A Multicellular Network Mechanism for Temperature-Robust Food Sensing.

Responsiveness to external cues is a hallmark of biological systems. In complex environments, it is crucial for organisms to remain responsive to specific inputs even as other internal or external factors fluctuate. Here, we show how the nematode Caenorhabditis elegans can discriminate between different food levels to modulate its lifespan despite temperature perturbations. This end-to-end robustness from environment to physiology is mediated by food-sensing neurons that communicate via transforming growth factor ß (TGF-ß) and serotonin signals to form a multicellular gene network. Specific regulations in this network change sign with temperature to maintain similar food responsiveness in the lifespan output. In contrast to robustness of stereotyped outputs, our findings uncover a more complex robustness process involving the higher order function of discrimination in food responsiveness. This process involves rewiring a multicellular network to compensate for temperature and provides a basis for understanding gene-environment interactions. Together, our findings unveil sensory computations that integrate environmental cues to govern physiology.

Quantification of Information Encoded by Gene Expression Levels During Lifespan Modulation Under Broad-range Dietary Restriction in C. elegans.

Sensory systems allow animals to detect, process, and respond to their environment. Food abundance is an environmental cue that has profound effects on animal physiology and behavior. Recently, we showed that modulation of longevity in the nematode Caenorhabditis elegans by food abundance is more complex than previously recognized. The responsiveness of the lifespan to changes in food level is determined by specific genes that act by controlling information processing within a neural circuit. Our framework combines genetic analysis, high-throughput quantitative imaging and information theory. Here, we describe how these techniques can be used to characterize any gene that has a physiological relevance to broad-range dietary restriction. Specifically, this workflow is designed to reveal how a gene of interest regulates lifespan under broad-range dietary restriction; then to establish how the expression of the gene varies with food level; and finally, to provide an unbiased quantification of the amount of information conveyed by gene expression about food abundance in the environment. When several genes are examined simultaneously under the context of a neural circuit, this workflow can uncover the coding strategy employed by the circuit.

Genetic control of encoding strategy in a food-sensing neural circuit.

Neuroendocrine circuits encode environmental information via changes in gene expression and other biochemical activities to regulate physiological responses. Previously, we showed that daf-7 TGFß and tph-1 tryptophan hydroxylase expression in specific neurons encode food abundance to modulate lifespan in Caenorhabditis elegans, and uncovered cross- and self-regulation among these genes (Entchev et al., 2015). Here, we now extend these findings by showing that these interactions between daf-7 and tph-1 regulate redundancy and synergy among neurons in food encoding through coordinated control of circuit-level signal and noise properties. Our analysis further shows that daf-7 and tph-1 contribute to most of the food-responsiveness in the modulation of lifespan. We applied a computational model to capture the general coding features of this system. This model agrees with our previous genetic analysis and highlights the consequences of redundancy and synergy during information transmission, suggesting a rationale for the regulation of these information processing features.

A gene-expression-based neural code for food abundance that modulates lifespan.

How the nervous system internally represents environmental food availability is poorly understood. Here, we show that quantitative information about food abundance is encoded by combinatorial neuron-specific gene-expression of conserved TGFß and serotonin pathway components in Caenorhabditis elegans. Crosstalk and auto-regulation between these pathways alters the shape, dynamic range, and population variance of the gene-expression responses of daf-7 (TGFß) and tph-1 (tryptophan hydroxylase) to food availability. These intricate regulatory features provide distinct mechanisms for TGFß and serotonin signaling to tune the accuracy of this multi-neuron code: daf-7 primarily regulates gene-expression variability, while tph-1 primarily regulates the dynamic range of gene-expression responses. This code is functional because daf-7 and tph-1 mutations bidirectionally attenuate food level-dependent changes in lifespan. Our results reveal a neural code for food abundance and demonstrate that gene expression serves as an additional layer of information processing in the nervous system to control long-term physiology.

Automated Processing of Imaging Data through Multi-tiered Classification of Biological Structures Illustrated Using Caenorhabditis elegans.

