Reduced expression of neuropeptide genes in a genome‐wide screen of a secretion‐deficient mouse

Abstract

Activity-dependent changes in synapses rely on functional changes in resident proteins and on gene expression. We addressed the relationship between synapse activity and the expression of synaptic genes by comparing RNA levels in the neocortex of normal mice versus secretion-deficient and therefore synaptically silent munc18-1 (mammalian homologue of Caenorhabditis elegans uncoordinated locomotion-18) null mutants, using microarray expression analysis, real-time quantitative PCR and northern blotting. We hypothesized that genes under the control of synaptic activity would be differentially expressed between mutants and controls. We found that few synaptic genes were differentially expressed. However, most neuropeptide genes with detectable expression on the microarray were differentially expressed, being expressed 3-20-fold higher in control cortex. Several other secreted proteins were also differentially expressed, but genes encoding their receptors and many other synaptic components were not. Differential expression was confirmed by real-time quantitative PCR analysis. In situ hybridization indicated that the difference in neuropeptide expression was uniform and not due to the loss of specific cells in the mutant. In primary sensory neurons, which do not depend on synaptic activity for their input, the differential expression of neuropeptides was not observed. These data argue against a general relationship between the activity of synapses and the expression of their resident proteins, but suggest a link between secretion and the expression of genes encoding the secreted products. Keywords: munc18 (mammalian homologue of Caenorhabditis elegans uncoordinated locomotion-18) null mutant, neocortex, synapse development. The strength of synaptic coupling between neurons is regulated by a number of different molecular mechanisms acting on different time scales, both during development and in mature networks. Most of the short-term changes probably depend on local, activity-dependent changes in synapse resident proteins (e.g. translocation, phosphorylation). Longterm changes in synapse efficacy probably involve changes in protein composition of the synapses involved. Indeed, gene expression is essential for some forms of synaptic plasticity (see, for instance, Nguyen et al. 2002). Several lines of evidence suggest a link between synapse use and the supply of synaptic components via activity-dependent gene expression. Firstly, interference with sensory input decreases Abbreviations used: BDNF, brain-derived neurotrophic growth factor; Cy3, cyanine 3; E0, embryonic day 0; HPRT, hypoxanthine phosphoribosyl transferase; munc18, mammalian homologue of Caenorhabditis elegans uncoordinated locomotion-18; PBS, phosphate-buffered saline; qPCR, real-time quantitative PCR; SSC, sodium chloride sodium citrate.

Munc18a Does Not Alter Fusion Rates Mediated by Neuronal SNAREs, Synaptotagmin, and Complexin

“…Interestingly, in neuroendocrine cells, transfection of syntaxin alone results in its mislocalization, whereas co-transfection of both Munc18a and syntaxin results in trafficking to the plasma membrane ( 37 , 38 ). Thus, one function of Munc18a may be to stabilize or “chaperone” syntaxin, which also agrees with the aforementioned knock-out studies of Munc18a in mice that showed a reduced level of syntaxin-1 in the plasma membrane ( 13 ).…”

“…Evidence that the neuronal SM protein Munc18a is involved in more than “just” facilitation or regulation comes primarily from Munc18a knock-out studies in mice ( 12 ) that resulted in complete loss of neurotransmitter release. However, Munc18a knockout also resulted in a 70% reduction in total syntaxin-1 protein level in brain lysates of the knock-out mice and reduced syntaxin-1 mRNA levels ( 13 ). Nonetheless, the residual syntaxin-1 was correctly targeted to the synapse and formed SDS-resistant SNARE complex ( 14 ), so the reduction in syntaxin-1 levels in the Munc18a knock-out mice may not explain the complete loss of neurotransmitter release.…”

Neuroendocrine transcriptional programs adapt dynamically to the supply and demand for neuropeptides as revealed in NSF mutant zebrafish

“…The neuroendocrine phenotype exhibited by the nsf st 53 zebrafish is partially mimicked in munc18-1 mutant mice, which exhibit a selective and stage-dependent (E18) decrease primarily in neuropeptide transcript expression [ 41 ]. Munc18-1 is essential for regulated exocytosis.…”

Mouse chromosome 17 candidate modifier genes for thrombosis

“…Zfp161 is expressed in neural tissue, is involved in transcription regulation (Numoto et al 1993), and is linked to a gene associated with an inheritable developmental disorder, holoprosencephaly (Sobek-Klocke et al 1997). Dlgap1 is expressed in the brain and neural tissues and functions in synaptic transmission and cell-cell signaling (Bouwman et al 2006; Nguyen et al 2005). Ralbp1 has GTPase activity, and homozygous and heterozygous mice have increased sensitivity to irradiation, oxidative stress, and impaired glutathione homeostasis (Singhal et al 2008).…”