 14 days after stroke, with the shortest length of 11 h at 2 days after stroke. The reduction of the T(C) resulted from a decrease of the G(1) phase because the G(2), M, and S phases were unchanged. In addition, during this period, reduction of the G(1) phase was concomitant with an increase in the P fraction, whereas an augmentation of the Q fraction was associated with lengthening of the G(1) phase. Furthermore, approximately 90% of cells that exited the cell cycle were neurons and the population of a pair of dividing daughter cells with a neuronal marker increased from 9% at 2 days to 26% at 14 days after stroke. These data suggest that stroke triggers early expansion of the progenitor pool via shortening the cell-cycle length and retaining daughter cells within the cell cycle, and the lengthening of G(1) leads to daughter cells exiting the cell cycle and differentiating into neurons.

COUP-TFI coordinates cortical patterning, neurogenesis, and laminar fate and modulates MAPK/ERK, AKT, and beta-catenin signaling

A major unsolved question in cortical development is how proliferation, neurogenesis, regional growth, regional identity, and laminar fate specification are coordinated. Here we provide evidence, using loss-of-function and gain-of-function manipulations, that the COUP-TFI orphan nuclear receptor promotes ventral cortical fate, promotes cell cycle exit and neural differentiation, regulates the balance of early- and late-born neurons, and regulates the balanced production of different types of layer V cortical projection neurons. We suggest that COUP-TFI controls these processes by repressing Mapk/Erk, Akt, and beta-catenin signaling.

The linkage of neural progenitor cell cycle profiles between embryonic and adult stroke models: Analytical approach II

Cell kinetics employed for embryonic models was modified and used to study the neuronogenesis in the subventricular zone (SVZ) in adult rats subjected to stroke. Enhanced analytical approaches were introduced and used to compare the cell cycle length (T(C)) and length in G(1) phase, T(G1), at various times after stroke to study the correlation between T(G1) and T(C) and to compare cell cycle evolution and proliferation profiles between the stroke and embryonic models. Our data indicate that cell cycle kinetics for the embryonic model can be applied to stroke in the adult. Significant reduction of T(G1) early after stroke (p<0.05) corresponds to an increase of neural progenitor cells remaining in the cycle at early times and cells exiting at later times. T(G1) correlates with T(C) (r=0.99, p<0.05). In conclusion, the analytical approaches proposed can be used to study the cell proliferation profiles in adult rats subjected to stroke with and without stroke therapy. The cell kinetics the cell proliferation profile differs between the stroke and embryonic models. T(C) evolution is three-fold slower in the cells and leave the cycle earlier and more frequently in the stroke model, compared to the embryonic model. T(C) is a surrogate measure of T(G1).

Decreased neurogenesis in aged rats results from loss of granule cell precursors without lengthening of the cell cycle

It is well established that neurogenesis in the dentate gyrus slows with aging, but it is unclear whether this change is due to slowing of the cell cycle, as occurs during development, or to loss of precursor cells. In the current study, we find that the cell cycle time of granule cell precursors in middle-aged male rats is not significantly different from that in young adults. The size of the precursor pool, however, was 3-4 times smaller in the middle-aged rats, as determined using both cumulative bromodeoxyuridine (BrdU) labeling as well as labeling with the endogenous marker of cell proliferation, proliferating cell nuclear antigen (PCNA). Loss of precursor cells was much greater in the granule cell layer than in the hilus, suggesting that dividing cells in the hilus belong to a distinct population, most likely glial progenitors, that are less affected by aging than neuronal precursors. BrdU-labeled precursor cells and young neurons, labeled with doublecortin, appeared to be lost equally from rostral and caudal, as well as suprapyramidal and infrapyramidal, subregions of the granule cell layer. However, doublecortin staining did show large parts of the caudal granule cell layer with few if any young neurons at both ages. Taken together, these findings indicate that precursor cells are not distributed evenly within the dentate gyrus in adulthood but that precursors are lost from throughout the dentate gyrus in old age with no concomitant change in the cell cycle time.

