Involvement of phosphatase and tensin homolog-induced putative kinase 1-Parkin-mediated mitophagy in septic acute kidney injury

BACKGROUND

Studies have reported mitophagy activation in renal tubular epithelial cells (RTECs) in acute kidney injury (AKI). Phosphatase and tensin homolog-induced putative kinase 1 (PINK1) and E3 ubiquitin-protein ligase Parkin are involved in mitophagy regulation; however, little is known about the role of PINK1-Parkin mitophagy in septic AKI. Here we investigated whether the PINK1-Parkin mitophagy pathway is involved in septic AKI and its effects on cell apoptosis in vitro and on renal functions in vivo.

METHODS

Mitophagy-related gene expression was determined using Western blot assay in human RTEC cell line HK-2 stimulated with bacterial lipopolysaccharide (LPS) and in RTECs from septic AKI rats induced by cecal ligation and perforation (CLP). Autophagy-related ultrastructural features in rat RTECs were observed using electron microscopy. Gain- and loss-of-function approaches were performed to investigate the role of the PINK1-Parkin pathway in HK-2 cell mitophagy. Autophagy activators and inhibitors were used to assess the effects of mitophagy modulation on cell apoptosis in vitro and on renal functions in vivo.

RESULTS

LPS stimulation could significantly induce LC3-II and BECN-1 protein expression (LC3-II: 1.72 ± 0.05 vs. 1.00 ± 0.05, P < 0.05; BECN-1: 5.33 ± 0.57 vs. 1.00 ± 0.14, P < 0.05) at 4 h in vitro. Similarly, LC3-II, and BECN-1 protein levels were significantly increased and peaked at 2 h after CLP (LC3-II: 3.33 ± 0.12 vs. 1.03 ± 0.15, P < 0.05; BECN-1: 1.57 ± 0.26 vs. 1.02 ± 0.11, P < 0.05) in vivo compared with those after sham operation. Mitochondrial deformation and mitolysosome-mediated mitochondria clearance were observed in RTECs from septic rats. PINK1 knockdown significantly attenuated LC3-II protein expression (1.35 ± 0.21 vs. 2.38 ± 0.22, P < 0.05), whereas PINK1 overexpression markedly enhanced LC3-II protein expression (2.07 ± 0.21 vs. 1.29 ± 0.19, P < 0.05) compared with LPS-stimulated HK-2 cells. LPS-induced proapoptotic protein expression remained unchanged in autophagy activator-treated HK-2 cells and was significantly attenuated in PINK1-overexpressing cells, but was remarkably upregulated in autophagy inhibitor-treated and in PINK1-depleted cells. Consistent results were observed in flow cytometric apoptosis assay and in renal function indicators in rats.

CONCLUSION

PINK1-Parkin-mediated mitophagy might play a protective role in septic AKI, serving as a potential therapeutic target for septic AKI.

Selection of mitochondria in female germline cells: is Balbiani body implicated in this process?

Early oocytes of nearly all animal species contain a transient organelle assemblage termed the Balbiani body. Structure and composition of this assemblage may vary even between closely related species. Despite this variability, the Balbiani body always comprises of numerous tightly clustered mitochondria and accumulations of nuage material. It has been suggested that the Balbiani body is an evolutionarily ancestral structure, which plays a role in various processes such as the localization of organelles and macromolecules to the germ plasm, lipidogenesis, as well as the selection/elimination of dysfunctional mitochondria from female germline cells. We suggest that the selection/elimination of mitochondria is a primary and evolutionarily ancestral function of Balbiani body, and that the other functions are secondary, evolutionarily derived additions. We propose a simple model explaining the role of the Balbiani body in the selection of mitochondria, i.e., in the mitochondrial DNA (mtDNA) bottleneck phenomenon.

Mitochondrial quality control by the Pink1/Parkin system

Mitochondrial dysfunction represents a prominent pathological feature in many neurodegenerative diseases, particularly in Parkinson's disease (PD). Mutations in the genes encoding the proteins Pink1 and Parkin have been identified as genetic risk factors in familiar cases of PD. Research during the last decade has identified both proteins as crucial components of an organellar quality control system that contributes to the maintenance of mitochondrial function in healthy cells. The Pink1/Parkin system acts as a sensor for mitochondrial quality and is activated, in particular, after the loss of the electric potential across the inner mitochondrial membrane. Pink1 molecules accumulate at the surface of damaged mitochondria to recruit and activate Parkin, which, in turn, elicits a signaling pathway eventually leading to the autophagic removal of the damaged organelles. This review summarizes recent advances in our knowledge of the functional role of the Pink1/Parkin system in preventing the accumulation of damaged mitochondria by mitophagy.

