Impaired osteoclast homeostasis in the cystatin B-deficient mouse model of progressive myoclonus epilepsy

Abstract

Progressive myoclonus epilepsy of Unverricht–Lundborg type (EPM1) is an autosomal recessively inherited disorder characterized by incapacitating stimulus-sensitive myoclonus and tonic-clonic epileptic seizures with onset at the age of 6 to 16 years. EPM1 patients also exhibit a range of skeletal changes, e.g., thickened frontal cranial bone, arachnodactyly and scoliosis. Mutations in the gene encoding cystatin B (CSTB) underlie EPM1. CSTB is an inhibitor of cysteine cathepsins, including cathepsin K, a key enzyme in bone resorption by osteoclasts. CSTB has previously been shown to protect osteoclasts from experimentally induced apoptosis and to modulate bone resorption in vitro. Nevertheless, its physiological function in bone and the cause of the bone changes in patients remain unknown. Here we used the CSTB-deficient mouse (Cstb−/−) model of EPM1 to evaluate the contribution of defective CSTB protein function on bone pathology and osteoclast differentiation and function. Micro-computed tomography of hind limbs revealed thicker trabeculae and elevated bone mineral density in the trabecular bone of Cstb−/− mice. Histology from Cstb−/− mouse bones showed lower osteoclast count and thinner growth plates in long bones. Bone marrow-derived osteoclast cultures revealed lower osteoclast number and size in the Cstb−/− group. Cstb−/− osteoclasts formed less and smaller resorption pits in an in vitro assay. This impaired resorptive capacity was likely due to a decrease in osteoclast numbers and size. These data imply that the skeletal changes in Cstb−/− mice and in EPM1 patients are a result of CSTB deficiency leading to impaired osteoclast formation and consequently compromised resorptive capacity. These results suggest that the role of CSTB in osteoclast homeostasis and modulation of bone metabolism extends beyond cathepsin K regulation.

Brain inflammation is accompanied by peripheral inflammation in Cstb −/− mice, a model for progressive myoclonus epilepsy

“…In immune cells, the function of CSTB has been linked to chemotaxis [8], expression and secretion of cytokines, and release of nitric oxide [10, 12, 13], implying a role in the immune response. CSTB function has also been associated with diverse cellular processes, such as regulation of apoptosis [14, 15], bone resorption [16, 17], protection of neurons from oxidative stress [18], and cell cycle progression [19]. …”

Epigenomic and Proteomic Changes in Fetal Spleens Persistently Infected with Bovine Viral Diarrhea Virus: Repercussions for the Developing Immune System, Bone, Brain, and Heart

“…A normal hypothesis for CSTB knockouts would be an increase in osteoclasts, due to a lack of available inhibition of cathepsin K ; however, that is not the case in vivo for CSTB deficient or knockout mice. In addition to epileptic phenotypes, CSTB knockout mice showed a significant decrease in osteoclasts and increased bone mineral densities [ 89 ]. A direct inhibition of CSTB has been shown in mice infected with ectromelia virus, which is hypothesized to enhance viral replication [ 90 ].…”

Comorbidities in patients with Unverricht–Lundborg disease ( EPM1 )

“…Cystatin B is associated with bone metabolism and particularly osteoclast function 7,8 . Moreover, patients with EPM1 have abnormal skeletal features that are probably associated with the CSTB mutation 6 .…”

“…Cystatin B is associated with bone metabolism and particularly osteoclast function. 7 , 8 Moreover, patients with EPM1 have abnormal skeletal features that are probably associated with the CSTB mutation. 6 The current data showed that bone fractures are very common among patients with EPM1 with 19% of the cohort having had at least one recorded.…”