Lifespan of Catalase-Decient Mice Decreases Due to Lysosomal Dysfunction

Background: Lysosomes are a central hub for cellular metabolism and are involved in the regulation of cell homeostasis through the degradation or recycling of unwanted or dysfunctional organelles through the autophagy pathway. Catalase, a peroxisomal enzyme, plays an important role in cellular antioxidant defense by decomposing hydrogen peroxide into water and oxygen. In accordance with pleiotropic signicance, both impaired lysosomes and catalase have been linked to many age-related pathologies with a decline in lifespan. Aging is characterized by progressive accumulation of macromolecular damage and the production of high levels of reactive oxygen species (ROS). Although lysosomes degrade the most long-lived proteins and organelles via the autophagic pathway, the role of lysosomes and their effect on peroxisomes during aging is not known. The present study investigated the role of catalase and lysosomal function in catalase-knockout (KO) mice. Results: We found that catalase-decient mice exhibited an aging phenotype faster than wild-type (WT) mice. We also found that aged catalase-KO mice induced leaky lysosomes by progressive accumulation of lysosomal contents, such as cathespin D, into the cytosol. Leaky lysosomes inhibited autophagosome formation and triggered impaired autophagy. The dysregulation of autophagy triggered mTORC1 (mechanistic target of rapamycin complex 1) activation, which plays a pivotal role in modulating aging. However, the antioxidant N-acetyl-L-cysteine (NAC) and mTORC1 inhibitor rapamycin rescued leaky lysosomes and aging phenotypes in catalase-decient aged mice. Conclusion: This study unveils the new role of catalase and its role in lysosomal function during aging.

Animals were sacri ced, and their blood was collected and allowed to clot for 20 min at room temperature (25°C). Serum was separated by centrifugation at 2000 ×g for 20 min at room temperature and stored at -80°C until further analysis. Liver tissue was removed from each mouse, weighed, frozen in liquid nitrogen, and stored at -80°C until further use or xed with formalin. Tissue sections (8-µm thick) were used for histochemical analysis.
All cell lines were con rmed to be negative for mycoplasma contamination.

Histological analysis
For histological analysis, tissues were xed in 10% formalin solution, embedded in para n, and sectioned. H&E staining was performed as previously described [46].
For IF staining of tissue and cells (both MEFs and HepG2), snap-frozen tissues were sectioned using a cryostat and then defrosted at room temperature for 1 h. Cells and tissue sections were xed in 4% paraformaldehyde (HT5014; Sigma-Aldrich) at room temperature for 20 min, washed with PBS, and incubated with 0.2% Triton X-100 for 5 min. The sections were blocked with 5% goat serum and 0.1% Triton X-100, whereas in vitro, cells were blocked with 3% bovine serum albumin (BSA) for 1 h at room temperature. After blocking, the cells were incubated with the target primary antibody (1:200) with their respective blocking solution overnight at 4°C. The next day, tissues and cells were washed with PBS and stained with Alexa Fluor-conjugated secondary antibody in blocking solution at room temperature in the dark for 1 h. After that, both tissue and cells were incubated with 10 µM DAPI (4′,6-diamidino-2phenylindole) in PBS at room temperature for 10 min and mounted with mounting media. Images were acquired and analyzed using an Olympus FluoView 1000 confocal laser scanning system. Slides were scored in a blinded manner.

4.Oil Red O (ORO) staining
Cryosectioned liver samples were xed with paraformaldehyde for 20 min and washed with distilled water. ORO solution (01391; Sigma-Aldrich) was added to the samples for 1 h at room temperature. Sections were washed with distilled water and counterstained with hematoxylin for 1 min. Images were acquired and analyzed using a light microscope. Slides were scored in a blinded manner.

Measurement of hepatic TG
To measure hepatic TG, 50 mg of liver tissue was homogenized, and TG was measured using a triglyceride colorimetric assay kit (#10010303, Cayman Chemical) according to the manufacturer's protocol.

