A molecular map of the locus coeruleus, the brain region where tau protein begins accumulating decades before memory loss in Alzheimer's disease, reveals that the APOE4 risk gene alters how astrocytes function near its neurons
A molecular map of the locus coeruleus, the brain region where tau protein begins accumulating decades before memory loss in Alzheimer's disease, reveals that the APOE4 risk gene alters how astrocytes function near its neurons
The Lieber Institute for Brain Development at Johns Hopkins University profiled the locus coeruleus in postmortem tissue from 33 neurologically healthy middle-aged donors. In carriers of the APOE4 risk variant, genes expressed by astrocytes (the support cells surrounding neurons) were less active than in non-carriers. Neuromelanin intensity, which neurons lose during aging and Alzheimer's disease, correlated with genes involved in cellular housekeeping and antioxidant defense.
The locus coeruleus is a thin strand of noradrenergic neurons in the brainstem, roughly a centimeter and a half long but no thicker than a grain of rice. Tau protein, the molecule that forms tangles in Alzheimer's disease, accumulates here earlier than anywhere else in the brain, decades before memory lapses appear. The neurons are pigmented with dark neuromelanin: MRI sequences sensitive to this pigment show that less of it corresponds to more severe disease. The region has been very little studied at the gene-expression level because it is small and buried deep in the brainstem.
A direct lead came from mouse experiments: E4 damages the vesicular monoamine transporter VMAT2, which packages noradrenaline into protective vesicles inside the neuron. Without that protection, toxic breakdown products of noradrenaline accumulate and tau pathology increases.
On September 11, a study was published in which the researchers applied spatial transcriptomics, a method that maps gene activity to specific locations and cell types in tissue rather than averaging across the whole sample as conventional bulk analysis does. To capture risk before late-stage cell death erases the signal, the authors studied 33 neurologically healthy donors of varying sex, ancestry, and APOE genotype (E4 versus E2).
In E4 carriers, the genes whose activity was reduced in the locus coeruleus belonged not to the neurons themselves but to the neighboring astrocytes that support them. The mouse model had pointed to damage inside the neuron; in humans, the risk gene appears to act also through the neuron's neighbors.
The effect appeared almost entirely in donors of European ancestry: in donors of African ancestry, E4 had little effect on gene expression, consistent with the lower Alzheimer's risk that E4 confers in that population. A subset of the same genes, however, was equally reduced in both groups, which means the risk gene operates through at least two mechanisms. A 2024 study in Nature Medicine illustrated the scale of the risk: by age 65, nearly all people carrying two copies of E4 already have Alzheimer's biomarkers, and their age at symptom onset is predictable with roughly the same precision as in rare inherited forms of the disease.
In mice, a causal test has already been performed in the hippocampus: switching off APOE4 in neurons abolished the hyperactivity it caused, while switching it off in astrocytes did not. In the locus coeruleus, by contrast, the risk signal sits in astrocyte genes.
Neuromelanin is a byproduct of noradrenaline synthesis and degradation, structurally similar to a lysosome, the organelle that digests cellular waste, including tau protein. Its amount increases until roughly age 50 to 60 and then declines, faster in Alzheimer's disease. Pale, degraded pigment correlated with high expression of the APOE gene, while dark, intact pigment correlated with genes of the same housekeeping pathway (autophagy) and antioxidant defense. The authors propose that these same genes explain the pigment loss that MRI detects in living patients.
The locus coeruleus also connects to Parkinson's disease: its neurons synthesize cholesterol on their own, a task that astrocytes normally handle, and this self-synthesis is more pronounced in men. The authors link this to the observation that men develop Alzheimer's less often but Parkinson's more often, and that cholesterol can accelerate the toxic aggregates seen in Parkinson's. The same resource, in other words, both protects and creates vulnerability.
The authors have released the data and analytical tools as an open-access resource, intended as a foundation for future efforts to protect the locus coeruleus before disease begins.