A subtle decline in the sense of smell could be among the earliest detectable changes linked to Alzheimer’s disease, potentially emerging well before clear memory problems. New research from Germany suggests the shift may be driven by the brain’s immune cells damaging key odor-processing connections.
The study, led by scientists at the German Center for Neurodegenerative Diseases (DZNE) and Ludwig Maximilian University of Munich, focuses on microglia, immune cells that help maintain brain health. Researchers report that in early Alzheimer’s, microglia may begin dismantling nerve fibers needed for normal smell perception.
How the brain’s smell circuit changes
The team examined communication between the olfactory bulb, which processes odor signals, and the locus coeruleus, a brainstem region involved in sensory regulation and other core functions. Long nerve fibers from the locus coeruleus help tune activity in the olfactory bulb, supporting normal smell processing.
According to the researchers, early Alzheimer’s-related alterations make these fibers appear abnormal to microglia. In response, microglia break down the connections, which could help explain why smell deficits can appear early in the disease course.
An eat-me signal on neurons
The study points to changes in the nerve fiber membrane as a likely trigger. A molecule called phosphatidylserine, typically kept on the inner side of the cell membrane, was observed on the outside, where it can act as an immune cue.
Microglia are known to respond to this kind of signal during normal synaptic pruning, a process that removes unused or impaired connections. The researchers suggest that in early Alzheimer’s, abnormal neuron activity may prompt this membrane shift, leading microglia to remove fibers that are still needed.
Evidence from mice, tissue, and PET scans
To support the mechanism, the scientists combined results from Alzheimer’s-like mouse models with analyses of human brain tissue and PET imaging data from people diagnosed with Alzheimer’s or mild cognitive impairment. Together, these lines of evidence point to immune-driven damage occurring at an early stage.
The findings also connect to a growing push for earlier diagnosis, as newer Alzheimer’s treatments are generally aimed at earlier phases of the disease. Researchers say a better understanding of smell-related changes could help identify people who should receive further testing before cognitive symptoms become pronounced.
Smell loss can have many causes, including aging, infections, allergies, and other neurological conditions, so it is not a stand-alone diagnostic sign. Still, the study strengthens the case that changes in olfaction may offer a practical early clue worth taking seriously in Alzheimer’s research and clinical follow-up.
Leave a Reply