Testis-enriched kinesin KIF9 is important for progressive motility in mouse spermatozoa

Expression pattern and functional analysis of kinesin-14 KIFC1 in spermatogenesis of Macaca mulatta

C-terminal kinesin motor KIFC1 is increasingly concerned with an essential role in germ cell development. During the spermatogenesis of mice, rats, and crustaceans, KIFC1 functions in regulating meiotic chromosome separation, acrosome vesicle transportation, and nuclear morphology maintenance. The expression pattern of KIFC1 is conservatively concentrated at the acrosome and nucleus of haploid sperm cells. However, whether KIFC1 has similar functions in non-human primates remains unknown. In this study, we constructed the testis-specific cDNA library and cloned different transcripts of KIFC1 based on the genomic sequence. New variants of KIFC1 were identified, and showed different functional domains from the predicted isoforms. The spatio-temporal expression of KIFC1 proteins in seminiferous tubules of rhesus monkeys showed an obvious nuclear localization, specifically expressed in the spermatocytes and early haploid spermatids. The transcripts of KIFC1 also exhibited considerable expression in the nucleus of rhesus LLC-MK2 cells. Besides, we demonstrated that KIFC1 located at the acrosome and microtubule flagella of the mature sperm, and KIFC1 inhibition resulted in sperm tail deformation as well as increased the instability of head-to-tail connection. In summary, this study filled a gap in the reproductive research of the KIFC1 gene in non-human primates.

Distinct roles of Kif6 and Kif9 in mammalian ciliary trafficking and motility

Ciliary beat and intraflagellar transport depend on dynein and kinesin motors. The kinesin-9 family members Kif6 and Kif9 are implicated in motile cilia motilities across protists and mammals. How they function and whether they act redundantly, however, remain unclear. Here, we show that Kif6 and Kif9 play distinct roles in mammals. Kif6 forms puncta that move bidirectionally along axonemes, whereas Kif9 appears to oscillate regionally on the ciliary central apparatus. Consistently, only Kif6 displays microtubule-based motor activity in vitro, and its ciliary localization requires its ATPase activity. Kif6 deficiency in mice disrupts coordinated ciliary beat across ependymal tissues and impairs cerebrospinal fluid flow, resulting in severe hydrocephalus and high mortality. Kif9 deficiency causes mild hydrocephalus without obviously affecting the ciliary beat or the lifespan. Kif6-/- and Kif9-/- males are infertile but exhibit oligozoospermia with poor sperm motility and defective forward motion of sperms, respectively. These results suggest Kif6 as a motor for cargo transport and Kif9 as a central apparatus regulator.

Two Tetrahymena kinesin-9 family members exhibit slow plus-end-directed motility in vitro

The kinesin-9 family comprises two subfamilies specific to ciliated eukaryotic cells, and has recently attracted considerable attention because of its importance in ciliary bending and formation. However, only scattered data are available on the motor properties of kinesin-9 family members; these properties have not been compared under identical experimental conditions using kinesin-9 motors from the same species. Here, we report the comprehensive motor properties of two kinesin-9 molecules of Tetrahymena thermophila, TtK9A (Kif9/Klp1 ortholog) and TtK9B1 (Kif6 ortholog), using microtubule-based in vitro assays, including single-motor and multi-motor assays and microtubule-stimulated ATPase assays. Both subfamilies exhibit microtubule plus-end-directed, extremely slow motor activity, both in single and multiple molecules. TtK9A shows lower processivity than TtK9B1. Our findings indicate that the considerable slow movement of kinesin-9 that corresponds to low ATP hydrolysis rates is a common feature of the ciliary kinesin-9 family.

