A new MIT study finds that Alzheimer’s disease disrupts at least one form of visual memory by degrading a newly identified circuit that connects the vision processing centers of each brain hemisphere.

The results of the study, published in Neuron by a research team based at The Picower Institute for Learning and Memory, come from experiments in mice, but provide a physiological and mechanistic basis for prior observations in human patients: the degree of diminished brain rhythm synchrony between counterpart regions in each hemisphere correlates with the clinical severity of dementia.

“We demonstrate that there is a functional circuit that can explain this phenomenon,” said lead author Chinnakkaruppan Adaikkan, a former Picower Institute postdoc who is now an assistant professor in the Centre for Brain Research at the Indian Institute of Science (IISc) in Bangalore. “In a way we uncovered a fundamental biology that was not known before.”

Specifically, Adaikkan’s work identified neurons that connect the primary visual cortex (V1) of each hemisphere and showed that when the cells are disrupted, either by genetic alterations that model Alzheimer’s disease or by direct laboratory perturbations, brain rhythm synchrony becomes reduced and mice become significantly less able to notice when a new pattern appeared on a wall in their enclosures. Such recognition of novelty, which requires visual memory of what was there the prior day, is an ability commonly disrupted in Alzheimer’s.

“This study demonstrates the propagation of gamma rhythm synchrony across the brain hemispheres via the cross hemispheric connectivity,” said study senior author Li-Huei Tsai, Picower Professor and director of The Picower Institute and MIT’s Aging Brain Initiative. “It also demonstrates that the disruption of this circuit in AD mouse models is associated with specific behavioral deficits.”

Cross-hemispheric cells

In the study, Adaikkan, Tsai, Thomas McHugh and co-authors discovered and traced V1 neurons that extended their axons all the way through the corpus callosum, which connects the brain’s hemispheres, to cells in the V1 on the brain’s other side. There, they found, the cross-hemispheric (CH) neurons forged connections, or synapses, with target cells, providing them with “excitatory” stimulation to drive their activity. Adaikkan also found that CH neurons were much more likely to be activated by a novelty discrimination task than V1 neurons in general or neurons in other regions heavily involved in memory such as the hippocampus or the prefrontal cortex.

Curious about how this might differ in Alzheimer’s disease, the team looked at the activity of the cells in two different Alzheimer’s mouse models. The found that CH cell activity was significantly lessened amid the disease. Unsurprisingly, Alzheimer’s mice fared much poorer in novelty discrimination tasks.

The team examined the CH cells closely and found that they gather incoming input from a variety of other cells within their V1 and other regions in their hemisphere that process visual information. When they compared the incoming connections of healthy CH neurons to those in CH cells afflicted with Alzheimer’s, they found that cells in the disease condition had significantly less infrastructure for hosting incoming connections (measured in terms of synapse-hosting spines protruding from the vine-like dendrites that sprawl out of the cell body).

Given the observations correlating reduced brain rhythm synchrony and memory performance in Alzheimer’s, the team wondered if that occurred in the mice, too. To find out, they custom-designed electrodes to measure rhythmic activity simultaneously in all cortical layers of each hemisphere’s V1. They observed that cross-hemispheric synchrony increased notably between the V1s when mice engaged in novelty discrimination but that the synchrony, both at high “gamma” and lower “theta” frequency rhythms, was significantly lower in the Alzheimer’s mice than it was in healthy mice.

Adaikkan’s evidence at that point was strong, but still only suggestive, that CH neurons provided the means by which the V1 regions on each side of the brain could coordinate to enable novelty discrimination, and that this ability became undermined by Alzheimer’s degradation of the CH cells’ connectivity. To more directly determine whether the CH circuit played such a causal, consequential role, the team directly intervened to disrupt them, testing what effect targeted perturbations had.

They found that chemically inhibiting CH cells disrupted rhythm synchrony between V1s, mirroring measures made in Alzheimer’s model mice. Moreover, disrupting CH activity undermined novelty discrimination ability. To further test whether it was the cells’ cross-hemispheric nature that mattered specifically, they engineered CH cells to be controllable with flashes of light (a technology called “optogenetics”). When they shined the light on the connections, they forged in the other hemisphere to inhibit those, they found that doing so again compromised visual discrimination ability.

All together, the study results show that CH cells in V1 connect with neurons in the counterpart area of the opposite hemisphere to synchronize neural activity needed for properly recognizing novelty, but that Alzheimer’s disease damages their ability to do that job.

Adaikkan said he is curious to now look at other potential cross-hemispheric connections and how they may be affected in Alzheimer’s disease, too. He said he also wants to study what happens to synchrony at other rhythm frequencies.

In addition to Adaikkan and Tsai, the study’s other authors are Jun Wang, Karim Abdelall, Steven Middleton, Lorenzo Bozzelli, and Ian Wickersham.

The JPB Foundation, The National Institutes of Health, and the Robert A. and Renee E. Belfer Foundation provided funding for the study.

One of the hallmarks of Alzheimer’s disease is a reduction in the firing of some neurons in the brain, which contributes to the cognitive decline that patients experience. A new study from MIT shows how a type of cells called microglia contribute to this slowdown of neuron activity.

