Early expression of the BM88 antigen during neuronal differentiation of P19 embryonal carcinoma cells

Cend1, a Story with Many Tales: From Regulation of Cell Cycle Progression/Exit of Neural Stem Cells to Brain Structure and Function

Neural stem/precursor cells (NPCs) generate the large variety of neuronal phenotypes comprising the adult brain. The high diversity and complexity of this organ have its origin in embryonic life, during which NPCs undergo symmetric and asymmetric divisions and then exit the cell cycle and differentiate to acquire neuronal identities. During these processes, coordinated regulation of cell cycle progression/exit and differentiation is essential for generation of the appropriate number of neurons and formation of the correct structural and functional neuronal circuits in the adult brain. Cend1 is a neuronal lineage-specific modulator involved in synchronization of cell cycle exit and differentiation of neuronal precursors. It is expressed all along the neuronal lineage, from neural stem/progenitor cells to mature neurons, and is associated with the dynamics of neuron-generating divisions. Functional studies showed that Cend1 has a critical role during neurogenesis in promoting cell cycle exit and neuronal differentiation. Mechanistically, Cend1 acts via the p53-dependent/Cyclin D1/pRb signaling pathway as well as via a p53-independent route involving a tripartite interaction with RanBPM and Dyrk1B. Upon Cend1 function, Notch1 signaling is suppressed and proneural genes such as Mash1 and Neurogenins 1/2 are induced. Due to its neurogenic activity, Cend1 is a promising candidate therapeutic gene for brain repair, while the Cend1 minimal promoter is a valuable tool for neuron-specific gene delivery in the CNS. Mice with Cend1 genetic ablation display increased NPC proliferation, decreased migration, and higher levels of apoptosis during development. As a result, they show in the adult brain deficits in a range of motor and nonmotor behaviors arising from irregularities in cerebellar cortex lamination and impaired Purkinje cell differentiation as well as a paucity in GABAergic interneurons of the cerebral cortex, hippocampus, and amygdala. Taken together, these studies highlight the necessity for Cend1 expression in the formation of a structurally and functionally normal brain.

Characterization of the BM88 promoter and identification of an 88 bp fragment sufficient to drive neurone-specific expression

BM88 is a neurone-specific protein implicated in cell cycle exit and differentiation of neuronal precursors. It is widely expressed in terminally differentiated neurones but also in neuronal progenitors, albeit in lower levels. Thus BM88 expression shows a tight correlation with the progression of progenitor cells towards neuronal differentiation. Here we report the genomic organization and proximal promoter characterization of the human and mouse BM88 genes. Both promoters lie in a CpG island, are TATA-less and have multiple transcription start sites. Deletion analysis performed on the human BM88 gene revealed an 88 bp minimal promoter fragment that is preferentially active in neural cells. Importantly, this minimal promoter is sufficient to confer specific transcriptional activity in primary neurones, but not in glial cells. Within the promoter region there are four functional Sp1-binding sites. Simultaneous mutations to all four Sp1 sites results in complete loss of promoter activity. Transactivation experiments revealed that Sp1 directly activates the BM88 promoter while activation also occurs in the presence of neurogenin-1. Characterization of the promoter elements that control neurone-specific and developmental expression of BM88 should contribute to the elucidation of the transcriptional networks that regulate the transition from a proliferative neural progenitor to a post-mitotic neurone.

BM88 is an early marker of proliferating precursor cells that will differentiate into the neuronal lineage

Progression of progenitor cells towards neuronal differentiation is tightly linked with cell cycle control and the switch from proliferative to neuron-generating divisions. We have previously shown that the neuronal protein BM88 drives neuroblastoma cells towards exit from the cell cycle and differentiation into a neuronal phenotype in vitro. Here, we explored the role of BM88 during neuronal birth, cell cycle exit and the initiation of differentiation in vivo. By double- and triple-labelling with the S-phase marker BrdU or the late G2 and M-phase marker cyclin B1, antibodies to BM88 and markers of the neuronal or glial cell lineages, we demonstrate that in the rodent forebrain, BM88 is expressed in multipotential progenitor cells before terminal mitosis and in their neuronal progeny during the neurogenic interval, as well as in the adult. Further, we defined at E16 a cohort of proliferative progenitors that exit S phase in synchrony, and by following their fate for 24 h we show that BM88 is associated with the dynamics of neuron-generating divisions. Expression of BM88 was also evident in cycling cortical radial glial cells, which constitute the main neurogenic population in the cerebral cortex. In agreement, BM88 expression was markedly reduced and restricted to a smaller percentage of cells in the cerebral cortex of the Small eye mutant mice, which lack functional Pax6 and exhibit severe neurogenesis defects. Our data show an interesting correlation between BM88 expression and the progression of progenitor cells towards neuronal differentiation during the neurogenic interval.

