Regulation of germline proteostasis by HSF1 and insulin/IGF-1 signaling

Abstract

Protein homeostasis (proteostasis) is essential for cellular function and organismal health and requires the concerted actions of protein synthesis, folding, transport, and turnover. In sexually reproducing organisms, the immortal germline lineage passes genetic information across generations. Accumulating evidence indicates the importance of proteome integrity for germ cells as genome stability. As gametogenesis involves very active protein synthesis and is highly energy-demanding, it has unique requirements for proteostasis regulation and is sensitive to stress and nutrient availability. The heat shock factor 1 (HSF1), a key transcriptional regulator of cellular response to cytosolic and nuclear protein misfolding has evolutionarily conserved roles in germline development. Similarly, insulin/insulin-like growth factor-1 (IGF-1) signaling, a major nutrient-sensing pathway, impacts many aspects of gametogenesis. Here, we focus on HSF1 and IIS to review insights into their roles in germline proteostasis and discuss the implications on gamete quality control during stress and aging.

Introduction

Proteostasis refers to the cellular state in which proteins are properly synthesized, folded into functional conformations, targeted to correct cellular compartments, and turned over when a mission is completed [1]. Players that function at those steps of the “on-pathway” protein lifecycle are collectively called the proteostasis network (PN) [1]. Proteostasis is essential for cellular functions but becomes compromised by environmental stresses (e.g. high temperature and oxidants) [2] and genetic mutations [3–6], which increase ‘off-pathway’ events including errors in translation, harmful modifications, protein misfolding and aggregation, and eventually cause proteome imbalance [7]. As part of a protein quality control system, cells have evolved robust stress response pathways that transcriptionally or translationally upregulate key PN genes encoding detoxification enzymes, molecular chaperones and protein clearance pathways (e.g. ubiquitin-proteasome system (UPS) and autophagy) to fix or remove damaged and misfolded proteins [8]. Among those proteotoxic stress responses are the heat shock response (HSR) that copes with protein misfolding in the cytosol and nuclei, unfolded protein response in the endoplasmic reticulum (UPR^ER) and mitochondria (UPR^mito) as well as antioxidant response (OxR).

While a healthy proteome is important for all cellular functions, increasing interest has been raised to understand the regulation of proteostasis in germ cells [9,10]. In sexually reproducing animals, gametogenesis refers to the processes that produce sperm, oocytes, or both (e.g., in the C. elegans hermaphrodites) from primordial stem cells via mitotic proliferation, meiotic differentiation and gamete maturation (Figure 1). Gametogenesis is essential for the passage of genetic information over generations while providing an opportunity to introduce genetic diversity via meiotic recombination. It is well known that germ cells employ robust mechanisms to maintain genetic integrity as indicated by the lower spontaneous mutation rate relative to somatic cells [11]. The recent discovery of lysosomal activation in C. elegans and Xenopus oocytes to resolve protein aggregates suggests that proteostasis may be reset before fertilization, implicating the importance of a healthy proteome to oocyte maturation and subsequent embryogenesis [12].

Figure 1. Diagrams of gametogenesis and fertilization in mammals and the C. elegans hermaphrodite.

A. The different steps of oogenesis (left) and spermatogenesis (right) in mammals and subsequent fertilization. Oocyte growth and maturation to meiosis I (MI) are highlighted. In the growth phase, primary oocytes at the germinal vesicle (GV) stage gain size and accumulate maternal factors through robust transcription and translation. With chromatin compaction, fully grown oocytes cease transcription before germinal vesicle breakdown (GVBD) and reenter meiosis [14,15]. The oocyte growth phase is where HSF1’s transcriptional activity contributes to postnatal oogenesis.

B. Germline development and fertilization in the gonad of C. elegans hermaphrodite. The proliferation of germ cells occurs continuously until they transition to meiosis prophase I. A similar oocyte growth phase occurs from the diplotene stage to pre-ovulatory oocytes, where transcription is suppressed. Oocyte maturation in worms is triggered by major sperm protein (MSP), which is secreted by sperm and serves as a hormone [118].

Unique Features of Proteostasis Control in the Germline

Characteristics of germline proteostasis

Despite obvious differences in spermatogenesis and oogenesis (as illustrated in Figure 1), both processes share several specific features of proteostasis regulation in common.

