Sequential steps in synaptic targeting of sensory afferents are mediated by constitutive and developmentally regulated glycosylations of CAMs

Gap junction proteins and the wiring (Rewiring) of neuronal circuits

The unique morphology and pattern of synaptic connections made by a neuron during development arise in part by an extended period of growth in which cell-cell interactions help to sculpt the arbor into its final shape, size, and participation in different synaptic networks. Recent experiments highlight a guiding role played by gap junction proteins in controlling this process. Ectopic and overexpression studies in invertebrates have revealed that the selective expression of distinct gap junction genes in neurons and glial cells is sufficient to establish selective new connections in the central nervous systems of the leech (Firme et al. [2012]: J Neurosci 32:14265-14270), the nematode (Rabinowitch et al. [2014]: Nat Commun 5:4442), and the fruit fly (Pézier et al., 2016: PLoS One 11:e0152211). We present here an overview of this work and suggest that gap junction proteins, in addition to their synaptic/communicative functions, have an instructive role as recognition and adhesion factors. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 575-586, 2017.

Overelaborated synaptic architecture and reduced synaptomatrix glycosylation in a Drosophila classic galactosemia disease model

Classic galactosemia (CG) is an autosomal recessive disorder resulting from loss of galactose-1-phosphate uridyltransferase (GALT), which catalyzes conversion of galactose-1-phosphate and uridine diphosphate (UDP)-glucose to glucose-1-phosphate and UDP-galactose, immediately upstream of UDP-N-acetylgalactosamine and UDP-N-acetylglucosamine synthesis. These four UDP-sugars are essential donors for driving the synthesis of glycoproteins and glycolipids, which heavily decorate cell surfaces and extracellular spaces. In addition to acute, potentially lethal neonatal symptoms, maturing individuals with CG develop striking neurodevelopmental, motor and cognitive impairments. Previous studies suggest that neurological symptoms are associated with glycosylation defects, with CG recently being described as a congenital disorder of glycosylation (CDG), showing defects in both N- and O-linked glycans. Here, we characterize behavioral traits, synaptic development and glycosylated synaptomatrix formation in a GALT-deficient Drosophila disease model. Loss of Drosophila GALT (dGALT) greatly impairs coordinated movement and results in structural overelaboration and architectural abnormalities at the neuromuscular junction (NMJ). Dietary galactose and mutation of galactokinase (dGALK) or UDP-glucose dehydrogenase (sugarless) genes are identified, respectively, as critical environmental and genetic modifiers of behavioral and cellular defects. Assaying the NMJ extracellular synaptomatrix with a broad panel of lectin probes reveals profound alterations in dGALT mutants, including depletion of galactosyl, N-acetylgalactosamine and fucosylated horseradish peroxidase (HRP) moieties, which are differentially corrected by dGALK co-removal and sugarless overexpression. Synaptogenesis relies on trans-synaptic signals modulated by this synaptomatrix carbohydrate environment, and dGALT-null NMJs display striking changes in heparan sulfate proteoglycan (HSPG) co-receptor and Wnt ligand levels, which are also corrected by dGALK co-removal and sugarless overexpression. These results reveal synaptomatrix glycosylation losses, altered trans-synaptic signaling pathway components, defective synaptogenesis and impaired coordinated movement in a CG neurological disease model.

Ectopic expression of select innexins in individual central neurons couples them to pre-existing neuronal or glial networks that express the same innexin

Fifteen of the 21 innexin (Inx) genes (Hve-inx) found in the genome of the medicinal leech, Hirudo verbana, are expressed in the CNS (Kandarian et al., 2012). Two are expressed pan-neuronally, while the others are restricted in their expression to small numbers of cells, in some cases reflecting the membership of known networks of electrically coupled and dye-coupled neurons or glial cells. We report here that when Hve-inx genes characteristic of discrete coupled networks were expressed ectopically in neurons known not to express them, the experimental cells were found to become dye coupled with the other cells in that network. Hve-inx6 is normally expressed by only three neurons in each ganglion, which form strongly dye-coupled electrical connections with each other [Shortening-Coupling interneuron (S-CI) network] (Muller and Scott, 1981; Dykes and Macagno, 2006). But when Hve-inx6 was ectopically expressed in a variety of central embryonic neurons, those cells became dye coupled with the S-CI network. Similarly, Hve-inx2 is normally uniquely expressed by the ganglion's large glial cells, but when it was ectopically expressed in different central neurons, they became dye coupled to the glial cells. In contrast, overexpression of the pan-neuronal Inx genes Hve-inx1 and Hve-inx14 did not yield any novel instances of dye coupling to pre-existent neuronal networks. These results reveal that expression of certain innexins is sufficient to couple individual neurons to pre-existing networks in the CNS. We propose that a primary determinant of selective neuronal connectivity and circuit formation in the leech is the surface expression of unique subsets of gap junctional proteins.

