Hepatic WDR23 proteostasis mediates insulin homeostasis by regulating insulin-degrading enzyme capacity

Maintaining insulin homeostasis is critical for cellular and organismal metabolism. In the liver, insulin is degraded by the activity of the insulin-degrading enzyme (IDE). Here, we establish a hepatic regulatory axis for IDE through WDR23-proteostasis. Wdr23KO mice have increased IDE expression, reduced circulating insulin, and defective insulin responses. Genetically engineered human cell models lacking WDR23 also increase IDE expression and display dysregulated phosphorylation of insulin signaling cascade proteins, IRS-1, AKT2, MAPK, FoxO, and mTOR, similar to cells treated with insulin, which can be mitigated by chemical inhibition of IDE. Mechanistically, the cytoprotective transcription factor NRF2, a direct target of WDR23-Cul4 proteostasis, mediates the enhanced transcriptional expression of IDE when WDR23 is ablated. Moreover, an analysis of human genetic variation in WDR23 across a large naturally aging human cohort in the US Health and Retirement Study reveals a significant association of WDR23 with altered hemoglobin A1C (HbA1c) levels in older adults, supporting the use of WDR23 as a new molecular determinant of metabolic health in humans.

Loss of WDR23 proteostasis impacts mitochondrial homeostasis in the mouse brain

Mitochondrial adaptation is important for stress resistance throughout life. Here we show that WDR23 loss results in an enrichment for genes regulated by nuclear respiratory factor 1 (NRF1), which coordinates mitochondrial biogenesis and respiratory functions, and an increased steady state level of several nuclear coded mitochondrial resident proteins in the brain. Wdr23KO also increases the endogenous levels of insulin degrading enzyme (IDE) and the relaxin-3 peptide (RLN3), both of which have established roles in mediating mitochondrial metabolic and oxidative stress responses. Taken together, these studies reveal an important role for WDR23 as a component of the mitochondrial homeostat in the murine brain.

WDR23 mediates NRF2 proteostasis and cytoprotective capacity in the hippocampus

Pathogenic brain aging and neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease are characterized by chronic neuroinflammation and the accumulation of dysfunctional or misfolded proteins that lead to progressive neuronal cell death. Here we demonstrate that a murine model with global loss of the CUL4-DDB1 substrate receptor WDR23 (Wdr23KO) results in changes in multiple age-related hippocampal-dependent behaviors. The behavioral differences observed in Wdr23KO animals accompany the stabilization of the NRF2/NFE2L2 protein, an increase in RNA transcripts regulated by this cytoprotective transcription factor, and an increase in the steady state level of antioxidant defense proteins. Taken together, these findings reveal a role for WDR23-proteostasis in mediating cytoprotective capacity in the hippocampus and reveal the potential for targeting WDR23-NRF2 signaling interactions for development of therapies for neurodegenerative disorders.

Disrupting the SKN-1 homeostat: mechanistic insights and phenotypic outcomes

The mechanisms that govern maintenance of cellular homeostasis are crucial to the lifespan and healthspan of all living systems. As an organism ages, there is a gradual decline in cellular homeostasis that leads to senescence and death. As an organism lives into advanced age, the cells within will attempt to abate age-related decline by enhancing the activity of cellular stress pathways. The regulation of cellular stress responses by transcription factors SKN-1/Nrf2 is a well characterized pathway in which cellular stress, particularly xenobiotic stress, is abated by SKN-1/Nrf2-mediated transcriptional activation of the Phase II detoxification pathway. However, SKN-1/Nrf2 also regulates a multitude of other processes including development, pathogenic stress responses, proteostasis, and lipid metabolism. While this process is typically tightly regulated, constitutive activation of SKN-1/Nrf2 is detrimental to organismal health, this raises interesting questions surrounding the tradeoff between SKN-1/Nrf2 cryoprotection and cellular health and the ability of cells to deactivate stress response pathways post stress. Recent work has determined that transcriptional programs of SKN-1 can be redirected or suppressed to abate negative health outcomes of constitutive activation. Here we will detail the mechanisms by which SKN-1 is controlled, which are important for our understanding of SKN-1/Nrf2 cytoprotection across the lifespan.

Oolonghomobisflavans from Camellia sinensis disaggregate tau fibrils across Alzheimer's disease models

Alzheimer's disease (AD) is a common debilitating neurodegenerative disease with limited treatment options. Amyloid-β (Aβ) and tau fibrils are well-established hallmarks of AD, which can induce oxidative stress, neuronal cell death, and are linked to disease pathology. Here, we describe the effects of Oolonghomobisflavan A (OFA) and Oolonghomobisflavan B (OFB) on tau fibril disaggregation and prionogenic seeding. Transcriptomic analysis of OF-treated animals reveals the induction of a proteostasis-enhancing and health-promoting signature. OFA treatment reduced the burden of Tau protein aggregation in a C. elegans model expressing pathogenic human tau ("hTau-expressing") and promoted Tau disaggregation and inhibited seeding in assays using ex vivo brain-derived paired helical filament tau protein fibrils from Alzheimer's disease brain donors. Correspondingly, treatment with OF improved multiple fitness and aging-related health parameters in the hTau-expressing C. elegans model, including reproductive output, muscle function, and importantly, reversed the shortened lifespan stemming from pathogenic Tau expression. Collectively, this study provides new evidence supporting the neuroprotective effects of OFs and reveal a new therapeutic strategy for targeting AD and other neurodegenerative diseases characterized by tauopathy.

Serotonin deficiency from constitutive SKN-1 activation drives pathogen apathy

When an organism encounters a pathogen, the host innate immune system activates to defend against pathogen colonization and toxic xenobiotics produced. C. elegans employ multiple defense systems to ensure survival when exposed to Pseudomonas aeruginosa including activation of the cytoprotective transcription factor SKN-1/NRF2. Although wildtype C. elegans quickly learn to avoid pathogens, here we describe a peculiar apathy-like behavior towards PA14 in animals with constitutive activation of SKN-1, whereby animals choose not to leave and continue to feed on the pathogen even when a non-pathogenic and healthspan-promoting food option is available. Although lacking the urgency to escape the infectious environment, animals with constitutive SKN-1 activity are not oblivious to the presence of the pathogen and display the typical pathogen-induced intestinal distension and eventual demise. SKN-1 activation, specifically in neurons and intestinal tissues, orchestrates a unique transcriptional program which leads to defects in serotonin signaling that is required from both neurons and non-neuronal tissues. Serotonin depletion from SKN-1 activation limits pathogen defense capacity, drives the pathogen-associated apathy behaviors and induces a synthetic sensitivity to selective serotonin reuptake inhibitors. Taken together, our work reveals new insights into how animals perceive environmental pathogens and subsequently alter behavior and cellular programs to promote survival.

KEY POINTS

Identify an apathy-like behavioral response for pathogens resulting from the constitutive activation of the cytoprotective transcription factor SKN-1.Uncover the obligate role for serotonin synthesis in both neuronal and non-neuronal cells for the apathy-like state and ability of serotonin treatment to restore normal behaviors.Characterize the timing and tissue specificity of SKN-1 nuclear localization in neurons and intestinal cells in response to pathogen exposure.Define the unique and context-specific transcriptional signatures of animals with constitutive SKN-1 activation when exposed to pathogenic environments.Reveal necessity for both neuronal and non-neuronal serotonin signaling in host survival from pathogen infection.

