Identifying molecular mechanisms of responding to gravity using the nematode C. elegans

ABSTRACT Impaired ability to respond to the earth’s gravitational field and loss of balance results in high risk of falls with adverse consequences for one’s health, quality of life, and health care costs to society. Falls are a particular risk for individuals with Parkinson’s disease as well as elderly humans in general. Despite the importance of gravity sensing to daily activities, the molecular, cellular, and circuit gravity sensing mechanisms are incompletely understood. To understand gravity sensing mechanisms at the molecular and neural circuit level, we use the simple microscopic nematode Caenorhabditis elegans. C. elegans offers many experimental advantages, including a small and simple nervous system, accessibility to rapid genetic manipulation and to optogenetic tools, and ease of cultivation. We have demonstrated that C. elegans senses and responds to gravity, and have used mutant analysis and pharmacology to show that one or more types of ciliated sensory neurons, as well as dopamine, play a role in gravitytaxis. Our preliminary data leads to the hypothesis, which we will test, that one or more of the three ciliated dopaminergic neurons types—CEP, ADE, and/or PDE— mediates the response to gravity. We will use genetically-encoded synaptic transmission toxins as well as genetically-encoded chemogenetic experiments to determine which of these 3 neuron types is required for the response to gravity. We will identify additional neurotransmitters required for gravitaxis by testing animals mutant for each of 4 neurotransmitter systems. We will test each of 58 C. elegans genes predicted to encode mechanosensitive ion channels for a role in gravitaxis. Finally, we will make use of the million mutation project to perform a pilot unbiased genetic screen for gravitaxis-defective mutants. In addition, we will develop a high throughput sorter to isolate gravitaxis – deficient mutants for a future high throughput genetic screens for additional gravitaxis defective mutants. At the completion of these studies, we will have identified one or more dopaminergic neuron type as well as other neurotransmitters required for gravitaxis and we will have developed the tools needed for further studies consistent with the objectives of this R21 exploratory/development funding mechanism. Our work will contribute to a comprehensive molecular understanding of animals’ response to gravity and set the stage for the development of novel therapeutics to reduce falls.