Molecular cloning of a novel Ca(2+)-binding protein (calmegin) specifically expressed during male meiotic germ cell development

Mixed mechanism of conformational selection and induced fit as a molecular recognition process in the calreticulin family of proteins

The fundamental question on the mechanism of molecular recognition during ligand binding has attracted a lot of scientific scrutiny. The two competing theories of ligand binding-"induced fit" and "conformational selection" have been proposed to explain biomolecular recognition. Since exploring a family of proteins with similar structural architectures and conserved functional roles can provide valuable insight into the significance of molecular structure and function, we performed molecular dynamics simulations on the calreticulin family of proteins, which specifically recognize monoglucosylated N-glycan during the protein folding process. Atomistic simulations of lectins in free and bound forms demonstrated that they exist in several conformations spanning from favorable to unfavorable for glycan binding. Our analysis was confined to the carbohydrate recognition domain (CRD) of these lectins to demonstrate the degree of conservation in protein sequence and structure and relate them with their function. Furthermore, we computed the lectin-glycan binding affinity using the mmPBSA approach to identify the most favorable lectin conformation for glycan binding and compared the molecular interaction fields in terms of noncovalent bond interactions. We also demonstrated the involvement of Tyr and Trp residues in the CRD with the non-reducing end glucose and central mannose residues, which contribute to some of the specific interactions. Furthermore, we analyzed the conformational changes in the CRD through SASA, RMSFs and protein surface topography mapping of electrostatic and hydrophobic potentials. Our findings demonstrate a hybrid mechanism of molecular recognition, initially driven by conformational selection followed by glycan-induced fluctuations in the key residues to strengthen the glycan binding interactions.

Dissecting the PRSS37 interactome and potential mechanisms leading to ADAM3 loss in PRSS37-null sperm

A disintegrin and metalloproteinase 3 (ADAM3) is a sperm membrane protein critical for sperm migration from the uterus into the oviduct and sperm-egg binding in mice. Disruption of PRSS37 results in male infertility concurrent with the absence of mature ADAM3 from cauda epididymal sperm. However, how PRSS37 modulates ADAM3 maturation remains largely unclear. Here, we determine the PRSS37 interactome by GFP immunoprecipitation coupled with mass spectrometry in PRSS37-EGFP knock-in mice. Three molecular chaperones (CLGN, CALR3 and PDILT) and three ADAM proteins (ADAM2, ADAM6B and ADAM4) were identified to be interacting with PRSS37. Coincidently, five of them (except ADAM4) have been reported to interact with ADAM3 precursor and regulate its maturation. We further demonstrated that PRSS37 also interacts directly with ADAM3 precursor and its deficiency impedes the association between PDILT and ADAM3. This could contribute to improper translocation of ADAM3 to the germ cell surface, leading to ADAM3 loss in PRSS37-null mature sperm. The understanding of the maturation mechanisms of pivotal sperm plasma membrane proteins will pave the way toward novel strategies for contraception and the treatment of unexplained male infertility.

Structural Analysis of Calreticulin, an Endoplasmic Reticulum-Resident Molecular Chaperone

Calreticulin (Calr) is an endoplasmic reticulum (ER) chaperone involved in protein quality control, Ca²⁺ regulation and other cellular processes. The structure of Calr is unusual, reflecting different functions of the protein: a proline-rich β-hairpin arm and an acidic C-terminal tail protrude from a globular core, composed of a β-sheet sandwich and an α-helix. The arm and tail interact in the presence of Ca²⁺ and cover the upper β-sheet, where a carbohydrate-binding site gives the chaperone glycoprotein affinity. At the edge of the carbohydrate-binding site is a conserved, strained disulphide bridge, formed between C¹⁰⁶ and C¹³⁷ of human Calr, which lies in a polypeptide-binding site. The lower β-sheet has several conserved residues, comprised of a characteristic triad, D¹⁶⁶-H¹⁷⁰-D¹⁸⁷, Tyr¹⁷² and the free C¹⁶³. In addition to its role in the ER, Calr translocates to the cell surface upon stress and functions as an immune surveillance marker. In some myeloproliferative neoplasms, the acidic Ca²⁺-binding C-terminal tail is transformed into a polybasic sequence.

Experimental induction and mathematical modeling of Ca2+ dynamics in rat round spermatids

Cytosolic Ca2+ concentration ([Ca2+ ]) has an important role in spermatozoa and hence it regulates fertilization. In male germinal cells, there are indirect evidences that this ion could regulate physiological processes in spermatogenesis. Since little is known about Ca2+ homeostasis in spermatogenic cells, in this work we propose a mathematical model that accounts for experimental [Ca2+ ] dynamics triggered by blockade of the SERCA transport ATPase with thapsigargin in round rat spermatids, without external Ca2+ and with different extracellular lactate concentrations. The model included three homogeneous calcium compartments and Ca2+-ATPase activities sensitive and insensitive to thapsigargin, and it adjusted satisfactorily the experimental calcium dynamic data. Moreover, an extended version of the model satisfactorily adjusted the stationary states of calcium modulated by extracellular lactate, which is consistent with the participation of a low affinity lactate transporter and further lactate metabolism in these cells. Further studies and modeling would be necessary to shed some light into the relation between Ca2+-lactate-ATP homeostasis and cell-cell interactions in the seminiferous tubules that are expected to modulate Ca2+ dynamics by hormonal factors or energetic substrates in meiotic and postmeiotic spermatogenic cells.

Identification of Novel Potential Type 2 Diabetes Genes Mediating β-Cell Loss and Hyperglycemia Using Positional Cloning

Type 2 diabetes (T2D) is a complex metabolic disease regulated by an interaction of genetic predisposition and environmental factors. To understand the genetic contribution in the development of diabetes, mice varying in their disease susceptibility were crossed with the obese and diabetes-prone New Zealand obese (NZO) mouse. Subsequent whole-genome sequence scans revealed one major quantitative trait loci (QTL), Nidd/DBA on chromosome 4, linked to elevated blood glucose and reduced plasma insulin and low levels of pancreatic insulin. Phenotypical characterization of congenic mice carrying 13.6 Mbp of the critical fragment of DBA mice displayed severe hyperglycemia and impaired glucose clearance at week 10, decreased glucose response in week 13, and loss of β-cells and pancreatic insulin in week 16. To identify the responsible gene variant(s), further congenic mice were generated and phenotyped, which resulted in a fragment of 3.3 Mbp that was sufficient to induce hyperglycemia. By combining transcriptome analysis and haplotype mapping, the number of putative responsible variant(s) was narrowed from initial 284 to 18 genes, including gene models and non-coding RNAs. Consideration of haplotype blocks reduced the number of candidate genes to four (Kti12, Osbpl9, Ttc39a, and Calr4) as potential T2D candidates as they display a differential expression in pancreatic islets and/or sequence variation. In conclusion, the integration of comparative analysis of multiple inbred populations such as haplotype mapping, transcriptomics, and sequence data substantially improved the mapping resolution of the diabetes QTL Nidd/DBA. Future studies are necessary to understand the exact role of the different candidates in β-cell function and their contribution in maintaining glycemic control.

