Background: Heart failure remains a major global health burden, driven largely by pathological cardiac hypertrophy. Mitochondrial dysfunction, particularly impaired mitochondrial complex I activity, is central to the disease progression, yet its regulatory mechanisms are poorly understood. Cross-species transcriptomic screening and UK Biobank analyses identified SBK2 (Src homology 3 domain-binding kinase 2) as a conserved, cardiac-enriched kinase potentially linked to heart failure risk. We hypothesized that SBK2 regulates cardiac hypertrophy by modulating mitochondrial complex I function.
Methods: Cross-species conserved genes were identified from Gene Expression Omnibus (GEO) data sets, and variants within ±10 kb of SBK2, ADAMTSL2 (ADAMTS-like protein 2), LOX (lysyl oxidase), and SPP1 (secreted phosphoprotein 1) were assessed for heart failure association in the UK Biobank cohort. SBK2 expression was examined in experimental models and publicly available human hypertrophic cardiomyopathy data sets, and its function was investigated through loss- and gain-of-function studies in cardiomyocytes and mice. The substrates and interacting partners of SBK2 were identified by proteomic and interactome analyses and validated by in vitro kinase assays and coimmunoprecipitation. Mitochondrial protein import and respiratory supercomplex assembly were assessed by biochemical fractionation and blue native PAGE.
Results: SBK2 expression was reduced in hypertrophic hearts and primary neonatal rat ventricular myocytes. Cardiomyocyte-specific overexpression of SBK2 attenuated hypertrophy and fibrosis, improved systolic function, and suppressed maladaptive gene expression. Conversely, SBK2 knockdown exacerbated these phenotypes. Mechanistically, SBK2 directly bound and phosphorylated NDUFV1 (NADH: ubiquinone oxidoreductase core subunit V1) at serine 251. This modification enhanced the interaction between NDUFV1 and the cytosolic chaperone HSPA1A (heat shock protein family A [Hsp70] member 1A) and facilitated TOM70 (translocase of outer mitochondrial membrane 70)-dependent mitochondrial import. Increased mitochondrial NDUFV1 promoted complex I activity, respiratory supercomplex assembly, oxidative phosphorylation, mitochondrial fusion, and redox homeostasis. Pharmacological inhibition of complex I or NDUFV1 silencing abolished SBK2-mediated protection. Moreover, a phospho-deficient NDUFV1 mutant (S251A) failed to rescue hypertrophic phenotypes in SBK2-deficient cardiomyocytes.
Conclusions: SBK2 is an upstream kinase that couples cytosolic signaling to mitochondrial protein import by phosphorylating NDUFV1, thereby sustaining complex I integrity and mitochondrial function to restrain pathological cardiac hypertrophy. These findings uncover a previously unrecognized SBK2-NDUFV1 signaling axis linking kinase signaling to mitochondrial proteostasis and identify a potential therapeutic target for heart failure.
Keywords: electron transport complex I; heart failure; mitochondria; phosphorylation; protein transport.