SA-β-Gal staining
For SA-β-Gal staining in vitro (MEFs and HepG2), cells were washed three times with PBS and stained with senescence β-galactosidase according to the manufacturer's protocol (#9860, Cell Signaling Technology). For in vivo experiments, both WT and catalase-KO mice at 9W and 53W were perfused through the heart initially with PBS, followed by 4% paraformaldehyde. Liver tissues were collected, frozen sectioned, and stored at −80°C for future use. Senescence β-galactosidase staining was performed according to the manufacturer's protocol (#9860, Cell Signaling Technology). Images were acquired and analyzed using a light microscope. Slides were scored in a blinded manner. SA-β-Gal staining intensity was determined using the ImageJ software, and the mean SA-β-Gal intensity in the region of interest was measured based on the cell shape. SA-β-Gal positivity was determined as fold-change relative to the control.

Lysotracker Red and MitoSOX Staining
Lysotracker red and MitoSOX staining were performed in live cells, according to the manufacturer's instructions. Images were acquired using a uorescence microscope, with the same xed exposure time for all samples in each experiment.

Western blot analysis
To determine the expression levels of target proteins, western blotting from whole cell analysis using tissue samples and primary adipocytes was performed as described previously [46]. The antibodies used and their sources are listed in Supplementary Table S1. 8. Cell fractionation Cell fractionation was performed to check cathespin protein (in either the membrane or cytosol) in both cell and liver tissues, as described previously [24]. In brief, treated cells and fresh liver tissues were washed with ice-cold PBS and homogenized with homogenization buffer [10 mM HEPES, 0.25 M sucrose, 10 mM Na 2 EDTA, adjusted to pH 7.0 with NaOH, and 1× protease inhibitor cocktail (GenDEPOT, P3100-001; P3200-001)]. After homogenization, both tissues and cells were centrifuged at 1,000 ×g for 10 min, and post-nuclear supernatants were further centrifuged at 100,000 ×g for 1 h to generate supernatant and pellet fractions. All the procedures were performed at 4°C. After measuring the protein concentration, the supernatant and pellet extracts were boiled at 97°C for 10 min and subjected to western blot analysis. 9. Determination of ROS production ROS production from tissue lysates and cells was performed as previously described (46).

ACOX1 activity
Liver tissues were homogenized in ice-cold PBS. After centrifugation, supernatants were collected, and the levels of acyl-CoA oxidase 1 (ACOX1) and activity were measured using ELISA assay kits (My Biosource).

Cathespin D activity
Treated cells and fresh liver tissues were washed with ice-cold PBS and homogenized with lysis buffer provided by a commercially available kit (#ab65302, Abcam). The activity of cathespin D was performed as previously described.

Statistical analysis
Data are presented as means ± standard deviation (SD). One-way analysis of variance was used to compare the means of two or more groups. Statistical signi cance was set at P < 0.05.

Results
1. Catalase-de cient mice induce aging phenotype faster than WT mice It has been well-documented that increased ROS and diminished antioxidant capacity induce cellular senescence, and catalase enzymes have been used to alleviate senescence through its antioxidant defense mechanism [6,8,15]. We investigated whether catalase is used to alleviate ROS and diminish aging in catalase-de cient mice. For this, WT and catalase-KO mouse embryonic broblasts (MEFs) were isolated and cultured. Catalase de ciency was con rmed by immunoblot analysis of MEFs (Fig. S1a). KO MEFs displayed attened and enlarged senescence phenotypic morphology at early passage (P2) and showed increased senescence phenotypic morphology with increasing passage (P4) (Fig. S1a,b). To determine whether the attened and enlarged MEFs were senescent, β-galactosidase staining was performed. As expected, KO MEFs showed positive staining for senescence-associated β-galactosidase, which signi cantly increased in P5 but not in WT cells ( Fig. 1b and S1c). To illustrate the senescenceinduced phenotype, immunoblot analysis was performed for WT and KO MEFs from P2 and P5. Notably, the expression levels of senescence-related proteins p21 and p16 increased in KO P5 MEFs (Fig. 1c). Furthermore, to con rm the aging phenotype, catalase-KO mice with WT littermates at the age of 9 weeks (9W) and 53 weeks (53W) were subjected to the experiment. β-galactosidase staining was performed in the liver of mice, which showed positive staining in KO mice livers at 53W (Fig. 1d). Immunoblot analysis of liver homogenates from mice showed the induction of senescence-related proteins in KO mice at 53W (Fig. 1e). Together, these data suggest that catalase de ciency induces an aging phenotype faster than WT mice.

