ABUNDANT CALCIUM HOMEOSTASIS MACHINERY IN RAT DENTAL ENAMEL CELLS - UP-REGULATION OF CALCIUM STORE PROTEINS DURING ENAMEL MINERALIZATION IMPLICATES THE ENDOPLASMIC-RETICULUM IN CALCIUM TRANSCYTOSIS

Enamel cells handle large amounts of calcium, particularly during the developmental phase (termed maturation) when dental enamel is hypermin eralized. The extent of intracellular calcium burden, and the nature o f calcium homeostasis machinery used to accommodate it, are largely un known. Here, the calcium-binding capacity of enamel cell cytosol was f ound to increase during development, in parallel with the putative tra nscellular flux of calcium. At maturation, the abundance of calcium-bi nding proteins in enamel cells exceeded that in brain and other establ ished calcium-oriented tissues, which implies a large calcium burden. A search for likely cytosolic calcium transporters revealed only one h igh-affinity calcium-binding protein (12 kDa, distinguished from alpha -parvalbumin) that was up-regulated during maturation, but its low abu ndance (0.02 % of soluble protein) precluded a major calcium transport or cytoprotective role. Two low-affinity calcium-binding proteins up- regulated during maturation (by 1.8-fold and 2.1-fold, respectively) w ere identified as calreticulin and endoplasmin, both residents of the endoplasmic reticulum. Together, calreticulin and endoplasmin constitu ted an exceptionally high proportion (5%) of soluble protein during ma turation, which gives an inferred calcium capacity 67-fold higher than that of the principal cytosolic calcium-binding protein, 28-kDa calbi ndin. Evidence that endoplasmin expression varied inversely with serum calcium concentration, and that the inositol trisphosphate receptor a lso was maturation, supported the novel hypothesis that non-mitochondr ial calcium stores play a major role in transcellular calcium transpor t. In conclusion: (a) enamel cells contain a general high abundance of calcium homeostasis proteins, consistent with a heavy intracellular c alcium burden; (b) the expression pattern (phenotype) of calcium-bindi ng proteins varies with enamel cell function; (c) enamel cells appear to contain unusually large non-mitochondrial calcium stores; (d) contr ary to the prevailing view that calcium passes mainly through the cyto sol of calcium-transporting cells, the findings imply a route through the endoplasmic reticulum. This study gives novel information about ho w a highly calcium-oriented tissue avoids calcium toxicity, and provid es a new focus for investigations into the mechanisms of transcellular calcium transport.