CPTX—A Synaptic “Glue” Restores Memory in Alzheimer’s Model

Neurexin proteins lie in the presynapse and pentraxins in the post, and never the twain shall meet. But what if they did? By designing a small protein that binds to both, researchers in Japan, Germany, and the U.K., have created an extracellular bridge that stabilizes synapses.

  • CPTX bridges presynaptic neurexins with postsynaptic AMPA receptors.
  • This induced excitatory synapses in hippocampal neurons.
  • Injected into 5xFAD mice, the chimera improved learning and memory.

When injected into the hippocampus of a mouse model of Alzheimer’s disease, this cerebellin-1/pentraxin-1 chimera increased synaptic density and improved neural plasticity and memory. It also restored function in mouse models of ataxia and spinal cord injury. “This approach may inspire the development of a variety of innovative molecular tools for basic neuroscience as well as the treatment of neurological disorders,” write the authors in the August 28 Science.

This was an international collaboration led by Alexander Dityatev, German Center for Neurodegenerative Disease, Magdeburg; Radu Aricescu, University of Oxford, England; and Michisuke Yuzaki, Keio University, Tokyo. Joint first authors Kunimichi Suzuki in Tokyo, Jonathan Elegheert in Oxford, Inseon Song in Magdeburg, and Hiroyuki Sasakura from Aichi Medical University, Japan, designed, synthesized, and tested the molecular bridge.

Three Tests. CPTX improved function in three models of neurodegeneration. [Image courtesy Suzuki et al., Science 2020.]

They combined the N-terminal cysteine-rich region of cerebellin 1 with the pentraxin domain of neuronal pentraxin 1 (NP1). Cerebellin 1 is an extrasynaptic scaffold protein that bridges presynaptic neurexin 4 in cerebellar granule cells with ionotropic glutamate receptor subunit GluD2 on the postsynapse of Purkinje cells. Other extrasynaptic scaffold proteins connect different pre- and postsynaptic proteins. Nuclear pentraxins, on the other hand, promote clustering of AMPA glutamate receptors on the postsynapse, but seem to have no influence on the presynapse. What if the scaffold and clustering properties were combined into one molecule?

Enter CPTX. Suzuki and colleagues connected the cysteine-rich repeat regions of cerebellin-1 with the pentraxin domains of NP1 via a small linker. The linker, a mutant of the yeast protein GCN4, naturally forms hexamers, and so did CPTX. Cerebellin 1 binds neurexins as a hexamer.

Synaptic Organizer. CPTX, a lab-made cerebellin-1/pentraxin-1 hybrid, induces and stabilizes synapses. It restores function in three different models of neurodegeneration. [Courtesy of Suzuki et al., Science 2020.]

First, the authors tested the hexameric CPTX in vitro. It bound neurexins containing a specific motif called splice sequence 4, just as cerebellin 1 does. In hippocampal neurons, it induced accumulation of presynaptic components, including synaptophysin and glutamate transporters. In hippocampal dendrites it coalesced AMPA receptor subunits GluA1-3. All told, this synaptic organizer seemed to do just what the scientists had predicted.

What about in vivo? The authors tested the molecule in mouse models of Alzheimer’s disease, ataxia, and spinal cord injury. Injected into the hippocampi of 11- to 12-month-old 5xFAD mice, which accumulate amyloid plaques in the brain, it increased co-localization of PSD95 and AMPA subunits, and restored spine density (see image below). Synapses strengthened, as judged by a boost in long-term potentiation of the Schaffer-collateral pathway in hippocampal slices.

More Spines. Neuronal spine deficits in 5xFAD mice (center) are restored (right) by injecting CPTX into the hippocampus. [Courtesy of Suzuki et al., Science 2020.]

Behaviorally, the mice improved as well. The AD mice better remembered the location of a food treat and the cage where they had received a foot shock when injected three days before with CPTX.

