Identification of a peptide mimic of the L2/HNK-1 carbohydrate epitope

Effects of local and systemic treatment with human natural killer-1 mimetic peptide (HNK-1) after ventral root avulsion and reimplantation in mice

Background

Spinal ventral root injuries generate significant motoneuron degeneration, which hinders full functional recovery. The poor prognosis of functional recovery can be attributed to the use or combination of different therapeutic approaches. Several molecules have been screened as potential treatments in combination with surgical reimplantation of the avulsed roots, the gold standard approach for such injuries. Among the studied molecules, human natural killer-1 (HNK-1) stands out as it is related to the stimulation of motor axon outgrowth. Therefore, we aimed to comparatively investigate the effects of local administration of an HNK-1 mimetic peptide (mp-HNK-1) and systemic treatment with ursolic acid (UA), another HNK-1 mimetic, after ventral root avulsion and reimplantation with heterologous fibrin biopolymer (HFB).

Methods

Female mice of the isogenic strain C57BL/6JUnib were divided into five experimental groups: Avulsion, Reimplantation, mp-HNK-1 (in situ), and UA (systemic treatment). Mice were evaluated 2 and 12 weeks after surgery. Functional assessment was performed every four days using the Catwalk platform. Neuronal survival was analyzed by cytochemistry, and glial reactions and synaptic coverage were evaluated by immunofluorescence.

Results

Treatment with UA elicited long-term neuroprotection, accompanied by a decrease in microglial reactions, and reactive astrogliosis. The neuroprotective effects of UA were preceded by increased glutamatergic and GABAergic inputs in the ventral spinal cord two weeks after injury. However, a single application of mp-HNK-1 had no significant effects. Functional analysis showed that UA treatment led to an improvement in motor and sensory recovery.

Conclusion

Overall, the results indicate that UA is neuroprotective, acting on glial cells and synaptic maintenance, and the combination of these findings led to a better functional recovery.

Peptide mimotopes to emulate carbohydrates

Glycoconjugates on animal cell surfaces are involved in numerous biological functions and diseases, especially the adhesion/metastasis of cancer cells, infection, and the onset of glycan-related diseases. In addition to glycoantigen detection, the regulation of glycan (carbohydrate)-protein interactions is needed to develop therapeutic strategies for glycan-related diseases. Preparation of a diverse range of glycan derivatives requires a massive effort, but the preparation and identification of alternative glycan-mimetic peptide mimotopes may provide a solution to this issue. Peptide mimotopes are recognized by glycan-binding proteins, such as lectins, enzymes, and antibodies, alternative to glycan ligands. Phage-display technology is the first choice in the selection of "glycan (carbohydrate)-mimetic peptide mimotopes" from a large repertoire of library sequences. This tutorial review describes the advantages of peptide mimotopes in comparison to glycan ligands, as well as their structural and functional mimicry. The detailed library design is followed by a description of the strategy used to improve affinity, and finally, an outline of the vaccine application of glycan-mimetic peptides is provided.

Neural glycomics: the sweet side of nervous system functions

The success of investigations on the structure and function of the genome (genomics) has been paralleled by an equally awesome progress in the analysis of protein structure and function (proteomics). We propose that the investigation of carbohydrate structures that go beyond a cell's metabolism is a rapidly developing frontier in our expanding knowledge on the structure and function of carbohydrates (glycomics). No other functional system appears to be suited as well as the nervous system to study the functions of glycans, which had been originally characterized outside the nervous system. In this review, we describe the multiple studies on the functions of LewisX, the human natural killer cell antigen-1 (HNK-1), as well as oligomannosidic and sialic (neuraminic) acids. We attempt to show the sophistication of these structures in ontogenetic development, synaptic function and plasticity, and recovery from trauma, with a view on neurodegeneration and possibilities to ameliorate deterioration. In view of clinical applications, we emphasize the need for glycomimetic small organic compounds which surpass the usefulness of natural glycans in that they are metabolically more stable, more parsimonious to synthesize or isolate, and more advantageous for therapy, since many of them pass the blood brain barrier and are drug-approved for treatments other than those in the nervous system, thus allowing a more ready access for application in neurological diseases. We describe the isolation of such mimetic compounds using not only Western NIH, but also traditional Chinese medical libraries. With this review, we hope to deepen the interests in this exciting field.

Functions of Small Organic Compounds that Mimic the HNK-1 Glycan

Because of the importance of the HNK-1 carbohydrate for preferential motor reinnervation after injury of the femoral nerve in mammals, we screened NIH Clinical Collection 1 and 2 Libraries and a Natural Product library comprising small organic compounds for identification of pharmacologically useful reagents. The reason for this attempt was to obviate the difficult chemical synthesis of the HNK-1 carbohydrate and its isolation from natural sources, with the hope to render such compounds clinically useful. We identified six compounds that enhanced neurite outgrowth from cultured spinal motor neurons at nM concentrations and increased their neurite diameter, but not their neurite branch points. Axons of dorsal root ganglion neurons did not respond to these compounds, a feature that is in agreement with their biological role after injury. We refer to the positive functions of some of these compounds in animal models of injury and delineate the intracellular signaling responses elicited by application of compounds to cultured murine central nervous system neurons. Altogether, these results point to the potential of the HNK-1 carbohydrate mimetics in clinically-oriented settings.

