Identification of novel genes involved in sarcopenia through RNAi screening in Caenorhabditis elegans

A Multifactorial Approach for Sarcopenia Assessment: A Literature Review

Simple Summary

Sarcopenia is characterized by an accelerated decline in skeletal muscle mass and strength, which results in poor quality of life, disability, and death. In the literature, sarcopenia is defined as the progressive breakdown of muscle tissue. The prevalence ranges from 5% to 13% in people 60–70 years old and from 11% to 50% in people older than 80 years. The comparison of risk factors associated with sarcopenia based on the European Working Group on Sarcopenia (1 and 2) in Older People, the Asian Working Group for Sarcopenia (1 and 2), the International Working Group on Sarcopenia, and the Foundation for the National Institutes of Health revealed no consistent patterns. Accordingly, the identification of a single risk factor for sarcopenia is unpredictable. Due to its “multifactorial” pathogenesis related to the involvement of a multitude of factors. In this review, we summarize 13 relevant risk factors associated with this disease that are important to consider prior to embarking on any related sarcopenia research. We suggest that researchers should concentrate on the biology of sarcopenia to develop a uniform consensus for screening this condition. In this review, we identify 50 biochemical markers across six pathways that have previously been investigated in subjects with sarcopenia. We suggest that these summarized biomarkers can be considered in future diagnosis to determine the biology of this disorder, thereby contributing to further research findings. As a result, a uniform consensus may also need to be established for screening and defining the disease. Sarcopenia is associated with a number of adverse economic and social outcomes, including disability, hospitalization, and death. In relation to this, we propose that we need to develop strategies including exercise interventions in the COVID-19 era to delay the onset and effects of sarcopenia. This suggestion should impact on sarcopenia’s primary and secondary outcomes, including physical, medical, social, and financial interactions.

Abstract

Sarcopenia refers to a progressive and generalized weakness of skeletal muscle as individuals age. Sarcopenia usually occurs after the age of 60 years and is associated with a persistent decline in muscle strength, function, and quality. A comparison of the risk factors associated with sarcopenia based on the European Working Group on Sarcopenia (1 and 2) in Older People, the Asian Working Group for Sarcopenia (1 and 2), the International Working Group on Sarcopenia, and the Foundation for the National Institutes of Health revealed no consistent patterns. Accordingly, the identification of a single risk factor for sarcopenia is unpredictable due to its “multifactorial” pathogenesis, with the involvement of a multitude of factors. Therefore, the first aim of this review was to outline and propose that the multiple factors associated with sarcopenia need to be considered in combination in the design of new experimentation in this area. A secondary aim was to highlight the biochemical risk factors that are already identified in subjects with sarcopenia to assist scientists in understanding the biology of the pathophysiological mechanisms affecting the old people with sarcopenia. We also briefly discuss primary outcomes (physical) and secondary outcomes (social and financial) of sarcopenia. For future investigative purposes, this comprehensive review may be useful in considering important risk factors in the utilization of a panel of biomarkers emanating from all pathways involved in the pathogenesis of this disease. This may help to establish a uniform consensus for screening and defining this disease. Considering the COVID-19 pandemic, its impact may be exacerbated in older populations, which requires immediate attention. Here, we briefly suggest strategies for advancing the development of smart technologies to deliver exercise in the COVID-19 era in an attempt regress the onset of sarcopenia. These strategies may also have an impact on sarcopenia’s primary and secondary outcomes.

Prediction of biological age by morphological staging of sarcopenia in Caenorhabditis elegans

Sarcopenia encompasses a progressive decline in muscle quantity and quality. Given its close association with ageing, it may represent a valuable healthspan marker. The commonalities with human muscle structure and facile visualization possibilities make Caenorhabditis elegans an attractive model for studying the relationship between sarcopenia and healthspan. However, classical visual assessment of muscle architecture is subjective and has low throughput. To resolve this, we have developed an image analysis pipeline for the quantification of muscle integrity in confocal microscopy images from a cohort of ageing myosin::GFP reporter worms. We extracted a variety of morphological descriptors and found a subset to scale linearly with age. This allowed establishing a linear model that predicts biological age from a morphological muscle signature. To validate the model, we evaluated muscle architecture in long-lived worms that are known to experience delayed sarcopenia by targeted knockdown of the daf-2 gene. We conclude that quantitative microscopy allows for staging sarcopenia in C. elegans and may foster the development of image-based screens in this model organism to identify modulators that mitigate age-related muscle frailty and thus improve healthspan.

Summary: A tool for quantitative image analysis of muscle deterioration that allows predicting healthspan in the nematode model Caenorhabditis elegans and may lead to the first C. elegans-based high-throughput sarcopenia screening platform.

