Global Proteome of LonP1+/− Mouse Embryonal Fibroblasts Reveals Impact on Respiratory Chain, but No Interdependence between Eral1 and Mitoribosomes

Research on healthy aging shows that lifespan reductions are often caused by mitochondrial dysfunction. Thus, it is very interesting that the deletion of mitochondrial matrix peptidase LonP1 was observed to abolish embryogenesis, while deletion of the mitochondrial matrix peptidase Caseinolytic Mitochondrial Matrix Peptidase Proteolytic Subunit (ClpP) prolonged survival. To unveil the targets of each enzyme, we documented the global proteome of LonP1+/− mouse embryonal fibroblasts (MEF), for comparison with ClpP−/− depletion. Proteomic profiles of LonP1+/− MEF generated by label-free mass spectrometry were further processed with the STRING (Search tool for the retrieval of interacting genes) webserver Heidelberg for protein interactions. ClpP was previously reported to degrade Eral1 as a chaperone involved in mitoribosome assembly, so ClpP deficiency triggers the accumulation of mitoribosomal subunits and inefficient translation. LonP1+/− MEF also showed Eral1 accumulation, but no systematic effect on mitoribosomal subunits. In contrast to ClpP−/− profiles, several components of the respiratory complex-I membrane arm, of the glutathione pathway and of lysosomes were accumulated, whereas the upregulation of numerous innate immune defense components was similar. Overall, LonP1, as opposed to ClpP, appears to have no effect on translational machinery, instead it shows enhanced respiratory dysfunction; this agrees with reports on the human CODAS syndrome (syndrome with cerebral, ocular, dental, auricular, and skeletal anomalies) caused by LonP1 mutations.


Introduction
Mitochondria are double membraned subcellular organelles which are implicated in multiple cellular processes. In addition to their main role in the production of energy through being the organelle housing oxidative phosphorylation and the citric acid cycle, mitochondria exert many other functions. These range from cellular calcium homeostasis [1], regulation of reactive oxygen species ClpP in the mitochondrial matrix is also a highly conserved serine peptidase, which is assembled in a barrel-like structure together with ClpX, the latter one providing energy via its ATPase function and assuring substrate specificity [41]. ClpP has been shown to play a role in the unfolded protein response in mitochondria (UPRmt) in C. elegans and may play a role in the degradation of unfolded proteins in rodents as well as in humans [42][43][44]. Mutations in ClpP in humans lead to the development of Perrault syndrome 3 (PRLTS3), which is modeled by ClpP −/− mice that mirror the infertility, sensorineural hearing loss, ataxia and growth retardation known from PRLTS3 patients [45,46]. In addition, mitochondrial DNA (mtDNA) content was shown to be present in excess in the absence of ClpP. It was recently shown that ClpP deficiency impairs the fidelity of mitochondrial translation and leads to an accumulation of mitoribosomes. This is thought to be caused by the pathological accumulation of the mitoribosomal assembly factor Eral1, which was claimed to be a direct proteolytic substrate of ClpP [47].
While the deficiency of LonP1 on the one hand and the subsequent accumulation of misfolded proteins in the mitochondria trigger an increase of Pink1 and the engulfment/elimination of mitochondria via the autophago-lysosomal pathway [48], the deficiency of ClpP, on the other hand, triggers selective changes in the cytosolic proteasome [46]. In both cases, the mitochondrial dysfunction caused by either LonP1 or ClpP depletion activates the Rig-I-like receptor pathway of the innate immune system, perhaps simply by altered binding of lactate and hexokinase-2 to the mitochondrial antiviral signaling protein Mavs [46,[49][50][51]. In order to elucidate the pathology of mitochondrial proteostasis and the compensatory efforts of the surrounding eukaryotic cell in further detail, we used LonP1 +/− mouse embryonic fibroblasts (MEFs) to document their global proteome profile. The comparison with previously reported ClpP −/− expression profiles may help to identify substrates that are selectively degraded by each of these proteases.