Catalase-de cient mice induce ROS through mitochondria and peroxisome
As reported earlier, ROS generation is the main cause of aging due to decreased cellular antioxidant capacity [6,8,15]. We hypothesized that ROS generation may be increased in catalase-KO mice. As expected, the uorescence intensity measured by 2′7′-dichloro uorescein diacetate (DCFH-DA) staining showed an increase in catalase-de cient MEFs at P5 (Fig. 2a). The uorescent signal of DCFH-DA staining, representing ROS, was quanti ed and showed that ROS generation was signi cantly higher in catalase-KO cells at P5 (Fig. 2a). Likewise, we examined the expression of another ROS marker, 4 hydroxynonenal (4-HNE), in MEFs by immuno uorescence staining (IF). Consistent with DCFH-DA staining, the uorescence intensity of 4-HNE in KO MEFs increased at P5 (Fig. 2b). In addition, to con rm ROS generation in vivo, intracellular ROS levels were measured in liver lysates of mice. Total ROS levels increased signi cantly in KO mice at 53W (Fig. 2c). The endogenous source of ROS contains different cellular locations, with mitochondria and peroxisomes being the major sites [2,16,17]. The principal source of ROS produced by mitochondria is the superoxide anion, a byproduct of the electron transport chain responsible for oxidative damage by aerobic energy metabolism [18,19]. To detect mitochondrial ROS, MitoSOX red, a mitochondrial superoxide indicator, was used in WT and KO MEFs at the P2 and P5 levels ( Fig. 2d). Catalase-KO cells at P5 showed increased levels of the red uorescence signal. In addition, in vivo, the level of ACOX1 (acyl-CoA oxidase 1), a major producer of ROS in the peroxisome and the rst and rate-limiting enzyme in fatty acid β-oxidation, increased signi cantly in KO mice at 53W (Fig.  2e). To con rm the induction of ROS in catalase-KO mice, MEFs were treated with the antioxidant Nacetyl-L-cysteine (NAC). Treatment with NAC, however, inhibited the level of ROS generation in KO MEFs at P5 (Fig. 2a, b, and d). Taken together, these data suggest that catalase-KO mice induce ROS production through both mitochondria and peroxisomes.
We hypothesized that ROS generation in catalase-KO mice may induce cellular senescence, as previously described [6,8,15]. Hence, β-galactosidase staining was again performed in WT and catalase-KO MEFs and co-treated with NAC (Fig. 2f). As expected, positive staining of senescence-associated βgalactosidase in KO MEFs was signi cantly diminished by treatment with NAC. Moreover, NAC treatment also decreased senescence-related protein in KO P5 MEFs (Fig. 2g). Together, these data suggest that catalase de ciency induces an aging phenotype through ROS generation.