A dicer-related helicase opposes the age-related pathology from SKN-1 activation in ASI neurons

Coordination of cellular responses to stress is essential for health across the lifespan. The transcription factor SKN-1 is an essential homeostat that mediates survival in stress-inducing environments and cellular dysfunction, but constitutive activation of SKN-1 drives premature aging thus revealing the importance of turning off cytoprotective pathways. Here, we identify how SKN-1 activation in two ciliated ASI neurons in Caenorhabditis elegans results in an increase in organismal transcriptional capacity that drives pleiotropic outcomes in peripheral tissues. An increase in the expression of established SKN-1 stress response and lipid metabolism gene classes of RNA in the ASI neurons, in addition to the increased expression of several classes of noncoding RNA, define a molecular signature of animals with constitutive SKN-1 activation and diminished healthspan. We reveal neddylation as a unique regulator of the SKN-1 homeostat that mediates SKN-1 abundance within intestinal cells. Moreover, RNAi-independent activity of the dicer-related DExD/H-box helicase, drh-1, in the intestine, can oppose the effects of aberrant SKN-1 transcriptional activation and delays age-dependent decline in health. Taken together, our results uncover a cell nonautonomous circuit to maintain organism-level homeostasis in response to excessive SKN-1 transcriptional activity in the sensory nervous system.

Comparative analysis of the molecular and physiological consequences of constitutive SKN-1 activation

Molecular homeostats play essential roles across all levels of biological organization to ensure a return to normal function after responding to abnormal internal and environmental events. SKN-1 is an evolutionarily conserved cytoprotective transcription factor that is integral for the maintenance of cellular homeostasis upon exposure to a variety of stress conditions. Despite the essentiality of turning on SKN-1/NRF2 in response to exogenous and endogenous stress, animals with chronic activation of SKN-1 display premature loss of health with age, and ultimately, diminished lifespan. Previous genetic models of constitutive SKN-1 activation include gain-of-function alleles of skn-1 and loss-of-function alleles of wdr-23 that impede the turnover of SKN-1 by the ubiquitin proteasome. Here, we define a novel gain-of-function mutation in the xrep-4 locus that results in constitutive activation of SKN-1 in the absence of stress. Although each of these genetic mutations results in continuously unregulated transcriptional output from SKN-1, the physiological consequences of each model on development, stress resistance, reproduction, lipid homeostasis, and lifespan are distinct. Here, we provide a comprehensive assessment of the differential healthspan impacts across multiple models of constitutive SKN-1 activation. Although our results reveal the universal need to reign in the uncontrolled activity of cytoprotective transcription factors, we also define the unique signatures of each model of constitutive SKN-1 activation, which provides innovative solutions for the design of molecular "off-switches" of unregulated transcriptional homeostats.

WDR23 mediates NRF2 proteostasis and cytoprotective capacity in the hippocampus

Pathogenic brain aging and neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease are characterized by chronic neuroinflammation and the accumulation of dysfunctional or misfolded proteins that lead to progressive neuronal cell death. Here we demonstrate that a murine model with global loss of the CUL4-DDB1 substrate receptor WDR23 ( Wdr23KO ) results in changes in multiple age-related hippocampal-dependent behaviors. The behavioral differences observed in Wdr23KO animals accompany the stabilization of the NRF2/NFE2L2 protein, an increase in RNA transcripts regulated by this cytoprotective transcription factor, and an increase in the steady state level of antioxidant defense proteins. Taken together, these findings reveal a role for WDR23-proteostasis in mediating cytoprotective capacity in the hippocampus and reveal the potential for targeting WDR23-NRF2 signaling interactions for development of therapies for neurodegenerative disorders.

HIGHLIGHTS

WDR23 regulates NRF2/NFE2L2 stability in the mouse hippocampus Loss of Wdr23 significantly increases the expression of NFE2L2/NRF2 target genes Global loss of WDR23 influences age-related behaviors differentially in males and females.

A dicer-related helicase opposes the age-related pathology from SKN-1 activation in ASI neurons

Coordination of cellular responses to stress are essential for health across the lifespan. The transcription factor SKN-1 is an essential homeostat that mediates survival in stress-inducing environments and cellular dysfunction, but constitutive activation of SKN-1 drives premature aging thus revealing the importance of turning off cytoprotective pathways. Here we identify how SKN-1 activation in two ciliated ASI neurons in C. elegans results in an increase in organismal transcriptional capacity that drives pleiotropic outcomes in peripheral tissues. An increase in the expression of established SKN-1 stress response and lipid metabolism gene classes of RNA in the ASI neurons, in addition to the increased expression of several classes of non-coding RNA, define a molecular signature of animals with constitutive SKN-1 activation and diminished healthspan. We reveal neddylation as a novel regulator of the SKN-1 homeostat that mediates SKN-1 abundance within intestinal cells. Moreover, RNAi-independent activity of the dicer-related DExD/H-box helicase, drh-1 , in the intestine, can oppose the e2ffects of aberrant SKN-1 transcriptional activation and delays age-dependent decline in health. Taken together, our results uncover a cell non-autonomous circuit to maintain organism-level homeostasis in response to excessive SKN-1 transcriptional activity in the sensory nervous system.

SIGNIFICANCE STATEMENT

Unlike activation, an understudied fundamental question across biological systems is how to deactivate a pathway, process, or enzyme after it has been turned on. The irony that the activation of a transcription factor that is meant to be protective can diminish health was first documented by us at the organismal level over a decade ago, but it has long been appreciated that chronic activation of the human ortholog of SKN-1, NRF2, could lead to chemo- and radiation resistance in cancer cells. A colloquial analogy to this biological idea is a sink faucet that has an on valve without a mechanism to shut the water off, which will cause the sink to overflow. Here, we define this off valve.

Different methods of killing bacteria diets differentially influence Caenorhabditis elegans physiology

Across species, diet plays a critical role in most, if not all life history traits. Caenorhabditis elegans is an important and facile organism for research across modalities, but the use of live bacteria as sources of nutrition can exert pleiotropic outcomes that stem from the action of host-pathogen defenses. Recently, a powerful new approach to readily generate dead and metabolically inactive Escherichia coli was developed that enabled reproducible measures of health across the lifespan. Here we further characterize additional comparisons of developmental and physiological parameters of animals fed either bacteria killed by treatment with ultraviolet (UV) light and bactericidal antibiotics or low-dose paraformaldehyde (PFA). Unlike bacteria killed by UV/Antibiotic treatment, PFA-killed diets resulted in a 25% reduction in body size just prior to adulthood and an overall reduction in stored intracellular lipids. Moreover, a small but reproducible number of animals fed PFA-killed bacteria display age-dependent depletion of somatic lipids, which does not normally occur on live bacteria or bacteria killed by UV/antibiotics. Lastly, animals fed PFA-treated, but not UV-antibiotic treated bacteria display a 10% increase in crawling speed. Taken together, these new data more thoroughly define the physiological impact two methodologies to prepare C. elegans diets that should be considered during experimental design.