Endoplasmic Reticulum Chaperone Calmegin Is Upregulated in Aldosterone-Producing Adenoma and Associates With Aldosterone Production

The endoplasmic reticulum (ER) plays a pivotal role in syntheses of proteins and steroid hormones and regulation of intracellular Ca²⁺ level. We aimed to investigate ER-associated genes in aldosterone-producing adenomas (APAs) and clarify their effect on aldosterone production. Microarray analysis targeting 288 ER-associated genes was conducted using nonfunctioning adrenocortical adenomas (n=5) and APAs (n=19). Immunohistochemistry and quantitative polymerase chain reaction analyses were performed with 13 nonfunctioning adrenocortical adenoma and 48 APA samples. Functional studies were performed with human adrenocortical carcinoma (HAC15) cells, some of which were genetically modified using lentiviruses. The ER chaperone calmegin (CLGN) was the most highly expressed ER-associated gene in APAs relative to nonfunctioning adrenocortical adenomas. Analysis with quantitative polymerase chain reaction revealed CLGN to be 9.5-fold upregulated in APAs relative to nonfunctioning adrenocortical adenomas. There were no differences among different APA genotypes affecting aldosterone production. Immunohistochemistry analysis revealed that CLGN was strongly expressed in APAs and aldosterone-producing cell clusters. Angiotensin II stimulation or KCNJ5 T158A overexpression in HAC15 cells did not affect CLGN mRNA levels. CLGN overexpression in HAC15 cells increased aldosterone levels but did not stimulate CYP11B2 mRNA levels. Pathway and gene ontology analyses using RNA sequencing results showed that tRNA aminoacyl metabolism was the most enriched pathway in CLGN-overexpressing cells. CYP11B2 (aldosterone synthase) and HSD3B2 (3 beta-hydroxysteroid dehydrogenase/delta 5->4-isomerase type 2) protein expression were more abundant in CLGN-overexpressing cells. CLGN knockdown using CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeat-associated 9) method in HAC15 cells that carry the KCNJ5 mutation did not affect aldosterone production. To summarize, CLGN was upregulated and associated with aldosterone production via translational regulation of CYP11B2 in APAs.

Evolving Evidence of Calreticulin as a Pharmacological Target in Neurological Disorders

Calreticulin (CALR), a lectin-like ER chaperone, was initially known only for its housekeeping function, but today it is recognized for many versatile roles in different compartments of a cell. Apart from canonical roles in protein folding and calcium homeostasis, it performs a variety of noncanonical roles, mostly in CNS development. In the past, studies have linked Calreticulin with various other biological components which are detrimental in deciding the fate of neurons. Many neurological disorders that differ in their etiology are commonly associated with aberrant levels of Calreticulin, that lead to modulation of apoptosis and phagocytosis, and impact on transcriptional pathways, impairment in proteostatis, and calcium imbalances. Such multifaceted properties of Calreticulin are the reason why it has been implicated in vital roles of the nervous system in recent years. Hence, understanding its role in the physiology of neurons would help to unearth its involvement in the spectrum of neurological disorders. This Review aims toward exploring the interplay of Calreticulin in neurological disorders which would aid in targeting Calreticulin for developing novel neurotherapeutics.

Comparative detection method of early onset cytokine-induced stress in β-cells (INS-1E)

β-Cells contain a prominent endoplasmic reticulum (ER), disrupting ER homeostasis and function, activating the unfolded protein response (UPR). Currently, no direct protocols measure the UPR initiation. Current methods to measure ER stress include the quantification of nitric oxide (NO) (indirect method), Western blotting, and qRT-PCR of downstream components. However, these methods do not account for the overlap with mitochondrial dysfunction. In this study, INS-1E cells were exposed to proinflammatory cytokines to induce ER stress, as determined using NO, thioflavin T (ThT) binding, and β-cell functionality (insulin production). ER stress was confirmed through the upregulation of CHOP. Cell viability was monitored using MTT, sulforhodamine B, and the xCELLigence system. Morphological changes were monitored using electron microscopy. IL-1β exposure-induced β-cell stress after 4 H, decreased insulin levels, and increased thioflavin binding were noted. Increased NO production was only detected after 10 H, highlighting its lack of sensitivity, and the need for a continuous, selective, rapid, convenient, and economical detection method for early onset of ER stress. Standard methods (MTT and NO) failed to detect early ER stress. The xCELLigence coupled with a functional assay such as the detection of insulin levels or ThT are better predictors of ER stress in INS-1E cells.

Role of Ca(2+) in the Stability and Function of TMEM16F and 16K

There are 10 transmembrane protein (TMEM) 16-family proteins in humans and mice. Among them, TMEM16F acts as a Ca(2+)-dependent phospholipid scramblase at the plasma membrane. However, how Ca(2+) activates TMEM16F's phospholipid-scramblase activity has not been elucidated. Here we found that in the presence of Ca(2+), TMEM16K (whose function is unknown) directly binds Ca(2+) to form a stable complex that can be detected by blue-native polyacrylamide gel electrophoresis. In the absence of Ca(2+), TMEM16K and TMEM16F aggregated, suggesting that their structure is stabilized by Ca(2+). Comprehensive mutagenesis of acidic residues in TMEM16K's cytoplasmic and transmembrane regions identified five residues that are critical for binding Ca(2+). These residues were well conserved between TMEM16F and 16K, and point mutations of these residues in TMEM16F reduced its ability to support Ca(2+)-dependent phospholipid scrambling. Our results suggest that Ca(2+) binds TMEM16F directly and induces conformational changes that support its stability and function.

Transcriptome analysis of highly purified mouse spermatogenic cell populations: gene expression signatures switch from meiotic-to postmeiotic-related processes at pachytene stage

Background

Spermatogenesis is a complex differentiation process that involves the successive and simultaneous execution of three different gene expression programs: mitotic proliferation of spermatogonia, meiosis, and spermiogenesis. Testicular cell heterogeneity has hindered its molecular analyses. Moreover, the characterization of short, poorly represented cell stages such as initial meiotic prophase ones (leptotene and zygotene) has remained elusive, despite their crucial importance for understanding the fundamentals of meiosis.

