Dysmorphic Mitochondria Research Articles

Nanoscale imaging of pT217‐tau in aged rhesus macaque entorhinal and dorsolateral prefrontal cortex: Evidence of interneuronal trafficking and early‐stage neurodegeneration

AbstractBackgroundAdvances in Alzheimer’s disease (AD) have revealed a novel fluid biomarker, tau phosphorylated at T217 (pT217‐tau), in CSF and plasma, that predicts AD prior to cognitive deficits. Understanding the role of pT217‐tau is important in assessing efficacy of novel treatments aimed at early‐stage disease. However, it is unknown why pT217‐tau is effective in predicting brain pathology, as little is known about early, soluble pT217‐tau brain expression. These questions are difficult to address in humans, as soluble p‐tau is rapidly dephosphorylated postmortem, and PET scans detect late‐stage, fibrillated tau. However, the etiology of pT217‐tau in aging brains can be probed in rhesus macaques, where perfusion fixation allows capture of phosphorylated proteins in their native state. Aging macaques naturally develop tau pathology with the same qualitative pattern and sequence as humans, including initial cortical pathology in layer II of the entorhinal cortex (ERC) evident early in aging, and later in layer III of the dorsolateral prefrontal cortex (dlPFC).MethodWe utilized multi‐label immunofluorescence and immunoelectron‐microscopy to examine the subcellular localization of early‐stage pT217‐tau in ERC and dlPFC of aged macaques with naturally occurring tau pathology and assayed pT217‐tau levels in plasma.ResultOur results show that pT217‐tau labeling is primarily observed in postsynaptic compartments, accumulating in: 1) dendritic spines on the calcium‐storing smooth endoplasmic reticulum spine apparatus near asymmetric glutamatergic‐like synapses, and 2) in dendritic shafts, where it aggregated on microtubules, often “trapping” endosomes associated with Aβ42. The dendrites expressing pT217‐tau were associated with autophagic vacuoles and dysmorphic mitochondria, indicative of early neurite degeneration. We observed trans‐synaptic pT217‐tau trafficking between neurons within omega‐shaped bodies and endosomes, specifically near excitatory, but not inhibitory synapses. We also examined pT217‐tau in blood plasma in macaques across age‐span and observed a statistically significant age‐related increase in pT217‐tau.ConclusionThese data provide the first direct evidence of pT217‐tau trafficking between neurons near synapses to “seed” tau pathology in higher brain circuits, interfacing with the extracellular space to become accessible to CSF and blood. The expression of pT217‐tau in dendrites with early signs of degeneration may help to explain why this tau species can herald future disease.

The actin binding protein profilin 1 localizes inside mitochondria and is critical for their function

The monomer-binding protein profilin 1 (PFN1) plays a crucial role in actin polymerization. However, mutations in PFN1 are also linked to hereditary amyotrophic lateral sclerosis, resulting in a broad range of cellular pathologies which cannot be explained by its primary function as a cytosolic actin assembly factor. This implies that there are important, undiscovered roles for PFN1 in cellular physiology. Here we screened knockout cells for novel phenotypes associated with PFN1 loss of function and discovered that mitophagy was significantly upregulated. Indeed, despite successful autophagosome formation, fusion with the lysosome, and activation of additional mitochondrial quality control pathways, PFN1 knockout cells accumulate depolarized, dysmorphic mitochondria with altered metabolic properties. Surprisingly, we also discovered that PFN1 is present inside mitochondria and provide evidence that mitochondrial defects associated with PFN1 loss are not caused by reduced actin polymerization in the cytosol. These findings suggest a previously unrecognized role for PFN1 in maintaining mitochondrial integrity and highlight new pathogenic mechanisms that can result from PFN1 dysregulation.

Severe kidney dysfunction in sialidosis mice reveals an essential role for neuraminidase 1 in reabsorption.

