Why the Connections Between Cells Could be New Targets for Managing Insect Pests
By John P. Roche, Ph.D.
Gap junctions are channels within organisms linking two adjoining cells. They control a wide array of biological functions, from electrical conduction, chemical conduction, and transport of small molecules to intercellular communication and detoxification. Because of their importance to insect survival, biologists are exploring gap junctions as potential targets for new insecticides to control insects that are agricultural pests and insects that spread disease.
Researchers at CSK Himachal Pradesh Agricultural University in Palampur, India, and Clemson University in South Carolina have provided a comprehensive review of the use of gap junctions for insect control in an article published in October in Annals of the Entomological Society of America. (The article is also included in a special collection of invited reviews in Annals of the ESA.)
With continued use of individual insecticides in pest control, insects often evolve insecticide resistance, creating a pressing need for new insecticides with different mechanisms of action. Gap junctions provide one such potential avenue to circumvent insecticide resistance. “Gap junctions are highly important in all animals for nervous system activity and virtually all other multicellular physiological processes,” says Jabez Raju Battu, a Ph.D. student in biological sciences at Clemson and one of the co-authors of the research review. “But the molecules that form gap junctions in insects are evolutionarily unrelated to those in vertebrate animals, including humans and livestock. This opens the potential to target these molecules in insects.”
Gap junctions interconnect the cytoplasm of adjacent cells. In vertebrates, gap junctions are made by proteins called connexins, while in invertebrates gap junctions are made by proteins called innexins. In an individual insect cell, a structure called an innexon is constructed in the cell membrane consisting of a channel of eight innexin proteins. When the innexons of two adjacent cells line up, they form a gap junction linking the two cells.
Research has shown that compounds that target innexins show potential as insecticides for mosquitoes. RNA interference (RNAi) is a technique in which double-stranded RNA that is complementary to messenger RNA (mRNA) is introduced to an organism. RNAi can be used to disable specific genes, and when RNAi of innexins was administered in yellow fever mosquitoes (Aedes aegypti), the mosquitoes’ survival was decreased. Controlling yellow fever mosquitoes is critical because they spread viruses such as dengue, yellow fever, Zika, and chikungunya.
Other research has shown that compounds such as carbenoxolone and meclofenamic acid that are inhibitors of gap junctions can successfully inhibit innexins in yellow fever mosquitoes.
Gap junctions are essential to the functioning of numerous organ systems, including the nervous, circulatory, gastrointestinal, reproductive, and immune systems. In terms of immunity, cells called hemocytes play a crucial role in insect innate immunity by engulfing bacteria with phagocytosis and neutralizing multicellular parasites by encapsulating them. Hemocytes form gap junctions when they perform encapsulation of parasites, so impairment of gap junctions may be an avenue to impair immunity in insect pests, which would be one potential route to create novel insecticides.
Other recent studies have examined how parasitoid wasps use viruses to increase their reproductive success. These viruses cause expression of proteins similar to innexins in insect hosts, and these proteins, dubbed vinnexins, disrupt the physiology of the hosts, including their immune systems. This allows the wasps to be successful in parasitizing their hosts. These findings suggest that vinnexins are a potential avenue for developing insecticides that target insect gap junctions.
Gap junctions also play a role in detoxification of cells and tissues. This function provides a potential target for insecticides, but research on this aspect of insect gap junctions is just beginning.
The challenge facing targeting gap junctions for pest management is currently a lack of knowledge. “More basic studies are needed in a variety of insects to determine the degree of diversity in the gap junction molecules,” Battu says. “Little work has been performed outside of Drosophila and a few other model systems. We need to understand better what gap junctions are formed from, where they are, what they are doing, and how to modify them.”
Targeting gap junctions offers clear promise as a mechanism of developing new insecticides, but the possibilities are just beginning to be studied. In terms of future research directions, Battu says, “We have numerous gap junction inhibitors, but nothing that is specific to insect innexins is known so far. A good start in this direction is to screen for molecules that selectively inhibit insect innexins.” With widespread future investigation, this chemical-control mechanism may provide a valuable management tool.
Annals of the Entomological Society of America
John P. Roche, Ph.D., is an author, biologist, and science writer with a Ph.D. in the biological sciences and a dedication to making rigorous science clear and accessible. He writes books and articles, and provides writing, editing, and curriculum development for universities, scientific societies, and publishers. Professional experience includes serving as a scientist and scientific writer at Indiana University, Boston College, and the University of Massachusetts Medical School, and as an editor-in-chief of science periodicals at Indiana University and Boston College.