Temporal and sequential transcriptional dynamics define lineage shifts in corticogenesis

The cerebral cortex contains billions of neurons, and their disorganization or misspecification leads to neurodevelopmental disorders. Understanding how the plethora of projection neuron subtypes are generated by cortical neural stem cells (NSCs) is a major challenge. Here, we focused on elucidating the transcriptional landscape of murine embryonic NSCs, basal progenitors (BPs), and newborn neurons (NBNs) throughout cortical development. We uncover dynamic shifts in transcriptional space over time and heterogeneity within each progenitor population. We identified signature hallmarks of NSC, BP, and NBN clusters and predict active transcriptional nodes and networks that contribute to neural fate specification. We find that the expression of receptors, ligands, and downstream pathway components is highly dynamic over time and throughout the lineage implying differential responsiveness to signals. Thus, we provide an expansive compendium of gene expression during cortical development that will be an invaluable resource for studying neural developmental processes and neurodevelopmental disorders.

Cyclins, Cyclin-Dependent Kinases, and Cyclin-Dependent Kinase Inhibitors in the Mouse Nervous System

Development and normal physiology of the nervous system require proliferation and differentiation of stem and progenitor cells in a strictly controlled manner. The number of cells generated depends on the type of cell division, the cell cycle length, and the fraction of cells that exit the cell cycle to become quiescent or differentiate. The underlying processes are tightly controlled and modulated by cyclin-dependent kinases (Cdks) and their interactions with cyclins and Cdk inhibitors (CKIs). Studies performed in the nervous system with mouse models lacking individual Cdks, cyclins, and CKIs, or combinations thereof, have shown that many of these molecules control proliferation rates in a cell-type specific and time-dependent manner. In this review, we will provide an update on the in vivo studies on cyclins, Cdks, and CKIs in neuronal and glial tissue. The goal is to highlight their impact on proliferation processes during the development of the peripheral and central nervous system, including and comparing normal and pathological conditions in the adult.

Stage-specific requirement for cyclin D1 in glial progenitor cells of the cerebral cortex

Despite the vast abundance of glial progenitor cells in the mouse brain parenchyma, little is known about the molecular mechanisms driving their proliferation in the adult. Here we unravel a critical role of the G1 cell cycle regulator cyclin D1 in controlling cell division of glial cells in the cortical grey matter. We detect cyclin D1 expression in Olig2-immunopositive (Olig2+) oligodendrocyte progenitor cells, as well as in Iba1+ microglia and S100β+ astrocytes in cortices of 3-month-old mice. Analysis of cyclin D1-deficient mice reveals a cell and stage-specific molecular control of cell cycle progression in the various glial lineages. While proliferation of fast dividing Olig2+ cells at early postnatal stages becomes gradually dependent on cyclin D1, this particular G1 regulator is strictly required for the slow divisions of Olig2+/NG2+ oligodendrocyte progenitors in the adult cerebral cortex. Further, we find that the population of mature oligodendrocytes is markedly reduced in the absence of cyclin D1, leading to a significant decrease in the number of myelinated axons in both the prefrontal cortex and the corpus callosum of 8-month-old mutant mice. In contrast, the pool of Iba1+ cells is diminished already at postnatal day 3 in the absence of cyclin D1, while the number of S100β+ astrocytes remains unchanged in the mutant.

Cyclin D1 is required for proliferation of Olig2-expressing progenitor cells in the injured cerebral cortex

Little is known about the molecular mechanisms driving proliferation of glial cells after an insult to the central nervous system (CNS). To test the hypothesis that the G1 regulator cyclin D1 is critical for injury-induced cell division of glial cells, we applied an injury model that causes brain damage within a well-defined region. For this, we injected the neurotoxin ibotenic acid into the prefrontal cortex of adult mice, which leads to a local nerve cell loss but does not affect the survival of glial cells. Here, we show that cyclin D1 immunoreativity increases drastically after neurotoxin injection. We find that the cyclin D1-immunopositive (cyclin D1+) cell population within the lesioned area consists to a large extent of Olig2+ oligodendrocyte progenitor cells. Analysis of cyclin D1-deficient mice demonstrates that the proliferation rate of Olig2+ cells diminishes upon loss of cyclin D1. Further, we show that cyclin-dependent kinase (cdk) 4, but not cdk6 or cdk2, is essential for driving cell division of Olig2-expressing cells in our injury model. These data suggest that distinct cell cycle proteins regulate proliferation of Olig2+ progenitor cells following a CNS insult.

