Biogenesis of stress granules and their role in the regulation of stress-induced male reproduction disorders

Abstract

Stress granules (SGs) are conserved messenger ribonucleoprotein (mRNP) granules that form through rapid coalescence in the cytoplasm of eukaryotic cells under stressful environments. These dynamic membrane-free organelles can respond to a variety of both intracellular and extracellular stressors. Studies have shown that stress conditions such as heat stress, arsenite exposure, and hypoxic stress can induce SGs formation. The formation of SGs helps mitigates the effects of environmental stimuli on cells, protects them from damage, and promotes cell survival. This paper focuses on the biogenesis of SGs and summarizes the role in regulating environmental stress-induced male reproductive disorders, with the aim of exploring SGs as a potential means of mitigating male reproduction disorders. Numerous studies have demonstrated that the detrimental effects of environmental stress on germ cells can be effectively suppressed by regulating the formation and timely disassembly of SGs. Therefore, regulating the phosphorylation of eIF2α and the assembly and disassembly of SGs could offer a promising therapeutic strategy to alleviate the impacts of environmental stress on male reproduction health.

Graphical abstract

Introduction

Stress granules (SGs) are a highly conserved cytoplasmic ribonucleoprotein (RNP) granules [1]. These unique mRNP granules form rapidly form within the cytoplasm of eukaryotic cells in response to stressful conditions such as heat shock, endoplasmic reticulum stress, and hypoxia. SGs varies in size, ranging from approximately 200 nm to over 5 μm [2], and SGs are dynamic, membrane-free organelles that respond to a range of intracellular and extracellular stressors, dissolving once the stress is alleviated [3]. RNP granules are membrane-free subcellular organelles formed by the condensation of RNA and proteins in eukaryotic cells. These structures, known as mRNP granules, consist of mRNA and proteins, and they influence the function and localization of mRNA [4] (Fig. 1). SGs play a crucial role in regulating stress responses, including hypoxia, heat shock, viral infections, and signaling pathway transduction. Proper formation of SGs is essential for cell survival under stress conditions. Improper SG formation can increase cellular sensitivity to stress, ultimately leading to cell death. Conversely, abnormal accumulation of SGs has been linked to neurodegenerative diseases and certain cancers.

Fig. 1

Diagram of stress granules patterns

During mammalian development, oocytes and sperm are derived from primordial germ cells (PGC). In contrast to somatic cells, germ cells are responsible for transmitting genetic information and undergo a precise and unique form of cell proliferation and division—meiosis—critical for their development. In contrast, somatic cells complete their growth through mitosis alone. The development of germ cells is a delicate and prolonged process. In most mammals, a complete spermatogenic cycle lasts approximately 40–90 days [5, 6], while human follicles take around 290 days to develop from primary follicles to mature secondary follicles [7]. Germ cells are highly sensitive to environmental stimuli, forming SGs in response, which will be the focus of this review.

Environmental factors pose numerous challenges to reproductive health, including infertility [8], menstrual irregularities [9], sexual dysfunction [10], and prostate issues [11]. Approximately 17.5% of the global adult population—about one-sixth of the global population—suffers from infertility [12]. These conditions not only reduce quality of life but also increase the risk of other chronic diseases. In addition, infertility, menstrual irregularities, and sexual dysfunction frequently lead to significant psychological stress, contributing to mental health issues such as anxiety and depression. Environmental factors exert a profound impact on reproductive health, either directly affecting the reproductive system or indirectly by disrupting the endocrine system and altering immune function. Therefore, effectively managing stress is essential for maintaining reproductive health.

The formation of SGs halts the synthesis of non-essential proteins, helping protect mRNA from damage and promoting cell survival. SGs can impact reproductive health both directly and indirectly. For example, during spermatogenesis, heat stress induces the formation of SGs, which protect normal spermatogenesis from germ cell apoptosis by phosphorylating eIF2α and recruiting RNA-binding proteins such as DAZL, BOULE, RACK [13, 14]. Although the specific role of SGs in reproductive health remains under investigation, their potential to maintain the normal function of germ cells and reproductive organs, as well as their response to various stressors, underscores their significance in reproductive medicine research. Data suggest that SGs formation can inhibit germ cell apoptosis and mitigate the effects of stressors on germ cells, thereby positively influencing reproductive health. These findings indicate that SGs play a crucial role in the reproductive systems’ response to environmental challenges and can effectively reduce the adverse effects of these stimuli on germ cells.

