Accumulation of the cyclin-dependent kinase inhibitor p27/Kip1 and the timing of oligodendrocyte differentiation

Loss of protein-tyrosine phosphatase α (PTPα) increases proliferation and delays maturation of oligodendrocyte progenitor cells

Tightly controlled termination of proliferation determines when oligodendrocyte progenitor cells (OPCs) can initiate differentiation and mature into myelin-forming cells. Protein-tyrosine phosphatase α (PTPα) promotes OPC differentiation, but its role in proliferation is unknown. Here we report that loss of PTPα enhanced in vitro proliferation and survival and decreased cell cycle exit and growth factor dependence of OPCs but not neural stem/progenitor cells. PTPα(-/-) mice have more oligodendrocyte lineage cells in embryonic forebrain and delayed OPC maturation. On the molecular level, PTPα-deficient mouse OPCs and rat CG4 cells have decreased Fyn and increased Ras, Cdc42, Rac1, and Rho activities, and reduced expression of the Cdk inhibitor p27Kip1. Moreover, Fyn was required to suppress Ras and Rho and for p27Kip1 accumulation, and Rho inhibition in PTPα-deficient cells restored expression of p27Kip1. We propose that PTPα-Fyn signaling negatively regulates OPC proliferation by down-regulating Ras and Rho, leading to p27Kip1 accumulation and cell cycle exit. Thus, PTPα acts in OPCs to limit self-renewal and facilitate differentiation.

Differential proliferation rhythm of neural progenitor and oligodendrocyte precursor cells in the young adult hippocampus

Oligodendrocyte precursor cells (OPCs) are a unique type of glial cells that function as oligodendrocyte progenitors while constantly proliferating in the normal condition from rodents to humans. However, the functional roles they play in the adult brain are largely unknown. In this study, we focus on the manner of OPC proliferation in the hippocampus of the young adult mice. Here we report that there are oscillatory dynamics in OPC proliferation that differ from neurogenesis in the subgranular zone (SGZ); the former showed S-phase and M-phase peaks in the resting and active periods, respectively, while the latter only exhibited M-phase peak in the active period. There is coincidence between different modes of proliferation and expression of cyclin proteins that are crucial for cell cycle; cyclin D1 is expressed in OPCs, while cyclin D2 is observed in neural stem cells. Similar to neurogenesis, the proliferation of hippocampal OPCs was enhanced by voluntary exercise that leads to an increase in neuronal activity in the hippocampus. These data suggest an intriguing control of OPC proliferation in the hippocampus.

Cdk6-dependent regulation of G(1) length controls adult neurogenesis

The presence of neurogenic precursors in the adult mammalian brain is now widely accepted, but the mechanisms coupling their proliferation with the onset of neuronal differentiation remain unknown. Here, we unravel the major contribution of the G(1) regulator cyclin-dependent kinase 6 (Cdk6) to adult neurogenesis. We found that Cdk6 was essential for cell proliferation within the dentate gyrus of the hippocampus and the subventricular zone of the lateral ventricles. Specifically, Cdk6 deficiency prevents the expansion of neuronally committed precursors by lengthening G(1) phase duration, reducing concomitantly the production of newborn neurons. Altogether, our data support G(1) length as an essential regulator of the switch between proliferation and neuronal differentiation in the adult brain and Cdk6 as one intrinsic key molecular regulator of this process.

Dynamic changes of PIRH2 and p27kip1 expression in injured rat sciatic nerve

p53-induced ring-H2 protein (PIRH2), a newly identified E3 ubiquitin ligase, has been reported to be interacted with p27Kip1 and promote ubiquitination of p27Kip1 independently of p53. p27kip1, a member of the Cip/Kip family of cyclin-dependent kinases inhibitors (CKIs), was shown to control cell cycle progression and promote cell proliferation. While the distribution and function of PIRH2 and p27kip1 in nervous system lesion and regeneration remains unclear. Here, we performed a sciatic nerve injury model in adult rats and studied the dynamic changes of PIRH2 and p27kip1 expression by western blot and RT-PCR in injured rat sciatic nerve. Sciatic nerve crush resulted in a significant up-regulation of PIRH2 and a down-regulation of p27kip1. Besides, we observed that they were expressed widely in both Schwann cells and axons in adult rat sciatic nerve by double immunofluorescence staining. Results obtained by coimmunoprecipitation and double labeling further showed their interaction in the regenerating process. Thus, these results indicate that PIRH2 and p27kip1 likely play an important role in peripheral nerve injury and regeneration.

Thyroid hormone and the neuroglia: both source and target

Thyroid hormone plays a crucial role in the development and function of the nervous system. In order to bind to its nuclear receptor and regulate gene transcription thyroxine needs to be activated in the brain. This activation occurs via conversion of thyroxine to T3, which is catalyzed by the type 2 iodothyronine deiodinase (D2) in glial cells, in astrocytes, and tanycytes in the mediobasal hypothalamus. We discuss how thyroid hormone affects glial cell function followed by an overview on the fine-tuned regulation of T3 generation by D2 in different glial subtypes. Recent evidence on the direct paracrine impact of glial D2 on neuronal gene expression underlines the importance of glial-neuronal interaction in thyroid hormone regulation as a major regulatory pathway in the brain in health and disease.

Mechanisms limiting body growth in mammals

Recent studies have begun to provide insight into a long-standing mystery in biology-why body growth in animals is rapid in early life but then progressively slows, thus imposing a limit on adult body size. This growth deceleration in mammals is caused by potent suppression of cell proliferation in multiple tissues and is driven primarily by local, rather than systemic, mechanisms. Recent evidence suggests that this progressive decline in proliferation results from a genetic program that occurs in multiple organs and involves the down-regulation of a large set of growth-promoting genes. This program does not appear to be driven simply by time, but rather depends on growth itself, suggesting that the limit on adult body size is imposed by a negative feedback loop. Different organs appear to use different types of information to precisely target their adult size. For example, skeletal and cardiac muscle growth are negatively regulated by myostatin, the concentration of which depends on muscle mass itself. Liver growth appears to be modulated by bile acid flux, a parameter that reflects organ function. In pancreas, organ size appears to be limited by the initial number of progenitor cells, suggesting a mechanism based on cell-cycle counting. Further elucidation of the fundamental mechanisms suppressing juvenile growth is likely to yield important insights into the pathophysiology of childhood growth disorders and of the unrestrained growth of cancer. In addition, improved understanding of these growth-suppressing mechanisms may someday allow their therapeutic suspension in adult tissues to facilitate tissue regeneration.

Looking back

In this Perspective, I review my scientific career, which began after I trained in medicine in Montreal and in neurology in Boston. I started in immunology in London with Avrion Mitchison, using antibodies against cell-surface antigens to study the development and functions of mouse T and B cells. The finding that antibody binding causes immunoglobulin on B cells to redistribute rapidly on the cell surface and be endocytosed transformed me from an immunologist into a cell biologist. I moved with Mitchison to University College London, where my colleagues and I used the antibody approach to study cells of the rodent nervous system, focusing on the intrinsic and extrinsic molecular mechanisms that control the development and behavior of myelinating glial cells-Schwann cells and oligodendrocytes. I retired from active research in 2002 and now spend much of my time on scientific advisory boards and thinking about autism.

