Long recognized for their pivotal role in cellular energy production through cellular respiration and oxidative phosphorylation, mitochondria respond in first line to the high-energy demand of kidneys, which required a higher amount of oxygen and abundance in functional mitochondria to sustain their functions.

Moreover, mitochondria are essential in several cellular mechanisms, including the maintenance of homeostasis in the redox state, the synthesis of several macromolecules, regulation of intracellular calcium, and release of pro- or anti-inflammatory signals and control of intrinsic pathway of apoptosis (Wirthensohn and Guder, 1986; Feucht et al., 2018).

Along nephrons, there is heterogeneity in generating energy; as a matter of fact, proximal tubular cells produce ATP via oxidative phosphorylation, while podocytes, endothelial, and mesangial cells through their glycolytic capacity (Wirthensohn and Guder, 1986). These differences may influence the impact of mitochondrial dysfunction in kidney and in the progression of renal diseases and aging-associated renal failure (Galvan et al., 2017).

In accordance, several studies have demonstrated that both acute and chronic insults lead to alterations in mitochondrial structure, inducing mitochondrial DNA (mtDNA) damage, decreased matrix density, and compromising the outer and inner membrane integrity (Che et al., 2014). The principal features of mitochondrial dysfunction, widely described in several renal diseases and aging, include several changes in mitochondrial formation (biogenesis), remodeling (fusion/fission), degradation (mitophagy), and mitochondrial impaired homeostasis. These alterations lead to a bioenergetic dysfunction, a decrease in ATP production and calcium signaling, and enhanced oxidative stress and apoptosis (Galvan et al., 2017; Eirin et al., 2016).

As is well known, mitochondrial biogenesis needs to meet specific metabolic and energetic cellular demands and this process is regulated through an interconnected set of transcription factors that controlled energy status and cellular adaptive responses. The peroxisome proliferator-activated receptor γ coactivator-1 (PGC-1) is defined as the principal factor regulating mitochondrial biogenesis and energy metabolism (Puigserver et al., 1998; Galvan et al., 2017). Spiegelman et al. were the first to discover PGC-1α, and only later, further studies demonstrated a higher expression of PGC-1α in several organs with high energy request such as kidneys and heart. PGC-1α exerts its regulatory effects on mitochondrial function by binding and co-activating several transcription factors such as nuclear respiratory factor-1 (NRF-1), NRF-2, and the estrogen-related receptors (ERR) (Scarpulla, 2008; Baldelli et al., 2013). These interactions mediate the control of some mitochondrial genes involved in mitochondrial biogenesis, lipid oxidation, glycolysis, and ATP synthesis. Moreover, the contribution of these transcriptional factors is cell-type dependent and explains the presence of different metabolic programs in several cell types. Different studies demonstrated that PGC-1α increases mitochondrial content, oxidative phosphorylation, and fatty acid oxidation.

Moreover, the decreased expression of PGC-1α is correlated with a reduced efficiency of mitochondrial biogenesis in AKI (Tran et al., 2011; Lynch et al., 2018) and in CKD (Lim et al., 2012; Galvan et al., 2017; Chung et al., 2018). Indeed, PGC‐1α expression is highly decreased in different preclinical models of AKI, including cisplatin‐induced renal damage (Morigi et al., 2015) (Portilla et al., 2002), folate (Ruiz-Andres et al., 2016), IRI‐ (Portilla et al., 2002; Lempiäinen et al., 2013), and lipopolysaccharide (LPS)‐induced AKI (Tran et al., 2011). In the I/R model, PGC‐1α‐knockout mice underwent worse pathological renal damage with increased fat deposits and clear signs of tubular damage compared to wild‐type (WT) mice (Tran et al., 2016).

