Purpose: HSF4 mutations are responsible for congenital cataract formation. Dysfunction of HSF4 leads to defects in lens terminal differentiation. We aimed to study the mechanism of how HSF4 promotes organelle degradation during lens differentiation.
Methods: HSF4del42 mutant mice that developed congenital cataracts were employed. The organelle degradation and autophagic function in lens fibers were detected by immunofluorescence and Immunoblotting. Transcriptome analysis was performed to investigate the differentially expressed genes in HSF4del42 lenses, whereas luciferase report assay and ChIP assay were used to confirm the directly transcriptional regulation of ATG9a by HSF4.
Results: HSF4del42 mice displayed delayed organelle clearance and impaired autophagic degradation function in lens fibers. Activation of autophagy by rapamycin ameliorated the defects in organelle clearance in HSF4del42 lenses ex vivo and in vivo. Depletion of HSF4 attenuated autophagic flux by disrupting autophagosome biogenesis and maturation in lens epithelial cells. HSF4 directly transcriptionally activated the core autophagy protein ATG9a. Instead of the canonical ATG9a isoform, the ATG9a-X2 isoform was predominantly expressed in the lens and alleviated autophagic defects in HSF4 KO lens epithelial cells. The ATG9a-X2 protein displayed a short half-life, and rapamycin treatment restored its levels in HSF4 KO lens epithelial cells and HSF4del42 lenses.
Conclusions: Our findings demonstrate that HSF4 facilitates organelle degradation probably by transcriptionally activating autophagy during lens terminal differentiation. We first report the involvement of HSF4 in autophagy and the tissue specific splicing of ATG9a. Our study indicates that autophagy activation is a possible therapeutic strategy for HSF4-related congenital cataracts.