Molecular mechanism of the transport and reduction pathway of vanadium in ascidians

Metal ions are required for physiologically essential functions such as metalloenzymatic reactions, redox reactions, electron transfer, regulation of transcription factor activity, and respiration in living cells. Generally, metal ions are homeostatically maintained at very low concentrations in the sub-micromolar to micromolar range in living cells. However, some organisms, called hyperaccumulators, collect extremely high levels of metal ions, thereby providing experimental systems in which to study the mechanisms underlying the selective accumulation of metal ions. Typical of such organisms are the ascidians, more commonly known as sea squirts or tunicates. They are sessile marine animals belonging to the chordates. Several species of ascidians are known to accumulate extremely high levels of vanadium ions in their blood cells. Vanadium is usually in the VV state in the natural environment, but in ascidians, most of vanadium is reduced to VIII via VIV during the assimilation process. In this review, we first summarize the history of the studies on vanadium accumulation in ascidians and then focus on the recent progress of molecular studies, especially on the transport and reduction of vanadium, using primarily two ascidian species, Ascidia sydneiensis samea and Ciona intestinalis. Several candidate genes for VIV transporters and enzymes catalyzing the redox reactions of VV/VIV are addressed in detail. The function of accumulated VIII is discussed in relation to redox reactions.