Stanford Synchrotron Radiation Lightsource

Figure 1 - Ribbon representation of the neuroligin-1/β-neurexin Complex. View from the pre-synapse - Green, neuroligin dimer; red, neurexin molecules. Residues found to be mutated in autism are shown as yellow amino acid side chains. N-linked glycosylation sites in neuroligin are also shown in red, position of alternative splicing are shown in cyan.

Autism spectrum disorders are among the most devastating disorders of early childhood in terms of prevalence, family impact, and cost to society. Several mutations of the coding regions of the neuroligins and neurexins have been implicated through genetic screens in the pathogenesis of autism and mental retardation (Jamain et al., 2003; Laumonnier et al., 2004; Yan et al., 2005). Neurexin and neuroligins are extracellular proteins that associate within the extracellular synaptic space and appear to be crucial for maintaining the integrity and functionality of brain circuitry through synaptic transmission (Varoqueaux et al., 2006). Many neurodevelopmental disorders involve abnormal synaptic function, the synapses providing the essential connections between nerve cells that enable signals to be transmitted. As the neuroligins and neurexins are thus far the only extracellular synaptic proteins clearly implicated in autism spectrum disorders and mental retardation, determining the structure of a neuroligin/neurexin complex represents a significant advance toward defining the molecular organization within the synapse. This structural framework is an essential foundation for linking genetic information with neuro-developmental disorders.

Glycosylation of the neuroligins and their unusual extended stalk regions (see below) have rendered a crystallographic solution of the structure of the extracellular portion of a neuroligin, as well as a neuroligin- neurexin complex, a challenging endeavor. We therefore turned to small angle X-ray and neutron solution scattering to determine the molecular shapes of neuroligin and neurexin, and their dispositions in their complex, in order to enhance our understanding of the complex formation within the synaptic space environment.

Figure 2 - Model of the neuroligin-1/β-neurexin assembly in the nerve synapse. The neurexin/neuroligin complex is tethered to the pre- and post-synaptic membranes through their respective O-linked glycosylated regions that have been drawn extended to position the complex in the synaptic space. Arrows specify the approximate sizes of the synaptic cleft and the complex.

Using small-angle X-ray scattering data from Beam Line 4-2 at SSRL, in combination with homology modeling, we first determined a three-dimensional structural model of the dimeric acetylcholinesterase-like domain for neuroligin-1. We then showed that all four neuroligin isoforms have similar overall shapes and dimensions as evidenced by their distance distribution functions, radius of gyration, maximum dimension, and three-dimensional shape reconstructions. The solution scattering data further indicate that the stalk region connecting the globular domain of the neuroligins with their transmembrane domain is elongated and projects away from the globular domain. Using the newly determined neuroligin-1 structure, and complementing our X-ray data with neutron contrast variation data from a complex of neuroligin-1 and a deuterated neurexin, we were able to determine the first three dimensional structural model of neuroligin-1 complexed with β-neurexin (Figure 1).

The combination of X-ray scattering with neutron contrast variation, sequence analysis, modeling, biochemical and mutagenesis data has allowed us to put forward a model of the neurexin and neuroligin complex in the synaptic space (Figure 2). This model provides an important structural framework for linking genetic information on mutated neurexins and neuroligins with neuro-developmental disorders.