Complexity of Sex Differences and Their Impact on Alzheimer's Disease

Abstract

Sex differences are present in brain morphology, sex hormones, aging processes and immune responses. These differences need to be considered for proper modelling of neurological diseases with clear sex differences. This is the case for Alzheimer's disease (AD), a fatal neurodegenerative disorder with two-thirds of cases diagnosed in women. It is becoming clear that there is a complex interplay between the immune system, sex hormones and AD. Microglia are major players in the neuroinflammatory process occurring in AD and have been shown to be directly affected by sex hormones. However, many unanswered questions remain as the importance of including both sexes in research studies has only recently started receiving attention. In this review, we provide a summary of sex differences and their implications in AD, with a focus on microglia action. Furthermore, we discuss current available study models, including emerging complex microfluidic and 3D cellular models and their usefulness for studying hormonal effects in this disease.

Keywords: Alzheimer’s disease; cerebral organoids; estrogen; hiPSC; microglia; sex differences; sex hormones.

Figure 1

Sex differences in the brain during development and aging. The female brain has a reduced volume compared to the male brain, and the major sex hormone in the female brain is estrogen compared to testosterone in the male brain. In females, there is an increase in estrogen after birth, another increase at puberty, varying levels of sex hormones during the menstrual cycle and a drastic drop in estrogen at menopause. In males, testosterone is increased in utero, at birth and during puberty, staying relatively stable throughout adulthood. During aging, females therefore experience a rapid estrogen loss compared to the gradual decline in testosterone in men. Moreover, females experience an earlier manifestation of age-related processes such as low energy production, transcriptomic changes and immune responses compared to males.

Figure 2

Sex differences in microglia. X inactivation should lead to an equal expression of X chromosome genes in both sexes. However, some genes escape this inactivation, which contributes to sex-specific gene signatures and intrinsic differences in female compared to male microglial immune responses. Sex differences in microglial transcriptomes are observed throughout development. Phenotypically, male microglia are generally more reactive with larger somas and increased motility, whereas female microglia are more phagocytic with a profile consistent with repair and inflammatory regulation. Male microglia further contribute to an increased density of dendritic spines and male-specific behavior.

Figure 3

Estrogen action and receptors. The main estrogen receptors are the nuclear estrogen receptors (ERα and ERβ). Upon estrogen binding, these can act as a transcription factor, directly influencing gene expression. Additionally, estrogen can act on membrane-bound receptors, such as G-protein coupled estrogen receptors (GPER), through non-genomic mechanisms via protein kinase cascades. Estrogen binding to either of these promotes phagocytosis and an anti-inflammatory microglia phenotype, mediated by ANXA1, ultimately possessing neuroprotective effects.

Figure 4

Advantages and disadvantages of commonly used AD model systems.

Figure 5

Advanced microfluidic system with a supporting vascularization to cerebral organoids would allow proper oxygen and nutrient flow to the whole organoid as well as removal of waste products, greatly facilitating long-term cultures. Additionally, incorporating microglia to have an immuno-competent cerebral organoid would lead to a complex model system. Investigating the role of sex hormones in development and disease would then be possible as female-derived organoids could be cultured until a post-natal stage when estrogen could be administrated to mimic in vivo exposure. This could further be compared to male-derived cerebral organoids exposed to testosterone in a manner also mimicking in vivo exposure.