Phylogenetic conservation of the cell-type-specific Lan3-2 glycoepitope in Caenorhabditis elegans

The biological function of a cell-type-specific glycosylation of an adhesion molecule belonging to the L1CAM immunoglobulin superfamily was previously determined in the nervous system of the embryonic leech, Hirudo medicinalis. The Lan3-2 glycoepitope is a surface marker of sensory afferent neurons and is required for their appropriate developmental collateral branching and synaptogenesis in the CNS. The chemical structure of the Lan3-2 glycoepitope consists of beta-(1,4)-linked mannopyranose. Here, we show the conservation of the cell-type-specific expression of this mannose polymer in Caenorhabditis elegans. The Lan3-2 glycoepitope is expressed on the cell surface of a subset of dissociated embryonic neurons and, in the adult worm, by the pharyngeal motor neuron, M5, and the chemosensory afferents, the amphids. Additionally, the vulval epithelium expresses the Lan3-2 glycoepitope in late L4 larvae and in adult hermaphrodites. To investigate proteins carrying this restrictively expressed glycoepitope, worm extract was immunoaffinity purified with Lan3-2 monoclonal antibody and Western blotted. A polyclonal antibody reactive with the cytoplasmic tail of LAD-1/SAX-7, a C. elegans member of the L1CAM family, recognizes a 270 kDa protein band while Lan3-2 antibody also recognizes a 190 kDa glycoform, its putative Lan3-2 ectodomain. Thus, in C. elegans, as in leech, the Lan3-2 epitope is located on a L1CAM homologue. The cell-type-specific expression of the Lan3-2 glycoepitope shared by leech and C. elegans will be useful for understanding how cell-type-specific glycoepitopes mediate cell-cell interactions during development.

Effects of nerve injury and segmental regeneration on the cellular correlates of neural morphallaxis

Functional recovery of neural networks after injury requires a series of signaling events similar to the embryonic processes that governed initial network construction. Neural morphallaxis, a form of nervous system regeneration, involves reorganization of adult neural connectivity patterns. Neural morphallaxis in the worm, Lumbriculus variegatus, occurs during asexual reproduction and segmental regeneration, as body fragments acquire new positional identities along the anterior-posterior axis. Ectopic head (EH) formation, induced by ventral nerve cord lesion, generated morphallactic plasticity including the reorganization of interneuronal sensory fields and the induction of a molecular marker of neural morphallaxis. Morphallactic changes occurred only in segments posterior to an EH. Neither EH formation, nor neural morphallaxis was observed after dorsal body lesions, indicating a role for nerve cord injury in morphallaxis induction. Furthermore, a hierarchical system of neurobehavioral control was observed, where anterior heads were dominant and an EH controlled body movements only in the absence of the anterior head. Both suppression of segmental regeneration and blockade of asexual fission, after treatment with boric acid, disrupted the maintenance of neural morphallaxis, but did not block its induction. Therefore, segmental regeneration (i.e., epimorphosis) may not be required for the induction of morphallactic remodeling of neural networks. However, on-going epimorphosis appears necessary for the long-term consolidation of cellular and molecular mechanisms underlying the morphallaxis of neural circuitry.

The atypical cadherin flamingo regulates synaptogenesis and helps prevent axonal and synaptic degeneration in Drosophila

The formation of synaptic connections with target cells and maintenance of axons are highly regulated and crucial for neuronal function. The atypical cadherin and G-protein-coupled receptor Flamingo and its orthologs in amphibians and mammals have been shown to regulate cell polarity, dendritic and axonal growth, and neural tube closure. However, the role of Flamingo in synapse formation and function and in axonal health remains poorly understood. Here we show that fmi mutations cause a significant increase in the number of ectopic synapses on muscles and result in the formation of novel en passant synapses along axons, and unique presynaptic varicosities, including active zones, within axons. The fmi mutations also cause defective synaptic responses in a small subset of muscles, an age-dependent loss of muscle innervation and a drastic degeneration of axons in 3rd instar larvae without an apparent loss of neurons. Neuronal expression of Flamingo rescues all of these synaptic and axonal defects and larval lethality. Based on these observations, we propose that Flamingo is required in neurons for synaptic target selection, synaptogenesis, the survival of axons and synapses, and adult viability. These findings shed new light on a possible role for Flamingo in progressive neurodegenerative diseases.