The study found that microglia that express the APOE4 gene, one of the strongest genetic risk factors for Alzheimer’s disease, cannot metabolize lipids normally. This leads to a buildup of excess lipids that interferes with nearby neurons’ ability to communicate with each other.

“APOE4 is a major genetic risk factor, and many people carry it, so the hope is that by studying APOE4, that will also provide a bigger picture of the fundamental pathophysiology of Alzheimer’s disease and what fundamental cell processes that have to go wrong to result in Alzheimer’s disease,” says Li-Huei Tsai, director of MIT’s Picower Institute for Learning and Memory and the senior author of the study.

The findings suggest that if researchers could find a way to restore normal lipid metabolism in microglia, that might help to treat some of the symptoms of the disease.

MIT postdoc Matheus Victor is the lead author of the paper, which appears today in Cell Stem Cell.

Lipid overload

About 14 percent of the population has the APOE4 variant, making it the most common genetic variant that has been linked to late-onset, nonfamilial Alzheimer’s disease. People who carry one copy of APOE4 have a threefold higher risk of developing Alzheimer’s, and people with two copies have a tenfold higher risk.

“If you look at this another way, if you look at the entire Alzheimer’s disease population, about 50 percent of them are APOE4 carriers. So, it’s a very significant risk, but we still don’t know why this APOE4 allele presents such a risk,” Tsai says.

The APOE gene also comes in two other forms, known as APOE2, which is considered protective against Alzheimer’s, and the most common form, APOE3, which is considered neutral. APOE3 and APOE4 differ by just one amino acid.

For several years, Tsai’s lab has been studying the effects of APOE4 on a variety of cell types in the brain. To do this, the researchers use induced pluripotent stem cells, derived from human donors, and engineer them to express a specific version of the APOE gene. These cells can then be stimulated to differentiate into brain cells, including neurons, microglia, and astrocytes.

In a 2018 study, they showed that APOE4 causes neurons to produce large quantities of amyloid beta peptide 42, an Alzheimer’s-linked molecule that causes the neurons to become hyperactive. That study found that APOE4 also affects the functions of microglia and astrocytes, leading to cholesterol accumulation, inflammation, and failure to clear amyloid beta peptides.

A 2021 follow-up showed that APOE4 astrocytes have dramatic impairments in their ability to process a variety of lipids, which leads to a buildup of molecules such as triglycerides, as well as cholesterol. In that paper, the researchers also showed that treating engineered yeast cells expressing APOE4 with choline, a dietary supplement that is a building block for phospholipids, could reverse many of the detrimental effects of APOE4.

In their new study, the researchers wanted to investigate how APOE4 affects interactions between microglia and neurons. Recent research has shown that microglia play an important role in modulating neuronal activity, including their ability to communicate within neural ensembles. Microglia also scavenge the brain looking for signs of damage or pathogens, and clear out debris.

The researchers found that APOE4 disrupts microglia’s ability to metabolize lipids and prevents them from removing lipids from their environment. This leads to a buildup of fatty molecules, especially cholesterol, in the environment. These fatty molecules bind to a specific type of potassium channel embedded in neuron cell membranes, which suppresses neuron firing.

“We know that in late stages of Alzheimer’s disease, there is reduced neuron excitability, so we may be mimicking that with this model,” Victor says.

The buildup of lipids in microglia can also lead to inflammation, the researchers found, and this type of inflammation is believed to contribute to the progression of Alzheimer’s disease.

Restoring function

The researchers also showed that they could reverse the effects of lipid overload by treating APOE4 microglia with a drug called Triacsin C, which interferes with the formation of lipid droplets. When APOE4 microglia were exposed to this drug, the researchers found that normal communication between neighboring neurons and microglia was restored.

“We can rescue the suppression of neuronal activity by APOE4 microglia, presumably through lipid homeostasis being restored, where now fatty acids are not accumulating extracellularly,” Victor says.

Triacsin C can be toxic to cells, so it would likely not be suitable to use as a drug to treat Alzheimer’s, but the researchers hope that other approaches to restore lipid homeostasis could help combat the disease. In Tsai’s 2021 APOE4 study, she showed that choline also helps to restore normal microglia activity.

“Lipid homeostasis is actually critical for a number of cell types across the Alzheimer’s disease brain, so it’s not singularly a microglia problem,” Victor says. “The question is, how do you restore lipid homeostasis across multiple cell types? It’s not an easy task, but we’re tackling that through choline, for example, which might be a really interesting angle.”

The researchers are now further studying how microglia transition from a healthy state to a “lipid-burdened,” inflammatory state, in hopes of discovering ways to block that transition. In previous studies in mice, they have shown that exposure to LED light flickering at a specific frequency can help to rejuvenate microglia, stimulating the cells to resume their normal functions.

The research was funded by the National Institutes of Health, the Howard Hughes Medical Institute Hanna H. Gray Postdoctoral Fellowship, The Robert A. and Renee E. Belfer Family Foundation, Carol and Eugene Ludwig Family Foundation, the Cure Alzheimer’s Fund, The JPB Foundation, Joseph P. DiSabato and Nancy E. Sakamoto, Donald A. and Glenda G. Mattes, Lester A. Gimpelson, the Halis Family Foundation, the Dolby family, David Emmes, and Alan and Susan Patricof.

