Active nuclear positioning and actomyosin contractility maintain leader cell integrity during gonadogenesis.

Proper distribution of organelles can play an important role in a moving cell's performance. During C. elegans gonad morphogenesis, the nucleus of the leading distal tip cell (DTC) is always found at the front, yet the significance of this localization is unknown. Here, we identified the molecular mechanism that keeps the nucleus at the front, despite a frictional force that pushes it backward. The Klarsicht/ANC-1/Syne homology (KASH) domain protein UNC-83 links the nucleus to the motor protein kinesin-1 that moves along a polarized acentrosomal microtubule network. Interestingly, disrupting nuclear positioning on its own did not affect gonad morphogenesis. However, reducing actomyosin contractility on top of nuclear mispositioning led to a dramatic phenotype: DTC splitting and gonad bifurcation. Long-term live imaging of the double knockdown revealed that, while the gonad attempted to perform a planned U-turn, the DTC was stretched due to the lagging nucleus until it fragmented into a nucleated cell and an enucleated cytoplast, each leading an independent gonadal arm. Remarkably, the enucleated cytoplast had polarity and invaded, but it could only temporarily support germ cell proliferation. Based on a qualitative biophysical model, we conclude that the leader cell employs two complementary mechanical approaches to preserve its integrity and ensure proper organ morphogenesis while navigating through a complex 3D environment: active nuclear positioning by microtubule motors and actomyosin-driven cortical contractility. [Display omitted] • LINC complex protein UNC-83 and kinesin motor position nucleus at the DTC forefront • Misplaced nucleus and low contractility cause DTC deformation during gonadal turn • Excessive strain during the turn leads to DTC fragmentation and gonad bifurcation • Enucleated DTC can invade but does not retain stem cell niche function for long Agarwal et al. combine cell-specific knockdown, long-term imaging, and physical modeling to show that the leader distal tip cell (DTC) employs two complementary mechanical strategies, active nuclear positioning by microtubule motors and actomyosin-driven cortical contractility, to prevent its deformation and ensure normal C. elegans gonadogenesis. [ABSTRACT FROM AUTHOR]