In replication of previous findings, we found higher CSF p-Tau levels in women versus men specifically among APOE4 carriers. Our novel finding was significant relationship between low testosterone levels and higher p-Tau among APOE4 carriers. Our hypothesis concerning a potential mechanistic role of testosterone in the sex difference in p-Tau was supported in that the significant sex difference in p-Tau levels among APOE4 carriers was eliminated when adjusting for testosterone levels. Findings suggest that the lower testosterone levels in women are a significant contributor to their higher levels of p-Tau compared to men. Previous animal and cell culture studies have described a protective role of testosterone against Tau pathology [15, 16, 43]; however, to the best of our knowledge, we are the first to report a testosterone and Tau link in a human sample.
Testosterone offers a number of neuroprotective effects including improvements in synaptic plasticity [44, 45] and synaptic density in hippocampal nerouns [46–49], heightened cerebral blood flow and glucose metabolism , reductions in inflammation and oxidative stress [51, 52], and prevention against Aβ plaque deposition and their neurotoxic effects [14, 53, 54]. Germane to Tau topography in AD, testosterone’s neuroprotective effects are concentrated in the hippocampus given this region’s high density of androgen receptors . Although the biological basis underlying the testosterone and tau link is unclear, most relevant to Tau pathogenesis may be testosterone’s anti-inflammatory actions . A role for gliosis and neuroinflammation in Tauopathy is evidenced by greater microglial activity and altered inflammatory pathway markers (e.g., interleukin-6, tumor necrosis factor-α) correlating with Tau [57–60] as well as inflammation-related AD risk factors that contribute to Tau pathogenesis such the genetic factors of TREM2  and APOE4  and the environmental factors of traumatic brain injury [63, 64] and viral infection [65, 66]. Evidence suggests bidirectional effects between neuroinflammation and Tau propagation whereby inflammation can initiate and propagate Tau pathology while Tau aggregates can directly activate microglia and secretion of pro-inflammatory cytokines [67–69]. In early AD, Aβ plaques stimulate microgliosis and release of inflammatory cytokines  suggesting that testosterone’s protection against Aβ plaque deposition may contribute to its anti-inflammatory properties and, in turn, decreased p-Tau. However, our results were unchanged after adjusting for Aβ suggesting that the mechanisms underlying the testosterone and p-Tau link are independent of Aβ. Research into the potential mediating role of neuroinflammation in the testosterone and Tau link is warranted.
Prior studies have also reported a testosterone by APOE4 interaction on cognitive function in animal models [37, 38] and on AD risk  and hippocampal volume  in humans. Similar to the majority of these studies, the pattern of interactive effects indicated an association between testosterone and Tau only among APOE4 carriers. There is biological plausibility for a testosterone by APOE interaction. In the brain, the APOE protein is secreted primarily by astrocytes and secondarily by microglia and neurons and serves as a key transporter of lipoproteins. Given testosterone’s role in triglyceride and high density lipoprotein cholesterol metabolism, as demonstrated in circulating testosterone [71, 72], the shared role of APOE and testosterone in this lipoprotein pathway offers interactive possibilities. The APOE4 allele is associated with an increased susceptibility to inflammation  and greater Aβ deposition . Thus, it is possible that APOE4 carriers are the most likely to benefit from the testosterone’s protective actions against inflammation and Aβ and, ultimately, Tau, although our results were independent of CSF Aβ levels. In animal studies, APOE4 is associated with a reduction in cytosolic androgen receptor (AR) levels in the neocortex  leading to the possibility that the adverse effects of low testosterone levels are further amplified in APOE4 carriers that have fewer or less efficient AR to support testosterone signaling. We extend previous findings of APOE by testosterone interactive effects by demonstrating their application to Tau. We also demonstrated the potential specificity of the APOE by testosterone interaction to women. Among women, the significant relationship between lower testosterone and higher p-Tau levels was specific to APOE4 carriers, whereas this was a trend-level relationship in men regardless of APOE4 status.
