COMMENTARY Mechanisms of Calcium Disposal from Osteoclastic

1 COMMENTARY Mechanisms of calcium disposal from osteoclastic resorption hemivacuole H K Datta and B R Horrocks1 School of Clinical and Laboratory Science, The Medical School, University of Newcastle, Framlington Place, Newcastle upon Tyne NE2 4HH, UK 1School of Natural Sciences, University of Newcastle, Framlington Place, Newcastle upon Tyne NE2 4HH, UK (Requests for offprints should be addressed to H K Datta; Email: [email protected]) Abstract One of the most remarkable but neglected aspects of evidence and theoretical considerations suggest that trans- osteoclast function is its unique adaptation that allows the cellular transport of Ca2+ and matrix protein is likely to cell to function despite its resorbing surface being exposed occur via distinct routes. In light of these considerations, to extremely high levels of ambient Ca2+. Recently our we are able to provide convincing explanations for studies have provided evidence of continuous transcellular the apparent anomalies of osteoclast intracellular [Ca2+] Ca2+ disposal, suggesting that osteoclasts are able to responses to a variety of endocrine stimuli. The under- prevent Ca2+ accumulation within the resorptive hemi- standing of the mechanisms involved in Ca2+ handling by vacuole. It has also been shown that matrix protein osteoclasts indicates the lack of a simple link between degradation products that accumulate within the osteo- osteoclast function and changes in overall cytosolic [Ca2+]. clast resorptive vacuole are also undergoing transcellular Journal of Endocrinology (2003) 176, 1–5 transport by transcytosis. However, both experimental Introduction the resorption hemivacuole by the osteoclast resorptive activity was continually transported out of the resorptive Osteoclasts are multinucleate cells that play a critical role site (Berger et al. 1999, 2001). These in situ studies have in bone morphogenesis and remodelling. A number of also suggested that in a bone-resorbing osteoclast a rela- metabolic bone diseases arise due purely to a net increase tively large amount of calcium enters from the resorption in osteoclastic activity; the increase in activity may be hemivacuole into the cell and is released in a constant subtle but insidious, as in osteoporosis, or acute and steady-state manner at the basolateral surface (Berger et al. aggressive, as in hypercalcaemia of malignancy. Despite its 2001). However, the nature of the mechanisms and central importance in the pathogenesis of a number of structures that are involved in the transport of Ca2+ from metabolic bone diseases, many aspects of osteoclast func- the hemivacuole to the basolateral surface are not tion remain unclear. During bone resorption, a large known. amount of Ca2+ is generated within the osteoclast resorptive hemivacuole and [Ca2+] in the resorptive hemivacuole can reach up to 40 mM (Silver et al. 1988). The Routes of Ca2+ disposal precise mechanisms involved in the disposal of Ca2+ are not clear. Understanding the mechanisms of Ca2+ dis- In the light of recent observations of continuous disposal of posal is of immense importance as it may lead to the Ca2+ from the resorptive site (Berger et al. 2001) and the development of novel therapeutic strategies for inhibiting demonstration of bulk trafficking of matrix protein colla- excessive osteoclast resorptive activity. gen by transcytosis (Nesbitt & Horton 1997, Salo et al. In order to address this problem we used an in vitro 1997), we have considered the possible routes of Ca2+ model system that employed scanning electrochemical disposal from osteoclast hemivacuole. The likely routes are microscopy and transparent resorbable matrices that as follows. closely mimic bone-resorbing osteoclast in vivo. This (1) Bulk transport by transcytosis, i.e. where the osteo- model system demonstrated that the Ca2+ produced in clast acts as a conduit allowing the calcium to pass as a part Journal of Endocrinology (2003) 176, 1–5 Online version via http://www.endocrinology.org 0022–0795/03/0176–001 2003 Society for Endocrinology Printed in Great Britain Downloaded from Bioscientifica.com at 10/02/2021 06:04:15PM via free access 2 H K DATTA and B R HORROCKS · Calcium mobilisation by osteoclasts of the bulk flow in manner akin to the one described as and pumps are involved, the most likely mode of func- ‘trafficking’ of collagen matrix (Nesbitt & Horton 1997, tioning is continuous rather than intermittent disposal. Salo et al. 1997). Indeed, our recent studies have suggested a continuous (2) Selective uptake involving specialised Ca2+ channels Ca2+ disposal via a transcellular route occurring at the in the cell plasma membrane at the site of the resorptive surface of bone-resorbing osteoclast. The data also show vacuole and involving intracellular channels traversing