New data suggest that TDP-43 from diseased human brains propagates along neuronal connections in mice. According to a report in the October 11 Nature Communications, extracts containing pathological TDP-43 from postmortem brains of patients with frontotemporal lobar degeneration, when injected into transgenic mouse brains, seeded new pathology, corrupting human TDP-43 protein that was expressed in the cytoplasm along its path.
The result places TDP-43 in the same vein as misfolded Aβ, tau, and α-synuclein, which also propagate from neuron to neuron in this type of mouse paradigm, said senior author Virginia Lee, Perelman School of Medicine, University of Pennsylvania, Philadelphia (see November 2013 news). “TDP-43 had never been shown to spread cell-to-cell from one brain region to another in animal models,” she told Alzforum. “We now show all neurodegenerative disease proteins spread this way, which allows us to generalize that cell-to-cell transmission is a common mechanism for the progression of all neurodegenerative diseases.” TDP-43 pathology afflicts motor neurons of patients with amyotrophic lateral sclerosis (ALS), meaning the results may have implications for that disease as well.
To look for spreading of TDP-43 in mouse brain, first author Sílvia Porta used the CamKIIa-hTDP43ΔNLSm model that the Lee group had generated to express human TDP-43 in forebrain neurons inducible by removing doxycycline from their chow (see Walker et al., 2015). TDP-43 is a nuclear protein, but this version had a mutation in its nuclear localization signal, meaning the protein wandered out into the cytoplasm.
Into this model, Porta and colleagues planned to inject brain extracts from the frontal cortices of patients who had died with FTLD. Those patients had either had sporadic disease, a causative repeat expansion in C9ORF72, or progranulin (GRN) mutation. Porta first evaluated the extracts’ seeding capacity in cultured QBI-293 cells that express human TDP-43 either in the nucleus or cytoplasm. Extracts seeded pathology in both cell types, but led to at least three times as much if TDP-43 appeared in the cytoplasm, suggesting mislocalization spurred seeding.
Extracts from patients with progranulin mutations seeded pathology faster in cultured cells than extracts from sporadic FTD. Extracts from patients with C9ORF72 expansions fell in between the two. Lee did not know why that is. It may hint that different mutations lead to different strains of misfolded TDP-43 that seed at different efficiencies, or it may be that progranulin mutations lead to more abundant pathology that seeds more effectively, Lee said, noting that her team tried to control for TDP-43 levels between samples.
Where did the researchers inject the human extract? Into the neocortices, hippocampi, and thalami of the mice. They then analyzed these brains immunohistochemically one to nine months later for phosphorylated TDP-43. After a month, pTDP-43 pathology turned up in the deep layers of the neocortex, hippocampus, subiculum, and cortex; it was most obvious on the side of the injection, but faint inclusions appeared on the contralateral side as well, indicating that TDP-43 pathology had begun to cross into the opposite hemisphere.
In mice sacrificed at later time points, inclusions appeared in more superficial layers of the neocortex, contralateral cortex, and hippocampus, as well as more distant brain regions such as the nucleus accumbens and lateral septum (see image above). These regions are all anatomically connected to the injection sites. Both gray and white matter became affected over time. As aggregates matured, they became more compact. Like inclusions in the brains of people with ALS or FTLD with TDP-43 pathology, the aggregates stained positive for markers of autophagy and proteolysis, suggesting activation of similar protein-degrading pathways between mice and humans.
Pathology was mostly neuronal in this model, with little to no accumulation in astrocytes, microglia, or oligodendrocytes. That is to be expected in a host that expresses cytoplasmic TDP-43 only in forebrain neurons, the authors wrote. Altogether, the results reveal that TDP-43 spread along the connectome of the brain, with inclusions maturing and condensing over time.
Propagation seemed to require the full-length protein. If extracts from patients were injected into the CamKIIa-208 mouse, which expresses only a C-terminal fragment of TDP-43, a bit of pathology did appear, but it was but a fraction of the inclusions found in mice expressing full-length cytoplasmic TDP-43 (see Walker et al., 2015).
Human brain extracts seeded much less pathology in non-transgenic mice that expressed only mouse TDP-43 in the nucleus, though it was detectable. One month post-injection, rare inclusions popped up in the cortex and hippocampus; they became more visible after nine months.
This study used only extracts from FTLD patients. TDP-43 pathology marks spinal cord neurons from most ALS patients, as well. Asked about ALS, Lee told Alzforum she assumes that the mechanism of spread is the same.
Masato Hasegawa, Tokyo Metropolitan Institute of Medical Science, told Alzforum the paper represents major progress.
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