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Regehr, Wade

It has been known for over 70 years that synaptic strength is dynamically regulated in a use-dependent manner1. At synapses with a low initial release probability, closely spaced presynaptic action potentials can result in facilitation, a short-term form of enhancement where each subsequent action potential evokes greater neurotransmitter release2. Facilitation can enhance neurotransmitter release manyfold and profoundly influence information transfer across synapses3, but the underlying mechanism remains a mystery. Among the proposed mechanisms is that a specialized calcium sensor for facilitation transiently increases the probability of release2,4 and is distinct from the fast sensors that mediate rapid neurotransmitter release. Yet such a sensor has never been identified, and its very existence has been disputed5,6. Here we show that synaptotagmin 7 (syt7) is a calcium sensor that is required for facilitation at multiple central synapses. In syt7 knockout mice, facilitation is eliminated even though the initial probability of release and presynaptic residual calcium signals are unaltered. Expression of wild-type syt7 in presynaptic neurons restored facilitation, whereas expression of a mutated syt7 with a calcium-insensitive C2A domain did not. By revealing the role of syt7 in synaptic facilitation, these results resolve a longstanding debate about a widespread form of short-term plasticity, and will enable future studies that may lead to a deeper understanding of the functional importance of facilitation.

SUMMARY Optical methods of interrogating neural circuits have emerged as powerful tools for understanding how the brain drives behaviors. Optogenetic proteins are widely used to control neuronal activity, while genetically encoded fluorescent reporters are used to monitor activity. These proteins are often expressed by injecting viruses, which frequently leads to inconsistent experiments due to misalignment of expression and optical components. Here, we describe how silk fibroin films simplify optogenetic experiments by providing targeted delivery of viruses. Films composed of silk fibroin and virus are applied to the surface of implantable optical components. After surgery, silk releases the virus to transduce nearby cells and provide localized expression around optical fibers and endoscopes. Silk films can also be used to express genetically encoded sensors in large cortical regions by using cranial windows coated with a silk/virus mixture. The ease of use and improved performance provided by silk make this a promising approach for optogenetic studies.