Mark Brown

Reproduction in female insects encompasses a highly regulated sequence of behavioral, metabolic, and synthetic processes that result in the production of eggs.  As in all other animals, peptide hormones provide precise regulation of physiological and metabolic processes during reproduction.  The primary objective of my research program is to characterize the structure and function of peptide hormones and their receptors involved in the regulation of key reproductive processes in two mosquito species: the yellow fever mosquito, Aedes aegypti, and the African malaria mosquito, Anopheles gambiae.

Our research contributes to two concepts shared by insect and vertebrate endocrinology. First, peptide hormones, as chemical messengers, are conserved to a high degree both in structure and function across the phyla of multicellular animals. Second, the nervous and digestive systems of animals use these messengers to coordinate metabolism and homeostasis, so that development and reproduction can occur. The elucidation of key regulatory pathways in mosquitoes can lead to stable and functional peptide mimics or to genetic transformation that may offer a new way to control their development or block pathogen transmission. Mosquitoes are exceptional model systems for this research because each successive cycle of egg maturation begins with a blood meal and ends with egg deposition two to three days later.  Blood provides females with nutrients for egg maturation and metabolic storage, thus enabling survival to initiate another cycle.  Understanding the regulation of reproduction in mosquitoes will give us insight into how pathogens, such as malaria and arboviruses, that are ingested in a blood meal from an infected host, can develop or multiply in the female’s body and be transmitted to a different host, days later.

We are also interested in understanding how the endocrine system responds to different nutrient states in mosquito larvae and how that response affects development and reproduction. Results from this work may provide insights into better ways to control mosquito populations in the field.

Current Research Interests

Insulin-Like Peptides (ILPs)
In vertebrates, insulin and related peptides are important growth factors and multifunctional hormones. This family of peptide hormones is structurally conserved across higher invertebrates and insect species. Up to eight ILPs are encoded in the genome of different mosquito species, which leads to the question, why are so many ILPs present in a mosquito? The objective of our collaborative research with Mike Strand is to define the expression, function, and signaling of specific ILPs in mosquito females.

A related project collaborative project with Mike Riehle and Shirley Luckhart investigates how insulin signaling affects mosquito longevity and immunity. Malaria parasites must develop for up to two weeks in the mosquito, and conceptually, this development can be disrupted by enhancing mosquito innate immunity or by shortening the mosquito’s lifespan. Our work shows that exogenous insulin in the blood meal not only modulates lifespan and oxidative stress response in female mosquitoes, but also Plasmodium development. We are characterizing the effects of exogenous human insulin and insulin-growth factors on key processes related to aging, innate immunity, and signaling in the mosquito Anopheles stephensi for comparison to transgenic mosquitoes expressing active proteins involved in insulin signaling.

Ovary Ecdysteroidogenic Hormone (OEH)
This neurohormone is the functional equivalent of follicle-stimulating hormone and luteinizing hormone in vertebrates. OEH was isolated from mosquito head extracts based on its indirect activation of egg maturation in blood-fed decapitated females and direct stimulation of mosquito ovaries to secrete ecdysteroid hormones. In turn, these hormones stimulate the production of yolk proteins, which are stored in mature eggs and used during embryonic development. The receptor for OEH and its mode of action are unknown. We are investigating this aspect of OEH endocrinology and how it and the ILPs coordinately regulate reproduction and other physiological processes in female mosquitoes.

Other Peptide Hormones of Interest

Head Peptide
This is the first neuropeptide to be isolated from mosquitoes, and it is a member of the extensive “RFamide” family of animal neuropeptides.   This peptide inhibits the host-seeking behavior of female Ae. aegypti, but its receptor and mode of action have yet to be identified.

Neuropeptide F (NPF)
Many peptide hormones are extensively distributed in both the brain and gut of vertebrates and coordinate appetite, digestion, and many other processes.  Neuropeptide Y and pancreatic peptide are good examples, and the related NPFs are known for many invertebrates.  We were the first to isolate authentic NPFs from different insect groups and to characterize the NPF receptor and its signaling.  In the fruit fly, NPF is an important regulator of feeding behavior.  In all insects, it likely has other functions, especially in the gut, but for now, NPF has no known function in mosquitoes and other insects.

Short neuropeptide F (sNPF)
Genes encoding sNPFs are expressed throughout the nervous system of mosquitoes, and multiple peptides are processed from the propeptide.  Although their cognate receptor was identified, nothing is known about the function of sNPF in mosquitoes.

Adipokinetic hormone (AKH)
This family of peptide hormones is well characterized for insects, but until recently, nothing was known about the distribution and function of AKH in mosquitoes.  There are two AKHs in mosquitoes, and we showed that the short AKH in fact mobilizes carbohydrate (glycogen) stores and not lipid stores.  Thus, it is a “hypertrehalosemic” hormone – an action similar to that of glucagon and opposite that of insulin in vertebrates.  The coordination of metabolism by this peptide and ILPs in sugar and blood-fed mosquitoes is yet to be explored.
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