Mapping mechanisms and charting the time course of premature cell senescence and apoptosis: lysosomal dysfunction and ganglioside accumulation in endothelial cells

Heart failure in diabetes

Heart failure and cardiovascular disorders represent the leading cause of death in diabetic patients. Here we present a systematic review of the main mechanisms underlying the development of diabetic cardiomyopathy. We also provide an excursus on the relative contribution of cardiomyocytes, fibroblasts, endothelial and smooth muscle cells to the pathophysiology of heart failure in diabetes. After having described the preclinical tools currently available to dissect the mechanisms of this complex disease, we conclude with a section on the most recent updates of the literature on clinical management.

The metabolic roots of senescence: mechanisms and opportunities for intervention

Cellular senescence entails a permanent proliferative arrest, coupled to multiple phenotypic changes. Among these changes is the release of numerous biologically active molecules collectively known as the senescence-associated secretory phenotype, or SASP. A growing body of literature indicates that both senescence and the SASP are sensitive to cellular and organismal metabolic states, which in turn can drive phenotypes associated with metabolic dysfunction. Here, we review the current literature linking senescence and metabolism, with an eye toward findings at the cellular level, including both metabolic inducers of senescence and alterations in cellular metabolism associated with senescence. Additionally, we consider how interventions that target either metabolism or senescent cells might influence each other and mitigate some of the pro-aging effects of cellular senescence. We conclude that the most effective interventions will likely break a degenerative feedback cycle by which cellular senescence promotes metabolic diseases, which in turn promote senescence.

Severity of COVID-19 Patients Predicted by Serum Sphingolipids Signature

The reason behind the high inter-individual variability in response to SARS-CoV-2 infection and patient’s outcome is poorly understood. The present study targets the sphingolipid profile of twenty-four healthy controls and fifty-nine COVID-19 patients with different disease severity. Sera were analyzed by untargeted and targeted mass spectrometry and ELISA. Results indicated a progressive increase in dihydrosphingosine, dihydroceramides, ceramides, sphingosine, and a decrease in sphingosine-1-phosphate. These changes are associated with a serine palmitoyltransferase long chain base subunit 1 (SPTLC1) increase in relation to COVID-19 severity. Severe patients showed a decrease in sphingomyelins and a high level of acid sphingomyelinase (aSMase) that influences monosialodihexosyl ganglioside (GM3) C16:0 levels. Critical patients are characterized by high levels of dihydrosphingosine and dihydroceramide but not of glycosphingolipids. In severe and critical patients, unbalanced lipid metabolism induces lipid raft remodeling, leads to cell apoptosis and immunoescape, suggesting active sphingolipid participation in viral infection. Furthermore, results indicated that the sphingolipid and glycosphingolipid metabolic rewiring promoted by aSMase and GM3 is age-dependent but also characteristic of severe and critical patients influencing prognosis and increasing viral load. AUCs calculated from ROC curves indicated ceramides C16:0, C18:0, C24:1, sphingosine and SPTLC1 as putative biomarkers of disease evolution.

MicroRNAs and obesity-induced endothelial dysfunction: key paradigms in molecular therapy

The endothelium plays a pivotal role in maintaining vascular health. Obesity is a global epidemic that has seen dramatic increases in both adult and pediatric populations. Obesity perturbs the integrity of normal endothelium, leading to endothelial dysfunction which predisposes the patient to cardiovascular diseases. MicroRNAs (miRNAs) are short, single-stranded, non-coding RNA molecules that play important roles in a variety of cellular processes such as differentiation, proliferation, apoptosis, and stress response; their alteration contributes to the development of many pathologies including obesity. Mediators of obesity-induced endothelial dysfunction include altered endothelial nitric oxide synthase (eNOS), Sirtuin 1 (SIRT1), oxidative stress, autophagy machinery and endoplasmic reticulum (ER) stress. All of these factors have been shown to be either directly or indirectly caused by gene regulatory mechanisms of miRNAs. In this review, we aim to provide a comprehensive description of the therapeutic potential of miRNAs to treat obesity-induced endothelial dysfunction. This may lead to the identification of new targets for interventions that may prevent or delay the development of obesity-related cardiovascular disease.

