Evidence of Chitin in the Ampullae of Lorenzini of Chondrichthyan Fishes

ABSTRACTChitin is synthesized by a variety of organisms using enzymes called chitin synthases and was recently discovered in a number of aquatic vertebrates. In our ongoing investigations into the presence of vertebrate chitin, we unexpectedly found evidence of the polysaccharide within the electrosensory organs, known as Ampullae of Lorenzini, of diverse chondrichthyan fishes. Experiments with histochemical reagents, chemical analyses, and enzymatic digestions suggested that chitin is a component of the hydrogel filling the structures. Further, in situ hybridization with a sequence from the little skate (Leucoraja erinacea) revealed that chitin synthase expression is localized to cells inside the organs. Collectively, these findings suggest that chondrichthyan fishes endogenously synthesize chitin and beg further investigation into the function of chitin in the electrosensory system.

Proton Conductivity of Glycosaminoglycans

Proton (H+) conductivity is important in many natural phenomena including oxidative phosphorylation in mitochondria and archea, uncoupling membrane potentials by the antibiotic Gramicidin, and proton actuated bioluminescence in dinoflagellate. In all of these phenomena, the conduction of H+ occurs along chains of hydrogen bonds between water and hydrophilic residues. These chains of hydrogen bonds are also present in many hydrated biopolymers and macromolecule including collagen, keratin, chitosan, and various proteins such as reflectin. All of these materials are also proton conductors. Recently, our group has discovered that the jelly found in the Ampullae of Lorenzini-shark’s electrosensing organs- is the highest naturally occurring proton conducting substance. The jelly has a complex composition, but we attributed the conductivity to the glycosaminoglycan keratan sulfate (KS). Here, we have measured the proton conductivity of hydrated keratan sulfate using PdHx contacts to be 0.50 ± 0.11 mS cm -1- consistent to that of Ampullae of Lorenzini jelly, 2 ± 1 mS cm -1. Proton conductivity, albeit with lower values, is also shared by other glycosaminoglycans with similar chemical structures including dermatan sulfate, chondroitin sulfate A, heparan sulfate, and hyaluronic acid. This observation confirms the structure property relationship between proton conductivity and the chemical structure of biopolymers.

Proton conductivity in ampullae of Lorenzini jelly

In 1678, Stefano Lorenzini first described a network of organs of unknown function in the torpedo ray—the ampullae of Lorenzini (AoL). An individual ampulla consists of a pore on the skin that is open to the environment, a canal containing a jelly and leading to an alveolus with a series of electrosensing cells. The role of the AoL remained a mystery for almost 300 years until research demonstrated that skates, sharks, and rays detect very weak electric fields produced by a potential prey. The AoL jelly likely contributes to this electrosensing function, yet the exact details of this contribution remain unclear. We measure the proton conductivity of the AoL jelly extracted from skates and sharks. The room-temperature proton conductivity of the AoL jelly is very high at 2 ± 1 mS/cm. This conductivity is only 40-fold lower than the current state-of-the-art proton-conducting polymer Nafion, and it is the highest reported for a biological material so far. We suggest that keratan sulfate, identified previously in the AoL jelly and confirmed here, may contribute to the high proton conductivity of the AoL jelly with its sulfate groups—acid groups and proton donors. We hope that the observed high proton conductivity of the AoL jelly may contribute to future studies of the AoL function.