Cell-specific spindle alignment in epithelial tissue via RASSF proteins

Abstract

Correct mitotic spindle alignment, which is essential to establish tissue architecture during development and maintain it during homeostasis, also plays an important role in cell fate specification through asymmetric cell division. Spindle alignment occurs through spindle tethering factors such as, Mud (NuMA in mammals), which are recruited to the cell cortex and capture astral microtubules, pulling the spindle in the correct orientation. These factors are often coupled to core polarity components shared by most cells. However, as seen in Drosophila symmetrically dividing epithelial cells and asymmetrically dividing sensory organ precursors (SOPs), precise spindle alignment between cell-types suggests a need for cell-specific factors that adapt the core spindle tethering machinery to each cell’s polarity and mode of division. In epithelia, the polarity protein Pins localises uniformly around the cortex and Pins recruits Mud, to ensure planar divisions. Conversely, SOP polarity results from the asymmetric localisation of Pins and the planar cell polarity protein Dishevelled to the anterior and posterior cortex, respectively. Subsequently, both promote Mud cortical recruitment, ultimately aligning the spindle along the anterior-posterior axis. SOP asymmetry results from SOP-specific RASSF protein, Meru, which is recruited by Dsh to the posterior cortex. Here, I show that Meru in turn recruits Mud to the posterior cortex, linking the spindle tethering and polarity machineries. In vitro, Meru is required to bridge the interaction between Dishevelled and Mud. In vivo, Meru loss results in a reduction of posterior Mud recruitment, spindle misalignment and sensory organ malformation. Interestingly, I also show that another RASSF protein, RASSF8, is expressed in epithelial cells, where it is required for Mud cortical recruitment and acts redundantly with Pins in mitotic spindle orientation. My results suggest a potentially conserved role for N-terminal RASSF proteins in mediating cell-specific spindle orientation.