Diethylstilbestrol alters the population dynamic of neural precursor cells in the neonatal male rat dentate gyrus

Little is known about how estrogens influence neurogenesis in the newborn male rodent. Herein, we examined the effects of neonatal diethylstilbestrol (DES) exposure on the proliferation and survival of type-1 and type-2 neural precursor cells (NPC) in the dentate gyrus of male rats. This was achieved by exposing newborn male pups to DES on postnatal day (PND) 1, PND3, PND5, and PND7, sacrificed at PND8 or PND21, followed by double immunohistochemistry and morphometric analysis of hippocampal dentate gyrus. Furthermore, vascular endothelial growth factor (VEGF) and brain-derived neurotrophic factor (BDNF) mRNA expression was evaluated in hippocampal tissue blocks by real time RT-PCR. At PND8, the density of total proliferating NPC decreased in DES-treated animals. This reduction was due to a significant decrease in the mitotic rate of type-2 cells only, since type-1 NPCs did not show changes in the proliferation index. Type-2 NPCs expressed the cell-cycle inhibitor p27(kip1) and its expression was clearly augmented in the DES-treated group. Furthermore, the number of apoptotic cells in the dentate gyrus of DES-treated rats decreased. Surprisingly, DES treatment enhanced cell survival and increased NPCs proliferation when animals were examined 14 days after treatment. VEGF mRNA expression showed a positive correlation with NPCs proliferation and BDNF mRNA levels were higher in DES-treated animals at both time points examined. Collectively, these results indicate that hippocampal NPCs proliferation and survival is a critical target of DES exposure during the early postnatal period. VEGF and BDNF are proposed as key mediators of DES-induced NPC mitotic response.

Computational models of neocortical neuronogenesis and programmed cell death in the developing mouse, monkey, and human

This paper presents a computational model allowing quantitative simulations of acquisition of neocortical neuronal number across mammalian species. When extrapolating scientific findings from rodents to humans, it is particularly pertinent to acknowledge the importance of the accelerated enlargement of the neocortex during human evolution. Neocortex development is marked by discrete stages of neural progenitor cell proliferation and death, neuronal differentiation, and neuronal programmed cell death. We have developed computational models of human and rhesus monkey neocortical neuronal cell acquisition based on experimentally derived parameters of cell cycle length, commitment to cell cycle exit, and cell death. Our model results agree with independent stereological studies estimating neocortical neuron number in adult and developing rhesus monkey and human. Comparisons of our primate models with previously developed rodent models suggest correlations between the lengthening of the duration of the neuronogenesis period and a lengthening of the cellular processes of cell cycle progression and death can account for the vast increase in size of the primate neocortex. Furthermore, when compared with rodents, we predict that cell death may play a larger role in shaping the primate neocortex. Our mathematical models of the development and evolution of the neocortex provide a quantitative, biologically based construct for extrapolation between rodent and humans. These models can assist in focusing future experimental research on the differing mechanisms of rodent versus human neocortical development.

Linkage of cell cycle kinetics between embryonic and adult stroke models: an analytical approach

In the adult brain, neurogenesis occurs in the subventricular zone (SVZ) of the lateral ventricle. The proliferating population and the cell cycle kinetics in the ventricular zone regulate cortical neurogenesis during the development. Using the embryonic model, we investigated kinetics of SVZ cells in adult rats after stroke, incorporating migration of SVZ cells to the ischemic boundary. Concurrent linear regressions were considered through iteration to improve precision of parameter estimation. We found no model-fit difference in stroke with and without the migration (p=0.31), suggesting no migration effect on assessment of the cell kinetics. Stroke increased SVZ cell proliferation (20% in non-stroke and 31% in stroke p<0.01). Cell cycle durations in stroke were reduced for the total cycle length (19h for non-stroke and 15.3h for stroke, p<0.05), in G1 phase (12.6 h for non-stroke and 9.6 h for stroke, p<0.01), but were the same in S, M2, and in G2 phases compared to non-stroke, indicating that stroke reduces the total cell cycle length, specially in G1 phase. We conclude that cell cycle kinetic models for cortical development can be adapted to the kinetics of adult SVZ cells after stroke. The analytical approach may be useful for studying neural progenitor cell proliferation under different treatments.

