Genes controlling neural fate and differentiation

BM88 is a dual function molecule inducing cell cycle exit and neuronal differentiation of neuroblastoma cells via cyclin D1 down-regulation and retinoblastoma protein hypophosphorylation

Control of cell cycle progression/exit and differentiation of neuronal precursors is of paramount importance during brain development. BM88 is a neuronal protein associated with terminal neuron-generating divisions in vivo and is implicated in mechanisms underlying neuronal differentiation. Here we have used mouse neuroblastoma Neuro 2a cells as an in vitro model of neuronal differentiation to dissect the functional properties of BM88 by implementing gain- and loss-of-function approaches. We demonstrate that stably transfected cells overexpressing BM88 acquire a neuronal phenotype in the absence of external stimuli, as judged by enhanced expression of neuronal markers and neurite outgrowth-inducing signaling molecules. In addition, cell cycle measurements involving cell growth assays, BrdUrd incorporation, and fluorescence-activated cell sorting analysis revealed that the BM88-transfected cells have a prolonged G(1) phase, most probably corresponding to cell cycle exit at the G(0) restriction point, as compared with controls. BM88 overexpression also results in increased levels of the cell cycle regulatory protein p53, and accumulation of the hypophosphorylated form of the retinoblastoma protein leading to cell cycle arrest, with concomitant decreased levels and, in many cells, cytoplasmic localization of cyclin D1. Conversely, BM88 gene silencing using RNA interference experiments resulted in acceleration of cell proliferation accompanied by impairment of retinoic acid-induced neuronal differentiation of Neuro 2a cells. Taken together, our results suggest that BM88 plays an essential role in regulating cell cycle exit and differentiation of Neuro 2a cells toward a neuronal phenotype and further support its involvement in the proliferation/differentiation transition of neural stem/progenitor cells during embryonic development.

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.

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.

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

Previous studies have shown that the BM88 antigen, a neuron-specific molecule, promotes the differentiation of mouse neuroblastoma cells [23] (Mamalaki A., Boutou E., Hurel C., Patsavoudi E., Tzartos S. and Matsas R. (1995) The BM88 antigen, a novel neuron-specific molecule, enhances the differentiation of mouse neuroblastoma cells. J. Biol. Chem. 270, 14201-14208). In particular, stably transfected with the BM88 cDNA, Neuro 2a cells over-expressing the BM88 antigen are morphologically distinct from their non-transfected counterparts; they exhibit enhanced process outgrowth and a slower rate of division. Moreover, they respond differentially to growth factors [10] (Gomez J., Boutou E., Hurel C., Mamalaki A., Kentroti S. , Vernadakis A. and Matsas R. (1998) Overexpression of the neuron-specific molecule BM88 in mouse neuroblastoma cells: Altered responsiveness to growth factors. J. Neurosci. Res. 51, 119-128). In order to further elucidate the role of the BM88 antigen in the differentiation of developing neurons we used the in vitro system of differentiating P19 cells which closely resembles early murine development in vivo. In this study, P19 cells were driven to the neuronal pathway with retinoic acid. We examined by immunofluorescence studies the expression of the BM88 antigen in these cells and we found that it correlates well with the expression of the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) which characterizes early differentiating post-mitotic neurons. In contrast, very few of the BM88 antigen-positive/PSA-NCAM-positive cells expressed neurofilament protein, a marker of more mature neurons. Our findings, in accordance with previously reported data, strongly suggest that the BM88 antigen is involved in the early stages of differentiation of neuronal cells.