This review focuses on the impact of environmental stress conditions on male reproductive health and summarizes and discusses the role of SGs in the regulating male reproductive disorders induced by environmental stimuli.

Formation, characterization, and role of SGs in cellular stress

The discovery of SGs

SGs were first observed in the cytoplasm of heat-shocked suspension cultures of Peruvian tomato (Lycopersicon peruvianum) in 1983. Similar granular aggregates were subsequently observed in the cytoplasm of heat-shocked tomato leaves, maize germ sheaths, and other plant cytoplasm. At that time, these transient structures were not yet named stress granules but were referred to as heat-shock granules, as most of the major small heat shock proteins were structural components of these newly identified granular aggregates [15]. In 1986, such similar granular aggregates were shown to be formed in chick embryo fibroblasts under heat stress [16], showing that the phenomena is conserved. Interestingly, in 1988, it was found that chicken embryo fibroblasts were able to form granular aggregates containing heat shock proteins when induced by a variety of stressors. Therefore their name was revised to stress granules (SG) [17]. In 1999, phosphorylation of eIF2α was linked to the assembly of mammalian SGs by the RNA-binding proteins TIA-1, TIAR [18]. In 2002, the accumulation of SGs was first observed in fission yeast under environmental stress conditions [19]. Since then, much of the research on the environmental induction of SG formation has been conducted using the yeast model [20].

Characterization of SGs and their comparison with other subcellular organelles

SGs contain numerous untranslated mRNAs, various translation initiation factors, and both RNA-binding and non-RNA-binding proteins. Analysis of the protein fractions within SGs, particularly the “core,” reveals that approximately 50% of the components are RNA-binding proteins [3]. This suggests that SGs are dynamic, heterogeneous structures, with a stable core rich in proteins and mRNAs, surrounded by a dynamic shell. The assembly, disassembly, and interaction between the core and shell are driven by weak multivalent interactions among the various protein and RNA components, which are rapidly and dynamically organized through phase separation and percolation [21]. The composition of SGs varies under different stress conditions, further emphasizing their complexity and variability [22] (Fig. 2A).

Fig. 2

Comparison of composition of stress granules with other subcellular bodies. A: Stress granule; B: Exosome [23]; C: Inflammasome [24]; D: Autophagosome [25]; E: Cajal body [26]; F: Migrasome [27]

Exosomes are formed within multivesicular bodies (MVBs) through endocytosis, resulting in small vesicles with a diameter of approximately 30–150 nm. These vesicles are eventually released into the extracellular matrix when MVBs fuse with the plasma membrane. The membrane structure of exosomes, similar to that of the plasma membrane, comprises a lipid bilayer and encapsulates a diverse array of biomolecules such as proteins, lipids, mRNA, miRNA, DNA, and other small molecules [28]. Particularly, exosomes secreted by the parietal plasma of epididymal epithelial cells can fuse with spermatozoa, promoting spermatid maturation by delivering paternally-derived sncRNAs [29]. These sncRNAs are then transferred to the oocyte, where they participate in regulating embryo development and zygote health [30] (Fig. 2B).

Inflammasomes were first described in 2002 as multiprotein complexes with a molecular weight of approximately 700 kDa [31]. They form in the cytoplasm when pattern-recognition receptors (PRRs) detect pathogen-associated molecular patterns (PAMPs) or host-derived danger-signaling molecules (DAMPs). Capable of sensing various stimuli, inflammasomes trigger inflammatory and immune responses. These complexes are primarily composed of sensor protein, adapter protein, and inflammatory caspases, which facilitate cytokine maturation and secretion, mediate inflammatory responses and induce cellular pyroptosis [32] (Fig. 2C).

Autophagosomes are essential intermediate structures in cellular autophagy, responsible for encapsulating damaged organelles, misfolded proteins, and other cellular debris, and ultimately transporting these materials to lysosomes for degradation and recycling. Their formation and function are critical for maintaining intracellular homeostasis, responding to stress, and regulating cell fate [33] (Fig. 2D).