Reduction of the cell cycle length by decreasing G1 phase and cell cycle reentry expand neuronal progenitor cells in the subventricular zone of adult rat after stroke

A critical determinant of proliferation of progenitor cells is the duration of the cell division cycle. Stroke increases proliferation of progenitor cells in the subventricular zone (SVZ). Using cumulative and single S-phase labeling with 5-bromo-2'-deoxyuridine, we examined cell cycle kinetics of neural progenitor cells in the SVZ after stroke. In nonstroke rats, 20% of the SVZ cell population was proliferating. However, stroke significantly increased dividing cells up to 31% and these cells had a cell cycle length (T(C)) of 15.3 h, significantly (P < 0.05) shorter than the 19 h Tc in nonstroke SVZ cells. Few terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling-positive cells were detected in the SVZ cells of nonstroke and stroke groups, suggesting that the majority of dividing cells in the SVZ do not undergo apoptosis. Cell cycle phase analysis revealed that stroke substantially shortened the length of the G1 phase (9.6 h) compared with the G1 phase of 12.6 h in nonstroke SVZ cells (P < 0.03). This reduction in G1 contributes to stroke-induced reduction of T(C) because no significant changes were detected on the length of S, G2 and M phases between two groups. Furthermore, compared with progenitor cells in nonstroke SVZ (10%), a greater proportion (14%) of progenitor cells in stroke SVZ reentered the cell cycle after mitosis (P < 0.05). These results show that an increase in proliferating progenitor cells in the SVZ contributes to stroke-induced neurogenesis and this increase is regulated by shortening the length of the cell cycle, decreasing the G1 phase and increasing cell cycle reentry.

Tissue inhibitor of metalloproteinase-2 promotes neuronal differentiation by acting as an anti-mitogenic signal

Although traditionally recognized for maintaining extracellular matrix integrity during morphogenesis, the function of matrix metallo-proteinases (MMPs) and their inhibitors, the tissue inhibitors of metalloproteinases (TIMPs), in the mature nervous system is essentially unknown. Here, we report that TIMP-2 induces pheochromocytoma PC12 cell-cycle arrest via regulation of cell-cycle regulatory proteins, resulting in differentiation and neurite outgrowth. TIMP-2 decreases cyclins B and D expression and increases p21Cip expression. Furthermore, TIMP-2 promotes cell differentiation via activation of the cAMP/Rap1/ERK (extracellular signal-regulated kinase) pathway. Expression of dominant-negative Rap1 blocks TIMP-2-mediated neurite outgrowth. Both the cell-cycle arrest and neurite outgrowth induced by TIMP-2 was independent of MMP inhibitory activity. Consistent with the PC12 cell data, primary cultures of TIMP-2 knock-out cerebral cortical neurons exhibit significantly reduced neurite length, which is rescued by TIMP-2. These in vitro results were corroborated in vivo. TIMP-2 deletion causes a delay in neuronal differentiation, as demonstrated by the persistence of nestin-positive progenitors in the neocortical ventricular zone. The interaction of TIMP-2 with alpha3beta1 integrin in the cerebral cortex suggests that TIMP-2 promotes neuronal differentiation and maintains mitotic quiescence in an MMP-independent manner through integrin activation. The identification of molecules responsible for neuronal quiescence has significant implications for the ability of the adult brain to generate new neurons in response to injury and neurological disorders, such as Alzheimer's and Parkinson's diseases.

Transforming growth factor beta 1 promotes cell cycle exit through the cyclin-dependent kinase inhibitor p21 in the developing cerebral cortex

During cortical neurogenesis, cell proliferation and cell cycle exit are carefully regulated to ensure that the appropriate numbers of cells are produced. The antiproliferative agent transforming growth factor beta1 (TGFbeta1) and its receptors are endogenously expressed in proliferative zones of the developing cerebral cortex, thus implicating the growth factor in cell cycle regulation. The present study tested the hypothesis that TGFbeta1 promotes cell cycle exit in the cortical ventricular zone (VZ) through modulation of cell cycle protein expression, in particular cyclin D1 and the cyclin-dependent kinase inhibitors p27 and p21. Although it did not affect the length of the cell cycle, TGFbeta1 decreased the fraction of VZ-cycling cells by 21% and increased the number of VZ cells exiting the cell cycle a commensurate 24%. TGFbeta1 selectively increased the expression of p21 in the VZ. In addition, high p21 expression levels were observed in VZ cells as they exited the cell cycle, and TGFbeta1 increased the number p21-positive cells exiting the cell cycle. Collectively, these data show the following: (1) TGFbeta1 promotes cell cycle exit, (2) p21 upregulation is correlated with cell cycle exit, and (3) TGFbeta1-induced cell cycle exit is mediated by p21.