The cyclin-dependent kinase inhibitor p27(Kip1) is a positive regulator of Schwann cell differentiation in vitro

Schwann cell precursors differentiating into a myelinating phenotype are critical for peripheral nerve development and regeneration. However, little is known about the underlying molecular mechanisms of Schwann cell differentiation. In the present study, we performed a cyclic adenosine monophosphate-induced Schwann cell differentiation model in vitro. Western blot analysis showed that p27(Kip1) expression was upregulated during the differentiation of Schwann cell, while the inhibition of p27(Kip1) expression by short hairpin RNA-mediated knockdown significantly abolished the expression of promyelinating markers and the alteration of cellular morphology. In addition, immunofluorescence revealed a decrease of p27(Kip1) nuclear staining and a concomitant increase of cytoplasmic staining in differentiated Schwann cells. In summary, our data indicated that p27(Kip1) was a positive regulator of Schwann cell differentiation in vitro.

The cyclin-dependent kinase inhibitor p57Kip2 regulates cell cycle exit, differentiation, and migration of embryonic cerebral cortical precursors

Mounting evidence indicates cyclin-dependent kinase (CDK) inhibitors (CKIs) of the Cip/Kip family, including p57(Kip2) and p27(Kip1), control not only cell cycle exit but also corticogenesis. Nevertheless, distinct activities of p57(Kip2) remain poorly defined. Using in vivo and culture approaches, we show p57(Kip2) overexpression at E14.5-15.5 elicits precursor cell cycle exit, promotes transition from proliferation to neuronal differentiation, and enhances process outgrowth, while opposite effects occur in p57(Kip2)-deficient precursors. Studies at later ages indicate p57(Kip2) overexpression also induces precocious glial differentiation, suggesting stage-dependent effects. In embryonic cortex, p57(Kip2) overexpression advances cell radial migration and alters postnatal laminar positioning. While both CKIs induce differentiation, p57(Kip2) was twice as effective as p27(Kip1) in inducing neuronal differentiation and was not permissive to astrogliogenic effects of ciliary neurotrophic factor, suggesting that the CKIs differentially modulate cell fate decisions. At molecular levels, although highly conserved N-terminal regions of both CKIs elicit cycle withdrawal and differentiation, the C-terminal region of p57(Kip2) alone inhibits in vivo migration. Furthermore, p57(Kip2) effects on neurogenesis and gliogenesis require the N-terminal cyclin/CDK binding/inhibitory domains, while previous p27(Kip1) studies report cell cycle-independent functions. These observations suggest p57(Kip2) coordinates multiple stages of corticogenesis and exhibits distinct and common activities compared with related family member p27(Kip1).

Non-stem cell origin for oligodendroglioma

Malignant astrocytic brain tumors are among the most lethal cancers. Quiescent and therapy-resistant neural stem cell (NSC)-like cells in astrocytomas are likely to contribute to poor outcome. Malignant oligodendroglial brain tumors, in contrast, are therapy sensitive. Using magnetic resonance imaging (MRI) and detailed developmental analyses, we demonstrated that murine oligodendroglioma cells show characteristics of oligodendrocyte progenitor cells (OPCs) and are therapy sensitive, and that OPC rather than NSC markers enriched for tumor formation. MRI of human oligodendroglioma also suggested a white matter (WM) origin, with markers for OPCs rather than NSCs similarly enriching for tumor formation. Our results suggest that oligodendroglioma cells show hallmarks of OPCs, and that a progenitor rather than a NSC origin underlies improved prognosis in patients with this tumor.

The oligodendrocyte-specific G protein-coupled receptor GPR17 is a cell-intrinsic timer of myelination

The bHLH transcription factor Olig1 promotes oligodendrocyte maturation and is required for myelin repair. In this report, we characterize an Olig1-regulated G-protein coupled receptor GPR17 whose function is to oppose the action of Olig1. GPR17 is restricted to oligodendrocyte lineage cells but downregulated during the peak period of myelination and in adulthood. Transgenic mice with sustained GPR17 expression in oligodendrocytes exhibit stereotypic features of myelinating disorders in the CNS. GPR17 overexpression inhibits oligodendrocyte differentiation and maturation both in vivo and in vitro. Conversely, GPR17 knockout mice display early onset of oligodendrocyte myelination. The opposing action of GPR17 on oligodendrocyte maturation reflects, at least partially, upregulation and nuclear translocation of the potent oligodendrocyte differentiation inhibitors ID2/4. Collectively, these findings suggest that GPR17 orchestrates the transition between immature and myelinating oligodendrocytes via an ID protein-mediated negative regulation, and may serve as a potential therapeutic target for CNS myelin repair.

Effect of human telomerase reverse transcriptase transfection on differentiation in BeWo choriocarcinoma cells

Arrest of proliferation is one of the prerequisites for differentiation of cytotrophoblasts into syncytiotrophoblasts, and thus during differentiation telomerase activity, as well as human telomerase reverse transcriptase (hTERT) expression, is down-regulated. Considering this, it is of interest to investigate whether syncytium formation can be delayed by prolonging the expression of telomerase in cytotrophoblasts. BeWo cells were transfected with pLPC-hTERT retroviral vector and the reverse transcription-polymerase chain reaction analysis for hTERT mRNA concentrations in the transfected cells revealed a several-fold increase in hTERT mRNA compared with the cells transfected with empty vector, and this confirmed that the transfection was successful. An increase in the proliferation, as assessed by bromodeoxyuridine incorporation assay, as well as an increase in mRNA and protein concentration of various cyclins and proliferating cell nuclear antigen, was noticed. The effect of hTERT transfection was also assessed after the addition of forskolin to induce differentiation and it was observed that cell-cell fusion was delayed and differentiation did not occur in hTERT-transfected cells. However, the effects seen were only transient as stable transfection was not possible and the cells were undergoing apoptosis after 72 h, which suggested that apart from hTERT other factors might be important for immortalization of BeWo cells.

p57Kip2 is a repressor of Mash1 activity and neuronal differentiation in neural stem cells

Mammalian central nervous system (CNS) development is a highly organized process involving the precise and coordinated timing of cell-cycle exit, differentiation, survival, and migration. These events require proper expression of pro-neuronal genes but also repression of alternative cell fates and restriction of cell-type-specific gene expression. Here, we show that the cyclin-dependent kinase (CDK) inhibitor p57Kip2 interacted with pro-neuronal basic helix-loop-helix (bHLH) factors such as Mash1, NeuroD, and Nex/Math2. Increased levels of p57Kip2 inhibited Mash1 transcriptional activity independently of CDK interactions and acted as a direct repressor in transcriptional assays. Proliferating telencephalic neural progenitors co-expressed basal levels of Mash1 and p57Kip2, and endogenous p57Kip2 accumulated transiently in the nuclei of neural stem cells (NSCs) during early stages of astrocyte differentiation mediated by ciliary neurotrophic factor (CNTF), independent of cell-cycle exit and at times when Mash1 expression was still prominent. In accordance with these observations, gain- and loss-of-function studies showed that p57Kip2 repressed neuronal differentiation after mitogen withdrawal, but exerted little or no effect on CNTF-mediated astroglial differentiation of NSCs. Our data suggest a novel role for p57Kip2 as a context-dependent repressor of neurogenic transcription factors and telencephalic neuronal differentiation.

