The bright and dark side of extracellular vesicles in the senescence-associated secretory phenotype

The role of lipid-based signalling in wound healing and senescence

Lipid-based signalling modulates several cellular processes and intercellular communication during wound healing and tissue regeneration. Bioactive lipids include but are not limited to the diverse group of eicosanoids, phospholipids, and extracellular vesicles and mediate the attraction of immune cells, initiation of inflammatory responses, and their resolution. In aged individuals, wound healing and tissue regeneration are greatly impaired, resulting in a delayed healing process and non-healing wounds. Senescent cells accumulate with age in vivo, preferably at sites implicated in age-associated pathologies and their elimination was shown to alleviate many age-associated diseases and disorders. In contrast to these findings, the transient presence of senescent cells in the process of wound healing exerts beneficial effects and limits fibrosis. Hence, clearance of senescent cells during wound healing was repeatedly shown to delay wound closure in vivo. Recent findings established a dysregulated synthesis of eicosanoids, phospholipids and extracellular vesicles as part of the senescent phenotype. This intriguing connection between cellular senescence, lipid-based signalling, and the process of wound healing and tissue regeneration prompts us to compile the current knowledge in this review and propose future directions for investigation.

Importance of stem cell culture conditions for their derived extracellular vesicles therapeutic effect

Stem cell-derived extracellular vesicles (EVs) could be enhanced by modifying specific in vitro parameters when culturing their originating stem cells. Controlling stem cell growth conditions with physical properties, oxygen tension and media preconditioning with soluble factors may influence EVs biogenesis and EVs biological function as well. Unfortunately, many misconceptions and methodological issues have hampered the progress in understanding the biological properties of EVs. In this review we will first discuss the major concerns involved in a suitable EVs production from stem cell culture. Then, we will describe the current techniques for EV isolation, focusing on their advantages and disadvantages, as well as their impact on EVs yield, recovery and functionality. Standardization of the methodology is a prerequisite to compare, to validate and to improve the reliability and credibility of all the different findings reported for the development of EV-based therapeutics.

Extracellular Vesicles for the Treatment of Radiation Injuries

Normal tissue injury from accidental or therapeutic exposure to high-dose radiation can cause severe acute and delayed toxicities, which result in mortality and chronic morbidity. Exposure to single high-dose radiation leads to a multi-organ failure, known as acute radiation syndrome, which is caused by radiation-induced oxidative stress and DNA damage to tissue stem cells. The radiation exposure results in acute cell loss, cell cycle arrest, senescence, and early damage to bone marrow and intestine with high mortality from sepsis. There is an urgent need for developing medical countermeasures against radiation injury for normal tissue toxicity. In this review, we discuss the potential of applying secretory extracellular vesicles derived from mesenchymal stromal/stem cells, endothelial cells, and macrophages for promoting repair and regeneration of organs after radiation injury.

p16-3MR: A Novel Model to Study Cellular Senescence in Cigarette Smoke-Induced Lung Injuries

Cellular senescence and lung aging are associated with the pathogenesis of chronic obstructive pulmonary disease (COPD). COPD progresses with aging, and chronic smoking is the key susceptibility factor in lung pathological changes concurrent with mitochondrial dysfunction and biological aging. However, these processes involving cigarette smoke (CS)-mediated lung cellular senescence are difficult to distinguish. One of the impediments to studying cellular senescence in relation to age-related lung pathologies is the lack of a suitable in vivo model. In view of this, we provide evidence that supports the suitability of p16-3MR mice to studying cellular senescence in CS-mediated and age-related lung pathologies. p16-3MR mice have a trimodal reporter fused to the promoter of the p16^INK4a gene that enables detection, isolation, and selective elimination of senescent cells, thus making them a suitable model to study cellular senescence. To determine their suitability in CS-mediated lung pathologies, we exposed young (12–14 months) and old (17–20 months) p16-3MR mice to 30 day CS exposure and studied the expression of senescent genes (p16, p21, and p53) and SASP-associated markers (MMP9, MMP12, PAI-1, and FN-1) in air- and CS-exposed mouse lungs. Our results showed that this model could detect cellular senescence using luminescence and isolate cells undergoing senescence with the help of tissue fluorescence in CS-exposed young and old mice. Our results from the expression of senescence markers and SASP-associated genes in CS-exposed young and old p16-3MR mice were comparable with increased lung cellular senescence and SASP in COPD. We further showed alteration in the; (i) tissue luminescence and fluorescence, (ii) mRNA and protein expressions of senescent markers and SASP genes, and (iii) SA-β-gal activity in CS-exposed young and old p16-3MR mice as compared to their air controls. Overall, we showed that p16-3MR is a competent model for studying the cellular senescence in CS-induced pathologies. Hence, the p16-3MR reporter mouse model may be used as a novel tool for understanding the pathobiology of cellular senescence and other underlying mechanisms involved in COPD and fibrosis.

Isolation methodology is essential to the evaluation of the extracellular vesicle component of the senescence-associated secretory phenotype

A hallmark of senescence is the acquisition of an enhanced secretome comprising inflammatory mediators and tissue remodelling agents - the senescence-associated secretory phenotype (SASP). Through the SASP, senescent cells are hypothesised to contribute to both ageing and pathologies associated with age. Whilst soluble factors have been the most widely investigated components of the SASP, there is growing evidence that small extracellular vesicles (EVs) comprise functionally important constituents. Thus, dissecting the contribution of the soluble SASP from the vesicular component is crucial to elucidating the functional significance of senescent cell derived EVs. Here, we take advantage of a systematic proteomics based approach to determine that soluble SASP factors co-isolate with EVs following differential ultracentrifugation (dUC). We present size-exclusion chromatography (SEC) as a method for separation of the soluble and vesicular components of the senescent secretome and thus EV purification. Furthermore, we demonstrate that SEC EVs isolated from senescent cells contribute to non-cell autonomous paracrine senescence. Therefore, this work emphasises the requirement for methodological rigor due to the propensity of SASP components to co-isolate during dUC and provides a framework for future investigations of the vesicular component of the SASP.