Catalase-de cient aged mice induce leaky lysosome
Lysosomes are the main catabolic organelles that play an essential role in cellular processes, including responses to nutrient availability, stress resistance, plasma membrane repair and development, and cellular differentiation [20]. In line with catabolic organelles, lysosomal activity is strongly in uenced by aging by altering the physical and chemical properties of these organelles and rendering them more sensitive to stress [12]. Considering this notion, immunoblot analysis of mice liver homogenates was performed to check the lysosomal marker protein LAMP1 (Fig. 3a). Lysosomal protein levels increased in the KO liver. Aging has also been reported to increase lysosomal volume [21]. Hence, to check the volume, the morphology of lysosomes was analyzed using IF. WT and KO MEFs were immunostained with the lysosomal marker LAMP1 (Fig. 3b). Catalase-de cient MEFs at the P5 level showed an enlarged cellular size, which signi cantly increased the red uorescence signal toward the cytoplasm of the cell. To con rm acidic lysosomal vesicles, MEFs were immunostained with lysotracker for labeling and tracking of the acidotropic probe for lysosomes (Fig. 3c). The acidic vesicle speci cally accumulating in lysosomes decreased, whereas cell size increased in KO MEFs at the P5 level. Although KO MEFs showed increased lysosome size, their acidic vesicles decreased by lysotracker (Fig. 3a-c). Lysosomal activity is highly in uenced by hydrolytic enzymes residing in the lumen of the lysosomal membrane, which is highly acidic [24]. As the acidotropic probe for lysosomes decreased in KO MEFs, we assumed that the resident hydrolytic enzymes in the lysosomal lumen were leaked. Hence, leaky lysosomal content may make the hydrolytic enzyme alkaline, causing lysosomes to fuse in the cytosol, which may increase lysosomal size. To check the lysosomal content, we analyzed the protein levels of cathepsin D (cathD) and cathepsin B (cathB), two major lysosomal hydrolases that can serve as molecular reporters for lysosomal functions [25], in WT and KO liver homogenates. Both lysosomal hydrolases were accumulated in the supernatant fraction in KO mice at 53W, whereas both cath D and B were normal in the pellet fraction in the other groups (Fig. 3d). Furthermore, cath D activity was measured in the liver lysates of mice (Fig. 3e). cath D activity decreased in KO mice at 53W, which further con rmed the presence of leaky lysosomes in KO aged mice. Together, these data suggest that catalase de ciency induces lysosomal damage through leaky lysosomes. 4. Leaky lysosome persuades alteration of autophagy through ROS in catalase-de cient aged mice Damaged lysosomes are selectively sequestered by autophagy [24]. Hence, we hypothesized that damaged lysosomes in KO mice may be recruited by autophagy machinery, which are then engulfed by autophagosomes. To con rm the autophagic process, MEFs were co-immunostained with LC3, an autophagy marker, with lysotracker ( Fig. 4a). In contrast to our assumption, the LC3 positive puncta increased but were not fused with lysosomes in KO MEFs due to decreased lysosomal acidic probe, which showed less fusion of autophagosomes with lysosomes. Further, immunoblot analysis was performed in WT and KO MEFs at the P2 and P5 levels. Notably, the expression levels of autophagy substrate marker P62 and autophagosome by LC3II were signi cantly increased in KO MEFs at P5 (Fig.  4b). To con rm autophagic dysregulation in vivo, liver tissues from mice were immunoblotted with an autophagy marker (Fig. 4c). Consistent with MEFs, liver tissues from KO mice at 53W showed dysregulation of basal autophagy in comparison to WT and KO 9W livers (Fig. 4c). ROS mediate leaky lysosomes [25]. Hence, to con rm this, WT and catalase-KO MEFs were treated with or without NAC at both passages (P2 and P5). Treatment with NAC slightly recovered the acidic probe of lysosomes, which fused with autophagosomes in KO MEFs at the P5 level (Fig. 4a). Hence, these data suggest that leaky lysosomes alter autophagy through ROS in catalase-de cient aged mice. 5. Leaky lysosomes induce lipofuscin accumulation through ROS in catalase-de cient aged mice Although leaky lysosomes are induced in catalase-de cient MEFs, the damaged lysosomes were not degraded through the autophagic process. Instead, basal autophagy was dysregulated in KO MEFs. To nd the mechanistic evidence for this, we examined the morphology of the liver by H&E staining in mice.
The liver morphology of 53W KO mice showed microvesicular steatosis (i.e., accumulation of small fat droplets) in the cytosol of hepatocytes (arrows), with golden-brown pigment (arrowhead), whereas the livers of WT and KO 9W mice showed normal lobular architecture with hepatocytes arranged in hepatic cords (Fig. 5a). To con rm the accumulation of fat droplets in hepatocytes, triacylglycerol (TG) was measured in the liver lysates of mice. Consistent with H&E staining, liver lysates of 53W KO mice showed a signi cant increase in liver TG compared to other groups (Fig. 5b). Furthermore, oil red O staining (ORO) also showed the induction of lipid droplets in the hepatocytes of 53W liver sections (Fig. 5c). During aging, the volume and structure of hepatocyte organelles change [26]. Although we observed a signi cant increase in body weight of 53W old KO mice, but there were no signi cant changes in liver weight ( Fig.  S2a-b). We hypothesized that the accumulation of small lipid droplets in the cytoplasm of hepatocytes in KO 53W old mice may be an undigested lipid, lipofuscin, which showed brown pigmentation on hepatocytes (Fig. 5a). Lipofuscin is a highly oxidized insoluble protein that fails to degrade damaged and denatured proteins [27]. Moreover, it is a chemically and morphologically polymorphous waste material that accumulates at the primary site of the lysosome and disturbs lysosomal degradation and causes lysosome leakage [28,29]. To examine the accumulation of lipofuscin or leaky lysosomes, MEFs were coimmunostained with LGALS1 (galectin-1), a leaky lysosome marker with lysotracker (Fig. 5d). Catalase-KO MEFs at P5 level showed a signi cant increase in LGALS1 puncta that were loaded on the lysosomes (Fig. 5d, arrow); although acidic vesicles in lysosomes by lysotracker were less, almost all LGALS1 puncta were localized to lysosomes. In contrast, LGALS1 puncta were less or not observed at all in the WT at P2, P5, and KO P2 levels. It is known that enhanced ROS results in the leakage and accumulation of lipofuscin in lysosomes [25,28]. Hence, to con rm this, WT and catalase-KO MEFs were treated with or without NAC at both passages (P2 and P5). Treatment with NAC slightly rescued the acidic vesicles of lysosomes and decreased the localization of