Systemic Glycosaminoglycan Clearance by HARE/Stabilin-2 Activates Intracellular Signaling

Scavenger receptors perform essential functions, critical to maintaining mammalian physiologic homeostasis by continuously clearing vast numbers of biomolecules from blood, interstitial fluid and lymph. Stabilin-2 (Stab2) and the Hyaluronic Acid Receptor for Endocytosis (HARE), a proteolytic isoform of Stab2, are important scavenger receptors responsible for the specific binding and internalization (leading to degradation) of 22 discrete molecules, macromolecular complexes and cell types. One-third of these ligands are glycosaminoglycans (GAGs). Full-length Stab2, but not HARE, mediates efficient phagocytosis of apoptotic cells and bacteria via binding to target surface ligands. HARE, the C-terminal half of Stab2, mediates endocytosis of all the known soluble ligands. HA was the first ligand identified, in 1981, prior to receptor purification or cloning. Seven other GAG ligands were subsequently identified: heparin, dermatan sulfate, chondroitin and chondroitin sulfates A, C, D and E. Synthetic dextran sulfate is also a GAG mimic and ligand. HARE signaling during HA endocytosis was first discovered in 2008, and we now know that activation of HARE/Stab2 signaling is stimulated by receptor-mediated endocytosis or phagocytosis of many, but not all, of its ligands. This review focuses on the HARE-mediated GAG activation of intracellular signaling, particularly the Extracellular Signal-Regulated Kinase 1/2 pathway.

Self-regenerating giant hyaluronan polymer brushes

Tailoring interfaces with polymer brushes is a commonly used strategy to create functional materials for numerous applications. Existing methods are limited in brush thickness, the ability to generate high-density brushes of biopolymers, and the potential for regeneration. Here we introduce a scheme to synthesize ultra-thick regenerating hyaluronan polymer brushes using hyaluronan synthase. The platform provides a dynamic interface with tunable brush heights that extend up to 20 microns - two orders of magnitude thicker than standard brushes. The brushes are easily sculpted into micropatterned landscapes by photo-deactivation of the enzyme. Further, they provide a continuous source of megadalton hyaluronan or they can be covalently-stabilized to the surface. Stabilized brushes exhibit superb resistance to biofilms, yet are locally digested by fibroblasts. This brush technology provides opportunities in a range of arenas including regenerating tailorable biointerfaces for implants, wound healing or lubrication as well as fundamental studies of the glycocalyx and polymer physics.

Discovery of the Liver Hyaluronan Receptor for Endocytosis (HARE) and Its Progressive Emergence as the Multi-Ligand Scavenger Receptor Stabilin-2

Since the discovery of a novel liver hyaluronan (HA) clearance receptor in 1981 by Laurent, Fraser and coworkers, 22 different ligands cleared by the renamed receptor (the Hyaluronan Receptor for Endocytosis (HARE); Stabilin-2 (Stab2)) were discovered over 37 years. Ligands fall into three groups: (1) 11 anionic polymers, (2) seven cleaved or modified proteins and (3) four types of cells. Seven synthetic ligands, not found normally in serum or tissues, likely mimic natural molecules cleared by the receptor. In 2002 we purified and cloned HARE, based on HA-binding activity, and two other groups cloned full-length receptor; FEEL-2 and Stab2. Macrophages likely require full-length Stab2 for efficient binding and phagocytosis of bacteria or apoptotic cells, since cell-binding domains are throughout the receptor. In contrast, all 16 known single-molecule binding sites are only within the C-terminal half (190HARE). The HARE isoform is generated by proteolysis, not mRNA splicing. The majority of circulating ligands is cleared by HARE, since sinusoidal endothelial cells of liver, spleen and lymph node express twice as many HARE half-receptors as full-length receptors. Based on their significant binding and functional differences, a modified receptor nomenclature is proposed that designates HARE as the C-terminal half-receptor isoform and Stab2 as the full-length receptor isoform.

Hyaluronan synthase assembles hyaluronan on a [GlcNAc(β1,4)]n-GlcNAc(α1→)UDP primer and hyaluronan retains this residual chitin oligomer as a cap at the nonreducing end

Class I hyaluronan synthases (HAS) assemble [GlcNAc(β1,4)GlcUA(β1,3)]n-UDP at the reducing end and also make chitin. Streptococcus equisimilis HAS (SeHAS) also synthesizes chitin-UDP oligosaccharides, (GlcNAc-β1,4)n-GlcNAc(α1→)UDP (Weigel et al. 2015). Here we determined if HAS uses chitin-UDPs as primers to initiate HA synthesis, leaving the non-HA primer at the nonreducing (NR) end. HA made by SeHAS membranes was purified, digested with streptomyces lyase, and hydrophobic oligomers were enriched by solid phase extraction and analyzed by MALDI-TOF MS. Jack bean hexosaminidase (JBH) and MS/MS were used to analyze 19 m/z species of possible GnHn ions with clustered GlcNAc (G) residues attached to disaccharide units (H): (GlcNAcβ1,4)2-5[GlcUA(β1,3)GlcNAc]2-6. JBH digestion sequentially removed GlcNAc from the NR-end of GnHn oligomers, producing successively smaller GnH2-3 series members. Since lyase releases dehydro-oligos (dHn; M-18), only the unique NR-end oligo lacks dehydro-GlcUA. Hn oligomers were undetectable in lyase digests, whereas JBH treatment created new H2-6m/z peaks (i.e. HA tetra- through dodeca-oligomers). MS/MS of larger GnHn species produced chitin (2-5 GlcNAcs), HA oligomers and multiple smaller series members with fewer GlcNAcs. All NR-ends (97%) started with GlcNAc, as a chitin trimer (three GlcNAcs), indicating that GlcNAc(β1,4)2GlcNAc(α1→)-UDP may be optimal for initiation of HA synthesis. Also, HA made by live S. pyogenes cells had G4Hn chitin-oligo NR-ends. We conclude that chitin-UDP functions in vitro and in live cells as a primer to initiate synthesis of all HA chains and these primers remain at the NR-ends of HA chains as residual chitin caps [(GlcNAc-β1,4)3-4].

A TLR/AKT/FoxO3 immune tolerance-like pathway disrupts the repair capacity of oligodendrocyte progenitors

Cerebral white matter injury (WMI) persistently disrupts myelin regeneration by oligodendrocyte progenitor cells (OPCs). We identified a specific bioactive hyaluronan fragment (bHAf) that downregulates myelin gene expression and chronically blocks OPC maturation and myelination via a tolerance-like mechanism that dysregulates pro-myelination signaling via AKT. Desensitization of AKT occurs via TLR4 but not TLR2 or CD44. OPC differentiation was selectively blocked by bHAf in a maturation-dependent fashion at the late OPC (preOL) stage by a noncanonical TLR4/TRIF pathway that induced persistent activation of the FoxO3 transcription factor downstream of AKT. Activated FoxO3 selectively localized to oligodendrocyte lineage cells in white matter lesions from human preterm neonates and adults with multiple sclerosis. FoxO3 constraint of OPC maturation was bHAf dependent, and involved interactions at the FoxO3 and MBP promoters with the chromatin remodeling factor Brg1 and the transcription factor Olig2, which regulate OPC differentiation. WMI has adapted an immune tolerance-like mechanism whereby persistent engagement of TLR4 by bHAf promotes an OPC niche at the expense of myelination by engaging a FoxO3 signaling pathway that chronically constrains OPC differentiation.

Planning, evaluating and vetting receptor signaling studies to assess hyaluronan size-dependence and specificity

Exciting discoveries in many diverse fields of hyaluronan (HA) biology over the last 40 years have centered around the ability of HA to bind cell surface HA receptors (e.g., CD44, Layilin, LYVE-1, HARE/Stab2 and RHAMM) and sometimes also to activate intracellular signal transduction pathways, frequently involving ERK1/2. Although perplexing, a major characteristic of HA-mediated signal pathway activation for some receptors has been a dependence on the size of the bound HA. Receptors that directly interact with HA, which may not include TLR2/4, bind very well to any HA molecule >8-20 sugars, depending on the receptor. Despite their ability to bind virtually any size HA, only HA chains of a particular mass range can activate receptor-mediated cell signaling. Many studies have demonstrated parts of this emerging story by utilizing different: HA receptors, cell types, animal models, HA sources, HA sizes, assays to assess HA mass and varying controls to verify HA specificity or HA size-dependence. Recent reports have highlighted issues with potential endotoxin contamination of HA fragments, especially those generated by hyaluronidase digestion. Also, researchers unfamiliar with HA polydispersity must adjust to working with, and interpreting data for, preparations without a unique molecular mass (molecular weight). The confusion, uncertainty and skepticism generated by these and other factors has hindered the development of a general consensus about HA-specific and HA-size dependent receptor activation. An overview of issues, suggested strategies and validating controls is presented to aid those planning an HA-mediated receptor signaling study or those trying to evaluate the literature.

What is special about 200 kDa hyaluronan that activates hyaluronan receptor signaling?

The polydispersity of hyaluronan (HA) presents challenges for analyzing its solution properties, such as the relationship between mass and particle size. The broad mass range of natural HA (≤50-fold) makes molecular characterization difficult and ambiguous compared to molecules with known molecular weights (e.g., proteins). Biophysical studies show that large >MDa HA behaves like a random coil, whereas very small (e.g., 10 kDa) HA behaves like a rod. However, the mass range for this conformational transition is not easily determined in natural polydisperse HA. Some HA receptors (e.g., CD44 and HARE) initiate signaling responses upon binding HA in the 100-300 kDa range, but not larger MDa HA. Size-dependent responses are studied using nonnatural HA: purified narrow-size range HA [Pandey MS, Baggenstoss BA, Washburn J, Harris EN, Weigel PH. 2013. The hyaluronan receptor for endocytosis (HARE) activates NF-κB-mediated gene expression in response to 40-400 kDa, but not smaller or sarger, hyaluronans. J Biol Chem. 288:14068-14079] and very narrow size range Select-HA made chemo-enzymatically [Jing W, DeAngelis PL. 2004. Synchronized chemoenzymatic synthesis of monodisperse hyaluronan polymers. J Biol Chem. 279:42345-42349]. Here, we used size exclusion chromatography and multiangle light scattering to determine the weight-average molar mass and diameter of ~60 very narrow size preparations from 29 to 1650 kDa. The ratio of HA mass to HA diameter showed a transition in the 150-250 kDa size range (~65 nm). The HA rod-to-coil transition occurs within the size range that specifically activates cell signaling by some receptors. Thus, size-specific signaling could be due to unique external receptor•HA conformation changes that enable transmembrane-mediated activation of cytoplasmic domains. Alternatively and more likely, transition-size HA may enable multiple receptors to bind the same HA, creating new internal signal-competent cytoplasmic domain complexes.

