Generating monoclonal antibodies against excised gel bands to correlate immunocytochemical and biochemical data

Minimal epitope for Mannitou IgM on paucimannose-carrying glycoproteins

Paucimannosidic glycans are restricted to the core structure [Man1-3GlcNAc2Fuc0-1] of N-glycans and are rarely found in mammalian tissues. Yet, especially [Man2-3GlcNAc2Fuc1] have been found significantly upregulated in tumors, including in colorectal and liver cancer. Mannitou IgM is a murine monoclonal antibody that was previously shown to recognise Man3GlcNAc2 with an almost exclusive selectivity. Here, we have sought the definition of the minimal glycan epitope of Mannitou IgM, initiated by screening on a newly designed paucimannosidic glycan microarray. Among the best binders were Man3GlcNAc2 and its α1,6 core-fucosylated variant, Man3GlcNAc2Fuc1. Unexpectedly and in contrast to earlier findings, Man5GlcNAc2-type structures bind equally well and a large tolerance was observed for substitutions on the α1,6 arm. It was confirmed that any substitution on the single α1,3-linked mannose completely abolishes binding. Surface plasmon resonance for kinetic measurements of Mannitou IgM binding, either directly on the glycans or as presented on omega-1 and kappa-5 soluble egg antigens from the helminth parasite Schistosoma mansoni, showed submicromolar affinities. To characterize the epitope in greater and atomic detail, saturation transfer difference nuclear magnetic resonance spectroscopy was performed with the Mannitou antigen-binding fragment. The STD-NMR data demonstrated the strongest interactions with the aliphatic protons H1 and H2 of the α1-3-linked mannose, and weaker imprints on its H3, H4 and H5 protons. In conclusion, Mannitou IgM binding requires a non-substituted α1,3-linked mannose branch of paucimannose also on proteins, making it a highly specific tool for the distinction of concurrent human tumor-associated carbohydrate antigens.

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.

Proteolytic cleavage of the ectodomain of the L1 CAM family member Tractin

Tractin is a member of the L1 family of cell adhesion molecules in leech. Immunoblot analysis suggests that Tractin is constitutively cleaved in vivo at a proteolytic site with the sequence RKRRSR. This sequence conforms to the consensus sequence for cleavage by members of the furin family of convertases, and this proteolytic site is shared by a majority of other L1 family members. We provide evidence with furin-specific inhibitor experiments, by site-specific mutagenesis of Tractin constructs expressed in S2 cells, as well as by Tractin expression in furin-deficient LoVo cells that a furin convertase is the likely protease mediating this processing. Cross-immunoprecipitations with Tractin domain-specific antibodies suggest that the resulting NH(2)- and COOH-terminal cleavage fragments interact with each other and that this interaction provides a means for the NH(2)-terminal fragment to be tethered to the membrane. Furthermore, in S2 cell aggregation assays we show that the NH(2)-terminal fragment is necessary for homophilic adhesion and that cells expressing only the transmembrane COOH-terminal fragment are non-adhesive. However, tethering of exogeneously provided Tractin NH(2)-terminal fragment to S2 cells expressing only the COOH-terminal fragment can functionally restore the adhesive properties of Tractin.

Leech photoreceptors project their galectin-containing processes into the optic neuropils where they contact AP cells

We characterized a subset of leech sensory afferents, the photoreceptors, in terms of their molecular composition, anatomical distribution, and candidate postsynaptic partners. For reagents, we used an antiserum generated against purified LL35, a 35 kD leech lactose-binding protein (galectin); monoclonal antibody (mAb) Lan3-2, which is specific for a mannose-containing epitope common to the full set of sensory afferents; and dye injections. Photoreceptors differ from other types of sensory afferents by their abundant expression of galectin. However, photoreceptors share in common with other sensory modalities the mannose-containing epitope recognized by mAb Lan3-2. Photoreceptors from a given segment project their axons directly into the CNS ganglion innervating the same segment. They assemble in a target region, the optic neuropil, which is separate from the target regions of other sensory modalities. They also extend their axons as an optic tract into the connective to innervate optic neuropils of other CNS ganglia, thereby providing extensive intersegmental innervation for the 33 CNS ganglia comprising the leech nerve cord. Because of its intimate contact with the optic neuropil, a central neuron, the AP effector cell, is a strong candidate second order visual neuron. In confocal images, the AP cell projects its primary axon for about 100 microns alongside the optic neuropil. In electron micrographs, spines emanating from the axon of the AP cell make contact with vesicle laden nerve terminals of photoreceptors. Leech photoreceptors and their second order visual neurons represent a simple visual system for studying the mechanisms of axonal targeting.