Results

We have developed a flow cytometry-based approach for obtaining highly pure stage-specific spermatogenic cell populations, including early meiotic prophase. Here we combined this methodology with next generation sequencing, which enabled the analysis of meiotic and postmeiotic gene expression signatures in mouse with unprecedented reliability. Interestingly, we found that a considerable number of genes involved in early as well as late meiotic processes are already on at early meiotic prophase, with a high proportion of them being expressed only for the short time lapse of lepto-zygotene stages. Besides, we observed a massive change in gene expression patterns during medium meiotic prophase (pachytene) when mostly genes related to spermiogenesis and sperm function are already turned on. This indicates that the transcriptional switch from meiosis to post-meiosis takes place very early, during meiotic prophase, thus disclosing a higher incidence of post-transcriptional regulation in spermatogenesis than previously reported. Moreover, we found that a good proportion of the differential gene expression in spermiogenesis corresponds to up-regulation of genes whose expression starts earlier, at pachytene stage; this includes transition protein-and protamine-coding genes, which have long been claimed to switch on during spermiogenesis. In addition, our results afford new insights concerning X chromosome meiotic inactivation and reactivation.

Conclusions

This work provides for the first time an overview of the time course for the massive onset and turning off of the meiotic and spermiogenic genetic programs. Importantly, our data represent a highly reliable information set about gene expression in pure testicular cell populations including early meiotic prophase, for further data mining towards the elucidation of the molecular bases of male reproduction in mammals.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2618-1) contains supplementary material, which is available to authorized users.

Influence of high-mannose glycan whose glucose moiety is substituted with 5-thioglucose on calnexin/calreticulin cycle

Our study first revealed that UDP-5-thioglucose functions as a glycosyl donor of UDP-glucose: glycoprotein glucosyltransferase to produce 5-thio-glucosylated Man9 (5S-G1M9).

Calreticulin is required for development of the cumulus oocyte complex and female fertility

Calnexin (CANX) and calreticulin (CALR) chaperones mediate nascent glycoprotein folding in the endoplasmic reticulum. Here we report that these chaperones have distinct roles in male and female fertility. Canx null mice are growth retarded but fertile. Calr null mice die during embryonic development, rendering indeterminate any effect on reproduction. Therefore, we conditionally ablated Calr in male and female germ cells using Stra8 (mcKO) and Zp3 (fcKO) promoter-driven Cre recombinase, respectively. Calr mcKO male mice were fertile, but fcKO female mice were sterile despite normal mating behavior. Strikingly, we found that Calr fcKO female mice had impaired folliculogenesis and decreased ovulatory rates due to defective proliferation of cuboidal granulosa cells. Oocyte-derived, TGF-beta family proteins play a major role in follicular development and molecular analysis revealed that the normal processing of GDF9 and BMP15 was defective in Calr fcKO oocytes. These findings highlight the importance of CALR in female reproduction and demonstrate that compromised CALR function leads to ovarian insufficiency and female infertility.

Multiple renal cyst development but not situs abnormalities in transgenic RNAi mice against Inv::GFP rescue gene

In this study we generated RNA interference (RNAi)-mediated gene knockdown transgenic mice (transgenic RNAi mice) against the functional Inv gene. Inv mutant mice show consistently reversed internal organs (situs inversus), multiple renal cysts and neonatal lethality. The Inv::GFP-rescue mice, which introduced the Inv::GFP fusion gene, can rescue inv mutant mice phenotypes. This indicates that the Inv::GFP gene is functional in vivo. To analyze the physiological functions of the Inv gene, and to demonstrate the availability of transgenic RNAi mice, we introduced a short hairpin RNA expression vector against GFP mRNA into Inv::GFP-rescue mice and analyzed the gene silencing effects and Inv functions by examining phenotypes. Transgenic RNAi mice with the Inv::GFP-rescue gene (Inv-KD mice) down-regulated Inv::GFP fusion protein and showed hypomorphic phenotypes of inv mutant mice, such as renal cyst development, but not situs abnormalities or postnatal lethality. This indicates that shRNAi-mediated gene silencing systems that target the tag sequence of the fusion gene work properly in vivo, and suggests that a relatively high level of Inv protein is required for kidney development in contrast to left/right axis determination. Inv::GFP protein was significantly down-regulated in the germ cells of Inv-KD mice testis compared with somatic cells, suggesting the existence of a testicular germ cell-specific enhanced RNAi system that regulates germ cell development. The Inv-KD mouse is useful for studying Inv gene functions in adult tissue that are unable to be analyzed in inv mutant mice showing postnatal lethality. In addition, the shRNA-based gene silencing system against the tag sequence of the fusion gene can be utilized as a new technique to regulate gene expression in either in vitro or in vivo experiments.

Rainbow trout (Oncorhynchus mykiss) contain two calnexin genes which encode distinct proteins

Calnexin (IP90/P88) is an integral membrane protein of the endoplasmic reticulum that binds newly synthesized N-linked glycoproteins during their folding in the ER including MHC class I molecule. This manuscript reports the identification of two unique cDNA clones of calnexin in rainbow trout. Both encode putative mature proteins of 579 and 592 aa respectively in addition to a 24 aa signal peptide. Sequence analysis revealed that only one of the two cDNA clones encodes a putative ER retention signal, K/QEDDL, followed by a serine phosphorylation site conserved with mammalian homologs. Amino acid sequence alignment illustrated conservation of the calnexin luminal domain, which consists of a globular and a P domain, in both copies. Southern blotting revealed that there are at least two copies of the calnexin gene in the trout genome and northern blotting showed a wide tissue distribution of an estimated 3 kbp calnexin transcript with an additional minor transcript of 2.3 kbp expressed only in head kidney, spleen PBLs and strongly in RTS11. Importantly, the smaller transcript was predominantly upregulated in RTS11 after a 24h treatment with the calcium ionophore A23187. In western blots, calnexin was detected primarily as a 120 kDa protein and upon A23187 treatment; a 100 kDa band was most prominently expressed. These results suggest that in salmonids there are two differentiated versions of the calnexin gene which encode proteins that may have diverged to perform unique biological functions.

JMJD1C, a JmjC domain-containing protein, is required for long-term maintenance of male germ cells in mice

JmjC domain-containing proteins are a class of enzymes responsible for histone demethylation. Previous studies revealed that the JmjC domain-containing protein KDM3A possesses intrinsic demethylase activity toward lysine 9 of histone H3 and plays essential roles in spermiogenesis. In contrast, the biological roles of JMJD1C, a KDM3A homolog in mice, are largely unknown. Here we present the crucial role of JMJD1C in male gametogenesis. Jmjd1c-deficient males became infertile due to the progressive reduction of germ cells after 3 mo of age. Importantly, Jmjd1c-deficient testes frequently contained abnormal tubules lacking developmentally immature germ cells. JMJD1C is most abundantly expressed in undifferentiated spermatogonia in mouse testis. The numbers of ZBTB16-positive spermatogonia and apoptotic germ cells in Jmjd1c-deficient testes decreased and increased in an age-dependent manner, respectively. Our studies demonstrated that JMJD1C contributes to the long-term maintenance of the male germ line.

