The two homologous chaperonin 60 proteins ofMycobacterium tuberculosishave distinct effects on monocyte differentiation into osteoclasts

Abstract

SummaryMycobacterium tuberculosis produces two homologous chaperonin (Cpn)60 proteins, Cpn60.1 and Cpn60.2 (Hsp65). Both proteins stimulate human and murine monocyte cytokine synthesis but, unlike Cpn60 proteins from other microbial species, fail to stimulate the breakdown of cultured murine bone. Here, we have examined the mechanism of action of these proteins on bone remodelling and osteoclastogenesis, induced in vitro in murine calvarial explants and the murine monocyte cell line RAW264.7. Additionally, we have determined their effect on bone remodelling in vivo in an animal model of arthritis. Recombinant Cpn60.1 but not Cpn60.2 inhibited bone breakdown both in vitro, in murine calvaria and in vivo, in experimental arthritis. Analysis of the mechanism of action of Cpn60.1 suggests that this protein works by directly blocking the synthesis of the key osteoclast transcription factor, nuclear factor of activated T cells c1. The detection of circulating immunoreactive intact Cpn60.1 in a small number of patients with tuberculosis but not in healthy controls further suggests that the skeleton may be affected in patients with tuberculosis. Taken together, these findings reveal that M. tuberculosis Cpn60.1 is a potent and novel inhibitor of osteoclastogenesis both in vitro and in vivo and a potential cure for bone-resorptive diseases like osteoporosis.

Unfolding the relationship between secreted molecular chaperones and macrophage activation states

“…This raised the question as to whether the Mtb Hsp60 proteins could inhibit bone resorption. Although the two Mtb Hsp60 proteins share over 60% sequence identity, the Hsp60.2 protein has no influence on the process of osteoclast formation, while the Hsp60.1 protein turns out to be a potent inhibitor of murine osteoclast formation both in vitro and in vivo (Winrow et al 2008). The process of osteoclast formation is multistaged and takes up to 7 days.…”

“…The process of osteoclast formation is multistaged and takes up to 7 days. It has been found that Mtb Hsp60.1 can block osteoclast formation in vitro even after the process has been going for 4 days (Winrow et al 2008). A partial explanation for the inhibitory effect of Mtb Hsp60.1 is that it inhibits the transcription of a key osteoclast transcription factor, nuclear factor of activated T cells (NFATc1).…”

Integrating the cell stress response: a new view of molecular chaperones as immunological and physiological homeostatic regulators

“…This possibly flies in the face of the evidence that 'Hsp70' is a pro-inflammatory factor. This was the situation with Hsp60 until more recent studies, described above, revealed that the human and M. tuberculosis Hsp60.1 proteins can inhibit leukocyte activation [see 29,69]. Thus it is possible that Hsp70…”

“…Moreover both proteins differ markedly in their ability to modulate the formation of the major cell population, the osteoclast, which is responsible for homeostatic bone resorption. Thus the chaperonin 60.1 protein is a potent inhibitor of osteoclast formation while the chaperonin 60.2 protein neither promotes nor inhibits osteoclastogenesis [29].…”

“…In contrast, a not inconsiderable proportion of bacteria have more than one Hsp60 gene [31]. The differences in the activities of the M. tuberculosis Cpn60 proteins has been described [28,29] and an even more distinct difference in folding function [32] and signalling activity [33] is seen with the three Cpn60 proteins of the soil bacterium Rhizobium leguminosarum. Drosophila melaogaster has four Hsp60 proteins and each seems to serve a distinct function [34].…”