Parkin silencing promotes OC activity in vitro
We sought to determine whether parkin plays a potential role in the differentiation of osteoblasts (OBs) or OCs by measuring its expression in these cells by quantitative PCR (qPCR) and western blot analysis. As shown in Fig. 1A-C, the mRNA and protein levels of parkin gradually increased during OB or OC differentiation. However, in early differentiating OBs or OCs, transfection with a retrovirus harboring the parkin gene did not affect their differentiation, as revealed by alizarin red S (AR) staining and alkaline phosphatase (ALP) activity for OBs and tartrate-resistant acid phosphatase (TRAP) staining for OCs (Fig. 1D, E). We then investigated whether parkin deficiency might be involved in bone destruction. Even though there were no significant differences in OC differentiation (Fig. 1F, G), the siRNA-mediated silencing of parkin in OCs induced robust bone resorption compared to a control siRNA (Fig. 1H), suggesting that a reduced expression of parkin may be linked to the bone pathology.
Parkin deficiency promotes OC activity and decreases bone mass in vivo
Next, we compared the bone phenotypes of WT and Parkin-deficient (Parkin−/−) mice to elucidate the bone-related function of parkin in vivo. A three-dimensional visualization of the distal femoral area revealed a significant loss of trabecular bone in Parkin−/− mice (Fig. 2A). The BMD levels of Parkin−/− mice were markedly reduced by 34% in comparison to WT mice (Fig. 2B). Parkin−/− mice also exhibited a lower bone volume per tissue volume (BV/TV), trabecular thickness (Tb. Th), and trabecular number (Tb. N) (Fig. 2C-E). By contrast, bone-resorbing parameters, such as trabecular separation (Tb. Sp), bone porosity, and the structure model index (SMI), tended to be higher in Parkin−/− mice (Fig. 2F-H), demonstrating spontaneous bone loss in these mice. We found larger OCs in the femoral bones of Parkin−/− mice compared to WT mice (Fig. 2I). In addition, we observed that the expression of serum-bone resorption (OC) marker, C-telopeptide of collagen type 1 (CTX-1), in the plasma of Parkin−/− mice was significantly higher than that of the control WT mice (Fig. 2J). Thus, we observed that Parkin-deficient mice display augmented OC activity and reduced bone mass. Moreover, we assessed the involvement of parkin in OC formation and function by in vitro culturing of bone marrow cells from Parkin−/− and WT mice with the OC-inducing cytokines macrophage colony-stimulating factor (M-CSF) and RANKL. Although the number of TRAP-positive OCs (MNCs ≥ 3 nuclei) exhibited no significant difference (Fig. 2L, left), the area of TRAP-positive MNCs (Fig. 2K, upper and Fig. 2L, middle) was potently augmented in Parkin−/− OCs compared with WT OCs. In particular, the area of resorption pits on the dentin slices was significantly higher in Parkin−/− OC cultures than in WT cultures (Fig. 2K, lower and Fig. 2L, right), further supporting the notion that a parkin deficiency leads to a noticeable increase in bone-resorbing activity.
