SAN FRANCISCO—Scientists at theGladstone Institutes of Neurological Disease (GIND) have made twoimportant breakthroughs in the study of Alzheimer's disease. On theone hand, they have added significantly to what researchers knowabout Apolipoprotein E4 (apoE4), the major genetic risk factor forAlzheimer's disease. Secondly, they suggest potential strategies totreat or prevent apoE4-related Alzheimer's disease.
Publishing their findings in TheJournal of Neuroscience, the Gladstone scientists note thatalthough biochemical, cell biological, transgenic animal and clinicalstudies have suggested potential explanations for apoE4'scontribution to the pathogenesis of Alzheimer's disease, itsunderlying mechanisms are still unclear—while at the same time,apoe4 is found in 65 to 80 percent of cases of sporadic and familialAlzheimer's disease, increases the occurrence and lowers the age ofonset of the disease in a gene dose-dependent manner. ApoE4 is alsoassociated with cognitive impairment in humans and causes learningand memory deficits in mice.
The neuropathological hallmarks ofAlzheimer's disease are—and much of current research focuseson—amyloid plaques, extracellular deposits of amyloid-beta peptidesand neurofibrillary tangles, which are intraneuronal filamentscomposed of hyperphosphorylated Tau. These amyloid plaques andneurofibrillary tangles colocalize with apoE4.
"We feel that apoE4 research, andAlzheimer's research in general, has been too focused on onetheory," says Dr. Yadong Huang, a senior author of the study. "ButAlzheimer's disease is a complex disease, with no single cause. Idon't think we need to put all of our eggs in one basket. We needto think about other possibilities.
The team at Gladstone previouslyreported that apoE4 impairs GABAergic interneuron in the hilus of thedentate gyrus in human apoE knock-in (KI) mice, likely due toincreased Tau phosphorylation mediated by neurotoxic apoE4 fragments.Interestingly, apoE4 is associated with subclinical epileptiformactivity under stress and increased brain activity at rest and inresponse to memory tasks in humans, probably reflecting impairedGABAergic inhibitory neuronal functions. Knowing that there isevidence of GABAergic interneuron impairment in Alzheimer's diseasepatients, including a decrease in GABA or somatostatin levels inbrains and CSF, the scientists hypothesized that apoE4 causeshippocampal GABAergic interneuron impairment, leading to learning andmemory deficits, and tested this hypothesis in different mouse modelsof apoE.
The Gladstone scientists found thatfemale apoE4 knock-in (KI) mice (which are more susceptible than malemice to apoE4-induced learning and memory deficits) had anage-dependent decrease in hilar GABAergic interneurons thatcorrelated with the extent of learning and memory deficits, asdetermined in the Morris water maze, in aged mice.
Treating apoE4-KI mice with dailyperitoneal injections of the GABAA receptor potentiator pentobarbitalat 20 mg/kg for four weeks rescued the learning and memory deficits.In neurotoxic apoE4 fragment transgenic mice, hilar GABAergicinterneuron loss was even more pronounced and also correlated withthe extent of learning and memory deficits. Neurodegeneration andtauopathy occurred earliest in hilar interneurons in apoE4 fragmenttransgenic mice; eliminating endogenous Tau prevented hilar GABAergicinterneuron loss and the learning and memory deficits. The GABAAreceptor antagonist picrotoxin abolished this rescue, whilepentobarbital rescued learning deficits in the presence of endogenousTau.
Thus, the researchers learned thatapoE4 causes age- and Tau-dependent impairment of hilar GABAergicinterneurons, leading to learning and memory deficits in mice. Thishighlights the importance of Tau in apoE4's contribution toAlzheimer's disease pathogenesis, Huang says.
"Here, Tau removal preventedGABAergic interneuron impairment and learning and memory deficitscaused by apoE4 fragments in transgenic mice, providing direct invivo evidence that apoE4 acts upstream of Tau in Alzheimer'sdisease pathogenesis," Huang says.
Consequently, the researchers foundthat reducing tau and enhancing GABA signaling are potentialstrategies to treat or prevent apoE4-related Alzheimer's disease.
"A-beta peptides may act upstream oftau, stimulating tau phosphorylation and neurofibrillary tangleformation in human mutant Tau transgenic mice overproducing orinjected with A-beta peptides," Huang says. "Knock-out of Taualso abolishes A-beta induced neurotoxicity in primary neuronalcultures and behavioral deficits in transgenic mice, suggesting thatTau is essential for A-beta-related detrimental effects both invitro and in vivo. Thus, acting downstream of apoE4 andA-beta peptides, Tau or p-Tau might be a general causative factor inAlzheimer's disease pathogenesis. Therefore, reducing Tau may be aneffective strategy to treat or prevent Alzheimer's disease."
Huang has received funding from Merckfor other research on apoE4 and its role in neurodegenerativedisorders, and Gladstone has a four-year research agreement withMerck to develop drugs based on apoE4 targets, but Huang says thatGladstone may or may not work with Merck on any commercialopportunities arising from the research.
The researchers noted in their studythat women with apoE4 have higher risk than men with apoE4 to developAlzheimer's disease, raising the question of whether male apoE4-KImice have a similar or less severe impairment of hilar GABAergicinterneurons. Experimentally addressing this question in futurestudies could help to understand better the gender difference indeveloping Alzheimer's disease in humans with apoE4, the team says.
"Clearly, further studies are neededto determine the potential contributions of other apoE4-relatedlosses of function or gains of negative function to GABAergicinterneuron impairment and learning and memory deficits," Huangsays.
The study, "Apolipoprotein E4 CausesAge- and Tau-Dependent Impairment of GABAergic Interneurons, Leadingto Learning and Memory Deficits in Mice," was published in theSept. 13 issue of The Journal of Neuroscience. The work wassupported in part by the J. David Gladstone Institutes and grantsfrom the National Institutes of Health.