A mimetic peptide of α2,6-sialyllactose promotes neuritogenesis

Oxidative stress contributes to the pathogenesis of neurodegenerative diseases. With the aim to find reagents that reduce oxidative stress, a phage display library was screened for peptides mimicking α2,6-sialyllactose (6'-SL), which is known to beneficially influence neural functions. Using Sambucus nigra lectin, which specifically binds to 6'-SL, we screened a phage display library and found a peptide comprising identical sequences of 12 amino acids. Mimetic peptide, reverse peptide and scrambled peptide were tested for inhibition of 6'-SL binding to the lectin. Indeed, lectin binding to 6'-SL was inhibited by the most frequently identified mimetic peptide, but not by the reverse or scrambled peptides, showing that this peptide mimics 6'-SL. Functionally, mimetic peptide, but not the reverse or scrambled peptides, increased viability and expression of neural cell adhesion molecule L1 in SK-N-SH human neuroblastoma cells, and promoted survival and neurite outgrowth of cultured mouse cerebellar granule neurons challenged by H₂O₂-induced oxidative stress. The combined results indicate that the 6'-SL mimetic peptide promotes neuronal survival and neuritogenesis, thus raising hopes for the treatment of neurodegenerative diseases. This study was approved by the Medical Ethics Committee of Shantou University Medical College, China (approval No. SUMC 2014-004) on February 20, 2014.

Hybrid ligands with calixarene and thiodigalactoside groups: galectin binding and cytotoxicity

Galectins have diverse functions and are involved in many biological processes because of their complex intra- and extracellular activities. Selective and potent inhibitors for galectins will be valuable tools to investigate the biological functions of these proteins. Therefore, we describe here the synthesis of galectin inhibitors with a potential "chelate effect". These compounds are designed to bind to two different binding sites on galectins simultaneously. In this paper a series of asymmetric "hybrid" compounds are prepared, which combine two galectin ligands (1) a substituted thiodigalactoside derivative and (2) an antagonist calixarene-based therapeutic agent. NMR spectroscopy was used to evaluate the interactions of these compounds with Galectin-1 and -3. In addition, cellular experiments were conducted to compare the cytotoxic effects of the hybrids with those of a calixarene derivative. While only the thiodigalactoside part of the hybrids showed strong binding, the calixarene part was responsible for observed cytoxoxicity effects, suggesting that the calixarene moiety may also be addressing a non-galectin target.

Glycoreplica peptides to investigate molecular mechanisms of immune-mediated physiological versus pathological conditions

Investigation of the role of saccharides and glycoconjugates in mechanisms of immune-mediated physiological and pathological conditions is a hot topic. In fact, in many autoimmune diseases cross-reactivity between sugar moieties exposed on exogenous pathogens and self-molecules has long been hinted. Several peptides have been reported as mimetics of glycans specifically interacting with sugar-binding antibodies. The seek for these glycoreplica peptides is instrumental in characterizing antigen mimicry pathways and their involvement in triggering autoimmunity. Therefore, peptides mimicking glycan-protein interactions are valuable molecular tools to overcome the difficulties of oligosaccharide preparations. The clinical impact of peptide-based probes for autoimmune diseases diagnosis and follow-up is emerging only recently as just the tip of the iceberg of an overlooked potential. Here we provide a brief overview of the relevance of the structural and functional aspects of peptide probes and their mimicry effect in autoimmunity mechanisms for promising applications in diagnostics and therapeutics.

Identification and characterization of synthetic chondroitin-4-sulfate binding peptides in neuronal functions

Chondroitin sulfate proteoglycans (CSPGs), up-regulated in and around the glial scar after mammalian spinal cord injury, have been suggested to be key inhibitory molecules for functional recovery by impeding axonal regrowth/sprouting and synaptic rearrangements. CSPG-mediated inhibition is mainly associated with the glycosaminoglycan chains of CSPGs, and chondroitin-4-sulfate (C4S) is the predominant sulfated structure that regulates axonal guidance and growth in the adult nervous system. With the aim to find molecules that neutralize the inhibitory functions of C4S, we screened a phage display library for peptides binding to C4S. From the phage clones binding to C4S we selected three peptides for further analysis. We observed that these peptides bind to C4S, but not chondroitin-6-sulfate, heparin sulfate or dermatan sulfate, in a concentration-dependent and saturable manner, whereas the scrambled peptides showed highly reduced or no binding to C4S. The C4S-binding peptides, but not their scrambled counterparts, when added to cultures of mouse cerebellar neurons and human neuroblastoma cells, neutralized the inhibitory functions of the C4S- and CSPG-coated substrate on cell adhesion, neuronal migration and neurite outgrowth. These results indicate that the C4S-binding peptides neutralize several inhibitory functions of CSPGs, suggesting that they may be beneficial in repairing mammalian nervous system injuries.

