Presynaptic calcium dynamics, neuronal excitability and synaptic vesicle Cycle in central synapses lacking cysteine string protein-alpha (CSP-ALPHA)

Abstract

Cysteine String Protein-α (CSP-α) is a molecular co-chaperone of the synaptic vesicles that prevents presynaptic degeneration. The role of CSP-α in the machinery of calcium dependent release of neurotransmitter from synaptic vesicles has been studied previously but some functional aspects remain still unknown, such as its role in exo- and endocytosis in central neurons and its role in the regulation of presynaptic calcium. To study this, we have measured the exo- and endocytosis of synaptic vesicles in hippocampal synapses lacking CSP-α using a pH-sensitive green fluorescent protein (GFP) fused to the luminal part of synaptic vesicles (pHluorin) showing no differences between wild type and mutant synapses. In addition, we have measured presynaptic cytosolic calcium changes in hippocampal cultures lacking CSP-α using a genetically encoded calcium indicator (GECI) called syGCaMP3, uncovering different type of responses in glutamatergic synapses: long duration responses (LDRs) and short duration responses (SDRs). These measurements show normal presynaptic calcium dynamics in neurons lacking CSP-α. On the other hand, highly active GABAergic synapses formed by fast spiking parvalbumin positive interneurons are extremely sensitive to the absence of CSP-α and they degenerate early postnatally in CSP-α KO mice. In contrast, glutamatergic hippocampal synapses do not show obvious signs of early neurodegeneration. Using electrophysiological and imaging techniques, in this thesis we demonstrate the propensity of hippocampal circuits of mice lacking CSP-α to hyperexcitability, probably produced by this excitatory/inhibitory imbalance caused by loss of inhibitory synapses.