Glycosaminoglycan-mediated loss of cathepsin K collagenolytic activity in MPS I contributes to osteoclast and growth plate abnormalities

Characterization of joint disease in mucopolysaccharidosis type I mice

Mucopolysaccharidoses (MPS) are lysosomal storage disorders characterized by mutations in enzymes that degrade glycosaminoglycans (GAGs). Joint disease is present in most forms of MPS, including MPS I. This work aimed to describe the joint disease progression in the murine model of MPS I. Normal (wild-type) and MPS I mice were sacrificed at different time points (from 2 to 12 months). The knee joints were collected, and haematoxylin-eosin staining was used to evaluate the articular architecture. Safranin-O and Sirius Red staining was used to analyse the proteoglycan and collagen content. Additionally, we analysed the expression of the matrix-degrading metalloproteinases (MMPs), MMP-2 and MMP-9, using immunohistochemistry. We observed progressive joint alterations from 6 months, including the presence of synovial inflammatory infiltrate, the destruction and thickening of the cartilage extracellular matrix, as well as proteoglycan and collagen depletion. Furthermore, we observed an increase in the expression of MMP-2 and MMP-9, which could conceivably explain the degenerative changes. Our results suggest that the joint disease in MPS I mice may be caused by a degenerative process due to increase in proteases expression, leading to loss of collagen and proteoglycans. These results may guide the development of ancillary therapies for joint disease in MPS I.

Binding of chondroitin 4-sulfate to cathepsin S regulates its enzymatic activity

Human cysteine cathepsin S (catS) participates in distinct physiological and pathophysiological cellular processes and is considered as a valuable therapeutic target in autoimmune diseases, cancer, atherosclerosis, and asthma. We evaluated the capacity of negatively charged glycosaminoglycans (heparin, heparan sulfate, chondroitin 4/6-sulfates, dermatan sulfate, and hyaluronic acid) to modulate the activity of catS. Chondroitin 4-sulfate (C4-S) impaired the collagenolytic activity (type IV collagen) and inhibited the peptidase activity (Z-Phe-Arg-AMC) of catS at pH 5.5, obeying a mixed-type mechanism (estimated Ki = 16.5 ± 6 μM). Addition of NaCl restored catS activity, supporting the idea that electrostatic interactions are primarly involved. Furthermore, C4-S delayed in a dose-dependent manner the maturation of procatS at pH 4.0 by interfering with the intermolecular processing pathway. Binding of C4-S to catS was demonstrated by gel-filtration chromatography, and its affinity was measured by surface plasmon resonance (equilibrium dissociation constant Kd = 210 ± 40 nM). Moreover, C4-S induced subtle conformational changes in mature catS as observed by intrinsic fluorescence spectroscopy analysis. Molecular docking predicted three specific binding sites on catS for C4-S that are different from those found in the crystal structure of the cathepsin K-C4-S complex. Overall, these results describe a novel glycosaminoglycan-mediated mechanism of catS inhibition and suggest that C4-S may modulate the collagenase activity of catS in vivo.

Effects of pharmacological inhibition of cathepsin K on fracture repair in mice

Cathepsin K inhibitors (CatK-I) have been developed and established to restore bone mass in both animal models of bone loss and postmenopausal osteoporotic patients. We investigated the effects of a CatK-I L-006235 on bone repair and compared to alendronate (ALN) for its known effects on fracture healing in preclinical models. Femoral fractures were performed on wild type mice that were given vehicle (CON), CatK-I or ALN from day 0 post-fracture until euthanasia. Radiologic and micro-CT analyses demonstrated that CatK-I enhanced mineralization within the calluses at day 21 post-fracture, but to a lesser degree than ALN. Histological analyses showed residual unmineralized and mineralized cartilage in the calluses of CatK-I and ALN treated groups at day 21 post-fracture compared to that in CON. CatK-I enhanced the number of tartrate-resistant acid phosphatase positive (TRAP+) osteoclasts in the fracture calluses compared to ALN and CON treated groups. However, relative levels of serum C-terminal telopeptides of type I collagen (CTX) normalized to the number of TRAP+ osteoclasts within the calluses were significantly decreased in both CatK-I and ALN groups compared to CON. Additionally, the percentages of osteoblast surface over mineralized calluses and levels of the bone formation marker serum N-terminal propeptide of type I procollagen (P1NP) were comparable between CatK-I versus CON groups, while these bone formation parameters were decreased by ALN. Taken together, these results indicate that unlike ALN, CatK-I inhibits osteoclastic activity without changing bone formation, and the inhibition of CatK delayed but did not abrogate callus remodeling during bone repair.