Cajal bodies are complex, membrane-free spherical structures formed by Liquid-liquid phase separation (LLPS) in the nuclei of higher eukaryotic cells, with diameters ranging from 0.2 to 2 μm. Their molecular composition is intricate, including various components of the nucleic acid-protein complexes of snRNPs necessary for mRNA splicing, as well as snoRNAs, scaRNAs, and other associated proteins. Cajal bodies play crucial roles in RNA metabolism, the formation of transcribed ribonucleoproteins, RNA splicing, and telomere maintenance [34] (Fig. 2E).

Migrasomes, a newly discovered subcellular structure first identified in 2014, form during cell migration when cells leave behind retraction fibers. Vesicles measuring 1–3 μm in diameter develop at the tips or intersections of these fibers and are referred to as “migrasomes” [35]. These structures contain mRNAs and proteins that accumulate damaged mitochondria, playing crucial roles in mediating signaling, regulating cellular homeostasis, and facilitating material transport [36,37,38,39] (Fig. 2F).

Unlike exosomes, inflammasomes, autophagosomes, Cajal bodies, and migrasomes, which are also subcellular structures, SGs are membrane-free organelles formed in the cytoplasm through liquid-liquid phase separation (LLPS) during cellular stress. These organelles regulate mRNA stability, translation, and degradation within the cell. Similarly to inflammasomes, SGs are multiprotein complexes, but are different because the protein components (translation factors, RNA-binding proteins, and other stress-related proteins) are primarily associated with untranslated mRNA. In contrast to autophagosomes, SGs lack a bilayer membrane and do not degrade damaged cellular components. The main characteristics and differences between SGs and other subcellular structures are listed in Table 1.

Table 1 Differences between stress granules and other subcellular structures

Spermatogenesis in the male

The process of spermatogenesis in the testis

Spermatogenesis is a complex process in the male reproductive system that involves the self-renewal and differentiation of spermatogonial stem cells into mature spermatozoa. In mammalian testis, the process of spermatogenesis consists of three stages: mitosis, meiosis and spermatogenesis [40]. First the spermatogonial stem cells (SSCs) undergo mitosis to produce new spermatogonia. One of the daughter cells becomes a new spermatogonial stem cell and remains in the stem cell pool, retaining stem cell characteristics and responsible for self-renewal to maintain the stability of the spermatogonial cell pool. The other one becomes an A1 spermatogonial cell and continues to differentiate and divides once to produce two A2 spermatogonial cells, which then undergoes successive mitotic divisions to produce four A3 spermatogonial cells, and eigh A4 spermatogonial cells. These A spermatogonial cells will divide, and with each division they will accumulate differentiation characteristics in preparation for transformation into type B spermatogonia. After the formation of type A4 spermatogonia, the type A4 spermatogonia will further divide to form sixteen intermediate spermatogonia. The intermediate spermatogonia further proliferate and finally differentiate into thirty two B spermatogonia, which marks the shift from A to B cell differentiation, and the B spermatogonia will undergo a final mitotic division and form sixty four primary spermatocytes, which will enter the next stage of spermatogenesis [41, 42]. Subsequently, type B spermatogonia will enter the prophase stage in preparation for meiosis initiated by retinoic acid (RA) and STRA8 [43]. Once the spermatogonia begin their first meiotic division they become primary spermatocytes, which complete their first meiotic division to become two secondary spermatocytes, which then undergo a second meiotic division, in which the chromosome number is halved to form four haploid spermatocytes [44, 45]. Therefore, according to the process of spermatogenesis, one type A1 spermatogonia is theoretically capable of producing two hundred and fifty-six spermatocytes. Moreover, the spermatogonia undergo a series of morphological changes while being nourished by the supporting cells, the spermatogonia further develop into spermatozoa in a process called spermatogenesis, followed by complex morphological changes in the round spermatogonia as well as the transformation and modification of the nucleoproteins, resulting in the formation of mature spermatozoa. However, in reality, 70–90% of spermatogonia are lost during spermatogenesis and only 10–30% of the cells are able to form spermatozoa [46, 47]. This suggests that the process of spermatogenesis is susceptible to environmental factors that reduce sperm quality.

