bunit-containing GABAA receptors . Discussion This study is based on the hypothesis that the pathogenetic mechanisms of EPM1 are reflected in altered gene expression patterns. To identify molecular defects associated with CSTB deficiency, we used cerebellum as a model which shows striking pathology in Cstb2/2 mice, and is also affected in human EPM1 patients. Global gene expression profiling from cerebella and cultured cerebellar granule cells revealed multiple affected biological pathways that could have a role in the pathogenesis of EPM1 not limited to cerebellum. Several modest changes in gene expression related to synapse maturation, development and function during postnatal maturation of the brain were detected already in P7 Cstb2/2 cerebellum. GABAergic signaling emerged as a pathway that showed most prominent changes manifesting, for example, as overexpression of GABAA receptor subunits Gabrd and Gabra6. GABA plays a central role in controlling neuronal development and connectivity, and defective GABAergic signaling in the cerebellum of Cstb2/2 mouse provides biological mechanisms for ataxia in these mice. However, when extrapolated to other brain regions affected in EPM1, alterations in GABAergic signaling could also contribute to hyperexcitability, manifested as severe myoclonus and seizures. In the cerebral cortex, alterations in GABA signaling may cause seizures, and vice versa, seizures can alter GABA signaling. Many GABA-related mutations are known to cause epilepsy in early life, and for example the analysis of a conditional mutant with disrupted GABAA signaling has implied a developmental period when disrupted GABA signaling may be critical for later ictogenesis and epileptogenesis in addition to having developmental consequences. Gene Expression Alterations in Cstb2/2 Mouse Another example of a synapse-related gene showing altered expression in Cstb2/2 cerebellum was the downregulated Epha7 that belongs to the ephrin receptor subfamily of the protein- tyrosine kinases, which through their surface associated ligands, ephrins, play a role in the initiation of new synaptic contacts, in dendritic spine morphology and in modulation of the mature 9 Gene Expression Alterations in Cstb2/2 Mouse synapses. The target recognition and axon guidance factor Slit2, which also showed reduced expression, is known to stimulate axonal elongation and branching, and to modulate neurogenesis in developing brain and may regulate axon-axon adhesion mediated PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19639654 by cadherins. Moreover, several genes encoding protocadherins were downregulated at P7. Synaptic protocadherins are central molecules in Ca2+-regulated cell-adhesion and they are expressed especially during neural development being implicated in synaptic recognition and AZ-6102 biological activity stabilization. While the microarray data in Cstb2/2 cerebellum at P7 pinpointed defects in processes related to synaptogenesis and synaptic maturation with alterations in the GABAergic signaling highlighted, Cstb2/2 neurons exposed upregulation of several cell cycle-related genes. Re-entry of neurons to cell cycle after differentiation would lead to apoptotic cell death and neurodegeneration, rather than cell division. PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19637192 We did not, however, detect elevated cell death in Cstb2/2 granule cell cultures, at least when the neurons were cultured for two days. We cannot, though, exclude the possibility that an activation of cell cycle related genes in Cstb2/2 granule cells reflects their dysfunction eventually 
leading to cell death.