Characterization of L-Glutamate Uptake in the freshwater sponge Ephydatia muelleri

Abstract

Animals use a variety of different means to integrate environmental cues and to coordinate responses to these cues. In sponges (phylum Porifera) this capacity for response is limited by the absence of nervous or endocrine systems. Despite this, sponges are capable of whole-body responses that require coordination of the action of multiple tissues and organs. The freshwater sponge Ephydatia muelleri undergoes an expansion-contraction “sneeze” response upon application of irritants or mechanical stimulation that flushes water out of its aquiferous system. Previous work has found that the amino acid L-glutamate specifically triggers this response at concentrations above 70 µM, and that incubation with GABA prevents the sneeze response from occurring. However, little is known regarding how L-glutamate passes into the body of E. muelleri. This thesis focused on characterizing the rate and functional characteristics of L-glutamate uptake by E. muelleri through use of radioisotope uptake. Uptake of L-glutamate by E. muelleri is specific at low L-glutamate concentrations with two putative transporters acting (Km = 8.77 µM, Jmax = 64.24 pmol L-Glu mg protein-1 min-1; Km = 2.87 µM, Jmax = 25.1 pmol L-Glu mg protein-1 min-1). L-glutamate uptake is reduced in the presence of D-glutamate and L-aspartate, but shows low or no dependence on sodium, calcium, or protons. Bioinformatic analysis identified two putative SLC1-like transporters and eight putative transporters resembling SLC17 transporters in the E. muelleri genome. These results support a case of uptake of an amino acid in a freshwater environment. The localization of uptake in the body of E. muelleri is currently unclear, but the rate of uptake suggests that the observed L-glutamate uptake does not directly contribute to triggering the sneeze response in E. muelleri.