Parasites have been called predators that consume their prey in portions of less than one.
Sea lice, as blood-feeding ectoparasites of fish, are no exception and because of this lifestyle, they are perceived by many mostly as a pest, vectors of disease, or simply a problem to be dealt with. But parasites might also provide novel insights into basal biology. Parasites can fulfill important roles in ecosystems and are - like the salmon louse - often beautifully adapted to their hosts by millions of years of co-evolution. Ecologists assume that almost all organisms are involved in a host-parasite relationship in one or the other role, or even both, but surprisingly little is known about how this intricate interplay works on a molecular level.
In our latest article, we asked the question what we can learn about the special way of feeding on blood, and there namely uptake of heme from a blood meal (Heggland et al. 2019). Heme is one of the most versatile bio-molecules and an essential iron-containing co-factor of many metalloproteins in almost all organisms. Even though heme is the most bioavailable source of iron for humans, how we exactly take it up from our food remains a mystery. The presence of a specific heme receptor in the intestine of mammals was proposed 40 years ago, but no clear candidate has emerged so far. Therefore, our new discovery might even have consequences for biomedical research in the future.
Using pathway analysis, we first discovered that the salmon louse is lacking almost all genes for the synthesis of heme, which makes it an ideal organism to search for genes involved in intestinal heme uptake. Then, by using genomics data in LiceBase, bioinformatics approaches and in silico modeling, we searched for suitable candidates to fill this role. Using RNA-sequencing data, we discovered, that a scavenger receptor was among the most highly expressed genes specific to the intestine, and our models predicted that it could be a heme binder. This gene and the encoded protein were then further characterized, among others, using heme-binding assays and RNA-interference mediated knock-down yielding 60% reduction in the parasite’s total heme content. We named the protein LsHSCARB (L. salmonis heme scavenger receptor class B-like).