Sex-specific Phenotypes in the Aging Mouse Heart and Consequences for Chronic Fibrosis

Normative Values of Echocardiographic Chamber Size and Function in Older Healthy Adults: The Multi-Ethnic Study of Atherosclerosis

BACKGROUND

Echocardiographic (2-dimensional echocardiography) thresholds indicating disease or impaired functional status compared with normal physiological aging in individuals aged ≥65 years are not clearly defined. In the present study, we sought to establish standard values for 2-dimensional echocardiography parameters related to chamber size and function in older adults without cardiopulmonary or cardiometabolic conditions.

METHODS

In this cross-sectional study of 3032 individuals who underwent 2-dimensional echocardiography at exam 6 in the MESA (Multi-Ethnic Study of Atherosclerosis), 608 participants fulfilled our inclusion criteria of healthy aging, with normative values defined as the mean ± 1.96 standard deviation and compared across sex and race and ethnicity. Functional status measures included NT-proBNP (N-terminal pro-B-type natriuretic peptide), 6-minute walk distance, and Kansas City Cardiomyopathy Questionnaire. Prognostic performance using MESA cutoffs was compared with established guideline cutoffs using time-to-event analysis.

RESULTS

The normative aging cohort (69.5±7.0 years, 46.2% male, 47.5% White) had lower NT-proBNP, higher 6-minute walk distance, and higher (better) Kansas City Cardiomyopathy Questionnaire summary values. Women had significantly smaller chamber sizes and better biventricular systolic function. White participants had the largest chamber dimensions, whereas Chinese participants had the smallest, even after adjustment for body size. Current guidelines identified 81.6% of healthy older adults in MESA as having cardiac abnormalities.

CONCLUSIONS

Among a large, diverse group of healthy older adults, we found significant differences in cardiac structure and function by sex and race/ethnicity, which may signal sex-specific cardiac remodeling with advancing age. It is crucial for existing guidelines to consider the observed and clinically significant differences in cardiac structure and function associated with healthy aging. Our study highlights that existing guidelines, which grade abnormalities in echocardiographic cardiac chamber size and function based on younger individuals, may not adequately address the anticipated changes associated with normal aging.

Sex as a biological variable for cardiovascular physiology

There have been ongoing efforts by federal agencies and scientific communities since the early 1990s to incorporate sex and/or gender in all aspects of cardiovascular research. Scientific journals provide a critical function as change agents to influence transformation by encouraging submissions for topic areas, and by setting standards and expectations for articles submitted to the journal. As part of ongoing efforts to advance sex and gender in cardiovascular physiology research, the American Journal of Physiology-Heart and Circulatory Physiology recently launched a call for papers on Considering Sex as a Biological Variable. This call was an overwhelming success, resulting in 78 articles published in this collection. This review summarizes the major themes of the collection, including Sex as a Biological Variable Within: Endothelial Cell and Vascular Physiology, Cardiovascular Immunity and Inflammation, Metabolism and Mitochondrial Energy, Extracellular Matrix Turnover and Fibrosis, Neurohormonal Signaling, and Cardiovascular Clinical and Epidemiology Assessments. Several articles also focused on establishing rigor and reproducibility of key physiological measurements involved in cardiovascular health and disease, as well as recommendations and considerations for study design. Combined, these articles summarize our current understanding of sex and gender influences on cardiovascular physiology and pathophysiology and provide insight into future directions needed to further expand our knowledge.

Fibroblasts and immune cells: at the crossroad of organ inflammation and fibrosis

The immune and fibrotic responses have evolved to work in tandem to respond to pathogen clearance and promote tissue repair. However, excessive immune and fibrotic responses lead to chronic inflammation and fibrosis, respectively, both of which are key pathological drivers of organ pathophysiology. Fibroblasts and immune cells are central to these responses, and evidence of these two cell types communicating through soluble mediators or adopting functions from each other through direct contact is constantly emerging. Here, we review complex junctions of fibroblast-immune cell cross talk, such as immune cell modulation of fibroblast physiology and fibroblast acquisition of immune cell-like functions, as well as how these systems of communication contribute to organ pathophysiology. We review the concept of antigen presentation by fibroblasts among different organs with different regenerative capacities, and then focus on the inflammation-fibrosis axis in the heart in the complex syndrome of heart failure. We discuss the need to develop anti-inflammatory and antifibrotic therapies, so far unsuccessful to date, that target novel mechanisms that sit at the crossroads of the fibrotic and immune responses.

