APOE4, NELL2, and the Hyperexcitable Brain: A New Mechanistic Window into Alzheimer’s Risk

What if APOE4 pathology does not begin with amyloid, inflammation, or memory loss—but with something far earlier and far more fundamental: a shift in how the brain fires?

A 2026 study from Tabuena et al out of the Gladstone Institute of Neurological Disease points in exactly that direction. LINK: https://www.nature.com/articles/s43587-026-01096-0

This work suggests something upstream may be setting the stage long before any pathological processes become visible. A change in hippocampal excitability.

Not as a byproduct of disease, but as an early organizing feature of the APOE4 brain.

And at the center of that shift is a single molecule: NELL2.

In young APOE4 knock-in mice, the hippocampus is already different before any memory impairment appears. The change is not diffuse or global. It is precise and region-specific. CA3 pyramidal neurons and dentate gyrus granule cells are hyperexcitable, while CA1 is largely unaffected. These same regions also show early interictal spike activity, a biomarker of network instability.

This shifts the timeline of risk. The signal is present long before obvious changes.

At the cellular level, the explanation is not just increased firing. It is structural.

APOE4 neurons in vulnerable hippocampal regions are smaller. They show reduced membrane capacitance, increased input resistance, lower rheobase, and higher firing gain. In simple terms, they require less input to fire and respond more strongly when they do.

This links structure directly to function. A smaller neuron is electrically more excitable. The system is biased toward firing before synapses or circuits are even considered.

To understand what drives this early state, the authors performed single-nucleus RNA sequencing across hippocampal regions and ages, followed by stringent filtering and functional validation.

One gene consistently stood out across the vulnerable populations: NELL2 (Neural Epidermal Growth Factor-Like Protein 2).

It was the only candidate shared between CA3 pyramidal neurons and dentate gyrus granule cells, the same populations showing early hyperexcitability.

This is where this “two-part” study moves from association to intervention.

When NELL2 expression was reduced in APOE4 neurons using CRISPR interference, the hyperexcitability phenotype was reversed. Neurons became larger. Membrane capacitance increased. Firing thresholds normalized. Excitability decreased. These changes were not partial or marginal. They broadly restored the electrophysiological profile toward an APOE3-like state.

The effect was cell-autonomous. Only neurons in which NELL2 was reduced were rescued. Neighboring APOE4 neurons in the same tissue slice remained hyperexcitable. That detail matters. It means the mechanism is embedded within the intrinsic biology of the neuron, not driven solely by surrounding network activity. It also implicates the cytoplasmic variant of NELL2 rather than its secreted form.

And critically, NELL2 rescued both structure and function. Reducing NELL2 alone was sufficient to normalize neuronal size and excitability—placing it upstream of the structural atrophy, not merely downstream of it. This links morphology, electrophysiology, and gene expression into a single axis.

Taken together, the study outlines a mechanistic sequence: APOE4 expression in neurons shifts transcriptional programs, including increased NELL2. This is associated with reduced neuronal size, which increases intrinsic excitability. That heightened excitability contributes to early network hyperactivity, which later aligns with cognitive impairment.

Two independent manipulations interrupt this sequence. Removing neuronal APOE4 eliminates the phenotype. Reducing NELL2 alone is sufficient to rescue it.

Now, NELL2 is not a new molecule. It has been studied for over two decades, and the existing literature makes the Tabuena finding far more significant.

NELL2 is highly expressed in the hippocampus and dentate gyrus, specifically in glutamatergic neurons—the exact region and cell type the study implicates. NELL2-knockout mice show enhanced long-term potentiation in the dentate gyrus and impaired hippocampus-dependent spatial learning. NELL2 signals through ERK1/2, a pathway replicated across multiple tissue types and experimental models.

Importantly, the connection extends to human disease. Increased NELL2 protein has been observed in the prefrontal cortex of patients with Alzheimer’s disease across three independent cohorts, where it negatively correlates with cognitive function and positively correlates with amyloid levels. Elevated NELL2 has also been reported in the cerebrospinal fluid of patients with Alzheimer’s disease. A recent study of cognitively unimpaired individuals over age 65 identified NELL2 as one of five proteins whose cerebrospinal fluid levels significantly associated with both total tau and phosphorylated tau.

In other words: NELL2 already had a known role in dentate gyrus excitability and human AD pathology before anyone connected it to APOE4.

There is also a built-in counterbalance worth noting. A cytoplasmic splice variant of NELL2 (cNELL2), expressed primarily in astrocytes, inhibits PKCβ1—a kinase involved in inflammatory signaling. So the brain may have a cell-type-specific balancing act: neuronal NELL2 driving excitability, astrocytic cNELL2 dampening it. Whether APOE4 disrupts that balance is unknown, but it suggests the system is not simply “NELL2 = bad.”

One of the most important implications of this work is temporal.
The system becomes altered early. Long before amyloid accumulation, long before degeneration, and long before symptoms, hippocampal circuits are already operating in a more excitable state.

This reframes APOE4 risk not as a late-stage cascade, but as an early shift in neural dynamics that persists over time.

The study also reveals a two-phase progression.

In the early phase, excitatory neurons in CA3 and the dentate gyrus are intrinsically hyperexcitable, while inhibitory signaling remains largely intact. Network instability is driven primarily by excitatory neuron properties.

In the later phase, inhibitory dysfunction emerges. Interneuron loss in the hippocampal hilus reduces inhibitory input, shifting the system toward a higher excitation-to-inhibition ratio. At that point, the system is no longer just biased toward excitation. It is also losing its ability to constrain it.