Sequential steps in axonal targeting are mediated by carbohydrate markers

Mannose and hybrid/complex-type oligosaccharides serve as markers for both the full set of peripheral sensory afferent neurons in the leech and also for disjoint subsets of these neurons. We have shown that these various surface carbohydrates play crucial roles in the multistep process by which afferents meet their synaptic partners in the central nervous system (CNS). The carbohydrate marker common to all these afferents allows their projections (which are fasciculated as they enter the CNS) to disperse and search out target regions. Carbohydrate markers specific for subsets of these afferents subsequently allow each subset to consolidate the position of its projections in appropriate regions of the CNS where it contacts its synaptic partners.

Targeting of neuronal subsets mediated by their sequentially expressed carbohydrate markers

The targeting of sensory afferent neurons in the leech CNS occurs in two discrete steps that are mediated via different carbohydrate recognitions, as shown by molecular perturbations of cultured embryos. A constitutive carbohydrate marker that is generic to all of these neurons mediates their initial defasciculation and arborization across the entire target region via mannose-specific recognition. Subsequently, two subsets of these same neurons can be differentiated by their expression of other markers that are located on hybrid or complex type carbohydrate chains. These developmentally regulated carbohydrate markers then mediate the target assembly of their respective neuronal subsets into discrete subregions. Thus, by performing opposing functions in a temporal sequence, constitutive and developmentally regulated carbohydrate markers collaborate in the targeting of neuronal subsets in the CNS.

The specificity of 130-kDa leech sensory afferent proteins is encoded by their carbohydrate epitopes

From early development through adulthood in the leech, sensory afferents, glial cells, and connective tissue express different epitopes located on a group of 130-kDa glycoproteins. The sensory epitope [reactive with monoclonal antibody (mAb) Lan3-2] is shared by the peripheral sensory afferents of different sensory modalities. In contrast, three other immunocytochemically distinct epitopes (reactive with mAbs Laz2-369, Laz7-79, and Laz6-212) differentiate these sensory afferents according to their sensory modalities. The glial epitope (mAb Laz6-297) is expressed on all macroglial processes, and the connective tissue epitope (mAb Laz9-84) is located on connective tissue surrounding the CNS, as well as in the peripheral tissues. The hydrophilic-hydrophobic nature of the 130-kDa sensory afferent and glial proteins was determined by phase separation with Triton X-114 and hypoosmotic extraction. They behave as peripheral membrane proteins. Deglycosylation of 130-kDa glycoproteins with N-Glycanase or preincubation of their respective mAbs with alpha-methylmannoside showed that the sensory epitope contains mannose, whereas the modality epitopes are of an undefined carbohydrate character. Immunoprecipitation and a peptide mapping experiment confirmed the existence of four distinct sensory afferent epitopes. Previous studies provided evidence that the mannose-containing Lan3-2 epitope mediates normal sensory afferent growth in the synaptic neuropile. We, therefore, postulate that the carbohydrate epitopes on sensory afferent glycoproteins participate in synapse formation.

