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
Mj. Hubbard, 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, European journal of biochemistry, 239(3), 1996, pp. 611-623
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.