Developmental expression of PC3 gene is correlated with neuronal cell birthday

[Ca2+]i fluctuation mediated by T-type Ca2+ channel is required for the differentiation of cortical neural progenitor cells

The fluctuation of intracellular calcium concentration ([Ca²⁺]_i) is known to be involved in various processes in the development of central nervous system, such as the proliferation of neural progenitor cells (NPCs), migration of intermediate progenitor cells (IPCs) from the ventricular zone (VZ) to the subventricular zone (SVZ), and migration of immature neurons from the SVZ to cortical plate. However, the roles of [Ca²⁺]_i fluctuation in NPC development, especially in the differentiation of the self-renewing NPCs into neuron-generating NPCs and immature neurons have not been elucidated. Using calcium imaging of acute cortical slices and cells isolated from mouse embryonic cortex, we examined temporal changes in the pattern of [Ca²⁺]_i fluctuations in VZ cells from E12 to E16. We observed intracellular Ca²⁺ levels in Pax6-positive self-renewing NPCs decreased with their neural differentiation. In E11, Pax6-positive NPCs and Tuj1-positive immature neurons exhibited characteristic [Ca²⁺]_i fluctuations; few Pax6-positive NPCs exhibited [Ca²⁺]_i transient, but many Tuj1-positive immature neurons did, suggesting that the change in pattern of [Ca²⁺]_i fluctuation correlate to their differentiation. The [Ca²⁺]_i fluctuation during NPCs development was mostly mediated by the T-type calcium channel and blockage of T-type calcium channel in neurosphere cultures increased the number of spheres and inhibited neuronal differentiation. Consistent with this finding, knockdown of Cav3.1 by RNAi in vivo maintained Pax6-positive cells as self-renewing NPCs, and simultaneously suppressing their neuronal differentiation of NPCs into Tbr1-positive immature neurons. These results reveal that [Ca²⁺]_i fluctuation mediated by Cav3.1 is required for the neural differentiation of Pax6-positive self-renewing NPCs.

BTG2 bridges PABPC1 RNA-binding domains and CAF1 deadenylase to control cell proliferation

While BTG2 plays an important role in cellular differentiation and cancer, its precise molecular function remains unclear. BTG2 interacts with CAF1 deadenylase through its APRO domain, a defining feature of BTG/Tob factors. Our previous experiments revealed that expression of BTG2 promoted mRNA poly(A) tail shortening through an undefined mechanism. Here we report that the APRO domain of BTG2 interacts directly with the first RRM domain of the poly(A)-binding protein PABPC1. Moreover, PABPC1 RRM and BTG2 APRO domains are sufficient to stimulate CAF1 deadenylase activity in vitro in the absence of other CCR4–NOT complex subunits. Our results unravel thus the mechanism by which BTG2 stimulates mRNA deadenylation, demonstrating its direct role in poly(A) tail length control. Importantly, we also show that the interaction of BTG2 with the first RRM domain of PABPC1 is required for BTG2 to control cell proliferation.

BTG2 promotes mRNA poly(A) tail shortening and regulates cellular differentiation. Here, Stupfler et al. show that the BTG2 APRO domain interacts with PABPC1 RRM1, allowing the former to recruit and stimulate the poly(A) tail shortening activity of CAF1 deadenylase and to control cell proliferation.

Retinoic acid signaling and neuronal differentiation

The identification of neurological symptoms caused by vitamin A deficiency pointed to a critical, early developmental role of vitamin A and its metabolite, retinoic acid (RA). The ability of RA to induce post-mitotic, neural phenotypes in various stem cells, in vitro, served as early evidence that RA is involved in the switch between proliferation and differentiation. In vivo studies have expanded this "opposing signal" model, and the number of primary neurons an embryo develops is now known to depend critically on the levels and spatial distribution of RA. The proneural and neurogenic transcription factors that control the exit of neural progenitors from the cell cycle and allow primary neurons to develop are partly elucidated, but the downstream effectors of RA receptor (RAR) signaling (many of which are putative cell cycle regulators) remain largely unidentified. The molecular mechanisms underlying RA-induced primary neurogenesis in anamniote embryos are starting to be revealed; however, these data have been not been extended to amniote embryos. There is growing evidence that bona fide RARs are found in some mollusks and other invertebrates, but little is known about their necessity or functions in neurogenesis. One normal function of RA is to regulate the cell cycle to halt proliferation, and loss of RA signaling is associated with dedifferentiation and the development of cancer. Identifying the genes and pathways that mediate cell cycle exit downstream of RA will be critical for our understanding of how to target tumor differentiation. Overall, elucidating the molecular details of RAR-regulated neurogenesis will be decisive for developing and understanding neural proliferation-differentiation switches throughout development.

Tis21 is required for adult neurogenesis in the subventricular zone and for olfactory behavior regulating cyclins, BMP4, Hes1/5 and Ids

Bone morphogenic proteins (BMPs) and the Notch pathway regulate quiescence and self-renewal of stem cells of the subventricular zone (SVZ), an adult neurogenic niche. Here we analyze the role at the intersection of these pathways of Tis21 (Btg2/PC3), a gene regulating proliferation and differentiation of adult SVZ stem and progenitor cells. In Tis21-null SVZ and cultured neurospheres, we observed a strong decrease in the expression of BMP4 and its effectors Smad1/8, while the Notch anti-neural mediators Hes1/5 and the basic helix-loop-helix (bHLH) inhibitors Id1-3 increased. Consistently, expression of the proneural bHLH gene NeuroD1 decreased. Moreover, cyclins D1/2, A2, and E were strongly up-regulated. Thus, in the SVZ Tis21 activates the BMP pathway and inhibits the Notch pathway and the cell cycle. Correspondingly, the Tis21-null SVZ stem cells greatly increased; nonetheless, the proliferating neuroblasts diminished, whereas the post-mitotic neuroblasts paradoxically accumulated in SVZ, failing to migrate along the rostral migratory stream to the olfactory bulb. The ability, however, of neuroblasts to migrate from SVZ explants was not affected, suggesting that Tis21-null neuroblasts do not migrate to the olfactory bulb because of a defect in terminal differentiation. Notably, BMP4 addition or Id3 silencing rescued the defective differentiation observed in Tis21-null neurospheres, indicating that they mediate the Tis21 pro-differentiative action. The reduced number of granule neurons in the Tis21-null olfactory bulb led to a defect in olfactory detection threshold, without effect on olfactory memory, also suggesting that within olfactory circuits new granule neurons play a primary role in odor sensitivity rather than in memory.