Moreover, Ruiz‐Andres et al. (2016) demonstrated that the increase in inflammatory mediators during AKI led to histone deacetylation that enhanced chromatin condensation, reducing accessibility of PGC-1α gene and suppressed its expression. Moreover, Smith et al. (2015) showed that in endotoxemic conditions, LPS systemic exposure promoted the activation of TLR-4 and MAPK/ERK pathway, inducing a significant decrease in PGC‐1α synthesis. Its gene expression is also reduced in various animal models of CKD, such as unilateral ureteral obstruction‐induced fibrosis (Han et al., 2017), db/db diabetic mice (Jianyin et al., 2016; Yuan et al., 2018), and streptozotocin‐induced diabetic mice (Kwon et al., 2017). Indeed, the inactivation of the PGC-1α signaling pathway has been demonstrated as a key mechanism of diabetic nephropathy (Dugan et al., 2013; Platt and Coward, 2017). Until now the role of this gene in renal aging is still unclear. Lim et al. (2012) showed that PGC‐1α expression was decreased in the kidneys of aged mice; moreover, a recent study demonstrated that the activation of SIRT1 and AMPK by an agonist of PPARα, known as fenofibrate, increased the expression of PGC‐1α and ERRα improving mitochondrial dysfunction in aged kidneys (Kim et al., 2016). Therefore, reestablishing PGC‐1α expression could be a promising therapeutic strategy to counteract senescence and progression of CKD (Lee et al., 2019).

The imbalance between mitochondrial fission and fusion has been described in several diseases including cardiovascular and neurodegenerative diseases, diabetes, and cancer (Eirin et al. 2016). Recently, two studies demonstrated increased mitochondrial fragmentation in renal tubular cells in CKD models (Brooks et al., 2009; Zhan et al., 2013). Therefore, there is an urgent need to discover a new treatment that could ameliorate mitochondrial dysfunction in CKD and other settings in which mitochondrial impairment has a key role (Che et al., 2014).

In addition, there are pieces of evidence that mitophagy significantly reduces during physiological aging (Sun et al., 2015; García-Prat et al., 2016). Therefore, the accumulation of damaged mitochondria modifies their functional homeostasis, increasing accumulation of oxygen reactive species (ROS) in aged organs. In several experimental animal models, the deletion of autophagy genes induced the accumulation of dysfunctional mitochondria and strongly increased ROS levels. The administration of antioxidant therapies mitigated these dysfunctional effects; therefore, the decline of autophagy and in particular mitophagy promotes a harmful increase in oxidative stress that also augments age-related tissue injury (Wu et al., 2009; Bratic and Larsson, 2013; Yan and Finkel, 2017) ( Figure 1 ).

Specifically, mitophagy has been implicated in several kidney diseases, including both AKI (Ishihara et al., 2013) and CKD (Namba et al., 2014). After IRI, the expression of BNIP3, a member of Bcl2 family that enhances mitophagy, is strongly reduced in renal tubular cells, suggesting a possible link between mitophagy and renal function. Moreover, the inhibition of mitophagy is also associated with CKD, indicating that a disruption of renal tubular mitochondrial quality control contributes to the pathogenesis of CKD (Tang et al., 2015). These studies imply that target therapies that improve the mitophagy process could induce several beneficial effects in counteracting age-related functional decline. In addition, some natural molecules such as the polyamine spermidine are able to induce both autophagy and mitophagy with an extension of lifespan and recovery in age-associated diseases (Eisenberg et al., 2016).

As is well known the increase in mitochondrial ROS has a strong impact on several pathways involved in apoptosis and senescence process. However, this traditional point of view has recently been changed with a new hypothesis that only the right amount of ROS is necessary to assure physiological cellular function (Eirin et al., 2016). The mutation rate in the mitochondrial genome is caused by a higher amount of ROS, leading to mitochondrial dysfunction, which promotes a further increase in ROS production that in a circle contributes to mtDNA damage. This progressive damage in the mitochondrial genome accelerates the aging process and has been associated with cardiovascular and CKD (Koyano et al., 2014). Generally, the mutations in mtDNA are mostly located in the genes implicated in mtDNA integrity, transcription, and RNA maturation (Bingol et al., 2014).

Therefore, the emerging literature highly supports the critical role of mitochondrial dysfunction in aging process and the development and progression of renal diseases. However, our knowledge of mitochondrial impairment in CKD has yet to be fully clarified. Therefore, additional studies are necessary to better delineate therapeutic strategies for recovery of mitochondrial function in the CKD setting (Eirin et al. 2016).

