Ratnappan, Ramesh et al. (2013) International Worm Meeting "Molecular Outsourcing: Reproductive Signals Deploy NHR-49/PPARa to Reorganize Lipid Homeostasis and Alter Lifespan."

Ratnappan, Ramesh, Ghazi, Arjumand, Gill, Hasreet, Ward, Jordan, Holden, Kyle, RG Amrit, Francis, Yamamoto, Keith

[International Worm Meeting, 2013]

Aging is an inherently entropic but remarkably plastic phenomenon. The rate of aging can be altered by modifications to an energy-intensive process such as reproduction. In worms, removal of the Germline-Stem Cells (GSCs) results in increased fat accumulation and the activation of a transcriptional network in intestinal cells that extends lifespan. Two key regulators in this network are DAF-16/FOXO and TCER-1/TCERG1. In a screen designed to identify nuclear hormone receptors (NHRs) that impact DAF-16/FOXO and TCER-1/TCERG1 function, one of the NHRs we isolated was NHR-49/PPARa, a conserved and crucial modulator of energy and fat metabolism. Independently, we also identified nhr-49 as a gene jointly upregulated by DAF-16/FOXO and TCER-1/TCERG1 through RNA-Seq transcriptomics. Predictably, nhr-49 mutation suppressed the longevity of GSC(-) worms, but did not appear to change the overall elevated fat content. A GFP-tagged translational reporter showed expression in the cytoplasm and nuclei of all somatic cells and rescued nhr-49(-) phenotypes. NHR-49/PPARa overexpression increased the lifespan of nhr-49 mutants substantially without a concomitant loss of fertility. NHR-49/PPARa regulated the expression of fatty-acid desaturases, specific mitochondrial b-oxidation pathway genes and acted in a feedback loop to increase DAF-16/FOXO and TCER-1/TCERG1 activity. At least part of these transcriptional changes may be brought about in co-operation with NHR-71, another NHR identified in our screen that interacted with both NHR-49/PPARa and TCER-1/TCERG1 in yeast two-hybrid experiments. Our results suggest that upon GSC loss, NHR-49/PPARa is recruited to fundamentally remodel the lipid metabolic profile of the worm. This reorganization not only allows the mobilization of fat that would otherwise have been transported to the oocytes, but also induces transcriptional and signaling events that direct the allotment of cellular resources towards somatic maintenance and longevity.

Doering, Kelsie et al. (2021) International Worm Meeting "Nuclear Hormone Receptor NHR-49 controls a HIF-1-independent hypoxia adaptation pathway in Caenorhabditis elegans"

Cheng, Xuanjin, Taubert, Stefan, Ghazi, Arjumand, Ratnappan, Ramesh, Doering, Kelsie, Miller, Dana, Milburn, Luke

[International Worm Meeting, 2021]

Cells encounter many harmful stresses, and their ability to mount responses specific to each stress is critical for survival. The response to insufficient oxygen (hypoxia) is orchestrated by the conserved Hypoxia-Inducible Factor (HIF). However, HIF-independent hypoxia response pathways exist that act in parallel to HIF to mediate the physiological hypoxia response. We have found a HIF-independent hypoxia response pathway controlled by Caenorhabditis elegans Nuclear Hormone Receptor NHR-49, an orthologue of mammalian lipid metabolism regulator Peroxisome Proliferator-Activated Receptor alpha (PPARalpha). We show that nhr-49 is required for worm survival in hypoxia and is synthetic lethal with hif-1 in this context, demonstrating that these factors act independently. Our RNA-seq analysis shows that in hypoxia nhr-49 regulates a set of genes that are hif-1-independent, including autophagy genes that promote hypoxia survival. We further show that Nuclear Hormone Receptor nhr-67 and Homeodomain-interacting Protein Kinase hpk-1 act in the NHR-49 pathway. The former acts during normoxia to repress NHR-49; however, during hypoxia, an increase in NHR-49 protein levels in turn represses nhr-67 levels, forming a feedback loop that may serve to reinforce NHR-49 activity. In contrast to nhr-67, the upstream kinase HPK-1 positively regulates the NHR-49 hypoxia response, as it is required to activate the NHR-49 regulated hypoxia response genes and to survive hypoxia. Together, our experiments define a new, essential hypoxia response pathway that acts in parallel to the well-known HIF-mediated hypoxia response.

