Paper 14: Complete Pelvic Organ Prolapse in Fibulin-5 Knockout Mice: Pregnancy Changes in Elastin and Fibulin-5 Protein in Mouse Vagina

OBJECTIVE: Elastic fibers are important in conferring elasticity, strength, and resilience to tissues. Tropoelastin, lysyl oxidase like 1 (LOXL1), and fibulin-5 (Fib5) are proteins involved in the synthesis and assembly of elastic fibers in the extracellular matrix. Recently, it was reported that mice with null mutations in the gene encoding LOXL1 develop pelvic organ prolapse. Fib5 knockout (Fib−/−) mice survive into adulthood but develop severe “elastinopathies,” including loose skin, vascular abnormalities and emphysema. In this investigation, we sought to characterize pelvic organ support in Fib5−/− mice and to determine changes in tropoelastin and Fib5 in normal mouse vagina during pregnancy. METHODS: Vaginal tissues were homogenized and the extracellular matrix protein components extracted with urea. Fib5 and tropoelastin protein contents were determined by immunoblot analysis. Samples were grouped as nulliparous nonpregnant (n=6), multiparous nonpregnant (n=4), gestation days 11–18 (n=11), in labor (n=3), postpartum <24 hours (n=14), postpartum 24–72 hours (n=15), and 1 week postpartum (n=6). ANOVA, Kruskal-Wallis ANOVA on ranks, and Bonferroni t-test were used to compare the groups, using nulliparous nonpregnant mouse vagina as controls. RESULTS: As early as 3 months of age, virginal Fib5−/− mice exhibited a genitourinary bulge of variable size surrounding the vaginal opening. Size of the perineal bulge increased significantly in older animals. Although there appeared to be no substantial change in the severity of prolapse during pregnancy, Fib5−/− mice demonstrated complete prolapse of the anterior and posterior vaginal walls at the time of parturition which could be reduced in some animals postpartum. To determine the regulation of Fib5 and tropoelastin in normal pregnancy and parturition, full-length vagina was harvested from 64 wild type mice in the nonpregnant state and at various time points during gestation, labor, and the puerperium. Compared with the nonpregnant state, tropoelastin and Fib5 contents were decreased 2.5- and 8-fold, respectively, in late gestation (P<0.001). Both proteins rebounded in the early postpartum period, but not to nonpregnant levels. After the initial postpartum rebound, there was a significant decrease in both proteins 1 week postpartum. Levels eventually returned to normal, however, because the amount of vaginal Fib5 and tropoelastin was similar in nulliparous and multiparous nonpregnant mice. CONCLUSION: Tropoelastin and Fib5 are downregulated in the mouse vagina during pregnancy but increase significantly after birth. These results, together with the phenotype of LOXL1 and Fib5 knockout mice, suggest that synthesis and assembly of elastic fibers are crucial for recovery of pelvic organ support after vaginal delivery, and that disordered elastic fiber homeostasis may contribute to pelvic organ prolapse.