Université de Bordeaux
BrainConf: Synaptic Plasticity27-30 September, 2022 - Bordeaux

Selected talks

Simon Chen
Functionally distinct NPAS4-expressing somatostatin interneuron ensembles critical for motor learning

Elva Diaz
Identification of endocytic signals in the proteins of the SynDIG/PRRT family responsible for AMPAR trafficking

Wouter Droogers
Mechanisms of nanoscale reorganization during synaptic potentiation

Serena Dudek
Mechanisms of mGluR-dependent plasticity in hippocampal area CA2

Julien Dupuis
Ketamine enhances NMDAR synaptic trapping and alleviates molecular and behavioral deficits elicited by anti-NMDAR encephalitis patient antibodies

Phlipppe Isope
Functional Diversity of Glutamate Release at Individual Granule Cell Terminals in the Cerebellar Cortex

Doyeon Kim
Visualizing potentiated synapses in vivo with pulse-chase HaloTag labeling of AMPA receptors

Mathieu Letellier
miR-124-dependent tagging of synapses by synaptopodin enables input-specific homeostatic plasticity

Agata Nowacka
The role of AMPA receptor surface mobility in high-frequency short-term synaptic plasticity

Antonio Rodriguez-Moreno
Astrocytes and adenosine control critical periods of plasticity

Courteney Westlake
Modulation of NMDA receptor synthesis and surface expression by sAPPα


Functionally Distinct NPAS4-Expressing Somatostatin Interneuron Ensembles Critical for Motor Skill Learning

Yang, J.1*, Serrano, P.2*, Yin, X.3*, Sun, X.4, Lin, Y.5, Chen, S.X6

1. Jungwoo Yang*, University of Ottawa, jyan7@uottawa.ca
2. Pablo Serrano*, University of Ottawa, pserr016@uottawa.ca
3. Xuming Yin*, University of Ottawa, yinxuming2009@gmail.com
4. Xiaochen Sun, SUNY Upstate Medical University, SunX@upstate.edu
5. Yingxi Lin, SUNY Upstate Medical University, linyi@upstate.edu
6. Simon X. Chen#, University of Ottawa,

* co-first authors

# corresponding author, presenter

Local GABAergic inhibitory neurons are known to play an essential role in memory formation and allocation by modulating level of inhibition to downstream excitatory neuronal ensembles. During motor learning, dendritic spines on pyramidal neurons (PN) undergo reorganization in motor cortex (M1), which coincides with subtype-specific axonal bouton changes in somatostatin-expressing inhibitory neurons (SOM-IN) and parvalbumin-expressing inhibitory neurons (PV-IN). Moreover, SOM-IN-mediated inhibition has been shown to regulate spine reorganization on PN. However, the molecular mechanisms that underlie changes in inhibition, and whether the changes arise from all SOM-IN remain unclear. Here, we identified that NPAS4, transcription factor, is selectively expressed in SOM-IN, but not in PV-IN or PN, during motor learning in M1. Combining in vivo two-photon imaging with a head-fixed pellet reaching task, we found that activity was reduced among NPAS4-expressing SOM-IN during task-related movements compared to non-NPAS4-expressing SOM-IN. Region- and cell-type specific deletion of Npas4 within SOM-IN in M1 disrupted the spine elimination process and impaired motor skill acquisition. Chemogenetic activation of NPAS4-expressing ensembles was sufficient to impair motor learning and alter the spine elimination process. Together, our results reveal an instructive role of NPAS4 within the microcircuits, in which it modulates the inhibition of a distinct subset of SOM-IN during motor learning to promote spine stabilization of downstream PN that are important for motor skill acquisition.

