Molecular Regulation of Copper Homeostasis in the Male Gonad during the Process of Spermatogenesis

Abstract

Owing to its redox properties, copper is a cofactor of enzymes that catalyze reactions in fundamental metabolic processes. However, copper-oxygen interaction, which is a source of toxic oxygen radicals generated by the Fenton reaction, makes copper a doubled-edged-sword in an oxygen environment. Among the microelements influencing male fertility, copper plays a special role because both copper deficiency and overload in the gonads worsen spermatozoa quality and disturb reproductive function in mammals. Male gametes are produced during spermatogenesis, a multi-step process that consumes large amounts of oxygen. Germ cells containing a high amount of unsaturated fatty acids in their membranes are particularly vulnerable to excess copper-mediated oxidative stress. In addition, an appropriate copper level is necessary to initiate meiosis in premeiotic germ cells. The balance between essential and toxic copper concentrations in germ cells at different stages of spermatogenesis and in Sertoli cells that support their development is handled by a network of copper importers, chaperones, recipient proteins, and exporters. Here, we describe coordinated regulation/functioning of copper-binding proteins expressed in germ and Sertoli cells with special emphasis on copper transporters, copper transporting ATPases, and SOD1, a copper-dependent antioxidant enzyme. These and other proteins assure copper bioavailability in germ cells and protection against copper toxicity.

Keywords: ATP7A; ATP7B; CTR1; copper; gametes production; spermatogenesis; testis.

Figure 1

Copper metabolism in polarized mammalian cells. Cupric ions (Cu²⁺) are reduced to cuprous ions (Cu⁺) by metaloreductase and can then be transported into the cell by the transmembrane transporters CTR1 or CTR2. In the cytosol, Cu⁺ ions can be reversibly bound by metallothionein (MT) and stored in the Cu-MT complex. In the cytoplasm Cu⁺ ions are bound by cytosolic metallochaperones: copper chaperone for superoxide dismutase 1 (CCS), COX17, and ATOX1, which deliver copper to various proteins such as superoxide dismutase 1 (SOD1), cytochrome c oxidase (CCO), ATP7A, and ATP7B. As a result, copper is delivered to different cellular compartments such as the cytoplasmic reticulum, mitochondria, or the Golgi apparatus. The ATP7A and ATP7B proteins located in the trans-Golgi network (TGN) incorporate Cu⁺ ions into the apoenzymes. ATP7B can transfer copper to apo-ceruloplasmin (CP) to form holo-ceruloplasmin. Then copper in the complex with ceruloplasmin can be transported to the blood vessels. Cu-transporting ATPases can remove the excess copper outside the cell: ATP7A across the basolateral membrane and ATP7B across the apical membrane of polarized cells. This figure was created with BioRender.com.

Figure 2

Localization of copper-related proteins in the seminiferous tubule and myoid cells. In the testes, gametes are produced during the multi-step process called spermatogenesis, which covers a complex network of regulations that occur in the seminiferous tubules. Each seminiferous tubule is surrounded by a few layers of myoid cells that border on blood vessels. The seminiferous epithelium contains the Sertoli cells and the germ cells and lies on the basal lamina. The lumen of the seminiferous tubule is filled with seminal fluid. The Sertoli cells are polarized cells that reach the lumen of the seminiferous tubule and support the individual stages of developing germ cells. Sertoli cells are linked by tight junctions that form a blood–testis barrier. This divides the seminiferous epithelium into the adluminal and basal compartments and allows Sertoli cells to control the environment for developing late germ cells. The first stages of the germ cell line are spermatogonia, which lie on the basal lamina. Above them, primary and secondary spermatocytes are successively situated. The next stages are round spermatids, which transform into elongated spermatids. After absorption of residual bodies, Sertoli cells release the elongated spermatids into the seminiferous tubule lumen where these spermatids become sperm. The transport of Cu ions during the process of spermatogenesis occurs between germ cells, Sertoli cells, and myoid cells. Spermatogonia, primary spermatocytes, and Sertoli cells absorb copper using CTR1. The excess copper is exported from the primary spermatocytes and Sertoli cells through the ATP7A protein. Presumably, a similar function is performed by ATP7B in the elongated spermatids. Removed Cu⁺ ions are imported to Sertoli cells through CTR1. In Sertoli cells and myoid cells, copper is delivered to the ceruloplasmin via ATP7B. In the complex with ceruloplasmin, Cu⁺ ions leave the myoid cells and enter the bloodstream. This figure was created with BioRender.com.