Modulating epigenetic mechanisms: the diverse functions of Ski during cortical development

In the developing forebrain, neural stem and progenitor cells generate a large variety of neurons with specific functions in the mature cortex. A central issue is to understand the roles of transcriptional networks and regulatory pathways that control these complex developmental processes. The proto-oncogene Ski is a transcriptional regulator linked to the human 1p36 deletion syndrome, which involves a set of phenotypes including nervous system defects. Ski shows a dynamic expression pattern during cortical development and, accordingly, the phenotype of Ski-deficient cortices is complex, involving altered cell cycle characteristics of neural progenitors, disturbed timing of neurogenesis and mis-specification of projection neurons. Ski is likely to play a role in various pathways by virtue of its ability to interact with a range of signaling molecules, thereby modulating transcriptional activity of corresponding target genes. Ski regulates proliferation and differentiation of various cell types, and more recent data from my laboratory demonstrates that Ski is also involved in the specification of cortical projection neurons. This Point-of-View elucidates the role of Ski as an essential linker between sequence-specific transcription factors and non-DNA binding cofactors with chromatin modifying activities. In particular, it puts forward the hypothesis that the diverse functions of Ski as a co-repressor might be related to its association with distinct HDAC-complexes.

Expression and localization of Ski determine cell type-specific TGFbeta signaling effects on the cell cycle

Transforming growth factor β (TGFβ) promotes epithelial cell differentiation but induces Schwann cell proliferation. We show that the protooncogene Ski (Sloan-Kettering viral oncogene homologue) is an important regulator of these effects. TGFβ down-regulates Ski in epithelial cells but not in Schwann cells. In Schwann cells but not in epithelial cells, retinoblastoma protein (Rb) is up-regulated by TGFβ. Additionally, both Ski and Rb move to the cytoplasm, where they partially colocalize. In vivo, Ski and phospho-Rb (pRb) appear to interact in the Schwann cell cytoplasm of developing sciatic nerves. Ski overexpression induces Rb hyperphosphorylation, proliferation, and colocalization of both proteins in Schwann cell and epithelial cell cytoplasms independently of TGFβ treatment. Conversely, Ski knockdown in Schwann cells blocks TGFβ-induced proliferation and pRb cytoplasmic relocalization. Our findings reveal a critical function of fine-tuned Ski levels in the control of TGFβ effects on the cell cycle and suggest that at least a part of Ski regulatory effects on TGFβ-induced proliferation of Schwann cells is caused by its concerted action with Rb.

Mice lacking protease nexin-1 show delayed structural and functional recovery after sciatic nerve crush

Multiple molecular mechanisms influence nerve regeneration. Because serine proteases were shown to affect peripheral nerve regeneration, we performed nerve crush experiments to study synapse reinnervation in adult mice lacking the serpin protease nexin-1 (PN-1). PN-1 is a potent endogenous inhibitor of thrombin, trypsin, tissue plasminogen activators (tPAs), and urokinase plasminogen activators. Compared with the wild type, a significant delay in synapse reinnervation was detected in PN-1 knock-out (KO) animals, which was associated with both reduced proliferation and increased apoptosis of Schwann cells. Various factors known to affect Schwann cells were also altered. Fibrin deposits, tPA activity, mature BDNF, and the low-affinity p75 neurotrophin receptor were increased in injured sciatic nerves of mutant mice. To test whether the absence of PN-1 in Schwann cells or in the axon caused delay in reinnervation, PN-1 was overexpressed exclusively in the nerves of PN-1 KO mice. Neuronal PN-1 expression did not rescue the delayed reinnervation. The results suggest that Schwann cell-derived PN-1 is crucial for proper reinnervation through its contribution to the autocrine control of proliferation and survival. Thus, the precise balance between distinct proteases and serpins such as PN-1 can modulate the overall impact on the kinetics of recovery.

