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Reduced life span of the osteoclast in osteopetrotic (mi and midi) mice
M J Marshall1, J A Rees, N W Nisbet
1Charles Salt Research Centre, Robert Jones and Agnes Hunt Orthopaedic Hospital, Shropshire, England.
Abstract:
Osteoclasts were enumerated on the parietal bones of mice carrying combinations of alleles (mi, midi and +) at the microphthalmic locus. Homozygous mutants at this locus show varying degrees of osteopetrosis due to defective bone resorption. As the severity of the bone resorption defect increased across the genotypes, there was an increase in the total number of osteoclasts and also an increase in the proportion of uninucleate osteoclasts. In parietal bones incubated for 24 h with parathyroid hormone (PTH) there was no significant difference between osteopetrotic and normal bones in terms of the number of osteoclast nuclei that had taken part in fusion. When bones were incubated for 24 h in the absence of PTH the percentage of osteoclasts remaining was correlated inversely with the severity of the bone resorption defect. Thus, we suggest that the increased proportion of uninucleate osteoclasts in these mutants results from a shortened life span, reducing the time-dependent accumulation of nuclei. This implies a reduced efficiency of uninucleate osteoclasts over multinucleate ones and we speculate on the reason for this.
Insights
Osteopetrotic mice with defective bone resorption show more osteoclasts. Increased uninucleate osteoclasts suggest a shortened lifespan, impacting bone remodeling.
Area of Science:
- Skeletal Biology
- Cell Biology
- Genetics
Background:
- Osteopetrosis is a bone disorder characterized by defective bone resorption.
- The microphthalmic (mi) locus in mice is associated with osteopetrosis and impaired osteoclast function.
- Osteoclasts are multinucleated cells responsible for bone breakdown.
Purpose of the Study:
- To investigate the characteristics of osteoclasts in mice with varying degrees of osteopetrosis due to mutations at the microphthalmic locus.
- To determine the relationship between bone resorption defects and osteoclast number and morphology.
- To explore the effect of parathyroid hormone (PTH) on osteoclast fusion and survival in these mutants.
Main Methods:
- Enumeration of osteoclasts on parietal bones of mice with different microphthalmic locus genotypes (mi, midi, +).
- Incubation of parietal bones with and without parathyroid hormone (PTH) for 24 hours.
- Assessment of osteoclast nuclei fusion and percentage of remaining osteoclasts.
Main Results:
- Homozygous microphthalmic mutants exhibited varying degrees of osteopetrosis due to defective bone resorption.
- Increased severity of bone resorption defect correlated with a higher total osteoclast number and an increased proportion of uninucleate osteoclasts.
- No significant difference in osteoclast fusion with PTH, but reduced osteoclast survival without PTH in mutants correlated with resorption defect severity.
Conclusions:
- The increased proportion of uninucleate osteoclasts in osteopetrotic mutants likely results from a shortened osteoclast lifespan.
- A shortened lifespan reduces the time for nuclear accumulation through fusion, leading to a higher proportion of uninucleate cells.
- This suggests that uninucleate osteoclasts may be less efficient than multinucleate osteoclasts in bone resorption, contributing to osteopetrosis.