Ether lipid biosynthesis promotes lifespan extension and enables diverse pro-longevity paradigms in Caenorhabditis elegans

Biguanides, including the world's most prescribed drug for type 2 diabetes, metformin, not only lower blood sugar, but also promote longevity in preclinical models. Epidemiologic studies in humans parallel these findings, indicating favorable effects of metformin on longevity and on reducing the incidence and morbidity associated with aging-related diseases. Despite this promise, the full spectrum of molecular effectors responsible for these health benefits remains elusive. Through unbiased screening in Caenorhabditis elegans, we uncovered a role for genes necessary for ether lipid biosynthesis in the favorable effects of biguanides. We demonstrate that biguanides prompt lifespan extension by stimulating ether lipid biogenesis. Loss of the ether lipid biosynthetic machinery also mitigates lifespan extension attributable to dietary restriction, target of rapamycin (TOR) inhibition, and mitochondrial electron transport chain inhibition. A possible mechanistic explanation for this finding is that ether lipids are required for activation of longevity-promoting, metabolic stress defenses downstream of the conserved transcription factor skn-1/Nrf. In alignment with these findings, overexpression of a single, key, ether lipid biosynthetic enzyme, fard-1/FAR1, is sufficient to promote lifespan extension. These findings illuminate the ether lipid biosynthetic machinery as a novel therapeutic target to promote healthy aging.

Riboflavin depletion promotes longevity and metabolic hormesis in Caenorhabditis elegans

Riboflavin is an essential cofactor in many enzymatic processes and in the production of flavin adenine dinucleotide (FAD). Here, we report that the partial depletion of riboflavin through knockdown of the C. elegans riboflavin transporter 1 (rft-1) promotes metabolic health by reducing intracellular flavin concentrations. Knockdown of rft-1 significantly increases lifespan in a manner dependent upon AMP-activated protein kinase (AMPK)/aak-2, the mitochondrial unfolded protein response, and FOXO/daf-16. Riboflavin depletion promotes altered energetic and redox states and increases adiposity, independent of lifespan genetic dependencies. Riboflavin-depleted animals also exhibit the activation of caloric restriction reporters without any reduction in caloric intake. Our findings indicate that riboflavin depletion activates an integrated hormetic response that promotes lifespan and healthspan in C. elegans.

Genetic variation in ALDH4A1 is associated with muscle health over the lifespan and across species

The influence of genetic variation on the aging process, including the incidence and severity of age-related diseases, is complex. Here, we define the evolutionarily conserved mitochondrial enzyme ALH-6/ALDH4A1 as a predictive biomarker for age-related changes in muscle health by combining Caenorhabditis elegans genetics and a gene-wide association scanning (GeneWAS) from older human participants of the US Health and Retirement Study (HRS). In a screen for mutations that activate oxidative stress responses, specifically in the muscle of C. elegans, we identified 96 independent genetic mutants harboring loss-of-function alleles of alh-6, exclusively. Each of these genetic mutations mapped to the ALH-6 polypeptide and led to the age-dependent loss of muscle health. Intriguingly, genetic variants in ALDH4A1 show associations with age-related muscle-related function in humans. Taken together, our work uncovers mitochondrial alh-6/ALDH4A1 as a critical component to impact normal muscle aging across species and a predictive biomarker for muscle health over the lifespan.

Ageing is inevitable, but what makes one person ‘age well’ and another decline more quickly remains largely unknown. While many aspects of ageing are clearly linked to genetics, the specific genes involved often remain unidentified.

Sarcopenia is an age-related condition affecting the muscles. It involves a gradual loss of muscle mass that becomes faster with age, and is associated with loss of mobility, decreased quality of life, and increased risk of death. Around half of all people aged 80 and over suffer from sarcopenia. Several lifestyle factors, especially poor diet and lack of exercise, are associated with the condition, but genetics is also involved: the condition accelerates more quickly in some people than others, and even fit, physically active individuals can be affected.

To study the genetics of conditions like sarcopenia, researchers often use animals like flies or worms, which have short generation times but share genetic similarities with humans. For example, the worm Caenorhabditis elegans has equivalents of several human muscle genes, including the gene alh-6. In worms, alh-6 is important for maintaining energy supply to the muscles, and mutating it not only leads to muscle damage but also to premature ageing. Given this insight, Villa, Stuhr, Yen et al. wanted to determine if variation in the human version of alh-6, ALDH4A1, also contributes to individual differences in muscle ageing and decline in humans.

Evaluating variation in this gene required a large amount of genetic data from older adults. These were taken from a continuous study that follows >35,000 older adults. Importantly, the study collects not only information on gene sequences but also measures of muscle health and performance over time for each individual. Analysis of these genetic data revealed specific small variations in the DNA of ALDH4A1, all of which associated with reduced muscle health.

Follow-up experiments in worms used genetic engineering techniques to test how variation in the worm alh-6 gene could influence age-related health. The resulting mutant worms developed muscle problems much earlier than their normal counterparts, supporting the role of alh-6/ALDH4A1 in determining muscle health across the lifespan of both worms and humans.

These results have identified a key influencer of muscle health during ageing in worms, and emphasize the importance of validating effects of genetic variation among humans during this process. Villa, Stuhr, Yen et al. hope that this study will help researchers find more genetic ‘markers’ of muscle health, and ultimately allow us to predict an individual’s risk of sarcopenia based on their genetic make-up.

Rapid Lipid Quantification in Caenorhabditis elegans by Oil Red O and Nile Red Staining

The ability to stain lipid stores in vivo allows for the facile assessment of metabolic status in individuals of a population following genetic and environmental manipulation or pharmacological treatment. In the animal model Caenorhabditis elegans, lipids are stored in and mobilized from intracellular lipid droplets in the intestinal and hypodermal tissues. The abundance, size, and distribution of these lipids can be readily assessed by two staining methods for neutral lipids: Oil Red O (ORO) and Nile Red (NR). ORO and NR can be used to quantitatively measure lipid droplet abundance, while ORO can also define tissue distribution and lipid droplet size. C. elegans are a useful animal model in studying pathways relating to aging, fat storage, and metabolism, as their transparent nature allows for easy microscopic assessment of lipid droplets. This is done by fixation and permeabilization, staining with NR or ORO, image capture on a microscope, and computational identification and quantification of lipid droplets in individuals within a cohort. To ensure reproducibility in lipid measurements, we provide a detailed protocol to measure intracellular lipid dynamics in C. elegans. Graphic abstract: Flow chart depicting the preparation of C. elegans for fat staining protocols.

