The fibrous sheath is a unique cytoskeletal structure surrounding the axoneme and outer dense fibers and defines the extent of the principal piece region of the sperm flagellum. It consists of two longitudinal columns connected by closely arrayed semicircular ribs that assemble from distal to proximal throughout spermiogenesis. The fibrous sheath is believed to influence the degree of flexibility, plane of flagellar motion, and the shape of the flagellar beat. Nearly half of the protein in fibrous sheaths isolated from mouse sperm is AKAP4. This protein and two others, AKAP3 and TAKAP-80, have anchoring sites for cAMP-dependent protein kinase. AKAP3 also anchors ropporin, a spermatogenic cell-specific protein that is linked through rhophilin to the small GTPase Rho. Other proteins associated with the fibrous sheath include two enzymes in the glycolytic pathway. Glyceraldehyde 3-phosphate dehydrogenase-s (GAPDS) is the product of a gene expressed only in spermatogenic cells, while hexokinase type 1-s (HK1-S) is derived from alternative transcripts present only in spermatogenic cells. Most of the other glycolytic enzymes in sperm have unique structural or functional properties. The fibrous sheath also contains a spermatogenic cell-specific member of the mu-class glutathione S-transferase family (GSTM5) and an intermediate filament-like protein (FS39). These and other observations indicate that the fibrous sheath functions as a scaffold for proteins in signaling pathways that might be involved in regulating sperm maturation, motility, capacitation, hyperactivation, and/or acrosome reaction and for enzymes in the glycolytic pathway that provide energy for the hyperactivated motility of sperm that allows them to penetrate the zona pellucida.

The acrosome is a unique organelle that plays an important role at the site of sperm-zona pellucida binding during the fertilization process, and is lost in globozoospermia, an inherited infertility syndrome in humans. Although the acrosome is known to be derived from the Golgi apparatus, molecular mechanisms underlying acrosome formation are largely unknown. Here we show that Golgi-associated PDZ-and coiled-coil motif-containing protein (GOPC), a recently identified Golgi-associated protein, is predominantly localized at the trans-Golgi region in round spermatids, and male mice in which GOPC has been disrupted are infertile with globozoospermia. The primary defect was the fragmentation of acrosomes in early round spermatids, and abnormal vesicles that failed to fuse to developing acrosomes were apparent. In later stages, nuclear malformation and an abnormal arrangement of mitochondria, which are also characteristic features of human globozoospermia, were observed. Interestingly, intracytoplasmic sperm injection (ICSI) of such malformed sperm into oocytes resulted in cleavage into blastocysts only when injected oocytes were activated. Thus, GOPC provides important clues to understanding the mechanisms underlying spermatogenesis, and the GOPC-deficient mouse may be a unique and valuable model for human globozoospermia.

Membrane fusion is an essential step in the encounter of two nuclei from sex cells-sperm and egg-in fertilization. However, aside from the involvement of two molecules, CD9 and Izumo, the mechanism of fusion remains unclear. Here, we show that spermegg fusion is mediated by vesicles containing CD9 that are released from the egg and interact with sperm. We demonstrate that the CD9 ؊/؊ eggs, which have a defective sperm-fusing ability, have impaired release of CD9-containing vesicles. We investigate the fusion-facilitating activity of CD9-containing vesicles by examining the fusion of sperm to CD9 ؊/؊ eggs with the aid of exogenous CD9-containing vesicles. Moreover, we show, by examining the fusion of sperm to CD9 ؊/؊ eggs, that hamster eggs have a similar fusing ability as mouse eggs. The CD9-containing vesicle release from unfertilized eggs provides insight into the mechanism required for fusion with sperm.

Hybrid breakdown is a type of reproductive failure that appears after the F 2 generation of crosses between different species or subspecies. It is caused by incompatibility between interacting genes. Genetic analysis of hybrid breakdown, particularly in higher animals, has been hampered by its complex nature (i.e., it involves more than two genes, and the phenotype is recessive). We studied hybrid breakdown using a new consomic strain, C57BL/6J-X MSM , in which the X chromosome of C57BL/6J (derived mostly from Mus musculus domesticus) is substituted by the X chromosome of the MSM/Ms strain (M. m. molossinus). Males of this consomic strain are sterile, whereas F 1 hybrids between C57BL/6J and MSM/Ms are completely fertile. The C57BL/6J-X MSM males showed reduced testis weight with variable defects in spermatogenesis and abnormal sperm head morphology. We conducted quantitative trait locus (QTL) analysis for these traits to map the X-linked genetic factors responsible for the sterility. This analysis successfully detected at least three distinct loci for the sperm head morphology and one for the testis weight. This study revealed that incompatibility of interactions of X-linked gene(s) with autosomal and/or Y-linked gene(s) causes the hybrid breakdown between the genetically distant C57BL/6J and MSM/Ms strains. H YBRIDS between individuals of two genetically di-Hybrid breakdown is another type of reproductive isolation, defined as inviability or sterility observed only verged populations show different extents of rein the F 2 or later generations of interspecific or intersubproductive failure; this failure is known as reproductive specific crosses, while F 1 hybrids are viable and fully isolation. This reduction of fecundity of the hybrids fertile. Hybrid breakdown may be due to disruption prevents gene flow across the two different populations, of interaction of genes at different loci as the genes accelerating genetic differentiation and eventually consegregate after the F 1 generation. Assuming that two tributing to speciation. Thus, the study of reproductive diverging populations have different alleles at each of isolation is essential for understanding the process of two loci that genetically interact, it is inferred that the speciation.proper interaction may occur only in the allele combinaIn mice, genetic studies on reproductive isolation tions that occur in each of the two diverging populahave focused mostly on male sterility in F 1 hybrids betions. Hybrid breakdown is thus hypothesized to arise tween subspecies or closely related species. This hybrid when genetic segregation causes alleles of each intersterility is likely caused by interallelic incompatibility at acting locus to become homozygous in an improper way. a given locus or at several different loci. In such cases, Therefore, the hybrid breakdown appears as a recessive one allele or a set of alleles is fixed or predominates trait (Muller 1940; Orr 1993). Little is known about in each genetically differentiating population. Hybrid the gene...

Spermatogenesis is a complex process that involves cooperation of germ cells and testicular somatic cells. Various genetic disorders lead to impaired spermatogenesis, defective sperm function and male infertility 1 . Here we show that Cnot7 -/-males are sterile owing to oligo-astheno-teratozoospermia, suggesting that Cnot7, a CCR4-associated transcriptional cofactor 2 , is essential for spermatogenesis. Maturation of spermatids is unsynchronized and impaired in seminiferous tubules of Cnot7 -/-mice. Transplantation of spermatogonial stem cells from male Cnot7 -/-mice to seminiferous tubules of Kit mutant mice (Kit W/W-v ) restores spermatogenesis, suggesting that the function of testicular somatic cells is damaged in the Cnot7 -/-condition. The testicular phenotypes of Cnot7 -/-mice are similar to those of mice deficient in retinoid X receptor beta (Rxrb) 3 . We further show that Cnot7 binds the AF-1 domain of Rxrb and that Rxrb malfunctions in the absence of Cnot7. Therefore, Cnot7 seems to function as a coregulator of Rxrb in testicular somatic cells and is thus involved in spermatogenesis.