Analysis of representative mutants for key DNA repair pathways on healthspan in Caenorhabditis elegans

Although the link between DNA damage and aging is well accepted, the role of different DNA repair proteins on functional/physiological aging is not well-defined. Here, using Caenorhabditis elegans, we systematically examined the effect of three DNA repair genes involved in key genome stability pathways. We assayed multiple health proxies including molecular, functional and resilience measures to define healthspan. Loss of XPF-1/ERCC-1, a protein involved in nucleotide excision repair (NER), homologous recombination (HR) and interstrand crosslink (ICL) repair, showed the highest impairment of functional and stress resilience measures along with a shortened lifespan. brc-1 mutants, with a well-defined role in HR and ICL are short-lived and highly sensitive to acute stressors, specifically oxidative stress. In contrast, ICL mutant, fcd-2 did not impact lifespan or most healthspan measures. Our efforts also uncover that DNA repair mutants show high sensitivity to oxidative stress with age, suggesting that this measure could act as a primary proxy for healthspan. Together, these data suggest that impairment of multiple DNA repair genes can drive functional/physiological aging. Further studies to examine specific DNA repair genes in a tissue specific manner will help dissect the importance and mechanistic role of these repair systems in biological aging.

Gluconeogenesis and PEPCK are critical components of healthy aging and dietary restriction life extension

High glucose diets are unhealthy, although the mechanisms by which elevated glucose is harmful to whole animal physiology are not well understood. In Caenorhabditis elegans, high glucose shortens lifespan, while chemically inflicted glucose restriction promotes longevity. We investigated the impact of glucose metabolism on aging quality (maintained locomotory capacity and median lifespan) and found that, in addition to shortening lifespan, excess glucose negatively impacts locomotory healthspan. Conversely, disrupting glucose utilization by knockdown of glycolysis-specific genes results in large mid-age physical improvements via a mechanism that requires the FOXO transcription factor DAF-16. Adult locomotory capacity is extended by glycolysis disruption, but maximum lifespan is not, indicating that limiting glycolysis can increase the proportion of life spent in mobility health. We also considered the largely ignored role of glucose biosynthesis (gluconeogenesis) in adult health. Directed perturbations of gluconeogenic genes that specify single direction enzymatic reactions for glucose synthesis decrease locomotory healthspan, suggesting that gluconeogenesis is needed for healthy aging. Consistent with this idea, overexpression of the central gluconeogenic gene pck-2 (encoding PEPCK) increases health measures via a mechanism that requires DAF-16 to promote pck-2 expression in specific intestinal cells. Dietary restriction also features DAF-16-dependent pck-2 expression in the intestine, and the healthspan benefits conferred by dietary restriction require pck-2. Together, our results describe a new paradigm in which nutritional signals engage gluconeogenesis to influence aging quality via DAF-16. These data underscore the idea that promotion of gluconeogenesis might be an unappreciated goal for healthy aging and could constitute a novel target for pharmacological interventions that counter high glucose consequences, including diabetes.

It is known that high levels of dietary sugar can negatively impact human health, but the mechanisms underlying this remain unclear. Here we use the facile Caenorhabditis elegans genetic model to extend understanding of the impact of glucose and glucose metabolism on health and aging. We show that the two opposing glucose metabolism pathways–glycolysis and gluconeogenesis–have dramatically opposite effects on health: glycolytic activity responsible for sugar catabolism is detrimental, but driving gluconeogenesis promotes healthy aging. The powerful longevity regulator DAF-16 is required for the healthspan effects of gluconeogenesis. Our data highlight the intriguing possibility that driving the biosynthetic gluconeogenesis pathway could be a novel strategy for healthspan promotion. Indeed, we find that increasing levels of the core gluconeogenic enzyme PEPCK (PCK-2) in just a few intestinal cells can increase overall health in a DAF-16-dependent manner. Dietary restriction, which can promote health and longevity across species, increases PCK-2 levels in the intestine via DAF-16, and PCK-2 is required for the health benefits seen when calories are limited. Our results define gluconeogenic metabolism as a key component of healthy aging, and suggest that interventions that promote gluconeogenesis may help combat the onset of age-related diseases, including diabetes.

