The paper that I chose to focus on is called “Farnesyltransferase Haplodeficiency Reduces Neuropathology and Rescues Cognitive Function in a Mouse Model of Alzheimer Disease.” This paper focuses on the extent that isoprenylation plays in the formation of amyloid-beta plaques in Alzheimer Disease (AD) and furthers knowledge from a paper that we discussed in class earlier. The authors were aware from previous studies that the use of statins reduced protein farnesylation and geranylgeranylation and therefore amyloid precursor protein (APP) metabolism through inhibition of HMG-CoA Reductase, however there was not much information about the relative importance of two proteins that perform the protein prenylation. These enzymes, called farnesyltransferase (FT) and geranylgeranyltransferase-1 (GGT), are involved in anchoring proteins to the membrane. This is important in AD because it effects the GTPase superfamily of Ras, Rab, and Rho proteins that are involved in APP metabolism. These GTPases are found in different parts of the brain and are involved in the signaling pathway to activate the processing of APP to make amyloid-beta (AB). The alternative splicing of APP to AB-42, a 42 amino acid protein, instead of AB-40, a 40 amino acid protein, has been associated with the physiological effects of AD. The accumulation of AB-42 on the surface of neurons eventually form plaques and increase phosphorylated Tau protein concentrations, which inhibits communication between neurons and causes neurodegeneration such as microgliosis.
To uncover more about the role of protein prenylation in AD, the researchers bred AD model mice to be haplodeficient for the gene encoding for either FT or GGT. Western blot analysis confirmed that the level of FT or GGT in the haplodeficient mice was lowered by ~50% as compared to WT AD mice. The authors also found that levels of AB-42 and AB-40 levels were decreased in the cerebrum through the analysis of brain lysates by ELISA (Figure 2). The same trend was seen in the hippocampal and cortical areas of the haplodeficient mice (Figure 3). To take these findings further, the authors went on to show that APP processing was trending towards the production of a non-amyloidogenic state in FT-haplodeficient mice. In other words, amyloid-alpha was being produced instead of amyloid beta. This is completed by an enzyme called a secretase. The version of the amyloid protein that is produced depends on the cleavage that is carried out by either alpha or beta-secretase. This produces small carboxyl (CTF) and amino terminal fragments (sAPP) that the authors could detect by immunoblot analysis. The data shows that in FT haplodeficient mice there is an increase of alpha-CTFs and sAPPs, which suggests that AB production is lowered since fewer beta fragments were seen after secretase splicing. There was no difference in the clearance of AB between WT and haplodeficient mice in data collected in a similar experiment targeting the apoE and LRP1 proteins. Neuroinflammation was also being influenced by less protein prenylation. Haplodeficient mice had lower levels of IBA-1 (a marker for microgliosis) and GFAP (a marker for astrocyte activiation), which are involved in neruoinflammation associated with AD. Next, what the authors referred to as a “battery of behavioral tests” were used to assess the implications of their work on a physiological level. Interestingly, the FT haplodeficient mice consistently performed as well as the WT mice, but the GGT deficient mice had the same performance as the AD mice with no gene knockouts. The authors go on to explain that since FT and GGT perform different roles in the progression of AD it is not surprising to see confounding results about the effect statins can have on AD. Targeting FT could prove to be an effective strategy for curing or treating patients with AD.