To date, accumulating evidence has facilitated our understanding of the role of autophagy in kidney physiology, diseases, and aging. Ischemic, toxic, immunological, and oxidative injury can enhance autophagy in proximal tubular cells and podocytes modifying the course of renal diseases (Huber et al., 2012). Autophagy is a highly conserved balancing mechanism for energy and resource; this process helps cells to “self-eat” endogenous material and recycle cellular components for maintaining cellular integrity and energy (Parzych and Klionsky, 2014; De Rechter et al., 2016; Lin et al., 2019). Dysregulation of autophagy is associated with the accumulation of autophagosomes, intracellular damaged proteins, and organelles that contribute to the progressive deterioration of the kidney’s functions (Huber et al., 2012).

As is well known, glomerular podocytes are terminally differentiated cells and their fate is strongly dependent on their capacity to build an efficient response against any insults. Moreover, proximal tubular cells require a large amount of mitochondria to assure protein reabsorption from glomerular filtrate (Christensen and Nielsen, 1991). Therefore, both podocytes and tubular cells have to assure their homeostasis and functions through the autophagy process avoiding the progression of kidney aging and CKD. Indeed, several studies have demonstrated a protective role of autophagy to counteract aging process in podocytes and tubular cells (Hartleben et al., 2010; Kimura et al., 2011; Weide and Huber, 2011).

Both rat and mouse models of renal aging showed that the accumulation of dysfunctional mitochondria and age-associated proteins, such as SQSTM1 (ubiquitin-binding protein p62), was associated with a decrease in autophagy activity (Kume et al., 2010; Cui et al., 2012). Therefore, aging kidneys lose their ability to induce autophagy and accelerate the progression of renal dysfunction.

Furthermore, autophagy-related protein (ATG) ATG5 has a critical role in converting LC3-I to LC3-II, which is crucial for autophagosome formation, and ATG5 knockout mice presented a decrease of autophagy in both podocytes and proximal tubular cells (Hartleben et al., 2010; Kimura et al., 2011). Specifically, podocytes accumulated dysfunctional mitochondria and ubiquitinated proteins aggregate, thereby increasing proteinuria and development of glomerulosclerosis (Hartleben et al., 2010). Also, proximal tubular cells increased abnormal mitochondria, ubiquitinated proteins such as SQSTM1, which significantly increased apoptosis. Therefore, the absence of basal autophagy is strongly correlated to an increase in kidney aging (Figure 1).

Recent findings demonstrated that energy restriction may provide a strategy to preserve the cellular autophagic process. Since calorie restriction induces physiological autophagy, it can counteract age-associated renal damage preventing albuminuria, glomerulosclerosis, and tubulointerstitial lesions (Wiggins et al., 2005; McKiernan et al., 2007; Colman et al., 2009; Kume et al., 2010). Moreover, calorie restriction counteracts hypoxic conditions, restoring autophagy activity and protecting the kidney against aging and CKD (Kume et al., 2010). Kume S. et al. showed in a mouse model of aging kidney that the molecular mechanism underlying the calorie restriction-mediated restoration of autophagy against hypoxia was SIRT-1. SIRT1, as its yeast homolog silent information regulator 2 (Sir2), plays a key role in promoting survival and restoring the autophagy in aging kidney by the deacetylation of FOXO3/FOXO3A, a transcription factor involved in the stress-oxidative pathway (Kume et al., 2010).

In addition to age-associated CKD, several findings demonstrated a major risk of AKI development in elderly population. Recently, numerous studies have demonstrated that the activation of autophagy protects proximal tubular cells from several insults during AKI. Indeed, Lai and colleagues showed in a rat model of IRI an increased expression of ATG proteins in tubular compartment, suggesting the autophagic activity as a key mechanism in renal pathology (Chien et al., 2007; Wu et al., 2009). Despite some controversial studies, emerging evidence has clearly demonstrated a reno-protective role of autophagy in renal tubular compartment during AKI. The insurgence of AKI is strictly related to increasingly aging population that presented a natural decrease of autophagy process. Although further studies are needed, the recovery of autophagic activity in the aging kidney would prevent tubular dysfunction against both acute and chronic damage.