LGALS1 puncta to the lysosomes in KO MEFs at P5 level (Fig. 5d). Hence, these data suggest that leaky lysosomes induce lipofuscin accumulation through ROS in catalase-KO aged mice. 6. Leaky lysosome affects lysosomal pH that activates mTORC1 (mechanistic target of rapamycin complex 1) and leads to cellular senescence Next, we constructed leaky lysosomes using the well-known lysosomal membrane permeabilization (LMP) marker L-leucyl-L-leucine methyl ester (LLOME) in hepatoma cells and questioned whether leaky lysosomes induce ROS and cellular senescence. For this, we treated HepG2 cells with LMP inducer LLOME for 24 h and examined the morphology of lysosomes by immunostaining with lysotracker. As expected, the speci c accumulation of acidic vesicles in lysosomes decreased in LLOME-treated cells (Fig. 6a). The disruption of acidic hydrolases in lysosomes or leaky lysosomes is induced through extensive ROS [25]. To con rm this, HepG2 cells were stained with DCFH-DA (Fig. 6b). Fluorescence intensity by DCFH-DA staining increased in LLOME-treated cells. Hence, to con rm the leaky lysosomes induced by ROS accumulation, HepG2 cells were co-treated with antioxidant NAC and LLOME. As expected, NAC recovered the acidic vesicles of lysosomes and abolished DCFH-DA uorescence intensity in HepG2 cells (Fig. 6a-b). LLOME treatment increased the cytosolic release of lysosomal hydrolases [24]. Hence, to check the lysosomal content, the protein levels of cath D and B were immunoblotted in HepG2 cells treated with LLOME (Fig. 6c). As expected, both lysosomal hydrolases (cath D and B) accumulated in the supernatant fraction in LLOME-treated cells, whereas they were normal in the pellet fraction of untreated cells. Further immunoblot analysis of HepG2 cells showed increased protein expression of the ROS marker 4-HNE and peroxisomal oxidase ACOX1 in LLOME-treated cells (Fig. S3a). However, we did not observe any changes in mitochondrial enzymes, including COX1, COX4, voltagedependent anion channel (VDAC), and antioxidant proteins, including SOD1 and SOD2.
The link between decreased lysosomal function and aging has been well studied [12,30]. Hence, we investigated whether LMP drug LLOME induces cellular senescence in cells. β-galactosidase staining was performed in HepG2 cells. As expected, cells treated with LLOME showed positive staining for senescence-associated β-galactosidase (Fig. S3b). However, treatment with NAC abolished the positive staining of senescence-associated β-galactosidase. Further immunoblot analysis was performed in HepG2 cells showing increased expression of aging-related proteins, including p16 and p21, in LLOMEtreated cells (Fig. S3c). During lysosomal damage, transcription factor EB (TFEB), a major regulator of autophagy and lysosomal biogenesis, has been shown to rapidly translocate to the nucleus and activate the transcription of its target gene for the activation of lysosomes [31]. Hence, immunostaining was performed for translocation of TFEB to HepG2 cells. As expected, LLOME-treated cells showed translocation of TFEB to the nucleus in LLOME-treated cells (Fig. 6d). Hence, lysosomal rupture induces the biogenesis of lysosomes through autophagy (Fig. S3d), as previously described [24,31]. Meanwhile, immunoblot analysis also showed that treatment with LLOME increased the expression of phosphorylated S6 (pS6), a marker of downregulation of mTORC1 (Fig. 6e). Further immunostaining with anti-mTORC1was performed in LLOME-treated cells, which showed increased expression of mTORC1 protein (Fig. S3e). Together, these data show that leaky lysosomes affect lysosomes and induce cellular senescence probably through mTORC1 activation. 7. Rapamycin attenuated cellular senescence induced by catalase-de cient cells mTOR is a key component of cellular metabolism that promotes cell growth and proliferation via nutrient sensing. In addition to cellular growth and proliferation, mTOR has also been associated as a lifespan regulator in mice [32][33][34][35]. The lifespan-enhancing effects of mTOR inhibitors have been linked to mTORC1 inhibition [36]. Hence, we treated cells with rapamycin, an mTORC1 inhibitor, to reverse cellular senescence induced by catalase-KO mice. For this, WT and KO MEFs were treated with rapamycin, and βgalactosidase staining was performed to check the senescence phenotype. KO MEFs displayed attened and enlarged senescence phenotypic morphology at early passage (P2) and showed increased senescence phenotypic morphology with increasing passage (P5) in addition to positive staining for senescence-associated β-galactosidase, but not in WT MEFs (Fig. 7a). However, treatment with rapamycin inhibited positive staining of senescence-associated β-galactosidase, but KO MEFs still displayed attened and enlarged senescence phenotypic morphology. Further immunoblot analysis of MEFs showed the induction of senescence-related proteins in KO cells that was suppressed by cotreatment with rapamycin (Fig. 7b). mTORC1 is also known to suppress autophagy, and activation of autophagy by suppression of mTORC1 can slow age by clearing the accumulating old and dysfunctional organelles [36,37]. Hence, to check the clearance of old and dysfunctional organelles by autophagy, rapamycin was used to treat MEFs, and immunoblot analysis was performed. Remarkably, the increased levels of autophagy substrate marker P62 and autophagosome marker LC3II were signi cantly decreased by rapamycin in KO MEFs at P5 level (Fig. 7c). Furthermore, phosphorylated S6, downstream of mTORC1 activity was decreased by rapamycin, suggesting that autophagy was initiated (Fig. 7c). In addition, accumulation of lysosomal content, such as cath D, in the supernatant fraction in KO MEFs at level P5 was suppressed by rapamycin (Fig. 7d). Similarly, cath D activity was also recovered in KO MEF following treatment with rapamycin (Fig. 7e). Additionally, we immunostained MEFs with lysotracker to measure the acidic probe of lysosomes by rapamycin. As expected, the decreased acidic puncta of lysosomes were recovered by rapamycin in KO MEFs at P5 (Fig. 7f). Taken together, we showed that mTORC1 depletion by rapamycin slightly attenuated the progression of aging in catalase-KO mice. We also tried to show aging progression by hyperactivation of mTOR by the point mutation S2215Y, identi ed in the human cancer genome database [38]. We transfected the FLAG-tagged mutated form of mTOR with its WT plasmid in MEFs, followed by immunostaining with lysotracker (Fig. S4). Consistent with the aged phenotype, the mutated form showed a diffuse form of lysotracker, whereas in WT cells, lysotracker puncta were quite distinctive. Together, these data showed that mTOR hyperactivation in catalase-KO mice may aggravate the aging phenotype faster than in WT mice.

