Distinct intracellular spatiotemporal expression of Calmodulin genes underlies functional diversity of Calmodulin-dependent signalling in cardiac myocytes

AIMS: This study aims to resolve the mechanisms underlying Calmodulin (CaM)'s signalling diversity by investigating whether the three CaM genes-Calm1, Calm2, and Calm3-play distinct or redundant roles in cardiac myocytes, focusing on their spatial mRNA localization and interactions with key targets. METHODS AND RESULTS: We utilized single-molecule mRNA detection and three-dimensional imaging to map the spatial distribution of Calm1, Calm2, and Calm3 mRNAs within ventricular myocytes. These mRNAs were found to be consistently positioned within specific cellular zones, overlapping with their target mRNAs and forming region-specific transcript conjunctions. This spatial organization aligns with two distinct protein synthesis pathways: centralized synthesis near the nucleus for proteins such as Cx43 and localized synthesis in more peripheral cytosolic areas for proteins like RyR2. Ablation of Calm1 triggered compensatory increases in Calm2 and Calm3; however, this compensation was insufficient to restore normal CaM transcript distribution, leading to disrupted Ca²⁺ handling. In the context of hypertrophic heart failure (HF), the distribution and spatial interactions of CaM transcripts, while potentially adaptive to support myocyte growth, become disrupted, leading to disorganized CaM signalling. CONCLUSION: Our findings reveal that Calm1, Calm2, and Calm3 fulfil distinct, non-redundant roles in cardiac myocytes through their spatially regulated mRNA localization (spatiotemporal coding). This precise spatial control of mRNA localization is critical for region-specific CaM signalling and is disrupted in hypertrophic HF, contributing to pathological remodelling.