Nociceptin/orphanin FQ opioid receptor (NOP) selective ligand MCOPPB links anxiolytic and senolytic effects

Accumulation of senescent cells may drive age-associated alterations and pathologies. Senolytics are promising therapeutics that can preferentially eliminate senescent cells. Here, we performed a high-throughput automatized screening (HTS) of the commercial LOPAC®Pfizer library on aphidicolin-induced senescent human fibroblasts, to identify novel senolytics. We discovered the nociceptin receptor FQ opioid receptor (NOP) selective ligand 1-[1-(1-methylcyclooctyl)-4-piperidinyl]-2-[(3R)-3-piperidinyl]-1H-benzimidazole (MCOPPB, a compound previously studied as potential anxiolytic) as the best scoring hit. The ability of MCOPPB to eliminate senescent cells in in vitro models was further tested in mice and in C. elegans. MCOPPB reduced the senescence cell burden in peripheral tissues but not in the central nervous system. Mice and worms exposed to MCOPPB also exhibited locomotion and lipid storage changes. Mechanistically, MCOPPB treatment activated transcriptional networks involved in the immune responses to external stressors, implicating Toll-like receptors (TLRs). Our study uncovers MCOPPB as a NOP ligand that, apart from anxiolytic effects, also shows tissue-specific senolytic effects.

Oolonghomobisflavans from Camellia sinensis increase Caenorhabditis elegans lifespan and healthspan

Tea polyphenols are widely considered as excellent antioxidant agents which can contribute to human health and longevity. However, the identification of the active biomolecules in complex tea extracts that promote health and longevity are not fully known. Here we used the nematode Caenorhabditis elegans to analyze the health benefits and longevity effects of Camellia sinensis oolong tea extracts (QFT, NFT, and CFT) and oolonghomobisflavan A and oolonghomobisflavan B, which are present in oolong tea extracts. Our results showed that oolong tea extracts and oolonghomobisflavans prolong lifespan and improved healthspan by curtailing the age-related decline in muscle activity and the accumulation of age pigment (lipofuscin). We found that the lifespan and healthspan promoting effects of oolong tea extracts and oolonghomobisflavans were positively correlated with the stress resistance via DAF-16/FOXO transcription factor. Furthermore, oolong tea extracts and oolonghomobisflavans displayed protective effects against Aβ- and polyQ-induced neuro/proteotoxicity. Overall, our study provides new evidence to support the health benefits of oolong tea and importantly identify oolonghomobisflavans as potent bioactive molecules that promote health when supplemented with a normal diet. As such, oolonghomobisflavans represent a valuable new class of compounds that promote healthy aging.

Metabolic Signatures of Life Span Regulated by Mating, Sex Peptide, and Mifepristone/RU486 in Female Drosophila melanogaster

Mating and transfer of male sex peptide (SP), or transgenic expression of SP, causes inflammation and decreased life span in female Drosophila. Mifepristone rescues these effects, yielding dramatic increases in life span. Here targeted metabolomics data were integrated with further analysis of extant transcriptomic data. Each of 7 genes positively correlated with life span were expressed in the brain or eye and involved regulation of gene expression and signaling. Genes negatively correlated with life span were preferentially expressed in midgut and involved protein degradation, amino acid metabolism, and immune response. Across all conditions, life span was positively correlated with muscle breakdown product 1/3-methylhistidine and purine breakdown product urate, and negatively correlated with tryptophan breakdown product kynurenic acid, suggesting a SP-induced shift from somatic maintenance/turnover pathways to the costly production of energy and lipids from dietary amino acids. Some limited overlap was observed between genes regulated by mifepristone and genes known to be regulated by ecdysone; however, mifepristone was unable to compete with ecdysone for activation of an ecdysone-responsive transgenic reporter. In contrast, genes regulated by mifepristone were highly enriched for genes regulated by juvenile hormone (JH), and mifepristone rescued the negative effect of JH analog methoprene on life span in adult virgin females. The data indicate that mifepristone increases life span and decreases inflammation in mated females by antagonizing JH signaling downstream of male SP. Finally, mifepristone increased life span of mated, but not unmated, Caenorhabditis elegans, in 2 of 3 trials, suggesting possible evolutionary conservation of mifepristone mechanisms.

Analysis of Caenorhabditis elegans Sperm Number, Size, Activation, and Mitochondrial Content

Infertility is a widespread and often unexplained issue. Studying reproduction using C. elegans males offers insight into the influence of individual factors on male fertility in humans. We have created a collection of protocols to assess several aspects of C. elegans sperm quality, including number, size, rate of activation, and mitochondrial morphology. Studying sperm biology in a model system such as C. elegans allows access to the wealth of resources and techniques that have been optimized for that organism while providing valuable biological information that may be applicable to other systems.

Graphic abstract:

Flowchart depicting the preparation of C. elegans males and subsequent sperm quality assays

Maf1 regulates intracellular lipid homeostasis in response to DNA damage response activation

Surveillance of DNA damage and maintenance of lipid metabolism are critical factors for general cellular homeostasis. We discovered that in response to DNA damage–inducing UV light exposure, intact Caenorhabditis elegans accumulate intracellular lipids in a dose-dependent manner. The increase in intracellular lipids in response to exposure to UV light utilizes mafr-1, a negative regulator of RNA polymerase III and the apical kinases atm-1 and atl-1 of the DNA damage response (DDR) pathway. In the absence of exposure to UV light, the genetic ablation of mafr-1 results in the activation of the DDR, including increased intracellular lipid accumulation, phosphorylation of ATM/ATR target proteins, and expression of the Bcl-2 homology region genes, egl-1 and ced-13. Taken together, our results reveal mafr-1 as a component the DDR pathway response to regulating lipid homeostasis following exposure to UV genotoxic stress.

Methods for Assessing Fertility in C. elegans from a Single Population

Reproductive senescence occurs in a wide range of species with mechanistic aspects that are conserved from Caenorhabditis elegans to humans. Genetic and environmental factors can influence fertility and reproductive output can impact rates of aging. The C. elegans Bristol N2 strain commonly used in laboratories is hermaphroditic, producing a defined number of sperm during larval development before switching exclusively to oogenesis. Here we show a method of assaying both oocyte and sperm quality from a single population of animals.

Incomplete proline catabolism drives premature sperm aging

Infertility is an increasingly common health issue, with rising prevalence in advanced parental age. Environmental stress has established negative effects on reproductive health, however, the impact of altering cellular metabolism and its endogenous reactive oxygen species (ROS) on fertility remains unclear. Here, we demonstrate the loss of proline dehydrogenase, the first committed step in proline catabolism, is relatively benign. In contrast, disruption of alh-6, which facilitates the second step of proline catabolism by converting 1-pyrroline-5-carboxylate (P5C) to glutamate, results in premature reproductive senescence, specifically in males. The premature reproductive senescence in alh-6 mutant males is caused by aberrant ROS homeostasis, which can be countered by genetically limiting the first committed step of proline catabolism that functions upstream of ALH-6 or by pharmacological treatment with antioxidants. Taken together, our work uncovers proline metabolism as a critical component of normal sperm function that can alter the rate of aging in the male reproductive system.

Bacterial diets differentially alter lifespan and healthspan trajectories in C. elegans

Diet is one of the more variable aspects in life due to the variety of options that organisms are exposed to in their natural habitats. In the laboratory, C. elegans are raised on bacterial monocultures, traditionally the E. coli B strain OP50, and spontaneously occurring microbial contaminants are removed to limit experimental variability because diet-including the presence of contaminants-can exert a potent influence over animal physiology. In order to diversify the menu available to culture C. elegans in the lab, we have isolated and cultured three such microbes: Methylobacterium, Xanthomonas, and Sphingomonas. The nutritional composition of these bacterial foods is unique, and when fed to C. elegans, can differentially alter multiple life history traits including development, reproduction, and metabolism. In light of the influence each food source has on specific physiological attributes, we comprehensively assessed the impact of these bacteria on animal health and devised a blueprint for utilizing different food combinations over the lifespan, in order to promote longevity. The expansion of the bacterial food options to use in the laboratory will provide a critical tool to better understand the complexities of bacterial diets and subsequent changes in physiology and gene expression.

