Sbk2, a Newly Discovered Atrium-Enriched Regulator of Sarcomere Integrity

Abstract

Background: Heart development relies on tight spatiotemporal control of cardiac gene expression. Genes involved in this intricate process have been identified using animals and pluripotent stem cell-based models of cardio(myo)genesis. Recently, the repertoire of cardiomyocyte differentiation models has been expanded with iAM-1, a monoclonal line of conditionally immortalized neonatal rat atrial myocytes (NRAMs), which allows toggling between proliferative and differentiated (ie, excitable and contractile) phenotypes in a synchronized and homogenous manner. Methods: In this study, the unique properties of conditionally immortalized NRAMs (iAMs) were exploited to identify and characterize (lowly expressed) genes with an as-of-yet uncharacterized role in cardiomyocyte differentiation. Results: Transcriptome analysis of iAM-1 cells at different stages during one cycle of differentiation and subsequent dedifferentiation identified ≈13 000 transcripts, of which the dynamic changes in expression upon cardiomyogenic differentiation mostly opposed those during dedifferentiation. Among the genes whose expression increased during differentiation and decreased during dedifferentiation were many with known (lineage-specific) functions in cardiac muscle formation. Filtering for cardiac-enriched low-abundance transcripts, identified multiple genes with an uncharacterized role during cardio(myo)genesis including Sbk2 (SH3 domain binding kinase family member 2). Sbk2 encodes an evolutionarily conserved putative serine/threonine protein kinase, whose expression is strongly up- and downregulated during iAM-1 cell differentiation and dedifferentiation, respectively. In neonatal and adult rats, the protein is muscle-specific, highly atrium-enriched, and localized around the A-band of cardiac sarcomeres. Knockdown of Sbk2 expression caused loss of sarcomeric organization in NRAMs, iAMs and their human counterparts, consistent with a decrease in sarcomeric gene expression as evinced by transcriptome and proteome analyses. Interestingly, co-immunoprecipitation using Sbk2 as bait identified possible interaction partners with diverse cellular functions (translation, intracellular trafficking, cytoskeletal organization, chromatin modification, sarcomere formation). Conclusions: iAM-1 cells are a relevant and suitable model to identify (lowly expressed) genes with a hitherto unidentified role in cardiomyocyte differentiation as exemplified by Sbk2: a regulator of atrial sarcomerogenesis.

Transient induction of actin cytoskeletal remodeling associated with dedifferentiation, proliferation, and redifferentiation stimulates cardiac regeneration

“…To show the full cycle of actin depolymerization and polymerization, NRVMs were first transfected with siRNA- Dmd for 0–48 h and then transfected with saRNA- Dmd for 48–96 h 18 . As expected, siRNA- Dmd triggered cardiomyocyte differentiation, as evidenced by the remodeling from a cross-striated pattern to a highly diffuse pattern for the sarcomeric Z-line marker Actn2 ( α -actinin) 19 , accompanied by the gradual increase in Ki67-positive cardiomyocytes, G-actin intensity, and percentage of disorganized F-actin. However, these changes were reversed by saRNA- Dmd ( Fig.…”

Building Haplotype‐Resolved 3D Genome Maps of Chicken Skeletal Muscle

“…Consistent with the boundary loss at E15, CD82 had more local interactions and higher expression at E15 than at D1 or D30 (Figure 3F ), a pattern likely related to its more active role in myogenesis before hatching. In contrast, CMBL (a key regulator of myoblast differentiation) [ 30 ] and SBK2 (essential for sarcomere integrity) [ 31 ] were both embedded in a TAD boundary at E15 and D1, and had more local interactions and higher expression levels at D30, at which point these genes were detected outside of the TADs (Figure 3G ), which aligns well with their roles in rapid growth and functional maintenance of SMTs in the post‐hatching D30 stage. These results highlight the possible role of TADs in shifting patterns of transcriptional regulation through modulation of local chromatin interactions during SMT development and growth.…”

Tbx5maintains atrial identity by regulating an atrial enhancer network

“…More broadly, we observed dysregulation of many atrial identity genes, suggesting that loss of atrial identity and altered expression of these signature atrial genes may also contribute to development of AF. Among signature atrial genes that were most strongly regulated by Tbx5 was Sbk2, a little-studied gene that was recently shown to be essential for aCM sarcomere integrity 33 .…”

“…4b). Sbk2, a gene recently shown to be essential for atrial sarcomere integrity 33 , was also more highly expressed in control compared to Tbx5 AKO aCMs, possibly contributing to the observed disorganization of Tbx5 AKO sarcomeres (Supp. Fig.…”