Identification of isoforms of a mitotic motor in mammalian spermatogenesis

Expression Dynamics Indicate Potential Roles of KIF17 for Nuclear Reshaping and Tail Formation during Spermiogenesis in Phascolosoma esculenta

Kinesin family member17 (KIF17), a homologous dimer of the kinesin-2 protein family, has important microtubule-dependent and -independent roles in spermiogenesis. Little is known about KIF17 in the mollusk, Phascolosoma esculenta, a newly developed mariculture species in China. Here, we cloned the open reading frame of Pe-kif17 and its related gene, Pe-act, and performed bioinformatics analysis on both. Pe-KIF17 and Pe-ACT are structurally conserved, indicating that they may be functionally conserved. The expression pattern of kif17/act mRNA performed during spermiogenesis revealed their expression in diverse tissues, with the highest expression level in the coelomic fluid of P. esculenta. The expressions of Pe-kif17 and Pe-act mRNA were relatively high during the breeding season (July-September), suggesting that Pe-KIF17/ACT may be involved in spermatogenesis, particularly during spermiogenesis. Further analysis of Pe-kif17 mRNA via fluorescence in situ hybridization revealed the continuous expression of this mRNA during spermiogenesis, suggesting potential functions in this process. Immunofluorescence showed that Pe-KIF17 co-localized with α-tubulin and migrated from the perinuclear cytoplasm to one side of the spermatid, forming the sperm tail. Pe-KIF17 and Pe-ACT also colocalized. KIF17 may participate in spermiogenesis of P. esculenta, particularly in nuclear reshaping and tail formation by interacting with microtubule structures similar to the manchette. Moreover, Pe-KIF17 with Pe-ACT is also involved in nuclear reshaping and tail formation in the absence of microtubules. This study provides evidence for the role of KIF17 during spermiogenesis and provides theoretical data for studies of the reproductive biology of P. esculenta. These findings are important for spermatogenesis in mollusks.

The potential function of KIF17 in large yellow croaker (Larimichthys crocea) spermatid remodeling: molecular characterization and expression pattern during spermiogenesis

KIF17, which belongs to the kinesin-2 protein family, plays an indispensable role in mammalian spermiogenesis. However, the role of KIF17 in fish spermatid remodeling during spermiogenesis remains poorly understood. Therefore, we aimed to study the role of KIF17 in spermatid remodeling during Larimichthys crocea (L. crocea) spermiogenesis. The kif17 cDNA sequence, 3247 bp in length, was cloned from L. crocea testis, which consisted of a 347-bp 5'-untranslated region (UTR), 413-bp 3' -UTR, and 2487-bp open reading frame. Bioinformatic analyses revealed that KIF17 obtained from L. crocea (Lc-KIF17) exhibited a high sequence identity compared with those from other teleosts and possessed the structural features of other kinesin-2 proteins. Based on structural similarity, we speculate that the role of Lc-KIF17 may be similar to that of KIF17 in other animals. Lc-kif17 mRNA was diffusely expressed in L. crocea tissues and was highly expressed in the testis, especially at stage IV testicular development. Immunofluorescence analysis revealed that Lc-KIF17 signals colocalized with β-tubulin signals and migrated from the perinuclear cytoplasm to the side of the nucleus where the tail forms during spermiogenesis. These findings revealed that KIF17 may be involved in L. crocea spermiogenesis. In particular, KIF17 may participate in spermatid remodeling by interacting with perinuclear microtubules during L. crocea spermiogenesis. Collectively, this study contributes to an improved understanding of the mechanism underlying L. crocea spermiogenesis and provides a basis for further research on L. crocea reproduction and development.

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.

Haploid male germ cells-the Grand Central Station of protein transport

BACKGROUND

The precise movement of proteins and vesicles is an essential ability for all eukaryotic cells. Nowhere is this more evident than during the remarkable transformation that occurs in spermiogenesis-the transformation of haploid round spermatids into sperm. These transformations are critically dependent upon both the microtubule and the actin cytoskeleton, and defects in these processes are thought to underpin a significant percentage of human male infertility.

OBJECTIVE AND RATIONALE

This review is aimed at summarising and synthesising the current state of knowledge around protein/vesicle transport during haploid male germ cell development and identifying knowledge gaps and challenges for future research. To achieve this, we summarise the key discoveries related to protein transport using the mouse as a model system. Where relevant, we anchored these insights to knowledge in the field of human spermiogenesis and the causality of human male infertility.

SEARCH METHODS

Relevant studies published in English were identified using PubMed using a range of search terms related to the core focus of the review-protein/vesicle transport, intra-flagellar transport, intra-manchette transport, Golgi, acrosome, manchette, axoneme, outer dense fibres and fibrous sheath. Searches were not restricted to a particular time frame or species although the emphasis within the review is on mammalian spermiogenesis.

OUTCOMES

Spermiogenesis is the final phase of sperm development. It results in the transformation of a round cell into a highly polarised sperm with the capacity for fertility. It is critically dependent on the cytoskeleton and its ability to transport protein complexes and vesicles over long distances and often between distinct cytoplasmic compartments. The development of the acrosome covering the sperm head, the sperm tail within the ciliary lobe, the manchette and its role in sperm head shaping and protein transport into the tail, and the assembly of mitochondria into the mid-piece of sperm, may all be viewed as a series of overlapping and interconnected train tracks. Defects in this redistribution network lead to male infertility characterised by abnormal sperm morphology (teratozoospermia) and/or abnormal sperm motility (asthenozoospermia) and are likely to be causal of, or contribute to, a significant percentage of human male infertility.

WIDER IMPLICATIONS

A greater understanding of the mechanisms of protein transport in spermiogenesis offers the potential to precisely diagnose cases of male infertility and to forecast implications for children conceived using gametes containing these mutations. The manipulation of these processes will offer opportunities for male-based contraceptive development. Further, as increasingly evidenced in the literature, we believe that the continuous and spatiotemporally restrained nature of spermiogenesis provides an outstanding model system to identify, and de-code, cytoskeletal elements and transport mechanisms of relevance to multiple tissues.

Sperm Differentiation: The Role of Trafficking of Proteins

Sperm differentiation encompasses a complex sequence of morphological changes that takes place in the seminiferous epithelium. In this process, haploid round spermatids undergo substantial structural and functional alterations, resulting in highly polarized sperm. Hallmark changes during the differentiation process include the formation of new organelles, chromatin condensation and nuclear shaping, elimination of residual cytoplasm, and assembly of the sperm flagella. To achieve these transformations, spermatids have unique mechanisms for protein trafficking that operate in a coordinated fashion. Microtubules and filaments of actin are the main tracks used to facilitate the transport mechanisms, assisted by motor and non-motor proteins, for delivery of vesicular and non-vesicular cargos to specific sites. This review integrates recent findings regarding the role of protein trafficking in sperm differentiation. Although a complete characterization of the interactome of proteins involved in these temporal and spatial processes is not yet known, we propose a model based on the current literature as a framework for future investigations.

Microtubular Dysfunction and Male Infertility

Microtubules are the prime component of the cytoskeleton along with microfilaments. Being vital for organelle transport and cellular divisions during spermatogenesis and sperm motility process, microtubules ascertain functional capacity of sperm. Also, microtubule based structures such as axoneme and manchette are crucial for sperm head and tail formation. This review (a) presents a concise, yet detailed structural overview of the microtubules, (b) analyses the role of microtubule structures in various male reproductive functions, and (c) presents the association of microtubular dysfunctions with male infertility. Considering the immense importance of microtubule structures in the formation and maintenance of physiological functions of sperm cells, this review serves as a scientific trigger in stimulating further male infertility research in this direction.