MYCBPAP is a central apparatus protein required for centrosome-nuclear envelope docking and sperm tail biogenesis in mice

The structure of the sperm flagellar axoneme is highly conserved across species and serves the essential function of generating motility to facilitate the meeting of spermatozoa with the egg. During spermiogenesis, the axoneme elongates from the centrosome, and subsequently the centrosome docks onto the nuclear envelope to continue tail biogenesis. Mycbpap is expressed predominantly in mouse and human testes and conserved in Chlamydomonas as FAP147. A previous cryo-electron microscopy analysis has revealed the localization of FAP147 to the central apparatus of the axoneme. Here, we generated Mycbpap-knockout mice and demonstrated the essential role of Mycbpap in male fertility. Deletion of Mycbpap led to disrupted centrosome-nuclear envelope docking and abnormal flagellar biogenesis. Furthermore, we generated transgenic mice with tagged MYCBPAP, which restored the fertility of Mycbpap-knockout males. Interactome analyses of MYCBPAP using Mycbpap transgenic mice unveiled binding partners of MYCBPAP including central apparatus proteins, such as CFAP65 and CFAP70, which constitute the C2a projection, and centrosome-associated proteins, such as CCP110. These findings provide insights into a MYCBPAP-dependent regulation of the centrosome-nuclear envelope docking and sperm tail biogenesis.

Development of functional spermatozoa in mammalian spermiogenesis

Infertility is a global health problem affecting one in six couples, with 50% of cases attributed to male infertility. Spermatozoa are male gametes, specialized cells that can be divided into two parts: the head and the flagellum. The head contains a vesicle called the acrosome that undergoes exocytosis and the flagellum is a motility apparatus that propels the spermatozoa forward and can be divided into two components, axonemes and accessory structures. For spermatozoa to fertilize oocytes, the acrosome and flagellum must be formed correctly. In this Review, we describe comprehensively how functional spermatozoa develop in mammals during spermiogenesis, including the formation of acrosomes, axonemes and accessory structures by focusing on analyses of mouse models.

Spermatogenic cell-specific type 1 hexokinase (HK1S) is essential for capacitation-associated increase in tyrosine phosphorylation and male fertility in mice

Hexokinase (HK) catalyzes the first irreversible rate-limiting step in glycolysis that converts glucose to glucose-6-phosphate. HK1 is ubiquitously expressed in the brain, erythrocytes, and other tissues where glycolysis serves as the major source of ATP production. Spermatogenic cell-specific type 1 hexokinase (HK1S) is expressed in sperm but its physiological role in male mice is still unknown. In this study, we generate Hk1s knockout mice using the CRISPR/Cas9 system to study the gene function in vivo. Hk1s mRNA is exclusively expressed in testes starting from postnatal day 18 and continuing to adulthood. HK1S protein is specifically localized in the outer surface of the sperm fibrous sheath (FS). Depletion of Hk1s leads to infertility in male mice and reduces sperm glycolytic pathway activity, yet they have normal motile parameters and ATP levels. In addition, by using in vitro fertilization (IVF), Hk1s deficient sperms are unable to fertilize cumulus-intact or cumulus-free oocytes, but can normally fertilize zona pellucida-free oocytes. Moreover, Hk1s deficiency impairs sperm migration into the oviduct, reduces acrosome reaction, and prevents capacitation-associated increases in tyrosine phosphorylation, which are probable causes of infertility. Taken together, our results reveal that HK1S plays a critical role in sperm function and male fertility in mice.

Protective effect of Petroselinum crispum methanolic extract against acrylamide-induced reproductive toxicity in male rats through NF-ĸB, kinesin, steroidogenesis pathways

This study examined the protective effects of a Petroselinum crispum (P. crispum) methanolic extract on reproductive dysfunction induced by acrylamide in male rats. A total of 40 rats were divided into four groups (n=10). The control group received distilled water, the acrylamide group received 10 mg/kg of acrylamide, the P. crispum group received 100 mg/kg of P. crispum extract, and the combined group was pretreated with P. crispum for two weeks before co-administration of P. crispum and acrylamide. All administrations were administered orally using a gastric tube for eight weeks. Acrylamide decreased testosterone levels but did not affect levels of FSH or LH. It also increased testicular levels of (MDA) malondialdehyde and reduced activity of (SOD) superoxide dismutase and impairment of sperm parameters. Furthermore, the administration of acrylamide resulted in an elevation of tumor necrosis factor-alpha (TNF-α) levels and a reduction in the levels of steroidogenic acute regulatory protein (STAR) and cytochrome P450scc (P450scc). Acrylamide negatively affected the histopathological outcomes, Johnsen's score, the diameter of seminiferous tubules, and the thickness of the germinal epithelium. It also upregulated the expression of NF-ĸB P65 and downregulated the expression of kinesin motor protein. In contrast, treatment with P. crispum extract restored the levels of antioxidant enzymes, improved sperm parameters, and normalized the gene expression of TNF-α, IL-10, IL-6, iNOS, NF-ĸB, STAR, CYP17A1, 17β-HSD and P450scc. It also recovered testicular histological parameters and immunoexpression of NF-ĸB P65 and kinesin altered by acrylamide. P. crispum showed protective effects against acrylamide-induced reproductive toxicity by suppressing oxidative damage and inflammatory pathways.