Exploring the Impact of BKCa Channel Function in Cellular Membranes on Cardiac Electrical Activity

This review paper delves into the current body of evidence, offering a thorough analysis of the impact of large-conductance Ca2+-activated K+ (BKCa or BK) channels on the electrical dynamics of the heart. Alterations in the activity of BKCa channels, responsible for the generation of the overall magnitude of Ca2+-activated K+ current at the whole-cell level, occur through allosteric mechanisms. The collaborative interplay between membrane depolarization and heightened intracellular Ca2+ ion concentrations collectively contribute to the activation of BKCa channels. Although fully developed mammalian cardiac cells do not exhibit functional expression of these ion channels, evidence suggests their presence in cardiac fibroblasts that surround and potentially establish close connections with neighboring cardiac cells. When cardiac cells form close associations with fibroblasts, the high single-ion conductance of these channels, approximately ranging from 150 to 250 pS, can result in the random depolarization of the adjacent cardiac cell membranes. While cardiac fibroblasts are typically electrically non-excitable, their prevalence within heart tissue increases, particularly in the context of aging myocardial infarction or atrial fibrillation. This augmented presence of BKCa channels' conductance holds the potential to amplify the excitability of cardiac cell membranes through effective electrical coupling between fibroblasts and cardiomyocytes. In this scenario, this heightened excitability may contribute to the onset of cardiac arrhythmias. Moreover, it is worth noting that the substances influencing the activity of these BKCa channels might influence cardiac electrical activity as well. Taken together, the BKCa channel activity residing in cardiac fibroblasts may contribute to cardiac electrical function occurring in vivo.

Resistance training up-regulates Smyd1 expression and inhibits oxidative stress and endoplasmic reticulum stress in the heart of middle-aged mice

Persistent oxidative stress and endoplasmic reticulum (ER) stress are the primary mechanisms of age-related cardiovascular diseases. Although exercise training is viewed as an effective anti-aging approach, further exploration is needed to identify the mechanisms and functional targets. In this study, the impact of resistance training (RT) on the expression of Smyd1, the levels of reactive oxygen species (ROS) and the expression of ER stress-related protein in the hearts of mice of different ages were assessed. In addition, the role of Smyd1 in the aging-induced oxidative stress and ER stress were evaluated in d-galactose (D-gal)-treated H9C2 cells. We demonstrated that RT in middle age increased the expression of Smyd1 and restricted heart aging-induced ER stress. Overexpression of Smyd1 restrained oxidative stress and ER stress in D-gal-treated H9C2 cells, while the inhibition of Nrf2 and Smyd1 escalated ER stress. These findings demonstrate that Smyd1 has significant impact in regulating age-related ER stress. RT in middle age can up-regulates Smyd1 expression and inhibits oxidative stress and ER stress in the heart.

Substrate stiffness modulates cardiac fibroblast activation, senescence, and proinflammatory secretory phenotype

In vitro cultures of primary cardiac fibroblasts (CFs), the major extracellular matrix (ECM)-producing cells of the heart, are used to determine molecular mechanisms of cardiac fibrosis. However, the supraphysiologic stiffness of tissue culture polystyrene (TCPS) triggers the conversion of CFs into an activated myofibroblast-like state, and serial passage of the cells results in the induction of replicative senescence. These phenotypic switches confound the interpretation of experimental data obtained with cultured CFs. In an attempt to circumvent TCPS-induced activation and senescence of CFs, we used poly(ethylene glycol) (PEG) hydrogels as cell culture platforms with low and high stiffness formulations to mimic healthy and fibrotic hearts, respectively. Low hydrogel stiffness converted activated CFs into a quiescent state with a reduced abundance of α-smooth muscle actin (α-SMA)-containing stress fibers. Unexpectedly, lower substrate stiffness concomitantly augmented CF senescence, marked by elevated senescence-associated β-galactosidase (SA-β-Gal) activity and increased expression of p16 and p21, which are antiproliferative markers of senescence. Using dynamically stiffening hydrogels with phototunable cross-linking capabilities, we demonstrate that premature, substrate-induced CF senescence is partially reversible. RNA-sequencing analysis revealed widespread transcriptional reprogramming of CFs cultured on low-stiffness hydrogels, with a reduction in the expression of profibrotic genes encoding ECM proteins, and an attendant increase in expression of NF-κB-responsive inflammatory genes that typify the senescence-associated secretory phenotype (SASP). Our findings demonstrate that alterations in matrix stiffness profoundly impact CF cell state transitions, and suggest mechanisms by which CFs change phenotype in vivo depending on the stiffness of the myocardial microenvironment in which they reside.NEW & NOTEWORTHY Our findings highlight the advantages and pitfalls associated with culturing cardiac fibroblasts on hydrogels of varying stiffness. The findings also define stiffness-dependent signaling and transcriptional networks in cardiac fibroblasts.

