Extensive Clear Zone and Defective Ruffled Border Formation in Osteoclasts of Osteopetrotic (ia/ia) Rats: Implications for Secretory Function

Involvement of PLEKHM1 in osteoclastic vesicular transport and osteopetrosis in incisors absent rats and humans

“…In accordance with the confocal microscopy data, osteoclasts from both the patient and the youngest brother were larger and more flattened, and showed little evidence of resorptive activity ( Figure 5A). Transmission EM demonstrated that the cytoplasm of these nonresorbing osteoclasts contained numerous vacuoles with electrondense material (Figure 5B) that were identical in appearance to the vesicles found in the ia rat ( Figure 5C), which have previously been shown to contain TRAP enzyme by immunoelectron microscopy (15). These vesicles were rarely seen in resorbing osteoclasts, which instead contained abundant multivesicular bodies, structures previously associated with transcytosis of resorbed material by the cell (S. Nesbitt, unpublished observations).…”

“…The in vitro osteoclast phenotype we observed was very similar to that seen with other osteopetrotic mutations (54,55). An interesting feature of osteoclasts from the patient with the PLEKHM1 mutation was an increased accumulation of the enzyme TRAP (which is highly expressed in osteoclasts), an effect also seen in the ia rat (15). Moreover, transmission EM of the osteoclasts from the 2 siblings homozygous for the PLEKHM1 mutation demonstrated the presence of electron-dense vesicles that were identical to those seen in the ia rat, which have previously been shown to contain TRAP (15).…”

“…An interesting feature of osteoclasts from the patient with the PLEKHM1 mutation was an increased accumulation of the enzyme TRAP (which is highly expressed in osteoclasts), an effect also seen in the ia rat (15). Moreover, transmission EM of the osteoclasts from the 2 siblings homozygous for the PLEKHM1 mutation demonstrated the presence of electron-dense vesicles that were identical to those seen in the ia rat, which have previously been shown to contain TRAP (15). These vesicles were not seen in actively resorbing osteoclasts from the heterozygous brother, which instead contained many multivesicular bodies near the ruffled border.…”

“…One of the spontaneous mutations is the incisors absent (ia) rat, first described in 1941 (14), which exhibits a generalized skeletal sclerosis and delay of tooth eruption. Histologic analysis shows that ia mutants have 2 to 3 times more osteoclasts than do normal littermates and that these cells exhibit an extended clear zone, an actin-rich area that facilitates tight attachment to the extracellular matrix (15). Furthermore, ia osteoclasts lack ruffled borders but contain numerous small cytoplasmic vesicles.…”

“…Furthermore, ia osteoclasts lack ruffled borders but contain numerous small cytoplasmic vesicles. The absence of extracellular tartrate-resistant acid phosphatase (TRAP), concomitant with accumulation of the enzyme in numerous intracellular vesicles, strongly suggests a dysfunction of the secretory pathways (15). Although a spontaneous partial recovery of the osteopetrotic phenotype is observed 30 to 50 days after birth (16), the defect is more rapidly reversed by transplantation of hematopoietic stem cells from normal littermates in irradiated ia rats.…”