Assistant Professor, Department of Biology
Ph.D. University of Oregon
Postdoc, Fred Hutchinson Cancer Research Center
Research Interests: Neural circuit wiring, synapse formation, and electrical synaptogenesis in zebrafish.
Overview: The human brain contains more connections between neurons than the Milky Way has stars! The brain is wired at a gross level into stereotyped neural circuits that underlie sensation, information processing, motor output, and ultimately, consciousness. Disrupted neural circuitry has been linked to many neurodevelopmental disorders, such as autism, epilepsy, and schizophrenia. How do the neurons of the brain connect and wire up into circuits? The goal of the research in the lab is to integrate genetics, biochemistry, cell biology, circuit function, and behavior, to understand how the brain creates functioning neural networks.
Neural circuits are defined by the connections made between neurons, and connections, termed synapses, come in two flavors: chemical, where transmission is mediated by neurotransmitters and receptors, and electrical, where neurons directly communicate with one another through gap junction channels. While the last decade has provided much insight into the developmental genetic mechanisms of building chemical synapses, electrical synapse formation is still not understood. However, it is known that electrical synapses are used by all animals both during development and in adulthood, and are found in sensory, central, and motor circuits. The goal of this project is to unlock the molecular mechanisms underlying electrical synaptogenesis.
Using zebrafish as a model system we have performed a forward genetic screen to identify mutations that cause defects in electrical synapse formation. Mapping mutations from forward genetic screens is challenging, particularly in large vertebrate genomes, but we have developed methods using on next generation sequencing which facilitate the identification of mutated genes (Genome Research). One of the mutations identified in the screen disrupted the autism-associated gene neurobeachin and we found that it was required for both electrical and chemical synapse formation, placing this gene as a critical lynchpin in all of synapse formation (Current Biology). We have also developed a novel CRISPR-based reverse genetic screening method to identify genes required for development – this was the first example that such an approach could be taken in a vertebrate (Nature Methods). The screen identified structural proteins that create the gap junction channel between the neurons and scaffolding that stabilize the synaptic structure. Ongoing work has revealed that electrical synapses can be asymmetric, with unique proteins on each side of the junction. This molecular asymmetry may underlie functional asymmetry and provide differential substrates for altering electrical synapse function.
Current projects focus on several diverse, but related, areas of electrical synaptogenesis:
1) Electrical synapse asymmetry – biochemistry, molecular biology, and genetics
How do the proteins of the synapse function at the molecular level to form the connection? What proteins interact and how do those interactions build the synapse? What other proteins are present at the synapse?
2) Electrical synapse formation – cell biology, development, and genetics
How are proteins trafficked to the synapse? How are they captured and stabilized once present? What are the cytoskeletal structures and motor proteins that facilitate movement? How long do proteins remain at the synapse and are they responsive to neuronal activity?
3) Electrical synapse function – behavior and physiology
Does the composition of the electrical synapse change based on circuit activity? Do molecular asymmetries produce effects on synapse function? How are molecular asymmetries integrated into circuit level function and behavioral output?
4) Electrical and chemical synapse interactions – physiology, development, and genetics
Are early-forming electrical synapses required for subsequent chemical synapse formation? What gap junction channels and scaffolds mediate early circuit activity? How are some early-forming electrical synapses removed as neural circuits mature? How are others retained?
Asymmetry of an Intracellular Scaffold at Vertebrate Electrical Synapses.
Curr Biol. 2017 Nov 20;27(22):3561-3567.e4
Authors: Marsh AJ, Michel JC, Adke AP, Heckman EL, Miller AC
Neuronal synaptic connections are either chemical or electrical, and these two types of synapses work together to dynamically define neural circuit function . Although we know a great deal about the molecules that support chemical synapse formation and function, we know little about the macromolecular complexes that regulate electrical synapses. Electrical synapses are created by gap junction (GJ) channels that provide direct ionic communication between neurons . Although they are often molecularly and functionally symmetric, recent work has found that pre- and postsynaptic neurons can contribute different GJ-forming proteins, creating molecularly asymmetric channels that are correlated with functional asymmetry at the synapse [3, 4]. Associated with the GJs are structures observed by electron microscopy termed the electrical synapse density (ESD) . The ESD has been suggested to be critical for the formation and function of the electrical synapse, yet the biochemical makeup of these structures is poorly understood. Here we find that electrical synapse formation in vivo requires an intracellular scaffold called Tight Junction Protein 1b (Tjp1b). Tjp1b is localized to the electrical synapse, where it is required for the stabilization of the GJs and for electrical synapse function. Strikingly, we find that Tjp1b protein localizes and functions asymmetrically, exclusively on the postsynaptic side of the synapse. Our findings support a novel model of electrical synapse molecular asymmetry at the level of an intracellular scaffold that is required for building the electrical synapse. We propose that such ESD asymmetries could be used by all nervous systems to support molecular and functional asymmetries at electrical synapses.
PMID: 29103941 [PubMed - indexed for MEDLINE]