The Tudor-domain protein TDRD7, mutated in congenital cataract, controls the heat shock protein HSPB1 (HSP27) and lens fiber cell morphology

Mutations of the RNA-granule component TDRD7 (OMIM: 611258) cause pediatric cataract. We applied an integrated approach to uncover the molecular pathology of cataract in Tdrd7−/− mice. Early postnatal Tdrd7−/− animals precipitously develop cataract suggesting a global-level breakdown/mis-regulation of key cellular processes. High-throughput RNA-sequencing integrated with iSyTE-bioinformatics analysis identified the molecular chaperone and cytoskeletal-modulator, HSPB1, among high-priority down-regulated candidates in Tdrd7−/− lens. A protein two-dimensional fluorescence difference in-gel electrophoresis-coupled mass spectrometry screen also identified HSPB1 down-regulation, offering independent support for its importance to Tdrd7−/− cataractogenesis. Lens fiber cells normally undergo nuclear degradation for transparency, posing a challenge: how is their cell morphology, also critical for transparency, controlled post-nuclear degradation? HSPB1 functions in cytoskeletal maintenance and its reduction in Tdrd7−/− lens precedes cataract, suggesting cytoskeletal defects may contribute to Tdrd7−/− cataract. In agreement, scanning electron microscopy revealed abnormal fiber cell morphology in Tdrd7−/− lenses. Further, abnormal phalloidin and WGA staining of Tdrd7−/− fiber cells, particularly those exhibiting nuclear degradation, reveal distinct regulatory mechanisms control F-actin cytoskeletal and/or membrane maintenance in post-organelle degradation maturation stage fiber cells. Indeed, RNA-immunoprecipitation identified Hspb1 mRNA in wild-type lens lysate TDRD7-pulldowns, and single-molecule RNA-imaging showed co-localization of TDRD7 protein with cytoplasmic Hspb1 mRNA in differentiating fiber cells, suggesting that TDRD7-ribonucleoprotein complexes may be involved in optimal build-up of key factors. Finally, Hspb1-knockdown in Xenopus causes eye/lens defects. Together, these data uncover TDRD7’s novel upstream role in elevation of stress-responsive chaperones for cytoskeletal maintenance in post-nuclear degradation lens fiber cells, perturbation of which causes early-onset cataracts.