Motor proteins, spermatogenesis and testis function

The role of motor proteins in supporting intracellular transports of vesicles and organelles in mammalian cells has been known for decades. On the other hand, the function of motor proteins that support spermatogenesis is also well established since the deletion of motor protein genes leads to subfertility and/or infertility. Furthermore, mutations and genetic variations of motor protein genes affect fertility in men, but also a wide range of developmental defects in humans including multiple organs besides the testis. In this review, we seek to provide a summary of microtubule and actin-dependent motor proteins based on earlier and recent findings in the field. Since these two cytoskeletons are polarized structures, different motor proteins are being used to transport cargoes to different ends of these cytoskeletons. However, their involvement in germ cell transport across the blood-testis barrier (BTB) and the epithelium of the seminiferous tubules remains relatively unknown. It is based on recent findings in the field, we have provided a hypothetical model by which motor proteins are being used to support germ cell transport across the BTB and the seminiferous epithelium during the epithelial cycle of spermatogenesis. In our discussion, we have highlighted the areas of research that deserve attention to bridge the gap of research in relating the function of motor proteins to spermatogenesis.

The kinesin motor Kif9 regulates centriolar satellite positioning and mitotic progression

Centrosomes are the principal microtubule-organizing centers of the cell and play an essential role in mitotic spindle function. Centrosome biogenesis is achieved by strict control of protein acquisition and phosphorylation prior to mitosis. Defects in this process promote fragmentation of pericentriolar material culminating in multipolar spindles and chromosome missegregation. Centriolar satellites, membrane-less aggrupations of proteins involved in the trafficking of proteins toward and away from the centrosome, are thought to contribute to centrosome biogenesis. Here we show that the microtubule plus-end directed kinesin motor Kif9 localizes to centriolar satellites and regulates their pericentrosomal localization during interphase. Lack of Kif9 leads to aggregation of satellites closer to the centrosome and increased centrosomal protein degradation that disrupts centrosome maturation and results in chromosome congression and segregation defects during mitosis. Our data reveal roles for Kif9 and centriolar satellites in the regulation of cellular proteostasis and mitosis.

Disruption of testis-enriched cytochrome c oxidase subunit COX6B2 but not COX8C leads to subfertility

Mammalian sperm flagellum contains the midpiece characterized by a mitochondrial sheath that packs tightly around the axoneme and outer dense fibers. Mitochondria are known as the "powerhouse" of the cell, and produce ATP through the tricarboxylic acid (TCA) cycle and oxidative phosphorylation (OXPHOS). However, the contribution of the TCA cycle and OXPHOS to sperm motility and male fertility is less clear. Cytochrome c oxidase (COX) is an oligomeric complex localized within the mitochondrial inner membrane, and the terminal enzyme of the mitochondrial electron transport chain in eukaryotes. Both COX6B2 and COX8C are testis-enriched COX subunits whose functions in vivo are poorly studied. Here, we generated Cox6b2 and Cox8c knockout (KO) mice using the CRISPR/Cas9 system. We examined their fertility and sperm mitochondrial function to determine the significance of testis-enriched COX subunits in male fertility. The mating test revealed that disrupting COX6B2 induces male subfertility, while disrupting COX8C does not affect male fertility. Cox6b2 KO spermatozoa showed low sperm motility, but mitochondrial function was normal according to oxygen consumption rates. Therefore, low sperm motility seems to cause subfertility in Cox6b2 KO male mice. These results also indicate that testis-enriched COX, COX6B2 and COX8C, are not essential for OXPHOS in mouse spermatozoa.