Hyaluronan synthase control of synthesis rate and hyaluronan product size are independent functions differentially affected by mutations in a conserved tandem B-X7-B motif

Hyaluronan synthases (HAS) normally make large (>MDa) hyaluronan (HA) products. Smaller HA fragments (e.g. 100-400 kDa) produced in vivo are associated with inflammation and cell signaling by HA receptors that bind small, but not large, HA. Although HA fragments can arise from breakdown by hyaluronidases, HAS might also be regulated directly to synthesize small HA. Here we examined the Streptococcus equisimilis HAS (SeHAS) C-terminus, which contains a tandem B-X₇-B motif (K³⁹⁸-X₇-R⁴⁰⁶-X₇-K⁴¹⁴), by testing the effects of 27 site-specific scanning mutations and 7 C-terminal truncations on HA synthesis activity and weight-average mass. Although HAS enzymes cannot be HA-binding proteins, these motifs are highly conserved within the Class I HAS family. Fifteen Arg⁴⁰⁶ mutants made large MDa HA (86-110% wildtype size), with specific activities from 70% to 177% of wildtype. In contrast, 10 of 12 Lys³⁹⁸ mutants made HA that was 8-14% of wildtype size (≤250-480 kDa), with specific activities from 14% to 64% of wildtype. Four nearly inactive (2% wildtype activity) C-terminal truncation mutants made MDa HA (56-71% wildtype). The results confirm earlier findings with Cys-mutants [Weigel PH, Baggenstoss BA. 2012. Hyaluronan synthase polymerizing activity and control of product size are discrete enzyme functions that can be uncoupled by mutagenesis of conserved cysteines. Glycobiology 22:1302-1310] that HAS uses two independent activities to control HA size and HA synthesis rate; these are two separate functions. We conclude that HAS regulatory modifications that alter tandem B-X₇-B motif conformation could mimic these mutagenesis-induced effects, allowing HAS in vivo to make small HA directly. The results also support a model in which the tandem-motif region is part of the intra-HAS pore and interacts directly with HA.

Activation of ERK and NF-κB during HARE-Mediated Heparin Uptake Require Only One of the Four Endocytic Motifs

Fifteen different ligands, including heparin (Hep), are cleared from lymph and blood by the Hyaluronan (HA) Receptor for Endocytosis (HARE; derived from Stabilin-2 by proteolysis), which contains four endocytic motifs (M1-M4). Endocytosis of HARE•Hep complexes is targeted to coated pits by M1, M2, and M3 (Pandey et al, Int. J. Cell Biol. 2015, article ID 524707), which activates ERK1/2 and NF-κB (Pandey et al J. Biol. Chem. 288, 14068-79, 2013). Here, we used a NF-κB promoter-driven luciferase gene assay and cell lines expressing different HARE cytoplasmic domain mutants to identify motifs needed for Hep-mediated signaling. Deletion of M1, M2 or M4 singly had no effect on Hep-mediated ERK1/2 activation, whereas signaling (but not uptake) was eliminated in HARE(ΔM3) cells lacking NPLY2519. ERK1/2 signaling in cells expressing WT HARE(Y2519A) or HARE(Y2519A) lacking M1, M2 and M4 (containing M3-only) was decreased by 75% or eliminated, respectively. Deletion of M3 (but not M1, M2 or M4) also inhibited the formation of HARE•Hep•ERK1/2 complexes by 67%. NF-κB activation by HARE-mediated uptake of Hep, HA, dermatan sulfate or acetylated LDL was unaffected in single-motif deletion mutants lacking M1, M2 or M4. In contrast, cells expressing HARE(ΔM3) showed loss of HARE-mediated NF-κB activation during uptake of each of these four ligands. NF-κB activation by the four signaling ligands was also eliminated in HARE(Y2519A) or HARE(M3-only;Y2519A) cells. We conclude that the HARE NPLY2519 motif is necessary for both ERK1/2 and NF-κB signaling and that Tyr2519 is critical for these functions.

Hyaluronan synthase assembles chitin oligomers with -GlcNAc(α1→)UDP at the reducing end

Class I hyaluronan synthases (HASs) assemble a polysaccharide containing the repeating disaccharide [GlcNAc(β1,4)GlcUA(β1,3)]n-UDP and vertebrate HASs also assemble (GlcNAc-β1,4)n homo-oligomers (chitin) in the absence of GlcUA-UDP. This multi-membrane domain CAZy GT2 family glycosyltransferase, which couples HA synthesis and translocation across the cell membrane, is atypical in that monosaccharides are incrementally assembled at the reducing, rather than the non-reducing, end of the growing polymer. Using Escherichia coli membranes containing recombinant Streptococcus equisimilis HAS, we demonstrate that a prokaryotic Class I HAS also synthesizes chitin oligomers (up to 15-mers, based on MS and MS/MS analyses of permethylated products). Furthermore, chitin oligomers were found attached at their reducing end to -4GlcNAc(α1→)UDP [i.e. (GlcNAcβ1,4)nGlcNAc(α1→)UDP]. These oligomers, which contained up to at least seven HexNAc residues, consisted of β4-linked GlcNAc residues, based on the sensitivity of the native products to jack bean β-N-acetylhexosaminidase. Interestingly, these oligomers exhibited mass defects of -2, or -4 for longer oligomers, that strictly depended on conjugation to UDP, but MS/MS analyses indicate that these species result from chemical dehydrogenations occurring in the gas phase. Identification of (GlcNAc-β1,4)n-GlcNAc(α1→)UDP as HAS reaction products, made in the presence of GlcNAc(α1→)UDP only, provides strong independent confirmation for the reducing terminal addition mechanism. We conclude that chitin oligomer products made by HAS are derived from the cleavage of these novel activated oligo-chitosyl-UDP oligomers. Furthermore, it is possible that these UDP-activated chitin oligomers could serve as self-assembled primers for initiating HA synthesis and ultimately modify the non-reducing terminus of HA with a chitin cap.

A hyaluronan receptor for endocytosis (HARE) link domain N-glycan is required for extracellular signal-regulated kinase (ERK) and nuclear factor-κB (NF-κB) signaling in response to the uptake of hyaluronan but not heparin, dermatan sulfate, or acetylated low density lipoprotein (LDL)

The human hyaluronan (HA) receptor for endocytosis (HARE; the 190-kDa C terminus of Stab2) is a major clearance receptor for multiple circulating ligands including HA, heparin (Hep), acetylated LDL (AcLDL), dermatan sulfate (DS), apoptotic debris, and chondroitin sulfate types A, C, D, and E. We previously found that HARE contains an N-glycan in the HA binding Link domain (at Asn(2280)), and cells expressing membrane-bound HARE(N2280A) bind and endocytose HA normally (Harris, E. N., Parry, S., Sutton-Smith, M., Pandey, M. S., Panico, M., Morris, H. R., Haslam, S. M., Dell, A., and Weigel, P. H. (2010) Glycobiology 20, 991-1001). Also, NF-κB-mediated signaling is activated by HARE-mediated endocytosis of HA, Hep, AcLDL, or DS but not by chondroitin sulfates (Pandey, M. S., and Weigel, P. H. (2014) J. Biol. Chem. 289, 1756-1767). Here we investigated the role of Link N-glycans in ligand uptake and NF-κB and ERK1/2 signaling. HA·HARE-mediated ERK1/2 activation was HA size- dependent, as found for NF-κB activation. HARE(N2280A) cells internalized HA, Hep, AcLDL, and DS normally. No ERK1/2 activation occurred during HA endocytosis by HARE(N2280A) cells, but activation did occur with Hep. Dual-luciferase recorder assays showed that NF-κB-mediated gene expression occurred normally in HARE(N2280A) cells endocytosing Hep, AcLDL, or DS but did not occur with HA. Activation of NF-κB by endogenous degradation of IκB-α was observed for HARE(N2280A) cells endocytosing Hep, AcLDL, or DS but not HA. We conclude that a Link domain complex N-glycan is required specifically for HARE·HA-mediated activation of ERK1/2 and NF-κB-mediated gene expression and that this initial activation mechanism is different from and independent of the initial mechanisms for HARE-mediated signaling in response to Hep, AcLDL, or DS uptake.

Hyaluronic acid receptor for endocytosis (HARE)-mediated endocytosis of hyaluronan, heparin, dermatan sulfate, and acetylated low density lipoprotein (AcLDL), but not chondroitin sulfate types A, C, D, or E, activates NF-κB-regulated gene expression

The hyaluronan (HA) receptor for endocytosis (HARE; Stab2) clears 14 systemic ligands, including HA and heparin. Here, we used NF-κB promoter-driven luciferase reporter assays to test HARE-mediated intracellular signaling during the uptake of eight ligands, whose binding sites in the HARE ectodomain were mapped by competition studies (Harris, E. N., and Weigel, P. H. (2008) Glycobiology 18, 638-648). Unique intermediate size Select-HA(TM), heparin, dermatan sulfate, and acetylated LDL stimulated dose-dependent HARE-mediated NF-κB activation of luciferase expression, with half-maximal values of 10-25 nM. In contrast, chondroitin sulfate types A, C, D, and E did not stimulate NF-κB activation. Moreover, degradation of endogenous IkB-α (an NF-κB inhibitor) was stimulated only by the signaling ligands. The stimulatory activities of pairwise combinations of the four signaling ligands were additive. The four nonstimulatory chondroitin sulfate types, which compete for HA binding, also effectively blocked HA-stimulated signaling. Clathrin siRNA decreased clathrin expression by ∼50% and completely eliminated NF-κB-mediated signaling by all four ligands, indicating that activation of signaling complexes occurs after endocytosis. These results indicate that HARE not only binds and clears extracellular matrix degradation products (e.g. released normally or during infection, injury, tumorigenesis, or other stress situations) but that a subset of ligands also serves as signaling indicator ligands. HARE may be part of a systemic tissue-stress sensor feedback system that responds to abnormal tissue turnover or damage as a danger signal; the signaling indicator ligands would reflect the homeostatic status, whether normal or pathological, of tissue cells and biomatrix components.

