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 intraflagellar 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.Reproduction (2016) 151 R43-R54

Male infertility is an increasing problem partly due to inherited genetic variations. Mutations in genes involved in formation of the sperm tail cause motility defects and thus male infertility. Therefore, it is crucial to understand the protein networks required for sperm differentiation. Sperm motility is produced through activation of the sperm flagellum, which core structure, the axoneme, resembles motile cilia. In addition to this, cytoskeletal axonemal structure sperm tail motility requires various accessory structures. These structures are important for the integrity of the long tail, sperm capacitation, and generation of energy during sperm passage to fertilize the oocyte. This review discusses the current knowledge of mechanisms required for formation of the sperm tail structures and their effect on fertility. The recent research based on animal models and genetic variants in relation to sperm tail formation and function provides insights into the events leading to fertile sperm production. Here we compile a view of proteins involved in sperm tail development and summarize the current knowledge of factors contributing to reduced sperm motility, asthenozoospermia, underline the mechanisms which require further research, and discuss related clinical aspects on human male infertility.

Background.The correct formation of the sperm tail and manchette are essential for male fertility. Sperm tail development is a complex process organized by intraflagellar transport (IFT), mechanism that is utilized to transport molecules along the axonemal microtubule doublets. Two motor proteins are responsible for the transport; kinesin II, the anterograde motor, carries particles from the base towards the site of tail assembly. The retrograde motor, dynein, restores particles and motorproteins back to the pool of IFT components. Manchette is a microtubule and F-actin containing structure expressed transiently during spermiogenesis. It serves as a platform for intramanchette transport (IMT) where particles are carried first from the cytosol or Golgi to the manchette and then IMT delivers proteins to the sperm head or basal body region. We are interest in one of the kinesin II motor protein subunits, KIF3A, which has been shown to be present during sperm tail development, but its specific functions during spermiogenesis are poorly understood. Results.We have localized KIF3A in wild type mice in the manchette, basal body and flagella of elongating spermatids and in the principal piece of mature sperm tail. The depletion of KIF3A results in defects at late spermatogenesis. Spermatogonia, spermatocytes and round spermatids appear normal, but elongating spermatids have short and immotile flagella with disorganized axoneme and accessory structures. Manchette was elongated and perinuclear ring seems to squeeze the developing head causing its knob-like appearance. We were able to identify meiosis-specific nuclear structural protein 1 (MNS1) as an interacting partner for KIF3A. These proteins co-localize in the manchette and principal piece of the sperm tail. MNS1 appears to be delivered through manchette to the sperm tail, where it is required for the assembly of the flagella. In KIF3A KO mice manchette clearance was delayed and MNS1 staining remained in the manchette. Conclusions.Depletion of KIF3A caused defects during sperm tail development, manchette function and head shaping. Its interaction with MNS1 indicates that KIF3A may be involved in the transport of MNS1 to the developing tail. MNS1 concentrates in the manchette in the KIF3A KO suggesting a delay in the transport through the IMT. In addition, MNS1 and KIF3A co-localize in principal piece indicating the possible interaction site in mature sperm. We suggest that KIF3A has a role in manchette formation and IMT in addition to its well defined role in IFT. This study also highlights the essential role of KIF3A and IFT during spermiogenesis.

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.