bioRxiv2026
BackgroundAdvancing age, the APOE{square}4 allele, and female sex are the top nonmodifiable risk factors for Alzheimers disease (AD). Female-specific experiences, such as parity and hormone therapy (HT) affect aging biomarkers such as metabolism and immune signaling and may even affect AD risk. Estradiol (E2), a common component of many HTs, affects cognition and brain health in aging females although this may vary depending on parity, genotype, and metabolic status. We hypothesized that prior parity influences brain and metabolic health, including response to E2, depending on APOE genotype. MethodsMiddle-aged female (10 month) wildtype (WT) or humanized (h) APOE{square}4 expressing rats, with different reproductive experience (nulliparous or primiparous) were fed a Western (WD) or standard diet (SD) for 2 months. In the second month, rats were given E2 or vehicle (oil) injections daily. Fear associative learning, plasma metabolic hormones, hippocampal inflammatory signalling, and neuroplasticity (neurogenesis, synaptic protein) were assessed. ResultsFemales fed a WD gained weight and displayed metabolic dysregulation, regardless of genotype. E2 treatment reduced WD-induced weight gain and reduced metabolic hormones, with stronger effects in WT rats. E2 treatment increased dorsal hippocampal inflammatory signalling selectively in primiparous hAPOE{square}4 females fed a WD. Previous parity increased neurogenesis and reduced certain cytokines in the hippocampus of middle-aged WT rats under a SD. Both E2 treatment and previous parity decreased dorsal neurogenesis in hippocampus of hAPOE{square}4 rats. In hAPOE{square}4 females, higher weight was associated with reduced contextual fear memory, an effect driven by primiparous females. In the cued fear conditioning task, hAPOE{square}4 females displayed better cued fear memory than WT, however, WD exposure reduced cued fear memory in this group. Together, this indicates that diet and weight gain may be more detrimental to associative memory in hAPOE{square}4 females and that E2 treatment has more favourable outcomes in WT rats. ConclusionsPrevious parity alters how females respond to E2 and metabolic stress in midlife. Primiparous hAPOE{square}4 females were especially vulnerable to the effects of WD and E2, exhibiting more inflammation, impaired memory, and reduced weight-loss. These findings highlight the importance of considering parity and genotype when evaluating midlife metabolic and cognitive risk. HighlightsO_LIEstradiol (E2) treatment reduced body weight gain under a Western diet (WD), with the most pronounced effects in wildtype primiparous rats. C_LIO_LIWD increased several metabolic hormones, and E2 treatment reduced several metabolic hormones after a WD in middle age, only in WT rats C_LIO_LIIncreased body weight impaired contextual associative memory in primiparous hAPOE{square}4 females. C_LIO_LIE2 treatment increased dorsal hippocampal inflammatory signalling in primiparous hAPOE{square}4 rats and the WD increased inflammatory signalling in primiparous WT rats. C_LIO_LIPrevious parity, but not E2 treatment, increased neurogenesis in WT rats only under standard diet conditions whereas a WD decreased neurogenesis based on genotype and previous parity C_LI Plain English SummaryAlzheimers disease (AD) causes general cognitive decline, and females are at higher risk than males, particularly those carrying the APOE{varepsilon}4 gene. Female-specific experiences, such as previous pregnancy (parity) and hormone therapy, as well as lifestyle factors like body weight and diet, may further influence AD risk. We examined how estradiol (E2), a hormone involved in the menstrual cycle, pregnancy, and some hormone therapies can affect memory, brain inflammation, synaptic plasticity (brain cell connectivity), and growth of new neurons (neurogenesis) in middle-aged female rats. We compared females with or without the APOE{varepsilon}4 gene and with or without prior parity, fed either a high-fat, high-sugar Western diet (WD) or a standard diet (SD). WD increased body weight and worsened metabolic health, but the strength of negative effects depended on genotype and pregnancy history. In females with the APOE{varepsilon}4 gene and prior parity, higher body weight was linked to poorer memory. hAPOE{varepsilon}4 females on SD showed better memory than non-carriers but this benefit disappeared on WD, suggesting WD and obesity are particularly detrimental for hAPOE{square}4 carriers with prior parity. WD-fed primiparous hAPOE{square}4 females treated with E2 also showed higher brain inflammation than other groups. Pregnancy history and E2 treatment were associated with more favourable outcomes in females without the genetic risk for AD and fed a standard diet. These findings highlight the importance of a personalized approach to hormone therapy and brain health in aging females. hAPOE{square}4 females with prior parity were most sensitive to the effects of diet. Together, this suggests genetic background, reproductive history, and diet interact to affect memory, inflammation, and neurogenesis, as well as the effects of E2 on these factors.