Parkin deficiency enhances the acetylation of α-tubulin dependently of RANKL-dependent ERK activation in OCs
As mentioned above, microtubules play a critical role in the expansion of podosomes into the belt during OC maturation for bone resorption (34), and those microtubules are regulated by phosphorylation and acetylation events (44). In addition, parkin has been shown to interact with tubulin and, in turn, stabilize microtubules (45). To investigate the regulatory role of parkin in controlling OC activity by interacting with microtubules, we assessed the localization of parkin in mature OCs. The assessment of parkin by confocal laser scanning microscopy showed that its staining signals were mostly confined to the internal tubulin border area of the OCs and were not evident in the specific actin-rich sealing zone (Fig. 3A). We also observed that the endogenous parkin colocalizes with β-tubulin but not F-actin, as revealed by the immunostaining of mature OCs (Fig. 3A and Figure S1). Parkin was further found to be strongly associated with tubulin heterodimers and microtubules (α- and β-tubulin), as evidenced by co-immunoprecipitation of tubulin and parkin using specific parkin antibodies (Fig. 3B). Although parkin interacted with both α- and β-tubulin (Fig. 3B), the binding to α-tubulin appeared to be relatively stronger. Given the literature findings of a close correlation between the stability of the podosome belt and the acetylation of microtubules (34, 38, 44), we next explored the possibility that parkin is involved in the acetylation of tubulin. An enhanced acetylation of α-tubulin was exhibited along the interior zone and also intensively detected around the internal border area of parkin-depleted OCs (Fig. 3C, left). In addition, the acetylation of α-tubulin was markedly augmented in parkin-deficient cells compared with WT cells, and the level of acetylated α-tubulin gradually increased in OCs (Fig. 3C, right). To provide a mechanistic explanation for the increase in the acetylation of α-tubulin by parkin deficiency, we compared the RANKL-induced signaling pathway, including activation of nuclear factor kappa B (NF-κB) and mitogen-activated protein kinases (MAPKs) (46) in bone marrow-derived macrophages (BMMs) isolated from WT or Parkin−/− mice (Fig. 3D, E). Western blot analysis revealed a slight increase in ERK and p38 activation in BMMs of Parkin−/− mice compared to WT mice and this increase was enhanced in response to RANKL (Fig. 3D). Thus, we next investigated whether acetylation of α-tubulin could be modulated through the MAPK signaling pathway. To this end, we added various signaling inhibitors in response to RANKL in OCs. We found that the inactivation of phosphorylated ERK (pERK) by PD98059 led to a significant reduction in the acetylated α-tubulin levels in Parkin−/− OCs (Fig. 3E) but not in WT OCs, suggesting that parkin might restrain the activity of α-tubulin via ERK. In comparison, p38 and c-Jun N-terminal kinase (JNK) inhibition did not significantly change in the RANKL-induced acetylation of α-tubulin. These observations suggested that the ERK-acetylated tubulin axis may act as a potential component mediator of OC activity downstream of the parkin pathway.
Parkin deficiency regulates the acetylation of α-tubulin by interacting with HDAC6 in OCs
Parkin and HDAC6, a known deacetylase, are reported to bind to tubulin α/β heterodimers (47, 48). Hence, we reasoned that the interaction of parkin with HDAC6 may affect the status of α-tubulin acetylation to regulate microtubule stability during OC activation. To test this possibility, we made mutant proteins with multiple interaction domains of parkin bound to the DD1 and/or DD2 domain of HDAC6 and then expressed parkin or its domains with HDAC6 in HEK 293 cells, as shown in Fig. 4A. The full-length parkin (WT), mutant 2, or mutant 3 of the parkin RING1 domain was binding with HDAC6 as shown in anti-FLAG (parkin) immunoprecipitation, while mutant 1 lacking RING1 of parkin did not interact with HDAC6 (Fig. 4B), indicating that parkin interacts with HDAC6 via its RING1 domain. This interaction was responsible for the deacetylation of α-tubulin in OCs in response to RANKL (Fig. 4C). HDAC6 knockdown substantially increased the acetylation of α-tubulin and was accompanied by the activation of cathepsin K and bone-resorption activity in mature OCs (Fig. 4D-F). Taken together, these results suggested that parkin may primarily regulate the acetylation of α-tubulin through its interaction with HDAC6 to exert its influence on the bone-resorbing capacity of OCs.