Cell-autonomous and non-autonomous roles of daf-16 in muscle function and mitochondrial capacity in aging C. elegans

Sarcopenia, defined as the loss of skeletal muscle mass and strength, contributes to disability and health-related conditions with aging. In vitro studies indicate that age-related mitochondrial dysfunction could play a central role in the development and progression of sarcopenia, but because of limitations in the methods employed, how aging affects muscle mitochondrial function in vivo is not fully understood. We use muscle-targeted fluorescent proteins and the ratiometric ATP reporter, ATeam, to examine changes in muscle mitochondrial mass and morphology, and intracellular ATP levels in C. elegans. We find that the preserved muscle function in aging daf-2 mutants is associated with higher muscle mitochondrial mass, preserved mitochondrial morphology, and higher levels of intracellular ATP. These phenotypes require the daf-16/FOXO transcription factor. Via the tissue-specific rescue of daf-16, we find that daf-16 activity in either muscle or neurons is sufficient to enhance muscle mitochondrial mass, whereas daf-16 activity in the muscle is required for the enhanced muscle function and mobility of the daf-2 mutants. Finally, we show through the use of drugs known to enhance mitochondrial activity that augmenting mitochondrial function leads to improved mobility during aging. These results suggest an important role for mitochondrial function in muscle aging.

Multiple Molecular Mechanisms Rescue mtDNA Disease in C. elegans

Genetic instability of the mitochondrial genome (mtDNA) plays an important role in human aging and disease. Thus far, it has proven difficult to develop successful treatment strategies for diseases that are caused by mtDNA instability. To address this issue, we developed a model of mtDNA disease in the nematode C. elegans, an animal model that can rapidly be screened for genes and biological pathways that reduce mitochondrial pathology. These worms recapitulate all the major hallmarks of mtDNA disease in humans, including increased mtDNA instability, loss of respiration, reduced neuromuscular function, and a shortened lifespan. We found that these phenotypes could be rescued by intervening in numerous biological pathways, including IGF-1/insulin signaling, mitophagy, and the mitochondrial unfolded protein response, suggesting that it may be possible to ameliorate mtDNA disease through multiple molecular mechanisms.

Relationship between the changes over time of bone mass and muscle health in children and adults: a systematic review and meta-analysis

Background

Various cross-sectional studies provide an abundance of evidence that shows a relationship between bone quantity and muscle health. However, one question remains, less-often studied: is their development - or decline – associated? The aim of the research was to conduct a systematic review and meta-analysis to summarize the studies exploring the association between changes in bone mineral density (BMD) and changes in muscle parameters (registration CRD42018093813).

Methods

We searched for prospective studies, both in children and adults, by consulting electronic databases (Ovid-MEDLINE, Ovid-AMED, Scopus). Each review steps were performed by two independent reviewers. For outcomes reported by less of 3 studies, we synthetized the results narratively. In other cases, a meta-analysis was performed, giving an overall r coefficient and its 95% confidence interval (CI).

Results

Fifteen papers were included. In connection with the change of BMD, 10 studies concerned the parallel change of lean mass, 4 were about grip strength, and 1 was about physical performance. Children were the population of interest for 5 studies, while the aging population was the focus of the other studies. The correlation between hip BMD and lean mass was significant, with an overall coefficient r = 0.37 (95% CI 0.23–0.49). High heterogeneity was observed between studies but the length of follow-up, sex and study quality did not seem to significantly influence results. The systematic review allowed some other highlights: a significant link between changes in BMD and changes in muscle strength was observed (p-value < 0.05 in the 4 studies), in addition to changes in performance (1 study, r = 0.21, p-value = 0.004).

Conclusion

Despite the heterogeneity between studies, we highlighted a significant association between the change of BMD and the change of various muscle parameters. Future studies should investigate preventive and therapeutic strategies that are based on a single entity: the ‘muscle-bone unit’.

Functional genomics applications and therapeutic implications in sarcopenia

The human genome contains around 20,000-25,000 genes coding for 30,000 proteins. Some proteins and genes represent therapeutic targets for human diseases. RNA and protein expression profiling tools allow the study of the molecular basis of aging and drug discovery validation. Throughout the life, there is an age-related and disease-related muscle decline. Sarcopenia is defined as a loss of muscle mass and a decrease in functional properties such as muscle strength and physical performance. Yet, there is still no consensus on the evaluation methods of sarcopenia prognosis. The main challenge of this complex biological phenomena is its multifactorial etiology. Thus, functional genomics methods attempt to shape the related scientific approaches via an innovative in-depth view on sarcopenia. Gene and drug high throughput screening combined with functional genomics allow the generation and the interpretation of a large amount of data related to sarcopenia and therapeutic progress. This review focuses on the application of selected functional genomics techniques such as RNA interference, RNA silencing, proteomics, transgenic mice, metabolomics, genomics, and epigenomics to better understand sarcopenia mechanisms.

Sarcopenia, Aging and Prospective Interventional Strategies

Sarcopenia, or age-related muscle decline, occurs in most organisms and burdens both human health and the healthcare system. As our population ages, additional options for treating sarcopenia are needed. Mitochondrial dysfunction is implicated in the onset of sarcopenia, so therapies directed at improving mitochondrial function in muscle should be considered. Many naturally-occurring compounds, derived from commonly consumed foods, possess anti-sarcopenic effects, such asnicotinamide riboside, tomatidine, and Urolithin A. These naturally-occurring compounds can improve mitochondrial health and efficiency by modulating mitochondrial biogenesis, cellular stress resistance, or mitophagy. Further research should assess whether compounds that improve mitochondrial health can attenuate sarcopenia in humans.

