Neurexin-Neuroligin Cell Adhesion Complexes Contribute to Synaptotropic Dendritogenesis via Growth Stabilization Mechanisms In Vivo

Abstract

Cell adhesion molecules are well characterized for mediating synapse initiation, specification, differentiation, and maturation, yet their contribution to directing dendritic arborization during early brain circuit formation remains unclear. Using two-photon time-lapse imaging of growing neurons within intact and awake embryonic Xenopus brain, we examine roles of β-neurexin (NRX) and neuroligin-1 (NLG1) in dendritic arbor development. Using methods of dynamic morphometrics for comprehensive 3D quantification of rapid dendritogenesis, we find initial trans-synaptic NRX-NLG1 adhesions confer transient morphologic stabilization independent of NMDA receptor activity, whereas persistent stabilization requires NMDA receptor-dependent synapse maturation. Disrupting NRX-NLG1 function destabilizes filopodia while reducing synaptic density and AMPA receptor mEPSC frequency. Altered dynamic growth culminates in reduced dendritic arbor complexity as neurons mature over days. These results expand the synaptotropic model of dendritogenesis to incorporate cell adhesion molecule-mediated morphological stabilization necessary for directing normal dendritic arborization, providing a potential morphological substrate for developmental cognitive impairment associated with cell adhesion molecule mutations.

Function directs form of neuronal architecture

“…This demonstrates that although NRX-NLG1 complexes can cluster PSD-95, which binds to other cytoskeletal scaffold proteins, this interaction is insufficient to promote transition of motile filopodia into persistently stabilized dendritic branches, and that glutamatergic synaptic transmission is required. 5 These findings are supported by previous work showing inhibition of NMDARs or AMPARs decreases tectal neuron dendritic arbor growth. [18][19][20][21] Interestingly, activity-dependent dendritogenesis appears to share molecular mechanisms with synapse plasticity underlying learning and memory in the mature brain.…”

“…3 Finally, creation of sophisticated computer software to track and measure dendritic arbor growth in 3D over short and long intervals has allowed comprehensive quantification of dendritogenesis. 4,5 Together, these new methods of labelling, imaging and quantification represent an emerging field termed dynamic morphometrics that allows precise characterization of neuronal growth behavior.…”

“…Recent in vivo two-photon rapid time-lapse imaging of growing neurons in Xenopus laevis tadpole optic tectum finds that the pre-and postsynaptic CAM ligand pair, neurexin (NRX) and neuroligin1 (NLG1), directly influence dendritic filopodial dynamic behavior with lasting effects on arbor structure. 5 Blocking trans-synaptic NRX-NLG1 interactions by applying extracellular soluble recombinant NRX (NRX-fc) prevents normal dendritic filopodial stabilization, while increasing NRX-NLG1 interactions by overexpression of NLG1 hyper-stabilizes filopodia. Trans-synaptic NRX-NLG interactions induce both during the next 2 days, short dendritic processes, or filopodia, extend with little net growth of the dendritic arbor.…”

“…Such imaging has revealed a surprising amount of motility and turnover of dendritic processes over periods of minutes, which could not be predicted from long interval imaging. 4,5,7 Short dendritic filopodia and longer branches are added and extend, retract and are eliminated. Remarkably, almost all dendritic filopodia retract within minutes of initial extension.…”

Inhibitory Synapse Dynamics: Coordinated Presynaptic and Postsynaptic Mobility and the Major Contribution of Recycled Vesicles to New Synapse Formation

“…Excitatory synapse formation on dendritic protrusions can stabilize those protrusions, promoting further dendrite elongation in a synaptotropic model of dendrite growth in some (Niell et al, 2004;Chen et al, 2010) but not all (Kerschensteiner et al, 2009) neuron types. In cultured rat hippocampal neurons, excitatory synapse formation can stabilize filopodia (Prange and Murphy, 2001), but most new synapses form on preexisting or newly formed dendritic protrusions (Okabe et al, 2001a) that develop on dendrite branches over several days following the branch's outgrowth (Ebihara et al, 2003).…”