Segregation of afferent projections in the central nervous system of the leech Hirudo medicinalis

Sensory axons originating in peripheral tissues converge onto each segmental ganglion in the central nervous system (CNS) of the leech, where they segregate into well-defined regions of the synaptic neuropil. Here we report on several aspects of the molecular and anatomical organizations of these afferent projections that bear upon the hypothesis that surface markers are involved in organizing these axons as they grow into the CNS. First, we show that the distribution of some surface markers in the adult is restricted to axons of peripheral origin and is not present on the neighboring axons of central neurons. Second, we demonstrate that the number of afferents increases postembryonically as the leech increases in size, suggesting that at least some of the cues employed by afferent axons to grow to appropriate central targets must be present throughout the life of the animal. We then show, using anterograde axonal tracing and immunohistochemistry, that there is both convergence and divergence of afferent axons into highly specific regions of the neuropil. Lastly, we examine the distribution of surface markers present on different subsets of afferents and show that axons having one type of marker segregate from those having the second type. Our results, considered together with previous observations in this system, provide new clues about the organization of afferent projections in the nervous system of the leech. They also suggest how a relatively small number of molecular markers might mediate fiber-fiber interactions to organize afferent axons as they grow into the CNS.

Carbohydrate epitopes involved in neural cell recognition are conserved between vertebrates and leech

We are reporting on the evolutionary conservation of carbohydrate epitope families from vertebrate to leech. 1) The sulfated L2/HNK-1 carbohydrate epitope (Abo T, Balch CM (1981): J Immunol 127:1024-1029; Kruse J, Mailhammer R, Wernecke H, Faissner A, Timpl R, Schachner M (1984): Nature 311:153-155) is detected on glycoproteins of leech neurons using monoclonal antibodies (mAbs) L2 (336) and HNK-1. 2) Three rat mAbs, L3, L4, and L5, bind to leech nerve and muscle. The L3, L4, and L5 epitopes are localized to a group of mannosidic leech glycoproteins originally identified through mAbs Lan3-2 (Hogg N, Flaster M, Zipser B (1983): J Neurosci Res 9:445-457 and Laz6-189 (McRorie JW III, Zipser B (1988): "Cell Culture Approaches to Invertebrate Neuroscience." London: Academie Press, pp 33-52. MAb Lan3-2, which binds to a mannosidic epitope of the 130 kD sensory protein, has recently been shown to perturb the penetration of sensory afferents into the synaptic area of the central neuropile (Zipser B, Morell R, Bajt ML (1989): Neuron 3:621-630). The L3, L4, and L5 mAbs have been described to recognize different mannosidic epitopes on glycoproteins, some of which have been identified as neural cell adhesion molecules, and on astrocyte-specific proteoglycan from mouse brain (Kücherer A, Faissner A, Schachner M (1987): J Cell Biol 104:1597-1602; Fahrig T, Schmitz B, Weber D, Kücherer-Ehret A, Faissner A, Schachner M (1990): Eur J Neurosci 2:153-161; Streit A, Faissner A, Gehrig B, Schachner M (1990): J Neurochem In Press). The superposition of five different mannosidic epitopes on the axons of sensory afferents suggests complex, concerted participation of mannosidic epitopes in neuronal pathfinding and target recognition.

Expression of surface glycoproteins early in leech neural development

Cell migration and axon growth during neural development rely upon cell-cell and cell-matrix interactions mediated by surface glycoproteins. The surface glycoprotein recognized on leech neurons by monoclonal antibody Lan3-2 has previously been implicated in the process of axon fasciculation during regeneration in adults. In adult leeches, Lan3-2 binds to a carbohydrate epitope of a 130 kD protein. The present study demonstrates that in embryos the antibody binds to the same carbohydrate epitope of glycoproteins with molecular weights of 130 kD and higher. As a first step in evaluating a possible role of the Lan3-2 glycoprotein or the cells that express it during neural development, we determined its distribution in the developing nervous system of the leech Hirudo medicinalis. In embryos, Lan3-2 epitope is expressed on fasciculated sensory afferents and it appears on the cell bodies before neurite outgrowth. The sensory fibers appear rostrally by embryonic day 10, less than halfway through development. Earlier, by 7 days of development at 20 degrees C, Lan3-2 binds to previously undocumented cell types: (1) cells appearing along the embryonic midline and (2) a cluster of cells located at the rostral edge of the germinal plate. These cells only transiently express this antigen and are present at critical left-right and rostrocaudal boundaries during a period of cell proliferation, movement, and migration that produces the nervous system. Thus the Lan3-2 surface glycoprotein or the cells expressing it are candidates for involvement in axon fasciculation, cell migration, and directed axonal growth.

