Maturation stage ameloblasts of rodents express vacuolar type-H-ATPase in the ruffled border of their plasma membrane in contact
with forming dental enamel, similar to osteoclasts that resorb bone. It has been proposed that in ameloblasts this v-H-ATPase
acts as proton pump to acidify the enamel space, required to complete enamel mineralization. To examine whether this v-H-ATPase
in mouse ameloblasts is a proton pump, we determined whether these cells express the lysosomal, T-cell, immune regulator 1
(Tcirg1, v-H-Atp6v0a3), which is an essential part of the plasma membrane proton pump that is present in osteoclasts. Mutation
of this subunit in Tcirg1 null (or oc/oc) mice leads to severe osteopetrosis. No immunohistochemically detectable Tcirg1 was
seen in mouse maturation stage ameloblasts. Strong positive staining in secretory and maturation stage ameloblasts however
was found for another subunit of v-H-ATPase, subunit b, brain isoform (v-H-Atp6v1b2). Mouse osteoclasts and renal tubular
epithelium stained strongly for both Tcirg1 and v-H-Atp6v1b2. In Tcirg1 null mice osteoclasts and renal epithelium were negative
for Tcirg1 but remained positive for v-H-Atp6v1b2. The bone in these mutant mice was osteopetrotic, tooth eruption was inhibited
or delayed, and teeth were often morphologically disfigured. However, enamel formation in these mutant mice was normal, ameloblasts
structurally unaffected and the mineral content of enamel similar to that of wild type mice.
We concluded that Tcirg1,
which is essential for osteoclasts to pump protons into the bone, is not appreciably expressed in maturation stage mouse ameloblasts.
Our data suggest that the reported v-H-ATPase in maturation stage ameloblasts is not the typical osteoclast-type plasma membrane
associated proton pump which acidifies the extracellular space, but rather a v-H-ATPase potentially involved in intracellular