Fast genetic mapping using insertion-deletion polymorphisms in Caenorhabditis elegans

COSA-1 mediated pro-crossover complex formation promotes meiotic crossing over in C. elegans

Accurate chromosome segregation during meiosis requires the establishment of at least one crossover (CO) between each pair of homologous chromosomes. CO formation depends on a group of conserved pro-CO proteins, which colocalize at CO-designated sites during late meiotic prophase I. However, it remains unclear whether these pro-CO proteins form a functional complex and how they promote meiotic CO formation in vivo. Here, we show that COSA-1, a key component required for CO formation, interacts with other pro-CO factors, MSH-5 and ZHP-3, via its N-terminal disordered region. Point mutations that impair these interactions do not affect CO designation, but they strongly hinder the accumulation of COSA-1 at CO-designated sites and result in defective CO formation. These defects can be partially bypassed by artificially tethering an interaction-compromised COSA-1 derivate to ZHP-3. Furthermore, we revealed that the accumulation of COSA-1 into distinct foci is required to assemble functional 'recombination nodules'. These prevent early CO-designated recombination intermediates from being dismantled by the RTEL-1 helicase and protect late recombination intermediates, such as Holliday junctions, until they are resolved by CO-specific resolvases. Altogether, our findings provide insight into COSA-1 mediated pro-CO complex assembly and its contribution to CO formation.

Domain 3a mutation of VPS33A suppresses larval arrest phenotype in the loss of VPS45 in Caenorhabditis elegans

The Sec1/Munc18 (SM) protein VPS45 is a key regulator of SNARE-mediated membrane fusion in endosomal trafficking, but its precise role remains unknown. To understand the function of VPS45 in vivo , we performed a genetic suppressor screen in Caenorhabditis elegans . We found that the temperature-sensitive lethality caused by the loss of VPS-45 can be suppressed by a mutation in another SM protein, VPS33A. The VPS33A M376I mutation is located in domain 3a, which is predicted to be essential for SNARE complex assembly. These results highlight the functional importance of domain 3a in endosomal SM proteins and its role in specific membrane fusion.

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.

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.