Epigenetic mechanisms are crucial in physiological condition and influence the gene expression without permanent modifications in the original DNA sequence (Nangaku et al., 2017); these changes include DNA methylation, histone posttranslational modifications, and miRNA pattern variations (Shiels et al., 2017). Epigenetics has recently been demonstrated to have a fundamental role in the development of CKD (Jones et al., 2015). Many factors, from the physiological to the pathological environment as the uremic toxins, oxidative stress, and inflammation, increase the occurrence of epigenetic changes as well as progression to CKD (Chu et al., 2017; Morgado-Pascual et al., 2018). Also miRNAs are involved in posttranscriptional regulation of gene expression and have been described as potential biomarkers in CKD progression (Zhao et al., 2019). The role of miRNAs in AKI has been also investigated, and several miRNAs have been associated with inflammation and fibrotic process (Lee et al., 2014).

Most of the knowledge about DNA methylation derives from experimental studies in diabetic nephropathy (Morgado-Pascual et al., 2018). DNA methylation patterns in specific genes involved in glucose metabolism or kidney fibrosis are considered potential biomarkers in diabetic nephropathy disease (Morgado-Pascual et al., 2018). Indeed, several experimental studies demonstrated that hypomethylation of the connective tissue growth factor (CTGF) gene promoter is associated with decreased glomerular filtration rate (eGFR) and declined kidney function (Yi et al., 2011; Zhang et al., 2014). In addition, emerging evidence showed that several methylated genes in whole blood samples of diabetic nephropathy subjects correlated to inflammation and apoptosis (Bell et al., 2010; Sapienza et al., 2011).

During kidney transplantation, the procedure itself with the unavoidable IRI, the cold ischemia, and acute rejection can worsen the graft outcome, inducing oxidative stress, inflammation, and vascular complications, which consequently predispose to aberrant DNA methylation changes and the acceleration of kidney aging (Parker et al., 2008; Franzin et al., 2020). Recently, several studies highlighted the relevance of epigenomic, transcriptomic, and proteomic signatures in renal graft that correlated to decline in kidney health and CKD progression (O’Sullivan et al., 2017). Experimental studies of rat and mouse models of IRI and CKD described altered DNA methylation as the crucial process for AKI to CKD transition (Franzin et al., 2020). In the pathological context of the UUO model, Shasha Yin et al. showed that TGFβ decreased Klotho protein expression through Klotho promoter hypermethylation, inducing tubule-interstitium fibrosis. Moreover, in a rat model IRI, Pratt et al. (2006) demonstrated that alterations in C3 methylated promoter region caused an increased activation of the complement system that is strongly involved in inflammation and renal damage.

In addition, aberrant hypermethylation of extracellular matrix laminin pattern genes has been found strongly involved in the development of glomerulosclerosis and tubulointerstitial fibrosis in older kidneys. Bechtel et al. showed that the aberrant methylation of RASAL1 gene induced an increased activation of Ras-GTPase pathway in fibroblasts, leading to proliferation and fibrosis. Complement activation has been associated with global tubular epithelial cell DNA hypomethylation (Castellano et al., 2019), which was present in premature and accelerated renal aging (Day et al., 2013; Johansson et al., 2013; Quach et al., 2017). Finally, we recently highlighted the role of complement component C5a in promoting DNA hypomethylation of several genes involved in premature senescence pathway, SASP phenotype, and cell cycle arrest (Castellano et al., 2019) in a swine model of IRI. In addition, different studies have found that posttranscriptional histone modifications are involved in experimental renal fibrosis in several pathological conditions such as CKD model, diabetic nephropathy, lupus nephritis, Adriamycin-induced renal damage, and AKI (Morgado-Pascual et al., 2018). Interestingly, there are ongoing clinical trials studying the use of therapeutic approaches in DNA methylation and histone modifications in renal cancer (Morgado-Pascual et al., 2018). All these epigenetic modifications represent a novel field of study to identify new therapeutic strategies and novel biomarkers to overcome the limitations related to the early diagnosis and the prevention of CKD and kidney aging.