Sialidosis is an ultra-rare multisystemic lysosomal disease caused by mutations in the neuraminidase 1 (NEU1) gene. The severe type II form of the disease manifests with a prenatal/infantile or juvenile onset, bone abnormalities, severe neuropathology, and visceromegaly. A subset of these patients present with nephrosialidosis, characterized by abrupt onset of fulminant glomerular nephropathy. We studied the pathophysiological mechanism of the disease in 2 NEU1-deficient mouse models, a constitutive Neu1-knockout, Neu1ΔEx3, and a conditional phagocyte-specific knockout, Neu1Cx3cr1ΔEx3. Mice of both strains exhibited terminal urinary retention and severe kidney damage with elevated urinary albumin levels, loss of nephrons, renal fibrosis, presence of storage vacuoles, and dysmorphic mitochondria in the intraglomerular and tubular cells. Glycoprotein sialylation in glomeruli, proximal distal tubules, and distal tubules was drastically increased, including that of an endocytic reabsorption receptor megalin. The pool of megalin bearing O-linked glycans with terminal galactose residues, essential for protein targeting and activity, was reduced to below detection levels. Megalin levels were severely reduced, and the protein was directed to lysosomes instead of the apical membrane. Together, our results demonstrated that desialylation by NEU1 plays a crucial role in processing and cellular trafficking of megalin and that NEU1 deficiency in sialidosis impairs megalin-mediated protein reabsorption.

Mitochondrial Dysfunction in Lung Resident Mesenchymal Stem Cells from Idiopathic Pulmonary Fibrosis Patients.

Idiopathic pulmonary fibrosis (IPF) is characterized by an aberrant repair response with uncontrolled turnover of extracellular matrix involving mesenchymal cell phenotypes, where lung resident mesenchymal stem cells (LRMSC) have been supposed to have an important role. However, the contribution of LRMSC in lung fibrosis is not fully understood, and the role of LRMSC in IPF remains to be elucidated. Here, we performed transcriptomic and functional analyses on LRMSC isolated from IPF and control patients (CON). Both over-representation and gene set enrichment analyses indicated that oxidative phosphorylation is the major dysregulated pathway in IPF LRMSC. The most relevant differences in biological processes included complement activation, mesenchyme development, and aerobic electron transport chain. Compared to CON LRMSC, IPF cells displayed impaired mitochondrial respiration, lower expression of genes involved in mitochondrial dynamics, and dysmorphic mitochondria. These changes were linked to an impaired autophagic response and a lower mRNA expression of pro-apoptotic genes. In addition, IPF TGFβ-exposed LRMSC presented different expression profiles of mitochondrial-related genes compared to CON TGFβ-treated cells, suggesting that TGFβ reinforces mitochondrial dysfunction. In conclusion, these results suggest that mitochondrial dysfunction is a major event in LRMSC and that their occurrence might limit LRMSC function, thereby contributing to IPF development.

Altered contractility, Ca2+ transients, and cell morphology seen in a patient-specific iPSC-CM model of Ebstein's anomaly with left ventricular noncompaction.

Patients with two congenital heart diseases (CHDs), Ebstein's anomaly (EA) and left ventricular noncompaction (LVNC), suffer higher morbidity than either CHD alone. The genetic etiology and pathogenesis of combined EA/LVNC remain largely unknown. We investigated a familial EA/LVNC case associated with a variant (p.R237C) in the gene encoding Kelch-like protein 26 (KLHL26) by differentiating induced pluripotent stem cells (iPSCs) generated from affected and unaffected family members into cardiomyocytes (iPSC-CMs) and assessing iPSC-CM morphology, function, gene expression, and protein abundance. Compared with unaffected iPSC-CMs, CMs containing the KLHL26 (p.R237C) variant exhibited aberrant morphology including distended endo(sarco)plasmic reticulum (ER/SR) and dysmorphic mitochondria and aberrant function that included decreased contractions per minute, altered calcium transients, and increased proliferation. Pathway enrichment analyses based on RNASeq data indicated that the "structural constituent of muscle" pathway was suppressed, whereas the "ER lumen" pathway was activated. Taken together, these findings suggest that iPSC-CMs containing this KLHL26 (p.R237C) variant develop dysregulated ER/SR, calcium signaling, contractility, and proliferation.NEW & NOTEWORTHY We demonstrate here that iPSCs derived from patients with Ebstein's anomaly and left ventricular noncompaction, when differentiated into cardiomyocytes, display significant structural and functional changes that offer insight into disease pathogenesis, including altered ER/SR and mitochondrial morphology, contractility, and calcium signaling.

Loss of the K+ channel Kv2.1 greatly reduces outward dark current and causes ionic dysregulation and degeneration in rod photoreceptors.