The C-terminal kinesin motor KIFC1 may participate in nuclear reshaping and flagellum formation during spermiogenesis of Larimichthys crocea

Spermatogenesis is a highly ordered process in the differentiation of male germ cells. Nuclear morphogenesis is one of the most fundamental cellular transformations to take place during spermatogenesis. These striking transformations from spermatogonia to spermatozoa are a result of phase-specific adaption of the cytoskeleton and its association with molecular motor proteins. KIFC1 is a C-terminal kinesin motor protein that plays an essential role in acrosome formation and nuclear reshaping during spermiogenesis in mammals. To explore its functions during the same process in Larimichthys crocea, we cloned and characterized the cDNA of a mammalian KIFC1 homolog (termed lc-KIFC1) from the total RNA of the testis. The 2481 bp complete lc-KIFC1 cDNA contained a 53 bp 5' untranslated region, a 535 bp 3' untranslated region, and a 1893 bp open reading frame that encoded a special protein of 630 amino acids. The predicted lc-KIFC1 protein possesses a divergent tail region, stalk region, and conserved carboxyl motor region. Protein alignment demonstrated that lc-KIFC1 had 73.2, 49.8, 49.3, 54.6, 56.5, 53.1, and 52.1% identity with its homologs in Danio rerio, Eriocheir sinensis, Octopus tankahkeei, Gallus gallus, Xenopus laevis, Mus musculus, and Homo sapiens, respectively. Tissue expression analysis revealed that lc-kifc1 mRNA was mainly expressed in the testis. The trend of lc-kifc1 mRNA expression at different growth stages of the testis showed that the expression increased first and then decreased, in the stage IV of testis, its expression quantity achieved the highest level. In situ hybridization and immunofluorescence results showed that KIFC1 was localized around the nucleus in early spermatids. As spermatid development progressed, the signals increased substantially. These signals peaked and were concentrated at one end of the nucleus when the spermatids began to undergo dramatic changes. In the mature sperm, the signal for KIFC1 gradually became weak and was mainly localized in the tail. In summary, evaluation of the expression pattern for lc-KIFC1 at specific stages of spermiogenesis has shed light on the potential functions of this motor protein in major cytological transformations. In addition, this study may provide a model for researching the molecular mechanisms involved in spermatogenesis in other teleost species, which will lead to a better understanding of the teleost fertilization process.

Dynamic transcription and expression patterns of KIF3A and KIF3B genes during spermiogenesis in the shrimp, Palaemon carincauda

Spermiogenesis is a highly ordered and complex process in the male germ cell differentiation. The microtubule-based motor proteins KIF3A and KIF3B are required for the progression of the stages of spermiogenesis. In this study, the main goal was to determine whether KIF3A and KIF3B have a key role in spermiogenesis in Palaemon carincauda. The complete cDNA of KIF3A/3B from the testis of P. carincauda was cloned by using PCR and rapid amplification of cDNA ends (RACE). The predicted secondary and tertiary structures of KIF3A/3B contained three domains which were the: a) head region, b) stalk region, and c) tail region. Real-time quantitative PCR (qPCR) results revealed that KIF3A and KIF3B mRNAs were obtained for all the tissues examined, with the greatest gene expression in the testis. In situ hybridization indicated the KIF3A and KIF3B mRNAs were distributed in the periphery of the nuclear in the early spermatid of spermiogenesis. In the middle and late spermatid stages, KIF3A and KIF3B mRNAs were gradually upregulated and assembled to one side where acrosome biogenesis begins. In the mature sperm, KIF3A and KIF3B mRNAs were distributed in the acrosome cap and spike. Immunofluorescence studies indicated that KIF3A, tubulin, mitochondria, and Golgi were co-localized in different stages during spermiogenesis in P. carincauda. The temporal and spatial gene expression dynamics of KIF3A/3B indicate that KIF3A and KIF3B proteins may be involved in acrosome formation and nucleus shaping. Moreover, these proteins can transport the mitochondria and Golgi that facilitate acrosome formation in P. carincauda.

SPEF2 functions in microtubule-mediated transport in elongating spermatids to ensure proper male germ cell differentiation

Sperm differentiation requires specific protein transport for correct sperm tail formation and head shaping. A transient microtubular structure, the manchette, appears around the differentiating spermatid head and serves as a platform for protein transport to the growing tail. Sperm flagellar 2 (SPEF2) is known to be essential for sperm tail development. In this study we investigated the function of SPEF2 during spermatogenesis using a male germ cell-specific Spef2 knockout mouse model. In addition to defects in sperm tail development, we observed a duplication of the basal body and failure in manchette migration resulting in an abnormal head shape. We identified cytoplasmic dynein 1 and GOLGA3 as novel interaction partners for SPEF2. SPEF2 and dynein 1 colocalize in the manchette and the inhibition of dynein 1 disrupts the localization of SPEF2 to the manchette. Furthermore, the transport of a known SPEF2-binding protein, IFT20, from the Golgi complex to the manchette was delayed in the absence of SPEF2. These data indicate a possible novel role of SPEF2 as a linker protein for dynein 1-mediated cargo transport along microtubules.

KIFC1: a promising chemotherapy target for cancer treatment?

The kinesin motor KIFC1 has been suggested as a potential chemotherapy target due to its critical role in clustering of the multiple centrosomes found in cancer cells. In this regard, KIFC1 seems to be non-essential in normal somatic cells which usually possess only two centrosomes. Moreover, KIFC1 is also found to initiatively drive tumor malignancy and metastasis by stabilizing a certain degree of genetic instability, delaying cell cycle and protecting cancer cell surviving signals. However, that KIFC1 also plays roles in other specific cell types complicates the question of whether it is a promising chemotherapy target for cancer treatment. For example, KIFC1 is found functionally significant in vesicular and organelle trafficking, spermiogenesis, oocyte development, embryo gestation and double-strand DNA transportation. In this review we summarize a recent collection of information so as to provide a generalized picture of ideas and mechanisms against and in favor of KIFC1 as a chemotherapy target. And we also drew the conclusion that KIFC1 is a promising chemotherapy target for some types of cancers, because the side-effects of inhibiting KIFC1 mentioned in this review are theoretically easy to avoid, while KIFC1 is functionally indispensable during mitosis and malignancy of multi-centrosome cancer cells. Further investigations of how KIFC1 is regulated throughout the mitosis in cancer cells are needed for the understanding of the pathways where KIFC1 is involved and for further exploitation of indirect KIFC1 inhibitors.

Formation and function of the manchette and flagellum during spermatogenesis

The last phase of spermatogenesis involves spermatid elongation (spermiogenesis), where the nucleus is remodeled by chromatin condensation, the excess cytoplasm is removed and the acrosome and sperm tail are formed. Protein transport during spermatid elongation is required for correct formation of the sperm tail and acrosome and shaping of the head. Two microtubular-based protein delivery platforms transport proteins to the developing head and tail: the manchette and the sperm tail axoneme. The manchette is a transient skirt-like structure surrounding the elongating spermatid head and is only present during spermatid elongation. In this review, we consider current understanding of the assembly, disassembly and function of the manchette and the roles of these processes in spermatid head shaping and sperm tail formation. Recent studies have shown that at least some of the structural proteins of the sperm tail are transported through the intra-manchette transport to the basal body at the base of the developing sperm tail and through the intra-flagellar transport to the construction site in the flagellum. This review focuses on the microtubule-based mechanisms involved and the consequences of their disruption in spermatid elongation.