Discussion
Aging comprises various probabilities and is usually treated as a discrete variable rather than a continuous variable. Thus, most studies focus on three speci c life phases: mature adult, middle age, and old. In mice, the mature adult (3-6 months) group is the reference for any age change, whether it is developmental, maturational, or senescent; middle age (10-14 months) refers to a stage in which senescent changes can be detected in some, but not all, biomarkers of aging; and old age (18-24 months) refers to a stage in which senescent changes can be detected in almost all biomarkers [13,14].
In this study, we used two groups of mice, viz.., younger (~2 months or 9W) that were not affected by senescent and middle-aged mice (12 months or 53W) that are on the way to senescence. Although the 53W mice breed normally, they exhibited the senescent phenotype faster, which decreased the lifespan of catalase-KO mice. In addition, during extensive culture passages, normal primary cells undergo malignant progression through cytostasis and contribute to cell senescence induction (39). Hence, primary MEFs in a recent study showed a senescent-like phenotype only after P5 levels. Thus, we mentioned MEFs for P2 as young cells and P5 as senescent cells.
Senescence refers to a cellular response characterized by a state of stable cell cycle arrest in which proliferating cells become resistant to growth but remain viable and metabolically active after prolonged time in culture [40]. However, senescence is not just a cell cycle arrest. When the cell cycle is arrested, an inappropriate growth-promotion pathway, such as mTOR, converts arrest into senescence and mTOR inhibitor rapamycin decelerates cell proliferation but preserves the re-proliferative potential of inactive cells that are lost during senescence [41,42]. Based on these ndings, we concluded that rapamycin treatment decelerates cell proliferation but inhibits positive staining of senescence-associated βgalactosidase in KO aged MEFs (Fig. 7).
Lipofuscin is comprised of highly oxidized proteins, lipids, and metal elements that accumulate primarily in the lysosomes and reduce its autophagic capacity [43,44]. These non-degradable lipids and ions or waste accumulation in the form of lipofuscin elevates the lysosomal pH, leading to a snowball effect on lysosomes [45]. Hence, the continuous accumulation of lipofuscin in lysosomes through ROS generation in catalase-KO mice increased the volume of lysosomes. Although lysosomal size increased, lysosomal hydrolytic activity decreased in catalase-KO aged mice (Fig. 3).
Although aging affects many cellular components, our study showed remarkable changes in lysosomes that induce mTORC hyperactivation and, thus, cause an aging phenotype faster in catalase-de cient mice. Our study may unveil the innovative roles of catalase and its relation to lysosomes and its role in aging. Availability of data and materials:

