Abstract P2006: S100A1 And STRIT1 Redundantly Governs Responsiveness Of The Heart To Hemodynamic Stress Via Modulation Of SERCA2a Activity

Background & Objectives: SR Ca2+ load, which is sustained by SERCA2a pump activity, is a critical determinant for cardiac performance regulation & adaptation. Independent studies identified both S100A1 & STRIT1 as molecular enhancers of SERCA2a activity in the heart. S100A1 & STRIT1 decline in post-myocardial infarction hearts aggravated the transition to adverse cardiac remodelling & contractile failure. We therefore hypothesized that S100A1 & STRIT1 could act as independent but potentially redundant molecular switches for SERCA2a activity.

Methods & Results: S100a1 knock-out (SKO) mice display no overt cardiac contractile or structural abnormalities in the absence of stress. RNA-seq transcriptomic analysis of left ventricle (LV) of SKO & wild type (WT) identified Strit1 amongst the top 3 most upregulated genes in SKO LV. We validated STRIT1 upregulation by RT-PCR as well as by immunoblotting (IB) that yielded a 15-fold increase compared with WT. Age lapse-resolved RT-PCR analysis showed Strit1 response to S100a1 knockout begins at post-partum day 5 & reaches plateau in adulthood. Next, we generated Strit1-S100a1 double knock out mice (StSKO), which showed only a mild decline in cardiac contractile performance. Interestingly, LV tissue serin-16 phospholamban (PLN) phosphorylation levels & PLNs pentameric state were found to be enhanced. WT, SKO & StSKO mice were then subjected to transaortic constriction (TAC) & followed for 60 days, which fully unmasked the mutually compensatory functions of S100A1 & STRIT1. In TAC-StSKO hearts showed significantly higher decline in LV %EF, significantly increased LV end-systolic volume & LV end-systolic diameter, & significantly increased cardiac hypertrophic growth together with concordant molecular markers. TAC-SKO mice did not show any decline in STRIT1 protein upregulation, while TAC-WT hearts showed a putatively compensatory increase in the S100A1/STRIT1 protein ratio.

Conclusion: Our first results indicates that STRIT1 & S100A1 can act as compensatory molecular switches securing sufficient SERCA2a activity. As such, our study further sheds new light onto the novel concept of “molecular redundancy” to secure & protect cardiac key effector activities to cope with distinct hemodynamic stressors

Abstract P1079: S100A1-SUMO Interaction Via SUMO Interaction Motif (SIM) Of The S100A1 C-terminus Domain Is Critical For S100A1 Post-translational Protein Stability

Background & Objectives: S100A1 protein is a 10 kDa Ca 2+ sensor & abundantly expressed in cardiomyocytes (CMs) of vertebrates. Marked reduction in S100A1 level is hallmark of diseased heart, vice versa, addition of S100A1 by gene transfer rescues diseased heart contractile dysfunction. Since our understanding of the molecular circuits that may contribute to the regulation of S100A1 protein levels are still scarce, we conducted a hybrid computational structural & experimental approach to unveil underlying molecular residues & mechanisms that may control S100A1’s protein stability.

Methods & Results: We employed in silico computational prediction & molecular docking tools, respectively, to inform experimental approaches in order to characterize residues within the 94 amino acid (aa) of S100A1. A web server-based GPS-SUMO 2.0 analysis of the human S100A1 sequence unveiled a putative SUMO interacting (SIM) motif (76-VVLVA-80) within the alpha-helical C-terminus of S100A1. Restrained docking with HADDOCK predicted a molecular interaction between SUMO-1 & the SIM-lined groove of the Ca2+-bound (holo) S100A1 homodimer that presents a potential novel type of interaction mode. We then performed an in vitro S100A1-SUMO interaction assay in the presence of 1 mM Ca 2+ or 1 mM EGTA & the assay revealed calcium dependent specific S100A1-SUMO proteins interaction. Overexpression of S100A1 together with SUMO1 increases S100A1 protein abundance in CMs & COS1 cells without changing the mRNA level of S100A1. Overexpression assays in COS1 cells & CMs involving S100A1 truncation mutant lacking SIM motif (S100A1-1-74) or site directed mutagenesis deleted (DSIM) or Alanine replaced SIM of S100A1 showed that S100A1 lacking SIM motif either via truncation & deletion or Alanine substitution led to, respectively, absence of detection or massively reduced overexpression of S100A1 protein without affecting the mRNA overexpression of the mutants. The aforementioned mutants could be rescued at protein level in CMs & COS1 cells by addition of proteasome inhibitor-MG-132.

Conclusion: Here we describe a yet unrecognized post-translational molecular checkpoint for S100A1’s protein stability involving a SIM-mediated interaction between S100A1 & SUMO

P5377A SUMO1-SIM motif within the C-terminal alpha-helical domain of S100A1 prevents its proteasomal degradation in cardiomyocytes

Acknowledgement/Funding

DZHK