The hyaluronan receptor for endocytosis (HARE) activates NF-κB-mediated gene expression in response to 40-400-kDa, but not smaller or larger, hyaluronans

The hyaluronan (HA) receptor for endocytosis (HARE; Stabilin-2) binds and clears 14 different ligands, including HA and heparin, via clathrin-mediated endocytosis. HA binding to HARE stimulates ERK1/2 activation (Kyosseva, S. V., Harris, E. N., and Weigel, P. H. (2008) J. Biol. Chem. 283, 15047-15055). To assess a possible HA size dependence for signaling, we tested purified HA fractions of different weight-average molar mass and with narrow size distributions and Select-HA(TM) for stimulation of HARE-mediated gene expression using an NF-κB promoter-driven luciferase reporter system. Human HARE-mediated gene expression was stimulated in a dose-dependent manner with small HA (sHA) >40 kDa and intermediate HA (iHA) <400 kDa. The hyperbolic dose response saturated at 20-50 nM with an apparent K(m) ~10 nM, identical to the Kd for HA-HARE binding. Activation was not detected with oligomeric HA (oHA), sHA <40 kDa, iHA >400 kDa, or large HA (lHA). Similar responses occurred with rat HARE. Activation by sHA-iHA was blocked by excess nonsignaling sHA, iHA, or lHA, deletion of the HA-binding LINK domain, or HA-blocking antibody. Endogenous NF-κB activation also occurred in the absence of luciferase plasmids, as assessed by degradation of IκB-α. ERK1/2 activation was also HA size-dependent. The results show that HA-HARE interactions stimulate NF-κB-activated gene expression and that HARE senses a narrow size range of HA degradation products. We propose a model in which optimal length HA binds multiple HARE proteins to allow cytoplasmic domain interactions that stimulate intracellular signaling. This HARE signaling system during continuous HA clearance could monitor the homeostasis of tissue biomatrix turnover throughout the body.

Digestion products of the PH20 hyaluronidase inhibit remyelination

OBJECTIVE

Oligodendrocyte progenitor cells (OPCs) recruited to demyelinating lesions often fail to mature into oligodendrocytes (OLs) that remyelinate spared axons. The glycosaminoglycan hyaluronan (HA) accumulates in demyelinating lesions and has been implicated in the failure of OPC maturation and remyelination. We tested the hypothesis that OPCs in demyelinating lesions express a specific hyaluronidase, and that digestion products of this enzyme inhibit OPC maturation.

METHODS

Mouse OPCs grown in vitro were analyzed for hyaluronidase expression and activity. Gain of function studies were used to define the hyaluronidases that blocked OPC maturation. Mouse and human demyelinating lesions were assessed for hyaluronidase expression. Digestion products from different hyaluronidases and a hyaluronidase inhibitor were tested for their effects on OPC maturation and functional remyelination in vivo.

RESULTS

OPCs demonstrated hyaluronidase activity in vitro and expressed multiple hyaluronidases, including HYAL1, HYAL2, and PH20. HA digestion by PH20 but not other hyaluronidases inhibited OPC maturation into OLs. In contrast, inhibiting HA synthesis did not influence OPC maturation. PH20 expression was elevated in OPCs and reactive astrocytes in both rodent and human demyelinating lesions. HA digestion products generated by the PH20 hyaluronidase but not another hyaluronidase inhibited remyelination following lysolecithin-induced demyelination. Inhibition of hyaluronidase activity lead to increased OPC maturation and promoted increased conduction velocities through lesions.

INTERPRETATION

We determined that PH20 is elevated in demyelinating lesions and that increased PH20 expression is sufficient to inhibit OPC maturation and remyelination. Pharmacological inhibition of PH20 may therefore be an effective way to promote remyelination in multiple sclerosis and related conditions.

Methods for measuring Class I membrane-bound hyaluronan synthase activity

Detecting and quantifying hyaluronan (HA) made by Class I HA synthase (HAS) and determining the level of activity of these membrane-bound enzymes is critical in studies to understand the normal biology of HA and how changes in HAS activity and HA levels or size are important in inflammatory and other diseases, tumorigenesis, and metastasis. Unlike the products made by the vast majority of glycosyltransferases, HA products are more complicated since they are made as a heterogeneous population of sizes spanning a broad mass range. Three radioactive and nonradioactive assay methods are described that can give the amount of HA made with or without information about the distribution of product sizes.

Hyaluronan turnover and hypoxic brown adipocytic differentiation are co-localized with ossification in calcified human aortic valves

The calcification process in aortic stenosis has garnered considerable interest but only limited investigation into selected signaling pathways. This study investigated mechanisms related to hypoxia, hyaluronan homeostasis, brown adipocytic differentiation, and ossification within calcified valves. Surgically explanted calcified aortic valves (n=14) were immunostained for markers relevant to these mechanisms and evaluated in the center (NodCtr) and edge (NodEdge) of the calcified nodule (NodCtr), tissue directly surrounding nodule (NodSurr); center and tissue surrounding small "prenodules" (PreNod, PreNodSurr); and normal fibrosa layer (CollFibr). Pearson correlations were determined between staining intensities of markers within regions. Ossification markers primarily localized to NodCtr and NodEdge, along with markers related to hyaluronan turnover and hypoxia. Markers of brown adipocytic differentiation were frequently co-localized with markers of hypoxia. In NodCtr and NodSurr, brown fat and ossification markers correlated with hyaluronidase-1, whereas these markers, as well as hypoxia, correlated with hyaluronan synthases in NodEdge. The protein product of tumor necrosis factor-α stimulated gene-6 strongly correlated with ossification markers and hyaluronidase in the regions surrounding the nodules (NodSurr, PreNodSurr). In conclusion, this study suggests roles for hyaluronan homeostasis and the promotion of hypoxia by cells demonstrating brown fat markers in calcific aortic valve disease.

Tumor necrosis factor-stimulated gene-6 (TSG-6) amplifies hyaluronan synthesis by airway smooth muscle cells

We tested the hypothesis that the artificial addition of heavy chains from inter-α-inhibitor to hyaluronan (HA), by adding recombinant TSG-6 (TNF-stimulated gene-6) to the culture medium of murine airway smooth muscle (MASM) cells, would enhance leukocyte binding to HA cables produced in response to poly(I:C). As predicted, the addition of heavy chains to HA cables enhanced leukocyte adhesion to these cables, but it also had several unexpected effects. (i) It produced thicker, more pronounced HA cables. (ii) It increased the accumulation of HA in the cell-associated matrix. (iii) It decreased the amount of HA in the conditioned medium. Importantly, these effects were observed only when TSG-6 was administered in the presence of poly(I:C), and TSG-6 did not exert any effect on its own. Increased HA synthesis occurred during active, poly(I:C)-induced HA synthesis and did not occur when TSG-6 was added after poly(I:C)-induced HA synthesis was complete. MASM cells derived from TSG-6(-/-), HAS1/3(-/-), and CD44(-/-) mice amplified HA synthesis in response to poly(I:C) + TSG-6 in a manner similar to WT MASM cells, demonstrating that they are expendable in this process. We conclude that TSG-6 increases the accumulation of HA in the cell-associated matrix, partially by preventing its dissolution from the cell-associated matrix into the conditioned medium, but primarily by inducing HA synthesis.

Hyaluronan synthase polymerizing activity and control of product size are discrete enzyme functions that can be uncoupled by mutagenesis of conserved cysteines

Streptococcus equisimilis hyaluronan (HA) synthase (SeHAS) contains four cysteines (C226, C262, C281 and C367) that are conserved in the mammalian HAS family. Previous studies of single Cys-to-Ser and all possible Cys-to-Ala mutants of SeHAS found that: the Cys-null mutant is active, Cys modification inhibits HAS activity and the conserved cysteines are clustered at the membrane-enzyme interface in substrate-binding sites (Kumari K, Weigel PH. 2005. Identification of a membrane-localized cysteine cluster near the substrate binding sites of the Streptococcus equisimilis hyaluronan synthase. Glycobiology. 15:529-539). We re-examined these Cys mutants using a single technique (size exclusion chromatography-multi-angle laser light scattering) that allows simultaneous assays on the same sample for both HA synthesis activity and HA product size. Among 18 mutants compared with wild type, 4 showed no change in either function and 3 showed changes in both (decreased activity and HA size). Only one of the two functions was altered in 11 other mutants, which showed either decreased polymerizing activity or product size. No mutants made larger HA, 8 made smaller HA and 10 showed no change in HA size. Nine mutants showed no change in activity and nine were less active. The mutants fell into four of nine possible groups in terms of changes in HA size or synthesis rate (i.e. none, increased or decreased). Specific Cys residues were associated with each mutant group and the pattern of effects on both functions. Thus, the four conserved Cys residues, individually and in specific combinations, influence the rate of sugar assembly by HAS and HA product size, but their participation in one function is independent of the other.