Loss of flavin adenine dinucleotide (FAD) impairs sperm function and male reproductive advantage in C. elegans

Exposure to environmental stress is clinically established to influence male reproductive health, but the impact of normal cellular metabolism on sperm quality is less well-defined. Here we show that impaired mitochondrial proline catabolism, reduces energy-storing flavin adenine dinucleotide (FAD) levels, alters mitochondrial dynamics toward fusion, and leads to age-related loss of sperm quality (size and activity), which diminishes competitive fitness of the animal. Loss of the 1-pyrroline-5-carboxylate dehydrogenase enzyme alh-6 that catalyzes the second step in mitochondrial proline catabolism leads to premature male reproductive senescence. Reducing the expression of the proline catabolism enzyme alh-6 or FAD biosynthesis pathway genes in the germline is sufficient to recapitulate the sperm-related phenotypes observed in alh-6 loss-of-function mutants. These sperm-specific defects are suppressed by feeding diets that restore FAD levels. Our results define a cell autonomous role for mitochondrial proline catabolism and FAD homeostasis on sperm function and specify strategies to pharmacologically reverse these defects.

CAENORHABDITIS ELEGANS AS A MODEL OF AIR POLLUTION TOXICITY DURING DEVELOPMENT AND LIFESPAN

Air pollution (AirPoll) is among the leading human mortality risk factors and yet little is known about the molecular mechanisms of this global environmental toxin. Our recent studies using mouse models even showed genetic variation and sex can alter biological responses to air pollution. To expand genetic studies of AirPoll toxicity throughout the lifespan, we introduced Caenorhabditis elegans as a new AirPoll exposure model which has a short lifespan, high throughput capabilities and shared longevity pathways with mammals. Acute exposure of C. elegans to airborne nanosized AirPoll matter (nPM) caused similar gene expression changes to our prior findings in cell culture and mouse models. Initial C. elegans responses to nPM included antioxidant, inflammatory and Alzheimer homolog genes. The magnitude of changes was dependent on the developmental stage of the worms. Even short term exposure of C. elegans to nPM altered developmental and lifespan hormetic effects, with pathways that included skn-1/Nrf family antioxidant responses. We propose C. elegans as a new and complementary model for mouse and cultured cells to study AirPoll across the lifespan. Future chronic nPM exposure and high throughput genetic screening of C. elegans can identify other major regulators of the developmental and lifespan effects of air pollution. This work was supported by grants R01AG051521 (CEF); R21AG05020 (CEF); Cure Alzheimer’s Fund (CEF); R01GM109028 (SPC), F31AG051382 (HMD) and T32AG000037 (HMD), T32AG052374 (AH).

Redirection of SKN-1 abates the negative metabolic outcomes of a perceived pathogen infection

Early host responses toward pathogens are essential for defense against infection. In Caenorhabditis elegans, the transcription factor, SKN-1, regulates cellular defenses during xenobiotic intoxication and bacterial infection. However, constitutive activation of SKN-1 results in pleiotropic outcomes, including a redistribution of somatic lipids to the germline, which impairs health and shortens lifespan. Here, we show that exposing C. elegans to Pseudomonas aeruginosa similarly drives the rapid depletion of somatic, but not germline, lipid stores. Modulating the epigenetic landscape refines SKN-1 activity away from innate immunity targets, which alleviates negative metabolic outcomes. Similarly, exposure to oxidative stress redirects SKN-1 activity away from pathogen response genes while restoring somatic lipid distribution. In addition, activating p38/MAPK signaling in the absence of pathogens, is sufficient to drive SKN-1-dependent loss of somatic fat. These data define a SKN-1- and p38-dependent axis for coordinating pathogen responses, lipid homeostasis, and survival and identify transcriptional redirection, rather than inactivation, as a mechanism for counteracting the pleiotropic consequences of aberrant transcriptional activity.

Air Pollution Alters Caenorhabditis elegans Development and Lifespan: Responses to Traffic-Related Nanoparticulate Matter

Air pollution is a heterogeneous environmental toxicant that impacts humans throughout their life. We introduce Caenorhabditis elegans as a valuable air pollution model with its short lifespan, medium-throughput capabilities, and highly conserved biological pathways that impact healthspan. We exposed developmental and adult life stages of C. elegans to airborne nano-sized particulate matter (nPM) produced by traffic emissions and measured biological and molecular endpoints that changed in response. Acute nPM did not cause lethality in C. elegans, but short-term exposure during larval stage 1 caused delayed development. Gene expression responses to nPM exposure overlapped with responses of mouse and cell culture models of nPM exposure in previous studies. We showed further that the skn-1/Nrf2 antioxidant response has a role in the development and hormetic effects of nPM. This study introduces the worm as a new resource and complementary model for mouse and cultured cell systems to study air pollution toxicity across the lifespan.

Hypodermal responses to protein synthesis inhibition induce systemic developmental arrest and AMPK-dependent survival in Caenorhabditis elegans

Across organisms, manipulation of biosynthetic capacity arrests development early in life, but can increase health- and lifespan post-developmentally. Here we demonstrate that this developmental arrest is not sickness but rather a regulated survival program responding to reduced cellular performance. We inhibited protein synthesis by reducing ribosome biogenesis (rps-11/RPS11 RNAi), translation initiation (ifg-1/EIF3G mutation and egl-45/EIF3A RNAi), or ribosome progression (cycloheximide treatment), all of which result in a specific arrest at larval stage 2 of C. elegans development. This quiescent state can last for weeks—beyond the normal C. elegans adult lifespan—and is reversible, as animals can resume reproduction and live a normal lifespan once released from the source of protein synthesis inhibition. The arrest state affords resistance to thermal, oxidative, and heavy metal stress exposure. In addition to cell-autonomous responses, reducing biosynthetic capacity only in the hypodermis was sufficient to drive organism-level developmental arrest and stress resistance phenotypes. Among the cell non-autonomous responses to protein synthesis inhibition is reduced pharyngeal pumping that is dependent upon AMPK-mediated signaling. The reduced pharyngeal pumping in response to protein synthesis inhibition is recapitulated by exposure to microbes that generate protein synthesis-inhibiting xenobiotics, which may mechanistically reduce ingestion of pathogen and toxin. These data define the existence of a transient arrest-survival state in response to protein synthesis inhibition and provide an evolutionary foundation for the conserved enhancement of healthy aging observed in post-developmental animals with reduced biosynthetic capacity.