Identification of endocytic signals in the proteins of the SynDIG/PRRT family responsible for AMPAR trafficking

David Speca, Chun-Wei He*, Christina Meyer*, Erin Scott*, Ricardo Cantua Pina, Elva Díaz (*equal contributions)

Department of Pharmacology, University of California, Davis

The transmembrane protein Synapse Differentiation Induced Gene 4 (SD4), also known as proline-rich transmembrane protein 1 (PRRT1), has recently been identified as an auxiliary factor of the AMPA-type glutamate receptor (AMPAR) necessary for maintaining extra-synaptic pools of GluA1, a subunit of AMPARs, required for synaptic plasticity. However, how SD4 establishes and maintains these pools is unclear. Previous studies suggested that endocytic machinery is important for maintaining a pool of mobile surface AMPARs, and that proteins associated with such cellular machinery are critical for proper trafficking and internalization. Additionally, SD4 co-localizes with GluA1 and resides in early and recycling endosomes. Therefore, identifying the sorting signal targeting SD4 to these organelles is essential to elucidate the role of SD4 in GluA1 trafficking. In this study, we report that SD4 possesses a YxxΦ sorting motif responsible for binding to the AP-2 complex cargo-sorting subunit μ2. This motif appears critical for proper SD4 internalization as mutation of this motif abolishes binding to μ2 and induces aberrant SD4 accumulation at the plasma-membrane of heterologous cells and primary rat hippocampal neurons. Intriguingly, surface accumulation of AMPARs at synapses is impaired when SD4 endocytic trafficking is blocked. Furthermore, we found that similar motifs exist in other proteins of the SynDIG/PRRT family. SynDIG/PRRT proteins belong to a larger superfamily that includes the interferon-induced transmembrane proteins (IFITMs). IFITMs are anti-viral proteins that inhibit membrane fusion of viral particles, including SARS-CoV-2, within endosomes of infected cells. Therefore, SD4 might establish reserve pools of AMPARs in endosomes by regulating delivery to the plasma membrane. In conclusion, we identify a sorting signal in SD4 important for understanding the SD4-dependent regulatory mechanism of GluA1 trafficking.  


Mechanisms of nanoscale reorganization during synaptic potentiation

W.J. Droogers, A.K. Serweta, A.A. Moerkerken, O.K. Klock, F.M. Berger, H.D. MacGillavry

Cell Biology, Neurobiology and Biophysics, Dept of Biology, Faculty of Science, Utrecht University, The Netherlands

Strengthening of excitatory synapses is pivotal for learning and memory. Long-term potentiation (LTP) of synaptic strength is broadly held to be mediated by an increase in the number of AMPA-type glutamate receptors (AMPARs). Nevertheless, super-resolution microscopy studies indicate that AMPARs concentrate in transsynaptic nanocolumns, aligned with presynaptic release sites. Redistribution of postsynaptic scaffolding proteins and AMPARs is thus predicted to underlie LTP. Therefore, unraveling the mechanisms that control AMPAR positioning and clustering within the postsynaptic density (PSD) is critical for fully understanding synaptic plasticity. Here, we used CRISPR-Cas9 genome editing and single-molecule localization microscopy to detect the nanoscale reorganization of endogenous scaffolding proteins. We observed an increase in the number and density of subsynaptic nanodomains marked by GKAP and PSD95, which resulted in higher postsynaptic currents in Monte Carlo simulations. Remarkably, knockdown of Shank proteins abolished the reorganization of PSD95 nanodomains, which was rescued by Shank3 re-expression. In addition, nanoscale reorganization of the PSD was influenced by actin dynamics. Lastly, we utilized Ten-fold Robust Expansion microscopy (TREx) to reveal the 3D transsynaptic nanocolumn organization of PSD95-GluA1 and presynaptic RIM1 during LTP. In summary, scaffolding proteins reorganize during synaptic potentiation, thereby concentrating AMPARs inside highly dense subsynaptic nanodomains.

Mechanisms of mGluR-dependent plasticity in hippocampal area CA2

Serena M. Dudek 1, Mahsa Samadi1, 2, 5, Daniel Lustberg3, Shannon Farris4, and Zafar I. Bashir5

1 National Institute of Environmental Health Sciences
2 Imperial College London
3 Emory University School of Medicine
4 Fralin Biomedical Research Institute, Virginia Tech.
5 University of Bristol

