Evolution of the cation chloride cotransporter family: ancient origins, gene losses, and subfunctionalization through duplication.

The cation chloride cotransporter (CCCs) family comprises of four subfamilies-K(+)-Cl(-) cotransporters (KCCs), Na(+)-K(+)-2Cl(-) cotransporters (NKCCs), and Na(+)-Cl(-) cotransporters (NCCs)-and possibly two additional members-CCC interacting protein (CIP1) and polyamine transporters (CCC9)-as well. Altogether, CCCs can play essential physiological roles in transepithelial ion reabsorption and secretion, cell volume regulation, and inhibitory neurotransmission and so are present across all domains of life. To gain insight into the evolution of this family, we performed a comprehensive phylogenetic analysis using publically available genomic information. Our results clearly support CIP1 as being a true CCC based on shared evolutionary history. By contrast, the status of CCC9 in this regard remains equivocal. We also reveal the existence of a single ancestral CCC gene present in Archaea, from which numerous duplication events at the base of archaeans and eukaryotes lead to the divergence and subsequent neofunctionalization of the paralogous CCC subfamilies. A diversity of ensuing gene-loss events resulted in the complex distribution of CCCs present across the different taxa. Importantly, the occurrence of KCCs in "basal" metazoan taxa like sponges would allow an early formation of fast hyperpolarizing neurotransmission in metazoans. Gene duplications within the CCC subfamilies in vertebrates (in particular, KCCs, NKCCs, and NCCs) lend further evidence to the 2R hypothesis of two rounds of genome duplication at the base of the vertebrate lineage, especially in concert with our syntenic cluster analyses. This increased number of KCCs, NKCCs, and NCCs isoforms facilitates their further, important subfunctionalization in the vertebrate lineage.