Aging is considered a progressive decline of cell functions, characterized by an insufficiency of response to insults that leads to the accumulation of DNA damage that in turn enhances cell death or the acquirement of a dysfunctional phenotype closely associated with aging processes (Lombard et al., 2005). Accordingly, mice that could not repair DNA damage presented clear signs of premature aging; indeed, young mice with defects in DNA helicase functions presented reduced survival and developed osteoporosis and cachexia, which are identified as aging features (De Boer et al., 2002). Defects in DNA repair in genetic diseases such as Hutchinson–Gilford Progeria, Werner syndrome, Nijmegen breakage syndrome, Fanconi anemia, and Bloom syndrome increased the development of premature aging (Fumagalli et al., 2014; Pan et al., 2016; Li et al., 2016; D’Alessandro et al., 2018). As is well known, DNA repair pathways include base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), and double-strand break repair (DSBR) (Maynard et al., 2015). DSBR is specific for repairing DSBs, mainly by either error-prone rejoining of the broken DNA ends (nonhomologous end joining [NHEJ]) or accurately repairing the DSB using information on the undamaged sister chromatid (homologous recombination [HR]) (Wyman and Kanaar, 2006; Iyama and Wilson, 2013) However, not all DNA modifications are associated with the aging process. For instance, some defects in MMR are involved in tumorigenesis and do not directly trigger senescence process (Papadopoulos and Lindblom., 1997). Otherwise, defects in other DNA repair mechanisms such as BER, NER, NHEJ, and HR are associated with aging-related diseases. For example, alterations in mitochondrial BER induced a decreased activity of four enzymes, DNA glycosylase, endonuclease, DNA polymerase, and DNA ligase that are involved in DNA repair, promoting aging process (Gorbunova et al., 2007; Maynard et al., 2009; Boesch et al., 2011). In addition, other studies showed that a reduced NHEJ was present in neurons from old rats. Telomere maintenance is another important way to assure genome stability and deficiency in this process is associated with cellular senescence and aging (Rodier et al., 2005). Therefore, the increase in DNA damage or the deficiency in DNA repair enhances several age-associated diseases (Pan et al., 2016).

Many features involved in AKI predispose patients to develop CKD, with an incidence of 10% in the world’s population (Bonventre and Yang, 2011; Kishi et al., 2019). Several studies showed a maladaptive response of tubular cells during AKI, characterized by the occurrence of modifications in DDR that could accelerate the aging process and the development of CKD (Ferenbach and Bonventre, 2016; Yang et al., 2010; Grgic et al., 2012). However, the recent literature has not yet demonstrated a direct correlation between DDR and specific renal compartment injury and long-term consequences in renal function (Kishi et al., 2019). Several studies defined DNA damage as a hallmark of several forms of renal damage (Ma et al., 2014; Pabla et al., 2008; Zhu et al., 2015); following DNA strand break (Figure 1), several sensor kinases such as ataxia telangiectasia mutated (ATM), ATM- and Rad3-related (ATR), and DNA-dependent protein kinase (DNA-PK) are activated and in turn enhance the activation of checkpoint kinases 1 and 2 (Chk1 and Chk2), which control cell-cycle progression through the G/S or G/M checkpoint (Brown and Baltimore, 2003; Sancar et al., 2004; Kidiyoor et al., 2016; Li et al., 2016). The DDR process is activated in response to IRI and ATP depletion (Ma et al., 2014). However, when DNA damage is severe, the DDR process is not sufficient to repair DNA and to overcome the insult; accordingly with other studies, we have recently shown that tubular cells arrested cell cycle and acquired a dysfunctional phenotype, known as SASP (Castellano et al., 2019), resulting in the generation of pro-inflammatory and pro-fibrotic cytokines, which promote the development of kidney fibrosis and the progression to CKD (Yang et al., 2010; Grgic et al., 2012).

Studies in mouse models with the deletion of the ATR gene led to an increase in DNA damage, alterations in tissue homeostasis, and the rapid development of age-related phenotypes (Ruzankina et al., 2007; Murga et al., 2009; McKinnon, 2009). Interestingly, cisplatin-stimulated renal tubular cells activated ATR gene (Pabla et al., 2008), but its contribution to the maladaptive response in these cells has to be clarified. The role of DDR in the aging process and in the progression of CKD cannot be ignored (Wang et al., 2017). Many factors that are involved in CKD can cause alterations in DDR response and accelerate aging process in renal parenchyma and, immune cells, endothelial cells, and progenitor and stem cells (Tasanarong et al., 2013; Goligorsky, 2014; Klinkhammer et al., 2014; Bonventre,; Bautista-Niño et al., 2016).

