A step closer to solving the Alzheimer’s puzzle

A study has found an early marker of Alzheimer’s among female mice, paving way for human trials and treatment techniques, writes Shree D N

Psychiatrist and neuropathologist Alois Alzheimer examined in 1901 a woman named Auguste Deter, plagued by insomnia, paranoia, and unwarranted suspicion. She also had severe memory problems. More than a century on, the field of Alzheimer’s research has remained largely fixated on memory, ignoring the area where the disease may actually begin.

A new study from the Centre for Brain Research (CBR) in Bengaluru, published in February in Alzheimer’s & Dementia: The Journal of the Alzheimer’s Association, a leading journal in the field, is shining light on a new area. The CBR researchers found something unexpected deep in the brainstem, in a mouse model study.

Smitha Karunakaran, senior author who led the study at CBR, says: “Our paper focuses on the locus coeruleus — it’s a tiny brainstem region which regulates attention, arousal, sleep and stress — the processes affected during the early stages of Alzheimer’s.”

The locus coeruleus is a cluster of roughly 50,000 neurons in the brainstem of vertebrates, from fish to humans. It governs some of the brain’s most basic functions: alertness, attention, stress responsiveness, and the sleep-wake cycle. 

It is also one of the earliest brain regions where the protein tangles associated with Alzheimer’s disease have been reported — before the disease becomes obvious in the hippocampus, the seat of memory, and long before a patient seeks help.

“Most of the studies so far on Alzheimer’s disease have focused on either the hippocampus or cortex,” Smitha says.

An early alarm, and a female one

Smitha's team worked with mice aged two to three months — early enough that neurons in the locus coeruleus were still intact and no plaques had yet formed, only the precursor molecules that researchers consider more toxic. Yet the brainstem was already showing changes, specifically in female mice. The support cells surrounding the locus coeruleus, known as astrocytes, had entered what researchers call a "primed" state.

The primed state is a non-reactive state in which there is no full inflammatory response or tissue destruction yet, but the tissue is no longer fully at baseline. This is like astrocytes quietly preparing for an upcoming war.

The researchers supported this picture using both molecular assays and live proton magnetic resonance spectroscopy, which detected an astrocyte-weighted metabolic signature in the brainstem of female mice that was not present in males.

Among humans, historically, women have been disproportionately affected by Alzheimer’s. “Our study points out that sex differences in Alzheimer’s may begin very early, even before neurons are lost,” Smitha says. “That is one of the major findings.”

One piece of a complex puzzle

Dr Sivakumar P Thangaraju, head of the Geriatric Psychiatry Unit, Department of Psychiatry at National Institute of Mental Health and Neurosciences (NIMHANS), Bengaluru, who reviewed the study, describes the female bias in Alzheimer’s as a problem with multiple, intersecting explanations. “The reasons could be biological, hormonal or psychosocial. There is no one single reason,” he says.

One of the more counterintuitive biological theories concerns cognitive reserve — the brain’s capacity to withstand damage before symptoms appear. “Women are also more resilient to withstand the pathology. They may develop symptoms much later,” Sivakumar says. “The damage has already occurred. They worsen faster when they develop symptoms.” In other words, the female brain may mask the disease longer, so that by the time it becomes visible, the damage is more advanced and the decline more rapid.

There is also the hormonal dimension. Estrogen deficiency after menopause has long been considered a candidate risk factor, and early studies testing hormone replacement therapy showed some promise. But a landmark randomised trial — the Women’s Health Initiative Memory Study, published in 2003 — found paradoxically that hormone replacement was associated with increased dementia risk in older women. 

The picture has since grown more nuanced: some researchers now argue that timing matters, and that hormone replacement initiated earlier — closer to menopause — may still be protective. “The final word has not been said,” Sivakumar notes.

Genetics adds another layer. The APOE4 gene variant is the strongest known genetic risk factor for Alzheimer’s, and studies suggest it operates more powerfully in women than in men — conferring a higher relative risk and a faster rate of disease progression in female carriers.

Sivakumar says that across much of India, lower educational attainment in women, combined with higher caregiver burden and limited access to healthcare, compounds the biological vulnerabilities.

CBR study adds its dimensions against this unclear research backdrop. “This is a well-conducted research,” Sivakumar says. “It adds to some of the scientific literature already existing to give some new insights into sex differences with respect to mechanisms for increased risk for Alzheimer’s disease.” But he is measured in his assessment of its immediate implications. “Translating from animal research to human research has a lot of steps, and this particular study, per se, is not going to yield a completely different understanding yet. It is just one piece of the puzzle.”

Smitha agrees. “We are not claiming that we have proved why women get Alzheimer’s,” she says. “It is an early-stage mouse model study that identifies one possible biological mechanism contributing to female-biased vulnerability.” A follow-up mechanistic paper, she adds, is already being written.

Asked about human trials, K V S Hari, the director of CBR, is careful not to exaggerate the study. “At this stage, it would be more accurate to speak of validating these findings in human cohorts rather than human trials. The locus coeruleus can be studied in living humans using advanced MRI approaches, but it remains technically challenging because it is a very small brainstem nucleus,” he explains the challenges.

“In addition, astrocyte states cannot be measured directly in living people as they can be in experimental tissue. Therefore, longitudinal cohorts that combine advanced imaging, fluid biomarkers, and clinical and cognitive follow-up will be important for testing whether similar vulnerability patterns are present in humans, including women at risk,” he adds.

Tricks to slow down degeneration?

There was another finding in the paper with translational importance. When the researchers exposed female mice to an “enriched environment” — more physical objects, more social interaction, more cognitive stimulation — several of these early brainstem changes moved back toward baseline. The primed astrocytes in the locus coeruleus returned towards baseline. The noradrenergic signalling was normalised.

This does not mean that environmental stimulation is a cure for Alzheimer’s. The mouse model is an early-stage model, the enrichment intervention is controlled, and tested at an early amyloid stage. Prior studies in other Alzheimer’s mouse models have found that enriched environments can influence astrocytes, consistent with the paper's findings in the brainstem. But it suggests that the locus coeruleus, precisely because it is an early site of vulnerability, may also be an early site of intervention — a window before the cascade of neuronal death becomes irreversible, feels Smitha.

She says the team is now following up on this among humans, in collaboration with Heidi Jacobs’ lab at Harvard. The researchers are using specialised MRI sequences to image the locus coeruleus in participants from an ongoing cohort study at the CBR. If the brainstem signature observed in mice can be detected in humans, it could serve as an earlier, non-invasive marker of Alzheimer’s-related vulnerability, before more widespread brain changes or cognitive symptoms become obvious.

Looking deep into hippocampus