Level and localization of polysialic acid is critical for early peripheral nerve regeneration

A brief review of polysialic acid-based drug delivery systems

Polysialic acid (PSA) is a straight-chain homoglycan linked by N-acetylneuraminic acid monomers via α-2, 8- or α-2, 9-glycosidic bonds. As a negatively charged non-glycosaminoglycan, PSA has the remarkable characteristics of non-immunogenicity and biodegradation. Although different in class, PSA is similar to poly(ethylene glycol), and was originally used to increase the stability of the delivery system in circulation to prolong the half-life. As research continues, PSA's application potential in the pharmaceutical field becomes increasingly prominent. It can be used as a biomaterial for protein polysialylation and tissue engineering, and it can be used alone or with other materials to develop multifunctional drug delivery systems. In this article, the results of the bioproduction and biofunction of PSA are introduced, the common strategies for chemical modification of PSA are summarized, and the application progress of PSA-based drug delivery systems is reviewed.

Engineering polysialic acid on Schwann cells using polysialyltransferase gene transfer or purified enzyme exposure for spinal cord injury transplantation

Polysialic acid (PolySia) is a critical post-translational modification on the neural cell adhesion molecule (NCAM, a.k.a., CD56), important for cell migration and axon growth during nervous system development, plasticity and repair. PolySia induction on Schwann cells (SCs) enhances their migration, axon growth support and ability to improve functional recovery after spinal cord injury (SCI) transplantation. In the current investigation two methods of PolySia induction on SCs, lentiviral vector transduction of the mouse polysialytransferase gene ST8SIA4 (LV-PST) or enzymatic engineering with a recombinant bacterial PST (PSTNm), were examined comparatively for their effects on PolySia induction, SC migration, the innate immune response and axon growth after acute SCI. PSTNm produced significant PolySia induction and a greater diversity of surface molecule polysialylation on SCs as evidenced by immunoblot. In the scratch wound assay, PSTNm was superior to LV-PST in the promotion of SC migration and gap closure. At 24 h after SCI transplantation, PolySia induction on SCs was most pronounced with LV-PST. Co-delivery of PSTNm with SCs, but not transient cell exposure, led to broader induction of PolySia within the injured spinal cord due to polysialylation upon both host cells and transplanted SCs. The innate immune response after SCI, measured by CD68 immunoreactivity, was similar among PolySia induction methods. LV-PST or PSTNm co-delivery with SCs provided a similar enhancement of SC migration and axon growth support above that of unmodified SCs. These studies demonstrate that LV-PST and PSTNm provide comparable acute effects on SC polysialation, the immune response and neurorepair after SCI.

Sensory axons inhibit motor axon regeneration in vitro

During mammalian embryonic development sensory and motor axons interact as an integral part of the pathfinding process. During regeneration, however, little is known of their interactions with one another. It is thus possible that sensory axons might influence motor axon regeneration in ways not currently appreciated. To explore this possibility we have developed an organotypic model of post-natal nerve regeneration in which sensory and motor axons are color-coded by modality. Motor axons that express yellow fluorescent protein (YFP) and sensory axons that express red fluorescent protein (RFP) are blended within a three-dimensional segment of peripheral nerve. This nerve is then transected, allowing axons to interact with one another as they grow out on a collagen/laminin gel that is initially devoid of directional cues. Within hours it is apparent that sensory axons extend more rapidly than motor axons and precede them during the early stages of regeneration, the opposite of their developmental order. Motor axons thus enter an environment already populated with sensory axons, and they adhere to these axons throughout most of their course. As a result, motor axon growth is reduced dramatically. Physical delay of sensory regeneration, allowing motor axons to grow ahead, restores normal motor growth; direct axonal interactions on the gel, rather than some other aspect of the model, are thus responsible for motor inhibition. Potential mechanisms for this inhibition are explored by electroporating siRNA to the neural cell adhesion molecule (NCAM) and the L1 adhesion molecule (L1CAM) into dorsal root ganglia (DRGs) to block expression of these molecules by regenerating sensory axons. Although neither maneuver improved motor regeneration, the results were consistent with early receptor-mediated signaling among axons rather than physical adhesion as the mechanism of motor inhibition in this model.