Opiates, psychostimulants, and adult hippocampal neurogenesis: Insights for addiction and stem cell biology

Once thought to produce global, nonspecific brain injury, drugs of abuse are now known to produce selective neuro-adaptations in particular brain regions. These neuro-adaptations are being closely examined for clues to the development, maintenance, and treatment of addiction. The hippocampus is an area of particular interest, as it is central to many aspects of the addictive process, including relapse to drug taking. A recently appreciated hippocampal neuro-adaptation produced by drugs as diverse as opiates and psychostimulants is decreased neurogenesis in the sub-granular zone (SGZ). While the role of adult-generated neurons is not clear, their functional integration into hippocampal circuitry raises the possibility that decreased adult SGZ neurogenesis may alter hippocampal function in such a way as to maintain addictive behavior or contribute to relapse. Here, we review the impact of opiates and psychostimulants on the different stages of cell development in the adult brain, as well as the different stages of the addictive process. We discuss how examination of drug-induced alterations of adult neurogenesis advances our understanding of the complex mechanisms by which opiates and psychostimulants affect brain function while also opening avenues for novel ways of assessing the functional role of adult-generated neurons. In addition, we highlight key discrepancies in the field and underscore the necessity to move "beyond BrdU"--beyond merely counting new hippocampal cells labeled with the S phase marker bromodeoxyuridine--so as to probe mechanistic questions about how drug-induced alterations in adult hippocampal neurogenesis occur and what the functional ramifications of alterations in neurogenesis are for addiction.

Ethanol disrupts cell cycle regulation in developing rat cortex interaction with transforming growth factor beta1

Ethanol is a potent teratogenic agent that disrupts several aspects of neuronogenesis, including the proliferation rate of cortical precursors. With regard to corticogenesis, possible targets of ethanol toxicity include soluble factors, like transforming growth factor beta1 (TGFbeta1), that regulate cortical growth and cell cycle proteins that control the kinetics of the cell cycle. The effect of ethanol on normal cell proliferation and TGFbeta1-regulated cell proliferation in the developing cortex was assessed using an organotypic slice culture model. Ethanol elongated the cell cycle, possibly through a decrease in the expression of G1 cell cycle protein cyclin D1. Further, ethanol exposure antagonized the anti-proliferative action of TGFbeta1 and blocked TGFbeta1-dependent increases in cell cycle inhibitor p21. Collectively, this evidence suggests that disruption of appropriate cell cycle protein expression and inhibition of TGFbeta1 activity are potential mechanisms underlying the effect of ethanol on cortical development.

Insulin-like growth factor-I accelerates the cell cycle by decreasing G1 phase length and increases cell cycle reentry in the embryonic cerebral cortex

Neurogenesis in the developing cerebral cortex of mice occurs in the dorsal telencephalon between embryonic day 11 (E11) and E17, during which time the majority of cortical projection neurons and some glia are produced from proliferating neuroepithelial cells in the ventricular zone. The number of cells produced by this process is governed by several factors, including cell cycle kinetics and the proportion of daughter cells exiting the cell cycle after a given round of cell division. The in vivo effects of IGF-I on cell cycle kinetics were investigated in nestin/IGF-I transgenic (Tg) embryos, in which IGF-I is overexpressed in the cerebral cortex and dorsal telencephalon. These Tg mice have been shown to exhibit increased cell number in the cortical plate by E16 and increased numbers of neurons and glia in the cerebral cortex during postnatal development. Cumulative S phase labeling with 5-bromo-2'-deoxyuridine revealed a decrease in total cell cycle length (TC) in Tg embryos on E14. This decrease in TC was found to result entirely from a reduction in the length of the G1 phase of the cell cycle from 10.66 to 8.81 hr, with no significant changes in the lengths of the S, G2, and M phases. Additionally, the proportion of daughter cells reentering the cell cycle was significantly increased by 15% in Tg embryos on E14-E15 compared with littermate controls. These data demonstrate that IGF-I regulates progenitor cell division in the ventricular zone by reducing G1 phase length and decreasing TC but increases cell cycle reentry.

