Developmental and regional expression of three new members of the ras-gene family in the mouse brain

Targeted disruption of Rab1a causes early embryonic lethality

Guanosine nucleotide diphosphate (GDP) dissociation inhibitor 2 (GDI2) regulates the GDP/guanosine triphosphate (GTP) exchange reaction of Rab proteins by inhibiting the dissociation of GDP and the subsequent binding of GTP. The present study aimed to determine the function of Rab1a in vivo, and thus generated mice with a trapped Rab1a gene. It was demonstrated that Rab1a is essential for embryonic development. It was also found that one functional Rab1a allele was sufficient for development in a heterozygous murine embryo, whereas a double mutant led to embryonic lethality. The dissection of uteri on embryonic day (E)10.5‑14.5 yielded no homozygous embryos, indicating that homozygotes die between E10.5 to E11.5. The gene trap construct contains a β‑galactosidase/neomycin reporter gene, allowing for heterozygotes to be stained for β‑galactosidase to determine the tissue‑specific expression of Rab1a. Rab1a was found to be highly expressed in the small intestine of both adult mice and embryos, although its expression levels were low in the brains of embryos. Moreover, there was no significant change in cytokine production and survival in wild‑type and heterozygous Rab1a+/‑ mice following a challenge with lipopolysaccharide. On the whole, the present study demonstrates that the disruption of the Rab1a gene causes embryonic lethality and homozygotes die between E10.5 and E11.5, suggesting that Rab1a is essential for the early development of mouse embryos.

Motifs and cis-regulatory modules mediating the expression of genes co-expressed in presynaptic neurons

An integrative strategy of comparative genomics, experimental and computational approaches reveals aspects of a regulatory network controlling neuronal-specific expression in presynaptic neurons.

Background

Hundreds of proteins modulate neurotransmitter release and synaptic plasticity during neuronal development and in response to synaptic activity. The expression of genes in the pre- and post-synaptic neurons is under stringent spatio-temporal control, but the mechanism underlying the neuronal expression of these genes remains largely unknown.

Results

Using unbiased in vivo and in vitro screens, we characterized the cis elements regulating the Rab3A gene, which is expressed abundantly in presynaptic neurons. A set of identified regulatory elements of the Rab3A gene corresponded to the defined Rab3A multi-species conserved elements. In order to identify clusters of enriched transcription factor binding sites, for example, cis-regulatory modules, we analyzed intergenic multi-species conserved elements in the vicinity of nine presynaptic genes, including Rab3A, that are highly and specifically expressed in brain regions. Sixteen transcription factor binding motifs were over-represented in these multi-species conserved elements. Based on a combined occurrence for these enriched motifs, multi-species conserved elements in the vicinity of 107 previously identified presynaptic genes were scored and ranked. We then experimentally validated the scoring strategy by showing that 12 of 16 (75%) high-scoring multi-species conserved elements functioned as neuronal enhancers in a cell-based assay.

Conclusions

This work introduces an integrative strategy of comparative genomics, experimental, and computational approaches to reveal aspects of a regulatory network controlling neuronal-specific expression of genes in presynaptic neurons.

Hyper immunoglobulin M syndrome due to CD40 deficiency: clinical, molecular, and immunological features

CD40 is a member of the tumor necrosis factor receptor family, which is expressed by a variety of cells including B cells, macrophages, dendritic cells, and other nonimmune cell types. CD40 activation is critical for B-cell proliferation, immunoglobulin (Ig)-isotype switching, and germinal center formation. In physiological conditions, the activation of CD40 occurs by binding to its natural ligand, CD154, which is expressed on activated T cells. The in vivo critical role of CD40-CD154 interaction on B-cell differentiation and isotype switching is provided by the discovery that mutations in either CD40 or CD154 gene cause the hyper IgM syndrome, termed HIGM3 or HIGM1, respectively, characterized by very low levels of serum IgG, IgA, and IgE, with normal or elevated IgM, associated with a defective germinal center formation. Originally considered humoral primary immunodeficiencies, the clinical features and the defect of T-cell priming, resulting from a defective T-B cell or dendritic cell interaction, is now considered as combined immunodeficiencies. In this article, we present a comprehensive overview of the clinical, genetic, and immunological features of patients with hyper IgM syndrome due to CD40 mutations.