Systemic blockade of the hyaluronan receptor for endocytosis prevents lymph node metastasis of prostate cancer

Tumor progression and metastasis are promoted by the remodeling of organized tissue architecture and engagement of molecular interactions that support tumor cell passage through endothelial barriers. Prostate tumor cells that secrete and turn over excessive quantities of pericellular hyaluronan (HA) exhibit accelerated growth kinetics and spontaneous lymph node metastasis in mice. The HA receptor for endocytosis (HARE) is an endocytic clearance receptor for HA in the liver that is also highly expressed in sinusoidal endothelium of lymph nodes and bone marrow, which are frequent sites of prostate cancer metastasis. In our study, we tested the hypothesis that HARE can act as an endothelial receptor for metastatic tumor cells with pericellular HA. In an orthotopic mouse model of prostate cancer, we delivered a monoclonal antibody against HARE that specifically blocks HA binding and internalization. This treatment fully blocked the formation of metastatic tumors in lymph nodes. No effects on primary tumor growth were observed and the antibody did not induce toxic outcomes in any other tissue. Our results implicate HARE for the first time in potentiation of tumor metastasis and suggest a novel mechanism by which tumor cell-associated HA could promote tissue-specific dissemination. "Published 2012 Wiley Periodicals, Inc. This article is a US Government work, and, as such, is in the public domain in the United States of America."

Hyaluronan synthase mediates dye translocation across liposomal membranes

Background

Hyaluronan (HA) is made at the plasma membrane and secreted into the extracellular medium or matrix by phospolipid-dependent hyaluronan synthase (HAS), which is active as a monomer. Since the mechanism by which HA is translocated across membranes is still unresolved, we assessed the presence of an intraprotein pore within HAS by adding purified Streptococcus equisimilis HAS (SeHAS) to liposomes preloaded with the fluorophore Cascade Blue (CB).

Results

CB translocation (efflux) was not observed with mock-purified material from empty vector control E. coli membranes, but was induced by SeHAS, purified from membranes, in a time- and dose-dependent manner. CB efflux was eliminated or greatly reduced when purified SeHAS was first treated under conditions that inhibit enzyme activity: heating, oxidization or cysteine modification with N-ethylmaleimide. Reduced CB efflux also occurred with SeHAS K48E or K48F mutants, in which alteration of K48 within membrane domain 2 causes decreased activity and HA product size. The above results used liposomes containing bovine cardiolipin (BCL). An earlier study testing many synthetic lipids found that the best activating lipid for SeHAS is tetraoleoyl cardiolipin (TO-CL) and that, in contrast, tetramyristoyl cardiolipin (TM-CL) is an inactivating lipid (Weigel et al, J. Biol. Chem. 281, 36542, 2006). Consistent with the effects of these CL species on SeHAS activity, CB efflux was more than 2-fold greater in liposomes made with TO-CL compared to TM-CL.

Conclusions

The results indicate the presence of an intraprotein pore in HAS and support a model in which HA is translocated to the exterior by HAS itself.

Clustered Conserved Cysteines in Hyaluronan Synthase Mediate Cooperative Activation by Mg2+ Ions and Severe Inhibitory Effects of Divalent Cations

Hyaluronan synthase (HAS) uses UDP-GlcUA and UDP-GlcNAc to make hyaluronan (HA). Streptococcus equisimilis HAS (SeHAS) contains four conserved cysteines clustered near the membrane, and requires phospholipids and Mg²⁺ for activity. Activity of membrane-bound or purified enzyme displayed a sigmoidal saturation profile for Mg²⁺ with a Hill coefficient of 2. To assess if Cys residues are important for cooperativity we examined the Mg²⁺ dependence of mutants with various combinations of Cys-to-Ala mutations. All Cys-mutants lost the cooperative response to Mg²⁺. In the presence of Mg²⁺, other divalent cations inhibited SeHAS with different potencies (Cu²⁺~Zn²⁺ >Co²⁺ >Ni²⁺ >Mn²⁺ >Ba²⁺ Sr²⁺ Ca²⁺). Some divalent metal ions likely inhibit by displacement of Mg²⁺-UDP-Sugar complexes (e.g. Ca²⁺, Sr²⁺ and Ba²⁺ had apparent K_i values of 2-5 mM). In contrast, Zn²⁺ and Cu²⁺ inhibited more potently (apparent K_i ≤ 0.2 mM). Inhibition of Cys-null SeHAS by Cu²⁺, but not Zn²⁺, was greatly attenuated compared to wildtype. Double and triple Cys-mutants showed differing sensitivities to Zn²⁺ or Cu²⁺. Wildtype SeHAS allowed to make HA prior to exposure to Zn²⁺ or Cu²⁺ was protected from inhibition, indicating that access of metal ions to sensitive functional groups was hindered in processively acting HA•HAS complexes. We conclude that clustered Cys residues mediate cooperative interactions with Mg²⁺ and that transition metal ions inhibit SeHAS very potently by interacting with one or more of these -SH groups.

N-Glycans on the link domain of human HARE/Stabilin-2 are needed for hyaluronan binding to purified ecto-domain, but not for cellular endocytosis of hyaluronan

The hyaluronic acid receptor for endocytosis (HARE)/Stabilin-2 is the primary systemic scavenger receptor for 13 ligands including hyaluronan (HA), heparin and chondroitin sulfates. Most ligand-binding sites are within the 190 kDa isoform, which contains approximately 25 kDa of N-glycans and is the C-terminal half of the full-length 315 kDa HARE. Glycoproteomic analyses of purified recombinant human 190-HARE ecto-domain identified a diverse population of glycans at 10 of 17 consensus sites. The most diversity (and the only sialylated structures) occurred at N(2280), within the HA-binding Link domain. To determine if these N-glycans are required for HA binding, we created human Flp-In 293 cell lines expressing membrane-bound or soluble ecto-domain variants of 190-HARE(N2280A). Membrane-bound HARE lacking Link domain N-glycans mediated rapid HA endocytosis, but purified 190-HARE(N2280A) ecto-domain showed little or no HA binding in ELISA-like, HA-HARE pull-down assays or by surface plasmon resonance analysis (which detected very high apparent affinity for 190-HARE ecto-domain binding to HA; K(d) = 5.2 nM). The results indicate that Link domain N-glycans stabilize interactions that facilitate HA binding to HARE.

Increased hyaluronan synthase-2 mRNA expression and hyaluronan accumulation with choroidal thickening: response during recovery from induced myopia

PURPOSE

Several studies have convincingly shown that in chicks, compensation for imposed focus involves immediate changes in choroid thickness. The molecular events associated with choroidal thickening and the regulation of the choroidal response are largely unknown.

METHODS

Form-deprivation myopia was induced in the right eyes of 2-day-old chicks by the application of translucent occluders for 10 days and was followed by unrestricted vision for an additional 1 to 20 days (recovery). Individual choroids were isolated from treated and control eyes and used for reverse transcription-quantitative PCR, hyaluronan (HA) localization with biotinylated hyaluronic acid binding protein (b-HABP), and analyses of HA size and concentration by size exclusion chromatography-multiangle laser light scattering (SEC-MALLS).

RESULTS

HAS2 gene expression increased significantly after 6 hours of unrestricted vision (>7-fold) and peaked at 24 hours (>9-fold). In untreated eyes, HA was localized to perivascular sheaths of larger choroidal blood vessels; however, after 4 to 15 days of recovery, intense labeling for HA was detected throughout the thickened choroidal stroma. Analyses of choroidal HA by SEC-MALLS indicated that HA concentration was significantly increased in recovering choroids compared with controls after 4 to 8 days of recovery (≈3.5-fold).

CONCLUSIONS

Newly synthesized HA accumulates in the choroidal stroma of recovering eyes and is most likely responsible for the stromal swelling observed during recovery from myopia. This HA accumulation is initiated by a rapid increase in choroidal expression of the HAS2 gene in response to myopic defocus.

Rat and human HARE/stabilin-2 are clearance receptors for high- and low-molecular-weight heparins

The human hyaluronic acid (HA) receptor for endocytosis (HARE/stabilin-2) is the primary clearance receptor for systemic HA, chondroitin sulfates, and heparin, but not for heparan sulfate or keratan sulfate (Harris EN, Weigel JA, Weigel PH. J Biol Chem 283: 17341-17350, 2008). HARE is expressed in the sinusoidal endothelial cells (SECs) of liver and lymph nodes where it acts as a scavenger for uptake and degradation of glycosaminoglycans, both as free chains and proteoglycan fragments. Unfractionated heparin (UFH; approximately 14 kDa) and low-molecular-weight heparin (LMWH; approximately 4 kDa) are commonly used in treatments for thrombosis and cancer and in surgical and dialysis procedures. The reported half-lives of UFH and LMWH in the blood are approximately 1 h and 2-6 h, respectively. In this study, we demonstrate that anti-HARE antibodies specifically block the uptake of LMWH and UFH by isolated rat liver SECs and by human 293 cells expressing recombinant human HARE (hHARE). hHARE has a significant affinity (K(d) = 10 microM) for LMWH, and higher affinity (K(d) = 0.06 microM) for the larger UFH. Rat liver SECs or cells expressing the recombinant 190-kDa HARE isoform internalized both UFH and LMWH, and both heparins cross-compete with each other, suggesting that they share the same binding sites. These cellular results were confirmed in ELISA-like assays using purified soluble 190-hHARE ectodomain. We conclude that both UFH and LMWH are cleared by HARE/Stab2 and that the differences in the affinities of HARE binding to LMWH and UFH likely explain the longer in vivo circulating half-life of LMWH compared with UFH.

