Mechanisms of axon regeneration and its inhibition: roles of sulfated glycans

Models and approaches to comprehend and address glial inflammation following spinal cord injury

Spinal cord injury (SCI) culminates in chronic inflammation and glial scar formation driven by the activation of microglia and astrocytes. Current anti-inflammatory strategies to treat glial activation associated with SCI have several limitations. Existing in vitro and ex vivo models studying molecular mechanisms associated with inflammation focus only on the acute phase. However, the progression of glial cell-derived inflammation over the acute-to-chronic phases has not been assessed. Understanding this progression will help establish a framework for evaluating therapeutic strategies. Additionally, new models could be useful as high-throughput screening (HTS) platforms. This review aims to highlight currently available models and future methods that could facilitate screening of novel therapeutics for SCI.

An insight into new glycotherapeutics in glial inflammation: Understanding the role of glycosylation in mitochondrial function and acute to the chronic phases of inflammation

INTRODUCTION

Glycosylation plays a critical role during inflammation and glial scar formation upon spinal cord injury (SCI) disease progression. Astrocytes and microglia are involved in this cascade to modulate the inflammation and tissue remodeling from acute to chronic phases. Therefore, understating the glycan changes in these glial cells is paramount.

METHOD AND RESULTS

A lectin microarray was undertaken using a cytokine-driven inflammatory mixed glial culture model, revealing considerable differential glycosylation from the acute to the chronic phase in a cytokine-combination generated inflamed MGC model. It was found that several N- and O-linked glycans associated with glia during SCI were differentially regulated. Pearson's correlation hierarchical clustering showed that groups were separated into several clusters, illustrating the heterogenicity among the control, cytokine combination, and LPS treated groups and the day on which treatment was given. Control and LPS treatments were observed to be in dense clusters. This was further confirmed with lectin immunostaining in which GalNAc, GlcNAc, mannose, fucose and sialic acid-binding residues were detected in astrocytes and microglia. However, the sialyltransferase inhibitor inhibited this modification (upregulation of the sialic acid expression), which indeed modulates the mitochondrial functions.

CONCLUSIONS

The present study is the first functional investigation of glycosylation modulation in a mixed glial culture model, which elucidates the role of the glycome in neuroinflammation in progression and identified potential therapeutic targets for future glyco therapeutics in neuroinflammation.

The Anti-inflammation Property of Olfactory Ensheathing Cells in Neural Regeneration After Spinal Cord Injury

Neural regeneration has troubled investigators worldwide in the past decades. Currently, cell transplantation emerged as a breakthrough targeted therapy for spinal cord injury (SCI) in the neurotrauma field, which provides a promising strategy in neural regeneration. Olfactory ensheathing cells (OECs), a specialized type of glial cells, is considered as the excellent candidate due to its unique variable and intrinsic regeneration-supportive properties. In fact, OECs could support olfactory receptor neuron turnover and axonal extension, which is essential to maintain the function of olfactory nervous system. Hitherto, an increasing number of literatures demonstrate that transplantation of OECs exerts vital roles in neural regeneration and functional recovery after neural injury, including central and peripheral nervous system. It is common knowledge that the deteriorating microenvironment (ischemia, hypoxia, scar, acute and chronic inflammation, etc.) resulting from injured nervous system is adverse for neural regeneration. Interestingly, recent studies indicated that OECs could promote neural repair through improvement of the disastrous microenvironments, especially to the overwhelmed inflammatory responses. Although OECs possess unusual advantages over other cells for neural repair, particularly in SCI, the mechanisms of OEC-mediated neural repair are still controversial with regard to anti-inflammation. Therefore, it is significant to summarize the anti-inflammation property of OECs, which is helpful to understand the biological characteristics of OECs and drive future studies. Here, we mainly focus on the anti-inflammatory role of OECs to make systematic review and discuss OEC-based therapy for CNS injury.