Structure-Activity Relationship Studies, SPR Affinity Characterization, and Conformational Analysis of Peptides That Mimic the HNK-1 Carbohydrate Epitope

The design of molecules that mimic biologically relevant glycans is a significant goal for understanding important biological processes and may lead to new therapeutic and diagnostic agents. In this study we focused our attention on the trisaccharide human natural killer cell-1 (HNK-1), considered the antigenic determinant of myelin-associated glycoprotein and the target of clinically relevant auto-antibodies in autoimmune neurological disorders such as IgM monoclonal gammopathy and demyelinating polyneuropathy. We describe a structure-activity relationship study based on surface plasmon resonance binding affinities aimed at the optimization of a peptide that mimics the HNK-1 minimal epitope. We developed a cyclic heptapeptide that shows an affinity of 1.09×10^-7  m for a commercial anti-HNK1 mouse monoclonal antibody. Detailed conformational analysis gave possible explanations for the good affinity displayed by this novel analogue, which was subsequently used as an immunological probe. However, preliminary screening indicates that patients' sera do not specifically recognize this peptide, showing that murine monoclonal antibodies cannot be used as a guide to select immunological probes for the detection of clinically relevant human auto-antibodies.

Porous and Nonporous Nerve Conduits: The Effects of a Hydrogel Luminal Filler With and Without a Neurite-Promoting Moiety

Nerve conduits prefilled with hydrogels are frequently explored in an attempt to promote nerve regeneration. This study examines the interplay in vivo between the porosity of the conduit wall and the level of bioactivity of the hydrogel used to fill the conduit. Nerve regeneration in porous (P) or nonporous (NP) conduits that were filled with either collagen only or collagen enhanced with a covalently attached neurite-promoting peptide mimic of the glycan human natural killer cell antigen-1 (m-HNK) were compared in a 5 mm critical size defect in the mouse femoral nerve repair model. Although collagen is a cell-friendly matrix that does not differentiate between neural and nonneural cells, the m-HNK-enhanced collagen specifically promotes axon growth and appropriate motor neuron targeting. In this study, animals treated with NP conduits filled with collagen grafted with m-HNK (CollagenHNK) had the best overall functional recovery, based on a range of histomorphometric observations and parameters of functional recovery. Our data indicate that under some conditions, the use of generally cell friendly fillers such as collagen may limit nerve regeneration. This finding is significant, considering the frequent use of collagen-based hydrogels as fillers of nerve conduits.

HNK-1 Carrier Glycoproteins Are Decreased in the Alzheimer's Disease Brain

The human natural killer-1 (HNK-1), 3-sulfonated glucuronic acid, is a glycoepitope marker of cell adhesion that participates in cell-cell and cell-extracellular matrix interactions and in neurite growth. Very little is known about the regulation of the HNK-1 glycan in neurodegenerative disease, particularly in Alzheimer's disease (AD). In this study, we investigate changes in the levels of HNK-1 carrier glycoproteins in AD. We demonstrate an overall decrease in HNK-1 immunoreactivity in glycoproteins extracted from the frontal cortex of AD subjects, compared with levels from non-demented controls (NDC). Immunoblotting of ventricular post-mortem and lumbar ante-mortem cerebrospinal fluid with HNK-1 antibodies indicate similar levels of carrier glycoproteins in AD and NDC samples. Decrease in HNK-1 carrier glycoproteins were not paralleled by changes in messenger RNA (mRNA) levels of the enzymes involved in the synthesis of the glycoepitope, β-1,4-galactosyltransferase (β4GalT), glucuronyltransferases GlcAT-P and GlcAT-S, or sulfotransferase HNK-1ST. Over-expression of amyloid precursor protein in Tg2576 transgenic mice and in vitro treatment of SH-SY5Y neuroblastoma cells with the amyloidogenic Aβ42 peptide resulted in a decrease in HNK-1 immunoreactivity levels in brain and cellular extracts, whereas the levels of soluble HNK-1 glycoproteins detected in culture media were not affected by Aβ treatment. HNK-1 levels remain unaffected in the brain extracts of Tg-VLW mice, a model of mutant hyperphosphorylated tau, and in SH-SY5Y cells over-expressing hyperphosphorylated wild-type tau. These results provide evidence that cellular levels of HNK-1 carrier glycoforms are decreased in the brain of AD subjects, probably influenced by the β-amyloid protein.

Novel GM1 ganglioside-like peptide mimics prevent the association of cholera toxin to human intestinal epithelial cells in vitro

Cholera is an acute diarrheal disease caused by infection in the gastrointestinal tract by the gram-negative bacterium, Vibrio cholerae, and is a serious public health threat worldwide. There has not been any effective treatment for this infectious disease. Cholera toxin (CT), which is secreted by V. cholerae, can enter host cells by binding to GM1, a monosialoganglioside widely distributed on the plasma membrane surface of various animal epithelial cells. The present study was undertaken to generate peptides that are conformationally similar to the carbohydrate epitope of GM1 for use in the treatment of cholera and related bacterial infection. For this purpose, we used cholera toxin B (CTB) subunit to select CTB-binding peptides that structurally mimic GM1 from a dodecamer phage-display library. Six GM1-replica peptides were selected by biopanning based on CTB recognition. Five of the six peptides showed inhibitory activity for GM1 binding to CTB. To test the potential of employing the peptide mimics for intervening with the bacterial infection, those peptides were examined for their binding capacity, functional inhibitory activity and in vitro effects using a human intestinal epithelial cell line, Caco-2 cells. One of the peptides, P3 (IPQVWRDWFKLP), was most effective in inhibiting cellular uptake of CTB and suppressing CT-stimulated cyclic adenosine monophosphate production in the cells. Our results thus provide convincing evidence that GM1-replica peptides could serve as novel agents to block CTB binding on epithelial cells and prevent the ensuing physiological effects of CT.

