Journal Facts

Journal
BoneKEy Knowledge Environment
ISSN
1533-4368
Volume
Year
2001

Article Facts

Title
Stressed out
DOI
10.1138/2001018
Author

Online Date
03/19/2001

Article Links

BoneKEy-Osteovision | Commentary

Stressed out


DOI:10.1138/2001018

A remarkable feature of the skeleton is its reactivity to a host of environmental cues, not the least of which are mechanical forces. There is no doubt that they play an integral role in mediating bone quantity and quality, as evidenced by the rapid bone loss induced by weightlessness or disuse. The sensing molecules that convey changes in mechanical forces to bone cells are not well delineated to date, but most certainly include cell surface integrins and their extracellular ligands. One of these ligands, osteopontin (OPN), is emerging as a major player in regulation of bone resorption, and as recently highlighted by Ishijima et al (), in bone resorption (and bone formation, although it will not be discussed here) instigated by changes in mechanical forces.

While not restricted to bone, the sialo/phosphoprotein, OPN (also described as 2ar, eta-1 and spp1), is highly enriched in bone matrix. The list of its potential functions range from matrix mineralization and immunity to tumor metastasis (). In a finding perhaps disappointing at first glance, transgenic mice null for OPN (OPN KO) were born overtly normal, albeit with slightly more dense bones than normal (). Hints of OPN's role in bone resorption were first brought to light when it was reported that OPN deficiency prevented the well documented bone loss that follows ovariectomy in mice (), and further solidified by the finding of resistance of OPN KO mice to bone resorption induced by mechanical unloading ().

While inhibition of induced bone resorption in OPN KO mice was a common feature of both studies, the results were somewhat puzzling. Osteoclast number was increased in the OPN KO compared to control, and did not change with ovariectomy, and yet the mice did not suffer from overly exuberant bone resorption. In the present study, markers of bone resorption were again not changed by mechanical unloading, but osteoclast formation was reduced compared to unloaded wild-type controls.

So the question is, how can OPN deficiency explain these two different scenarios? It is clear that osteopontin is intimately involved in bone resorption induced by hormonal and mechanical changes, but what are the potential mechanisms, and what do these varying results tell us about OPN metabolism?

To put the present results into context, it is worth examining results of previous studies in the OPN KO mice. More osteoclasts were formed by the OPN KO cells in vitro using a stromal cell-based system (), a result that was later confirmed histomorphometrically in vivo (). A possible explanation for the increased osteoclastogenesis may be related to the transient expression of CD44 by osteoclast progenitor cells. CD44 is known to bind not only hyaluronan, but also to osteopontin, binding that is reported to inhibit the generation of multi-nucleated osteoclast-like cells (). It may be that OPN occupancy of the CD44 receptor is a critical branch point as a precursor mononuclear cell proceeds towards its final mature state. Without OPN binding, commitment to osteoclastogenesis may occur. However, in the recent study, it was stated (data not presented) that osteoclast formation in vitro was equivalent between OPN KO cells and control cells taken from unloaded animals of both types, although osteoclast number was clearly decreased in vivo in the unloaded OPN KO mice. Interestingly, the authors note that in this study, they avoided the use of a stromal cell layer by using soluble RANKL and M-CSF. This is worthy of comment in that it suggests that the function of OPN in osteoclast formation may be upstream of the function of RANKL. Therefore, the branch point in osteoclast formation regulated by OPN (possibly by binding to CD44-bearing cells) may have been bypassed by the addition of soluble RANKL to the culture system.

Irrespective of the question of variability of osteoclast generation in vitro, it is clear that once formed, OPN deficient osteoclasts are not fully competent to respond to certain signals that elicit bone resorption in vivo (). Based on what is known about binding of RGD-containing proteins to the αv β3 integrin on the surface of developing osteoclasts, it would not be hard to imagine that an intracellular signal elicited by OPN binding stimulates osteoclast activity. Thus, in this case, cell-matrix interaction and subsequent intracellular signaling appear to be the mechanism by which OPN is regulating bone resorption.

The finding of decreased osteoclast formation in mechanically unloaded mice was unexpected. But some clues may come from observations of OPN expression during orthodontic tooth movement. Upon initiation of the motive force, the number of osteocytes expressing OPN dramatically increased in bone on the side of the tooth feeling the compressive pressure, and this increase preceded the likewise dramatic increase in osteoclast number in the same region (). It has long been speculated that osteocytes are the mechanoreceptors in bone, and that they signal information to the endosteal and periosteal surfaces via their cell processes in canaliculi that form a continuous network. Taking all of this into account along with the newly published results suggests that OPN may be a factor that is utilized by osteocytes to signal for mechanically induced bone resorption. In the case of unloading, osteocytes would signal for the removal of redundant bone; that is, bone not needed to bear the existing load. Osteocyte signaling via OPN would explain the lack of induced osteoclast formation in the unloaded OPN KO mice. (One is curious to know what happened in the forelimbs of the tail suspended animals!). Consequently, mechanical force impacts on OPN KO animals at the very earliest phase of bone resorption: the signal from the osteocyte. If true, it would be of interest to determine the nature of the intercellular communications. Is it integrin mediated? Would OPN in fact have to be secreted? And what cell receives this signal that will result in increased osteoclastogenesis?

From these studies, it appears that OPN is both stimulatory or inhibitory in different cell types, depending on the different phases of several pathways that lead to bone resorption. Ovariectomy-induced bone resorption is signaled by cells in the marrow responding to a change in hormonal and cytokine microenvironment. In the face of OPN deficiency, signals initiated in this fashion do not prevent the increase in osteoclast formation seen independent of ovariectomy, but can inhibit the final function of osteoclasts. This is quite different from what may be happening in the mechanically unloaded OPN KO mice where the inability to up-regulate OPN leads to an osteocyte's inability to initiate bone resorption.

From these studies, a pathway that contains three possible points at which OPN regulates bone resorption can be contemplated (Fig. 1), which includes: 1) signaling by osteocytes to initiate bone resorption, 2) regulation of fusion of pre-osteoclastic cells upstream of regulation mediated by RANKL, and 3) induction of intracellular signaling of a mature osteoclast. Clearly, there is much to be learned. However, this increased understanding of OPN's role in bone resorption in both osteoclastic and osteoblastic cells is a major step forward.

Figure 1: Based on the data by Ishijima and previous work utilizing the OPN KO mice, it can be postulated that OPN plays both a stimulatory and inhibitory role in bone resorption, depending on the process that elicits it, and the cell types involved at various different steps. In mechanically unloading, it may be that OPN is the molecule by which osteocytes signal for the recruitment of osteoclasts to resorb bone (A). Based on the increased osteoclastogenesis seen in ovariectomized OPN KO, it may be that OPN binds to the CD44 receptor on osteoclast precursors and prevents fusion, and that this may be upstream of the RANK/RANKL interaction (B). Finally, even with the increased number of osteoclasts seen in ovariectomized mice, it is clear that they are not active in the OPN KO mice, suggesting that interaction of cell surface integrins with matrix initiate important signaling pathways to initiate resorption (C).

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