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. The nuclear envelope 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 nuclear envelope dynamics we initiated the C. elegans nuclear architecture project. This project has two main components: (a) to examine in C. elegans the role of proteins identified in a yeast screen as being involved in nuclear architecture;and (b) to screen for additional gene/proteins involved in determining nuclear shape and size, and in affecting nuclear envelope breakdown and reassembly. In the past year we have made significant progress on both these fronts, as described below. (a) We have previously shown that the lipin pathway, which regulates lipid biosythesis, is involved in maintaining proper nuclear structure. Since yeast nuclei and nuclei of higher eukaryotes differ in several important aspects (e.g. nuclear envelope breakdown, which does not occur in budding yeast, and the presence of lamins, which are absent in yeast), we determined whether lipin plays a similar role in maintaining nuclear integrity in C. elegans. Our findings indicate that down regulating lipin leads to aberrant ER structure (as in yeast) and to defects in nuclear envelope assembly and disassembly. These findings validate our approach for using yeast as a starting point for uncovering proteins involved in nuclear architecture in higher eukaryotes, and it contributes to our understanding of the function of lipin in metazoa. We are currently adopting the same approach to study additional genes that may affect nuclear architecture in both yeast and C. elegans. (b) We constructed a C. elegans strain with which we can monitor nuclear dynamics. To screen for proteins that affect nuclear architecture, we treated this strain with RNAi from an RNAi collection of roughly 2000 genes known to cause embryonic lethality when inactivated. This screen resulted in over 300 genes that when down regulated by RNAi cause a defect in nuclear morphology. Two types of morphologies are being pursued: paired nuclei, a phenotype that is similar to that caused by lipin inactivation, and uneven distribution of nuclear pore complexes. We are in the process of characterizing these morphologies and understanding how the genes we isolated contribute to these processes.
