Mutations in the gene for neural cell adhesion molecule L1 (L1CAM) result in a debilitating X-linked congenital disorder of brain development. At the neuronal cell surface L1 may interact with a variety of different molecules including itself and two other CAMs of the immunoglobulin superfamily, axonin-1 and F11. However, whether all of these interactions are relevant to normal or abnormal development has not been determined. Over one-third of patient mutations are single amino acid changes distributed across 10 extracellular L1 domains. We have studied the effects of 12 missense mutations on binding to L1, axonin-1 and F11 and shown for the first time that whereas many mutations affect all three interactions, others affect homophilic or heterophilic binding alone. Patient pathology is therefore due to different types of L1 malfunction. The nature and functional consequence of mutation is also reflected in the severity of the resultant phenotype with structural mutations likely to affect more than one binding activity and result in early mortality. Moreover, the data indicate that several extracellular domains of L1 are required for homophilic and heterophilic interactions.

SummaryThe expression of genes encoding patatin, a major tuber protein, is highly tissue-specific but is also modulated by exogenous sucrose. The patterns of transcription observed in potato plants could be due to mechanisms conferring tuber-specificity or they could reflect the concentrations of sucrose found in different tissues. To distinguish between these possibilitles, a datalled examination was made of the function of a region of the promoter previously impliceted in conferring tissue-specific and sucrose-inducible expression. Internal deletions of this region revealed three separate functional domains regulating expression. The B repeat region acted as a positive activator of transcription in the tuber and was also responsible for a degree of sucrose-inducibility. The dlatal region of the A repeat repressed transcription in leaf and tuber tissue, while the proximal region of the A repeat was able to confer sucrose-responsiveness. Each of these regions specifically bound nuclear proteins which may be putative transcription factors involved in conferring these responses. The region found to confer sucrose-inducible expression was conserved among some other genes that are also regulated by exogenous sucrose.

The L1 cell adhesion molecule has six domains homologous to members of the immunoglobulin superfamily and five homologous to fibronectin type III domains. We determined the outline structure of the L1 domains by showing that they have, at the key sites that determine conformation, residues similar to those in proteins of known structure. The outline structure describes the relative positions of residues, the major secondary structures and residue solvent accessibility. We use the outline structure to investigate the likely effects of 22 mutations that cause neurological diseases. The mutations are not randomly distributed but cluster in a few regions of the structure. They can be divided into those that act mainly by changing conformation or denaturing their domain and those that alter its surface properties.

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We have produced a number of astrocytic cell lines, some of which promote abundant neurite outgrowth, some of which are poor promoters of neurite outgrowth. The critical difference between these lines lies in the extracellular matrix, cell lines that are good promoters of axon growth producing a matrix that promotes axon growth, cell lines that are poor promoters of axon growth producing a non-permissive matrix. We were unable to find any consistent correlations between promotion of axon growth and production of proteases, protease inhibitors, N-cadherin, growth cone collapsing activity, and several extracellular matrix molecules. In the present study we have compared the least permissive of our cell lines, Neu7, with the most permissive, A7. Medium conditioned by the cell lines has the same properties as the matrix, since dorsal root ganglia (DRGs) grown in conditioned medium from the Neu7 line grow axons poorly, while DRGs grown in medium conditioned by A7 or primary astrocytes grow many long axons. Since matrix produced by all the cell lines contains large amounts of laminin, we looked to see whether the cells were producing laminin-blocking activity. Medium from the Neu7 line blocked laminin, while that from the A7 and primary astrocytes did not. However, when the conditioned media were heat-treated to remove neurite-promoting activity, they all had laminin-blocking activity: the blocking activity is heat stable. The neurite-promoting properties of the conditioned media therefore probably reflect a balance between promoting molecules and blockers.(ABSTRACT TRUNCATED AT 250 WORDS)