Regulatory Roles of Tonicity-responsive MicroRNAs in Osmoregulatory Functions of the gills of Japanese Eels.

Homeostatic regulation of fluid and electrolyte balance is a vital process for guiding many cellular functions. It is crucial for upholding osmotic stabilization of living cells. Perturbations of body-fluid tonicity can cause distress in biological activities of macromolecules, the functions of cells and organs and putting threats to the health of animals. This may lead to a diverse group of pathological consequences, from renal failure, mild neurological symptoms to death. In the past decades, molecular and physiological factors related to osmotic homeostasis have been thoroughly studied. However our understanding on the molecular mechanisms that is involved in the progress of osmolar disorders, and the identification of therapeutic targets to the related diseases has yet to be fully elucidated. Nonetheless recent advances in the understanding of small non-coding RNAs (microRNAs) on regulatory circuit in physiological and pathophysiological functions shed light on a novel concept of disease cure. Given the emerging roles of microRNAs in osmotic stress responses, it warrants deeper investigations into how tonicity-inducible microRNAs modulate membrane transport in native tissues under anisosmotic conditions.

The catadromous fish Japanese eels actively engage physiological responses to oppose osmotic perturbations to stabilize body osmolarity. Gills of these fishes possess great physiological and structural plasticity to adapt to large changes in external osmolality and to participate in ion uptake/excretion, which is critical for the re-establishment of fluid and electrolyte homeostasis. Therefore their fish gills provide an excellent model to study the role of microRNAs in direct osmotic stress responses. In the past 20 years, our laboratory has established both animal and primary gill cell culture models to investigate the interactions of hormones, tissue remodeling and molecular pathways in relation to osmoregulation of Japanese eels. Major knowledge gap exists in our understanding on how microRNAs are involved in osmotic perturbations and their functional roles to define cell specific functions in transepithelial transport. The study described here signifies a revolutionary step in the field as it aims to explore the importance of this fundamental and evolutionary conserved regulation. We target to identify tonicity-sensitive microRNAs and to define their expression and functions in the two specialized gill ion-transporting cells, pavement and mitochondria-rich cells. The results will open up a new approach for the investigation of fish osmoregulation and reveal how this fundamental process evolves in animals. The study will unravel a novel layer of regulatory network mechanism for cellular osmosignaling and osmotic homeostasis in animals.