Richard Oko

Secretion and endocytosis in the male reproductive tract: a role in sperm maturation

Publisher Summary The epithelial cells of the entire male reproductive duct system, from the testis to the vas deferens, contribute to a proper milieu for sperm maturation through two distinct activities: secretion and endocytosis. Examples are provided in the chapter of these activities by following the origin and fate of SGP-1, SGP-2, and immobilin in the excurrent duct system. These proteins typify the regional variations that exist for the secretion and endocytosis of proteins along the reproductive duct. The reasons for such regional variations in secretion and endocytosis of different proteins ultimately lies in the genetic regulatory factors for each protein, the type of association of each protein with the spermatozoa, if any, and the functional contributions that each protein plays in the final maturation of spermatozoa. An important concept discussed in this chapter is that the spermatozoon itself may contribute to its own maturation (i.e., glycosylation) providing that the appropriate conditions of its external milieu are met by the secretory and endocytic activities along the excurrent reproductive duct system.

The immunolocalization of small nuclear ribonucleoprotein particles in testicular cells during the cycle of the seminiferous epithelium of the adult rat

The objective of this study was to determine the cellular and subcellular distribution of small nuclear ribonucleoprotein particles (snRNPs) in the adult rat testis in relation to the different cell types at the various stages of the cycle of the seminiferous epithelium. The distribution of snRNPs in the nucleus and cytoplasm of germ cells was quantitated in an attempt to correlate RNA processing with morphological and functional changes occurring during the development of these cells. Light-microscopic immunoperoxidase staining of rat testes with polyclonal anti-Sm and monoclonal anti-Y12 antibodies localized spliceosome snRNPs in the nuclei and cytoplasm of germ cells up to step 10 spermatids. Nuclear staining was intense in Sertoli cells, spermatogonia, spermatocytes, and in the early steps of round spermatid development. Although comparatively weaker, cytoplasmic staining for snRNPs was strongest in mid and late pachytene spermatocytes and early round spermatids. Quantitative electron-microscopic immunogold labeling of Lowicryl embedded testicular sections confirmed the light-microscopic observations but additionally showed that the snRNP content peaked in the cytoplasm of midpachytene spermatocytes and in the nuclei of late pachytene spermatocytes. The immunogold label tended to aggregate into distinct loci over the nuclear chromatin. The chromatoid body of spermatids and spermatocytes and the finely granular material in the interstices of mitochondrial aggregates of spermatocytes were found to be additional sites of snRNP localization and were intensely labeled. This colocalization suggests that these dense cytoplasmic structures may be functionally related. Anti-U1 snRNP antibodies applied to frozen sections showed the same LM localization pattern as spliceosome snRNPs. Anti-U3 snRNP antibodies applied to frozen sections stained nucleoli of germ cells where pre-rRNA is spliced.

Biogenesis of Specialized Cytoskeletal Elements of Rat Spermatozoa

Our results on the formation of the ODF and perforatorium are diagrammatically summarized in Figures 30 and 31. The developmental expression of proteins making up these two cytoskeletal elements differs in timing, duration and intracellular localization. The ODF proteins are synthesized exclusively during the latter part of spermiogenesis, well after transcriptional activity in the haploid germ cell nucleus has ended. This implies that these major integral proteins of the tail are translationally regulated and that mechanisms must exist for the storage and eventual release of the mRNAs encoding these proteins. The perforatorial proteins, on the other hand, begin to be synthesized during the meiotic prophase reaching a peak of production in early spermiogenesis just before the initiation of the condensation of the spermatids nucleus, at which time RNA synthesis stops. Another major difference between ODF and perforatorial protein production is that there seems to be a coordinated activity between the synthesis and the assembly of the ODF proteins, whereas there appears to be an almost 25 day delay between the initial meiotic synthesis and final condensation of perforatorial proteins in the subacrosomal space at the end of spermiogenesis. As for the intracellular localization of ODF and perforatorial proteins both have unprecedented distributions. The ODF proteins appear to be concentrated in a particular type of granular body which is especially abundant in the elongated spermatid at the time of peak ODF assembly. The perforatorial proteins, on the other hand, appear to be concentrated in the nuclei of pachytene spermatocytes and round spermatids until their displacement into the cytoplasm during nuclear condensation. Both forms of localization suggest a storage role for these proteins uniquely adapted by the spermatid to regulate the assemblies of the respective cytoskeletal elements.

Molecular and Cellular Biology of Novel Cytoskeletal Proteins in Spermatozoa

Most of the integral components of the mature mammalian spermatozoon are made up of cytoskeletal proteins that are synthesized and assembled during the haploid phase of spermatogenesis. With the exception of various isoforms of tubulin composing the microtubules of the sperm tail (1,2) and filamentous (3–5) and nonfilamentous (6, 7) forms of actin localized in diverse regions of the sperm head and tail of various species, the majority of sperm cytoskeletal proteins appear to have no protein or structural counterparts in somatic cells. Specialized cytoskeletal elements found in the sperm tail are the outer dense fibers (ODF), the fibrous sheath (FS), the submitochondrial reticulum (8), the annulus, and the striated collar and capitulum of the neck piece [reviewed by Oko and Clermont (9)]. In the sperm head are found the perinuclear theca (PT), the outer periacrosomal layer (OPL), and the basal plate [reviewed by Oko (10)]. The isolation or extraction of many of these sperm elements is made possible by their differential resistance to protein solubilizing agents (11–21). Compositional studies have revealed that most of these elements are made up of a heterogeneous mixture of proteins of various concentrations (11–21).