Results
The analysis of previously generated [6] LonP1 +/− MEFs versus wildtype littermate MEFs regarding their global proteome profile by quantitative label-free mass spectrometry detected a total of 5929 proteins. A high number exhibited significant changes in abundance (Supplementary Table S1). Among them, 463 proteins were upregulated and 328 downregulated with nominal significance. Thus, despite the deficiency of the main mitochondrial protease in these cells, there was no strong bias towards protein accumulations in the profile. However, the effects were clearly non-random, with highly significant enrichments of specific pathways and an overall protein-protein-interaction enrichment p-value of <1.0 × 10 −16 upon bioinformatics analysis at the STRING (search tool for the retrieval of interacting genes) webpage in Heidelberg. Interestingly, known LonP1 degradation targets such as Tfam and Aco2 [6,40,52] did not show significantly elevated abundance in this heterozygous LonP1 depletion state. It is also noteworthy that indicators of mitochondrial biogenesis such as Ppargc1a, Nrf1, Nrf2, Tfam [53], or components of the selective autophagy for mitochondria such as Pink1, Park2, Bnip3, Bnip3l, Fundc1, Ambra1, Mul1, Arih1, March5, gp78/Amfr, Mgrn1, and Huwe1 [54,55] did not show dysregulated abundance. With regard to the selectivity of proteolytic degradation by Lonp1 versus ClpXP in the mitochondrial matrix, it is important to note that mitoribosomal subunits were not accumulated in LonP1 +/− MEFs, in contrast to ClpP −/− proteome profiles.
The distribution of dysregulations and the most important factors are illustrated in a Volcano plot (Figure 1), where the genetically reduced abundance of LonP1 is illustrated with a blue dot, other significant downregulations are shown in green, and the relevant upregulations in red color. This Volcano plot represents all significant dysregulations in the global proteome of LonP1 +/− mouse embryonal fibroblasts (MEFs) (heterozygous mutant (HET)) in green color (downregulated), red color (upregulated) or blue color (genetically deleted). The two-fold deficiency of LonP1 is represented by log2 ratio as -1 on the X-axis. Significance values are shown on the Y-axis. Peptides with nonsignificant changes are shown as grey dots. Factors of high significance despite moderate fold-change such as LonP1 can be easily distinguished in this diagram from other factors of massive fold-changes with higher variability such as the longevity and health span factor Fmo2 [59].

Dysregulated Mitochondrial Factors in LonP1 +/− MEFs are Enriched for Oxidation Processes
To understand how LonP1 deficiency triggers these cellular responses, we selected among all significant dysregulations only those factors that are enriched in mitochondria, based on their classification in the STRING database and the localization predictions in the GeneCards database (see Table 1).

Figure 1.
This Volcano plot represents all significant dysregulations in the global proteome of LonP1 +/− mouse embryonal fibroblasts (MEFs) (heterozygous mutant (HET)) in green color (downregulated), red color (upregulated) or blue color (genetically deleted). The two-fold deficiency of LonP1 is represented by log2 ratio as -1 on the X-axis. Significance values are shown on the Y-axis. Peptides with non-significant changes are shown as grey dots. Factors of high significance despite moderate fold-change such as LonP1 can be easily distinguished in this diagram from other factors of massive fold-changes with higher variability such as the longevity and health span factor Fmo2 [59].
Given that our further analyses focus on pathways with strong enrichment at the FDR significance level, we used less stringency for their individual components, referring to their nominal p-values.