Kif9 is an active kinesin motor required for ciliary beating and proximodistal patterning of motile axonemes

Most motile cilia have a stereotyped structure of nine microtubule outer doublets and a single central pair of microtubules. The central pair of microtubules are surrounded by a set of proteins, termed the central pair apparatus. A specific kinesin, Klp1 projects from the central pair and contributes to ciliary motility in Chlamydomonas. The vertebrate ortholog, Kif9, is required for beating in mouse sperm flagella, but the mechanism of Kif9/Klp1 function remains poorly defined. Here, using Xenopus epidermal multiciliated cells, we show that Kif9 is necessary for ciliary motility and the proper distal localization of not only central pair proteins, but also radial spokes and dynein arms. In addition, single-molecule assays in vitro reveal that Xenopus Kif9 is a long-range processive motor, although it does not mediate long-range movement in ciliary axonemes in vivo. Together, our data suggest that Kif9 is integral for ciliary beating and is necessary for proper axonemal distal end integrity.

Kif6 regulates cilia motility and polarity in brain ependymal cells

Ependymal cells, lining brain ventricular walls, display tufts of cilia that beat in concert promoting laminar Cerebrospinal fluid (CSF) flow within brain ventricles. The ciliary axonemes of multiciliated ependymal cells display a 9+2 microtubule array common to motile cilia. Dyneins and kinesins are ATPase microtubule motor proteins that promote the rhythmic beating of cilia axonemes. Despite common consensus about the importance of axonemal dynein motor proteins, little is known about how Kinesin motors contribute to cilia motility. Here, we define the function of Kinesin family member 6 (Kif6) using a mutation that lacks a highly conserved C-terminal tail domain ( Kif6 p.G555fs ) and which displays progressive hydrocephalus in mice. An analogous mutation was isolated in a proband displaying macrocephaly, hypotonia, and seizures implicating an evolutionarily conserved function for Kif6 in neurodevelopment. We find that loss of Kif6 function caused decreased ependymal cilia motility and subsequently decreased fluid flow on the surface of brain ventricular walls. Kif6 protein was localized at ependymal cilia and displayed processive motor movement (676 nm/s) on microtubules in vitro . Loss of the Kif6 C-terminal tail domain did not affect the initial ciliogenesis in vivo , but did result in defects in cilia orientation, the formation of robust apical actin networks, and stabilization of basal bodies at the apical surface. This suggests a novel role for the Kif6 motor in maintenance of ciliary homeostasis of ependymal cells.

Summary statement

We found that Kif6 is localized to the axonemes of ependymal cells. In vitro analysis shows that Kif6 moves on microtubules and that its loss mice decrease cilia motility and cilia-driven flow, resulting in hydrocephalus.

Identification of bi-allelic KIF9 loss-of-function variants contributing to asthenospermia and male infertility in two Chinese families

Introduction

Asthenozoospermia (AZS) is a leading cause of male infertility, affecting an estimated 18% of infertile patients. Kinesin proteins function as molecular motors capable of moving along microtubules. The highly conserved kinesin family member 9 (KIF9) localizes to the central microtubule pair in the flagella of Chlamydomonas cells. The loss of KIF9 expression in mice has been linked to AZS phenotypes.

Methods

Variant screening was performed by whole exome sequencing from 92 Chinese infertile patients with AZS. Western blot was used to was used for analyzing of candidate proteins expression. Patients' sperm samples were stained with immunofluorescent to visualise proteins localization and were visualised by transmission electron microscopy (TEM) to determine axoneme structures. Co-immunoprecipitation assay was used to verify the binding proteins of KIF9. In vitro fertilization (IVF) was used to evaluate the efficiency of clinical treatment.

Results

Bi-allelic KIF9 loss-of-function variants were identified in two unrelated Chinese males exhibiting atypical sperm motility phenotypes. Both of these men exhibited typical AZS and suffered from infertility together with the complete absence of KIF9 expression. In contrast to these KIF9-deficient patients, positive KIF9 staining was evident throughout the flagella of sperm from normal control individuals. KIF9 was able to interact with the microtubule central pair (CP) component hydrocephalus-inducing protein homolog (HYDIN) in human samples. And KIF9 was undetectable in spermatozoa harboring CP deletions. The morphologicy of KIF9-deficient spermatozoa appeared normal under gross examination and TEM. Like in mice, in vitro fertilization was sufficient to overcome the fertility issues for these two patients.