Close association of polarization and LC3, a marker of autophagy, in axon determination in mouse hippocampal neurons

The autophagy-lysosome pathway is a cellular clearance system for intracellular organelles, macromolecules and microorganisms. It is indispensable for cells not only to maintain their homeostasis but also to achieve more active cellular processes such as differentiation. Therefore, impairment or disruption of the autophagy-lysosome pathway leads to a wide spectrum of human diseases, ranging from several types of neurodegenerative diseases to malignancies. In elongating axons, autophagy preferentially occurs at growth cones, and disruption of autophagy is closely associated with incapacity for axonal regeneration after injury in the central nervous system. However, the roles of autophagy in developing neurons remain elusive. In particular, whether autophagy is involved in axon-dendrite determination is largely unclear. Using primary cultured mouse embryonic hippocampal neurons, we here showed the polarized distribution of autophagosomes among minor processes of neurons at stage 2. Time-lapse observation of neurons from GFP-LC3 transgenic mice demonstrated that an "LC3 surge"-i.e., a rapid accumulation of autophagic marker LC3 that continues for several hours in one minor process-proceeded the differentiation of neurons into axons. In addition, pharmacological activation and inhibition of autophagy by trehalose and bafilomycin, respectively, accelerated and delayed axonal determination. Taken together, our findings revealed the close association between LC3, a marker of autophagy, and axon determination in developing neurons.

Dermatan sulfate is an activating ligand of anaplastic lymphoma kinase

Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase (RTK) that harbors a tyrosine kinase domain in its intracellular region and is expressed in both central and peripheral nervous systems. RTKs are activated upon ligand binding and receptor clustering; however, ALK remains an orphan receptor despite its pathological significance, especially in malignancy. Recent biochemical work showed that heparan sulfate (HS), an unbranched sulfated glycan, acts as a ligand for and activates ALK. Here we show that dermatan sulfate (DS, chondroitin sulfate B) directly interacts with the extracellular N-terminal region of ALK as well as HS. The tetrasaccharide of DS was required and was sufficient for inducing autophosphorylation of ALK at tyrosine 1604, a marker for activated ALK. Interestingly, longer oligosaccharides caused enhanced activation of ALK, as was the case for HS. Our results provide a novel example of glycans as signaling molecules and shed light on the pathophysiological roles of ALK.

Enoxaparin promotes functional recovery after spinal cord injury by antagonizing PTPRσ

The receptor-type protein tyrosine phosphatase sigma (PTPRσ) regulates axonal regeneration/sprouting as a molecular switch in response to glycan ligands. Cell surface heparan sulfate oligomerizes PTPRσ and inactivates its enzymatic activity, which in turn promotes axonal growth. In contrast, matrix-associated chondroitin sulfate monomerizes PTPRσ and activates it. This leads to dephosphorylation of its specific substrates, such as cortactin, resulting in a failure of axonal regeneration after injury. However, this molecular switch model has never been challenged in a clinical situation. In this study, we demonstrated that enoxaparin, a globally approved anticoagulant consisting of heparin oligosaccharides with an average molecular weight of 45 kDa, induced clustering and inactivated PTPRσ in vitro. Enoxaparin induced PTPRσ clustering, and counteracted PTPRσ-mediated dephosphorylation of cortactin, which was shown to be important for inhibition of axonal regeneration. Systemic administration of enoxaparin promoted anatomical recovery after both optic nerve and spinal cord injuries in rats at clinically tolerated doses. Moreover, enoxaparin promoted recovery of motor function without obvious hemorrhage. Collectively, our data provide a new strategy for the treatment of traumatic axonal injury.

