Abstract
Scorpions are among the oldest terrestrial arthropods and they have passed through small morphological changes during their evolutionary history on land. They are efficient predators capable of capturing and consuming large preys and due to envenomation these animals can become a human health challenge. Understanding the physiology of scorpions can not only lead to evolutionary insights but also is a crucial step in the development of control strategies. However, the digestive process in scorpions has been scarcely studied. In this work, we describe the combinatory use of next generation sequencing, proteomic analysis and biochemical assays in order to investigate the digestive process in the yellow scorpion Tityus serrulatus, mainly focusing in the initial protein digestion. The transcriptome generated database allowed the quantitative identification by mass spectrometry of different enzymes and proteins involved in digestion. All the results suggested that cysteine cathepsins play an important role in protein digestion. Two digestive cysteine cathepsins were isolated and characterized presenting acidic characteristics (pH optima and stability), zymogen conversion to the mature form after acidic activation and a cross-class inhibition by pepstatin. A more elucidative picture of the molecular mechanism of digestion in a scorpion was proposed based on our results from Tityus serrulatus. The midgut and midgut glands (MMG) are composed by secretory and digestive cells. In fasting animals, the secretory granules are ready for the next predation event, containing enzymes needed for alkaline extra-oral digestion which will compose the digestive fluid, such as trypsins, astacins and chitinase. The digestive vacuoles are filled with an acidic proteolytic cocktail to the intracellular digestion composed by cathepsins L, B, F, D and legumain. Other proteins as lipases, carbohydrases, ctenitoxins and a chitolectin with a perithrophin domain were also detected. Evolutionarily, a large gene duplication of cathepsin L occurred in Arachnida with the sequences from ticks being completely divergent from other arachnids probably due to the particular selective pressures over this group.
Highlights
Scorpions are ancient arthropods which have the oldest known fossil record among the living arachnids dating from the Silurian period 428 Ma [1]
We investigated the molecular physiology of digestion in the scorpion Tityus serrulatus by using a combination of transcriptomic, proteomic and enzymological approaches, mainly focusing on protein digestion
The enzymological assays allowed the inference of zymogens from cysteine peptidases activated under acidic conditions and that acidic initial protein digestion, which is mainly performed by cathepsin L, seems to be quantitatively more important in contrast to the alkaline one
Summary
Scorpions are ancient arthropods which have the oldest known fossil record among the living arachnids dating from the Silurian period 428 Ma [1] They are efficient predators presenting a varied diet (e.g., insects, spiders, solifugae, scorpions, isopods, gastropods, snakes, lizards, rodents) and it has been reported that scorpions can have their mass largely increased after one single meal [2] and survive up to one year starvation [3]. Digestive enzymes are released by the secretory cells in prosomal midgut, anterior intestine and its respective digestive glands to be regurgitated into the pre-oral cavity where the liquefaction of the chewed food starts. The predigested food is absorbed by pinocytosis and the intracellular digestion is performed inside the digestive cells from the midgut and midgut glands [4]
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