Our results suggest that higher levels of p-Tau in women versus men is likely capturing an association between the low testosterone levels that are commonly seen in women and higher p-Tau. In fact, we found that the higher p-Tau levels in female APOE4 carriers versus male APOE4 carriers was eliminated when adjusting for testosterone suggesting that differences in testosterone between men and women is a central mechanism underlying this sex difference. These findings may have implications for the well-evidenced higher AD risk in women considering that Tau pathology is closely tied to neurodegeneration and clinical symptomology. Our findings also challenge the concept that testosterone is a ‘male hormone’ in which the implications of low levels on AD-related outcomes are mostly circumscribed to men. The effects of aging-related decreases in testosterone levels on AD-related outcomes in men had been repeatedly studied with mixed results, whereas these associations have been minimally examined in women despite women having lower testosterone levels than men overall as well as age-related declines.
Our results offer a potential mechanism for the strongly, yet not consistently (Neu et al., 2017) supported finding of a stronger effect of APOE4 in women versus men on AD risk [6–8, 74]. If APOE4 has a stronger effect on AD-related outcomes in the context of low testosterone levels, as suggested by our data, then this would lead to a greater susceptibility of women to these effects. Our findings may also help to explain inconsistencies in the literature regarding an effect of APOE4 on Tau. Other biomarker , neuroimaging  and autopsy  studies found a more robust association between APOE4 and Tau in women versus men, whereas studies that did not compare by sex have shown inconsistent findings in the APOE4 and Tau link. Specifically, some, but not all [78–80], studies found greater Tau burden among APOE4 carriers [75, 81, 82]. If the Tau and APOE4 relationship is dependent on testosterone, as our results suggest, the presence of this relationship may be related to the proportion of men versus women in a sample. In non-sex-stratified analyses, an association between APOE4 and Tau may be obscured in samples that are predominantly male and, thus, likely characterized by higher testosterone levels.
Our findings have clinical relevance in that low testosterone is a potentially modifiable risk factor. Although numerous studies have investigated the effects of testosterone supplementation on cognitive function and AD risk with mixed findings (Wolf et al., 1999), very few studies have examined the effects of testosterone supplementation in women and with regard to APOE4 status. Resistance exercise and diet have been shown to modulate testosterone levels [83–85] indicating other opportunities for intervention through lifestyle or behavioral means. Our results raise the possibility that pharmacological and behavioral interventions to elevate testosterone may be particularly beneficial in female APOE4 carriers and, upon replication, warrant consideration for intervention studies in women at-risk for AD.
This study has limitations. Our smaller sample size likely limited statistical power particularly when examining the testosterone and APOE4 interaction in sex-stratified analyses. Levels of circulating estradiol were not available in the ADNI, which precluded us from examining whether it is testosterone or the aromatization of testosterone to estradiol that is responsible for the observed association. However, previous animal work found testosterone’s neuroprotective effects against Aβ  and p-Tau  to be independent of estradiol levels suggesting that androgenic mechanisms are implicated in these effects . CSF levels of testosterone may be more reflective of testosterone activity in the brain; however, only plasma-based levels were available to us. Because of our cross-sectional design, we were precluded from determining the temporal relationship between testosterone and Tau. Although previous findings suggest that testosterone’s effects predate AD outcomes, there is potential for bidirectional given evidence that AD pathology may negatively feedback on testosterone levels by hindering production of sex steroid hormones [87, 88]. Lastly, ADNI is a convenience sample of mostly white and well-educated volunteers compared with the general US population, which limits generalizability of results.
In conclusion, we found a relationship between lower testosterone levels and higher CSF p-Tau that was specific to APOE4 carriers. The specificity of this relationship to APOE4 carriers seemed to be driven by women. We replicated a consistent finding of higher p-Tau levels in women versus men at-risk for AD; however, this difference was eliminated after adjusting for testosterone. Results suggest that testosterone has a protective role against Tau particularly among APOE4 carriers, and that low testosterone levels that are more characteristic of women than men may predispose one to Tau.
Perspectives and Significance
Our findings inform a knowledge gap in our understanding of greater Tauopathy in women versus men on the AD trajectory and in the repeated demonstration of a stronger APOE4 effect in women. Our findings may also help to enlighten disparities in the literature regarding an APOE4 and Tau relationship. This study represents a call to researchers and clinicians that it is equally important to examine the effects of testosterone on AD-related outcomes in women as it is in men, if not more. Our findings stress the need to examine the effects of testosterone on AD-related outcomes in women in addition to men. Follow-up studies include should investigate (a) the association between testosterone levels and cortical Tau as measured by PET, (b) the effect of testosterone supplementation on Tau burden and (c) the mediating role of neuroinflammation in the testosterone and Tau link.