TOKYO —

Millions of people carry APOE4, the strongest known genetic risk factor for Alzheimer’s disease. New research suggests the gene variant may begin altering brain activity well before memory problems become noticeable. Researchers at Gladstone Institutes have now mapped out a molecular sequence that could help explain these early effects, findings that also point to a possible way to reverse some of the detrimental changes. This groundbreaking study, published in the prestigious journal Nature Aging, offers a critical new perspective on the preclinical stages of Alzheimer’s and introduces Nell2 as a promising avenue for future therapeutic interventions.

The discovery represents a significant leap forward in understanding how a common genetic predisposition can subtly, yet profoundly, influence brain function years, even decades, before the clinical hallmarks of Alzheimer’s disease manifest. For individuals who carry the APOE4 variant, this research provides both a deeper understanding of their elevated risk and a beacon of hope for potential preventative or early-stage treatments.

APOE4: A Predominant Genetic Driver of Alzheimer’s Risk

The apolipoprotein E (APOE) gene plays a crucial role in transporting cholesterol and other lipids in the brain and body. It exists in three common forms: APOE2, APOE3, and APOE4. While APOE2 is associated with a reduced risk of Alzheimer’s disease, and APOE3 is considered neutral, APOE4 stands out as the most significant genetic risk factor. Approximately one in four people worldwide carry at least one copy of the APOE4 allele, and its prevalence dramatically increases among individuals diagnosed with Alzheimer’s, estimated to be present in 60 to 75 percent of cases.

Historically, research into APOE4’s role in Alzheimer’s has focused on its impact on amyloid-beta plaque and tau tangle accumulation, the hallmark pathological protein aggregates found in the brains of Alzheimer’s patients. However, the new research from Gladstone Institutes shifts the focus to the gene’s effects on neuronal function at a much earlier stage, suggesting that altered electrical activity in the brain may precede the widespread protein pathology.

Unveiling the Molecular Cascade: Nell2 and Neuronal Hyperactivity

The core of the new research lies in the identification of a specific molecular pathway initiated by APOE4 within neurons. Using sophisticated mouse models, the Gladstone team meticulously traced the consequences of APOE4’s presence on brain cells. Their findings revealed that APOE4 significantly increases the production of a protein known as Nell2. This elevation in Nell2 levels has a direct and observable impact on neuronal structure and function.

The study demonstrated that increased Nell2 levels lead to neurons becoming smaller in size and exhibiting unusually high levels of electrical activity, a phenomenon termed hyperactivity. Crucially, the researchers observed a direct correlation: mice exhibiting greater brain hyperactivity at a young age were more prone to developing severe memory deficits later in life. This establishes a clear link between early neuronal dysfunction and the eventual onset of cognitive decline characteristic of Alzheimer’s disease.

"To the best of our knowledge, this is the first study that has directly examined what APOE4 does to the function of neurons at different ages," stated Dr. Misha Zilberter, PhD, principal staff research scientist at Gladstone and a senior author of the study. "We found fundamental changes in brain circuits occurring in young mice that still had normal learning and memory, and importantly, that those changes predicted the development of cognitive deficits at older ages." This statement underscores the novelty of the research in pinpointing functional brain alterations at a preclinical stage.

The Timeline of Dysfunction: Early Changes, Later Consequences

The research provides a compelling chronological narrative of APOE4’s impact. Previous studies had hinted at increased brain activity in human APOE4 carriers before middle age, associating this early hyperactivity with later cognitive impairment. However, the precise cellular mechanisms and the reasons behind this connection remained elusive.

The Gladstone team’s work fills this critical knowledge gap. By analyzing brain activity recordings and individual neurons in young mice, they observed excessive neuronal firing in key memory-related areas of the hippocampus in APOE4-carrying mice. These findings are particularly striking because similar hyperactivity has been detected in the hippocampi of human APOE4 carriers before middle age.

"We found that the extent of hyperactivity in young mice predicted how poorly they performed on spatial learning and memory tests later in life," explained Dr. Dennis Tabuena, PhD, a scientist co-mentored by Zilberter and Huang, and the first author of the new paper. This predictive capacity of early neuronal hyperactivity is a significant discovery, offering a potential biomarker for Alzheimer’s risk.

Furthermore, the study revealed structural differences. Neurons in the affected hippocampal regions of APOE4 mice were smaller compared to their counterparts in mice carrying APOE3, a gene variant associated with lower Alzheimer’s risk. Smaller neurons are generally more susceptible to stimulation, increasing their likelihood of excessive firing. In contrast, while hippocampal neurons in APOE3 mice did eventually become more excitable, this shift occurred much later in their lifespan, suggesting that APOE4 significantly accelerates a process akin to normal aging.

