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.
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
Research Models Citations
- 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.
No Available Further Reading
- 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.