Discussion

These findings indicate that KIF9 associates with the central microtubules in human sperm and that it functions to specifically regulate flagellar swinging. Overall, these results offer greater insight into the biological functions of KIF9 in the assembly of the human flagella and its role in male fertility.

Islet primary cilia motility controls insulin secretion

Primary cilia are specialized cell-surface organelles that mediate sensory perception and, in contrast to motile cilia and flagella, are thought to lack motility function. Here, we show that primary cilia in human and mouse pancreatic islets exhibit movement that is required for glucose-dependent insulin secretion. Islet primary cilia contain motor proteins conserved from those found in classic motile cilia, and their three-dimensional motion is dynein-driven and dependent on adenosine 5'-triphosphate and glucose metabolism. Inhibition of cilia motion blocks beta cell calcium influx and insulin secretion. Human beta cells have enriched ciliary gene expression, and motile cilia genes are altered in type 2 diabetes. Our findings redefine primary cilia as dynamic structures having both sensory and motile function and establish that pancreatic islet cilia movement plays a regulatory role in insulin secretion.

Kinesins in Mammalian Spermatogenesis and Germ Cell Transport

In mammalian testes, the apical cytoplasm of each Sertoli cell holds up to several dozens of germ cells, especially spermatids that are transported up and down the seminiferous epithelium. The blood-testis barrier (BTB) established by neighboring Sertoli cells in the basal compartment restructures on a regular basis to allow preleptotene/leptotene spermatocytes to pass through. The timely transfer of germ cells and other cellular organelles such as residual bodies, phagosomes, and lysosomes across the epithelium to facilitate spermatogenesis is important and requires the microtubule-based cytoskeleton in Sertoli cells. Kinesins, a superfamily of the microtubule-dependent motor proteins, are abundantly and preferentially expressed in the testis, but their functions are poorly understood. This review summarizes recent findings on kinesins in mammalian spermatogenesis, highlighting their potential role in germ cell traversing through the BTB and the remodeling of Sertoli cell-spermatid junctions to advance spermatid transport. The possibility of kinesins acting as a mediator and/or synchronizer for cell cycle progression, germ cell transit, and junctional rearrangement and turnover is also discussed. We mostly cover findings in rodents, but we also make special remarks regarding humans. We anticipate that this information will provide a framework for future research in the field.

Cryo-EM structure of an active central apparatus

Accurately regulated ciliary beating in time and space is critical for diverse cellular activities, which impact the survival and development of nearly all eukaryotic species. An essential beating regulator is the conserved central apparatus (CA) of motile cilia, composed of a pair of microtubules (C1 and C2) associated with hundreds of protein subunits per repeating unit. It is largely unclear how the CA plays its regulatory roles in ciliary motility. Here, we present high-resolution structures of Chlamydomonas reinhardtii CA by cryo-electron microscopy (cryo-EM) and its dynamic conformational behavior at multiple scales. The structures show how functionally related projection proteins of CA are clustered onto a spring-shaped scaffold of armadillo-repeat proteins, facilitated by elongated rachis-like proteins. The two halves of the CA are brought together by elastic chain-like bridge proteins to achieve coordinated activities. We captured an array of kinesin-like protein (KLP1) in two different stepping states, which are actively correlated with beating wave propagation of cilia. These findings establish a structural framework for understanding the role of the CA in cilia.

The motor domain of testis-enriched kinesin KIF9 is essential for its localization in the mouse flagellum

Kinesin is a molecular motor that moves along microtubules. Testis-enriched kinesin KIF9 (Kinesin family member 9) is localized in the mouse sperm flagellum and is important for normal sperm motility and male fertility; however, it is unclear if the motor domain of KIF9 is involved in these processes. In this study, we substituted threonine of the ATP binding motif in the KIF9 motor domain to asparagine (T100N) in mice using the CRISPR/Cas9 system, which is known to impair kinesin motor activity. T100N mutant mice exhibit reduced sperm motility and male fertility consistent with Kif9 knockout mice. Further, KIF9 was depleted in the spermatozoa of T100N mutant mice although the amounts of KIF9 were comparable between wild-type and T100N mutant testes. These results indicate that the motor domain of KIF9 is essential for its localization in the sperm flagellum.

