Neuronal Protein Farnesylation Regulates Hippocampal Synaptic Plasticity and Cognitive Function

Protein prenylation is a post-translational lipid modification that governs a variety of important cellular signaling pathways, including those regulating synaptic functions and cognition in the nervous system. Two enzymes, farnesyltransferase (FT) and geranylgeranyltransferase type I (GGT), are essential for the prenylation process. Genetic reduction of FT or GGT ameliorates neuropathology but only FT haplodeficiency rescues cognitive function in transgenic mice of Alzheimer's disease. A follow-up study showed that systemic or forebrain neuron-specific deficiency of GGT leads to synaptic and cognitive deficits under physiological conditions. Whether FT plays different roles in shaping neuronal functions and cognition remains elusive. This study shows that in contrast to the detrimental effects of GGT reduction, systemic haplodeficiency of FT has little to no impact on hippocampal synaptic plasticity and cognition. However, forebrain neuron-specific FT deletion also leads to reduced synaptic plasticity, memory retention, and hippocampal dendritic spine density. Furthermore, a novel prenylomic analysis identifies distinct pools of prenylated proteins that are affected in the brain of forebrain neuron-specific FT and GGT knockout mice, respectively. Taken together, this study uncovers that physiological levels of FT and GGT in neurons are essential for normal synaptic/cognitive functions and that the prenylation status of specific signaling molecules regulates neuronal functions.

Systemic or Forebrain Neuron-Specific Deficiency of Geranylgeranyltransferase-1 Impairs Synaptic Plasticity and Reduces Dendritic Spine Density

Isoprenoids and prenylated proteins regulate a variety of cellular functions, including neurite growth and synaptic plasticity. Importantly, they are implicated in the pathogenesis of several diseases, including Alzheimer's disease (AD). Recently, we have shown that two protein prenyltransferases, farnesyltransferase (FT) and geranylgeranyltransferase-1 (GGT), have differential effects in a mouse model of AD. Haplodeficiency of either FT or GGT attenuates amyloid-β deposition and neuroinflammation but only reduction in FT rescues cognitive function. The current study aimed to elucidate the potential mechanisms that may account for the lack of cognitive benefit in GGT-haplodeficient mice, despite attenuated neuropathology. The results showed that the magnitude of long-term potentiation (LTP) was markedly suppressed in hippocampal slices from GGT-haplodeficient mice. Consistent with the synaptic dysfunction, there was a significant decrease in cortical spine density and cognitive function in GGT-haplodeficient mice. To further study the neuron-specific effects of GGT deficiency, we generated conditional forebrain neuron-specific GGT-knockout (GGT^f/fCre+) mice using a Cre/LoxP system under the CAMKIIα promoter. We found that both the magnitude of hippocampal LTP and the dendritic spine density of cortical neurons were decreased in GGT^f/fCre+ mice compared with GGT^f/fCre- mice. Immunoblot analyses of cerebral lysate showed a significant reduction in cell membrane-associated (geranylgeranylated) Rac1 and RhoA but not (farnesylated) H-Ras, in GGT^f/fCre+ mice, suggesting that insufficient geranylgeranylation of the Rho family of small GTPases may underlie the detrimental effects of GGT deficiency. These findings reinforce the critical role of GGT in maintaining spine structure and synaptic/cognitive function in development and in the mature brain.