We studied the distribution of the ubiquitin-specific protease Usp9x in mouse brain as it relates to the potential role of ubiquitin proteasome system in synaptic plasticity. Usp9x is the mouse homolog of faf, known for its function in synaptic development in Drosophila. In adults, high levels of expression of Usp9x protein were found in layer V of neocortex, Purkinje cells in cerebellum, and specific hippocampal subfields. In hippocampal pyramidal cells, Usp9x expression was higher in CA3 than in CA1. This regional specificity was detected at postnatal day 22 but not at postnatal day 15. In adult mice, the CA1-CA3 difference was partially accounted for by a difference in the level of Usp9x mRNA, suggesting that transcription of Usp9x was differentially regulated between hippocampal subfields. Two synaptic marker proteins, synaptotagmin and spinophilin, were both more abundant in the striatum oriens of CA3 than in the similar region of CA1, correlating with the distribution of Usp9x, a result compatible with a role for Usp9x in synaptic development in mouse hippocampus. Ube1x, the enzyme responsible for the initial step in ubiquitin conjugation, was preferentially concentrated in the dendrites of the CA1 neurons instead of the CA3 neurons, suggesting a reciprocal relationship between ubiquitin conjugation and deubiquitination in CA3 and CA1. This spatial and temporal specificity in expression of Usp9x and Ube1x protein raises interesting questions about the roles of these ubiquitin enzymes in the differential functions of CA1 and CA3.
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