Gene Expression Profiling in the Intact and Injured Brain Following Environmental Enrichment

An enriched environment promotes structural changes in both injured and intact brain and improves behavioral performance. In 2 different experimental approaches, the effects of enriched surroundings were analyzed utilizing DNA microarrays. First, gene expression patterns of the sensorimotor cortex and the hippocampus of noninjured adult rats with enriched housing were compared with analogous regions of rats kept in standard cages. Second, circumscribed infarcts affecting the forelimb area of the sensorimotor cortex were induced, and gene expression patterns of the non-necrotic ipsilesional as well as the contralesional homotopic cortex of rats (postlesionally enriched housing versus standard) were analyzed. In the intact brain, the hippocampus, which had 43 upregulations and 15 downregulations showed more changes than the sensorimotor cortex, which had 13 upregulations and 4 downregulations, indicating a greater responsiveness of the hippocampus to environmental stimuli. In the injured brain, enrichment led ipsilesionally to 28 downregulations and 14 upregulations, while in the contralesional cortex, upregulations prevailed with 46 upregulations and 13 downregulations. The larger number of genes responsive to enrichment in the contralesional cortex (59 gene regulations) as compared to the analogous area (i.e. sensorimotor cortex) of the intact brain (17 gene regulations) likely reflects increased susceptibility for plastic changes due to injury. With the exception of the perilesional cortex, similar functional groups of genes were differentially regulated in different brain regions/paradigms, suggesting basically similar molecular cascades being involved in reorganizing the brain following external stimuli. Many of the genes detected here correspond to molecular pathways known to be involved in neuroplasticity, whereas others provide new and hitherto unrecognized entry points.

CD40 is expressed and functional on neuronal cells

We show here that CD40 mRNA and protein are expressed by neuronal cells, and are increased in differentiated versus undifferentiated N2a and PC12 cells as measured by RT-PCR, western blotting and immunofluorescence staining. Additionally, immunohistochemistry reveals that neurons from adult mouse and human brain also express CD40 in situ. CD40 ligation results in a time-dependent increase in p44/42 MAPK activation in neuronal cells. Furthermore, ligation of CD40 opposes JNK phosphorylation and activity induced by NGF-beta removal from differentiated PC12 cells or serum withdrawal from primary cultured neurons. Importantly, CD40 ligation also protects neuronal cells from NGF-beta or serum withdrawal-induced injury and affects neuronal differentiation. Finally, adult mice deficient for the CD40 receptor demonstrate neuronal dysfunction as evidenced by decreased neurofilament isoforms, reduced Bcl-x(L):Bax ratio, neuronal morphological change, increased DNA fragmentation, and gross brain abnormality. These changes occur with age, and are clearly evident at 16 months. Taken together, these data demonstrate a role of CD40 in neuronal development, maintenance and protection in vitro and in vivo.

Expression of rab GTP-binding proteins during oligodendrocyte differentiation in culture

Oligodendrocytes (OLs) synthesize and transport vast amounts of proteins and lipids from the cell body to the morphologically and biochemically distinct domains of the myelin membrane. From our prediction that regulators of vesicular transport should be up-regulated at the time of myelin production, we hypothesized that the up-regulated and unidentified small GTPases found by Huber et al. [1994a] may be Rab proteins. We have analyzed the mRNA expression of rabs in OLs, and have detected rabs 10, 11b, 18, 24, 26, and 28 in addition to rabs that were found previously. Our data show that among the Rabs so far detected during differentiation, only Rabs 5a and 8a exhibited up-regulation in addition to the previously published Rab3a (Madison et al. [1999], J. Neurochem. 72:988-998). We discuss the limited extent of up-regulation of rabs in the context of the presumed necessity for an increase in Rab activity during myelin assembly.

Specific expression of the ras-related rab3A gene in human normal and malignant neuroendocrine cells

BACKGROUND

The authors originally demonstrated the tissue-specific expression of the rab3A gene in the mouse brain. In the current study, they analyze the activity of this gene in fresh human tumors associated with neuronal phenotype compared with normal and malignant cells from other origins.

METHODS

The authors studied the transcription levels of the rab3A gene by Northern blot in 81 fresh tumors.

RESULTS

A high rab3A gene expression was observed in tumor samples derived from the neural tube (i.e., neuroblastomas, ganglioneuroblastomas, and adult nervous system neoplasms). In addition, this tissue-specific expression extended to neuroendocrine tumors of the gut, small cell lung cancers, and pheochromocytomas.

CONCLUSIONS

These results suggest a specific restriction pattern to human cells derived from the neural tube and neural crests. The GTP/GDP-binding rab3A protein may be a useful differentiation marker of neuro-endocrine cells in the characterization of undifferentiated neoplasms.

