Cysteine String Protein- Prevents Activity-Dependent Degeneration in GABAergic Synapses

Abstract

The continuous release of neurotransmitter could be seen to place a persistent burden on presynaptic proteins, one that could compromise nerve terminal function. This supposition and the molecular mechanisms that might protect highly active synapses merit investigation. In hippocampal cultures from knock-out mice lacking the presynaptic cochaperone cysteine string protein-_ (CSP-_),weobserve progressive degeneration of highly active synaptotagmin 2 (Syt2)-expressing GABAergic synapses, but surprisingly not of glutamatergic terminals. In CSP-_ knock-out mice, synaptic degeneration of basket cell terminals occurs in vivo in the presence of normal glutamatergic synapses onto dentate gyrus granule cells. Consistent with this, in hippocampal cultures from these mice, the frequency of miniature IPSCs, caused by spontaneous GABA release, progressively declines, whereas the frequency of miniature excitatory AMPA receptormediated currents (mEPSCs), caused by spontaneous release of glutamate, is normal. However, the mEPSC amplitude progressively decreases. Remarkably, long-term block of glutamatergic transmission in cultures lacking CSP-_ substantially rescues Syt2-expressing GABAergic synapses from neurodegeneration. These findings demonstrate that elevated neural activity increases synapse vulnerability and that CSP-_ is essential to maintain presynaptic function under a physiologically high-activity regimen.