The QKI-6 and QKI-7 RNA binding proteins block proliferation and promote Schwann cell myelination

Background

The quaking viable (qk^v) mice have uncompacted myelin in their central and peripheral nervous system (CNS, PNS). The qk gene encodes 3 major alternatively spliced isoforms that contain unique sequence at their C-terminus dictating their cellular localization. QKI-5 is a nuclear isoform, whereas QKI-6 and QKI-7 are cytoplasmic isoforms. The qk^v mice harbor an enhancer/promoter deletion that prevents the expression of isoforms QKI-6 and QKI-7 in myelinating cells resulting in a dysmyelination phenotype. It was shown that QKI regulates the differentiation of oligodendrocytes, the myelinating cells of the CNS, however, little is known about the role of the QKI proteins, or RNA binding proteins in PNS myelination.

Methodology/Principal Findings

To define the role of the QKI proteins in PNS myelination, we ectopically expressed QKI-6 and QKI-7 in primary rat Schwann cell/neuron from dorsal root ganglia cocultures. We show that the QKI isoforms blocked proliferation and promoted Schwann cell differentiation and myelination. In addition, these events were coordinated with elevated proteins levels of p27^KIP1 and myelin basic protein (MBP), markers of Schwann cell differentiation. QKI-6 and QKI-7 expressing co-cultures contained myelinated fibers that had directionality and contained significantly thicker myelin, as assessed by electron microscopy. Moreover, QKI-deficient Schwann cells had reduced levels of MBP, p27^KIP1 and Krox-20 mRNAs, as assessed by quantitative RT-PCR.

Conclusions/Significance

Our findings suggest that the QKI-6 and QKI-7 RNA binding proteins are positive regulators of PNS myelination and show that the QKI RNA binding proteins play a key role in Schwann cell differentiation and myelination.

Patterns of p57Kip2 expression in embryonic rat brain suggest roles in progenitor cell cycle exit and neuronal differentiation

In developing central nervous system, a variety of mechanisms couple cell cycle exit to differentiation during neurogenesis. The cyclin-dependent kinase (CDK) inhibitor p57Kip2 controls the transition from proliferation to differentiation in many tissues, but roles in developing brain remain uncertain. To characterize possible functions, we defined p57Kip2 protein expression in embryonic (E) day 12.5 to 20.5 rat brains using immunohistochemistry combined with markers of proliferation and differentiation. The p57Kip2 was localized primarily in cell nuclei and positive cells formed two distinct patterns including wide dispersion and laminar aggregation that were brain region-specific. From E12.5 to E16.5, p57Kip2 expression was detected mainly in ventricular zone (VZ) and/or mantle zone of hippocampus, septum, basal ganglia, thalamus, hypothalamus, midbrain, and spinal cord. After E18.5, p57Kip2 was detected in select regions undergoing differentiation. The p57Kip2 expression was also compared with regional transcription factors, including Ngn2, Nkx2.1, and Pax6. Time course studies performed in diencephalon showed that p57Kip2 immunoreactivity colocalized with BrdU at 8 hr in nuclei exhibiting the wide dispersion pattern, whereas colocalization in the laminar pattern occurred only later. Moreover, p57Kip2 frequently colocalized with neuronal marker, beta-III tubulin. Finally, we characterized relationships of p57Kip2 to CDK inhibitor p27Kip1: in proliferative regions, p57Kip2 expression preceded p27Kip1 as cells underwent differentiation, though the proteins colocalized in substantial numbers of cells, suggesting potentially related yet distinct functions of Cip/Kip family members during neurogenesis. Our observations that p57Kip2 exhibits nuclear expression as precursors exit the cell cycle and begin expressing neuronal characteristics suggests that the CDK inhibitor contributes to regulating the transition from proliferation to differentiation during brain development.

p27(KIP1) regulates neurogenesis in the rostral migratory stream and olfactory bulb of the postnatal mouse

Neuronal progenitor cells of the anterior subventricular zone (SVZa) migrate along the rostral migratory stream (RMS) to the olfactory bulb, where they exit the cell cycle and differentiate. The molecular mechanisms that regulate SVZa progenitor proliferation and cell-cycle exit are largely undefined. We investigated the role of p27(KIP1) in regulating cell proliferation and survival in the RMS and olfactory bulb between postnatal day 1 (P1) and P14, the peak period of olfactory bulb neuron generation. A large proportion of cells in the RMS and the olfactory bulb express cytoplasmic p27(KIP1), but a small percentage display high nuclear p27(KIP1) immunostaining, which exhibit a caudal(low)-rostral(high) gradient: lowest in the SVZa and highest in the glomerular layer of the olfactory bulb. p27(KIP1) is also present in the nucleus and/or the cytoplasm of neuron-specific type III beta-tubulin(+) cells. Cells with strong nuclear p27(KIP1) expression are BrdU(-) and Ki67(-). The percentage of BrdU(+) cells in the SVZa, RMS, and olfactory bulb is higher in p27(KIP1) null than wild-type (WT) mice at all ages analyzed. Consistent with these findings, p27(KIP1) overexpression in cultured p27(KIP1) null and WT SVZ cells reduced cell proliferation and self-renewal. Finally, in p27(KIP1) null mice, the diameter of the horizontal limb of the RMS is larger than in WT mice, and development of the olfactory bulb granule cell layer is delayed, together with increased apoptotic cell density. Our results indicate that in the postnatal brain, p27(KIP1) regulates the proliferation and survival of neuronal cells in the RMS and olfactory bulb.

The anaphase promoting complex induces substrate degradation during neuronal differentiation

The anaphase promoting complex (APC) is an E3 ubiquitin ligase required for the metaphase-to-anaphase transition and mitotic exit. However, APC also plays roles in G(1), where it is regulated by Cdh1, and APC activity has also been detected in differentiated and non-proliferating cells, suggesting that it may play roles outside the cell cycle. Here, we report that disrupting APC(Cdh1) activity inhibits neurite outgrowth of both PC12 pheochromocytoma cells and primary cerebellar granule cells. APC(Cdh1) activity dramatically increases as PC12 cells differentiate in response to nerve growth factor. Furthermore, a key target degraded by APC(Cdh1) following nerve growth factor treatment is the F-box protein Skp2, and APC(Cdh1)-mediated destruction of Skp2 is essential for proper terminal differentiation of neuronal precursors.

Antitumor bioactivity of adenovirus-mediated p27mt in colorectal cancer cell line SW480

AIM: To explore the antitumor bioactivity of adenovirus-mediated mutant type p27^kip1 gene in a colorectal cancer cell line SW480.

METHODS: We constructed recombinant adenovirus vector expressing a mutant type p27^kip1 gene (ad-p27mt), with mutation of Thr-187/Pro-188 (ACGCCC) to Met-187/Ile-188 (ATGATC), and transduced into SW480 cells. Then we detected expression of p27, Bcl-2 and Bax protein in the transductants by Western blotting, cell cycle of transductants by a digital flow cytometric system, migrating potential with Boyden Chamber and SW480 tumor cell growth inhibition in vitro and in vivo.