Defasciculation as a neuronal pathfinding strategy: involvement of a specific glycoprotein

Leech sensory afferents change their growth behavior as they enter the CNS. Arriving from the periphery in fasciculated tracts, they abruptly defasciculate and expand into diffuse trees before reassembling into four distinct central tracts. In the organ-cultured germinal plate, growing sensory afferents were incubated with monovalent Fab fragments of the Lan3-2 antibody, which recognizes a 130 kd sensory neuron protein by its mannose epitope. Very low concentrations of Lan3-2 (6 and 12 nM) specifically inhibited the central defasciculation of sensory afferents, which then continued growing as a single tract. In contrast, monoclonal antibody Lan3-6, which binds to an internal sensory antigen, failed to yield the same effect. These observations suggest that this sensory neuron 130 kd surface glycoprotein participates in a developmentally significant heterophilic interaction specific for the CNS.

Glial processes, identified through their glial-specific 130 kD surface glycoprotein, are juxtaposed to sites of neurogenesis in the leech germinal plate

Glial processes, bearing a unique 130 kD surface protein, are located at key sites of morphogenic movement and neuronal differentiation in the leech germinal plate. A midline glial fascicle resides at the primary axis of embryonic symmetry, alongside which teloblasts move as they generate their bandlets of stem cells. The n-bandlets straddle the midline glia and are known to produce most of the central neuroblasts. The midline glia then defasciculates as neuroblasts begin to aggregate into neuromeres. The defasciculated processes expand into these neuromeres, molding the future central neuropile. Neuroblasts will initiate primary axons toward the midline glia. As the neuromeres mature, midline glial process thin out to demarcate the orientation of the future connectives, which are the major longitudinal axon tracts along the midline. Next, segmental but still primordial glia appear in the neuromeres. Initially, they also project longitudinally, then transversely, demarcating the other two major axonal pathways--the central commissures and peripheral roots. Finally, macroglial processes proliferate as massive axon growth invades the central and peripheral nervous system. Thus, glial processes with different developmental histories accompany different aspects of leech neurogenesis. In other systems, glia have been shown to promote the differentiation and the guidance of neurons. It remains to be seen whether the glial-specific 130 kD protein is a receptor mediating these typical glial functions in the leech germinal plate.

A group of related surface glycoproteins distinguish sets and subsets of sensory afferents in the leech nervous system

The distribution of 4 surface glycoproteins on axons of peripheral neurons was studied in the leech Hirudo medicinalis through monoclonal antibodies. All 4 glycoproteins have a similar molecular weight of 130 kDa. Immunohistochemical localization of these glycoproteins on tissue sections of nerves and neuropil reveals tracts of afferent axons organized as nested sets. Their distribution suggests a possible role for these molecules in mediating axon fasciculation.

The macroglial cells of the leech are molecularly heterogeneous

Monoclonal antibodies derived from fusions employing either whole leech nerve cords or fractionated proteins (gel bands) bind to the macroglial cells of the nerve cord. Three different antibodies bind to either one, two, or three of the four macroglial cell types in the leech CNS, serving as markers and showing that these four cell types, which differ primarily by anatomical position, all differ molecularly as well. Conventional microscopy confirms the existence of a novel macroglial cell type first noted because it binds antibody. Western (immunoblot) blot analyses demonstrate a polypeptide antigen of 77 kD in the macroglial cells of the connectives and a 130-kD polypeptide antigen associated with the macroglial cells of the connectives, of the root nerves, and of the ganglionic neuropil. The extensive molecular heterogeneity of leech neurons demonstrated by monoclonal antibody techniques is shared by macroglia.

