Sulfation of glycosaminoglycans depends on catalytic activity of a lithium-inhibited phosphatase

Golgi-resident bisphosphate nucleotidase 2 (BPNT2) is a member of a family of magnesium-dependent/lithium-inhibited phosphatases that share a three-dimensional structural motif that directly coordinates metal binding to effect phosphate hydrolysis. BPNT2 is responsible for the breakdown of 3’-phosphoadenosine-5’-phosphate (PAP), a by-product of glycosaminoglycan (GAG) sulfation. Disruption of BPNT2 in mice leads to skeletal abnormalities due to impaired GAG sulfation, especially chondroitin-4-sulfation. Mutations in BPNT2 have also been found to underlie a chondrodysplastic disorder in humans. The precise mechanism by which loss of BPNT2 impairs sulfation remains unclear. Here, we utilize an in vitro approach using mouse embryonic fibroblasts (MEFs) to test the hypothesis that catalytic activity of BPNT2 is required for GAG sulfation. We show that a catalytic-dead Bpnt2 construct (D108A) does not rescue impairments in intracellular or secreted sulfated GAG, including decreased chondroitin-4-sulfate, present in Bpnt2-knockout MEFs. We also demonstrate that missense mutations in Bpnt2 which are adjacent to the catalytic site (and known to cause chondrodysplasia in humans) recapitulate defects in overall GAG sulfation and chondroitin-4-sulfation in MEF cultures. We further show that treatment of MEFs with lithium inhibits GAG sulfation, and that this effect depends on the presence of BPNT2. This work demonstrates that the catalytic activity of an enzyme potently inhibited by lithium can modulate GAG sulfation and therefore extracellular matrix composition, revealing new insights into lithium pharmacology and the pathophysiology of psychiatric disorders responsive to lithium.