Regulated expression and neural functions of human natural killer-1 (HNK-1) carbohydrate

Human natural killer-1 (HNK-1) carbohydrate, comprising a unique trisaccharide HSO(3)-3GlcAβ1-3Galβ1-4GlcNAc, shows well-regulated expression and unique functions in the nervous system. Recent studies have revealed sophisticated and complicated expression mechanisms for HNK-1 glycan. Activities of biosynthetic enzymes are controlled through the formation of enzyme-complexes and regulation of subcellular localization. Functional aspects of HNK-1 carbohydrate were examined by overexpression, knockdown, and knockout studies of these enzymes. HNK-1 is involved in several neural functions such as synaptic plasticity, learning and memory, and the underlying molecular mechanisms have been illustrated upon identification of the target carrier glycoproteins of HNK-1 such as the glutamate receptor subunit GluA2 or tenascin-R. In this review, we describe recent findings about HNK-1 carbohydrate that provide further insights into the mechanism of its expression and function in the nervous system.

Potential of peptides as inhibitors and mimotopes: selection of carbohydrate-mimetic peptides from phage display libraries

Glycoconjugates play various roles in biological processes. In particular, oligosaccharides on the surface of animal cells are involved in virus infection and cell-cell communication. Inhibitors of carbohydrate-protein interactions are potential antiviral drugs. Several anti-influenza drugs such as oseltamivir and zanamivir are derivatives of sialic acid, which inhibits neuraminidase. However, it is very difficult to prepare a diverse range of sugar derivatives by chemical synthesis or by the isolation of natural products. In addition, the pathogenic capsular polysaccharides of bacteria are carbohydrate antigens, for which a safe and efficacious method of vaccination is required. Phage-display technology has been improved to enable the identification of peptides that bind to carbohydrate-binding proteins, such as lectins and antibodies, from a large repertoire of peptide sequences. These peptides are known as “carbohydrate-mimetic peptides (CMPs)” because they mimic carbohydrate structures. Compared to carbohydrate derivatives, it is easy to prepare mono- and multivalent peptides and then to modify them to create various derivatives. Such mimetic peptides are available as peptide inhibitors of carbohydrate-protein interactions and peptide mimotopes that are conjugated with adjuvant for vaccination.

The effect of glycomimetic functionalized collagen on peripheral nerve repair

Increasing evidence suggests that the improper synaptic reconnection of regenerating axons is a significant cause of incomplete functional recovery following peripheral nerve injury. In this study, we evaluate the use of collagen hydrogels functionalized with two peptide glycomimetics of naturally occurring carbohydrates-polysialic acid (PSA) and human natural killer cell epitope epitope (HNK-1)-that have been independently shown to encourage nerve regeneration and axonal targeting. Our novel biomaterial was used to bridge a critical gap size (5 mm) in a mouse femoral nerve injury model. Functional recovery was assessed using gait and hind limb extension, and was significantly better in all glycomimetic peptide-coupled collagen conditions versus non-functional scrambled peptide-coupled collagen, native collagen, and saline controls. Analysis of cross-sections of the regenerated nerve demonstrated that hydrogels coupled with the PSA glycomimetic, but not HNK, had significant increases in the number of myelinated axons over controls. Conversely, hydrogels coupled with HNK, but not PSA, showed improvement in myelination. Additionally, significantly more correctly projecting motoneurons were observed in groups containing coupled HNK-1 mimicking peptide, but not PSA mimicking peptide. Given the distinct morphological outcomes between the two glycomimetics, our study indicates that the enhancement of recovery following peripheral nerve injury induced by PSA- and HNK-functionalized collagen hydrogels likely occurs through distinct mechanisms.

Glycomimetic improves recovery after femoral injury in a non-human primate

In adult mammals, restoration of function after peripheral nerve injury is often poor and effective therapies are not available. Previously we have shown in mice that a peptide which functionally mimics the human natural killer cell (HNK)-1 trisaccharide epitope significantly improves the outcome of femoral nerve injury. Here we evaluated the translational potential of this treatment using primates. We applied a linear HNK-1 mimetic or a functionally inactive control peptide in silicone cuffs used to reconstruct the cut femoral nerves of adult cynomolgus monkeys (Macaca fascicularis). Functional recovery was evaluated using video-based gait analysis over a 160-day observation period. The final outcome was further assessed using force measurements, H-reflex recordings, nerve histology, and ELISA to assess immunoreactivity to HNK-1 in the treated monkeys. Gait deficits were significantly reduced in HNK-1 mimetic-treated compared with control peptide-treated animals between 60 and 160 days after injury. Better outcome at 160 days after surgery in treated versus control animals was also confirmed by improved quadriceps muscle force, enhanced H-reflex amplitude, decreased H-reflex latency, and larger diameters of regenerated axons. No adverse reactions to the mimetic, in particular immune responses resulting in antibodies against the HNK-1 mimetic or immune cell infiltration into the damaged nerve, were observed. These results indicate the potential of the HNK-1 mimetic as an efficient, feasible, and safe adjunct treatment for nerve injuries requiring surgical repair in clinical settings.