Protein synthesis is an essential cellular process, but post-developmental reduction of protein synthesis across multiple species leads to improved health- and lifespan. To better understand the physiological responses to impaired protein synthesis, we characterize a novel developmental arrest state that occurs when reducing protein synthesis during C. elegans development. Arrested animals have multiple survival-promoting phenotypes that are all dependent on the cellular energy sensor, AMP kinase. This survival response acts through the hypodermis and causes a reduction in pharyngeal pumping, indicating that the animal is responding to a perceived external threat, even in adults. Furthermore, exposing animals to pathogens, or xenobiotics they produce, can recapitulate these phenotypes, providing a potential evolutionary explanation for how a beneficial response in adults could evolve through the inhibition of an essential biological process such as protein synthesis.

Genome-wide RNAi Screen for Fat Regulatory Genes in C. elegans Identifies a Proteostasis-AMPK Axis Critical for Starvation Survival

Organisms must execute metabolic defenses to survive nutrient deprivation. We performed a genome-wide RNAi screen in Caenorhabditis elegans to identify fat regulatory genes indispensable for starvation resistance. Here, we show that opposing proteostasis pathways are principal determinants of starvation survival. Reduced function of cytoplasmic aminoacyl tRNA synthetases (ARS genes) increases fat mass and extends starvation survival, whereas reduced proteasomal function reduces fat and starvation survival. These opposing pathways converge on AMP-activated protein kinase (AMPK) as the critical effector of starvation defenses. Extended starvation survival in ARS deficiency is dependent upon increased proteasome-mediated activation of AMPK. When the proteasome is inhibited, neither starvation nor ARS deficiency can fully activate AMPK, leading to greatly diminished starvation survival. Thus, activity of the proteasome and AMPK are mechanistically linked and highly correlated with starvation resistance. Conversely, aberrant activation of the proteostasis-AMPK axis during nutritional excess may have implications for obesity and cardiometabolic diseases.

Quantification of Lipid Abundance and Evaluation of Lipid Distribution in Caenorhabditis elegans by Nile Red and Oil Red O Staining

Caenorhabditis elegans is an exceptional model organism in which to study lipid metabolism and energy homeostasis. Many of its lipid genes are conserved in humans and are associated with metabolic syndrome or other diseases. Examination of lipid accumulation in this organism can be carried out by fixative dyes or label-free methods. Fixative stains like Nile red and oil red O are inexpensive, reliable ways to quantitatively measure lipid levels and to qualitatively observe lipid distribution across tissues, respectively. Moreover, these stains allow for high-throughput screening of various lipid metabolism genes and pathways. Additionally, their hydrophobic nature facilitates lipid solubility, reduces interaction with surrounding tissues, and prevents dissociation into the solvent. Though these methods are effective at examining general lipid content, they do not provide detailed information about the chemical composition and diversity of lipid deposits. For these purposes, label-free methods such as GC-MS and CARS microscopy are better suited, their costs notwithstanding.

The C-Box Region of MAF1 Regulates Transcriptional Activity and Protein Stability

MAF1 is a conserved negative regulator of RNA polymerase (pol) III and intracellular lipid homeostasis across species. Here, we show that the MAF1 C-box region negatively regulates its activity. Mutations in Caenorhabditis elegans mafr-1 that truncate the C-box retain the ability to inhibit the transcription of RNA pol III targets, reduce lipid biogenesis, and lower reproductive output. In human cells, C-box deletion of MAF1 leads to increased MAF1 nuclear localization and enhanced repression of ACC1 and FASN, but with impaired repression of RNA pol III targets. Surprisingly, C-box mutations render MAF1 insensitive to rapamycin, further defining a regulatory role for this region. Two MAF1 species, MAF1_L and MAF1_S, are regulated by the C-box YSY motif, which, when mutated, alters species stoichiometry and proteasome-dependent turnover of nuclear MAF1. Our results reveal a role for the C-box region as a critical determinant of MAF1 stability, activity, and response to cellular stress.

Gene-diet interactions and aging in C. elegans

Diet is the most variable aspect of life history, as most individuals have a large diversity of food choices, varying in the type and amount that they ingest. In the short-term, diet can affect metabolism and energy levels. However, in the long run, the net deficiency or excess of calories from diet can influence the progression and severity of age-related diseases. An old and yet still debated question is: how do specific dietary choices impact health- and lifespan? It is clear that genetics can play a critical role - perhaps just as important as diet choices. For example, poor diet in combination with genetic susceptibility can lead to metabolic disorders, such as obesity and type 2 diabetes. Recent work in Caenorhabditis elegans has identified the existence of diet-gene pairs, where the consequence of mutating a specific gene is only realized on specific diets. Many core metabolic pathways are conserved from worm to human. Although only a handful of these diet-gene pairs has been characterized, there are potentially hundreds, if not thousands, of such interactions, which may explain the variability in the rates of aging in humans and the incidence and severity of age-related diseases.

Omega-3 and -6 fatty acids allocate somatic and germline lipids to ensure fitness during nutrient and oxidative stress in Caenorhabditis elegans

Animals in nature are continually challenged by periods of feast and famine as resources inevitably fluctuate, and must allocate somatic reserves for reproduction to abate evolutionary pressures. We identify an age-dependent lipid homeostasis pathway in Caenorhabditis elegans that regulates the mobilization of lipids from the soma to the germline, which supports fecundity but at the cost of survival in nutrient-poor and oxidative stress environments. This trade-off is responsive to the levels of dietary carbohydrates and organismal oleic acid and is coupled to activation of the cytoprotective transcription factor SKN-1 in both laboratory-derived and natural isolates of C. elegans. The homeostatic balance of lipid stores between the somatic and germ cells is mediated by arachidonic acid (omega-6) and eicosapentaenoic acid (omega-3) precursors of eicosanoid signaling molecules. Our results describe a mechanism for resource reallocation within intact animals that influences reproductive fitness at the cost of somatic resilience.

The SKN-1 hunger games: May the odds be ever in your favor

Animals must continually assess nutrient availability to develop appropriate strategies for survival and reproductive success. It is no secret that nutritional state plays a large role in both aging and health.(1-7) Appropriate cellular energy usage is not only crucial for animal starvation survival, but is also important for diseases such as obesity and cancer, which characteristically have metabolic dysfunction.(8-10) C. elegans are exceptionally well poised to handle bouts of starvation as resource availability in the wild varies greatly.(11,12) We recently discovered an evolutionarily conserved pathway, regulated by the cytoprotective transcription factor SKN-1/Nrf2, which integrates diet composition and availability with utilization for survival.(13,14) These responses have potent impact on organismal physiology and remarkably are influenced by current and parental life history events, including choice of diet. In this commentary we will focus on recent insights concerning dietary intake and the impact that this can have throughout the life history of the nematode, Caenorhabditis elegans. In light of the strong impact that diet plays throughout life we urge caution when interpreting previous studies that make use of only one diet and suggest a reinvestigation utilizing a different diet is warranted.

Emerging Roles for Maf1 beyond the Regulation of RNA Polymerase III Activity

Maf1 was first identified in yeast, and studies in metazoans have primarily focused on examining its role in the repression of transcription that is dependent on RNA polymerase III. Recent work has revealed a novel and conserved function for Maf1 in the maintenance of intracellular lipid pools in Caenorhabditis elegans, mice, and cancer cell lines. Although additional Maf1 targets are likely, they have not been identified, and these recent findings begin to define specific activities for Maf1 in multicellular organisms beyond the regulation of RNA polymerase III transcription and suggest that Maf1 plays a more diverse role in organismal physiology. We will discuss these newly defined physiological roles of Maf1 that point to its placement as an important new player in lipid metabolism with implications in human metabolic diseases such as obesity and cancer, which display prominent defects in lipid homeostasis.

