Martin Chalfie

As a basic unit of the nervous system, the sensory-motor reflex circuit is fast and robust. However, it is not entirely clear how this robustness is achieved, given that various genetic perturbations can alter the function of the sensory neurons. By mapping the molecular basis of neuronal connections in the touch response circuit of Caenorhabditis elegans , we found prevalent genetic redundancy at neural pathway, synaptic, and molecular levels, which ensures that sensory signals can be relayed to command interneurons that control motor output. We also found developmental remodeling of the anterior circuit, which leads to the pruning of larval synapses, establishment of a second pathway that activates additional interneurons, and lateralization of the circuit. Finally, we found that the synapses that appeared to be functionally redundant in a simple touch assay contribute to the extent of reversal response in an additive manner, which may help the organism escape from predators.

Experimental organisms such as the nematode Caenorhabditis elegans are fundamental to biological discovery. The success of C. elegans research has been greatly enabled by infrastructure that allows thousands of scientists to share and access research materials and unpublished information efficiently. Here, we celebrate the worm by interweaving vignettes describing four Nobel Prize–winning discoveries with descriptions of how the major NIH-supported research resources—the Caenorhabditis Genetics Center, WormBase, and WormAtlas—provide invaluable support for all C. elegans research. The synergy between investigation and the availability of shared resources for the C. elegans community is a paradigm for all model organism research, and the continued support of such community research resources will be essential for maximizing impactful discoveries in the future.

SUMMARY: As a basic unit of the nervous system, the sensory-motor reflex circuit is fast and robust. However, it is not entirely clear how this robustness is achieved, given that various genetic perturbations can disrupt the function of the sensory neurons. By mapping the molecular basis of neuronal connections in the touch response circuit of Caenorhabditis elegans , we found prevalent genetic redundancy at neural pathway, synaptic, and molecular levels, which ensures that sensory signals can be relayed to command interneurons that control motor output. We also discovered developmental remodeling of the anterior circuit, which leads to the pruning of larval synapses, establishment of a second pathway that activates additional interneurons, and lateralization of the circuit. Finally, we found that the synapses that appeared to be functionally redundant in a simple touch assay contribute to the extent of reversal response in an additive manner, which may help the organism escape from predators.

This article is based on an interview between Prof. Martin Chalfie and Noa Segev.Scientists investigate things by observation. They look at a phenomenon that interests them and try to understand it, using the most advanced tools they have. It is often challenging for scientists to see and measure what they want to study, often because they want to go beyond what had been previously seen. The development of modern imaging techniques has allowed scientists to see things that they could not see before. In this article, I will tell you about one of those breakthroughs in imaging, based on a wonderful glowing protein called green fluorescent protein (GFP). GFP not only changed my life, but the lives of many other scientists, and ultimately of many non-scientists as well. Among other things, GFP allows us to detect and observe the activity of proteins and whole cells in living animals, and to detect the activity of genes that code for specific proteins. By the end of this article, I hope you will understand much more about GFP and how it lights up science.

Mutations in tubulins affect microtubule (MT) dynamics and functions during neuronal differentiation and their genetic interaction provides insights into the regulation of MT functions. We previously used C. elegans touch receptor neurons to analyze the cellular impact of tubulin mutations and reported the phenotypes of 67 tubulin missense mutations, categorized into three classes: loss-of-function (lf), antimorphic (anti), and neomorphic (neo) alleles. In this study, we isolated 54 additional tubulin alleles through suppressor screens in sensitized backgrounds that caused excessive neurite growth. These alleles included 32 missense mutations not analyzed before, bringing the total number of mutations in our collection to 99. Phenotypic characterization of these newly isolated mutations identified three new types of alleles: partial lf and weak neo alleles of mec-7/β-tubulin that had subtle effects and strong anti alleles of mec-12/α-tubulin. We also discovered complex genetic interactions among the tubulin mutations, including the suppression of neo mutations by intragenic lf and anti alleles, additive and synthetic effects between mec-7 neo alleles, and unexpected epistasis, in which weaker neo alleles masked the effects of stronger neo alleles in inducing ectopic neurite growth. We also observed balancing between neo and anti alleles, whose respective MT-hyperstablizing and -destabilizing effects neutralized each other.

A slide taped to a window at the Woods Hole Marine Biology Laboratory was my first introduction to the touch receptor neurons of the nematode Caenorhabditis elegans. Studying these cells as a postdoc with Sydney Brenner gave me a chance to work with John Sulston on a fascinating set of neurons. I would never have guessed then that 43 years later I would still be excited about learning their secrets.

Molecular chaperones often work collaboratively with the ubiquitination-proteasome system (UPS) to facilitate the degradation of misfolded proteins, which typically safeguards cellular differentiation and protects cells from stress. In this study, however, we report that the Hsp70/Hsp90 chaperone machinery and an F-box protein, MEC-15, have opposing effects on neuronal differentiation and that the chaperones negatively regulate neuronal morphogenesis and functions. Using the touch receptor neurons (TRNs) of Caenorhabditis elegans, we find that mec-15(-) mutants display defects in microtubule formation, neurite growth, synaptic development, and neuronal functions, and these defects can be rescued by the loss of Hsp70/Hsp90 chaperones and cochaperones. MEC-15 likely functions in a SCF complex to degrade DLK-1, which is an Hsp90 client protein stabilized by the chaperones. The abundance of DLK-1, and likely other Hsp90 substrates, is fine-tuned by the antagonism between MEC-15 and chaperones; this antagonism regulates TRN development as well as synaptic functions of GABAergic motor neurons. Therefore, a balance between UPS and chaperones tightly controls neuronal differentiation.

Molecular chaperones often work collaboratively with the ubiquitination-proteasome system (UPS) to facilitate the degradation of misfolded proteins, which typically safeguards cellular differentiation and protects cells from stress. In this study, however, we report that the Hsp70/Hsp90 chaperone machinery and an F-box protein, MEC-15, have opposing effects on neuronal differentiation and that the chaperones negatively regulate neuronal morphogenesis and functions. Using the touch receptor neurons (TRNs) of Caenorhabditis elegans , we find that mec-15(−) mutants display defects in microtubule formation, neurite growth, synaptic development, and neuronal functions, and these defects can be rescued by the loss of Hsp70/Hsp90 chaperones and cochaperones. MEC-15 likely functions in a SCF complex to degrade DLK-1, which is an Hsp90 client protein stabilized by the chaperones. The abundance of DLK-1, and likely other Hsp90 substrates, is fine-tuned by the antagonism between MEC-15 and chaperones; this antagonism regulates TRN development as well as synaptic functions of GABAergic motor neurons. Therefore, a balance between UPS and chaperones tightly controls neuronal differentiation. Summary statement: Molecular chaperones are known to protect cells from stress. However, in this study the authors showed that the Hsp90 chaperone negatively regulates neuronal differentiation when the ubiquitination-proteasome system is compromised.