Genetic control of adult neurogenesis: interplay of differentiation, proliferation and survival modulates new neurons function, and memory circuits

Within the hippocampal circuitry, the basic function of the dentate gyrus is to transform the memory input coming from the enthorinal cortex into sparse and categorized outputs to CA3, in this way separating related memory information. New neurons generated in the dentate gyrus during adulthood appear to facilitate this process, allowing a better separation between closely spaced memories (pattern separation). The evidence underlying this model has been gathered essentially by ablating the newly adult-generated neurons. This approach, however, does not allow monitoring of the integration of new neurons into memory circuits and is likely to set in motion compensatory circuits, possibly leading to an underestimation of the role of new neurons. Here we review the background of the basic function of the hippocampus and of the known properties of new adult-generated neurons. In this context, we analyze the cognitive performance in mouse models generated by us and others, with modified expression of the genes Btg2 (PC3/Tis21), Btg1, Pten, BMP4, etc., where new neurons underwent a change in their differentiation rate or a partial decrease of their proliferation or survival rate rather than ablation. The effects of these modifications are equal or greater than full ablation, suggesting that the architecture of circuits, as it unfolds from the interaction between existing and new neurons, can have a greater functional impact than the sheer number of new neurons. We propose a model which attempts to measure and correlate the set of cellular changes in the process of neurogenesis with the memory function.

PC3 is involved in the shift from proliferation to differentiation and maturation in spiral ganglion neurons

PC3 is a member of the BTG/Tob family of antiproliferative genes. Here, we report the results of an analysis of PC3 protein expression in spiral ganglion neurons of the rat cochlea at embryonic days 16 (E16) and 20 (E20), and postnatal days 4 (P4) and 7 (P7). PC3 expression was observed in the cytoplasm of ganglion neurons at E16 and E20, and this protein had translocated to the nucleus by P4. The expression of Ki-67, a nuclear antigen expressed by dividing cells, was detected in ganglion neurons at E16 and E20, but not at P4 or P7. These results suggest that PC3 is involved in the shift from proliferation to differentiation and maturation in the ganglion neurons of the rat cochlea.

Impaired terminal differentiation of hippocampal granule neurons and defective contextual memory in PC3/Tis21 knockout mice

Neurogenesis in the dentate gyrus of the adult hippocampus has been implicated in neural plasticity and memory, but the molecular mechanisms controlling the proliferation and differentiation of newborn neurons and their integration into the synaptic circuitry are still largely unknown. To investigate this issue, we have analyzed the adult hippocampal neurogenesis in a PC3/Tis21-null mouse model. PC3/Tis21 is a transcriptional co-factor endowed with antiproliferative and prodifferentiative properties; indeed, its upregulation in neural progenitors has been shown to induce exit from cell cycle and differentiation. We demonstrate here that the deletion of PC3/Tis21 causes an increased proliferation of progenitor cells in the adult dentate gyrus and an arrest of their terminal differentiation. In fact, in the PC3/Tis21-null hippocampus postmitotic undifferentiated neurons accumulated, while the number of terminally differentiated neurons decreased of 40%. As a result, PC3/Tis21-null mice displayed a deficit of contextual memory. Notably, we observed that PC3/Tis21 can associate to the promoter of Id3, an inhibitor of proneural gene activity, and negatively regulates its expression, indicating that PC3/Tis21 acts upstream of Id3. Our results identify PC3/Tis21 as a gene required in the control of proliferation and terminal differentiation of newborn neurons during adult hippocampal neurogenesis and suggest its involvement in the formation of contextual memories.

Tis21 expression marks not only populations of neurogenic precursor cells but also new postmitotic neurons in adult hippocampal neurogenesis

During embryonic cortical development, expression of Tis21 is associated with cell cycle lengthening and neurogenic divisions of progenitor cells. We here investigated if the expression pattern of Tis21 also correlates with the generation of new neurons in the adult hippocampus. We used Tis21 knock-in mice expressing green fluorescent protein (GFP) and studied Tis21-GFP expression together with markers of adult hippocampal neurogenesis in newly generated cells. We found that Tis21-GFP 1) was absent from the radial glia–like putative stem cells (type-1 cells), 2) first appeared in transient amplifying progenitor cells (type-2 and 3 cells), 3) did not colocalize with markers of early postmitotic maturation stage, 4) was expressed again in maturing neurons, and 5) finally decreased in mature granule cells. Our data show that, in the course of adult neurogenesis, Tis21 is expressed in a phase additional to the one of the embryonic neurogenesis. This additional phase of expression might be associated with a new and different function of Tis21 than during embryonic brain development, where no Tis21 is expressed in mature neurons. We hypothesize that this function is related to the final functional integration of the newborn neurons. Tis21 can thus serve as new marker for key stages of adult neurogenesis.

Estradiol downregulation of the tumor suppressor gene BTG2 requires estrogen receptor-alpha and the REA corepressor

B-cell Translocation Gene 2 (BTG2/TIS21/PC3) is an anti-proliferative tumor suppressor gene whose expression is significantly reduced in breast carcinomas, and in MCF-7 and T-47D breast cancer cell lines treated with estradiol (E2). In this study the mechanisms involved in E2 down regulation of BTG2 gene expression were examined. Depletion of ERalpha by siRNA indicated that the receptor is required for E2 down regulation of BTG2 mRNA levels, and cycloheximide experiments indicated that the effect of E2 on BTG2 expression was independent of intermediary protein synthesis. Chromatin immunoprecipitation analyses revealed that ERalpha interacts with the BTG2 promoter in a ligand-independent fashion whereas transfection experiments indicated that ERalpha's DNA and ligand binding domains are required for E2 repression of BTG promoter activity. Surprisingly, histone deacetylase (HDACs) activity is essential for basal expression as evidenced by trichostatin A inhibition of BTG2 mRNA levels. Estradiol treatment did not alter histone H3 acetylation although it did induce displacement of RNA polymerase II from the BTG2 gene. Depletion of the ER specific corepressor REA (Repressor of Estrogen Receptor Activity) significantly abrogated E2-mediated BTG2 repression. Taken together, our results reveal a requirement of HDAC activity for basal BTG2 expression and the ERalpha-REA interaction for estrogen repression of the BTG2 gene. The ability of E2-bound ERalpha and REA to suppress BTG2 expression indicates a positive role for this corepressor in regulation of breast cancer cell proliferation.

