Electron tomography of microtubule end-morphologies in C. elegans embryos

Ultrastructural organization of NompC in the mechanoreceptive organelle of Drosophila campaniform mechanoreceptors

Mechanoreceptive organelles (MOs) are specialized subcellular entities in mechanoreceptors that transform extracellular mechanical stimuli into intracellular signals. Their ultrastructures are key to understanding the molecular nature and mechanics of mechanotransduction. Campaniform sensilla detect cuticular strain caused by muscular activities or external stimuli in Drosophila Each campaniform sensillum has an MO located at the distal tip of its dendrite. Here we analyzed the molecular architecture of the MOs in fly campaniform mechanoreceptors using electron microscopic tomography. We focused on the ultrastructural organization of NompC (a force-sensitive channel) that is linked to the array of microtubules in these MOs via membrane-microtubule connectors (MMCs). We found that NompC channels are arranged in a regular pattern, with their number increasing from the distal to the proximal end of the MO. Double-length MMCs in nompC ^29+29ARs confirm the ankyrin-repeat domain of NompC (NompC-AR) as a structural component of MMCs. The unexpected finding of regularly spaced NompC-independent linkers in nompC ³ suggests that MMCs may contain non-NompC components. Localized laser ablation experiments on mechanoreceptor arrays in halteres suggest that MMCs bear tension, providing a possible mechanism for why the MMCs are longer when NompC-AR is duplicated or absent in mutants. Finally, mechanical modeling shows that upon cuticular deformation, sensillar architecture imposes a rotational activating force, with the proximal end of the MO, where more NOMPC channels are located, being subject to larger forces than the distal end. Our analysis reveals an ultrastructural pattern of NompC that is structurally and mechanically optimized for the sensory functions of campaniform mechanoreceptors.

Compartmentalization of the endoplasmic reticulum in the early C. elegans embryos

Lee et al. show that the ER in the C. elegans embryo is continuous, but its membrane is compartmentalized, as found in budding yeast and mouse NSCs. This compartmentalization plays a potential role in the polarity of the early embryo.

The one-cell Caenorhabditis elegans embryo is polarized to partition fate determinants between the cell lineages generated during its first division. Using fluorescence loss in photobleaching, we find that the endoplasmic reticulum (ER) of the C. elegans embryo is physically continuous throughout the cell, but its membrane is compartmentalized shortly before nuclear envelope breakdown into an anterior and a posterior domain, indicating that a diffusion barrier forms in the ER membrane between these two domains. Using mutants with disorganized ER, we show that ER compartmentalization is independent of the morphological transition that the ER undergoes in mitosis. In contrast, compartmentalization takes place at the position of the future cleavage plane in a par-3–dependent manner. Together, our data indicate that the ER membrane is compartmentalized in cells as diverse as budding yeast, mouse neural stem cells, and the early C. elegans embryo.

Three-dimensional reconstruction methods for Caenorhabditis elegans ultrastructure

The roundworm Caenorhabditis elegans is one of the major model organisms in modern cell and developmental biology. Here, we present methods for the three-dimensional (3D) reconstruction of the worm ultrastructure. We describe the use of (1) serial-section analysis, (2) electron tomography, and (3) serial block face imaging by scanning electron microscopy (SEM). Sample preparation for high-pressure freezing/freeze substitution (HPF/FS) has been extensively covered in a previous volume of this "Methods in Cell Biology" series and will only be described briefly. We will discuss these 3D methods in light of recent research activities related to worm and early embryo biology.