A pH probe inhibits senescence in mesenchymal stem cells

BACKGROUND

Bone marrow-derived mesenchymal stem cells (BMSCs) are gradually getting attention because of its multi-directional differentiation potential, hematopoietic support, and promotion of stem cell implantation. However, cultured BMSCs in vitro possess a very limited proliferation potential, and the presence of stem cell aging has substantially restricted the effect together with the efficiency in clinical treatment. Recently, increasing attention has been paid to the connection between cellular aging and lysosomal acidification as new reports indicated that vacuolar H⁺-ATPase (v-ATPase) activity was altered and lysosomal pH was dysregulated in the process of cellular aging. Therefore, promoting lysosomal acidification might contribute to inhibition of cell senescence. Our previous studies showed that a novel small molecule, 3-butyl-1-chloro imidazo [1, 5-a] pyridine-7-carboxylic acid (SGJ), could selectively and sensitively respond to acidic pH with fast response (within 3 min), but whether SGJ can promote lysosomal acidification and inhibit senescence in BMSCs is unknown.

METHODS

Rat BMSCs were cultured based on our system that had been already documented. BMSCs were treated with SGJ and/or Bafilomycin-A1 (Baf-A1). The co-localization between SGJ and lysosomes was assessed by a confocal microscope. Acridine orange (AO) staining and the Lysosensor™ Green DND-189 reagents were used for indicating changes in lysosomal concentration of H⁺. Changes of senescence were detected by immunoblotting of p21 and senescence-associated beta-galactosidase (SA-β-gal) staining as well as immunofluorescence assay of senescence-associated heterochromatin foci (SAHF). Changes of autophagy were detected by immunoblotting of MAP1LC3 (LC3B) and SQSTM1 (p62). Cell proliferation was determined by flow cytometry. Cell viability was calculated by sulforhodamine B assay (SRB). The V0 proton channel of v-ATPase was knocked down by transfecting with its small interfering RNA (si-ATP6V0C).

RESULTS

Our work showed that SGJ can promote lysosomal acidification and inhibit senescence in BMSCs. Firstly, SGJ and lysosomes were well co-located in senescent BMSCs with the co-localization coefficient of 0.94. Secondly, SGJ increased the concentration of H⁺ and the protein expression of lysosome-associated membrane protein 1 (LAMP1) and lysosome-associated membrane protein 2 (LAMP2). Thirdly, SGJ suppressed the expression of p21 in the senescent BMSCs and reduced SA-β-gal positive cells. Fourthly, SGJ promoted senescent BMSCs' proliferation and protein level of LC3B but reduced the p62/SQSTM1 protein level. Furthermore, experimental group pretreated with 20 μM SGJ showed a stronger red fluorescent intensity, thinner cell morphology, less SA-β-gal positive cell, and less p21 protein level as well as higher cell viability in the presence of Baf-A1. Notably, ATP6V0C knockdown decreased the activity of v-ATPase and SGJ increased the concentration of H⁺.

CONCLUSION

Our work showed that SGJ could inhibit senescence in BMSCs and protect lysosomes by promoting expression of LAMP1 and LAMP2. Meanwhile, SGJ could promote autophagy. Furthermore, our study also suggested that SGJ was a new Baf-A1 antagonist because SGJ could target and occupy the V0 proton channel of v-ATPase.

Vascular endothelium in diabetes

Three decades ago a revolutionary idea was born that ascribed to dysfunctional endothelia some manifestations of diabetes, the Steno hypothesis, so named after the Steno Diabetes Center, Gentofte, in Denmark. Here I briefly outline the accomplishments accrued in the past 15 years to buttress this hypothesis. Those include development of novel technological platforms to examine microcirculatory beds, deeper understanding of patterns of microvascular derangement in diabetes, pathophysiology of nitric oxide synthesis and availability, nitrosative and oxidative stress in diabetes, premature senescence of endothelial cells and the role of sirtuin 1 and lysosomal dysfunction in this process, and the state of endothelial glycocalyx and endothelial progenitor cells in diabetes. These pathophysiological findings may yield some therapeutic benefits.