As described previously, when these cells become senescent, they are metabolically active and have a distinct secretome termed SASP (Coppé et al., 2008). The acquirement of SASP is accompanied by genomic damage and epigenetic abnormalities (Castellano et al., 2019). The principal causes include radiation agents, cytotoxic drugs, topoisomerase inhibitors, ROS accumulation, and other agents (Hayflick, 1965; Larsson, 2011; Demaria et al., 2017). These stimuli provoke single- or double-strand breaks in DNA and DDR response cannot recover DNA damage (Campisi, 2013; Johmura et al., 2016). The increase and persistence of DNA damage extended cycle arrest in G1 and G2 phases by the activation of the DDR pathway (Malaquin et al., 2015). Therefore, DDR promotes a prolonged cell cycle arrest through the accumulation and activation of cyclin-dependent kinase inhibitors (CKIs) such as tumor protein p53 (TP53 or p53), p21CIP1 (p21), and p16INK4a (p16) (El-Deiry et al., 1993; Wade Harper et al., 1993). These CKIs inactivate cyclin-dependent kinases (CDKs), which are necessary for the activation and progression of cell cycle. Therefore, CDKs are not able to mediate the phosphorylation of the retinoblastoma tumor suppressor (Rb), which in turn cannot activate E2F protein complex, arresting G1/S progression and DNA replication, or G2/M transition and mitosis, avoiding cellular capability of proliferation and inducing senescent phenotype. In other cases, some oncogenes, such as H-RAS, the mitogen-activated protein kinase (MAPK) signaling pathway, and tumor suppressors p53 and retinoblastoma (Rb), are strongly activated, inducing additional alterations in DNA sequences and contributing to cellular senescence (Larsson, 2011; Campisi, 2013).

Under certain conditions, in the absence of DNA damage, DDR response can be activated promoting the acquisition of senescent phenotype. Indeed, the DDR arrests cell cycle through the activation of the p53/p21 and p16INK4a/ Rb signaling, induces senescence growth-arrested state, and maintains SASP in senescent cells (Acosta et al., 2008; Kuilman et al., 2008; Campisi, 2013). The synthesis and release of SASP factors is associated with the activation of the transcription factors NF-κB and CCAAT enhancer binding protein β (C/EBPβ) (Acosta et al., 2008; Kuilman et al., 2008). Thus, both DDR response and NF-κB activation significantly contribute to establishing and maintaining SASP condition. Taken together, the increase in SASP exacerbates DDR and influences senescent cells to produce and release more senescent secretome, contributing to functional deterioration of neighboring cells and accelerating the process of cellular aging.

The importance of p16INK4a signaling to arrest cell cycle in G1/S checkpoint has already been described. Of interest, the number of p16-positive cells increased in both physiological and pathological aging (Valentijn et al., 2018).

In recent seminal studies, Baker et al. (2011); Baker et al. (2016) demonstrated the safety and efficacy of senescent cell depletion in vivo. The authors generated transgenic mice (INK-ATTAC) where the senescence biomarker p16INK4a promoter was associated with a “suicide” gene triggering caspase-dependent apoptosis (Baker et al., 2011). A recent improvement of this technology allowed the selective elimination of p16INK4a-positive cells by the administration of the AP20187 molecule. The treatment of transgenic mice with AP20187 led to increased lifespan, providing strong evidence that senescent cell depletion increases healthy lifespan by postponing the onset of several age-associated pathologies. In addition, the clearance of p16Ink4a-positive cells delayed tumorigenesis and attenuated age-related deterioration of several organs, including kidney, where clearance preserved the functionality of glomeruli, reduced glomerulosclerosis, and improved BUN. Importantly, such benefits were not correlated with increased cancer incidence (Baker et al., 2016).