A MEIG1/PACRG complex in the manchette is essential for building the sperm flagella

A key event in the process of spermiogenesis is the formation of the flagella, which enables sperm to reach eggs for fertilization. Yeast two-hybrid studies revealed that meiosis-expressed gene 1 (MEIG1) and Parkin co-regulated gene (PACRG) interact, and that sperm-associated antigen 16, which encodes an axoneme central apparatus protein, is also a binding partner of MEIG1. In spermatocytes of wild-type mice, MEIG1 is expressed in the whole germ cell bodies, but the protein migrates to the manchette, a unique structure at the base of elongating spermatid that directs formation of the flagella. In the elongating spermatids of wild-type mice, PACRG colocalizes with α-tubulin, a marker for the manchette, whereas this localization was not changed in the few remaining elongating spermatids of Meig1-deficient mice. In addition, MEIG1 no longer localizes to the manchette in the remaining elongating spermatids of Pacrg-deficient mice, indicating that PACRG recruits MEIG1 to the manchette. PACRG is not stable in mammalian cells, but can be stabilized by MEIG1 or by inhibition of proteasome function. SPAG16L is present in the spermatocyte cytoplasm of wild-type mice, and in the manchette of elongating spermatids, but in the Meig1 or Pacrg-deficient mice, SPAG16L no longer localizes to the manchette. By contrast, MEIG1 and PACRG are still present in the manchette of Spag16L-deficient mice, indicating that SPAG16L is a downstream partner of these two proteins. Together, our studies demonstrate that MEIG1/PACRG forms a complex in the manchette and that this complex is necessary to transport cargos, such as SPAG16L, to build the sperm flagella.

Summary: In the manchette, a structure at the base of the elongating spermatid, the proteins MEIG1 and PACRG act in a complex to control cargo transport and direct formation of the flagellum.

Gene expression pattern of KIFC3 during spermatogenesis of the skink Eumeces chinensis

Kinesin superfamily is a class of microtubule-dependent motors that play crucial roles in acrosome biogenesis, nuclear reshaping and flagellum formation during spermiogenesis. We have cloned kinesin-like gene kifc3 (termed ec-kifc3) from the total RNA of the testis of the skink Eumeces chinensis. The cDNA sequence of ec-kifc3 had a full-length of 3033bp, including a 260bp 5'-untranslated region (5'UTR), a 445bp 3'-untranslated region (3'UTR) and an open reading frame that encoded a 775-amino-acid protein. Additionally, the calculated molecular weight of the putative ec-KIFC3 was 87kDa and its estimated isoelectric point was 6.18. Structurally, the putative ec-KIFC3 had three domains: head domain, neck domain and tail domain. Protein alignment demonstrated that ec-KIFC3 had 47.2%, 67.8%, 68.8%, 69.3% and 76.8% identity with its homologues in Xenopus laevis, Mus musculus, Cricetulus griseus, Homo sapiens, and Gallus gallus. The phylogenetic analysis showed that ec-KIFC3 was more related to KIFC3 in vertebrates than invertebrates. Tissue expression results showed the presence of ec-KIFC3 in various tissues with its highest expression in the testis. In situ hybridization demonstrated that ec-KIFC3 mRNA was distributed around the nucleus in early and middle stage spermatids and expressed in the nucleus in the elongating spermatids during spermiogenesis. Besides, the ec-KIFC3 mRNA was expressed in the acrosome of the developmental spermatids. From the results of in situ hybridization and previous researches, we speculated that ec-KIFC3 may play a role in nuclear morphogenesis and acrosome formation during spermiogenesis of E. chinensis.

Acroframosome-dependent KIFC1 facilitates acrosome formation during spermatogenesis in the caridean shrimp Exopalaemon modestus

Background

Acrosome formation and nuclear shaping are the main events in spermatogenesis. During spermiogenesis in Exopalaemon modestus, a unique microtubular structure called the acroframosome (AFS) forms in spermatids. The AFS links to a temporary organelle called the lamellar complex (LCx) leading to the formation of an everted umbrella-shaped acrosome and a dish-shaped nucleus in the mature sperm. These morphological changes require complex cell motility in which the C-terminal kinesin motor protein called KIFC1 is involved. In this study, we demonstrate that KIFC1 moves along the AFS and plays an important role in acrosome formation and nuclear shaping during spermatogenesis in E. modestus.

Methodology/Principal Findings

We cloned a 3125 bp complete cDNA of kifc1 from the testis of E. modestus by PCR. The predicted secondary and tertiary structures of E. modestus KIFC1 contain three domains: a) the C-terminus, b) the stalk region, and the c) N-terminusl. Semi-quantitative RT-PCR detected the expression of kifc1 mRNA in different tissues of E. modestus. In situ hybridization demonstrated the temporal and spatial expression profile of kifc1 during spermiogenesis. Western blot identified the expression of KIFC1 in different tissues of E. modestus, including the testis. Immunofluorescence localized KIFC1, tubulin, GM130, and mitochondria in order to elucidate their role during spermiogenesis in E. modestus.

Conclusion/Significance

Our results indicate that KIFC1 transports the Golgi complex, mitochondria, and other cellular components that results in acrosome formation and nuclear shaping in E. modestus. The KIFC1 transport function depends upon the microtubular structure called the acroframosome (AFS). This study describes some of the molecular mechanisms involved in the acrosome formation and nuclear shaping in E. modestus. In addition, this study may provide a model for studying the molecular mechanisms involved in spermatogenesis in other crustacean species and lead to a better understanding of the fertilization process in crustaceans.

Characterization and expression pattern of KIFC1-like kinesin gene in the testis of the Macrobrachium nipponense with discussion of its relationship with structure lamellar complex (LCx) and acroframosome (AFS)

Spermiogenesis is a developmental process undergoing continuous differentiation to drive a diploid spermatogonium towards a haploid sperm cell. This striking transformation from spermatogonium to spermatozoa is made possible by the stage-specific adaption of cytoskeleton and associated molecular motor proteins. KIFC1 is a C-terminal kinesin motor found to boast essential roles in acrosome biogenesis and nuclear reshaping during spermiogenesis in rat. To explore its functions during the same process in Macrobrachium nipponense, we have cloned and sequenced the cDNA of a mammalian KIFC1 homologue (termed mn-KIFC1) from the total RNA of the testis. The 2,296 bp mn-KIFC1 cDNA contained a 87 bp 5' untranslated region, a 211 bp 3' untranslated region and a 1,998 bp open reading frame. Protein alignment demonstrated that mn-KIFC1 had 37.7, 58.7, 38.4, 37.2, 38.9 and 37.8% identity with its homologues in Salmo salar, Eriocheir sinensis, Homo sapiens, Mus musculus, Danio rerio and Xenopus laevis respectively. The phylogenetic tree revealed that mn-KIFC1 is most related to E. Sinensis KIFC1 among the examined species. Tissue expression analysis showed the presence of mn-KIFC1 in the testis, hepatopancreas, gill, muscle and heart. In situ hybridization showed that the mn-KIFC1 mRNA was localized at the periphery of the nuclear membrane and in the proacrosomal vesicle in early and middle spermatids. In late spermatids and spermatozoa, mn-KIFC1 was expressed in the acrosome and in the spike. In situ hybridization also indicated that KIFC1 works together with lamellar complex (LCx) and acroframosome (AFS) to drive acrosome formation and cellular transformation. LCx and AFS have both been previously proved to have essential roles during spermiogenesis in M. nipponense. In conclusion, the expression of mn-kifc1 at specific stages of spermiogenesis suggests a role in cellular transformations in M. nipponense.