Postnatal progression of bone disease in the cervical spines of mucopolysaccharidosis I dogs

INTRODUCTION

Mucopolysaccharidosis I (MPS I) is a lysosomal storage disorder characterized by deficient α-l-iduronidase activity leading to accumulation of poorly degraded dermatan and heparan sulfate glycosaminoglycans (GAGs). MPS I is associated with significant cervical spine disease, including vertebral dysplasia, odontoid hypoplasia, and accelerated disk degeneration, leading to spinal cord compression and kypho-scoliosis. The objective of this study was to establish the nature and rate of progression of cervical vertebral bone disease in MPS I using a canine model.

METHODS

C2 vertebrae were obtained post-mortem from normal and MPS I dogs at 3, 6 and 12 months-of-age. Morphometric parameters and mineral density for the vertebral trabecular bone and odontoid process were determined using micro-computed tomography. Vertebrae were then processed for paraffin histology, and cartilage area in both the vertebral epiphyses and odontoid process were quantified.

RESULTS

Vertebral bodies of MPS I dogs had lower trabecular bone volume/total volume (BV/TV), trabecular thickness (Tb.Th), trabecular number (Tb.N) and bone mineral density (BMD) than normals at all ages. For MPS I dogs, BV/TV, Tb.Th and BMD plateaued after 6 months-of-age. The odontoid process appeared morphologically abnormal for MPS I dogs at 6 and 12 months-of-age, although BV/TV and BMD were not significantly different from normals. MPS I dogs had significantly more cartilage in the vertebral epiphyses at both 3 and 6 months-of-age. At 12 months-of-age, epiphyseal growth plates in normal dogs were absent, but in MPS I dogs they persisted.

CONCLUSIONS

In this study we report reduced trabecular bone content and mineralization, and delayed cartilage to bone conversion in MPS I dogs from 3 months-of-age, which may increase vertebral fracture risk and contribute to progressive deformity. The abnormalities of the odontoid process we describe likely contribute to increased incidence of atlanto-axial subluxation observed clinically. Therapeutic strategies that enhance bone formation may decrease incidence of spine disease in MPS I patients.

Brain and spinal MR imaging findings in mucopolysaccharidoses: a review

MPS represents a group of rare hereditary disorders characterized by multisystem involvement due to intralysosomal GAG accumulation. Among various tissues, both the central and peripheral nervous system are affected in almost all types of the disease. Thus, brain and spinal MR imaging are valuable tools for the assessment of neurologic involvement, and there is evidence that they might be reliable markers demonstrating disease severity and efficacy of treatment options currently used in patients with MPS. We aimed to review the most prominent MR imaging features of patients with MPS, paying attention to the physiopathologic mechanisms responsible for these alterations. Along with the description of neuroimaging findings, existing data in relation to their correlation with the severity of neurologic involvement is discussed, while another topic of great importance is the effect of various therapeutic regimens in the progression of brain and spinal MR imaging alterations. Finally, recent data concerning MR spectroscopy studies in MPS are also critically discussed.

