Disease relevance of SLC24A2

• PURPOSE: To study the possible involvement of the rod (SLC24A1) and cone (SLC24A2) Na-Ca+K exchanger (NCKX) genes in retinal diseases [1].

High impact information on SLC24A2

• These findings are the first report of a regulatory kinetic state of Ca(2+) transport via NCKX2 Na(+)/Ca(2+)-K(+) exchangers that may play a prominent role in regulation of Ca(2+) extrusion in cellular environments such as neuronal synapses that experience frequent and dynamic Ca(2+) fluxes [2].
• With the use of the ionophore gramicidin to control alkali cation concentrations across the plasma membrane, application of high intracellular Na(+) promoted large NCKX2-mediated increases in intracellular free Ca(2+) in the 15-20 mum range; this also resulted in inactivation of NCKX2 transport, the first description of this novel kinetic state [2].
• Na+-dependent Inactivation of the Retinal Cone/Brain Na+/Ca2+-K+ Exchanger NCKX2 [2].
• Surprisingly, there was no obvious loss of photoreceptor function in cones, where expression of the NCKX2 protein had been reported previously [3].
• NCKX1 is found in retinal rod photoreceptors, while NCKX2 is found in retinal cone photoreceptors and is also widely expressed in the brain [4].

Biological context of SLC24A2

• The single-strand conformation polymorphism (SSCP) technique and direct sequencing were used to screen the patients' DNA for mutations in SLC24A1 and SLC24A2 [1].
• METHODS: DNA was collected from unrelated patients with retinal disease, mainly from North America. A human genomic library was screened with the cone NCKX cDNA, and hybridizing clones were sequenced to determine the genomic organization of the SLC24A2 gene [1].
• Twenty-five residues were identified for which mutagenesis reduced NCKX function to <20% of wild-type cone NCKX2 activity, while protein expression and plasma membrane targeting were not affected [5].
• The Na(+)/Ca(2+)-K(+) exchanger plays a critical role in Ca(2+) homeostasis in retinal rod (NCKX1) and cone (NCKX2) photoreceptors by extruding Ca(2+) that enters rod and cone cells via the cGMP-gated channels [6].
• The Na+/Ca2+ exchange activities of NCKX2 and NCX1 are considered to be regulated differentially via the respective binding site domains that have distinct sensitivities to the external monovalent cations [7].

Anatomical context of SLC24A2

• The NCKX1 isoform is found in rods, while the NCKX2 isoform is found in cones, in retinal ganglion cells, and in various parts of the brain [8].
• Sensitivities of the reverse-mode Na+/Ca2+ exchange activity measured as the Na+i-dependent Ca2+ uptake to extracellular monovalent cations K+, Li+, and Na+ were compared between the K+ -dependent (NCKX2) and the K+ -independent Na+/Ca2+ exchanger (NCX1) overexpressed in a fibroblast cell [7].

Associations of SLC24A2 with chemical compounds

• Charge-removing replacement of Asp(575) by either asparagine or cysteine rendered the mutant NCKX2 proteins independent of K(+), whereas the charge-conservative substitution of Asp(575) to glutamate resulted in a nonfunctional mutant NCKX2 protein, accentuating the critical nature of this residue [4].
• Here we developed a simple fluorometric method to analyze changes in K(+) and Ca(2+) dependences of mutant NCKX2 proteins in which candidate residues within membrane-spanning domains were substituted [9].

Enzymatic interactions of SLC24A2

• In this study we have placed a peptide tag at different positions of the NCKX sequence to examine whether the putative signal sequence is indeed cleaved in either NCKX1 or NCKX2 proteins expressed in heterologous systems [10].

Other interactions of SLC24A2

• Cysteine residues were also not critical for the formation of NCKX2 homo-oligmers or NCKX2 hetero-oligomers with the CNGA subunit of the cGMP-gated channel [6].
• NCKX2 is expressed in the brain, in retinal cone photoreceptors and in retinal ganglion cells, while NCKX3 and NCKX4 show a broader expression pattern [11].

Analytical, diagnostic and therapeutic context of SLC24A2

• In this study we have examined the roles of endogenous cysteine residues in the rat brain K(+)-dependent Na(+)/Ca(2+) exchanger protein, NCKX2, by site-directed mutagenesis [12].

References