SPATA33 localizes calcineurin to the mitochondria and regulates sperm motility in mice

Calcineurin is a target of immunosuppressive drugs such as cyclosporine A and tacrolimus. In the immune system, calcineurin interacts with NFAT via the PxIxIT motif to activate T cells. In contrast, little is known about the proteins that interact with a testis-enriched calcineurin that is essential for sperm motility and male fertility. Here, we discovered that calcineurin interacts with SPATA33 via a PQIIIT sequence in the testis. Further analyses reveal that SPATA33 plays critical roles in sperm motility and male fertility. Our finding sheds new light on the molecular mechanisms of sperm motility regulation and the etiology of human male fertility. Furthermore, it may help us not only understand reproductive toxicities but also develop nonhormonal male contraceptives.

Calcineurin is a calcium-dependent phosphatase that plays roles in a variety of biological processes including immune responses. In spermatozoa, there is a testis-enriched calcineurin composed of PPP3CC and PPP3R2 (sperm calcineurin) that is essential for sperm motility and male fertility. Because sperm calcineurin has been proposed as a target for reversible male contraceptives, identifying proteins that interact with sperm calcineurin widens the choice for developing specific inhibitors. Here, by screening the calcineurin-interacting PxIxIT consensus motif in silico and analyzing the function of candidate proteins through the generation of gene-modified mice, we discovered that SPATA33 interacts with sperm calcineurin via a PQIIIT sequence. Spata33 knockout mice exhibit reduced sperm motility because of an inflexible midpiece, leading to impaired male fertility, which phenocopies Ppp3cc and Ppp3r2 knockout mice. Further analysis reveals that sperm calcineurin disappears from the mitochondria in the Spata33 knockout testis. In addition, immunoprecipitation analysis indicates that sperm calcineurin interacts with not only SPATA33 but also the mitochondrial protein VDAC2. These results indicate that SPATA33 localizes calcineurin to the mitochondria and regulates sperm motility.

Structural and Functional Characterization of a Testicular Long Non-coding RNA (4930463O16Rik) Identified in the Meiotic Arrest of the Mouse Topaz1 -/- Testes

Spermatogenesis involves coordinated processes, including meiosis, to produce functional gametes. We previously reported Topaz1 as a germ cell-specific gene highly conserved in vertebrates. Topaz1 knockout males are sterile with testes that lack haploid germ cells because of meiotic arrest after prophase I. To better characterize Topaz1^–/– testes, we used RNA-sequencing analyses at two different developmental stages (P16 and P18). The absence of TOPAZ1 disturbed the expression of genes involved in microtubule and/or cilium mobility, biological processes required for spermatogenesis. Moreover, a quarter of P18 dysregulated genes are long non-coding RNAs (lncRNAs), and three of them are testis-specific and located in spermatocytes, their expression starting between P11 and P15. The suppression of one of them, 4939463O16Rik, did not alter fertility although sperm parameters were disturbed and sperm concentration fell. The transcriptome of P18-4939463O16Rik^–/– testes was altered and the molecular pathways affected included microtubule-based processes, the regulation of cilium movement and spermatogenesis. The absence of TOPAZ1 protein or 4930463O16Rik produced the same enrichment clusters in mutant testes despite a contrasted phenotype on male fertility. In conclusion, although Topaz1 is essential for the meiosis in male germ cells and regulate the expression of numerous lncRNAs, these studies have identified a Topaz1 regulated lncRNA (4930463O16Rik) that is key for both sperm production and motility.

Testis-expressed protein 33 is not essential for spermiogenesis and fertility in mice