Vertebrate retinal photoreceptors signal light by suppressing a circulating "dark current" that maintains their relative depolarization in the dark. This dark current is composed of an inward current through CNG channels and NCKX transporters in the outer segment that is balanced by outward current exiting principally from the inner segment. It has been hypothesized that Kv2.1 channels carry a predominant fraction of the outward current in rods. We examined this hypothesis by comparing whole cell, suction electrode, and electroretinographic recordings from Kv2.1 knockout (Kv2.1-/-) and wild-type (WT) mouse rods. Single cell recordings revealed flash responses with unusual kinetics, and reduced dark currents that were quantitatively consistent with the measured depolarization of the membrane resting potential in the dark. A two-compartment (outer and inner segment) physiological model based on known ionic mechanisms revealed that the abnormal Kv2.1-/- rod photoresponses arise principally from the voltage dependencies of the known conductances and the NCKX exchanger, and a highly elevated fraction of inward current carried by Ca2+ through CNG channels due to the aberrant depolarization. Kv2.1-/- rods had shorter outer segments than WT and dysmorphic mitochondria in their inner segments. Optical coherence tomography of knockout animals demonstrated a slow photoreceptor degeneration over a period of 6 mo. Overall, these findings reveal that Kv2.1 channels carry 70-80% of the non-NKX outward dark current of the mouse rod, and that the depolarization caused by the loss of Kv2.1 results in elevated Ca2+ influx through CNG channels and elevated free intracellular Ca2+, leading to progressive degeneration.

High‐Throughput Imaging of Kidney Cell Function to Elucidate Unknown Mechanisms of Antiretroviral Drug Toxicity

Globally, the World Health Organization estimates that 38 million people live with human immunodeficiency virus (HIV) infection and 257 million with hepatitis B virus (HBV). Toxicity from antiretroviral drugs is a major cause of kidney disease in these individuals. Tenofovir disoproxil fumarate (TDF) is a first line therapy for both HIV and HBV. TDF induces functional defects in the kidney proximal tubule (PT) for reasons that are unknown, partly due to a lack of appropriate experimental models to investigate this. TDF nephrotoxicity is characterized by two major clinical phenotypes: approximately 25% of patients develop defects in PT solute transport (Labarga et al., 2009); whilst 1% develop severe damage characterized by the appearance of enlarged and dysmorphic mitochondria in the PT (Hall et al., 2011). The aim of our study was to establish realistic in vitro models of TDF toxicity, to further investigate the underlying metabolic phenotypes.Experiments were performed on human‐derived PT cells (RPTEC/TERT1) grown to confluence. A high‐throughput imaging pipeline was established using automated microscopy and machine learning based image analysis. Solute transport was assessed by measuring the number/size of domes formed in the monolayer. Metabolism was assessed by antibody staining for mitochondrial morphology and a marker of autophagy (LC3).We screened a number of different treatment regimes, and identified two that produced phenotypes matching those reported in patients. Treatment with 300 μM TDF for 24 hours induced a decrease in solute transport, but without changing cell number or producing mitochondrial defects. In contrast, treatment with 500 μM TDF for 24 hours caused mitochondrial enlargement and activated autophagy. Further experiments were then performed with these treatment protocols to investigate the underlying metabolic phenotypes. TDF produced a dose dependent decrease in ATP, despite an increase in glycolysis. Baseline oxygen consumption rate (OCR) was also decreased, with a blunted response to oligomycin. However, a substantial increase in OCR occurred in response to the uncoupler FCCP. Moreover, metabolomics analysis revealed no evidence of major defects in the citric acid cycle, beta‐oxidation or urea cycle.Taken together, these findings are consistent with a partial inhibition of the mitochondrial ATP synthase (complex V). Of note, TDF is phosphorylated within cells to produce Tenofovir diphosphate (TFVpp), a structural analogue of ATP. In subsequent experiments using an established in vitro assay of complex V activity, we observed a dose dependent inhibition with TFVpp.In summary, we have developed a high‐throughput imaging pipeline of human‐derived PT cells to generate realistic in vitro models of functional TDF toxicity. Metabolic characterization of these models revealed a phenotype consistent with complex V inhibition, which might explain toxicity observed in patients, since PT solute transport in vivo is almost entirely dependent on mitochondrial ATP production.Support or Funding InformationSwiss National Science Foundation and Swiss National Centre of Competence in Research (NCCR) Kidney Control of Homeostasis.

The clinical and pathological features of adefovir dipivoxil-related renal impairment .

