PARENT SESSION
OVARIAN FOLLICLE
Laurel
7:30 AM-10:00 AM

(91) DISTINCT FORMS OF FOLLISTATIN IN HUMAN SERUM AND FOLLICULAR FLUID.

Schneyer, Alan¹, Sidis, Yisrael¹, Sluss, Patrick¹, ¹ Reproductive Endocrine Unit, Boston, MA

ABSTRACT- Follistatin (FS) is an activin binding protein found in many tissues that also synthesize activin. Alternate splicing results in mRNA species coding for mature proteins of 288 (FS288) or 315 (FS315) residues. In addition, FS purified from follicular fluid was predominantly 300 or 303 residues, indicating that FS315 can be proteolytically processed at its C-terminus. While immunoassays have detected circulating FS, its concentration does not appear to depend on gonadal FS production. Thus, while we had previously demonstrated that FS in serum and human follicular fluid (hFF) is biochemically and immunologically distinct, the precise nature and source of serum FS remain unknown. To address these issues, we developed a 2-site monoclonal antibody based assay that specifically recognizes FS315. This immunoassay is calibrated against recombinant human FS315 (rhFS315) and has a 1.5 ng/ml detection limit. Human serum diluted in parallel to the rhFS315 standard, and rhFS315 was completely recoverable from serum. While FS315 was detectable in >85% of all human serum samples tested, serum levels did not reflect antral follicle development in normal women nor were they different from post-menopausal women. However, mean FS315 levels were consistent with those detected using a previously developed total FS RIA, suggesting that the vast majority of FS in serum is FS315. In contrast, no FS315 was detectable in hFF samples, including those from untreated normal women and gonadotropin-treated IVF patients, despite the fact that FS was measurable in all samples using our previously described FS SPICA which detects all forms of FS. These results demonstrate that while the majority of FS in serum is FS315, there is no detectable full-length FS315 in hFF during antral follicle development and are consistent with the concept that FS is compartmentalized within the body. Furthermore, since; 1) there is no FS315 in hFF, 2) the predominant form is longer than FS288, and 3) the major mRNA in granulosa cells is FS315, these results indicate that proteolytic processing of FS by granulosa cells, which increases the affinity of FS for cell-surface proteoglycans, is a potential mechanism whereby FS distribution is regulated. Supported by NIH grants U54HD29164 and R01DK55838.

KEY WORDS: follistatin, activin, follicle, human