3D imaging reveals gradual degeneration of liver sympathetic nerves in human and experimental nonalcoholic fatty liver disease.

Although the basic molecular components that promote regulated neurotransmitter release are well established, the contribution of these proteins as regulators of the plasticity of neurotransmission and refinement of synaptic connectivity during development is elaborated less fully. For example, during the period of synaptic growth and maturation in brain, the expression of synaptosomal protein 25 kDa (SNAP-25), a neuronal t-SNARE (solubleN-ethylmaleimide-sensitive factor attachment protein receptor) essential for action potential-dependent neuroexocytosis, is altered through alternative splicing of pre-mRNA transcripts. We addressed the role of the two splice-variant isoforms of SNAP-25 with a targeted mouse mutation that impairs the shift from SNAP-25a to SNAP-25b. Most of these mutant mice die between 3 and 5 weeks of age, which coincides with the time when SNAP-25b expression normally reaches mature levels in brain and synapse formation is essentially completed. The altered expression of these SNAP-25 isoforms influences short-term synaptic function by affecting facilitation but not the initial probability of release. This suggests that mechanisms controlling alternative splicing between SNAP-25 isoforms contribute to a molecular switch important for survival that helps to guide the transition from immature to mature synaptic connections, as well as synapse regrowth and remodeling after neural injury.

Cyclin-dependent kinase 5 (Cdk5) is widely expressed although kinase activity has been described preferentially in neuronal systems. Cdk5 has an impact on actin polymerization during neuronal migration and neurite outgrowth and deregulation of the kinase has been implicated in the promotion of neurodegeneration. Recently it was shown that Cdk5 modulates dopamine signaling in neurons by regulating DARPP-32 function. In addition, Cdk5 phosphorylates munc-18 and synapsin I, two essential components of the exocytotic machinery. We have shown by reverse transcriptase-polymerase chain reaction, immunocytochemistry, and Western blotting that Cdk5 is present in the insulin-secreting pancreatic β-cell. Subcellular fractionation of isolated β-cells revealed a glucose-induced translocation of membrane-bound Cdk5 protein to lower density fractions. Inhibition of Cdk5 with roscovitine reduced insulin secretion with ∼35% compared with control after glucose stimulation and with ∼65% after depolarization with glucose and KCl. Capacitance measurements performed on single β-cells that expressed a dominant-negative Cdk5 mutant showed impaired exocytosis. The effect on exocytosis by Cdk5 appeared to be independent of changes in free cytoplasmic Ca²⁺ concentration. Taken together these results show that Cdk5 is present in β-cells and acts as a positive regulator of insulin exocytosis.