The Jekyll and Hyde of Cellular Senescence in Cancer

Cellular senescence is a state of stable cell cycle arrest that can be triggered in response to various insults and is characterized by distinct morphological hallmarks, gene expression profiles, and the senescence-associated secretory phenotype (SASP). Importantly, cellular senescence is a key component of normal physiology with tumor suppressive functions. In the last few decades, novel cancer treatment strategies exploiting pro-senescence therapies have attracted considerable interest. Recent insight, however, suggests that therapy-induced senescence (TIS) elicits cell-autonomous and non-cell-autonomous implications that potentially entail detrimental consequences, reflecting the Jekyll and Hyde nature of cancer cell senescence. In essence, the undesirable manifestations that generally culminate in inflammation, cancer stemness, senescence reversal, therapy resistance, and disease recurrence are dictated by the persistent accumulation of senescent cells and the SASP. Thus, mitigating these pro-tumorigenic effects by eliminating these cells or inhibiting their SASP production holds great promise for developing innovative therapeutic strategies. In this review, we describe the fundamental aspects and dynamics of cancer cell senescence and summarize the comprehensive research on the adverse outcomes of TIS. Furthermore, we underline the rationale and motivation of emerging senotherapeutic modalities surrounding the removal of senescent cells and the SASP to help maximize the overall efficacy of cancer therapies.

A guide to assessing cellular senescence in vitro and in vivo

Cellular senescence is a physiological mechanism whereby a proliferating cell undergoes a stable cell cycle arrest upon damage or stress and elicits a secretory phenotype. This highly dynamic and regulated cellular state plays beneficial roles in physiology, such as during embryonic development and wound healing, but it can also result in antagonistic effects in age-related pathologies, degenerative disorders, ageing and cancer. In an effort to better identify this complex state, and given that a universal marker has yet to be identified, a general set of hallmarks describing senescence has been established. However, as the senescent programme becomes more defined, further complexities, including phenotype heterogeneity, have emerged. This significantly complicates the recognition and evaluation of cellular senescence, especially within complex tissues and living organisms. To address these challenges, substantial efforts are currently being made towards the discovery of novel and more specific biomarkers, optimized combinatorial strategies and the development of emerging detection techniques. Here, we compile such advances and present a multifactorial guide to identify and assess cellular senescence in cell cultures, tissues and living organisms. The reliable assessment and identification of senescence is not only crucial for better understanding its underlying biology, but also imperative for the development of diagnostic and therapeutic strategies aimed at targeting senescence in the clinic.

Cellular senescence as a response to multiwalled carbon nanotube (MWCNT) exposure in human mesothelial cells

Cellular senescence is a stable cell cycle arrest induced by diverse triggers, including replicative exhaustion, DNA damaging agents, oncogene activation, oxidative stress, and chromatin disruption. With important roles in aging and tumor suppression, cellular senescence has been implicated also in tumor promotion. Here we show that certain multiwalled carbon nanotubes (MWCNTs), as fiber-like nanomaterials, can trigger cellular senescence in primary human mesothelial cells. Using in vitro approaches, we found manifestation of several markers of cellular senescence, especially after exposure to a long and straight MWCNT. These included inhibition of cell division, senescence-associated heterochromatin foci, senescence-associated distension of satellites, LMNB1 depletion, γH2A.X nuclear panstaining, and enlarged cells exhibiting senescence-associated β-galactosidase activity. Furthermore, genome-wide transcriptome analysis revealed many differentially expressed genes, among which were genes encoding for a senescence-associated secretory phenotype. Our results clearly demonstrate the potential of long and straight MWCNTs to induce premature cellular senescence. This finding may find relevance in risk assessment of workplace safety, and in evaluating MWCNT's use in medicine such as drug carrier, due to exposure effects that might prompt onset of age-related diseases, or even carcinogenesis.

Targeting Molecular Mechanism of Vascular Smooth Muscle Senescence Induced by Angiotensin II, A Potential Therapy via Senolytics and Senomorphics

Cardiovascular disease (CVD) is a prevalent issue in the global aging population. Premature vascular aging such as elevated arterial stiffness appears to be a major risk factor for CVD. Vascular smooth muscle cells (VSMCs) are one of the essential parts of arterial pathology and prone to stress-induced senescence. The pervasiveness of senescent VSMCs in the vasculature increases with age and can be further expedited by various stressing events such as oxidative stress, mitochondria dysfunction, endoplasmic reticulum stress, and chronic inflammation. Angiotensin II (AngII) can induce many of these responses in VSMCs and is thus considered a key regulator of VSMC senescence associated with CVD. Understanding the precise mechanisms and consequences of senescent cell accumulation may uncover a new generation of therapies including senolytic and senomorphic compounds against CVD. Accordingly, in this review article, we discuss potential molecular mechanisms of VSMC senescence such as those induced by AngII and the therapeutic manipulations of senescence to control age-related CVD and associated conditions such as by senolytic.