RESULTS: We found that a recombinant adenovirus vector of expressing ad-p27mt, with mutation of Thr-187/Pro-188 (ACGCCC) to Met-187/Ile-188 (ATGATC) has potent inhibition of SW480 tumor cell growth in vitro and in vivo. Furthermore, ad-p27mt induced cell apoptosis via regulating bax and bcl-2 expressions, and G₁/S arrest in SW480 cells and inhibited cell migration.

CONCLUSION: ad-p27mt has a strong anti-tumor bioactivity and has the potential to develop into new therapeutic agents for colorectal cancer.

Mutant p27(Kip1) and its potential effect as hepatocellular gene therapy

BACKGROUND

The cyclin-dependent kinase (cdk) inhibitor p27(Kip1) is an important regulator of cell cycle progression as it negatively regulates G(0/1) progression and plays a major role in controlling the cell cycle. The screening of the p27(Kip1) sequence identified many potential phosphorylation sites. To investigate the effects of the overexpression of exogenous p27(Kip1) protein lacking the Thr157 sites on subcellular localization, cell cycle, and proliferation, a plasmid was constructed containing mutations of p27(Kip1) at Thr157 (T157A p27), and transfected into the SMMC7721 cell line with Lipofectamine. Wild-type and mutant p27 plasmids T157A were transfected separately as control groups.

METHODS

We detected the proliferation of SMMC7721 cells by the Cell Counting Kit and FACS/Calibur Flow Cytometer and analyzed the expression and localization of p27(Kip1) by Western blotting analysis and cell fractionation. The cdk2 dependent kinase activity was determined by in vitro kinase assay.

RESULTS

Proliferation of SMMC7721 cells was greatly inhibited and cell cycle was arrested in G(0/1) phase after exogenous p27(Kip1) mutant expression much more than wild-type p27(Kip1). The expressed T157A p27(Kip1) proteins were translocated from the cytoplasm into nucleus much more compare with wild-type. Compared with pcDNA3.1-Myc control, transient transfection of T157A p27(Kip1) decreased expression of cyclin D1 and the phosphorylated form of retinoblastoma protein.

CONCLUSIONS

These findings support the potential effectiveness of a PI3K/Akt-resistant phosphorylated form of p27 in hepatocellular carcinoma gene therapy.

Dynamic changes of p27(kip1) and Skp2 expression in injured rat sciatic nerve

S phase kinase-associated protein 2 (Skp2), an F-box protein, is required for the ubiquitination and consequent degradation of p27(kip1). Previous reports have showed that p27(kip1 )played important roles in cell cycle regulation and neurogenesis in the developing central nervous system. But the distribution and function of p27(kip1 )and Skp2 in nervous system lesion and regeneration remains unclear. In this study, we observed that they were expressed mainly in both Schwann cells and axons in adult rat sciatic nerve. Sciatic nerve crush and transection resulted in a significant up-regulation of Skp2 and a down-regulation of p27(kip1). By immunochemistry, we found that in the distal stumps of transected nerve from the end to the edge, the appearance of Skp2 in the edge is coincided with the decrease in p27(kip1) levels. Changes of them were inversely correlated. Results obtained by coimmunoprecipitation and double labeling further showed their interaction in the regenerating process. Thus, these results indicate that p27(kip1 )and Skp2 likely play an important role in peripheral nerve injury and regeneration.

The human ubiquitin conjugating enzyme, UBE2E3, is required for proliferation of retinal pigment epithelial cells

PURPOSE

Cell cycle progression is governed by the coordinated activities of kinases, phosphatases, and the ubiquitin system. The entire complement of ubiquitin pathway components that mediate this process in retinal pigment epithelial (RPE) cells remains to be identified. This study was undertaken to determine whether the human ubiquitin-conjugating enzyme, UBE2E3, is essential for RPE cell proliferation.

METHODS

UBE2E3 expression and localization in telomerase-immortalized, human RPE cells was determined with a UBE2E3-specific antibody. The necessity for UBE2E3 in RPE proliferation was determined using small interfering (si)RNA to target the expression of the enzyme. Cell counts and immunolabeling for the proliferation marker Ki-67 and the cyclin-dependent kinase inhibitor p27(Kip1) were performed to assess the consequences of UBE2E3 depletion. A mouse strain harboring a disrupted allele of UbcM2 (the mouse counterpart of UBE2E3) with the coding sequence for beta-galactosidase was used to track the developmental expression of the enzyme in murine RPE cells.

RESULTS

UBE2E3 localized in the nucleus of the immortalized RPE cells. Depletion of the enzyme by siRNA resulted in a cell-cycle exit accompanied by a loss of Ki-67, an increase in p27(Kip1), and a doubling in cell area. Rescue experiments confirmed the specificity of the RNA interference. In vivo, UbcM2 was transcriptionally downregulated during RPE development in the mouse.

CONCLUSIONS

UBE2E3 is essential for the proliferation of RPE-1 cells and is downregulated during RPE layer maturation in the developing mouse eye. These findings indicate that UBE2E3 is a major enzyme in modulating the balance between RPE cell proliferation and differentiation.

Progression elevated gene-3 promoter (PEG-Prom) confers cancer cell selectivity to human polynucleotide phosphorylase (hPNPase(old-35))-mediated growth suppression

The poor prognosis of pancreatic cancer patients using currently available therapies mandates novel therapeutics that combine anti-neoplastic potency with toxicity-minimizing cancer specificity. Employing an overlapping pathway screen to identify genes exhibiting coordinated expression as a consequence of terminal cell differentiation and replicative senescence, we identified human polynucleotide phosphorylase (hPNPase(old-35)), a 3',5'-exoribonuclease that exhibits robust growth-suppressing effects in a wide spectrum of human cancers. A limitation to the anti-neoplastic efficacy of hPNPase(old-35) relates to its lack of cancer specificity. The promoter of Progression Elevated Gene-3 (PEG-Prom), discovered in our laboratory via subtraction hybridization in a transformation progression rodent tumor model functions selectively in a diverse array of human cancer cells, with limited activity in normal cells. An adenovirus constructed with the PEG-Prom driving expression of hPNPase(old-35) containing a C-terminal Hemaglutinin (HA)-tag (Ad.PEG.hPNPase(old-35)) was shown to induce robust transgene expression, growth suppression, apoptosis, and cell-cycle arrest in a broad panel of pancreatic cancer cells, with minimal effects in normal immortalized pancreatic cells. hPNPase(old-35) expression correlated with arrest in the G(2)/M phase of the cell cycle and up-regulation of the cyclin-dependent kinase inhibitors (CDKI) p21(CIP1/WAF-1/MDA-6) and p27(KIP1). In a nude mouse xenograft model, Ad.PEG.hPNPase(old-35) injections effectively inhibited growth of human pancreatic cancer cells in vivo. These findings support the potential efficacy of combining a cancer-specific promoter, such as the PEG-Prom, with a novel anti-neoplastic agent, such as hPNPase(old-35), to create a potent, targeted cancer therapeutic, especially for a devastating disease like pancreatic cancer.

New implications for the QUAKING RNA binding protein in human disease

The use of spontaneously occurring mouse models has proved to be a valuable tool throughout the years to delineate the signals required for nervous system development. This is especially true in the field of myelin biology, with a large number of different models available. The quaking viable mouse models dysmyelination in the nervous system and links the QUAKING RNA binding proteins to myelination and cell fate decisions. In this Mini-Review, we highlight the biological functions attributed to this KH-type RNA binding protein and the recent achievements linking it to human disorders.

