Central to male reproductive function, the mammalian testis is a highly organized and complex organ. Its architecture features winding seminiferous tubules, where sperm production occurs with supportive Sertoli cells lining the interior. These tubules are encased by peritubular myoid cells. Nestled between the tubules, the interstitial compartment houses a diverse cellular population, including testosterone-producing Leydig cells, blood vessel-forming endothelial cells, and connective tissue-forming stromal cells (Magre and Jost, 1991; Nel-Themaat et al., 2010). Leydig cells secrete testosterone in response to luteinizing hormone (LH) stimulation through a complex signaling cascade involving G protein-coupled receptors (Newton et al., 2011; Gorowska-Wojtowicz et al., 2018). This process activates adenylate cyclase and phospholipase C pathways, leading to increased levels of cyclic AMP (cAMP) and inositol 1,4,5-trisphosphate (IP₃), respectively, which modulate cytosolic calcium concentration with a crucial factor for steroidogenesis (Costa et al., 2010).

Research has confirmed that Leydig cells not only possess these receptors but also use them functionally. When LH stimulates these cells, it initiates a two-step calcium signaling process. First, calcium enters the cell through T-type calcium channels. This influx then triggers a larger release of calcium from the endoplasmic reticulum, the cell’s internal calcium store. The CaV3 subtype of T-type calcium channels is particularly crucial in this process, acting as the initial trigger for the LH-induced rise in intracellular Ca²⁺ levels (Costa and Varanda, 2007; Costa et al., 2010). While protein kinase C (PKC)-mediated Ca²⁺ can stimulate testosterone production and secretion (Manna et al., 1999; Xu et al., 2018), protein kinase A (PKA) appears to play a more significant role than PKC in regulating Ca²⁺ dynamics in mouse Leydig cells (Costa et al., 2011). Additionally, purinergic P₂ receptors have been shown to modulate intracellular Ca²⁺ levels (Pérez-Armendariz et al., 1996; de Deus et al., 2018). However, some controversies exist regarding Ca²⁺ signaling mechanisms in Leydig cells, with some studies suggesting independence from extracellular Ca²⁺ (Tomić et al., 1995). To fully understand how calcium signaling influences testosterone synthesis in Leydig cells, additional scientific investigation is necessary.

TASK (TWIK-related acid-sensitive K⁺) channels, a subfamily of the two-pore domain potassium (K2P) channel family, consist of five members, with TASK-1, TASK-3, and TASK-5 sharing close structural similarities. These channels generate immediate and sustained K⁺ currents that contribute to background potassium conductance and are characteristically inhibited by acidic extracellular conditions (Duprat et al., 1997; Morton et al., 2003; Cui et al., 2007; Enyedi and Czirják, 2010; Rinné et al., 2024). TASK channels are not confined to a single tissue type but are extensively distributed throughout the body. They can be found in diverse locations, ranging from neurons in the nervous system to an array of peripheral tissues. These channels are present in both cells that generate electrical signals and those that do not, highlighting their versatile roles in cellular function (Wiedmann et al., 2023; Csáki et al., 2024; Hegner et al., 2024).

Transient receptor potential (TRP) channels are key mediators of a wide variety of sensory experiences, responding to stimuli including heat, cold pain, stress, and taste, converting these into physiological responses (Peier et al., 2002; Zhang et al., 2023). Research has demonstrated that TRPC channels are crucial for the proper functioning of gonadotrope cells, which are key players in the endocrine system (Beck et al., 2017). In particular, research has identified TRPC5 as a critical component in the cellular response to GnRH stimulation, contributing to plasma membrane depolarization and raising intracellular Ca²⁺ levels. This occurs through its calcium permeability and the activation of voltage-gated Ca²⁺ channels (Beck et al., 2017; Götz et al., 2017). During the process of steroid hormone production, or steroidogenesis, in both the gonads and adrenal glands, hormone cues trigger complex cellular responses. These responses involve two key signaling pathways, one mediated by cAMP and another by Ca²⁺. Both pathways play vital roles in controlling the expression and activity of the steroidogenic acute regulatory protein. This protein is particularly crucial in Leydig cells, where it facilitates steroid production in response to LH stimulation (Abdou et al., 2013).