RanBPM, a scaffolding protein for gametogenesis

RanBPM is a multimodular scaffold protein that interacts with a great variety of molecules including nuclear, cytoplasmic, and membrane proteins. By building multiprotein complexes, RanBPM is thought to regulate various signaling pathways, especially in the immune and nervous system. However, the diversity of these interactions does not facilitate the identification of its precise mechanism of action, and therefore the physiological role of RanBPM still remains unclear. Recently, RanBPM has been shown to be critical for the fertility of both genders in mouse. Although mechanistically it is still unclear how RanBPM affects gametogenesis, the data collected so far suggest that it is a key player in this process. Here, we examine the RanBPM sterility phenotype in the context of other genetic mutations affecting mouse gametogenesis to investigate whether this scaffold protein affects the function of other known proteins whose deficiency results in similar sterility phenotypes.

Essential roles of the chromatin remodeling factor BRG1 in spermatogenesis in mice

Mammalian spermatogenesis is a complex process that involves spatiotemporal regulation of gene expression and meiotic recombination, both of which require the modulation of chromatin structure. Proteins important for chromatin regulation during spermatogenesis remain poorly understood. Here we addressed the role of BRG1, the catalytic subunit of the mammalian Swi/Snf-like BAF chromatin-remodeling complex, during spermatogenesis in mice. BRG1 expression is dynamically regulated in the male germline, being weakly detectable in spermatogonia, highly expressed in pachytene spermatocytes, and turned off in maturing round spermatids. This expression pattern overlaps that of Brm, the Brg1 homolog. While Brm knockout males are known to be fertile, germline-specific Brg1 deletion completely arrests spermatogenesis at the midpachytene stage, which is associated with spermatocyte apoptosis and apparently also with impaired homologous recombination and meiotic sex chromosome inactivation. However, Brg1 is dispensable for gammaH2AX formation during meiotic recombination, contrary to its reported role in DNA repair in somatic cells. Our study reveals the essential role of Brg1 in meiosis and underscores the differences in the mechanisms of DNA repair between germ cells and somatic cells.

Protein disulfide isomerase homolog PDILT is required for quality control of sperm membrane protein ADAM3 and male fertility [corrected]

A disintegrin and metalloproteinase 3 (ADAM3) is a sperm membrane protein critical for both sperm migration from the uterus into the oviduct and sperm primary binding to the zona pellucida (ZP). Here we show that the testis-specific protein disulfide isomerase homolog (PDILT) cooperates with the testis-specific calreticulin-like chaperone, calsperin (CALR3), in the endoplasmic reticulum and plays an indispensable role in the disulfide-bond formation and folding of ADAM3. Pdilt(-/-) mice were male infertile because ADAM3 could not be folded properly and transported to the sperm surface without the PDILT/CALR3 complex. Peculiarly we find that not only Pdilt(-/-), but also Adam3(-/-), spermatozoa effectively fertilize eggs when the eggs are surrounded in cumulus oophorus. These findings reveal that ADAM3 requires testis-specific private chaperones to be folded properly and that the principle role of ADAM3 is for sperm migration into the oviduct but not for the fertilization event. Moreover, the importance of primary sperm ZP binding, which has been thought to be a critical step in mammalian fertilization, should be reconsidered.

The tissue-specific Rep8/UBXD6 tethers p97 to the endoplasmic reticulum membrane for degradation of misfolded proteins

The protein known as p97 or VCP in mammals and Cdc48 in yeast is a versatile ATPase complex involved in several biological functions including membrane fusion, protein folding, and activation of membrane-bound transcription factors. In addition, p97 plays a central role in degradation of misfolded secretory proteins via the ER-associated degradation pathway. This functional diversity of p97 depends on its association with various cofactors, and to further our understanding of p97 function it is important that these cofactors are identified and analyzed. Here, we isolate and characterize the human protein named Rep8 or Ubxd6 as a new cofactor of p97. Mouse Rep8 is highly tissue-specific and abundant in gonads. In testes, Rep8 is expressed in post-meiotic round spermatids, whereas in ovaries Rep8 is expressed in granulosa cells. Rep8 associates directly with p97 via its UBX domain. We show that Rep8 is a transmembrane protein that localizes to the ER membrane with its UBX domain facing the cytoplasm. Knock-down of Rep8 expression in human cells leads to a decreased association of p97 with the ER membrane and concomitantly a retarded degradation of misfolded ER-derived proteasome substrates. Thus, Rep8 tethers p97 to the ER membrane for efficient ER-associated degradation.

RanBPM is essential for mouse spermatogenesis and oogenesis

RanBPM is a recently identified scaffold protein that links and modulates interactions between cell surface receptors and their intracellular signaling pathways. RanBPM has been shown to interact with a variety of functionally unrelated proteins; however, its function remains unclear. Here, we show that RanBPM is essential for normal gonad development as both male and female RanBPM(-/-) mice are sterile. In the mutant testis there was a marked decrease in spermatogonia proliferation during postnatal development. Strikingly, the first wave of spermatogenesis was totally compromised, as seminiferous tubules of homozygous mutant animals were devoid of post-meiotic germ cells. We determined that spermatogenesis was arrested around the late pachytene-diplotene stages of prophase I; surprisingly, without any obvious defect in chromosome synapsis. Interestingly, RanBPM deletion led to a remarkably quick disappearance of all germ cell types at around one month of age, suggesting that spermatogonia stem cells are also affected by the mutation. Moreover, in chimeric mice generated with RanBPM(-/-) embryonic stem cells all mutant germ cells disappeared by 3 weeks of age suggesting that RanBPM is acting in a cell-autonomous way in germ cells. RanBPM homozygous mutant females displayed a premature ovarian failure due to a depletion of the germ cell pool at the end of prophase I, as in males. Taken together, our results highlight a crucial role for RanBPM in mammalian gametogenesis in both genders.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 2: changes in spermatid organelles associated with development of spermatozoa

Spermiogenesis is a long process whereby haploid spermatids derived from the meiotic divisions of spermatocytes undergo metamorphosis into spermatozoa. It is subdivided into distinct steps with 19 being identified in rats, 16 in mouse and 8 in humans. Spermiogenesis extends over 22.7 days in rats and 21.6 days in humans. In this part, we review several key events that take place during the development of spermatids from a structural and functional point of view. During early spermiogenesis, the Golgi apparatus forms the acrosome, a lysosome-like membrane bound organelle involved in fertilization. The endoplasmic reticulum undergoes several topographical and structural modifications including the formation of the radial body and annulate lamellae. The chromatoid body is fully developed and undergoes structural and functional modifications at this time. It is suspected to be involved in RNA storing and processing. The shape of the spermatid head undergoes extensive structural changes that are species-specific, and the nuclear chromatin becomes compacted to accommodate the stream-lined appearance of the sperm head. Microtubules become organized to form a curtain or manchette that associates with spermatids at specific steps of their development. It is involved in maintenance of the sperm head shape and trafficking of proteins in the spermatid cytoplasm. During spermiogenesis, many genes/proteins have been implicated in the diverse dynamic events occurring at this time of development of germ cells and the absence of some of these have been shown to result in subfertility or infertility.