First, germline development experiences periods of very active protein synthesis, thus demanding high capacities of nascent folding. Spermatogenesis starts from mitotic proliferation of spermatogonial stem cells (SCCs) to replenish the stem cell pool and produce primary spermatocytes in massive quantities [13]. Robust translation is required to make proteomes for new cells and for subsequent meiosis. On the other hand, oogenesis in mammals has temporally separated mitosis and meiosis [14,15]. All primary oocytes are already made at the time of birth and arrested at meiotic prophase I. Following puberty, primary oocytes undergo a growth phase, in which the oocytes increase in volume and are highly active in transcription and translation. Messenger RNAs synthesized at this growth stage are either used for immediate translation or stored in repressive complexes for protein synthesis in the subsequent oocyte maturation after meiosis is resumed [16,17]. The robust translation during oocyte growth and maturation phases is necessary to make proteins that function in meiosis and fertilization and to prepare maternal factors for early embryogenesis. The massive protein synthesis in both spermatogenesis and oogenesis consequently requires sufficient folding capacities for nascent polypeptides aided by molecular chaperones and adds pressure on the protein quality control system.

Second, germ cells cease transcription and/or translation and lack the canonical stress responses at certain stages of gametogenesis. Transcription is suppressed when primary oocytes re-enter meiosis until zygotic gene activation [14,16]. Therefore, the synthesis of PN players is solely dependent on the mRNAs made during the oocyte growth phase. In addition, the lack of transcriptional stress responses including the HSR and UPRs raises unique challenges to oocyte proteostasis, especially during oocyte maturation when translation is highly active [14,18]. Similarly, spermatozoa made from primary spermatocytes through meiosis and spermiogenesis are transcriptionally inactive. In addition, these sperm cells lack cytoplasmic content and translation [13], therefore must maintain proteome integrity with very limited protein renewal.

Finally, multiple PN players in protein quality control (e.g., molecular chaperones and UPS) are engaged in specialized roles in meiosis and germline function. This multitasking adds complication to proteostasis control in germ cells. As an example, the HSP90 chaperone and its co-chaperone CDC37 are associated with many key kinases that control cell cycle and meiosis and are required for their stability and/or activation [19–21]. Inhibition of HSP90 in mouse oocytes destabilizes cyclin-dependent kinase 1 (CDK1) and impairs the mitogen-activated protein kinase (MAPK) signaling, which underlie the delayed resumption of meiosis and abnormal cytokinesis respectively [21]. Similarly, HSP90/CDC37 contributes to human sperm capacitation by promoting the activation of extracellular signal-regulated kinase 1/2 (ERK1/2) in the MAPK pathway [20]. It is noteworthy that HSP90 could modulate different phases of gametogenesis via different clients. In C. elegans, HSP90 functions in MAPK activation during oocyte growth [22] and maintains the proper prophase I arrest of fully grown oocytes through stabilizing WEE-1.3 kinase [23]. Furthermore, HSP90 and HSC70 chaperone machinery suppresses transposon activation in both Drosophila ovaries and testes by promoting PIWI-interacting RNA (piRNA) biogenesis [24,25] suggesting a role of proteostasis in germline genome stability. Like molecular chaperones, UPS is involved in the regulation of meiosis besides its quality control function of removing misfolded proteins. It has been reported that proteasome is recruited to meiotic chromosomes during mouse spermatogenesis and UPS-mediated proteolysis is required for the selection of crossover sites [26]. UPS also functions in oocyte maturation. Proteasome degradation of protein phosphatase 2A (PP2A) and CDK1 is important for meiosis resumption and first polar body extrusion respectively [27,28]. It is well established that different client proteins and substrates compete for the limited capacity of molecular chaperones and proteasomes in cells [29–32]. Germ cells are expected to be hypersensitive to damaged and misfolded proteins as they titrate molecular chaperones and UPS from their specific targets in gametogenesis. Therefore, a pristine proteome is necessary to ensure gamete quality.

Different challenges to proteostasis in male and female germlines

Besides the above-discussed common features of germline proteostasis, the different physiological properties of spermatogenesis and oogenesis put additional challenges to proteome health in these processes.