Prevalence and development of orthopaedic symptoms in the dutch hurler patient population after haematopoietic stem cell transplantation

Hurler syndrome (MPS-IH) is a rare autosomal recessive lysosomal storage disease. Besides a variety of other features, Hurler syndrome is characterized by a range of skeletal abnormalities known as dysostosis multiplex. Despite the successful effect of haematopoietic stem cell transplantation on the other features, dysostosis remains a disabling symptom of the disease. This study analyzed the status and development of the orthopaedic manifestations of 14 Dutch Hurler patients after stem cell transplantation.Data were obtained retrospectively by reviewing patients' charts, radiographs and MRIs. Existing methods to measure the deficiencies were modified to optimally address the dysostosis. These measurements were done by two of the authors independently. The odontoïd/body ratio, kyphotic angle, scoliotic angle and parameters for hip dysplasia and genu valgum were measured and plotted against age. The degree of progression was determined. The intraclass correlation coefficient (ICC) was calculated to determine the reliability of the measurements.All patients showed hypoplasia of the odontoïd, which significantly improved during growth. Kyphosis in the thoracolumbar area was present in 13 patients and proved to be progressive. Scoliosis was observed in eight patients. Hip dysplasia was present in all patients and showed no tendency of improvement. In all but one patient, knee valgus remained more than two standard deviations above normal.Dysostosis remains a major problem after haematopoietic stem cell transplantation in Hurler patients. Moreover, except for dens hypoplasia, it appears to be progressive and therefore surgical interventions may be necessary in the majority of these patients.

Pathogenesis of lumbar spine disease in mucopolysaccharidosis VII

Mucopolysaccharidosis type VII (MPS VII) is characterized by deficient β-glucuronidase (GUSB) activity, which leads to accumulation of chondroitin, heparan and dermatan sulfate glycosaminoglycans (GAGs), and multisystemic disease. MPS VII patients can develop kypho-scoliotic deformity and spinal cord compression due to disease of intervertebral disks, vertebral bodies, and associated tissues. We have previously demonstrated in MPS VII dogs that intervertebral disks degenerate, vertebral bodies have irregular surfaces, and vertebral body epiphyses have reduced calcification, but the pathophysiological mechanisms underlying these changes are unclear. We hypothesized that some of these manifestations could be due to upregulation of destructive proteases, possibly via the binding of GAGs to Toll-like receptor 4 (TLR4), as has been proposed for other tissues in MPS models. In this study, the annulus fibrosus of the intervertebral disk of 6-month-old MPS VII dogs had cathepsin B and K activities that were 117- and 2-fold normal, respectively, which were associated with elevations in mRNA levels for these cathepsins as well as TLR4. The epiphyses of MPS VII dogs had a marked elevation in mRNA for the cartilage-associated gene collagen II, consistent with a developmental delay in the conversion of the cartilage to bone in this region. The spine obtained at autopsy from a young man with MPS VII exhibited similar increased cartilage in the vertebral bodies adjacent to the end plates, disorganization of the intervertebral disks, and irregular vertebral end plate morphology. These data suggest that the pathogenesis of destructive changes in the spine in MPS VII may involve upregulation of cathepsins. Inhibition of destructive proteases, such as cathepsins, might reduce spine disease in patients with MPS VII or related disorders.

Hurler disease bone marrow stromal cells exhibit altered ability to support osteoclast formation

Mucopolysaccharidosis type I (MPS IH; Hurler syndrome) is a rare genetic disorder that is caused by mutations in the α-L-iduronidase (IDUA) gene, resulting in the deficiency of IDUA enzyme activity and intra-cellular accumulation of glycosaminoglycans. A characteristic skeletal phenotype is one of the many clinical manifestations in Hurler disease. Since the mechanism(s) underlying these skeletal defects are not completely understood, and bone and cartilage are mesenchymal lineages, we focused on the characterization of mesenchymal cells isolated from the bone marrow (BM) of 5 Hurler patients. IDUA-mutated BM stromal cells (BMSC) derived from MPS IH patients exhibited decreased IDUA activity, consistent with the disease genotype. The expansion rate, phenotype, telomerase activity, and differentiation capacity toward adipocytes, osteoblasts, chondrocytes, and smooth muscle cells in vitro of the MPS I BMSC lines were similar to those of BMSC from age-matched normal control donors. MPS I BMSC also had a similar in vivo osteogenic capacity as normal BMSC. However, MPS I BMSC displayed an increased capacity to support osteoclastogenesis, which may correlate with the up-regulation of the RANKL/RANK/OPG molecular pathway in MPS I BMSC compared with normal BMSC.

