DEFINITION
A neurexin (NRXN) is a presynaptic protein that helps to make connections between neurons at the synapse. Neurexins (located presynaptically) act as receptors for neuroligin (located postsynaptically).
Neurexin and neuroligin also play a role in angiogenesis.
Neurexins and neuroligins, which are transmembrane synaptic proteins of the central nervous system, are codified in humans by families of 3 and 5 genes, respectively
- Neurexins, produced in long (α) and short (β) forms, have been widely studied because of their extended alternative splicing and have been localized indirectly at the presynaptic membrane
- Neuroligins are localized at the postsynaptic membrane and interact with neurexins from the opposite side (in trans) of the synaptic cleft in a calcium-dependent manner
Both proteins display a strong and selective synapse formation-promoting activity in vitro. Nonetheless, knockdown of the expression of
- 3 α forms of neurexins
- neuroligins 1–3
in the mouse demonstrates that these proteins have a more fundamental role in the modulation of synaptic transmission than in the early adhesive steps of synapse formation. The same studies revealed a redundancy of function between isoforms of the same gene family. Neurexins and neuroligins are part of a large set of synaptic cell adhesion molecules whose elimination in mice surprisingly results in an overall maintenance of synaptic structures
Working Hypothesis: Neurexins corresponds to Growth Factor Receptors (VEGFR), Neuroligins to Growth Factors (VEGF)
CHEMICAL STRUCTURE AND IMAGES
When relevant for the function
- Primary structure
- Secondary structure
- Tertiary structure
- Quaternary structure
"Protein Aminoacids Percentage
Model (Width 600 px)
SYNTHESIS AND TURNOVER
mRNA synthesis
protein synthesis
Papers De Jaco A
post-translational modifications
degradation
CELLULAR FUNCTIONS
cellular localization,
The extracellular segment of α-neurexins contains six LNS domains (laminin, neurexin, sex-hormone-binding protein domain) organized with epidermal growth factor (EGF)-like domains into three homologous modules (Cell adhesion and synaptogenesis, 2007
Alternative splicing of neurexins and neuroligins determines the assembly of excitatory or inhibitory synapses.
Neurexins and neuroligins contain complex extracellular domains that are modified at several positions by alternative splicing (indicated by the arrowheads). Alternative splicing at position 4 (S4) of -neurexin and position B of neuroligin 1 alters the proteins' binding specificity for their neuroligin or neurexin partners and alters their ability to induce glutamatergic versus GABA (-aminobutyric acid)-mediated synaptogenesis. b | Each neurexin isoform will pair only with certain neuroligin isoforms, which leads to the formation of specialized synapses. Only -neurexin lacking an alternative exon inserted at S4 (neurexin-1 (-S4)) binds neuroligin 1 (Ref. 190), and the neuroligin 1 splice variant must have an exon at position B (making it neuroligin 1 (+B)). Inclusion of the alternative exon at the S4 position of -neurexin inhibits neuroligin-1-binding and reduces clustering of postsynaptic proteins that are specific to glutamatergic synapses. However, the -neurexin (+S4) form maintains high affinity for neuroligin 2 and triggers clustering of neuroligin 2 and gephyrin at GABA-signalling synapses191. On the postsynaptic side, neuroligin 1 isoforms that carry the B insert (neuroligin 1 (+B)) interact with -neurexin, but not with -neurexin. In contrast, the -B isoforms bind both - and -neurexins. The alternative exon at position B inserts eight amino-acid residues that form an N-glycosylation site — this blocks -neurexin binding and controls neuroligin localization60. Neuroligin 1 (+B) is preferentially targeted to glutamatergic synapses. Most neuroligin 2, which does not have a corresponding B site, contains an insert at position A (neuroligin 2 (+A)) that leads to preferential localization at GABA-signalling terminals61. C, carboxyl terminus; N, amino terminus; PSD95, postsynaptic density protein 95; TM, transmembrane domain. (Neuronal regulation of alternative pre-mRNA splicing, 2007)
Neurexins/Neuroligins at synaptic terminal.
