An osteoclast is a large cell that is characterized by multiple nuclei and a cytoplasm with a homogeneous, "foamy" appearance. This appearance is due to a high concentration of vesicles and vacuoles. At a site of active bone resorption, the osteoclast forms a specialized cell membrane, the "ruffled border", which touches the surface of the bone tissue. The ruffled border, which facilitates removal of the bony matrix, is a morphologic characteristic of an osteoclast that is actively resorbing bone. The ruffled border increases surface area interface for bone resorption. The mineral portion of the matrix (called hydroxyapatite) includes calcium and phosphate ions. These ions are absorbed into small vesicles (see endocytosis) which move across the cell and eventually are released into the extracellular fluid, thus increasing levels of the ions in the blood.
M-CSF acts through its receptor on the osteoclast, c-fms (colony stimulating factor 1 receptor), a transmembrane tyrosine kinase-receptor, leading to secondary messenger activation of tyrosine kinase Src. Both of these molecules are necessary for osteoclastogenesis and are widely involved in the differentiation of monocyte/macrophage derived cells.
RANKL is a member of the tumour necrosis family (TNF), and is essential in osteoclastogenesis. RANKL knockout mice exhibit a phenotype of osteopetrosis and defects of tooth eruption, along with an absence or deficiency of osteoclasts. RANKL activates NF-κβ (nuclear factor-κβ) and NFATc1 (nuclear factor of activated t cells, cytoplasmic, calcineurin-dependent 1) through RANK. NF-κβ activation is stimulated almost immediately after RANKL-RANK interaction occurs, and is not upregulated. NFATc1 stimulation, however, begins ~24-48 hours after binding occurs and its expression has been shown to be RANKL dependent.
Osteoclast differentiation is inhibited by osteoprotegerin (OPG), which binds to RANKL thereby preventing interaction with RANK.
Cathepsin K has an optimal enzymatic activity in acidic conditions. It is synthesized as a proenzyme with a molecular weight of 37kDa, and upon activation by autocatalytic cleavage, is transformed into the mature, active form with a molecular weight of ~27kDa.
In the osteoclast, cathepsin K functions in the resorptive process. Upon polarization of the osteoclast over the site of resorption, cathepsin K is secreted from the ruffled border into the resorptive pit. Here, it is the major protease involved in the degradation of type I collagen and other noncollagenous proteins, which have been demineralized by the acidic environment of the resorptive pit. From the resorptive pit, cathepsin K transmigrates across the ruffled border, through the osteoclast via intercellular vesicles and is then released by the functional secretory domain. Within these intercellular vesicles, cathepsin K, along with ROS generation by TRAP further degrades bone resorption products.
Numerous other cathepsins are expressed in osteoclasts. These include cathepsin B, C, D, E, G, and L. The function of these cysteine and aspartic proteases is generally unknown within bone, and they are expressed at much lower levels that cathepsin K.
Studies on cathepsin L knockout mice have been mixed, with a report of reduced trabecular bone in homozygous and heterozygous cathepsin L knockout mice compared to wild-type and another report finding no skeletal abnormalities.
MMP-9 is associated with the bone microenvironment. It is expressed by osteoclasts, and is known to be required for osteoclast migration and is a powerful gelatinase. Transgenic mice lacking MMP-9 develop defects in bone development, intraosseous angiogenesis, and fracture repair.
MMP-13 is believed to be involved in bone resorption and in osteoclast differentiation, as knockout mice revealed decreased osteoclast numbers, osteopetrosis, and decreased bone resorption.
MMPs expressed by the osteoclast include MMP-9, -10, -12, and -14. apart from MMP-9, little is know about their relevance to the osteoclast, however, high levels of MMP-14 are found at the sealing zone.