Stress-Induced Premature Senescence of Endothelial and Endothelial Progenitor Cells

This brief overview of premature senescence of dysfunctional endothelial and endothelial progenitor cells provides information on endothelial cell differentiation and specialization, their ontogeny, and controversies related to endothelial stem and progenitor cells. Stressors responsible for the dysfunction of endothelial and endothelial progenitor cells, as well as cellular mechanisms and consequences of endothelial cell dysfunction are presented. Metabolic signatures of dysfunctional endothelial cells and senescence pathways are described. Emerging strategies to rejuvenate endothelial and endothelial progenitor cells conclude the review.

eEOC-mediated modulation of endothelial autophagy, senescence, and EnMT in murine diabetic nephropathy

Diabetic nephropathy is the most frequent single cause of end-stage renal disease in our society. Microvascular damage is a key event in diabetes-associated organ malfunction. Early endothelial outgrowth cells (eEOCs) act protective in murine acute kidney injury. The aim of the present study was to analyze consequences of eEOC treatment of murine diabetic nephropathy with special attention on endothelial-to-mesenchymal transdifferentiation, autophagy, senescence, and apoptosis. Male C57/Bl6N mice (8–12 wk old) were treated with streptozotocin for 5 consecutive days. Animals were injected with untreated or bone morphogenetic protein (BMP)-5-pretreated syngeneic murine eEOCs on days 2 and 5 after the last streptozotocin administration. Four, eight, and twelve weeks later, animals were analyzed for renal function, proteinuria, interstitial fibrosis, endothelial-to-mesenchymal transition, endothelial autophagy, and senescence. In addition, cultured mature murine endothelial cells were investigated for autophagy, senescence, and apoptosis in the presence of glycated collagen. Diabetes-associated renal dysfunction (4 and 8 wk) and proteinuria (8 wk) were partly preserved by systemic cell treatment. At 8 wk, antiproteinuric effects were even more pronounced after the injection of BMP-5-pretreated cells. The latter also decreased mesenchymal transdifferentiation of the endothelium. At 8 wk, intrarenal endothelial autophagy (BMP-5-treated cells) and senescence (native and BMP-5-treated cells) were reduced. Autophagy and senescence in/of cultured mature endothelial cells were dramatically reduced by eEOC supernatant (native and BMP-5). Endothelial apoptosis decreased after incubation with eEOC medium (native and BMP-5). eEOCs act protective in diabetic nephropathy, and such effects are significantly stimulated by BMP-5. The cells modulate endothelial senescence, autophagy, and apoptosis in a protective manner. Thus, the renal endothelium could serve as a therapeutic target in diabetes-associated kidney dysfunction.

Acidic intracellular Ca(2+) stores and caveolae in Ca(2+) signaling and diabetes

Acidic Ca(2+) stores, particularly lysosomes, are newly discovered players in the well-orchestrated arena of Ca(2+) signaling and we are at the verge of understanding how lysosomes accumulate Ca(2+) and how they release it in response to different chemical, such as NAADP, and physical signals. Additionally, it is now clear that lysosomes play a key role in autophagy, a process that allows cells to recycle components or to eliminate damaged structures to ensure cellular well-being. Moreover, lysosomes are being unraveled as hubs that coordinate both anabolism via insulin signaling and catabolism via AMPK. These acidic vesicles have close contact with the ER and there is a bidirectional movement of information between these two organelles that exquisitely regulates cell survival. Lysosomes also connect with plasma membrane where caveolae are located as specialized regions involved in Ca(2+) and insulin signaling. Alterations of all these signaling pathways are at the core of insulin resistance and diabetes.

Endothelial sirtuin 1 deficiency perpetrates nephrosclerosis through downregulation of matrix metalloproteinase-14: relevance to fibrosis of vascular senescence

Sirtuin 1 (SIRT1) depletion in vascular endothelial cells mediates endothelial dysfunction and premature senescence in diverse cardiovascular and renal diseases. However, the molecular mechanisms underlying these pathologic effects remain unclear. Here, we examined the phenotype of a mouse model of vascular senescence created by genetically ablating exon 4 of Sirt1 in endothelial cells (Sirt1(endo-/-)). Under basal conditions, Sirt1(endo-/-) mice showed impaired endothelium-dependent vasorelaxation and angiogenesis, and fibrosis occurred spontaneously at low levels at an early age. In contrast, induction of nephrotoxic stress (acute and chronic folic acid-induced nephropathy) in Sirt1(endo-/-) mice resulted in robust acute renal functional deterioration followed by an exaggerated fibrotic response compared with control animals. Additional studies identified matrix metalloproteinase-14 (MMP-14) as a target of SIRT1. In the kidneys of Sirt1(endo-/-) mice, impaired angiogenesis, reduced matrilytic activity, and retention of the profibrotic cleavage substrates tissue transglutaminase and endoglin accompanied MMP-14 suppression. Furthermore, restoration of MMP-14 expression in SIRT1-depeleted mice improved angiogenic and matrilytic functions of the endothelium, prevented renal dysfunction, and attenuated nephrosclerosis. Our findings establish a novel mechanistic molecular link between endothelial SIRT1 depletion, downregulation of MMP-14, and the development of nephrosclerosis.

