Neuronal expression and interaction with the synaptic protein CASK suggest a role for Neph1 and Neph2 in synaptogenesis

Formation, differentiation, and plasticity of synapses require interactions between pre- and postsynaptic partners. Recently, it was shown that the transmembrane immunoglobulin superfamily protein SYG-1 is required for providing synaptic specificity in C. elegans. However, it is unclear whether the mammalian orthologs of SYG-1 are also involved in local cell interactions to determine specificity during synapse formation. We used in situ hybridization, immunohistochemistry, and immunogold electron microscopy to study the temporal and spatial expression of Neph1 and Neph2 in the developing and adult mouse brain. Both proteins show similar patterns with neuronal expression starting around embryonic days 12 and 11, respectively. Expression is strongest in areas of high migratory activity. In the adult brain, Neph1 and Neph2 are predominantly seen in the olfactory nerve layer and the glomerular layer of the olfactory bulb, in the hippocampus, and in Purkinje cells of the cerebellum. At the ultrastructural level, Neph1 and Neph2 are detectable within the dendritic shafts of pyramidal neurons. To a lesser extent, there is also synaptic localization of Neph1 within the stratum pyramidale of the hippocampal CA1 and CA3 region on both pre- and postsynaptic sites. Here it colocalizes with the synaptic scaffolder calmodulin-associated serin/threonin kinase (CASK), and both Neph1 and Neph2 interact with the PDZ domain of CASK via their cytoplasmic tail. Our results show that Neph proteins are expressed in the developing nervous system of mammals and suggest that these proteins may have a conserved function in synapse formation or neurogenesis.