Wnt/BMP signal integration regulates the balance between proliferation and differentiation of neuroepithelial cells in the dorsal spinal cord

Multiple signaling pathways regulate proliferation and differentiation of neural progenitor cells during early development of the central nervous system (CNS). In the spinal cord, dorsal signaling by bone morphogenic protein (BMP) acts primarily as a patterning signal, while canonical Wnt signaling promotes cell cycle progression in stem and progenitor cells. However, overexpression of Wnt factors or, as shown here, stabilization of the Wnt signaling component beta-catenin has a more prominent effect in the ventral than in the dorsal spinal cord, revealing local differences in signal interpretation. Intriguingly, Wnt signaling is associated with BMP signal activation in the dorsal spinal cord. This points to a spatially restricted interaction between these pathways. Indeed, BMP counteracts proliferation promoted by Wnt in spinal cord neuroepithelial cells. Conversely, Wnt antagonizes BMP-dependent neuronal differentiation. Thus, a mutually inhibitory crosstalk between Wnt and BMP signaling controls the balance between proliferation and differentiation. A model emerges in which dorsal Wnt/BMP signal integration links growth and patterning, thereby maintaining undifferentiated and slow-cycling neural progenitors that form the dorsal confines of the developing spinal cord.

Beta1-integrin signaling mediates premyelinating oligodendrocyte survival but is not required for CNS myelination and remyelination

Previous reports, including transplantation experiments using dominant-negative inhibition of beta1-integrin signaling in oligodendrocyte progenitor cells, suggested that beta1-integrin signaling is required for myelination. Here, we test this hypothesis using conditional ablation of the beta1-integrin gene in oligodendroglial cells during the development of the CNS. This approach allowed us to study oligodendroglial beta1-integrin signaling in the physiological environment of the CNS, circumventing the potential drawbacks of a dominant-negative approach. We found that beta1-integrin signaling has a much more limited role than previously expected. Although it was involved in stage-specific oligodendrocyte cell survival, beta1-integrin signaling was not required for axon ensheathment and myelination per se. We also found that, in the spinal cord, remyelination occurred normally in the absence of beta1-integrin. We conclude that, although beta1-integrin may still contribute to other aspects of oligodendrocyte biology, it is not essential for myelination and remyelination in the CNS.

ErbB2 signaling in Schwann cells is mostly dispensable for maintenance of myelinated peripheral nerves and proliferation of adult Schwann cells after injury

Neuregulin/erbB signaling is critically required for survival and proliferation of Schwann cells as well as for establishing correct myelin thickness of peripheral nerves during development. In this study, we investigated whether erbB2 signaling in Schwann cells is also essential for the maintenance of myelinated peripheral nerves and for Schwann cell proliferation and survival after nerve injury. To this end, we used inducible Cre-loxP technology using a PLP-CreERT2 allele to ablate erbB2 in adult Schwann cells. ErbB2 expression was markedly reduced after induction of erbB2 gene disruption with no apparent effect on the maintenance of already established myelinated peripheral nerves. In contrast to development, Schwann cell proliferation and survival were not impaired in mutant animals after nerve injury, despite reduced levels of MAPK-P (phosphorylated mitogen-activated protein kinase) and cyclin D1. ErbB1 and erbB4 do not compensate for the loss of erbB2. We conclude that adult Schwann cells do not require major neuregulin signaling through erbB2 for proliferation and survival after nerve injury, in contrast to development and in cell culture.

