Inducible knockdown of pregnancy-associated plasma protein-A gene expression in adult female mice extends life span

Response eQTLs, chromatin accessibility, and 3D chromatin structure in chondrocytes provide mechanistic insight into osteoarthritis risk

Osteoarthritis (OA) poses a significant healthcare burden with limited treatment options. While genome-wide association studies (GWAS) have identified over 100 OA-associated loci, translating these findings into therapeutic targets remains challenging. Integrating expression quantitative trait loci (eQTL), 3D chromatin structure, and other genomic approaches with OA GWAS data offers a promising approach to elucidate disease mechanisms; however, comprehensive eQTL maps in OA-relevant tissues and conditions remain scarce. We mapped gene expression, chromatin accessibility, and 3D chromatin structure in primary human articular chondrocytes in both resting and OA-mimicking conditions. We identified thousands of differentially expressed genes, including those associated with differences in sex and age. RNA-seq in chondrocytes from 101 donors across two conditions uncovered 3782 unique eGenes, including 420 that exhibited strong and significant condition-specific effects. Colocalization with OA GWAS signals revealed 13 putative OA risk genes, 10 of which have not been previously identified. Chromatin accessibility and 3D chromatin structure provided insights into the mechanisms and conditional specificity of these variants. Our findings shed light on OA pathogenesis and highlight potential targets for therapeutic development.

Highlights

∘ Comprehensive analysis of sex- and age-related global gene expression in human chondrocytes revealed differences that correlate with osteoarthritis ∘ First response eQTLs in chondrocytes treated with an OA-related stimulus ∘ Deeply sequenced Hi-C in resting and activated chondrocytes helps connect OA risk variants to their putative causal genes ∘ Colocalization analysis reveals 13 (including 10 novel) putative OA risk genes.

The Pregnancy-Associated Plasma Protein-A (PAPP-A) Story

Pregnancy-associated plasma protein-A (PAPP-A) was first identified in the early 1970s as a placental protein of unknown function, present at high concentrations in the circulation of pregnant women. In the mid-to-late 1990s, PAPP-A was discovered to be a metzincin metalloproteinase, expressed by many nonplacental cells, that regulates local insulin-like growth factor (IGF) activity through cleavage of high-affinity IGF binding proteins (IGFBPs), in particular IGFBP-4. With PAPP-A as a cell surface-associated enzyme, the reduced affinity of the cleavage fragments results in increased IGF available to bind and activate IGF receptors in the pericellular environment. This proteolytic regulation of IGF activity is important, since the IGFs promote proliferation, differentiation, migration, and survival in various normal and cancer cells. Thus, there has been a steady growth in investigation of PAPP-A structure and function outside of pregnancy. This review provides historical perspective on the discovery of PAPP-A and its structure and cellular function, highlights key studies of the first 50 years in PAPP-A research, and introduces new findings from recent years.

Heritability and circulating concentrations of pregnancy-associated plasma protein-A and stanniocalcin-2 in elderly monozygotic and dizygotic twins

Introduction

Pregnancy-associated plasma protein-A (PAPP-A) is an IGF-activating enzyme suggested to influence aging-related diseases. However, knowledge on serum PAPP-A concentration and regulation in elderly subjects is limited. Therefore, we measured serum PAPP-A in elderly same-sex monozygotic (MZ) and dizygotic (DZ) twins, as this allowed us to describe the age-relationship of PAPP-A, and to test the hypothesis that serum PAPP-A concentrations are genetically determined. As PAPP-A is functionally related to stanniocalcin-2 (STC2), an endogenous PAPP-A inhibitor, we included measurements on STC2 as well as IGF-I and IGF-II.

Methods

The twin cohort contained 596 subjects (250 MZ twins, 346 DZ twins), whereof 33% were males. The age ranged from 73.2 to 94.3 (mean 78.8) years. Serum was analyzed for PAPP-A, STC2, IGF-I, and IGF-II by commercial immunoassays.

Results

In the twin cohort, PAPP-A increased with age (r=0.19; P<0.05), whereas IGF-I decreased (r=-0.12; P<0.05). Neither STC2 nor IGF-II showed any age relationship. When analyzed according to sex, PAPP-A correlated positively with age in males (r=0.18; P<0.05) and females (r=0.25; P<0.01), whereas IGF-I correlated inversely in females only (r=-0.15; P<0.01). Males had higher levels of PAPP-A (29%), STC2 (18%) and IGF-I (19%), whereas serum IGF-II was 28% higher in females (all P<0.001). For all four proteins, within-pair correlations were significantly higher for MZ twins than for DZ twins, and they demonstrated substantial and significant heritability, which after adjustment for age and sex averaged 59% for PAPP-A, 66% for STC2, 58% for IGF-I, and 52% for IGF-II.