Pathogenesis of skeletal and connective tissue involvement in the mucopolysaccharidoses: glycosaminoglycan storage is merely the instigator

The mucopolysaccharidoses (MPSs) are a series of rare genetic disorders in which progressive bone and joint disease represents a key source of morbidity for patients. The recent introduction of enzyme replacement therapy for many of the MPSs has led to a need for increased physician awareness of these rare conditions in order to ensure that treatment is initiated at a time that leads to optimal benefit for patients. In addition, the current experiences of the clinical responsiveness of patient's symptoms to enzyme replacement approaches have also fuelled an interest in the development of alternative and adjunctive therapeutic approaches directed particularly to the rheumatological aspects of disease. Understanding the underlying pathogenesis of the MPSs is a key element for advancements in both of these areas. This review highlights the current knowledge underlying the pathophysiology of disease symptoms in the MPSs and underscores the importance and role of pathogenic cascades.

Mass spectrometry-based proteomics strategies for protease cleavage site identification

Protease-catalyzed hydrolysis of peptide bonds is one of the most pivotal post-translational modifications fulfilling manifold functions in the regulation of cellular processes. Therefore, dysregulation of proteolytic reactions plays a central role in many pathophysiological events. For this reason, understanding the molecular mechanisms in proteolytic reactions, in particular the knowledge of proteases involved in complex processes, expression levels and activity of protease and knowledge of the targeted substrates are an indispensable prerequisite for targeted drug development. The present review focuses on mass spectrometry-based proteomic methods for the analysis of protease cleavage sites, including the identification of the hydrolyzed bonds as well as of the surrounding sequence. Peptide- and protein-centric approaches and bioinformatic tools for experimental data interpretation will be presented and the major advantages and drawbacks of the different approaches will be addressed. The recent applications of these approaches for the analysis of biological function of different protease classes and potential future directions will be discussed.

Closed Meningo(encephalo)cele: a new feature in Hunter syndrome

BACKGROUND AND PURPOSE

Hunter syndrome (MPS type II) is a rare X-linked recessive disease caused by lysosomal enzyme iduronate-2-sulfatase deficiency, characterized by frequent and variable brain and skull involvement. Our objective was determine the frequency of closed cephaloceles in a large cohort of subjects affected with Hunter syndrome and to investigate possible correlations with clinical and neuroradiologic findings.

MATERIALS AND METHODS

Brain MR imaging of 33 patients (32 males and 1 female, age range 2.5-30.8 years, mean age 10.4 years) affected with Hunter syndrome were retrospectively evaluated. Eleven (age range 3.6-30.8 years; mean age 15.1) presented with an "attenuated" phenotype, while 22 (age range 2.5-19.1 years; mean age 8.2) had a "severe" phenotype.

RESULTS

A closed cephalocele was detected in 9/33 patients (27%) at the level of anterior and middle fossa in 6 and 3 cases, respectively; 6/9 subjects were affected with the attenuated phenotype and 1/9 suffered from epilepsy. Closed cephaloceles did not show a significant association with other brain and spine MR imaging features of Hunter disease, such as enlargement of perivascular spaces, cisterna magna, pituitary sella, ventricles and subarachnoid spaces, craniosynostosis, dens hypoplasia, white matter abnormalities, spinal stenosis due to periodontoid cap, platyspondylia, or intervertebral disk anomalies.

CONCLUSIONS

Closed cephaloceles are frequent in Hunter syndrome and should be considered a neuroradiologic feature of this disease.