Localization of the ras-like rab3A protein in the adult rat brain

Rab3A is a small GTP-binding synaptic vesicle protein, shown to dissociate from synaptic vesicle membranes upon depolarization-induced exocytosis. Using an antiserum raised against rab3A, we found that the antigen was localized to the neuropil of specific brain regions, but was not present in major fiber tracts or most cell bodies. For example, the neuropil of several thalamic nuclei (i.e., dorsal lateral geniculate nucleus, lateral posterior nucleus, ventroposterior nucleus), cerebral cortex, upper layers of the superior colliculus and matrix zones of the neostriatum, were strongly immunoreactive, while the anterior commissure, corpus callosum, optic tract and internal capsule were devoid of staining. The hippocampus, regions of cerebral cortex and the cerebellum exhibited striking laminar distributions of rab3A immunoreactivity. In the hippocampus, dark staining was observed in the stratum oriens, stratum radiatum and molecular layer of the dentate gyrus, while the pyramidal, stratum lacunosum moleculare and dentate granule layers were not stained. In cerebellum the molecular layer and to a lesser extent, the underlying granule cell layer showed enhanced immunoreactivity. Seven days after excitotoxic lesions of the cerebral cortex, rab3A immunoreactivity was diminished in the mirror locus in the contralateral cortical hemisphere and in certain thalamic nuclei ipsilateral to the injection site. These results show that rab3A is localized to a number of specific regions. Its absence from other areas suggests that this synaptic vesicle protein is not universal to all neuronal terminals and pathways. In addition, our lesion studies indicate that for some brain regions, much of the antigen originates in cortical neurons and is distributed within specific axonal projections.

Expression of the Raf-1 protein in rat brain during development and its hormonal regulation in hypothalamus

To study mechanisms involved in the sexual differentiation of the rat brain, the expression of the protein product of the proto-oncogene c-raf-1 (Raf-1) was examined. Biochemical and immunocytochemical analyses localized Raf-1 in embryonic rat brain regions and demonstrated hormonally induced changes in Raf-1 expression. For this study an affinity-purified anti-peptide antiserum specific for Raf-1 (NH-44) was used. Western blots revealed an approximately 77 kD polypeptide isolated in the cytosol of developing rat brains. Raf-1 levels were highest in the embryonic (E) day 22 female hypothalamus (HYP), and approximately twofold higher than levels detected in male HYP at E22 as determined by quantitative protein dot blot and semiquantitative Western blot analyses. Raf-1 levels in HYP were greater than those in either brain stem (BS) or cortex. Immunocytochemical analysis revealed high levels of Raf-1 in selective brain regions (e.g., the ventromedial nucleus in the HYP, the mitral cell layers in the main and accessory olfactory bulbs (OB), and the locus coeruleus) at E22 and postnatal (P) day 1. Lower levels of immunoreactivity were observed in many areas of the perinatal neuraxis. To test hormonal regulation of Raf-1, testosterone propionate (TP) was administered to pregnant rats on E17; male and female fetuses were examined on E22. This treatment significantly decreased Raf-1 levels in female HYP, but not in male HYP, as determined by Western blot analysis. No significant sex difference or response to prenatal hormone treatments were observed in either brain stem or cortex. No significant sex difference was noted postnatally, and administration of TP 3 h after birth did not change Raf-1 levels examined 24 h later. In summary, Raf-1 was localized within selective regions of the rat brain, and its expression was altered by exogenous prenatal hormonal stimulation. One role for Raf-1 in signal transduction may be to delimit hormonal critical periods in sexual differentiation of the brain.

Ontogeny of Ha-ras and c-myc mRNA levels in rabbit embryo and extraembryonic tissues by quantitative in situ hybridization

A large variety of proto-oncogenes are known to be of key importance in cellular growth and differentiation during embryonic development. Using quantitative in situ hybridization, we studied in detail the levels of the proto-oncogenes Ha-ras and c-myc mRNA in embryos and extraembryonic tissues (maternal and embryonic placentas, trophoblast, and endometrial epithelium) during prenatal life of rabbit. cDNA probes encoding for Ha-ras (fragment Kpn 1-BstE II of 883 bp) and c-myc (fragment Pst 1-Pst 1 of 490 bp) were used to detect specific transcripts in fixed cryostat sections. High levels of Ha-ras and c-myc mRNA were detected in the rabbit embryo as well as in the decidua and in the trophoblast as early as day 9 of gestation. At 12 and 15 days of gestation, Ha-ras and c-myc mRNA levels decreased in both embryonic and maternal placenta while in the embryo a significant increase of Ha-ras and c-myc expression was detected with particular evidence in the central nervous system. Finally, at 25 days of gestation the expression of the two proto-oncogenes, Ha-ras and c-myc, was greatly decreased in both the embryo and extraembryonic tissues, and was undetectable by 30 days of gestation. These results show that in rabbit the expression of the two proto-oncogenes Ha-ras and c-myc is localized in the same tissues with similar intensity and follows an unparallel temporal modulation in the embryo and in the extraembryonic tissues during prenatal development.