The timing of differentiation of adult hippocampal neurons is crucial for spatial memory

Adult neurogenesis in the dentate gyrus plays a critical role in hippocampus-dependent spatial learning. It remains unknown, however, how new neurons become functionally integrated into spatial circuits and contribute to hippocampus-mediated forms of learning and memory. To investigate these issues, we used a mouse model in which the differentiation of adult-generated dentate gyrus neurons can be anticipated by conditionally expressing the pro-differentiative gene PC3 (Tis21/BTG2) in nestin-positive progenitor cells. In contrast to previous studies that affected the number of newly generated neurons, this strategy selectively changes their timing of differentiation. New, adult-generated dentate gyrus progenitors, in which the PC3 transgene was expressed, showed accelerated differentiation and significantly reduced dendritic arborization and spine density. Functionally, this genetic manipulation specifically affected different hippocampus-dependent learning and memory tasks, including contextual fear conditioning, and selectively reduced synaptic plasticity in the dentate gyrus. Morphological and functional analyses of hippocampal neurons at different stages of differentiation, following transgene activation within defined time-windows, revealed that the new, adult-generated neurons up to 3-4 weeks of age are required not only to acquire new spatial information but also to use previously consolidated memories. Thus, the correct unwinding of these key memory functions, which can be an expression of the ability of adult-generated neurons to link subsequent events in memory circuits, is critically dependent on the correct timing of the initial stages of neuron maturation and connection to existing circuits.

Crystal structures of human BTG2 and mouse TIS21 involved in suppression of CAF1 deadenylase activity

BTG2 is the prototypical member of the TOB family and is known to be involved in cell growth, differentiation and DNA repair. As a transcriptional co-regulator, BTG2 interacts with CCR4-associated factor 1 (CAF1) and POP2 (CALIF), which are key components of the general CCR4/NOT multi-subunit transcription complex, and which are reported to play distinct roles as nucleases involved in mRNA deadenylation. Here we report the crystal structures of human BTG2 and mouse TIS21 to 2.3 Å and 2.2 Å resolution, respectively. The structures reveal the putative CAF1 binding site. CAF1 deadenylase assays were performed with wild-type BTG2 and mutants that disrupt the interaction with CAF1. The results reveal the suppressive role of BTG2 in the regulation of CAF1 deadenylase activity. Our study provides insights into the formation of the BTG2-CAF1 complex and the potential role of BTG2 in the regulation of CAF1.

Btg1 and Btg2 gene expression during early chick development

Btg/Tob genes encode for a new family of proteins with antiproliferative functions, which are also able to stimulate cell differentiation. Btg1 and Btg2 are the most closely related members in terms of gene sequence. We analyzed their expression patterns in avian embryos by in situ hybridization, from embryonic day 1 to 3. Btg1 was distinctively expressed in the Hensen's node, the notochord, the cardiogenic mesoderm, the lens vesicle, and in the apical ectodermal ridge and mesenchyme of the limb buds. On the other hand, Btg2 expression domains included the neural plate border, presomitic mesoderm, trigeminal placode, and mesonephros. Both genes were commonly expressed in the myotome, epibranchial placodes, and dorsal neural tube. The results suggest that Btg1 and Btg2 are involved in multiple developmental processes. Overlapping expression of Btg1 and Btg2 may imply redundant functions, but unique expression patterns suggest also differential regulation and function.

Inhibition of medulloblastoma tumorigenesis by the antiproliferative and pro-differentiative gene PC3

Medulloblastoma, the most common brain tumor in childhood, appears to originate from cerebellar granule cell precursors (GCPs), located in the external granular layer (EGL) of the cerebellum. The antiproliferative gene PC3 (Tis21/BTG2) promotes cerebellar neurogenesis by inducing GCPs to shift from proliferation to differentiation. To assess whether PC3 can prevent the neoplastic transformation of GCPs and medulloblastoma development, we crossed transgenic mice conditionally expressing PC3 (TgPC3) in GCPs with Patched1 heterozygous mice (Ptc(+/-)), a model of medulloblastoma pathogenesis characterized by hyperactivation of the Sonic Hedgehog pathway. Perinatal up-regulation of PC3 in Ptc(+/-)/TgPC3 mice results in a decrease of medulloblastoma incidence of approximately 40% and in a marked reduction of preneoplastic abnormalities, such as hyperplastic EGL areas and lesions. Moreover, overexpression of cyclin D1, hyperproliferation, and defective differentiation--observed in Ptc(+/-) GCPs--are restored to normality in Ptc(+/-)/TgPC3 mice. The PC3-mediated inhibition of cyclin D1 expression correlates with recruitment of PC3 to the cyclin D1 promoter, which is accompanied by histone deacetylation. Remarkably, down-regulation of PC3 is observed in preneoplastic lesions, as well as in human and murine medulloblastomas. As a whole, this indicates that PC3 may prevent medulloblastoma development by controlling cell cycle and promoting differentiation of GCPs.

Decoding NMDA Receptor Signaling: Identification of Genomic Programs Specifying Neuronal Survival and Death

NMDA receptors promote neuronal survival but also cause cell degeneration and neuron loss. The mechanisms underlying these opposite effects on neuronal fate are unknown. Whole-genome expression profiling revealed that NMDA receptor signaling is decoded at the genomic level through activation of two distinct, largely nonoverlapping gene-expression programs. The location of the NMDA receptor activated specifies the transcriptional response: synaptic NMDA receptors induce a coordinate upregulation of newly identified pro-survival genes and downregulation of pro-death genes. Extrasynaptic NMDA receptors fail to activate this neuroprotective program, but instead induce expression of Clca1, a putative calcium-activated chloride channel that kills neurons. These results help explain the opposing roles of synaptic and extrasynaptic NMDA receptors on neuronal fate. They also demonstrate that the survival function is implemented in neurons through a multicomponent system of functionally related genes, whose coordinate expression is controlled by specific calcium signal initiation sites.

The role of XBtg2 in Xenopus neural development

In early neural development, active cell proliferation and apoptosis take place concurrent with cell differentiation, but how these processes are coordinated remains unclear. In this study, we characterized the role of XBtg2 in Xenopus neural development. XBtg2 transcripts were detected at the edge of the anterolateral neural plate and the neural crest region at the midneurula stage, and in eyes and in part of the neural tube at the tailbud stage. Translational inhibition of XBtg2 affected anterior neural development and impaired eye formation. XBtg2 depletion altered the expression patterns of the early neural genes, Zic3 and SoxD, at the midneurula stage, but not at the early neurula stage. At the midneurula stage, XBtg2-depleted embryos exhibited a marked decrease in the expression of anterior neural genes, En2, Otx2, and Rx1, without any changes in neural crest genes, Slug and Snail, or an epidermal gene, XK81. These results suggest that XBtg2 is required for the differentiation of the anterior neural plate from the midneurula stage, but not for the specification of the fate and patterning of the neural plate. XBtg2-depleted embryos also exhibited an increase in both proliferation and apoptosis in the anterior neural plate; however, the altered expression patterns of neural markers were not reversed by inhibition of either the cell cycle or apoptosis. Based on these data, we propose that XBtg2 plays an essential role in the anterior neural development, by regulating neural cell differentiation, and, independently, cell proliferation and survival.