Pyramidal cells in hippocampal area CA2 have synaptic properties that are distinct from the other CA subregions. Notably, this includes a lack of ability to express typical long-term potentiation of stratum radiatum synapses (Zhao, 2007). CA2 neurons express high levels of several known and potential regulators of metabotropic glutamate receptor (mGluR)-dependent signaling including Striatal-Enriched Tyrosine Phosphatase (STEP) and several Regulator of G-protein Signaling (RGS) proteins (Boulanger,1995; Lee, 2010), yet what is known about mGluR-dependent synaptic plasticity in CA2 is limited to that influenced by Group III mGluRs (Dasgupta, et al., 2020). Thus, the aim of this study was to examine the effects of an mGluR I agonist (DHPG; Palmer, et al., 1997) on synaptic transmission in CA2 and to determine whether STEP and the RGS proteins RGS4 and RGS14 are involved in any plasticity induced there. Using whole cell voltage-clamp recordings from pyramidal cells in slices from male and female mice, we found that mGluR agonist-induced long-term depression (mGluR-LTD) is greater in CA2 compared with that in CA1 and CA3, congruent with ACPD-stimulated PI turnover reported decades ago (Hwang, et al. 1990). This mGluR-LTD in CA2 was mechanistically similar to that induced in CA1 in that it was protein synthesis and STEP dependent, suggesting that CA2 mGluR-LTD shares processes with those seen in CA1 (Huber, et al., 2000; Zhang, et al., 2008). In addition, however, RGS14, but not RGS4, was essential for mGluR-LTD in CA2. The lack of mGluR-LTD in CA2 of RGS14 KO mice was likely due to regulation of STEP protein, which was decreased in RGS14 KO tissue. We found that inclusion of active, but not inactive STEP in the patch pipette could rescue mGluR-LTD in RGS14 KO slices. These results highlight possible roles for mGluRs, RGS14, and STEP in synaptic plasticity in CA2, perhaps biasing the dominant form of plasticity away from LTP and toward LTD.

Boulanger, et al., 1995 J Neurosci 15:1532.

Dasgupta, et al., 2020 eLife 9:e55344.

Palmer, et al., 1997 Neuropharmacology 36:1517.

Huber, Kayser, & Bear, 2000 Science 288:1254.

Hwang, Bredt, & Snyder, 1990. Science 249:802.

Lee, et al., 2010. PNAS 107:16994.

Zhang Y, et al., 2008 J Neurosci 28:10561.

Zhao, Choi, Obrietan, & Dudek, 2007. J Neurosci 27:12025.

Ketamine enhances NMDAR synaptic trapping and alleviates molecular and behavioral deficits elicited by anti-NMDAR encephalitis patient antibodies

Frédéric Villéga1,2,#, Alexandra Fernandes1,2,#, Julie Jézéquel1,2,#, Floriane Uyttersprot1,2, Nathan Benac1,2, Sarra Zenagui1,2, Laurine Bastardo1,2, Delphine Bouchet1,2, Véronique Rogemond3,4,5, Jérôme Honnorat3,4,5, Julien P. Dupuis1,2,£, Laurent Groc1,2,£

1University of Bordeaux, Interdisciplinary Institute for Neuroscience, UMR 5297, 33077 Bordeaux, France. 2CNRS, IINS UMR 5297, 33077 Bordeaux, France. 3Institut NeuroMyoGene INSERM U1217/CNRS, UMR 5310, Lyon 69007, France. 4Hospices Civils de Lyon, Hôpital Neurologique, 69677 Bron, France. 5Université de Lyon-Université Claude Bernard Lyon 1, 69008 Lyon, France. #Equal contribution. £Shared seniority.

