Sex-Dependent Synaptic Alterations in a Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a devastating memory disorder affecting 32 million people worldwide (Drew et al., 2023). AD is characterized by aggregation of misfolded amyloid and tau proteins, disruption of neural circuits, and death of neural cells (Wang and Holtzman, 2020). Circuit dysfunction is a critical feature of disease progression in AD, and work in mouse AD models has demonstrated both hyperactivity (e.g., neocortex) and a reduction in the number of synaptic inputs (e.g., the hippocampus; Ye et al., 2022; Meftah and Gan, 2023).

Interestingly, the degree of neural circuitry impairment in AD may be sex-dependent. In humans, females are at a higher risk of developing AD compared with males (Nebel et al., 2018), and mouse studies have demonstrated sex-dependent changes to neural circuitry (e.g., in the hippocampus; Ye et al., 2022). However, few investigations have examined sex differences across brain regions in AD, leaving the extent of these variations largely unknown.

An area of keen interest is the subiculum, because this region is impaired early in AD pathogenesis (Kampmann, 2024) and is connected with many areas throughout the brain. Not only are neurons of the subiculum vulnerable to AD pathology, but AD-related changes in the interactions between this region and the rest of the brain vary by sex (Ye et al., 2022). Given this, a recent paper published in The Journal of Neuroscience aimed to document sex-dependent synaptic inputs to the subiculum in the most widely used mouse model of AD, the 5xFAD mouse line, that overexpress a form of the AD-linked amyloid protein harboring five mutations linked to familial AD.

Ye et al. (2024) compared AD pathology and inputs with the subiculum across four groups of 5XFAD mice: young and middle-aged, male and female. Young female 5xFAD mice had more severe amyloid pathology than young males, and middle-aged female 5xFAD mice had significantly higher amyloid plaque density, size, and intensity than the three other groups. Thus, females were more susceptible to amyloid burden regardless of age.

Direct projections to the subiculum were examined using a monosynaptic rabies tracer. Middle-aged 5xFAD of both sexes had fewer projections to the subiculum than young 5xFAD mice. Specifically, inputs from hippocampal areas CA1 and CA2, medial septum, and the retrosplenial, visual, auditory, and temporal association cortices were reduced. Interestingly, the subiculum of young 5xFAD received more inputs from the visual cortex compared with age-matched wild–type mice. But in middle-aged 5xFAD mice, there were fewer inputs to the subiculum from several regions, including sublayers of the CA1, medial septum, and visual and auditory cortices compared with middle-aged wild–type mice. These results not only demonstrate that input to the subiculum is weakened in the context of AD but that at young ages, the subiculum of 5xFAD mice receives stronger-than-normal input from the visual cortex.

Finally, Ye et al. examined sex differences in regional inputs to the subiculum in young and old 5xFAD mice. In wild-type mice, middle-aged males had more inputs from the hippocampus to the subiculum than female 5xFAD mice. Moreover, in young 5xFAD mice, males received more synaptic inputs from the CA1 oriens and CA3 pyramidal layer of the hippocampus but also exhibited fewer synaptic inputs to the postsubiculum region and medial entorhinal cortex. At middle age, 5xFAD male mice showed more overall inputs to the subiculum compared with females, including inputs from the perirhinal cortex and both lateral and medial portions of the entorhinal cortex. Overall, these findings suggest that changes in input to neurons of the subiculum vary by sex, with female wild-type and female 5xFAD mice exhibiting greater reductions in synaptic input to the subiculum relative to male wild-type and male 5xFAD mice, respectively. The decreased amount of inputs to the subiculum observed in female 5xFAD mice might be a product of higher amyloid burden.

The findings from Ye et al. reveal novel sex- and age-dependent reductions in synaptic inputs to the subiculum, a region known to be affected early in disease (Kampmann, 2024) and highly involved in memory formation (Bicanski and Burgess, 2020). Synaptic alterations are reliable predictors of cognitive dysfunction in AD and are known to precede the onset of dementia (Meftah and Gan, 2023). Consequently, Ye et al.'s findings suggest that reduced input to the subiculum be an underlying mechanism in memory impairment that worsens throughout disease progression.

Ye et al.'s results show a sex-dependent change to synaptic inputs to the subiculum in 5xFAD mice, demonstrating both increased AD pathology and weakened synaptic connections in females (Fig. 1). These findings are consistent with clinical data reporting a greater incidence of AD in women (Nebel et al., 2018). In addition, the authors’ observation that young female mice exhibit fewer synaptic inputs to the subiculum compared with younger males is in line with clinical findings demonstrating more severe hippocampal volume loss in female patients with mild cognitive impairment compared with men (Nebel et al., 2018). Thus, Ye et al.'s demonstration of age- and sex-dependent changes to synaptic inputs to the subiculum may highlight an underlying mechanism of heightened vulnerability to AD pathology in women compared with that in men.

The authors additionally note that the visual cortex sends a greater density of projections to the subiculum at early ages in 5xFAD mice than in wild-type, but these are reduced later in life. Both the visual cortex and the subiculum are associated with spatial awareness, and deficits in spatial navigation are one of the first cognitive symptoms observed in AD patients (Bicanski and Burgess, 2020). This is supported by previous work identifying a working pathway between the visual cortex and subiculum that modulates the formation of spatial representations and object–place associations (Bicanski and Burgess, 2020). Given that other regions exhibit hyperexcitability preceding hypoexcitability in AD (Meftah and Gan, 2023), it would be of interest for future investigations to further characterize the firing patterns between the visual cortex and subiculum over time using methods such as two-photon calcium imaging, along with behavioral measures of spatial memory. This would help determine how changes in circuit excitability relate to behavioral changes in AD.

Ye et al.'s observation that female 5xFAD mice show more severe pathology in the subiculum opens new avenues of research related to AD progression. One interesting sex difference in AD relates to sleep disturbances, a common symptom of this disorder. Women have more sleep disturbances at older ages compared with men, and sleep loss is more severe in female 5xFAD mice compared with males (Drew et al., 2023; Johnson et al., 2024). A recent study revealed that the subiculum is an important wake-promoting structure (Zhang et al., 2024). Therefore, changes in synaptic inputs projecting either to or within the subiculum—as confirmed here by Ye et al.—may disrupt normal sleep–wake regulation. Importantly, increased wakefulness influences the levels of pathological amyloid in the CSF of human patients, as well as the hippocampus of AD mouse models (Wang and Holtzman, 2020). Insufficient sleep also leads to cognitive impairment as well as worsening cognitive symptoms in patients with AD (Johnson et al., 2024). Hence, it would be of interest to investigate whether amyloid pathology or decreased inputs to the subiculum contribute to the sleep impairment observed in women suffering from AD.

The results reported by Ye et al. are an important contribution to the field of AD. The findings here clearly demonstrate how plaque density and synaptic inputs both differ depending on sex, thus opening the door for future investigations to elucidate early- and late-stage disease circuit dysfunction that include sex-dependent research.

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