Melatonin rescues zebrafish embryos from the parkinsonian phenotype restoring the parkin/PINK1/DJ-1/MUL1 network

Multiple studies reporting mitochondrial impairment in Parkinson's disease (PD) involve knockout or knockdown models to inhibit the expression of mitochondrial-related genes, including parkin, PINK1, and DJ-1 ones. Melatonin has significant neuroprotective properties, which have been related to its ability to boost mitochondrial bioenergetics. The meaning and molecular targets of melatonin in PD are yet unclear. Zebrafish are an outstanding model of PD because they are vertebrates, their dopaminergic system is comparable to the nigrostriatal system of humans, and their brains express the same genes as mammals. The exposure of 24 hpf zebrafish embryos to MPTP leads to a significant inhibition of the mitochondrial complex I and the induction of sncga gene, responsible for enhancing γ-synuclein accumulation, which is related to mitochondrial dysfunction. Moreover, MPTP inhibited the parkin/PINK1/DJ-1 expression, impeding the normal function of the parkin/PINK1/DJ-1/MUL1 network to remove the damaged mitochondria. This situation remains over time, and removing MPTP from the treatment did not stop the neurodegenerative process. On the contrary, mitochondria become worse during the next 2 days without MPTP, and the embryos developed a severe motor impairment that cannot be rescued because the mitochondrial-related gene expression remained inhibited. Melatonin, added together with MPTP or added once MPTP was removed, prevented and recovered, respectively, the parkinsonian phenotype once it was established, restoring gene expression and normal function of the parkin/PINK1/DJ-1/MUL1 loop and also the normal motor activity of the embryos. The results show, for the first time, that melatonin restores brain function in zebrafish suffering with Parkinson-like disease.

[PINK1 and the related diseases]

As a kind of mitochondrial membrane protein with protein kinase activity, phosphatase and tensin homolog deleted on chromosome ten induced kinase 1 (PINK1) is involved in many biological metabolic processes. Since PINK1 had been found to be associated with Parkinson's disease, researchers have been exploring its biological function. PINK1 localizes in the outer mitochondrial membrane and regulates cell function through phosphorylating proteins. PINK1 is involved in mitochondrial function, mitochondrial morphology and mitochondrial autophagy, but the regulatory pathway is not yet clear. PINK1 is expressed widely in many tissues with a variety of biological activity, especially in tissues with high energy consumption. It may therefore be involved in the development and regulation of many diseases. Mutations in PINK1 were originally discovered to cause autosomal recessive Parkinson's disease. Recently some research has revealed that PINK1 is related to the development of neonatal hypoxic-ischemic encephalopathy, cancer, diabetes and other diseases. Studying and exploring the biological functions of PINK1 will facilitate the identification of the targets for therapeutic intervention for its related diseases. This review article mainly focuses on recent studies about the biological function and related diseases of PINK1.

Phloretin-induced cytoprotective effects on mammalian cells: A mechanistic view and future directions

Phloretin (C15 H14 O5 ), a dihydrochalcone flavonoid, is mainly found in fruit, leaves, and roots of apple tree. Phloretin exerts antioxidant, anti-inflammatory, and anti-tumor activities in mammalian cells through mechanisms that have been partially elucidated throughout the years. Phloretin bioavailability is well known in humans, but still remains to be better studied in experimental animals, such as mouse and rat. The focus of the present review is to gather information regarding the mechanisms involved in the phloretin-elicited effects in different in vitro and in vivo experimental models. Several manuscripts were analyzed and data raised by authors were described and discussed here in a mechanistic manner. Comparisons between the effects elicited by phloretin and phloridzin were made whenever possible, as well as with other polyphenols, clarifying questions about the use of phloretin as a potential therapeutic agent. Toxicological aspects associated to phloretin exposure were also discussed here. Furthermore, a special section containing future directions was created as a suggestive guide towards the elucidation of phloretin-related actions in mammalian cells and tissues.