Histone H1 improves regeneration after mouse spinal cord injury and changes shape and gene expression of cultured astrocytes

BACKGROUND

We have shown that histone H1 is a binding partner for polysialic acid (PSA) and that it improves functional recovery, axon regrowth/sprouting, and target reinnervation after mouse femoral nerve injury.

OBJECTIVE

Here, we analyzed whether histone H1 affects functional recovery, axon regrowth/sprouting, and target reinnervation after spinal cord injury of adult mice. Furthermore, we tested in vitro histone H1's effect on astrocytic gene expression, cell shape and migration as well as on cell survival of cultured motoneurons.

METHODS

We applied histone H1 to compressed spinal cord and determined functional recovery and number of fibrillary acidic protein (GFAP)- and neuron-glial antigen 2 (NG2)- positive glial cells, which contribute to glial scarring. Histone H1's effect on migration of astrocytes, astrocytic gene expression and motoneuronal survival was determined using scratch-wounded astroglial monolayer cultures, astrocyte cultures for microarray analysis, and motoneuron cell culture under oxidative stress conditions, respectively.

RESULTS

Histone H1 application improves locomotor functions and enhances monoaminergic and cholinergic reinnervation of the spinal cord. Expression levels of GFAP and NG2 around the lesion site were decreased in histone H1-treated mice relative to vehicle-treated mice six weeks after injury. Histone H1 reduced astrocytic migration, changed the shape of GFAP- and NG2-positive glial cells and altered gene expression. Gene ontology enrichment analysis indicated that in particular genes coding for proteins involved in proliferation, differentiation, migration and apoptosis are dysregulated. The up- and down-regulation of distinct genes was confirmed by qPCR and Western blot analysis. Moreover, histone H1 reduced hydrogen peroxide-induced cell death of cultured motoneurons.

CONCLUSIONS

The combined observations indicate that histone H1 locally applied to the lesion site, improves regeneration after spinal cord injury. Some of these beneficial functions of histone H1 in vivo and in vitro can be attributed to its interaction with PSA-carrying neural cell adhesion molecule.

The polysialic acid mimetics idarubicin and irinotecan stimulate neuronal survival and neurite outgrowth and signal via protein kinase C

Polysialic acid (PSA) is a large, negatively charged, linear homopolymer of alpha2-8-linked sialic acid residues. It is generated by two polysialyltransferases and attached to N- and/or O-linked glycans, and its main carrier is the neural cell adhesion molecule (NCAM). PSA controls the development and regeneration of the nervous system by enhancing cell migration, axon pathfinding, synaptic targeting, synaptic plasticity, by regulating the differentiation of progenitor cells and by modulating cell-cell and cell-matrix adhesions. In the adult, PSA plays a role in the immune system, and PSA mimetics promote functional recovery after nervous system injury. In search for novel small molecule mimetics of PSA that are applicable for therapy, we identified idarubicin, an antineoplastic anthracycline, and irinotecan, an antineoplastic agent of the topoisomerase I inhibitor class, as PSA mimetics using a competition enzyme-linked immunosorbent assay. Idarubicin and irinotecan compete with the PSA-mimicking peptide and colominic acid, the bacterial analog of PSA, for binding to the PSA-specific monoclonal antibody 735. Idarubicin and irinotecan stimulate neurite outgrowth and survival of cultured cerebellar neurons after oxidative stress via protein kinase C and Erk1/2 in a similar manner as colominic acid, whereas Fyn, casein kinase II and the phosphatase and tensin homolog are only involved in idarubicin and irinotecan-stimulated neurite outgrowth. These novel results show that the structure and function of PSA can be mimicked by the small organic compounds irinotecan and idarubicin which trigger the same signaling cascades as PSA, thus introducing the possibility of retargeting these drugs to treat nervous system injuries.