XBtg2 is required for notochord differentiation during early Xenopus development

The notochord is essential for normal vertebrate development, serving as both a structural support for the embryo and a signaling source for the patterning of adjacent tissues. Previous studies on the notochord have mostly focused on its formation and function in early organogenesis but gene regulation in the differentiation of notochord cells itself remains poorly defined. In the course of screening for genes expressed in developing notochord, we have isolated Xenopus homolog of Btg2 (XBtg2). The mammalian Btg2 genes, Btg2/PC3/TIS21, have been reported to have multiple functions in the regulation of cell proliferation and differentiation but their roles in early development are still unclear. Here we characterized XBtg2 in early Xenopus laevis embryogenesis with focus on notochord development. Translational inhibition of XBtg2 resulted in a shortened and bent axis phenotype and the abnormal structures in the notochord tissue, which did not undergo vacuolation. The XBtg2-depleted notochord cells expressed early notochord markers such as chordin and Xnot at the early tailbud stage, but failed to express differentiation markers of notochord such as Tor70 and 5-D-4 antigens in the later stages. These results suggest that XBtg2 is required for the differentiation of notochord cells such as the process of vacuolar formation after determination of notochord cell fate.

Fibroblast growth factor 2 negatively regulates the induction of neuronal progenitors from neural stem cells

Fibroblast growth factor 2 (FGF2) exhibits pleiotropic functions during embryogenesis. In neural development, both pro- and antineurogenic activities of FGF2 have been described in the differentiation of neuronal progenitors into postmitotic neurons. We used cultured neural stem cells (NSCs) derived from rat embryonic day 14.5 cortex to determine the FGF2 effect on the induction of early neuronal progenitors. Our data showed that the presence of FGF2 during serum-induced differentiation of NSCs reduced the number of Tuj1(+) neurons. A bromodeoxyuridine (BrdU)/Tuj1 double-labeling assay and expression analyses of the pro- and antineurogenic basic helix-loop-helix (bHLH) factors showed that FGF2 blocked the generation of early neuronal progenitors, but not the cell-cycle exit of dividing neurons. This negative regulation of neuronal induction by FGF2 was associated with the persistent expression of an antineurogenic bHLH, hairy and enhancer of split (HES)-1. A gene-profiling study demonstrated that the developmental programs underlying neuronal differentiation were altered as a whole and identified several developmentally regulated, neural-enriched genes. This work shows that FGF2 exerts an antineurogenic effect during the developmental window when neuronal progenitors are first induced from NSCs. It also provides a novel experimental system that can be used to prospectively identify genes expressed at different stages of neuronal differentiation.

The neurogene BTG2TIS21/PC3 is transactivated by DeltaNp73alpha via p53 specifically in neuroblastoma cells

The p53 gene and its homologue p73 are rarely mutated in neuroblastoma. In recent studies, we showed that overexpression of DeltaNp73alpha, an isoform lacking the N-terminal transactivation (TA) domain, surprisingly induces p53 protein accumulation in the wild-type (wt) p53 human neuroblastoma line SH-SY5Y. As can be expected owing to its dominant-negative effect, DeltaNp73alpha inhibits Waf1/p21 gene expression, but equally importantly, it upregulates BTG2TIS21/PC3, another p53 target gene. This effect is not observed in neuroblastoma cells that express a mutated p53. To better understand the DeltaNp73-mediated transactivation of the BTG2TIS21/PC3 gene we performed luciferase assays with two reporter plasmids harboring long and short BTG2 promoter sequences in three human neuroblastoma cell lines and one breast cancer cell line. Our results demonstrate that BTG2TIS21/PC3 transactivation by DeltaNp73alpha depends on both p53 status (as it is not observed in a p53-/- neuroblastoma cell line) and cellular context (as it occurs in a p53+/+ neuroblastoma cell line but not in a p53+/+ breast tumor cell line). The fact that DeltaNp73alpha may either inhibit or stimulate wt-p53 transcriptional activity, depending on both the p53 target gene and the cellular context, was confirmed by real-time quantitative PCR. Moreover, transactivation of the BTG2TIS21/PC3 promoter requires a complete DeltaNp73alpha C-terminus sequence as it is not observed with DeltaNp73beta, which lacks most of the C-terminal domain. We have previously shown that DeltaNp73alpha is the only p73 isoform expressed in undifferentiated neuroblastoma tumors. In light of all these findings, we propose that DeltaNp73alpha not only acts as an inhibitor of p53/TAp73 functions in neuroblastoma tumors, but also cooperates with wt-p53 in playing a physiological role through the activation of BTG2TIS21/PC3 gene expression.

Dual control of neurogenesis by PC3 through cell cycle inhibition and induction of Math1

Growing evidence indicates that cell cycle arrest and neurogenesis are highly coordinated and interactive processes, governed by cell cycle genes and neural transcription factors. The gene PC3 (Tis21/BTG2) is expressed in the neuroblast throughout the neural tube and inhibits cell cycle progression at the G1 checkpoint by repressing cyclin D1 transcription. We generated inducible mouse models in which the expression of PC3 was upregulated in neuronal precursors of the neural tube and of the cerebellum. These mice exhibited a marked increase in the production of postmitotic neurons and impairment of cerebellar development. Cerebellar granule precursors of PC3 transgenic mice displayed inhibition of cyclin D1 expression and a strong increase in the expression of Math1, a transcription factor required for their differentiation. Furthermore, PC3, encoded by a recombinant adenovirus, also induced Math1 in postmitotic granule cells in vitro and stimulated the Math1 promoter activity. In contrast, PC3 expression was unaffected in the cerebellar primordium of Math1 null mice, suggesting that PC3 acts upstream to Math1. As a whole, our data suggest that cell cycle exit of cerebellar granule cell precursors and the onset of cerebellar neurogenesis are coordinated by PC3 through transcriptional control of cyclin D1 and Math1, respectively.