The spermatogenic cycle of different animals

Studies have shown that under stress conditions SGs are distributed at specific stages of spermatogenesis, primarily occurring prior to meiosis rather than throughout the entire process. This suggests that mitigating the effects during critical stages of spermatogenesis can benefit overall spermatogenesis. The duration of one spermatogenic epithelial cycle and a complete spermatogenic cycle in different animal species are listed in Table 2.

Table 2 A complete spermatogenic cycle and seminiferous epithelium cycle in different animals

SGs biogenesis and mechanism of action

Formation and decomposition of SGs

The Integrated Stress Response (ISR) is a complex signaling pathway in eukaryotic cells, activated in response to various physiological changes and pathological conditions, including extracellular factors such as hypoxia, amino acid deprivation, glucose deprivation, and viral infection [54,55,56]. Typically, cellular stress leads to translational arrest via ISR activation. This activation is mediated by the phosphorylation of eIF2α (eukaryotic translation initiation factor 2α) by one or more of four specific kinases: HRI (heme-regulated eIF2 kinase), PKR (viral infection-regulated eIF2 kinase), PERK (endoplasmic reticulum stress-regulated eIF2 kinase), and GCN2 (amino acid deprivation-regulated eIF2 kinase). Phosphorylation of eIF2α is a critical step in SGs formation, as it converts eIF2α from a substrate of eIF2B into an inhibitor, thus blocking the formation of the eIF2-GTP-Met-tRNA ternary complex and halting the initiation of mRNA translation initiation [57]. An alternative pathway for SGs formation, independent of eIF2α phosphorylation, involves the disruption of the eIF4 complex. This can be achieved through pharmacological inhibition of eIF4A or eIF4E [58], leading to disassembly of the multimer and preventing the assembly of the translation initiation complex. This disruption marks the first step in SG formation: translation initiation arrest.

Liquid-liquid phase separation (LLPS) is a density transition characterized by the segregation of molecules in solution into distinct light and dense liquid fractions [59]. Weak multivalent interactions between proteins, protein-RNA, RNA-RNA, and protein intrinsically disordered regions (IDRs) promote the formation of phase-separated states in SGs [60, 61]. Research indicates that the UBAP2L core forms first, upstream of the G3BP1 core, and facilitates its formation [62]. SGs are RNP granules that dynamically incorporate translationally stalled mRNAs, releasing significant amounts of unbound mRNA into the cytosol, which is essential for SG formation and stability. G3BP1 acts as a central node and molecular switch, triggering RNA-dependent LLPS in response to increased intracellular free RNA concentration, leading to SG assembly [60]. Caprin1 binds to G3BP, promoting SG formation [63], while stalled translational complexes associate with core SG proteins such as TIA-1 [64], TIAR [65], G3BP [63], and PABPC1 [66]. These proteins contribute to the aggregate of mRNPs and form the SG core, storing translation initiation factors, RNA-binding proteins, and signaling molecules [67]. This aggregation process constitutes the second step in SG formation: the nucleation and assembly of mRNPs into oligomers.

When SG formation is initiated, small granules appear simultaneously. These small SGs subsequently fuse to create mature clustered SGs in a microtubule-dependent manner [2]. This fusion and maturation process constitutes the third step in SG formation: involves further aggregation, leading to the development of fully mature SGs. These series of events are crucial for the proper formation and function of SGs.

SGs formed under stressful conditions do not dissolve rapidly; the dissolution of intact SGs typically takes several hours [68]. During disassembly, larger SGs break down into smaller SGs, which then dissolve individually, without further fragmentation [69]. This process likely reflects the progressive reversal of multivalent interactions involving proteins and RNAs. Interestingly, translation is restored before the complete dissolution of SGs, with nearly half of the transcripts being actively translated during stress recovery while SGs are still present [70]. Dephosphorylation also contributes to SG clearance; ATF4, a transcription factor, induces the expression of GADD34, a phosphatase regulatory subunit that recruits protein phosphatase 1 (PP1) to eIF2α, triggering its dephosphorylation and thus alleviating translation arrest [71]. Studies have shown that autophagy or molecular chaperones, such as the HSPB8-BAG3-HSP70 complex, can mediate SG clearance [72]. The autophagy pathway specifically removes persistent or aberrant SGs [73]. The solubilization of SGs is influenced by the duration and severity of stress conditions: SGs formed from acute heat stress are not dependent on autophagy for catabolism, whereas those induced by prolonged heat stress rely on autophagy for clearance [74, 75]. Additionally, several deaggregation enzymes, such as VCP [76] and the endoplasmic reticulum-associated protein FAF2 [77], are essential for SG clearance. Damage to these components significantly delays SG dissolution and may lead to the formation of abnormal SGs (Fig. 3).

