Abstract 12293: Intercalated Disk Nanostructure and Molecular Organization Underlie Conduction Differences Between Atria and Ventricles

The intercalated disc (ID) is a complex, heterogeneous structure that affords electrical (gap junctions; GJ) and mechanical (adherence junctions [AJ], desmosomes [Des]) coupling between cardiomyocytes. Electrogenic proteins underlying the action potential upstroke (cardiac sodium channels [Na V 1.5], inward-rectifier potassium channels [K ir 2.1] and sodium potassium ATPase [NKA]) enriched within ID nanodomains are emerging as vital machinery for cardiac impulse propagation. ID structure is thus a critical determinant of cardiac conduction. We used indirect correlative light and electron microscopy ( iCLEM ) to assess ID ultrastructure (transmission electron microscopy; TEM) and molecular organization (confocal, STORM super resolution microscopy) in mouse atria and ventricles. TEM uncovered structural differences between atrial and ventricular IDs from the micro- through nano-scales including key factors that may underlie faster atrial conduction: larger interplicate regions and more numerous GJs with associated perinexi. Confocal microscopy revealed significant ID enrichment of Na V 1.5, K ir 2.1 and NKA in both atrial and ventricular myocytes. All three displayed more intense immunosignals in atrial myocytes, suggestive of greater expression relative to ventricle. STORM defined the distribution of Na V 1.5, K ir 2.1, NKA within the ID relative to AJ, Des, and GJ: In the ventricle, Na V 1.5 associated most closely with GJ (median intercluster distance: 110 nm), K ir 2.1 with Des (163 nm), and NKA with both GJ (150 nm) and AJ (135 nm). Next, the % of each electrogenic protein localized within 100 nm from AJ, Des, and GJ were calculated: 47% of Na V 1.5 around GJs, 35% of K ir 2.1 around Des and 36% and 39% of NKA near GJ and AJ respectively were the most prominent subpopulations. Protein organization within atria ID was broadly similar with some notable differences: Na V 1.5 was shifted closer to MJs relative to GJs with K ir 2.1 showing the opposite pattern. These data provide the first ever comprehensive quantitative picture of ID structure and molecular organization. We are now incorporating these data into our recently published modeling pipeline to generate realistic 3D finite-element meshes of IDs to uncover the structural underpinnings of conduction.