N-Propionylmannosamine stimulates axonal elongation in a murine model of sciatic nerve injury

Increasing evidence indicates that sialic acid plays an important role during nerve regeneration. Sialic acids can be modified in vitro as well as in vivo using metabolic oligosaccharide engineering of the N-acyl side chain. N-Propionylmannosamine (ManNProp) increases neurite outgrowth and accelerates the reestablishment of functional synapses in vitro. We investigated the influence of systemic ManNProp application using a specific in vivo mouse model. Using mice expressing axonal fluorescent proteins, we quantified the extension of regenerating axons, the number of regenerating axons, the number of arborising axons and the number of branches per axon 5 days after injury. Sciatic nerves from non-expressing mice were grafted into those expressing yellow fluorescent protein. We began a twice-daily intraperitoneal application of either peracetylated ManNProp (200 mg/kg) or saline solution 5 days before injury, and continued it until nerve harvest (5 days after transection). ManNProp significantly increased the mean distance of axonal regeneration (2.49 mm vs. 1.53 mm; P < 0.005) and the number of arborizing axons (21% vs. 16%; P = 0.008) 5 days after sciatic nerve grafting. ManNProp did not affect the number of regenerating axons or the number of branches per arborizing axon. The biochemical glycoengineering of the N-acyl side chain of sialic acid might be a promising approach for improving peripheral nerve regeneration.

In vivo stimulation of early peripheral axon regeneration by N-propionylmannosamine in the presence of polysialyltransferase ST8SIA2

The key enzyme of sialic acid (Sia) biosynthesis is the bifunctional UDP-N-acetylglucosamine 2-epimerase/ManNAc kinase (GNE/MNK). It metabolizes the physiological precursor ManNAc and N-acyl modified analogues such as N-propionylmannosamine (ManNProp) to the respective modified sialic acid. Polysialic acid (polySia) is a crucial compound for several functions in the nervous system and is synthesized by the polysialyltransferases ST8SIA2 and ST8SIA4. PolySia can be modified in vitro and in vivo by metabolic glycoengineering of the N-acyl side chain of Sia. In vitro studies show that the application of ManNProp increases neurite outgrowth and accelerates the re-establishment of functional synapses. In this study, we investigate in vivo how ManNProp application might benefit peripheral nerve regeneration. In mice expressing axonal fluorescent proteins (thy-1-YFP), we transected the sciatic nerve and then replaced part of it with a sciatic nerve graft from non-expressing mice (wild-type mice or St8sia2(-/-) mice). Analyses conducted 5 days after grafting showed that systemic application of ManNProp (200 mg/kg, twice a day, i.p.), but not of physiological ManNAc (1 g/kg, twice a day, i.p.), significantly increased the extent of axonal elongation, the number of arborizing axons and the number of branches per regenerating axon within the grafts from wild-type mice, but not in those from St8sia2(-/-) mice. The results demonstrate that the application of ManNProp has beneficial effects on early peripheral nerve regeneration and indicate that the stimulation of axon growth depends on ST8SIA2 activity in the nerve graft.

Physiological adaptations of key oral bacteria

Oral colonising bacteria are highly adapted to the various environmental niches harboured within the mouth, whether that means while contributing to one of the major oral diseases of caries, pulp infections, or gingival/periodontal disease or as part of a commensal lifestyle. Key to these infections is the ability to adhere to surfaces via a range of specialised adhesins targeted at both salivary and epithelial proteins, their glycans and to form biofilm. They must also resist the various physical stressors they are subjected to, including pH and oxidative stress. Possibly most strikingly, they have developed the ability to harvest both nutrient sources provided by the diet and those derived from the host, such as protein and surface glycans. We have attempted to review recent developments that have revealed much about the molecular mechanisms at work in shaping the physiology of oral bacteria and how we might use this information to design and implement new treatment strategies.

Tegaserod mimics the neurostimulatory glycan polysialic acid and promotes nervous system repair

Glycans attached to the cell surface via proteins or lipids or exposed in the extracellular matrix affect many cellular processes, including neuritogenesis, cell survival and migration, as well as synaptic activity and plasticity. These functions make glycans attractive molecules for stimulating repair of the injured nervous system. Yet, glycans are often difficult to synthesize or isolate and have the disadvantage to be unstable in a complex tissue environment. To circumvent these issues, we have screened a library of small organic compounds to search for structural and functional mimetics of the neurostimulatory glycan polysialic acid (PSA) and identified the 5-HT₄ receptor agonist tegaserod as a PSA mimetic. The PSA mimicking activity of tegaserod was shown in cultures of central and peripheral nervous system cells of the mouse and found to be independent of its described function as a serotonin (5-HT₄) receptor agonist. In an in vivo model for peripheral nerve regeneration, mice receiving tegaserod at the site of injury showed enhanced recovery compared to control mice receiving vehicle control as evidenced by functional measurements and histology. These data indicate that tegaserod could be repurposed for treatment of nervous system injuries and underscores the potential of using small molecules as mimetics of neurostimulatory glycans.