Fig. 3

Formation and decomposition of stress granules. In cells under stress conditions translation initiation is halted and RNA-dependent LLPS occurs in the cytoplasm in response to increased intracellular free RNA concentration, which in turn leads to the assembly of SGs. mRNPs are assembled into oligomers, at which point the SGs are initially formed, and then through further aggregation, fusion, and maturation ultimately mature SGs are formed. After the stress conditions subside, the larger SGs break down into smaller SGs, which then dissolve individually

Functions and mechanisms of action of SGs

SGs in cells help mitigate responses to stressors such as viral invasion, chemical damage, and oxidative stress. Research indicates that SGs can inhibit viral replication independently of the RLR pathway, thereby acting outside immune control. This highlight their role as multifunctional cellular “shock absorbers” that protect homeostasis by curbing toxic immune responses and viral replication [77]. APE1 promotes phosphorylation of YBX1 at the S174 and S176 sites, enhancing SG formation and promoting cell survival [78]. In KRAS-mutated cells, SGs significantly increase in response to various stress stimuli, controlling prostaglandin metabolism and providing cytoprotection through both cell-autonomous and non-autonomous mechanisms [79]. SGs can inhibit apoptosis by suppressing reactive oxygen species (ROS) levels, a function regulated by two SG components: G3BP1 and USP10. G3BP1 increases ROS levels by inhibiting the antioxidant activity of USP10. When SGs are formed under stress conditions, G3BP1 and USP10 are recruited into SGs, and this inhibition is inactivated. Therefore, SGs formation weakens the ability of G3BP1 to inhibit the antioxidant activity of USP10, which in turn reduces ROS levels and inhibits apoptosis [80]. Additionally, SGs can alleviate endoplasmic reticulum stress (ERS)-mediated apoptosis, protecting hepatocytes from hypoxia-induced injury during acute liver failure (ALF) [81].

Numerous studies have demonstrated that the anti- apoptosis effects in cells are associated with the formation of SGs. Exposure to various stress conditions can result in either cell recovery or apoptosis, depending on the type and intensity of the stress. Under conditions such as nitrite exposure, hypoxia, and heat stress, the formation of SGs is promoted. These SGs inhibit stress-induced apoptosis by recruiting key proteins in the relevant signaling pathways. For instance, SGs regulate cellular signaling by recruiting proteins such as TOR, RACK1, and TRAF2. When DYRK3 is inactivated, it prevents the disassembly of SGs and the release of mTORC1, which is then recruited into SGs, inhibiting mTOR signaling [82, 83]. RACK1, a pro- apoptosis protein, is known to activate apoptosis pathways, but SGs inhibit caspase-3 activation by recruiting RACK1, thereby promoting cell survival [84]. Additionally, stressed cells recruit translation factors and TRAF2 into SGs through protein-protein interactions, blocking tumor necrosis factor signaling and thus the pro-inflammatory response [85]. Previous research has shown that TM4SF1-AS1 associates with various SG-associated proteins, including G3BP2, RACK1, and DDX3. TM4SF1-AS1 promotes SGs formation and inhibits apoptosis in gastric cancer cells by sequestering RACK1 within SGs [86] (Fig. 4).

