Trailblazers: Investigating Chemotaxis with C.elegans
neurons are adapted to their function. Most CS neurons have many hair-like organs, called cilia, at one tip in order to maximize environmental contact and a high concentration of presynaptic connections at the other tip in order to send out multiple chemical messages. ODR-3, G PROTEINS, AND THE GENETICS BEHIND CHEMOSENSATION Compared to other organisms C.elegans has a small number of CS neurons and yet is able to detect a diversity of chemicals and respond in complex ways. This is because each neuron expresses a large and unique subset of signal transductions molecules. Signal transduction molecules enable the transmission of a molecular signal from a cell’s exterior to its interior and include transmembrane receptors, g proteins, ion channels, and regulators. Genomics studies suggest that the C.elegans genome contains over 1,500 genes that code for signal transduction molecules and other che- mosensory related proteins. In this experiment, you will examine one of these – the odr-3 gene. The odr-3 gene encodes part of a Guanine nucleotide-binding (G) protein (Figure 5). G proteins consist of three subunits (alpha, beta, and gamma) that together act as molecular switches within cells. When “off” the three subunits plus a GDP molecule are bound together. When “on” the
GDP is exchanged for a GTP molecule and the beta and gamma units separate from the alpha unit. An “on”, or active, G protein can trigger a cell to produce thousands of secondary messenger mol- ecules that in turn create a large and diverse cellular response. Eventually the GTP degrades into GDP, and this switches the G protein back to its “off” configuration. Expressions studies in C.elegans indicate that the protein ODR-3 is active in several CS neurons including AWC, AWA, AWB, ASH, and ADF. Worms with odr-3 muta-
Extracellular signals
Alpha
Beta
Alpha
Beta
GTP
Gamma
Gamma
GDP
Intercellular signals
Inactive G Protein
Active G Protein
Figure 5: G Proteins
tions show reduced responses to volatile and waters soluble odorants as well as dysfunctional osmotic and touch avoidance. In addition, the shape of certain neurons or neural cillia are mal- formed in worms with odr-3 mutations. In this experiment, you will use both volatile (olfactory) and water-soluble (gustatory) chemical cues to compare the responses of C.elegans mutants with a partial loss of function odr-3 gene and wild-type C.elegans . To do this you will use a two-quadrant chemotaxis assay. In this type of assay, a population of worms is introduced to the center of an agar plate that has been previously spiked with a test compound and a control compound at polar ends. (In some cases these compounds will be mixed with an undetectable anesthetic like Sodium Azide to immobilize individuals once they have navigated towards a side.) The response of each strain to the test chemical will be quantified by calculating the difference between how many worms move towards the attractant/ repellent versus the control.
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