Expression of p27kip1 and Skp2 in the adult spinal cord following sciatic nerve injury

Skp2 (S-phase-associated kinase protein-2) is involved in ubiquitination and proteasome-mediated degradation of p27kip1, which plays important roles in cell cycle regulation and neurogenesis in the developing central nervous system (CNS). But their distribution and function in the nervous system lesion and regeneration remains unclear. In this study, we examined expression and relationship of p27kip1 and Skp2 in adult rat spinal cord following sciatic nerve injury. It was illustrated that they localized mainly in neurons and astrocytes of spinal cord, and might also expressed in other glial cells according to the results of immunohistochemistry. Sciatic nerve crush and transection resulted in a significant up-regulation of Skp2 and a down-regulation of p27kip1 in spinal cord. Statistical analysis indicated negative correlation between the number of p27kip1 and Skp2 positive cells in the ventral horn following the sciatic nerve lesion. Immunoprecipitation further showed that they interacted with each other in the regenerating process. Thus, p27kip1 and Skp2 likely play an important role in spinal cord regeneration after peripheral nerve injury.

The transcription factor Yin Yang 1 is essential for oligodendrocyte progenitor differentiation

The progression of progenitors to oligodendrocytes requires proliferative arrest and the activation of a transcriptional program of differentiation. While regulation of cell cycle exit has been extensively characterized, the molecular mechanisms responsible for the initiation of differentiation remain ill-defined. Here, we identify the transcription factor Yin Yang 1 (YY1) as a critical regulator of oligodendrocyte progenitor differentiation. Conditional ablation of yy1 in the oligodendrocyte lineage in vivo induces a phenotype characterized by defective myelination, ataxia, and tremor. At the cellular level, lack of yy1 arrests differentiation of oligodendrocyte progenitors after they exit from the cell cycle. At the molecular level, YY1 acts as a lineage-specific repressor of transcriptional inhibitors of myelin gene expression (Tcf4 and Id4), by recruiting histone deacetylase-1 to their promoters during oligodendrocyte differentiation. Thus, we identify YY1 as an essential component of the transcriptional network regulating the transition of oligodendrocyte progenitors from cell cycle exit to differentiation.

A crucial role for p57(Kip2) in the intracellular timer that controls oligodendrocyte differentiation

The intracellular molecular mechanism that controls the timing of oligodendrocyte differentiation remains unknown. Temple and Raff (1986) previously showed that an oligodendrocyte precursor cell (OPC) can divide a maximum of approximately eight times before its daughter cells simultaneously cease proliferating and differentiate into oligodendrocytes. They postulated that over time the level of an intracellular molecule might synchronously change in each daughter cell, ultimately reaching a level that prohibited additional proliferation. Here, we report the discovery of such a molecule, the cyclin-dependent kinase inhibitor p57(Kip2) (Cdkn1c). We show in vitro that all daughters of a clone of OPCs express similar levels of p57(Kip2), that p57(Kip2) levels increase over time in proliferating OPCs, and that p57(Kip2) levels regulate how many times an OPC can divide before differentiating. These findings reveal a novel part of the mechanism by which OPCs measure time and are likely to extend to similar timers in many other precursor cell types.

The selective RNA-binding protein quaking I (QKI) is necessary and sufficient for promoting oligodendroglia differentiation

Quaking I (QKI) is a selective RNA-binding protein essential for myelination of the central nervous system. Three QKI isoforms with distinct C termini and subcellular localization, namely QKI-5, QKI-6, and QKI-7, are expressed in oligodendroglia progenitor cells (OPCs) prior to the initiation of myelin formation and implicated in promoting oligodendrocyte lineage development. However, the functional requirement for each QKI isoform and the mechanisms by which QKI isoforms govern OPC development still remain elusive. We report here that exogenous expression of each QKI isoform is sufficient to enhance differentiation of OPCs with different efficiency, which is abolished by a point mutation that abrogates the RNA binding activity of QKI. Reciprocally, small interfering RNA-mediated QKI knockdown blocks OPC differentiation, which can be partly rescued by QKI-5 and QKI-6 but not by QKI-7, indicating the differential requirement of QKI isoform function in advancing OPC differentiation. Furthermore, we found that abrogation of OPC differentiation, as a result of QKI deficiency, is not due to altered proliferation capacity or cell cycle progression. These results indicate that QKI isoforms are necessary and sufficient for promoting OPC development, which must involve direct influence of QKI on differentiation/maturation of OPCs independent of cell cycle exit, likely via regulating the expression of the target mRNAs of QKI that support OPC differentiation.

Galectin-4 is involved in p27-mediated activation of the myelin basic protein promoter

Our previous studies have found that expression of p27 in oligodendrocytes enhances myelin basic protein (MBP) gene expression through a mechanism that involves the transcription factor Sp1. In this study we show that this activation only requires the N-terminal 45 amino acids of p27 containing a functional cyclin-binding motif. In an effort to identify other cofactors that are involved in the p27-mediated activation of MBP gene expression, a yeast two-hybrid assay was performed using an N-terminal truncated p27 and a mouse embryo cDNA library. Galectin-4 was found to interact with p27 in the yeast two-hybrid assay. This novel interaction was also confirmed using a glutathione-S-transferase interaction assay and immunoprecipitation assays. Expression of galectin-4 in primary oligodendrocytes was confirmed by western blot. Additionally, the MBP promoter could be activated by expression of galectin-4 in CG4 oligodendrocytes, similar to the effects of increased p27 levels. We also show that Sp1 and galectin-4 interact in cells, while a complex of all three proteins could not be found. We conclude that galectin-4 is involved in the p27-mediated activation of the MBP gene, possibly through modulation of the glycosylation status of the transcription factor Sp1.

Licensing regulators Geminin and Cdt1 identify progenitor cells of the mouse CNS in a specific phase of the cell cycle

Nervous system formation integrates control of cellular proliferation and differentiation and is mediated by multipotent neural progenitor cells that become progressively restricted in their developmental potential before they give rise to differentiated neurons and glial cells. Evidence from different experimental systems suggests that Geminin is a candidate molecule linking proliferation and differentiation during nervous system development. We show here that Geminin and its binding partner Cdt1 are expressed abundantly by neural progenitor cells during early mouse neurogenesis. Their expression levels decline at late developmental stages and become undetectable upon differentiation. Geminin and Cdt1 expressing cells also express Sox2 while no overlap is detected with cells expressing markers of a differentiated neuronal phenotype. A fraction of radial glial cells expressing RC2 and Pax6 are also immunoreactive for Geminin and Cdt1. The majority of the Geminin and Cdt1 expressing cell populations appears to be distinct from fate-restricted precursor cells expressing Mash1 or Neurogenin2. Bromo-deoxy-uridine (BrdU) incorporation experiments reveal a cell cycle specific expression in neural progenitor cells, with Geminin being present from S to M phase, while Cdt1 expression characterizes progenitor cells in G1 phase. Furthermore, in vitro differentiation of adult neurosphere cultures shows downregulation of Geminin/Cdt1 in the differentiated state, in line with our data showing that Geminin is present in neural progenitor cells of the CNS during mouse embryogenesis and adulthood and becomes downregulated upon cell fate specification and differentiation. This suggests a role for Geminin in the formation and maintenance of the neural progenitor cells.

