toca-1 Is in a Novel Pathway That Functions in Parallel with a SUN-KASH Nuclear Envelope Bridge to Move Nuclei in Caenorhabditis elegans

Abstract

Moving the nucleus to an intracellular location is critical to many fundamental cell and developmental processes, including cell migration, differentiation, fertilization, and establishment of cellular polarity. Bridges of SUN and KASH proteins span the nuclear envelope and mediate many nuclear positioning events, but other pathways function independently through poorly characterized mechanisms. To identify and characterize novel mechanisms of nuclear migration, we conducted a nonbiased forward genetic screen for mutations that enhanced the nuclear migration defect of unc-84, which encodes a SUN protein. In Caenorhabditis elegans larvae, failure of hypodermal P-cell nuclear migration results in uncoordinated and egg-laying-defective animals. The process of P-cell nuclear migration in unc-84 null animals is temperature sensitive; at 25°migration fails in unc-84 mutants, but at 15°the migration occurs normally. We hypothesized that an additional pathway functions in parallel to the unc-84 pathway to move P-cell nuclei at 15°. In support of our hypothesis, forward genetic screens isolated eight emu (enhancer of the nuclear migration defect of unc-84) mutations that disrupt nuclear migration only in a null unc-84 background. The yc20 mutant was determined to carry a mutation in the toca-1 gene. TOCA-1 functions to move P-cell nuclei in a cell-autonomous manner. TOCA-1 is conserved in humans, where it functions to nucleate and organize actin during endocytosis. Therefore, we have uncovered a player in a previously unknown, likely actindependent, pathway that functions to move nuclei in parallel to SUN-KASH bridges. The other emu mutations potentially represent other components of this novel pathway.

Nuclear migration events throughout development

“…Since the in vitro nature of these studies is limiting it is essential to study nuclear squeezing events in vivo. One possible model is nuclear migration in C. elegans larval P-cells, in which the nucleus migrates from lateral to ventral through a constricted space of about 200 nm between the muscle and the cuticle (Chang et al, 2013b;Sulston, 1976). Such future in vivo studies are likely to expand our understanding of disease.…”

Cell Biology of theCaenorhabditis elegansNucleus

“…Based on detailed time-lapse microscopy including analysis of microtubule dynamics, it has been proposed that the involvement of two different molecular motors increases the efficiency of nuclear migration by accommodating for bidirectional movements to bypass "roadblocks" . Most studies on nuclear migration have focused on the UNC-84/UNC-83 and UNC-84/ANC-1 LINC complexes, but it is interesting to note that a screen for enhancers of unc-84 phenotypes uncovered a potential parallel pathway involving the conserved protein TOCA-1 (Chang et al 2013). TOCA-1 presumably interacts with actin but it remains unclear how it regulates nuclear migration.…”

Nuclei migrate through constricted spaces using microtubule motors and actin networks in C. elegans hypodermal cells

“…1, green, tan and blue, respectively) -are involved in this rearrangement of the ventral hypodermis. To characterize P-cell morphology changes, we imaged cell-cell junctions using the adherens junction marker AJM-1 and labeled P cell nuclei with an NLS: tdTomato marker driven by the P-cell-specific promoter for hlh-3 Chang et al, 2013;Doonan et al, 2008;Koppen et al, 2001) (Fig. 1).…”