Fig. 4

Role of stress granules in cellular stress response. SGs protect cells from stress by recruiting proteins such as TOR, RACK1, or TRAF2 that affect signaling in the apoptosis pathway under stress conditions

Effects of SGs produced by environmental stimuli on germ cells

Environmental stimuli are first detected by cell surface receptors, which then activate a series of cascading signaling pathways, with the phosphorylation of eIF2α being central. Under stress conditions such as heat shock, oxidative stress, nutrient deprivation, or viral infection, the phosphorylation level of eIF2α increases, leading to a global inhibition of protein synthesis. This triggers the aggregation of mRNAs with RNA-binding proteins to form SGs, temporarily sequestering mRNA until the stress is alleviated, thereby allowing normal translation to resume. This process ensures that cells maintain basic functions and viability under adverse conditions. Arsenite induces oxidative stress in cells, promoting eIF2α phosphorylation and SG assembly [87], which in turn reduces cellular damage caused by arsenite. Physical stressors, such as extreme temperatures, also impact cellular physiology; high temperatures trigger a heat shock response, increasing the expression of heat shock proteins (HSPs) and promoting SG formation [88].

Research has shown that SGs are developmental stage-specific. After heat stress, SGs are primarily found in the testicular lumen during stages I-VIII, with the highest distribution in stages IV-VI, and are almost absent in stages IX-XII. SGs are widely distributed in spermatogonial cells, early spermatocytes, and late spermatocytes but are not formed in round or elongated spermatids [13]. Because germ cells carry the function of gene transmission, it has been shown that the effects caused by environmental factors may be transmitted epigenetically through germ cells to offspring [89, 90], so we need to pay extra attention to the effects of environmental factors on germ cells. This mechanism is crucial for protecting germ cells from heat-induced DNA damage and protein misfolding. In reproductive health, high temperature exposure affects spermatogenesis and oocyte maturation, with SGs potentially acting as a buffer, reducing direct damage to germ cells [13, 14, 91].

SGs participate in the regulation of reproductive disorders under environmental stimulation

Role of SGs in reproductive dysfunction following chemical exposure

Endocrine-disrupting chemicals (EDCs) are widespread and can accumulate in the environment as well as in everyday household products, including cosmetics, plastic food packaging, and pharmaceuticals. In the male reproductive system, EDCs are associated with various health issues, such as cryptorchidism [92], testicular cancer [93], and reduced semen quality [94]. Heavy metals are introduced into the environment through natural sources like rock weathering andvolcanic eruptions, as well as through human activities, including industrial emissions, mineral extraction, and automobile exhaust. These metals disrupt multiple aspects of reproductive functions in both women and men, including hormonal imbalance [95], sperm count, viability and spermatogenesis [96].

Arsenic (As) is the 20th most abundant element in the Earths’ crust and is widely distributed in the environment. It predominantly exists in the terrestrial environment in inorganic forms, including arsenate (AsIII) and arsenite (AsV), both of which are toxic to humans and animals, Inorganic arsenic is water-soluble, leading to its accumulation in biological cells [97]. Exposure to and accumulation of arsenic predisposes individuals to cancerous and non-cancerous diseases. Chronic arsenic exposure can also impact reproductive health and alter responses to environmental stressors by affecting estrogenic and androgenic signaling processes [98, 99]. Arsenite, in particular, acts as an endocrine disruptor, inducing the formation of SGs. Studies have shown that heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNPA2B1) is involved in the formation of SGs under arsenite-induced stress. Deletion of hnRNPA2B1 promotes the disassembly of SGs, leading to support-cell-only syndrome (SCOS) and complete male sterility [100]. Additionally, arsenite induces the formation of SGs containing SERPINE1 mRNA-binding protein 1 (SERBP1), which inhibits apoptosis caused by arsenite stress and promotes the clearance of SGs to prevent their abnormal accumulation [101]. These findings suggest that many ribonucleoproteins may be present in SGs formed under arsenite-induced stress conditions, and some may regulate the assembly and disassembly of SGs, thereby supporting the survival of germ cells.