The testes are located outside the body cavity and express many testis-specific genes [33]. The lower ambient temperature and highly specialized proteome are expected to require a tailored PN [9]. Consistent with this idea, several testis-enriched molecular chaperones and UPS components have been discovered in mice that play essential roles in spermatogenesis. These include the HSP70-family chaperone HSPA2, which is involved in the synaptonemal complex function and CDC2/cyclinB1 complex formation during meiosis I [34,35] and contributes to sperm-oocyte recognition via regulation of sperm surface receptors [36]. Another example is the testis-specific proteasome subunit PSMA8, which is incorporated in the 20S core particle [37], located to the synaptonemal complex, and responsible for the turnover of several key meiotic players [38].

A prominent challenge to oocyte proteostasis is the long cell-cycle arrest of primary oocytes before the resumption of meiosis, which provides a time window for accumulation of protein damage and misfolding [39,40]. It was discovered in C. elegans that a lysosomal pathway is activated in oocytes to clear the protein aggregates, providing a mechanism of protein renewal before meiotic maturation and fertilization [12]. Notably, players in protein synthesis, folding, degradation and ER proteostasis are also required for lysosomal protein clearance in oocytes [41], highlighting the importance of concerted activities of PN. Lysosomal activation in oocyte maturation is likely conserved in lower vertebrates [12] and remains to be tested in mammals. Furthermore, it is known that certain proteins critical for meiosis have a limited capacity of turnover during oogenesis (e.g., subunits of the cohesion complex) [42,43], therefore relying on the maintenance of protein integrity through the prophase I arrest.

Multifaceted Roles of HSF1 in Germline Proteostasis

The HSR-dependent and independent roles of HSF1 in proteostasis

HSF1 (HSF-1 in C. elegans) is best known as a key regulator of the heat shock response (HSR). In the classic model of the HSR, HSF1 is activated upon thermal stress and rapidly induces transcription of a group of molecular chaperones initially discovered as heat shock proteins (HSPs) [44]. The activation and attenuation cycle of HSF1 is subject to the regulation of HSF1 trimerization, chromatin accessibility, post-translational modifications, proteasomal degradation, and interaction with chaperones. The molecular mechanisms of the HSR have been discussed in detail in other reviews [44–46]. Members of the small HSPs and inducible HSP70s (iHSP70s) are among those with the highest fold induction by HSF1 in the HSR and have important roles in handling misfolded proteins [47,48]. The small HSPs co-aggregate with misfolded proteins and hold them in a reversible state for disaggregation and refolding through the coordinated actions of HSP70 with the HSP104 disaggregase in yeast or with HSP40/DNAJ and HSP110 co-chaperones in metazoans [49]. Accumulating evidence indicates broader functions of the HSR than coping with heat shock. It has been accepted that the HSR is induced by diverse cellular stresses that cause protein misfolding in the cytosol and nuclei, and the direct targets of HSF1 in the HSR include detoxification enzymes and components of protein clearance machinery as well [45]. On the other hand, genomic studies in the past decades have revealed context-dependent HSF1 transcriptional programs that are distinct from the HSR, most prominently in animal development, carcinogenesis, and energy metabolism [50]. Among the increasing number of HSF1 target genes, a compact yet important network of chaperones and co-chaperones utilize HSF1 to activate their expression in the absence of stress in yeast and human cancer cells as well as during C. elegans larval development [51–53], suggesting a role of HSF1 in proteostasis beyond coping with environmental stress.

Specialized HSRs during gametogenesis

HSF1 is important for gametogenesis in both vertebrates and invertebrates [54–57]. Despite that only a subset of the germline is transcriptionally competent, the HSF1-mediated HSR has a profound consequence in cell fate decisions upon proteotoxic stress. In C. elegans, the maternal HSR is important for the survival of progenies upon thermal stress [58]. Interestingly, the effects of HSR are beyond oocyte quality control as HSF-1 in C. elegans germline could mediate transgenerational stress resistance through epigenetic remodeling [59]. It is noteworthy, the HSR in germ cells is quite different from that in somatic tissues and has divergent roles in the male and female germline. In C. elegans, HSF-1 shows different binding preferences in the somatic and germline cells upon heat stress. While HSF1 binds to the promoters and drives the expression of HSP genes in the soma, not all transcriptionally active germ cells can induce the canonical HSR. Instead, HSF-1 binding is enriched at loci of Helitron DNA transposons in the germline upon heat stress [60]. Whether HSF-1 impacts the activity of those transposable elements is yet to be determined but it is proposed that Helitron insertion rewires HSF1 binding sites in the genome and underlies species- and strain-specific HSR [61]. In mice, the classic HSR is considered to be pro-survival in primary oocytes [62,63]. On the contrary, HSF1 promotes apoptosis in pachytene spermatocytes upon thermal stress [64]. Instead of inducing HSPs to protect those spermatocytes, HSF1 activates the expression of the proapoptotic gene Tdag51 [65], which is proposed as a selective mechanism to eliminate damaged germ cells from further spermatogenesis. The distinct roles of HSF1 in proteotoxic stress response during oogenesis and spermatogenesis are linked to their different physiology. While spermatocytes are made in large quantities and continuously during adult life, the pool of primary oocytes is determined at birth in mammals and is limited.

