Linkage of Alzheimer’s gene to early synaptic loss provides evidence for a new focus in Alzheimer’s research and treatment.
In a groundbreaking new study, researchers from the Blanchette Rockefeller Neurosciences Institute (BRNI) have discovered that the Apolipoprotein E4 (ApoE4) gene, the major genetic risk factor for the vast majority of late, age-dependent Alzheimer’s patients, can reduce the number of mature, functional synapses in the brain by interfering with the DNA responsible for synapse formation and maintenance. Synaptic loss, a key element of Alzheimer’s disease, often occurs before the onset of amyloid plaques or tangles in Alzheimer’s patients. This new finding could potentially shift current thinking around Alzheimer’s disease—from treatment of the disease to prevention.
In the study published May 13 in the Journal of Neuroscience, scientists found that the ApoE4 gene increases nuclear translocation and activity of histone deacetylases (HDACs) in human neurons. (HDACs are enzymes that act like on/off buttons for genes.) This activity reduces levels of DNA-programmed brain-derived neurotrophic factor (BDNF), which is a critical component in the formation, repair and plasticity of synapses between brain cells. The loss of mature, functional synapses is a key element of early Alzheimer’s disease and its associated cognitive deficits.
“We know that people with the complete ApoE4 genes are 10 times more likely to suffer from the most common form of late, age-dependent Alzheimer’s disease. We also know from previous autopsy studies that Alzheimer’s patients have deficits of BDNF, Protein Kinase C (PKC) epsilon and synapses,” said Dr. Daniel Alkon, Scientific Director of BRNI. “Now, in the present study, the brains of Alzheimer’s patients were found to have increased levels of HDACs. These findings, taken together, suggest that substituting the abnormal ApoE4 gene for the ApoE3 gene is one of the earliest causes of synaptic loss in Alzheimer’s disease.”
A Shift in Thinking around Alzheimer’s
To date, every single late-phase clinical trial for Alzheimer’s drugs—even those that held promise in preclinical studies—has failed. The majority of these studies has focused on the pathologic hallmarks of brains of Alzheimer’s sufferers, particularly on sticky extracellular clumps of proteins and cellular debris known as “amyloid plaques,” and twisted intracellular tau fibers, often referred to as “tangles.”
“Our study provides evidence for a major shift in current thinking around Alzheimer’s disease and research,” said Dr. Alkon. “Synaptic loss often occurs before the onset of amyloid plaques or tangles in Alzheimer’s patients, so our latest findings suggest that many of today’s trials that only focus on plaques and tangles aren’t targeting a critical pathway responsible for early synaptic loss and, therefore, Alzheimer’s disease.”
A Pathway to Prevention
But there may be hope for prevention. BRNI has shown in pre-clinical studies that activating PKC with potent activators such as Bryostatin can prevent ApoE4 from inhibiting BDNF production. Bryostatin, by increasing PKC epsilon, has also been shown in previous pre-clinical studies to lower soluble A Beta oligomers that lead to plaque formation. Since A Beta oligomers also function like ApoE4 to interfere with DNA-controlled BDNF production through HDACs, Bryostatin could potentially block this A Beta oligomer effect as well. This would offer further Alzheimer’s disease prevention potential. Elevated HDACs, lower PKC epsilon, reduced BDNF and increased A Beta oligomers, working together, compromise synaptic function, growth and maintenance in the absence of amyloid plaques and tangles.
Discoveries from the present study suggest that these same PKC activators in trials to treat Alzheimer’s disease patients could potentially be given to healthy individuals who have the ApoE4 genes – even before Alzheimer’s disease begins – thereby preventing the onset of debilitating dementia and brain degeneration.
“We are excited and encouraged by these results,” said Alkon. “In essence, our findings suggest that Bryostatin could be used in some patients to prevent Alzheimer’s disease before it ever begins.”
This neuroscience article was submitted to NeuroscienceNews.com by Alissa Momberg Lawver. We would like to thank Alissa for her contribution.
Source: Alissa Momberg Lawver – Advocom Group
Image Credit: The image is credited to ADEAR/NIh and is in the public domain
Original Research: Abstract for “ApoE4 and Aβ Oligomers Reduce BDNF Expression via HDAC Nuclear Translocation” by Abhik Sen, Thomas J. Nelson, and Daniel L. Alkon in Journal of Neuroscience. Published online May 13 2015 doi:10.1523/JNEUROSCI.0260-15.2015
Abstract
ApoE4 and Aβ Oligomers Reduce BDNF Expression via HDAC Nuclear Translocation
Apolipoprotein E4 (ApoE4) is a major genetic risk factor for several neurodegenerative disorders, including Alzheimer’s disease (AD). Epigenetic dysregulation, including aberrations in histone acetylation, is also associated with AD. We show here for the first time that ApoE4 increases nuclear translocation of histone deacetylases (HDACs) in human neurons, thereby reducing BDNF expression, whereas ApoE3 increases histone 3 acetylation and upregulates BDNF expression. Amyloid-β (Aβ) oligomers, which have been implicated in AD, caused effects similar to ApoE4. Blocking low-density lipoprotein receptor-related protein 1 (LRP-1) receptor with receptor-associated protein (RAP) or LRP-1 siRNA abolished the ApoE effects. ApoE3 also induced expression of protein kinase C ε (PKCε) and PKCε retained HDACs in the cytosol. PKCε activation and ApoE3 supplementation prevented ApoE4-mediated BDNF downregulation. PKCε activation also reversed Aβ oligomer- and ApoE4-induced nuclear import of HDACs, preventing the loss in BDNF. ApoE4 induced HDAC6–BDNF promoter IV binding, which reduced BDNF exon IV expression. Nuclear HDAC4 and HDAC6 were more abundant in the hippocampus of ApoE4 transgenic mice than in ApoE3 transgenic mice or wild-type controls. Nuclear translocation of HDA6 was also elevated in the hippocampus of AD patients compared with age-matched controls. These results provide new insight into the cause of synaptic loss that is the most important pathologic correlate of cognitive deficits in AD.
“ApoE4 and Aβ Oligomers Reduce BDNF Expression via HDAC Nuclear Translocation” by Abhik Sen, Thomas J. Nelson, and Daniel L. Alkon in Journal of Neuroscience. Published online May 13 2015 doi:10.1523/JNEUROSCI.0260-15.2015
