New Biomarker Reveals When Alzheimer's Inflammation Begins Before Symptoms Appear

The study showed that the protein TSPO, used as a marker of brain inflammation, increases very early in Alzheimer's disease, even before memory problems appear. This increase first appears in a region called the subiculum and is linked to the defense cells (microglia) that try to deal with beta-amyloid plaques. The results were confirmed both in genetically modified mice and in the brains of people with early-onset hereditary Alzheimer's. This suggests that TSPO may be an important target for early diagnosis and even new treatments.

Alzheimer's disease is a progressive condition that causes the gradual loss of neurons and brain function. One of the most important processes in this condition is neuroinflammation, the overactivation of the nervous system's defense cells.

This begins early, when proteins called beta-amyloid begin to accumulate in the brain. These proteins form plaques between neurons and attract cells called microglia, which attempt to eliminate them. This inflammation process occurs even before other typical changes in the disease, such as tau protein tangles, loss of brain volume, and symptoms of impaired memory and thinking.

One of the markers used to study neuroinflammation is a protein called TSPO. This protein appears in small amounts in the healthy brain but increases significantly when inflammation is present. Researchers can measure TSPO levels using imaging tests (such as PET scans), and it is already known that they are increased in both the brains of people with Alzheimer's and in animal models of the disease.

Despite this, it was still unclear when exactly this increase begins, in which brain regions it appears first, and what it means for disease progression.

Most previous studies were conducted with transgenic mice carrying human mutations associated with early-onset Alzheimer's. These animals are useful, but they have a more rapid and aggressive form of the disease than the more common human cases, which typically begin later and progress slowly.

This can make direct comparisons difficult. To address this limitation, the researchers decided to compare the results from the mice with the brains of people with early-onset hereditary Alzheimer's, caused by a genetic mutation called PSEN1-E280A, which also causes an aggressive and early progression of the disease.

In the study, scientists at Florida International University, USA, used the 5XFAD animal model, a type of mouse carrying five human mutations associated with Alzheimer's. They followed these animals from very young to adulthood, which allowed them to observe how TSPO levels changed throughout life.

Researchers Daniel Martínez Pérez (L) and Dr. Tomás R. Guilarte (R) in the Brain, Behavior, and Environment Laboratory at Florida International University. Guilarte and Martínez Pérez published a study finding that TSPO, a key biomarker of brain inflammation, may help detect Alzheimer's disease years before the onset of memory loss and other symptoms. Credit: Chris Necuze, Florida International University

To do this, they analyzed different brain regions at various ages, assessed the mice's cognitive performance on memory tests, evaluated biomarkers of the disease in their blood, and investigated how brain cells responded to beta-amyloid plaques.

The results showed that the increase in TSPO appeared very early, around 1.5 months of age, in a region of the brain called the subiculum. This increase occurred in both males and females and appeared before any signs of memory loss or significant changes in blood tests.

The increase in TSPO levels was linked to an increase in the number of activated microglia, especially those in direct contact with the amyloid beta plaques. Furthermore, it was observed that microglia in contact with the plaques had higher TSPO levels than those not attached to the plaques. In contrast, another type of brain cell, astrocytes, were also activated but showed no increase in TSPO.

Analysis of microglia number, classification of microglia relevant to contact with amyloid beta (Aβ) plaques, and TSPO volume per microglia in WT and 5XFAD mice. The images show immunofluorescently labeled cells of Aβ (cyan), TSPO (red), and microglia (Iba-1; blue) in the subiculum of a 1.5-month-old 5XFAD female. The middle panel is an Imaris render showing Aβ (cyan), TSPO (red), microglia not in contact with Aβ plaques (white), and microglia in contact with Aβ plaques (blue), with the white square indicating the enlarged image shown in the right panel.

Finally, the findings in mice were compared with analyses of brains from individuals with hereditary Alzheimer's disease due to the PSEN1-E280A mutation. The results confirmed that the same increase in TSPO, particularly in microglia near beta-amyloid plaques, also occurs in these human cases. This reinforces the idea that TSPO may be an early marker of inflammation in the brain in Alzheimer's disease.

Analysis of beta-amyloid (Aβ) plaque burden in the subiculum and hippocampus of mice with the 5XFAD Alzheimer's disease mutation as a function of age.

One of the greatest challenges in diagnosing Alzheimer's disease and developing effective therapies is early patient identification. These findings in a preclinical model of Alzheimer's disease suggest the possibility of implementing TSPO as an early biomarker of neuroinflammation, complementing other established biomarkers.

Furthermore, the finding that TSPO levels initially increased in the subiculum with early beta-amyloid aggregation suggests that the subiculum should be a focus area for future TSPO-PET imaging studies in Alzheimer's disease.

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Amyloid-β plaque-associated microglia drive TSPO upregulation in Alzheimer’s disease

Daniel A. Martinez-Perez, Jennifer L. McGlothan, Alexander N. Rodichkin, Karam Abilmouna, Zoran Bursac, Francisco Lopera, Carlos Andres Villegas-Lanau and Tomás R. Guilarte.

Acta Neuropathologica. Acta Neuropathol 150, 6 (2025).DOI: 10.1007/s00401-025-02912-4

Abstract: 

Translocator protein 18 kDA (TSPO) imaging using positron emission tomography (PET) is widely used to assess neuroinflammation in Alzheimer’s disease (AD). However, the significance of the increase in brain TSPO levels in AD pathophysiology is not known. Here, we show that in the 5XFAD transgenic mouse model, brain TSPO levels increase in an age-, brain region-, and sex-dependent fashion. TSPO levels were first increased in the subiculum at 1.5 months of age in male and female 5XFAD mice compared to wildtype mice. The TSPO increase in the subiculum of 1.5-month 5XFAD mice coincided with the appearance of Aβ aggregation and increased serum Aβ1-42/Aβ1-40 ratio which occurred prior to increased serum neurofilament light chain (Nfl) levels and well before cognitive function deficits. We also discovered that the brain TSPO increase was driven by an expansion of activated microglia in contact with Aβ-plaques, that also expressed higher TSPO levels per microglia than microglia not in contact with plaques. While overall, astrocytes were highly activated, the increased TSPO signal in the 5XFAD mouse brain did not increase in astrocytes. We also compared the 5XFAD mouse findings to postmortem human brain tissue from early-onset autosomal-dominant Presenilin 1 (PSEN1)-E280A mutation AD cases. The results in PSEN1-E280A cases confirmed the 5XFAD mouse findings relevant to increased TSPO levels and an increase in TSPO per microglia contacting Aβ-plaques. In summary, TSPO is an early biomarker of neuroinflammation in the AD brain that first increases in the subiculum simultaneously with increased Aβ aggregation and serum Aβ1-42/Aβ1-40 ratio. The increased TSPO response in the 5XFAD mouse brain and in the brain from PSEN1-E280A mutation AD cases reflects Aβ-plaque-associated microglia with a high TSPO content. This microglia subtype is likely to promote the progression of AD pathology, neurodegeneration, and cognitive decline and their high TSPO content may serve as a target for TSPO ligand-based therapy.