Protein quality control in the ER: the recognition of misfolded proteins

The mechanism, in molecular terms of protein quality control, specifically of how the cell recognizes and discriminates misfolded proteins, remains a challenge. In the secretory pathway the folding status of glycoproteins passing through the endoplasmic reticulum is marked by the composition of the N-glycan. The different glycoforms are recognized by specialized lectins. The folding sensor UGGT acts as an unusual molecular chaperone and covalently modifies the Man9 N-glycan of a misfolded protein by adding a glucose moiety and converts it to Glc1Man9 that rebinds the lectin calnexin. However, further links between the folding status of a glycoprotein and the composition of the N-glycan are unclear. There is little unequivocal evidence for other proteins in the ER recognizing the N-glycan and also acting as molecular chaperones. Nevertheless, based upon a few examples, we suggest that this function is carried out by individual proteins in several different complexes. Thus, calnexin binds the protein disulfide isomerase ERp57, that acts upon Glc1Man9 glycoproteins. In another example the protein disulfide isomerase ERdj5 binds specifically to EDEM (which is probably a mannosidase) and a lectin OS9, and reduces the disulfide bonds of bound glycoproteins destined for ERAD. Thus the glycan recognition is performed by a lectin and the chaperone function performed by a specific partner protein that can recognize misfolded proteins. We predict that this will be a common arrangement of proteins in the ER and that members of protein foldase families such as PDI and PPI will bind specifically to lectins in the ER. Molecular chaperones BiP and GRp94 will assist in the folding of proteins bound in these complexes as well as in the folding of non-glycoproteins.

EPAS1 Is Required for Spermatogenesis in the Postnatal Mouse Testis

Spermatogenesis, a process involving the differentiation of spermatogonial stem cells into mature spermatozoa, takes place throughout masculine life. A complex system in the testis, including endocrine signaling, physical interactions between germ and somatic cells, spermatocyte meiosis, and timely release of spermatozoa, controls this cycle. We demonstrate herein that decreased O(2) levels and Epas1 activation are critical components of spermatogenesis. Postnatal Epas1 ablation leads to male infertility, with reduced testis size and weight. While immature spermatogonia and spermatocytes are present in Epas1(Delta/Delta) testes, spermatid and spermatozoan numbers are dramatically reduced. This is not due to germ cell-intrinsic defects. Rather, Epas(Delta/Delta) Sertoli cells exhibit decreased ability to form tight junctions, thereby disrupting the blood-testis barrier necessary for proper spermatogenesis. Reduced numbers of tight junction complexes are due to decreased expression of multiple genes encoding tight junction proteins, including TJP1 (ZO1), TJP2 (ZO2), and occludin. Furthermore, Epas1(Delta/Delta) testes exhibit disrupted basement membranes surrounding the seminiferous tubules, causing the premature release of incompletely differentiated germ cells. We conclude that low O(2) levels in the male gonad regulate germ cell homeostasis in this organ via EPAS1.

Calnexin phosphorylation: linking cytoplasmic signalling to endoplasmic reticulum lumenal functions

Calnexin is an abundant integral membrane phosphoprotein of the endoplasmic reticulum (ER) of eukaryotic cells. The role of the luminal domain as an N-glycoprotein specific lectin has been well-established. Cytosolic C-terminal domain phosphorylation of calnexin has recently been elucidated in glycoprotein folding and quality control. Signalling of the presence of unfolded proteins from the lumen of the ER is mediated by the three ER membrane sensor proteins Ire1, ATF6 and PERK. The observation that the C-terminus of calnexin is differentially phosphorylated when glycoproteins are misfolded initiated our search for functional roles of calnexin phosphorylation. Recent studies have defined a role for phosphorylation at a proline-directed kinase site (Ser563) in ER protein quality control, while phosphorylation at a casein kinase 2 site (Ser534, Ser544) may be linked to transport functions. There are also four other abundant integral membrane phosphoproteins in the ER, and these may be components of other signalling pathways that link and coordinate other ER functions with the rest of the cell.

A spontaneous smc1b mutation causes cohesin protein dysfunction and sterility in mice

In this paper, we describe a novel spontaneous mutation of the Smc1b gene coding a cohesin component, which causes female and male sterility. We have discovered an ICR male mouse with a novel autosomal recessive gene that causes small gonads and sterility in both sexes. Mutant female and male mice homozygous for the novel sterility gene had normal body weights and showed normal mating behavior, but did not produce any offspring. Histological examination showed that Sertoli cells and spermatogonia were present in the testicular seminiferous tubules in 8-week-old mutant male mice, but no spermatids or spermatozoa were observed. Mutant females showed a markedly reduced number of oocytes with age. The novel sterility gene mapped between D15Mit105 (47.9 cM) and D15Mit171 (54.5 cM) on chromosome 15. Sequences of three candidate sterility genes, Dmc1, Mei1 and Smc1b, which are closely linked to these microsatellite markers, were compared between normal and mutant mice. The Dmc1 and Mei1 genes showed the same sequences in both normal and mutant mice, but the Smc1b gene had a deletion of 16 nucleotides in exon 5, in the mutant mice. We concluded that this deletion led to a frame-shift, which generated a stop codon at position 761 (amino acid 247) of the Smc1b cDNA in mutant mice.

A conserved E2F6-binding element in murine meiosis-specific gene promoters

During gametogenesis, germ cells must undergo meiosis in order to become viable haploid gametes. Successful completion of this process is dependent upon the expression of genes whose protein products function specifically in meiosis. Failure to express these genes in meiotic cells often results in infertility, whereas aberrant expression in somatic cells may lead to mitotic catastrophe. The mechanisms responsible for regulating the timely expression of meiosis-specific genes have not been fully elucidated. Here we demonstrate that E2F6, a member of the E2F family of transcription factors, is essential for the repression of the newly identified meiosis-specific gene, Slc25a31 (also known as Ant4, Aac4), in somatic cells. This discovery, along with previous studies, prompted us to investigate the role of E2F6 in the regulation of meiosis-specific genes in general. Interestingly, the core E2F6-binding element (TCCCGC) was highly conserved in the proximal promoter regions of 19 out of 24 (79.2%) meiosis-specific genes. This was significantly higher than the frequency found in the promoters of all mouse genes (15.4%). In the absence of E2F6, only a portion of these meiosis-specific genes was derepressed in somatic cells. However, endogenous E2F6 bound to the promoters of these meiosis-specific genes regardless of whether they required E2F6 for their repression in somatic cells. Further, E2F6 overexpression was capable of reducing their transcription. These findings indicate that E2F6 possesses a broad ability to bind to and regulate the meiosis-specific gene population.

