Processing of the synaptic cell adhesion molecule neurexin-3β by Alzheimer disease α- and γ-secretases
ABSTRACT Neurexins (NRXNs) are synaptic cell adhesion molecules having essential roles in the assembly and maturation of synapses into fully functional units. Immunocytochemical and electrophysiological studies have shown that specific binding across the synaptic cleft of the ectodomains of presynaptic NRXNs and postsynaptic neuroligins have the potential to bidirectionally coordinate and trigger synapse formation. Moreover, in vivo studies as well as genome-wide association studies pointed out implication of NRXNs in the pathogenesis of cognitive disorders including autism spectrum disorders and different types of addictions including opioid and alcohol dependences, suggesting an important role in synaptic function. Despite extensive investigations, the mechanisms by which NRXNs modulate the properties of synapses remain largely unknown. We report here that α- and γ-secretases can sequentially process NRXN3β, leading to the formation of two final products, an ∼80-kDa N-terminal extracellular domain of Neurexin-3β (sNRXN3β) and an ∼12-kDa C-terminal intracellular NRXN3β domain (NRXN3β-ICD), both of them being potentially implicated in the regulation of NRXNs and neuroligins functions at the synapses or in yet unidentified signal transduction pathways. We further report that this processing is altered by several PS1 mutations in the catalytic subunit of the γ-secretase that cause early-onset familial Alzheimer disease.
- SourceAvailable from: Yoo Sung Song
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- "A large number of γ-secretase substrates have recently been identified and the number is now approaching approximately 90 [83–85]. Notably, many γ-secretase substrates are synaptic surface proteins [83–85], including N-cadherin , ErbB4 [87,88], nectin-1α , syndecan-1/2/3 [85,90], GluR3 (GluA3) , ephrinB1/2 [92–94], EphA4/B2 [95,96], LAR , neurexin-1/3β [98,99] and neuroligin-1 [30,31]. This suggests that γ-secretase could act on these proteins to regulate synapse structure and function. "
ABSTRACT: Long-term depression (LTD) reduces the functional strength of excitatory synapses through mechanisms that include the removal of AMPA glutamate receptors from the postsynaptic membrane. LTD induction is also known to result in structural changes at excitatory synapses, including the shrinkage of dendritic spines. Synaptic adhesion molecules are thought to contribute to the development, function and plasticity of neuronal synapses largely through their trans-synaptic adhesions. However, little is known about how synaptic adhesion molecules are altered during LTD. We report here that NGL-3 (netrin-G ligand-3), a postsynaptic adhesion molecule that trans-synaptically interacts with the LAR family of receptor tyrosine phosphatases and intracellularly with the postsynaptic scaffolding protein PSD-95, undergoes a proteolytic cleavage process. NGL-3 cleavage is induced by NMDA treatment in cultured neurons and low-frequency stimulation in brain slices and requires the activities of NMDA glutamate receptors, matrix metalloproteinases (MMPs) and presenilin/γ-secretase. These results suggest that NGL-3 is a novel substrate of MMPs and γ-secretase and that NGL-3 cleavage may regulate synaptic adhesion during LTD.Philosophical Transactions of The Royal Society B Biological Sciences 01/2014; 369(1633):20130158. DOI:10.1098/rstb.2013.0158 · 6.31 Impact Factor
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- "The effect of these mutations on NRXN3 processing has been elucidated by overexpressing them in Chinese hamster ovary cells stably expressing NRXN3. The mutated proteins increased NRXN3b-CTF levels and decreased NRXN3b-ICD formation (Bot et al., 2011). These data suggest that mutated forms of PS1/g-secretase impair NRXN3 processing and may cause the accumulation of the intracellular NRXN3 C-terminal fragment. "
ABSTRACT: Synaptic damage is a critical hallmark of Alzheimer's disease, and the best correlate with cognitive impairment ante mortem. Synapses, the loci of communication between neurons, are characterized by signature protein combinations arrayed at tightly apposed pre- and post-synaptic sites. The most widely studied trans-synaptic junctional complexes, which direct synaptogenesis and foster the maintenance and stability of the mature terminal, are conjunctions of presynaptic neurexins and postsynaptic neuroligins. Fluctuations in the levels of neuroligins and neurexins can sway the balance between excitatory and inhibitory neurotransmission in the brain, and could lead to damage of synapses and dendrites. This review summarizes current understanding of the roles of neurexins and neuroligins proteolytic processing in synaptic plasticity in the human brain, and outlines their possible roles in β-amyloid metabolism and function, which are central pathogenic events in Alzheimer's disease progression.Neurobiology of aging 11/2013; 35(4). DOI:10.1016/j.neurobiolaging.2013.09.032 · 4.85 Impact Factor
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- "A second concern is that we did not examine the effects of neurexin and neuroligin shedding on BLINC or ID-PRIME signal in this work. Recent studies have shown that the ectodomains of both NRX ,  and NLG ,  may be cleaved by membrane-anchored metalloproteases in a potentially activity-dependent manner. This could complicate the interpretation of BLINC and ID-PRIME data if some fusion constructs of NRX and NLG are more prone to cleavage than wild-type (giving false negatives), or if labeling signal on the cleaved NLG ectodomain that ought to have escaped into the medium is trapped by full-length NRX (giving false positives). "
ABSTRACT: Neurexin and neuroligin are transmembrane adhesion proteins that play an important role in organizing the neuronal synaptic cleft. Our lab previously reported a method for imaging the trans-synaptic binding of neurexin and neuroligin called BLINC (Biotin Labeling of INtercellular Contacts). In BLINC, biotin ligase (BirA) is fused to one protein while its 15-amino acid acceptor peptide substrate (AP) is fused to the binding partner. When the two fusion proteins interact across cellular junctions, BirA catalyzes the site-specific biotinylation of AP, which can be read out by staining with streptavidin-fluorophore conjugates. Here, we report that BLINC in neurons cannot be reproduced using the reporter constructs and labeling protocol previously described. We uncover the technical reasons for the lack of reproducibilty and then re-design the BLINC reporters and labeling protocol to achieve neurexin-neuroligin BLINC imaging in neuron cultures. In addition, we introduce a new method, based on lipoic acid ligase instead of biotin ligase, to image trans-cellular neurexin-neuroligin interactions in human embryonic kidney cells and in neuron cultures. This method, called ID-PRIME for Interaction-Dependent PRobe Incorporation Mediated by Enzymes, is more robust than BLINC due to higher surface expression of lipoic acid ligase fusion constructs, gives stronger and more localized labeling, and is more versatile than BLINC in terms of signal readout. ID-PRIME expands the toolkit of methods available to study trans-cellular protein-protein interactions in living systems.PLoS ONE 04/2013; 8(2):e52823. DOI:10.1371/journal.pone.0052823 · 3.23 Impact Factor