The mechanism of cell size regulation by polycystins

The mechanism of mechanosensing by polycystins during cell growth
Cytokinesis is the last stage of cell division when two daughter cells separate, but it is equally important for the
transition to cell growth including cell size expansion. The mechanism regulating such a transition is poorly
understood. We identified the role of calcium and the polycystin channel Pkd2p in this process while studying
cytokinesis of the model organism fission yeast. Pkd2p is essential to regulate the cell size and it mediates
calcium influx. Polycystins are evolutionally conserved ion channels. Loss of function mutations of human
polycystins lead to the genetic disorder, Autosomal Polycystic Kidney Disorder (ADPKD). The cellular function
of this highly conserved family of channels is not well understood. This study will determine how Pkd2p and
calcium regulate the transition to cell growth.
Aim 1. Determine how the Hippo signaling pathways regulate Pkd2p. Our genetic screen has identified the
fission yeast Hippo pathways SIN and MOR as crucial for the regulation of Pkd2p. Both are highly conserved
kinase cascades that are essential for cell proliferation. SIN antagonizes Pkd2p activity, while MOR synergizes
with Pkd2p. However, the mechanism is unclear. We will 1) Determine how MOR and SIN regulate cellular
calcium during cytokinesis through calcium-imaging. 2) Determine how MOR and SIN regulate Pkd2p in cell size
expansion. 3) Determine how MOR promotes the cell cycle-dependent localization of Pkd2p. 4) Determine
whether Pkd2p is a direct substrate of the MOR kinase Orb6p. Aim 2. Determine how the Pkd2p channel is
activated. Pkd2 channel opens in response to mechanical force in vitro, but the mechanism is unclear. We will
test the proposal that Pkd2p is a channel sensitive to the force driving the yeast cell growth. We will 1) Determine
how Pkd2p regulates calcium when the cells are stimulated by external force. 2) Probe how Pkd2p senses
osmotic stimuli and the lipid environment in vitro through a collaboration with Allen Liu’s group (University of
Michigan). 3) Determine whether Pkd2p channel allows ions other than calcium to pass through, using patch
clamp, through a collaboration with Du Jianyang’s group (University of Tennessee). Aim 3 Determine how
Pkd2p regulates the actin re-organization during cytokinesis. Calcium can activate two highly conserved
molecules Cam1p and Ppb1p. We will determine how they contribute to the role of Pkd2p in cytokinesis. We will
determine 1) how Pkd2p regulates the Cam1p-dependent endocytosis during cytokinesis. 2) how Pkd2p
regulates the Cam1p-dependent intracellular transport. 3) how Pkd2p regulates the enzymatic activity of Ppb1p.
Through this study, we expect to uncover a novel cell size regulation pathway mediated by Pkd2p channel. We
will demonstrate how both internal signaling pathways and external environment play a vital role in activating
this channel in cytokinesis. We will employ novel imaging methods combined with innovative in vitro techniques
in our study. Our works shall help us better understand the cellular functions of the human polycystins.

Grant Number: 5R01GM144652-04
NIH Institute/Center: NIH
Principal Investigator: Qian Chen