Tissue Remodeling in the Female Reproductive Tract—A Complex Process Becomes More Complex: The Role of Hox Genes

Homeobox (Hox/HOX) genes are conserved genes that encode master transcription factors regulating genes that in turn regulate other genes in large networks. They play important roles in organogenesis and morphogenesis during mammalian embryonic growth [1–5]. In mammals, Hox/HOX genes are contained in four clusters: a, b, c, and d. Hox/HOX genes are also expressed in adult cells, where they are associated with extracellular matrix (ECM)-dependent alterations in gene expression and cell phenotype by promoting cell proliferation, cell adhesion, apoptosis, and cell migration [2–4]. These processes themselves depend on the highly dynamic ECM remodeling and rapid transient cell-cell and cell-matrix interactions [6]. Furthermore, expression of specific Hox/ HOX genes is influenced by the ECM-cell interactions [6]. In addition to the ECM regulation of these genes, components of the ECM are also targets of Hox/HOX gene regulation [7]. This phenomenon has been called dynamic reciprocity [6]. In mice, Hoxa9, Hoxa10, Hoxa11, and Hoxa13 are expressed in the female reproductive tract [2], and HOXA11 has been shown to promote fibroblast proliferation in the uterosacral ligaments of women [6]. In this current issue of Biology of Reproduction, Yan Ma and colleagues report the development of a mouse model of in vivo Hoxa11 silencing within the female genital tract and abrogate gene effects on interstitial collagens and the gelatinase class of matrix metalloproteinases (MMPs) [7]. The uterosacral ligaments, supporting structures for the uterus, are condensed bands of tissue formed from the endopelvic fascia attached to the posterior uterine cervix and the anterior face of the sacrum. Composed of the ECM proteins collagen, elastin, and proteoglycans, as well as smooth muscle and nerves, these ligaments are capable of undergoing tissue remodeling [8]. Studies report differentially expressed ECM proteins in uterosacral ligaments and vagina from women with pelvic organ prolapse (POP) [8–14]. POP of uterus, bladder, or rectum occurs in 41% of women aged 50–79, and uterus-only prolapse is found in 14.2% of this age group [14]. Some POP risk factors include childbirth trauma and genetic susceptibility [15, 16]. Ma and colleagues report that the knockdown of Hoxa11 in mice results in alterations of the mRNA and protein expression of types I and III interstitial collagen [7]. The ECM itself plays a vital role in the extensive remodeling of insect and amphibian metamorphosis, mammalian embryo development, the mammary gland, and the female reproductive tract and other remodeling in other organ systems [17–19]. The uterus, vagina, and supporting tissues undergo massive growth and remodeling throughout gestation, parturition, and the postpartum period. No other adult organ system undergoes such rapid growth and involution due to cellular hyperplasia, hypertrophy, and apoptosis as well as synthesis and degradation of the ECM [20, 21]. The nonpregnant human uterus weighs 50–60 g and increases 11-fold in weight during gestation [22]. There is a 7-fold increase in collagen and a 5–6fold increase in elastin during this time [22]. By 8 days postpartum, the human uterus has undergone an involution of 75% toward baseline [22]. The ECM not only has a structural role in tissues, it creates an environment that regulates and modulates a large, highly diverse number of cellular functions involving cell proliferation, adhesion, migration, differentiation, cell death, and tissue remodeling [23]. In humans there are 30 types of collagens and 23 MMPs; 19 of the disintegrin and MMPs with thrombospondin motifs (ADAMTS) family degrade ECM and other proteins [23]. In humans, numerous specific inhibitors, such as the four tissue inhibitors of MMPs (TIMPs) and a2macroglobulin, are players in tissue remodeling. Thus, tissue remodeling involves both cellular turnover and ECM synthesis and degradation; it is an extremely complex process [19, 23– 26]. Mechanical forces, growth factors, cytokines, hormones, MMPs, and ADAMTS are all involved in the numerous cell signals that affect this important biological process [23–26]. MMPs, zinc-containing multiple-domain enzymes, target a number of ECM proteins and other proteins sequestered in the ECM and are secreted as inactivate enzymes. Individual MMPs have prominent roles in tissues that do not always directly involve degradation of the ECM. For instance, MMP3 activates MMP1. MMP1 preferentially cleaves collagen III into 3/4 and 1/4 fragments, whereas MMP2 and MMP9 degrade denatured collagen or gelatin, acting as collaborators in ECM degradation by digesting collagenase-clipped collagen [23–26]. MMP2 is able to digest native collagen types I, II, and III. However, its collagenolytic activity is weaker than that of other collagenases. Interestingly, pro-MMP2 is recruited to the cell surface and activated by the membrane-bound MT-MMPs and therefore probably has very local collagenase activity [24]. The study by Ma et al. [7] utilizes gelatin zymography to demonstrate that pro-MMP2 and pro-MMP9 and active MMP2 are increased in Hoxa11 KD mice. In gelatin zymography, the active forms of the MMPs are separated from their latent forms by molecular mass, as the active forms of the enzymes do not possess the 10kDa inhibitory N-terminal domain and are therefore smaller Correspondence: Phyllis C. Leppert, Duke University SOM, Obstetrics and Gynecology, DUMC 103206, Durham, NC 27710. E-mail: phyllis.leppert@duke.edu