Physiological roles for mafr-1 in reproduction and lipid homeostasis

Maf1 is a conserved repressor of RNA polymerase (Pol) III transcription; however, its physiological role in the context of a multicellular organism is not well understood. Here, we show that C. elegans MAFR-1 is functionally orthologous to human Maf1, represses the expression of both RNA Pol III and Pol II transcripts, and mediates organismal fecundity and lipid homeostasis. MAFR-1 impacts lipid transport by modulating intestinal expression of the vitellogenin family of proteins, resulting in cell-nonautonomous defects in the developing reproductive system. MAFR-1 levels inversely correlate with stored intestinal lipids, in part by influencing the expression of the lipogenesis enzymes fasn-1/FASN and pod-2/ACC1. Animals fed a high carbohydrate diet exhibit reduced mafr-1 expression and mutations in the insulin signaling pathway genes daf-18/PTEN and daf-16/FoxO abrogate the lipid storage defects associated with deregulated mafr-1 expression. Our results reveal physiological roles for mafr-1 in regulating organismal lipid homeostasis, which ensure reproductive success.

Adaptive capacity to bacterial diet modulates aging in C. elegans

Diet has a substantial impact on cellular metabolism and physiology. Animals must sense different food sources and utilize distinct strategies to adapt to diverse diets. Here we show that Caenorhabditis elegans lifespan is regulated by their adaptive capacity to different diets, which is controlled by alh-6, a conserved proline metabolism gene. alh-6 mutants age prematurely when fed an Escherichia coli OP50 but not HT115 diet. Remarkably, this diet-dependent aging phenotype is determined by exposure to food during development. Mechanistically, the alh-6 mutation triggers diet-induced mitochondrial defects and increased generation of ROS, likely due to accumulation of its substrate 1-pyrroline-5-carboxylate. We also identify that neuromedin U receptor signaling is essential for diet-induced mitochondrial changes and premature aging. Moreover, dietary restriction requires alh-6 to induce longevity. Collectively, our data reveal a homeostatic mechanism that animals employ to cope with potential dietary insults and uncover an example of lifespan regulation by dietary adaptation.

Prediction of C. elegans longevity genes by human and worm longevity networks

Intricate and interconnected pathways modulate longevity, but screens to identify the components of these pathways have not been saturating. Because biological processes are often executed by protein complexes and fine-tuned by regulatory factors, the first-order protein-protein interactors of known longevity genes are likely to participate in the regulation of longevity. Data-rich maps of protein interactions have been established for many cardinal organisms such as yeast, worms, and humans. We propose that these interaction maps could be mined for the identification of new putative regulators of longevity. For this purpose, we have constructed longevity networks in both humans and worms. We reasoned that the essential first-order interactors of known longevity-associated genes in these networks are more likely to have longevity phenotypes than randomly chosen genes. We have used C. elegans to determine whether post-developmental inactivation of these essential genes modulates lifespan. Our results suggest that the worm and human longevity networks are functionally relevant and possess a high predictive power for identifying new longevity regulators.

Mitochondrial SKN-1/Nrf mediates a conserved starvation response

SKN-1/Nrf plays multiple essential roles in development and cellular homeostasis. We demonstrate that SKN-1 executes a specific and appropriate transcriptional response to changes in available nutrients, leading to metabolic adaptation. We isolated gain-of-function (gf) alleles of skn-1, affecting a domain of SKN-1 that binds the transcription factor MXL-3 and the mitochondrial outer membrane protein PGAM-5. These skn-1(gf) mutants perceive a state of starvation even in the presence of plentiful food. The aberrant monitoring of cellular nutritional status leads to an altered survival response in which skn-1(gf) mutants transcriptionally activate genes associated with metabolism, adaptation to starvation, aging, and survival. The triggered starvation response is conserved in mice with constitutively activated Nrf and may contribute to the tumorgenicity associated with activating Nrf mutations in mammalian somatic cells. Our findings delineate an evolutionarily conserved metabolic axis of SKN-1/Nrf, further establishing the complexity of this pathway.

Longevity and the long arm of epigenetics: acquired parental marks influence lifespan across several generations

A recent study reported that longevity in Caenorhabditits elegans can be inherited over several generations. This is probably achieved through the following epigenetic mechanism: inherited demethylated histones at some central loci, such as miRNA, transcription factors or signaling regulators affect the expression of certain genes leading to the longevity phenotype.

A soma-to-germline transformation in long-lived Caenorhabditis elegans mutants

Unlike the soma, which ages during the lifespan of multicellular organisms, the germ line traces an essentially immortal lineage. Genomic instability in somatic cells increases with age, and this decline in somatic maintenance might be regulated to facilitate resource reallocation towards reproduction at the expense of cellular senescence. Here we show that Caenorhabditis elegans mutants with increased longevity exhibit a soma-to-germline transformation of gene expression programs normally limited to the germ line. Decreased insulin-like signalling causes the somatic misexpression of the germline-limited pie-1 and pgl family of genes in intestinal and ectodermal tissues. The forkhead boxO1A (FOXO) transcription factor DAF-16, the major transcriptional effector of insulin-like signalling, regulates pie-1 expression by directly binding to the pie-1 promoter. The somatic tissues of insulin-like mutants are more germline-like and protected from genotoxic stress. Gene inactivation of components of the cytosolic chaperonin complex that induce increased longevity also causes somatic misexpression of PGL-1. These results indicate that the acquisition of germline characteristics by the somatic cells of C. elegans mutants with increased longevity contributes to their increased health and survival.

Lifespan regulation by evolutionarily conserved genes essential for viability

Evolutionarily conserved mechanisms that control aging are predicted to have prereproductive functions in order to be subject to natural selection. Genes that are essential for growth and development are highly conserved in evolution, but their role in longevity has not previously been assessed. We screened 2,700 genes essential for Caenorhabditis elegans development and identified 64 genes that extend lifespan when inactivated postdevelopmentally. These candidate lifespan regulators are highly conserved from yeast to humans. Classification of the candidate lifespan regulators into functional groups identified the expected insulin and metabolic pathways but also revealed enrichment for translation, RNA, and chromatin factors. Many of these essential gene inactivations extend lifespan as much as the strongest known regulators of aging. Early gene inactivations of these essential genes caused growth arrest at larval stages, and some of these arrested animals live much longer than wild-type adults. daf-16 is required for the enhanced survival of arrested larvae, suggesting that the increased longevity is a physiological response to the essential gene inactivation. These results suggest that insulin-signaling pathways play a role in regulation of aging at any stage in life.