Neural cell type-specific responses to glycomimetic functionalized collagen

Despite their noted functional role, glycans have had limited therapeutic use due to difficulties in synthesis and quick degradation in vivo. The recent discovery of glycomimetics has provided new opportunities for their application. In this study, we have functionalized type I collagen with peptide mimics of two glycans: (1) polysialic acid (PSA) and (2) an epitope first discovered on human natural killer cells (HNK-1). These glycans and their glycomimetic counterparts have been shown to be important regulators of repair following injury through their unique and phenotypically specific effects on neural behavior. We show that these molecules retain their bioactivity following functionalization to the collagen backbone. Grafted HNK-1 encouraged motor neuron outgrowth, while grafted PSA encouraged sensory and motor neuron outgrowth and enhanced Schwann cell proliferation and process extension. These data support the potential of glycomimetic-functionalized collagen as a biomaterial strategy to increase the efficiency of synaptic reconnection following nervous system injury.

Synapsin I is an oligomannose-carrying glycoprotein, acts as an oligomannose-binding lectin, and promotes neurite outgrowth and neuronal survival when released via glia-derived exosomes

Oligomannosidic glycans play important roles in nervous system development and function. By performing a phage display screening with oligomannose-specific antibodies, we identified an oligomannose-mimicking peptide that was functionally active in modulating neurite outgrowth and neuron-astrocyte adhesion. Using the oligomannose-mimicking peptide in crosslinking experiments, synapsin I was identified as a novel oligomannose-binding protein in mouse brain. Further analyses not only verified that synapsin I is an oligomannose-binding lectin, but also indicated that it is a glycoprotein carrying oligomannose and Lewis(x). We also found that synapsin I is expressed in glia-enriched cultures and is released from glial cells via exosomes. Incubation of glial-derived exosomes in the presence of high KCl concentrations or subjecting glial cell cultures to either oxygen/glucose deprivation or hydrogen peroxide resulted in release of synapsin I from exosomes. Application of synapsin I promoted neurite outgrowth from hippocampal neurons and increased survival of cortical neurons upon hydrogen peroxide treatment or oxygen/glucose deprivation. Coculture experiments using wild-type hippocampal neurons and wild-type or synapsin-deficient glial cells showed enhanced neurite outgrowth when synapsin was expressed by glial cells. Synapsin-induced neurite outgrowth was dependent on oligomannose on synapsin I and the neural cell adhesion molecule NCAM at the neuronal cell surface. The data indicate that, under conditions of high neuronal activity and/or oxidative stress, synapsin can be released from glial-derived exosomes and promotes neurite outgrowth and neuronal survival by modulating the interactions between glia and neurons.

Identification of the glycosylation pattern of meat proteins in tissues from bovine myelin

A variant form of Creutzfeld-Jacob disease in humans is associated with the consumption of food contaminated with the bovine central neural system. This has focused attention on the need for procedures to detect tissues of the neural system in meat and meat products. A method was developed for the identification of myelin glycoproteins in bovine neural tissue. The glycosylated structures of glycoproteins in different protein mixtures from central nervous system (CNS) and peripheral nervous system (PNS) isolated from bovine myelin were identified by the specific lectin-glycoprotein reactions. Digoxigenin-labeled lectins bonded to the terminal glycoconjugate sequences of nine CNS glycoproteins ranging from 15 to 200 kDa and four PNS glycoproteins ranging from 22 to 105 kDa. Datura stramonium (DSA) recognized the epitope Galbeta1-4GlcNAc by two CNS and three PNS glycoproteins. Maackia amurensis (MAA) recognized the epitope NeuAcalpha2-3Gal by four CNS and two PNS glycoproteins. The peanut Arachis hypogaea (PNA) reacted with the high molecular CNS glycoprotein (200 kDa) with the sequence Galbeta1-3GalNAc. Galanthus nivalis (GNA) bonded to mannose subunits linked alpha1-3 and alpha1-6 by six CNS glycoproteins with molecular weights between 17 and 200 kDa. Four of these glycoproteins were recognized from monoclonal antibodies against the "HNK-1 epitope".

