G Protein signalling in C. elegans

One of the most widely used signal transduction pathways in eukaryotic cells employs serpentine receptors and heterotrimeric G proteins. We use the nematode Caenorhabditis elegans to analyse G protein mediated signal transduction in vivo. We focus our research on G protein signalling in smell and taste and sensory neuron development.

G protein signal transduction
Signalling through serpentine receptors and heterotrimeric G proteins is one of the main means of transducing extracellular signals in the cell. G protein signalling has been the subject of intense research over the last 2 decades. This has yielded extensive biochemical knowledge about some of the players of these cascades. Binding of a ligand to a specific G protein coupled receptor (GPCR) results in the activation of a G protein complex. In the inactive state the G protein complex consists of a Ga (GDP-bound), Gb and g subunit. Upon activation GDP is exchanged for GTP, which results in dissociation of the GTP bound a subunit and the bg dimer. Both entities can activate effector molecules, such as adenylate or guanylate cyclases, cGMP phosphodiesterases, phospholipase C, phosphoinositide 3-kinases, ion channels selective for K⁺, Ca²⁺ and Na⁺, tyrosine kinases and MAP kinases. Ga subunits have an intrinsic GTPase activity, which in time results in hydrolysis of the GTP to GDP and reassociation of the inactive heterotrimeric complex. Accessory proteins, termed regulators of G protein signalling (RGS) proteins, regulate this GTPase activity and the association of the a- with the bg-subunits. Despite the extensive characterisation of these signalling cascades, many questions remain, such as how different signals are integrated, segregated and insulated from another within a cell.

The nematode C. elegans
Previous studies have made it clear that many molecular and cellular processes are often conserved between humans and simple organisms such as worms. This enables us to use genetic analyses in the nematode Caenorhabditis elegans to analyse the fundamental mechanisms of G protein signal transduction. C. elegans has been a very successful model system for the in vivo analysis of many processes. Despite its small body size, simple body plan and short life cycle C. elegans still shows many behaviours that make life interesting, such as directed movements, feeding, responses to various environmental cues and sexual reproduction.

G proteins in C. elegans
The genome of C. elegans contains 21 Ga, 2 Gb and 2 Gg genes. Functionally, these proteins can be divided into 2 groups, G protein subunits that regulate muscle and neuron activity in general and sensory specific G proteins. We focus on the latter group of G proteins. C. elegans responds to a wide range of chemicals, including salts, amino acids, bitter and sweet compounds, water-soluble repellents and volatile chemicals. The nematode uses 11 bilateral symmetric pairs of chemosensory neurons, the amphid neurons, to detect compounds in its environment (see figure).

The function of each of these neurons has been determined. With just 11 pairs of chemosensory cells to detect at least 100 water-soluble attractants and repellents and at least 60 odorants, C. elegans seems significantly restricted in its possibilities to discriminate between chemical compounds. Still, the animal can discriminate between Na+ and Cl-, sensed by the ASE neurons, and it can discriminate between various odorants sensed by the AWC neurons. The functional analysis of all G protein genes in C. elegans showed that each amphid neuron expresses multiple Ga subunits (e.g. gpa-13::GFP expression in the amphid neurons ADF, ASH and AWC and in the phasmid sensory neurons, there is aspecific fluorescence in the gut), suggesting extensive modulation of signalling. The analysis of all olfactory Ga subunits confirmed this: We found that olfactory signalling is mediated by 1 main signal (ODR-3), which is modulated by 2 to 5 Ga subunits, with either stimulatory or inhibitory functions. These results have shown that sensory signalling in C. elegans offers an excellent model system to analyse G protein signal transduction and, particularly, determine how several different signalling pathways can function within one cell, enabling cross-talk and modulation while preserving specificity.