In C. elegans, gametogenesis also has profound impacts on maternal stress response and proteostasis in somatic tissues. It is well known that germline stem cell (GSC) arrest enhances stress responses including the HSR and extends lifespan [66–68]. Recent studies suggest that GSC proliferation and embryo integrity regulate the age-associated decline of the HSR [69,70]. In addition, germline proteostasis could have systemic effects on protein aggregation in the soma through the regulation of mitochondria [71]. Therefore, it is important to take into consideration cell-non-autonomous effects of gametogenesis and germline proteostasis in stress resistance and organismal health.

Regulatory targets of HSF1 in germline proteostasis and beyond

In addition to the HSR, HSF1’s activities are required for germline development in physiological conditions. It has been shown during mouse gametogenesis that HSF1 binds to promoters and regulates the transcription of genes that are not involved in the canonical HSR. These include meiotic genes in mouse oocytes [72] and the sex chromosomal multicopy genes from mouse testis [56]. On the other hand, expression of Hsp90α, the major isoform of HSP90 genes in mouse oocytes, is activated by HSF1 in the absence of environmental stress [21]. Inhibition of HSP90 phenocopied diverse meiotic defects in HSF1 knock-out oocytes including delayed germinal vesicle breakdown, partial meiosis I block and defective asymmetric division [21], implicating Hsp90α as a primary target of HSF1 in oogenesis. A systematic analysis of the HSF-1 transcriptional program in gametogenesis has been done in C. elegans [60]. HSF-1 activates the expression of HSP90, HSC70 and their co-chaperones as well as subunits of the CCT/TRiC complex during C. elegans germline development. Though some members of this chaperone network can be induced in the HSR, in comparison to small HSPs and iHSP70s, they have substantial expression in physiological conditions and limited induction during stress. In addition, they have essential roles in ATP-dependent protein folding, especially in the folding and maturation of newly synthesized proteins [73–75]. HSF-1 also binds to the promoters and activates other pro-reproduction genes in the C. elegans germline. However, kinetic analyses upon loss of germline HSF-1 indicate that the chaperone network is the most sensitive to HSF-1 activities. This evidence suggests a vital role of HSF-1 in germline proteostasis.

Insulin/IGF-1 Signaling (IIS) in Proteostasis and Gametogenesis

Roles of IIS in proteostasis

Insulin/IGF-1 signaling (IIS) is a highly conserved nutrient-sensing pathway that has broad impacts on energy homeostasis, stress response, reproduction, and longevity [76]. In mammals, insulin secretion is regulated by circulating nutrients, primarily glucose [77]. Binding of insulin or IGF-1 to its receptors (IR/IGF-1R) activates the tyrosine kinase receptor and initiates downstream signaling through two kinase cascades (Figure 2A): phosphoinositide 3-kinase (PI3K) that further activates the serine/threonine kinase AKT (PI3K/AKT pathway) and the small GTPase Ras that leads to activation of MAPK signaling (Ras/MAPK pathway) [78,79]. Through different substrates of the two kinase pathways, IIS modulates diverse cellular processes including multiple steps in the protein lifecycle that impacts proteostasis [76].

Figure 2. Models of proteostasis regulation by IIS and HSF1.