Characterizations of Hirudo medicinalis DNA promoters for targeted gene expression

The expression of exogenous genes in neurons and other cells has become a powerful means for studying the function of encoded proteins. We report here on the isolation and functional analysis of three Hirudo medicinalis actin gene promoters and the 5' UTR of a leech elongation factor-1alpha (HmEF-1alpha) gene. In situ hybridization labeling revealed that the EF-1alpha gene and one of the actins had pan-neuronal expression, whereas, the other two actin genes were expressed by the embryo's body wall musculature. Comparative analysis shows that they all display many features typical of actin and EF-1alpha promoters from other species, including canonical TATA box sequences and predicted general transcription factor binding sites (such as CCATT, CarB boxes and CG-rich regions). The ability of these 5' UTR sequences to drive expression of the enhanced green fluorescent protein (EGFP), leech cytoplasmic actin and leech synaptobrevin was examined. Direct intracellular nuclear, but not cytoplasmic, microinjection of each of the promoter sequences was found to produce reliably cellular expression of the reporter construct in both neuronal and muscle cells. These results introduce reliable and effective methods to selectively express genes in individual cells of the leech in vivo during embryonic development.

Regeneration and asexual reproduction share common molecular changes: upregulation of a neural glycoepitope during morphallaxis in Lumbriculus

Neural morphallaxis is a regenerative process characterized by wide-spread anatomical and physiological changes in an adult nervous system. During segmental regeneration of the annelid worm, Lumbriculus variegatus, neural morphallaxis involved a reorganization of sensory, interneuronal, and motor systems as posterior fragments gained a more anterior body position. A monoclonal antibody, Lan 3-2, which labels a neural glyco-domain in the leech, was reactive in Lumbriculus. In the worm, this antibody labeled neural structures, particularly axonal tracts and giant fiber pathways of the central nervous system. A 60kDa protein, possessing a lumbriculid mannose-rich glycoepitope, was upregulated during neural morphallaxis, peaking in its expression at 3 weeks post-amputation. Peak upregulation of the Lan 3-2 epitope, or the protein possessing it, corresponded to the time of major neurobehavioral plasticity during regeneration. Analyses of asexually reproducing animals also revealed induction of the Lan 3-2 epitope. In this developmental context, Lan 3-2 epitope upregulation was also confined to segments expressing both changes in positional identity and neurobehavioral plasticity, but these molecular and behavioral changes occurred prior to body fragmentation. These results suggest that the lumbriculid Lan 3-2 glycoepitope and proteins that bear them have been co-opted for neural morphallactic programs, induced both in anticipation of reproductive fragmentation and in compensation for injury-induced fragmentation.

Glycosylation patterns are sexually dimorphic throughout development of the olfactory system in Manduca sexta

In the moth Manduca sexta, development of the adult olfactory system depends on complex interactions between olfactory receptor neurons in the antenna, antennal-lobe neurons in the brain, and several classes of glial cells. As one approach to characterizing molecules that may play roles in these interactions, we used lectins to screen antennae and antennal lobes at different stages of adult development. We find that each of the major neural cell types has a distinct pattern of labeling by lectins. Effects of enzymatic and other treatments on lectin labeling lead us to conclude that the predominant lectin ligands are: glycosphingolipids and an O-linked, fucose-containing glycoprotein on axons of olfactory receptor neurons, O-linked glycoproteins on antennal-lobe neurons, and N-linked glycoproteins on all classes of glial cells in the primary olfactory pathway. Wheat germ agglutinin labels all olfactory axons uniformly during much of development, but labeling becomes restricted to the pheromone-responsive olfactory receptor neurons in the adult male. Succinylated WGA reveals differences in these axon classes earlier, as glomerului develop from protoglomeruli. The adult female displays a less pronounced difference in labeling of axons targeting ordinary and sexually dimorphic glomeruli. Differences in labeling of receptor axons targeted to ordinary and sexually dimorphic glomeruli may be correlated with differences in function or connectivity in different regions of the antennal lobe.