Polyunsaturated fatty acid deficiency affects sulfatides and other sulfated glycans in lysosomes through autophagy-mediated degradation

Metabolic changes in sulfatides and other sulfated glycans have been related to various diseases, including Alzheimer's disease (AD). However, the importance of polyunsaturated fatty acids (PUFA) in sulfated lysosomal substrate metabolism and its related disorders is currently unknown. We investigated the effects of deficiency or supplementation of PUFA on the metabolism of sulfatides and sulfated glycosaminoglycans (sGAGs) in sulfatide-rich organs (brain and kidney) of mice. A PUFA-deficient diet for over 5 weeks significantly reduced the sulfatide expression by increasing the sulfatide degradative enzymes arylsulfatase A and galactosylceramidase in brain and kidney. This sulfatide degradation was clearly associated with the activation of autophagy and lysosomal hyperfunction, the former of which was induced by suppression of the Erk/mTOR pathway. A PUFA-deficient diet also activated the degradation of sGAGs in the brain and kidney and that of amyloid precursor proteins in the brain, indicating an involvement in general lysosomal function and the early developmental process of AD. PUFA supplementation prevented all of the above abnormalities. Taken together, a PUFA deficiency might lead to sulfatide and sGAG degradation associated with autophagy activation and general lysosomal hyperfunction and play a role in many types of disease development, suggesting a possible benefit of prophylactic PUFA supplementation.

Keratan Sulphate in the Tumour Environment

Keratan sulphate (KS) is a bioactive glycosaminoglycan (GAG) of some complexity composed of the repeat disaccharide D-galactose β1→4 glycosidically linked to N-acetyl glucosamine. During the biosynthesis of KS, a family of glycosyltransferase and sulphotransferase enzymes act sequentially and in a coordinated fashion to add D-galactose (D-Gal) then N-acetyl glucosamine (GlcNAc) to a GlcNAc acceptor residue at the reducing terminus of a nascent KS chain to effect chain elongation. D-Gal and GlcNAc can both undergo sulphation at C6 but this occurs more frequently on GlcNAc than D-Gal. Sulphation along the developing KS chain is not uniform and contains regions of variable length where no sulphation occurs, regions which are monosulphated mainly on GlcNAc and further regions of high sulphation where both of the repeat disaccharides are sulphated. Each of these respective regions in the KS chain can be of variable length leading to KS complexity in terms of chain length and charge localization along the KS chain. Like other GAGs, it is these variably sulphated regions in KS which define its interactive properties with ligands such as growth factors, morphogens and cytokines and which determine the functional properties of tissues containing KS. Further adding to KS complexity is the identification of three different linkage structures in KS to asparagine (N-linked) or to threonine or serine residues (O-linked) in proteoglycan core proteins which has allowed the categorization of KS into three types, namely KS-I (corneal KS, N-linked), KS-II (skeletal KS, O-linked) or KS-III (brain KS, O-linked). KS-I to -III are also subject to variable addition of L-fucose and sialic acid groups. Furthermore, the GlcNAc residues of some members of the mucin-like glycoprotein family can also act as acceptor molecules for the addition of D-Gal and GlcNAc residues which can also be sulphated leading to small low sulphation glycoforms of KS. These differ from the more heavily sulphated KS chains found on proteoglycans. Like other GAGs, KS has evolved molecular recognition and information transfer properties over hundreds of millions of years of vertebrate and invertebrate evolution which equips them with cell mediatory properties in normal cellular processes and in aberrant pathological situations such as in tumourogenesis. Two KS-proteoglycans in particular, podocalyxin and lumican, are cell membrane, intracellular or stromal tissue-associated components with roles in the promotion or regulation of tumour development, mucin-like KS glycoproteins may also contribute to tumourogenesis. A greater understanding of the biology of KS may allow better methodology to be developed to more effectively combat tumourogenic processes.