Ca2+ disposal from basolateral surface that can be detected the cell and opening at the ventral surface. The release of within minutes of resorption, favouring an explanation the calcium could be facilitated by the presence of Ca2+ based on the ‘selective’ route rather than ‘bulk trafficking’ pumps at the basolateral surface, thereby accelerating or ‘leakage’. However, our data do not rule out a quantal calcium disposal at the surface. disposal where the size of the quanta may be small and we (3) Ca2+ may in fact ‘leak’ around the sealing zone of see a net flux averaged over many quanta and also the resorptive hemivacuole, though this seems inconsistent broadened in time by the diffusional blurring as the with the requirement for the osteoclast to form a tight seal calcium transits between the basolateral surface and the to create a local microenvironment conducive to initiating microelectrode detector. resorption. Finally, ‘bulk’ flow of Ca2+ disposal by transcytosis It is also possible that all the above-mentioned mech- would involve cytosolic vacuoles traversing the osteoclast anisms (1–3) may operate, making variable contributions laden with high concentration of the cation. Such excep- to the Ca2+ disposal process. Here we consider exper- tionally high vacuolar [Ca2+] have not been reported, and imental evidence and theoretical arguments to demon- are likely to be extremely hazardous to the cell. strate that the ‘selective’ rather than the ‘bulk transcytosis’ or ‘leakage’ is likely to be the main mode of Ca2+ disposal. Lessons from other cells We believe that possibility (3) is the least likely since the generation of a low pH in the hemivacuole to initiate There are various documented anomalies and peculiarities dissolution of the inorganic component of the matrix of Ca2+ handling by osteoclasts, such as erratic and would be frustrated by the rapid diffusion of protons out of inconsistent hormone-induced intracellular [Ca2+] ff 2+ the hemivacuole. It is worth noting that the di usion ([Ca ]i) mobilisation, heterogeneity in hormone-induced ffi 5 2+ 2+ 2+ coe cient of protons in aqueous media (9·3 10 Ca signals, high resting [Ca ]i and rather active Ca cm2/s) is much larger than that of any other small ion, e.g. surface efflux (Zaidi et al. 1990, MacIntyre et al. 1991, Ca2+ (7·910 6 cm2/s). Bizzari et al. 1994, Rathod et al. 1995, Berger et al. 2001). We here propose a hypothesis that is based on recent findings in other cell types and readily explains these Experimental evidence for selective transport anomalies and peculiarities (Berridge 1993, Parekh & Penner 1997, Zhang et al. 1999, Mogami et al. 2000, Park Interestingly, fluorescence microscopic experiments have et al. 2000). One such interesting observation is the role of indicated transport of collagen through the osteoclast, a endoplasmic reticulum (ER) as a functional pool for Ca2+ process termed transcytosis, during resorption (Nesbitt & in pancreatic acinar cells, which shows that ER is one Horton 1997, Salo et al. 1997). These studies also provided functionally continuous unit, providing a homogeneous evidence for the transcytosis of the inorganic phosphate, environment for the lumenal Ca2+concentrations thereby suggesting one possible mechanism for the dis- (Mogami et al. 2000). The transcellular Ca2+ transport in posal of Ca2+ within the osteoclast hemivacuole. How- acinar cells is thought to involve uptake through store- ever, a close analysis of the data on the kinetics of Ca2+ operated Ca2+ channels (SOC) (Parekh & Penner 1997) in disposal at the surface of the bone-resorbing osteoclast and the basal membrane and is pumped into the basal ER by its comparison with fluorescence microscopic experiments Ca2+ ATPases (Mogami et al. 2000). Ca2+ then diffuses reveals some critical differences between the mechanism from the base to the apical region of the ER lumen, where of Ca2+ disposal (Berger et al. 1999, 2001) and transcytosis it is believed to be released into cytosol via specific Ca2+ (Nesbitt & Horton 1997, Salo et al. 1997). Perhaps the channels (Mogami et al. 2000, Park et al. 2000). Plasma most important and obvious difference is that, unlike membrane Ca2+ ATPases, concentrated in the apical collagen trafficking that occurs after hours, Ca2+ flux membrane, then pump Ca2+ into the apical lumen. at the basolateral surface is seen within minutes following Interestingly, such regional-specific specialisation of the ‘seeding’ of osteoclast on bone and occurs at an Ca2+stores has been observed in parotid acinar cells, where approximately constant rate. ryanodine- and cyclic ADP-ribose (cADPR)-sensitive In cases (1) and (3) stated above, the flow would be stores are localised in the basal region whilst inositol expected to take place continuously during

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