Discussion

This twin study confirms our hypothesis that the heritability of PAPP-A serum concentrations is substantial, and the same is true for STC2. As regards the age relationship, PAPP-A increases with age, whereas STC2 remains unchanged, thereby supporting the idea that the ability of STC2 to inhibit PAPP-A enzymatic activity decreases with increasing age.

Senescent Cells: A Therapeutic Target in Cardiovascular Diseases

Senescent cell accumulation has been observed in age-associated diseases including cardiovascular diseases. Senescent cells lack proliferative capacity and secrete senescence-associated secretory phenotype (SASP) factors that may cause or worsen many cardiovascular diseases. Therapies targeting senescent cells, especially senolytic drugs that selectively induce senescent cell removal, have been shown to delay, prevent, alleviate, or treat multiple age-associated diseases in preclinical models. Some senolytic clinical trials have already been completed or are underway for a number of diseases and geriatric syndromes. Understanding how cellular senescence affects the various cell types in the cardiovascular system, such as endothelial cells, vascular smooth muscle cells, fibroblasts, immune cells, progenitor cells, and cardiomyocytes, is important to facilitate translation of senotherapeutics into clinical interventions. This review highlights: (1) the characteristics of senescent cells and their involvement in cardiovascular diseases, focusing on the aforementioned cardiovascular cell types, (2) evidence about senolytic drugs and other senotherapeutics, and (3) the future path and clinical potential of senotherapeutics for cardiovascular diseases.

Senescence induces proteolytically-active PAPP-A secretion and association with extracellular vesicles in human pre-adipocytes

Senescence is a cellular response to various stressors characterized by irreversible cell cycle arrest, resistance to apoptosis and expression of a senescence-associated secretory phenotype (SASP). Interestingly, studies where senescent cells were deleted in mice produced beneficial effects similar to those where the zinc metalloproteinase, PAPP-A, was deleted in mice. In this study, we investigated the effect of senescence on PAPP-A secretion and activity in primary cultures of adult human pre-adipocytes. Cultured pre-adipocytes were isolated from subcutaneous (Sub) and omental (Om) fat. Senescence was induced with low dose etoposide. PAPP-A protein was measured by an ultrasensitive PAPP-A ELISA. PAPP-A proteolytic activity was measured by a specific substrate cleavage assay. Senescence significantly increased PAPP-A levels in both Sub and Om conditioned medium (CM) 8- to 15-fold over non-senescent CM. Proteolytic activity reflected PAPP-A protein with 12- to 18-fold greater activity in senescent CM versus non-senescent CM. Furthermore, PAPP-A was found at high levels on the surface of extracellular vesicles secreted by senescent pre-adipocytes and was proteolytically active. In conclusion, we identified enzymatically active PAPP-A as a component of human pre-adipocyte SASP. This recognition warrants further investigation of PAPP-A as a new biomarker for senescence and a potential therapeutic target to control of the spread of senescence in adipose tissue.

Early Life Exposure to a Diet With a Supramolecular Lipid Structure Close to That of Mammalian Milk Improves Early Life Growth, Skeletal Development, and Later Life Neurocognitive Function in Individually and Socially Housed Male C57BL/6J Mice

Breastfeeding (duration) can be positively associated with infant growth outcomes as well as improved cognitive functions during childhood and later life stages. (Prolonged) exposure to optimal lipid quantity and quality, i.e., the supramolecular structure of lipids, in mammalian milk, may contribute to these beneficial effects through nutritional early-life programming. In this pre-clinical study, we exposed male C57BL/6J mice from post-natal Days 16 to 42 (i.e., directly following normal lactation), to a diet with large lipid droplets coated with bovine milk fat globule membrane-derived phospholipids, which mimic more closely the supramolecular structure of lipid droplets in mammalian milk. We investigated whether exposure to this diet could affect growth and brain development-related parameters. As these outcomes are also known to be affected by the post-weaning social environment in mice, we included both individually housed and pair-wise housed animals and studied whether effects of diet were modulated by the social environment. After Day 42, all the animals were fed standard semi-synthetic rodent diet. Growth and body composition were assessed, and the mice were subjected to various behavioral tests. Individual housing attenuated adolescent growth, reduced femur length, and increased body fat mass. Adult social interest was increased due to individual housing, while cognitive and behavioral alterations as a result of different housing conditions were modest. The diet increased adolescent growth and femur length, increased lean body mass, reduced adolescent anxiety, and improved adult cognitive performance. These effects of diet exposure were comparable between individually and socially housed mice. Hence, early life exposure to a diet with lipid droplets that mimic the supramolecular structure of those in mammalian milk may improve adolescent growth and alters brain function in both socially and individually housed mice. These findings suggest that lipid structure in infant milk formula may be a relevant target for nutritional solutions, targeting both healthy infants and infants facing growth challenges.

