Comparison of bioactivities, binding properties and intrafollicular levels of bovine follistatins.

Five isoforms of follistatin (FST) (Mr 31, 33, 35, 37, and 41  kDa) were purified from bovine follicular fluid (bFF). Comparison of their activin and heparan sulphate proteoglycan (HSP) binding properties and biopotencies in the neutralisation of activin A action in vitro revealed that all five isoforms bound activin A, but they did so with different affinities. Only the 31  kDa isoform (FST-288) bound to HSP. FST-288 also showed the greatest biopotency, and the 35 and 41  kDa isoforms were the least potent. To determine whether bovine follicle development is associated with changing intrafollicular FST and activin profiles, we analysed bFF from dominant follicles (DFs) and subordinate follicles (SF) collected at strategic times during a synchronised oestrous cycle. Total FST, activin A and activin AB were measured by immunoassay, whereas individual FST isoforms were quantified by immunoblotting. Follicle diameter was positively correlated with oestrogen:progesterone ratio (r=0.56) in bFF but negatively correlated with activin A (r=-0.34), activin AB (r=-0.80) and 'total' FST (r=-0.70) levels. Follicle diameter was positively correlated with the abundance of the 41  kDa isoform (r=0.59) but negatively correlated with the abundance of the 33 and 31  kDa isoforms (r=-0.56 and r=-0.41 respectively). Both follicle statuses (DF and SF) and cycle stage affected total FST, activin A and activin B levels, whereas follicle status, but not cycle stage, affected the abundance of the 41, 37, 33 and 31  kDa FST isoforms. Collectively, these findings indicate that intrafollicular FST isoforms, which differ in their ability to bind and neutralise activins and to associate with cell-surface proteoglycans, show divergent changes during follicle development. Enhanced FST production may play an important negative role, either directly or via the inhibition of the positive effects of activins, on follicle growth and function during follicular waves.