Differential Effects of Two Hydrocephalus/MASA Syndrome-related Mutations on the Homophilic Binding and Neuritogenic Activities of the Cell Adhesion Molecule L1

Abstract

The cell adhesion molecule L1 plays an important role in neural development. We have previously demonstrated that the second immunoglobulin-like domain (Ig2) of L1 contains both homophilic binding and neuritogenic activities (Zhao, X., and Siu, C.-H. (1995) J. Biol. Chem. 270, 29413-29421). Recently, two mutations (R184Q and H210Q) within the Ig2 region of the human L1 gene have been shown to be responsible for X-linked hydrocephalus and the related MASA (mental retardation, aphasia, shuffling gait, and adducted thumbs) syndrome. Glutathione S-transferase-Ig2 fusion proteins containing these mutations were used to evaluate their effects on L1. The homophilic binding activity of fusion proteins and their ability to promote neurite outgrowth from retinal cells were examined. The R184Q mutation led to a complete loss of both homophilic binding and neuritogenic activities, while the H210Q mutation resulted only in a partial loss. These results provide, for the first time, direct demonstration of the deleterious effects of hydrocephalus/MASA mutations on two intrinsic properties of L1.The cell adhesion molecule L1 is expressed primarily in postmitotic neurons and has been implicated in neural migration, neurite outgrowth, and fasciculation during brain development (for a review, see Ref. 1). L1 is a 200-kDa transmembrane glycoprotein and a member of the immunoglobulin (Ig) superfamily of cell adhesion molecules. It contains six Ig-like domains in the amino-terminal region, followed by five fibronectin type III repeats, one transmembrane domain, and a cytoplasmic domain (2, 3). L1 can undergo homophilic interactions with L1 (4, 5), as well as heterophilic interactions with other adhesion molecules, such as NCAM 1 (6), TAG-1/axonin-1 (7, 8), F3/F11 (9), glia (10), and components of the extracellular matrix (11,12). In addition to cell adhesion, substrate-coated L1 is a potent inducer of neurite outgrowth from primary neurons (4, 5). The human L1 cDNA has been cloned (13), and the gene has been mapped to chromosome Xq28 (14). Several recent reports show that a group of heterogeneous mutations in L1 are responsible for X-linked hydrocephalus and two related neurological disorders, MASA (mental retardation, aphasia, shuffling gait, and adducted thumbs) syndrome, and spastic paraplegia type 1 (15-17). Most of them are missense mutations, resulting in amino acid changes in the extracellular and cytoplasmic domains, while others are nonsense, deletion, or splicing mutations resulting in the truncation or secretion of L1. However, little is known about how these mutations give rise to these related neurological diseases. An investigation of the role of the mutated residues in L1 function is, therefore, crucial to our understanding of these defects.We recently demonstrated that the Ig2 domain of L1 harbors both homophilic binding and neuritogenic activities (18). Interestingly, two missense mutations have been localized to Ig2. One results in the replacement of Arg-184 with Gln. This mutation is found in patients with severe h...

Cytoplasmic Domain Mutations of the L1 Cell Adhesion Molecule Reduce L1–Ankyrin Interactions

“…These results suggested that the recruitment of ankyrin to the plasma membrane by L1 in this assay was independent of the L1 extracellular domain interactions affected by these extracellular domain mutations associated with CRASH syndrome. Furthermore, because the mutations R184Q and H210Q previously had been shown to reduce L1-L1 trans-interactions substantially (Zhao and Siu, 1996;De Angelis et al, 1999), these results confirm the independence of L1-L1 trans-interaction and L1-ankyrin interaction in this assay.…”

“…In addition, L1 mutations in the extracellular domain (R184Q, H210Q, E309K, Y784C, and Y1070C) effectively recruited ankyrin to the plasma membrane. The L1 R184Q and H210Q mutations previously have been shown to disrupt L1-L1 trans-interactions substantially (Zhao and Siu, 1996;De Angelis et al, 1999). Furthermore, these two mutations as well as the mutations E309K and Y1070C previously have been shown to alter L1-heterophilic interactions with the L1-binding partners F11/F3/contactin and axonin I/TAG1 (De Angelis et al, 1999).…”

Transient expression of neurofascin by oligodendrocytes at the onset of myelinogenesis: Implications for mechanisms of axon-glial interaction

“…The importance of the signalling function of CAMs has been underlined by recent studies on the effects of mutations in the cytoplasmic domain on L1 function, which have shown that some mutants have normal adhesive properties but display defects in their ability to provoke intracellular signalling (Zhao and Siu, 1996). Adhesive interactions involving L1, NCAM, and Ncadherin are known to activate secondary message pathways, such as those mediated by the FGF receptors, and these signalling capabilities influence a number of important aspects of neuronal development such Fig.…”

The Drosophila L1CAM homolog Neuroglian signals through distinct pathways to control different aspects of mushroom body axon development

“…5E,F). This corresponds to the effects on homophilic interaction of a point mutation of the corresponding amino acid in human L1-CAM (H201Q) in an individual with 1599 RESEARCH ARTICLE Neuroglian in mushroom body development MASA syndrome (Zhao and Siu, 1996). The Nrg 892 D203N mutation has no effect on homophilic interaction, as transfected cells can form aggregates (Fig.…”