In mouse models of Alzheimer’s, tickets for retrograde transport to neuronal cell bodies may become scarce, leaving BACE1 stranded at synapses. So hints a paper in the March 8 Journal of Neuroscience. Researchers led by Qian Cai of Rutgers University in Piscataway, New Jersey, also reported that restoring retrograde endosomal trafficking cleared excess BACE1, and Aβ, out of synapses, prevented their loss, and restored learning and memory. The researchers proposed the endosomal gridlock plays a pivotal role in synaptic destruction and neurodegeneration in AD.
Swollen axons, aka dystrophic neurites, are a hallmark of AD. Researchers have reported that the bulging processes, frequently found in the vicinity of amyloid plaques, are chock-full of BACE1 and one of its products, Aβ, as well as endosomal vesicles (see Kandalepas et al., 2013; Oct 2016 conference news). Previous studies have described the slowing of retrograde transport of BACE1-laden endosomes out of axons, hinting that malfunctions in the pathway could explain the synaptic overcrowding (see Buggia-Prévot et al., 2014; Sadleir et al., 2016). Cai’s previous studies detailed how synaptic endosomes hitch a ride to the soma by latching onto snapin, an adaptor that hooks up to dynein motors (see Cai et al., 2010). More recently, Cai and colleagues reported that this snapin-dynein transport malfunctioned in neurons from AD mice in culture, leading to a buildup of BACE1 and Aβ in distal axons (see Ye and Cai, 2014).
For the current study, first author Xuan Ye and colleagues looked into the synaptic, cellular, and cognitive consequences of this transport hitch. Dovetailing with what Cai and other researchers had observed previously, the researchers spotted BACE1 in clusters congregating in dystrophic neurites in the hippocampus of hAPP-J20 mice, and determined that the enzyme co-localized with late endosomes crammed into presynaptic terminals. They observed elevated synaptic BACE1 in postmortem samples from AD patients as well.
To investigate the role of snapin-dynein retrograde transport of BACE1 in normal animals, the researchers next generated conditional knockout mice lacking snapin expression in neurons of the frontal cortex and hippocampus. Similar to hAPP mice, BACE1 co-localized with late endosomal and synaptic markers in these cKOs, and appeared to accumulate in presynaptic terminals of mossy fibers in the hippocampus. Electron microscopy revealed that BACE1 density shot up by sevenfold in the fibers of snapin-deficient mice compared to wild-type animals, and that nearly 90 percent of BACE1 resided in presynapses. Perhaps due to BACE1’s activity within these acidic organelles, mouse Aβ40 concentrations topped those in normal mice by more than 100 percent. The findings suggested that snapin-mediated retrograde transport of BACE1-laden endosomes played a key role in ridding synapses of the enzyme, along with its potentially damaging products.
The researchers hypothesized that snapin-mediated transport was defective in AD neurons, and that boosting snapin expression would restore it. Live imaging of hAPP neurons revealed that indeed, the flow of late endosomal BACE1 along axons moved predominantly in the anterograde direction: from the soma to the axon. In wild-type neurons, the traffic moved both ways. Overexpressing snapin in hAPP neurons partially restored retrograde transport, but did not affect anterograde. This boost of traffic out of the axon reduced the accumulation of endosomal BACE1 in presynaptic terminals.
Would snapin overexpression restore retrograde BACE1 trafficking in hAPP mice as well? To find out, the researchers injected adeno-associated viruses expressing snapin directly into the hippocampi of two- to three-month-old hAPP mice. Compared with animals injected with a control virus, these mice had more BACE1 in the soma of hippocampal neurons, and less in the mossy fiber axons, suggesting that transport from the axons was improved. Strikingly, snapin overexpression staved off synapse loss as well, restoring to wild-type levels the density of presynaptic terminals adorning the mossy fibers.
Snapin overexpression also reduced Aβ40 by more than 60 percent in preparations of hippocampal synaptosomes. Staining hippocampal slices with the A11 antibody, which recognizes soluble Aβ oligomers, revealed that snapin overexpression cut intracellular oligomers by more than half. Snapin’s benefits extended into the extracellular space as well, halving plaque area in the hippocampus, as measured by immunostaining with the 6E10 antibody.
Finally, the researchers asked whether snapin overexpression would stave off cognitive deficits in hAPP mice. By injecting AAV with the dynein adaptor gene they restored hAPP animals’ preference for novel, as opposed to familiar, objects, indicating that snapin corrected nonspatial learning and memory deficits. Judging by their ability to learn the location of a hidden platform, and their freezing behavior when placed into a cage where they had previously received a foot shock, the treated mice also performed at or near wild-type levels on spatial and contextual memory tests. Notably, snapin overexpression in wild-type mice had no effects on the animals’ cognition.