To investigate the clinicopathological features and outcomes of adefovir dipivoxil (ADV)-related renal impairment in Chinese patients. Clinical, pathological, and follow-up data from 15 patients with ADV-related renal impairment were studied. Proximal renal tubular dysfunction (PRTD) was defined by the presence of at least two of the following four abnormalities: hypophosphatemia, hypouricemia, nondiabetic glucosuria, and proteinuria. All patients were treated for 3 - 15 (mean 6.7) years with daily ADV of 10 mg. Renal impairment manifested as PRTD (12, 80%), elevated serum creatinine (12, 80%), and hematuria (2, 13.3%). Mild to moderate tubulointerstitial injury primarily affecting the proximal tubules by light microscopy, and enlarged, dysmorphic mitochondria with loss and disorientation of cristae by electron microscope were identified in all of our cases. Four patients had pathological evidence of IgA nephropathy. The phosphorus, serum uric acid, and creatinine levels were normalized after ADV cessation in 66.7% (8/12) of affected patients, 27.3% (3/11) of affected patients, and 25% (3/12) of affected patients, respectively; proteinuria was eliminated in 7 of 13 affected patients (53.8%); and glucosuria and hematuria both disappeared in all affected patients. These abnormalities had hardly any recovery, and even aggravated with new-onset glucosuria, new-onset hematuria in 3 patients who replaced ADV with tenofovir. Nephrotoxicity developed in patients undergoing long-term ADV treatment and was partially reversible after drug cessation. Tubulointerstitial lesions and heteromorphic mitochondria were the predominant pathological changes. Patients with ADV-induced renal impairment should replace ADV with other antiviral agents other than tenofovir. .

A novel ADCK4 mutation in a Chinese family with ADCK4-Associated glomerulopathy

AarF domain-containing kinase 4 (ADCK4)-associated glomerulopathy (ADCK4-GN) is an inherited mitochondrial nephropathy caused by mutations in the ADCK4 gene. Herein, we report a case of ADCK4-GN. The patient, a 14-year-old Chinese male, presented with asymptomatic proteinuria and steroid resistance. A renal biopsy showed focal segmental glomerulosclerosis (FSGS) and dysmorphic mitochondria in podocytes. Coenzyme Q10 treatment was partially effective. No adverse events were observed. Next generation and Sanger sequencing analyses revealed compound heterozygous mutations (c.625C > G, p.D209H and c.918G > T, p.C306X). Both inherited mutations are located in the highly conserved ABC1 domain. The unreported nonsense mutation causes a loss of 197 amino acids from the C-terminal portion of ADCK4, suggesting a deleterious effect of the truncating mutation by abolishing ADCK4 function. In conclusion, we identified a novel ABC1 domain-localized pathogenic mutation responsible for ADCK4-GN, further supporting the importance of the C-terminal portion of ADCK4. Next generation sequencing facilitated the early diagnosis. CoQ10 treatment may reduce proteinuria and postpone ADCK4-GN progression.

Loss of the Drosophilam-AAA mitochondrial protease paraplegin results in mitochondrial dysfunction, shortened lifespan, and neuronal and muscular degeneration

The progressive accumulation of dysfunctional mitochondria is implicated in aging and in common diseases of the elderly. To oppose this occurrence, organisms employ a variety of strategies, including the selective degradation of oxidatively damaged and misfolded mitochondrial proteins. Genetic studies in yeast indicate that the ATPase Associated with diverse cellular Activities (AAA+) family of mitochondrial proteases account for a substantial fraction of this protein degradation, but their metazoan counterparts have been little studied, despite the fact that mutations in the genes encoding these proteases cause a variety of human diseases. To begin to explore the biological roles of the metazoan mitochondrial AAA+ protease family, we have created a CRISPR/Cas9 allele of the Drosophila homolog of SPG7, which encodes an inner membrane-localized AAA+ protease known as paraplegin. Drosophila SPG7 mutants exhibited shortened lifespan, progressive locomotor defects, sensitivity to chemical and environmental stress, and muscular and neuronal degeneration. Ultrastructural examination of photoreceptor neurons indicated that the neurodegenerative phenotype of SPG7 mutants initiates at the synaptic terminal. A variety of mitochondrial defects accompanied the degenerative phenotypes of SPG7 mutants, including altered axonal transport of mitochondria, accumulation of electron-dense material in the matrix of flight muscle mitochondria, reduced activities of respiratory chain complexes I and II, and severely swollen and dysmorphic mitochondria in the synaptic terminals of photoreceptors. Drosophila SPG7 mutants recapitulate key features of human diseases caused by mutations in SPG7, and thus provide a foundation for the identification of Drosophila paraplegin substrates and strategies that could be used to ameliorate the symptoms of these diseases.

