Nuclear architecture in C. elegans

National Institute Of Diabetes And Digestive And Kidney Diseases

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Abstract

The nucleus is arguably one of the most important organelles in the cell, and yet surprisingly little is known about how its shape is maintained and what determines its size. The importance of nuclear shape is underscored by the fact that various diseases, including cancer and premature aging, are associated with changes in nuclear shape, and yet the relationship between nuclear shape and nuclear function is poorly understood. In most metazoan cells, the nuclear envelope (NE) undergoes cycles of assembly and disassembly in each and every cell cycle, and one of the key outstanding questions in the field is which proteins facilitate these processes. To gain insight into NE dynamics, we initiated the C. elegans nuclear architecture project. This project has three main components: (a) to examine in C. elegans the role of proteins identified in a yeast screen as being involved in nuclear architecture; (b) to screen for additional gene/proteins involved in determining nuclear shape and size, in affecting NE breakdown and reassembly, and in promoting pronuclear fusion after fertilization; and (c) understanding the function of the membrane reticulum, the centriculum, that surrounds the centrosome. Ultimately, are goal is to understand the mechanisms by which the nuclear envelope is remodeled during the cell cycle and during development. Following our observation in budding yeast that Cdc5, the yeast homolog of polo kinase, affects nuclear morphology, we examined the role of the C. elegans polo kinase, PLK-1, in NE dynamics. We discovered that PLK-1 is required for nuclear envelope breakdown during early embryogenesis. In particular, PLK-1 is essential for breaching the NEs that separate the maternal and paternal chromosomes. Following fertilization in C. elegans, the maternal and paternal chromosomes become encased in two distinct NEs, forming two pronuclei. These pronuclei migrate towards each other and become juxtaposed such that between them there are 4 closely apposed membranes (2 from each NE). During any mitosis, the NE disassembles and the nuclear membrane retreats into the rest of the ER. We found that during the first embryonic mitosis, the disassembly of the NEs between the two pronuclei involves the formation of membrane junctions that lead to a membrane gap that spans all 4 membranes. This gap is critical for the mixing of the parental genomes and its formation is dependent on PLK-1. We are currently investigating the mechanism by which this gap is formed. Specifically, we identified a number of genes that, when down-regulated, fail to form the aforementioned membrane gap and we are exploring their role in gap formation. Additionally, we are using focused ion beam/scanning electron microscopy (FIB/SEM) to understand the structure of the gap between the two pronuclei. Finally, we uncovered a novel membrane reticulum surrounding the centrosome, which we call the centriculum. Our data suggest that the centriculum affect centrosome function, serves as a microtubule filter and contributes to nuclear envelope breakdown.