The system goes from too much gas to…too much gas with failing brakes.

This two-phase model matters because it defines two different intervention windows: the early excitatory neuron pathology, driven by NELL2, and the later inhibitory failure, driven by interneuron loss.

Now, although the study did not examine hormones, I believe its framework changes how hormonal effects should be interpreted.

No sex hormone controls the APOE4 excitability set point. NELL2-driven hyperexcitability is established by neuronal APOE4 expression itself. But all major sex hormones enter this system and interact with it. The evidence suggests those interactions are not straightforward.

Estrogen directly regulates NELL2 transcription. It increases NELL2 expression in brain tissue. Ovariectomy reduces it. Estrogen replacement restores it. Estrogen receptors bind regulatory elements in the NELL2 promoter.

Separately, blocking NELL2 eliminates several of estrogen’s neuroprotective effects in hippocampal neurons, suggesting that NELL2 functions as a required downstream effector for a subset of estrogen’s actions in this circuit rather than a simple transcriptional target. In other words, estrogen signaling appears to depend on NELL2 to execute specific hippocampal protective programs. In an APOE4 brain where NELL2 is already elevated and associated with hyperexcitability, this creates a state-dependent tension: the same molecular node that is necessary for estrogen-linked neuroprotection may also sit within a system already biased toward increased excitability.

Testosterone is also relevant. In APOE4 carrier men, low free testosterone interacts with ε4 status to predict worse episodic memory and smaller hippocampal volume. But the relationship is not simple. One study found that higher free testosterone was associated with worse executive function specifically in ε4 carriers, while benefiting non-carriers. Testosterone also signals through ERK, the same pathway NELL2 activates.

Both estrogen and testosterone have downstream neurosteroid products that support inhibition. Progesterone is metabolized to allopregnanolone. Testosterone is converted to androstanediol. Both enhance GABA-A receptor function—the brain’s primary inhibitory brake. Both decline with aging, making them directly relevant to the second phase of the model, where inhibitory failure accelerates network instability.

The pattern across all three hormones is the same: each is broadly neuroprotective in many contexts, but each enters a system whose excitability baseline has already been set. The outcome appears to depend not on the hormone itself, but on the state of the brain receiving it.

The APOE4 excitability phenotype is established early—well before perimenopause or andropause. Hormonal transitions do not create the vulnerability; they interact with one that is already there, which may help explain heterogeneous outcomes in hormone therapy studies and why timing of intervention appears critical.

APOE4 carriers are known to show reduced brain DHA uptake and higher arachidonic acid levels. NELL2 signals through ERK, which can activate lipid-remodeling enzymes like cPLA2. This specific chain has not been tested in APOE4 models. But if it holds, the downstream effect would depend on what the membranes are made of. DHA-rich membranes yield pro-resolving mediators. AA-rich membranes yield pro-inflammatory ones.

In this framework, lipid changes are not initiating events. They are downstream expressions of a system whose electrical baseline has already shifted. But, we can influence our membrane-lipid composition, I believe. We cannot influence Nell2, in humans, yet.

What This Means for to me, as an APOE4 Carrier

This research does not offer a ready-made intervention. There is currently no supplement, drug, or clinical test that safely targets NELL2. But it provides a map of timing, mechanism, and leverage points.

The disease likely starts decades earlier than symptoms. Not as amyloid or memory loss, but as a subtly hyperexcitable hippocampal network—present in young brains, silent clinically, predictive of later decline.

Excitability is upstream—and modifiable through tractable systems. The framework suggests thinking in terms of preserving inhibitory tone and preventing chronic overexcitation:

- Sleep: deep sleep reduces hippocampal hyperactivity; sleep deprivation increases excitability and impairs inhibition

- Metabolic stability: stable glucose and insulin signaling reduce excitatory stress

- GABAergic support: progesterone (via allopregnanolone), testosterone (via androstanediol), magnesium, and other inhibitory modulators become more mechanistically relevant in this framework—not as “calming supplements” but as inputs to a system that is losing its braking capacity. I wrote an article about research suggesting the BALANCE of estrogen to progesterone is as important as either hormone on it’s own.

Understanding Hormonal Balance and Brain Health in Perimenopause: New Mechanistic Insights

- Lipid environment: if excitability is a relatively fixed input, membrane composition may shape the output—DHA-rich membranes shift downstream signaling toward resolution rather than inflammation

The biggest mindset shift: as an APOE4 carrier I am not trying to “prevent plaques.”. The goal is to keep my system from drifting into a self-amplifying unstable state—where overexcitation drives interneuron loss, which removes inhibition, which worsens overexcitation.

Prepare the terrain—optimize membrane lipids, support inhibitory tone, address metabolic and inflammatory load—and hormonal signals may become allies. Ignore the terrain, and the same signals may amplify the very processes APOE4 carriers need to prevent.


What emerges from this work is not a single pathway, but an organizing principle.

APOE4 shifts the brain toward a more excitable baseline state. NELL2 appears to be a validated molecular effector of that shift, at least in this model. Once that baseline is established, every downstream signal—metabolic, hormonal, inflammatory—acts on top of it.

Important note: The Tabuena study was conducted in mice. The CRISPRi and transcriptomic experiments used female mice; the electrophysiology and behavioral cohorts included both sexes. The study did not examine hormones or hormone therapy. The hormonal and lipid sections above connect NELL2 findings to independently established biology. This is my mechanistic synthesis, not Tabuena et al’s direct experimental conclusion. And as always, my articles are not medical advice. That are for educational purposes only!