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Heparan sulfate proteoglycans (HSPGs) have essential functions during embryonic development and throughout postnatal life. To exert these functions, they undergo a series of processing reactions by heparan-sulfate-modifying enzymes (HSMEs), which endows them with highly modified heparan sulfate (HS) domains that provide specific docking sites for a large number of bioactive molecules. The development and antigen-dependent differentiation of normal B lymphocytes, as well as the growth and progression of B-lineage malignancies, are orchestrated by an array of growth factors, cytokines and chemokines many of which display HS binding. As discussed in this review, tightly regulated HSPG expression is a requirement for normal B cell maturation, differentiation and function. In addition, the HSPG syndecan-1 functions as a versatile co-receptor for signals from the bone marrow microenvironment, essential for the survival of long-lived plasma cells and multiple myeloma (MM) plasma cells. Targeting of HSMEs or HS chains on MM cells increases their sensitivity to drugs currently used in MM treatment, including bortezomib, lenalidomide or dexamethasone. Taken together, these findings render the HS biosynthetic machinery a promising target for MM treatment., (© 2013 The Authors Journal compilation © 2013 FEBS.)

The development and antigen-dependent differentiation of B lymphocytes are orchestrated by an array of growth factors, cytokines, and chemokines that require tight spatiotemporal regulation. Heparan sulfate proteoglycans specifically bind and regulate the bioavailability of soluble protein ligands, but their role in the immune system has remained largely unexplored. Modification of heparan sulfate by glucuronyl C5-epimerase (Glce) controls heparan sulfate-chain flexibility and thereby affects ligand binding. Here we show that Glce deficiency impairs B-cell maturation, resulting in decreased plasma cell numbers and immunoglobulin levels. We demonstrate that C5-epimerase modification of heparan sulfate is critical for binding of a proliferation inducing ligand (APRIL) and that Glce-deficient plasma cells fail to respond to APRIL-mediated survival signals. Our results identify heparan sulfate proteoglycans as novel players in B-cell maturation and differentiation and suggest that heparan sulfate conformation is crucial for recruitment of factors that control plasma cell survival.

The development of lymphoid organs depends on cross talk between hematopoietic cells and mesenchymal stromal cells and on vascularization of the lymphoid primordia. These processes are orchestrated by cytokines, chemokines, and angiogenic factors that require tight spatiotemporal regulation. Heparan sulfate (HS) proteoglycans are molecules designed to specifically bind and regulate the bioactivity of soluble protein ligands. Their binding capacity and specificity are controlled by modification of the HS side chain by HS-modifying enzymes. Although HS proteoglycans have been implicated in the morphogenesis of several organ systems, their role in controlling lymphoid organ development has thus far remained unexplored. In this study, we report that modification of HS by the HS-modifying enzyme glucuronyl C5-epimerase (Glce), which controls HS chain flexibility, is required for proper lymphoid organ development. Glce(-/-) mice show a strongly reduced size of the fetal spleen as well as a spectrum of defects in thymus and lymph node development, ranging from dislocation to complete absence of the organ anlage. Once established, however, the Glce(-/-) primordia recruited lymphocytes and developed normal architectural features. Furthermore, Glce(-/-) lymph node anlagen transplanted into wild-type recipient mice allowed undisturbed lymphocyte maturation. Our results indicate that modification of HS by Glce is required for controlling the activity of molecules that are instructive for early lymphoid tissue morphogenesis but may be dispensable at later developmental stages and for lymphocyte maturation and differentiation.