The mystery of intracellular developmental programmes and timers

There has been a revolution in understanding animal development in the last 25 years or so, but there is at least one area of development that has been relatively neglected and therefore remains largely mysterious. This is the intracellular programmes and timers that run in developing precursor cells and change the cells over time. The molecular mechanisms underlying these programmes are largely unknown. My colleagues and I have studied such programmes in two types of rodent neural precursor cells: those that give rise to oligodendrocytes, which make myelin in the CNS (central nervous system), and those that give rise to the various cell types in the retina.

Intracellular developmental timers

One of the most poorly understood aspects of animal development is how the timing of developmental events is controlled. In most vertebrate cell lineages, for example, precursor cells divide a limited number of times before they stop and terminally differentiate, but it is not known what controls when the cells stop dividing and differentiate. There is increasing evidence, however, that intracellular timers play an important part. Such cell-intrinsic timers are examples of intracellular developmental programs that change precursor cells over time. My colleagues and I have studied such intracellular timers and programs in rodent oligodendrocyte precursor cells (OPCs), as reviewed here.

Evidence for cytosolic p27(Kip1) ubiquitylation and degradation during adipocyte hyperplasia

OBJECTIVE

Subcellular localization has been shown to play an important role in determining activity and accumulation of p27 protein during cell cycle progression. The purpose of this study was to examine p27 localization and ubiquitylation in relation to E3 ligase expression during adipocyte hyperplasia.

RESEARCH METHODS AND PROCEDURES

This study used the murine 3T3-L1 preadipocyte model to examine p27 regulation during synchronous cell cycle progression. Cell lysates were isolated over time after hormonal stimulation, fractionated to cytosolic and nuclear compartments, and immunoblotted for relative protein determinations.

RESULTS

Data presented in this study show that p27 was present in the cytosol and nucleus in density-arrested preadipocytes and that abundance in both compartments decreased in a phase-specific manner as preadipocytes synchronously re-entered the cell cycle during early phases of adipocyte differentiation. Blocking CRM1-mediated nuclear export did not prevent degradation, nor did it cause nuclear accumulation of p27, suggesting that distinct mechanisms mediating cytosolic and nuclear p27 degradation were involved. Treating preadipocytes with a potent and specific proteasome inhibitor during hormonal stimulation prevented Skp2 accumulation and p27(187) phosphorylation, which are essential events for SCF(Skp2) E3 ligase activity and nuclear p27 ubiquitylation during S/G(2) phase progression. Proteasome blockade also resulted in the first evidence of cytosolic p27 ubiquitylation during late G(1) phase as preadipocytes undergo the transition from quiescence to proliferation.

DISCUSSION

These data are consistent with the postulate that p27 is ubiquitylated and targeted for degradation by the 26S proteasome in a phase-specific manner by distinct ubiquitin E3 ligases localized to the cytosol and nucleus during adipocyte hyperplasia.

The Yin and Yang of cell cycle progression and differentiation in the oligodendroglial lineage

In white matter disorders such as leukodystrophies (LD), periventricular leucomalacia (PVL), or multiple sclerosis (MS), the hypomyelination or the remyelination failure by oligodendrocyte progenitor cells involves errors in the sequence of events that normally occur during development when progenitors proliferate, migrate through the white matter, contact the axon, and differentiate into myelin-forming oligodendrocytes. Multiple mechanisms underlie the eventual progressive deterioration that typifies the natural history of developmental demyelination in LD and PVL and of adult-onset demyelination in MS. Over the past few years, pathophysiological studies have mostly focused on seeking abnormalities that impede oligodendroglial maturation at the level of migration, myelination, and survival. In contrast, there has been a strikingly lower interest for early proliferative and differentiation events that are likely to be equally critical for white matter development and myelin repair. This review highlights the Yin and Yang principles of interactions between intrinsic factors that coordinately regulate progenitor cell division and the onset of differentiation, i.e. the initial steps of oligodendrocyte lineage progression that are obviously crucial in health and diseases.

Rescuing qkV dysmyelination by a single isoform of the selective RNA-binding protein QKI

Alternative splicing of the qkI transcript generates multiple isoforms of the selective RNA-binding protein QKI, which play key roles in controlling the homeostasis of their mRNA targets. QKI deficiency in oligodendrocytes of homozygous quakingviable (qkV/qkV) mutant mice results in severe hypomyelination, indicating the essential function of QKI in myelinogenesis. However, the molecular mechanisms by which QKI controls myelination remain elusive. We report here that QKI-6 is the most abundant isoform in brain and is preferentially reduced in the qkV/qkV mutant during normal myelinogenesis. To test whether QKI-6 is the predominant isoform responsible for advancing CNS myelination, we developed transgenic mice that express Flag-QKI-6 specifically in the oligodendroglia lineage, driven by the proteolipid protein (PLP) promoter. When introduced into the qkV/qkV mutant, the QKI-6 transgene rescues the severe tremor and hypomyelination phenotype. Electron microscopic studies further revealed that the Flag-QKI-6 transgene is sufficient for restoring compact myelin formation with normal lamellar periodicity and thickness. Interestingly, Flag-QKI-6 preferentially associates with the mRNA encoding the myelin basic protein (MBP) and rescues MBP expression from the beginning of myelinogenesis. In contrast, Flag-QKI-6 binds the PLP mRNA with lower efficiency and has a minimal impact on PLP expression until much later, when the expression level of QKI-6 in the transgenic animal significantly exceeds what is needed for normal myelination. Together, our results demonstrate that QKI-6 is the major isoform responsible for CNS myelination, which preferentially promotes MBP expression in oligodendrocytes.

Downregulation of the Cyclin-Dependent Kinase Inhibitor p27kip1 Might Correlate with Poor Disease-Free and Overall Survival in Inflammatory Breast Cancer

PURPOSE

The objective of this study was to evaluate whether p27kip1 downregulation is a prognostic factor in patients with inflammatory breast carcinoma (IBC).

PATIENTS AND METHODS

Fifty-eight patients with IBC were treated between January 1994 and July 2002. Median age was 49 years. Thirty-eight patients had baseline biopsy specimens. Patients received preoperative chemotherapy with FAC (5-fluorouracil/doxorubicin/cyclophosphamide; 34%) or FAC followed by a taxane (66%). All patients underwent mastectomies. All patients received radiation therapy and hormonal treatment when indicated. Expression level of p27kip1 was evaluated by indirect immunoperoxidase procedure. The p27kip1 was considered downregulated if nuclear staining was present in < 50% of the neoplastic cells.

RESULTS

Thirty-two patients (84%) had p27kip1-downregulated tumors, and 6 patients (17%) had p27kip1-normal tumors. Six patients (16%) exhibited a pathologic complete response. At a median follow-up of 43 months, 25 recurrences (66%) and 27 deaths (71%) occurred. Patients with p27kip1-downregulated tumors had fewer pathologic complete responses (9% vs. 50%; P = 0.03) and had lower 4-year recurrence-free survival (23% vs. 83%; P = 0.03) and overall survival rates (36% vs. 83%; P = 0.01).

CONCLUSION

The p27kip1 deregulation manifested by low protein cellular concentration might represent an adverse prognostic marker in IBC and could provide a valuable tool for selecting treatment for this aggressive disease.

