SUMMARY Intercalated disks (ICDs) connect the termini of adjacent cardiomyocytes (CMs) physically, electrically, and chemically. The structural role of ICDs to preserve CM integrity in the face of billions of cycles of forceful con- traction and relaxation is well appreciated; however, the function of ICDs as essential CM signaling hubs is only now emerging. Arrhythmogenic cardiomyopathy (ACM) provides a unique window into the function of ICDs and specifically desmosomes. ACM is a potentially lethal disorder characterized by high arrhythmia bur- den, loss of contractile myocardium, and replacement by fibro-fatty tissue. Mutations of desmosome genes (PKP2, DSG2, DSC2, DSP, JUP) occur in approximately half of ACM patients. Despite growing knowledge about ACM disease pathogenesis, the mechanistic links between desmosome mutations and arrhythmias, my- ocardial dysfunction, and fibrofatty replacement remain poorly understood. The overall goal of this proposal is to gain insights into the mechanisms by which desmosome mutations cause arrhythmia and myocardial dysfunction; Our overarching hypothesis is that desmosomes are inte- gral for maintaining normal cardiomyocyte homeostasis through both their structural and signaling activities. ACM mutations disrupt these activities to cause both loss of structural integrity and aberrant signaling. We will test these hypotheses through four parallel but complementary Specific Aims: (1) We will examine cell composition and gene regulation of human ACM myocardium, using concurrent single nucleus RNA-seq and ATAC-seq, and spatial transcriptomics (snMulti-seq) with massively parallel single molecule fluo- rescent in situ hybridization (MERFISH); (2) We will use mosaic, adult, cardiomyocyte specific inactivation of Dsp to probe the cell autonomous functions of desmosomes. This model will be studied using snMulti-seq and MERFISH, followed by interrogation of key predicted regulators using in vivo gain- and loss-of-function ap- proaches; (3) Using proximity proteomics of ICD component N-cadherin, we identified novel ICD components and ICD components that are altered by Dsp ablation. We will use in vivo gain- and loss-of-function ap- proaches to study the function of selected candidates identified by this screen; (4) Define the contributions of WNT and GSK3 signaling to ACM phenotypes in DSP mutant hiPSC-CMs. Using genetic approaches in bioen- gineered hiPSC-CMs, we will dissect the involvement of GSK3 and WNT signaling to ACM pathogenesis. Impact: This proposal will advance our understanding of the function of desmosomes and ICDs in CM homeostasis and the molecular pathogenesis of ACM. This knowledge will accelerate efforts to develop targeted ACM therapies.