Quantitative imaging has become a vital technique in biological discovery and clinical diagnostics; a plethora of tools have recently been developed to enable new and accelerated forms of biological investigation. Increasingly, the capacity for high-throughput experimentation provided by new imaging modalities, contrast techniques, microscopy tools, microfluidics and computer controlled systems shifts the experimental bottleneck from the level of physical manipulation and raw data collection to automated recognition and data processing. Yet, despite their broad importance, image analysis solutions to address these needs have been narrowly tailored. Here, we present a generalizable formulation for autonomous identification of specific biological structures that is applicable for many problems. The process flow architecture we present here utilizes standard image processing techniques and the multi-tiered application of classification models such as support vector machines (SVM). These low-level functions are readily available in a large array of image processing software packages and programming languages. Our framework is thus both easy to implement at the modular level and provides specific high-level architecture to guide the solution of more complicated image-processing problems. We demonstrate the utility of the classification routine by developing two specific classifiers as a toolset for automation and cell identification in the model organism Caenorhabditis elegans. To serve a common need for automated high-resolution imaging and behavior applications in the C. elegans research community, we contribute a ready-to-use classifier for the identification of the head of the animal under bright field imaging. Furthermore, we extend our framework to address the pervasive problem of cell-specific identification under fluorescent imaging, which is critical for biological investigation in multicellular organisms or tissues. Using these examples as a guide, we envision the broad utility of the framework for diverse problems across different length scales and imaging methods.

Steroid hormones controlling the life cycle of the nematode Caenorhabditis elegans: stereoselective synthesis and biology.

Cholesterol-derived hormones, the dafachronic acids, play a major role in controlling the life cycle and initiating dauer larva formation of the nematode Caenorhabditis elegans. This Perspective describes recent progress in the synthesis of these steroid hormones and their biological function.

Methylation of the sterol nucleus by STRM-1 regulates dauer larva formation in Caenorhabditis elegans.

In response to pheromone(s), Caenorhabditis elegans interrupts its reproductive life cycle and enters diapause as a stress-resistant dauer larva. This decision is governed by a complex system of neuronal and hormonal regulation. All the signals converge onto the nuclear hormone receptor DAF-12. A sterol-derived hormone, dafachronic acid (DA), supports reproductive development by binding to DAF-12 and inhibiting its dauer-promoting activity. Here, we identify a methyltransferase, STRM-1, that modulates DA levels and thus dauer formation. By modifying the substrates that are used for the synthesis of DA, STRM-1 can reduce the amount of hormone produced. Loss of STRM-1 function leads to elevated levels of DA and inefficient dauer formation. Sterol methylation was not previously recognized as a mechanism for regulating hormone activity. Moreover, the C-4 sterol nucleus methylation catalyzed by STRM-1 is unique to nematodes and thus could be a target for therapeutic strategies against parasitic nematode infections.

4Alpha-bromo-5alpha-cholestan-3beta-ol and nor-5alpha-cholestan-3beta-ol derivatives-stereoselective synthesis and hormonal activity in Caenorhabditis elegans.

We describe the stereoselective synthesis of 4alpha-bromo-5alpha-cholestan-3beta-ol, 21-nor-5alpha-cholestan-3beta-ol, 27-nor-5alpha-cholestan-3beta-ol and 21,27-bisnor-5alpha-cholestan-3beta-ol. In order to clarify the in vivo metabolism of cholesterol, these compounds have been used for feeding experiments in Caenorhabditis elegans. Our preliminary results provide important insights into the metabolism of cholesterol in worms.

Two cytochrome P450s in Caenorhabditis elegans are essential for the organization of eggshell, correct execution of meiosis and the polarization of embryo.

The role of lipids in the process of embryonic development of Caenorhabditis elegans is still poorly understood. Cytochrome P450s, a class of lipid-modifying enzymes, are good candidates to be involved in the production or degradation of lipids essential for development. We investigated two highly similar cytochrome P450s in C. elegans, cyp-31A2 and cyp-31A3, that are homologs of the gene responsible for Bietti crystalline corneoretinal dystrophy in humans. Depletion of both cytochromes either by RNAi or using a double deletion mutant, led to the failure of establishing the correct polarity of the embryo and to complete the extrusion of the polar bodies during meiosis. In addition, the egg became osmotic sensitive and permeable to dyes. The phenotype of cyp-31A2 or cyp-31A3 is very similar to a class of mutants that have polarization and osmotic defects (POD), thus the genes were renamed to pod-7 and pod-8, respectively. Electron microscopic analysis demonstrated that the activity of pod-7/pod-8 is crucial for the proper assembly of the eggshell and, in particular, for the production of its lipid-rich layer. Using a complementation with lipid extracts, we show that POD-7/POD-8 function together with a NADPH cytochrome P450 reductase, coded by emb-8, and are involved in the production of lipid(s) required for eggshell formation.

Synthesis and biological activity of the (25R)-cholesten-26-oic acids--ligands for the hormonal receptor DAF-12 in Caenorhabditis elegans.