A. The PI3K/AKT and Ras/MAPK pathways downstream from the insulin/IGF-1 receptor and their effectors that regulate protein metabolism. RSK, a downstream kinase of MAPK/ERK, phosphorylates the regulatory associated protein of mTOR (raptor) and directly stimulates mTORC1 activity [81]. In addition, both AKT and RSK phosphorylate and inactivate the tumor-suppressor function of tuberous sclerosis complex-1/2 (TSC1/2). As TSC1/2 inhibits the small GTPase Ras homolog enriched in the brain (Rheb), an activator of mTORC1, ATK and RSK indirectly activate mTORC1 through TSC1/2 and Rheb [80]. Phosphorylation of 4E-BP1 by mTORC1 releases 4E-BP1 from the translation initiation factor eIF4E, allowing cap-dependent translation to occur [83]. mTORC1 also promotes translation through S6K, which phosphorylates ribosomal S6 (rpS6), translation initiation factor eIF4B and eukaryotic elongation factor 2 kinase (eEF2K) to regulate translation initiation and elongation [82,84]. MAPK can also regulate translation via mTORC1-independent mechanisms. Similar to S6K, RSK phosphorylates rpS6 and eIF4B [80]. MNK mediates the phosphorylation of two translation initiation factors, eIF4E and eIF4G [119]. AKT can also regulate proteostasis via stress-responsive transcription factors, NRF2 and FOXO [68,87,88]. NRF2 activates antioxidant response (OxR), while FOXO regulates OxR and additional PN genes. AKT-mediated phosphorylation prevents NRF2 and FOXO from localizing to the nucleus and activating their target genes.

B. Working model of germline HSF1 activation in response to IIS. Chaperones including HSC70, HSP90 and the TRiC/CCT complex interact with HSF1 and inhibit its activity [115–117]. When IIS is high, newly synthesized proteins titrate chaperones from HSF1, resulting in HSF1 activation that upregulates chaperone expression to support protein anabolism. When IIS is low, translation is less robust. Chaperones are in excess, which keeps HSF1 in the inert state and downregulates HSF1-dependent chaperone expression.

IIS regulates proteostasis via transcriptional control of PN players and transcription-independent mechanisms. Phosphorylation events by AKT and MAPK pathways converge on multiple points to promote protein synthesis and suppress protein turnover. Both AKT and p90 ribosomal protein S6 kinase (RSK) in the MAPK pathway, activate mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) through direct or indirect mechanisms [78–81]. mTORC1 promotes both translation initiation and elongation by phosphorylating p70 ribosomal protein S6 kinase (S6K) and eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1) [82–84]. Conversely, mTORC1 suppresses autophagy through multiple substrates [85]. In addition, the MAPK pathway can also activate translation through mTORC1 independent mechanisms via its downstream kinases, RSK and MNK1 [78,80].

IIS also has a central role in proteotoxic stress response and controls the expression of many PN players [86]. IIS represses the stress-responsive transcription factors forkhead box class O (FOXO) and nuclear factor erythroid 2-related factor 2 (NRF2) through AKT-mediated phosphorylation [68,87,88]. 4E-BP1 is a direct target gene of FOXO in Drosophila and mammals [89,90] providing another way for IIS to regulate mTORC1 signaling. Mammalian FOXO transcription factors also promote protein degradation by activating genes in autophagic/lysosomal and proteasomal pathways [91–93]. The C. elegans ortholog of FOXO, DAF-16 promotes the expression of diverse proteostatic genes including proteasome components, lysosomal genes, small HSPs, and those involved in antioxidant response and ER proteostasis [94–99]. Similarly, NRF2 and its C. elegans ortholog, SKN-1 serve as the master regulator of antioxidant response that induces genes encoding proteins in detoxification and elimination of reactive oxygen species (ROS) [100]. Reduction of IIS leads to constitutive activation of FOXO/DAF-16 and NRF2/SKN-1 and enhanced stress resistance [68,87]. Reduction of IIS also promotes the HSR in C. elegans upon stress [101]. However, different from the regulation of FOXO/DAF-16 and NRF2/SKN-1, IIS activates rather than represses the germline HSF-1 transcriptional program in physiological conditions [60].

Collectively, through both transcriptional and post-transcriptional regulations, IIS promotes protein synthesis and suppresses protein turnover, which favors cell proliferation and growth. On the other hand, reduced IIS enhances proteotoxic stress response, activates protein degradation, and increases proteostatic capacity in adverse growth conditions.