Biomarker analysis of Morquio syndrome: identification of disease state and drug responsive markers

Background

This study was conducted to identify potential biomarkers that could be used to evaluate disease progression and monitor responses to enzyme replacement therapy (ERT) in patients with mucopolysaccharidosis (MPS) IVA.

Methods

Levels of 88 candidate biomarkers were compared in plasma samples from 50 healthy controls and 78 MPSIVA patients not receiving ERT to test for significant correlations to the presence of MPSIVA. MPSIVA samples were also tested for correlations between candidate biomarkers and age, endurance, or urinary keratin sulfate (KS) levels. Then, levels of the same 88 analytes were followed over 36 weeks in 20 MPSIVA patients receiving ERT to test for significant correlations related to ERT, age, or endurance.

Results

Nineteen candidate biomarkers were significantly different between MPSIVA and unaffected individuals. Of these, five also changed significantly in response to ERT: alpha-1-antitrypsin, eotaxin, lipoprotein(a), matrix metalloprotein (MMP)-2, and serum amyloid P. Three of these were significantly lower in MPSIVA individuals versus unaffected controls and were increased during ERT: alpha-1-antitrypsin, lipoprotein(a), and serum amyloid P.

Conclusions

Candidate biomarkers alpha-1-antitrypsin, lipoprotein(a), and serum amyloid P may be suitable markers, in addition to urinary KS, to follow the response to ERT in MPSIVA patients.

Brain and spine MRI features of Hunter disease: frequency, natural evolution and response to therapy

BACKGROUND

Hunter disease is a rare X-linked mucopolysaccharidosis. Despite frequent neurological involvement, characterizing the severe phenotype, neuroimaging studies are scarce.

OBJECTIVES

To determine frequency and severity of neuroradiological mucopolysaccharidosis-related features; to correlate them with clinical phenotype; to evaluate their natural evolution and the impact of intravenous enzymatic replacement therapy (ERT).

METHODS

Sixty nine brain MRI examinations of 36 Italian patients (mean-age 10.4 years; age-range 2.2-30.8; severe phenotype in 22 patients) were evaluated. Twenty patients had multiple MRIs (median follow-up 3.1 years, range 1-16.9): among them 15 had MRIs before and after ERT, six had repeated MRIs without being on ERT and five while on ERT. Perivascular, subarachnoid and ventricle space enlargement, white matter abnormality (WMA) burden, pituitary sella/skull/posterior fossa abnormalities, periodontoid thickening, spinal stenosis, dens hypoplasia, myelopathy, vertebral and intervertebral disc abnormalities were graded by means of dedicated scales.

RESULTS

Perivascular spaces enlargement (89%), WMAs (97%), subarachnoid space enlargement (83%), IIIrd-ventricle dilatation (100%), pituitary sella abnormalities (80%), cranial hyperostosis (19%), craniosynostosis (19%), enlarged cisterna magna (39%), dens hypoplasia (66%), periodontoid thickening (94%), spinal stenosis (46%), platyspondylia (84%) and disc abnormalities (79%) were frequently detected. WMAs, IIIrd-ventricle dilatation and hyperostosis correlated with the severe phenotype (p < 0.05). Subarachnoid spaces and ventricle enlargement, WMAs and spinal stenosis progressed despite ERT, while other MR features showed minimal or no changes.

CONCLUSIONS

The spectrum of brain and spine MRI abnormalities in Hunter disease is extremely wide and requires a thorough evaluation. WMAs, atrophy/communicating hydrocephalus and spinal stenosis progress over time and might represent possible disease severity markers for new treatment efficacy assessment.