Mice with gene alterations in the GH and IGF family

Much of our understanding of GH's action stems from animal models and the generation and characterization of genetically altered or modified mice. Manipulation of genes in the GH/IGF1 family in animals started in 1982 when the first GH transgenic mice were produced. Since then, multiple laboratories have altered mouse DNA to globally disrupt Gh, Ghr, and other genes upstream or downstream of GH or its receptor. The ability to stay current with the various genetically manipulated mouse lines within the realm of GH/IGF1 research has been daunting. As such, this review attempts to consolidate and summarize the literature related to the initial characterization of many of the known gene-manipulated mice relating to the actions of GH, PRL and IGF1. We have organized the mouse lines by modifications made to constituents of the GH/IGF1 family either upstream or downstream of GHR or to the GHR itself. Available data on the effect of altered gene expression on growth, GH/IGF1 levels, body composition, reproduction, diabetes, metabolism, cancer, and aging are summarized. For the ease of finding this information, key words are highlighted in bold throughout the main text for each mouse line and this information is summarized in Tables 1, 2, 3 and 4. Most importantly, the collective data derived from and reported for these mice have enhanced our understanding of GH action.

Molecular mechanisms of dietary restriction promoting health and longevity

Dietary restriction with adequate nutrition is the gold standard for delaying ageing and extending healthspan and lifespan in diverse species, including rodents and non-human primates. In this Review, we discuss the effects of dietary restriction in these mammalian model organisms and discuss accumulating data that suggest that dietary restriction results in many of the same physiological, metabolic and molecular changes responsible for the prevention of multiple ageing-associated diseases in humans. We further discuss how different forms of fasting, protein restriction and specific reductions in the levels of essential amino acids such as methionine and the branched-chain amino acids selectively impact the activity of AKT, FOXO, mTOR, nicotinamide adenine dinucleotide (NAD⁺), AMP-activated protein kinase (AMPK) and fibroblast growth factor 21 (FGF21), which are key components of some of the most important nutrient-sensing geroprotective signalling pathways that promote healthy longevity.

Growth hormone receptor gene disruption in mature-adult mice improves male insulin sensitivity and extends female lifespan

Studies in multiple species indicate that reducing growth hormone (GH) action enhances healthy lifespan. In fact, GH receptor knockout (GHRKO) mice hold the Methuselah prize for the world's longest-lived laboratory mouse. We previously demonstrated that GHR ablation starting at puberty (1.5 months), improved insulin sensitivity and female lifespan but results in markedly reduced body size. In this study, we investigated the effects of GHR disruption in mature-adult mice at 6 months old (6mGHRKO). These mice exhibited GH resistance (reduced IGF-1 and elevated GH serum levels), increased body adiposity, reduced lean mass, and minimal effects on body length. Importantly, 6mGHRKO males have enhanced insulin sensitivity and reduced neoplasms while females exhibited increased median and maximal lifespan. Furthermore, fasting glucose and oxidative damage was reduced in females compared to males irrespective of Ghr deletion. Overall, disrupted GH action in adult mice resulted in sexual dimorphic effects suggesting that GH reduction at older ages may have gerotherapeutic effects.

Brain-specific PAPP-A knock-out mice?

