Abstract Body: Background
Leucine-Rich Repeat Containing 14B (LRRC14B) is a striated muscle-specific protein highly enriched in the heart. LRRC14B has been implicated in heart failure (HF), a condition affecting 6.7 million adults in the U.S. Yet, its function remains completely unknown. Thus, filling this knowledge gap will lay a conceptual foundation for improving our understanding of its role in HF.

Aim
To define the functional and mechanistic role of LRRC14B in the heart.

Methods & Results
To investigate the function of LRRC14B, we generated Lrrc14b knockout (Lrrc14b-KO) mice (n = 14; 7 males, 7 females) with wild-type littermates as controls. By six months, echocardiography revealed systolic dysfunction in Lrrc14b-KO mice, with reduced ejection fraction (males: P < 0.0001; females: P = 0.001) and fractional shortening (males: P < 0.0001; females: P = 0.0008). Histological analysis showed no sizeable changes in morphology and fibrosis. Notably, AAV-mediated overexpression of Lrrc14b in Lrrc14b-KO mice rescues their systolic function. These findings indicate that Lrrc14b is required for normal post-natal heart function.

Mechanistically, all LRRC proteins contain LRR motifs, which are well-established mediators of protein- protein interactions. Thus, LRRC14B is likely to exert its effects through similar interactions. BioGRID screening identified LRRC14B’s interactions with BAG3 and HSP70—key regulators of chaperone- assisted selective autophagy, a pathway critical for sarcomeric protein quality control. Co- immunoprecipitation (Co-IP) and immunoblot confirmed these interactions, and protein levels of Bag3 and Hsp70 were reduced in Lrrc14b-KO hearts.

Additionally, prior studies show that LRRC14B correlates with Nebulette (NEBL) and Tropomyosin 3 (TPM3), sarcomeric proteins that regulate contractility. Hence, we hypothesized that LRRC14B stabilizes the BAG3-HSP70 complex to regulate their quality control. Co-IP and immunoblot confirmed Lrrc14b interacts with Nebl and Tpm3.

Conclusion
These findings demonstrate LRRC14B is required for postnatal cardiac function, likely by stabilizing autophagy-mediated sarcomeric protein quality control. Although further studies are needed, LRRC14B represents a novel potential target for HF research.