Abstract 285: Osteopontin Deficiency Ameliorates Heart Failure with Preserved Ejection Fraction <HFpEF> Pathology by Upregulating Mitochondrial 2-Oxoglutarate Dehydrogenase Like <OGDHL> Enzyme

HFpEF is an increasingly prevalent syndrome associated with impaired myocardial energetics, for which no etiologic therapy is available. Osteopontin (OPN) is a matricellular protein that is upregulated in the circulation of HFpEF patients, and reported to induce mitochondrial stress in rodent cardiomyocytes. Here we evaluate the role of circulating OPN in regulating myocardial function in the nephrotic Col4a3 -/- mouse model of HFpEF. We performed extensive cardiac, biochemical and mitochondrial analyses of the Col4a3 -/- mouse and found a striking HFpEF phenotype. We showed OPN levels were elevated in Col4a3 -/- mice (FC=2.1, n=6; p<.01). Col4a3 -/- mice were hypertensive, had diastolic dysfunction, myocyte hypertrophy and interstitial fibrosis - all of which were ameliorated in Col4a3 -/- OPN -/- mice (n=5-20; p<.05). Col4a3 -/- hearts had dysmorphic mitochondria (EM), lowered antioxidant capacity as a 50% reduction in GSH/GSSG ratio (n=6; p<.05) and lower protein levels of mitochondrial respiratory complexes I, II and IV (p<.05). Flux assay in adult cardiomyocytes showed that maximal respiration was reduced in Col4a3 -/- hearts (575.84±37.6 vs 322.34±25.48 pmol/min in WT, n=9; p<.0001). Microarray data (validated by mitochondrial blot) implicated OGDHL as decreased in Col4a3 -/- hearts but increased in double knockout Col4a3 -/- OPN -/- hearts compared to WT (n=3; p<.05). OGDH activity was also lower in Col4a3 -/- hearts (17.1±7.3 vs 2.5±1.1 mU/mg in WT; n=6; p<.05). In Col4a3 -/- mice, heart-specific AAV9-mediated overexpression of OGDHL, similar to global OPN KO, improved survival by ~50-100% (p<.0001). Isovolumetric relaxation time, a marker of diastolic dysfunction, which is prolonged in Col4a3 -/- mice (26.17 vs 15.30±1 ms, n=26; p<.001) was decreased in Col4a3 -/- OPN -/- mice (18.1±1 ms, n=37; p<.01) as well as in AAV9-cTnT-OGDHL-treated Col4a3 -/- mice (16.7±2.5 ms, n=8; p<.05). In conclusion, we present a new mouse model for HFpEF in which diastolic function and lifespan can be improved by genetic deletion of OPN or cardiac OGDHL gene therapy. Our results elucidate for the first time the pivotal roles of circulating OPN and cardiac OGDHL in HFpEF pathophysiology and present two related potential therapeutic targets for HFpEF.

A Personalized Model of COQ2 Nephropathy Rescued by the Wild-Type COQ2 Allele or Dietary Coenzyme Q10 Supplementation.

Clinical studies have identified patients with nephrotic syndrome caused by mutations in genes involved in the biosynthesis of coenzyme Q10 (CoQ10), a lipid component of the mitochondrial electron transport chain and an important antioxidant. However, the cellular mechanisms through which these mutations induce podocyte injury remain obscure. Here, we exploited the striking similarities between Drosophila nephrocytes and human podocytes to develop a Drosophila model of these renal diseases, and performed a systematic in vivo analysis assessing the role of CoQ10 pathway genes in renal function. Nephrocyte-specific silencing of Coq2, Coq6, and Coq8, which are genes involved in the CoQ10 pathway that have been associated with genetic nephrotic syndrome in humans, induced dramatic adverse changes in these cells. In particular, silencing of Coq2 led to an abnormal localization of slit diaphragms, collapse of lacunar channels, and more dysmorphic mitochondria. In addition, Coq2-deficient nephrocytes showed elevated levels of autophagy and mitophagy, increased levels of reactive oxygen species, and increased sensitivity to oxidative stress. Dietary supplementation with CoQ10 at least partially rescued these defects. Furthermore, expressing the wild-type human COQ2 gene specifically in nephrocytes rescued the defective protein uptake, but expressing the mutant allele derived from a patient with COQ2 nephropathy did not. We conclude that transgenic Drosophila lines carrying mutations in the CoQ10 pathway genes are clinically relevant models with which to explore the pathogenesis of podocyte injury and could serve as a new platform to test novel therapeutic approaches.