Proteomic investigation of 1,6-dimethoxyhexane testicular toxicity

The aliphatic ether 1,6-dimethoxyhexane (DMH) was previously identified as a testicular toxicant. Testis protein extracts from control and DMH-treated rats were subjected to two-dimensional gel electrophoresis for comparison of protein expression profiles. MALDI-ToF peptide mass fingerprinting of differentially expressed proteins resulted in the conclusive identification of heat shock-related 70kDa protein 2 (HSP70.2), 60kDa heat shock protein, mitochondrial precursor (HSP60) and protein disulfide isomerase A3 precursor (ERp60). The potential involvement of these proteins in chemically induced perturbation of spermatogenesis and their utility as biomarkers of testicular toxicity are discussed in light of the knowledge currently available from the literature.

A developmentally regulated chaperone complex for the endoplasmic reticulum of male haploid germ cells

Glycoprotein folding is mediated by lectin-like chaperones and protein disulfide isomerases (PDIs) in the endoplasmic reticulum. Calnexin and the PDI homologue ERp57 work together to help fold nascent polypeptides with glycans located toward the N-terminus of a protein, whereas PDI and BiP may engage proteins that lack glycans or have sugars toward the C-terminus. In this study, we show that the PDI homologue PDILT is expressed exclusively in postmeiotic male germ cells, in contrast to the ubiquitous expression of many other PDI family members in the testis. PDILT is induced during puberty and represents the first example of a PDI family member under developmental control. We find that PDILT is not active as an oxido-reductase, but interacts with the model peptide Delta-somatostatin and nonnative bovine pancreatic trypsin inhibitor in vitro, indicative of chaperone activity. In vivo, PDILT forms a tissue-specific chaperone complex with the calnexin homologue calmegin. The identification of a redox-inactive chaperone partnership defines a new system of testis-specific protein folding with implications for male fertility.

Mammalian Polycomb Scmh1 mediates exclusion of Polycomb complexes from the XY body in the pachytene spermatocytes

The product of the Scmh1 gene, a mammalian homolog of Drosophila Sex comb on midleg, is a constituent of the mammalian Polycomb repressive complexes 1 (Prc1). We have identified Scmh1 as an indispensable component of the Prc1. During progression through pachytene, Scmh1 was shown to be excluded from the XY body at late pachytene, together with other Prc1 components such as Phc1, Phc2, Rnf110 (Pcgf2), Bmi1 and Cbx2. We have identified the role of Scmh1 in mediating the survival of late pachytene spermatocytes. Apoptotic elimination of Scmh1(-/-) spermatocytes is accompanied by the preceding failure of several specific chromatin modifications at the XY body, whereas synapsis of homologous autosomes is not affected. It is therefore suggested that Scmh1 is involved in regulating the sequential changes in chromatin modifications at the XY chromatin domain of the pachytene spermatocytes. Restoration of defects in Scmh1(-/-) spermatocytes by Phc2 mutation indicates that Scmh1 exerts its molecular functions via its interaction with Prc1. Therefore, for the first time, we are able to indicate a functional involvement of Prc1 during the meiotic prophase of male germ cells and a regulatory role of Scmh1 for Prc1, which involves sex chromosomes.

Expression profiling reveals meiotic male germ cell mRNAs that are translationally up- and down-regulated

Gametes rely heavily on posttranscriptional control mechanisms to regulate their differentiation. In eggs, maternal mRNAs are stored and selectively activated during development. In the male, transcription ceases during spermiogenesis, necessitating the posttranscriptional regulation of many paternal mRNAs required for spermatozoan assembly and function. To date, most of the testicular mRNAs known to be translationally regulated are initially transcribed in postmeiotic cells. Because protein synthesis occurs on polysomes and translationally inactive mRNAs are sequestered as ribonucleoproteins (RNPs), movement of mRNAs between these fractions is indicative of translational up- and down-regulation. Here, we use microarrays to analyze mRNAs in RNPs and polysomes from testis extracts of prepuberal and adult mice to characterize the translation state of individual mRNAs as spermatogenesis proceeds. Consistent with published reports, many of the translationally delayed postmeiotic mRNAs shift from the RNPs into the polysomes, establishing the validity of this approach. In addition, we detect another 742 mouse testicular transcripts that show dramatic shifts between RNPs and polysomes. One subgroup of 35 genes containing the known, translationally delayed phosphoglycerate kinase 2 (Pgk2) is initially transcribed during meiosis and is translated in later-stage cells. Another subgroup of 82 meiotically expressed genes is translationally down-regulated late in spermatogenesis. This high-throughput approach defines the changing translation patterns of populations of genes as male germ cells differentiate and identifies groups of meiotic transcripts that are translationally up- and down-regulated.

Versatility of the endoplasmic reticulum protein folding factory

The endoplasmic reticulum (ER) is dedicated to import, folding and assembly of all proteins that travel along or reside in the secretory pathway of eukaryotic cells. Folding in the ER is special. For instance, newly synthesized proteins are N-glycosylated and by default form disulfide bonds in the ER, but not elsewhere in the cell. In this review, we discuss which features distinguish the ER as an efficient folding factory, how the ER monitors its output and how it disposes of folding failures.

The role of molecular chaperones in mouse sperm-egg interactions

Fertilization is a unique and exquisitely choreographed cellular interaction between the male and female gamete that results in the creation of a genetically unique individual. Despite the fundamental importance of fertilization, there remains a dearth of information about the basic biochemical mechanisms that underpin this process. One of the key issues that remain unresolved is the molecular basis of sperm-egg recognition. From the female perspective, it is well established that the sperm recognition sites reside in the zona pellucida (ZP), an acellular coat that surrounds the oocyte. In contrast, numerous studies into the cognate zona receptors residing on the sperm surface have failed to shed significant light on the biochemical identity of these molecules. Such difficulties may, in part, have arisen because investigations have traditionally been based on the precept that the zona receptor represents a single molecular entity that is constitutively expressed on the sperm surface. While such a view holds obvious appeal, it fails to account for growing evidence that gamete interaction is not mediated by a simple lock-and-key mechanism. In this review, we present a novel hypothesis in which the zona recognition site is portrayed as a multimeric molecular structure that is assembled into a functional complex during a maturation process known as 'capacitation'. Furthermore, we consider the possibility that this previously cryptic complex is assembled and delivered to the outer surface of the sperm plasma membrane through the concerted action of several members of the molecular chaperone family of proteins.

