Complexin has opposite effects on two modes of synaptic vesicle fusion.

BACKGROUND: Synaptic transmission can occur in a binary or graded fashion, depending on whether transmitter release is triggered by action potentials or by gradual changes in membrane potential. Molecular differences of these two types of fusion events and their differential regulation in a physiological context have yet to be addressed. Complexin is a conserved SNARE-binding protein that has been proposed to regulate both spontaneous and stimulus-evoked synaptic vesicle (SV) fusion. RESULTS: Here we examine complexin function at a graded synapse in C. elegans. Null complexin (cpx-1) mutants are viable, although nervous system function is significantly impaired. Loss of CPX-1 results in a 3-fold increase in the rate of tonic synaptic transmission at the neuromuscular junction, whereas stimulus-evoked SV fusion is decreased 10-fold. A truncated CPX-1 missing its C-terminal domain can rescue stimulus-evoked synaptic vesicle exocytosis but fails to suppress tonic activity, demonstrating that these two modes of exocytosis can be distinguished at the molecular level. A CPX-1 variant with impaired SNARE binding also rescues evoked, but not tonic, neurotransmitter release. Finally, tonic, but not evoked, release can be rescued in a syntaxin point mutant by removing CPX-1. Rescue of either form of exocytosis partially restores locomotory behavior, indicating that both types of synaptic transmission are relevant. CONCLUSION: These observations suggest a dual role for CPX-1: suppressing SV exocytosis, driven by low levels of endogenous neural activity, while promoting synchronous fusion of SVs driven by a depolarizing stimulus. Thus, patterns of synaptic activity regulate complexin's inhibitory and permissive roles at a graded synapse.

Fibrinogen and beta-amyloid association alters thrombosis and fibrinolysis: a possible contributing factor to Alzheimer's disease.

Alzheimer's disease (AD) is a neurodegenerative disorder in which vascular pathology plays an important role. Since the beta-amyloid peptide (Abeta) is a critical factor in this disease, we examined its relationship to fibrin clot formation in AD. In vitro and in vivo experiments showed that fibrin clots formed in the presence of Abeta are structurally abnormal and resistant to degradation. Fibrin(ogen) was observed in blood vessels positive for amyloid in mouse and human AD samples, and intravital brain imaging of clot formation and dissolution revealed abnormal thrombosis and fibrinolysis in AD mice. Moreover, depletion of fibrinogen lessened cerebral amyloid angiopathy pathology and reduced cognitive impairment in AD mice. These experiments suggest that one important contribution of Abeta to AD is via its effects on fibrin clots, implicating fibrin(ogen) as a potential critical factor in this disease.