Wani, Khursheed A. et al. (2021) International Worm Meeting "NHR-49/PPAR-alpha and HLH-30/TFEB cooperate for C. elegans host defense via a flavin-containing monooxygenase"

Goswamy, Debanjan, Irazoqui, Javier E., Wani, Khursheed A., Ratnappan, Ramesh, Ghazi, Arjumand, Taubert, Stefan

[International Worm Meeting, 2021]

During bacterial infection, the host is confronted with multiple overlapping signals that are integrated at the organismal level to produce defensive host responses. How multiple infection signals are sensed by the host and how they elicit the transcription of host defense genes is much less understood at the whole-animal level than at the cellular level. The model organism Caenorhabditis elegans is known to mount transcriptional defense responses against intestinal bacterial infections that elicit overlapping starvation and infection responses, but the regulation of such responses is not well understood. Direct comparison of C. elegans that were starved or infected with Gram-positive pathogen Staphylococcus aureus revealed a large infection-specific transcriptional signature. Both the starvation response and the infection-specific signature were largely dependent on the transcription factor, HLH-30/TFEB, highlighting its key role as a transcriptional integrator of organismal stress during infection. Interestingly, we identified six genes that were specifically induced during infection even in the absence of HLH-30/TFEB, potentially revealing an alternative transcriptional host response signaling pathway. The induction of two of the six genes, fmo-2/FMO5 (encodes flavin-containing monooxygenase 2) and K08C7.4 (an uncharacterized gene), was entirely dependent on nuclear hormone receptor, NHR-49/PPAR-alpha. NHR-49/PPAR-alpha was required non cell-autonomously for fmo-2/FMO5 induction and host defense against S. aureus. Moreover, functional characterization of FMO-2/FMO5 suggested that its enzymatic activity is specifically required for host defense against S. aureus, revealing that FMO-2/FMO5 is a key host defense effector. Further, fmo-2/FMO5 was specifically induced by Gram-positive pathogens and Gram-positive natural microbiota of C. elegans. These findings for the first time reveal an infection-specific host response to S. aureus, identify HLH-30/TFEB as its main regulator, reveal that FMOs are important innate immunity effectors in animals, and identify the mechanism of FMO regulation through NHR-49/PPAR-alpha in C. elegans, with important implications for innate host defense in higher organisms.

Amrit, Francis RG et al. (2015) International Worm Meeting "Transcription Factors that Coordinate Lipid Synthesis and Breakdown to Ensure Metabolic Homeostasis and Longevity."

Steenkiste, Elizabeth, Ratnappan, Ramesh, Amrit, Francis RG, Olsen, Carissa P, Yanowitz, Judith L, McClendon, T Brooke, Chen, Shaw-Wen

[International Worm Meeting, 2015]

The coordination of lipid production and degradation is essential for animals' health - lipid imbalances are characteristic of obesity and a feature of many reproductive pathologies and age-related diseases. But how these processes are balanced in multicellular organisms is poorly understood. In C. elegans, removing the germline, using mutations in genes such as glp-1, extends lifespan. Germline loss is a major metabolic challenge that compels the animal to stop fat deposition into eggs and remodel its lipid reserves. This phenomenon provides a unique platform to understand how complex metazoans retain metabolic homeostasis when challenged with alterations in fertility and age. Recent studies, including ours, have shown that germline loss activates conserved transcription factors such as DAF-16/FOXO and TCER-1/TCERG1 in the intestine, the worms' main fat depot. We recently demonstrated that the nuclear hormone receptor, NHR-49/PPARalpha, under partial regulation by DAF-16/FOXO and TCER-1/TCERG1, promotes glp-1 mutants' longevity by elevating fatty-acid beta-oxidation and desaturation- pathways involved in fat buildup and breakdown, respectively. We have now discovered that both lipid synthesis and degradation are widely and concurrently enhanced in response to germline loss. Using transcriptomics, molecular-genetic analyses and biochemical approaches, we demonstrate that these transcription factors activate multiple lipid anabolic (de novo lipid synthesis, fatty-acid desaturation and elongation and triglyceride production) and catabolic (triglyceride hydrolysis, fatty-acid oxidation) pathways in glp-1 mutants. Our data suggest that the coordinated enhancement of lipid synthesis and breakdown, by NHR-49, DAF-16 and TCER-1, facilitates the adaptation to germline loss by ensuring lipid homeostasis, and mediates longevity.