# Overactivation of Cdc42 GTPase Impairs the Cytotoxic Function of NK Cells From Old Individuals Towards Senescent Fibroblasts

## TL;DR

Old NK cells fail to kill senescent fibroblasts due to overactive Cdc42, which can be partially reversed with a drug called CASIN.

## Contribution

Identifies overactivation of Cdc42 GTPase as a novel mechanism for impaired NK cell function in aging.

## Key findings

- Aged NK cells show reduced perforin and granzyme B release due to disrupted microtubular organization.
- Cdc42 overactivation in aged NK cells is linked to impaired mitochondrial ATP production.
- CASIN treatment restores NK cell cytotoxicity by rebalancing Cdc42 activity.

## Abstract

Senescent fibroblasts accumulate in the connective tissue of all organs and promote organ aging and aging‐related diseases. The underlying mechanisms for the accumulation of senescent fibroblasts are poorly understood. Natural killer (NK) cells of innate immunity play a critical role in the removal of tissue resident senescent cells. We here show that NK cells from old adults and old mice fail to efficiently remove senescent fibroblasts. This is due to severely reduced perforin and granzyme B release from aged NK cells where perforin is responsible for inducing holes in the membrane of senescent fibroblasts through which granzyme B enters enforcing cell death of senescent fibroblasts. We demonstrate elevated activation of the small Cdc42 Rho GTPase in aged NK cells to be responsible for the disruption of the microtubular organization which is essential for the proper release of perforin and granzyme B and for energy homeostasis. Attenuation of the elevated activity of Cdc42 in aged human NK cells with CASIN, a small molecule Cdc42 inhibitor, rebalances Cdc42 activity to a young level. Rebalancing of Cdc42 restores proper perforin and granzyme B release and attenuates reduced ATP levels in aged NK cells resulting in an attenuated “youthful” cytotoxicity of aged NK cells against senescent cells. Collectively, we identified a previously unreported molecular mechanism underlying functional impairment of NK cells from older adults. In perspective, our data hold promise to develop novel strategies against age‐related disorders driven by tissue‐resident senescent fibroblasts.

Unrestrained Cdc42 activity causes failure of cytotoxic function of old NK cells. This is associated with disruption of microtubular network and impaired mitochondrial ATP resulting in reduced conjugation and impaired degranulation of lytic vesicles. Pharmacological inhibition of Cdc42 by CASIN attenuates all these and in part rejuvenates the cytotoxicity.