Cell cycle inhibitors p21 and p16 are required for the regulation of Schwann cell proliferation

Regulated cell proliferation is a crucial prerequisite for Schwann cells to achieve myelination in development and regeneration. In the present study, we have investigated the function of the cell cycle inhibitors p21 and p16 as potential regulators of Schwann cell proliferation, using p21- or p16-deficient mice. We report that both inhibitors are required for proper withdrawal of Schwann cells from the cell cycle during development and following injury. Postnatal Schwann cells express p21 exclusively in the cytoplasm, first detectable at postnatal day 7. This cytoplasmic p21 expression is necessary for proper Schwann cell proliferation control in the late development of peripheral nerves. After axonal damage, p21 is found in Schwann cell nuclei during the initiation of the proliferation period. This stage is critically regulated by p21, since loss of p21 leads to a strong increase in Schwann cell proliferation. Unexpectedly, p21 levels are upregulated in this phase suggesting that the role of p21 may be more complex than purely inhibitory for the Schwann cell cycle. However, inhibition of Schwann cell proliferation is the overriding crucial function of p21 and p16 in peripheral nerves as revealed by the consequences of loss-of-function in development and after injury. Different mechanisms appear to underlie the inhibitory function, depending on whether p21 is cytoplasmic or nuclear.

An animal model for Charcot–Marie–Tooth disease type 4B1

Charcot-Marie-Tooth disease (CMT) comprises a family of clinically and genetically very heterogeneous hereditary peripheral neuropathies and is one of the most common inherited neurological disorders. We have generated a mouse model for CMT type 4B1 using embryonic stem cell technology. To this end, we introduced a stop codon into the Mtmr2 locus within exon 9, at the position encoding amino acid 276 of the MTMR2 protein (E276X). Concomitantly, we have deleted the chromosomal region immediately downstream of the stop codon up to within exon 13. The resulting allele closely mimics the mutation found in a Saudi Arabian CMT4B1 patient. Animals homozygous for the mutation showed various degrees of complex myelin infoldings and outfoldings exclusively in peripheral nerves, in agreement with CMT4B1 genetics and pathology. Mainly, paranodal regions of the myelin sheath were affected, with a high degree of quantitative and qualitative variability between individuals. This pathology was progressive with age, and axonal damage was occasionally observed. Distal nerve regions were more affected than proximal parts, in line with the distribution in CMT. However, we found no significant electrophysiological changes, even in aged (16-month-old) mice, suggesting that myelin infoldings and outfoldings per se are not invariably associated with detectable electrophysiological abnormalities. Our animal model provides a basis for future detailed molecular and cellular studies on the underlying disease mechanisms in CMT4B1. Such an analysis will reveal how the disease develops, in particular, the enigmatic myelin infoldings and outfoldings as well as axonal damage, and provide mechanistic insights that may aid in the development of potential therapeutic approaches.

The Protooncogene Ski Controls Schwann Cell Proliferation and Myelination

Schwann cell proliferation and subsequent differentiation to nonmyelinating and myelinating cells are closely linked processes. Elucidating the molecular mechanisms that control these events is key to the understanding of nerve development, regeneration, nerve-sheath tumors, and neuropathies. We define the protooncogene Ski, an inhibitor of TGF-beta signaling, as an essential component of the machinery that controls Schwann cell proliferation and myelination. Functional Ski overexpression inhibits TGF-beta-mediated proliferation and prevents growth-arrested Schwann cells from reentering the cell cycle. Consistent with these findings, myelinating Schwann cells upregulate Ski during development and remyelination after injury. Myelination is blocked in myelin-competent cultures derived from Ski-deficient animals, and genes encoding myelin components are downregulated in Ski-deficient nerves. Conversely, overexpression of Ski in Schwann cells causes an upregulation of myelin-related genes. The myelination-regulating transcription factor Oct6 is involved in a complex modulatory relationship with Ski. We conclude that Ski is a crucial signal in Schwann cell development and myelination.

Tamoxifen-inducible glia-specific Cre mice for somatic mutagenesis in oligodendrocytes and Schwann cells

Inducible transgenesis provides a valuable technique for the analysis of gene function in vivo. We report the generation and characterization of mouse lines carrying glia lineage-specific transgenes expressing an improved variant of the tamoxifen-inducible Cre recombinase, CreERT2, where the recombinase is fused to a mutated ligand binding domain of the human estrogen receptor. Using a PLP-CreERT2 transgene, we have generated mice that show specific inducible Cre function, as analyzed by cross-breeding experiments into the Rosa26 Cre-LacZ reporter line, in developing and adult Schwann cells, in mature myelinating oligodendrocytes, and in undifferentiated NG2-positive oligodendrocyte precursors in the adult. Using a P0Cx-CreERT2 transgene, we have also established mouse lines with inducible Cre function specifically in the Schwann cell lineage. These tamoxifen-inducible CreERT2 lines will allow detailed spatiotemporally controlled analysis of gene functions in loxP-based conditional mutant mice in both developing and adult Schwann cells and in the oligodendrocyte lineage.