The resistance of senescent cells to apoptosis relies on pro-survival BCL-2, BCL-xl, and BCL-w proteins activation. The Bcl-2 family consists of many evolutionarily conserved proteins that share Bcl-2 homology (BH3) domains. During senescence, increased Bcl-2 expression acts as a disruptor of the interaction between the pro-apoptotic proteins Bad and Bax, which cannot form oligomers, inhibiting the mitochondrial outer membrane permeabilization that normally leads to the intrinsic pathway of apoptosis. This property of senescent cells has been used in the establishment of BH3 mimetics, which is able to bind and control different BCL-2 family members.

In an elegant study, Chang et al. (2016) identified Navitoclax (ABT263), a specific inhibitor of BCL-2 and BCL-xL, as a potent senolytic drug able to selectively kill senescent cells in culture by inducing apoptosis. Oral administration of ABT263 to normally aged mice effectively depleted senescent cells, at level of both bone marrow hematopoietic stem cells and muscle stem cells. More importantly, Navitoclax was shown to reduce viability of senescent human umbilical vein epithelial cells, IMR90 human lung fibroblasts, and murine embryonic fibroblasts, but not preadipocytes isolated from lean kidney transplant donors suggesting mechanisms of resistance in obese patients (Zhu et al., 2016).

The great opportunity represented by Navitoclax therapy in chronic fibrosis has recently been documented by Pan et al. in 2017. In a mice model of ionizing radiation-induced pulmonary fibrosis the authors demonstrated that chronic pulmonary fibrosis could be reversed by Navitoclax after it became a progressive disease and offered a new strategy also in other contexts of irreversible fibrosis (Pan et al., 2017). Currently, Navitoclax is under evaluation in several trials to overcome the chemotherapy-induced apoptosis resistance in solid tumors (i.e., melanoma NCT01989585, small-cell lung cancer NCT03366103) and lymphoid tumors (Wilson et al., 2010; Gandhi et al., 2011; Roberts et al., 2015). Despite encouraging results, the use of Navitoclax has been associated with several adverse effects; the most serious are thrombocytopenia due to the effect of the drug on apoptosis in circulating platelets.

Other senolytics drugs were recently discovered from the natural product from the class of flavonoids (Panche et al., 2016). Like most flavonoids, quercetin is a polyphenol with potent antioxidant activity, abundant in fruits and vegetables (García-Barrado et al., 2020). Quercetin is involved in several biological functions such as cancer prevention, antiviral activity (Wang et al., 2019; Mrityunjaya et al., 2020), and selectively clearing senescent cells, inhibiting PI3K/AKT and p53/p21/serpines and inducing apoptosis (Russo et al., 2014; Myrianthopoulos et al., 2019). Quercetin has an important anti-oxidative effect, since it is able to target ROS. In addition, quercetin can affect also Nrf2 pathway, which is particularly important in senescent cells. Recently, quercetin has been linked to SIRT1, a NAD deacetylase with important antiaging effect mediated by Klotho, p53, and mitochondrial dysfunction modulation (Wakino et al., 2015). The pivotal role of Klotho signaling as renoprotective function in tubular cells has been well described elsewhere. (Dalton et al., 2017; Wang et al., 2018). Quercetin has been investigated in combination with a tyrosine kinase inhibitor called dasatinib. Several pieces of evidence demonstrated that quercetin and dasatinib, alone and in combination, cause apoptosis in senescent cells without significant effects in quiescent or proliferating cells (Zhu et al., 2015; Xu et al., 2018).

In vivo, this combination delays natural aging and age-related symptoms such as obesity disease, atherosclerosis, pulmonary fibrosis, and transplantation of senescent cells. In an elegant study, Xu et al. provide evidence that senescent cells can induce physical dysfunction and decreased survival even in young mice, whereas senolytics enhanced health and increased lifespan in old mice (Xu et al., 2018). The authors demonstrated that transplantation of a few of senescent cells into young mice not only triggered physical dysfunction, but also propagated senescence to other tissues. The transplantation of fewer senescent cells in older recipients further decreased lifespan. In the same study, in explants of human adipose tissue, the combination of dasatinib plus quercetin appeared effective in reducing the number of naturally occurring senescent cells and their secretion of frailty-related pro-inflammatory cytokines. Finally, the authors also showed a protective effect of senolytics when orally administered intermittently, showing a reduction in mortality hazard to 65% (Xu et al., 2018).