Dysregulated Mitochondrial Factors in LonP1 +/− MEFs are Enriched for Oxidation Processes
To understand how LonP1 deficiency triggers these cellular responses, we selected among all significant dysregulations only those factors that are enriched in mitochondria, based on their classification in the STRING database and the localization predictions in the GeneCards database (see Table 1). Table 1. List of mitochondrial factors with significant dysregulation, ordered by direction of change (dark green color for downregulations, red for upregulations) and by fold-changes. In response to the deficiency of the AAA+ domain containing peptidase LonP1 (purple), several AAA+ domain containing factors and proteases (highlighted in yellow), as well as two chaperones (orange), were altered. Several subunits of the respiratory complex-I stood out (green), with upregulations of three subunits in the membrane arm (P module) [60], while one subunit in the matrix arm of iron/sulfur and flavoproteins (NQ modules) showed a minor downregulation. In a STRING analysis to identify regulated pathways within mitochondria (enrichment p-value <1.0 × 10 −16 ), the changes concerned inner membrane factors (FDR = 3.81 × 10 −15 ) even more strongly than the mitochondrial matrix factors (FDR = 3.37 × 10 −12 ), in particular the Biological Process GO term "oxidation-reduction" (FDR = 5.33 × 10 −10 ). Among the KEGG and REACTOME pathways, the "Complex-I biogenesis" (FDR = 5.14 × 10 −5 ) and the "Processing of SMDT1" (FDR = 5.52 × 10 −5 ) as inner membrane processes were affected more significantly than the matrix processes "citrate cycle" (FDR = 4.30 × 10 −4 ) and gluconeogenesis (FDR = 2.50 × 10 −4 ). Also in the matrix, the one-carbon metabolism was affected with "formate-tetrahydrofolate ligase" being prominent among PFAM protein domains (FDR = 9.60 × 10 −4 ), while membrane processes again dominated with "mitochondrial carrier protein superfamily" (FDR = 2.0 × 10 −4 ) among INTERPRO protein domains, and the dysregulation of several AAA+ disaggregases appeared as "ATPases associated with a variety of cellular activities" (FDR =2.09 × 10 −2 ) among SMART protein domains, in response to deficiency of the AAA+ domain containing LonP1. Figure 2 illustrates these factors with various colors for each pathway, and with connecting lines that represent different evidence for interactions among them. Importantly, respiratory complex I subunits in the ND5, ND4 and ND2 module of the membrane arm accumulate, in contrast to a subunit from the Q module in the hydrophilic peripheral arm in the mitochondrial matrix. Overall, complex-I assembly stoichiometry appears to be more strongly affected by LonP1 deficiency than the previously reported LonP1-dependent degradation substrates such as Tfam or Aco2. the mitochondrial matrix factors (FDR = 3.37 × 10 −12 ), in particular the Biological Process GO term "oxidation-reduction" (FDR = 5.33 × 10 −10 ). Among the KEGG and REACTOME pathways, the "Complex-I biogenesis" (FDR = 5.14 × 10 −5 ) and the "Processing of SMDT1" (FDR = 5.52 × 10 −5 ) as inner membrane processes were affected more significantly than the matrix processes "citrate cycle" (FDR = 4.30 × 10 −4 ) and gluconeogenesis (FDR = 2.50 × 10 −4 ). Also in the matrix, the one-carbon metabolism was affected with "formate-tetrahydrofolate ligase" being prominent among PFAM protein domains (FDR = 9.60 × 10 −4 ), while membrane processes again dominated with "mitochondrial carrier protein superfamily" (FDR = 2.0 × 10 −4 ) among INTERPRO protein domains, and the dysregulation of several AAA+ disaggregases appeared as "ATPases associated with a variety of cellular activities" (FDR =2.09 × 10 −2 ) among SMART protein domains, in response to deficiency of the AAA+ domain containing LonP1. Figure 2 illustrates these factors with various colors for each pathway, and with connecting lines that represent different evidence for interactions among them. Importantly, respiratory complex I subunits in the ND5, ND4 and ND2 module of the membrane arm accumulate, in contrast to a subunit from the Q module in the hydrophilic peripheral arm in the mitochondrial matrix. Overall, complex-I assembly stoichiometry appears to be more strongly affected by LonP1 deficiency than the previously  In parallel with the deficiency of LonP1, strongly significant reductions appeared (Table 1) for the chaperone Hspd1, which is responsible of folding mitochondrial proteins to their native state [61], and for the peptidase Pmpcb (beta-MPP), which catalyzes the cleavage of mitochondrial precursor proteins upon import, before they are folded [62,63]. In addition, a strongly significant reduction of the Tomm70a import receptor in the outer membrane translocase suggests that the recruitment and folding of precursor proteins decrease notably. Three components of the nucleobase biosynthetic process were also diminished with strong significance, namely Dhodh as the only mitochondrial factor involved in de novo pyrimidine synthesis, as well as Mthfd1l and Mthfd1 that represent mitochondrial factors involved in de novo purine synthesis. These data suggest that RNA and DNA processes in LonP1 +/− mitochondria become unbalanced.

