Differential dynamics and activity-dependent regulation of α- and β-neurexins at developing GABAergic synapses

Abstract

Neurexins (NRXs) and neuroligins are key synaptic adhesion molecules that also recruit synaptic signaling machineries. Neurexins consist of α-and β-isoforms, but how they couple synaptic transmission and adhesion to regulate activity-dependent synapse development remains unclear, in part because of poor understanding of their cell biology and regulation in the relevant neurons. Here, we examined the subaxonal localization, dynamics, and regulation of NRX1α and NRX1β in cortical perisomatic inhibitory synapses. Both isoforms are delivered to presynaptic terminals but show significant and different turnover rate at the membrane. Although NRX1α is highly diffuse along developing axons and filopodia, NRX1β is strictly anchored at terminals through binding to postsynaptic ligands. The turnover rate of NRX1β is attenuated by neural activity and presynaptic GABA B receptors. NRXs, thus, are intrinsically dynamic but are stabilized by local transmitter release. Such an activity-adjusted adhesion system seems ideally suited to rapidly explore and validate synaptic partners guided by synaptic transmission.surface dynamics | cell adhesion molecules S ynapse formation is a crucial component of neural circuit assembly. In the developing vertebrate CNS, synapse formation involves multiple steps (1-3). The initial contact of an axon with a potential postsynaptic target often leads to rapid initiation of transient synapse formation (4), which triggers the accumulation of adhesion molecules and recruitment of pre-and postsynaptic signaling machinery (1). These nascent synapses undergo an extensive process of validation (e.g., the matching of synapse types and transmitter and receptor types) and competition (for limited pre-and postsynaptic resources); only a subset of nascent contacts mature into more stable and functional synapses. A key mechanism for synapse validation and competition is the strength of synaptic transmission itself, but how synaptic activity regulates synaptic adhesion remains poorly understood. In particular, whether and how GABAergic transmission regulates synaptic adhesion at developing inhibitory synapses are largely unknown.Neurexins (NRXs) and neuroligins (NLs) are arguably the best characterized synaptic adhesion molecules and have been implicated in the synapse formation process (5-7). Recent studies suggest that NLs contribute to activity-dependent specification and validation of synapses, with NL1 and NL2 acting on excitatory and inhibitory synapses, respectively (8). Because NRXs and NLs also bind and recruit pre-and postsynaptic signaling molecules, they seem ideally suited to couple synaptic signaling and adhesion. Current evidence implicates NRXs as a key mechanism that nucleates transsynaptic signaling, but whether and how neural activity regulates NRX property and function are unknown.Vertebrate NRXs are encoded by three genes with extensive alternative splicing (9). Each gene contains two promoters that direct the synthesis of the longer α-NRX and shorter β-NRX, which share identical cytoplasm...

Modulation of synaptic function through the α-neurexin–specific ligand neurexophilin-1

“…4G) and at extrasynaptic sites of the cell surface (Fig. 4H), consistent with the mobile behavior of its cognate receptor α-Nrxn on axonal membranes (36). Intracellularly, we observed gold-labeled Nxph1 over vesicular structures in the presynaptic terminal ( Fig.…”

“…In addition, research has identified several molecules, mostly ion channels and receptors, which are functionally impaired when Nrxn expression is altered. Nrxn variants have been found to affect voltage-dependent calcium channels (9,10), GABA A R (7, 37), NMDAR (44), GABA B R (36,45), nicotinergic acetylcholine receptors (69), and AMPAR (47). These ionotropic and metabotropic receptors represent "target molecules" of Nrxn that may include some physical association, but it appears unlikely that the functional link to all these requires stable protein-protein interactions.…”

Synapse-specific IL-1 receptor subunit reconfiguration augments vulnerability to IL-1β in the aged hippocampus

“…Next, to identify potentiated synapses, our method uses antibodies specific for extracellular epitopes on GluR1 and neurexin-1β (Nrx1β) (Fig. 6D), a presynaptic adhesion molecule stabilized at the membrane surface by synaptic activity (43)…”

GABA Signaling Promotes Synapse Elimination and Axon Pruning in Developing Cortical Inhibitory Interneurons

“…Thus, only a rigorous quantitative analysis with simultaneous presynaptic and postsynaptic labeling could reveal whether the cellular and subcellular specificity of synapse formation is compromised by transmission blockade in basket cell axons. This possibility warrants investigation because it would provide evidence that although synapse specificity might be guided by molecular cues, the execution of molecular cues in developing vertebrate circuits may be intertwined with and dependent on synaptic transmission, such as shown for the neurexin-neuroligin system (Fu and Huang, 2010).…”

“…In addition, we examined the effect of release blockade on the localization of the essential synaptic adhesion molecule neurexin1␤. We recently developed a method to detect the membrane-inserted form of neurexin1 and showed that neurexin1␤ is specifically localized to presynaptic boutons of basket interneurons (Fu and Huang, 2010). In control cells, Nrx1␤-SEP (a neurexin1␤-synaptopHlorin fusion protein) was often concentrated to a restricted area within a bouton with variable sizes among different boutons (Fig.…”