Sex hormones, the gut microbiome, and neurodegenerative diseases: Lifespan perspective

The gut-brain axis represents a highly integrated communication network, connecting the gastrointestinal tract and the central nervous system via neural, immune, endocrine, and metabolic pathways. Steroid hormones, such as estrogens, androgens, and glucocorticoids, play a pivotal role in modulating these interactions across the lifespan. These hormones influence the composition of microbiota, intestinal permeability, and neuroimmune responses, thereby shaping brain function and behavior. Emerging evidence suggests a correlation between disruptions in the gut-brain axis and the onset and progression of neurodegenerative diseases, including Parkinson's disease, Alzheimer's disease, and multiple sclerosis. The diseases exhibit distinct sex-specific patterns in terms of prevalence, symptomatology, and progression. These patterns are often the consequence of differences in steroid hormone levels, receptor distribution, and immune responses. Despite these differences, the role of sex as a biological variable remains underrepresented in experimental and clinical research. This review synthesizes current evidence on how steroid hormones modulate gut-brain axis interactions and how these mechanisms contribute to neurodegeneration in a sex-specific manner. We highlight recent findings on hormonal regulation of the gut microbiome and its impact on neuroinflammation and neuronal vulnerability. This overview focuses not only on Parkinson's disease, in which genetic variations in the gene for brain-derived neurotrophic factor have been observed among others as triggers for dopaminergic neurodegeneration. In addition, Alzheimer's disease and multiple sclerosis are also considered, in which the prevalence of intestinal dysbiosis and impaired intestinal barrier function have been identified as significant influencing factors. This review provides a comprehensive framework for understanding the gender-specific neurobiology of gut-brain axis by integrating perspectives from the fields of endocrinology, neuroimmunology, and microbiome research. It is argued that a targeted investigation of the interactions between hormones and gut-brain axis is essential for the development of sex-specific therapeutic strategies for neurodegenerative diseases.