Resveratrol Protects Against Pulmonary Arterial Hypertension in Rats via Activation of Silent Information Regulator 1

Background/Objectives: The polyphenol resveratrol (Rev) has been found to exhibit various beneficial effects including prevention of pulmonary arterial hypertension (PAH). The present study was designed to investigate the action and potential mechanism of Rev on PAH, focusing on the role of SIRT1 (Silent Information Regulator 1) in apoptosis of pulmonary artery smooth muscle cells (PASMCs). Methods: PAH rats were established by exposure to hypoxia for 21 days. Rev and SRT1720 (a selective SIRT1 activator) were used to reverse PAH by gavaging rats. PASMCs were confronted with hypoxia for 24 h or 48 h and were then treated with Rev or SRT1720 in vitro. Western blot was performed to detect the protein expression of SIRT1. CCK-8 and scratch wound experiments were carried out to verify cell proliferation. In addition, the TUNEL positive assay and flow cytometry assay were used to measure PASMC apoptosis. Mitochondrial permeability transition (mPT) was identified by confocal microscopy. Right ventricular systolic pressure (RVSP) was determined with a Gould pressure transducer, and right ventricular hypertrophy (RVH) was determined by weighing the cardiac muscle. Results: We demonstrated that Rev could reverse the remodelling of the pulmonary vasculature, thus contributing to alleviating the severity of PAH. Down-regulation of SIRT1 was observed in PAH, but administration of Rev had no obvious effect on the protein expression of SIRT1. In addition, Rev could induce mitochondrial swelling and nuclear pyknosis, leading to small, dense, and dysmorphic mitochondria in rats exposed to hypoxia alone. Rev treatment inhibited PASMC proliferation in a dose-dependent manner in vitro. Incubation with SRT1720, a specific activator of SIRT1, significantly retarded PASMC proliferation and promoted PASMC apoptosis in vitro. The mechanism could be associated with inducing mPT damage in PASMCs. Rev and SRT1720 treatment mitigated RVSP and reduced RVH. Conclusion: Rev produced a beneficial effect partially by enhancing the activation of SIRT1, thus improving RVSP and reducing RVH. SIRT1 activation increased PASMC apoptosis by inducing mPT dysfunction, which might be a novel future strategy for the treatment of PAH.

Abstract 4: Macrophage p62/SQSTM1 Ameliorates Atherosclerosis by Sequestering Inclusion Bodies and Mediating Mitophagy

Protein and organelle turnover is critical for cellular homeostasis and is prominently mediated by autophagy. Disruptions in autophagy lead to accumulation of protein aggregates and dysfunctional organelles such as mitochondria. Recent evidence suggests that the chaperone protein p62 is a critical link for targeting polyubiquitinated protein aggregates/damaged mitochondria to autophagosomes for degradation. Herein we describe a p62-centric mechanism of handling protein aggregates and dysfunctional mitochondria in atherosclerosis. Macrophages deficient in autophagy (ATG5-/-) or rendered deficient by incubation with atherogenic lipids have significantly increased levels of p62. This coincides with 1) the accumulation of polyubiquitinated proteins co-localizing with p62 and present as cytoplasmic inclusion bodies, and 2) p62 co-localization with mitochondrial markers. Aortas from atherosclerotic (ApoE-/-) mice also have progressive and marked elevations in p62, polyubiquitinated proteins, and mitochondrial reactive oxygen species that predominantly co-localize with plaque macrophages, a process further exacerbated in the autophagy-deficient setting. The formation of cytoplasmic inclusions and maintenance of adequate mitochondrial function appears to be dependent on p62. Lipid-loaded p62-null macrophages show polyubiquitinated protein accumulation present in a diffuse/disrupted cytoplasmic pattern. These macrophages also develop larger dysmorphic mitochondria with increased polarization and decreased oxidative phosphorylation capacity. As a result, p62-null macrophages display apoptotic susceptibility to atherogenic lipids and increased IL-1β secretion likely through mitochondrial-dependent inflammasome activation. Consistent with our in vitro observations, mice with either whole-body p62-deficiency or transplanted with p62-deficient bone marrow show significantly increased atherosclerotic plaque burden and lesion complexity with increased apoptosis and necrotic cores. Taken together, these data demonstrate a previously unrecognized atheroprotective role for macrophage p62 by facilitating the formation of inclusion bodies and maintaining healthy mitochondria.

