Non-Coding RNAs Steering the Senescence-Related Progress, Properties, and Application of Mesenchymal Stem Cells

The thirst to postpone and even reverse aging progress has never been quenched after all these decades. Unequivocally, mesenchymal stem cells (MSCs), with extraordinary abilities such as self-renewal and multi-directional differentiation, deserve the limelight in this topic. Though having several affable clinical traits, MSCs going through senescence would, on one hand, contribute to age-related diseases and, on the other hand, lead to compromised or even counterproductive therapeutical outcomes. Notably, increasing evidence suggests that non-coding RNAs (ncRNAs) could invigorate various regulatory processes. With even a slight dip or an uptick of expression, ncRNAs would make a dent in or even overturn cellular fate. Thereby, a systematic illustration of ncRNAs identified so far to steer MSCs during senescence is axiomatically an urgent need. In this review, we introduce the general properties and mechanisms of senescence and its relationship with MSCs and illustrate the ncRNAs playing a role in the cellular senescence of MSCs. It is then followed by the elucidation of ncRNAs embodied in extracellular vesicles connecting senescent MSCs with other cells and diversified processes in and beyond the skeletal system. Last, we provide a glimpse into the clinical methodologies of ncRNA-based therapies in MSC-related fields. Hopefully, the intricate relationship between senescence and MSCs will be revealed one day and our work could be a crucial stepping-stone toward that future.

An Optimized Protocol for Histochemical Detection of Senescence-associated Beta-galactosidase Activity in Cryopreserved Liver Tissue

Senescence-associated beta-galactosidase (SA-β-gal) activity assay is commonly used to evaluate the increased beta-galactosidase (β-gal) activity in senescent cells related to enhanced lysosomal activity. Although the optimal pH for β-gal is 4.0, this enzymatic activity has been most commonly investigated at a suboptimal pH by using histochemical reaction on fresh tissue material. In the current study, we optimized a SA-β-gal activity histochemistry protocol that can also be applied on cryopreserved hepatic tissue. This protocol was developed on livers obtained from control rats and after bile duct resection (BDR). A significant increase in β-gal liver activity was observed in BDR rats vs controls after 2 hr of staining at physiological pH 4.0 (6.98 ± 1.19% of stained/total area vs 0.38 ± 0.22; p<0.01) and after overnight staining at pH 5.8 (24.09 ± 6.88 vs 0.12 ± 0.08; p<0.01). Although we noticed that β-gal activity staining decreased with cryopreservation time (from 4 to 12 months of storage at -80C; p<0.05), the enhanced staining observed in BDR compared with controls remained detectable up to 12 months after cryopreservation (p<0.01). In conclusion, we provide an optimized protocol for SA-β-gal activity histochemical detection at physiological pH 4.0 on long-term cryopreserved liver tissue.

Mouse Models of Accelerated Cellular Senescence

Senescent cells accumulate in multiple tissues as virtually all vertebrate organisms age. Senescence is a highly conserved response to many forms of cellular stress intended to block the propagation of damaged cells. Senescent cells have been demonstrated to play a causal role in aging via their senescence-associated secretory phenotype and by impeding tissue regeneration. Depletion of senescent cells either through genetic or pharmacologic methods has been demonstrated to extend murine lifespan and delay the onset of age-related diseases. Measuring the burden and location of senescent cells in vivo remains challenging, as there is no marker unique to senescent cells. Here, we describe multiple methods to detect the presence and extent of cellular senescence in preclinical models, with a special emphasis on murine models of accelerated aging that exhibit a more rapid onset of cellular senescence.

Bone marrow mesenchymal stem cells: Aging and tissue engineering applications to enhance bone healing

Bone has well documented natural healing capacity that normally is sufficient to repair fractures and other common injuries. However, the properties of bone change throughout life, and aging is accompanied by increased incidence of bone diseases and compromised fracture healing capacity, which necessitate effective therapies capable of enhancing bone regeneration. The therapeutic potential of adult mesenchymal stem cells (MSCs) for bone repair has been long proposed and examined. Actions of MSCs may include direct differentiation to become bone cells, attraction and recruitment of other cells, or creation of a regenerative environment via production of trophic growth factors. With systemic aging, MSCs also undergo functional decline, which has been well investigated in a number of recent studies. In this review, we first describe the changes in MSCs during aging and discuss how these alterations can affect bone regeneration. We next review current research findings on bone tissue engineering, which is considered a promising and viable therapeutic solution for structural and functional restoration of bone. In particular, the importance of MSCs and bioscaffolds is highlighted. Finally, potential approaches for the prevention of MSC aging and the rejuvenation of aged MSC are discussed.

Cellular senescence: Molecular mechanisms and pathogenicity

Cellular senescence is the arrest of normal cell division. Oncogenic genes and oxidative stress, which cause genomic DNA damage and generation of reactive oxygen species, lead to cellular senescence. The senescence-associated secretory phenotype is a distinct feature of senescence. Senescence is normally involved in the embryonic development. Senescent cells can communicate with immune cells to invoke an immune response. Senescence emerges during the aging process in several tissues and organs. In fact, increasing evidence shows that cellular senescence is implicated in aging-related diseases, such as nonalcoholic fatty liver disease, obesity and diabetes, pulmonary hypertension, and tumorigenesis. Cellular senescence can also be induced by microbial infection. During cellular senescence, several signaling pathways, including those of p53, nuclear factor-κB (NF-κB), mammalian target of rapamycin, and transforming growth factor-beta, play important roles. Accumulation of senescent cells can trigger chronic inflammation, which may contribute to the pathological changes in the elderly. Given the variety of deleterious effects caused by cellular senescence in humans, strategies have been proposed to control senescence. In this review, we will focus on recent studies to provide a brief introduction to cellular senescence, including associated signaling pathways and pathology.

Identification of HSP90 inhibitors as a novel class of senolytics

Aging is the main risk factor for many chronic degenerative diseases and cancer. Increased senescent cell burden in various tissues is a major contributor to aging and age-related diseases. Recently, a new class of drugs termed senolytics were demonstrated to extending healthspan, reducing frailty and improving stem cell function in multiple murine models of aging. To identify novel and more optimal senotherapeutic drugs and combinations, we established a senescence associated β-galactosidase assay as a screening platform to rapidly identify drugs that specifically affect senescent cells. We used primary Ercc1^−/− murine embryonic fibroblasts with reduced DNA repair capacity, which senesce rapidly if grown at atmospheric oxygen. This platform was used to screen a small library of compounds that regulate autophagy, identifying two inhibitors of the HSP90 chaperone family as having significant senolytic activity in mouse and human cells. Treatment of Ercc1^−/∆ mice, a mouse model of a human progeroid syndrome, with the HSP90 inhibitor 17-DMAG extended healthspan, delayed the onset of several age-related symptoms and reduced p16^INK4a expression. These results demonstrate the utility of our screening platform to identify senotherapeutic agents as well as identified HSP90 inhibitors as a promising new class of senolytic drugs.

The accumulation of senescent cells is thought to contribute to the age-associated decline in tissue function. Here, the authors identify HSP90 inhibitors as a new class of senolytic compounds in an in vitro screening and show that administration of a HSP90 inhibitor reduces age-related symptoms in progeroid mice.