Molecular characterization of a KIF3B-like kinesin gene in the testis of Octopus tankahkeei (Cephalopoda, Octopus)

KIF3B is known for maintaining and assembling cilia and flagellum. To date, the function of KIF3B and its relationship with KIF3A during spermiogenesis in the cephalopod Octopus tankahkeei remains unknown. In the present study, we characterized a gene encoding a homologue of rat KIF3B in the O. tankahkeei testis and examined its temporal and spatial expression pattern during spermiogenesis. The cDNA of KIF3B was obtained with degenerate and RACE PCR and the distribution pattern of ot-kif3b were observed with RT-PCR. The morphological development during spermiogenesis was illustrated by histological and transmission electron microscopy and mRNA expression of ot-kif3b was observed by in situ hybridization. The 2,365 nucleotides cDNA consisted of a 102 bp 5' untranslated region (UTR), a 2,208 bp open reading frame (ORF) encoding a protein of 736 amino acids, and a 55 bp 3' UTR. Multiple alignments revealed that the putative Ot-KIF3B shared 68, 68, 69, 68, and 67% identity with that of Homo sapiens, Mus musculus, Gallus gallus, Danio rerio, and Xenopus laevis, respectively, along with high identities with Ot-KIF3A in fundamental structures. Ot-kif3b transcripts appeared gradually in early spermatids, increased in intermediate spermatids and maximized in drastically remodeled and final spermatids. The kif3b gene is identified and its expression pattern is demonstrated for the first time in O. tankahkeei. Compared to ot-kif3a reported by our laboratory before, our data suggested that the putative heterodimeric motor proteins Ot-KIF3A/B may be involved in intraspermatic transport and might contribute to structural changes during spermiogenesis.

KIFC1-like motor protein associates with the cephalopod manchette and participates in sperm nuclear morphogenesis in Octopus tankahkeei

Background

Nuclear morphogenesis is one of the most fundamental cellular transformations taking place during spermatogenesis. In rodents, a microtubule-based perinuclear structure, the manchette, and a C-terminal kinesin motor KIFC1 are believed to play crucial roles in this process. Spermatogenesis in Octopus tankahkeei is a good model system to explore whether evolution has created a cephalopod prototype of mammalian manchette-based and KIFC1-dependent sperm nuclear shaping machinery.

Methodology/Principal Findings

We detected the presence of a KIFC1-like protein in the testis, muscle, and liver of O. tankahkeei by Western Blot. Then we tracked its dynamic localization in spermatic cells at various stages using Immunofluorescence and Immunogold Electron Microscopy. The KIFC1-like protein was not expressed at early stages of spermatogenesis when no significant morphological changes occur, began to be present in early spermatid, localized around and in the nucleus of intermediate and late spermatids where the nucleus was dramatically elongated and compressed, and concentrated at one end of final spermatid. Furthermore, distribution of the motor protein during nuclear elongation and condensation overlapped with that of the cephalopod counterpart of manchette at a significant level.

Conclusions/Significance

The results support the assumption that the protein is actively involved in sperm nuclear morphogenesis in O. tankahkeei possibly through bridging the manchette-like perinuclear microtubules to the nucleus and assisting in the nucleocytoplasmic trafficking of specific cargoes. This study represents the first description of the role of a motor protein in sperm nuclear shaping in cephalopod.

The presence of kinesin superfamily motor proteins KIFC1 and KIF17 in normal and pathological human placenta

Kinesin superfamily proteins (KIFs) are motor proteins that participate in chromosomal and spindle movements during mitosis and meiosis, and transport membranous organelles and macromolecules fundamental for cellular functions. Although the roles of KIFs in axonal and dendritic transports have been studied extensively, their role in intracellular transport in general is less well known. The diversity of kinesins suggests that each kinesin may have a specific function. Therefore, in this study we aimed to investigate the presence and cellular localization of KIFC1 and KIF17 in normal and pathological human placentas. First-trimester (22-56 days) and normal, preeclamptic (PE), and diabetic-term placental tissues were obtained and further studied by immunohistochemistry (IHC) and Western blot methods. KIFC1 was mainly localized to the syncytiotrophoblast both in early and term placental samples. However, a stronger immunoreactivity was observed both in PE and diabetic placentas compared to normal-term placentas. KIF17 was most intensively localized in developing vascular endothelium in early pregnancy. Even though KIF17 was moderately stained in the endothelium of villi from normal human-term placentas, stronger immunoreactivity was observed in all types of villi of both PE and diabetic placentas. Western blotting of tissue extracts confirmed the IHC results. Here, we demonstrate the presence of KIFC1 and KIF17 in human placenta for the first time. The intense expression of KIFC1 in syncytiotrophoblast and KIF17 in vascular endothelium suggests that both the proteins might be important in a cargo-transport system. An increased expression pattern of both KIFC1 and KIF17 in PE and diabetes might suggest that these proteins may be involved in complex trophoblast functions and placental pathologies. Further studies will clarify the physiological role of KIFs in human placental transport and development.

KIFC1 participates in acrosomal biogenesis, with discussion of its importance for the perforatorium in the Chinese mitten crab Eriocheir sinensis

Spermatogenesis is a complicated process during which spermatogonia undergo proliferation and divisions leading, after a series of dramatic changes, to the production of mature spermatozoa. Many molecular motors are involved in this process. KIFC1, a C-terminal kinesin motor, participates in acrosome biogenesis and nuclear shaping. We report here the expression profile of KIFC1 during spermatogenesis in the Chinese mitten crab, Eriocheir sinensis. KIFC1 mainly localizes around the nucleus but is also present within the nucleus of the spermatogonium and spermatocyte. At the early spermatid stage, KIFC1 begins to be distributed on the nuclear membrane at the region where the proacrosomal vesicle is located. By the late spermatid stage, KIFC1 is found on the acrosome. Immunocytochemical and ultrastructural analyses have shown that KIFC1 localizes on the perforatorium, which is composed of an apical cap and an acrosomal tubule. We demonstrate that, during spermatogenesis in E. sinensis, KIFC1 probably plays important roles in the biogenesis of the acrosome and in its maintenance. KIFC1 may also be essential for the eversion of the acrosome during fertilization.

Relocation of myosin and actin, kinesin and tubulin in the acrosome reaction of bovine spermatozoa

The mammalian acrosome reaction is a specialised exocytotic event. Although molecular motors are known to be involved in exocytosis in many cell types, their potential involvement in the acrosome reaction has remained unknown. Here, it has been shown that actin is localised within the equatorial segment and in the marginal acrosomal ridge of the heads of unreacted bull spermatozoa. Myosins IIA and IIB are found within the anterior acrosomal margins of virtually all sperm cells and, less prominently, within the equatorial segment. Tubulin was detected in the equatorial segment and around the periphery of the acrosome while kinesin was prominent in the equatorial segment. After induction of the acrosome reaction by means of the calcium ionophore A23187, the number of cells exhibiting actin fluorescence intensity in the anterior acrosomal margin decreased four-fold and those displaying equatorial segment fluorescence decreased 3.5-fold; myosin IIA immunofluorescence decreased in intensity with most spermatozoa losing equatorial staining, whereas there was little change in the distribution or intensity of myosin IIB immunofluorescence, except for approximately 20% decrease in the number of cells exhibiting acrosomal staining. Tubulin became largely undetectable within the head and kinesin staining spread rostrally over the main acrosome region. A possible sequence of events that ties in these observations of molecular motor involvement with the known participation of SNARE proteins is provided.

Inhibition of rat testicular nuclear kinesins (krp2; KIFC5A) by acrylamide as a basis for establishing a genotoxicity threshold

Acrylamide is a toxic substance that induces a variety of cellular responses including neurotoxicity, male reproductive toxicity, tumorigenicity, clastogenicity, and DNA alkylation. Evidence is provided that inhibition of the microtubule motor protein kinesin is responsible for acrylamide-induced clastogenicity and aneuploidy. Two kinesin motors, KIFC5A and KRP2, which are responsible for spindle assembly and disassembly of kinetochore MT, respectively, are inhibited by acrylamide. The inhibitory concentration for a response is below the levels shown to adversely affect the cytogenetic parameters. The relative contribution of these inhibitions compared to DNA alkylation is considered. The implications of inhibition of these kinesins as the site of action of acrylamide with regard to risk assessment are substantial as this event will have a threshold and a safe level of acrylamide can be determined.