Patterns that Define the Four Domains Conserved in Known and Novel Isoforms of the Protein Import Receptor Tom20

Tom20 is the master receptor for protein import into mitochondria. Analysis of motifs present in Tom20 sequences from fungi and animals found several highly conserved regions, including features of the transmembrane segment, the ligand-binding domain and functionally important flexible segments at the N terminus and the C terminus of the protein. Hidden Markov model searches of genome sequence data revealed novel isoforms of Tom20 in vertebrate and invertebrate animals. A three-dimensional comparative model of the novel type I Tom20, based on the structurally characterized type II isoform, shows important differences in the amino acid residues lining the ligand-binding groove, where the type I protein from animals is more similar to the fungal form of Tom20. Given that the two receptor types from mouse interact with the same set of precursor protein substrates, comparative analysis of the substrate-binding site provides unique insight into the mechanism of substrate recognition. No Tom20-related protein was found in genome sequence data from plants or protozoans, suggesting the receptor Tom20 evolved after the split of animals and fungi from the main lineage of eukaryotes.

Defective mitochondrial protein translocation precludes normal Caenorhabditis elegans development

We demonstrate biochemically that the genes identified by sequence similarity as orthologs of the mitochondrial import machinery are functionally conserved in Caenorhabditis elegans. Specifically, tin-9.1 and tin-10 RNA interference (RNAi) treatment of nematodes impairs import of the ADP/ATP carrier into isolated mitochondria. Developmental phenotypes are associated with gene knock-down of the mitochondrial import components. RNAi of tomm-7 and ddp-1 resulted in mitochondria with an interconnected morphology in vivo, presumably due to defects in the assembly of outer membrane fission/fusion components. RNAi of the small Tim proteins TIN-9.1, TIN-9.2, and TIN-10 resulted in a small body size, reduced number of progeny produced, and partial embryonic lethality. An additional phenotype of the tin-9.2(RNAi) animals is defective formation of the somatic gonad. The biochemical demonstration that the protein import activity is reduced, under the same conditions that yield the defects in specific tissues and lethality in a later generation, suggests that the developmental abnormalities observed are a consequence of defects in mitochondrial inner membrane biogenesis.

The Role of Hot13p and Redox Chemistry in the Mitochondrial TIM22 Import Pathway

The small Tim proteins in the mitochondrial intermembrane space participate in the TIM22 import pathway for assembly of the inner membrane. Assembly of the small TIM complexes requires the conserved "twin CX3C" motif that forms juxtapositional intramolecular disulfide bonds. Here we identify a new intermembrane space protein, Hot13p, as the first component of a pathway that mediates assembly of the small TIM complexes. The small Tim proteins require Hot13p for assembly into a 70-kDa complex in the intermembrane space. Once assembled the small TIM complexes escort hydrophobic inner membrane proteins en route to the TIM22 complex. The mechanism by which the small Tim proteins bind and release substrate is not understood, and we investigated the affect of oxidant/reductant treatment on the TIM22 import pathway. With in organello import studies, oxidizing agents arrest the ADP/ATP carrier (AAC) bound to the Tim9p-Tim10p complex in the intermembrane space; this productive intermediate can be chased into the inner membrane upon subsequent treatment with reductant. Moreover, AAC import is markedly decreased by oxidant treatment in Deltahot13 mitochondria and improved when Hot13p is overexpressed, suggesting Hot13p may function to remodel the small TIM complexes during import. Together these results suggest that the small TIM complexes have a specialized assembly pathway in the intermembrane space and that the local redox state of the TIM complexes may mediate translocation of inner membrane proteins.

The role of Tim9p in the assembly of the TIM22 import complexes

Tim9p is located in the soluble 70-kDa Tim9p-Tim10p complex and the 300-kDa membrane complex in the mitochondrial TIM22 protein import system, which mediates the import of inner membrane proteins. From a collection of temperature-sensitive mutants, we have analyzed two in detail. tim9-3 contained two mutations and tim9-19 contained one mutation, all located near the 'twin CX3C' motif that is conserved in the small Tim proteins. As a result, the import components in the tim9-3 mutant mitochondria were severely reduced and assembled into complexes of aberrant sizes. Protein import was severely reduced and Tim9p and Tim10p binding to in vitro imported ADP/ATP carrier was impaired. In the tim9-19 mutant mitochondria, the 300-kDa membrane complex was assembled, although the soluble 70-kDa Tim9p-Tim10p complex was not detectable. Protein import was decreased only two-fold. When coexpressed in Escherichia coli, tim9-19 and TIM10 proteins failed to assemble into a 70-kDa complex. Our findings suggest that residues near the 'twin CX3C' motif are important for the assembly of Tim9p in both the Tim9p-Tim10p complex and the 300-kDa membrane complex.

The role of the Tim8p-Tim13p complex in a conserved import pathway for mitochondrial polytopic inner membrane proteins

Tim23p is imported via the TIM (translocase of inner membrane)22 pathway for mitochondrial inner membrane proteins. In contrast to precursors with an NH2-terminal targeting presequence that are imported in a linear NH2-terminal manner, we show that Tim23p crosses the outer membrane as a loop before inserting into the inner membrane. The Tim8p-Tim13p complex facilitates translocation across the intermembrane space by binding to the membrane spanning domains as shown by Tim23p peptide scans with the purified Tim8p-Tim13p complex and crosslinking studies with Tim23p fusion constructs. The interaction between Tim23p and the Tim8p-Tim13p complex is not dependent on zinc, and the purified Tim8p-Tim13p complex does not coordinate zinc in the conserved twin CX3C motif. Instead, the cysteine residues seemingly form intramolecular disulfide linkages. Given that proteins of the mitochondrial carrier family also pass through the TOM (translocase of outer membrane) complex as a loop, our study suggests that this translocation mechanism may be conserved. Thus, polytopic inner membrane proteins, which lack an NH2-terminal targeting sequence, pass through the TOM complex as a loop followed by binding of the small Tim proteins to the hydrophobic membrane spanning domains.

Human deafness dystonia syndrome is caused by a defect in assembly of the DDP1/TIMM8a-TIMM13 complex

Mohr-Tranebjaerg syndrome (MTS/DFN-1) or deafness/dystonia syndrome results from a mutation in deafness/dystonia protein 1/translocase of mitochondrial inner membrane 8a (DDP1/TIMM8a). DDP1/TIMM8a is similar to a family of yeast proteins in the mitochondrial intermembrane space which mediate the import and insertion of inner membrane proteins. We now show that TIMM8a assembles in a 70 kDa complex in the intermembrane space with TIMM13. DDP1/TIMM8a is not detectable in fibroblasts derived from a patient with a missense mutation in the DDP1/TIMM8a gene; the point mutation results in cysteine-66 being changed to tryptophan-66 in the conserved 'twin CX(3)C' motif. The corresponding mutation in yeast translocase of inner membrane 8p (Tim8p) yields an unstable protein that does not assemble with yeast Tim13p. DDP1/TIMM8a, when expressed with TIMM13 in yeast mitochondria lacking the Tim8p-Tim13p complex, restores Tim23p import, and TIMM8a and TIMM13 can be cross-linked to the hTim23 import intermediate in rat and yeast mitochondria. In a similar manner to Tim8p, TIMM8a seemingly mediates the import of hTim23. Deafness/dystonia syndrome thus may be caused by decreased levels of Tim23 in the mitochondrial inner membrane in affected tissues.