Profiling, Bioinformatic, and Functional Data on the Developing Olfactory/GnRH System Reveal Cellular and Molecular Pathways Essential for This Process and Potentially Relevant for the Kallmann Syndrome

During embryonic development, immature neurons in the olfactory epithelium (OE) extend axons through the nasal mesenchyme, to contact projection neurons in the olfactory bulb. Axon navigation is accompanied by migration of the GnRH+ neurons, which enter the anterior forebrain and home in the septo-hypothalamic area. This process can be interrupted at various points and lead to the onset of the Kallmann syndrome (KS), a disorder characterized by anosmia and central hypogonadotropic hypogonadism. Several genes has been identified in human and mice that cause KS or a KS-like phenotype. In mice a set of transcription factors appears to be required for olfactory connectivity and GnRH neuron migration; thus we explored the transcriptional network underlying this developmental process by profiling the OE and the adjacent mesenchyme at three embryonic ages. We also profiled the OE from embryos null for Dlx5, a homeogene that causes a KS-like phenotype when deleted. We identified 20 interesting genes belonging to the following categories: (1) transmembrane adhesion/receptor, (2) axon-glia interaction, (3) scaffold/adapter for signaling, (4) synaptic proteins. We tested some of them in zebrafish embryos: the depletion of five (of six) Dlx5 targets affected axonal extension and targeting, while three (of three) affected GnRH neuron position and neurite organization. Thus, we confirmed the importance of cell-cell and cell-matrix interactions and identified new molecules needed for olfactory connection and GnRH neuron migration. Using available and newly generated data, we predicted/prioritized putative KS-disease genes, by building conserved co-expression networks with all known disease genes in human and mouse. The results show the overall validity of approaches based on high-throughput data and predictive bioinformatics to identify genes potentially relevant for the molecular pathogenesis of KS. A number of candidate will be discussed, that should be tested in future mutation screens.

Expression and localization of neural cell adhesion molecule and polysialic acid during chick corneal development

PURPOSE

To assay for expression and localization of neural cell adhesion molecule (NCAM) and polysialic acid (polySia) in the chick cornea during embryonic and postnatal development.

METHODS

Real time quantitative PCR and Western blot analyses were used to determine NCAM expression and polysiaylation in embryonic, hatchling, and adult chick corneas. Immunofluorescence staining for NCAM and polySia was conducted on cryosections of embryonic and adult corneas, whole embryonic corneas, and trigeminal neurons.

RESULTS

NCAM and ST8SiaII mRNA transcripts peaked by embryonic day (E)9, remained steady between E10 and E14 and slowly decreased thereafter during embryonic development. Both gene transcripts showed > 190-fold decline in the adult chick cornea compared with E9. In contrast, ST8SiaIV expression gradually decreased 26.5-fold from E6 to E19, increased thereafter, and rose to the early embryonic level in the adult cornea. Western blot analysis revealed NCAM was polysialylated and its expression developmentally changed. Other polysiaylated proteins aside from NCAM were also detected by Western blot analysis. Five NCAM isoforms including NCAM-120, NCAM-180 and three soluble NCAM isoforms with low molecular weights (87-96 kDa) were present in chick corneas, with NCAM-120 being the predominate isoform. NCAM was localized to the epithelium, stroma, and stromal extracellular matrix (ECM) of the embryonic cornea. In stroma, NCAM expression shifted from anterior to posterior stroma during embryonic development and eventually became undetectable in 20-week-old adult cornea. Additionally, both NCAM and polySia were detected on embryonic corneal and pericorneal nerves.

CONCLUSIONS

NCAM and polySia are expressed and developmentally regulated in chick corneas. Both membrane-associated and soluble NCAM isoforms are expressed in chick corneas. The distributions of NCAM and polySia in cornea and on corneal nerves suggest their potential functions in corneal innervation.

