Impaired geranylgeranyltransferase‐I regulation reduces membrane‐associated Rho protein levels in aged mouse brain

Synaptic impairment rather than neuronal loss may be the leading cause of cognitive dysfunction in brain aging. Certain small Rho‐GTPases are involved in synaptic plasticity, and their dysfunction is associated with brain aging and neurodegeneration. Rho‐GTPases undergo prenylation by attachment of geranylgeranylpyrophosphate (GGPP) catalyzed by GGTase‐I. We examined age‐related changes in the abundance of Rho and Rab proteins in membrane and cytosolic fractions as well as of GGTase‐I in brain tissue of 3‐ and 23‐month‐old C57BL/6 mice. We report a shift in the cellular localization of Rho‐GTPases toward reduced levels of membrane‐associated and enhanced cytosolic levels of those proteins in aged mouse brain as compared with younger mice. The age‐related reduction in membrane‐associated Rho proteins was associated with a reduction in GGTase‐Iβ levels that regulates binding of GGPP to Rho‐GTPases. Proteins prenylated by GGTase‐II were not reduced in aged brain indicating a specific targeting of GGTase‐I in the aged brain. Inhibition of GGTase‐I in vitro modeled the effects of aging we observed in vivo. We demonstrate for the first time a decrease in membrane‐associated Rho proteins in aged brain in association with down‐regulation of GGTase‐Iβ. This down‐regulation could be one of the mechanisms causing age‐related weakening of synaptic plasticity. Rho‐GTPases are geranylgeranylated by transferase GGTase‐I. Their prenylation is essential for their localization in membranes, the site of their activation and function. Despite elevated GGPP levels in brains of aged (23 months) mice compared to younger (3 months) mice as well as in GGTI‐2133‐treated SH‐SY5Y cells, the amount of total (homogenate) Rho‐GTPases (Rac1, RhoA, and Cdc42) was unchanged. Treatment with the GGTaseI‐inhibitor GGTI‐2133 decreased prenylation of Rho‐GTPases in membrane preparations of aged mice and SH‐SY5Y, correlating with the reduction of relative GGTase activity, GGTaseIß protein, and mRNA levels. As Rac1, RhoA, and Cdc42 are associated with synaptogenesis, we examined the synaptic marker proteins GAP43 and synaptophysin. GAP43 and synaptophysin declined in an age‐related manner in the mouse brain and were also reduced in our in vitro model. Faulty regulation of Rho proteins in aged brain is associated with a specific deficit in GGTase‐Iβ, which could contribute to age‐related deficits in neuronal outgrowth. Rho‐GTPases are geranylgeranylated by transferase GGTase‐I. Their prenylation i