Over the past decade, the spatiotemporal regulation of N-methyl-D-aspartate glutamate receptor (NMDAR) organization at synapses emerged as a critical feature controlling excitatory synaptic transmissions and cognitive functions. Moreover, several studies reported alterations of NMDAR synaptic stability and organization in experimental models of neuropsychiatric conditions. Yet, whether acting on receptor stability and organization at synapses has therapeutic potential for the treatment of brain disorders remains an open question. Using a combination of single molecule and FLIM-FRET imaging, we report that ketamine - a dissociative anesthetic targeting NMDAR - promotes the cytosolic interaction of NMDAR with PDZ domain-containing scaffolding proteins and enhances the trapping of receptors at synapses. We further show that enhanced NMDAR synaptic trapping upon ketamine binding compensates for alterations in synaptic anchoring and mitigates depletion in receptors triggered by autoantibodies from patients with anti-NMDAR encephalitis, a severe brain condition in which immunoglobulins directed at NMDAR cause signaling impairments resulting in neurological and psychiatric symptoms. Thereby, ketamine attenuates impairments in NMDAR-associated Ca2+/calmodulin-dependent protein kinase II (CaMKII) signaling and alleviates subsequent anxiety- and sensorimotor gating-related behavioral deficits caused by autoantibodies. Altogether, these findings highlight an unexpected action of ketamine on the physiology of NMDAR and advocate for further investigations exploring the potency of targeting receptor synaptic anchoring in the treatment of brain disorders.



Functional Diversity of Glutamate Release at Individual Granule Cell Terminals in the Cerebellar Cortex

Théo Rossi, Bernard Poulain, Frédéric Doussau and Philippe Isope

Institut of Cellular and Integrative Neuroscience, CNRS UPR 3212, University of Strasbourg, France

The cerebellar cortex is organized in functional modules that process sensorimotor information and coordinate movements. Information is conveyed to the cerebellar cortex by mossy fibers (MFs) that target granule cells (GCs), the most numerous cell type in the brain. The long T-shaped axon of GCs makes parallel fibers (PFs) that contact Purkinje cells (PCs), the sole output of the cerebellar cortex. PFs also contact molecular layer interneurons (MLIs) providing feed-forward inhibition onto PCs. The MF-GC-MLI-PC pathway enables communication between cerebellar modules by selecting specific subsets of GC-PC synapses while silencing others (Spaeth et al., 2022, Valera et al., 2016). In this pathway, bursts of activity trigger several forms of short-term synaptic plasticity (STP) that dynamically and differentially shape synaptic strengths in the millisecond timescale and the PC discharge (Grangeray-Vilmint et al., 2018).

While heterogeneous STP properties have been observed at GC-MLI synapses (Dorgans et al., 2019), STP organization at PF synaptic boutons along a single PF is still unknown. We therefore performed ex-vivo patch-clamp recordings and two-photon imaging of a genetically-encoded fluorescent glutamate reporter (iGluSnFR-S72A) expressed in GCs to monitor STP at single terminals. Using high-frequency electrical stimulations of GCs, we demonstrate that nearly all PF boutons release glutamate suggesting that synapses are postsynaptically silent. STP profiles are heterogeneous at terminals on a single PF, and we identified four classes of GC terminals. Using transgenic mice expressing tdTomato spcifically in PCs, we compared PF-MLI vs and PF-PC contact, and showed that STP profiles are target independent. Under high [Ca2+]e condition, STP profiles vanished, all terminals displayed similar STP, still facilitating. Lastly, forskolin, which activates adenylate cyclase, yield complex effects, increasing or decreasing release at identified terminals, suggesting heterogeneous presynaptic molecular signalling pathways between terminals.

Altogether, our results suggest that vesicular release at GCs terminals is highly dynamic and versatile allowing a fine and temporal tuning of PC excitatory and inhibitory synaptic integration without any change in postsynaptic weight. These properties may enable complex and precise temporal information processing at the millisecond timescale in the cerebellar cortex.

Spaeth L, Bahuguna J, Gagneux T, Dorgans K, Sugihara I, Poulain B, Battaglia D, Isope P. Cerebellar connectivity maps embody individual adaptive behavior in mice. Nat Commun. 2022 Jan 31;13(1):580.

Dorgans K, Demais V, Bailly Y, Poulain B, Isope P, Doussau F. Short-term plasticity at cerebellar granule cell to molecular layer interneuron synapses expands information processing. Elife. 2019 May 13;8:e41586.

Grangeray-Vilmint A, Valera AM, Kumar A, Isope P. Short-Term Plasticity Combines with Excitation-Inhibition Balance to Expand Cerebellar Purkinje Cell Dynamic Range. J Neurosci. 2018 May 30;38(22):5153-5167.