Expression and localization of smg p25A (= rab3A) in cultured rat hippocampal cells

We have studied the expression of smg p25A and synaptophysin in cultured hippocampal neurons isolated from 5-day-old rat brain by an immunocytochemical technique. In a dispersed cell culture seeded on astrocyte monolayers, well-branching neurite proliferation was observed along with age in culture. The synaptophysin immunoreactivity was present in the neuronal cell bodies and neurites at 1 and 5 days in vitro (DIV) and was eventually localized to discrete areas along neurites at 15 DIV while the immunoreactivity in cell bodies became less prominent. On the other hand, the smg p25A immunoreactivity was observed in the neuronal cell bodies and neurites at 1 through 15 DIV. The immunoreactivity for smg p25A or synaptophysin was not observed in astrocytes and this finding was confirmed by an immunoblot analysis. These results indicate that smg p25A as well as synaptophysin is present exclusively in neurons and suggest that these two synapse-associated proteins have different sites of function and different kinetics of synthesis, transport, and/or turnover in cultured hippocampal neurons.

Expression of c-myc and c-fos oncogenes in different rat brain regions during postnatal development

We have studied the expression of c-myc and c-fos proto-oncogenes in various areas of the central nervous system during postnatal development. c-myc mRNA levels increased during the first 5 days and then decreased over the next 15 days in all nervous regions studied. c-fos mRNA levels changed in a different way in four brain areas. While in cerebral cortex and cerebellum there was a sharp decrease during the first 10 days, in white matter and hypothalamus c-fos transcript levels remained high during the same period, decreasing at a later stage. Changes in oncogenes mRNA levels are related to various developmental events, such as neurite growth, myelination and cell proliferation. The dissimilar patterns of c-myc and c-fos expression suggests that they play different functions in CNS maturation. c-myc mRNA levels are temporally related to active neurite growth and to cell proliferation. Changes in c-fos mRNA correlate in time with early developmental processes and also with those occurring at later stages, such as myelination.

Association of the GTP-binding protein Rab3A with bovine adrenal chromaffin granules

The Rab3A protein belongs to a large family of small GTP-binding proteins that are present in eukaryotic cells and that share amino acid identities with the Ras proteins (products of the ras protooncogenes). Rab3A, which is specifically located in nervous and endocrine tissues, is suspected to play a key role in secretion. Its localization was investigated in bovine adrenal gland by using a polyclonal antibody. Rab3A was detected in adrenal medulla but not in adrenal cortex. In cultured adrenal medulla cells. Rab3A was specifically expressed in the catecholamine-secreting chromaffin cells. Subcellular fractionation suggested that Rab3A is about 30% cytosolic and that particulate Rab3A is associated with the membrane of chromaffin granules (the catecholamine storage organelles) and with a second compartment likely to be the plasma membrane. The Rab3A localization on chromaffin granule membranes was confirmed by immunoadsorption with an antibody against dopamine beta-hydroxylase. Rab3A was not extracted from this membrane by NaCl or KBr but was partially extracted by urea and totally solubilized by Triton X-100, suggesting either an interaction with an intrinsic protein or a membrane association through fatty acid acylation. This study suggests that Rab3A, which may also be located on other secretory vesicles containing noncharacterized small GTP-binding proteins, is involved in their biogenesis or in the regulated secretion process.

The product of rab2, a small GTP binding protein, increases neuronal adhesion, and neurite growth in vitro

The rab genes code for small GTP binding proteins that share with p21ras the ability to bind and hydrolyze GTP. They present significant sequence homologies with the products of YPT1 and SEC4, two small GTP binding proteins involved in the regulation of secretion in the yeast. Several rab genes are expressed in the developing and adult mouse brain. To test directly the possible involvement of these genes in neuronal differentiation, purified rab proteins produced in E. coli were introduced into neurons dissociated from E15 rat midbrain. The most striking effects were obtained with rab2 protein (rab2p). Compared with untreated cells, neurons loaded with rab2p presented an enhanced adhesion to the culture substratum. This phenomenon was visible 3 hr after seeding and was followed within 24 hr by a dramatic increase in neurite growth. Loading the same population of neurons with the products of four other rab genes either decreased neuronal adhesion and neurite growth or had no effect. These experiments suggest that the expression of rab2p plays an important role in neuronal differentiation.

Expression of the ras-related rap genes in human tumors

The expression of the recently described rap genes, closely related in the effector region to the ras proto-oncogenes, was examined by Northern blot analysis in 41 primary human tumors. The structural and in vitro biological properties of the rap gene products suggest their possible antagonistic action in the same effector pathway as the ras proteins. In order to determine whether a deregulation in the rap transcription levels could be involved per se in the multistep carcinogenic process, we chose to analyze tumors for which the ras mutation rate was previously reported to be extremely rare or unknown, i.e., non-Hodgkin's lymphomas, certain types of carcinoma, sarcomas, germinal neoplasms of the testes and various tumors of the nervous system. A severe decrease in the expression of the rap1A gene was shown in the fibrosarcomas and the adenocarcinoma of the salivary gland studied, as compared to their normal counterparts, whereas no rap2 expression was found in the polyadenylated RNA of sarcoma samples.