Dumping toxic Aβ oligomers onto cultured neurons causes a reduction in synapse number. The assumption, long held but never fully proven, was that Aβ precipitates the destruction of synapses. Now, a study from Zachary Wills’ lab at the University of Pittsburgh proffers a different idea, namely that Aβ oligomers prevent new synapses from forming. In the October 11 Neuron, Wills and colleagues describe how Aβ binds to the Nogo receptor (NgR), a key inhibitor of synapse formation during development. In Wills’ experiments, the Aβ-NgR union kicks off a signaling pathway that shuts down T-type calcium channels, leading to persistent dysregulation of calcium in dendrites. Consequently, new dendritic spines no longer assemble, even in adult mouse brain. “NgR functions as a plasticity restrictor, and our data suggest that Aβ affects plasticity by tapping into NgR signaling,” Wills said.
- Toxic Aβ oligomers ablate synapses on hippocampal neurons.
- This may partly stem from inhibition of new synapses.
- Aβ activates signaling through Nogo receptors that suppress synaptogenesis.
The study adds to a long list of proposed Aβ receptors. Alas, none of them have drawn robust converging evidence thus far, leaving the field with little consensus on which, if any, might be relevant to Aβ species in the Alzheimer’s brain. Commenters raised questions about how relevant the Nogo receptor pathway might be in old mice or people, and whether synthetic Aβ oligomers as used here are a reasonable proxy for amyloid species present in brain.
Nonetheless, because learning and memory depend on new synapse formation, the findings could help explain why Alzheimer's disease takes an early toll on the ability to make new memories. Supporting this idea, injecting Aβ oligomers into the hippocampus inhibited learning and memory in wild-type mice but not NgR knockouts, report the authors.
The idea that Aβ might put the kibosh on synaptogenesis is not new. Recently, Jochen Herms of Ludwig-Maximilians-Universität Munich and colleagues reported that assembly of new dendritic spines, which bear new synapses, stalls in two different AD mouse models (Zou et al, 2015). However, no one had looked for a mechanism by which Aβ might interfere with spine formation.
To do that, first author Yanjun Zhao imaged spine dynamics in hippocampal slice cultures prepared from postnatal day six rat pups. He exposed the slices to Aβ-derived diffusible ligands (ADDLs), prepared from purified synthetic Aβ (Nicoll et al., 2013). In control cultures, spine density doubled in 96 hours, but cultures exposed to Aβ saw no increase. When he tracked individual spines, Zhao found Aβ did not affect spine elimination compared to control cultures. Instead, in the Aβ-treated cultures fewer new spines appeared during the imaging period.
Wills suspected a role for Nogo receptors, a family of cell surface proteins that bind myelin-associated glycoproteins. NgRs also bind APP and Aβ peptides, and previously have been implicated in APP processing, amyloid deposition, and learning deficits in AD mouse models (Zhou et al., 2011; Park et al, 2006; Park et al., 2006). Because Wills previously reported that NgR1 signaling inhibited the assembly of new synapses in hippocampal neurons, the researchers asked if NgR1 mediated Aβ’s actions on spine formation (Wills et al., 2012). After determining that Aβ oligomers bound to NgR1 on cells, Zhao knocked down NgR1 expression in the hippocampal neurons from the P6 rat pups. This drove up spine density, unperturbed by Aβ. In addition, while Aβ inhibited synaptic long-term potentiation in hippocampal slices from six- to nine-month-old wild-type mice, it had no such effect on slices from NgR1 knockouts.
Looking more closely into the signaling changes evoked by Aβ, Zhao found that oligomers elicited activation of Rho GTPase, and a reduction of dendritic calcium levels that were dependent on NgR1. In experiments with knockouts and pharmacological inhibitors, the researchers defined a pathway whereby Aβ binding to NgR1 on dendrites led to activation of the Rho GTPase, which in turn unleased a Rho-dependent kinase (ROCK)-mediated phosphorylation of the T-type calcium channel, CaV3.1. Phosphorylation inactivated the channel, lowered baseline calcium, and reduced the calcium transients required for dendritic spine assembly and synaptic plasticity. The researchers saw the same cascade when they used Aβ oligomers derived from 7PA2 cells, which secrete a variety of Aβ species, including oligomers. The results suggest that Aβ peptides may co-opt a developmental function of the NgR, acting as receptor agonists and suppressing spine formation.
Because learning and memory require new spines, Wills wanted to know if inhibiting spine assembly affected behavior in an animal model. They injected Aβ oligomers directly into brains of young wild-type or NgR knockout mice, and measured the animals’ performance on a novel object-recognition task. As in the slice studies, Aβ decreased spine density and calcium currents in wild-type mice and hampered their ability to recognize the novel object. NgR knockouts were unfazed.
Other researchers questioned the relevance to AD of pathways found in young mice. “The perinatal and developmentally young neurons harvested for much of these studies express a distinct population of channels and signaling systems uniquely designed for structural plasticity, harboring the ability to extend, retract, and respond to cues that aren’t fully replicated in adult, or even aged, neuronal populations. It is not yet clear from this study how much of the developmental machinery present only in the perinatal stages contribute to some of the mechanisms proposed for synapse loss in AD,” wrote Grace Stutzmann from the Rosalind Franklin University in North Chicago (see full comment below).
Yet synapse formation associated with learning does occur in adulthood. Robert Sweet, who is also at the University of Pittsburgh but was not involved in the new study, said he hopes the work will call attention to disruption of spine assembly as a potential mechanism for synapse loss and learning deficits in AD. “People have focused on loss of existing spines that occur with neuronal death. This study suggests a more subtle process by which Aβ could impair learning and memory by subtly accelerating the reduced formation of new synapses that occurs with normal aging,” he said.
It also remains to be seen if the NgR pathway triggered by synthetic Aβ or cell-derived Aβ plays a role in AD, where toxic species of Aβ may be different. Even with soluble Aβ isolated from brain, the identity of the toxic species remains a matter of debate (Jan 2017 news). Wills said he’s planning to examine if this mechanism is at play in people with AD, using phosphorylation of Cav3.1 as a signature for NgR-mediated effects. “We’ve developed phospho-specific antibodies for the 3.1 channel, and we hope to use mass spec to quantitate phosphorylation levels in postmortem tissue,” he said.—Pat McCaffrey
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No Available Further Reading
- Zhao Y, Sivaji S, Chiang MC, Ali H, Zukowski M, Ali S, Kennedy B, Sklyar A, Cheng A, Guo Z, Reed AK, Kodali R, Borowski J, Frost G, Beukema P, Wills ZP. Amyloid Beta Peptides Block New Synapse Assembly by Nogo Receptor-Mediated Inhibition of T-Type Calcium Channels. Neuron. 2017 Oct 11;96(2):355-372.e6. PubMed.