Neurexins and Neuroligins are distinct families of cell adhesion molecules both of which are composed of relatively large extracellular regions, transmembrane regions and short cytoplasmic tails with PDZ binding sequences. Neurexins primarily localize at presynaptic terminals where they bind postsynaptic Neuroligins between their LNS and acetylcholinesterase-like domains, respectively, in the presence of calcium. This interaction is considered to trigger the signals required for synapse formation and maturation through modification of intracellular adapter proteins. (Katsuhiko Tabuchi, M.D., Ph.D.)
biological function
Folding anomalies of neuroligin3 caused by a mutation in the alpha/beta-hydrolase fold domain. 2010
Chem Biol Interact. 2010 Mar 12. [Epub ahead of print]
De Jaco A, Dubi N, Comoletti D, Taylor P.
Department of Pharmacology, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego, La Jolla, CA 92093, United States.
Abstract
Proteins of the alpha/beta-hydrolase fold family share a common structural fold, but perform a diverse set of functions. We have been studying natural mutations occurring in association with congenital disorders in the alpha/beta-hydrolase fold domain of neuroligin (NLGN), butyrylcholinesterase (BChE), acetylcholinesterase (AChE). Starting from the autism-related R451C mutation in the alpha/beta-hydrolase fold domain of NLGN3, we had previously shown that the Arg to Cys substitution is responsible for endoplasmic reticulum (ER) retention of the mutant protein and that a similar trafficking defect is observed when the mutation is inserted at the homologous positions in AChE and BChE. Herein we show further characterization of the R451C mutation in NLGN3 when expressed in HEK-293, and by protease digestion sensitivity, we reveal that the phenotype results from protein misfolding. However, the presence of an extra Cys does not interfere with the formation of disulfide bonds as shown by reaction with PEG-maleimide and estimation of the molecular mass changes. These findings highlight the role of proper protein folding in protein processing and localization. Copyright © 2010. Published by Elsevier Ireland Ltd.
- Enzymes
- Cell signaling and Ligand transport
Neurons
Cell adhesion molecules detected at inhibitory synapses. α-neurexins specifically induce local accumulations of neuroligin 2, gephyrin and the GABAA receptor γ2-subunit, but not of components of excitatory synapses. Knockout of all three α-neurexins reduces the number of inhibitory synapses in the brain stem, and knockout of neuroligin 2 impairs inhibitory synaptic transmission. β-neurexins induce the accumulation of induce accumulation of essential components of both excitatory and inhibitory synapse components. Triple knockout of neuroligins 1, 2 and 3, impairs both excitatory and inhibitory synaptic transmission, and leads to a reduction in the number of inhibitory receptor clusters in the brain stem. Neurofascin, a member of the L1-family of adhesion proteins, is implicated in the generation of gephyrin clusters, as well as for the targeting these clusters and of inhibitory synapse formation to the axon initial segment in Purkinje cells. The isoforms SynCAM 1 and 2 have been detected at inhibitory and excitatory. Not shown is N-Cadherin, which is localized to excitatory and inhibitory synapses in immature neurons and later becomes restricted to excitatory synapses
Architecture of the trans-synaptic neurexin–neuroligin complex. 2009
Neuroligins and neurexins link synaptic function to cognitive disease 2008
Papers neuroligin 2 brain
Bioinformation. 2006 Dec 6;1(8):281-4.
An analysis of acetylcholinesterase sequence for predicting mechanisms of its non-catalytic actions. 2006
Srivatsan M.