HSP72 is a mitochondrial stress sensor critical for Parkin action, oxidative metabolism, and insulin sensitivity in skeletal muscle.

Increased heat shock protein (HSP) 72 expression in skeletal muscle prevents obesity and glucose intolerance in mice, although the underlying mechanisms of this observation are largely unresolved. Herein we show that HSP72 is a critical regulator of stress-induced mitochondrial triage signaling since Parkin, an E3 ubiquitin ligase known to regulate mitophagy, was unable to ubiquitinate and control its own protein expression or that of its central target mitofusin (Mfn) in the absence of HSP72. In wild-type cells, we show that HSP72 rapidly translocates to depolarized mitochondria prior to Parkin recruitment and immunoprecipitates with both Parkin and Mfn2 only after specific mitochondrial insult. In HSP72 knockout mice, impaired Parkin action was associated with retention of enlarged, dysmorphic mitochondria and paralleled by reduced muscle respiratory capacity, lipid accumulation, and muscle insulin resistance. Reduced oxygen consumption and impaired insulin action were recapitulated in Parkin-null myotubes, confirming a role for the HSP72-Parkin axis in the regulation of muscle insulin sensitivity. These data suggest that strategies to maintain HSP72 may provide therapeutic benefit to enhance mitochondrial quality and insulin action to ameliorate complications associated with metabolic diseases, including type 2 diabetes.

Increased Mitochondrial Fatty Acid Oxidation Is Sufficient to Protect Skeletal Muscle Cells from Palmitate-induced Apoptosis

The mechanisms underlying the protective effect of monounsaturated fatty acids (e.g. oleate) against the lipotoxic action of saturated fatty acids (e.g. palmitate) in skeletal muscle cells remain poorly understood. This study aimed to examine the role of mitochondrial long-chain fatty acid (LCFA) oxidation in mediating oleate's protective effect against palmitate-induced lipotoxicity. CPT1 (carnitine palmitoyltransferase 1), which is the key regulatory enzyme of mitochondrial LCFA oxidation, is inhibited by malonyl-CoA, an intermediate of lipogenesis. We showed that expression of a mutant form of CPT1 (CPT1mt), which is active but insensitive to malonyl-CoA inhibition, in C2C12 myotubes led to increased LCFA oxidation flux even in the presence of high concentrations of glucose and insulin. Furthermore, similar to preincubation with oleate, CPT1mt expression protected muscle cells from palmitate-induced apoptosis and insulin resistance by decreasing the content of deleterious palmitate derivates (i.e. diacylglycerols and ceramides). Oleate preincubation exerted its protective effect by two mechanisms: (i) in contrast to CPT1mt expression, oleate preincubation increased the channeling of palmitate toward triglycerides, as a result of enhanced diacylglycerol acyltransferase 2 expression, and (ii) oleate preincubation promoted palmitate oxidation through increasing CPT1 expression and modulating the activities of acetyl-CoA carboxylase and AMP-activated protein kinase. In conclusion, we demonstrated that targeting mitochondrial LCFA oxidation via CPT1mt expression leads to the same protective effect as oleate preincubation, providing strong evidence that redirecting palmitate metabolism toward oxidation is sufficient to protect against palmitate-induced lipotoxicity.

Heterozygous Mutation of Opa1 in Drosophila Shortens Lifespan Mediated through Increased Reactive Oxygen Species Production

Optic atrophy 1 (OPA1) is a dynamin-like GTPase located in the inner mitochondrial membrane and mutations in OPA1 are associated with autosomal dominant optic atrophy (DOA). OPA1 plays important roles in mitochondrial fusion, cristae remodeling and apoptosis. Our previous study showed that dOpa1 mutation caused elevated reactive oxygen species (ROS) production and resulted in damage and death of the cone and pigment cells in Drosophila eyes. Since ROS-induced oxidative damage to the cells is one of the primary causes of aging, in this study, we examined the effects of heterozygous dOpa1 mutation on the lifespan. We found that heterozygous dOpa1 mutation caused shortened lifespan, increased susceptibility to oxidative stress and elevated production of ROS in the whole Drosophila. Antioxidant treatment partially restored lifespan in the male dOpa1 mutants, but had no effects in the females. Heterozygous dOpa1 mutation caused an impairment of respiratory chain complex activities, especially complexes II and III, and reversible decreased aconitase activity. Heterozygous dOpa1 mutation is also associated with irregular and dysmorphic mitochondria in the muscle. Our results, for the first time, demonstrate the important role of OPA1 in aging and lifespan, which is most likely mediated through augmented ROS production.