Identification of a novel Leucine-rich repeat protein and candidate PP1 regulatory subunit expressed in developing spermatids

Background

Spermatogenesis is comprised of a series of highly regulated developmental changes that transform the precursor germ cell into a highly specialized spermatozoon. The last phase of spermatogenesis, termed spermiogenesis, involves dramatic morphological change including formation of the acrosome, elongation and condensation of the nucleus, formation of the flagella, and disposal of unnecessary cytoplasm. A prominent cytoskeletal component of the developing spermatid is the manchette, a unique microtubular structure that surrounds the nucleus of the developing spermatid and is thought to assist in both the reshaping of the nucleus and redistribution of spermatid cytoplasm. Although the molecular motor KIFC1 has been shown to associate with the manchette, its precise role in function of the manchette and the identity of its testis specific protein partners are unknown. The purpose of this study was to identify proteins in the testis that interact with KIFC1 using a yeast 2 hybrid screen of a testis cDNA library.

Results

Thirty percent of the interacting clones identified in our screen contain an identical cDNA encoding a 40 kD protein. This interacting protein has 4 leucine-rich repeats in its amino terminal half and is expressed primarily in the testis; therefore we have named this protein testis leucine-rich repeat protein or TLRR. TLRR was also found to associate tightly with the KIFC1 targeting domain using affinity chromatography. In addition to the leucine-rich repeats, TLRR contains a consensus-binding site for protein phosphatase-1 (PP1). Immunocytochemistry using a TLRR specific antibody demonstrates that this protein is found near the manchette of developing spermatids.

Conclusion

We have identified a previously uncharacterized leucine-rich repeat protein that is expressed abundantly in the testis and associates with the manchette of developing spermatids, possibly through its interaction with the KIFC1 molecular motor. TLRR is homologous to a class of regulatory subunits for PP1, a central phosphatase in the reversible phosphorylation of proteins that is key to modulation of many intracellular processes. TLRR may serve to target this important signaling molecule near the nucleus of developing spermatids in order to control the cellular rearrangements of spermiogenesis.

A kinesin is present at unique sertoli/spermatid adherens junctions in rat and mouse testes

During spermatogenesis, spermatids undergo a "down and up" translocation event in the seminiferous epithelium. This event has been proposed to result from the movement of ectoplasmic specializations, which are formed in Sertoli cells at sites of adhesion to spermatids, along adjacent microtubule tracts. To test the hypothesis that a kinesin is associated with ectoplasmic specializations, we generated antibodies to conserved kinesin sequences and detected kinesins on fixed frozen testis sections and fixed seminiferous epithelial fragments. The antibodies reacted with ectoplasmic specializations related to spermatids, in addition to reacting with other structures in the epithelium known to contain kinesins. At the electron microscopy level, the antibodies reacted with the cytoplasmic face of the endoplasmic reticulum component of ectoplasmic specializations. Based on mRNA transcript screens using mouse GeneChip arrays of testis and Sertoli cells, we identified KIF20 as a candidate kinesin at ectoplasmic specializations. Antibodies generated against a peptide sequence unique to this kinesin reacted at ectoplasmic specializations in testis sections and epithelial fragments, as well as with the endoplasmic reticulum component of ectoplasmic specializations when analyzed by electron microscopy. The antibody reacted on Western blots with full-length KIF20. On Western blots of testis lysates, the antibody reacted with a protein that is not present in other tissues and which migrates at a higher molecular weight than that predicted for KIF20. Our results demonstrate that a kinesin is associated with apical ectoplasmic specializations in Sertoli cells and that the motor may be an isoform of KIF20.

Acrylamide effects on kinesin-related proteins of the mitotic/meiotic spindle

The microtubule (MT) motor protein kinesin is a vital component of cells and organs expressing acrylamide (ACR) toxicity. As a mechanism of its potential carcinogenicity, we determined whether kinesins involved in cell division are inhibited by ACR similar to neuronal kinesin [Sickles, D.W., Brady, S.T., Testino, A.R., Friedman, M.A., and Wrenn, R.A. (1996). Direct effect of the neurotoxicant acrylamide on kinesin-based microtubule motility. Journal of Neuroscience Research 46, 7-17.] Kinesin-related genes were isolated from rat testes [Navolanic, P.M., and Sperry, A.O. (2000). Identification of isoforms of a mitotic motor in mammalian spermatogenesis. Biology of Reproduction 62, 1360-1369.], their kinesin-like proteins expressed in bacteria using recombinant DNA techniques and the effects of ACR, glycidamide (GLY) and propionamide (a non-neurotoxic metabolite) on the function of two of the identified kinesin motors were tested. KIFC5A MT bundling activity, required for mitotic spindle formation, was measured in an MT-binding assay. Both ACR and GLY caused a similar concentration-dependent reduction in the binding of MT; concentrations of 100 microM ACR or GLY reduced its activity by 60%. KRP2 MT disassembling activity was assayed using the quantity of tubulin disassembled from taxol-stabilized MT. Both ACR and GLY inhibited KRP2-induced MT disassembly. GLY was substantially more potent; significant reductions of 60% were achieved by 500 microM, a comparable inhibition by ACR required a 5 mM concentration. Propionamide had no significant effect on either kinesin, except KRP2 at 10 mM. This is the first report of ACR inhibition of a mitotic/meiotic motor protein. ACR (or GLY) inhibition of kinesin may be an alternative mechanism to DNA adduction in the production of cell division defects and potential carcinogenicity. We conclude that ACR may act on multiple kinesin family members and produce toxicities in organs highly dependent on microtubule-based functions.

Differential expression of molecular motors in the motor cortex of sporadic ALS

The molecular mechanisms underlying the selective neurodegeneration of motor neurons in amyotrophic lateral sclerosis (ALS) are inadequately understood. Recent breakthroughs have implicated impaired axonal transport, mediated by molecular motors, as a key element for disease onset and progression. The current work identifies the expression of 15 kinesin-like motors in healthy human motor cortex, including three novel isoforms. Our comprehensive quantitative mRNA analysis in control and sporadic ALS (SALS) motor cortex specimens detects SALS-specific down-regulation of KIF1Bbeta and novel KIF3Abeta, two isoforms we show to be enriched in the brain, and also of SOD1, a key enzyme linked to familial ALS. This is accompanied by a marked reduction of KIF3Abeta protein levels. In the motor cortex KIF3Abeta localizes in cholinergic neurons, including upper motor neurons. No mutations causing splicing defects or altering protein-coding sequences were identified in the genes of the three proteins. The present study implicates two motor proteins as possible candidates in SALS pathology.

Kif5B and Kifc1 interact and are required for motility and fission of early endocytic vesicles in mouse liver

Early endocytic vesicles loaded with Texas Red asialoorosomucoid were prepared from mouse liver. These vesicles bound to microtubules in vitro, and upon ATP addition, they moved bidirectionally, frequently undergoing fission into two daughter vesicles. There was no effect of vanadate (inhibitor of dynein) on motility, whereas 5'-adenylylimido-diphosphate (kinesin inhibitor) was highly inhibitory. Studies with specific antibodies confirmed that dynein was not associated with these vesicles and that Kif5B and the minus-end kinesin Kifc1 mediated their plus- and minus-end motility, respectively. More than 90% of vesicles associated with Kifc1 also contained Kif5B, and inhibition of Kifc1 with antibody resulted in enhancement of plus-end-directed motility. There was reduced vesicle fission when either Kifc1 or Kif5B activity was inhibited by antibody, indicating that the opposing forces resulting from activity of both motors are required for fission to occur. Immunoprecipitation of native Kif5B by FLAG antibody after expression of FLAG-Kifc1 in 293T cells indicates that these two motors can interact with each other. Whether they interact directly or through a complex of potential regulatory proteins will need to be clarified in future studies. However, the present study shows that coordinated activity of these kinesins is essential for motility and processing of early endocytic vesicles.