The cytoplasmic domain of the hyaluronan receptor for endocytosis (HARE) contains multiple endocytic motifs targeting coated pit-mediated internalization

The hyaluronic acid (HA) receptor for endocytosis (HARE) is the primary scavenger receptor for HA and chondroitin sulfates in mammals. The two human isoforms of HARE (full-length 315-kDa and a 190-kDa proteolytic cleavage product), which are type I single-pass membrane proteins, are highly expressed in sinusoidal endothelial cells of lymph nodes, liver, and spleen. Their identical HARE cytoplasmic domains contain four candidate AP-2/clathrin-mediated endocytic signaling motifs as follows: YSYFRI(2485), FQHF(2495), NPLY(2519), and DPF(2534) (315-HARE numbering). Stably transfected cells expressing 190-HARE(DeltaYSYFRI), 190-HARE(DeltaFQHF), or 190-HARE(DeltaNPLY) (lacking Motifs 1, 2, or 3) had decreased (125)I-HA endocytosis rates of approximately 49, approximately 39, and approximately 56%, respectively (relative to wild type). In contrast, 190-HARE(DeltaDPF) cells (lacking Motif 4) showed no change in HA endocytic rate. Deletions of motifs 1 and 2 or of 1, 2, and 4 decreased the rate of HA endocytosis by only approximately 41%. Endocytosis was approximately 95% decreased in mutants lacking all four motifs. Cells expressing a 190-HARE(Y2519A) mutant of the NPLY motif retained 85-90% of wild type endocytosis, whereas this mutation in the triple motif deletant decreased endocytosis to approximately 7% of wild type. Tyr in NPLY(2519) is thus important for endocytosis. All HARE mutants showed similar HA binding and degradation of the internalized HA, indicating that altering endocytic motifs did not affect ectodomain binding of HA or targeting of internalized HA to lysosomes. We conclude that, although NPLY may be the most important motif, it functions together with two other endocytic motifs; thus three signal sequences (YSYFRI, FQHF, and NPLY) provide redundancy to mediate coated pit targeting and endocytosis of HARE.

The ligand-binding profile of HARE: hyaluronan and chondroitin sulfates A, C, and D bind to overlapping sites distinct from the sites for heparin, acetylated low-density lipoprotein, dermatan sulfate, and CS-E

The hyaluronic acid receptor for endocytosis (HARE)/ Stabilin-2 is the primary systemic scavenger receptor for hyaluronan (HA), the chondroitin sulfates (CS), dermatan sulfate (DS), and nonglycosaminoglycan (GAG) ligands such as acetylated low-density lipoprotein (AcLDL), pro-collagen propeptides, and advanced glycation end products. We recently discovered that HARE is also a systemic scavenger receptor for heparin (Hep) (Harris EN, Weigel JA, Weigel PH. 2008. The human hyaluronan receptor for endocytosis [HARE/Stabilin-2] is a systemic clearance receptor for heparin. J Biol Chem. 283:17341-17350). Our goal was to map the binding sites of eight different ligands within HARE. We used biotinylated GAGs and radio-iodinated streptavidin or AcLDL to assess the binding activities of ligands directly or indirectly (by competition with unlabeled ligands) in endocytosis assays using stable cell lines expressing the 315 or 190 kDa HA receptor for endocytosis (315- or 190-HARE) isoforms, and ELISA-like assays, with purified recombinant soluble 190-HARE ecto-domain. For example, Hep binding to HARE was competed by DS, CS-E, AcLDL, and dextran sulfate, but not by other CS types, HA, dextran, or heparosan. (125)I-AcLDL binding to HARE was partially competed by Hep and dextran sulfate, but not competed by HA. Two ligands, DS and CS-E, competed with both Hep and HA to some degree. Hep and HA binding or endocytosis is mutually inclusive; binding of these two GAGs occurs with functionally separate, noncompetitive, and apparently noninteracting domains. Thus, HARE binds to HA and Hep simultaneously. Although the domain(s) responsible for Hep binding remains unknown, the Link domain was required for HARE binding to HA, CS-A, CS-C, and CS-D. These results enable us to outline, for the first time, a binding activity map for multiple ligands of HARE.

The human hyaluronan receptor for endocytosis (HARE/Stabilin-2) is a systemic clearance receptor for heparin

The hyaluronic acid receptor for endocytosis (HARE; also designated Stabilin-2) mediates systemic clearance of hyaluronan and chondroitin sulfates from the vascular and lymphatic circulations. The internalized glycosaminoglycans are degraded in lysosomes, thus completing their normal turnover process. Sinusoidal endothelial cells of human liver, lymph node, and spleen express two HARE isoforms of 315 and 190 kDa. Here we report that the 190- and 315-kDa HARE isoforms, expressed stably either in Flp-In 293 cell lines or as soluble ectodomains, specifically bind heparin (Hep). The K(d) for Hep binding to purified 190- and 315-kDa HARE ectodomains was 17.2 +/- 4.9 and 23.4 +/- 5.3 nm, respectively. Cells expressing HARE readily and specifically internalized (125)I-streptavidin-biotin-Hep complexes, which was inhibited >70% by hyperosmolar conditions, confirming that uptake is mediated by the clathrin-coated pit pathway. Internalization of Hep occurred for many hours with an estimated HARE recycling time of approximately 12 min. Internalized fluorescent streptavidin-biotin-Hep was present in a typical endocytic vesicular pattern and was delivered to lysosomes. We conclude that HARE in the sinusoidal endothelial cells of lymph nodes and liver likely mediates the efficient systemic clearance of Hep and many different Hep-binding protein complexes from the lymphatic and vascular circulations.

The hyaluronan receptor for endocytosis mediates hyaluronan-dependent signal transduction via extracellular signal-regulated kinases

The hyaluronan (HA) receptor for endocytosis (HARE) mediates the endocytotic clearance of HA and other glycosaminoglycans from lymph and blood. Two isoforms of human HARE, 315- and 190-kDa, are highly expressed in sinusoidal endothelial cells of liver, lymph node, and spleen; HARE is also in specialized cells in the eye, heart, brain, and kidney. Here we determined whether HA binding to HARE initiates intracellular signaling in Flp-In 293 cells stably expressing either the 315- and 190-kDa HARE or the 190-kDa HARE alone. HARE was co-immunoprecipitated with extracellular signal-regulated kinase 1 and 2 (ERK1/2), c-Jun N-terminal protein kinase (JNK), and p38 members of the mitogen-activated protein kinase signaling cascade. ERK phosphorylation increased in a dose- and time-dependent manner when HA was added to cells expressing full-length or 190-kDa HARE, but not cells with vector-only or a HARE(DeltaLink) construct with greatly decreased ( approximately 90%) HA uptake. HA did not induce phosphorylation of JNK or p38. A maximum increase in phospho-ERK1/2 occurred within 30 min at 5 mug/ml HA, and the response was dampened at >20 mug/ml HA. HA binding did not increase the level of HARE-ERK complexes, but did increase HARE phosphorylation. These findings demonstrate a novel functional response, when HARE binds HA, that leads to activation of ERK1/2, important mediators of intracellular signal transduction.

Hyaluronan synthases: a decade-plus of novel glycosyltransferases

Hyaluronan synthases (HASs) are glycosyltransferases that catalyze polymerization of hyaluronan found in vertebrates and certain microbes. HASs transfer two distinct monosaccharides in different linkages and, in certain cases, participate in polymer transfer out of the cell. In contrast, the vast majority of glycosyltransferases form only one sugar linkage. Although our understanding of HAS biochemistry is still incomplete, very good progress has been made since the first genetic identification of a HAS in 1993. New enzymes have been discovered, and some molecular details have emerged. Important findings are the lipid dependence of Class I HASs, the function of HASs as protein monomers, and the elucidation of mechanisms of synthesis by Class II HAS. We propose three classes of HASs based on differences in protein sequences, predicted membrane topologies, potential architectures, mechanisms, and direction of polymerization.

The specific interaction between fibrin(ogen) and hyaluronan: possible consequences in haemostasis, inflammation and wound healing

We have proposed that fibrin and hyaluronan (HA) are macromolecular regulators during inflammation and wound healing. Here we extend our studies to characterize the specific interaction between fibrin(ogen) and HA. 125I-labelled HA (Mr approximately 32,000) was bound by plastic surfaces coated with human fibrinogen but not bovine serum albumin, ovalbumin, beta-lactoglobin or rabbit immunoglobulin G. 125I-labelled fibrinogen bound to a unique hexylamine derivative of HA coupled to Sepharose and was eluted specifically by HA oligosaccharides in a size-dependent manner. A dot blot assay, in which proteins are adsorbed to nitrocellulose and probed with 125I-HA, also showed specific binding to human fibrinogen. This assay was used to examine fibrinogens from other mammalian species. No specific 125I-HA binding was observed with the protein from horse, rat or cow. Significant binding was detected with human, sheep, rabbit, dog, baboon, goat and pig fibrinogens. Thrombin-induced formation of fibrin clots is also affected by HA, which decreases the lag time before clotting and increases the rate of clot formation. The rate of fibrin polymerization increased over 500% in the presence of 60 microM HA. Furthermore, the structure of the fibrin gel, as assessed by light scattering, was altered by HA or chondroitin sulphate in a concentration-dependent manner. The results support the proposed wound-healing model and indicate that an increase in circulating HA levels could adversely affect haemostasis and increase the risk of thrombosis or bleeding. The interaction between HA and fibrinogen emphasizes the importance of the liver endothelial cell HA receptor in the removal of glycosaminoglycans from the blood. Cultured cells continuously endocytosing 125I-HA for 4 h reutilized their total cellular HA receptors at least once every 50 min even in the presence of cycloheximide. This endocytotic receptor was therefore shown to be part of a recycling system.

An enzyme capture assay for analysis of active hyaluronan synthases

We describe a sensitive assay for detection of active hyaluronan synthases (HASs) capable of synthesizing hyaluronan (HA) without use of radioactive uridine 5'-diphosphate sugar precursors. The HAS capture assay is based on the binding of a biotinylated HA binding protein (bHABP) to HA chains that are associated with HAS and the subsequent capture of bHABP-HA-HAS complexes with streptavidin-agarose. Specific HAS proteins (e.g., HAS1, not HAS2 or HAS3) captured in this pull-down approach are readily immunodetected by Western blot analysis using appropriate antibodies. The assay was used to detect active HAS proteins in cell membranes, purified recombinant Streptococcus equisimilis HAS (SeHAS), and in vitro translated human HAS1 or SeHAS. The HAS capture assay was also used to assess the fraction of HAS molecules that were active, which cannot be done using standard assays for synthase activity. Assay sensitivity for detection of purified SeHAS is <1 pmol.

