Mucopolysaccharidosis (MPS) consists of a group of inherited lysosomal storage disorders that are caused by a defect of certain enzymes that participate in the metabolism of glycosaminoglycans (GAGs). The abnormal accumulation of GAGs leads to progressive dysfunctions in various tissues and organs during childhood, contributing to premature death. As the current therapies are limited and inefficient, exploring the molecular mechanisms of the pathology is thus required to address the unmet needs of MPS patients to improve their quality of life. Lysosomal cysteine cathepsins are a family of proteases that play key roles in numerous physiological processes. Dysregulation of cysteine cathepsins expression and activity can be frequently observed in many human diseases, including MPS. This review summarizes the basic knowledge on MPS disorders and their current management and focuses on GAGs and cysteine cathepsins expression in MPS, as well their interplay, which may lead to the development of MPS-associated disorders.

Human cathepsin S (CatS) constitutes a key orchestrator of extracellular matrix (ECM) remodeling. Recent evidence suggest that CatS inhibition is a valuable therapeutic option for inflammatory diseases that are characterized by an excessive degradation of insoluble elastin, a process in which CatS contributes significantly when it is overexpressed. Here, we report novel findings that highlight the inhibition of CatS by two low-molecular weight highly sulphated exopolysaccharides (EPS), named diabolican (Dia_9-Hsulf) and infernan (Inf_19-Hsulf), derived from two marine bacterial strains Vibrio diabolicus and Alteromonas infernus, respectively. Dia_9-Hsulf and to a lesser extent Inf_19-Hsulf were selective and potent uncompetitive inhibitors of CatS (nanomolar range), which represents an unprecedented mechanism of CatS inhibition. In addition, both EPS selectively inhibited the elastolytic activity of CatS, without affecting the degradation of its other biological substrates (e.g. collagen, thyroglobulin). Combined experimental and in silico approaches provided new insights on how highly sulfated diabolican prevented the CatS elastolytic activity by hindering its elastin-binding exosite. These results evidenced EPS as promising molecules to preserve the elastin integrity during inflammatory events implying CatS.

Cathepsin S (Cat S) is a cysteine protease involved in several human diseases (i.e., autoimmune, inflammatory and cardiovascular disorders, cancer, and psoriasis) and is an important target in drug development. Emerging evidence highlights the potential of inhibiting Cat S by glycosaminoglycans, particularly chondroitin sulfates (CSs), as a promising therapeutic strategy. Given the limited and heterogeneous GAG materials from animal sources, a series of synthetic biotinylated non- or sulfated chondroitin oligomers were synthesized and assessed for their ability to inhibit Cat S. The biotinylated disaccharide C4S displayed in vitro potent inhibitory activity toward Cat S with IC₅₀ value in the micromolar range and showed selectivity over cathepsins K and L. Molecular modeling studies suggested that only C4S dp2 but not C6S, C4,6S or non-sulfated chondroitin binds selectively to the active site of Cat S. In addition, a synthetic multivalent C4S dp2 glycosylated BSA was shown to be more efficient towards Cat S inhibition (nanomolar range) than the monovalent parent C4S dp2. Our findings also indicated that this new neoglycoconjugate displayed selectivity for Cat S vs. cysteine cathepsins expressed by differentiated THP-1 cells. This study reports a new approach for designing selective and potent inhibitors of Cat S using multivalent C4S derivatives as a molecular scaffold.

Pulmonary fibrosis is characterized by extracellular matrix (ECM) deposition driven by fibroblast-to-myofibroblast transition (FMT) and by an altered proteolytic balance. While the roles of several cysteine proteases have been documented, the specific contribution of cathepsin V (CatV) and legumain (LGMN) remains poorly explored. LGMN, CatV and their dual inhibitor cystatin M/E (CysM/E) are significantly increased in lung specimens and bronchoalveolar lavage fluids from patients with idiopathic pulmonary fibrosis. TGF-β1 triggered CysM/E expression and LGMN transcription, intracellular maturation, enzymatic activity, and pro-LGMN secretion via the Smad-3 pathway, whereas CatV was downregulated in human lung fibroblasts (CCD-19Lu and primary HPF cells) undergoing myodifferentiation. Genetic silencing of LGMN or CatV, and pharmacological inhibition of LGMN, led to accumulation of fibronectin and elastin, implying that both proteases contribute to ECM remodeling. LGMN cleaved fibronectin, while CatV predominantly regulated elastin levels. Conversely, broad-spectrum inhibitor cystatin C (hCC) markedly _r educed elastin and fibronectin degradation, whereas CysM/E exerted a weaker effect, mainly on elastin turnover. LGMN inhibition transiently delayed fibroblast wound closure, establishing a functional role in tissue repair through fibronectin remodeling. Neither LGMN nor CatV influenced α-SMA expression, distinguishing them from CatB, which participates in FMT. Altogether, LGMN was identified as an effector of matrix remodeling rather than myodifferentiation, acting in concert with CatV. Within the proteolytic network governing fibrosis progression, the present findings identify a cystatin-regulated LGMN/CatV partnership, participating in ECM turnover and cell migration. Present results also provide new perspectives on potential therapeutic protease-based strategies targeting ECM turnover underlying lung fibrosis.