ALDH Enzyme Expression Is Independent of the Spermatogenic Cycle, and Their Inhibition Causes Misregulation of Murine Spermatogenic Processes1

Abstract

Perturbations in the vitamin A metabolism pathway could be a significant cause of male infertility, as well as a target toward the development of a male contraceptive, necessitating the need for a better understanding of how testicular retinoic acid (RA) concentrations are regulated. Quantitative analyses have recently demonstrated that RA is present in a pulsatile manner along testis tubules. However, it is unclear if the aldehyde dehydrogenase (ALDH) enzymes, which are responsible for RA synthesis, contribute to the regulation of these RA concentration gradients. Previous studies have alluded to fluctuations in ALDH enzymes across the spermatogenic cycle, but these inferences have been based primarily on qualitative transcript localization experiments. Here, we show via various quantitative methods that the three well-known ALDH enzymes (ALDH1A1, ALDH1A2, and ALDH1A3), and an ALDH enzyme previously unreported in the murine testis (ALDH8A1), are not expressed in a stage-specific manner in the adult testis, but do fluctuate throughout juvenile development in perfect agreement with the first appearance of each advancing germ cell type. We also show, via treatments with a known ALDH inhibitor, that lowered testicular RA levels result in an increase in blood-testis barrier permeability, meiotic recombination, and meiotic defects. Taken together, these data further our understanding of the complex regulatory actions of RA on various spermatogenic events and, in contrast with previous studies, also suggest that the ALDH enzymes are not responsible for regulating the recently measured RA pulse.

Roles of Retinoic Acid in Germ Cell Differentiation

“…Keeping in mind the possible artefactual effects of BMS-204493 mentioned above, these data may indicate a role for endogenous ATRA in the stable commitment to the meiotic cell cycle in the testis. A recent report further suggests that ATRA may also be required for meiotic recombination (Kent et al, 2016).…”

“…The expression patterns of Aldh1a1 to Aldha3 genes has been extensively investigated: RALDH1 and RALDH2 are the main ATRA synthesizing enzymes in Sertoli cells and GC respectively, while RALDH3 is not detected in the SE (Vernet et al, 2006a, Bowles et al, 2009Sugimoto, Nabeshima, & Yoshida, 2012;Kent et al, 2016). In Sertoli cells of the adult testis, Aldh1a1 transcripts are expressed in a SE cycle-dependent manner with the highest levels being found at stages II-VII (Fig.…”

Differential RA responsiveness directs formation of functionally distinct spermatogonial populations at the initiation of spermatogenesis in the mouse

“…RA levels are generally tightly controlled by the combined action of enzymes regulating its synthesis (retinol dehydrogenases and aldehyde dehydrogenases) and catabolism (CYP26 enzymes) (reviewed by Mark et al, 2015;Griswold, 2016). RA is synthesized in target tissues by converting circulating or stored retinol via a two-step reaction, and the proposed source of RA in the developing testis is Sertoli cells; Sertoli cell expression of the synthesizing enzymes aldehyde dehydrogenase family 1 (ALDH1A1-3) and retinol dehydrogenase 10 (RDH10) is essential for initiating the first wave of spermatogenesis (Kent et al, 2016;Raverdeau et al, 2012;Tong et al, 2013). From the site of biosynthesis, RA then diffuses and can act either through binding its cognate receptors or by being converted into inert polar metabolites (Niederreither et al, 2002) by the intracellular cytochrome P450 enzymes CYP26A1, CYP26B1 and CYP26C1 (reviewed by Nelson et al, 2013).…”

Testicular Architecture Is Critical for Mediation of Retinoic Acid Responsiveness by Undifferentiated Spermatogonial Subtypes in the Mouse

“…Lastly, the hierarchical model proposed by Ikami et al. eliminates an influence from the soma in modulating the RA response in spermatogonia, which is at odds with the well-established dynamic role that multiple testis somatic cell populations play in the biosynthesis and clearance of RA ( Hogarth et al., 2015 , Kent et al., 2015 , Tong et al., 2013 ).…”