Guidelines on the use of sex and gender in cardiovascular research

In cardiovascular research, sex and gender have not typically been considered in research design and reporting until recently. This has resulted in clinical research findings from which not only all women, but also gender-diverse individuals have been excluded. The resulting dearth of data has led to a lack of sex- and gender-specific clinical guidelines and raises serious questions about evidence-based care. Basic research has also excluded considerations of sex. Including sex and/or gender as research variables not only has the potential to improve the health of society overall now, but it also provides a foundation of knowledge on which to build future advances. The goal of this guidelines article is to provide advice on best practices to include sex and gender considerations in study design, as well as data collection, analysis, and interpretation to optimally establish rigor and reproducibility needed to inform clinical decision-making and improve outcomes. In cardiovascular physiology, incorporating sex and gender is a necessary component when optimally designing and executing research plans. The guidelines serve as the first guidance on how to include sex and gender in cardiovascular research. We provide here a beginning path toward achieving this goal and improve the ability of the research community to interpret results through a sex and gender lens to enable comparison across studies and laboratories, resulting in better health for all.

Fibroblasts under pressure: cardiac fibroblast responses to hypertension and antihypertensive therapies

Approximately 50% of Americans have hypertension, which significantly increases the risk of heart failure. In response to increased peripheral resistance in hypertension, intensified mechanical stretch in the myocardium induces cardiomyocyte hypertrophy and fibroblast activation to withstand increased pressure overload. This changes the structure and function of the heart, leading to pathological cardiac remodeling and eventual progression to heart failure. In the presence of hypertensive stimuli, cardiac fibroblasts activate and differentiate to myofibroblast phenotype capable of enhanced extracellular matrix secretion in coordination with other cell types, mainly cardiomyocytes. Both systemic and local renin-angiotensin-aldosterone system activation lead to increased angiotensin II stimulation of fibroblasts. Angiotensin II directly activates fibrotic signaling such as transforming growth factor β/SMAD and mitogen-activated protein kinase (MAPK) signaling to produce extracellular matrix comprised of collagens and matricellular proteins. With the advent of single-cell RNA sequencing techniques, heterogeneity in fibroblast populations has been identified in the left ventricle in models of hypertension and pressure overload. The various clusters of fibroblasts reveal a range of phenotypes and activation states. Select antihypertensive therapies have been shown to be effective in limiting fibrosis, with some having direct actions on cardiac fibroblasts. The present review focuses on the fibroblast-specific changes that occur in response to hypertension and pressure overload, the knowledge gained from single-cell analyses, and the effect of antihypertensive therapies. Understanding the dynamics of hypertensive fibroblast populations and their similarities and differences by sex is crucial for the advent of new targets and personalized medicine.

Normative Values of Echocardiographic Chamber Size and Function in Older Healthy Adults: The Multi-Ethnic Study of Atherosclerosis

Background

Echocardiographic (2DE) thresholds indicating disease or impaired functional status compared to normal physiologic aging in individuals ≥ 65 years are not clearly defined. In the present study, we sought to establish standard values for 2DE parameters related to chamber size and function in older adults without cardiopulmonary or cardiometabolic conditions.

Methods

In this cross-sectional study of 3032 individuals who underwent 2DE at Exam 6 in the Multi-Ethnic Study of Atherosclerosis (MESA), 608 participants fulfilled our inclusion criteria, with normative values defined as the mean value ± 1.96 standard deviations and compared across sex and race/ethnicity. Functional status measures included NT-proBNP, 6-minute walk distance [6MWD], and Kansas City Cardiomyopathy Questionnaire [KCCQ]. Prognostic performance using MESA cutoffs was compared to established guideline cutoffs using time-to-event analysis.

Results

Participants meeting our inclusion criteria (69.5 ± 7.0 years, 46.2% male, 47.5% White) had lower NT-proBNP, higher 6MWD, and higher (better) KCCQ summary values. Women had significantly smaller chamber sizes and better biventricular systolic function. White participants had the largest chamber dimensions, while Chinese participants had the smallest, even after adjustment for body size. Current guidelines identified 81.6% of healthy older adults in MESA as having cardiac abnormalities.

Conclusions

Among a large, diverse group of healthy older adults, we found significant differences in cardiac structure and function across sexes and races/ethnicities, which may signal sex-specific cardiac remodeling with advancing age. It is crucial for existing guidelines to consider the observed and clinically significant differences in cardiac structure and function associated with healthy aging. Our study highlights that existing guidelines, which grade abnormalities in echocardiographic cardiac chamber size and function based on younger individuals, may not adequately address the anticipated changes associated with normal aging.