Mitochondrial Upregulations in LonP1 +/− MEFs
The most prominent increase (p = 9.25 × 10 −4 ) among mitochondrial factors (Table 1) concerned the transaminase Abat, which cooperates with the succinate-CoA ligases Suclg1 and Sucla2 to convert dNDPs to dNTPs, while also affecting the glutamate-glutamine cycle and mediating the GABA-shunt as a complement to the citric acid cycle that is crucial for anoxia and acid tolerance [64][65][66]. The second most significant upregulation was noted for the mitoribosome assembly factor Eral1. This was a surprising finding, given that Eral1 was recently claimed to be a selective substrate of ClpXP-mediated degradation [47]. ClpP protein levels were unchanged in LonP1 +/− MEF (Supplementary Table S1), ruling out that this effect was still dependent on ClpP. In a subsequent paragraph, we performed meta-analyses of two published proteome profiles in comparison to this proteome profile to assess the questions, whether mitoribosome accumulation is a ClpXP-specific effect, whether Eral1 accumulation is a ClpXP-specific effect (or also depends on the presence of LonP1), and whether mitoribosome and Eral1 levels depend on each other.
Several other upregulated factors in LonP1 +/− MEFs reflect enhanced oxoacid metabolism in the mitochondrial matrix, namely Pcx, Pck2, Pccb, Idh2, Cpt2, Marc2, and Rars2. The increased levels of Fech and Oxr1 presumably imply efforts to compensate oxidative stress within mitochondria. Importantly, the deficiency of the AAA+ domain containing peptidase LonP1 triggered elevated levels for two factors with a role in protein processing: Firstly, Bcsl1 as an AAA+ domain-containing assembly factor of respiratory complex-III [67] showed significant accumulation. Secondly, an accumulation was found for the AAA+ domain containing peptidase Afg3l2 (mAAA-subunit-2), which modulates assembly of respiratory complex-IV and of the MCU complex [68,69]. Both are highlighted in yellow in Table 1. Overall, many upregulations refer to oxoacid metabolism, oxidative stress, and assembly of the oxidative phosphorylation/respiratory chain.

Eral1 and Mitoribosomal Factors as Substrates of LonP1 Versus ClpXP-Two Meta-Analyses
In an effort to understand, to what degree the degradation of the RNA-chaperone Eral1 is due to LonP1 or ClpXP, and whether Eral1 accumulation is prominently responsible for the assembly of the 28S ribosomal subunit in mitochondria, we reassessed previously published global proteome data from the heart of ClpP −/− mice [47]. Highly significant peptide accumulations were observed not only for Eral1, but also for several other mitochondrial matrix chaperones. In particular, a massive increase was observed for ClpX (91.8-fold, p = 3.8 × 10 −5 ). Substantially elevated levels existed also for Trap1 (4.8-fold, p = 7.11 × 10 −6 ), Grpel1 (3.5-fold, p = 7.03 × 10 −6 ), Hspa9 (2.5-fold, p = 1.4 × 10 −5 ), and Dnaja3 (2.1-fold, p = 3.6 × 10 −4 ). It remained unclear to what degree excessive transcriptional induction versus deficient proteolytic turnover is responsible for these chaperone accumulations [47]. Thus, a host of chaperones show elevated levels in parallel to the mitoribosome accumulation.
In view of the massive ClpX accumulation above, it is also interesting to re-assess a recent study where ClpX had been overexpressed 1.8-fold in C2C12 mouse myoblast cells and the global proteome profile was documented [42]. While ClpX is elevated in both experiments, ClpP as an interactor protein showed a 1.4-fold co-accumulation upon ClpX overexpression, in contrast to the previous ClpP deletion study. Thus, a comparison of both profiles permits the identification of selective consequences of ClpP deficiency versus ClpP overactivity, separating them from indirect effects. We performed an analogous bioinformatics assessment of this published dataset, selecting the proteins with >1.5-fold accumulation and testing their pathway interaction profile in STRING enrichment statistics and diagrams. Indeed, the STRING diagram of upregulations upon ClpX overexpression (Supplementary Figure S1) detects similarly strong accumulations of mitoribosomal subunits as in ClpP −/− tissues with their massive ClpX accumulation. Thus, strong mitoribosomal accumulation occurs in both cases together with elevated ClpX levels. In comparison, Eral1 levels showed only 1.1-fold upregulation in ClpX overexpression myoblasts. The most significant findings in ClpX overexpression myoblasts (see Supplementary Table S2) reflect mitochondrial and nucleolar anomalies, with altered ribosome biogenesis. Relevant similarities include the co-accumulation of Noa1 (1.7-fold), Hspa9 (1.5-fold), and Grpel2 (1.4-fold) with ClpX. The increased levels of many chaperones might simply be due to co-accumulation with misfolded mitochondrial ribosome complexes. Specifically, Eral1 may simply show retarded turnover upon the accumulation of its binding partners, mitoribosomes and RNAs. Overall, the available data suggest that the accumulation of mitoribosomes in ClpP −/− tissues occurs when ClpX levels are elevated and is independent from Eral1 accumulation. In contrast to mitoribosomes, Eral1 levels are not determined by ClpXP-mediated degradation alone, but elevated in ClpP deficiency, in ClpX overexpression, and in LonP1 deficiency.