Collagenolytic activities of the major secreted cathepsin L peptidases involved in the virulence of the helminth pathogen, Fasciola hepatica

Background

The temporal expression and secretion of distinct members of a family of virulence-associated cathepsin L cysteine peptidases (FhCL) correlates with the entry and migration of the helminth pathogen Fasciola hepatica in the host. Thus, infective larvae traversing the gut wall secrete cathepsin L3 (FhCL3), liver migrating juvenile parasites secrete both FhCL1 and FhCL2 while the mature bile duct parasites, which are obligate blood feeders, secrete predominantly FhCL1 but also FhCL2.

Methodology/Principal Findings

Here we show that FhCL1, FhCL2 and FhCL3 exhibit differences in their kinetic parameters towards a range of peptide substrates. Uniquely, FhCL2 and FhCL3 readily cleave substrates with Pro in the P2 position and peptide substrates mimicking the repeating Gly-Pro-Xaa motifs that occur within the primary sequence of collagen. FhCL1, FhCL2 and FhCL3 hydrolysed native type I and II collagen at neutral pH but while FhCL1 cleaved only non-collagenous (NC, non-Gly-X-Y) domains FhCL2 and FhCL3 exhibited collagenase activity by cleaving at multiple sites within the α1 and α2 triple helix regions (Col domains). Molecular simulations created for FhCL1, FhCL2 and FhCL3 complexed to various seven-residue peptides supports the idea that Trp67 and Tyr67 in the S2 subsite of the active sites of FhCL3 and FhCL2, respectively, are critical to conferring the unique collagenase-like activity to these enzymes by accommodating either Gly or Pro residues at P2 in the substrate. The data also suggests that FhCL3 accommodates hydroxyproline (Hyp)-Gly at P3-P2 better than FhCL2 explaining the observed greater ability of FhCL3 to digest type I and II collagens compared to FhCL2 and why these enzymes cleave at different positions within the Col domains.

Conclusions/Significance

These studies further our understanding of how this helminth parasite regulates peptidase expression to ensure infection, migration and establishment in host tissues.

Fasciola hepatica is a helminth parasite that causes liver fluke disease (fasciolosis) in domestic animals (sheep and cattle) and humans worldwide. In order to infect their mammalian hosts, F. hepatica larvae must penetrate and traverse the intestinal wall of the duodenum, move through the peritoneum and penetrate the liver. After migrating through the liver, causing extensive tissue damage, the parasites move to their final niche in the bile ducts where they mature and feed on host haemoglobin to support the production of eggs. To achieve these tasks, F. hepatica secretes a number of distinct cathepsin L cysteine peptidases (FhCL). Thus, the infective larvae that penetrate the host gut secrete cathepsin L3 (FhCL3), the migrating liver-stage juvenile parasites secrete both FhCL1 and FhCL2 while mature bile duct parasites that feed on host blood secrete predominantly FhCL1 but also FhCL2. Here we show that the major cathepsin L peptidases secreted by F. hepatica (FhCL1, FhCL2 and FhCL3) display differential ability to degrade host collagen (an important component of host tissues) and investigate this phenomenon at the molecular level.

Structure-activity analysis of cathepsin K/chondroitin 4-sulfate interactions

In the presence of oligomeric chondroitin 4-sulfate (C4-S), cathepsin K (catK) forms a specific complex that was shown to be the source of the major collagenolytic activity in bone osteoclasts. C4-S forms multiple contacts with amino acid residues on the backside of the catK molecule that help to facilitate complex formation. As cathepsin L does not exhibit a significant collagenase activity in the presence or in the absence of C4-S, we substituted the C4-S interacting residues in catK with those of cathepsin L. Variants revealed altered collagenolytic activities with the largest inhibitory effect shown by the hexavariant M5. None of the variants showed a reduction in their gelatinolytic and peptidolytic activities when compared with wild-type catK, indicating no structural alteration within their active sites. However, the crystal structure of the M5 variant in the presence of oligomeric C4-S revealed a different binding of chondroitin 4-sulfate. C4-S is not continuously ordered as it is in the wild-type catK·C4-S complex. The orientation and the direction of the hexasaccharide on the catK surface have changed, so that the hexasaccharide is positioned between two symmetry-related molecules. Only one M5 variant molecule of the dimer that is present in the asymmetric unit interacts with C4-S. These substitutions have changed the mode of catK binding to C4-S and, as a result, have likely affected the collagenolytic potential of the variant. The data presented here support our hypothesis that distinct catK/C4-S interactions are necessary for the collagenolytic activity of the enzyme.