Within the diverse TRP channel family, the vanilloid subfamily member TRPV stands out for its crucial role in pain perception and regulation. When activated, TRPV channels can trigger responses associated with both acute pain sensation and chronic neuropathic pain conditions. Notably, TRPV1 interacts with inflammatory molecules in a particularly intriguing manner. This interaction can enhance TRPV1 expression in various tissues, including neurons, prostate cells, and the nerve fibers that supply the prostate gland (Dinis et al., 2005; Riera et al., 2014; Jiang et al., 2024). The increased presence of TRPV1 in response to inflammation is not just limited to the nervous system; it plays a critical role in the development and progression of prostate inflammation, or prostatitis (Jiang et al., 2024).

In our previous studies on ion channels in the male reproductive systems, we identified TASK-1 and TRPV1, respectively. This review aims to highlight the significance of these ion channels, with a focus on TASK-1 and TRPV1 in the male reproductive systems, emphasizing their expression, hormonal regulation, and cellular responsiveness.

Table 1. The expression of TRPV1 and TASK-1 in the male reproductive system.

TRPV1 channel
Organism	Sample types	Methods	Localization	Reference
Mouse	Testicular tissue section, mouse Leydig cell line (TM3)	Immunohistochemistry (IHC) RT-PCR, western blot	Leydig cells	Kim et al., 2021
	Testicular tissue section, whole testis homogenates	IHC RT-PCR	Leydig cells of young mice, entire region of testes (germ, Sertoli, Leydig cells) of old mice	Siregar et al., 2019
Rat	Whole prostate homogenates	Western blot	Prostate	Zhang et al., 2019
	Spermatogonial stem cell lines (Gc-5spg, Gc-6spg), germ cells, testicular tissue section	IHC, immunocytochemistry, western blot	Germ cell, seminiferous epithelium	Mizrak et al., 2008
Human	Testis biopsy (entire prostate gland)	IHC	Nerve fiber in prostate	Dinis et al., 2005
	Whole genitourinary tract homogenates	RT-PCR	Genitourinary tract tissue	Stein et al., 2004
	Testicular tissue section, btestis needle aspiration	RT-PCR, western blot, IHC	Sertoli cells, germ-line cells, seminiferous tubule, ejaculated spermatozoa	De Toni et al., 2016
TASK-1 channel
Organism	Sample types	Methods	Localization	Reference
Mouse	Testicular tissue section, TM3	IHC, RT-PCR, western blot	Sertoli cell, spermatogenic cells, Leydig cells, TM3 Leydig cells	Woo et al., 2023
	Testicular tissue section	IHC	Leydig cells, peritubular myoid cells	Woo et al., 2024

The comprehensive table provides an overview of TRPV1 and TASK-1 expression in mouse, rat, and human male reproductive systems. It details the specific tissue or cell types examined, the experimental techniques employed for detection, and the precise cellular localization of these channels across the three species..

Studies utilizing genetically modified mice have shed light on the critical importance of the tyrosine kinase receptor c-kit and its binding partner, the stem cell factor. These investigations have demonstrated that both molecules are indispensable for the proper development and functioning of the testis (Loveland and Schlatt, 1997; Zhang et al., 2020; Bhartiya and Kaushik, 2021). Our previous research has expanded on this knowledge, demonstrating that two key proteins, c-kit, and anoctamin 1 (ANO1), are abundantly present in both primary mouse Leydig cells and a mouse Leydig cell line, TM3. Further research has illuminated the cellular response to LH stimulation. When exposed to LH, Leydig cells exhibit increased intracellular Ca²⁺, depolarization of membrane potential, and the secretion of testosterone. Notably, these responses were inhibited when c-kit and ANO1 inhibitors were applied, indicating that these proteins are not merely present in Leydig cells but actively participate in the process of testosterone secretion (Ko et al., 2022).

The platelet-derived growth factor (PDGF) family is composed of four distinct genes that encode four polypeptide chains. These chains combine to form five dimeric isoforms through homo- and heterodimerization (Fredriksson et al., 2004; Andrae et al., 2008). The PDGF family exhibits diverse molecular configurations. PDGF-A and PDGF-B can create homodimers, PDGF-AA and PDGF-BB as well as a heterodimer, PDGF-AB. In contrast, PDGF-C and PDGF-D are found solely as homodimers, PDGF-CC, and PDGF-DD (Heldin et al., 1986; Li et al., 2000; Bergsten et al., 2001). The platelet-derived growth factor receptor (PDGFR) is widely distributed across various cell types within the testis, contributing to testicular growth and development. For example, PDGFRα has a broader ligand specificity compared to PDGFRβ, binding to PDGF-A, -B, -C, and AB heterodimers. From the earliest stages of development and continuing throughout maturity, this receptor maintains a consistent presence in two key testicular cell types peritubular cells and Leydig cells (Gnessi et al., 1995; Ge et al., 2005; Stanley et al., 2011). Studies have shown that PDGFR signaling plays a crucial role in Leydig cell development and differentiation. Genetic studies also have revealed the crucial role of PDGF-A in Leydig cell development. When this gene is eliminated, Leydig cells fail to form, underscoring the potential significance of PDGFR signaling in stem Leydig cells (Gnessi et al., 2000; Brennan et al., 2003).