Cathepsin cleavage of sirtuin 1 in endothelial progenitor cells mediates stress-induced premature senescence

Stress-induced premature senescence (SIPS) of endothelial cells (ECs) has emerged as a contributor to global EC dysfunction. One of the cellular abnormalities mechanistically linked to SIPS is lysosomal dysfunction. In this study, we examined the impact of a range of cardiovascular risk factors on the expression of sirtuin 1 (SIRT1), SIPS, and apoptosis, and we documented the role of SIRT1 in reduced EC and endothelial progenitor cell (EPC) viability. These findings were confirmed in mice with selective endothelial SIRT1 knockout. The effects of stressors could be partially mimicked by inducing lysosomal membrane permeabilization or inhibiting autophagy, and were reversed by a cathepsin inhibitor. We provide evidence that SIRT1 is an important substrate of cysteine cathepsins B, S, and L. An antioxidant/peroxynitrite scavenger, ebselen, prevented stress-induced SIRT1 depletion and subversion of autophagy by mitigating lysosomal dysfunction. In conclusion, our data advance the concept of "stem cell aging" by establishing the critical role of lysosomal dysfunction in the development of SIPS through the cathepsin-induced proteolytic cleavage of SIRT1, a mechanism linking cell stress to apoptosis and SIPS. Ebselen potently protects lysosomal membrane integrity, preventing cathepsin-induced cleavage of SIRT 1 in EPCs and blunting SIPS and apoptotic cell death induced by relevant cardiovascular stressors. The proposed mechanism of SIRT1 depletion in stress has all of the attributes of being a paradigm of SIPS of EPCs.

Tunneling nanotubes mediate rescue of prematurely senescent endothelial cells by endothelial progenitors: exchange of lysosomal pool

Although therapeutic effect of adoptive transfer of endothelial progenitor cells (EPC) has been well-substantiated, the actual engraftment is relatively low compared to a robust functional improvement of vasculopathy. Cellular mechanisms governing this action remain elusive. A recently discovered cell-cell communication via tunneling nanotube (TNT) formation is capable of transferring mitochondria and lysosomes between the cells - "organellar diakinesis". Based on the previous demonstration of lysosomal dysfunction in endothelial cells exposed to AGE-modified collagen I, we inquired whether TNT mechanism may be involved in EPC-mediated repair of stressed endothelial cells. Here we demonstrate that EPC selectively and multiplicatively establish TNT communication with stressed endothelia. The guidance cues for the selectivity are provided by exofacially exposed phosphatidylserine moieties. Lysosomal transfer is associated with the preservation of lysosomal pH gradient, functionally reconstituting lysosomal pool of stressed cells and improving endothelial cell viability, reducing premature senescence and apoptosis. In vivo, adoptive transfer of EPC to streptozotocin-diabetic mice results in a TNT-dependent reduction of senescent endothelial cells and correction of endothelium-dependent vasorelaxation. Collectively, these data establish a selective multiplicative effect of TNT between EPC and stressed endothelia, reconstitution of the lysosomal pool, and improved viability and function of stressed endothelia.

Fluorescence-based detection and quantification of features of cellular senescence

Cellular senescence is a spontaneous organismal defense mechanism against tumor progression which is raised upon the activation of oncoproteins or other cellular environmental stresses that must be circumvented for tumorigenesis to occur. It involves growth-arrest state of normal cells after a number of active divisions. There are multiple experimental routes that can drive cells into a state of senescence. Normal somatic cells and cancer cells enter a state of senescence upon overexpression of oncogenic Ras or Raf protein or by imposing certain kinds of stress such as cellular tumor suppressor function. Both flow cytometry and confocal imaging analysis techniques are very useful in quantitative analysis of cellular senescence phenomenon. They allow quantitative estimates of multiple different phenotypes expressed in multiple cell populations simultaneously. Here we review the various types of fluorescence methodologies including confocal imaging and flow cytometry that are frequently utilized to study a variety of senescence. First, we discuss key cell biological changes occurring during senescence and review the current understanding on the mechanisms of these changes with the goal of improving existing protocols and further developing new ones. Next, we list specific senescence phenotypes associated with each cellular trait along with the principles of their assay methods and the significance of the assay outcomes. We conclude by selecting appropriate references that demonstrate a typical example of each method.