Failure to express GAP-43 leads to disruption of a multipotent precursor and inhibits astrocyte differentiation

The nervous system-specific protein GAP-43 is significantly upregulated in neurons and glia that are differentiating. In P19 EC cells that do not express GAP-43, neurogenesis is inhibited; many immature neurons apoptose and the survivors do not mature morphologically. Here we show that the initial defect is in an early precursor with characteristics of a neural stem cell, which failed to respond normally to retinoic acid (RA). As a consequence, its progeny had altered cell fates: In addition to the neuronal defects previously reported, RC1-labeled radial glia failed to exit the cell cycle, accumulated, and failed to acquire GFAP immunoreactivity. However, leukemia inhibitory factor (LIF) could stimulate GFAP expression suggesting that astrocytes not derived from radial glia are less affected by absence of GAP-43. Differentiation of radial glia-derived astrocytes was also inhibited in glial cultures from GAP-43 (-/-) cerebellum, and in GAP-43 (-/-) telencephalon in vivo, differentiation of astrocytes derived from both radial and nonradial glia lineages were both affected: In the glial wedge, GFAP-labeled radial glia-derived astrocytes were reduced consistent with the interpretation that they may be unable to deflect GAP-43 (-/-) commissural axons toward the midline. At the midline, both radial and nonradial glia-derived astrocytes were also decreased although it fused normally. The results demonstrate that GAP-43 expressed in multipotent precursors is required for appropriate cell fate commitment, and that its absence affects astrocyte as well as neuronal differentiation.

Ectopic expression of the catalytic subunit of telomerase protects against brain injury resulting from ischemia and NMDA-induced neurotoxicity

The catalytic subunit of telomerase reverse transcriptase (TERT) protects dividing cells from replicative senescence in vitro. Here, we show that expression of TERT mRNA is induced in the ipsilateral cortical neurons after occlusion of the middle cerebral artery in adult mice. Transgenic mice that overexpress TERT showed significant resistance to ischemic brain injury. Among excitotoxicity, oxidative stress, and apoptosis comprising of routes of ischemic neuronal death, NMDA receptor-mediated excitotoxicity was reduced in forebrain cell cultures overexpressing TERT. NMDA-induced accumulation of cytosolic free Ca2+ ([Ca2+]c) was reduced in forebrain neurons from TERT transgenic mice, which was attributable to the rapid flow of [Ca2+]c into the mitochondria from the cytosol without change in Ca2+ influx and efflux through the plasma membrane. The present study provides evidence that TERT is inducible in postmitotic neurons after ischemic brain injury and prevents NMDA neurotoxicity through shift of the cytosolic free Ca2+ into the mitochondria, and thus plays a protective role in ameliorating ischemic neuronal cell death.

Cloning, expression and localization of human BM88 shows that it maps to chromosome 11p15.5, a region implicated in Beckwith-Wiedemann syndrome and tumorigenesis

Porcine BM88 is a neuron-specific protein that enhances neuroblastoma cell differentiation in vitro and may be involved in neuronal differentiation in vivo. Here we report the identification, by Western blotting, of homologous proteins in human and mouse brain and the isolation of their respective cDNAs. Several human and mouse clones were identified in the EST database using porcine BM88 cDNA as a query. A human and a mouse EST clone were chosen for sequencing and were found both to predict a protein of 149 amino acids, with 79.9% reciprocal identity, and 76.4% and 70.7% identities to the porcine protein, respectively. This indicated that the clones corresponded to the human and mouse BM88 homologues. In vitro expression in a cell-free system as well as transient expression in COS7 cells yielded polypeptide products that were recognized by anti-BM88 antibodies and were identical in size to the native BM88 protein. Northern-blot analysis showed a wide distribution of the gene in human brain whereas immunohistochemistry on human brain sections demonstrated that the expression of BM88 is confined to neurons. The initial mapping assignment of human BM88 to chromosome 11p15.5, a region implicated in Beckwith-Wiedemann syndrome and tumorigenesis, was retrieved from the UniGene database maintained at the National Centre for Biotechnology Information (NCBI, Bethesda, MD, U.S.A.). We confirmed this localization by performing fluorescence in situ hybridization on BM88-positive cosmid clones isolated from a human genomic library. These results suggest that BM88 may be a candidate gene for genetic disorders associated with alterations at 11p15.5.

Isolation of a mouse brain cDNA expressed in developing neuroblasts and mature neurons

A previously uncharacterized 4.5-kb mouse cDNA clone, designated mc7, was isolated and found to be predominantly expressed in brain. This cDNA predicts a 1035-bp open reading frame that encodes for a 345-amino acid polypeptide especially rich in glutamic acid residues located in the region from residues 80 to 174. Computational analysis revealed among other features, putative zinc-finger motifs and coiled-coil regions. The corresponding mc7 gene is detected in mouse, rat, pig and human genomes. In mouse the mc7 mRNA is expressed predominantly in brain and to a much lesser extent in kidney, lung and spleen. In brain it is detectable as early as embryonic day 14 while it is retained in the adult. In situ hybridization studies revealed that mc7 mRNA is widely, albeit unevenly, expressed in neurons throughout the adult brain. Developmental in situ hybridization studies in the cerebellar cortex demonstrated that at postnatal day 5 mc7 mRNA is mainly expressed in neuroblasts of the external granular layer and in developing neurons of the internal granular layer. Some staining is also present in purkinje cells becoming particularly pronounced at postnatal day 10, the time of arborarization of their dendritic tree. In the adult cerebellar cortex expression is mainly confined in purkinje cells and to a lesser extent in granule neurons. The early expression of mc7 in neuroblasts and developing neurons as well as its retention in a wide variety of mature neurons suggest that it may play a role in the process of differentiation and maturation of these cells in the brain.