Discovery of potent antiosteoporotic cyclic depsipeptides with an unusual nitrile hydroxy acid from Microascus croci.

Two cyclic octadepsipeptides, microascusins A and B (1 and 2), were identified from the marine sponge-associated Microascus croci IMB19-064 co-cultivated with Escherichia coli. Their structures and conformations in solution were determined by comprehensive spectroscopic data analysis. The absolute configurations of amino and hydroxy acids were determined by the advanced Marfey's and O-Marfey's methods, respectively, as well as chiral-phase HPLC analysis. Microascusins A (1) and B (2) represent a new type of cyclic octadepsipeptides characterized by the presence of the unique hydroxy acid l-2-hydroxy-4-cyanobutyric acid. Compound 1 exhibited potent in vitro antiosteoporotic activity with dual regulation effects of promoting the osteoblast-mediated bone formation as well as inhibiting the RANKL-induced osteoclast differentiation at the concentrations of 0.1-10 nM. Further in vivo study showed that compound 1 could improve bone mineralization after dexamethasone-induced bone damage in zebrafish through up-regulating the expression of the osteoblast-specific genes Bmp2 and Oc.