New biotechnological and nanomedicine strategies for treatment of lysosomal storage disorders

Lysosomal enzyme transport. After their biosynthesis in the endoplasmic reticulum (ER), lysosomal enzymes are transported to the Golgi apparatus where they are modified by addition of sugar residues and, in some cases, phosphorylation. Certain enzyme residues, such as mannose‐6‐phosphate (M6P) are then recognized by M6P receptors (M6PR), which mediate intracellular trafficking to lysosomes. Lysosomal enzymes are also secreted to the extracellular milieu, where they can bind to M6PR on the cell surface of the same cell or a different cell. This mediates clathrin‐mediated uptake and endocytic transport to lysosomes. This pathway constitutes the foundation for delivery of recombinant enzymes in replacement therapy protocols.

Chimeric lysosomal enzymes. To improve delivery of replacement therapies for treatment of lysosomal storage disorders, the recombinant enzymes can be produced as multifunctional fusion proteins or chimeras comprising the enzymatic module and an affinity peptide, which provides affinity to the cell surface. These peptides can be designed to target receptors of endogenous lysosomal enzymes, such as the mannose‐6‐phosphate receptor (M6PR), which can be targeted by M6P and insulin‐like growth factor II (IGF‐II). Chimeric enzymes can also be targeted to alternative receptors of endocytosis, such as low‐density lipoprotein receptor (LDLR) family, which is targeted by receptor‐associated protein (RAP), transferrin receptor (TfR), targeted by transferrin (Tf), or insulin receptor (InsR), targeted by a peptide derived from an antibody to InsR (anti‐InsR). These fusion enzymes can also be addressed to negatively charged domains of the plasma membrane, which can be targeted by Tat peptides. Chimeric enzymes targeted by these peptides can enter cells and traffic to lysosomes via clathrin‐mediated endocytosis (those targeted to the receptors described above) or unspecific endocytosis (those targeted by Tat). (Reproduced with permission from Ref 33. Copyright 2006 CRC Press).

Brain targeting of recombinant lysosomal enzymes. To improve delivery of replacement therapies to the brain in the case of lysosomal storage disorders affecting the central nervous system, recombinant enzymes can be designed to target receptors involved in transcellular transport of substances across the endothelium in the blood–brain barrier. This transport mediated by vesicular transcytosis is relatively safe, as it does not cause opening of the junctions between adjacent endothelial cells maintaining the permeability barrier (the paracellular route). M6PR, mannose‐6‐phosphate receptor; TfR, transferrin receptor; InsR, insulin receptor.

Intercellular adhesion molecule 1 (ICAM‐1) structure. ICAM‐1 is characterized by a long glycosylated extracellular region with five immunoglobulin‐like domains (D1–D5), a transmembrane region, and a short cytosolic tail. The extracellular domains mediate interaction with leukocyte β₂ integrins (leukocyte function‐associated antigen (LFA) and Mac‐1) and fibrin(ogen). The cytosolic domain mediates signal transduction upon engagement of the extracellular domains. ICAM‐1‐mediated signaling includes tyrosine phosphorylation of SHP‐2 helped by Grb2, PKC activation, phosphorylation of Src kinases, signaling through MAP kinases, Rho‐dependent remodeling of the cytoskeleton, activation of cortactin and focal adhesion kinase (FAK), and interaction with α‐actinin, ezrin, and moesin, and the microtubule‐related protein β‐tubulin.99.

Intercellular adhesion molecule 1 (ICAM‐1), CAM‐mediated endocytosis and lysosomal transport. CAM‐endocytosis is induced upon ICAM‐1 engagement by multivalent ligands, such as 100‐nm polystyrene or poly(lactic‐co‐glycolic acid) nanocarriers coated by anti‐ICAM. This pathway involves PKC, Src kinases, and Rho‐dependent kinase (ROCK) and formation of actin stress fibers. ICAM‐1 interacts with the amiloride‐sensitive, sodium/proton exchanger NHE1, which contributes to cell surface deformability and is an adaptor of the cytoskeleton. Dynamin is involved in budding of the nascent vesicles. These events contribute to the progression of membrane invaginations and intracellular vesicular transport. After internalization, ICAM‐1/anti‐ICAM carrier complexes traffic to endosomes that are positive for EEA1 and PKC‐regulated NHE6. ICAM‐1 recycles then to the plasmalemma, and anti‐ICAM nanocarriers traffic to LAMP1‐positive lysosomes. (Reproduced with permission from Ref 95. Copyright 2006 ASPET).

Enzyme replacement therapy by Intercellular adhesion molecule 1 (ICAM‐1)‐targeted nanocarriers. Model polymer nanocarriers (FITC‐labeled polystyrene, 100‐nm diameter) were coated by surface absorption with recombinant acid sphingomyelinase (acid sphingomyelinase (ASM), Schuchman—Mount Sinai School of Medicine) and monoclonal anti‐ICAM or control immunoglobulin G (IgG). Upon injection in vivo in mice, only ICAM‐1‐targeted carriers (lower panel, left), but not control IgG carriers (upper panel, left), showed efficient binding to the vasculature irrigating all organs (mesentery is shown).118 At the cellular level, incubation of skin fibroblasts from type B Niemann–Pick disease patients with non‐fluorescent carriers for 3 h (lower panel, right) significantly attenuated lysosomal sphingomyelin (labeled by a green‐fluorescent BODIPY derivative, upper panel, right).108 (Reproduced with permission from Ref 118. Copyright 2008 ASPET).