Hematopoietic differentiation of induced pluripotent stem cells from patients with mucopolysaccharidosis type I (Hurler syndrome)

Mucopolysaccharidosis type I (MPS IH; Hurler syndrome) is a congenital deficiency of α-L-iduronidase, leading to lysosomal storage of glycosaminoglycans that is ultimately fatal following an insidious onset after birth. Hematopoietic cell transplantation (HCT) is a life-saving measure in MPS IH. However, because a suitable hematopoietic donor is not found for everyone, because HCT is associated with significant morbidity and mortality, and because there is no known benefit of immune reaction between the host and the donor cells in MPS IH, gene-corrected autologous stem cells may be the ideal graft for HCT. Thus, we generated induced pluripotent stem cells from 2 patients with MPS IH (MPS-iPS cells). We found that α-L-iduronidase was not required for stem cell renewal, and that MPS-iPS cells showed lysosomal storage characteristic of MPS IH and could be differentiated to both hematopoietic and nonhematopoietic cells. The specific epigenetic profile associated with de-differentiation of MPS IH fibroblasts into MPS-iPS cells was maintained when MPS-iPS cells are gene-corrected with virally delivered α-L-iduronidase. These data underscore the potential of MPS-iPS cells to generate autologous hematopoietic grafts devoid of immunologic complications of allogeneic transplantation, as well as generating nonhematopoietic cells with the potential to treat anatomical sites not fully corrected with HCT.

Chondroitin sulfate and growth factor signaling in the skeleton: Possible links to MPS VI

Mucopolysaccharidosis type VI (MPS VI), also called Maroteaux-Lamy syndrome, is an autosomal recessive lysosomal storage disorder caused by deficiency of a specific enzyme required for glycosaminoglycan catabolism. Deficiency in the N-acetylgalactosamine-4-sulfatase (4S) enzyme, also called arylsulfatase B (ARSB), may have profound skeletal consequences. In MPS VI, partially degraded glycosaminoglycans (GAGs) such as dermatan sulfate and chondroitin sulfate accumulate within lysosomes. Through mechanisms that remain unclear, the abnormal GAG metabolism impacts several aspects of cellular function, particularly in the growth plate. This article explores the hypothesis that accrued partially degraded GAGs may contribute to deregulation of signaling pathways that normally orchestrate skeletal development, with a focus on members of the transforming growth factor-β (TGF-β) family. Understanding the molecular mechanisms disrupted by MPS VI may yield insight to improve the efficacy of MPS VI therapies, including bone marrow transplantation and enzyme replacement therapies.

Potential role of cathepsin K in the pathophysiology of mucopolysaccharidoses

Cathepsin K, a papain-like cysteine protease, is highly expressed in osteoclasts and plays a critical role in bone resorption. Dysfunction of the enzyme leads to various skeletal abnormalities. The recent knowledge that the collagenolytic activity of cathepsin K depends on interactions with bone and cartilage-resident glycosaminoglycans (GAGs) may shed some light on diseases such as mucopolysaccharidoses (MPSs). MPSs are a group of lysosomal storage diseases characterized by the accumulation of GAGs in tissues including bone. Typical pathological features of these diseases include skeletal abnormalities such as dysostosis multiplex, short stature, and multiple irregularities in bone development. We describe how further investigation of the cathepsin K/GAG complexes could provide valuable insights into the bone pathology associated with MPS diseases. In this review, we discuss the inhibition of osteoclast function through altered activity of cathepsin K by GAGs and offer insight into a mechanism for the bone pathology seen in MPS patients.