Primary cardiac fibroblast cell culture: methodological considerations for physiologically relevant conditions

Primary cardiac fibroblast (CF) tissue culture is a necessary tool for interrogating specific signaling mechanisms that dictate the phenotypic heterogeneity observed in vivo in different disease states. Traditional approaches that use tissue culture plastic and nutrient-rich medium have been shown to induce CF activation and, therefore, alter CF subpopulation composition. This shift away from in vivo phenotypes complicate the interpretation of results through the lens of the animal model. As the field works to identify CF diversity, these methodological flaws have begun to be addressed and more studies are focused on the dynamic interaction of CFs with their environment. This review focuses on the aspects of tissue culture that impact CF activation and, therefore, require consideration when designing in vitro experiments. The complexity of CF biology overlaid onto diverse model systems highlight the need for study-specific optimization and validation.

A defective mechanosensing pathway affects fibroblast-to-myofibroblast transition in the old male mouse heart

The cardiac fibroblast interacts with an extracellular matrix (ECM), enabling myofibroblast maturation via a process called mechanosensing. Although in the aging male heart, ECM is stiffer than in the young mouse, myofibroblast development is impaired, as demonstrated in 2-D and 3-D experiments. In old male cardiac fibroblasts, we found a decrease in actin polymerization, α-smooth muscle actin (α-SMA), and Kindlin-2 expressions, the latter an effector of the mechanosensing. When Kindlin-2 levels were manipulated via siRNA interference, young fibroblasts developed an old-like fibroblast phenotype, whereas Kindlin-2 overexpression in old fibroblasts reversed the defective phenotype. Finally, inhibition of overactivated extracellular regulated kinases 1 and 2 (ERK1/2) in the old male fibroblasts rescued actin polymerization and α-SMA expression. Pathological ERK1/2 overactivation was also attenuated by Kindlin-2 overexpression. In contrast, old female cardiac fibroblasts retained an operant mechanosensing pathway. In conclusion, we identified defective components of the Kindlin/ERK/actin/α-SMA mechanosensing axis in aged male fibroblasts.

Female cardiovascular biology and resilience in the setting of physiological and pathological stress

For years, females were thought of as smaller men with complex hormonal cycles; as a result, females have been largely excluded from preclinical and clinical research. However, in the last ten years, with the increased focus on sex as a biological variable, it has become clear that this is not the case, and in fact, male and female cardiovascular biology and cardiac stress responses differ substantially. Premenopausal women are protected from cardiovascular diseases, such as myocardial infarction and resultant heart failure, having preserved cardiac function, reduced adverse remodeling, and increased survival. Many underlying biological processes that contribute to ventricular remodeling differ between the sexes, such as cellular metabolism; immune cell responses; cardiac fibrosis and extracellular matrix remodeling; cardiomyocyte dysfunction; and endothelial biology; however, it is unclear how these changes afford protection to the female heart. Although many of these changes are dependent on protection provided by female sex hormones, several of these changes occur independent of sex hormones, suggesting that the nature of these changes is more complex than initially thought. This may be why studies focused on the cardiovascular benefits of hormone replacement therapy in post-menopausal women have provided mixed results. Some of the complexity likely stems from the fact that the cellular composition of the heart is sexually dimorphic and that in the setting of MI, different subpopulations of these cell types are apparent. Despite the documented sex-differences in cardiovascular (patho)physiology, the underlying mechanisms that contribute are largely unknown due to inconsistent findings amongst investigators and, in some cases, lack of rigor in reporting and consideration of sex-dependent variables. Therefore, this review aims to describe current understanding of the sex-dependent differences in the myocardium in response to physiological and pathological stressors, with a focus on the sex-dependent differences that contribute to post-infarction remodeling and resultant functional decline.

High-impact opportunities to address ischemia: a focus on heart and circulatory research

Myocardial ischemic injury and its resolution are the key determinants of morbidity or mortality in heart failure. The cause and duration of ischemia in patients vary. Numerous experimental models and methods have been developed to define genetic, metabolic, molecular, cellular, and pathophysiological mechanisms, in addition to defining structural and functional deterioration of cardiovascular performance. The rapid rise of big data, such as single-cell analysis techniques with bioinformatics, machine learning, and neural networking, brings a new level of sophistication to our understanding of myocardial ischemia. This mini-review explores the multifaceted nature of ischemic injury in the myocardium. We highlight recent state-of-the-art findings and strategies to show new directions of high-impact approach to understanding myocardial tissue remodeling. This next age of heart and circulatory physiology research will be more comprehensive and collaborative to uncover the origin, progression, and manifestation of heart failure while strengthening novel treatment strategies.