The ESCRT-II proteins are involved in shaping the sarcoplasmic reticulum in C. elegans

The sarcoplasmic reticulum is a network of tubules and cisternae localized in close association with the contractile apparatus, and regulates Ca(2+)dynamics within striated muscle cell. The sarcoplasmic reticulum maintains its shape and organization despite repeated muscle cell contractions, through mechanisms which are still under investigation. The ESCRT complexes are essential to organize membrane subdomains and modify membrane topology in multiple cellular processes. Here, we report for the first time that ESCRT-II proteins play a role in the maintenance of sarcoplasmic reticulum integrity inC. elegans ESCRT-II proteins colocalize with the sarcoplasmic reticulum marker ryanodine receptor UNC-68. The localization at the sarcoplasmic reticulum of ESCRT-II and UNC-68 are mutually dependent. Furthermore, the characterization of ESCRT-II mutants revealed a fragmentation of the sarcoplasmic reticulum network, associated with an alteration of Ca(2+)dynamics. Our data provide evidence that ESCRT-II proteins are involved in sarcoplasmic reticulum shaping.

Regulation of structure and function of sarcomeric actin filaments in striated muscle of the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has been used as a valuable system to study structure and function of striated muscle. The body wall muscle of C. elegans is obliquely striated muscle with highly organized sarcomeric assembly of actin, myosin, and other accessory proteins. Genetic and molecular biological studies in C. elegans have identified a number of genes encoding structural and regulatory components for the muscle contractile apparatuses, and many of them have counterparts in mammalian cardiac and skeletal muscles or striated muscles in other invertebrates. Applicability of genetics, cell biology, and biochemistry has made C. elegans an excellent system to study mechanisms of muscle contractility and assembly and maintenance of myofibrils. This review focuses on the regulatory mechanisms of structure and function of actin filaments in the C. elegans body wall muscle. Sarcomeric actin filaments in C. elegans muscle are associated with the troponin-tropomyosin system that regulates the actin-myosin interaction. Proteins that bind to the side and ends of actin filaments support ordered assembly of thin filaments. Furthermore, regulators of actin dynamics play important roles in initial assembly, growth, and maintenance of sarcomeres. The knowledge acquired in C. elegans can serve as bases to understand the basic mechanisms of muscle structure and function.

Meta-analysis of global metabolomic data identifies metabolites associated with life-span extension

The manipulation of distinct signaling pathways and transcription factors has been shown to influence life span in a cell-non-autonomous manner in multicellular model organisms such as Caenorhabditis elegans. These data suggest that coordination of whole-organism aging involves endocrine signaling, however, the molecular identities of such signals have not yet been determined and their potential relevance in humans is unknown. Here we describe a novel metabolomic approach to identify molecules directly associated with extended life span in C. elegans that represent candidate compounds for age-related endocrine signals. To identify metabolic perturbations directly linked to longevity, we developed metabolomic software for meta-analysis that enabled intelligent comparisons of multiple different mutants. Simple pairwise comparisons of long-lived glp-1, daf-2, and isp-1 mutants to their respective controls resulted in more than 11,000 dysregulated metabolite features of statistical significance. By using meta-analysis, we were able to reduce this number to six compounds most likely to be associated with life-span extension. Mass spectrometry-based imaging studies suggested that these metabolites might be localized to C. elegans muscle. We extended the metabolomic analysis to humans by comparing quadricep muscle tissue from young and old individuals and found that two of the same compounds associated with longevity in worms were also altered in human muscle with age. These findings provide candidate compounds that may serve as age-related endocrine signals and implicate muscle as a potential tissue regulating their levels in humans.

The C. elegans healthspan and stress-resistance assay toolkit

A wealth of knowledge on the genetic mechanisms that govern aging has emerged from the study of mutants that exhibit enhanced longevity and exceptional resilience to adverse environmental conditions. In these studies, lifespan has been an excellent proxy for establishing the rate of aging, but it is not always correlated with qualitative measures of healthy aging or 'healthspan'. Although the attributes of healthspan have been challenging to define, they share some universal features that are increasingly being incorporated into aging studies. Here we describe methods used to determine Caenorhabditis elegans healthspan. These include assessments of tissue integrity and functionality and resistance to a variety of biotic and abiotic stressors. We have chosen to include simple, rapid assays in this collection that can be easily undertaken in any C. elegans laboratory, and can be relied on to provide a preliminary but thorough insight into the healthspan of a population.