Adam Miller

Assistant Professor, Department of Biology
Member, ION

Ph.D. University of Oregon
Postdoc, Fred Hutchinson Cancer Research Center

Office:
Huestis 314A
541-346-4523
Lab:
Huestis 314
541-346-4587

 

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?

RECENT PUBLICATIONS

Commun Biol. 2021 Jun 3;4(1):676. doi: 10.1038/s42003-021-02185-z.

ABSTRACT

Myopia is the most common developmental disorder of juvenile eyes, and it has become an increasing cause of severe visual impairment. The GJD2 locus has been consistently associated with myopia in multiple independent genome-wide association studies. However, despite the strong genetic evidence, little is known about the functional role of GJD2 in refractive error development. Here, we find that depletion of gjd2a (Cx35.5) or gjd2b (Cx35.1) orthologs in zebrafish, cause changes in the biometry and refractive status of the eye. Our immunohistological and scRNA sequencing studies show that Cx35.5 (gjd2a) is a retinal connexin and its depletion leads to hyperopia and electrophysiological changes in the retina. These findings support a role for Cx35.5 (gjd2a) in the regulation of ocular biometry. Cx35.1 (gjd2b) has previously been identified in the retina, however, we found an additional lenticular role. Lack of Cx35.1 (gjd2b) led to a nuclear cataract that triggered axial elongation. Our results provide functional evidence of a link between gjd2 and refractive error.

PMID:34083742 | DOI:10.1038/s42003-021-02185-z

Elife. 2021 Apr 28;10:e66898. doi: 10.7554/eLife.66898.

ABSTRACT

Electrical synaptic transmission relies on neuronal gap junctions containing channels constructed by Connexins. While at chemical synapses neurotransmitter-gated ion channels are critically supported by scaffolding proteins, it is unknown if channels at electrical synapses require similar scaffold support. Here, we investigated the functional relationship between neuronal Connexins and Zonula Occludens 1 (ZO1), an intracellular scaffolding protein localized to electrical synapses. Using model electrical synapses in zebrafish Mauthner cells, we demonstrated that ZO1 is required for robust synaptic Connexin localization, but Connexins are dispensable for ZO1 localization. Disrupting this hierarchical ZO1/Connexin relationship abolishes electrical transmission and disrupts Mauthner cell-initiated escape responses. We found that ZO1 is asymmetrically localized exclusively postsynaptically at neuronal contacts where it functions to assemble intercellular channels. Thus, forming functional neuronal gap junctions requires a postsynaptic scaffolding protein. The critical function of a scaffolding molecule reveals an unanticipated complexity of molecular and functional organization at electrical synapses.

PMID:33908867 | DOI:10.7554/eLife.66898