freshidea-AdobeStock-ef12.jpeg)
It has been proven for many years: there is a link between hearing and perception. But the nature of this association has never been clarified and only hypotheses have been put forward. In a historical article [1]Timothy Griffiths' team identified several scenarios, including one associated with Alzheimer's disease.
He pointed out that there could be a common cause between the two phenomena. Researchers working on Alzheimer's disease have already noticed that their mouse models — genetically modified to mimic the disease — have early hearing problems. In addition, hearing loss has been shown to be a major risk factor for Alzheimer's disease.
Biomarker of Alzheimer's disease
Recent studies also indicate that hearing loss is one of the first symptoms of Alzheimer's disease, and that it could be used as a biomarker for the disease. This is the path followed by Hongbo Zhao's team at Yale University (USA). In 2020, these researchers worked on mouse models of Alzheimer's disease [2]. They observed cortical auditory stimulation potentials in these animals and in wild lines. The curves were different: among other things, the amplitude of the P1 wave was lower in mice with Alzheimer's disease.
However, this work does not say much about the nature of the relationship between Alzheimer's disease and hearing loss. Many researchers are working on this topic and the results are sometimes contradictory. But the Chinese team from Wuhan University, led by neurologist Jintao Zhang, made an important discovery. The key role of the GDF1 protein has just been highlighted [3]. The gene it codes for is not sufficiently expressed in the brains of mice with hearing loss.
Indirect causation
In these mice, hearing loss was associated with an increase in amyloid plaques—a marker of Alzheimer's disease—in the hippocampus, auditory cortex, and association cortex. These rodents also display poorer cognitive performance. Cause: Decreased expression of the gene that encodes GDF1. Furthermore, when these mice are supplemented with GDF1, the effects are attenuated. Conversely, if we induce decreased gene expression in normal mice, these effects appear. For the authors, this suggests that hearing loss can lead to a decrease in articulation gdf1Promote the development of Alzheimer's disease.
The authors' work also shows that decreased expression of gdf1 activates an enzyme (б-secretase), leading to synaptic dysfunction and cognitive disorders.
For Jintao Zhang's team, these findings open the way to a GDF1-based treatment that could prevent the harmful effects of hearing loss. For the hearing world, the study is further evidence of the importance of prevention.
