# Rac1 and the actin cytoskeleton in renal tubular repair

> **NIH NIH K08** · VANDERBILT UNIVERSITY MEDICAL CENTER · 2024 · $158,544

## Abstract

Polarized epithelial tubes are critical for an intact kidney. They control water, electrolyte, and nutrient
homeostasis, all of which are deranged in chronic kidney disease. These specialized epithelia require a highly
organized actin cytoskeleton that determines cellular shape and function. During renal epithelial repair, the actin
cytoskeleton of proliferating cells is rapidly re-organized to form the complex polarized architecture of an
epithelial tube. The small Rho GTPase Rac1 is a multifunctional molecular switch and a master regulator of the
actin cytoskeleton. We recently demonstrated that Rac1 is required to maintain actin cytoskeletal integrity,
epithelial polarity, and cell shape of the mature collecting duct (CD) epithelium. It is still unknown how the actin
cytoskeleton is regulated in epithelial cell repair and what role Rac1 plays in this process. For coordinated
epithelial tube regeneration, epithelial cells need to rapidly progress through the cell cycle and divide in the
correct direction along the tissue plane by undergoing oriented mitosis. A critical step in renal epithelial repair is
the activation of the master mitotic kinase cyclin B–CDK1 complex which drives the G2/M transition and prepares
the actin cytoskeleton for mitosis. Undergoing mitosis correctly in a confined tight epithelial space is a challenging
process, which requires the cells to round up against their neighbors. This so-called mitotic rounding depends
on the actin cytoskeleton forming a dense contractile actomyosin cortex, which is tethered to the cell membrane
by ERM Proteins (Ezrin), a known target of Rac1 and its main effector p21-activated kinase (Pak1). Defects in
mitotic morphology lead to cell cycle delays, mechanical defects, and cell death and abnormal repair. How actin
cytoskeletal dynamics are molecularly coordinated and by what means cell shape is connected to cell cycle
control during these critical biological processes in kidney repair is not known. We deleted Rac1 in the collecting
duct (AQP2Cre) or proximal tubule (γGTCre) and performed reversible unilateral ureteric obstruction (R-UUO)
or ischemia-reperfusion induced acute kidney injury (AKI-IRI). Functional and histological assessment indicated
that Rac1 is required for repair in both models. Optical clearing and 3D high-resolution imaging revealed that the
mutant epithelium was unable to restore normal morphology and actin organization post-injury. Repair in Rac1
mutants showed defects in actin-dependent mitotic morphology, misplaced mitotic figures, and the inability to
proliferate due to a G2/M cell cycle defect. 3D high-resolution live imaging of cell division in vitro revealed that
Rac1 is required for normal mechanical progression of mitosis and Ezrin-associated mitotic rounding in renal
epithelial cells. This forms the basis for our hypothesis that Rac1 controlled actin-dependent mitotic rounding
mediated via its effector Pak1 and the linker protein Ezrin is required to promote cell...

## Key facts

- **NIH application ID:** 10870056
- **Project number:** 5K08DK134879-02
- **Recipient organization:** VANDERBILT UNIVERSITY MEDICAL CENTER
- **Principal Investigator:** Fabian Maximilian Bock
- **Activity code:** K08 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2024
- **Award amount:** $158,544
- **Award type:** 5
- **Project period:** 2023-07-01 → 2028-05-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10870056

## Citation

> US National Institutes of Health, RePORTER application 10870056, Rac1 and the actin cytoskeleton in renal tubular repair (5K08DK134879-02). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/10870056. Licensed CC0.

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