Effects of accumulated lysosomal products on cellular and tissue homeostasis. (1) Accumulation of GAGs in the lysosome leads to enlargement of the lysosome, followed by a loss of membrane integrity. Leakage of H⁺ out of the lysosome results in a higher pH and may lead to compromised activity of lysosomal hydrolases and secondary lysosomal storage. Furthermore, the higher pH results in impaired fusion of endolysosomes and autolysosomes and a reduced recycling of the M6P receptor. Finally, enlarged lysosomes have decreased diffusion ability inside the cell. (2) Enlargement of the lysosome ultimately results in lysosomal rupture with the escape of cathepsins and proteases in the cell. Simultaneously, lysosomal rupture leads to increased concentration of H⁺ and Ca²⁺ in the cytosol. Together, this results in increased reactive oxygen species (ROS) production, necrosis, and apoptosis of cells. (3) The dysfunctional lysosome leads to impaired autophagy and fusion with endolysosomes. Therefore, increased lifespans are seen for autolysosomes and endolysosomes. Impaired autophagy subsequently results in mitochondrial dysfunction. (4) The increased concentration of cytosolic Ca²⁺ and cellular stress stimulates the release of extracellular vesicles (EVs). This is an alternative protective route for the cell to maintain cellular homeostasis. (5) The accumulation of GAGs in the cell and the secondary affected mechanisms result in tissue alterations and destruction. Connective tissue is the most affected tissue in MPS patients. Studies show altered proteoglycan turnover, altered fibril spacing and alignment, and altered chondrocyte distribution and disruption leading to increased concentrations of matrix metalloproteinase-2 (MMP-2), MMP-9, and tissue inhibitor of metalloproteinase-1 (TIMP-1). Depending on the type of GAG, undegraded GAGs lead to inhibition or activation of cathepsin K, which is necessary for the degradation of aggrecan and collagen I and II. Finally, HS and cellular stress lead to inflammation via activation of the Toll-like receptor 4 (TLR4) pathway. DAMPs, damage associated molecular patterns.