sign in sign up
<<
Home , Aggrecan

seminars in & DEVELOPMENTAL BIOLOGY, Vol. 12, 2001: pp. 65–67 doi:10.1006/scdb.2000.0237, available online at http://www.idealibrary.com on

Cellular functions of —an overview

Norbert Perrimon a and Merton Bernfield b

Introduction valently associated with . But it is considered here because it is highly abundant in vertebrate em- bryos and tightly associates with a wide variety of pro- This issue of Seminars in Cell and Developmental Biology teins in performing major developmental functions. contains a number of articles reviewing the cellular The structural diversities of proteoglycans under- functions of proteoglycans. Proteoglycans are pro- lie their various functions. For example, via their HS teins, substituted with (GAGs), chains, syndecans can bind growth factors, extracellu- that generally either decorate the cell membrane or lar matrix components, enzymes, inhibitors, fill the extracellular space. GAGs are linear polysac- charides consisting of a repeating disaccharide, chemokines among other extracellular constituents, generally of an acetylated amino sugar alternating and play an active role in signal transduction. Others, with a uronic acid. Although there are intracellular such as aggrecan or , are structural compo- types, most proteoglycans are destined for the ex- nents of the that associate tightly tracellular space. These are very diverse molecules, with both HA and . and various combinations of both different types Extensive biochemical studies of proteoglycans of proteins [matrix, cell surface transmembrane have provided a wealth of information on the struc- or covalently linked to membrane glycosylphos- ture of these molecules. Much of the early structural phatidylinositol (GPI)] and classes of GAG chains work was done by Karl Meyer on HA and by Jorpes [hyaluronan (HA), (CS), keratan and Gardell on HS. The pioneering work of Hascall sulfate (KS), (DS) and heparan sul- and Sajdera taught us much about the structure fate (HS)] are found in vertebrates. For example, the of proteoglycans, and that of Kraemer syndecan transmembrane proteins can be decorated showed that every adherent cell contains HS at their with two HS and one CS chains, while the aggrecan surfaces. Dorfman, Silbert, and Lindahl performed cartilage matrix protein is typically decorated by now classic studies on GAG biosynthesis. Despite about 100 CS and 30 KS chains, and , a major their widespread distribution and this wealth of of the interstitial matrix, contains a information, until relatively recently little was known single DS chain. Further, GAGs may decorate only about proteoglycan function beyond their structural a proportion of a type of protein. The function of role in organizing cartilage and the basal lamina. the GAG substituent in these so-called ‘part-time In recent years, understanding of proteoglycan proteoglycans’ is rarely understood. functions has been obtained from cell biological Proteoglycans can be very abundant. At the surface and genetic studies. These studies have provided of epithelial cells, it has been estimated that there many surprises and have propelled the study of may be as many as 1 million syndecan-1 molecules, proteoglycans to the forefront of modern biomedical and cartilage matrix is a several mg per ml composite research. In particular, the importance of these of aggrecan with HA. HA, the structurally most simple molecules has been highlighted by the discovery that GAG, is not an authentic proteoglycan, as it is not co- a number of diseases, such as the Simpson- Golabi-Behmel syndrome (SGBS), a rare pre- and post-natal overgrowth, birth defect and tumor sus- ceptibility syndrome associated with mutations in a a From the Department of Genetics, Harvard Medical School, Boston, GPI-linked HS proteoglycan, and multiple hereditary MA 02115, USA and bDepartments of Pediatrics and Cell Biology, Harvard Medical School, Children’s Hospital, Boston, MA 02115, USA. exostoses (EXTs), bony tumors that can undergo c 2001Academic Press malignant transformation, associated with mutations 1084–9521/01/020065 03/$35.00 in an HS biosynthetic enzyme. Further, genetic + 65 N. Perrimon and M. Bernfield analyses in the mouse and Drosophila model systems Cellular function of proteoglycans have provided novel insights into the function of these molecules during development. The various chapters illustrate: (1) the role of some All cellular processes that involve molecular interac- proteoglycans in organization of the extracellular tions at the cell surface, such as cell–matrix, cell–cell matrix (Knudson and Knudson, Toole); (2) the and ligand–receptor interactions, likely involve role of proteoglycans in metastasis and invasion of proteoglycans because these molecules avidly bind tumor cells (Sanderson) and cell differentiation proteins and are quite abundant at this site. This in the nervous system (Yamaguchi); (3) the role diversity of interactions is well illustrated during the of membrane attached proteogly- development of the nervous system and concomitant cans (HSPGs) in regulation of cell signaling events with tumor invasion and metastasis, instances where (Rapraeger, De Cat and David); and (4) Genetic cell movements and cell extensions are the major evidence for proteoglycan function in invertebrate cellular behaviors. models (Selleck). Yamaguchi describes the role of HSPGs during three critical phases of the development of the Proteoglycans and extracellular matrix mammalian nervous system. He reviews their roles in the generation and differentiation of from stem cells, in axonal guidance and in synapse devel- Proteoglycans constitute a major component of the opment. In particular, the roles of cell surface HSPGs extracellular matrix and have been particularly well and growth factors in these processes are described. analyzed in the context of cartilage where the matrix The critical aspects of tumor cell behavior that constitutes more than 90% of the dry weight of the influence the health of an organism are local invasion tissue. Knudson and Knudson review the role of pro- and distant metastasis. These behaviors depend on teoglycans in organizing cartilage and point out that , motility and growth, each readily it is the unique mixing of several proteoglycans to- affected by HSPGs. Sanderson reviews these effects gether with their organization within the extracellular of proteoglycans, and emphasizes that HSPGs can matrix that gives cartilage its unique physical proper- either promote or inhibit these processes depending ties. In particular, they discuss the interactions that on the tissue type, the pathophysiological state of the illustrate the complex mode of association between a tumor, and the step within the metastatic cascade that proteoglycan (aggrecan), a GAG (HA), and a protein is affected. (link protein) in the cartilage extracellular matrix. HA is a linear GAG composed of alternating residues of glucuronic acid and N-acetylglucosamine which is Proteoglycans and signal transduction found at the cell surface, in the extracellular matrix and within cells. It interacts with many proteins (hyal- adherins) that modify its structural and physiologi- Two major families of cell surface HSPGs, syndecans cal properties. Knudson and Knudson also provides and , have been identified. These bind a a thorough description of other proteoglycans found multitude of growth factors and extracellular matrix in cartilage matrix. molecules, and have been implicated in several signal The developmental roles of HA are further transduction pathways that regulate cell proliferation discussed by the paper of Toole that reviews the and cell shape. While syndecans are transmembrane function of this GAG at the surface of mesenchy- proteins with an intracellular cytoplasmic domain, mal cells. Toole describes the motifs found in the glypicans are attached to discrete cell membrane multiple hyaladherins that interact with HA, and regions by a GPI lipid anchor. then describes how changes in the hydration of the Rapraeger details the function and various binding pericellular matrix influence the physical nature of partners of the four known mammalian synde- HA, and regulate signal transduction events as well cans. The syndecan core proteins are structurally as cell behaviors such as mitosis and cell migration. quite similar but have distinct regions and cellular Altogether, the reviews of Knudson and Kundson distributions, but share conserved cytoplasmic, jux- and Toole illustrate how changes in the physical tamembrane and transmembrane domains. Both characteristics of a GAG can influence a variety of the extracellular domain and a region within the cellular events. cytoplasmic domain are divergent underlying the