Role of SGs in reproductive dysfunction in heat stress exposure

The reproductive system of male mammals is more susceptible to the adverse effects of elevated ambient temperatures due to a lower physiological temperature than a constant body temperature resulting from its distance from the body cavity. The normal spermatogenesis process in male mammals requires temperatures that are 2–7 °C lower than the core temperature, and sperm quality decreases significantly as scrotal temperature increases [102]. The main effects of heat stress on cells include increased membrane fluidity, changes in cellular morphology including rounding and shrinkage due to cytoskeletal modifications and reorganization of microfilaments [103, 104] and heat stress causes significant morphological changes in many intracellular materials such as mitochondrial swelling, increase in visible size and enlargement of cristae space, fragmentation of the endoplasmic reticulum and the Golgi apparatus, and under conditions of heat stress leads to the nucleolus disassembly and consequently the release of a variety of proteins, resulting in damage to the ultrastructure of the nucleus [105]. Spermatogonia, on the other hand, are more susceptible to heat stress than other testicular cells due to their delicate and prolonged development, during which there are multiple divisions [106]. The results of existing studies indicate that heat stress induces germ cell apoptosis and spermatogenic disorders in male mammals, and the scrotum can be affected by heat stress to produce damage such as apoptosis, autophagy, lipid peroxidation, oxidative stress, DNA fragmentation, and damage to the blood-testis barrier which can result in the destruction of testicular structure, cell death, and gamete development abnormalities, which ultimately leads to decreased fertility in males and male animals and even infertility [107, 108].

SGs can form in germ cells under heat stress conditions. Studies have shown that SGs are widely distributed in spermatogonia, early spermatocytes, and late spermatocytes under heat stress, whereas they are absent in round and elongated spermatids, SGs formation inhibits heat stress-induced apoptosis [13] (Fig. 5). It has been previously demonstrated that SGs in male germ cells contain the spermatogonial cell-specific marker DAZL, which protects against heat-induced apoptosis by sequestering specific signaling molecules like RACK1 to block apoptosis MAPK pathways [14]. The testis-specific protein MAGE-B2 regulates the translation of the SGs core protein G3BP by displacing DDX5, enhancing cellular tolerance to heat stress [109]. Under heat stress, NEDD4 controls spermatogonial stem cell homeostasis by regulating the involvement of messenger ribonucleoprotein complexes (mRNPs) in SG synthesis and promoting SG disassembly during recovery to prevent pathological accumulation [110]. The TIAR-1 protein facilitates SG formation in Caenorhabditis elegans under heat stress, protecting female germ cells from heat shock [91]. These findings suggest that heat stress induces SGs formation by phosphorylating eIF2α and recruiting RNA-binding proteins, such as DAZL, BOULE, RACK1, ultimately protecting normal spermatogenesis and inhibiting germ cell apoptosis. Additionally, heat-induced SGs can protect female germ cell formation and prevent apoptosis.

Fig. 5

Spermatogenesis and the stages of distribution of SGs under stress conditions. Cross-sections of the seminiferous tubules and specific periods of formation of SGs during spermatogenesis

Role of SGs in reproductive dysfunction influenced by other factors

Traumatic brain injury (TBI) can significantly affect male reproductive function, which relies on the brain as well as the hypothalamic and pituitary centers. The intact hypothalamic-pituitary-gonadal (HPG) axis is crucial for male reproduction. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones stimulate the interstitial and supporting cells in the testis respectively. The interstitial cells produce most of the body’s testosterone, while the supporting cells release androgen-binding proteins. These process is essential for spermatogenesis [111]. Studies have shown that TBI can induce the formation of SGs in vivo, with the extent of SG formation directly related to the severity of the trauma [112]. For example, a 20-year-old taekwondo athlete developed reproductive dysfunction following a competition [113]. Other studies have documented various changes in hypopituitarism caused by exercise-associated TBI, such as growth hormone deficiency and central dysuria [114,115,116]. Sports commonly associated with head trauma and consequently with TBI risk to the HPG axis include soccer, cycling, boxing, and skiing [111]. These findings suggest that various environmental factors in life, such as TBI, may induce the formation of SGs, which protect the cells to mitigate the effects of stimuli in life, and thus enhance the stress tolerance of the cells to protect germ cells from stress damage. The role of stress granules-associated proteins in reproductive dysfunction under environmental stimulation are listed in Table 3.

