At the end of 2019, a novel coronavirus, named SARS-CoV-2, was found in patients with severe pneumonia and has become a worldwide pandemic with a rapidly growing number of infection cases up to now. SARS-CoV-2 and SARS-CoV-1 belong to the same coronavirus subfamily named the beta coronaviruses (1, 2). SARS-CoV-2 and SARS-CoV-1 share the same receptors named ACE2 residing in many different human organs (3). Previously, several research groups have revealed that SARS-CoV-1 patients have post-infection reproductive system complications (4, 5). Thus, in the present review, we demonstrate the molecular mechanism of the viral tropism, post-infected pathological features, and potentially detrimental effects in the male reproductive system in the COVID-19 patients. The review aims to draw appropriate conclusions about the impact of COVID-19 on male reproduction and put forward some new ideas for further research.

In early 2020, it was demonstrated that the transmission mode of SARS-CoV-2 is mainly by droplets containing pathogens in the air (6). Droplets enter the nose and mouth and then carry pathogens to infect the upper respiratory tract. Firstly, SARS-CoV-2 entry into the host cells depends on the recognition of the angiotensin-converting enzyme 2 (ACE2) receptor (7). Since viral spike protein binds to the extracellular domain of ACE2, another critical factor, transmembrane protease serine protease-2 (TMPRSS2), mediates cleavage at the S1/S2 site of S protein, which is indispensable for viral membrane fusion (7). The viral RNA genome begins to replicate and instruct host-cell ribosomes to translate its structural proteins and polyproteins for viral capsid after SARS-CoV-2 enters the host cells (8). Viral particles are assembled in reticulum-Golgi intermediate compartment (ERGIC) (9) and subsequently are released by means of exocytosis for the spread of the infection (8).

ACE2 is regarded as the most essential key for viral entry; thus, expression patterns of ACE2 in different tissues and organs have become a focus in many research works, which may suggest potential routes of SARS-CoV-2 infection in humans. Previous studies have shown that ACE2 expression is abundant in the testis (10, 11). The proportion of ACE2-positive cells in the testis is more than that in the lung, which indicated that the testis might serve as a high-risk potential infection organ (12). Using single-cell RNA sequencing (scRNA-seq), it is reported that ACE2 is primarily expressed in Sertoli cells, which have the highest expression level, and then Leydig cells (LCs) and spermatogonia (13). It suggests a possible tropism of SARS-CoV-2 to the testis. The virus may infect Sertoli cells, disturbing the physiological process in which Sertoli cells control the germ cell environment by the secretion and transport of nutrients and regulatory factors, which is strongly related to spermatogenesis. Sertoli cells also play an important role in constituting the blood–testis barrier (BTB) (14). The BTB gives testes immune privilege separating autoantigens and host immune cells from germ cells and Sertoli cells. The Leydig cells are located in the interstitium of the testis where they are most vulnerable within the testis against the virus. LCs secrete androgen, which is indispensable in spermatogenesis and in maintaining secondary sex characteristics (15). Moreover, LCs also regulate testicular macrophage and lymphocyte numbers (16). Collectively, if SARS-CoV-2 infects these types of cells, the disruption of spermatogenesis will occur. In addition, a human sperm proteomic database reveals the presence of ACE2 in the human sperm while it is not detected through scRNA-seq (17). Furthermore, TMPRSS2 is expressed abundantly in prostasomes and is released into the seminal fluid from the prostate gland at ejaculation (18), which makes it possible for sperm to be vulnerable to SARS-CoV-2 infection.

Besides, the viral infection of cells requires cofactors, such as TMPRSS2 (19) and CD147 (20), to promote its invasion. TMPRSS2 is highly expressed in the kidney, epididymis, prostate, and seminal vesicles. TMPRSS2 expression was concentrated in spermatogonia and spermatids with relatively low levels in other cell types of testes (13). Stanley et al. used the scRNA-seq method to find that the proportion of co-expression of ACE2 and TMPRSS2 in testicular cells is less than 0.05% (21), which indicated that human testis is not susceptible to viral attack. However, transcript-level expression cannot represent the protein expression profiles of the human reproductive system. Additionally, most studies evaluated the risk of viral infection by examining the expression of ACE2 and TMPRSS2, which is sort of arbitrary, because other co-receptors promote the entry of SARS-CoV-2 in cells of the respiratory system. That is why the lung is the most susceptible organ to SARS-CoV-2, while single-cell sequencing data show that ACE2 expression was expressed in fewer than 0.1% of cells in the lung (22). Therefore, further studies are warranted to focus on protein expression of more viral receptors in testicular cells so that we could better elucidate the tropism of SARS-CoV-2 in the human reproduction system.