Overall, Cai proposes that the buildup of Aβ in BACE1-loaded synapses contributes to their subsequent demise in hAPP mice, and also to learning and memory problems and neurodegeneration. What kick-starts this destructive cascade remains unclear, although Cai speculated that cytoplasmic, soluble Aβ oligomers are to blame because she had previously found that they interfered with the coupling of snapin adaptors to endosomes (see Tammineni et al., 2017). She believes that Aβ oligomers could arise inside the cell, or from outside. The subsequent trafficking defects would then lead to a buildup of endosomes in axons and eventually to dystrophic neurites, where BACE1 accumulates to produce more Aβ and drive the whole process in a destructive feed-forward loop. Given that overexpressing snapin reduced plaque load, Cai also proposed that the synaptic pool of Aβ contributed to plaques.
Claudia Almeida of NOVA University in Lisbon, Portugal, raised the possibility of a slightly different scenario, commenting that snapin’s role in preventing Aβ accumulation could be more closely tied to transport of APP, rather than BACE1, to lysosomes for degradation.
Jochen Herms of the German Center for Neurodegenerative Diseases in Munich said Cai’s findings were impressive but he prefers the idea that Aβ fibrils, rather than soluble oligomers, harm neurites from the outside first, and that this damage ultimately triggers transport problems within. The subsequent accumulation of endosomes in synapses would ultimately lead to the release of intracellular Aβ, which could further contribute to plaques, he said. Herms’s outside-in concept stems from his recent data suggesting that dystrophic neurites only appear after the detection of fibrillar Aβ deposits (see Blazquez-Llorca et al., 2017).
Cai pointed out that a failure of late endosomes to return to the soma, where they fuse with lysosomes, could also affect lysosomal degradation of proteins more broadly. In her other recent study, which revealed that Aβ oligomers interfere with dynein transport, Cai also reported that other digestive vesicles—autophagosomes—get stuck in synapses, leading to autophagic stress and an accumulation of damaged mitochondria in neurons from AD mice. She proposed that therapies aimed at restoring normal axonal trafficking, rather than solely blocking BACE1, have the potential to correct several cellular malfunctions in AD, and in other neurodegenerative diseases.—Jessica Shugart
Research Models Citations
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- Sadleir KR, Kandalepas PC, Buggia-Prévot V, Nicholson DA, Thinakaran G, Vassar R. Presynaptic dystrophic neurites surrounding amyloid plaques are sites of microtubule disruption, BACE1 elevation, and increased Aβ generation in Alzheimer's disease. Acta Neuropathol. 2016 Aug;132(2):235-56. Epub 2016 Mar 18 PubMed.
- Cai Q, Lu L, Tian JH, Zhu YB, Qiao H, Sheng ZH. Snapin-regulated late endosomal transport is critical for efficient autophagy-lysosomal function in neurons. Neuron. 2010 Oct 6;68(1):73-86. PubMed.
- Ye X, Cai Q. Snapin-mediated BACE1 retrograde transport is essential for its degradation in lysosomes and regulation of APP processing in neurons. Cell Rep. 2014 Jan 16;6(1):24-31. Epub 2013 Dec 27 PubMed.
- Tammineni P, Ye X, Feng T, Aikal D, Cai Q. Impaired retrograde transport of axonal autophagosomes contributes to autophagic stress in Alzheimer's disease neurons. Elife. 2017 Jan 13;6 PubMed.
- Blazquez-Llorca L, Valero-Freitag S, Rodrigues EF, Merchán-Pérez Á, Rodríguez JR, Dorostkar MM, DeFelipe J, Herms J. High plasticity of axonal pathology in Alzheimer's disease mouse models. Acta Neuropathol Commun. 2017 Feb 7;5(1):14. PubMed.
- Buggia-Prévot V, Thinakaran G. Significance of transcytosis in Alzheimer's disease: BACE1 takes the scenic route to axons. Bioessays. 2015 Aug;37(8):888-98. Epub 2015 Jun 30 PubMed.
- Ye X, Feng T, Tammineni P, Chang Q, Jeong YY, Margolis DJ, Cai H, Kusnecov A, Cai Q. Regulation of Synaptic Amyloid-β Generation through BACE1 Retrograde Transport in a Mouse Model of Alzheimer's Disease. J Neurosci. 2017 Mar 8;37(10):2639-2655. Epub 2017 Feb 3 PubMed.