Cell-autonomous and non-autonomous regulation of gametogenesis by IIS

As an essential coordinator of energy metabolism, growth and maintenance in response to nutrient abundance, IIS has evolutionarily conserved roles in germline development and quality control in both cell-autonomous and non-autonomous manners [76]. In C. elegans and Drosophila, IIS promotes germline stem cell proliferation and vitellogenesis that occurs in the intestine and fat body respectively [102]. IIS also contributes to the later stages of oogenesis. In C. elegans, IIS couples nutrient signals with meiotic progression via the MAPK pathway [103]. In Drosophila, IIS regulates oocyte maturation by modulating mitochondrial activity [104]. Mammals have evolved more complicated endocrine systems for reproduction, in which IIS works cooperatively with gonadotropins to impact gametogenesis. IIS contributes to steroidogenesis from stroma and follicular cells in the ovary [102], which promotes folliculogenesis and meiotic maturation of oocytes. Similarly, IIS is required for androgenesis by the Leydig cells in the testis [105] and has essential roles in the Sertoli cell proliferation and sperm production [106].

While IIS in general has positive roles in gametogenesis, its activity needs to be fine-tuned for gamete quality. Hyperactivation of IIS in mouse oocytes leads to premature activation and depletion of functional oocytes [107]. On the contrary, Ames dwarf mice that have low circulating IGF-1, better preserve the pool of primordial follicles in aging and delay reproductive senescence [108]. The impact of IIS in mammalian ovarian aging is not only in regulating the quantity but also the quality of oocytes. High levels of insulin during oocyte growth have detrimental effects on meiotic chromatin remodeling and induce chromosome condensation errors in mice [109]. Similarly, in C. elegans, reduction of IIS not only extends the reproductive span but also improves the oocyte morphology and decreases chromosomal segregation errors in maternal aging [110,111]. However, the mechanisms underlying IIS on oocyte quality control are not well understood.

Genetic Interactions between HSF-1 and IIS in C. elegans and its Implications in Germline Proteostasis.

Loss of proteostasis is a hallmark of aging and underlies many age-related diseases [112]. As two important proteostatic regulators, the interaction between HSF-1 and IIS has been extensively studied in C. elegans, especially in the context of longevity. It is well accepted that the HSR is enhanced by reduced IIS, which serves as an important mechanism for longevity in IIS mutants [96,113]. Our recent study provides another view of IIS-HSF-1 interaction in reproduction [60]. First, we found that HSF-1 activates the expression of a chaperone network that is different from the HSR in the germline. Importantly, HSF-1 is not an on-off switch but tunes the expression (from the basal level of HSF-1-independent expression) in response to IIS activity. Second, despite the essentiality of HSF1 in oogenesis in flies and mice, and the requirement of HSF-1 for reproduction in wildtype worms, reducing IIS rescues the sterility caused by HSF-1 depletion in the germline. Given the prominent role of IIS in stimulating protein synthesis and the function of HSF-1-dependent chaperone network in nascent folding, it is attempting to think that protein folding capacity controlled by HSF-1 needs to be coupled with protein synthesis controlled by IIS (Figure 2B). Evidence from cancer cells supports this hypothesis, where HSF1 binding and transcriptional activities in unstressed cells are tightly linked to protein translation [114]. As the chaperones (HSC70, HSP90 and TRiC/CCT) in the HSF-1 germline transcriptional program can bind and inhibit HSF-1 activity when in excess [115–117], it provides an elegant feedback mechanism to titrate HSF-1 activity against the folding requirement. On the other hand, despite slowing down gametogenesis, reduced IIS is favored by conditions when folding capacity is limited as upon HSF-1 depletion [60] and in maternal aging [111] that accumulate misfolded proteins. It remains to be determined if reduced IIS enhances germline proteostasis by reducing translation and thus lowering the demand for protein folding and/or promoting protein turnover that improves protein quality control. In addition, DAF-16 is required in somatic tissues upon IIS reduction for reproduction in the absence of germline HSF-1 [60] and for improved oocyte quality during maternal aging [111]. Future work will understand the cell-non-autonomous mechanism that regulates germline function by somatic IIS.

Perspectives

1. Due to the unique features of proteostasis control in germ cells, gametogenesis is hypersensitive to protein misfolding. Maintaining a pristine proteome is essential for reproductive success and gamete quality.

2. HSF1 contributes to germline proteostasis through germline-specific HSR and an HSR-independent chaperone network. Insulin/IGF-1 signaling (IIS) has prominent roles in both gametogenesis and proteostasis and interacts with HSF-1 in the germline development of C. elegans.

3. Future work will understand the cell-non-autonomous regulation of germline proteostasis by somatic IIS. It is interesting to determine if the interaction of HSF-1 and IIS is conserved in mammals and if enhanced proteostasis underlies delayed oocyte aging upon IIS reduction.