Table 3 The role of stress granules-associated proteins in reproductive dysfunction under environmental stimulation

SGs could be a potential therapeutic target

Phosphorylation of eIF2α is a crucial step in the formation of SGs, positioning its upstream kinases and downstream phosphatases as potential therapeutic targets. Modulating these proteins using small molecule inhibitors or activators can regulate SGs formation, thereby influencing disease processes. Studies have shown that antioxidants, such as cyanidin-3-O-glucoside (C3G) and its metabolites, PCA and vitamin C, can mitigate the effects of environmental stress on testicular tissues by modulating eIF2α phosphorylation. This reduces the translation-blocking process, thereby decreasing SG formation and enhancing the stress tolerance of male germ cells [117]. Consequently, exploring small molecule inhibitors or activators that regulate eIF2α phosphorylation, both upstream and downstream, could bolster stress tolerance and mitigate germ cell damage caused by environmental stressors by modulating SGs formation.

Research has demonstrated that numerous stress granule-associated proteins play pivotal roles in SGs formation and mitigating cellular damage. Proteins such as hnRNPA2B1, SERBP1, and NEDD4 are crucial for the timely clearance of SGs, thus protecting germ cells from damage, preventing Support Cell Only Syndrome (SCOS), and reducing male infertility and aberrant protein accumulation [100, 101, 110] (Fig. 6). Key RNA-binding proteins like G3BP1 and TIA-1/TIAR are essential in SGs assembly. Investigating molecules that interfere with protein-protein interactions or target specific abnormally accumulating RNA-binding proteins could offer new therapeutic avenues. This approach would promote the timely disassembly of SGs and restore normal protein synthesis. Future research should focus on proteins that regulate SGs formation and timely disassembly, as well as the interactions among these proteins, to dynamically manage SGs and improve reproductive health. In cancer cells, SGs formation is accelerated in response to stress from radiotherapy and chemotherapy, significantly reducing the efficacy of these treatments. APE1 has been shown to promote SGs formation through YBX1 phosphorylation [78], suggesting that inhibiting YBX1 phosphorylation could suppress SGs formation in ovarian cancer, thereby enhancing the effects of chemotherapy and radiotherapy. Studies have shown that PKI-402, a dual PI3K/mTOR inhibitor, promotes the formation of SGs in ovarian cancer cells, which play a protective role by intercepting the signaling factor ATF5, inducing changes in localization and modulating mitochondrial unfolded proteins, activating adaptive stress responses and increasing drug resistance [118]. Moreover, an abnormal increase in SGs has been observed in SPOP-mutant prostate cancers cells, where SG assembly enhances cancer cell resistance to chemotherapeutic agents [119]. These findings underscore the importance of exploring molecular targeted drugs that affect SGs in cancer cells. Identifying proteins and factors that promote SG clearance could lead to the development of drugs that enhance the effectiveness of radiotherapy and chemotherapy in ovarian cancer, prostate cancer, and other cancers.

While SGs hold significant promise as therapeutic targets, several challenges persist. The function of SGs varies across different cell types and stress conditions, requiring a precise understanding of their roles in specific diseases. Furthermore, regulating SGs without disrupting normal cellular functions or inducing unwanted side effects is crucial when investigating their therapeutic potential.

Fig. 6

Formation of stress granules in germline stem cells (GSCs) induced by heat stress (42 °C, 20 min) and in the recovery group (33 °C, 3 h) after heat stress induction [110]. A-B: Scale bar, 10 μm. C: *GSCs with SGs

Summary

SGs are being studied in full swing as membrane-free organelles that sense and resist changes in the external environment. Their main feature and function is that as a dynamic membrane-free cytoplasmic organelle they can respond to a variety of stress factors both inside and outside the cell and dissolve after the stress is relieved. SGs can promote cell survival, by blocking signaling and inhibiting apoptosis. Current research on SGs primarily focuses on neurodegenerative diseases, cancer and inflammatory diseases. The unique division process and prolonged development timeline of germ cells make them particularly vulnerable to environmental factors, which can be harmful. Environmental stimuli can trigger the formation of SGs during gametogenesis, affecting the process of gametogenesis itself. Since gametogenesis is essential for intergenerational transmission, changes induced by environmental stressors may be passed to the next generation via the gametes, leading to potential epigenetic inheritance. Therefore, SGs hold significant importance in the study of reproductive disorders and fertility regulation.