In the adult Leydig cell lineage, PDGFRα has emerged as a key identifier. Researchers have successfully used this marker to isolate stem Leydig cells from the testis of prepubertal rats. These stem cells are distinguished by the presence of PDGFRα and the absence of the LH receptor, which is typically found in more mature Leydig cells (Ge et al., 2006; Stanley et al., 2011). DNA array analysis of purified Leydig cells has provided insights into the expression patterns of PDGFR and c-kit genes. They exhibit their highest levels of activity in the progenitor stage of Leydig cells. However, as these cells progress through their development and transform into fully mature adult Leydig cells, the expression of these genes gradually diminishes (Ge et al., 2005). Our previous research demonstrated that within adult testicular tissues, PDGFRα expression is confined to two specific cell types, Leydig cells and peritubular myoid cells (Woo et al., 2024). Importantly, PDGFRα is co-expressed with other key proteins, including c-Kit, ANO-1, and TASK-1 (Woo et al., 2023). These co-expression patterns indicate the presence of complex regulatory mechanisms that may play significant roles in modulating testicular function.

Maintaining appropriate levels of free Ca²⁺ in the cytosol involves the intricate interplay of various cellular mechanisms, including the functions of intracellular membranes and diverse ion channels. Among these, TRPV1 channels emerge as particularly crucial players. TRPV1, part of the TRP channel family, responds to a range of stimuli, including heat, oxidative stress, and capsaicin (Caterina et al., 1997). This non-selective cation channel, known for its high Ca²⁺ permeability, initiates a pronounced Ca²⁺ response upon activation. It allows an influx of Ca²⁺ from the extracellular space while simultaneously prompting the release of Ca²⁺ from intracellular storage sites. This results in elevated intracellular Ca²⁺ levels, leading to cell depolarization, action potential generation, and the initiation of complex intracellular signaling cascades (Bujak et al., 2019). TRPV1 mRNA has been detected throughout the male genitourinary tract, indicating its potential importance in these tissues (Stein et al., 2004). Research has revealed diverse roles for TRPV1 in testicular function. In rat Sertoli cells, TRPV1 has been found to modulate the activity of a chloride channel that responds to acidic conditions (Auzanneau et al., 2008). More significantly, TRPV1 has emerged as a key player in human sperm a temperature-guided movement of sperm cells crucial for fertilization and an important aspect of reproductive biology (De Toni et al., 2016; Xiao and Chen, 2022; Amaya-Rodriguez et al., 2023). When activated by temperature gradients, TRPV1 channels in sperm cells trigger a Ca²⁺ influx, directly impacting sperm motility and enhancing their movement capabilities. This TRPV1-mediated Ca²⁺ signaling is essential for sperm navigation through temperature differences in the female reproductive tract, underscoring its significance in reproduction (De Toni et al., 2021). Interestingly, the absence of TRPV1 has been associated with extended longevity and the maintenance of a youthful metabolic profile into old age, attributed to the inactivation of a Ca²⁺-signaling cascade (Riera et al., 2014). Additionally, the activation of TRPV1 in germ cells has been linked to apoptosis in immortalized cell lines, suggesting a direct connection between TRPV1 activation and germ cell death. Germ cells appear to undergo apoptosis in response to capsaicin, with both the duration of exposure and the concentration of capsaicin influencing this process. This apoptosis is likely mediated through TRPV1 channels located in these cells (Mizrak et al., 2008).