Sequences prior to conserved catalytic motifs of polysialyltransferase ST8Sia IV are required for substrate recognition

Polysialic acid on the neural cell adhesion molecule (NCAM) modulates cell-cell adhesion and signaling, is required for proper brain development, and plays roles in neuronal regeneration and the growth and invasiveness of tumor cells. Evidence indicates that NCAM polysialylation is highly protein-specific, requiring an initial polysialyltransferase-NCAM protein-protein interaction. Previous work suggested that a polybasic region located prior to the conserved polysialyltransferase catalytic motifs may be involved in NCAM recognition, but not overall enzyme activity (Foley, D. A., Swartzentruber, K. G., and Colley, K. J. (2009) J. Biol. Chem. 284, 15505-15516). Here, we employ a competition assay to evaluate the role of this region in substrate recognition. We find that truncated, catalytically inactive ST8SiaIV/PST proteins that include the polybasic region, but not those that lack this region, compete with endogenous ST8SiaIV/PST and reduce NCAM polysialylation in SW2 small cell lung carcinoma cells. Replacing two polybasic region residues, Arg(82) and Arg(93), eliminates the ability of a full-length, catalytically inactive enzyme (PST H331K) to compete with SW2 cell ST8SiaIV/PST and block NCAM polysialylation. Replacing these residues singly or together in ST8SiaIV/PST substantially reduces or eliminates NCAM polysialylation, respectively. In contrast, replacing Arg(82), but not Arg(93), substantially reduces the ability of ST8SiaIV/PST to polysialylate neuropilin-2 and SynCAM 1, suggesting that Arg(82) plays a general role in substrate recognition, whereas Arg(93) specifically functions in NCAM recognition. Taken together, our results indicate that the ST8SiaIV/PST polybasic region plays a critical role in substrate recognition and suggest that different combinations of basic residues may mediate the recognition of distinct substrates.

Functional role of the interaction between polysialic acid and extracellular histone H1

Polysialic acid (PSA) is a large and highly negatively charged glycan that plays crucial roles in nervous system development and function in the adult. It has been suggested to facilitate cell migration, neurite outgrowth, and synaptic plasticity because its hydration volume could enhance flexibility of cell interactions. Evidence for receptors of PSA has so far been elusive. We now identified histone H1 as binding partner of PSA via a single-chain variable fragment antibody using an anti-idiotypic approach. Histone H1 directly binds to PSA as shown by ELISA. Surface biotinylation of cultured cerebellar neurons indicated an extracellular localization of histone H1. Immunostaining of live cerebellar neurons and Schwann cells confirmed that an extracellular pool of histone H1 colocalizes with PSA at the cell surface. Histone H1 was also detected in detergent-insoluble synaptosomal membrane subfractions and postsynaptic densities. When applied in vitro, histone H1 stimulated neuritogenesis, process formation and proliferation of Schwann cells, and migration of neural precursor cells via a PSA-dependent mechanism, further indicating that histone H1 is active extracellularly. These in vitro observations suggested an important functional role for the interaction between histone H1 and PSA not only for nervous system development but also for regeneration in the adult. Indeed, histone H1 improved functional recovery, axon regrowth, and precision of reinnervation of the motor branch in adult mice with femoral nerve injury. Our findings encourage investigations on the therapeutic potential of histone H1 in humans.

Fiber scaffolds of polysialic acid via electrospinning for peripheral nerve regeneration

Fiber scaffolds of bioactive polysialic acid have been prepared via electrospinning for peripheral nerve regeneration. The diameter, morphology and alignment of fibers in scaffolds were adjusted by variation of electrospinning parameters, which are decisive for the cell-scaffold interaction. Due to the high water solubility of polysialic acid (poly-alpha-2,8-N-acetylneuraminic acid) a photoactive derivative (poly-alpha-2,8-N-pentenoylneuraminic acid) was used to obtain stable fiber scaffolds in water by photochemical crosslinking. At the optimized fiber scaffolds good cell viability and directed cell proliferation along the fibers was achieved by cell tests with immortalized Schwann cells.

Brain development needs sugar: the role of polysialic acid in controlling NCAM functions

Polysialic acid (polySia) is a major regulator of cell-cell interactions in the developing nervous system and a key factor in maintaining neural plasticity. As a polyanionic molecule with high water binding capacity, polySia increases the intercellular space and creates conditions that are permissive for cellular plasticity. While the prevailing model highlights polySia as a non-specific regulator of cell-cell contacts, this review concentrates on recent studies in knockout mice indicating that a crucial function of polySia resides in controlling interactions mediated by its predominant protein carrier, the neural cell adhesion molecule NCAM.