Ultrastructural analysis of kinesin-related motor proteins during spermatogenesis

Kinesins are a superfamily of microtubule-based motors that power intracellular traffic and play important roles in many fundamental cellular and developmental processes. Kinesins move on microtubules from their minus to plus end (conventional kinesin) or from plus to minus end (C-terminal kinesins), carrying cargoes to different destinations. A variety of cargoes such as vesicles, proteins, lipid drops, pigments, and the nucleus are moved by kinesins along cytoplasmic microtubules. Multiple mitotic kinesins and microtubule-associated proteins (MAPs) also have direct functions in spindle formation, chromosome segregation, and cytokinesis. Spermatogenesis provides an excellent model system to study the role of kinesin motor proteins during the dramatic cytoskeletal rearrangements that take place during male germ cell development. This chapter describes how to identify the multiple functions of kinesin motors during spermatogenesis by using ultrastructural analysis. Testis perfusion is described in detail, including how to anesthetize animals and how to select seminiferous tubules under transilluminated microscopy. Practical immunocytochemical staining is also described in detail in this chapter, especially methods to enhance staining and avoid contamination.

Regulation of gene expression in post-meiotic male germ cells: CREM-signalling pathways and male fertility

Spermatogenesis is a remarkably complex process in which diploid spermatogonial stem cells undergo a series of mitotic and meiotic cell divisions to give rise to haploid round spermatids. These haploid cells then go through a dramatic morphological remodelling involving extensive chromatin condensation, reduction in nuclear and cytoplasmic volume, formation of an acrosome system and tail, all of which contribute to the formation of a mature spermatozoon fully capable of fertilizing the oocyte and passing along its genetic information to the next generation. To accomplish such a complex program, an intricate and efficient mechanism is required to finely tune the levels of expression of specific genes necessary for this process. Accordingly, the regulation of gene expression in post-meiotic male germ cells is governed by specific mechanisms unique to these cells. The cyclic adenosine monophosphate (cAMP) response element modulator (CREM) is an essential component of this program, and its activity is regulated through interactions with a germ cell-specific, CREM phosphorylation-independent transcriptional co-activator, activator of CREM in testis (ACT). In turn, the ability of ACT to regulate CREM activity is controlled by a germ cell-specific kinesin, Kif17b, which regulates the subcellular distribution of ACT. Further, the mRNA from CREM target genes interacts with several germ cell-specific RNA-binding proteins, which function to transport and stabilize these mRNAs. This sophisticated and complex regulation of gene expression in post-meiotic germ cells is governed by unique mechanisms specific to these cells and is fundamental to male fertility.

Motor protein KIFC5A interacts with Nubp1 and Nubp2, and is implicated in the regulation of centrosome duplication

Inhibition of motor protein activity has been linked with defects in the formation of poles in the spindle of dividing cells. However, the molecular mechanisms underlying the functional relationship between motor activity and centrosome dynamics have remained uncharacterised. Here, we characterise KIFC5A, a mouse kinesin-like protein that is highly expressed in dividing cells and tissues, and is subject to developmental and cell-type-specific regulation. KIFC5A is a minus-end-directed, microtubule-dependent motor that produces velocities of up to 1.26 μm minute-1 in gliding assays and possesses microtubule bundling activity. It is nuclear in interphase, localises to the centre of the two microtubule asters at the beginning of mitosis, and to spindle microtubules in later mitotic phases. Overexpression of KIFC5A in mouse cells causes the formation of aberrant, non-separated microtubule asters and mitotic arrest in a prometaphase-like state. KIFC5A knockdown partly rescues the phenotype caused by inhibition of plus-end-directed motor Eg5 by monastrol on the mitotic spindle, indicating that it is involved in the balance of forces determining bipolar spindle assembly and integrity. Silencing of KIFC5A also results in centrosome amplification detectable throughout the cell cycle. Supernumerary centrosomes arise primarily as a result of reduplication and partly as a result of cytokinesis defects. They contain duplicated centrioles and have the ability to organise microtubule asters, resulting in the formation of multipolar spindles. We show that KIFC5A interacts with nucleotide-binding proteins 1 and 2 (Nubp1 and Nubp2), which have extensive sequence similarity to prokaryotic division-site-determining protein MinD. Nubp1 and Nubp2 also interact with each other. Knockdown of Nubp1 or double knockdown of Nubp1 and Nubp2 (Nubp1&Nubp2) both phenocopy the KIFC5A silencing effect. These results implicate KIFC5A and the Nubp proteins in a common regulatory pathway involved in the control of centrosome duplication in mammalian cells.

The molecular motor KIFC1 associates with a complex containing nucleoporin NUP62 that is regulated during development and by the small GTPase RAN

KIFC1 is a C-terminal kinesin motor associated with the nuclear membrane and acrosome in round and elongating spermatids. This location in developing spermatids is consistent with possible roles in acrosome elongation and manchette motility or both. Here we describe the association of the KIFC1 motor with a complex containing the nucleoporin NUP62. Formation of this complex is developmentally regulated, being absent before puberty and appearing only after nuclear elongation has begun. In addition, the integrity of this complex is dependent on GTP hydrolysis and the GTP state of the small GTPase RAN. Concomitant with the association of this motor with the NUP62-containing complex is an apparent reorganization of the nuclear pore with loss of NUP62 from larger complexes containing other nucleoporins. The association of KIFC1 with a component of the nuclear membrane is more consistent with a role for this motor in acrosome/manchette transport along the nuclear membrane than for a role for this motor in transport of vesicles along the outer face of the manchette.

KIF2Abeta: A kinesin family member enriched in mouse male germ cells, interacts with translin associated factor-X (TRAX)

Translin associated factor X (TRAX) is a binding partner of TB-RBP/Translin. A cDNA encoding the 260 C-terminal amino acids of KIF2Abeta was isolated from mouse testis cDNAs in a yeast two-hybrid library screen for specific TRAX-interacting proteins. KIF2Abeta was expressed predominantly in the mouse testis and enriched in germ cells. The interaction of full-length KIF2Abeta or its C-terminus with TRAX was verified using in vitro synthesized fusion proteins. Deletion mapping of the TRAX-binding region of KIF2Abeta indicated that amino acids 514-659 were necessary and sufficient for the interaction in vivo. Confocal microscopy studies using GFP-fusion proteins demonstrated that KIF2Abeta colocalizes with TRAX in a perinuclear location. KIF2Abeta does not interact with TB-RBP, suggesting that either TRAX can function as an adaptor molecule for motor proteins and TB-RBP, or that this interaction reveals an undescribed role for TRAX in germ cells. The interaction with KIF2Abeta suggests a role for TRAX in microtubule-based functions during spermatogenesis.