Besides the murine model of physiological aging, the combination of quercetin plus dasanitib has been associated with reduction of obesity-induced metabolic dysfunction and recently, in mitigated radiation ulcers (Wang et al., 2019). Indeed, senescent cells accumulate in adipose tissue of obese and diabetic humans and mice (Schafer et al., 2017; Sierra-Ramirez et al., 2020); however, their role is not well understood. For instance, components of the SASP secreted by adipose‐derived senescent cells have been postulated to confer insulin resistance upon metabolic tissues, inhibit adipogenesis, and attract immune cells that can exacerbate insulin resistance (Palmer et al., 2019). In a recent study, Palmer et al. demonstrated that quercetin plus dasanitib treatment in obese mice improved insulin sensitivity, lowered circulating inflammatory mediators, and promoted adipogenesis. More interestingly, also the renal function, one of the most important complications of type 2 diabetes, was significantly improved by microalbuminuria and podocyte function restoration (Palmer et al., 2015; Palmer et al., 2019).

Interestingly, Kim et al. (2019) demonstrated that obesity and dyslipidemia induced renal senescence that was modulated by quercetin treatment. In particular, the authors found that mice fed with high-fat diets showed impaired renal function, cortical oxygenation, and presented glomerulomegaly. Obese hypercholesterolemic mice displayed augmented level of p16, p19, and p53 expression, whereas in quercetin-treated mice senescence was significantly modulated. Quercetin treatment also increased renal cortical oxygenation and decreased plasma creatinine levels in obese mice.

The senolytic effect of quercetin has been analyzed in combination with other compounds such as resveratrol in human kidney cell culture under hyperglycemia condition. Resveratrol is a polyphenol of the stilbene class and is abundant in foods such as wine and fruit. This natural compound has been involved in cancer chemoprevention, protection against cardiovascular diseases, and anti-inflammatory activity. The molecule interferes with several critical pathways such as those of NF-kB, IGF-1R/Akt/Wnt, and PI3K and more importantly can target SIRT1, a member of the class III histone deacetylases (Abharzanjani et al., 2017), strongly involved in kidney functionality and aging (Zhuo et al., 2011; Ugur et al., 2015).

The quercetin and resveratrol were associated with increased expression levels of antioxidants and can reduce aging markers in embryonic kidney cell line in hyperglycemia conditions; furthermore, the gene expression of Sirtuin 1 and thioredoxin (Trx) in all treated groups in comparison with control group increased in a dose-dependent fashion (Abharzanjani et al., 2017).

Recently, these results have been translated in a clinical trial. In the first, the combination of quercetin plus dasanitib was associated with improved physical function in patients with idiopathic pulmonary fibrosis (IPF), a fatal senescence-associated disease (Justice et al., 2019). Last, the same cocktail was investigated in an open label phase 1 pilot study, in which the two drugs were administered for 3 days in subjects with diabetic kidney disease. The authors showed a reduced number of adipose tissue senescent cells, with overall decreases in p16INK4A- and p21CIP1-expressing cells. Interestingly, also adipose tissue macrophages, which are attracted, anchored, and activated by senescent cells, and presented crown-like structures were decreased. Skin epidermal p16INK4A+ and p21CIP1+ cells were reduced, as were circulating SASP factors, including IL-1α, IL-6, and MMPs-9 and MMP-12 (NCT02848131) (Hickson et al., 2019). Moreover, quercetin appears as a promising senolytic treatment in renal aging, also because it is much more selective for senescent endothelial cells during oxidative stress.

Besides interventions that directly target senescent cells, in the armamentarium of senotherapeutics other agents offer new strategies for the modulation of senescent cells induced SASP. These senostatic/senomorphic drugs include not only molecules such as Janus kinase (JAK)/STAT pathway inhibitors, inhibitors of NF-kB, and other compounds, but also the caloric restriction diets, sirtuin activators, antioxidants, anti-inflammatory agents, autophagy activators, and mTOR inhibitors. Here, we will review rapamycin and metformin given their key role in preventing kidney aging during CKD.