Gene expression analyses have revealed that there are >2,300 testis-enriched genes in mice, and gene knockout models have shown that a number of them are responsible for male fertility. However, the functions of numerous genes have yet to be clarified. The aim of the present study was to identify the expression pattern of testis-expressed protein 33 (TEX33) in mice and explore the role of TEX33 in male reproduction. Reverse transcription-polymerase chain reaction and western blot assays were used to investigate the mRNA and protein levels of TEX33 in mouse testes during the first wave of spermatogenesis. Immunofluorescence analysis was also performed to identify the cellular and structural localization of TEX33 protein in the testes. Tex33 knockout mice were generated by CRISPR/Cas9 gene-editing. Histological analysis with hematoxylin and eosin or periodic acid-Schiff (PAS) staining, computer-assisted sperm analysis (CASA) and fertility testing, were also carried out to evaluate the effect of TEX33 on mouse spermiogenesis and male reproduction. The results showed that Tex33 mRNA and protein were exclusively expressed in mouse testes and were first detected on postnatal days 21–28 (spermiogenesis phase); their expression then remained into adulthood. Immunofluorescence analysis revealed that TEX33 protein was located in the spermatids and sperm within the seminiferous tubules of the mouse testes, and exhibited specific localization to the acrosome, flagellum and manchette during spermiogenesis. These results suggested that TEX33 may play a role in mouse spermiogenesis. However, Tex33 knockout mice presented no detectable difference in testis-to-body weight ratios when compared with wild-type mice. PAS staining and CASA revealed that spermatogenesis and sperm quality were normal in mice lacking Tex33. In addition, fertility testing suggested that the Tex33 knockout mice had normal reproductive functions. In summary, the findings of the present study indicate that TEX33 is associated with spermiogenesis but is not essential for sperm development and male fertility.

Central Apparatus, the Molecular Kickstarter of Ciliary and Flagellar Nanomachines

Motile cilia and homologous organelles, the flagella, are an early evolutionarily invention, enabling primitive eukaryotic cells to survive and reproduce. In animals, cilia have undergone functional and structural speciation giving raise to typical motile cilia, motile nodal cilia, and sensory immotile cilia. In contrast to other cilia types, typical motile cilia are able to beat in complex, two-phase movements. Moreover, they contain many additional structures, including central apparatus, composed of two single microtubules connected by a bridge-like structure and assembling numerous complexes called projections. A growing body of evidence supports the important role of the central apparatus in the generation and regulation of the motile cilia movement. Here we review data concerning the central apparatus structure, protein composition, and the significance of its components in ciliary beating regulation.

Motor Proteins and Spermatogenesis

Unlike the intermediate filament- and septin-based cytoskeletons which are apolar structures, the microtubule (MT) and actin cytoskeletons are polarized structures in mammalian cells and tissues including the testis, most notable in Sertoli cells. In the testis, these cytoskeletons that stretch across the epithelium of seminiferous tubules and lay perpendicular to the basement membrane of tunica propria serve as tracks for corresponding motor proteins to support cellular cargo transport. These cargoes include residual bodies, phagosomes, endocytic vesicles and most notably developing spermatocytes and haploid spermatids which lack the ultrastructures of motile cells (e.g., lamellipodia, filopodia). As such, these developing germ cells require the corresponding motor proteins to facilitate their transport across the seminiferous epithelium during the epithelial cycle of spermatogenesis. Due to the polarized natures of these cytoskeletons with distinctive plus (+) and minus (-) end, directional cargo transport can take place based on the use of corresponding actin- or MT-based motor proteins. These include the MT-based minus (-) end directed motor proteins: dyneins, and the plus (+) end directed motor proteins: kinesins, as well as the actin-based motor proteins: myosins, many of which are plus (+) end directed but a few are also minus (-) end directed motor proteins. Recent studies have shown that these motor proteins are essential to support spermatogenesis. In this review, we briefly summarize and evaluate these recent findings so that this information will serve as a helpful guide for future studies and for planning functional experiments to better understand their role mechanistically in supporting spermatogenesis.

The developmental biology of kinesins

Kinesins are microtubule-based motor proteins that are well known for their key roles in cell biological processes ranging from cell division, to intracellular transport of mRNAs, proteins, vesicles, and organelles, and microtubule disassembly. Interestingly, many of the ~45 distinct kinesin genes in vertebrate genomes have also been associated with specific phenotypes in embryonic development. In this review, we highlight the specific developmental roles of kinesins, link these to cellular roles reported in vitro, and highlight remaining gaps in our understanding of how this large and important family of proteins contributes to the development and morphogenesis of animals.

