**1. Introduction**

In a life expectancy that decreases with the aging process, cardiovascular, mental health, and neurodegenerative disorders strongly contribute to the total disability-adjusted life years (DALYs) with significant sex differences observed [1,2]. On the other hand, the morbidity/mortality paradox points to females having greater longevity but worse healthy life expectancy (HALE) than males [3]. Neurodegenerative disorders such as dementia are associated with increased mortality compared to aged control populations [4–6]. Besides, in recent years, the high degree of heterogeneity in the clinical and temporal patterns

**Citation:** Muntsant, A.;

Jiménez-Altayó, F.; Puertas-Umbert, L.; Jiménez-Xarrie, E.; Vila, E.; Giménez-Llort, L. Sex-Dependent End-of-Life Mental and Vascular Scenarios for Compensatory Mechanisms in Mice with Normal and AD-Neurodegenerative Aging. *Biomedicines* **2021**, *9*, 111. https://doi.org/10.3390/ biomedicines9020111

Academic Editor: Masaru Tanaka Received: 26 October 2020 Accepted: 20 January 2021 Published: 24 January 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of advanced stages of Alzheimer's disease (AD) in the elderly population evidences the existence of several subgroups of patients and demands clinical prognosticators of end-oflife dementia [7]. At the translational level, the shorter life span of animal models provides a particular scenario for studying and long-term monitoring of the factors relevant for health/disease and those factors involved in its fine-tuning modulation, from genetic and epigenetic to morphological, structural, and functional levels.

Among the animal models of AD, we have proposed long-term survivors of the widely used 3xTg-AD mice as a model for mortality selection bias and heterogeneity in end-of-life dementia [8]. This model, homozygous for the familial AD mutations PS1/M146V and APPSwe, also harboring the tauP301L human transgene, progressively develops temporal- and regional-specific neuropathological patterns and other hallmarks of the human disease [9–11]. The mortality rates of 3xTg-AD mice at 15 months of age, an advanced stage of amyloid and tau neuropathology, are higher than in the nontransgenic (NTg) counterparts [12]. Thus, survivors could be used to investigate the frailty/survival paradigm in normal and pathological aging, since a small number of animals overcome advanced neuropathological stages of the disease. Across the literature, most experimental research of our and other laboratories has shown higher mortality rates in homozygous [12,13] and heterozygous [14] male 3xTg-AD mice, despite female 3xTg-AD mice exhibiting worse neuropathological status than males [15].

Peripheral cardiovascular dysfunction as a risk factor in AD is among the promising emerging fields, since bidirectional interactions have been reported in these patients. Thus, amyloid pathology affects patients' hearts [16], and the impact of cerebrovascular dysfunction independently of cerebral amyloid angiopathy has also been demonstrated [17]. The arterial function is crucial to regulate blood pressure and flow through the body by the contraction and relaxation of the vascular smooth muscle cells. Increasing evidence obtained in different mouse models suggests that AD is associated with vascular dysfunction affecting different circulation arterial beds [18–21]. Angiogenesis, or growth of new blood vessels from pre-existent vessels, is a fundamental process, such as during development, wound healing, and restoring blood flow from hypoxic regions. Previous studies support a role of angiogenesis as a compensatory response to impaired cerebral blood flow (CBF) in AD [22]. However, extensive angiogenesis can lead to increased vascular permeability and subsequent hypervascularization and brain damage in AD [23,24]. Overall, although the "vascular hypothesis" of AD is increasingly being understood, the influence of sex on the vascular features present in AD has been a neglected topic.

Cardiovascular disease resulting from oxidative stress inflammation can exacerbate Alzheimer's disease. We recently reported the first evidence of sex-dependent worse physiologically relevant structural (increased passive external and internal diameters, cross sectional area) and functional (increased active internal diameters) properties in small peripheral mesenteric resistance arteries (MRA) in 15-month-old 3xTg-AD mice (advanced stages of disease) compared to age-matched mice with normal aging [21]. Thus, at both physiological and high intraluminal pressures, vascular alterations of female 3xTg-AD mice were found more pronounced than those found in age-matched male 3xTg-AD mice. Besides, a correlation between MRA properties and anxiety-like behavioral profile was found in both 3xTg-AD mice and age-matched non-transgenic counterparts with normal aging, pointing at the relevant interaction between vascular and mental health in the aging process.

The present work aimed to further explore the cardiovascular–brain interactions in normal and AD-neurodegenerative aging models using a life span paradigm. For that purpose, we studied two cohorts of NTg and 3xTg-AD mice, including both sexes, until the end of life. Survival, monitored from birth, showed exceptional worse mortality rates in females than males, independently of the genotype. This mortality selection provided a "survivors" sample that could unveil brain–cardiovascular interaction mechanisms relevant for normal and neurodegenerative aging processes in long-lived animals. The results show sex-dependent distinct physical (worse in 3xTg-AD males), neuropsychiatric-like and cognitive phenotypes (worse in 3xTg-AD females), and hypothalamic–pituitary–adrenal (HPA) axis activation (higher in females), with higher cerebral blood flow and improved cardiovascular phenotype in 3xTg-AD female mice survivors. The present study suggests a potential compensatory hemodynamic mechanism in end-of-life dementia, which is sexdependent and can be a target for pharmacological and non-pharmacological interventions.

### **2. Materials and Methods**
