Nitric Oxide and Cyclic Nucleotides: Their Roles in Junction Dynamics and Spermatogenesis

[Oxidative Medicine and Cellular Longevity 1:1, 25-32; October/November/December 2008]; ©2008 Landes Bioscience Review Nitric oxide and cyclic nucleotides Their roles in junction dynamics and spermatogenesis Nikki P.Y. Lee1,* and C. Yan Cheng2,* 1Departments of Medicine/Surgery; University of Hong Kong; Queen Mary Hospital; Hong Kong China; 2Population Council; New York, New York USA Key words: spermatogenesis, nitric oxide, nitric oxide synthase, integral membrane proteins, testes Spermatogenesis is a highly complicated process in which functional spermatozoa (haploid, 1n) are generated from primi‑ tive mitotic spermatogonia (diploid, 2n). This process involves the differentiation and transformation of several types of germ cells as spermatocytes and spermatids undergo meiosis and differentiation. Due to its sophistication and complexity, testis possesses intrinsic mechanisms to modulate and regulate different stages of germ cell development under the intimate and indirect cooperation with Sertoli and Leydig cells, respectively. Furthermore, developing germ cells must translocate from the basal to the apical (adluminal) compartment of the seminiferous epithelium. Thus, extensive junc‑ tion restructuring must occur to assist germ cell movement. Within the seminiferous tubules, three principal types of junctions are found namely anchoring junctions, tight junctions, and gap junc‑ tions. Other less studied junctions are desmosome‑like junctions and hemidesmosome junctions. With these varieties of junction types, testes are using different regulators to monitor junction turn‑ over. Among the uncountable junction modulators, nitric oxide (NO) is a prominent candidate due to its versatility and extensive downstream network. NO is synthesized by nitric oxide synthase (NOS). Three traditional NOS, specified as endothelial NOS (eNOS), inducible NOS (iNOS), and neuronal NOS (nNOS), and one testis‑specific nNOS (TnNOS) are found in the testis. For these, eNOS and iNOS were recently shown to have putative junc‑ tion regulation properties. More important, these two NOSs likely rely on the downstream soluble guanylyl cyclase/cGMP/protein kinase G signaling pathway to regulate the structural components at the tight junctions and adherens junctions in the testes. Apart from the involvement in junction regulation, NOS/NO also participates in controlling the levels of cytokines and hormones in the testes. On the other hand, NO is playing a unique role in modulating germ cell viability and development, and indirectly acting on some aspects of male infertility and testicular pathological conditions. *Correspondence to: Nikki P.Y. Lee; Departments of Medicine and Surgery; University of Hong Kong, Queen Mary Hospital, Hong Kong, China. Email: nikkilee@ hkucc.hku.hk / C. Yan Cheng; Population Council, 1230 York Avenue; New York, New York 10021 USA; Email: Previously published online as an Oxidative Medicine and Cellular Longevity E-publication: www.landesbioscience.com/journals/oximed/article/6856 Reprinted from: Molecular Mechanisms in Spermatogenesis, edited by C.Y. Cheng © 2007 Landes Bioscience. www.landesbioscience.com Thus, NOS/NO bears an irreplaceable role in maintaining the homeostasis of the microenvironment in the seminiferous epithe‑ lium via its different downstream signaling pathways. Introduction Among the organs in the mammalian body, testis is one of the exceptional organs having complex cellular structures and organization. After puberty, testis functions as a sperm producing factory, generating up to millions of spermatozoa on a daily basis through the entire adulthood.1 In order to fulfill its reproductive function, testis is compartmentalized into two broad partitions, the seminiferous tubules and the inter‑tubular areas (Fig. 1).2,3 In the seminiferous tubules, the epithelium is physically divided into the adluminal compartment and basal compartment by the blood‑testis barrier (BTB) which is constituted by adjacent Sertoli cells near the basement membrane.4,5 Different cell types situate in specialized testicular locations. Sertoli cells and assorted germ cell types namely spermatogonia, spermatocytes, and spermatids are found in the seminiferous epithelium.6 Myoid cells locate adjacent to the tubules and Leydig cells reside in the inter‑tubular space known as the interstitium.2 Each cell type performs different function, however they are communicating with each other to share the core role in sperm production during spermatogenesis. Apart from that, the male sex hormone level namely testosterone in the systemic circulation is also produced and regulated by the Leydig cells in the testis via steroidogenesis. These processes cannot be fully executed, if they are not equipped with precisely regulated interactive mechanisms during spermatogenesis. In rodents, the germ‑line lineage spermatogonia, initially residing on the basement membrane of the seminiferous epithelium must differentiate into preleptotene spermatocytes, which in turn, traverse the BTB at stages VII‑VIII of the epithelial cycle to gain entry into the adluminal compartment for further development7 (Fig. 1). During this event of germ cell movement, Sertoli cells also play a paramount role in determining the molecular events of germ cell development, including mitosis, meiosis, cellular differentiation and transformation.8 Sertoli cells accomplish this in part by monitoring the assembly and disassembly of inter‑Sertoli junctions in the testes and partly by initiating cross‑talk with germ cells.9 For instance, Sertoli cells are equipped with certain architectural machineries, such as microtubules, that interact with the movement‑associated germ cell proteins (e.g., motor proteins) to provide Oxidative Medicine and Cellular Longevity 25 NO and spermatogenesis this coordination.9 As such, premature germ cell release from the epithelium will be prohibitive. To sustain the optimal germ cell population in the testis, more than half (~75%) of the germ cells that are produced including spermatogonia, spermatocytes and spermatids undergo apoptosis, and are phagocytosed by Sertoli cells, thereby restricting the numbers of germ cells in the seminiferous epithelium. This spontaneous removal mechanism ensures that the limited resources from Sertoli cells (note: the number of Sertoli cells in adult rats remain the same throughout adulthood) are sufficient for germ cell development, and are within the capacity of the testes, by eliminating excessive germ cells to maintain germ cell quality. Nitric oxide (NO) is a free radical synthesized by nitric oxide synthase (NOS). NOS is composed of two identical monomers with molecular weights ranging from 130 to 160 kDa.10,11 Three isoforms of NOS, NOS I, II, and III, are known to date, which are the alternate names for neuronal NOS (nNOS) (Mr, ~320 kDa), induc- Figure 1. Cellular localization of different NOS/NO signaling pathway components in the testes. ible NOS (iN (...truncated)