Neurons arise in the basal neuroepithelium of the early mammalian telencephalon: a major site of neurogenesis

Neurons of the mammalian CNS are thought to originate from progenitors dividing at the apical surface of the neuroepithelium. Here we use mouse embryos expressing GFP from the Tis21 locus, a gene expressed throughout the neural tube in most, if not all, neuron-generating progenitors, to specifically reveal the cell divisions that produce CNS neurons. In addition to the apical, asymmetric divisions of neuroepithelial (NE) cells that generate another NE cell and a neuron, we find, from the onset of neurogenesis, a second population of progenitors that divide in the basal region of the neuroepithelium and generate two neurons. Basal progenitors are most frequent in the telencephalon, where they outnumber the apically dividing neuron-generating NE cells. Our observations reconcile previous data on the origin and lineage of CNS neurons and show that basal, rather than apical, progenitors are the major source of the neurons of the mammalian neocortex.

Bone growth stimulators. New tools for treating bone loss and mending fractures

In the new millennium, humans will be traveling to Mars and eventually beyond with skeletons that respond to microgravity by self-destructing. Meanwhile in Earth's aging populations growing numbers of men and many more women are suffering from crippling bone loss. During the first decade after menopause all women suffer an accelerating loss of bone, which in some of them is severe enough to result in "spontaneous" crushing of vertebrae and fracturing of hips by ordinary body movements. This is osteoporosis, which all too often requires prolonged and expensive care, the physical and mental stress of which may even kill the patient. Osteoporosis in postmenopausal women is caused by the loss of estrogen. The slower development of osteoporosis in aging men is also due at least in part to a loss of the estrogen made in ever smaller amounts in bone cells from the declining level of circulating testosterone and is needed for bone maintenance as it is in women. The loss of estrogen increases the generation, longevity, and activity of bone-resorbing osteoclasts. The destructive osteoclast surge can be blocked by estrogens and selective estrogen receptor modulators (SERMs) as well as antiosteoclast agents such as bisphosphonates and calcitonin. But these agents stimulate only a limited amount of bone growth as the unaffected osteoblasts fill in the holes that were dug by the now suppressed osteoclasts. They do not stimulate osteoblasts to make bone--they are antiresorptives not bone anabolic agents. (However, certain estrogen analogs and bisphosphates may stimulate bone growth to some extent by lengthening osteoblast working lives.) To grow new bone and restore bone strength lost in space and on Earth we must know what controls bone growth and destruction. Here we discuss the newest bone controllers and how they might operate. These include leptin from adipocytes and osteoblasts and the statins that are widely used to reduce blood cholesterol and cardiovascular damage. But the main focus of this article is necessarily the currently most promising of the anabolic agents, the potent parathyroid hormone (PTH) and certain of its 31- to 38-aminoacid fragments, which are either in or about to be in clinical trial or in the case of Lilly's Forteo [hPTH-(1-34)] tentatively approved by the Food and Drug Administration for treating osteoporosis and mending fractures.

BTG2(TIS21/PC3) induces neuronal differentiation and prevents apoptosis of terminally differentiated PC12 cells

The p53-transcriptional target, BTG2(TIS21/PC3), was previously identified as an antiproliferative gene. However, the precise biological functions of the protein product remain to be elucidated. BTG2(TIS21/PC3) expression is induced in vivo during neurogenesis, and the gene is transiently expressed in vitro in rat pheochromocytoma PC12 cells after induction of neuronal differentiation by addition of nerve growth factor (NGF). These observations suggest that BTG2(TIS21/PC3) is functionally significant during the neuronal differentiation process. To test this hypothesis, a vector that expressed BTG2(TIS21/PC3) under the control of an inducible promoter was introduced into PC12 cells. Growth arrest and differentiation in response to NGF were greatly enhanced by BTG2(TIS21/PC3) overexpression. Furthermore, an antisense oligonucleotide complementary to BTG2(TIS21/PC3) mRNA, which was able to inhibit endogenous BTG2(TIS21/PC3) expression, triggered programmed cell death in differentiated PC12 cells. These observations confirm that BTG2(TIS21/PC3) expression promotes neuronal differentiation and that it is required for survival of terminally differentiated cells.

PC3 potentiates NGF-induced differentiation and protects neurons from apoptosis

PC3TIS21/BTG2 is member of a novel family of antiproliferative genes (BTG1, ANA/BTG3, PC3B, TOB, and TOB2) that play a role in cellular differentiation. We have previously shown that PC3TIS21/BTG2 is induced by nerve growth factor (NGF) at the onset of neuronal differentiation in the neural crest-derived PC12 cell line, and is a marker for neuronal birth. We now observe that PC3TIS21/BTG2 ectopically expressed in PC12 cells synergises with NGF, similarly to the cyclin-dependent kinase inhibitor p21, potentiating the induction of the neuronal markers tyrosine hydroxylase and neurofilament 160 kDa. Furthermore, PC3TIS21/BTG2 protects from apoptosis elicited by NGF deprivation in terminally differentiated PC12 cultures. Such effects might be a consequence of the arrest of cell cycle exerted by PC3TIS21/BTG2, or expression of a sensitizing (neurogenic) property of the molecule.

Parathyroid Hormone, Its Fragments and Their Analogs for the Treatment of Osteoporosis

The susceptibility to traumatic fracturing of osteopenic bones, and the spontaneous fracturing of osteoporotic bones by normal body movements caused by the microstructural deterioration and loss of bone, are currently treated with antiresorptive drugs, such as the bisphosphonates, calcitonin, estrogens, and selective estrogen receptor modulators. These antiresorptive agents target osteoclasts and, as their name indicates, reduce or stop bone resorption. They cannot directly stimulate bone formation, increase bone mass above normal values in ovariectomized rat models, or improve microstructure. However, there is a family of agents - the parathyroid hormone (PTH) and some of its fragments and their analogs - which directly stimulate bone growth and improve microstructure independently from impairing osteoclasts. These drugs are about to make their clinical debut in treating patients with osteoporosis and, probably not too far in the future, for accelerating fracture healing. They stimulate osteoblast accumulation and bone formation in three ways via signals from the type 1 PTH/PTH-related protein (PTHR1) receptors on proliferatively inactive preosteoblasts, osteoblasts, osteocytes and bone-lining cells. The receptor signals shut down the proliferative machinery in preosteoblasts and push their maturation to osteoblasts, cause the osteoblastic cells to make and secrete several factors that stimulate the extensive proliferation of osteoprogenitors without PTHRI receptors, stimulate the reversion of bone-lining cells to osteoblasts, and extend osteoblast lifespan and productivity by preventing them from suicidally initiating apoptosis. The first of the PTHs to reach the clinic will be teriparatide [recombinant human (h)PTH-(1-34)], which was recommended for approval in 2001 by the US Food and Drug Administration Endocrinology and Metabolic Drugs Advisory Committee for the treatment of postmenopausal osteoporosis. Teriparatide has been shown to considerably increase cancellous and cortical bone mass, improve bone microstructure, prevent fractures and thus provide benefits that cannot be provided by current antiresorptive drugs, when administered subcutaneously at a daily dose of 20 microg for no longer than 2 years to patients with osteoporosis.