Homeodomain transcription factor Phox2a, via cyclic AMP-mediated activation, induces p27Kip1 transcription, coordinating neural progenitor cell cycle exit and differentiation

Mechanisms coordinating neural progenitor cell cycle exit and differentiation are incompletely understood. The cyclin-dependent kinase inhibitor p27(Kip1) is transcriptionally induced, switching specific neural progenitors from proliferation to differentiation. However, neuronal differentiation-specific transcription factors mediating p27(Kip1) transcription have not been identified. We demonstrate the homeodomain transcription factor Phox2a, required for central nervous system (CNS)- and neural crest (NC)-derived noradrenergic neuron differentiation, coordinates cell cycle exit and differentiation by inducing p27(Kip1) transcription. Phox2a transcription and activation in the CNS-derived CAD cell line and primary NC cells is mediated by combined cyclic AMP (cAMP) and bone morphogenetic protein 2 (BMP2) signaling. In the CAD cellular model, cAMP and BMP2 signaling initially induces proliferation of the undifferentiated precursors, followed by p27(Kip1) transcription, G(1) arrest, and neuronal differentiation. Small interfering RNA silencing of either Phox2a or p27(Kip1) suppresses p27(Kip1) transcription and neuronal differentiation, suggesting a causal link between p27(Kip1) expression and differentiation. Conversely, ectopic Phox2a expression via the Tet-off expression system promotes accelerated CAD cell neuronal differentiation and p27(Kip1) transcription only in the presence of cAMP signaling. Importantly, endogenous or ectopically expressed Phox2a activated by cAMP signaling binds homeodomain cis-acting elements of the p27(Kip1) promoter in vivo and mediates p27(Kip1)-luciferase expression in CAD and NC cells. We conclude that developmental cues of cAMP signaling causally link Phox2a activation with p27(Kip1) transcription, thereby coordinating neural progenitor cell cycle exit and differentiation.

p27kip1 independently promotes neuronal differentiation and migration in the cerebral cortex

The generation of neurons by progenitor cells involves the tight coordination of multiple cellular activities, including cell cycle exit, initiation of neuronal differentiation, and cell migration. The mechanisms that integrate these different events into a coherent developmental program are not well understood. Here we show that the cyclin-dependent kinase inhibitor p27(Kip1) plays an important role in neurogenesis in the mouse cerebral cortex by promoting the differentiation and radial migration of cortical projection neurons. Importantly, these two functions of p27(Kip1) involve distinct activities, which are independent of its role in cell cycle regulation. p27(Kip1) promotes neuronal differentiation by stabilizing Neurogenin2 protein, an activity carried by the N-terminal half of the protein. p27(Kip1) promotes neuronal migration by blocking RhoA signaling, an activity that resides in its C-terminal half. Thus, p27(Kip1) plays a key role in cortical development, acting as a modular protein that independently regulates and couples multiple cellular pathways contributing to neurogenesis.

Heterogeneity of NG2-expressing cells in the newborn mouse cerebellum

The function and origin of NG2+ cells in the adult brain are still controversial. A large amount of data is available which strongly indicates that adult NG2-expressing cells form a heterogeneous population, constituted by oligodendrocyte precursor cells (OPCs) and a fourth novel type of glial cells named the synantocytes. Whether these two populations derive from the progressive maturation of perinatal NG2+ OPCs or are generated as separate populations is not known. We used organotypic cultures of newborn mouse cerebellum depleted, by anti-mitotic drug treatment, of their NG2+ cells with perinatal features (high proliferating rate and high oligodendrocytic differentiation ability). In these cultures, despite the lack of myelin after 14 days in vitro, numerous NG2+ cells remained. We show that these BrdU-resistant cells were able to slowly divide, as adult NG2+ cells do. Although many of these cells expressed O4, only a very small fraction of them was further engaged in oligodendrocyte lineage, as they had an extremely poor capacity to generate myelin sheaths to the Purkinje cell axons. These results support the view that at least two distinct populations of NG2+ cells coexist in the cerebellum from birth: one with the young OPC characteristics, another with adult NG2+ cell characteristics. Thus, a fraction of adult NG2+ cells do not derive from the maturation of perinatal OPCs.

A novel function of OLIG2 to suppress human glial tumor cell growth via p27Kip1 transactivation

The basic helix-loop-helix transcription factor OLIG2 is specifically expressed in cells of the oligodendrocyte lineage. It is also expressed in various tumors originating from glial cells; however, the expression of OLIG2 is rare or weak in glioblastomas, the most malignant gliomas. The role of OLIG2 in glioma remains unclear. To investigate the function of OLIG2 in glial tumor cells, we have established a glioblastoma cell line, U12-1, in which the expression of OLIG2 is induced by the Tet-off system. Induction of OLIG2 resulted in suppression of both the proliferation and anchorage-independent growth of U12-1. It also resulted in an increase in the expression of p27Kip1. A luciferase assay revealed that the CTF site of the p27Kip1 gene promoter was essential for OLIG2-dependent activation of p27Kip1 gene transcription. Electrophoretic mobility shift assays confirmed that a nuclear extract of OLIG2-expressing U12-1 cells contained a protein complex that binds to the CTF site of the p27Kip1 gene promoter. Furthermore, siRNA against p27Kip1 rescued the OLIG2-mediated growth and DNA synthesis inhibition of U12-1 cells. These results indicate that OLIG2 suppresses the proliferation of U12-1 and that this effect is mediated by transactivation of the p27Kip1 gene, and low expression of OLIG2 may be related to the malignant behavior of human glioblastoma.

Genetic mosaics reveal both cell-autonomous and cell-nonautonomous function of murine p27Kip1

Loss of the cyclin-dependent kinase inhibitor p27(Kip1) leads to an overall increase in animal growth, pituitary tumors, and hyperplasia of hematopoietic organs, yet it is unknown whether all cells function autonomously in response to p27(Kip1) activity or whether certain cells take cues from their neighbors. In addition, there is currently no genetic evidence that tumor suppression by p27(Kip1) is cell-autonomous because biallelic gene inactivation is absent from tumors arising in p27(Kip1) hemizygous mice. We have addressed these questions with tissue-specific targeted mouse mutants and radiation chimeras. Our results indicate that the suppression of pars intermedia pituitary tumors by p27(Kip1) is cell-autonomous and does not contribute to overgrowth or infertility phenotypes. In contrast, suppression of spleen growth and hematopoietic progenitor expansion is a consequence of p27(Kip1) function external to the hematopoietic compartment. Likewise, p27(Kip1) suppresses thymocyte hyperplasia through a cell-nonautonomous mechanism. The interaction of p27(Kip1) loss with epithelial cell-specific cyclin-dependent kinase 4 overexpression identifies the thymic epithelium as a relevant site of p27(Kip1) activity for the regulation of thymus growth.