Oxidative Stress and Glutathione Pathways in LonP1 +/− MEFs
Beyond the mainly mitochondrial factors, relevant enrichments among GO terms Biological processes were noted for "response to oxidative stress" (FDR = 0.00060) and "glutathione metabolic process" (FDR = 0.00055) in the LonP1 +/− MEF proteome profile, with an upregulation for each of these enzymes except the glutathione synthase Gss, the spermine synthase Sms and the hypoxia-inducible factor Egln1 (Table 2). These upregulations were strongest for Cat and Mgst1, which are present both in mitochondria and the cytosol and small for Gstm5 and Gstm2 whose mitochondrial presence is weak, according to localization predictions in the GeneCards database. It seems likely that these changes are secondary to the altered oxidative processes within mitochondria.

The Strongest Upregulations in LonP1 +/− MEFs can be Seen in the Lysosomal Pathway
In the STRING analysis of all enrichments, "lysosome" was prominent among KEGG pathways (FDR = 1.19 × 10 −18 ), also among UNIPROT keywords (FDR = 1.18 × 10 −9 ) and among GO terms Molecular Function (FDR = 5.27 × 10 −9 ). Almost all dysregulations consisted of increased abundance of lysosomal factors, suggesting elevated degradation activity (Table 4). This overactivity seemed directed towards specific targets, given that the strongest upregulations concerned the lysosomal cathepsin Ctsc as an activator of many serine proteases in the immune system, and lysosomal cathepsin Ctsz also as a component of innate immune responses. Conversely, the strongest downregulation concerned the lysosomal cathepsin Ctsh, which is responsible for the overall degradation of proteins in lysosomes. While there was a strong increase for Tmem59, which directs endosomal LC3 labelling and lysosomal targeting, there was a reciprocal decrease for Sqstm1/p62, which directs protein aggregates to autophago-lysosomal degradation. It is also noteworthy that a dysregulation of the proteasomal pathways was not significant in STRING enrichment analysis. These findings could be interpreted as activated selective capacity for the vesicle-mediated elimination of mitochondrial fragments and other immune-response triggering bacteria by lysosomes. Indeed, among the GO terms Biological Process, there was a significant enrichment for "phagocytosis" (FDR = 0.0047), with upregulation of Mesdc2, Colec12, Lrp1, Rab22a, B2m, Itgb3, Cdc42ep2 versus downregulation of Mfge8, Axl, Thbs1, Rab31, Tgm2, Kif5b, Flnb, Anxa1, Anxa3, Myo1c, Cd302, Myh9, Gsn, and Vim. As expected during innate immune responses, these changes were accompanied by massive changes in the REACTOME pathway "extracellular matrix organization" (FDR = 3.27 × 10 −15 ), in the KEGG pathway "focal adhesion" (9.94 × 10 −12 ) and in the INTERPRO protein domains "growth factor receptor cysteine-rich domain superfamily" (1.54 × 10 −5 ). Table 3. List of dysregulated factors in the innate immune system, ordered by direction of change (red for upregulations, dark green for downregulations) and by fold-changes. Among the many existing receptors for damage-associated-patterns, components of the pathways for detection of toxic DNA (highlighted by orange color) and double-stranded RNA (yellow) with their downstream nuclear transcription factors in the Stat-family (sky blue) were prominent among the upregulations. Plasma membrane epitopes (light green color) within the cluster of differentiation CD* superfamily stood out among the downregulations.