Cfap97d1 is important for flagellar axoneme maintenance and male mouse fertility

The flagellum is essential for sperm motility and fertilization in vivo. The axoneme is the main component of the flagella, extending through its entire length. An axoneme is comprised of two central microtubules surrounded by nine doublets, the nexin-dynein regulatory complex, radial spokes, and dynein arms. Failure to properly assemble components of the axoneme in a sperm flagellum, leads to fertility alterations. To understand this process in detail, we have defined the function of an uncharacterized gene, Cfap97 domain containing 1 (Cfap97d1). This gene is evolutionarily conserved in mammals and multiple other species, including Chlamydomonas. We have used two independently generated Cfap97d1 knockout mouse models to study the gene function in vivo. Cfap97d1 is exclusively expressed in testes starting from post-natal day 20 and continuing throughout adulthood. Deletion of the Cfap97d1 gene in both mouse models leads to sperm motility defects (asthenozoospermia) and male subfertility. In vitro fertilization (IVF) of cumulus-intact oocytes with Cfap97d1 deficient sperm yielded few embryos whereas IVF with zona pellucida-free oocytes resulted in embryo numbers comparable to that of the control. Knockout spermatozoa showed abnormal motility characterized by frequent stalling in the anti-hook position. Uniquely, Cfap97d1 loss caused a phenotype associated with axonemal doublet heterogeneity linked with frequent loss of the fourth doublet in the sperm stored in the epididymis. This study demonstrates that Cfap97d1 is required for sperm flagellum ultra-structure maintenance, thereby playing a critical role in sperm function and male fertility in mice.

Analysis of the sperm flagellar axoneme using gene-modified mice

Infertility is a global health issue that affects 1 in 6 couples, with male factors contributing to 50% of cases. The flagellar axoneme is a motility apparatus of spermatozoa, and disruption of its structure or function could lead to male infertility. The axoneme consists of a "9+2" structure that contains a central pair of two singlet microtubules surrounded by nine doublet microtubules, in addition to several macromolecular complexes such as dynein arms, radial spokes, and nexin-dynein regulatory complexes. Molecular components of the flagellar axoneme are evolutionally conserved from unicellular flagellates to mammals, including mice. Although knockout (KO) mice have been generated to understand their function in the formation and motility regulation of sperm flagella, the majority of KO mice die before sexual maturation due to impaired ciliary motility, which makes it challenging to analyze mature spermatozoa. In this review, we introduce methods that have been used to overcome premature lethality, focusing on KO mouse lines of central pair components.

CIB4 is essential for the haploid phase of spermatogenesis in mice†

Spermatogenesis is a complex developmental process that involves the proliferation of diploid cells, meiotic division, and haploid differentiation. Many genes are shown to be essential for male fertility using knockout (KO) mice; however, there still remain genes to be analyzed to elucidate their molecular mechanism and their roles in spermatogenesis. Calcium- and integrin-binding protein 1 (CIB1) is a ubiquitously expressed protein that possesses three paralogs: CIB2, CIB3, and CIB4. It is reported that Cib1 KO male mice are sterile due to impaired haploid differentiation. In this study, we discovered that Cib4 is expressed strongly in mouse and human testis and begins expression during the haploid phase of spermatogenesis in mice. To analyze the function of CIB4 in vivo, we generated Cib4 KO mice using the CRISPR/Cas9 system. Cib4 KO male mice are sterile due to impaired haploid differentiation, phenocopying Cib1 KO male mice. Spermatogenic cells isolated from seminiferous tubules demonstrate an essential function of CIB4 in the formation of the apical region of the sperm head. Further analysis of CIB4 function may shed light on the etiology of male infertility caused by spermatogenesis defects, and CIB4 could be a target for male contraceptives because of its dominant expression in the testis.

Testis-enriched kinesin KIF9 is important for progressive motility in mouse spermatozoa

Kinesin is a molecular motor that moves along microtubules. Kinesin family member 9 (KIF9) is evolutionarily conserved and expressed strongly in mouse testis. In the unicellular flagellate Chlamydomonas, KLP1 (ortholog of KIF9) is localized to the central pair microtubules of the axoneme and regulates flagellar motility. In contrast, the function of KIF9 remains unclear in mammals. Here, we mutated KIF9 in mice using the CRISPR/Cas9 system. Kif9 mutated mice exhibit impaired sperm motility and subfertility. Further analysis reveals that the flagella lacking KIF9 showed an asymmetric waveform pattern, which leads to a circular motion of spermatozoa. In spermatozoa that lack the central pair protein HYDIN, KIF9 was not detected by immunofluorescence and immunoblot analysis. These results suggest that KIF9 is associated with the central pair microtubules and regulates flagellar motility in mice.