Protocols to detect senescence-associated beta-galactosidase (SA-betagal) activity, a biomarker of senescent cells in culture and in vivo

Normal cells can permanently lose the ability to proliferate when challenged by potentially oncogenic stress, a process termed cellular senescence. Senescence-associated beta-galactosidase (SA-betagal) activity, detectable at pH 6.0, permits the identification of senescent cells in culture and mammalian tissues. Here we describe first a cytochemical protocol suitable for the histochemical detection of individual senescent cells both in culture and tissue biopsies. The second method is based on the alkalinization of lysosomes, followed by the use of 5-dodecanoylaminofluorescein di-beta-D-galactopyranoside (C12FDG), a fluorogenic substrate for betagal activity. The cytochemical method takes about 30 min to execute, and several hours to a day to develop and score. The fluorescence methods take between 4 and 8 h to execute and can be scored in a single day. The cytochemical method is applicable to tissue sections and requires simple reagents and equipment. The fluorescence-based methods have the advantages of being more quantitative and sensitive.

Stress-induced premature senescence of endothelial cells: a perilous state between recovery and point of no return

PURPOSE OF REVIEW

To discuss most recently published studies on morphologic patterns and molecular mechanisms of stress-induced premature senescence (SIPS) of vascular endothelial cells.

RECENT FINDINGS

Lysosomal dysfunction and impaired autophagy, which have been well established in replicative senescence, were also described in SIPS induced by advanced glycation end products (AGEs). Recently, strides were made to prevent and reverse senescence. The role of lysosomal dysfunction and Lamp-2A deficiency has been demonstrated in aging. Molecular analyses identified the role of sirtuin 1 in preventing cell senescence; shed light on the role of polycomb group (PcG) protein Bmi-1 in senescence. Additionally, intriguing data on the role of caveolin-1 in cell senescence have emerged.

SUMMARY

In aging organisms and chronic diseases properly functioning tissue is replaced by senescent cells. Comparison between replicative senescence and SIPS indicates that replicative senescence is almost exclusively associated with the reduction of telomerase activity and attrition of telomeres, whereas SIPS does not require these events, thus conferring potential reversibility onto this process.

Lipid mediators of autophagy in stress-induced premature senescence of endothelial cells

Our group (Patschan S, Chen J, Gealekman O, Krupincza K, Wang M, Shu L, Shayman JA, Goligorsky MS; Am J Physiol Renal Physiol 294: F100-F109, 2008) previously observed an accumulation of gangliosides coincident with development of cell senescence and demonstrated lysosomal permeabilization in human umbilical vein endothelial cells exposed to glycated collagen I (GC). Therefore, we investigated whether the lysosome-dependent, caspase-independent or type 2-programmed cell death (autophagy) is involved in development of premature senescence of endothelial cells. The cleaved microtubule-associated protein 1 light-chain 3 (LC3), a marker of autophagosome formation, was overexpressed within 24 h of GC treatment; however, by 4-5 days, it was nearly undetectable. Early induction of autophagosomes was associated with their fusion with lysosomes, a phenomenon that later became subverted. Autophagic cell death can be triggered by the products of damaged plasma membrane, sphingolipids, and ceramide. We observed a clustering of membrane rafts shortly after exposure to GC; later, after 24 h, we observed an internalization, accompanied by an increased acid sphingomyelinase activity and accumulation of ceramide. Pharmacological inhibition of autophagy prevented development of premature senescence but did lead to the enhanced rate of apoptosis in human umbilical vein endothelial cells exposed to GC. Pharmacological induction of autophagy resulted in reciprocal changes. These observations appear to represent a mechanistic molecular cascade whereby advanced glycation end products like GC induce sphingomyelinase activity, accumulation of ceramide, clustering, and later internalization of lipid rafts.