Keratan sulfate (KS)-proteoglycans and neuronal regulation in health and disease: the importance of KS-glycodynamics and interactive capability with neuroregulatory ligands

Compared to the other classes of glycosaminoglycans (GAGs), that is, chondroitin/dermatan sulfate, heparin/heparan sulfate and hyaluronan, keratan sulfate (KS), have the least known of its interactive properties. In the human body, the cornea and the brain are the two most abundant tissue sources of KS. Embryonic KS is synthesized as a linear poly-N-acetyllactosamine chain of d-galactose-GlcNAc repeat disaccharides which become progressively sulfated with development, sulfation of GlcNAc is more predominant than galactose. KS contains multi-sulfated high-charge density, monosulfated and non-sulfated poly-N-acetyllactosamine regions and thus is a heterogeneous molecule in terms of chain length and charge distribution. A recent proteomics study on corneal KS demonstrated its interactivity with members of the Slit-Robbo and Ephrin-Ephrin receptor families and proteins which regulate Rho GTPase signaling and actin polymerization/depolymerization in neural development and differentiation. KS decorates a number of peripheral nervous system/CNS proteoglycan (PG) core proteins. The astrocyte KS-PG abakan defines functional margins of the brain and is up-regulated following trauma. The chondroitin sulfate/KS PG aggrecan forms perineuronal nets which are dynamic neuroprotective structures with anti-oxidant properties and roles in neural differentiation, development and synaptic plasticity. Brain phosphacan a chondroitin sulfate, KS, HNK-1 PG have roles in neural development and repair. The intracellular microtubule and synaptic vesicle KS-PGs MAP1B and SV2 have roles in metabolite transport, storage, and export of neurotransmitters and cytoskeletal assembly. MAP1B has binding sites for tubulin and actin through which it promotes cytoskeletal development in growth cones and is highly expressed during neurite extension. The interactive capability of KS with neuroregulatory ligands indicate varied roles for KS-PGs in development and regenerative neural processes.

[Inhibitors of CNS regeneration, their physiological role and participation in pathogenesis of diseases]

The review is devoted to axon growth inhibitors in the CNS, including a physiological role of myelin-associated proteins (Nogo-A, MAG, OMgp) and their involvement in the pathogenesis of various diseases (spinal injuries, stroke, neurodegenerations).

Profiling of glycan alterations in regrowing limb tissues of Cynops orientalis

Glycans are known to play important roles in molecular recognition, cell-cell adhesion, molecular trafficking, receptor activation, and signal transduction during development and regeneration. Despite numerous investigations of regenerating salamander limbs, global analysis of the precise variation of glycans during the limb regeneration process has received little attention. Here, we have used lectin microarrays and lectin histochemistry to analyze the alterations and distribution of glycans during the early stages leading to blastema formation during Cynops orientalis limb regeneration in response to limb amputation. Compared with the control group, analysis at several time points (3, 7, and 14 days postamputation) using microarrays containing 37 lectins showed that limb tissues expressed significantly different complements of glycans recognized by 9 different lectins. Postamputation limb tissues showed higher expression of two glycan structures recognized by the lectins STL and LTL and lower expression of seven glycan structures recognized by PHA-E, MAL-I, SNA, UEA-I, PHA-E + L, VVA, and GNA. We also observed significant changes in glycans specifically at 7 days postamputation, including higher binding capacity by WFA, GSL-I, and NPA and lower binding capacity by PNA, HHL, ConA, LCA, GSL-II, and PWM. Next, we validated our lectin microarray data using lectin histochemistry in limb tissues. Glycans recognized by STL and GNA showed similar changes in signal intensity to those found in the lectin microarrays, with STL staining in the cytoplasm and GNA in the cytoplasm and nucleus. Our findings are the first report of significant postamputation changes in glycans in limb tissues and suggest that those glycans perform potentially important functions during the early stages of C. orientalis limb regeneration.

Formation and remodeling of the brain extracellular matrix in neural plasticity: Roles of chondroitin sulfate and hyaluronan

BACKGROUND

The extracellular matrix (ECM) of the brain is rich in glycosaminoglycans such as chondroitin sulfate (CS) and hyaluronan. These glycosaminoglycans are organized into either diffuse or condensed ECM. Diffuse ECM is distributed throughout the brain and fills perisynaptic spaces, whereas condensed ECM selectively surrounds parvalbumin-expressing inhibitory neurons (PV cells) in mesh-like structures called perineuronal nets (PNNs). The brain ECM acts as a non-specific physical barrier that modulates neural plasticity and axon regeneration.

