Homophilic interaction of the L1 family of cell adhesion molecules plays a pivotal role in regulating neurite outgrowth and neural cell networking in vivo. Functional defects in L1 family members are associated with neurological disorders such as X-linked mental retardation, multiple sclerosis, low-IQ syndrome, developmental delay, and schizophrenia. Various human tumors with poor prognosis also implicate the role of L1, a representative member of the L1 family of cell adhesion molecules, and ectopic expression of L1 in fibroblastic cells induces metastasis-associated gene expression. Previous studies on L1 homologs indicated that four N-terminal immunoglobulin-like domains form a horseshoe-like structure that mediates homophilic interactions. Various models including the zipper, domain-swap, and symmetry-related models are proposed to be involved in structural mechanism of homophilic interaction of the L1 family members. Recently, cryo-electron tomography of L1 and crystal structure studies of neurofascin, an L1 family protein, have been performed. This review focuses on recent discoveries of different models and describes the possible structural mechanisms of homophilic interactions of L1 family members. Understanding structural mechanisms of homophilic interactions in various cell adhesion proteins should aid the development of therapeutic strategies for L1 family cell adhesion molecule-associated diseases.
Cell Adhesion ; Crystallography, X-Ray ; Escherichia coli ; Humans ; Immunoglobulins/chemistry ; Neural Cell Adhesion Molecule L1/*chemistry/*metabolism ; *Neurites/chemistry/metabolism ; Protein Conformation ; *Protein Interaction Domains and Motifs

Neurogenin1 (Ngn1) is a basic helix-loop-helix (bHLH) transcription factor expressed in neuronal precursors in the developing nervous system. The function of Ngn1 in neurogenesis has been shown in various aspects. In this study, we investigated the neurogenic potential of Ngn1 using neuroblastoma cell line, F11, which could be induced to differentiate into neurons in the presence of cAMP. To investigate the expression of Ngn1, expression vectors for the full-length and the C- terminal deletion mutant of Ngn1 were constructed and their transactivation potential was verified using reporter gene containing the E-box sequence. Overexpression of the full-length Ngn1 induced neurite outgrowth in F11 cells in the absence of cAMP. A C-terminal deletion mutant, Ngn1(1-197), inhibited neurite outgrowth induced by cAMP in F11 cells. These results indicate that the Ngn1 plays an important role in differentiation of neuroblastoma cells and the C terminus of Ngn1 is essential for the efficient differentiation.
Blotting, Western ; Cell Differentiation ; Cell Line, Tumor ; Cloning, Molecular ; Cyclic AMP/metabolism ; DNA, Complementary/genetics ; Gene Expression Regulation, Neoplastic ; Helix-Loop-Helix Motifs ; Human ; Nerve Tissue Proteins/chemistry/*genetics/*metabolism ; Neurites/*metabolism ; Neuroblastoma/genetics/*metabolism/*pathology ; Trans-Activation (Genetics) ; Transcription Factors/chemistry/*genetics/*metabolism