PAPP-A knock-out (KO) mice are a valuable model for investigating the effects of down-regulating localized insulin-like growth factor (IGF) action, which has been shown to extend lifespan and healthspan when the PAPP-A gene is globally deleted. Based on previous mouse models of brain-specific reduction in IGF signaling associated with longevity, we sought to generate brain-specific PAPP-A KO mice and determine effects on metabolism and lifespan. Mice with the PAPP-A gene floxed (fPAPP-A) were crossed with Nestin promoter-driven Cre recombinase transgenic mice. This cross-breeding of mice for Nestin-Cre and mice with other floxed target alleles has been used extensively to investigate brain-specific effects. Our cross-breeding generated four genotypes for study: fPAPP-A/Nestin positive (brain-specific PAPP-A KO); fPAPP-A/Nestin negative (Control for floxed PAPP-A); WT/Nestin positive (Control for Nestin-Cre); WT/Nestin negative (Wild-type Control). The basic genotype screen of neonatal tail snip DNA clearly indicated PAPP-A gene status and the presence (pos) or absence (neg) of Nestin-Cre. We then determined tissue specificity of PAPP-A gene excision. We had expected fPAPP-A/pos mice to be relatively brain-specific for PAPP-A gene deletion and the controls (fPAPP-A/neg, WT/neg and WT/pos mice) to show no effect on PAPP-A expression in brain or other tissues. However, in fPAPP-A/neg mice we found evidence of PAPP-A excision in all tissues examined, i.e., in the presumed absence of Nestin-Cre, indicating germline recombination. We further found that fPAPP-A/pos mice showed near complete excision of the PAPP-A gene in brain, but some also showed germline recombination affecting all tissues tested. To determine if the level of excision indicated by tissue genotyping approximated PAPP-A mRNA expression, we performed RT-qPCR. fPAPP-A/pos mice that showed markedly decreased whole brain PAPP-A mRNA expression (~80%), with little or no effect on expression in the other tissues tested, were designated as "brain-specific" PAPP-A KO. fPAPP-A/pos mice that showed germline recombination had similar decreases in PAPP-A expression in brain but also showed 40-65% decreased PAPP-A mRNA expression in other tissues as well, which was especially striking in kidney, tibia, thymus and spleen. These were designated as "non-specific" PAPP-A KO mice. With unknown and unpredictable specificity until harvest, we chose to assess a surrogate marker of lifespan i.e., thymic involution, in 15- to 18-month-old fPAPP-A/pos and WT/pos mice, the latter an important control for a possible effect of Nestin-Cre per se. Diminished thymic involution as indicated by increased thymic weight (135%, P = 0.035) and decreased histological disruption was seen in "non-specific" PAPP-A KO mice, similar to what was previously reported in 18-month-old global PAPP-A KO mice. There was no significant difference between "brain-specific" PAPP-A KO and control mice. This study highlights the importance of thorough characterization of assumed tissue-specific mouse models and awareness of potential germline recombination for proper data interpretation.

Extending lifespan by modulating the growth hormone/insulin-like growth factor-1 axis: coming of age

Progress made in the years of aging research have allowed the opportunity to explore potential interventions to slow aging and extend healthy lifespan. Studies performed in yeast, worms, flies and mice subjected to genetic and pharmacological interventions have given insight into the cellular and molecular mechanisms associated with longevity. Furthermore, it is now possible to effectively modulate pathways that slow aging at different stages of life (early life or at an adult age). Interestingly, interventions that extend longevity in adult mice have had sex-specific success, suggesting a potential link between particular pathways that modulate aging and sex. For example, reduction of the growth hormone (GH)/insulin-like growth factor-1 (IGF-1) axis at an adult age extends lifespan preferentially in females. Moreover, several postnatal dietary interventions tested by the 'Intervention Testing Program (ITP)' from the National Institute of Aging (NIA) have shown that while pharmacological interventions like rapamycin affect the IGF-1/insulin pathway and preferentially extend lifespan in females; dietary compounds that target other cellular pathways are effective only in male mice-indicating mutually exclusive sex-specific pathways. Therefore, a combination of interventions that target non-overlapping aging-related pathways appears to be an effective approach to further extend healthy lifespan in both sexes. Here, we review the germline and postnatal mouse lines that target the GH/IGF-1 axis as a mechanism to extend longevity as well as the dietary compounds that tested positive in the NIA program to increase lifespan. We believe that the interventions reviewed in this paper could constitute feasible combinations for an extended healthy lifespan in both male and female mice.

Inhibition of longevity regulator PAPP-A modulates tissue homeostasis via restraint of mesenchymal stromal cells

Pregnancy‐associated plasma protein‐A (PAPP‐A) is a secreted metalloprotease that increases insulin‐like growth factor (IGF) availability by cleaving IGF‐binding proteins. Reduced IGF signaling extends longevity in multiple species, and consistent with this, PAPP‐A deletion extends lifespan and healthspan; however, the mechanism remains unclear. To clarify PAPP‐A’s role, we developed a PAPP‐A neutralizing antibody and treated adult mice with it. Transcriptomic profiling across tissues showed that anti‐PAPP‐A reduced IGF signaling and extracellular matrix (ECM) gene expression system wide. The greatest reduction in IGF signaling occurred in the bone marrow, where we found reduced bone, marrow adiposity, and myelopoiesis. These diverse effects led us to search for unifying mechanisms. We identified mesenchymal stromal cells (MSCs) as the source of PAPP‐A in bone marrow and primary responders to PAPP‐A inhibition. Mice treated with anti‐PAPP‐A had reduced IGF signaling in MSCs and dramatically decreased MSC number. As MSCs are (1) a major source of ECM and the progenitors of ECM‐producing fibroblasts, (2) the originating source of adult bone, (3) regulators of marrow adiposity, and (4) an essential component of the hematopoietic niche, our data suggest that PAPP‐A modulates bone marrow homeostasis by potentiating the number and activity of MSCs. We found that MSC‐like cells are the major source of PAPP‐A in other tissues also, suggesting that reduced MSC‐like cell activity drives the system‐wide reduction in ECM gene expression due to PAPP‐A inhibition. Dysregulated ECM production is associated with aging and drives age‐related diseases, and thus, this may be a mechanism by which PAPP‐A deficiency enhances longevity.

