Effects of RhoA and Rac1 prenylation on Alzheimer's disease proteins

Alzheimer’s disease (AD) is a progressive neurodegenerative disease characterized by brain extracellular amyloid plaques and intracellular neurofibrillary tangles.  amyloid (Aβ) plaques are produced from the cleavage of amyloid precursor protein (APP) by β-amyloid cleavage enzyme (BACE1) and -secretase. Intracellular neurofibrillary tangles are formed from the hyperphosphorylation of tau accomplished by increased kinase and/or decreased phosphatase activity. Clinically, the cholesterol-depleting drugs, statins, which also decrease the isoprenoid production, are associated with decreased incidence of AD. Geranylgeranyltransferase I activity adds a 20-carbon geranylgeranyl group (a type of prenylation) to proteins and its activity decreases with age or AD. This enzyme prenylates Rho guanosine triphosphatases (GTPases), including RhoA and Rac1. Some evidence indicates that dysregulation of Rho GTPase or activation or prenylation is associated with altered AD pathological markers A or tau. Here, we investigated how altered prenylation of Rac1 or RhoA affects AD pathological makers. We hypothesized that overexpressing Rac1 or RhoA that cannot be prenylated would increase APP and tau by increasing their processing secretases and kinases. To address this, we overexpressed emerald green fluorescently protein (EmGFP)-tagged prenylatable (wild-type) Rac1 or RhoA or non-prenylatable Rac1 or RhoA and measured APP, A, BACE1, -secretase, tau, phosphorylated tau, and levels of one kinase that phohsphorylates tau, glycogen synthesis kinase 3 (GSK3β), and activation of BACE1. In whole cell lysates of B35 neuroblastoma cells, overexpressing prenylatable or non-prenylatable Rac1 did not alter APP, A BACE1, presenilin 2 (the active subunit of  secretase), or GSK3β levels. However, APP levels in membranes were decreased by overexpressing either prenylatable or non-prenylatable Rac1, with the latter decreasing membrane-associated APP less than overexpressed wild-type Rac1. Cells overexpressing prenylatable or non-prenylatable RhoA increased total levels of tau, but only overexpression of wild-type RhoA increased levels of tau phosphorylation. We interpret results showing increased Rac1 and decreased RhoA prenylation increases tau phosphorylation to indicate a possible contribution to neurofibrillary tangle formation. However, since altering Rac1 or RhoA prenylation did not affect Aβ formation and evidence indicates that Rho GTPases function both upstream and downstream of AD marker production, RhoA GTPase prenylation, regardless of activation, may result from AD marker production instead of synthesis regulation.