Discussion
This manuscript provides the first documentation of the global proteome of LonP1 +/− MEFs. While a strong effect on oxidative processes in the mitochondria was expected and did not require validation, novel observations included the lack of mitoribosomal accumulations despite Eral1 accumulation. The LonP1 +/− proteome profile of all mitochondrially localized factors showed Eral1 as second most significant upregulation effect (p = 1.97 × 10 −3 ), after the Lonp1 heterozygous loss itself (p = 1.92 × 10 −4 ) and the 2.3-fold accumulation of Abat (p = 9.25 × 10 −4 ), as the crucial enzyme in the generation of succinate via breakdown of amino acids. The Eral1 increase was an unexpected finding, given that Eral1 was recently proposed as a ClpXP degradation substrate [47]. Eral1 is a mitochondrial matrix chaperone, which associates with the small ribosomal subunit and contributes to mitoribosome assembly [72]. Our novel proteome profile and two meta-analyses indicate that mitoribosome accumulation occurs only upon CLPX mutations and correlates with ClpX levels, while Eral1 accumulates upon Lonp1 mutation, ClpP loss or ClpX overexpression quite unspecifically.
In the bioinformatic analysis of the LonP1 +/− MEF global proteome, the previously published degradation substrates such as Aco2 and Tfam did not show changes in their protein abundances. Also, total mitochondrial biogenesis and mitophagy did not seem to be altered significantly, and proteasomal degradation was not activated. However, the lysosomal degradation of immunological targets was strongly upregulated together with phagocytosis and endosomal pathways, with the strongest upregulations concerning Ctsz and Ctsc which act in innate immune pathways [73,74]. Furthermore, it seemed credible that LonP1 deficiency affects the import of mitochondrial precursor proteins, since the mitochondrial import receptor Tomm70a, the import peptidase Pmpcb and the folding chaperone Hspd1 had significantly altered abundances. This is in good agreement with the previously established role of LonP1 in the unfolded protein response [75,76].
Several mitochondrial factors were significantly downregulated, which are involved in purine and pyrimidine synthesis, such as Mthfd1 and Dhodh, respectively [77,78]. This contrasted with the strong upregulation of Abat, which is known for its role in the conversion of dNDPs to dNTPs. These findings again hint towards an altered RNA and DNA homeostasis in LonP1-deficient cells. In ClpP −/− mice, we previously observed an accumulation of mtDNA [46], whereas mtDNA was lower upon knockdown of LonP1 in B16F10 melanoma cells [12]. Both changes in the content of mitochondrial DNA are accompanied by alterations of the innate immune system. In the ClpP −/− mice, upregulations occur for several factors that are important in Rig-I signaling, which responds to viral RNA [46]. In LonP1-deficient cells, there were also upregulations of cytosolic factors responding to toxic RNA or DNA, such as Ifih1, Trim25, Mnda, and Ifi204. The observations in both ClpP −/− and LonP1 +/− mouse mutants might reflect responses to the dysfunctional mitochondria and their altered state of nucleic acid content. It was recently shown that mitochondrial DNA or double-stranded RNA can escape to the cytosol and lead to cellular antiviral reactions [70,71]. Since both systems that lack a mitochondrial matrix protease show features of changes of the innate immune system, this may be a general mechanism with mitochondrial dysfunction as an underlying cause. In contrast to ClpP −/− mice, the LonP1 +/− MEFs exhibited a significant downregulation of Trim44, which was shown to stabilize the mitochondrial antiviral signaling protein Mavs [57]. In regard to the lower amount of mtDNA upon LonP1 deficiency, this downregulation could be a response towards the upregulation of other cytosolic nucleic acid sensors, as an effort of the cell to minimize the inflammatory state. This innate immune system activation was also documented in another model of mitochondrial dysfunction, where mice with deletion of the mitochondrial transcription factor Tfam display a massive induction of inflammatory signatures [71].
In the analysis of the global proteome of LonP1 +/− MEF beyond mitochondria, mostly upregulations were detected for pathways involved in oxidative stress and glutathione. The strongest upregulation was seen for Catalase, a factor known to play a major role in oxidative stress responses [79]. The second highest upregulation was seen for Sod3, which is also established as a marker for oxidative stress [80]. Elevated levels also were observed for Mgst1, Oxr1 and many components of the glutathione pathway, which is involved in the neutralization of reactive oxidative species [81]. All these changes may reflect secondary cytoplasmic adaptations to altered oxidative processes in mitochondria.
Mitochondrial dysfunction is closely related to respiratory function. In chronically ClpP-deleted mice and cells, there are functional reductions in complex-I of the respiratory chain, but this might be an indirect effect upon chronic tissue pathology [46,47,82]. In the acute knockdown of ClpP or ClpX, respiratory function of complex-II was selectively reduced [82,83] and an accumulation of the subunits of complex-II was observed. In contrast, deficiency of LonP1 led to decreased activities of mitochondrial complex-I, III, and IV in heart tissue of LonP1 conditional knockout mice [84]. In LonP1 +/− mouse heart complex-I levels and activities were decreased [85]. Mutations of LonP1 in humans lead to a rare multi-system developmental disorder-CODAS syndrome. Patients show swollen mitochondria with abnormal inner membrane morphology and reduced respiratory capacity [12]. Our results revealed that LonP1 +/− MEF exhibited upregulations for three proteins in the HP membrane arm of complex-I (in contrast to ClpP-deleted tissues), namely Ndufa10, Ndufb7, and Ndufb4, whereas a slight downregulation was detected for one protein (Ndufa5) in the peripheral matrix arm (Q-module) [86]. Given that LonP1 is localized in the matrix, the accumulation of the membrane arm may be due to an indirect effect when a preassembled subcomplex consisting of the joint ND5, ND4, and ND2 module of the membrane arm cannot be docked onto an improperly folded Q module in the IP/FP matrix arm.
Lon protease function decreases with old age in mice. It was also seen that overexpression of the Lon protease orthologue in P. anserina resulted in a prolonged lifespan [30,87]. This hints towards its role in the regulation of stress and survival [88]. Mitochondrial mutants and altered life expectancy are intimately related and have been most studied in C. elegans [89]. The homozygous deficiency of LonP1 shows one of the most drastic effects on lifespan since mice die very early in utero and even the heterozygous depletion results in strong changes in various cellular processes. Here, we could show that there are similarities between the absence of ClpP and Lonp1, highlighting their crucial roles for protein homeostasis within mitochondria. But we also defined LonP1-deletion-specific consequences, so the relative targets of each protease may be inferred. Still, in order to understand all functions of ClpP and Lonp1, much future research will be needed.