Why structurally different cyclic peptides can be glycomimetics of the HNK-1 carbohydrate antigen

The cyclic peptides c-(LSETTl) and c-(RTLPFS) are of potential clinical interest--they stimulate neurite outgrowth in a way that is similar to the effects of the HNK-1 (human natural killer cell-1) antigenic carbohydrate chains, which are terminated by 3'-sulfated glucuronic acid attached to an N-acetyllactosamine unit. To investigate the structure-activity relationships of the ability of the cyclic peptides to mimic HNK-1 carbohydrates, conformational analysis and examination of hydrophobic and hydrophilic patterns were performed and compared with the characteristics of a synthetic HNK-1 trisaccharide derivative. Data obtained demonstrate that both the trisaccharide and the glycomimetic peptide c-(LSETTl) exhibit a similar relationship between their hydrophobic moieties and their negatively charged sites. However, the second cyclic glycomimetic peptide investigated here, c-(RTLPFS), has a positively charged group as a potential contact point due to its Arg residue. Therefore, we studied the amino acid composition of all known receptor structures in the Protein Data Bank that are in contact with uronic acid and/or sulfated glycans. Interactions of the HNK-1 trisaccharide, c-(LSETTl), and c-(RTLPFS) with a laminin fragment involved in HNK-1 carbohydrate binding (i.e., the 21mer peptide: KGVSSRSYVGCIKNLEISRST) were also analyzed. Because the structure of the HNK-1-binding laminin domain is not available in the Protein Data Bank, we used the HNK-1-binding 21mer peptide fragment of laminin for the construction of a model receptor that enabled us to compare the molecular interplay of the HNK-1 trisaccharide and the two cyclopeptides c-(LSETTl) and c-(RTLPFS) with a reliable receptor structure in considerable detail.

Development of a novel therapy for Lipo-oligosaccharide-induced experimental neuritis: use of peptide glycomimics

Recent etiological studies have revealed that molecular mimicry between the lipo-oligosaccharide (LOS) component of Campylobacter jejuni and gangliosides of peripheral nervous system plays an important role in the pathogenesis of Guillain-Barré syndrome (GBS). Previously, we demonstrated GD3 ganglioside molecular mimicry in a model of GBS in Lewis rats by sensitization with GD3-like LOS (LOS(GD3)) from C. jejuni. Since the neuropathophysiological consequences were due largely to the anti-GD3-like antibodies, we subsequently focused our effort upon eliminating the pathogenic antibodies using several strategies to mimic GD3 in this model. Here, we have validated this strategy by the use of peptide glycomimics based on epitopic mimicry between carbohydrates and peptides. We treated rats by i.p. administration of phage-displayed GD3-like peptides. One GD3-like peptide (P(GD3)-4; RHAYRSMAEWGFLYS) induced in treated rats a remarkable restoration of motor nerve functions, as evidenced by improved histopathology, rotarod performance, and motor nerve conduction velocity. P(GD3)-4 effectively decreased the titer of anti-GD3/anti-LOS(GD3) antibodies and ameliorated peripheral nerve dysfunction in the sera of treated rats. The data suggest that peptide glycomimics of ganglioside may be potential powerful reagents for therapeutic intervention in GBS by neutralizing specific pathogenic anti-ganglioside antibodies.

CD57+ T lymphocytes and functional immune deficiency

CD57(+) expression in T lymphocytes has been recognized for decades as a marker of in vitro replicative senescence. In recent years, accumulating evidences have pointed on the utility of this marker to measure functional immune deficiency in patients with autoimmune disease, infectious diseases, and cancers. We review here the relevant literature and implications in clinical settings.

Polysialic acid glycomimetic promotes functional recovery and plasticity after spinal cord injury in mice

Regeneration after injury of the central nervous system is poor due to the abundance of molecules inhibiting axonal growth. Here we pursued to promote regeneration after thoracic spinal cord injury in young adult C57BL/6J mice using peptides which functionally mimic polysialic acid (PSA) and human natural killer cell-1 (HNK-1) glycan, carbohydrate epitopes known to promote neurite outgrowth in vitro. Subdural infusions were performed with an osmotic pump, over 2 weeks. When applied immediately after injury, the PSA mimetic and the combination of PSA and HNK-1 mimetics, but not the HNK-1 mimetic alone, improved functional recovery as assessed by locomotor rating and video-based motion analysis over a 6-week observation period. Better outcome in PSA mimetic-treated mice was associated with higher, as compared with control mice, numbers of cholinergic and glutamatergic terminals and monaminergic axons in the lumbar spinal cord, and better axonal myelination proximal to the injury site. In contrast to immediate post-traumatic application, the PSA mimetic treatment was ineffective when initiated 3 weeks after spinal cord injury. Our data suggest that PSA mimetic peptides can be efficient therapeutic tools improving, by augmenting plasticity, functional recovery when applied during the acute phase of spinal cord injury.

Structural and functional studies of Peptide-carbohydrate mimicry

Certain peptides act as molecular mimics of carbohydrates in that they are specifically recognizedby carbohydrate-binding proteins. Peptides that bind to anti-carbohydrate antibodies, carbohydrate-processingenzymes, and lectins have been identified. These peptides are potentially useful as vaccines andtherapeutics; for example, immunologically functional peptide molecular mimics (mimotopes) can strengthenor modify immune responses induced by carbohydrate antigens. However, peptides that bind specificallyto carbohydrate-binding proteins may not necessarily show the corresponding biological activity, andfurther selection based on biochemical studies is always required. The degree of structural mimicryrequired to generate the desired biological activity is therefore an interesting question. This reviewwill discuss recent structural studies of peptide-carbohydrate mimicry employing NMR spectroscopy,X-ray crystallography, and molecular modeling, as well as relevant biochemical data. These studiesprovide insights into the basis of mimicry at the molecular level. Comparisons with other carbohydrate-mimeticcompounds, namely proteins and glycopeptides, will be drawn. Finally, implications for the designof new therapeutic compounds will also be presented.