Valera AM, Binda F, Pawlowski SA, Dupont JL, Casella JF, Rothstein JD, Poulain B, Isope P. Stereotyped spatial patterns of functional synaptic connectivity in the cerebellar cortex. Elife. 2016 Mar 16;5:e09862.

Visualizing potentiated synapses in vivo with pulse-chase HaloTag labeling of AMPA receptor

Doyeon Kim, Xiuyuan Li, Pojeong Park, He Tian, J.D. Wong-Campos, Adam E. Cohen

Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States

Memories are stored, at least in part, in the strength of synaptic connections. The α-amino-3- hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) plays a major role in determining the strength of these connections, and changes in the number of AMPARs at the post synaptic density are implicated in memory formation (Lynch, 2004; Takeuchi et al., 2014). In vivo 2-photon microscopy has been used to observe modulation of surface AMPARs levels at individual synapses during memory formation (Zhang et al., 2015; Tan et al., 2020; Roth et al., 2020; Graves et al., 2021). However, the highly scattering property of the brain has limited the observable area to superficial brain regions, and it is technically challenging to cover large brain volumes with real-time imaging. Here we developed an approach to tag strengthened synapses by sequential pulse-chase labeling of surface AMPARs with different colors of membrane-impermeable fluorescent dyes. Subsequent ex vivo multi-color imaging revealed maps of AMPAR exocytosis with single-synapse resolution across large volumes of brain tissue and in multiple brain regions. 

To label newly exposed AMPARs , we fused the AMPAR subunit GluA1 with a HaloTag, a self-labeling protein domain that can be covalently modified with an exogenous ligand (HaloTag-ligand) (England et al., 2015). After expressing HaloTag-fused AMPAR in vivo, we saturated surface-exposed AMPARs with a membrane-impermeable HaloTag-ligand dye. A second dye of a different color was then added to label newly exposed AMPARs. We investigated the spatial distribution of potentiated synapses upon the formation of perceptual memories in mouse barrel cortex layer 2/3 and fear memories in hippocampal CA1. We also mapped the background turnover rates of surface AMPARs in vivo and observed substantial cell-to-cell and dendritic branch-to-branch variation in AMPAR turnover rates. Multi-color labeling of AMPARs can be a powerful tool to study the spatiotemporal dynamics of AMPARs in the brain. We expect that this method will give us deeper understanding on the nature of memory formation and storage.

miR-124-dependent tagging of synapses by synaptopodin enables input-specific homeostatic synaptic plasticity

Sandra Dubes1, Anaïs Soula1, Sébastien Benquet1, Béatrice Tessier1, Christel Poujol2, Alexandre Favereaux1, Olivier Thoumine1*, Mathieu Letellier1*#

1. Univ. Bordeaux, CNRS, Interdisciplinary Institute for Neuroscience, IINS, UMR 5297, F-33000 Bordeaux, France
2. Univ. Bordeaux, CNRS, INSERM, Bordeaux Imaging Center, BIC, UMS 3420, US 4, F-33000 Bordeaux, France
#lead contact: mathieu.letellier@u-bordeaux.fr

Homeostatic synaptic plasticity is a process by which neurons adjust their synaptic strength to compensate for perturbations in neuronal activity. Whether the highly diverse synapses on a neuron respond uniformly to the same perturbation remains unclear. Moreover, the molecular determinants that underlie synapse-specific homeostatic synaptic plasticity are unknown. Here, we report a synaptic tagging mechanism in which the ability of individual synapses to increase their strength in response to activity-deprivation depends on the local expression of the spine-apparatus protein synaptopodin under the regulation of miR-124. By using genetic manipulations to alter synaptopodin expression or regulation by miR-124 in hippocampal neurons, we show that synaptopodin behaves as a "postsynaptic tag" whose translation is derepressed in a subpopulation of synapses and allows for non-uniform homeostatic strengthening and synaptic AMPAR stabilization. By genetically silencing individual connections in pairs of neurons, we demonstrate that this process operates in an input-specific manner. Overall, our study shifts the current view that homeostatic synaptic plasticity affects all synapses uniformly to a more complex paradigm where the ability of individual synapses to undergo homeostatic changes depends on their own functional and biochemical state.