Expression, processing, and glycosaminoglycan binding activity of the recombinant human 315-kDa hyaluronic acid receptor for endocytosis (HARE)

The hyaluronic acid (HA) receptor for endocytosis (HARE; also designated stabilin-2 and FEEL-2) mediates systemic clearance of glycosaminoglycans from the circulatory and lymphatic systems via coated pit-mediated uptake. HARE is primarily found as two isoforms (315- and 190-kDa) in sinusoidal endothelial cells of the liver, lymph node, and spleen. Here we characterize the ligand specificity and function of the large stably expressed 315-HARE isoform in Flp-In 293 cell lines. Like human spleen sinusoidal endothelial cells, Flp-In 293 cell lines transfected with a single cDNA encoding the full-length 315-HARE express both the 315-kDa and the proteolytically truncated 190-kDa isoforms in a ratio of approximately 3-4:1. The 190-kDa HARE isoform generated from the 315-kDa HARE and the 315-kDa HARE specifically bound 125I-HA. Like the 190-kDa HARE expressed alone (Harris, E. N., Weigel, J. A., and Weigel, P. H. (2004) J. Biol. Chem. 279, 36201-36209), the 190- and 315-kDa HARE isoforms expressed in 315-HARE cell lines were recognized by anti-HARE monoclonal antibodies 30, 154, and 159. All 315-HARE cell lines could endocytose and degrade 125I-HA. Competition studies with live cells indicate that 190-HARE and 315-HARE bind HA with higher apparent affinity (Kd approximately 10-20 nM) than chondroitin sulfate (CS) types A, C, D, or E. Only slight competition of HA endocytosis was observed with CS-B (dermatan sulfate) and chondroitin. Direct binding assays with the 315-HARE ectodomain revealed high affinity HA binding, and lower binding affinities for CS-C, CS-D, and CS-E. A majority of each HARE isoform was intracellular, within the endocytic system, suggesting transient surface residency typical of an active endocytic recycling receptor.

Phospholipid Dependence and Liposome Reconstitution of Purified Hyaluronan Synthase

Previous radiation inactivation and enzyme characterization studies demonstrated that the Streptococcus equisimilis hyaluronan synthase (seHAS) is phospholipid-dependent and that cardiolipin (CL) is the best phospholipid for enzyme activation. Here we investigated the ability of seHAS, purified in the absence of added lipid, to be activated by synthetic phosphatidic acid (PA), phosphatidylserine, or CL lipids containing fatty acyl chains of different length or different numbers of double bonds. The most effective lipid was tetraoleoyl CL (TO-CL), whereas tetramyristoyl CL (TM-CL) was ineffective. None of the phosphatidylserine species tested gave significant activation. PAs containing C10 to C18 saturated acyl chains were not effective activators, and neither were oleoyl lyso PA, dilinoleoyl PA, or PA containing one oleoyl chain and either a palmitoyl or stearoyl chain. In contrast, dioleoyl PA stimulated seHAS approximately 10-fold, to approximately 20% of the activity observed with TO-CL. The tested acidic lipids such as PA and CL activated the enzyme most efficiently if they contained only oleic acid. Mixing experiments showed that the enzyme interacts preferentially with TO-CL in the presence of TM-CL. Similarly, seHAS incorporated into phosphotidylcholine-based liposomes showed increasing activity with increasing TO-CL, but not TM-CL, content. Inactivation of membrane-bound seHAS by solubilization with Nonidet P-40 was prevented by TO-CL, but not TM-CL. The pH dependence of seHAS in the presence of synthetic or naturally occurring CLs showed the same pattern of lipid preference between pH 6 and 10.5. Unexpectedly, HAS showed lipid-independent activity at pH 11.5. The results suggest that Class I HAS enzymes are lipid-dependent and that assembly of active seHAS-lipid complexes has high specificity for the phospholipid head group and the nature of the fatty acyl chains.

Hyaluronan and its receptors in mucoepidermoid carcinoma

BACKGROUND

Hyaluronan (HA) is a prominent extracellular matrix component undergoing continuous production and degradation. Increased HA levels have been described in a variety of tumors. The objective of this study was to examine the staining patterns of HA and two of its associated receptors (CD44 and HARE) in relation to the metastatic potential of mucoepidermoid carcinoma (MC). Immunohistochemical staining of preserved surgical specimens was used.

METHODS

Tissues from 12 patients with a histologic diagnosis of salivary MC (10 parotid, one submandibular gland, one minor salivary gland) were studied. Half (six of 12) of the patients had regional metastases. Tumor, normal salivary tissue, and regional lymph nodes were stained for HA, CD44, and HARE expression. Specimens were graded for staining intensity and a percent of the specimen stained.

RESULTS

Normal salivary tissue did not demonstrate epithelial cell surface HA expression, whereas HA was expressed on tumor cells and in regional lymph nodes containing metastases. These differences were both significant using Student's t test (p < .00002, and p < .0022, respectively). Tumors with positive nodes tended to have greater cell surface HA. Decreased expression or downregulation of HARE was also noted in involved lymph nodes. No differences in CD44 expression were seen between primary specimens and lymph nodes. The observed staining patterns for CD44 and HARE were not reflective of the metastatic potential of the primary MC.

CONCLUSIONS

Increased HA expression was seen on mucoepidermoid carcinoma cells compared with adjacent normal salivary gland epithelium. This observation may assist in explaining the development of regional metastasis in these tumors. We did not identify specific HA, CD44, or HARE staining patterns in primary lesions that were predictive of regional metastases.

Mutation of Two Intramembrane Polar Residues Conserved within the Hyaluronan Synthase Family Alters Hyaluronan Product Size

We identified two conserved polar amino acids within different membrane domains (MD) of Streptococcus equisimilis hyaluronan synthase (seHAS), Lys48 in MD2 and Glu327 in MD4. In eukaryotic HASs, the position of the Glu is very similar and the Lys is replaced by a conserved polar Gln. To assess whether Lys48 and Glu327 interact or influence seHAS activity, we investigated the effects of changing Lys48 to Arg or Glu and Glu327 to Lys, Asp, or Gln. Mutants, including a double switch variant with Lys48 and Glu327 exchanged, were expressed and assayed in Escherichia coli membranes. SeHASE327Q and seHASE327K were expressed at low levels, whereas seHASE327D and the Lys48 mutants were expressed well. The specific enzyme activities (relative to wild type) were 17 and 7% for the K48R and K48E mutants and 26 and 38% for the E327Q and E327D mutants, respectively. In contrast, seHAS(E327K) showed only 0.16% of wild-type activity but was rescued over 46-fold by changing Lys48 to Glu. Expression of the seHASE327K,K48E protein was also rescued to near wild-type levels. Based on size exclusion chromatography coupled to multiangle laser light scattering analysis, all the variants synthesized hyaluronan (HA) of smaller weight-average molar mass than wild-type enzyme (3.6 MDa); the smallest HA (approximately 0.6 MDa) was made by seHASE327K,K48E and seHASK48E. The results indicate that Glu327 within MD4 is a critical residue for the stability of seHAS, that it may interact with Lys48 within MD2, and that these residues are involved in the ability of HAS to synthesize very large HA.

Size exclusion chromatography-multiangle laser light scattering analysis of hyaluronan size distributions made by membrane-bound hyaluronan synthase

Size exclusion chromatography-multiangle laser light scattering (SEC-MALLS) analyses of Escherichia coli membranes expressing Streptococcus equisimilis hyaluronan synthase (seHAS) demonstrated an inherent artifact (10-100 MDa) that coeluted with hyaluronan (HA) and skewed the apparent weight-average mass of HA to erroneously high values. Briefly heating samples to 65-75 degrees C eliminated this artifact and increased the yield of recovered HA due to the release of HA chains that were attached to membrane-bound HAS. Inclusion of alkaline phosphatase, which removed uridine 5'-diphosphate (UDP) produced during the reaction, improved the linearity of HA synthesis-even at high substrate use. Surprisingly, the addition of EDTA, to chelate Mg(2+) ions, did not completely stop the HAS reaction at 30 degrees C or at 4 degrees C. The best conditions for stopping the reaction without altering SEC-MALLS profiles of the product HA were to chill samples on ice in the presence of both EDTA and UDP. Even with excess substrate, the maximum size of product HA decreased as the enzyme concentration increased. Therefore, the maximum HA size made by HAS was determined by extrapolation to zero enzyme concentration. Using the above conditions, membrane-bound seHAS synthesized a cohort of HA products that steadily increased in weight-average molar mass, reaching a final maximal steady-state size of 4 to 6 MDa within 2-4 h.

Hyaluronic acid production in Bacillus subtilis

The hasA gene from Streptococcus equisimilis, which encodes the enzyme hyaluronan synthase, has been expressed in Bacillus subtilis, resulting in the production of hyaluronic acid (HA) in the 1-MDa range. Artificial operons were assembled and tested, all of which contain the hasA gene along with one or more genes encoding enzymes involved in the synthesis of the UDP-precursor sugars that are required for HA synthesis. It was determined that the production of UDP-glucuronic acid is limiting in B. subtilis and that overexpressing the hasA gene along with the endogenous tuaD gene is sufficient for high-level production of HA. In addition, the B. subtilis-derived material was shown to be secreted and of high quality, comparable to commercially available sources of HA.

Hyaluronan biosynthesis by class I streptococcal hyaluronan synthases occurs at the reducing end

Previous studies reached different conclusions about whether class I hyaluronan synthases (HASs) elongate hyaluronic acid (HA) by addition to the reducing or the nonreducing end. Here we used two strategies to determine the direction of HA synthesis by purified class I HASs from Streptococcus equisimilis and Streptococcus pyogenes. In the first strategy we used each of the two UDP-sugar substrates separately to pulse label either the beginning or the end of HA chains. We then quantified the relative rates of radioactive HA degradation by treatment with beta-glycosidases that act at the nonreducing end. The results with both purified HASs demonstrated that HA elongation occurred at the reducing end. In the second strategy, we used purified S. equisimilis HAS, UDP-glucuronic acid, and UDP[beta-32P]-Glc-NAc to radiolabel nascent HA chains. Under conditions of limiting substrate, the 32P-labeled products were separated from the substrates by paper chromatography and identified as HA-[32P]UDP saccharides based on their degradation by snake venom phosphodiesterase or hyaluronidase and by their binding to a specific HA-binding protein. The 32P radioactivity was chased (released) by incubation with unlabeled UDP-sugars, showing that the HA-UDP linkages turn over during HA biosynthesis. In contrast, HA-[32P]UDP products made by the purified class II Pasteurella multocida HAS were not released by adding unlabeled UDP-sugars, consistent with growth at the nonreducing end for this enzyme. The results demonstrate that the streptococcal class I HAS enzymes polymerize HA chains at the reducing end.

