Procollagen IIA mediates positive feedback control of the mouse cardiogenic transcriptional network

Cardiogenesis relies on the integrated interplay between cardiac transcription factors and signaling pathways. Here, we uncover a role for type IIA procollagen (IIA), an extracellular matrix (ECM) protein encoded by an alternatively spliced Col2a1 transcript, encoding a N-terminal cysteine-rich domain, as a critical regulator in a cardiac gene regulatory feedback loop. The cysteine-rich domain of IIA protein was previously reported to interact with bone morphogenetic proteins (BMPs) and transforming growth factors-beta (TGFβ) in in vitro binding assays and acts as a BMP antagonist in amphibian embryo assays. We show that the Col2a1 gene in mice is activated in the developing heart by core cardiogenic factors (NKX2-5, GATA4, MEF2, and SRF) via cis-regulatory enhancer elements. IIA loss (ΔIIA) in mice results in depletion of Isl1- and Nkx2-5-expressing progenitors, causing outflow tract defects resembling disrupted BMP/TGFβ-SMAD signaling, alongside reduced nuclear pSMAD1/5/8 in cardiac tissues. Compound +/ΔIIA; Smad4+/- mutants exhibit aggravated malformations. IIA enhances BMP-responsive reporter activity in cells in transactivation assays. We propose that IIA supports a positive functional role on SMAD4-dependent signaling, fine-tuning BMP/TGFβ signaling, thereby regulating GATA4 and NKX2-5 activity during second heart field progenitor specification. These findings position IIA procollagen as a key ECM component that integrates BMP/TGFβ signaling with cardiac transcription factors such as NKX2-5, revealing a feedback loop essential for cardiogenesis. Given its role in cardiac development, IIA emerges as a potential congenital heart disease risk factor.