The POU Geneceh-18Promotes Gonadal Sheath Cell Differentiation and Function Required for Meiotic Maturation and Ovulation inCaenorhabditis elegans

Abstract

In Caenorhabditis elegans, specialized contractile myoepithelial cells of the somatic gonad, the gonadal sheath cells, are closely apposed to oocytes and are required for normal meiotic maturation and ovulation. Previously we found that mutations in the ceh-18 gene, which encodes a POU-class homeoprotein expressed in sheath cells, result in oocyte defects. To determine the basis for these oocyte defects, we have used time-lapse video Nomarski microscopy to observe meiotic maturation, ovulation, and early embryogenesis in ceh-18 mutants. In ceh-18 mutants sheath cell contractions are weaker, less frequent, and uncoordinated throughout the sequence of ovulation events, and ovulation is defective. Defective ovulation can result in the formation of endomitotic oocytes in the gonad, the formation of haploid embryos, and reversals in embryonic polarity. ceh-18 mutant oocytes exhibit defects prior to nuclear envelope breakdown, suggesting that they are physiologically different from the wild type. We observed delays in meiotic maturation, as well as maturation out of the normal spatial and temporal sequence, suggesting that proximal sheath cells directly or indirectly promote and spatially restrict meiotic maturation. Analysis of sheath cell differentiation in ceh-18 mutants using antibodies to proteins of the contractile apparatus reveals that although contractile proteins are expressed, the sheath cells appear disorganized. Transmission electron microscopy reveals that ceh-18 mutant sheath cells are morphologically irregular and only loosely cover oocytes. Taken together, these observations indicate that ceh-18 is a crucial determinant of sheath cell differentiation, a function required for normal meiotic maturation and ovulation.

Structural components of the nonstriated contractile apparatuses in the Caenorhabditis elegans gonadal myoepithelial sheath and their essential roles for ovulation

“…Previous studies by others have shown that the myoepithelial sheath cells express MYO-3 (myoA; Fig. 4A-C) and UNC-54 (myoB) myosin heavy chains (Ardizzi and Epstein, 1987) that are also expressed in the striated body wall muscle and that UNC-54 localizes to the outer regions, whereas MYO-3 is limited to the central part of the thick filaments (Rose et al, 1997) as observed in the thick filaments in the striated body wall muscle (Miller et al, 1983). UNC-15 paramyosin was also expressed in the myoepithelial sheath (Ardizzi and Epstein, 1987;Fig.…”

“…Previous studies have shown that unc-54 myosin heavy chain (Rose et al, 1997), lev-11 tropomyosin (Ono and Ono, 2004), pat-10 troponin C (Ono and Ono, 2004), and mup-2 troponin T (Myers et al, 1996) are required for ovulation. Here, we found that the Emo phenotype (22-51%; n ϭ 100) was induced by RNAi treatments in wild-type background for myo-3 myosin heavy chain, unc-15 paramyosin, pat-3 ␤-integrin, deb-1 vinculin, or unc-52 perlecan (Fig.…”

“…1)-on poly-lysinecoated slides as described previously (Rose et al, 1997). These samples were fixed by an optimal method and stained with antibodies as listed in Table 1.…”

“…Contraction of the myoepithelial sheath is periodically enhanced upon maturation of the most proximal oocytes (McCarter et al, 1997(McCarter et al, , 1999. Sheath contraction is negatively regulated by ceh-18 encoding a POU-class homeoprotein (Rose et al, 1997;Miller et al, 2003) and gap junctions that are formed between the myoepithelial sheath and maturing oocytes (Whitten and Miller, 2006). On the other hand, it is enhanced by major sperm protein (Miller et al, 2001;Kosinski et al, 2005) by means of the VAB-1 Eph receptor (Miller et al, 2003).…”

TetraThymosinβ Is Required for Actin Dynamics inCaenorhabditis elegansand Acts via Functionally Different Actin-binding Repeats

“…Adult worms were stained as described (Finney and Ruvkun, 1990). Worm embryos (obtained by cutting gravid adults; Epstein et al, 1993) and dissected gonads (prepared as in Rose et al, 1997) were attached on poly-lysine-coated slides by freeze-cracking, fixed with methanol at Ϫ20°C for 5 min, and stained with antibodies. Rabbit polyclonal anti-tetraThymosin␤ antibody raised against full-length recombinant protein at the Centre d'Economie rurale, Laboratoire d'Hormonologie (Marloie, Belgium) was enriched by affinity purification over tetraThymosin␤ coupled to CNBr-Sepharose 4B (Pharmacia, Piscataway, NJ).…”