Roles of SNAP-25 isoforms in activity-dependent long-term synaptic plasticity

Abstract

SNARE proteins, SNAP-25, syntaxin 1A and VAMP2 constitute the functional units which join together to form the core SNARE complex. The SNARE complex carries out the vital function of membrane fusion of intracellular vesicles with plasma membranes, leading to the release of neurotransmitters in brain neuronal circuits and of hormones in endocrine glands. SNAP-25 exists as two alternatively spliced isoforms, resulting in two similar but distinct proteins, SNAP-25a and SNAP-25b. The distribution of these two proteins in brain and periphery are regulated developmentally. In this thesis, the focus has been on SNAP-25 in hippocampus. We evaluated the roles of SNAP-25 isoforms, and SNAP-25 mutants in activity-dependent long-term potentiation (LTP) and depression (LTD) at hippocampal Schaffer collateral-CA1 synapses.

We utilized gene targeted mouse models, i) the first only expressing SNAP-25a (the SNAP-25b-deficient mouse) and ii) the second having a mutated C-terminus of SNAP-25 (the SNAP-25∆3 mouse), to investigate alterations in synaptic plasticity. SNAP-25b-deficient mice displayed a reduced magnitude of LTP at Schaffer collateral-CA1 synapses and an enhanced magnitude of LTD at similar synapses at similar age. These mice exhibited abnormalities in basal synaptic transmission, short-term synaptic plasticity (STP) and faster neurotransmitter release kinetics. Abnormalities in synaptic transmission were evident as deficits in learning and memory formation in a behavioral task of active avoidance. Mutations in the C-terminus of SNAP-25 reduce the ability of inhibitory Gβγ subunits to interact with SNAP-25, and we show here that SNAP-25∆3 mice exhibit enhanced LTP at Schaffer collateral-CA1 synapses.

Lack of SNAP-25b causes hyperinsulinemia and, combined with Western diet, results in a diabetic phenotype. We investigated if a metabolic phenotype triggered by SNAP-25b-deficiency, or Western diet alone, affected higher cognitive functions of the brain. SNAP-25b-deficient mice and wild type mice with diet-induced metabolic syndrome performed poorly in brain region-specific behavioral tasks. Proteins quantification in the specific brain areas revealed changes in the expression levels of the SNARE proteins.

In conclusion, SNAP-25a and SNAP-25b play specialized and different roles in synaptic transmission. The roles of SNAP-25b appear to be more suited to a mature brain with stronger synaptic connectivity, and the work in this thesis clarifies the presynaptic contributions of the SNAP-25 isoforms to activity-dependent synaptic plasticity.