We describe the stereoselective transformation of diosgenin (4a) to (25R)-Delta(4)-dafachronic acid (1a),(25R)-Delta(7)-dafachronic acid (2a), and (25R)-cholestenoic acid (3a), which represent potential ligands forthe hormonal receptor DAF-12 in Caenorhabditis elegans. Key-steps of our synthetic approach are amodified Clemmensen reduction of diosgenin (4a) and a double bond shift from the 5,6- to the 7,8-position. In the 25R-series, the Delta(7)-dafachronic acid 2a exhibits the highest hormonal activity.

Stereoselective synthesis of the hormonally active (25S)-delta7-dafachronic acid, (25S)-Delta4-dafachronic acid, (25S)-dafachronic acid, and (25S)-cholestenoic acid.

We report a stereoselective synthesis of the (25S)-cholestenoic-26-acids which are highly efficient ligands for the hormonal receptor DAF-12 in Caenorhabditis elegans.

LET-767 is required for the production of branched chain and long chain fatty acids in Caenorhabditis elegans.

LET-767 from Caenorhabditis elegans belongs to a family of short chain dehydrogenases/reductases and is homologous to 17beta-hydroxysterol dehydrogenases of type 3 and 3-ketoacyl-CoA reductases. Worms subjected to RNA interference (RNAi) of let-767 displayed multiple growth and developmental defects in the first generation and arrested in the second generation as L1 larvae. To determine the function of LET-767 in vivo, we exploited a biochemical complementation approach, in which let-767 (RNAi)-arrested larvae were rescued by feeding with compounds isolated from wild type worms. The arrest was only rescued by the addition of triacylglycerides extracted from worms but not from various natural sources, such as animal fats and plant oils. The mass spectrometric analyses showed alterations in the fatty acid content of triacylglycerides. Essential for the rescue were odd-numbered fatty acids with monomethyl branched chains. The rescue was improved when worms were additionally supplemented with long chain even-numbered fatty acids. Remarkably, let-767 completely rescued the yeast 3-ketoacyl-CoA reductase mutant (ybr159Delta). Because worm ceramides exclusively contain a monomethyl branched chain sphingoid base, we also investigated ceramides in let-767 (RNAi). Indeed, the amount of ceramides was greatly reduced, and unusual sphingoid bases were observed. Taken together, we conclude that LET-767 is a major 3-ketoacyl-CoA reductase in C. elegans required for the bulk production of monomethyl branched and long chain fatty acids, and the developmental arrest in let-767 (RNAi) worms is caused by the deficiency of the former.

Requirement of sterols in the life cycle of the nematode Caenorhabditis elegans.

The nematode Caenorhabditis elegans represents an excellent model for studying many aspects of sterol function on the level of a whole organism. Recent studies show that especially two processes in the life cycle of the worm, dauer larva formation and molting, depend on sterols. In both cases, cholesterol or its derivatives seem to act as hormones rather than being structural components of the membrane. Investigations on C. elegans could provide information on the etiology of human diseases that display defects in the transport or metabolism of sterols.

Sterol-derived hormone(s) controls entry into diapause in Caenorhabditis elegans by consecutive activation of DAF-12 and DAF-16.

Upon starvation or overcrowding, Caenorhabditis elegans interrupts its reproductive cycle and forms a specialised larva called dauer (enduring). This process is regulated by TGF-beta and insulin-signalling pathways and is connected with the control of life span through the insulin pathway components DAF-2 and DAF-16. We found that replacing cholesterol with its methylated metabolite lophenol induced worms to form dauer larvae in the presence of food and low population density. Our data indicate that methylated sterols do not actively induce the dauer formation but rather that the reproductive growth requires a cholesterol-derived hormone that cannot be produced from methylated sterols. Using the effect of lophenol on growth, we have partially purified activity, named gamravali, which promotes the reproduction. In addition, the effect of lophenol allowed us to determine the role of sterols during dauer larva formation and longevity. In the absence of gamravali, the nuclear hormone receptor DAF-12 is activated and thereby initiates the dauer formation program. Active DAF-12 triggers in neurons the nuclear import of DAF-16, a forkhead domain transcription factor that contributes to dauer differentiation. This hormonal control of DAF-16 activation is, however, independent of insulin signalling and has no influence on life span.

Morphogen gradient formation and vesicular trafficking.

Morphogens are secreted signaling molecules which form spatial concentration gradients while moving away from a restricted source of production. A simple model of gradient formation postulates that the morphogens dilute as they diffuse between cells. In this review we discuss recent data supporting the idea that movement of the morphogen could also occur via vesicular trafficking through the cells. We explore the implications of these results for the control of gradient formation and the determination of the gradient slope which ultimately encodes the coordinates of positional information.