Comparative analysis of two C-terminal kinesin motor proteins: KIFC1 and KIFC5A

We have taken advantage of the close structural relationship between two C-terminal motors, KIFC5A and KIFC1, to examine the sequence requirements for targeting of these two motors within the cell. Although KIFC5A and KIFC1 are almost identical in their motor and stalk domains, they differ in well-defined regions of their tail domains. Specific antisera to these motors were used to determine their localization to distinct subcellular compartments, the spindle for KIFC5A or membranous organelles for KIFC1. In addition to defining the intracellular localization of KIFC1, the reactivity of the KIFC1 antibody demonstrates that this motor contains a frame shift with respect to KIFC5A and is likely the product of a separate gene. The divergent tail domains of these motors are predicted to harbor specific information that directs them to their correct intracellular targets. In order to define the sequences responsible for the differential localization of these two motors, GFP was fused to motors with various tail deletions and their localization visualized after transfection. We were able to identify distinct sequences in each motor responsible for its unique cellular localization. The KIFC5A tail contains a 43 amino acid sequence with both nuclear localization and microtubule binding activity while KIFC1 contains a 19 amino acid sequence sufficient to target this motor to membrane-bounded organelles.

Rat kinesin light chain 3 associates with spermatid mitochondria

We recently discovered that in rat spermatids, kinesin light chain KLC3 can associate with outer dense fibers, major sperm tail components, and accumulates in the sperm midpiece. Here, we show that mitochondria isolated from rat-elongating spermatids have bound KLC3. Immunoelectron microscopy indicates that the association of KLC3 with mitochondria coincides with the stage in spermatogenesis when mitochondria move from the plasma membrane to the developing midpiece. KLC3 is able to bind in vitro to mitochondria from spermatids as well as somatic cells employing a conserved kinesin light chain motif, the tetratrico-peptide repeats. Expression of KLC3 in fibroblasts results in formation of large KLC3 clusters close to the nucleus, which also contain mitochondria: no other organelles were present in these clusters. Mitochondria are not present in KLC3 clusters after deletion of KLC3's tetratrico-peptide repeats. Our results indicate that the rat spermatid kinesin light chain KLC3 can associate with mitochondria.

Novel aspect of perinuclear theca assembly revealed by immunolocalization of non-nuclear somatic histones during bovine spermiogenesis

The perinuclear theca (PT) is an important accessory structure of the sperm head, yet its biogenesis is not well defined. To understand the developmental origins of PT-derived somatic histones during spermiogenesis, we used affinity-purified antibodies against somatic-type histones H3, H2B, H2A, and H4 to probe bovine testicular tissue using three different immunolocalization techniques. While undetectable in elongating spermatid nuclei, immunoperoxidase light microscopy showed all four somatic histones remained associated to the caudal head region of spermatids from steps 11 to 14 of the 14 steps in bovine spermiogenesis. Immunogold electron microscopy confirmed the localization of somatic histones on two nonnuclear structures, namely transient manchette microtubules of step-9 to step-11 spermatids and the developing postacrosomal sheath of step-13 and -14 spermatids. Immunofluorescence demonstrated somatic histone immunoreactivity in the developing postacrosomal sheath, and on anti-beta-tubulin decorated manchette microtubules of step-12 spermatids. Focal antinuclear pore complex labeling on the base of round spermatid nuclei was detected by electron microscopy and immunofluorescence, occurring before the nucleoprotein transition period during spermatid elongation. This indicated that, if nuclear histone export precedes their degradation, this process could only occur in this region, thereby questioning the proposed role of the manchette in nucleocytoplasmic trafficking. Somatic histone immunodetection on the manchette during postacrosomal sheath formation supports a role for the manchette in PT assembly, signifying that some PT components have origins in the distal spermatid cytoplasm. Furthermore, these findings suggest that somatic histones are de novo synthesized in late spermiogenesis for PT assembly.

The kinesin KIF17b and RNA-binding protein TB-RBP transport specific cAMP-responsive element modulator-regulated mRNAs in male germ cells

Testis brain RNA-binding protein (TB-RBP), the mouse orthologue of the human protein Translin, is a widely expressed and highly conserved protein with proposed functions in chromosomal translocations, mitotic cell division, and mRNA transport, stabilization, and storage. Targeted inactivation of TB-RBP leads to abnormalities in fertility and behavior. A testis-enriched kinesin KIF17b coimmunoprecipitates with TB-RBP in a RNA-protein complex containing specific cAMP-responsive element modulator (CREM)-regulated mRNAs. The specificity of this interaction is confirmed by in vivo RNA-protein crosslinking and transfections of hippocampal neurons. Combining in situ hybridization and immunohistochemistry at the electron microscope level, a temporally sequential dissociation of KIF17b and TB-RBP from specific mRNAs is detected with TB-RBP release coincident with the time of mRNA translation, indicating a separation of the processes of transport and translation. We conclude that KIF17b serves as a molecular motor component of a TB-RBP-mouse ribonucleoprotein complex transporting a group of specific CREM-regulated mRNAs in mammalian male postmeiotic germ cells. Because KIF17b has been reported to control CREM-dependent transcription in male germ cells by regulating the intracellular location of the transcriptional coactivator activator of CREM in testis, this indicates that one kinesin links the processes of transcription and transport of specific mRNAs in mammalian male germ cells.

C-terminal kinesin motor KIFC1 participates in acrosome biogenesis and vesicle transport

We have identified a possible role for the KIFC1 motor protein in formation of the acrosome, an organelle unique to spermatogenesis. KIFC1, a C-terminal kinesin motor, first appears on membrane-bounded organelles (MBOs) in the medulla of early spermatids followed by localization to the acrosomal vesicle. KIFC1 continues to be present on the acrosome of elongating spermatids as it flattens on the spermatid nucleus; however, increasing amounts of KIFC1 are found at the caudal aspect of the spermatid head and in distal cytoplasm. The KIFC1 motor is also found in the nucleus of very immature round spermatids just prior to its appearance on the acrosome. In some cases, KIFC1 appears localized just below the nuclear membrane adjacent to the subacrosomal membrane. We demonstrate that KIFC1 is associated with importin beta and colocalizes with this nuclear transport factor on curvilinear structures associated with the spermatid nuclei. These data support a model in which KIFC1, perhaps in association with nuclear factors, assists in the formation and/or elongation of the spermatid acrosome. This article represents the first demonstration of a direct association of a molecular motor with the spermatid acrosome, the formation of which is essential for fertilization.

Importin alpha/beta and Ran-GTP regulate XCTK2 microtubule binding through a bipartite nuclear localization signal

The small GTPase Ran is essential for spindle assembly. Ran is proposed to act through its nuclear import receptors importin alpha and/or importin beta to control the sequestration of proteins necessary for spindle assembly. To date, the molecular mechanisms by which the Ran pathway functions remain unclear. Using purified proteins, we have reconstituted Ran-regulated microtubule binding of the C-terminal kinesin XCTK2, a kinesin important for spindle assembly. We show that the tail of XCTK2 binds to microtubules and that this binding is inhibited in the presence of importin alpha and beta (alpha/beta) and restored by addition of Ran-GTP. The bipartite nuclear localization signal (NLS) in the tail of XCTK2 is essential to this process, because mutation of the NLS abolishes importin alpha/beta-mediated regulation of XCTK2 microtubule binding. Our data show that importin alpha/beta directly regulates the activity of XCTK2 and that one of the molecular mechanisms of Ran-regulated spindle assembly is identical to that used in classical NLS-driven nuclear transport.

New insights into the assembly of the periaxonemal structures in mammalian spermatozoa

Disruption of Ube2b in the mouse has revealed that the regular and symmetric organization of the fibrous sheath of the sperm flagella is dependent on expression of the ubiquitin-conjugating enzyme UBE2B. These data could cast light on how a component of the ubiquitin-proteasome pathway participates in the assembly of flagellar periaxonemal structures. Data in the literature support the notion of involvement of ubiquitin-proteasome pathways in the assembly of cytoskeletal components in somatic cells. This review attempts to integrate recent knowledge regarding flagellar components that could be related to proteasome components and, therefore, could be targets of UBE2B in the spermatid. An attempt is made to characterize the human flagellar anomalies of infertile patients, which are the closest to those of Ube2b-deficient mice. These new insights regarding the assembly of mammalian sperm flagella provide a basis for studying the ontogenesis of flagellar accessory structures and suggest leads for medical and genetic investigations.

