Analysis of the Na ⁺/Ca ²⁺ Exchanger Gene Family within the Phylum Nematoda

Abstract

Na ⁺/Ca ²⁺ exchangers are low affinity, high capacity transporters that rapidly transport calcium at the plasma membrane, mitochondrion, endoplasmic (and sarcoplasmic) reticulum, and the nucleus. Na ⁺/Ca ²⁺ exchangers are widely expressed in diverse cell types where they contribute homeostatic balance to calcium levels. In animals, Na ⁺/Ca ²⁺ exchangers are divided into three groups based upon stoichiometry: Na ⁺/Ca ²⁺ exchangers (NCX), Na ⁺/Ca ²⁺/K ⁺ exchangers (NCKX), and Ca ²⁺/Cation exchangers (CCX). In mammals there are three NCX genes, five NCKX genes and one CCX (NCLX) gene. The genome of the nematode Caenorhabditis elegans contains ten Na ⁺/Ca ²⁺ exchanger genes: three NCX; five CCX; and two NCKX genes. Here we set out to characterize structural and taxonomic specializations within the family of Na ⁺/Ca ²⁺ exchangers across the phylum Nematoda. In this analysis we identify Na ⁺/Ca ²⁺ exchanger genes from twelve species of nematodes and reconstruct their phylogenetic and evolutionary relationships. The most notable feature of the resulting phylogenies was the heterogeneous evolution observed within exchanger subtypes. Specifically, in the case of the CCX exchangers we did not detect members of this class in three Clade III nematodes. Within the Caenorhabditis and Pristionchus lineages we identify between three and five CCX representatives, whereas in other Clade V and also Clade IV nematode taxa we only observed a single CCX gene in each species, and in the Clade III nematode taxa that we sampled we identify NCX and NCKX encoding genes but no evidence of CCX representatives using our mining approach. We also provided re-annotation for predicted CCX gene structures from Heterorhabditis bacteriophora and Caenorhabditis japonica by RT-PCR and sequencing. Together, these findings reveal a complex picture of Na ⁺/Ca ²⁺ transporters in nematodes that suggest an incongruent evolutionary history of proteins that provide central control of calcium dynamics.