Estrogen receptor mediated neuroprotection in models of Alzheimer's disease

<jats:p>&lt;p dir="ltr"&gt;Alzheimer's disease (AD) shows a strong sex bias, with women representing close to two-thirds of patients, a difference only partly explained by their greater longevity. The loss of neuroprotective estrogen at menopause has been proposed as contributing factor. However, estrogen therapy acting through estrogen receptor alpha (ERα) carries systemic risks that limit its clinical use. In contrast, estrogen receptor beta (ERβ) mediates neuroprotective and anti-inflammatory actions in the brain without these effects, making it an attractive target. Yet the protective cellular and molecular pathways through which ERß modulates amyloid pathology, neuroinflammation, and neurovascular function remain poorly understood.&lt;/p&gt;&lt;p dir="ltr"&gt;This thesis aimed to elucidate how ERβ signaling and its upstream circulating sex hormones modulate sex-specific AD-like pathology, using App knock-in mouse models combined with behavioral, biochemical, and single-cell transcriptomic approaches.&lt;/p&gt;&lt;p dir="ltr"&gt;In Paper I, we treated male and female AppNL-G-F mice with the selective ERβ agonist LY500307 and ovariectomized a subset of females to model surgical menopause. Untreated females had higher levels of soluble amyloid-beta (Aβ) and more activated microglia than males. ERβ activation improved memory in the Y- maze and contextual fear conditioning tests and lowered soluble and insoluble Aβ and plaque load in both sexes, without altering amyloid precursor protein (APP) processing. The protection was associated with fewer activated CD68+ microglia and greater microglia-plaque association, most strongly in males, whereas ovariectomy increased cortical soluble and hippocampal insoluble Aβ and sustained neuroinflammation.&lt;/p&gt;&lt;p dir="ltr"&gt;In Paper II, we crossed ERβ knockout (Esr2KO) mice with AppNL-G-F mice to generate AppNL-G-F/Esr2KO offspring and assessed pathology and behavior in male and female AppNL-G-F and AppNL-G-F/Esr2KO mice at six months of age, in combination with single-cell transcriptomic analysis of hippocampal tissue. Loss of ERβ impaired associative memory and increased hippocampal amyloid burden selectively in males, while females remained largely resilient. Profiling of 35,703 hippocampal cells showed that ERβ loss in males rewired intercellular communication, shifting it away from endothelial and vascular-trophic programs toward amyloid- and stress-associated pathways, with sex-divergent transcriptional responses across astrocytes and the vasculature. Transthyretin (Ttr) emerged as the most strongly sex-modulated gene, reduced specifically in ERβ-deficient males in parallel with their elevated amyloid levels, identifying an ERβ-TTR axis as a candidate male-specific clearance mechanism.&lt;/p&gt;&lt;p dir="ltr"&gt;In Paper III, we depleted the circulating sex hormones by gonadectomy in two models with different progression rates, the aggressive AppNL-G-F and the slower AppNL-F. In AppNL-G-F mice, gonadectomy improved learning and reduced hippocampal amyloid burden in males while having little effect in females. Gonadectomy depleted circulating estradiol in females and testosterone in males, with brain estradiol partly preserved. The slower AppNL-F model showed the opposite: gonadectomy worsened spatial memory and increased hippocampal amyloid burden in both sexes, indicating that the consequences of hormone withdrawal depend on the pace of pathology and on local brain steroidogenesis.&lt;/p&gt;&lt;p dir="ltr"&gt;Together, these studies define ERβ as a proximal, sex-biased regulator of neuroprotection in App knock-in amyloidosis, acting through microglial amyloid clearance and associated with astrocyte and neurovascular signaling and a candidate TTR-dependent route that appears most critical in the male brain, while circulating hormones contribute in a model- and timing-dependent manner. This clarifies the cellular basis of sex differences in AD and identifies ERβ-TTR signaling as a sex-specific therapeutic candidate, underscoring the need to treat sex as a biological variable in AD research.&lt;/p&gt;&lt;h3 dir="ltr"&gt;List of scientific papers&lt;/h3&gt;&lt;p dir="ltr"&gt;I. Aphrodite Demetriou, Birgitta Lindqvist, &lt;b&gt;Heba G. Ali&lt;/b&gt;, Mohamed M. Shamekh, Mukesh Varshney, Jose Inzunza, Silvia Maioli, Per Nilsson and Ivan Nalvarte. ERβ mediates sex-specific protection in the AppNL-G-F mouse model of Alzheimer's disease. Biology of sex Differences. 2025 Apr 29;16(1):29. &lt;a href="https://doi.org/10.1186/s13293-025-00711-w" target="_blank" rel="noreferrer"&gt;https://doi.org/10.1186/s13293-025-00711-w&lt;/a&gt;&lt;/p&gt;&lt;p dir="ltr"&gt;II. &lt;b&gt;Heba G. Ali&lt;/b&gt;, Aphrodite Demetriou, Michael P. Rossiter, Mukesh Varshney, Birgitta Lindqvist, Per Antonson, Per Nilsson, Ivan Nalvarte. Single cell transcriptomics identifies male selective vulnerability to estrogen receptor beta loss across amyloid, glial, and vascular pathways in the AppNL-G-F mouse model of Alzheimer's disease. [Manuscript]&lt;/p&gt;&lt;p dir="ltr"&gt;III. Patricia Muñoz, &lt;b&gt;Heba G. Ali&lt;/b&gt;, Aphrodite Demetriou, Maria Latorre- Leal, Makoto Shimozawa, Ljerka Delac, Tudor-Fabian Troncea-Sandu, Jose Inzunza, Per Nilsson, Silvia Maioli, Ivan Nalvarte. Effects of gonadectomy on brain sex hormone levels and amyloid pathology in male and female AppNL-G-F and AppNL-F mice. Journal of Neuroendocrinology. 2026 Mar;38(3):e70161. &lt;a href="https://doi.org/10.1111/jne.70161" target="_blank" rel="noreferrer"&gt;https://doi.org/10.1111/jne.70161&lt;/a&gt;&lt;/p&gt;</jats:p>