Ran, a GTP-binding protein involved in nucleocytoplasmic transport and microtubule nucleation, relocates from the manchette to the centrosome region during rat spermiogenesis

Ran, a Ras-related GTPase, is required for transporting proteins in and out of the nucleus during interphase and for regulating the assembly of microtubules. cDNA cloning shows that rat testis, like mouse testis, expresses both somatic and testis-specific forms of Ran-GTPase. The presence of a homologous testis-specific form of Ran-GTPase in rodents implies that the Ran-GTPase pathway plays a significant role during sperm development. This suggestions is supported by distinct Ran-GTPase immunolocalization sites identified in developing spermatids. Confocal microscopy demonstrates that Ran-GTPase localizes in the nucleus of round spermatids and along the microtubules of the manchette in elongating spermatids. When the manchette disassembles, Ran-GTPase immunoreactivity is visualized in the centrosome region of maturing spermatids. The circumstantial observation that fractionated manchettes, containing copurified centrin-immunoreactive centrosomes, can organize a three-dimensional lattice in the presence of taxol and GTP, points to the role of Ran-GTPase and associated factors in microtubule nucleation as well as the potential nucleating function of spermatid centrosomes undergoing a reduction process. Electron microscopy demonstrates the presence in manchette preparations of spermatid centrosomes, recognized as such by their association with remnants of the implantation fossa, a dense plate observed only at the basal surface of developing spermatid and sperm nuclei. In addition, we have found importin beta1 immunoreactivity in the nucleus of elongating spermatids, a finding that, together with the presence of Ran-GTPase in the nucleus of round spermatids and the manchette, suggest a potential role of Ran-GTPase machinery in nucleocytoplasmic transport. Our expression and localization analysis, correlated with functional observations in other cell systems, suggest that Ran-GTPase may be involved in both nucleocytoplasmic transport and microtubules assembly, two critical events during the development of functional sperm. In addition, the manchette-to-centrosome Ran-GTPase relocation, together with the similar redistribution of various proteins associated to the manchette, suggest the existence of an intramanchette molecular transport mechanism, which may share molecular analogies with intraflagellar transport.

Expression and potential function of Rho family small G proteins in cells of the mammalian seminiferous epithelium

Dynamic cellular rearrangements involving the actin cytoskeleton are required of both Sertoli and germ cells during spermatogenesis. Rho family small G proteins have been implicated in the control of the actin cytoskeleton in numerous cell types. Therefore, RhoA and Rac1 were investigated in Sertoli and germ cells. RhoA and Rac1 have been detected at both the mRNA and protein levels in these cells. In addition, Sertoli cell L-selectin is shown to interact with actin binding proteins, potentially providing a link between L-selectin and Rac1 signaling. Finally, inactivation of Sertoli cell Rho family proteins yields disruption of the actin cytoskeleton.

KRP3A and KRP3B: candidate motors in spermatid maturation in the seminiferous epithelium

We have identified KRP3, a novel kinesin-related protein expressed in the mammalian testis, and have examined the tissue distribution and subcellular localization of isoforms of this protein. Isolation of KRP3 clones, using the head domain identified in a previous PCR screen as probe, identified at least two KRP3 isoforms in the rat. We have isolated coding sequences of two highly related cDNAs from the rat testis that we have termed KRP3A and KRP3B (kinesin-related protein 3, A and B). Both cDNAs code for predicted polypeptides with the three-domain structure typical of kinesin superfamily members; namely a conserved motor domain, a region capable of forming a limited coiled-coil secondary structure, and a globular tail domain. Although almost identical in their head and stalk domains, these motors diverge in their tail domains. This group of motors is found in many tissues and cell types. The KRP3B motor contains DNA-binding motifs and an RCC1 (regulator of chromosome condensation 1) consensus sequence in its tail domain. Despite this similarity, KRP3B is not associated with the same structures as RCC1. Instead, KRP3 isoforms localize with the nuclei of developing spermatids, and their immunolocalization in the testis overlaps with that of the small GTPase Ran. Like Ran, KRP3 motors are associated in a polarized fashion with the nucleus of maturing spermatids at various stages of elongation. Our findings suggest a possible role for KRP3 motor isoforms in spermatid maturation mediated by possible interaction with the Ran GTPase.

All kinesin superfamily protein, KIF, genes in mouse and human

Intracellular transport is essential for morphogenesis and functioning of the cell. The kinesin superfamily proteins (KIFs) have been shown to transport membranous organelles and protein complexes in a microtubule- and ATP-dependent manner. More than 30 KIFs have been reported in mice. However, the nomenclature of KIFs has not been clearly established, resulting in various designations and redundant names for a single KIF. Here, we report the identification and classification of all KIFs in mouse and human genome transcripts. Previously unidentified murine KIFs were found by a PCR-based search. The identification of all KIFs was confirmed by a database search of the total human genome. As a result, there are a total of 45 KIFs. The nomenclature of all KIFs is presented. To understand the function of KIFs in intracellular transport in a single tissue, we focused on the brain. The expression of 38 KIFs was detected in brain tissue by Northern blotting or PCR using cDNA. The brain, mainly composed of highly differentiated and polarized cells such as neurons and glia, requires a highly complex intracellular transport system as indicated by the increased number of KIFs for their sophisticated functions. It is becoming increasingly clear that the cell uses a number of KIFs and tightly controls the direction, destination, and velocity of transportation of various important functional molecules, including mRNA. This report will set the foundation of KIF and intracellular transport research.

Functional analysis of mouse C-terminal kinesin motor KifC2

Proteins of the kinesin superfamily define a class of microtubule-dependent motors that play crucial roles in cell division and intracellular transport. In the mouse, several kinesin motors have been characterized and are suggested to play roles in axonal and/or dendritic transport. One such kinesin is KifC2. Sequence and secondary structure analysis revealed that KifC2 is a member of the C-terminal motor family. Northern and Western blot analyses indicated that KifC2 is specifically expressed in both the central and peripheral nervous systems. The cellular locations of the KifC2 proteins were found to be mainly in neural cell bodies and dendrites but also in axons. To understand the in vivo function of the KifC2 gene, we used homologous recombination in embryonic stem cells to construct knockout mouse strains for the KifC2 gene. Homozygous KifC2 mutants were viable and reproduced normally, and their development was apparently normal. These results suggest that KifC2 is dispensable for normal neural development and behavior in the mouse.

Molecular cloning and functional analysis of mouse C-terminal kinesin motor KifC3

Proteins of the kinesin superfamily define a class of microtubule-dependent motors that play crucial roles in cell division and intracellular transport. To study the molecular mechanism of intracellular transport involving microtubule-dependent motors, a cDNA encoding a new kinesin-like protein called KifC3 was cloned from a mouse brain cDNA library. Sequence and secondary structure analysis revealed that KifC3 is a member of the C-terminal motor family. In contrast to other mouse C-terminal motors, KifC3 is apparently ubiquitous and may have a general role in intracellular transport. To understand the in vivo function of the KifC3 gene, we used homologous recombination in embryonic stem cells to construct knockout mouse strains for the KifC3 gene. Homozygous mutants of the KifC3 gene are viable, reproduce normally, and apparently develop normally. These results suggest that KifC3 is dispensable for normal development and reproduction in the mouse.