Taste bud cell dynamics during normal and sodium-restricted development

Taste bud volume increases over the postnatal period to match the number of neurons providing innervation. To clarify age-related changes in fungiform taste bud volume, the current study investigated developmental changes in taste bud cell number, proliferation rate, and life span. Taste bud growth can largely be accounted for by addition of cytokeratin-19-positive taste bud cells. Examination of taste bud cell kinetics with 3H-thymidine autoradiography revealed that cell life span and turnover periods were not altered during normal development but that cells were produced more rapidly in young rats, a prominent modification that could lead to increased taste bud size. By comparison, dietary sodium restriction instituted during pre- and postnatal development results in small taste buds at adulthood as a result of fewer cytokeratin-19-positive cells. The dietary manipulation also had profound influences on taste bud growth kinetics, including an increased latency for cells to enter the taste bud and longer life span and turnover periods. These studies provide fundamental, new information about taste bud development under normal conditions and after environmental manipulations that impact nerve/target matching.

HB-GAM inhibits proliferation and enhances differentiation of neural stem cells

Proliferation of neural stem cells in the embryonic cerebral cortex is regulated by many growth factors and their receptors. Among the key molecules stimulating stem cell proliferation are FGF-2 and the FGF receptor-1. This ligand-receptor system is highly dependent on the surrounding heparan sulfates. We have found that heparin-binding growth-associated molecule (HB-GAM, also designated as pleiotrophin) regulates neural stem cell proliferation in vivo and in vitro. Deficiency of HB-GAM results in a pronounced, up to 50% increase in neuronal density in the adult mouse cerebral cortex. This phenotype arises during cortical neurogenesis, when HB-GAM knockout embryos display an enhanced proliferation rate as compared to wild-type embryos. Further, our in vitro studies show that exogenously added HB-GAM inhibits formation and growth of FGF-2, but not EGF, stimulated neurospheres, restricts the number of nestin-positive neural stem cells, and inhibits FGF receptor phosphorylation. We propose that HB-GAM functions as an endogenous inhibitor of FGF-2 in stem cell proliferation in the developing cortex.

Global analysis of gene expression in neural progenitors reveals specific cell-cycle, signaling, and metabolic networks

The genetic programs underlying neural stem cell (NSC) proliferation and pluripotentiality have only been partially elucidated. We compared the gene expression profile of proliferating neural stem cell cultures (NS) with cultures differentiated for 24 h (DC) to identify functionally coordinated alterations in gene expression associated with neural progenitor proliferation. The majority of differentially expressed genes (65%) were upregulated in NS relative to DC. Microarray analysis of this in vitro system was followed by high throughput screening in situ hybridization to identify genes enriched in the germinal neuroepithelium, so as to distinguish those expressed in neural progenitors from those expressed in more differentiated cells in vivo. NS cultures were characterized by the coordinate upregulation of genes involved in cell cycle progression, DNA synthesis, and metabolism, not simply related to general features of cell proliferation, since many of the genes identified were highly enriched in the CNS ventricular zones and not widely expressed in other proliferating tissues. Components of specific metabolic and signal transduction pathways, and several transcription factors, including Sox3, FoxM1, and PTTG1, were also enriched in neural progenitor cultures. We propose a putative network of gene expression linking cell cycle control to cell fate pathways, providing a framework for further investigations of neural stem cell proliferation and differentiation.

Lack of the cell-cycle inhibitor p27Kip1 results in selective increase of transit-amplifying cells for adult neurogenesis

The subventricular zone (SVZ) is the largest germinal layer in the adult mammalian brain and comprises stem cells, transit-amplifying progenitors, and committed neuroblasts. Although the SVZ contains the highest concentration of dividing cells in the adult brain, the intracellular mechanisms controlling their proliferation have not been elucidated. We show here that loss of the cyclin-dependent kinase inhibitor p27Kip1 has very specific effects on a population of CNS progenitors responsible for adult neurogenesis. Using bromodeoxyuridine and [(3)H]thymidine incorporation to label cells in S phase and cell-specific markers and electron microscopy to identify distinct cell types, we compared the SVZ structure and proliferation characteristics of wild-type and p27Kip1-null mice. Loss of p27Kip1 had no effect on the number of stem cells but selectively increased the number of the transit-amplifying progenitors concomitantly with a reduction in the number of neuroblasts. We conclude that cell-cycle regulation of SVZ adult progenitors is remarkably cell-type specific, with p27Kip1 being a key regulator of the cell division of the transit-amplifying progenitors.

