An Ancient Duplication of Exon 5 in the Snap25 Gene Is Required for Complex Neuronal Development/Function

Alternative splicing is an evolutionary innovation to create functionally diverse proteins from a limited number of genes. SNAP-25 plays a central role in neuroexocytosis by bridging synaptic vesicles to the plasma membrane during regulated exocytosis. The SNAP-25 polypeptide is encoded by a single copy gene, but in higher vertebrates a duplication of exon 5 has resulted in two mutually exclusive splice variants, SNAP-25a and SNAP-25b. To address a potential physiological difference between the two SNAP-25 proteins, we generated gene targeted SNAP-25b deficient mouse mutants by replacing the SNAP-25b specific exon with a second SNAP-25a equivalent. Elimination of SNAP-25b expression resulted in developmental defects, spontaneous seizures, and impaired short-term synaptic plasticity. In adult mutants, morphological changes in hippocampus and drastically altered neuropeptide expression were accompanied by severe impairment of spatial learning. We conclude that the ancient exon duplication in the Snap25 gene provides additional SNAP-25-function required for complex neuronal processes in higher eukaryotes

Cyclin-Dependent Kinase 5 and Insulin Secretion

Cyclin-dependent kinase 5 (Cdk5) is emerging as a multifunctional kinase involved in regulating numerous cellular processes. Lately, Cdk5 has also emerged as a key controller of regulated membrane fusion in secretory cells. The pancreatic β-cell is highly specialized to secrete insulin in response to elevated glucose concentrations in the blood. The final biochemical events leading to insulin release from the β-cell are governed by a secretion apparatus that is similar to the presynaptic machinery mediating synaptic transmission in neuronal networks. We now summarize recent developments in the field of Cdk5 and regulated exocytosis and also present some novel findings regarding Cdk5’s effect on insulin secretion

Munc18-1 and Munc18-2 Proteins Modulate β-Cell Ca2+ Sensitivity and Kinetics of Insulin Exocytosis Differently

Fast neurotransmission and slower hormone release share the same core fusion machinery consisting of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins. In evoked neurotransmission, interactions between SNAREs and the Munc18-1 protein, a member of the Sec1/Munc18 (SM) protein family, are essential for exocytosis, whereas other SM proteins are dispensable. To address if the exclusivity of Munc18-1 demonstrated in neuroexocytosis also applied to fast insulin secretion, we characterized the presence and function of Munc18-1 and its closest homologue Munc18-2 in β-cell stimulus-secretion coupling. We show that pancreatic β-cells express both Munc18-1 and Munc18-2. The two Munc18 homologues exhibit different subcellular localization, and only Munc18-1 redistributes in response to glucose stimulation. However, both Munc18-1 and Munc18-2 augment glucose-stimulated hormone release. Ramp-like photorelease of caged Ca2+ and high resolution whole-cell patch clamp recordings show that Munc18-1 and Munc18-2 overexpression shift the Ca2+ sensitivity of the fastest phase of insulin exocytosis differently. In addition, we reveal that Ca2+ sensitivity of exocytosis in β-cells depends on the phosphorylation status of the Munc18 proteins. Even though Munc18-1 emerges as the key SM-protein determining the Ca2+ threshold for triggering secretory activity in a stimulated β-cell, Munc18-2 has the ability to increase Ca2+ sensitivity and thus mediates the release of fusion-competent granules requiring a lower cytoplasmic-free Ca2+ concentration, [Ca2+]i. Hence, Munc18-1 and Munc18-2 display distinct subcellular compartmentalization and can coordinate the insulin exocytotic process differently as a consequence of the actual [Ca2+]i