Abstract
The core axoneme structure of both the motile cilium and sperm tail has the same ultrastructural 9 + 2 microtubular arrangement. Thus, it can be expected that genetic defects in motile cilia also have an effect on sperm tail formation. However, recent studies in human patients, animal models and model organisms have indicated that there are differences in components of specific structures within the cilia and sperm tail axonemes. Primary ciliary dyskinesia (PCD) is a genetic disease with symptoms caused by malfunction of motile cilia such as chronic nasal discharge, ear, nose and chest infections and pulmonary disease (bronchiectasis). Half of the patients also have situs inversus and in many cases male infertility has been reported. PCD genes have a role in motile cilia biogenesis, structure and function. To date mutations in over 40 genes have been identified cause PCD, but the exact effect of these mutations on spermatogenesis is poorly understood. Furthermore, mutations in several additional axonemal genes have recently been identified to cause a sperm-specific phenotype, termed multiple morphological abnormalities of the sperm flagella (MMAF). In this review, we discuss the association of PCD genes and other axonemal genes with male infertility, drawing particular attention to possible differences between their functions in motile cilia and sperm tails.
Highlights
Motile cilia and flagella have a conserved axonemal structure consisting of a ring of nine microtubular doublets and a central pair of microtubules, giving the classical 9 + 2 microtubular arrangement (Fig. 1a)
The primary ciliary dyskinesia (PCD) protein RSPH4A seems to be dispensable for spermatogenesis, since male PCD patients with RSPH4A mutations have been reported to be fertile and its expression levels in the testis are very low ([130], Fig. 4). These contrasting effects on male fertility suggest that differences between sperm flagella and cilia exist for the radial spoke head proteins, since mutations in any of the RSPH genes seems to have a similar consequence in the cilia; central pair loss and motility defects
Further studies on the effects of specific gene mutations on male fertility and intracytoplasmic sperm injection (ICSI) outcome are required to improve the diagnosis and treatment of male infertility in PCD patients. It appears that many PCD genes have an effect on male fertility, it is clear that the comparative disease phenotype between cilia and sperm does not correlate in all cases
Summary
Motile cilia and flagella have a conserved axonemal structure consisting of a ring of nine microtubular doublets and a central pair of microtubules, giving the classical 9 + 2 microtubular arrangement (Fig. 1a). The force for motility is produced by inner and outer dynein arms (IDA and ODA, respectively), which are carried by the A-type tubule and project toward the B-tubule of the adjacent doublet. The sperm flagellum has an ultrastructurally comparable axonemal structure, but in addition the sperm tail contains accessory structures: the mitochondrial sheath (MS), fibrous sheath (FS) and outer dense fibres (ODFs) (Fig. 1b). These structures are specific to the sperm tail and are required for fertile sperm production providing, e.g. additional rigidity and energy for the movement of the sperm in the female reproductive tract [58]. Sperm tail is connected to the head by the head tail coupling apparatus (HTCA) and annulus forms a diffusion barrier between the
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