Hippocampal metaplasticity induced by deficiency in the extracellular matrix glycoprotein tenascin-R

Predisposition of synapses to undergo plastic changes can be dynamically adjusted according to the history of synaptic activity (i.e., synapses are metaplastic). In search of a molecular mechanism underlying metaplasticity, we investigated mice deficient in the glycoprotein tenascin-R (TNR), based on the observations that this mutant exhibits elevated basal excitatory synaptic transmission and reduced perisomatic GABAergic inhibition. TNR is a major extracellular matrix glycoprotein of the CNS and carries the HNK-1 carbohydrate (human natural killer cell glycan), which has been identified as the functional epitope mediating regulation of GABAergic transmission via GABA(B) receptors. Here, we used patch-clamp recordings in hippocampal slices to determine the critical levels of postsynaptic neuron depolarization necessary for induction of long-term potentiation (LTP) at CA3-CA1 synapses and found that deficiency in TNR leads to a metaplastic increase in the threshold for induction of LTP. Reconstitution of slices from TNR-deficient mice with an HNK-1 glycomimetic or pharmacological treatment with either a GABA(A) receptor agonist, a GABA(B) receptor antagonist, an L-type voltage-dependent Ca2+ channel blocker, or an inhibitor of protein serine/threonine phosphatases restored LTP to the levels seen in wild-type mice. We propose that a chain of events initiated by reduced GABAergic transmission and proceeding via Ca2+ entry into cells and elevated activity of phosphatases mediates homeostatic adjustment of hippocampal plasticity in the absence of TNR. These data uncover a novel mechanism by which an extracellular matrix molecule and its associated carbohydrate provide conditions beneficial for induction of LTP in the CA1 region of the hippocampus.

Selection of a phage-displayed peptide recognized by monoclonal antibody directed blocking the site of hepatitis C virus E2 for human CD81

The human CD81 (hCD81) molecule has been identified as a putative receptor for hepatitis C virus (HCV). HCV envelope glycoprotein 2 (E2) most likely plays a pivotal role in binding to host cells by interacting with the hCD81 molecule. In this study, a phage-displayed peptide library was used to select small peptides with anti-hCD81 monoclonal antibody JS-81. The output/input ratio of phages increased about 91 fold after the third round of selection. Eight of the 30 phage clones selected from the phage library showed specific binding to the anti-hCD81 by enzyme linked immunosorbent assay (ELISA). Competitive inhibition test further demonstrated that HCV E2 could significantly inhibit the binding of a positive phage clone to anti-hCD81 JS-81. Exogenous small peptide ATWVCGPCT contained by the positive phage clones showed aligned with the hCD81 sequence from 153-161 by sequence analyses. These results suggest that the selected ATWVCGPCT is a novel hCD81-like small peptide, which can block the binding site of HCV E2 for hCD81. It may be of further application on development of antiviral agents targeting the stage of HCV entry.

Effects of motor and sensory nerve transplants on amount and specificity of sciatic nerve regeneration

Nerve regeneration after complete transection does not allow for adequate functional recovery mainly because of lack of selectivity of target reinnervation. We assessed if transplanting a nerve segment from either motor or sensory origin may improve specifically the accuracy of sensory and motor reinnervation. For this purpose, the rat sciatic nerve was transected and repaired with a silicone guide containing a predegenerated segment of ventral root (VR) or dorsal root (DR), compared to a silicone guide filled with saline. Nerve regeneration and reinnervation was assessed during 3 months by electrophysiologic and functional tests, and by nerve morphology and immunohistochemistry against choline acetyltransferase (ChAT) for labeling motor axons. Functional tests showed that reinnervation was successful in all the rats. However, the two groups with a root allotransplant reached higher degrees of reinnervation in comparison with the control group. Group VR showed the highest reinnervation of muscle targets, whereas Group DR had higher levels of sensory reinnervation than VR and saline groups. The total number of regenerated myelinated fibers was similar in the three groups, but the number of ChAT+ fibers was slightly lower in the VR group in comparison with DR and saline groups. These results indicate that a predegenerated root nerve allotransplant enhances axonal regeneration, leading to faster and higher levels of functional recovery. Although there is not clear preferential reinnervation, regeneration of motor axons is promoted at early times by a motor graft, whereas reinnervation of sensory pathways is increased by a sensory graft.

Carbohydrate mimics promote functional recovery after peripheral nerve repair

OBJECTIVE

The outcome of peripheral nerve repair is often unsatisfactory, and efficient therapies are not available. We tested the therapeutic potential of functional mimics of the human natural killer cell glycan (3-sulfoglucuronyl beta1-3 galactoside) (HNK-1) epitope, a carbohydrate indicated to favor specificity of motor reinnervation in mice.