Functional characterization of Drosophila sialyltransferase

Sialylation is an important carbohydrate modification of glycoconjugates in the deuterostome lineage of animals. By contrast, the evidence for sialylation in protostomes has been scarce and somewhat controversial. In the present study, we characterize a Drosophila sialyltransferase gene, thus providing experimental evidence for the presence of sialylation in protostomes. This gene encodes a functional alpha2-6-sialyltransferase (SiaT) that is closely related to the vertebrate ST6Gal sialyltransferase family, indicating an ancient evolutionary origin for this family. Characterization of recombinant, purified Drosophila SiaT revealed a novel acceptor specificity as it exhibits highest activity toward GalNAcbeta1-4GlcNAc carbohydrate structures at the non-reducing termini of oligosaccharides and glycoprotein glycans. Oligosaccharides are preferred over glycoproteins as acceptors, and no activity toward glycolipid acceptors was detected. Recombinant Drosophila SiaT expressed in cultured insect cells possesses in vivo and in vitro autosialylation activity toward beta-linked GalNAc termini of its own N-linked glycans, thus representing the first example of a sialylated insect glycoconjugate. In situ hybridization revealed that Drosophila SiaT is expressed during embryonic development in a tissue- and stage-specific fashion, with elevated expression in a subset of cells within the central nervous system. The identification of a SiaT in Drosophila provides a new evolutionary perspective for considering the diverse functions of sialylation and, through the powerful genetic tools available in this system, a means of elucidating functions for sialylation in protostomes.

In vivo dynamics of CNS sensory arbor formation: a time-lapse study in the embryonic leech

In the embryo of the leech Hirudo medicinalis, afferent projections of peripheral sensory neurons travel along common nerve tracts to the CNS, where they defasciculate, branch, and arborize into separate, modality-specific synaptic laminae. Previous studies have shown that this process requires, at least in part, the constitutive and then modality-specific glycosylations of tractin, a leech L1 homologue. We report here on the dynamics of growth of these projections as obtained by examining the morphology of single growing dye-filled sensory afferents as a function of time. Using 2-photon laser-scanning microscopy of the intact developing embryo, we obtained images of individual sensory projections at 3 to 30 min intervals, over several hours of growth, and at different stages of development. The time-lapse series of images revealed a highly dynamic and maturation-state-dependent pattern of growth. Upon entering the CNS, the growth cone-tipped primary axon sprouted numerous long filopodial processes, many of which appeared to undergo repeated cycles of extension and retraction. The growth cone was transformed into a sensory arbor through the formation of secondary branches that extended within the ganglionic neuropil along the anterior-posterior axis of the CNS. Numerous tertiary and quaternary processes grew from these branches and also displayed cycles of extension and retraction. The motility of these higher-order branches changed with age, with younger afferents displaying higher densities and greater motility than older, more mature sensory arbors. Finally, coincident with a reduction in higher order projections was the appearance of concavolar structures on the secondary processes. Rows of these indentations suggest the formation of presynaptic en-passant specializations accompanying the developmental onset of synapse formation.

Extracellularly applied horseradish peroxidase increases the number of dense core vesicles in leech sensory neurons

The uptake of horseradish peroxidase (HRP), applied as an extracellular tracer, is a classical method for studying endo/exocytosis of synaptic vesicles at the ultrastructural level. It is generally not considered that HRP may affect neuronal function. Reported here is the finding that extracellularly applied HRP (0.1%) perturbs dense core vesicles in the synaptic processes of leech neurons. The strength of the effect varies with neuronal class. In sensory afferents, the number of dense core vesicles increases 5-fold, while there is only a 2-fold increase in central neurons.