Proliferation of Schwann cells and regulation of cyclin D1 expression in an animal model of Charcot-Marie-Tooth disease type 1A

Overexpression of PMP22 is responsible for the most common form of inherited neuropathy, Charcot-Marie-Tooth disease (CMT) type 1A. The PMP22-transgenic rat (CMT rat) is an animal model of CMT1A, and its peripheral nerves show the characteristic features of ongoing demyelination and remyelination that is also seen in CMT1A patients. Since Schwann cell proliferation is a prominent feature of peripheral nerves in inherited peripheral neuropathies, we examined proliferation and the expression of cyclin D1 in CMT rats. D-type cyclins are required for the initial steps in cell division and nuclear import is crucial for the function of cyclin D1 in promoting cell proliferation. Like normal myelinating Schwann cells in wild-type rats, remyelinating Schwann cells in CMT rats show perinuclear cyclin D1 expression. Schwann cells with nuclear cyclin D1 expression, as well as proliferating Schwann cells, were both associated with demyelinated axonal segments. Supernumerary onion bulb Schwann cells, however, do not express cyclin D1 and were not proliferating. Thus, cyclin D1 expression and its subcellular localization correlate directly with distinct physiological states of Schwann cells in this animal model of CMT1A.

Differential cyclin D1 requirements of proliferating Schwann cells during development and after injury

Neurons regulate Schwann cell proliferation, but little is known about the molecular basis of this interaction. We have examined the possibility that cyclin D1 is a key regulator of the cell cycle in Schwann cells. Myelinating Schwann cells express cyclin D1 in the perinuclear region, but after axons are severed, cyclin D1 is strongly upregulated in parallel with Schwann cell proliferation and translocates into Schwann cell nuclei. During development, cyclin D1 expression is confined to the perinuclear region of proliferating Schwann cells and the analysis of cyclin D1-null mice indicates that cyclin D1 is not required for this type of Schwann cell proliferation. As in the adult, injury to immature peripheral nerves leads to translocation of cyclin D1 to Schwann cell nuclei and injury-induced proliferation is impaired in both immature and mature cyclin D1-deficient Schwann cells. Thus, our data indicate that the molecular mechanisms regulating proliferation of Schwann cells during development or activated by axonal damage are fundamentally different.

Progressive segregation of unmyelinated axons in peripheral nerves, myelin alterations in the CNS, and cyst formation in the kidneys of myelin and lymphocyte protein-overexpressing mice

Myelin and lymphocyte protein (MAL) is a putative tetraspan proteolipid that is highly expressed by Schwann cells and oligodendrocytes as a component of compact myelin. Outside of the nervous system, MAL is found in apical membranes of epithelial cells, mainly in the kidney and stomach. Because MAL is associated with glycosphingolipids, it is thought to be involved in the organization, transport, and maintenance of glycosphingolipid-enriched membrane microdomains. In this report, we describe the generation and analysis of transgenic mice with increased MAL gene dosage. Immunohistochemical analysis revealed that the localization of MAL overexpression in the transgenic animals corresponded closely to the MAL expression pattern observed in wildtype animals, indicating correct spatial regulation of the transgene. Phenotypically, MAL overexpression led to progressive dissociation of unmyelinated axons from bundles in the PNS, a tendency to hypomyelination and aberrant myelin formation in the CNS, and the formation of large cysts in the tubular region of the kidney. Thus, increased expression of MAL appears to be deleterious to membranous structures in the affected tissues, indicating a requirement for tight control of endogenous MAL expression in Schwann cells, oligodendrocytes, and kidney epithelial cells.

