Rose KL et al. (1997) Dev Biol "The POU gene ceh-18 promotes gonadal sheath cell differentiation and function required for meiotic maturation and ovulation in Caenorhabditis elegans."

Hoffman LH, Rose KL, Winfrey VP, Hall DH, Greenstein D, Furuta T

[Dev Biol, 1997]

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.

Hall DH et al. (1999) Dev Biol "Ultrastructural features of the adult hermaphrodite gonad of Caenorhabditis elegans: relations between the germ line and soma."

Hall DH, Winfrey VP, Hobert O, Rose KL, Blaeuer G, Furuta T, Greenstein D, Hoffman LH

[Dev Biol, 1999]

Genetic and embryological experiments have established the Caenorhabditis elegans adult hermaphrodite gonad as a paradigm for studying the control of germline development and the role of soma-germline interactions. We describe ultrastructural features relating to essential germline events and the soma-germline interactions upon which they depend, as revealed by electron and fluorescence microscopy. Gap junctions were observed between oocytes and proximal gonadal sheath cells that contract to ovulate the oocyte. These gap junctions must be evanescent since individual oocytes lose contact with sheath cells when they are ovulated. In addition, proximal sheath cells are coupled to each other by gap junctions. Within proximal sheath cells, actin/myosin bundles are anchored to the plasma membrane at plaque-like structures we have termed hemi-adherens junctions, which in turn are closely associated with the gonadal basal lamina. Gap junctions and hemi-adherens junctions are likely to function in the coordinated series of contractions required to ovulate the mature oocyte. Proximal sheath cells are fenestrated with multiple small pores forming conduits from the gonadal basal lamina to the surface of the oocyte, passing through the sheath cell. In most instances where pores occur, extracellular yolk particles penetrate the gonadal basal lamina to directly touch the underlying oocytes. Membrane-bounded yolk granules were generally not found in the sheath cytoplasm by either electron microscopy or fluorescence microscopy. Electron microscopic immunocytochemistry was used to confirm and characterize the appearance of yolk protein in cytoplasmic organelles within the oocyte and in free particles in the pseudocoelom. The primary route of yolk transport apparently proceeds from the intestine into the pseudocoelom, then through sheath pores to the surface of the oocyte, where endocytosis occurs. Scanning electron microscopy was used to directly visualize the distal tip cell which extends tentacle-like processes that directly contact distal germ cells. These distal tip cell processes are likely to play a critical role in promoting germline mitosis. Scanning electron microscopy also revealed thin filopodia extending from the distal sheath cells. Distal sheath filopodia were also visualized using a green fluorescent protein reporter gene fusion and confocal microscopy. Distal sheath filopodia may function to stretch the sheath over the distal arm.

Miller DM et al. (1995) Development "Expression of the unc-4 homeoprotein in Caenorhabditis elegans motor neurons specifies presynaptic input."

Miller DM, Niemeyer CJ

[Development, 1995]

In the nematode, Caenorhabditis elegans, VA and Vp motor neurons arise from a common precursor cell but adopt different morphologies and synapse with separate sets of interneurons in the ventral nerve cord. A mutation that inactivates the unc-4 homeodomain gene causes VA motor neurons to assume the VB pattern of synaptic input while retaining normal axonal polarity and output; the disconnection of VA motor neurons from their usual presynaptic partners blocks backward locomotion. We show that expression of a functional unc-4-beta-galactosidase chimeric protein in VA motor neurons restores wild-type movement to an unc-4 mutant. We propose that unc-4 controls a differentiated characteristic of the VA motor neurons that distinguishes them from their VB sisters, thus dictating recognition by the appropriate interneurons. Our results show that synaptic choice can be controlled at the level of transcription in the post-synaptic neuron and identify a homeoprotein that defines a subset of cell-specific traits required for this choice.