Rap1b GTPase Ameliorates Glucose-Induced Mitochondrial Dysfunction

The role of tubular injury in diabetic nephropathy is relatively unknown, despite that apoptosis of tubular epithelial cells is commonly observed in human renal biopsies. The GTPase Ras-proximate-1 (Rap1b) is upregulated in the hyperglycemic state and is known to increase B-Raf, an antiapoptotic effector protein. In this study, the effects of high glucose on renal tubular apoptosis and the potential ability for Rap1b to ameliorate these effects were investigated. In the kidneys of diabetic mice, apoptotic tubular cells and dysmorphic mitochondria were observed, Bcl-2 expression was decreased, and Bax expression was increased. Total Rap1b expression was slightly increased, but its associated GTPase activity was significantly decreased. In vitro, high extracellular glucose led to decreased Bcl-2 expression, reduced Rap1b GTPase activity, and increased levels of both Bax and GTPase activating protein in a proximal tubular cell line (HK-2). These changes were accompanied by increased DNA fragmentation, decreased high molecular weight mitochondrial DNA, altered mitochondrial morphology and function, disrupted Bcl-2-Bax and Bcl-2-Rap1b interactions, and reduced cell survival. Overexpression of Rap1b partially prevents these abnormalities. Furthermore, the BH4 domain of Bcl-2 was found to be required for successful protein-protein interaction between Bcl-2 and Rap1b. In summary, these data suggest that Rap1b ameliorates glucose-induced mitochondrial dysfunction in renal tubular cells.

The kd/kd Mouse Is a Model of Collapsing Glomerulopathy

Collapsing glomerulopathy (CG) is associated with disorders that markedly perturb the phenotype of podocytes. The kd/kd mouse has been studied for immune and genetic causes of microcystic tubulointerstitial nephritis with little attention to its glomerular lesion. Because histologic examination revealed classic morphologic features of CG, the question arises whether podocytes in kd/kd mice exhibit additional phenotypic criteria for CG. Utilizing Tg26 mice as a positive control, immunohistochemical profiling of the podocyte phenotype was conducted simultaneously on both models. Similar to Tg26 kidneys, podocytes in kd/kd kidneys showed de novo cyclin D1, Ki-67, and desmin expression with loss of synaptopodin and WT-1 expression. Electron micrographs showed collapsed capillaries, extensive foot process effacement, and dysmorphic mitochondria in podocytes. These results indicate that the kd/kd mouse is a model of CG and raise the possibility that human equivalents of the kd susceptibility gene may exist in patients with CG.

Mutant prenyltransferase-like mitochondrial protein (PLMP) and mitochondrial abnormalities in kd/kd mice

Mice that are homozygous for the kidney disease (kd) mutation are apparently healthy for the first 8 weeks of life, but spontaneously develop a severe form of interstitial nephritis that progresses to end-stage renal disease (ESRD) by 4 to 8 months of age. By testing for linkage to microsatellite markers, we previously localized the kd gene to a YAC/BAC contig. The sequence of the entire critical region was examined, and candidate genes were identified. These candidate genes were sequenced in both mutant (kd/kd) mice and normal controls. The phenotype was further characterized by immunohistochemistry and electron microscopy. Transgenic mice were constructed that carried the wild-type allele of the prime candidate gene, and this transgene was transferred to a kd/kd background by breeding. We have obtained evidence that kd is a mutant allele of a novel gene for a prenyltransferase-like mitochondrial protein (PLMP). This gene is alternatively spliced, with the larger gene product having one domain that resembles transprenyltransferase and another that is similar to geranylgeranyl pyrophosphate synthase. The smaller gene product includes only the first domain. An antiserum to PLMP localizes to mitochondria, and ultrastructural defects are present in the mitochondria of renal tubular epithelial cells, and to a lesser extent, hepatocytes and heart cells from kd/kd mice. In a line of kd/kd mice that carried the wild-type PLMP allele as a transgene, only 1 out of 13 animals expressed the disease by 120 days of age. The kd allele codes for a novel protein that localizes to the mitochondria, and the kd/kd mouse has dysmorphic mitochondria in the renal tubular epithelial cells. This mouse is therefore a unique animal model for studying mechanisms that lead to tubulointerstitial nephritis.