METHODS

We applied a linear HNK-1 mimic peptide, scrambled peptide, or vehicle substances in polyethylene cuffs used to reconstruct the severed femoral nerves of adult mice. We used video-based motion analysis and morphological and tracing techniques to monitor the outcome of nerve repair.

RESULTS

After glycomimetic application, quadriceps muscle function recovered to 93% of normal within 3 months. Restoration of function was less complete (71-76%) in control groups. Better functional recovery was associated with larger motoneuron somata, better axonal myelination in the quadriceps nerve, and enhanced precision of target reinnervation. Lesion-induced death of motoneurons was reduced by 20 to 25%. The glycomimetic enhanced survival and neurite outgrowth of both mouse and human motoneurons in vitro by 30 to 75%. Application of a novel cyclic glycomimetic also enhanced functional recovery in vivo.

INTERPRETATION

The improved outcome of nerve repair after glycomimetic application may be attributed to neurotrophic effects. Our results hold promise for therapeutic use in humans.

Correlation between target reinnervation and distribution of motor axons in the injured rat sciatic nerve

Peripheral nerve injuries are rarely followed by complete return of function. Deficits are particularly important for motor function, resulting in paralysis and muscle atrophy. In different groups, the sciatic nerve was either crushed or transected and repaired by direct suture or by tube repair using silicone or collagen tubes. After 60 days, nerve regeneration was assessed by electrophysiological and functional tests, nerve morphology and immunohistochemistry against choline acetyltransferase (ChAT) for labeling motor axons. Suture and tube repair resulted in similar levels of muscle reinnervation, but significantly lower than after nerve crush. Recovery of walking track pattern was poor in all groups after nerve section. The numbers of regenerated myelinated fibers and of ChAT+ fibers were similar to control values after nerve crush, but increased after section and repair. The normal fascicular architecture and grouping of ChAT+ fibers were maintained after nerve crush, but lost after section and repair, where motor fibers were scattered within small regenerated fascicles throughout the nerve. The loss of fascicular organization was related to the deficient recovery of locomotor function. Thus, labeling of motor axons by ChAT immunohistochemistry provides useful information for the study of the degree and specificity of nerve regeneration.

Antiglycolipid antibodies in Guillain-Barré syndrome and related diseases: Review of clinical features and antibody specificities

Guillain-Barré syndrome (GBS) is an acute inflammatory polyradiculoneuropathy that usually develops following a respiratory or intestinal infection. Although the pathogenic mechanisms of GBS have not been fully established, both humoral and cell-mediated immune factors have been shown to contribute to the disease process. Several antiglycosphingolipid (anti-GSL) antibodies have been found in the sera of patients with GBS or related diseases. Measurements of these antibody titers are very important in the diagnosis of GBS and in evaluating the effectiveness of treatments in clinical trials. The most common treatment strategies for these disorders involve plasmapheresis and the use of steroids for reducing anti-GSL antibody titers to ameliorate patients' clinical symptoms. Administration of intravenous immunoglobulin may also be beneficial in the treatment of neuropathies by suppressing the immune-mediated processes that are directed against antigenic targets in myelin and axons. In certain demyelinating neuropathies, the destruction or malfunctioning of the blood-nerve barrier, which results in the leakage of circulating antibodies into the peripheral nerve parenchyma, has been considered to be an initial step in development of the disease process. In addition, anti-GSL antibodies, such as anti-GM1, may cause nerve dysfunction and injury by interfering with the ion channel function at the nodes of Ranvier, where carbohydrate epitopes of glycoconjugates are located. These malfunctions thus contribute to the pathogenic mechanisms of certain demyelinating neuropathies.

Selection of Poly-α 2,8-Sialic Acid Mimotopes from a Random Phage Peptide Library and Analysis of Their Bioactivity

Poly-alpha 2-8 sialic acid (PSA), attached to the neural cell adhesion molecule, is a permissive determinant for numerous morphogenetic and neural plasticity processes, making it a potential therapeutic target. Here, using a monoclonal antibody specific for PSA, we screened a phage-display library and identified two cyclic nine-amino acid peptides (p1, p2) that are PSA epitope analogues. We evaluated their bioactivity in vitro and in vivo. In culture, micromolar concentrations of the peptides promoted axon growth, defasciculation, and migration of neural progenitors. When injected into developing chicken retina, the peptides modified the trajectory of retinal ganglion cell axons. Moreover, they enhanced migration of grafted neuroblasts in mouse brain. These effects were selective and dependent upon the presence of PSA on transplanted cells. Our results demonstrate the feasibility and therapeutic potential of enhancing PSA biological activity.

Glycans and neural cell interactions

Carbohydrate-carrying molecules in the nervous system have important roles during development, regeneration and synaptic plasticity. Carbohydrates mediate interactions between recognition molecules, thereby contributing to the formation of a complex molecular meshwork at the cell surface and in the extracellular matrix. The tremendous structural diversity of glycan chains allows for immense combinatorial possibilities that might underlie the fine-tuning of cell-cell and cell-matrix interactions.