EGFR controls IQGAP basolateral membrane localization and mitotic spindle orientation during epithelial morphogenesis

Abstract

Establishing the correct orientation of the mitotic spindle is an essential step in epithelial cell division in order to ensure that epithelial tubules form correctly during organ development and regeneration. While recent findings have identified some of the molecular mechanisms that underlie spindle orientation, many aspects of this process remain poorly understood. Here, we have used the 3D-MDCK model system to demonstrate a key role for a newly identified protein complex formed by IQGAP1 and the epithelial growth factor receptor (EGFR) in controlling the orientation of the mitotic spindle. IQGAP1 is a scaffolding protein that regulates many cellular pathways, from cell-cell adhesion to microtubule organization, and its localization in the basolateral membrane ensures correct spindle orientation. Through its IQ motifs, IQGAP1 binds to EGFR, which is responsible for maintaining IQGAP1 in the basolateral membrane domain. Silencing IQGAP1, or disrupting the basolateral localization of either IQGAP1 or EGFR, results in a non-polarized distribution of NuMA, mitotic spindle misorientation and defects in single lumen formation.

IQGAPs choreograph cellular signaling from the membrane to the nucleus

“…Signal transduction from the receptor at the plasma membrane to downstream MAPK components requires coordination among signaling proteins. IQGAP1 associates with several MAPK proteins, including EGFR [42, 43], K-Ras [44], Raf [45, 46], MEK [14] and ERK [13, 14] (Figure 1Ai) (Table 1), providing a scaffold which promotes EGF-induced MAPK signaling [13, 45]. IQGAP3, which interacts with H-Ras [47] and ERK1 [48], also augments EGF-stimulated MAPK signaling.…”

“…Evidence reveals that IQGAP1 localizes to the basolateral membrane of epithelial cells to correctly orient the mitotic spindle [42]. Interestingly, EGFR is required for this localization of IQGAP1 as well as for the polarized anchoring of microtubules during epithelial cell division and to control spindle orientation.…”

Spindle orientation: What if it goes wrong?

“…RNAi-mediated depletion of IQGAP1 leads to spindle misorientation, as does treatment with EGF [71]. Although the relationship between these proteins and the canonical spindle machinery remains unclear, it has been observed that mitotic NuMA localization is affected in siRNA IQGAP1 cells.…”

FAK transduces extracellular forces that orient the mitotic spindle and control tissue morphogenesis

“…Fink et al 7 , elegantly showed that external forces transmitted through the retraction fibres (RFs) can guide mitotic spindle orientation in adherent cells, whereas external forces have also been shown to control spindle orientation in vivo in the zebrafish EVL 8 . Despite an abundance of molecules identified as regulators of mitotic spindle positioning and orientation, including microtubule-and actin-associated proteins, kinases, cell polarity proteins, adherens junction proteins, transmembrane receptors and phosphoinositides 3,5,[7][8][9][10][11][12][13][14][15] , how are external forces transmitted to the cell cortex and bias cortical cues is still poorly understood.…”