Structural basis of selective inhibition toward gap junction channels involved in vision.

Project Summary Gap junction channels are critical to vision by maintaining homeostasis in the lens and propagating electrical signals in the retina. Age-related stress and genetic mutations in connexins 46 and 50 (Cx46/50) in the lens and connexin 36 (Cx36) in the retina have been linked to cataracts, glaucoma, and retinopathies. Despite their vital importance to vision biology, we still lack effective pharmacological tools to elucidate the physiological and pathophysiological roles of gap junctions and their potential as therapeutic targets. Current small molecule modulators of gap junctions lack potency and isoform specificity, often interacting with and inhibiting other ion channels, eliciting unwanted off-target effects. Drug development has been slow in this field due to the lack of high-resolution structures of gap junctions in complex with a drug-like molecule. Recent advances in single particle Cryo-EM have enabled us to solve the structures of ion channels to near-atomic resolution and capture inhibitor-bound states. The Aims of this proposal will leverage these recent advances, in combination with fragment antigen-binding (Fab) technologies to deconvolute the mechanism of inhibitory action against native heteromeric gap junctions. To glean insight into the selectivity potential of gap junction inhibitors, this proposal will target the structural effects of mefloquine (MFQ) against the major gap junctions in the visual system. MFQ is one of the few drugs that exhibits gap junction selectivity against Cx50 and Cx36 isoforms. Remarkably, MFQ does not show appreciable binding to the highly related isoform, Cx46. In Aim 1, I will resolve the atomic structures of homomeric Cx36 and Cx50 in complex with MFQ. Comparative analysis to structures of Cx46 and Cx26 (no binding) and Cx43 (semi-inhibitory) will be used to identify interactions that are critical for MFQ selectivity and drive structure-guided mutation studies to validate the selectivity mechanism in vitro. Remarkably, MFQ inhibits native heteromeric gap junction channels composed of Cx46/50. In Aim 2, I will utilize high-affinity Fabs to identify co-assembly patterns of native mammalian lens gap junctions in the presence of MFQ to understand how this inhibition is achieved, and potentially elucidate the proposed cooperativity. Mechanistic insights garnered from these works will aid in designing the next generation of gap junction pharmacological probes to better understand the precise roles of gap junctions in vision health and disease.