CYP26 Enzymes Are Necessary Within the Postnatal Seminiferous Epithelium for Normal Murine Spermatogenesis1

Abstract

The active metabolite of vitamin A, retinoic acid (RA), is known to be essential for spermatogenesis. Changes to RA levels within the seminiferous epithelium can alter the development of male germ cells, including blocking their differentiation completely. Excess RA has been shown to cause germ cell death in both neonatal and adult animals, yet the cells capable of degrading RA within the testis have yet to be investigated. One previous study alluded to a requirement for one of the RA degrading enzymes, CYP26B1, in Sertoli cells but no data exist to determine whether germ cells possess the ability to degrade RA. To bridge this gap, the roles of CYP26A1 and CYP26B1 within the seminiferous epithelium were investigated by creating single and dual conditional knockouts of these enzymes in either Sertoli or germ cells. Analysis of these knockout models revealed that deletion of both Cyp26a1 and Cyp26b1 in either cell type resulted in increased vacuolization within the seminiferous tubules, delayed spermatid release, and an increase in the number of STRA8-positive spermatogonia, but spermatozoa were still produced and the animals were found to be fertile. However, elimination of CYP26B1 activity within both germ and Sertoli cells resulted in severe male subfertility, with a loss of advanced germ cells from the seminiferous epithelium. These data indicate that CYP26 activity within either Sertoli or germ cells is essential for the normal progression of spermatogenesis and that its loss can result in reduced male fertility.

Undifferentiated spermatogonia regulateCyp26b1expression through NOTCH signaling and drive germ cell differentiation

“…Because both an excess and lack of RA are detrimental for most tissues, regulation of its concentration by CYP26 enzymes needs to be appropriately controlled (56)(57)(58)(59)(60). In the seminiferous epithelium, the major CYP26 enzyme that hydrolyzes RA is CYP26B1 (18,32). However, despite its importance for RA degradation in the testis, the molecular control of spatiotemporal CYP26B1 expression is still poorly understood.…”

“…In the adult testis, CYP26B1 is produced by peritubular myoid cells (68) and at a lower level by Sertoli cells and germ cells. Elimination of its activity within both germ cells and Sertoli cells induces loss of germ cells and severe subfertility, demonstrating a clear function for this enzyme in the adult seminiferous epithelium (32). Although the question of which cells produce RA in the adult seminiferous epithelium is still debated, it has been recently demonstrated that Sertoli cells produce RA necessary for the transition between A undiff and A diff spermatogonia, whereas midpachytene spermatocytes produce the RA necessary for induction of the meiotic prophase, spermatid elongation, Figure 6.…”

“…8B). Lastly, it is important to note that SSCs and their direct progeny also express some levels of CYP26B1 to possibly prevent early differentiation (32), which might counteract the effects of NOTCH signaling on RA availability in these cells. Further understanding of how Cyp26b1 expression is regulated in subsets of germ cells across the stages of the seminiferous epithelium is therefore an important future endeavor.…”

“…As the availability of RA within the stem cell niche increases after birth, CYP26B1 in Sertoli cells must be down-regulated to allow spermatogonial differentiation and the first wave of spermatogenesis to take place. Further, low levels of CYP26B1 are still expressed and functionally relevant in adult Sertoli cells and must be modulated to allow the transition from A undiff to A 1 spermatogonia at stage VIII of the seminiferous epithelium (31,32). However, the mechanisms that modulate CYP26B1 enzyme production in the postnatal testis are poorly understood.…”

Mechanisms Regulating Spermatogonial Differentiation

“…Independent of the nuclear RAR/RXR response, it appears that a primary consequence of RA stimulation of progenitor spermatogonia is to induce translation of mRNAs encoding the KIT tyrosine kinase and STRA8, via a mechanism involving P13K/AKT/mTORC1 signaling (Busada et al 2015a, b). Recent conditional mutants of CYP26A1 and CYP26B1 within either Sertoli cells (Amh-Cre) or differentiating spermatogonia (Stra8-iCre) suggest that neither enzyme is essential for spermatogenesis, since these animals were fertile (Hogarth et al 2015b). It is not clear why only some undifferentiated spermatogonia respond to RA, although it is possible that expression of CYP26A1 and CYP26B1 enzymes, which catalyze the degradation of RA, may restrict RA action to subpopulations of progenitor spermatogonia.…”