Identification of a membrane-localized cysteine cluster near the substrate-binding sites of the Streptococcus equisimilis hyaluronan synthase

The membrane-bound hyaluronan synthase (HAS) from Streptococcus equisimilis (seHAS), which is the smallest Class I HAS, has four cysteine residues (positions 226, 262, 281, and 367) that are generally conserved within this family. Although Cys-null seHAS is still active, chemical modification of cysteine residues causes inhibition of wild-type enzyme. Here we studied the effects of N-ethylmaleimide (NEM) treatment on a panel of seHAS Cys-mutants to examine the structural and functional roles of the four cysteine residues in the activity of the enzyme. We found that Cys226, Cys262, and Cys281 are reactive with NEM, but Cys367 is not. Substrate protection studies of wild-type seHAS and a variety of Cys-mutants revealed that binding of UDP-GlcUA, UDP-GlcNAc, or UDP can protect Cys226 and Cys262 from NEM inhibition. Inhibition of the six double Cys-mutants of seHAS by sodium arsenite, which can cross-link vicinyl sulfhydryl groups, also supported the conclusion that Cys262 and Cys281 are close enough to be cross-linked. Similar results indicated that Cys281 and Cys367 are also very close in the active enzyme. We conclude that three of the four Cys residues in seHAS (Cys262, Cys281, and Cys367) are clustered very close together, that these Cys residues and Cys226 are located at the inner surface of the cell membrane, and that Cys226 and Cys262 are located in or near a UDP binding site.

Injured brain endothelial cells release neurotoxic thrombin

The multifunctional serine protease thrombin has been shown to be neurotoxic in vitro and in vivo and is demonstrable in the Alzheimer disease (AD) brain. We have documented that in AD the cerebral microvasculature is a source of inflammatory and neurotoxic proteins. The objective of this study was to determine if injured brain endothelial cells could be a source of neurotoxic thrombin. Brain endothelial cells were incubated with either sodium nitroprusside (SNP, 10 microM), inflammatory proteins (IL-1beta, IL-6, TNFalpha, LPS, IFNgamma) or the PKC inhibitor bisindolymaleimide (1 microM) for 24 h and conditioned media collected. Endothelial cell conditioned medium was incubated with purified apolipoprotein E4 (apoE4) for 24 h, and then analyzed for neurotoxic activity against primary cortical cultures and for apoE4 fragments by western blot. Endothelial cell conditioned medium collected after treatment with either SNP, inflammatory proteins, or the PKC inhibitor bisindolymaleimide, demonstrated a significant (p < 0.005) level of thrombin activity, the presence of apoE4 fragments, and was capable of evoking neuronal cell death. These data demonstrate that endothelial cell injury results in thrombin release and suggest that the brain microcirculation could be a source of neurotoxic factors in AD.

Characterization of the purified hyaluronan synthase from Streptococcus equisimilis

Hyaluronan synthase (HAS) utilizes UDP-GlcUA and UDP-GlcNAc in the presence of Mg(2+) to form the GAG hyaluronan (HA). The purified HAS from Streptococcus equisimilis (seHAS) shows high fidelity in that it only polymerizes the native substrates, UDP-GlcNAc and UDP-GlcUA. However, other uridinyl nucleotides and UDP-sugars inhibited enzyme activity, including UDP-GalNAc, UDP-Glc, UDP-Gal, UDP-GalUA, UMP, UDP, and UTP. Purified seHAS was approximately 40% more active in 25 mM, compared to 50 mM, PO(4) in the presence of either 50 mM NaCl or KCl, and displayed a slight preference for KCl over NaCl. The pH profile was surprisingly broad, with an effective range of pH 6.5-11.5 and the optimum between pH 9 and 10. SeHAS displayed two apparent pK(a) values at pH 6.6 and 11.8. As the pH was increased from approximately 6.5, both K(m) and V(max) increased until pH approximately 10.5, above which the kinetic constants gradually declined. Nonetheless, the overall catalytic constant (120/s) was essentially unchanged from pH 6.5 to 10.5. The enzyme is temperature labile, but more stable in the presence of substrate and cardiolipin. Purified seHAS requires exogenous cardiolipin for activity and is very sensitive to the fatty acyl composition of the phospholipid. The enzyme was inactive or highly activated by synthetic cardiolipins containing, respectively, C14:0 or C18:1(Delta9) fatty acids. The apparent E(act) for HA synthesis is 40 kJ (9.5 kcal/mol) disaccharide. Increasing the viscosity by increasing concentrations of PEG, ethylene glycol, glycerol, or sucrose inhibited seHAS activity. For PEGs, the extent of inhibition was proportional to their molecular mass. PEGs with average masses of 2.7, 11.7, and 20 kg/mol caused 50% inhibition of V(max) at 21, 6.5, and 3.5 mM, respectively. The apparent K(i) values for ethylene glycol, glycerol, and sucrose were, respectively, 4.5, 3.3, and 1.2 mM.

Endocytic function, glycosaminoglycan specificity, and antibody sensitivity of the recombinant human 190-kDa hyaluronan receptor for endocytosis (HARE)

The human hyaluronan receptor for endocytosis (hHARE) mediates the endocytic clearance of hyaluronan (HA) and chondroitin sulfate from lymph fluid and blood. Two hHARE isoforms (190 and 315 kDa) are present in sinusoidal endothelial cells of liver, spleen, and lymph nodes (Zhou, B., McGary, C. T., Weigel, J. A., Saxena, A., and Weigel, P. H. (2003) Glycobiology 13, 339-349). Here we report the specificity and function of the 190-kDa HARE, expressed without the larger isoform, in Flp-In 293 cell lines (190hHARE cells). Like the native protein, recombinant hHARE contains approximately 25 kDa of N-linked oligosaccharides, binds HA in a ligand blot assay, cross-reacts with three anti-rat HARE monoclonal antibodies, and is inactivated by reduction. The 190hHARE cell lines mediated rapid, continuous (125)I-HA endocytosis and degradation for >1 day. About 30-50% of the total cellular receptors were on the cell surface, and their recycling time for reutilization was approximately 8.5 min. The average K(d) for the binding of HA to the 190-kDa hHARE at 4 degrees C was 7 nm with 118,000 total HA binding sites per cell. Competition studies at 37 degrees C indicated that the 190-kDa hHARE binds HA and chondroitin better than dermatan sulfate and chondroitin sulfates A, C, D, and E, but it does not bind to heparin, heparan sulfate, or keratan sulfate. Although competition was observed at 37 degrees C, none of the glycosaminoglycans tested, except HA, competed for (125)I-HA binding by 190hHARE cells at 4 degrees C. Anti-HARE monoclonal antibodies #30 and #154, which do not inhibit (125)I-HA uptake mediated by the 175-kDa rat HARE, partially blocked HA endocytosis by the 190-kDa hHARE. We conclude that the 190-kDa hHARE can function independently of other hHARE isoforms to mediate the endocytosis of multiple glycosaminoglycans. Furthermore, the rat and human small HARE isoforms have different glycosaminoglycan specificities and sensitivities to inhibition by cross-reacting antibodies.

Characterization of the recombinant rat 175-kDa hyaluronan receptor for endocytosis (HARE)

Hyaluronan (HA) and chondroitin sulfate (CS) clearance from lymph and blood in mammals is mediated by the HA receptor for endocytosis (HARE), which is present as two isoforms in rat and human (175/300 kDa and 190/315 kDa, respectively) in the sinusoidal endothelial cells of liver, spleen, and lymph nodes (Zhou, B., McGary, C. T., Weigel, J. A., Saxena, A., and Weigel, P. H. (2003) Glycobiology 13, 339-349). The small rat and human HARE proteins are not encoded directly by mRNA but are derived from larger precursors. Here we characterize the specificity and function of the 175-kDa HARE, expressed in the absence of the 300-kDa species, in stably transfected SK-Hep-1 cells. The HARE cDNA was fused with a leader sequence to allow correct orientation of the membrane protein. The recombinant rHARE contained approximately 25 kDa of N-linked oligosaccharides and, like the native protein, was able to bind HA in a ligand blot assay, even after de-N-glycosylation. SK-HARE cell lines demonstrated specific 125I-HA endocytosis, receptor recycling, and delivery of HA to lysosomes for degradation. The Kd for the binding of HA (number-average molecular mass approximately 133 kDa) to the 175-kDa HARE at 4 degrees C was 4.1 nm with 160,000 to 220,000 HA-binding sites per cell. The 175-kDa rHARE binds HA, dermatan sulfate, and chondroitin sulfates A, C, D, and E, but not chondroitin, heparin, heparan sulfate, or keratan sulfate. Surprisingly, recognition of glycosaminoglycans (GAGs) other than HA by native or recombinant HARE was temperature-dependent. Although competition was observed at 37 degrees C, none of the other GAGs competed for 125I-HA binding to SK-HARE cells at 4 degrees C. Anti-HARE monoclonal antibody-174 showed a similar temperature-dependence in its ability to block HA endocytosis. These data suggest that temperature-induced conformational changes may alter the GAG specificity of HARE. The results confirm that the 175-kDa rHARE does not require the larger HARE isoform to mediate endocytosis of multiple GAGs.