Oligodendrocyte specification in zebrafish requires notch-regulated cyclin-dependent kinase inhibitor function

Cyclin-dependent kinase inhibitors (Cdkis) influence both cell-cycle progression and differentiation of neural cells. However, the precise roles of Cdkis in coordinating formation of neurons and glia and the mechanisms that regulate expression of genes that encode Cdkis in the vertebrate CNS remain unknown. Here, we report that, in zebrafish, expression of the Cdki gene cyclin-dependent kinase inhibitor 1c (cdkn1c), a p57 homolog, is negatively regulated by Delta-Notch signaling and that Cdkn1c function is required for neural plate cells to stop dividing and differentiate as neurons on schedule, even in the absence of Notch signaling activity. Furthermore, Cdkn1c function is required for specification of oligodendrocytes from ventral spinal cord precursors. We propose that levels of cdkn1c expression are an important factor in regulating neural development: high levels of Cdkn1c promote cell-cycle exit and neuronal development, whereas, during late embryogenesis, neural cells that have low but functional levels of Cdkn1c, regulated by Notch activity, are specified for oligodendrocyte fate.

Immunohistochemical Detection of p27 and p21 Proteins in Canine Hair Follicle and Epidermal Neoplasms

Trichoblastomas, trichoepitheliomas, and squamous cell carcinomas in the skin of dogs were analysed by immunohistochemistry for the nuclear expression of p27, p21 and proliferating cell nuclear antigen (PCNA). High levels of p27 were present in trichoepitheliomas and trichoblastomas compared with squamous cell carcinomas. Detectable p21 was found in trichoepitheliomas and squamous cell carcinomas, but trichoblastomas had low level of p21 nuclear reactivity. Low levels of PCNA were detected in trichoepitheliomas and trichoblastomas compared with squamous cell carcinomas. The results suggested that nuclear p27 acts as a cyclin-dependent kinase (CDK) inhibitor in trichoepitheliomas and trichoblastomas. Nuclear p21 expression is involved in the induction of epithelial differentiation and seems to be unrelated to CDK inhibition.

Interferon-gamma inhibits cell cycle exit in differentiating oligodendrocyte progenitor cells

The developmental processes of the oligodendrocyte progenitor cell (OPC) lineage that are targeted by interferon-gamma (IFN-gamma) were studied in primary rat OPC cultures. Under conditions of thyroid hormone-mediated oligodendrocyte differentiation, IFN-gamma produced a dose-dependent apoptotic response in OPCs. The lowest dose tested (15 ng/ml or 75 U/ml) was nonapoptotic, but activated detectable STAT1 DNA-binding. At this dose, IFN-gamma reduced the percentage of mature O1+ cells and increased the percentage of immature A2B5+ OPCs. This was observed without significant change in total cell number and cytotoxicity, and was accompanied by an increase in BrdU-labeled A2B5+ and O4+ cells. FACS analysis confirmed a lack of apoptotic sub-G1 cells and revealed a greater percentage of S- and G2/M-phase OPCs with IFN-gamma treatment. Dual immunostaining with Ki-67 and Olig2 showed a smaller percentage of Olig2+ cells in G0 phase in IFN-gamma-treated OPCs, indicating loss of G1 control. Instead, increased levels and phosphorylation of the checkpoint protein p34cdc2 by IFN- suggested increased partial arrest in G2. IFN-gamma not only sustained expression of PCNA and the G1-S regulators retinoblastoma protein, cyclin D1, cyclin E, and cdk2, but also decreased p27 levels. In addition to changes in cell proliferation and differentiation, IFN-gamma attenuated myelin basic protein (MBP) expression significantly, which was associated with decreased expression of both MBP and Sox10 RNAs. These findings indicate that IFN-gamma not only maintains cell cycle activity that could predispose OPCs to apoptosis, but also overrides G1-G0 signals leading to thyroid hormone-mediated terminal differentiation and myelin gene expression.

G1 phase regulation, area-specific cell cycle control, and cytoarchitectonics in the primate cortex

We have investigated the cell cycle-related mechanisms that lead to the emergence of primate areas 17 and 18. These areas are characterized by striking differences in cytoarchitectonics and neuron number. We show in vivo that (1) area 17 precursors of supragranular neurons exhibit a shorter cell cycle duration, a reduced G1 phase, and a higher rate of cell cycle reentry than area 18 precursors; (2) area 17 and area 18 precursors show contrasting and specific levels of expression of cyclin E (high in area 17, low in area 18) and p27Kip1 (low in area 17, high in area 18); (3) ex vivo up- and downmodulation of cyclin E and p27Kip1 show that both regulators influence cell cycle kinetics by modifying rates of cell cycle progression and cell cycle reentry; (4) modeling the areal differences in cell cycle parameters suggests that they contribute to areal differences in numbers of precursors and neuron production.

RB and RB2/p130 genes demonstrate both specific and overlapping functions during the early steps of in vitro neural differentiation of marrow stromal stem cells

Marrow stromal stem cells (MSCs) are stem-like cells that are currently being tested for their potential use in cell therapy for a number of human diseases. MSCs can differentiate into both mesenchymal and nonmesenchymal lineages. In fact, in addition to bone, cartilage and fat, it has been demonstrated that MSCs are capable of differentiating into neurons and astrocytes. RB and RB2/p130 genes are involved in the differentiation of several systems. For this reason, we evaluated the role of RB and RB2/p130 in the differentiation and apoptosis of MSCs under experimental conditions that allow for MSC differentiation toward the neuron-like phenotype. To this end, we ectopically expressed either RB or RB2/p130 and monitored proliferation, differentiation and apoptosis in rat primary MSC cultures induced to differentiate toward the neuron-like phenotype. Both RB and RB2/P130 decreased cell proliferation rate. In pRb-overexpressing cells, the arrest of cell growth was also observed in the presence of the HDAC-inhibitor TSA, suggesting that its antiproliferative activity does not rely upon the HDAC pathway, while the addition of TSA to pRb2/p130-overexpressing cells relieved growth inhibition. TUNEL reactions and studies on the expression of genes belonging to the Bcl-2 family showed that while RB protected differentiating MSCs from apoptosis, RB2/p130 induced an increase of apoptosis compared to controls. The effects of both RB and RB2/p130 on programmed cell death appeared to be HDAC- independent. Molecular analysis of neural differentiation markers and immunocytochemistry revealed that RB2/p130 contributes mainly to the induction of generic neural properties and RB triggers cholinergic differentiation. Moreover, the differentiation potentials of RB2/p130 and RB appear to rely, at least in part, on the activity of HDACs.

GABAergic Excitation Promotes Neuronal Differentiation in Adult Hippocampal Progenitor Cells

Hippocampal activity influences neurogenesis in the adult dentate gyrus; however, little is known about the involvement of the hippocampal circuitry in this process. In the subgranular zone of the adult dentate gyrus, neurogenesis involves a series of differentiation steps from radial glia-like stem/progenitor (type-1) cells, to transiently amplifying neuronal progenitor (type-2) cells, to postmitotic neurons. In this study, we conducted GFP-targeted recordings of progenitor cells in fresh hippocampal slices from nestin-GFP mice and found that neuronal progenitor (type-2) cells receive active direct neural inputs from the hippocampal circuitry. This input was GABAergic but not glutamatergic. The GABAergic inputs depolarized type-2 cells because of their elevated [Cl(-)](i). This excitation initiated an increase of [Ca(2+)](i) and the expression of NeuroD. A BrdU-pulse labeling study with GABA(A)-R agonists demonstrated the promotion of neuronal differentiation via this GABAergic excitation. Thus, it appears that GABAergic inputs to hippocampal progenitor cells promote activity-dependent neuronal differentiation.