The expression of calpain 1 and calpain 2 in spermatogenic cells and spermatozoa of the mouse

There is some evidence suggesting that Ca2+is involved in processes that occur during the development and function of spermatozoa. Calcium-dependent proteins, such as calmodulin, are expressed during mammalian spermatogenesis further suggesting that Ca2+takes part in its regulation. However, the precise roles of Ca2+in spermatogenesis remain to be elucidated. Calpains are a family of Ca2+-dependent cysteine proteases whose members are expressed ubiquitously or in a tissue-specific manner. Calpain has been demonstrated to mediate specific Ca2+-dependent processes including cell fusion, mitosis and meiosis. We herein followed the expression pattern of calpain’s ubiquitous isoforms, 1 and 2, throughout spermatogenesis at the RNA and protein levels by RT-PCR and Western blotting analysis. Both RNA and protein studies revealed that these isoforms are expressed in all spermatogenic cells. The expression of calpain 1 levels is slightly higher in spermatocytes entering the meiotic phase. Both calpain isoforms are also expressed in mouse spermatozoa and are localized to the acrosomal cap. Inducing capacitated spermatozoa to undergo the acrosome reaction in the presence of a selective calpain inhibitor significantly reduced the acrosome reaction rate in a dose-dependent manner. Thus, calpain, a pluripotential protease with numerous substrates, may serve as an effector in more than one pathway in the complex process of spermatogenesis and in the events preceding fertilization, such as the acrosome reaction.

PDILT, a Divergent Testis-specific Protein Disulfide Isomerase with a Non-classical SXXC Motif That Engages in Disulfide-dependent Interactions in the Endoplasmic Reticulum

Protein disulfide isomerase (PDI) is the archetypal enzyme involved in the formation and reshuffling of disulfide bonds in the endoplasmic reticulum (ER). PDI achieves its redox function through two highly conserved thioredoxin domains, and PDI can also operate as an ER chaperone. The substrate specificities and the exact functions of most other PDI family proteins remain important unsolved questions in biology. Here, we characterize a new and striking member of the PDI family, which we have named protein disulfide isomerase-like protein of the testis (PDILT). PDILT is the first eukaryotic SXXC protein to be characterized in the ER. Our experiments have unveiled a novel, glycosylated PDI-like protein whose tissue-specific expression and unusual motifs have implications for the evolution, catalytic function, and substrate selection of thioredoxin family proteins. We show that PDILT is an ER resident glycoprotein that liaises with partner proteins in disulfide-dependent complexes within the testis. PDILT interacts with the oxidoreductase Ero1alpha, demonstrating that the N-terminal cysteine of the CXXC sequence is not required for binding of PDI family proteins to ER oxidoreductases. The expression of PDILT, in addition to PDI in the testis, suggests that PDILT performs a specialized chaperone function in testicular cells. PDILT is an unusual PDI relative that highlights the adaptability of chaperone and redox function in enzymes of the endoplasmic reticulum.

Functional characterization of a mouse testicular olfactory receptor and its role in chemosensing and in regulation of sperm motility

Although a subset of the olfactory receptor (OR) gene family is expressed in testis, neither their developmental profile nor their physiological functions have been fully characterized. Here, we show that MOR23 (a mouse OR expressed in the olfactory epithelium and testis) functions as a chemosensing receptor in mouse germ cells. In situ hybridization showed that MOR23 was expressed in round spermatids during stages VI-VIII of spermatogenesis. Lyral, a cognate ligand of MOR23, caused an increase in intracellular Ca2+ in a fraction of spermatogenic cells and spermatozoa. We also generated transgenic mice that express high levels of MOR23 in the testis and examined the response of their germ cells to lyral. The results provided evidence that lyral-induced Ca2+ increases were indeed mediated by MOR23. In a sperm accumulation assay, spermatozoa migrated towards an increasing gradient of lyral. Tracking and sperm flagellar analyses suggest that Ca2+ increases caused by MOR23 activation lead to modulation of flagellar configuration, resulting in chemotaxis. By contrast, a gradient of a cAMP analog or K8.6 solution, which elicit Ca2+ influx in spermatozoa, did not cause sperm accumulation, indicating that chemosensing and regulation of sperm motility was due to an OR-mediated local Ca2+ increase. The present studies indicate that mouse testicular ORs might play a role in chemoreception during sperm-egg communication and thereby regulate fertilization.

Cloning, Characterization, and Functional Expression of Taenia Solium Calreticulin

Calreticulin is an endoplasmic reticulum protein involved in the homeostasis of intracellular Ca++ and other physiological processes. A complementary DNA clone containing the complete coding sequence of Taenia solium calreticulin (TsCRT) was isolated and characterized. Recombinant TsCRT was expressed in bacteria as a 50-kDa protein that specifically bound calcium when tested in a radioassay. The deduced amino acid sequence has 47-50% identity with other reported calreticulins. Poor recognition of TsCRT by human and pig sera with confirmed cysticercosis discourages its use for diagnosis of the disease. However, further characterization and localization studies could provide insights into the role of TsCRT in T. solium physiology and host-parasite interactions.

Mili, a mammalian member ofpiwifamily gene, is essential for spermatogenesis

The piwi family genes, which are defined by conserved PAZ and Piwi domains, play important roles in stem cell self-renewal, RNA silencing, and translational regulation in various organisms. To reveal the function of the mammalian homolog of piwi, we produced and analyzed mice with targeted mutations in the Mili gene, which is one of three mouse homologs of piwi. Spermatogenesis in the MILI-null mice was blocked completely at the early prophase of the first meiosis, from the zygotene to early pachytene, and the mice were sterile. However, primordial germ cell development and female germ cell production were not disturbed. Furthermore,MILI bound to MVH, which is an essential factor during the early spermatocyte stage. The similarities in the phenotypes of the MILI- and MVH-deficient mice and in the physical binding properties of MILI and MVH indicate a functional association of these proteins in post-transcriptional regulation. These data indicate that MILI is essential for the differentiation of spermatocytes.

A new sperm-specific Na+/H+ exchanger required for sperm motility and fertility

It has long been speculated that intracellular pH is a critical regulator of both invertebrate and vertebrate sperm motility, and sodium-hydrogen exchange has been suggested as a mediator of such pH(i) regulation in various instances. Two sodium-hydrogen exchangers (NHE1 and NHE5) are expressed in spermatozoa. However, elimination of the NHE1 gene fails to cause infertility, suggesting that normal sperm function is maintained in NHE1-null animals. Here, we used a functionally unbiased signal peptide trap screen to identify a novel sperm-specific NHE. The NHE contains 14 predicted transmembrane segments, including a potential voltage sensor and a consensus cyclic nucleotide-binding motif. Testis histology, sperm numbers and morphology were normal, but NHE-null males were completely infertile with severely diminished sperm motility. The addition of ammonium chloride, which elevates intracellular pH, partially rescued the motility and fertility defects. Surprisingly, cyclic AMP analogues almost completely rescued the motility and infertility phenotypes. The existence of this new sperm NHE provides an attractive contraceptive target, given its cell-specific expression and absolute requirement for fertility.