Purification and molecular identification of the human hyaluronan receptor for endocytosis

The clearance of hyaluronan (HA) and chondroitin sulfates from the circulating blood and lymph in the body is mediated by the membrane-bound HA receptor for endocytosis (HARE). Previously, we found that two HARE species of approximately 175 kDa and approximately 300 kDa are abundant in the sinusoidal endothelial cells in rat liver, spleen, and lymph nodes (Zhou et al. [2000], J. Biol. Chem., 275, 37733-37741). In the present study, immunocytochemical analysis of human tissues showed a similar pattern with abundant expression of HARE in the sinusoidal endothelial cells of human liver, spleen, and lymph nodes. The two human HARE proteins were immunoaffinity-purified from human spleen. Each protein was recognized in western blots using several anti-rat HARE monoclonal antibodies and was able to bind 125I-HA specifically. In nonreducing SDS-PAGE, these two human HARE species migrated at approximately 190 kDa and approximately 315 kDa; both proteins are approximately 15 kDa larger than the corresponding rat HAREs, although the de-N-glycosylated core proteins are essentially the same mass. After reduction, the human 190-kDa HARE gave a single 196-kDa species, which was not seen in the approximately 315-kDa HARE after reduction. The reduced approximately 315-kDa HARE yielded two major proteins at approximately 250 kDa and approximately 220 kDa. We determined the sequence of the human 190-kDa HARE cDNA based on analysis of internal tryptic peptides, as well as RT-PCR and 5' RACE analyses using human spleen and lymph node cDNA libraries. The human gene that encodes HARE is on chromosome 12.

A blocking antibody to the hyaluronan receptor for endocytosis (HARE) inhibits hyaluronan clearance by perfused liver

Hyaluronan (HA) and chondroitin sulfate clearance from lymph and blood is mediated by the hyaluronan receptor for endocytosis (HARE). The purification and molecular cloning (Zhou, B., Weigel, J. A., Saxena, A., and Weigel, P. H. (2002) Mol. Biol. Cell 13, 2853-2868) of this cell surface receptor were finally achieved after we developed monoclonal antibodies (mAbs) against HARE. There are actually two independent isoreceptors for HA, which in rat are designated the 175-kDa HARE and 300-kDa HARE. Only one mAb (number 174) effectively and completely blocked the specific uptake of 125I-HA at 37 degrees C by rat liver sinusoidal endothelial cells. 125I-HA binding to both the 175-kDa and 300-kDa HARE proteins in a ligand blot assay was almost completely inhibited by <1 microg/ml mAb-174, whereas mouse IgG had little or no effect. MAb-174 also performed very well in Western analysis, indirect fluorescence microscopy, and a variety of immuno-procedures. Immunohistochemistry using mAb-174 localized HARE to the sinusoidal cells of rat liver, spleen, and lymph node. Western analysis using mAb-174 revealed that the sizes of both HARE glycoproteins were the same in these three tissues. 125I-HA was taken up and degraded by excised rat livers that were continuously perfused ex vivo with a recirculating medium. This HA clearance and metabolism by liver, which is a physiological function of HARE, was very effectively blocked by mAb-174 but not by mouse IgG. The results indicate that mAb-174 will be a useful tool to study the functions of HARE and the physiological significance of HA clearance.

Use of Iodinated Hyaluronan Derivatives to Study Hyaluronan Binding, Endocytosis, and Metabolism by Cultured Cells

We have described several simple procedures to determine whether cells have specific receptors or binding proteins for HA. The advantage of using the uniquely modified 125I-labeled HA as a probe is that low levels of binding can be detected. It has been straightforward and productive to use 125I-labeled HA to quantitate cell surface and intracellular binding sites to determine affinity constants for HA binding and to determine whether the cell type being studied can efficiently endocytose and degrade the HA. The 125I-labeled HA preparations and assays described here and in Chapter 26 provide a very useful array of methods for studying the biochemistry and cell biology of HA.

Study of Hyaluronan-Binding Proteins and Receptors Using Iodinated Hyaluronan Derivatives

This chapter detailed methodology for the purification of high molecular weight HA, as well as procedures to fragment the HA to prepare large oligosaccharides in the range of 40,000-80,000 Da. The aforementioned procedures used to prepare HA-alkylamine and HA-Bolton-Hunter adducts, as well as 125I-labeled HA, have been very reproducible, and the latter preparations are of adequate length to retain high-affinity interactions and specific binding, e.g., to human fibrinogen and HARE. For example, we were able to isolate, characterize, and clone the rat HARE using 125I-labeled HA initially with the dot blot assay to monitor solubilization and partial purification, and later with the ligand blot assay, to identify the protein after SDS-PAGE. The ligand blot assay enabled us to determine that HARE is actually present as two discrete isoreceptors of different molecular masses. These techniques should provide a means to analyze purification strategies and to characterize additional HA receptors and binding proteins involved in a variety of physiologic processes.

The position of cysteine relative to the transmembrane domain is critical for palmitoylation of H1, the major subunit of the human asialoglycoprotein receptor

The mammalian hepatic asialoglycoprotein receptor (ASGP-R) is an endocytic recycling receptor that mediates the internalization of desialylated glycoproteins and their delivery to lysosomes where they are degraded. The human ASGP-R is a hetero-oligomeric complex composed of two subunits designated H1 and H2. Both subunits are palmitoylated at the cytoplasmic Cys residues near their transmembrane domains (TMD). The cytoplasmic Cys(36) in H1 is located at a position that is five amino acids from the transmembrane junction. Because the sequences of subunits in all mammalian ASGP-R species are highly conserved especially at the region near the palmitoylated Cys, we sought to identify a recognition signal for the palmitoylation of H1. Various types of H1 mutants were created by site-directed or deletion mutagenesis including alteration of the amino acids surrounding Cys(36), replacing portions of the TMD with that of a different protein and partial deletion of the cytoplasmic domain as well as transposing the palmitoylated Cys to positions further away from the TMD. Mutant H1 cDNAs were transiently expressed in COS-7 cells, and the H1 proteins were analyzed after metabolic labeling with [(3)H]palmitate. The results indicate that neither the native amino acid sequence surrounding Cys(36) nor the majority of the cytoplasmic domain sequence is critical for palmitoylation. Palmitoylation was also not dependent on the native TMD of H1. In contrast, the attachment of palmitate was abolished if the Cys residue was transposed to a position that was 30 amino acids away from the transmembrane border. We conclude that the spacing of a Cys residue relative to the TMD in the primary protein sequence of H1 is the major determinant for successful palmitoylation.

Nonpalmitoylated human asialoglycoprotein receptors recycle constitutively but are defective in coated pit-mediated endocytosis, dissociation, and delivery of ligand to lysosomes

The hepatic asialoglycoprotein receptor (ASGP-R) internalizes desialylated glycoproteins via the clathrin-coated pit pathway and mediates their delivery to lysosomes for degradation. The human ASGP-R contains two subunits, H1 and H2. Cytoplasmic residues Cys(36) in H1, as well as Cys(54) and Cys(58) in H2 are palmitoylated (Zeng, F.-Y., and Weigel, P. H. (1996) J. Biol. Chem. 271, 32454). In order to study the function(s) of ASGP-R palmitoylation, we mutated these Cys residues to Ser and generated stably transfected SK-Hep-1 cell lines expressing either wild-type or nonpalmitoylated ASGP-Rs. Compared with wild-type ASGP-Rs, palmitoylation-defective ASGP-Rs showed normal ligand binding, intracellular distribution and trafficking patterns, and pH-induced dissociation profiles in vitro. However, continuous ASOR uptake, and the uptake of prebound cell surface ASOR were slower in cells expressing palmitoylation-defective ASGP-Rs than in cells expressing wild-type ASGP-Rs. Unlike native ASGP-Rs in hepatocytes or hepatoma cells, which mediate endocytosis via the clathrin-coated pit pathway and are almost completely inhibited by hypertonic medium, only approximately 40% of the ASOR uptake in SK-Hep-1 cells expressing wild-type ASGP-Rs was inhibited by hyperosmolarity. This result suggests the existence of an alternate nonclathrin-mediated internalization pathway, such as transcytosis, for the entry of ASGP-R.ASOR complexes into these cells. In contrast, ASOR uptake mediated by cells expressing palmitoylation-defective ASGP-Rs showed only a marginal difference under hypertonic conditions, indicating that most of the nonpalmitoylated ASGP-Rs were not internalized and processed normally through the clathrin-coated pit pathway. Furthermore, cells expressing wild-type ASGP-Rs were able to degrade the internalized ASOR, whereas ASOR dissociation was impaired and degradation was barely detectable in cells expressing nonpalmitoylated ASGP-Rs. We conclude that palmitoylation of the ASGP-R is required for its efficient endocytosis of ligand by the clathrin-dependent endocytic pathway and, in particular, for the proper dissociation and delivery of ligand to lysosomes.

Palmitoylation-defective asialoglycoprotein receptors are normal in their cellular distribution and ability to bind ligand, but are defective in ligand uptake and degradation

The hepatic asialoglycoprotein receptor (ASGP-R) is an endocytic recycling receptor that mediates the endocytosis of desialylated glycoproteins. The human ASGP-R is composed of two homologous subunits, H1 and H2, and the cytoplasmic Cys residues in both subunits are palmitoylated. To study the effects of palmitoylation on ASGP-R activity and function, we generated four types of stably transfected cell lines in SK-Hep-1 hepatoma cells, expressing wild-type, or partially or completely palmitoylation-defective ASGP-Rs containing Cys-to-Ser mutations in either one or both subunits. Scatchard analysis showed that all four stable cell lines expressed a similar number of binding sites for asialo-orosomucoid, with comparable dissociation constants of approximately 1-3nM. Immunofluorescence confocal microscopy indicated a normal distribution of the palmitoylation-defective H1 and H2 subunits compared to the wild-type. However, cell lines expressing palmitoylation-defective ASGP-Rs had markedly reduced rates of ligand uptake and degradation compared to cells expressing wild-type ASGP-Rs. We conclude that failure to palmitoylate Cys residues in either or both subunits of human ASGP-Rs results in very inefficient uptake and degradation of ligands.