During the treatment of COVID-19, the use of antiviral drugs often neglects the potential reproductive toxicity. The following are the drugs that may be used/exploited for treatment referring to current guidelines (70). Remdesivir is approved by the FDA for the treatment of COVID-19 in hospitalized adult and pediatric patients (70). Up to now, there have been no reports of Remdesivir concerning the adverse effect on the human productive system despite one investigation suggesting the reproductive toxicity of Remdesivir, which was withdrawn by the authors for better experimental design (71). IFN-α is known for antiviral, antiproliferative, and immunoregulatory activities (72). de Lima Rosa et al. concluded that IFN-α within the normal dose range did not significantly influence sperm production, maturation, and motility, as well as levels of gonadal hormones (73). Ribavirin is used widely as an antiviral drug, which is a candidate for COVID-19 treatment due to the inhibition of viral RNA-dependent RNA polymerase (74). It is generally accepted that ribavirin exposure should be avoided during pregnancy due to the potential teratogen (75). Besides, the impact of ribavirin on spermatogenesis should be emphasized. Narayana et al. revealed that ribavirin significantly decreased the sperm count in a dose- and time-dependent pattern in rats. Additionally, ribavirin could cause a reversible decrease in sperm parameters including decreased sperm motility, DNA packaging abnormalities, and increase in sperm DNA fragmentation up to 8 months after drug discontinuation (76), which indicates that mandatory contraception should be taken after treatment discontinuation. Corticosteroids are used widely in the clinic for the reason that their potent anti-inflammatory effects might prevent or mitigate these deleterious effects. There is no evidence supporting adverse effects on human sperm, while corticosteroids possibly indirectly affect spermatogenesis and oocyte competence through the hypothalamic–pituitary–gonadal axis (77, 78). Broad-spectrum antibiotics are being widely used in patients with COVID-19 (79). Although most relevant studies are inconclusive, the potential reproductive toxicity of antibiotics should be considered (80). Hargreaves et al. found that amoxicillin impaired sperm viability at high doses (81). Another group demonstrated that therapeutic doses of penicillin G resulted in spermatogenic arrest in rats after treatment for 8 days (82). Moreover, high doses of nitrofurantoin can cause spermatogenic arrest, reduced sperm counts, and sperm immobilization, which is probably due to the failure of testicular cells to use carbohydrates and oxygen (83, 84). There are two drugs mispresented as “miracle drugs” by media misinformation and forged studies. However, both are not approved to treat COVID-19 patients. Chloroquine, an immunomodulant drug, is effective in reducing viral replication in SARS-CoV-2 infections, supported by in vitro data and clinical studies involving humans (85). Rat models showed that chloroquine reduces motility and fertilizing capacity of sperm (86, 87). However, there is a lack of sufficient data to make recommendations. Ivermectin has been shown to inhibit the replication of SARS-CoV-2 in vitro (88). Moreira et al. found a significant decrease in striatal dopaminergic system activity including dopamine release and lower testosterone levels in male rats, leading to a reduction in motor coordination (89). Above all, couples who have used related drugs are advised to seek professional advice and fertility check before planning to conceive.

Humanized ACE2 (hACE2) mice, which have overcome the natural insensitivity of mice to SARS-CoV-2 infection, are widely exploited for infection models and drug development. However, they are not an economical tool due to their high breeding fee and the lack of a qualified animal laboratory. Organoids are increasingly utilized for drug screening, toxicity assessment, and viral infection progression. Existing methods to evaluate the potential reproductive toxicity of drugs and viral infection require a large amount of animal sacrifice and could not ignore the individual differences. Organoids could provide more controllable, high-throughput, and faster evaluation techniques to simulate the microenvironment in vivo. In COVID-19, multiple research groups have resorted to organoid approaches to understand the tissue tropism of SARS-CoV-2. Penninger et al. established the capillary organoids and kidney organoids from human iPSCs and demonstrated that SARS-CoV-2 could directly infect cells in the capillary and kidney, which explains the spread of the virus throughout the body and the loss of kidney function in several severely infected cases (90). Another research group utilized human ASC-derived intestinal organoids to prove that SARS-CoV-2 could infect intestinal epithelium, the enterocyte, and replicate in intestines, suggesting that the intestine is a susceptible site of SARS-CoV-2 (91). To note, Lancaster and colleagues used brain organoids to investigate viral neurotropism and discovered that SARS-CoV-2 mainly infects the choroid plexus leading to damage to this brain barrier (92), and it should be emphasized that testis has a similar structure composed of cell–cell tight junctions. There is a restricted number of research groups that have reported and characterized human testicular organoids. Daniel and his colleagues developed the human testicular organoid to investigate the impact of ZIKV infection on testis (93). Until now, testicular organoids have not been used in studies about COVID-19 possibly because of difficulties of building human testicular organoids. Thus, further improvement of strategies to establish testicular organoids is needed. There is no denying that testicular organoid is a novel and efficient tool to investigate the susceptibility of testis to SARS-CoV-2 and to understand the alterations of post-infected reproductive capacity.

It appears that COVID-19 influences different aspects of male reproduction including reproductive tracts, hormone, gametes, and sexual function. COVID-19 may trigger orchitis or epididymitis, thus impairing testis integrity and spermatogenesis. COVID-19 patients have decreased sperm concentration and motility. However, there seems to be a lack of consensus in the presence of SARS-COV-2 in semen and testis. Moreover, COVID-19 patients are exposed to a high risk of ED and thus we suggested Angiotensin-(1-7) as a novel drug target for ED in COVID-19. The current review also discusses the reproductive tract toxicity of drugs targeting COVID-19, and it also sheds new light on research on related fields and how the emerging model, such as the organoid, can be used to accelerate understanding of the related topic. Taken together, the impact of COVID-19 on the reproductive system still has outstanding unsolved questions. We need extensive clinical research and more efforts to investigate the long-term influence on male reproduction.

JG wrote the manuscript. KS performed the figure design and gave valuable advice. SW and HC contributed to collect relevant materials. All authors listed have made a substantial, direct, and intellectual contribution to the work, and approved it for publication.

Our work was supported by National Key Research & Development grant (No. 2018YFC1003603) and the National Nature Science Fund of China (No. 81971445).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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