Increased RhoA Prenylation in the loechrig (loe) Mutant Leads to Progressive Neurodegeneration

Abstract

The Drosophila mutant loechrig (loe) shows age-dependent degeneration of the nervous system and is caused by the loss of a neuronal isoform of the AMP-activated protein kinase (AMPK) γ-subunit (also known as SNF4Aγ). The trimeric AMPK complex is activated by low energy levels and metabolic insults and regulates multiple important signal pathways that control cell metabolism. A well-known downstream target of AMPK is hydroxyl-methylglutaryl-CoA reductase (HMGR), a key enzyme in isoprenoid synthesis, and we have previously shown that HMGR genetically interacts with loe and affects the severity of the degenerative phenotype. Prenylation of proteins like small G-proteins is an important posttranslational modification providing lipid moieties that allow the association of these proteins with membranes, thereby facilitating their subsequent activation. Rho proteins have been extensively studied in neuronal outgrowth, however, much less is known about their function in neuronal maintenance. Here we show that the loe mutation interferes with isoprenoid synthesis, leading to increased prenylation of the small GTPase Rho1, the fly orthologue of vertebrate RhoA. We also demonstrate that increased prenylation and Rho1 activity causes neurodegeneration and aggravates the behavioral and degenerative phenotypes of loe. Because we cannot detect defects in the development of the central nervous system in loe, this suggests that loe only interferes with the function of the RhoA pathway in maintaining neuronal integrity during adulthood. In addition, our results show that alterations in isoprenoids can result in progressive neurodegeneration, supporting findings in vertebrates that prenylation may play a role in neurodegenerative diseases like Alzheimer’s Disease.

Drosophila Lung Cancer Models Identify Trametinib plus Statin as Candidate Therapeutic

“…5A), we reduced activity of Drosophila orthologs of Farnesyl Pyrophosphate Synthase (FPPS) and geranylgeranyl diphosphate synthase (GGPS) and examined toxic levels of trametinib or everolimus. The FPPS mutant fpps k03514 , previously demonstrated to decrease levels of Rho prenylation (Cook et al 2012), was strongly insensitive to trametinib toxicity and mildly insensitive to everolimus toxicity (Fig. 7B,C).…”

Increased Actin Polymerization and Stabilization Interferes with Neuronal Function and Survival in the AMPKγ Mutant Loechrig

“…We previously showed that loe mutant flies show increased prenylation and genetically interact with RhoA, whereby reducing RhoA levels suppressed loe -induced phenotypes [14]. Rho GTPases are key regulators of the actin network through their downstream targets PAK and ROCK [17].…”

“…Well-known targets of isoprenylation are small GTPases, like Ras, Rab, Rac, and Rho, which through the addition of these lipid moieties can associate with membranes and subsequently be activated [13]. The connection between loe and the isoprenoid pathway was confirmed by experiments showing that altering isoprenoid levels, either genetically or by pharmacological means, affected the severity of loe associated phenotypes and loe flies show increased levels of isoprenylated RhoA [14]. In addition, increased RhoA levels enhanced the degeneration observed in loe , whereas decreased amounts suppressed it.…”

The Role of AMPK in Drosophila melanogaster

“…This reduced AMPK function allows adult flies to develop past the earlier lethality found in AMPK-null mutations, but results in a severe vacuolar pathology and widespread neuronal death in the adult fly brain (Kazgan et al 2010; Tschape et al 2002). More recently, Kretzschmar and colleagues have demonstrated that loe mutants have increased prenylation of Rho1 (Rho1 is orthologous to mammalian RhoA) (Cook et al 2012). Although loechrig does not model the etiology of AD, Cook et al stress the translational value of their research by discussing the link between activated RhoA/Rho-associated protein kinase (ROCK) signaling and AD (Cook et al 2012).…”