Zhaoyu Li

Dissecting a central flip-flop circuit that integrates contradictory sensory cues in C. elegans feeding regulation

Feeding behaviour is modulated by both environmental cues and internal physiological states. Appetite is commonly boosted by the pleasant smell (or appearance) of food and destroyed by a bad taste. In reality, animals sense multiple environmental cues at the same time and it is not clear how these sensory inputs are integrated and a decision is made to regulate feeding behaviour accordingly. Here we show that feeding behaviour in Caenorhabditis elegans can be either facilitated by attractive odours or suppressed by repellents. By identifying mutants that are defective for sensory-mediated feeding regulation, we dissected a central flip-flop circuit that integrates two contradictory sensory inputs and generates bistable hormone output to regulate feeding behaviour. As feeding regulation is fundamental to animal survival, we speculate that the basic organizational logic identified here in C. elegans is likely convergent throughout different phyla.

Glial Expression of the Caenorhabditis elegans Gene swip-10 Supports Glutamate Dependent Control of Extrasynaptic Dopamine Signaling

Glial cells play a critical role in shaping neuronal development, structure, and function. In a screen for Caenorhabditis elegans mutants that display dopamine (DA)-dependent, Swimming-Induced Paralysis (Swip), we identified a novel gene, swip-10, the expression of which in glia is required to support normal swimming behavior. swip-10 mutants display reduced locomotion rates on plates, consistent with our findings of elevated rates of presynaptic DA vesicle fusion using fluorescence recovery after photobleaching. In addition, swip-10 mutants exhibit elevated DA neuron excitability upon contact with food, as detected by in vivo Ca2+ monitoring, that can be rescued by glial expression of swip-10. Mammalian glia exert powerful control of neuronal excitability via transporter-dependent buffering of extracellular glutamate (Glu). Consistent with this idea, swip-10 paralysis was blunted in mutants deficient in either vesicular Glu release or Glu receptor expression and could be phenocopied by mutations that disrupt the function of plasma membrane Glu transporters, most noticeably glt-1, the ortholog of mammalian astrocytic GLT1 (EAAT2). swip-10 encodes a protein containing a highly conserved metallo-β-lactamase domain, within which our swip-10 mutations are located and where engineered mutations disrupt Swip rescue. Sequence alignments identify the CNS-expressed gene MBLAC1 as a putative mammalian ortholog. Together, our studies provide evidence of a novel pathway in glial cells regulated by swip-10 that limits DA neuron excitability, DA secretion, and DA-dependent behaviors through modulation of Glu signaling.

Fabrication of size-controllable ultrasmall-disk electrode: Monitoring single vesicle release kinetics at tiny structures with high spatio-temporal resolution

Size-controllable micron or nano-disk carbon fiber electrode (CFE) is prepared and demonstrated to be excellent for extra-cellular transmitter release detection at tiny structures and vesicle fusion kinetics analysis with high spatio-temporal resolution. An improved electrochemical etching procedure was employed, for the first time, to fabricate cylindrical fiber with controlled micron or nano-diameter. Afterwards, a facile insulation with polypropylene sheath was employed to completely insulate the whole body of the thinned fiber, and an ultrasmall-disk sensing area was finally produced by cutting of the insulated fibers. Scanning electron microscopy (SEM) was employed to characterize the ultrasmall geometry size of the fabricated electrode and to show the tight adherence of the insulation sheath on the fiber. The cut ends of the electrodes were also shown to be smooth, clean and without obvious jagged layer. The fabricated micron or nano-disk carbon electrodes show ideal steady-state voltammetric behavior with satisfying reversibility. Subsequently, the performance of the ultrasmall-disk CFE for amperometric detection of cell secretion was characterized. Results showed that, compared to the conventional micro-disk CFE, the etched small disk CFE possesses higher sensitivity due to its obviously improved signal-to-noise level, which enables minute amounts of 3000 oxidizable molecules to be detectable. The nano-disk CFE was shown to be particularly ideal for analysis of fusion kinetics, due to its avoidance of diffusion broadening of the detected spikes, which is the inherent defect of the conventional micro-CFE technique.

PKG and NHR-49 signalling co-ordinately regulate short-term fasting-induced lysosomal lipid accumulation in C. elegans

Lysosomes act as terminal degradation organelles to hydrolyse macromolecules derived from both the extracellular space and the cytoplasm. In Caenorhabditis elegans fasting induces the lysosomal compartment to expand. However, the molecular and cellular mechanisms for this stress response remain largely unclear. In the present study, we find that short-term fasting leads to increased accumulation of polar lipids in lysosomes. The fasting response is co-ordinately regulated by EGL-4, the C. elegans PKG (protein kinase G) orthologue, and nuclear hormone receptor NHR-49. Further results demonstrate that EGL-4 acts in sensory neurons to enhance lysosomal lipid accumulation through inhibiting the DAF-3/SMAD pathway, whereas NHR-49 acts in intestine to inhibit lipids accumulation via activation of IPLA-2 (intracellular membrane-associated calcium-independent phospholipase A2) in cytoplasm and other hydrolases in lysosomes. Remarkably, the lysosomal lipid accumulation is independent of autophagy and RAB-7-mediated endocytosis. Taken together, our results reveal a new mechanism for lysosomal lipid metabolism during the stress response, which may provide new clues for investigations of lysosome function in energy homoeostasis.

cGMP Signalling Mediates Water Sensation (Hydrosensation) and Hydrotaxis in Caenorhabditis elegans

Animals have developed the ability to sense the water content in their habitats, including hygrosensation (sensing humidity in the air) and hydrosensation (sensing the water content in other microenvironments), and they display preferences for specific water contents that influence their mating, reproduction and geographic distribution. We developed and employed four quantitative behavioural test paradigms to investigate the molecular and cellular mechanisms underlying sensing the water content in an agar substrate (hydrosensation) and hydrotaxis in Caenorhabditis elegans. By combining a reverse genetic screen with genetic manipulation, optogenetic neuronal manipulation and in vivo Ca2+ imaging, we demonstrate that adult worms avoid the wetter areas of agar plates and hypo-osmotic water droplets. We found that the cGMP signalling pathway in ciliated sensory neurons is involved in hydrosensation and hydrotaxis in Caenorhabditis elegans.