Erections and eyesight both have cyclic guanosine monophosphate (cGMP) in common. This chemical messenger helps intervene in a wide range of cellular responses such as phototransduction, the transformation of light energy into electrical potential in the eyes rod and cone cells, and triggering eclosion,the developmental program where insects progress on to their next stage of life. Many cellular signals, including nitric oxide (NO), boost cellular cGMP levels by activating enzymes called guanylyl cyclases.
David Morton has been interested in the role of cGMP in eclosion in the hawk moth since he and Jim Truman discovered that the behaviour was triggered by cGMP 20 years ago. Since then he has been interested in understanding the exact cellular pathways that control the insect's life change. In the current issue of the J. Exp. Biol., Morton and colleagues describe how they have identified two new classes of guanylyl cyclase enzymes in the tobacco hornworm Manduca sexta. They have discovered that one of these novel groups, MsGC-β3, may actually dampen the NO-dependent effects of its chemical brethren (p. 937).“Understanding more about the normal regulation of cGMP levels will undoubtedly have an impact,” Morton said.
There are two main guanylyl cyclases in cells, one found in membranes and the other in the cytoplasm. The soluble enzymes are activated by NO and are made up of two protein subunits; the α and the β subunits. However,MsGC-β3 differs from its soluble brethren; it is not activated by NO, and the MsGC-β3 protein functions as a homodimer without the need for additional subunits. However, Morton and Anderson suspected that an MsGC-β3 subunit might also be able to form a dimer with either of the classical NO-sensitive α and β subunits, which the investigators thought could lead to enzymatic activity.
The team used a combination of chromatography and molecular techniques to discover which guanylyl cyclase subunits can interact to produce a functional enzyme. They found that both MsGC-β3 subunits indeed bind to the otherα and β subunits as predicted, but surprisingly the new forms of the enzyme are inactive. This means that if MsGC-β3 is generated along with its brethren, “it will act as a dominant negative, reducing the effect of NO in the cells,” Morton explains.
There are counterparts of MsGC-β3 in mammals, fruitflies and nematodes. In insects, Morton suspects it may play a crucial role in eclosion. In mammals, he explains that the orthologous β subunit also does not require a partner for activity, with some circumstantial evidence that raised levels of this subunit reduces NO-stimulated cGMP levels in the kidney.“Hopefully, our findings will encourage other investigators to study this further,” Morton said.
Of course, these findings have to be confirmed in vivo, he adds,as opposed to simply being an in vitro phenomenon. What needs to be shown first is whether these NO-sensitive and NO-insensitive subunits are expressed together in the same cells. “If they are, it could point to a novel method of regulating cGMP levels in a cell,” Morton said.“There are many uncovered aspects of cGMP regulation left to be revealed.”