In vivo PC3 overexpression by retroviral vector affects cell differentiation of rat cortical precursors

The PC3 gene is a marker of dividing neuroepithelial (NE) cells. We transduced single cortical precursors of the ventricular zone (VZ) with a PC3-carrying retroviral vector at E16 stage, and analysed the effects of transgene expression on their progeny in 3-week-old animals. Unlike control-transduced cells, all viable PC3-transduced cells remained close to the ventricle and displayed a round-shaped, undifferentiated morphology.

Association of ANA, a member of the antiproliferative Tob family proteins, with a Caf1 component of the CCR4 transcriptional regulatory complex

A 35-kDa protein, ANA, belongs to an emerging family of antiproliferative proteins consisting of Tob, Tob2, ANA / BTG3, PC3B, PC3 / TIS21 / BTG2, and BTG1. All of these, except ANA and PC3B, have been shown to interact with the CCR4 transcription factor-associated protein Caf1. Here we show that ANA also associates with Caf1, ANA being the preferred partner of Caf1 among the Tob family proteins. Although ANA is likely to interact with Caf1 at its amino-terminal half, which is conserved among the family members, our data suggest that the carboxyl-terminal half of ANA plays a role in the interaction. Finally, in situ hybridization experiments revealed that expression of Caf1 overlaps at least in part with that of ANA. Thus, ANA could function through its interaction with Caf1.

In search of a function for the TIS21/PC3/BTG1/TOB family

The Btg family of anti-proliferative gene products includes Pc 3/Tis 21/Btg 2, Btg 1, Tob, Tob2, Ana/Btg3, Pc3k and others. These proteins are characterized by similarities in their amino-terminal region: the Btg1 homology domain. However, the pleiotropic nature of these family proteins has been observed and no common physiological function among family members was suggested from the history of their identification. Recent progress in the search for Btg family functions has come from the analysis of cell regulation and of cell differentiation. It is now emerging that every member of this family has a potential to regulate cell growth. We would like to propose here to use a nomenclature APRO as a new term for the family.

The gene PC3(TIS21/BTG2), prototype member of the PC3/BTG/TOB family: regulator in control of cell growth, differentiation, and DNA repair?

PC3(TIS21/BTG2) is the founding member of a family of genes endowed with antiproliferative properties, namely BTG1, ANA/BTG3, PC3B, TOB, and TOB2. PC3 was originally isolated as a gene induced by nerve growth factor during neuronal differentiation of rat PC12 cells, or by TPA in NIH3T3 cells (named TIS21), and is a marker for neuronal birth in vivo. This and other findings suggested its implication in the process of neurogenesis as mediator of the growth arrest before differentiation. Remarkably, its human homolog, named BTG2, was shown to be p53-inducible, in conditions of genotoxic damage. PC3(TIS21/BTG2) impairs G(1)-S progression, either by a Rb-dependent pathway through inhibition of cyclin D1 transcription, or in a Rb-independent fashion by cyclin E downregulation. PC3(TIS21/BTG2) might also control the G(2) checkpoint. Furthermore, PC3(TIS21/BTG2) interacts with carbon catabolite repressor protein-associated factor 1 (CAF-1), a molecule that associates to the yeast transcriptional complex CCR4 and might influence cell cycle, with the transcription factor Hoxb9, and with the protein-arginine methyltransferase 1, that might control transcription through histone methylation. Current evidence suggests a physiological role of PC3(TIS21/BTG2) in the control of cell cycle arrest following DNA damage and other types of cellular stress, or before differentiation of the neuron and other cell types. The molecular function of PC3(TIS21/BTG2) is still unknown, but its ability to modulate cyclin D1 transcription, or to synergize with the transcription factor Hoxb9, suggests that it behaves as a transcriptional co-regulator.

Expression of the extra-large G protein alpha-subunit XLalphas in neuroepithelial cells and young neurons during development of the rat nervous system

XLalphas ('extra large' alpha) is a 78 kDa splice variant of the alpha-subunit of the heterotrimeric G protein, Gs (Nature 372 (1994) 804). Prompted by its neuroendocrine-specific tissue distribution in the adult (J. Biol. Chem. 275 (2000) 33622) and its ability to activate adenylyl cyclase (J. Biol. Chem. 275 (2000) 33633), we investigated the expression of XLalphas in the developing rat nervous system using immunofluorescence. Remarkably, XLalphas expression in the neural tube was found to begin at the onset of neurogenesis, being observed in a subset of mitotic neuroepithelial cells as well as in young neurons. At later developmental stages, XLalphas was associated with a subset of neurons in certain regions of the nervous system such as diencephalon, midbrain, hindbrain, spinal cord and sympathetic trunk. These results suggest a role of XLalphas in neuronal differentiation.

Relationships of the antiproliferative proteins BTG1 and BTG2 with CAF1, the human homolog of a component of the yeast CCR4 transcriptional complex: involvement in estrogen receptor alpha signaling pathway

We have reported previously the physical interaction of B-cell translocation gene proteins (BTG)1 and BTG2 with the mouse protein CAF1 (CCR4-associated factor 1) and suggested that these proteins may participate, through their association with CAF1, in transcription regulation. Here we describe the in vitro and in vivo association of these proteins with hPOP2, the human paralog of hCAF1. The physical and functional relationships between the BTG proteins and their partners hCAF1 and hPOP2 were investigated to find out how these interactions affect cellular processes, and in particular transcription regulation. We defined their interaction regions and examined their expression in various human tissues. We also show functional data indicating their involvement in estrogen receptor alpha (ERalpha)-mediated transcription regulation. We found that BTG1 and BTG2, probably through their interaction with CAF1 via a CCR4-like complex, can play both positive or negative roles in regulating the ERalpha function. In addition, our results indicate that two LXXLL motifs, referred to as nuclear receptor boxes, present in both BTG1 and BTG2, are involved in the regulation of ERalpha-mediated activation.

