Abstract Body: Therapies for end-stage heart failure are lacking and the number of patients awaiting heart transplants far outweighs availability. Left ventricular assist devices (LVADs) take over cardiac pump function and are used as a bridge-to-transplant. Notably, some patients on LVADs experience recovery of cardiac function and reversal of pathological cardiac remodeling; however, the molecular mechanisms underlying these favorable outcomes are unknown. In this study, we studied 19 paired pre- and post-LVAD samples with 10 clinically categorized as responders and 9 as non-responders. Bulk mRNA sequencing identified 486 differentially expressed genes post-LVAD, including reduced pro-inflammatory gene expression and increased expression of genes involved in metal ion binding; however, just 23 total genes were differentially expressed between responders and non-responders. To determine whether LVAD differentially impacted the proteome in responders and non-responders, we next performed quantitative proteomics on post-LVAD samples. This analysis identified 154 differentially expressed proteins and revealed that factors involved in mRNA processing were higher expressed in LVAD responders. Re-analysis of our mRNA sequencing data for gene isoforms identified that alternative splicing of calcium and calmodulin-dependent protein kinase 2-delta (CAMK2D) was the most significantly impacted event with non-responders having increased inclusion of exon 14 (denoting the B isoform), which encodes a nuclear localization sequence. Notably, there was a strong negative correlation between exon 14 inclusion and ΔLVEF post-LVAD. We validated these findings using targeted exon-specific quantitative PCR, which confirmed that expression of CAMK2D_B was higher both pre- and post-LVAD in non-responders, while responders had post-LVAD levels matching non-failing controls. We next investigated whether responders and non-responders differed at the phospho-proteome level via mass spectrometry and found three serines in CaMK2δ exon 14 were the only differentially abundant phosphosites and all increased in non-responders. Localization studies in vitro with adenoviruses expressing phospho-null or phospho-mimetic CaMK2D_B constructs revealed that these phosphorylation events precluded nuclear localization. In conclusion, we identified differential alternative splicing and phosphorylation of CaMK2δ in LVAD responders, which may represent novel therapeutic targets for heart failure recovery.