Mouse with ataxia due to knockout of cerebellar glutamate receptor GluD2 partially restored connections between granule and Purkinje cells and were better able to run after CPTX injection into the cerebellum. Finally, CPTX strengthened neuronal connections in semi-severed mouse spinal cords.

Super-resolution microscopy showed the chimera working as a go-between among presynaptic VgluT2 and postsynaptic GluA4, which are expressed by most excitatory motor neurons in the spinal cord.

The synaptic organizer improved locomotion as judged by the Basso mouse scale, when given one week after the injury (Basso et al., 2006). In contrast, chondroitinase ABC, which promotes nerve regeneration, was less effective if the researchers waited a week. CPTX also restored locomotion when the mice’s spines were injured by contusion, which is more typical of the spinal injuries people suffer. The researchers could not detect CPTX in the spinal cord or brain seven days after injection, though locomotion continued to improve for at least another seven weeks. Suzuki and colleagues think that endogenous synaptic organizers may take over to further stabilize connections.

Taisuke Tomita, University of Tokyo, noted that synaptic organizers have been studied mainly in the context of existing synapses. “The effectiveness of CPTX in AD and SCI models suggests that it may be possible to use synaptic organizers to induce synaptic recovery in neurodegenerative diseases as well,” he wrote to Alzforum. “This is a very elegant and profound paper that introduces a new therapeutic concept to the field of neurological diseases.”—Tom Fagan

Comments

  1. This is a very elegant and profound paper that introduces a new therapeutic concept to the field of neurological diseases. Based on previously known molecular neurobiological and structural information, the authors have successfully generated an artificial synaptic organizer molecule, CPTX, which restored synaptic and behavioral abnormalities in mouse models of Alzheimer’s disease (AD) and spinal cord injury (SCI).

    Since mutations in genes encoding synaptic organizer molecules have been identified in psychiatric disorders, where neuronal death is not evident, these molecules have been studied mainly for their relationship to the function of existing synapses. The effectiveness of CPTX in AD and SCI models suggests that it may be possible to use synaptic organizers to induce synaptic recovery in neurodegenerative diseases as well. Considering further applications in therapeutics, it will be important to examine the feasibility of such treatments for neurodegeneration caused not just by extracellular toxins such as amyloid-β, but by intracellular aggregates such as tau, α-synuclein, and TDP-43.

    The synaptic induction potency and stability of CPTX would be critical, as this synaptic organizer approach might not alter the formation/maintenance of such protein aggregates, which affect neuronal viability over long periods of time in people. Nevertheless, results of SCI model clearly indicate that CPTX is potent enough to induce synapses in a neurodegenerative condition. This study opens a new avenue for the treatment of neurological disorders.

    View all comments by Taisuke Tomita

  2. A synthetic synaptic organizer protein restores glutamatergic neuronal circuits

    This is a remarkable study that proposes the novel concept that a synthetic molecule that assembles pre- and postsynaptic regions in central synapses can promote the formation of active glutamatergic synapses and restore damaged neural circuits. Based on protein structural analysis, the researchers engineered a synaptic bridge protein (CPTX), consisting on key domains of extracellular synaptic proteins cerebellin-1 (Cbln1) and pentraxin-1 (NP1), that induces synapses in vitro and in vivo, and improves the function of excitatory synapses in mouse models of neurodegeneration. The investigation opens the possibility that molecules designed to potentiate synaptic connections could be useful as therapeutic drugs for synaptopathies, including neuropsychiatric and neurological disorders. The idea that therapeutics based on synapse recovery in neurological diseases is not novel, but this study represents certainly a proof of principle that potentiating synapse function could be useful to improve and/or recover memory and behavioral alterations in neurological and neurodegenerative diseases, including Alzheimer disease (AD) among others. In support of this idea, the authors demonstrate that injection of CPTX promotes synapse formation and functionality of neural circuits in AD (hippocampus), ataxia (cerebellum) and spinal cord injury (SCI) mouse models.