Cloning of PC3B, a novel member of the PC3/BTG/TOB family of growth inhibitory genes, highly expressed in the olfactory epithelium

We identified in the EST database murine and human sequences similar, but not identical, to the members of the PC3/BTG/TOB family of cell cycle inhibitors. A conserved domain (aa 50-68) of the PC3 protein, the prototype member of the family, was used as a query. That domain has been shown by us to be necessary for the antiproliferative activity of PC3. A murine EST clone and a highly homologous human EST clone, containing the entire ORF, were chosen for sequencing. Comparison to databases and a phylogenetic tree analysis indicated that these EST clones are the mouse and human homologues of a gene that represents a novel member of the PC3/BTG/TOB family. This gene, named PC3B, is endowed with marked antiproliferative activity, being able to induce G(1) arrest, and is highly expressed in testis, in oocyte, and in preimplantation embryos. Analysis of its expression during murine development indicated a specific localization in the olfactory epithelium at midgestation, suggesting that PC3B might be involved in the differentiation of this neuronal structure. Human PC3B mapped to chromosome 11q23, as indicated by radiation hybrid analysis.

Arrest of G(1)-S progression by the p53-inducible gene PC3 is Rb dependent and relies on the inhibition of cyclin D1 transcription

The p53-inducible gene PC3 (TIS21, BTG2) is endowed with antiproliferative activity. Here we report that expression of PC3 in cycling cells induced accumulation of hypophosphorylated, growth-inhibitory forms of pRb and led to G(1) arrest. This latter was not observed in cells with genetic disruption of the Rb gene, indicating that the PC3-mediated G(1) arrest was Rb dependent. Furthermore, (i) the arrest of G(1)-S transition exerted by PC3 was completely rescued by coexpression of cyclin D1 but not by that of cyclin A or E; (ii) expression of PC3 caused a significant down-regulation of cyclin D1 protein levels, also in Rb-defective cells, accompanied by inhibition of CDK4 activity in vivo; and (iii) the removal from the PC3 molecule of residues 50 to 68, a conserved domain of the PC3/BTG/Tob gene family, which we term GR, led to a loss of the inhibition of proliferation as well as of the down-regulation of cyclin D1 levels. These data point to cyclin D1 down-regulation as the main factor responsible for the growth inhibition by PC3. Such an effect was associated with a decrease of cyclin D1 transcript and of cyclin D1 promoter activity, whereas no effect of PC3 was observed on cyclin D1 protein stability. Taken together, these findings indicate that PC3 impairs G(1)-S transition by inhibiting pRb function in consequence of a reduction of cyclin D1 levels and that PC3 acts, either directly or indirectly, as a transcriptional regulator of cyclin D1.

The leukemia-associated protein Btg1 and the p53-regulated protein Btg2 interact with the homeoprotein Hoxb9 and enhance its transcriptional activation

BTG1 and BTG2 belong to a family of functionally related genes involved in the control of the cell cycle. As part of an ongoing attempt to understand their biological functions, we used a yeast two-hybrid screening to look for possible functional partners of Btg1 and Btg2. Here we report the physical and functional association between these proteins and the homeodomain protein Hoxb9. We further show that Btg1 and Btg2 enhance Hoxb9-mediated transcription in transfected cells, and we report the formation of a Hoxb9.Btg2 complex on a Hoxb9-responsive target, and the fact that this interaction facilitates the binding of Hoxb9 to DNA. The transcriptional activity of the Hoxb9.Btg complex is essentially dependent on the activation domain of Hoxb9, located in the N-terminal portion of the protein. Our data indicate that Btg1 and Btg2 act as transcriptional cofactors of the Hoxb9 protein, and suggest that this interaction may mediate their antiproliferative function.

PC3 overexpression affects the pattern of cell division of rat cortical precursors

The PC3 gene is transiently expressed during neurogenesis in precursor cells of the telencephalic ventricular/subventricular zone, and is rapidly downregulated before cell migration and differentiation. It is thought to have a role in controlling cell proliferation, but its precise function is not known. Here we present evidence that PC3, when overexpressed in vitro by retroviral-mediated gene transfer, acts by interfering with the normal pattern of cell division. Firstly, we report evidence that PC3 overexpression reduces the rate of cell proliferation in both NIH 3T3 cells and embryonic precursor cells from the rat cerebral cortex. Secondly, when studying the pattern of BrdU dilution in clones of cortical precursors, we observe that clones transduced with PC3 show an asymmetric pattern of BrdU dilution more frequently than clones transduced with a control vector. We discuss the hypothesis that the higher number of PC3 transduced clones showing an asymmetric pattern of BrdU dilution may be due to an increase in asymmetric cell divisions.

A novel member of the tob family of proteins controls sexual fate in Caenorhabditis elegans germ cells

Although many cell fates differ between males and females, probably the most ancient type of sexual dimorphism is the decision of germ cells to develop as sperm or as oocytes. Genetic analyses of Caenorhabditis elegans suggest that fog-3 might directly control this decision. We used transformation rescue to clone the fog-3 gene and show that it produces a single major transcript of approximately 1150 nucleotides. This transcript is predicted to encode a protein of 263 amino acids. One mutation causes a frame shift at the sixth codon and is thus likely to define the null phenotype of fog-3. Although the carboxyl-terminus of FOG-3 is novel, the amino-terminal domain is similar to that of the Tob, BTG1, and BTG2 proteins from vertebrates, which might suppress proliferation or promote differentiation. This domain is essential for FOG-3 activity, since six of eight missense mutations map to this region. Furthermore, this domain of BTG1 and BTG2 interacts with a transcriptional regulatory complex that has been conserved in all eukaryotes. Thus, one possibility is that FOG-3 controls transcription of genes required for germ cells to initiate spermatogenesis rather than oogenesis. This model implies that FOG-3 is required throughout an animal's life for germ cells to initiate spermatogenesis. We used RNA-mediated interference to demonstrate that fog-3 is indeed required continuously, which is consistent with this model.