Department of Biological Sciences, Arkansas State University, Jonesboro, AR 72401, USA. msrivatsan@astate.edu
The enzyme acetylcholinesterase (AChE) which belongs to the family of alpha/beta hydrolases is well known for hydrolyzing the neurotransmitter acetylcholine (ACh). In addition to its catalytic function, AChE appears to play a significant non-catalytic role in development, regeneration and modulation of properties of neurons. However the mechanisms underlying these important actions of AChE are unknown. It prompted the analysis of the sequence of fetal bovine serum AChE to seek clue(s) for the mechanisms of AChE non-catalytic actions. The searches for motifs, finger prints and domains demonstrated the presence of a highly conserved carboxylesterase type B signature in AChE from slime molds to human. Interestingly, the presence of finger prints for a metabotropic glutamate receptor, gap junction protein connexin, a calcium binding motif, several phosphorylation sites, and a motif similar to nicotinic acetylcholine receptor were also found in the sequence of AChE suggesting a role for AChE in cell signalling. The similarity search using gapped BLAST retrieved previously known similar proteins, namely neurotactin, neuroligin and thyroglobulin. When aligned neurotactin, neuroligin and bovine AChE showed the presence of three stretches of consensus, one of which is carboxylesterase B signature. The identity of the other two are not known. Amino acids in these stretches in AChE may be involved in possible growth factor like action and awaits experimental verification. Thus sequence analysis can be very useful for biologists while searching for a possible mechanism of action of a protein. Further, these results underscore the significance of performing both local and global searches to identify possible functional domains as well as consensus sequence.
Vessels
The synaptic proteins neurexins and neuroligins are widely expressed in the vascular system and contribute to its functions. 2009
Differential gene expression of primary cultured lymphatic and blood vascular endothelial cells. 2007 Neoplasia. 2007 Dec;9(12):1038-45. Fulltext
Nelson GM, Padera TP, Garkavtsev I, Shioda T, Jain RK.
Blood vascular endothelial cells (BECs) and the developmentally related lymphatic endothelial cells (LECs) create complementary, yet distinct vascular networks. Each endothelial cell type interacts with flowing fluid and circulating cells, yet each vascular system has evolved specialized gene expression programs and thus both cell types display different phenotypes. BECs and LECs express distinct genes that are unique to their specific vascular microenvironment. Tumors also take advantage of the molecules that are expressed in these vascular systems to enhance their metastatic potential. We completed transcriptome analyses on primary cultured LECs and BECs, where each comparative set was isolated from the same individual. Differences were resolved in the expression of several major categories, such as cell adhesion molecules (CAMs), cytokines, and cytokine receptors. We have identified new molecules that are associated with BECs (e.g., claudin-9, CXCL11, neurexin-1, neurexin-2, and the neuronal growth factor regulator-1) and LECs (e.g., claudin-7, CD58, hyaluronan and proteoglycan link protein 1 (HAPLN1), and the poliovirus receptor-related 3 molecule) that may lead to novel therapeutic treatments for diseases of lymphatic or blood vessels, including metastasis of cancer to lymph nodes or distant organs.
Open questions
- the complete overlap between cells expressing neuroligin and cells expressing VEGF receptor 2 (VEGFR2) in the sprouted region
- β-neurexin bands (in a single discrete form, not as a stack of bands) co-precipitate with neuroligin, indicating a selective interaction of the 2 proteins in this tissue
REGULATION
NLGN2 regulated by steroids (E2 or ??)
Gene expression analysis of the pro-oestrous-stage rat uterus reveals neuroligin 2 as a novel steroid-regulated gene. 2004
This gene is expressed at high levels in the central nervous system and acts as a nerve cell adhesion factor. According to Northern blot analysis, neuroligin 2 gene expression was higher during the pro-oestrus and metoestrus stages than during the oestrus and dioestrus stages of the oestrous cycle. In addition, neuroligin 2 mRNA levels were increased by both 17beta-oestradiol (E(2)) and P(4), although P(4) administration upregulated gene expression to a greater extent than injection of E(2).
DIAGNOSTIC USE
- As a result of mapping the structure of the protein complex implicated in autism spectrum disorders, a research team led by scientists at the Skaggs School of Pharmacy at UC San Diego, and aided by computational modeling at the San Diego Supercomputer Center (SDSC) at UCSD, has discovered how particular genetic mutations affect this complex and contribute to the developmental abnormalities found in children with autism.