Declarations
All data generated and/or analyzed in this study are available from the corresponding author upon reasonable request.
Ethics approval and consent to participate:     Catalase-de cient aged mice induce alteration of autophagy through ROS (a) MEFs from WT and catalase-KO mice at P2 and P5 levels were treated with 5 mM NAC, xed, and immunostained with anti-LC3 (green) and Lysotracker (red). Scale bar represents 10 μm. (b) Proteins were extracted from MEFs as in a. Immunoblot analysis was performed using whole-cell lysates with the indicated antibodies. (c) Protein was extracted from liver lysates from WT and catalase-KO mice at 9W and 53W. Immunoblot analysis was performed using whole-cell lysates with the indicated antibodies. Catalase-de cient aged mice induce lipofuscin accumulation through ROS (a) Representative hematoxylin and eosin (H&E) staining of livers from WT and catalase-KO mice at the age of 9W and 53W, respectively. The livers of 53W KO mice showed microvesicular steatosis (i.e., accumulation of small fat droplets) in the cytosol of hepatocytes (arrows) with a golden-brown pigment (arrowhead) known as lipofuscin. Scale bar represents 50 μm. (b) Liver samples from mice as mentioned in a were homogenized, and TG levels were analyzed. Bar graph represents mean ± SD (n = 3 experiments). *P < 0.05, WT 53W vs. KO 53W. (c) Cryosectioned liver tissues from mice were stained with ORO. Scale bar represents 50 μm. (d) Representative uorescence images of MEFs at P2 and P5 treated with 5 mM NAC, xed, and immunostained with Lysotracker (red), LGALS1 (green), and DAPI (blue). Scale bar represents 5 μm.

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download.