RESUMO
Alzheimer's disease (AD) is characterized by accumulation of misfolded proteins. Genetic studies implicate microglia, brain-resident phagocytic immune cells, in AD pathogenesis. As positive effectors, microglia clear toxic proteins, whereas as negative effectors, they release proinflammatory mediators. An imbalance of these functions contributes to AD progression. Polymorphisms of human CD33, an inhibitory microglial receptor, are linked to AD susceptibility; higher CD33 expression correlates with increased AD risk. CD33, also called Siglec-3, is a member of the sialic acid-binding immunoglobulin-type lectin (Siglec) family of immune regulatory receptors. Siglec-mediated inhibition is initiated by binding to complementary sialoglycan ligands in the tissue environment. Here, we identify a single sialoglycoprotein in human cerebral cortex that binds CD33 as well as Siglec-8, the most abundant Siglec on human microglia. The ligand, which we term receptor protein tyrosine phosphatase zeta (RPTPζ)S3L, is composed of sialylated keratan sulfate chains carried on a minor isoform/glycoform of RPTPζ (phosphacan) and is found in the extracellular milieu of the human brain parenchyma. Brains from human AD donors had twofold higher levels of RPTPζS3L than age-matched control donors, raising the possibility that RPTPζS3L overexpression limits misfolded protein clearance contributing to AD pathology. Mice express the same structure, a sialylated keratan sulfate RPTPζ isoform, that binds mouse Siglec-F and crossreacts with human CD33 and Siglec-8. Brains from mice engineered to lack RPTPζ, the sialyltransferase St3gal4, or the keratan sulfate sulfotransferase Chst1 lacked Siglec binding, establishing the ligand structure. The unique CD33 and Siglec-8 ligand, RPTPζS3L, may contribute to AD progression.
Assuntos
Doença de Alzheimer , Lectinas Semelhantes a Imunoglobulina de Ligação ao Ácido Siálico , Doença de Alzheimer/genética , Doença de Alzheimer/metabolismo , Animais , Encéfalo/metabolismo , Humanos , Sulfato de Queratano/metabolismo , Ligantes , Camundongos , Microglia/metabolismo , Isoformas de Proteínas/metabolismo , Proteínas Tirosina Fosfatases Classe 5 Semelhantes a Receptores/metabolismo , Lectina 3 Semelhante a Ig de Ligação ao Ácido Siálico/genética , Lectina 3 Semelhante a Ig de Ligação ao Ácido Siálico/metabolismo , Lectinas Semelhantes a Imunoglobulina de Ligação ao Ácido Siálico/genética , Lectinas Semelhantes a Imunoglobulina de Ligação ao Ácido Siálico/metabolismoRESUMO
Pleiotrophin (PTN) is a secreted heparin-binding growth factor that through its receptor protein tyrosine phosphatase beta/zeta (RPTPß/ζ) has a significant regulatory effect on angiogenesis and cancer. PTN and RPTPß/ζ are over-expressed in several types of human cancers and regulate important cancer cell functions in vitro and cancer growth in vivo. This review begins with a brief introduction of PTN and the regulation of its expression. PTN receptors are described with special emphasis on RPTPß/ζ, which also interacts with and/or affects the function of other important targets for cancer therapy, such as vascular endothelial growth factor A, ανß3 and cell surface nucleolin. PTN biological activities related to angiogenesis and cancer are extensively discussed. Finally, up to date approaches of targeting PTN or RPTPß/ζ for cancer treatment are presented. Insights into the regulatory role of PTN/RPTPß/ζ on angiogenesis will be extremely beneficial for future development of alternative anti-angiogenic approaches in cancer therapy.