A procedure of sequential extractions of cerebellar tissue was set up, which allowed specific solubilization of endogenous lectins by mannose. Two cerebellar soluble lectins, CSL1 (Mr = 33,000) and CSL2 (Mr = 31,500), were isolated. They appeared to consist of structurally and immunologically related polypeptides chains. By immunoaffinity, another minor component (Mr = 45,000) was isolated. Immunological studies suggested that the minor component is the precursor of the two other, i.e., CSL1 and CSL2, subunits. CSL1 (mainly lysosomal) possesses an additional peptide compared with CSL2 (mainly cytoplasmic and extracellular), which seems to be implicated in the signal for secretion and release.
In the central nervous system, postmitotic neurons migrate along astrocytic processes to reach their adult position. The molecular mechanisms of this guided migration are not clearly defined, although some steps have been shown to involve proteases and cell adhesion molecules. We report that monovalent antibodies (Fab fragments) raised against an endogenous cerebellar soluble lectin (CSL) completely inhibit neuronal migration in cultures of cerebellar explants at concentrations as low as 50 ,ug/ml. A similar inhibition pattern was obtained with Fab fragments prepared against one of the endogenous glycoprotein ligands of CSL, the 31-kDa glycoprotein (this glycoprotein is a membrane-bound glycoprotein specifically occurring, in the cerebellum, at the surface of immature neurons). We propose that this lectin-glycoprotein interaction supports the adhesion between neurons and the astrocyte guide during the migration of cerebellar immature neurons.
Cultures of rat oligodendrocytes were used to test the possible role of the cerebellar soluble lectin (CSL) in myelin formation. Immunocytochemistry at the ultrastructural level showed that the lectin is present in the cytoplasm of the perikaryon of cultured oligodendrocytes and also on the plasma membrane of the cell body and processes. It is present in compact myelin and in the zones of contacts between different myelin sheaths or oligodendrocyte membranes. Staining of blots of the cultures with iodinated CSL indicated that endogenous glycoprotein ligands for CSL are present in the culture, rendering probable the hypothesis that cell contacts between different oligodendrocytes or between adjacent lamellae in myelin are mediated by lectin-glycoprotein interactions. This hypothesis was demonstrated by two effects of anti-CSL Fab fragments (4 µg/ml) on oligodendrocyte cultures: (1) the almost complete detachment of the cell layer from the culture substratum, and (2) the loss of myelin compaction by a separation of lamellae at the intraperiod line. The present findings could explain the complexity of the contacts between cultured oligodendrocyte processes by the formation of CSL bridges between glycoproteins of the membranes of these cells. CSL seems to be a key molecule in adhesion both for intercellular contacts and fixation of cells to the substratum. The small number of glycoprotein subunits found in oligodendrocytes that interact with CSL suggests that CSL-mediated cell adhesion involves a special class of glycoprotein glycans.
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