SCOPE OF REVIEW

Here, we review recent progress in understanding of the molecular basis of organization and remodeling of the brain ECM, and the involvement of several types of experience-dependent neural plasticity, with a particular focus on the mechanism that regulates PV cell function through specific interactions between CS chains and their binding partners. We also discuss how the barrier function of the brain ECM restricts dendritic spine dynamics and limits axon regeneration after injury.

MAJOR CONCLUSIONS

The brain ECM not only forms physical barriers that modulate neural plasticity and axon regeneration, but also forms molecular brakes that actively controls maturation of PV cells and synapse plasticity in which sulfation patterns of CS chains play a key role. Structural remodeling of the brain ECM modulates neural function during development and pathogenesis.

GENERAL SIGNIFICANCE

Genetic or enzymatic manipulation of the brain ECM may restore neural plasticity and enhance recovery from nerve injury. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Mechanisms of axon regeneration: The significance of proteoglycans

BACKGROUND

Therapeutics specific to neural injury have long been anticipated but remain unavailable. Axons in the central nervous system do not readily regenerate after injury, leading to dysfunction of the nervous system. This failure of regeneration is due to both the low intrinsic capacity of axons for regeneration and the various inhibitors emerging upon injury. After many years of concerted efforts, however, these hurdles to axon regeneration have been partially overcome.

SCOPE OF REVIEW

This review summarizes the mechanisms regulating axon regeneration. We highlight proteoglycans, particularly because it has become increasingly clear that these proteins serve as critical regulators for axon regeneration.

MAJOR CONCLUSIONS

Studies on proteoglycans have revealed that glycans not only assist in the modulation of protein functions but also act as main players-e.g., as functional ligands mediating intracellular signaling through specific receptors on the cell surface. By regulating clustering of the receptors, glycans in the proteoglycan moiety, i.e., glycosaminoglycans, promote or inhibit axon regeneration. In addition, proteoglycans are involved in various types of neural plasticity, ranging from synaptic plasticity to experience-dependent plasticity.

GENERAL SIGNIFICANCE

Although studies on proteins have progressively facilitated our understanding of the nervous system, glycans constitute a new frontier for further research and development in this field. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Mild Hypothermia Promotes Pericontusion Neuronal Sprouting via Suppressing Suppressor of Cytokine Signaling 3 Expression after Moderate Traumatic Brain Injury

Mild therapeutic hypothermia is a candidate for the treatment of traumatic brain injury (TBI). However, the role of mild hypothermia in neuronal sprouting after TBI remains obscure. We used a fluid percussion injury (FPI) model to assess the effect of mild hypothermia on pericontusion neuronal sprouting after TBI in rats. Male Sprague-Dawley rats underwent FPI or sham surgery, followed by mild hypothermia treatment (33°C) or normothermia treatment (37°C) for 3 h. All the rats were euthanized at 7 days after FPI. Neuronal sprouting that was confirmed by an increase in growth associated protein-43 (GAP-43) expression was evaluated using immunofluorescence and Western blot assays. The expression levels of several intrinsic and extrinsic sprouting-associated genes such as neurite outgrowth inhibitor A (NogoA), phosphatase and tensin homolog (PTEN), and suppressor of cytokine signaling 3 (SOCS3) were analyzed by quantitative real-time polymerase chain reaction (RT-PCR). Our results revealed that mild hypothermia significantly increased the expression level of GAP-43 and dramatically suppressed the expression level of interleukin-6 (IL-6) and SOCS3 at 7 days after FPI in the ipsilateral cortex compared with that of the normothermia TBI group. These data suggest that post-traumatic mild hypothermia promotes pericontusion neuronal sprouting after TBI. Moreover, the mechanism of hypothermia-induced neuronal sprouting might be partially associated with decreased levels of SOCS3.