ARDD 2020: from aging mechanisms to interventions

Aging is emerging as a druggable target with growing interest from academia, industry and investors. New technologies such as artificial intelligence and advanced screening techniques, as well as a strong influence from the industry sector may lead to novel discoveries to treat age-related diseases. The present review summarizes presentations from the 7th Annual Aging Research and Drug Discovery (ARDD) meeting, held online on the 1st to 4th of September 2020. The meeting covered topics related to new methodologies to study aging, knowledge about basic mechanisms of longevity, latest interventional strategies to target the aging process as well as discussions about the impact of aging research on society and economy. More than 2000 participants and 65 speakers joined the meeting and we already look forward to an even larger meeting next year. Please mark your calendars for the 8th ARDD meeting that is scheduled for the 31st of August to 3rd of September, 2021, at Columbia University, USA.

Effects of suppressing bioavailability of insulin-like growth factor on age-associated intervertebral disc degeneration

Suppression of the insulin-like growth factor-1 (IGF-1) signaling pathway reduces age-related disorders and increases lifespan across species, making the IGF-1 pathway a key regulator of aging. Previous in vitro intervertebral disc cell studies have reported the pro-anabolic effect of exogenously adding IGF-1 on matrix production. However, the overall effects of suppressing IGF-1 signaling on age-related intervertebral disc degeneration (IDD) is not known. Here, the effects of suppressing IGF-1 signaling on age-related IDD in vivo were examined using PAPPA -/- mice. These are animals with targeted deletion of pregnancy-associated plasma protein A (PAPPA), the major protease that cleaves inhibitory IGF binding proteins that control bioavailability of IGF-1 for cell signaling. Compared to age-matched wild-type (Wt) littermates, reduced levels of matrix proteoglycan (PG) and aggrecan were seen in discs of 23-month old PAPPA -/- mice. Decreased aggrecanolysis and expression of two key catabolic markers, matrix metalloproteinase-3 and a disintegrin and metalloproteinase with thrombospondin motifs-4, were also observed in discs of old PAPPA -/- mice compared to Wt littermates. Suppressing IGF-1 signaling has been implicated to shift cellular metabolism toward maintenance rather than growth and decreasing cellular senescence. Along this line, discs of old PAPPA -/- mice also exhibited lower cellular senescence, assessed by p53 and lamin B1 markers. Collectively, the data reveal complex regulation of disc matrix homeostasis by PAPPA/IGF-1 signaling during chronologic aging, that is, reduced IGF-1 bioavailability confers the benefit of decreasing disc cellular senescence and matrix catabolism but also the disadvantage of decreasing disc PG matrix anabolism. This pathway requires further mechanistic elucidation before IGF-1 could be considered as a therapeutic growth factor for treating IDD.

Growth Hormone and Aging: Updated Review

Role of growth hormone (GH) in mammalian aging is actively explored in clinical, epidemiological, and experimental studies. The age-related decline in GH levels is variously interpreted as a symptom of neuroendocrine aging, as one of causes of altered body composition and other unwelcome symptoms of aging, or as a mechanism of natural protection from cancer and other chronic diseases. Absence of GH signals due to mutations affecting anterior pituitary development, GH secretion, or GH receptors produces an impressive extension of longevity in laboratory mice. Extension of healthspan in these animals and analysis of survival curves suggest that in the absence of GH, aging is slowed down or delayed. The corresponding endocrine syndromes in the human have no consistent impact on longevity, but are associated with remarkable protection from age-related disease. Moreover, survival to extremely old age has been associated with reduced somatotropic (GH and insulin-like growth factor-1) signaling in women and men. In both humans and mice, elevation of GH levels into the supranormal (pathological) range is associated with increased disease risks and reduced life expectancy likely representing acceleration of aging. The widely advertised potential of GH as an anti-aging agent attracted much interest. However, results obtained thus far have been disappointing with few documented benefits and many troublesome side effects. Possible utility of GH in the treatment of sarcopenia and frailty remains to be explored.