Extracellular matrix glycoproteins inhibit neurite production by cultured neurons

We have analyzed the role of extracellular matrix glycoproteins in the formation of a bipolar outgrowth pattern of identified leech neurons in culture. Adult anterior pagoda (AP) neurons cultured on the inner surface of the ganglion capsules that surround central nervous system, generate two processes oriented in opposite directions. This pattern differs from those produced by these neurons cultured on other substrates, and is similar to the pattern of developing AP neurons at embryonic day 10. We used different lectins to identify subsets of glycoproteins in the extracellular matrix (ECM) of the capsules and to study their contribution to the formation of the bipolar outgrowth pattern. ECM glycoproteins binding to peanut agglutinin (PNA) or Galanthus nivalis aglutinin (GNA) lectins were detected in ganglion capsules and in ganglion extracts that had been separated by electrophoresis and blotted to nitrocellulose membranes. Four protein bands bound to PNA lectin and six other bands, including laminin subunits, bound to GNA lectin. Other lectins failed to recognize any of the proteins. For AP neurons cultured on capsules, addition of PNA lectin to the culture medium produced a dose-dependent increase in the number of primary neurites without affecting their shape, length or number of branch points. However, PNA lectin used as substrate did not affect sprouting of AP neurons. Our results suggest that PNA-binding extracellular matrix glycoproteins regulate the formation of the bipolar pattern of AP neurons by inhibiting the formation of neurites.

Expression of specific glycoconjugates in both primary and secondary olfactory pathways in BALB/C mice

Binding of cell surface carbohydrates to their receptors specifically promotes axon growth and synaptogenesis in select regions of the developing nervous system. In some cases these interactions depend upon cell-cell adhesion mediated by the same glycoconjugates present on the surface of apposing cells or their processes. We have previously shown that the plant lectin Dolichos biflorus agglutinin (DBA) binds to a subpopulation of mouse primary olfactory neurons whose axons selectively fasciculate prior to terminating in the olfactory bulb. In the present study, we investigated whether these glycoconjugates were also expressed by postsynaptic olfactory neurons specifically within the olfactory pathway. We show here for the first time that DBA ligands were expressed both by a subset of primary olfactory neurons as well as by the postsynaptic mitral/tufted cells in BALB/C mice. These glycoconjugates were first detected on mitral/tufted cell axons during the early postnatal period, at a time when there is considerable synaptogenesis and synaptic remodelling in the primary olfactory cortex. This is one of the few examples of the selective expression of molecules in contiguous axon tracts in the mammalian nervous system. These results suggest that glycoconjugates recognized by DBA may have a specific role in the formation and maintenance of neural connections within a select functional pathway in the brain.

Posttranslational processing and differential glycosylation of Tractin, an Ig-superfamily member involved in regulation of axonal outgrowth

Tractin is a novel member of the Ig-superfamily which has a highly unusual structure. It contains six Ig domains, four FNIII-like domains, an acidic domain, 12 repeats of a novel proline- and glycine-rich motif with sequence similarity to collagen, a transmembrane domain, and an intracellular tail with an ankyrin and a PDZ domain binding motif. By generating domain-specific antibodies, we show that Tractin is proteolytically processed at two cleavage sites, one located in the third FNIII domain, and a second located just proximal to the transmembrane domain resulting in the formation of four fragments. The most NH(2)-terminal fragment which is glycosylated with the Lan3-2, Lan4-2, and Laz2-369 glycoepitopes is secreted, and we present evidence which supports a model in which the remaining fragments combine to form a secreted homodimer as well as a transmembrane heterodimer. The extracellular domain of the dimers is mostly made up of the collagen-like PG/YG-repeat domain but also contains 11/2 FNIII domain and the acidic domain. The collagen-like PG/YG-repeat domain could be selectively digested by collagenase and we show by yeast two-hybrid analysis that the intracellular domain of Tractin can interact with ankyrin. Thus, the transmembrane heterodimer of Tractin constitutes a novel protein domain configuration where sequence that has properties similar to that of extracellular matrix molecules is directly linked to the cytoskeleton through interactions with ankyrin.

Differential glycosylation and proteolytical processing of LeechCAM in central and peripheral leech neurons

LeechCAM is a recently described member of the Ig-superfamily which has five Ig-domains, two FNIII-domains, a transmembrane domain, and a cytoplasmic domain. Phylogenetic analysis indicated that LeechCAM is the leech homolog of apCAM, FasII, and vertebrate NCAM. Using a leechCAM-specific monoclonal antibody we show by immunoblot analysis and by Triton X-114 phase separation experiments that in addition to existing in a transmembrane version LeechCAM is likely to be proteolytically cleaved into a secreted form without the transmembrane domain and the intracellular tail. Furthermore, by immunoprecipitation we demonstrate that LeechCAM is glycosylated with the Laz2-369 glycoepitope, an epitope that has been specifically implicated in regulation of axonal outgrowth and synapse formation.