Spns1-dependent endocardial lysosomal function drives valve morphogenesis through Notch1-signaling

Autophagy-lysosomal degradation is an evolutionarily conserved process key to cellular homeostasis, differentiation, and stress survival, which is particularly important for the cardiovascular system. Furthermore, experimental and clinical observations indicate it affects cardiac morphogenesis, including valve development. However, the cell-specificity and functional role of autophagic processes during heart development remain unclear. Here, we introduce novel zebrafish models to visualize autophagic vesicles in vivo and follow their temporal and cellular localization in the larval heart. We observed a significant accumulation of lysosomal vesicles in the developing atrioventricular and bulboventricular regions and their respective valves. Next, we addressed the role of lysosomal degradation using a Spinster homolog 1 ( spns1 ) mutant. spns1 mutants displayed morphological and functional cardiac defects, including abnormal endocardial organization, impaired valve formation and retrograde blood flow. Single-nuclear transcriptome analysis revealed endocardial-specific differences in the expression of lysosome-related genes and alterations of notch1- signalling in the mutant. Endocardial-specific overexpression of spns1 and notch1 rescued features of valve formation and function. Altogether, our study reveals a cell-autonomous role of lysosomal processing during cardiac valve formation upstream of notch1- signalling. ### Competing Interest Statement The authors have declared no competing interest.