Sexually dimorphic roles of toll-like receptors in the central nervous system

Many neurological and psychiatric diseases and disorders show sex differences in prevalence, incidence, disease manifestation and response to treatment. Yet, historically, most clinical and pre-clinical studies have been conducted disproportionately or exclusively in male subjects. In recent years, this research bias has been increasingly addressed through human and animal studies where both sexes are appropriately represented. These investigations have identified sex-specific disease mechanisms driven by a combination of distinct genetic, anatomical, physiological, hormonal and neural factors in males and females. Sexual dimorphism in immune function has long been recognized. Toll-like receptors (TLRs), important mediators of the innate immune response to pathogens and endogenous danger signals, play sex-biased roles in peripheral immunity. Toll-like receptors are also expressed in cells intrinsic to the central nervous system (CNS). They initiate, not only neuroinflammation in CNS infections and disease and injuries, but also influence neurodevelopment and normal aging. Emerging evidence indicates that TLRs expressed in CNS cells contribute to neural pathology in a sex-specific manner, a research area that warrants further investigations. The aim of the present review is to highlight the sex-specific contribution of TLRs expressed in the CNS to chronic pain, neurodegenerative diseases including Alzheimer's disease (AD) and Parkinson's Disease and psychiatric disorders including major depressive disorder (MDD). Major findings are highlighted, essential concepts, controversies and knowledge gaps are discussed, and potential future directions are proposed. Attention is drawn to the importance of advancing this research area given that neuroinflammation is a key player in many CNS pathologies and TLRs are the essential drivers of neuroinflammation.