BIOLOGICAL OVERVIEW

Interactions between embryonic neural cells generate the specific patterns of connectivity observed in mature nervous systems. It has been proposed that cell surface carbohydrates function in cellular recognition events, guiding such interactions. Carbohydrate-binding proteins (lectins) that recognize specific oligosaccharide ligands in embryonic neural tissue provide a molecular mechanism for carbohydrate-mediated cell-cell interactions in neural development.

Gliolectin is a novel carbohydrate-binding protein, expressed by a subset of glia in the embryonic Drosophila nervous system. Since the procedure used to isolate Gliolectin is in itself, of interest, this procedure will be described first, before dealing with the biology of the protein.

Drosophila Gliolectin was cloned by an enrichment procedure selecting for mammalian cells expressing cell adhesion proteins. An embryonic Drosophila cDNA library was generated from poly(A)+ mRNA prepared from Drosophila embryos aged 7-13 hours after egg-laying, and was ligated into a vector for expression in mammalian tissue culture cells. Adherent cells were separated from non adherent cells by attachment to culture plates, to which purified Drosophila glycolipids (lipids with attached carbohydrate residues) had been adsorbed. Plasmid was released from adherent cells, subject to purification and subjected to additional rounds of cell infection and selection. After three rounds of selection, bacteria were transformed with plasmid DNA and individual plasmids were cloned. Three clones imparted to cultured cells the ability to adhere to Drosophila neutral/zwitterionic lipids. cDNA was inserted into M13 phage, then replicated and sequenced. Sequence was gathered from 65 M13 clones. Four of these clones yielded continuous sequence spanning the Gliolectin gene (Tienmeyer, 1996).

Gliolectin mediates cell adhesion in two contexts. Expressed in mammalian cells, Gliolectin generates adhesion to immobilized NZ-fraction glycolipids purified from Drosophila embryos. When expressed in the Drosophila cell line S2, Gliolectin mediates cellular aggregation. The heterophilic nature of Gliolectin was demonstrated by the formation of aggregates composed of mixed expressing and non-expressing S2 cells. This demonstrates that Gliolectin expressed on one cell binds ligands expressed on another.

Gliolectin is found in the ventral nervous system intimately associated with the anterior commissure in a position consistent with that of the identified midline glial cell pairs: MGA (anterior midline glia) and MGM (medial midline glia). Until stage 14, these glial cells remain tightly juxtaposed. A distinct separation between the two pair of cells develops at stage 14/15 as the MGM cells migrate posteriorly, consequently separating anterior from posterior commissure. The temporally and spatially restricted expression of Gliolectin indicates the potential cellular interactions in which it may be involved during neural development. The anteriormost midline glial cells begin to express Gliolectin just before the neural processes of the forming commissure encounter the MGA cell surface. A glial-glial interaction also occurs within the spatial domain and during the time of Gliolectin expression. While maintaining intimate contact with the MGA cells, the MGM cells migrate posteriorly. This results in the separation of anterior and posterior commissures. Gliolectin is, therefore, positioned to mediate glia-glia and/or glia-neuron interaction (Tiemeyer, 1996).

There are many examples of specific cell surface proteins recognizing surface oligosaccharides to mediate cellular interactions in vertebrates. One vertebrate lectin, RL-14.5, is homologous to other soluble lectins, and is expressed both in developing neurons but also in non-neural tissue. RL-14.5 is expressed in both neural and non-neural tissues on embryonic day 13. High levels of RL-14.5 are present in primary sensory neurons and motoneurons in the spinal cord and brain stem of newborn rats. Expression in both sensory neurons and motoneurons is detectable soon after neural differentiation. Within the brain stem, cranial nerve motoneurons, including the motor nucleus of the trigeminal nerve and the facial motor nucleus are intensely labeled by in situ hybridization. Oligosaccharide ligands for RL-14-5 are selectively expressed on the same neurons, suggesting that carbohydrate-mediated interactions contribute to the development of this subset of mammalian neurons (Hynes, 1990).

The novel sequence of Gliolectin suggests that current classification schemes for animal lectins are too confining for the growing cadre of described carbohydrate-binding activities. Lectin activity has been associated with some unexpected molecules. For instance, the endoplasmic reticulum proteins calnexin and calreticulin are neither C-type, I-type nor galectin but bind high-mannose N-linked oligosaccharides of newly synthesized proteins (Hammond, 1994). The identification of a novel carbohydrate-binding protein in Drosophila indicates that additional undefined lectin families await discovery in many organisms (Tiemeyer, 1996).

Gliolectin positively regulates Notch signalling during wing-vein specification in Drosophila

Notch signalling is essential for animal development. It integrates multiple pathways controlling cell fate and specification. This paper reports the genetic characterization of Gliolectin, presumably a lectin, a cytoplasmic protein, significantly enriched in Golgi bodies. Its expression overlaps with regions where Notch is activated. Loss of gliolectin function results in ectopic veins, while gain of its function causes loss of wing veins. It positively regulates Enhancer of split mβ, a target of Notch signalling. These observations suggest that it is a positive regulator of Notch signalling during wing development in Drosophila (Prasad, 2015).

GENE STRUCTURE

Bases in 5' UTR - 316

Bases in 3' UTR - 753

PROTEIN STRUCTURE

Amino Acids - 228

Structural Domains

Two stretches of hydrophobicity exist, one at the N-terminus and another, flanked by basic residues, approximately halfway through the polypeptide. There are five consensus N-linked glycosylation sites, all on the carboxy-terminal side of the second hydrophobic stretch. A very short stretch of OPA repeat, a Drosophila repetitive element, is also present at the carboxy terminus. Gliolectin is detected at the surface of cells by staining live cells with monoclonal antibody raised against the amino-terminal half of the molecule. The carboxy-terminal is also accessible on the exterior of the cell. Taken together, the C-terminal distribution of the glycosylation consensus sequences and the lack of a clear N-terminal signal-cleavage site suggest that Gliolectin is a type II transmembrane protein, with the C-terminal region found on the outside of the cell. There are no significant similarites between Gliolectin and any previously catalogued protein (Tiemeyer, 1996).

date revised: 9 Jan 97 

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