–From MIT News

The urgency to remember a dangerous experience requires the brain to make a series of potentially dangerous moves: Neurons and other brain cells snap open their DNA in numerous locations—more than previously realized, according to a new study—to provide quick access to genetic instructions for the mechanisms of memory storage.

The extent of these DNA double-strand breaks (DSBs) in multiple key brain regions is surprising and concerning, said study senior author Li-Huei Tsai, Picower Professor of Neuroscience at MIT and director of The Picower Institute for Learning and Memory, because while the breaks are routinely repaired, that process may become more flawed and fragile with age. Tsai’s lab has shown that lingering DSBs are associated with neurodegeneration and cognitive decline and that repair mechanisms can falter.

“We wanted to understand exactly how widespread and extensive this natural activity is in the brain upon memory formation because that can give us insight into how genomic instability could undermine brain health down the road,” said Tsai, who is also a professor in the Department of Brain and Cognitive Sciences and a leader of MIT’s Aging Brain Initiative. “Clearly memory formation is an urgent priority for healthy brain function but these new results showing that several types of brain cells break their DNA in so many places to quickly express genes is still striking.”

Tracking breaks

In 2015, Tsai’s lab provided the first demonstration that neuronal activity caused DSBs and that they induced rapid gene expression. But those findings, mostly made in lab preparations of neurons, did not capture the full extent of the activity in the context of memory formation in a behaving animal and did not investigate what happened in cells other than neurons.

IIn the new study published July 1 in PLOS ONE, lead author and former graduate student Ryan Stott and co-author and former research technician Oleg Kritsky sought to investigate the full landscape of DSB activity in learning and memory. To do so, they gave mice little electrical zaps to the feet when they entered a box, to condition a fear memory of that context. They then used several methods to assess DSBs and gene expression in the brains of the mice over the next half hour, particularly among a variety of cell types in the prefrontal cortex and hippocampus, two regions essential for the formation and storage of conditioned fear memories. They also made measurements in the brains of mice who did not experience the foot shock to establish a baseline of activity for comparison.

The creation of a fear memory doubled the number of DSBs among neurons in the hippocampus and the prefrontal cortex, affecting more than 300 genes in each region. Among 206 affected genes common to both regions, the researchers then looked at what those genes do. Many were associated with the function of the connections neurons make with each other, called synapses. This makes sense because learning arises when neurons change their connections (a phenomenon called “synaptic plasticity”) and memories are formed when groups of neurons connect together into ensembles called engrams.

“Many genes essential for neuronal function and memory formation, and significantly more of them than expected based on previous observations in cultured neurons…are potentially hotspots of DSB formation,” the authors wrote in the study.

In another analysis, the researchers confirmed through measurements of RNA that the increase in DSBs indeed correlated closely with increased transcription and expression of affected genes, including ones affecting synapse function, as quickly as 10-30 minutes after the foot shock exposure.

“Overall, we find transcriptional changes are more strongly associated with [DSBs] in the brain than anticipated,” they wrote. “Previously we observed 20 gene-associated [DSB] loci following stimulation of cultured neurons, while in the hippocampus and prefrontal cortex we see more than 100-150 gene associated [DSB] loci that are transcriptionally induced.”

Snapping with stress

In the analysis of gene expression, the neuroscientists looked at not only neurons but also non-neuronal brain cells, or glia, and found that they also showed changes in expression of hundreds of genes after fear conditioning. Glia called astrocytes are known to be involved in fear learning, for instance, and they showed significant DSB and gene expression changes after fear conditioning.

Among the most important functions of genes associated with fear conditioning-related DSBs in glia was the response to hormones. The researchers therefore looked to see which hormones might be particularly involved and discovered that it was glutocortocoids, which are secreted in response to stress. Sure enough, the study data showed that in glia, many of the DSBs that occurred following fear conditioning occurred at genomic sites related to glutocortocoid receptors. Further tests revealed that directly stimulating those hormone receptors could trigger the same DSBs that fear conditioning did and that blocking the receptors could prevent transcription of key genes after fear conditioning.

Tsai said the finding that glia are so deeply involved in establishing memories from fear conditioning is an important surprise of the new study.

“The ability of glia to mount a robust transcriptional response to glutocorticoids suggest that glia may have a much larger role to play in the response to stress and its impact on the brain during learning than previously appreciated,” she and her co-authors wrote.

Damage and danger?

More research will have to be done to prove that the DSBs required for forming and storing fear memories are a threat to later brain health, but the new study only adds to evidence that it may be the case, the authors said.

“Overall we have identified sites of DSBs at genes important for neuronal and glial functions, suggesting that impaired DNA repair of these recurrent DNA breaks which are generated as part of brain activity could result in genomic instability that contribute to aging and disease in the brain,” they wrote.

The National Institutes of Health, The Glenn Foundation for Medical Research and the JPB Foundation provided funding for the research.