66 diverse functions of these molecules. Rapraeger also They differ from proteins and other describes both the conserved and divergent partner by their GAG chain substituents. While in some proteins associated with syndecan core proteins instances their functions reflect the unique chemical and relates these to the cellular and developmental properties of the linear anionic polysaccharide functions of these proteoglycans. GAGs (e.g. binding of water by aggrecan to provide DeCat and David review the structure and function cartilage with the ability to resist compression), in of the glypicans. Recent studies in Drosophila, other instances the rationale for Nature employing and mice have implicated these cell surface molecules a GAG chain is not clear (e.g. the co-receptor role in the control of cell growth and differentiation. In of cell surface HSPGs when several cell surface particular they have been implicated in regulating the proteins similarly serve as co-receptors). Further, signaling pathways of molecules such as FGFs, BMPs, the evolutionary advantage for synthesizing GAG Wnts, Hhs and IGFs. Although the detailed molecu- chains, which appear to require specific sequences lar mechanisms by which these molecules operate is to interact selectively with proteins, by an enzymatic, not yet understood, one attractive possibility is that non-template mechanism is quite unclear. The they activate or determine the activity-ranges of mor- resulting sequence complexity of GAG chains has phogens and growth factors. all the characteristics of an informational code, one which we cannot as yet read, but which is used by the very large variety of extracellular proteins that Proteoglycan function in model organisms bind to GAG chains. Importantly, except for HA, no GAG-containing proteoglycans exist in prokaryotes Much of the enhanced current interest in proteo- or in yeast, and it appears that proteoglycans arose glycans results from genetic studies of the fruit fly, evolutionarily with the emergence of multicellularity Drosophila melanogaster, and the nematode worm, in the metazoans. A critical role for proteoglycans Caenorhabditis elegans. Identification of a syndecan in survival is inferred from the retention of so many homolog in the fly was initially a surprise, but proteoglycan encoding in higher vertebrates. subsequent work, reviewed by Selleck, has revealed These considerations suggest that the key cellular multiple physiological roles for cell surface HSPGs role of these molecules is to mediate cell interactions, in these organisms. Despite major differences in whether direct, via soluble mediators or via the physiology and development, the functions of the insoluble extracellular matrix. Once we understand two Drosophila glypicans and single syndecan are the rationale for these molecules and decipher the often mimicked in mammals by their six glypicans code, the reasons for their ubiquity and abundance and four syndecans, providing a clear example of should become apparent and we should be able to conservation of function during the two genome modulate their interactions for therapeutic purposes. duplications that transpired between the times when With so much more to learn, the next several years these organisms emerged. will be exciting for proteoglycan research.

Concluding remarks: whither proteoglycans? Acknowledgement

Proteoglycans are ubiquitous, highly abundant, We thank Charlotte McManus for organizing and chemically complex and functionally disparate. compiling these essays.

67