Proteomics
Protein abundance of cell pellets from LonP1 +/− MEF lines and their matching controls (n = 3, passage 4) were analyzed by label-free quantitative proteomics as recently described [90]. Mass spectrometry data were analyzed by Max Quant [91] and extended statistics were done with Perseus [92]. Quantified proteins were quality filtered for at least 3 label-free quantification values in one experimental group. Missing values were randomly replaced from normal distribution. Common contaminants and reverse identifications were excluded. For statistical comparison Student's t-tests and permutation based False Discovery Rate with 250 randomizations were used.

Bioinformatic Analyses
For protein-protein-interaction (PPI) network analysis, the software tool STRING (Search tool for the retrieval of interacting genes) v.11.0 (https://string-db.org/) with standard settings was used to visualize networks among factors with nominally significant difference between HET and WT status, using three biological replicates [93]. Automated network statistics were performed; significant functional enrichments of GO (Gene Ontology terms regarding biological processes, molecular functions, cellular components), KEGG pathways, REACTOME pathways, PFAM protein domains, INTERPRO Protein Domains and Features, and SMART protein domains were exported into Excel files. Pathway findings were processed further into tables for dysregulated (p-values < 0.05) mitochondrial factors, oxidative stress, and glutathione factors, innate immunity factors as well as lysosomal factors.