TMEM165 a new player in proteoglycan synthesis: loss of TMEM165 impairs elongation of chondroitin- and heparan-sulfate glycosaminoglycan chains of proteoglycans and triggers early chondrocyte differentiation and hypertrophy.

TMEM165 deficiency leads to skeletal disorder characterized by major skeletal dysplasia and pronounced dwarfism. However, the molecular mechanisms involved have not been fully understood. Here, we uncover that TMEM165 deficiency impairs the synthesis of proteoglycans by producing a blockage in the elongation of chondroitin-and heparan-sulfate glycosaminoglycan chains leading to the synthesis of proteoglycans with shorter glycosaminoglycan chains. We demonstrated that the blockage in elongation of glycosaminoglycan chains is not due to defect in the Golgi elongating enzymes but rather to availability of the co-factor Mn2+. Supplementation of cell with Mn2+ rescue the elongation process, confirming a role of TMEM165 in Mn2+ Golgi homeostasis. Additionally, we showed that TMEM165 deficiency functionally impairs TGFß and BMP signaling pathways in chondrocytes and in fibroblast cells of TMEM165 deficient patients. Finally, we found that loss of TMEM165 impairs chondrogenic differentiation by accelerating the timing of Ihh expression and promoting early chondrocyte maturation and hypertrophy. Collectively, our results indicate that TMEM165 plays an important role in proteoglycan synthesis and underline the critical role of glycosaminoglycan chains structure in the regulation of chondrogenesis. Our data also suggest that Mn2+ supplementation may be a promising therapeutic strategy in the treatment of TMEM165 deficient patients.

Neurochemistry in shiverer mouse depicted on MR spectroscopy.

PURPOSE: To evaluate the neurochemical changes associated with hypomyelination, especially to clarify whether increased total N-acetylaspartate (tNAA) with decreased choline (Cho) observed in the thalamus of msd mice with the plp1 mutation is a common finding for hypomyelinating disorders. MATERIALS AND METHODS: We performed magnetic resonance imaging (MRI) and proton MR spectroscopy ((1) H-MRS) of the thalamus and cortex of postnatal 12-week shiverer mice devoid of myelin basic protein (mbp), heterozygous and wild-type mice with a 7.0T magnet. Luxol Fast Blue staining and immunohistochemical analysis with anti-Mbp, Gfap, Olig2, and NeuN antibodies were also performed. RESULTS: In the thalamus, decreased Cho and normal tNAA were observed in shiverer mice. In the cortex, tNAA, Cho, and glutamate were decreased in shiverer mice. Histological and immunohistochemical analysis of shiverer mice brains revealed hypomyelination in the thalamus, white matter, and cortex; astrogliosis and an increased number of total oligodendrocytes in the white matter; and a decreased number of neurons in the cortex. CONCLUSION: The reduction of Cho on (1) H-MRS might be a common marker for hypomyelinating disorders. A normal tNAA level in the thalamus of shiverer mice might be explained by the presence of mature oligodendrocytes, which enable neuron-to-oligodendrocyte NAA transport or NAA catabolism.