Diabetic kidney disease (DKD) is the major cause of end-stage renal disease (ESRD) [1]. Current clinical therapies include controlling blood glucose, blood pressure and blood lipids and improving circulation, but these strategies cannot dramatically delay the progression of DKD dramatically [2]. Tubulointerstitial changes are considered as early features of DKD and play a pivotal role in its pathogenesis independent of glomerular lesions [[3], [4], [5]]. The kidneys have high oxygen consumption and metabolic rates. As the center of energy metabolism, mitochondria are abundant in the renal tubules, of which the proximal tubules are critical [6]. Accumulating evidence has demonstrated that dysfunctional kidney mitochondria are the pathological mediators of DKD. Abnormalities in mitochondrial morphology and function, including mitochondrial oxidative stress, mitochondrial biogenesis, mitochondrial dynamics including fusion and fission, and mitochondrial autophagy, are strongly correlated with the development and progression of DKD [7]. The renal proximal convoluted tubule in DKD animal models and the high glucose (HG) induced tubular epithelium exhibit decreased ATP content, increased mitochondrial fragmentation, abnormal membrane potential, and accumulated mitochondrial reactive oxygen species (mtROS), which are closely related to tubulointerstitial injury [[8], [9], [10]]. Our previous studies revealed that mitoQ, which is a mitochondrial antioxidant, can alleviate apoptosis and interstitial fibrosis in DKD tubules by improving mitochondrial function [11]. It is reflected that abnormal mitochondrial quality control (MQC) is critical sufficiently to contribute to tubular damage and interstitial fibrosis in DKD.

The mitochondrial unfolded protein response (UPRmt) is a stress-adaptive transcriptional response mediated by the mitochondrial-to-nuclear retrograde signal transduction pathway. Under mitochondrial stress conditions, transcription factors are activated to intensely induce the transcription of mitochondrial proteases and chaperone genes encoded by nuclear DNA [12]. Molecular chaperones located primarily in the mitochondrial matrix facilitate nascent polypeptide folding or repair misfolded proteins, such as heat shock protein 60 (HSP60) and heat shock protein 10 (HSP10), which are regarded as markers of UPRmt. The proteases Caseinolytic peptidase P (ClpP) and Lon peptidase 1 (LonP1) facilitate misfolded or unrepairable protein hydrolysis. Transcription factors include activating transcription factor associated with stress-1 (ATFS-1), activating transcription factor 5 (ATF5, the human orthologue of ATFS-1), activating transcription factor 4 (ATF4) and C/EBP homology protein (CHOP) [13,14]. The reaction restores and maintains mitochondrial proteins. UPRmt is closely associated with a variety of diseases, including metabolic diseases, age-related degenerative disease, neurodegenerative diseases, heart disease and cancers [15,16]. It affects metabolic homeostasis and its signaling has both beneficial and detrimental outcomes [17]. The UPRmt is present in the diabetic state [[18], [19], [20], [21]]. DKD-related proteomic analysis showed that HSP60, HSP10 and other stress response genes were abnormally expressed and misfolded proteins had accumulated within tubule cells cultured with HG [18]. Short-term temporal feeding (TRF) enhanced UPRmt in renal mitochondria in the acute phase of ischemia and reperfusion (IR) [22]. It is suggested that UPRmt plays a considerable role in the kidney.

The human ClpP gene was assigned to chromosome 19. ClpP is a highly conserved serine protease expressed in multiple tissues with different levels and is present in the mitochondrial matrix [23]. As the main ATP-dependent protease in the UPRmt, ClpP bines with the AAA+ molecular chaperone Caseinolytic peptidase X (ClpX) to form the active protease complex ClpXP, which degrades short-lived, misfolded and damaged proteins to exert powerful proteolytic function and maintain proteostasis [24]. ClpXP substrates include proteins involved in electron transport, metabolism, the Krebs cycle and mitochondrial genome stabilization [25,26]. Several studies have demonstrated that ClpP affects mitochondrial function by regulating the mitochondrial respiratory chain, oxidative phosphorylation, mitochondrial translation and mitochondrial fission, which affects cell metabolism, cell proliferation, migration, and apoptosis [[26], [27], [28], [29]]. Downregulation of the mitochondrial protease ClpP aggravated mitochondrial dysfunction caused by HG in the pancreatic islet β cell line Min6 in T2DM mice [30]. On the contrary, ClpP-deficient mice exhibited increased white adipose tissue mitochondrial respiration and avoided HFD-induced obesity and insulin resistance [31]. ClpP participates in the UPRmt; it hydrolyzes misfolded and damaged proteins into short peptides and initiates a transcriptional response dominated by the transcription factor ATFS1, and then activates the UPRmt in nematodes [32,33]. When ClpP was downregulated, UPRmt activation was inhibited in muscle cells [34], while the absence of ClpP triggered compensatory responses in mice and promoted mitochondrial chaperone protein expression [31]. We noted that ClpP had different effects on mitochondrial function and UPRmt under various states and tissues. Therefore, given the importance of ClpP in the maintenance of mitochondrial quality control and the development of metabolic diseases, we investigated the impact of ClpP on apoptosis and interstitial injury in DKD tubular cells and explored UPRmt to reveal the potential mechanism of this process.