Probing structural homologies in cell-specific glycoproteins in the leech CNS

Three monoclonal antibodies (mAbs) raised against the leech CNS recognize surface antigens on small sets and subsets of neurons or on glial cells. On immunoblots, they all recognize proteins of 130 kDa molecular weight. In addition, they each bind up to several different lower molecular weight forms. The 130 kDa polypeptides recognized by these mAbs are not major proteins on Coomassie blue-stained gels. They behave as glycoproteins on lentil lectin columns but are not major Concanavalin A-binding molecules. These molecules therefore represent a group of lower abundance, cell-type-specific antigens. Structural relationships between these antigens were explored using immunoprecipitation. The glial cell antigen was immunopurified, however, a fraction of the neuronal antigens co-precipitate. The co-precipitation of neuronal antigens raises the possibility that different neuronal antigenic determinants are carried on the same protein molecule. Such a protein may be modified to carry either one or both neuronal determinants, and could serve as a tag to physiologically delineate subsets of neurons nested within larger neuronal sets.

Expression of surface antigens recognized by the monoclonal antibody lan 3-2 during embryonic development in the leech

The monoclonal antibody lan 3-2 was used as a marker for the developmental expression of surface antigens which are specific for the two pairs of nociceptive neurons and a subset of axons in the adult CNS of the leech. In Haemopis embryos labeling of both nerve fibers and cell bodies with the antibody appears as expected for a metameric animal in a rostrocaudal temporal gradient from about day 5-6. Surprisingly, all central cell bodies are stained by the antibody in early development. However, later in embryogenesis around day 17 the staining intensity of most cells decreases except for the nociceptive cells, which remain antibody-positive, and the adult staining pattern gradually emerges. In addition to describing the central staining pattern, we show that specific peripheral neurons associated with the segmental sensilla also are antibody-positive during development. The distribution and developmental expression of the lan 3-2-positive antigens are compared between two phylogenetically different species of leeches and the diversity of the staining pattern of the monoclonal antibody is discussed.

Immunoblot specificity of monoclonal antibodies assayed against complex extracts

Monoclonal antibodies of known specificity, and target cells of known antigen expression, were used to evaluate the specificity of the immunoblot assay against complex extracts. The antibody panel included 10 previously characterized monoclonals to antigens of known structure: HLA-A,B,C; Ia; and actin. In addition, a commercial reagent was shown to react with HLA chains in immunoblots, and not to cross-react with actin. Target cells known to have strong, weak, or negative expression of the antigens were examined. In all cases, the results were in complete accord with the established properties of the antibodies, antigens, and target cells. For example, HLA bands were detected in lymphoid cells with strong HLA-A,B,C expression, and also in a neuronal cell line with 0.5% of the lymphoid activity. Cytoskeletal cross-reactions, or other unexpected specific bands, were not observed. This work lays a foundation for interpreting immunoblot analyses of monoclonal antibodies to newly defined proteins of complex tissue.

Immunological approaches to the nervous system

Immunology has had a major impact on neurobiology, expanding dramatically the number of subjects amenable to investigation. Studies with antibodies to neuropeptides, transmitters, and transmitter enzymes have disclosed a great heterogeneity among neurons and have provided clues for interpreting anatomical connections. Monoclonal antibodies are being used to identify functionally related subpopulations of neurons and cell lineages in development and to study mechanisms by which axons grow along stereotypic pathways to reach their targets. Other antibodies have identified molecules that appear to participate in cell aggregation, cell migration, cell position, and axon growth. Antibodies have revealed that many proteins are concentrated in anatomically distinct regions of the neuron. Moreover, these studies have suggested that individual proteins have different antigenic epitopes shielded or modified in different parts of the same neuron. Antibodies to membrane proteins crucial for neuronal function, such as ion pumps, ion-selective channels, and receptors, have been used to map their distributions and to study their structures at high resolution.