Comparison of munc-18 and cdk5 expression in the nervous system during mouse embryogenesis

Genetic and microstructural differences in the cortical plate of gyri and sulci during gyrification in fetal sheep

Gyrification of the cerebral cortex is a developmentally important process, but the mechanisms that drive cortical folding are not fully known. Theories propose that changes within the cortical plate (CP) cause gyrification, yet differences between the CP below gyri and sulci have not been investigated. Here we report genetic and microstructural differences in the CP below gyri and sulci assessed before (at 70 days of gestational age [GA] 70), during (GA 90), and after (GA 110) gyrification in fetal sheep. The areal density of BDNF, CDK5, and NeuroD6 immunopositive cells were increased, and HDAC5 and MeCP2 mRNA levels were decreased in the CP below gyri compared with sulci during gyrification, but not before. Only the areal density of BDNF-immunopositive cells remained increased after gyrification. MAP2 immunoreactivity and neurite outgrowth were also increased in the CP below gyri compared with sulci at GA 90, and this was associated with microstructural changes assessed via diffusion tensor imaging and neurite orientation dispersion and density imaging at GA 98. Differential neurite outgrowth may therefore explain the localized changes in CP architecture that result in gyrification.

MUNC18-1 gene abnormalities are involved in neurodevelopmental disorders through defective cortical architecture during brain development

While Munc18–1 interacts with Syntaxin1 and controls the formation of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) complex to regulate presynaptic vesicle fusion in developed neurons, this molecule is likely to be involved in brain development since its gene abnormalities cause early infantile epileptic encephalopathy with suppression-burst (Ohtahara syndrome), neonatal epileptic encephalopathy and other neurodevelopmental disorders. We thus analyzed physiological significance of Munc18–1 during cortical development. Munc18–1-knockdown impaired cortical neuron positioning during mouse corticogenesis. Time-lapse imaging revealed that the mispositioning was attributable to defects in radial migration in the intermediate zone and cortical plate. Notably, Syntaxin1A was critical for radial migration downstream of Munc18–1. As for the underlying mechanism, Munc18–1-knockdown in cortical neurons hampered post-Golgi vesicle trafficking and subsequent vesicle fusion at the plasma membrane in vivo and in vitro, respectively. Notably, Syntaxin1A-silencing did not affect the post-Golgi vesicle trafficking. Taken together, Munc18–1 was suggested to regulate radial migration by modulating not only vesicle fusion at the plasma membrane to distribute various proteins on the cell surface for interaction with radial fibers, but also preceding vesicle transport from Golgi to the plasma membrane. Although knockdown experiments suggested that Syntaxin1A does not participate in the vesicle trafficking, it was supposed to regulate subsequent vesicle fusion under the control of Munc18–1. These observations may shed light on the mechanism governing radial migration of cortical neurons. Disruption of Munc18–1 function may result in the abnormal corticogenesis, leading to neurodevelopmental disorders with MUNC18–1 gene abnormalities.

Synaptic Effects of Munc18-1 Alternative Splicing in Excitatory Hippocampal Neurons

The munc18-1 gene encodes two splice-variants that vary at the C-terminus of the protein and are expressed at different levels in different regions of the adult mammalian brain. Here, we investigated the expression pattern of these splice variants within the brainstem and tested whether they are functionally different. Munc18-1a is expressed in specific nuclei of the brainstem including the LRN, VII and SOC, while Munc18-1b expression is relatively low/absent in these regions. Furthermore, Munc18-1a is the major splice variant in the Calyx of Held. Synaptic transmission was analyzed in autaptic hippocampal munc18-1 KO neurons re-expressing either Munc18-1a or Munc18-1b. The two splice variants supported synaptic transmission to a similar extent, but Munc18-1b was slightly more potent in sustaining synchronous release during high frequency stimulation. Our data suggest that alternative splicing of Munc18-1 support synaptic transmission to a similar extent, but could modulate presynaptic short-term plasticity.

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

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.