Putative stem cells and the lineage of rod photoreceptors in the mature retina of the goldfish

The retinas of teleost fish grow continuously, in part, by neuronal hyperplasia and when lesioned will regenerate. Within the differentiated retina, the growth-associated hyperplasia results in the generation of new rod photoreceptors only, whereas injury-induced neurogenesis results in the regeneration of all retinal cell types. It is believed, however, that both new rod photoreceptors and regenerated neurons originate from the same populations of intrinsic progenitors. Experiments are described here that attempt to identify in the normal retina of goldfish neuronal progenitors intrinsic to the retina, particularly those which have remained cryptic because they divide infrequently. Long-term, systemic exposure to bromodeoxyuridine (BrdU) was used to label these cells. Five populations of proliferative cells were labeled: microglia, which are briefly described but not studied further; retinal progenitors in the circumferential germinal zone (CGZ); and rod precursors in the outer nuclear layer (ONL), both of which have been well characterized previously; and two populations of slowly-dividing cells in the inner nuclear layer (INL). The majority of these cells have a fusiform morphology, whereas the remaining ones are spherical. Longitudinal BrdU labeling suggests that the fusiform cells migrate to the ONL to replenish the pool of rod precursors. A subset of the spherical cells express pax6, although none are stained with markers of differentiated amacrine or bipolar cells. It is hypothesized that these rare, pax6-expressing cells are retinal stem cells, which give rise to the pax6-negative fusiform cells. Based on these data, two models are proposed: the first describes the lineage of rod photoreceptors in goldfish; the second is a consensus model of neurogenesis in the retinas of all teleosts.

Developmental mechanisms in the pathogenesis of neurodegenerative diseases

Cellular genes that are mutated in neurodegenerative diseases code for proteins that are expressed throughout neural development. Genetic analysis suggests that these genes are essential for a broad range of normal neurodevelopmental processes. The proteins they code for interact with numerous other cellular proteins that are components of signaling pathways involved in patterning of the neural tube and in regional specification of neuronal subtypes. Further, pathogenetic mutations of these genes can cause progressive, sublethal alterations in the cellular homeostasis of evolving regional neuronal subpopulations, culminating in late-onset cell death. Therefore, as a consequence of the disease mutations, targeted cell populations may retain molecular traces of abnormal interactions with disease-associated proteins by exhibiting changes in a spectrum of normal cellular functions and enhanced vulnerability to a host of environmental stressors. These observations suggest that the normal functions of these disease-associated proteins are to ensure the fidelity and integration of developmental events associated with the progressive elaboration of neuronal subtypes as well as the maintenance of mature neuronal populations during adult life. The ability to identify alterations within vulnerable neuronal precursors present in pre-symptomatic individuals prior to the onset of irrevocable cellular injury may help foster the development of effective therapeutic interventions using evolving pharmacologic, gene and stem cell technologies.

Neocortical malformation as consequence of nonadaptive regulation of neuronogenetic sequence

Variations in the structure of the neocortex induced by single gene mutations may be extreme or subtle. They differ from variations in neocortical structure encountered across and within species in that these "normal" structural variations are adaptive (both structurally and behaviorally), whereas those associated with disorders of development are not. Here we propose that they also differ in principle in that they represent disruptions of molecular mechanisms that are not normally regulatory to variations in the histogenetic sequence. We propose an algorithm for the operation of the neuronogenetic sequence in relation to the overall neocortical histogenetic sequence and highlight the restriction point of the G1 phase of the cell cycle as the master regulatory control point for normal coordinate structural variation across species and importantly within species. From considerations based on the anatomic evidence from neocortical malformation in humans, we illustrate in principle how this overall sequence appears to be disrupted by molecular biological linkages operating principally outside the control mechanisms responsible for the normal structural variation of the neocortex. MRDD Research Reviews 6:22-33, 2000.