A Cell Surface Biotinylation Assay to Reveal Membrane-associated Neuronal Cues: Negr1 Regulates Dendritic Arborization

Abstract

A complex and still not comprehensively resolved panel of transmembrane proteins regulates the outgrowth and the subsequent morphological and functional development of neuronal processes. In order to gain a more detailed description of these events at the molecular level, we have developed a cell surface biotinylation assay to isolate, detect, and quantify neuronal membrane proteins. When we applied our assay to investigate neuron maturation in vitro, we identified 439 differentially expressed proteins, including 20 members of the immunoglobulin superfamily. Among these candidates, we focused on Negr1, a poorly described cell adhesion molecule. We demonstrated that Negr1 controls the development of neurite arborization in vitro and in vivo. Given the tight correlation existing among synaptic cell adhesion molecules, neuron maturation, and a number of neurological disorders, our assay results are a useful tool that can be used to support the understanding of the molecular bases of physiological and pathological brain function. Genetic analysis indicates that 20% to 30% of the total open reading frame encodes for integral membrane proteins (1). Although less abundant than cytosolic proteins, membrane-passing proteins contribute to the regulation of all major cell processes and signaling pathways. In particular, membrane proteins play an important role in the establishment of functional neuronal circuitries during development. This process initially entails the growth, guidance, and stabilization of neuronal processes (axons and dendrites) in a timely, ordered manner involving cell surface molecules that sense the extracellular surroundings and activate signaling cascades (2).Then, specialized cell-to-cell connections, the synapses, are formed. These connections allow information to flow from one neuron to another and relay the precise juxtaposition and interactions between the pre-and postsynaptic membrane proteins to support their final functional establishment. Several families of synaptic transmembrane or membrane proteins, such as semaphorin, neuroligin, neurexin, and the immunoglobulin superfamily (IgSF), 1 are implicated in neurite formation and synapse establishment (3). However, the picture of membrane proteins expressed in neurons is still far from being completely resolved, and it is expected that many other key molecules are awaiting identification (4). Thus, uncovering the nature of the dynamic multiprotein complexes expressed at the plasma membrane will possibly strongly support the understanding of the mechanism controlling structural and functional neuron development. Here, we describe a biochemical approach to isolate and quantify proteins exposed at the extracellular side of the plasma membrane. Our assay utilized affinity purification on streptavidin resin of biotinylated membrane proteins extracted from a crude synaptosomal preparation. We combined this cell surface biotinylation assay with MS/MS analysis and label-free quantification to investigate protein patterns characterizing immature and m...

IgLON Cell Adhesion Molecules Are Shed from the Cell Surface of Cortical Neurons to Promote Neuronal Growth

“…Axonal LSAMP expression plays an important role in the specific targeting of dopaminergic axons to the lateral subdomain of the habenula (41). Furthermore, down-regulation of NEGR1 expression both in vitro and in vivo was observed to decrease the overall complexity of neurite arborization suggesting a role in axon targeting and stability rather than overall outgrowth (42). IgLON family members are also expressed at synaptic sites (22,40,43).…”

“…IgLON family members are also expressed at synaptic sites (22,40,43). Overexpression of individual IgLON family members differentially affects the number of dendritic synapses that form in cultured hippocampal neurons, whereas NEGR1 and OBCAM loss of function results in a reduction in synapse formation on dendrites (42,44,45). Proteolytic cleavage of IgLON family members could thus have critical roles in specific targeting and synaptogenesis of cortical neurons similar to roles of other synaptic cell adhesion molecules, including NCAM, L1-CAM, and N-cadherin (46 -48).…”

Delineating the Genetic Component of Gene Expression in Major Depression

“…NEGR1 plays a role in axon extension, synaptic plasticity, and synapse formation, processes key to neuronal functioning (28)(29)(30). Moreover, it includes variants found to be related to obesity, a trait repeatedly correlated with MD, both at phenotypic and biological levels (Milaneschi et al, 2019).…”

Common risk variants identified in autism spectrum disorder

“…Shared ASD-MDD locus Locus also significant in depression 29,78 , obesity and BMI [84][85][86][87][88] NEGR1 is the only protein-coding gene in the locus NEGR1 is supported by brain Hi-C and eQTL analyses 29 NEGR1 (neuronal growth regulator 1) is an adhesion molecule modulating synapse formation in hippocampal neurons 89,90 and neurite outgrowth 91,92 . It is member of the IgLON protein family implicated in synaptic plasticity and axon extension [93][94][95] .…”