A pair of studies published in the July 6 Neuron show that in two different mouse models of Down syndrome, the cause of neuron death boils down to a lack of neurotrophic factor support. One study, from Ahmad Salehi, Jean-Dominque Delcroix, William Mobley and colleagues at Stanford University in California, puts the blame on the amyloid precursor protein (APP)—its increased expression disrupts nerve growth factor (NGF) retrograde transport and leads to degeneration of cholinergic neurons, they report. The second paper, from Susan Dorsey and Lino Tessarollo of the National Cancer Institute in Frederick, Maryland, concludes that it is the upregulation of a truncated TrkB receptor that spells the demise of hippocampal neurons in a mouse trisomy model—the dominant-negative receptor isoform inhibits the action of another neurotrophin, brain-derived neurotrophic factor (BDNF).
Down syndrome (DS) leads to early onset AD-like pathology that many researchers believe is caused by the increased dose of the APP gene, present on the triplicated chromosome 21. Though the findings of Salehi and colleagues support this view, Dorsey and colleagues’ work suggests that APP is only part of a bigger picture. But together the two papers show in concrete detail how interrupting neurotrophic factor action causes neurodegeneration in DS models, and make the case that similar perturbations could contribute to neuron loss in Alzheimer disease and other neurodegenerative disorders. It is already known, for example, that there are defects in axonal transport early in the neurodegenerative processes in AD (see ARF related news story) and the work from Salehi and colleagues suggests that this could be linked to APP. Coincidentally, a third report last week, this one in Nature Neuroscience from the lab of Moses Chao of New York University, shows that the basis for another action of BDNF—its ability to stimulate neurotransmitter release—is also linked to transport. The actin-dependent motor Myo6 and an adaptor protein, GIPC1, which complex with the TrkB receptor, are essential for BDNF-stimulated release of glutamate.
The study by Salehi, Delcroix and Mobley, along with collaborators from across the U.S., continues their work on the degeneration of basal forebrain cholinergic neurons (BFCNs) in Ts65DN Down syndrome mice. These same cells degenerate early in AD, and in the DS model can be saved by application of NGF. The Ts65DN mice are trisomic for a portion of chromosome 16, which is analogous to human chromosome 21. Work from this group so far suggests that increased levels of APP contribute to neurodegeneration by interfering with the retrograde transport of the neurotrophic factor NGF from nerve endings in the hippocampus back to BFCN cell bodies (see Cooper et al., 2001, and coverage of Delcroix’s presentation at the SfN meeting in 2004). With their supply of NGF cut off, the BFCNs atrophy and die.
The new report shows this data, as well as additional work on the mechanism by which APP inhibits NGF transport. Experiments show that while there is no elevation of Aβ peptides in the mice, APP and its C-terminal cleavage fragments are elevated, and localized in part to signaling endosomes that also contain NGF and its receptor—after endocytosis at nerve terminals, NGF is transported as a signaling complex with its receptor in these early endosomes. In the Ts56Dn mice, there was no indication that NGF binding or internalization was changed. By immunostaining, Salehi and coworkers showed that the levels of APP and APP-CTF were increased in early endocytic vesicles in cholinergic nerve terminals. They also detected NGF and its receptors in the same vesicles, and showed that cholinergic terminals in the Ts56Dn mice contained enlarged vesicles containing APP, APP-CTF, and NGF. Further work will be needed to understand how vesicle enlargement might be related to the failure of transport. Importantly, they showed that disruption of transport was selective for NGF, since there was no change in general retrograde transport as measured by movement of Fluoro-Gold.
Since triplication of the APP gene in the context of trisomy 16 in the Ts65DN mice caused a 90 percent reduction in NGF transport and loss of BFCNs, the researchers checked transport in mice overexpressing the human APP gene, or the APP gene with the AD-causing Swedish mutation. In this case, mice expressing either wild-type APP or the Swedish mutation experienced a slowing of NGF transport by 40-60 percent, but no neuron loss. These results suggest either that more severe deficits in transport are necessary to trigger neurodegeneration, or that other genes present on the triplicated region also play a part. Mice overexpressing APP along with presenilin1 (PS1) had decreased NGF transport, but mice overexpressing only PS1 instead experienced an increase in NGF transport, for reasons that are still mysterious.
One of those other genes could code for TrkB. In a different mouse model of DS, Dorsey, Tessarollo, and colleagues from the National Cancer Institute found that alterations not in transport but instead in TrkB-mediated receptor signaling of BDNF lead to the early death of hippocampal and cortical neurons. They had shown previously that hippocampal neurons from the trisomy 16 mouse model of DS died quickly in culture, and had a defective response to their normal trophic factor, BDNF. Now, the group shows that the reason for this is that the cells overexpress a dominant negative form of the TrkB receptor, TrkB.T1. By reducing the levels of this truncated receptor to normal, they prevented cell death in the trisomy 16 neurons. That prevention was associated with restored signaling in response to BDNF, including a normalization of AKT kinase activity and intracellular calcium levels, both of which are known to contribute to cell survival. When they measured apoptosis in vivo in developing mouse brain, they found high levels of cell death in the cortex of Ts16 mice, which were decreased to normal by lowering expression the TrkB.T1 isoform. “These data suggest that neurodegeneration may not only be the result of a diminished supply of neurotrophins and provide direct evidence that neurons must express the correct set of receptor isoforms to transduce a proper survival signal in response to neurotrophins,” the authors conclude.
“The common message of both of these papers is that the inhibition of signaling or transport of a single neurotrophic factor may be partially responsible for the neuronal pathology observed in DS mouse models, and the same mechanisms may be affected also in AD,” write Finnish researchers Eero Castren of the University of Helsinki and Heikki Tanila of the University of Kuopio, in their accompanying preview. “The papers…underline the notion that the primary aim of treatment of neurodegenerative disorders is not to keep the neurons alive, but to keep them connected.”
Castren and Tanila outline other important avenues that need to be explored, including elucidating the processing steps for APP, the role of α- versus β- secretase processing, and the importance of various APP products in NGF transport. Adults with Down syndrome invariably develop AD, and recently a duplication of APP gene alone has been shown to cause AD (see ARF related news story). While the mechanisms of neurodegeneration in DS and AD are not exactly the same (there was no elevation of Aβ peptides in the Ts56Dn mice, for one), the same cell types are involved, and some commonalities, such as failure of axonal transport, may turn up based on this work. In addition, overexpression of truncated TrkB.T1 has been reported in AD (Ferrer et al., 1999), and when added to observations of BDNF deficiency in AD brain (e.g., Peng et al., 2005), the current work should spur research into the role of TrkB isoforms in AD.
In addition to its functions in keeping neurons alive and connected, BDNF released in response to synaptic activity acts through the TrkB receptor to acutely regulate neurotransmitter release and synaptic plasticity. In the third paper, Chao and colleagues show that the presynaptic complex of the actin motor Myo6 and the GIPC1 adaptor protein links TrkB to neurotransmitter release. Writing in the July 2 Nature Neuroscience, first author Hiroko Yano and coworkers report that the TrkB receptor directly associates with Myo6-GIPC1. Either Myo6 or GIPC1 knockout mice showed defects in BDNF-induced LTP in very young mice, while BDNF-independent LTP in adults was normal. The complex was required for BDNF-induced glutamate release, and lack of either protein compromised vesicle recycling in response to BDNF. Their conclusion that BDNF can use GIPC1 and the Myo6 actin motor to modulate neurotransmitter release at presynaptic locations lends an important role to actin filaments in neurotransmitter release, and sheds light on another function of BDNF in brain health.—Pat McCaffrey