The role of AMPA receptor surface mobility in high-frequency short-term synaptic plasticity

Nowacka A, Getz A, Breillat, Daburon S, Bessa-Neto, Lemoigne C, Sainlos M, Choquet D

Univ. Bordeaux, CNRS, IINS, UMR 5297, F-33000 Bordeaux, France

Activity-dependent plasticity of synaptic transmission is a key mechanism underlying learning and memory. During high-frequency short-term synaptic plasticity (HF-STP) the amplitude of synaptic responses changes upon presynaptic stimulation on a timescale of seconds. HF-STP is important for information processing in the brain, serving particularly for temporal integration. Yet, the precise functions of STP, and its impact on information processing, remain unknown. It is widely accepted that STP is regulated primarily by presynaptic mechanisms. However, postsynaptic mechanisms have been shown to regulate HF-STP, although their role here remains to be fully understood. Previously we demonstrated that AMPA receptors (AMPARs), the main excitatory receptors in the brain, are trafficked in and out of synapses by surface diffusion that complements endo- and exocytosis. Here, we study the functional role of AMPAR lateral mobility in HF-STP in integrated slice tissue models with intact synaptic connectivity. We use the AP-GluA2 knock-in (KI) mouse model, developed in the lab, where GluA2 subunits of AMPARs are tagged with a 15 amino acid biotinylation acceptor peptide (AP-tag) and can be specifically biotinylated when co-expressed with an endoplasmic reticulum resident biotin ligase (BirAER), and immobilized on the cell surface with a biotin-binding protein NeutrAvidin. With this, we demonstrate that immobilization of endogenous AMPARs modulates STP by increasing synaptic depression in the Schaffer collateral-CA1 synapse of organotypic hippocampal slices. This effect is reversed when desensitization blockers of AMPAR are applied, suggesting that the modulation of HF-STP is achieved by preventing the replacement of desensitized AMPARs in the synapse. Moreover, with iGluSnFr imaging, we find no change in presynaptic glutamate release upon AMPAR immobilization. Altogether this strongly suggests a postsynaptic contribution of AMPAR mobility in regulating HF-STP. We aim to determine exhaustively the respective contributions of presynaptic transmitter release and postsynaptic AMPAR biophysics and mobility in HF-STP and to identify physiological processes which act upon AMPAR kinetics and mobility to regulate HF-STP in the brain. Moreover, AMPAR cross-link is a promising tool to achieve cell-specific blockade of STP that may allow the transition from modeling-based evidence of HF-STP roles in brain function to experimental evidence.

Astrocytes and adenosine control critical periods of plasticity

Mikel Pérez-Rodríguez, Irene Martínez-Gallego, José Prius Mengual, Rafael Falcón-Moya, and Antonio Rodríguez-Moreno

Laboratorio de Neurociencia Celular y Plasticidad. Universidad Pablo de Olavide. Sevilla, Spain.

During development, critical periods of plasticity facilitate the reordering and refining of neural connections, allowing correct adult physiology to be established. The L4-L2/3 synapses in the somatosensory cortex and the CA3-CA1 synapses of the hippocampus exhibit a presynaptic form plasticity (long-term depression -LTD) that probably fulfils a role in synaptic refinement. This form of plasticity is present until the 3rd-4rd postnatal week in mice, disappearing thereafter. The mechanisms that are responsible for this loss of plasticity are not clear. We describe here these mechanisms and those involved in the switch from LTD to LTP observed as the brain matures. Defining these events responsible for closing (and opening) plasticity windows may be important for brain repair, sensorial recovery, the treatment of neurodevelopmental disorders and for educational policy. When we investigated the mechanisms underlying this maturation-related loss of t-LTD in either sex mouse slices, we found that it could be completely recovered by antagonizing adenosine type 1 receptors (A1R). By contrast, an agonist of A1R impeded the induction of t-LTD in juvenile mice. Furthermore, we found that the adenosine that mediated the loss of t-LTD at the end of the 3rd- 4th week of development is most probably supplied by astrocytes. At more mature stages, we found that the protocol used to induce t-LTD provokes t-LTP. We characterized the mechanisms underlying the induction of this form of LTP and we found it to be expressed presynaptically, as witnessed by paired-pulse and coefficient of variation analysis. In addition, this form of presynaptic t-LTP requires the activation of NMDARs) in the cortex but not in the hippocampus) and mGlu1Rs, and the entry of Ca2+ into the postsynaptic neuron through L-type voltage-dependent Ca2+ channels, as are the adenosine and glutamate that are released in association with astrocyte signaling. These results provide direct evidence of the mechanisms that close the window of plasticity associated with t-LTD and that drive the switch in synaptic transmission from t-LTD to t-LTP at L4-L2/3 and CA3-CA1 synapses, in which astrocytes play a central role.

