Research on healthy ageing 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 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 webserver Heidelberg for protein interactions. ClpP was previously reported to degrade Eral1 as a chaperone involved in mitoribosome assembly, so ClpP deficiency triggers 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 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 caused by LonP1 mutations.

Mitochondrial dysfunctions, e.g. abnormal handling of mitochondrial DNA in TFAM mutants or in altered mitophagy, activate innate immunity. Recent reports also showed that deletion of mitochondrial matrix peptidase ClpP in mice transcriptionally upregulates inflammatory factors. Here, we studied ClpP-null mouse brain at two ages and embryonal fibroblasts, to identify which signaling pathways are responsible, employing mass spectrometry, immunoblots, and reverse transcriptase polymerase chain reaction. Anomalies in the mitochondrial unfolded protein responses pathway were prominent for the co-chaperone DNAJA3, and for its known interactor STAT1. Their mitochondrial dysregulation affected also their extra-mitochondrial abundance, as possible innate immune modulators. Increased expression was observed not only for the transcription factors Stat1/2, but also for two interferon-stimulated genes (Ifi44, Gbp3). Inflammatory responses were strongest for RLR pattern recognition receptors (Ddx58, Ifih1, Oasl2, Trim25) and several cytosolic nucleic acid sensors (Ifit1, Ifit3, Oas1b, Ifi204, Mnda). They can be explained by the accumulation of mitoribosomes and mitochondrial nucleoids in ClpP-null cells, which may act as damage-associated molecular patterns. The consistent dysregulation of these factors from early age might influence also human Perrault syndrome, where ClpP loss-of-function leads to early infertility and deafness, with subsequent widespread neurodegeneration.

Biallelic pathogenic variants in CLPP, encoding mitochondrial matrix peptidase ClpP cause a rare autosomal recessive condition, Perrault syndrome type 3 (PRLTS3). It is characterized by primary ovarian insufficiency and early sensorineural hearing loss, often associated with progressive neurological deficits. Mouse models showed that accumulations of (i) its main protein interactor, the substrate-selecting AAA+ ATPase ClpX, (ii) mitoribosomes, and (iii) mtDNA nucleoids are main cellular consequences of ClpP absence. However, the sequence of these events and their validity in human remain unclear. Here, we studied global proteome profiles to define ClpP substrates among mitochondrial ClpX interactors, which accumulated consistently in ClpP-null mouse embryonal fibroblasts and brain. Validation work included novel ClpP-mutant patient fibroblast proteomics. ClpX co-accumulated in mitochondria with POLDIP2 as nucleoid component, LRPPRC as mitochondrial poly(A) mRNA granule element, GFM1 (in mouse also GRSF1) as tRNA processing factors. Only in mouse, accumulated ClpX, GFM1 and GRSF1 appeared in nuclear fractions. Mitoribosomal accumulation was minor. Consistent accumulations in murine and human fibroblasts also affected multimerizing factors not known as ClpX interactors, namely OAT, ASS1, ACADVL, STOM, PRDX3, PC, MUT, ALDH2, PMPCB, UQCRC2 and ACADSB, but the impact on downstream metabolites was marginal. Our data demonstrate the primary impact of ClpXP on the assembly of proteins with nucleic acids, and show nucleoid enlargement in human as a key consequence.