Phtf1 is an integral membrane protein localized in an endoplasmic reticulum domain in maturing male germ cells

Phtf1 is a gene evolutionarily conserved from Drosophila to human that is abundantly expressed in testis. In adult rat, transcripts were abundant in germinal meiotic and postmeiotic cells. Phtf1-specific antibodies revealed weak activity in a juxtanuclear region of early pachytene spermatocytes. Labeling progressively extended to the entire cytoplasm of step 2-3 spermatids, became intense from step 4, and persisted until the end of spermiogenesis, when it was eliminated in the residual bodies. Phtf1 displayed the properties of an integral membrane protein. In transfected cells and haploid cells of rat seminiferous epithelium, it colocalized with ER markers (calnexin and calmegin, respectively). By using both ER and Golgi markers (TGN-38, p58), we were able to show that, in pachytene spermatocytes and in Golgi phase spermatids, phtf1 labeled a region neighboring the cis-Golgi that probably corresponded to the peripheral Golgi region. Phtf1 staining was not related to beta-COP, AP1, or AP2 aptamers, indicating that it was not transported between Golgi saccules or between the Golgi complex and plasma membrane. However, aptamer labeling showed that chlatrin vesicles could be engaged in a new traffic route, raising the possibility of a meiotic proacrosomal vesicle origin. Colocalization between phtf1 and calmegin decreased during the acrosomal phase. During the maturation phase, phtf1 was able to identify different ER domains, as described previously for the peripheral Golgi region. Phtf1 provides a potential new marker for Golgi modifications as well as for many of the obscure transformations undergone by the endoplasmic reticulum. It could help to elucidate the morphogenic events connected with the transformation of spermatogenic cells.

Essential role of citron kinase in cytokinesis of spermatogenic precursors

During spermatogenesis, the first morphological indication of spermatogonia differentiation is incomplete cytokinesis, followed by the assembly of stable intercellular cytoplasmic communications. This distinctive feature of differentiating male germ cells has been highly conserved during evolution, suggesting that regulation of the cytokinesis endgame is a crucial aspect of spermatogenesis. However, the molecular mechanisms underlying testis-specific regulation of cytokinesis are still largely unknown. Citron kinase is a myotonin-related protein acting downstream of the GTPase Rho in cytokinesis control. We previously reported that Citron kinase knockout mice are affected by a complex neurological syndrome caused by cytokinesis block and apoptosis of specific neuronal precursors. In this report we show that, in addition, these mice display a dramatic testicular impairment, with embryonic and postnatal loss of undifferentiated germ cells and complete absence of mature spermatocytes. By contrast, the ovaries of mutant females appear essentially normal. Developmental analysis revealed that the cellular depletion observed in mutant testes is caused by increased apoptosis of undifferentiated and differentiating precursors. The same cells display a severe cytokinesis defect, resulting in the production of multinucleated cells and apoptosis. Our data indicate that Citron kinase is specifically required for cytokinesis of the male germ line.

Calmegin is required for fertilin alpha/beta heterodimerization and sperm fertility

Loss of the endoplasmic reticulum resident chaperone calmegin leads to the production of sterile sperm that do not bind to the egg zona pellucida (M. Ikawa et al., 1997, Nature 387, 607-611). In the present study, we demonstrate that calmegin -/- sperm were defective in migrating into the oviducts and in binding to the egg plasma membrane. Despite the impaired adhesive function, calmegin -/- sperm could fertilize eggs when zonae pellucidae were partially dissected, and eggs fertilized in this manner could develop normally to term. Since these sperm characteristics were similar to those found in fertilin beta -/- sperm, we investigated the interaction of calmegin with fertilin beta. Using immunoprecipitation techniques, calmegin was found to bind to sperm membrane proteins, fertilin alpha and beta, during spermatogenesis. The binding was specific to calmegin: another endoplasmic reticulum chaperone calnexin, a calmegin homologue, was not able to bind to fertilin alpha and beta. In the calmegin -/- mice, a loss of heterodimerization of fertilin alpha and beta was observed and fertilin beta was not detectable in mature sperm. The data not only explain why the calmegin and fertilin beta knockout mouse lines share a common infertile phenotype, but also reveal the importance of the maturation of sperm membrane proteins in the endoplasmic reticulum.

Major histocompatibility class I folding, assembly, and degradation: a paradigm for two-stage quality control in the endoplasmic reticulum

Protein folding in living cells is a complex process involving many interdependent factors. The primary site for folding of nascent proteins destined for secretion is the endoplasmic reticulum (ER). Several disease states, including cystic fibrosis, are brought about because of irregularities in protein folding. Under normal cellular conditions, "quality control" mechanisms ensure that only correctly folded proteins are exported from the ER, with incorrectly folded or incompletely assembled proteins being degraded. Quality control mechanisms can be divided into two broad processes: (1) Primary quality control involves general mechanisms that are not specific for individual proteins; these monitor the fidelity of nascent protein folding in the ER and mediate the destruction of incompletely folded proteins. (2) Partially folded or assembled proteins may be subject to secondary quality control mechanisms that are protein- or protein-family-specific. Here we use the folding and assembly of major histocompatibility complex (MHC) class I as an example to illustrate the processes of quality control in the ER. MHC class I, a trimeric complex assembled in the ER of virally infected or malignant cells, presents antigenic peptide to cytotoxic T lymphocytes; this mediates cell killing and thereby prevents the spread of infection or malignancy. The folding and assembly of MHC class I is subjected to both primary and secondary quality control mechanisms that lead either to correct folding, assembly, and secretion or to degradation via a proteasome-associated mechanism.

The Structure of calnexin, an ER chaperone involved in quality control of protein folding

The three-dimensional structure of the lumenal domain of the lectin-like chaperone calnexin determined to 2.9 A resolution reveals an extended 140 A arm inserted into a beta sandwich structure characteristic of legume lectins. The arm is composed of tandem repeats of two proline-rich sequence motifs which interact with one another in a head-to-tail fashion. Identification of the ligand binding site establishes calnexin as a monovalent lectin, providing insight into the mechanism by which the calnexin family of chaperones interacts with monoglucosylated glycoproteins.

Germ cell differentiation and synaptonemal complex formation are disrupted in CPEB knockout mice

CPEB is a sequence-specific RNA binding protein that regulates translation during vertebrate oocyte maturation. Adult female CPEB knockout mice contained vestigial ovaries that were devoid of oocytes; ovaries from mid-gestation embryos contained oocytes that were arrested at the pachytene stage. Male CPEB null mice also contained germ cells arrested at pachytene. The germ cells from the knockout mice harbored fragmented chromatin, suggesting a possible defect in homologous chromosome adhesion or synapsis. Two CPE-containing synaptonemal complex protein mRNAs, which interact with CPEB in vitro and in vivo, contained shortened poly(A) tails and mostly failed to sediment with polysomes in the null mice. Synaptonemal complexes were not detected in these animals. CPEB therefore controls germ cell differentiation by regulating the formation of the synaptonemal complex.