Parkin deficiency increases the susceptibility to inflammatory arthritis but not ovariectomy (OVX)-induced bone loss
To clarify the role of parkin in bone metabolism, Parkin−/− mice underwent an exaggerated state of estrogen deficiency-induced bone remodeling and were compared with WT mice. WT and Parkin−/− mice were either OVX or sham-operated and subjected to micro-computed tomography (micro-CT) assessment at 4 weeks post-operation (Fig. 5A). Micro-CT analysis showed that OVX successfully induced a bone loss in WT and Parkin−/− mice, but there was no significant increase in ovariectomized Parkin−/− mice compared to ovariectomized WT mice (Fig. 5). It was previously reported that lack of parkin increases inflammation induced by IL-1β, which leads to accumulation of damaged mitochondria in human articular chondrocytes (49), implying an association between parkin and IL-1β-associated signaling. In agreement with these observations (50), IL-1β stimulation reduced parkin expression in mouse primary BMMs and mature OCs (Figure S2). These findings supported the evidence for the decreased expression of parkin in monocytes under the inflammatory condition. Given that IL-1β is increased in the inflamed joint of the K/BxN-induced arthritis model due to its critical role in inflammatory bone loss (51), we determined whether parkin is involved in the pathogenesis of inflammatory arthritis and bone density using a K/BxN serum-transfer arthritis mouse model. Both WT and Parkin−/− mice began to develop arthritis 4 days after immunization with K/BxN (Fig. 6). Nonetheless, with regards to inflammatory arthritis, the increase in the mean arthritis scores (Fig. 6A) and paw thickness (Fig. 6B) of the Parkin−/− mice was more profound than that of the WT mice. In addition, synovitis, pannus, and erosion scores were higher in Parkin−/− mice than in WT mice in the K/BxN serum-transfer model (Fig. 6C, D). Moreover, bone destruction and inflammatory F4/80-positive macrophages were abundantly observed in Parkin−/− mice (Fig. 6C). Concomitantly, a three-dimensional visualization of the distal femur in these animals indicated a massive loss of trabecular bone in K/BxN Parkin−/− mice (Fig. 6E). In addition, analysis of the micro-CT data revealed a markedly reduced BV/TV (%) in K/BxN Parkin−/− mice (Fig. 6E, left), whereas the SMI and total bone porosity were significantly increased in K/BxN Parkin−/− mice compared with K/BxN WT mice (Fig. 6E, middle and right). Collectively, these results suggested that parkin may protect against bone destruction mediated by inflammatory stimuli.
Ectopic parkin suppresses IL-1β-mediated OC activity
To provide the mechanistic explanation for aggravated inflammatory bone loss by parkin deficiency, we next explored the functional association of parkin with IL-1β-mediated signaling in OCs. IL-1β stimulation in parkin-depleted OCs compared to WT OCs significantly reinforced phosphorylation of ERK and acetylation of α-tubulin and was accompanied by HDAC6 reduction (Fig. 7A). Additionally, in line with the IL-1β-mediated increase in the number of TRAP-positive MNCs with actin rings and the area of TRAP-positive MNCs in the absence of parkin (Figure S3A-C), Parkin−/− cells treated with IL-1β produced an approximately 3.5-fold greater resorption pit area in dentin slices (Fig. 7B). Furthermore, in parallel with the augmented acetylation of tubulin, cathepsin K, an OC resorptive marker protein, was enhanced in IL-1β-treated Parkin−/− cells relative to IL-1β-treated WT cells (Fig. 7C). Parkin depletion increased IL-1β-induced ERK activation accompanied by a reduction in the interaction with endogenous HDAC6 and tubulin (Fig. 7D). Moreover, the acetylated α-tubulin level was intensively increased by the IL-1β response in parkin-deficient cells, and this increase was ameliorated by an ERK inhibitor (Fig. 7E). IL-1β also enhanced the enzymatic activity of cathepsin K in the conditioned medium from OCs from both WT and Parkin−/− mice (Fig. 7F). The blockade of this IL-1β-induced cathepsin K activity by a pERK inhibitor (PD98059) (Fig. 7E, F) suggests that IL-1β-induced parkin-deficient OCs have an increased bone-resorptive capacity that is mediated through pERK signaling.
Next, to examine whether ectopic parkin expression could reverse the effects of IL-1β on OC activity, we employed a retroviral system to overexpress parkin in Parkin−/− BMMs. The expression levels of acetylated tubulin and cathepsin K were selectively restrained by parkin overexpression upon IL-1β stimulation (Fig. 7G). Furthermore, parkin overexpression in the homozygous deficient osteoclast precursor (OCP) cells decreased the number of resorption pits induced by IL-1β in mature OCs in association with inhibition of cathepsin K activity (Fig. 7H, I).