Citrate Transporter NaCT and Enamel Mineralization: The Slc13a5^R337* Mouse Model

Solute Carrier Family 13 Member 5 (SLC13A5) encodes the sodium-dependent citrate cotransporter NaCT, which mediates citrate transport across cell membranes. Pathogenic variants in SLC13A5 cause developmental and epileptic encephalopathy 25 with amelogenesis imperfecta, DEE25; OMIM #615905, a debilitating autosomal recessive disorder. To better define the role of NaCT in ameloblast function and enamel mineralization, we used CRISPR/Cas9 genome editing to generate Slc13a5^R337* knock-in mice that terminate NaCT translation at the Arg337 codon, which is homologous to the human SLC13A5^R333* variant associated with DEE25. We compared enamel phenotypes among wild-type, Slc13a5^+/+; heterozygous, Slc13a5^+/R337*; and homozygous, Slc13a5^R337*/R337* mice using light microscopy, in situ hybridization, immunohistochemistry, backscattered scanning electron microscopy (bSEM); and focused ion beam-scanning electron microscopy (FIB-SEM) with quantitative imaging of organelles and matrix. Citrate bioassays were performed on serum, long bones, such as the femur and tibia, and developing mouse first molars, including enamel organ epithelium, mineralized tooth matrix, and pulp mesenchyme, to assess citrate levels during the presecretory, secretory, and maturation stages of enamel formation. In addition, first molars collected at postnatal days 0, 3, 5, and 12 were analyzed to characterize glycolytic and TCA cycle-related metabolic signatures. Homozygous Slc13a5^R337*/R337* mice exhibited severe defects during the secretory and maturation stages of amelogenesis. Most notably, Slc13a5^R337*/R337* ameloblasts failed to develop a Tomes' process, detached from the enamel matrix surface, and produced a thin, poorly mineralized crust on the dentin surface rather than organized enamel ribbons. Despite the absence of normal enamel deposition, ameloblasts initially appeared viable and did not become dysplastic until the late secretory stage. Cellular and subcellular analyses revealed increased secondary lysosomes and intracellular accumulation of enamel matrix proteins, consistent with impaired matrix processing or secretion. Citrate concentrations were elevated in serum and long bones at both 7 and 35 weeks of age. Citrate was elevated in secretory-stage Slc13a5^R337*/R337* molars at days 0 and 3, the enamel organ epithelium (including ameloblasts), the pulp mesenchyme (including odontoblasts), and mineralizing dentin and enamel matrices. These levels gradually declined at day 5 and into the enamel maturation stage (day 12). GC-MS-based analysis of central carbon metabolites revealed increased intracellular accumulation of citrate, malate, and pyruvate, suggesting altered energy metabolism and reduced metabolic efficiency in Slc13a5^R337*/R337* mice. Together, these findings indicate that loss of NaCT function in the ameloblasts causes citrate accumulation, which impairs hydroxyapatite formation. Consequently, only a thin, structurally defective mineral crust forms on the dentin surface, while mineral nodules develop ectopically within the maturation-stage enamel organ epithelium. We conclude that regulating citrate concentration is essential for proper appositional growth of enamel.