Testicular aging is characterized by a range of structural and functional changes, including decreased volume, reduced cell populations (Leydig, Sertoli, and germ cells), thickening of seminiferous tubule walls, vascular alterations, and fibrosis (Zitzmann, 2013; Gunes et al., 2016; Li et al., 2024; Tysoe, 2024). Earlier studies have shown a notable decrease in TRPV1 expression in the sperm of men with various fertility problems, including sub-fertile individuals, those with idiopathic infertility, and normozoospermic infertile males with elevated levels of reactive oxygen species (Swain et al., 2022). This finding suggests a potentially crucial role for TRPV1 in sperm function, specifically as a sensor for oxidative stress within sperm cells. Our previous studies have demonstrated a link between TRPV1 and testicular apoptosis, with significantly increased expression levels observed in the testis of aged mice (Siregar et al., 2019). Interestingly, TRPV1 knockout elderly mice exhibited extended lifespans and reduced testicular apoptosis compared to their wild-type counterparts. In Leydig cells, we found that TRPV1 channel activation triggers cell death. However, testosterone appears to play a protective role by reducing TRPV1 expression levels and inhibiting cell death induced by capsaicin, a TRPV1 activator (Kim et al., 2021). This suggests a protective effect of testosterone in regulating Leydig cell survival through modulation of TRPV1 expression.

The regulation of Ca²⁺ influx plays a pivotal role in steroidogenesis within Leydig cells. This process is primarily controlled by luteinizing hormone (LH), which upon binding to its receptor, triggers an increase in intracellular cAMP concentration (Costa et al., 2010). This, in turn, leads to a rise in intracellular Ca²⁺ levels, ultimately upregulating testosterone production. The cytosolic free Ca²⁺ concentration is tightly regulated by various intracellular membrane functions and ion channels, with TRPV1 being a key player in this process (Song et al., 2024). Steroid hormones exert a significant influence on Ca²⁺ channels across various tissues, with profound physiological implications. Testosterone and its derivatives modulate L-type Ca²⁺ channels in cardiomyocytes (Curl et al., 2009) and also impacts Ca²⁺ homeostasis by inhibiting the expression of Ca²⁺ transport proteins, particularly TRPV5 (Méndez-Reséndiz et al., 2020). One of the TRPV channel subfamily, TRPV5, a highly Ca²⁺-selective ion channel, is believed to play a significant role in spermatogenesis. Testosterone, the primary product of Leydig cells, exhibits complex effects on Ca²⁺ channels. It directly inhibits L-type Ca²⁺ channels at physiological concentrations (IC50 = 38 nM) and T-type Ca²⁺ channels at higher concentrations in vascular cells (Loh et al., 2013). Interestingly, testosterone also induces relaxation in human-isolated internal spermatic veins by activating ATP-sensitive K⁺ channels (Seyrek et al., 2011). TRPV1 is a Ca²⁺-permeable channel and can modulate Ca²⁺ influx making it a crucial contributor to cellular Ca²⁺ homeostasis and related physiological function. In tumor Leydig cells, activation of TRPV1 channels leads to increased caspase 3/9 activities, elevated ROS production, oxidative stress, and Ca²⁺ influx, all associated with an increase in mitochondrial membrane depolarization (Defo Deeh et al., 2019). Consequently, it is postulated that testosterone may inhibit Ca²⁺ increases or membrane potential depolarization mediated by TRPV1 in Leydig cells, suggesting a potential regulatory mechanism in these cells.

TASK-1 and TRPV1 channels exert opposing effects on membrane potential and cell excitability. TASK-1 generally promotes hyperpolarization and reduced excitability, while TRPV1 typically causes depolarization and increased excitability in their respective cellular contexts (Limberg et al., 2011; Zhang et al., 2023). However, our study did not explore potential interactions between these two channel types. To fully understand any functional interplay between TASK-1 and TRPV1 channels, further targeted research is required. This represents crucial area for future investigation to elucidate potential crosstalk between these channels and its physiological implications. The exploration of functional divergence and genetic variability in TASK-1 and TRPV1 channels within testicular cells offers promising avenues for advancing our understanding of male reproductive biology. This research has the potential to uncover novel therapeutic approaches for addressing male infertility, hormonal imbalances, and age-related deterioration of testicular function.

None.

Conceptualization, E-A.K., D.K.; data curation, D.K.; formal analysis, D.K.; investigation, D.K.; methodology, D.K.; project administration, E-A.K., D.K.; supervision, E-A.K.; writing - original draft, E-A.K., D.K.; writing - review & editing, E-A.K., D.K.

This work was supported by the 2024 education, research and student guidance grant funded by Jeju National University.

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No potential conflict of interest relevant to this article was reported.