SNAP25 disease mutations change the energy landscape for synaptic exocytosis due to aberrant SNARE interactions

The complex molecular epileptogenesis landscape of glioblastoma

The cortical microenvironment surrounding malignant glioblastoma is a source of depolarizing crosstalk favoring hyperexcitability, tumor expansion, and immune evasion. Neosynaptogenesis, excess glutamate, and altered intrinsic membrane currents contribute to excitability dyshomeostasis, yet only half of the cases develop seizures, suggesting that tumor and host genomics, along with location, rather than mass effect, play a critical role. We analyzed the spatial contours and expression of 358 clinically validated human epilepsy genes in the human glioblastoma transcriptome compared to non-tumor adult and developing cortex datasets. Nearly half, including dosage-sensitive genes whose expression levels are securely linked to monogenic epilepsy, are strikingly enriched and aberrantly regulated at the leading edge, supporting a complex epistatic basis for peritumoral epileptogenesis. Surround hyperexcitability induced by complex patterns of proepileptic gene expression may explain the limited efficacy of narrowly targeted antiseizure medicines and the persistence of epilepsy following tumor resection and clarify why not all brain tumors provoke seizures.

Single residues in the complexin N-terminus exhibit distinct phenotypes in synaptic vesicle fusion

The release of neurotransmitters at central synapses is dependent on a cascade of protein interactions, specific to the presynaptic compartment. Amongst those dedicated molecules the cytosolic complexins play an incompletely defined role as synaptic transmission regulators. Complexins are multidomain SNARE complex binding proteins which confer both inhibitory and stimulatory functions. Using systematic mutagenesis and combining reconstituted in vitro membrane fusion assays with electrophysiology in neurons, we deciphered the function of the N-terminus of complexin II (Cpx). The N-terminus (amino acid 1 - 27) starts with a region enriched in hydrophobic amino acids (1-12), which can lead to lipid binding. In contrast to mutants which maintain the hydrophobic character and the stimulatory function of Cpx, non-conservative exchanges largely perturbed spontaneous and evoked exocytosis. Mutants in the downstream region (amino acid 11-18) show differential effects. Cpx-A12W increased spontaneous release without affecting evoked release whereas replacing D15 with amino acids of different shapes or hydrophobic properties (but not charge) not only increased spontaneous release, but also impaired evoked release and surprisingly reduced the size of the readily releasable pool, a novel Cpx function, unanticipated from previous studies. Thus, the exact amino acid composition of the Cpx N-terminus fine tunes the degree of spontaneous and evoked neurotransmitter release.

Significance Statement

We describe in this work the importance of the N-terminal domain of the small regulatory cytosolic protein complexin in spontaneous and evoked glutamatergic neurotransmitter release at hippocampal mouse neurons. We show using a combination of biochemical, imaging and electrophysiological techniques that the binding of the proximal region of complexin (amino acids 1-10) to lipids is crucial for spontaneous synaptic vesicular release. Furthermore, we identify a single amino acid at position D15 which is structurally important since it not only is involved in spontaneous release but, when mutated, also decreases drastically the readily releasable pool, a function that was never attributed to complexin.