BTG1: a triiodothyronine target involved in the myogenic influence of the hormone

The product of the B-cell translocation gene 1 (BTG1), a member of an antiproliferative protein family including Tis-21/PC3 and Tob, is thought to play an important role in the regulation of cell cycle progression. We have shown in a previous work that triiodothyronine (T3) stimulates quail myoblast differentiation, partly through a cAMP-dependent mechanism involved in the stimulation of cell cycle withdrawal. Furthermore, we found that T3 or 8-Br-cAMP increases BTG1 nuclear accumulation in confluent myoblast cultures. In this study, we report that BTG1 is essentially expressed at cell confluence and in differentiated myotubes. Whereas neither T3 nor cAMP exerted a direct transcriptional control upon BTG1 expression, we found that AP-1 activity, a crucial target involved in the triiodothyronine myogenic influence, repressed BTG1 expression, thus probably explaining the low BTG1 expression level in proliferating myoblasts. In transient transfection studies, we demonstrated that an AP-1-like sequence located in the BTG1 promoter was involved in this negative regulation. Our present data also bring evidence that the stimulation of BTG1 nuclear accumulation by T3 or 8-Br-cAMP probably results from an increased nuclear import or retention in the nucleus. Lastly, BTG1 overexpression in quail myoblasts mimicked the T3 or 8-Br-cAMP myogenic influence: (i) inhibition of myoblast proliferation due to an increased rate of myoblast withdrawal from the cell cycle; and (ii) stimulation of terminal differentiation. These data suggest that BTG1 is probably involved in T3 and cAMP myogenic influences. In conclusion, BTG1 is a T3 target involved in the regulation of myoblast differentiation.

Expression of the antiproliferative gene TIS21 at the onset of neurogenesis identifies single neuroepithelial cells that switch from proliferative to neuron-generating division

At the onset of mammalian neurogenesis, neuroepithelial (NE) cells switch from proliferative to neuron-generating divisions. Understanding the molecular basis of this switch requires the ability to distinguish between these two types of division. Here we show that in the mouse ventricular zone, expression of the mRNA of the antiproliferative gene TIS21 (PC3, BTG2) (i) starts at the onset of neurogenesis, (ii) is confined to a subpopulation of NE cells that increases in correlation with the progression of neurogenesis, and (iii) is not detected in newborn neurons. Expression of the TIS21 mRNA in the NE cells occurs transiently during the cell cycle, i.e., in the G1 phase. In contrast to the TIS21 mRNA, the TIS21 protein persists through the division of NE cells and is inherited by the neurons, where it remains detectable during neuronal migration and the initial phase of differentiation. Our observations indicate that the TIS21 gene is specifically expressed in those NE cells that, at their next division, will generate postmitotic neurons, but not in proliferating NE cells. Using TIS21 as a marker, we find that the switch from proliferative to neuron-generating divisions is initiated in single NE cells rather than in synchronized neighboring cells.

Interaction of BTG1 and p53-regulated BTG2 gene products with mCaf1, the murine homolog of a component of the yeast CCR4 transcriptional regulatory complex

Both BTG1 and BTG2 are involved in cell-growth control. BTG2 expression is regulated by p53, and its inactivation in embryonic stem cells leads to the disruption of DNA damage-induced G2/M cell-cycle arrest. In order to investigate the mechanism underlying Btg-mediated functions, we looked for possible functional partners of Btg1 and Btg2. Using yeast two-hybrid screening, protein-binding assays, and transient transfection assays in HeLa cells, we demonstrated the physical in vitro and in vivo interaction of both Btg1 and Btg2 with the mouse protein mCaf1 (i.e. mouse CCR4-associated factor 1). mCaf1 was identified through its interaction with the CCR4 protein, a component of a general transcription multisubunit complex, which, in yeast, regulates the expression of different genes involved in cell-cycle regulation and progression. These data suggest that Btg proteins, through their association with mCaf1, may participate, either directly or indirectly, in the transcriptional regulation of the genes involved in the control of the cell cycle. Finally, we found that box B, one of two conserved domains which define the Btg family, plays a functional role, namely that it is essential to the Btg-mCaf1 interaction.

Sequence and developmental expression of AmphiTob, an amphioxus homolog of vertebrate Tob in the PC3/BTG1/Tob family of tumor suppressor genes

Tob is a member of the PC3/ BTG1/Tob family of vertebrate tumor suppressor genes; its expression is known to inhibit proliferation of cells in vitro, but its possible roles during normal development have not been investigated previously. The present study concerns the structure and developmental expression of AmphiTob in an invertebrate chordate, amphioxus. This is the first investigation of any Tob gene during embryological development. The 311 amino acid AmphiTob protein is similar to vertebrate Tob but lacks the C-terminal PQ-rich domain of the latter. In early embryos of amphioxus, in situ hybridization first reveals AmphiTob expression in the hypoblast at the gastrula stage on the likely dorsal side of the embryo. During subsequent development, expression is seen in several tissues of the ectoderm, mesoderm, and endoderm. The most striking expression domains are in the developing somitic musculature and dorsal nerve cord. In the medial wall of each somite, AmphiTob is expressed strongly by cells destined to differentiate into the axial trunk muscles; this pattern persists until late in the larval stage, evidently because undifferentiated cells are continually becoming myogenic as the muscles grow. Nerve cord cells conspicuously transcribe AmphiTob from the late neurula until the early larval stage: Expression occurs in a few cells scattered along the nerve cord and in a group of cells located in the cerebral vesicle (in a region presumably homologous to the vertebrate diencephalic forebrain). During development, an intense and transitory transcription of AmphiTob may be an early event in cells exiting the cell cycle in preparation for differentiation.

Expression of the PC4 gene in the developing rat nervous system

PC4 is an early NGF-inducible gene, transiently expressed during the in vitro differentiation of PC12 cells toward a neuronal phenotype. By in situ hibridization analysis, we found that PC4 is expressed at high levels along the whole neural tube of early rat embryos. PC4 mRNA expression is not uniform across the wall of the neural tube, the autoradiographic signal being most intense on the ventricular layer. At later stages, when the rate of proliferation and production of postmitotic neurons decreases, PC4 gene expression also decreases and becomes restricted to the telencephalon, that is the last region to complete neurogenesis. Thus the expression of PC4 gene, although not exclusive of proliferating cells, appears to be correlated to the time span of proliferation of neuronal and glial precursors.

TrkA, TrkB and p75 mRNA expression is developmentally regulated in the rat retina

We examined the cellular distribution of mRNAs coding for the neurotrophin receptors TrkA, TrkB and p75 in the rat retina during early postnatal development. At P0 (postnatal day 0), mRNAs coding for each of the three receptors were detected in the ganglion cell layer (GCL) and in the inner plexiform layer (IPL), the latter structure essentially containing retinal ganglion cell processes at this developmental stage. At P5, the innermost part of the inner nuclear layer (INL) also expressed TrkA, TrkB and p75 mRNAs. Finally, the GCL, IPL and the whole INL of P10 retinae were labeled by the three probes. The developmentally regulated expression of these receptors underlies a possible role for neurotrophins in the differentiation and survival of retinal cells.