The Chemorepulsive Protein Semaphorin 3A and Perineuronal Net-Mediated Plasticity

During postnatal development, closure of critical periods coincides with the appearance of extracellular matrix structures, called perineuronal nets (PNN), around various neuronal populations throughout the brain. The absence or presence of PNN strongly correlates with neuronal plasticity. It is not clear how PNN regulate plasticity. The repulsive axon guidance proteins Semaphorin (Sema) 3A and Sema3B are also prominently expressed in the postnatal and adult brain. In the neocortex, Sema3A accumulates in the PNN that form around parvalbumin positive inhibitory interneurons during the closure of critical periods. Sema3A interacts with high-affinity with chondroitin sulfate E, a component of PNN. The localization of Sema3A in PNN and its inhibitory effects on developing neurites are intriguing features and may clarify how PNN mediate structural neural plasticity. In the cerebellum, enhanced neuronal plasticity as a result of an enriched environment correlates with reduced Sema3A expression in PNN. Here, we first review the distribution of Sema3A and Sema3B expression in the rat brain and the biochemical interaction of Sema3A with PNN. Subsequently, we review what is known so far about functional correlates of changes in Sema3A expression in PNN. Finally, we propose a model of how Semaphorins in the PNN may influence local connectivity.

Neuron-Glia Interactions in Neural Plasticity: Contributions of Neural Extracellular Matrix and Perineuronal Nets

Synapses are specialized structures that mediate rapid and efficient signal transmission between neurons and are surrounded by glial cells. Astrocytes develop an intimate association with synapses in the central nervous system (CNS) and contribute to the regulation of ion and neurotransmitter concentrations. Together with neurons, they shape intercellular space to provide a stable milieu for neuronal activity. Extracellular matrix (ECM) components are synthesized by both neurons and astrocytes and play an important role in the formation, maintenance, and function of synapses in the CNS. The components of the ECM have been detected near glial processes, which abut onto the CNS synaptic unit, where they are part of the specialized macromolecular assemblies, termed perineuronal nets (PNNs). PNNs have originally been discovered by Golgi and represent a molecular scaffold deposited in the interface between the astrocyte and subsets of neurons in the vicinity of the synapse. Recent reports strongly suggest that PNNs are tightly involved in the regulation of synaptic plasticity. Moreover, several studies have implicated PNNs and the neural ECM in neuropsychiatric diseases. Here, we highlight current concepts relating to neural ECM and PNNs and describe an in vitro approach that allows for the investigation of ECM functions for synaptogenesis.

Genome-wide Association Study of Postburn Scarring Identifies a Novel Protective Variant

OBJECTIVE

To identify genetic variants associated with the severity of postburn hypertrophic scarring (HTS) using a genome-wide approach.

BACKGROUND

Risk of severe postburn HTS is known to depend on race, but the genetic determinants of HTS are unknown.

METHODS

We conducted a genome-wide association study (GWAS) in a prospective cohort of adults admitted with deep-partial-thickness burns from 2007 through 2014. Scar severity was assessed over time using the Vancouver Scar Scale (VSS), and DNA was genotyped with a >500,000-marker array. We performed association testing of single-nucleotide polymorphisms (SNPs) with minor allele frequency (MAF) >0.01 using linear regression of VSS height score on genotype adjusted for patient and injury characteristics as well as population genetic structure. Array-wide significance was based on Bonferroni correction for multiple testing.

RESULTS

Of 538 patients (median age 40 years, median burn size 6.0% of body surface area), 71% were men and 76% were White. The mean VSS height score was 1.2 (range: 0-3). Of 289,639 SNPs tested, a variant in the CUB and Sushi multiple domains 1 (CSMD1) gene (rs11136645; MAF = 0.49), was significantly associated with decreased scar height (regression coefficient = -0.23, P = 7.9 × 10).

