Synapsin I is a neuron-specific protein consisting of two isoforms Ia and Ib. It is thought to play a role in the regulation of neurotransmitter release. In this study the structure and expression of two classes of synapsin I mRNA have been examined. The two mRNA classes have molecular sizes of 3.4 and 4.5 kb, respectively. Both classes translate into synapsin I polypeptides and display a high degree of base sequence homology. Utilizing an oligonucleotide-directed RNase H assay we have shown that both mRNA classes have a common start site of transcription and differ from one another toward their 3' ends. The expression of the two synapsin I mRNA classes is differentially regulated during the development of the rat brain and cerebellum. In the cerebellum the 4.5-kb transcript is expressed until postnatal day 7, after which it decreases to an undetectable level. The 3.4-kb mRNA is found throughout cerebellar development and in the adult. This suggests that the 3.4-kb mRNA class consists of messages which can encode both synapsin I polypeptides. Using quantitative Northern blot analysis a peak in the expression of this mRNA was observed at postnatal day 20. The maximum expression of the 3.4-kb class coincides with the period of synaptogenesis in the cerebellum. In addition to the developmental time course of synapsin mRNA expression a description of its spatial distribution throughout the cerebellum was performed using in situ hybridization histochemistry. From postnatal day 15 onwards, with a maximum at postnatal day 20, synapsin mRNA was localized in the internal granule cell layer of the cerebellum. On a cellular level, the granule cells, but not the neighboring Purkinje cells, express high levels of synapsin mRNA. These observations implicate developmentally coordinated differential RNA splicing in the regulation of neuron-specific gene expression and substantiate the correlation of synapsin gene expression with the period of synaptogenic differentiation of neurons.

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