Genome sequence may lead to better methods to target lice

June 21, 2010

Photo of a louse (CDC photo/Frank Collins)

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WEST LAFAYETTE, Ind. - A Purdue University researcher hopes a better understanding of the neurological system of the body louse through the mapping of its genome will lead to better control or elimination of the human parasite.

Catherine Hill, an associate professor of entomology, with postdoctoral researchers Jason M. Meyer and Janice Pagel VanZee, and former undergraduate student Emily Krause contributed to the overall genome-mapping effort led by the University of Illinois and published online Monday (June 21) in the Proceedings of the National Academy of Sciences. The body louse genome is the smallest known genome of any insect, said University of Illinois entomology professor Barry Pittendrigh, who led the drive to fund the project and coordinated the international team of scientists who analyzed the sequence.

Purdue researchers described G protein-coupled receptors, responsible for neurological signaling and vision of the body louse (Pediculus humanus humanus).

"Because they're a part of the nervous system function, these receptors make great targets for new insecticides," Hill said.

The body louse is a close relative of the head louse, better known because of its association with schoolchildren. Both are human parasites that spread through contact among hosts and feed on human blood. Hill believes information gleaned from the body louse genome will be useful for controlling head lice as well, which have begun to resist current insecticides.

While lice are more of an embarrassment and do not spread diseases in the United States, they have been transmitters of epidemic typhus, relapsing fever and trench fever elsewhere. Lice can have serious effects in less-developed nations and have been known to spread in places where there is overcrowding and lack of sanitation, such as areas that have experienced a natural disaster.

In terms of the G protein-coupled receptors, Hill said the louse genome is quite basic. The louse has only 104 of the receptors, the smallest number seen in any sequenced insect. None of those receptors seem to be unique to just the louse.

"We were really excited because the louse genome enabled us to identify this core set of receptors that are conserved in many other insects," Hill said. "This suggests they have been retained throughout millions of years of evolution in multiple insects - they must perform important functions in the insect nervous system."

The receptors act as a message relay system, sitting half in and half out of the cell's membrane. Incoming messages in the form of molecules or light attach to the receptor, causing the receptor to pass the message into the cell. The cell then carries out a function based on that message.

The receptors make ideal targets for insecticide development since they control nervous system function. Insecticides can attach to and block or overstimulate those receptors, disrupting the relay of neurological messages and killing the louse.

Purdue researchers also found that the body louse has only three receptors related to vision, compared with seven to 12 found in most insects. Hill said the louse doesn't need finely tuned senses to survive.

"We think this reflects their parasitic lifestyle and the fact that they live in close association with their human host," Hill said. "Because body lice live in human clothing or bedding, they do not need to travel very far to seek out a blood meal from their host or to find a site to lay their eggs."

Hill's laboratory will use the receptors to design new insecticides that are specific to controlling lice, have low risk to humans and the environment, and can be used where other products have become ineffective due to resistance. The National Institutes of Health funded the louse genome project.

Writer: Brian Wallheimer, 765-496-2050, bwallhei@purdue.edu

Source: Catherine Hill, 765-496-6157, hillca@purdue.edu

Note to Journalists: Go to http://delicious.com/bwallhei/Hill for more information on Catherine Hill's work and lice.

Ag Communications: (765) 494-2722;
Keith Robinson, robins89@purdue.edu
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ABSTRACT

Genome Sequences of the Human Body Louse and Its Primary Endosymbiont Provide Insights Into the Permanent Parasitic Lifestyle

Ewen F. Kirknessa, Brian J. Haas, Weilin Sun, Henk R. Braig, M. Alejandra Perotti, John M. Clark, Si Hyeock Lee, Hugh M. Robertson, Ryan C. Kennedy, Eran Elhaik, Daniel Gerlach, Evgenia V. Kriventseva, Christine G. Elsik, Dan Graur, Catherine A. Hill, Jan A. Veenstra, Brian Walenz, José Manuel C. Tubío,, José M. C. Ribeiro, Julio Rozas, J. Spencer Johnston, Justin T. Reese, Aleksandar Popadic, Yoshi Tomoyasu, Marta Tojo, Didier Raoult, David L. Reed, Emily Krause, Omprakash Mittapalli, Venu M. Margam, Hong-Mei Li, Jason M. Meyer, Reed M. Johnson, Jeanne Romero-Severson, Janice Pagel VanZee, David Alvarez-Ponce, Filipe G. Vieira, Montserrat Aguadé, Sara Guirao-Rico, Juan M. Anzola, Kyong S. Yoon, Joseph P. Strycharz, Maria F. Unger, Scott Christley, Neil F. Lobo, Manfredo J. Seufferheld, NaiKuan Wang, Gregory A. Dasch, Claudio J. Struchiner, Greg Madey, Linda I. Hannick, Shelby Bidwell, Vinita Joardar, Elisabet Caler, Renfu Shao, Stephen C. Barker, Stephen Cameron, Robert V. Bruggner, Allison Regier, Justin Johnson, Lakshmi Viswanathan, Terry R. Utterback, Granger G. Sutton, Daniel Lawson, Robert M. Waterhouse, J. Craig Venter, Robert L. Strausberg, May Berenbaum, Frank H. Collins, Evgeny M. Zdobnov, and Barry R. Pittendrigh

As an obligatory parasite of humans, the body louse (Pediculus humanus humanus) is an important vector for human diseases, including epidemic typhus, relapsing fever, and trench fever. Here, we present genome sequences of the body louse and its primary bacterial endosymbiont Candidatus Riesia pediculicola. The body louse has the smallest known insect genome, spanning 108Mb. Despite its status as an obligate parasite, it retains a remarkably complete basal insect repertoire of 10,773 protein-coding genes and 57 microRNAs. Representing hemimetabolous insects, the genome of the body louse thus provides a reference for studies of holometabolous insects. Compared with other insect genomes, the body louse genome contains significantly fewer genes associated with environmental sensing and response, including odorant and gustatory receptors and detoxifying enzymes. The unique architecture of the 18 minicircular mitochondrial chromosomes of the body louse may be linked to the loss of the gene encoding the mitochondrial single-stranded DNA binding protein. The genome of the obligatory louse endosymbiont Candidatus Riesia pediculicola encodes less than 600 genes on a short, linear chromosome and a circular plasmid. The plasmid harbors a unique arrangement of genes required for the synthesis of pantothenate, an essential vitamin deficient in the louse diet. The human body louse, its primary endosymbiont, and the bacterial pathogens that it vectors all possess genomes reduced in size compared with their free-living close relatives. Thus, the body louse genome project offers unique information and tools to use in advancing understanding of co-evolution among vectors, symbionts, and pathogens.