CONCLUSIONS

In the first published GWAS of HTS, we report that a common intronic variant in the CSMD1 gene is associated with reduced severity of postburn HTS. If this association is confirmed in an independent cohort, investigating the potential role of CSMD1 in wound healing may elucidate HTS pathophysiology.

A combination of keratan sulfate digestion and rehabilitation promotes anatomical plasticity after rat spinal cord injury

Functional recovery after neuronal injuries relies on neuronal network reconstruction which involves many repair processes, such as sealing of injured axon ends, axon regeneration/sprouting, and construction and refinement of synaptic connections. Chondroitin sulfate (CS) is a major inhibitor of axon regeneration/sprouting. It has been reported that the combination of task-specific rehabilitation and CS-digestion is much more effective than either treatment alone with regard to the promotion of functional and anatomical plasticity for dexterity in acute and chronic spinal cord injury models. We previously reported that keratan sulfate (KS) is another inhibitor and has a potency equal to CS. Here, we compared the effects of KS- or CS-digestion plus rehabilitation on recovery from spinal cord injury. Keratanase II or chondroitinase ABC was locally administered at the lesion after spinal cord injury at C3/4. Task-specific rehabilitation training, i.e., a single pellet reaching task using a Whishaw apparatus, was done for 3 weeks before injury, and then again at 1-6 weeks after injury. The combination of KS-digestion and rehabilitation yielded a better rate of pellet removal than either KS-digestion alone or rehabilitation alone, although these differences were not statistically significant. The combination of CS-digestion and rehabilitation showed similar results. Strikingly, both KS-digestion/rehabilitation and CS-digestion/rehabilitation showed significant increases in neurite growth in vivo as estimated by 5-hydroxytryptamine and GAP43 staining. Thus, KS-digestion and rehabilitation exerted a synergistic effect on anatomical plasticity, and this effect was comparable with that of CS-digestion/rehabilitation. KS-digestion might widen the therapeutic window of spinal cord injury if combined with rehabilitation.

Probing the acceptor active site organization of the human recombinant β1,4-galactosyltransferase 7 and design of xyloside-based inhibitors

Among glycosaminoglycan (GAG) biosynthetic enzymes, the human β1,4-galactosyltransferase 7 (hβ4GalT7) is characterized by its unique capacity to take over xyloside derivatives linked to a hydrophobic aglycone as substrates and/or inhibitors. This glycosyltransferase is thus a prime target for the development of regulators of GAG synthesis in therapeutics. Here, we report the structure-guided design of hβ4GalT7 inhibitors. By combining molecular modeling, in vitro mutagenesis, and kinetic measurements, and in cellulo analysis of GAG anabolism and decorin glycosylation, we mapped the organization of the acceptor binding pocket, in complex with 4-methylumbelliferone-xylopyranoside as prototype substrate. We show that its organization is governed, on one side, by three tyrosine residues, Tyr¹⁹⁴, Tyr¹⁹⁶, and Tyr¹⁹⁹, which create a hydrophobic environment and provide stacking interactions with both xylopyranoside and aglycone rings. On the opposite side, a hydrogen-bond network is established between the charged amino acids Asp²²⁸, Asp²²⁹, and Arg²²⁶, and the hydroxyl groups of xylose. We identified two key structural features, i.e. the strategic position of Tyr¹⁹⁴ forming stacking interactions with the aglycone, and the hydrogen bond between the His¹⁹⁵ nitrogen backbone and the carbonyl group of the coumarinyl molecule to develop a tight binder of hβ4GalT7. This led to the synthesis of 4-deoxy-4-fluoroxylose linked to 4-methylumbelliferone that inhibited hβ4GalT7 activity in vitro with a K_i 10 times lower than the K_m value and efficiently impaired GAG synthesis in a cell assay. This study provides a valuable probe for the investigation of GAG biology and opens avenues toward the development of bioactive compounds to correct GAG synthesis disorders implicated in different types of malignancies.