"This suggests APOE4 accelerates a process that resembles normal aging, and could explain why people with the gene variant are more prone to develop Alzheimer’s disease earlier in life," commented Dr. Yadong Huang, MD, PhD, associate director of the Gladstone Institute of Neurological Disease and a senior author of the study. This acceleration of age-related changes is a critical insight into the increased vulnerability conferred by APOE4.

Neuronal Autonomy: APOE4’s Impact Within the Neuron

A prevailing hypothesis in Alzheimer’s research has been that APOE4’s detrimental effects are primarily mediated by astrocytes, a type of glial cell that supports neuronal function. This was largely due to the observation that astrocytes produce the majority of APOE in a healthy brain. However, the new Gladstone findings challenge this long-held assumption.

The research unequivocally demonstrates that the APOE4-driven hippocampal hyperactivity is almost entirely orchestrated by APOE4 produced within the neurons themselves. When the researchers experimentally removed the APOE4 gene from astrocytes in their mouse models, no significant changes in neuronal activity were observed. Conversely, deleting APOE4 from neurons led to their normalization in size and firing patterns.

"When we deleted the APOE4 gene from astrocytes, nothing changed," Dr. Zilberter elaborated. "But when we deleted it from neurons, the cells became larger and started functioning normally again." This crucial distinction highlights the direct role of APOE4 within neurons as the primary driver of early dysfunction, opening new avenues for targeted therapeutic strategies.

Nell2: A Pivotal Target for Intervention

With the molecular culprit identified—increased Nell2 production driven by APOE4 within neurons—the researchers then investigated whether targeting Nell2 could reverse these detrimental effects. Employing CRISPRi, a gene-editing technique that can lower gene activity without altering the underlying DNA sequence, they reduced Nell2 levels in hippocampal neurons of adult APOE4 mice.

The results were remarkably encouraging. Lowering Nell2 levels restored neurons to their normal size and reduced their excessive excitability. This unequivocally demonstrates that elevated Nell2 is the direct cause of the neuronal hyperactivity associated with APOE4.

While Nell2 had not been previously studied in direct connection with APOE4, earlier independent research had noted elevated levels of this protein in the brains of Alzheimer’s patients, with higher concentrations correlating with poorer cognitive function. This prior evidence lends further support to Nell2’s involvement in the disease process.

The potential therapeutic implications are substantial. "What’s exciting about Nell2 is that we were able to reverse the disease manifestations in adult mice by lowering its level," Dr. Huang emphasized. "That tells us the damage is not irreversible, and that there may be a window for intervention even after disease processes have been triggered." This suggests that therapies targeting Nell2 could potentially halt or even reverse early Alzheimer’s-related changes, offering a critical window for intervention.

Broader Implications and Future Directions

The implications of this research extend far beyond a deeper understanding of APOE4. It provides a tangible molecular target for the development of novel therapeutic agents. Future drug development could focus on inhibitors that specifically reduce Nell2 production or block its activity within neurons. Such interventions could be administered early in life to individuals identified as APOE4 carriers, potentially preventing the cascade of neuronal dysfunction that leads to Alzheimer’s.

This study also validates the importance of studying preclinical disease stages. By identifying and understanding the earliest cellular changes, scientists can develop more effective diagnostic tools and preventative strategies. The predictive power of early neuronal hyperactivity, as demonstrated in this research, could pave the way for early screening methods, allowing for timely interventions.

Furthermore, the research’s reliance on mouse models offers a robust platform for preclinical testing of potential Nell2-targeting therapies. The ability to reverse disease manifestations in adult mice by lowering Nell2 levels provides a strong rationale for advancing such approaches into human clinical trials.

"This study is a big breakthrough for the field of Alzheimer’s research," Dr. Huang concluded. "It opens the door to a better understanding of how APOE4 alters the function of neurons at a young age to increase risk of cognitive decline, and to the development of therapies that could block the detrimental effects of APOE4 early on."

The research was generously supported by grants from the National Institute on Aging (NIA), the National Institute of Neurological Disorders and Stroke (NINDS), and the National Center for Research Resources (NCRR). This collaborative effort highlights the significant investment and multidisciplinary approach required to tackle complex diseases like Alzheimer’s. As the scientific community digests these findings, the focus will undoubtedly shift towards translating this fundamental discovery into tangible clinical benefits for the millions at risk.