    The beneficial effects of CPTX are observed only few days after treatment, suggesting a rapid recovery of functional synapses in the damaged regions. However, many relevant questions arise. For instance, which are the long-term CPTX effects of potentiating the number and/or function of synapses, particularly excitatory synapses, in memory and motor neural circuits? Could CPTX treatment lead to overexcitation and/or excitotoxicity resulting in further neurodegeneration? This question is important because increased synaptic excitability and excitatory/inhibitory imbalance has been associated with changes of brain activity at early stages of AD (Saura et al., 2015; Styr and Slutsky, 2018). The authors argue that, at least in vivo, CPTX increases and potentiates specifically excitatory glutamatergic synapses. However, and according to the study, CPTX not only increases postsynaptic GluA1-3 glutamate receptors in hippocampal neurons but also GluA4 in parvalbumin interneurons, which suggests effects on both excitatory and inhibitory synapses. Considering that neurexins and their ligands neuroligins, and Cbln isoforms, regulate inhibitory synaptic activity and other neurotransmitter systems (Südhof, 2008), adverse drug effects cannot ruled out. Future studies are needed to reveal the possible collateral pre- and postsynaptic effects of CPTX on key synapse homeostasis and mechanisms, including synaptic signaling, plasticity, synapse-to-cell signaling, etc. … The stability and side effects of these synthetic molecules may dictate their real use and effectiveness in future therapeutics.

    An important challenge is to translate these interesting results to medical applications. Can these synthetic synaptic molecules, or similar extracellular scaffolding proteins, be effective for treating synapse dysfunction in brain disorders caused by different etiologies? Are they beneficial to restore damaged neural circuits in human neurodegenerative/injury (e.g., AD, SCI …) and developmental neuropsychiatric (e.g., autism, intellectual disability…) diseases? Could they improve brain function in non-pathological conditions? If this is the case, as suggested by the study, how do these treatments affect other classical pathological hallmarks of these neurological diseases, including neurodegeneration, neuroinflammation and accumulation of misfolded proteins? In order to be effective in the central nervous system, these synthetic "synapse bridge" molecules should cross the blood-brain barrier, target specific neural circuits and, ideally, modulate other classical disease hallmarks. Further developments may improve the design of future “synapse bridge” molecules, shedding light onto both the beneficial and adverse effects of potentiating glutamatergic neurotransmission in humans.

    References:

    Saura CA, Parra-Damas A, Enriquez-Barreto L. Gene expression parallels synaptic excitability and plasticity changes in Alzheimer's disease. Front Cell Neurosci. 2015;9:318. Epub 2015 Aug 25 PubMed.

    Styr B, Slutsky I. Imbalance between firing homeostasis and synaptic plasticity drives early-phase Alzheimer's disease. Nat Neurosci. 2018 Apr;21(4):463-473. Epub 2018 Feb 5 PubMed.

    Südhof TC. Neuroligins and neurexins link synaptic function to cognitive disease. Nature. 2008 Oct 16;455(7215):903-11. PubMed.

    View all comments by Carlos Saura View all comments by Arnaldo Parra-Damas

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References

Research Models Citations

  1. 5xFAD (C57BL6)

Paper Citations

  1. Basso DM, Fisher LC, Anderson AJ, Jakeman LB, McTigue DM, Popovich PG. Basso Mouse Scale for locomotion detects differences in recovery after spinal cord injury in five common mouse strains. J Neurotrauma. 2006 May;23(5):635-59. PubMed.

Further Reading

No Available Further Reading

Primary Papers

  1. Suzuki K, Elegheert J, Song I, Sasakura H, Senkov O, Matsuda K, Kakegawa W, Clayton AJ, Chang VT, Ferrer-Ferrer M, Miura E, Kaushik R, Ikeno M, Morioka Y, Takeuchi Y, Shimada T, Otsuka S, Stoyanov S, Watanabe M, Takeuchi K, Dityatev A, Aricescu AR, Yuzaki M. A synthetic synaptic organizer protein restores glutamatergic neuronal circuits. Science. 2020 Aug 28;369(6507) PubMed.

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