N-Oct 5 is generated by in vitro proteolysis of the neural POU-domain protein N-Oct 3

The neural POU-domain proteins N-Oct 3 and N-Oct 5 were first identified in electrophoretic mobility retardation assays through their ability to bind to the octamer sequence ATGCAAAT. These two N-Oct factors are detected in extracts from tumor-derived and normal neural cells. They are present differentially, however, in extracts from melanocytes and melanoma cells: N-Oct 3 is present in extracts from both melanocytes and melanoma cells, whereas N-Oct 5 is more evident in extracts from metastatic melanoma cells. We show here that a cDNA encoding N-Oct 3 directs synthesis of both the N-Oct 3 and N-Oct 5 proteins and that the N-Oct 5 protein in neural and melanoma-cell extracts is also related to N-Oct 3. N-Oct 5, however, is apparently not expressed in vivo: It is not detected if cells are rapidly lysed in SDS or if extracts are prepared with a cocktail of protease inhibitors that includes the serine-protease inhibitor 4-(2-Aminoethyl)benzenesulfonyl fluoride hydrochloride (AEBSF). These data suggest that N-Oct 5 is a specific in vitro proteolytic cleavage product of N-Oct 3 and is not directly related to melanocyte malignancy.

Isolation of the human genomic brain-2/N-Oct 3 gene (POUF3) and assignment to chromosome 6q16

N-Oct 3 is a human POU domain transcription factor that binds to the octamer sequence ATGCAAAT. The protein is expressed in the central nervous system during development and in adult brain. We have isolated and characterized genomic clones encoding the human N-Oct 3 gene (HGMW-approved symbol POUF3). Comparison of the structure of these clones with the N-Oct 3 cDNA revealed that POUF3 is an intronless gene. Sequencing of 650 bp of the promoter region showed 84% sequence identity of POUF3 with its murine homologue, the brain-2 (designated brn-2) gene. Whereas both POUF3 and brn-2 lack a TATA box, consensus sequences for AP-2, GCF, and SP1 transcription factors were identified within the highly conserved 5'-flanking region. These sequences may play a crucial role for the tissue-specific transcription activation of the POUF3 gene. Southern blotting and in situ hybridization localized the human POUF3 gene to chromosome 6q16.

Ethanol-induced enhancement of cocaine bioactivation and irreversible protein binding: evidence against a role of cytochrome P-450IIE1

Chronic ethanol consumption potentiates cocaine-induced liver injury in rodents. Since cocaine has to be bioactivated by a cytochrome P-450-dependent N-oxidative pathway to exert its hepatotoxic effects, we studied the role of the ethanol-inducible P-450IIE1 for cocaine metabolism. Male Sprague-Dawley rats were pretreated with either a liquid diet containing ethanol (30% of calories) for 4 weeks or injected with pyrazole (200 mg/kg/day, ip, for 3 days). Both agents induced microsomal p-nitrophenol hydroxylation which is a probe for the catalytic activity of P-450IIE1. However, only ethanol, but not pyrazole, increased both microsomal cocaine N-demethylase activity (by 47%) and the extent of irreversible binding of [3H]-cocaine to microsomal proteins (by 100%), which was taken as a quantitative endpoint for the formation of a reactive metabolite. Cocaine N-demethylation and irreversible protein binding of cocaine were not inhibited by P-450IIE1 isozyme-selective substrates, nor was the rate of cocaine metabolism and binding decreased by functionally active polyclonal anti-rat P-450IIE1 antibodies. Furthermore, pyrazole pretreatment sensitized cultured hepatocytes to the glutathione-dependent cytotoxic effects of nontoxic concentrations of cocaine. These results indicate that (a) cocaine is not a major substrate for the ethanol-inducible P-450IIE1, (b) the enhancing effects of ethanol on cocaine bioactivation may be due to induction of other P-450 isoforms, and (c) induction of P-450IIE1 may potentiate cocaine-induced hepatocellular toxicity in vitro independently of cocaine metabolism, e.g., by P-450IIE1-dependent oxidative stress.