Structural Characteristics and Proton Conductivity of the Gel Within the Electrosensory Organs of Cartilaginous Fishes

The Ampullae of Lorenzini (AoL) of cartilaginous fishes are sensory organs used to detect environmental electric fields. The proximal ends of the organs are externally visible as pores in the skin that lead into gel-filled tubular canals which terminate in rounded chambers filled with highly specialized electrosensory cells. The viscoelastic gel that fills the organs is composed of proteins and polysaccharides that are not yet completely characterized but are thought to play a critical role in the electrosensing mechanism. Although recent studies have identified various components of AoL gel, it has remained unclear how the component molecules are structurally arranged and how their structure influences the overall function of the AoL. Here we present the first microscopic descriptions and x-ray scattering data from AoL gel extracted from spotted ratfish (Hydrolagus colliei). Our results suggest that AoL gel is colloidal in nature and composed of spherical globules that are approximately 10-100 nm in size. We investigated the structural influence of the protein components of the gel specifically by analyzing gel that had been digested in situ via enzymatic proteolysis. By comparing gel before and after digestion using microscopy, x-ray scattering analyses, and proton conductivity measurements, we directly observed the structural and functional influence of proteins in AoL gel. The findings described here represent the first detailed structural analysis of AoL gel and lay the groundwork for more detailed studies into the specific interactions of molecules inside AoL gel at the nanoscale, with particular reference to their mechanistic role in electrosensing.