SPATIAL AND PLANAR DYSREGULATION OF RHOGTPASES IN HUMAN AND ALZHEIMER’S  DISEASE MOUSE MODEL

Nik Akhtar, Shayan

SPATIAL AND PLANAR DYSREGULATION OF RHOGTPASES IN HUMAN AND ALZHEIMER’S DISEASE MOUSE MODEL

Date

2023-11-27

Access

2025-12-01

Authors

Nik Akhtar, Shayan

Publisher

East Carolina University

Abstract

Rho proteins belong to the Ras superfamily of small GTPases. In humans, the Rho family of GTPases contains 20 members out of which the most studied Rho proteins are RhoA, Rac1, and Cdc42. Rho signaling is known to regulate actin cytoskeleton dynamics in neuronal cells which are important for growth cone dynamics, axongenesis, neuronal polarity, synaptic plasticity, and neuronal survival. Rho GTPases are considered molecular switches due to their dual state of activation and inactivation. Briefly, in the inactive state, the GTPase is bound to Guanosine-5’- diphosphate (GDP) and in the active state, the GTPase is bound to Guanosine-5’-Triphosphate (GTP). The active and inactive forms of GTPase are controlled by three main types of proteins: Guanine nucleotide exchange factors (GEF), GTPase activating proteins (GAP), and Guanine nucleotide- dissociation inhibitors (GDI). GEFs promote inactive to active Rho GTPase. GAPs promote active to inactive Rho GTPase. The GDI does not allow the GDP to dissociate from the GTPase resulting in an inactive form of GTPase.A vast number of studies have implicated Rho-GTPases dysregulation in promoting neurodegenerative diseases such as Alzheimer’s disease (AD). RhoA GTPase and ROCK1, its downstream effector protein, increased in AD brains compared to non-AD brains. In Tg2575 AD mouse model, RhoA levels are reduced in the synaptic terminal while they were elevated in dystrophic neurites. Treatment of cultured neurons with beta-amyloid (Aβ) increased RhoA activity which leads to dendritic spine loss. Rac1 expression was shown to decrease in AD cortex compared to control. In 3xTg-AD mouse model, Rac1 levels were shown to be decreased by 7 months. Inhibition of Rac1 resulted in reduced expression of Amyloid Precursor Protein (APP) via decreasing the level of APP mRNA. Cdc42 was increased in frontal cortex in patients suffering from Frontotemporal dementia (FTLD) compared to control. Cdc42 conditional knockout mice demonstrated impaired synaptic plasticity. Cdc42 knockout neurons lead to abnormal axonal morphology. Aside from the studies mentioned there are numerous other studies highlighting the importance of Rho GTPase signaling homeostasis in neurodegeneration. Despite their importance it has been a challenge to elucidate the exact nature of dysregulation of Rho GTPases in AD. This can be attributed to several factors such as the model of choice, the specific region of the brain, as well as the different states of Rho GTPases, which is the focus of this study, such as expression vs activity. We hypothesize that there are different regional expressions of Rho-GTPases and their activation and inactivation states in the brain.In this study we have demonstrated that there is a regional and spatial dependent change in expression and activity for both RhoA and Rac/Cdc42 activity. We demonstrated that Rho GTPase activity changes in the different subregions of the hippocampus, such as CA3, CA2, CA1 and dentate gyrus, and cortex. We also demonstrated that there is a dynamic nuclear to cytoplasmic redistribution of both Rho and Rac/Cdc42 proteins in a plane dependent manner. In human brain sample Rho GTPases are also differentially expressed in the cortex and cytoplasm. Overall, we demonstrated that Rho GTPases are differentially dysregulated in both human AD brain samples and in 3xTg-AD mouse model.Modulation of Cdc42 through small molecules such as ZCL278, ZCL279 and ZCL367 in 3xTg AD mice showed an improvement in mental status as well as burrowing behavior. ZCL278 reduced tau in 3xTg-AD mice compared to nontreated 3xTg-AD mice. Knowing the potential of small molecules to modulate protein-protein interaction, they can be potentially used to target specific regions of the brain along the rostral to caudal dimensions to perturb a unique protein-protein interaction.