As misfolded proteins corrupt their natively folded counterparts, many neurodegenerative diseases spread from one part of the brain to the next in a predictable fashion. To take a crack at formally categorizing this sequence for the neuropathology of TDP-43, Virginia Lee of the University of Pennsylvania in Philadelphia presented a four-part staging scheme at “RNA Metabolism in Neurological Disease,” a conference held November 7-8 in San Diego. “We can add ALS to the growing list of neurodegenerative diseases where the pathology spreads in a stereotypical manner,” said Lee. Published last July, the scheme should help clinicians and researchers classify pathology at autopsy, much as Braak staging has done for Alzheimer’s disease (Brettschneider et al., 2013). That pathology progresses in a defined pattern also indicates a potential therapeutic avenue: treatments that block TDP-43 spread might freeze disease at an early stage, suggested Robert Brown of the University of Massachusetts Medical School in Worcester, who was not involved with the work.
Misfolded Aβ, tau, and α-synuclein all spread from cell to cell in the brain in predictable patterns. Might TDP-43 do the same? Studies indicate that misfolded TDP-43 infiltrates cells in culture and seeds new misfolding and aggregation (see Apr 2011 news story; Nonaka et al., 2013). To find out if brain pathology supports a cell-to-cell pathway, Johannes Brettschneider from the University of Ulm, Germany, analyzed human autopsy tissue in a study begun while he was on sabbatical in the laboratory of John Trojanowski at UPenn. Brettschneider continued the work in Ulm along with co-first author Kelly Del Tredici and co-senior author Heiko Braak.
Brettschneider examined the brains and spinal cords from 76 people who died of ALS. The researchers did not examine brains of ALS patients who died early in the disease of other causes. People with ALS typically die when their diaphragm no longer supports breathing unless they use a respirator; this can happen at different stages of disease, so the scientists believe their sample reflects various degrees of severity of ALS. They found that TDP-43 antibodies bound dash-, dot-, and skein-shaped inclusions in the cytoplasm of neurons and oligodendrocytes. Based on the patterns of these aggregates, the researchers defined four different stages.
Stage 1 cases exhibited TDP-43 aggregates only in the motor cortex, brainstem, and spinal cord. In stage 2, aggregates also dotted the prefrontal neocortex, precerebellar nuclei, and the red nucleus, a midbrain structure involved in motor control. By stage 3, TDP-43 pathology spread to the postcentral neocortex and striatum. In those few people who reached stage 4, the pathology also occurred in the temporal lobe. Overall, TDP-43 pathology appears to start in the motor cortex at the top of the brain and radiate downward to the spinal cord, as well as forward to the frontal cortices.
Since autopsy tissue provides only a static snapshot, the precise directionality of any TDP-43 transfer remains uncertain, Trojanowski said. Presumably, TDP-43 turns bad at a single point and spreads from there, Trojanowski speculated, via axons (reviewed in Braak et al., 2013).
Eleven of the people in the study had ALS due to a repeat expansion in the C9ORF72 gene. Brettschneider and colleagues noted that in those cases, the direction of spread was the same, but the density of TDP-43 lesions was greater at any given stage. This makes sense, Trojanowski said, since doctors have observed that C9ORF72 expansions create a more aggressive disease.
“It is a useful construct for understanding the [progression] of disease in the brain,” commented Brown, who called the paper “brilliant.” Even so, researchers at the meeting agreed there is more work to be done. Brown would like to see more details on TDP-43 spread in the spinal cord, because ALS is a disease primarily of spinal motor neurons, not the brain. Trojanowski said the scientists are now taking a closer look at spinal-cord pathology. They also are examining frontotemporal dementia cases to complement the ALS work, because TDP-43 proteinopathy can cause either condition or a combination of the two. That prefrontal cortex pathology was a regular feature of the ALS cases in this study could explain why cognitive dysfunction is a common symptom, the authors suggest.
Staging systems provide a useful shorthand for clinical researchers, Trojanowski said. Instead of describing dozens of places where pathology occurs, they can label a case by stage and easily communicate the extent of disease. In living persons, there is no sure way to identify stages; rather, physicians typically estimate what stage a patient is at by assessing their symptoms and deducing which brain areas are likely affected. This could prove useful if someday there are different treatments appropriate to different stages, Brown speculated.
The predictable spread of TDP-43 indicates that this misfolded protein, like others, transfers pathology down the axon and across synapses through connected neural networks. Scientists are actively working on tracing this cell-to-cell spread in animal models, as has been done already for Aβ, tau and α-synuclein (see Jul 2012 news story; Jun 2009 news story; Mougenot et al., 2011).
“The evidence of cell-to-cell spread is transformative in how we think of therapy,” Trojanowski said. An extracellular antibody to each protein might halt that spread, effectively isolating the pathology in one part of the nervous system. Scientists are already testing this new kind of passive immunotherapy in tauopathy (see Aug 2013 news story), and Trojanowski suggests a similar treatment could work for TDP-43 proteinopathy as well.
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