Modulation of NMDA receptor synthesis and surface expression by sAPPα.

Westlake C.M.1,2, Spoelstra, H.E.1,2, Livingstone, R.W.1,2, Williams, J.M.1,2

1Department of Anatomy,
2Brain Health Research Centre, University of Otago, Dunedin, New Zealand

Secreted amyloid precursor protein-alpha (sAPPα), a neuromodulator derived from the amyloid precursor protein, has demonstrated neuroprotective and memory enhancing qualities, including rescue of impaired synaptic plasticity in rodent models of Alzheimer’s disease (AD). In order  to understand the molecular mechanisms underpinning these positive effects, our previous work has demonstrated that sAPPα acts via enhancing protein synthesis, including synthesis of amino-3-hydroxy-5-methyl4-isoxazole-propionic acid receptor (AMPAR) subunit GluA1, which are rapidly trafficked to extrasynaptic membranes1. Alongside this we have shown that sAPPα promotes rapid insertion of GluN2B-containing N-methyl-d-aspartate receptors (NMDARs)2 to extrasynaptic membranes. As activation of NMDARs results in Ca2+ influx that is pivotal to synaptic plasticity and aberrant NMDAR function has been implicated in neurodegenerative diseases, including AD, we hypothesized that sAPPα fundamentally alters the complement of peri-synaptic glutamate receptors which may become available to participate in synaptic transmission. To advance these studies we have explored whether sAPPα-induced enhancement of GluN1 persists at the neuronal surface, and whether sAPPα promotes de novo synthesis of the plasticity-associated NMDAR subunit, GluN2B.

Utilizing fluorescent non-canonical amino acid tagging with proximity ligase assay (FUNCAT-PLA), we found that treatment of primary hippocampal cultures with sAPPα (1 nM) induced an increase in dendritic de novo GluN2B expression at 30 min (1.41 ­­± 0.12, mean ± SEM; p <0.005) and 60 min (2.09 ± 0.25; p <0.005) before returning to baseline by 120 min (1.15 ­­± 0.10; p = 0.2). Conversely, in the soma we found a more delayed increase in de novo GluN2B at 60 min (2.81 ­­± 0.49; p <0.001) that was sustained for at least 120 min (1.73 ± 0.32; p <0.05). Curiously, immunocytochemistry experiments examining the expression of surface GluN1, revealed that at 120 min, sAPPαsignificantly decreased both somatic (0.56 ­­± 0.04; p <0.0001) and dendritic (0.78 ­­± 0.04; p <0.0001) surface expression.

Together these results suggest that sAPPα may exert its neuroprotective effects by withdrawing NMDARs from the surface, potentially preventing Ca2+-induced excitotoxicity. This, alongside enhancement of GluN2B synthesis could be a mechanism through which sAPPα balances both plasticity and neuroprotection. Ongoing work aims to elucidate sAPPα-induced changes in the surface expression of GluN2A and 2B subunits and further examine the expression characteristics of these de novo NMDAR.

1 Livingstone, R.W. et al. Secreted Amyloid Precursor Protein-Alpha Enhances LTP Through the Synthesis and Trafficking of Ca2+-Permeable AMPA Receptors. Front. Mol. Neurosci. 14, 1–20 (2021).

2 Mockett, B.G. et al. Glutamate receptor trafficking and protein synthesis mediate the facilitation of LTP by secreted amyloid precursor protein-alpha. J. Neurosci. 39, 3188–3203 (2019).