Vps34/PIK3C3 deletion in thyroid impairs thyroid hormonogenesis and autophagic flux

Abstract

BACKGROUNDThe production of thyroid hormones (T3, T4) depends on thyroid organization in follicles, lined by a monolayer of thyrocytes with strict apico-basal polarity. Polarization supports vectorial transport of thyroglobulin for storage into, and recapture from, the colloid. It also allows selective addressing of channels, transporters, pumps and enzymes to their appropriate basolateral (NIS and Na+/K+-ATPase) or apical membrane domain (pendrin, anoctamin, DUOX2, DUOXA2 and TPO). How these actors of T3/T4 synthesis reach their final destination remains poorly understood. Vps34/PIK3C3 is now recognized as a main component in the general control of vesicular trafficking and of cell homeostasis via autophagy. We recently reported that conditional Vps34 inactivation in kidney proximal tubular cells by Pax8-driven excision prevents normal addressing of apical membrane proteins and causes abortive macroautophagy. METHODSVps34 was inactivated using a Pax8-driven Cre recombinase system. The impact of Vps34 inactivation in thyrocytes was analyzed by histological, immunolocalization and mRNA expression profiling. Thyroid hormone synthesis was assayed by 125I injection and by serum plasma analysis. RESULTSVps34cKO mice were born at the expected Mendelian ratio and showed normal growth until postnatal day 14, then stopped growing and died at around 1 month of age. We therefore analyzed thyroid Vps34cKO before postnatal day 14. We found that loss of Vps34 in thyrocytes causes: (i) disorganization of thyroid parenchyma with abnormal thyrocyte and follicular shape and reduced PAS+ colloidal spaces; (ii) impaired 125I organification at comparable uptake and frequent occurrence of follicles with luminal thyroglobulin but non-detectable T4-bearing thyroglobulin; (iii) severe non-compensated hypothyroidism with extremely low T4 levels (<0.25 {+/-} 1.5 g/dL) and huge TSH plasma levels (19,300 {+/-} 10,500 mU/L); (iv) intense signal in thyrocytes for the lysosomal membrane marker, LAMP-1, as well as thyroglobulin and the autophagy marker, p62, indicating defective proteolysis. CONCLUSIONSWe conclude that Vps34 is crucial for thyroid hormonogenesis, at least by controlling delivery of apical actors responsible for biogenesis of thyroid hormones on Tg as well as defective proteolytic T3/T4 excision in lysosomes.