First Symptom Of Alzheimer’s Is Triggered By This Signal

A fading sense of smell may do more than hint at aging—it could be one of the earliest warning signs of Alzheimer’s disease. New research from DZNE and Ludwig-Maximilians-Universität München reveals that immune cells in the brain mistakenly target and destroy nerve fibers critical for odor perception, offering fresh clues into how the disease begins and how it might be diagnosed sooner.Patients and families may notice that scents seem muted or distorted, sometimes years before other symptoms. Until now, the exact cause behind this early warning sign remained elusive.A new study led by researchers at the German Center for Neurodegenerative Diseases (DZNE) and Ludwig-Maximilians-Universität München (LMU) sheds fresh light on the mystery. The findings, published in Nature Communications, suggest that the brain’s own immune system mistakenly destroys nerve fibers vital for processing odors. This breakthrough could open the door to earlier diagnosis—and possibly earlier treatment—of Alzheimer’s.How the Brain Processes Smell?Smell perception begins in the olfactory bulb, a small but complex structure in the forebrain that receives input from sensory receptors in the nose. But the olfactory bulb doesn’t work alone. It relies on nerve fibers extending from the locus coeruleus, a brainstem region that helps regulate attention, blood flow, and sensory processing.The new study shows that in early Alzheimer’s disease, this communication line is disrupted. Microglia—immune cells that normally act as the brain’s cleanup crew—start dismantling the nerve fibers linking the locus coeruleus and olfactory bulb. This immune-driven attack deprives the brain of crucial odor-processing pathways, leading to smell loss.“Our study suggests that in early Alzheimer’s disease, changes occur in the nerve fibers linking the locus coeruleus to the olfactory bulb. These alterations signal to the microglia that affected fibers are defective or superfluous. Consequently, the microglia break them down,” explained Dr. Lars Paeger of DZNE and LMU.What Is The “Eat-Me” Signal That Triggers Alzheimer’s?At the heart of this immune misfire lies an unusual molecular change. Researchers observed that phosphatidylserine, a fatty acid normally tucked inside the protective membrane of neurons, shifts to the cell’s outer surface in affected nerve fibers.When this happens, microglia interpret it as an “eat-me” signal. Under normal conditions, this signal supports a healthy process called synaptic pruning, where unnecessary or damaged connections are cleared away. But in Alzheimer’s, this mechanism seems to go awry.Paeger explained that hyperactive neurons—cells firing abnormally due to early disease changes—appear to trigger this membrane shift. Once flagged as dysfunctional, these otherwise critical smell-related fibers are targeted and destroyed.PET imaging scans in living patients, which confirmed damage to smell-related nerve circuits in people with Alzheimer’s or mild cognitive impairment.“Smell issues in Alzheimer’s disease and damage to the associated nerves have been discussed for some time. However, the causes were unclear until now. Our findings point to an immunological mechanism as cause for such dysfunctions – and, in particular, that such events already arise in the early stages of Alzheimer’s disease,” said Prof. Joachim Herms, senior researcher on the study.Why Smell Loss Is Important in Diagnosis of Alzheimer’s?More than 6 million Americans currently live with Alzheimer’s disease, a number projected to rise sharply as populations age. Yet diagnosis often comes late, when memory loss and cognitive decline are already advanced.Smell loss offers an earlier red flag. If the biological mechanisms behind it can be mapped and measured, doctors could potentially screen for Alzheimer’s years before symptoms interfere with daily life. This could be especially critical as new treatments, such as amyloid-beta antibodies, are designed to work best in the earliest phases of disease progression.“Our findings could pave the way for the early identification of patients at risk of developing Alzheimer’s, enabling them to undergo comprehensive testing to confirm the diagnosis before cognitive problems arise,” Herms noted. “This would allow earlier intervention with amyloid-beta antibodies, increasing the probability of a positive response.”The study also highlights the often-overlooked locus coeruleus, a small cluster of neurons deep in the brainstem. Beyond smell, this structure regulates blood flow, sleep-wake cycles, and stress responses through widespread connections.Damage to the locus coeruleus is one of the earliest detectable signs of Alzheimer’s, even before amyloid plaques and tau tangles spread widely. The new findings underscore that its breakdown may not only affect memory and attention but also disrupt sensory pathways, making smell loss one of the first noticeable symptoms.Can Sense of Smell Be Preserved?While the research answers critical questions, it also raises new ones. If immune cells misidentify smell-related fibers as expendable, can this process be slowed or stopped? Could therapies aimed at stabilizing neuron membranes prevent the fatal “eat-me” signal from being displayed in the first place?Future studies will likely explore whether drugs that regulate microglial activity—or protect the integrity of phosphatidylserine positioning—could preserve nerve connections in early Alzheimer’s.Meanwhile, incorporating smell testing into routine checkups for older adults may become a more practical step. Inexpensive scratch-and-sniff exams already exist and could serve as a noninvasive screening tool to identify people who may need further evaluation.For families watching loved ones struggle with subtle changes—whether misplaced keys, unusual forgetfulness, or an inability to smell morning coffee—the findings offer clarity. Smell loss is not just an odd, isolated symptom but a biological signal that Alzheimer’s is affecting the brain long before dementia sets in.Recognizing this signal early could give patients and clinicians a window of opportunity: time to prepare, time to plan, and perhaps in the future, time to intervene with treatments that slow or alter the course of disease.Alzheimer’s remains one of the greatest public health challenges of our time. While there is still no cure, understanding its earliest signals brings us closer to meaningful change. By uncovering how immune cells mistakenly dismantle smell pathways, this study not only solves a long-standing puzzle but also lays the foundation for earlier, more effective care strategies.The sense of smell may be more than just a window to the world—it may be a window into the earliest changes of Alzheimer’s disease.

First Symptom Of Alzheimer’s Is Triggered By This Signal

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