Pathological TDP-43 appears to follow a set route through the nervous system, and what that route is depends on the disease at hand. Two new papers in Acta Neuropathologica add TDP-43 itineraries for Alzheimer’s disease and frontotemporal lobar degeneration to a previously published staging scheme for amyotrophic lateral sclerosis (see Nov 2013 news story). While the starting points and paths taken differ, the disease-specific routes suggest that TDP-43 travels from neuron to neuron along axonal highways. The stages should also provide convenient shorthand for pathologists to describe a patient’s pathological burden.
The TDP-43 stages fit with the ongoing theme in neurodegeneration research that these diseases are progressive not only over time, but also in space, as pathological proteins spread throughout the nervous system, according to John Ravits of the University of California in San Diego, who was not involved in the current studies. Researchers have defined stages of Aβ, tau, and α-synuclein proteinopathy by examining autopsy tissue from people who died at various points in the progression of their disease (Braak et al., 2006; Braak and Braak, 1991; Braak et al., 2003). Rather than flowing freely through the nervous system, these proteins appear to follow defined neuroanatomical pathways (see Jun 2009 news story; Apr 2012 news story).
The FTLD study, published online January 10, comes from the same group that performed the ALS staging study. Senior authors Heiko Braak of the University of Ulm, Germany, and John Trojanowski of the University of Pennsylvania in Philadelphia led the project. Joint first authors Johannes Brettschneider and Kelly Del Tredici of Ulm and David Irwin of Pennsylvania examined 39 cases of behavioral variant frontotemporal dementia to define a pattern of sequential pathology. In the lowest-burden cases, TDP-43 inclusions occurred in the basal forebrain (see image below). In the next stage, the proteinopathy progressed to the prefrontal area. In the third pattern defined by the group, TDP-43 pathology also occurred in the brainstem, motor cortex, and spinal cord grey matter. In the most severe cases, it hit occipital areas, too.
Overall, Trojanowski summed up, the FTLD pathology progressed from the front of the brain to the back. This contrasted with the ALS staging system, which began in the motor cortex at the brain’s apex and moved downward and forward from there. “The spreading mechanisms could be very similar, but the early focus of pathology seems to be different [between ALS and FTLD],” Brettschneider said. Trojanowski added, “This reflects the different clinical manifestations. Why this happens is mysterious.” The researchers selected relatively homogeneous cases of clinically pure FTLD or pure ALS to obtain clear staging; cases with a mixture of symptoms would likely show different pathological pathways, Trojanowski said.
Looking to TDP-43’s spread in Alzheimer’s, senior author Dennis Dickson of the Mayo Clinic in Jacksonville, Florida, and colleagues report their findings in a study published online November 16, 2013. First author Keith Josephs of the Clinic’s branch in Rochester, Minnesota, identified TDP-43 pathology in 195 out of 342 autopsy AD cases—a “staggering” proportion that suggests TDP-43 inclusions appear in more than half of Alzheimer’s patients, he said. The authors defined a five-stage progression of TDP-43 inclusion pathology, starting with the amygdala, then moving to the entorhinal cortex and subiculum, followed by the dentate gyrus and occipitotemporal cortex, and thereafter the inferior temporal cortex. In the final stage, the middle frontal cortex and basal ganglia were also affected.
The pathology moves from the amygdala to memory-controlling areas to the cortex, Josephs summarized. This pathway differs from the routes taken by Aβ and tau. Notably, amyloid does not arise in the amygdala. Josephs was uncertain why TDP-43 pathology began there, but noted the amygdala often contains proteinopathy in neurodegenerative disease.
The brain areas examined by the two research groups were not entirely identical, and some overlapping locales could have been missed. Like tau, TDP-43 occurs in different places in different conditions.
How can the same misfolded protein appear in AD, FTLD, and ALS? “I ask myself this question all the time, but I cannot answer it,” Brettschneider said. He speculated that perhaps pathological TDP-43 occurs in different “strains,” as has been shown for alpha-synuclein and prions, and each strain occurs in a different disease (see Aug 2013 conference story; Jul 2013 news story; Colby and Prusiner, 2011).
When pathological TDP-43 does occur, does it cause the disease, or simply mark degenerating neurons? “I personally think it does both,” Josephs said. TDP-43 may indicate general neurodegeneration, but must also drive the process in cases where it is the only proteinopathy present, he said (see also Jan 2014 news story).
Though these studies point to axonal routes for the spread of TDP-43 from region to region, the evidence provided for this by autopsy tissue is at best indirect. Another possibility, Ravits suggested, would be that TDP-43 diffuses indiscriminately across the brain, but only certain neural types are susceptible to the traveling pathology. Trojanowski said studies in mouse and cell culture models are up next to clinch the mechanism of TDP-43 transfer.—Amber Dance
- The Four Stages of TDP-43 Proteinopathy
- Traveling Tau—A New Paradigm for Tau- and Other Proteinopathies?
- Synthetic Synuclein Corrupts Native Along Mouse Brain Networks
- Are Protein Strains The Cause of Different Tauopathies?
- An Extra Strain on the Brain—α-Synuclein Seeds Tau Aggregation
- Lower Motor Neuron Pathology Links ALS and FTLD
- Braak H, Alafuzoff I, Arzberger T, Kretzschmar H, Del Tredici K. Staging of Alzheimer disease-associated neurofibrillary pathology using paraffin sections and immunocytochemistry. Acta Neuropathol. 2006 Oct;112(4):389-404. PubMed.
- Braak H, Del Tredici K, Rüb U, De Vos RA, Jansen Steur EN, Braak E. Staging of brain pathology related to sporadic Parkinson's disease. Neurobiol Aging. 2003 Mar-Apr;24(2):197-211. PubMed.
- Colby DW, Prusiner SB. De novo generation of prion strains. Nat Rev Microbiol. 2011 Nov;9(11):771-7. PubMed.
- Amador-Ortiz C, Lin WL, Ahmed Z, Personett D, Davies P, Duara R, Graff-Radford NR, Hutton ML, Dickson DW. TDP-43 immunoreactivity in hippocampal sclerosis and Alzheimer's disease. Ann Neurol. 2007 May;61(5):435-45. PubMed.
- Hu WT, Josephs KA, Knopman DS, Boeve BF, Dickson DW, Petersen RC, Parisi JE. Temporal lobar predominance of TDP-43 neuronal cytoplasmic inclusions in Alzheimer disease. Acta Neuropathol. 2008 Aug;116(2):215-20. PubMed.
- Mackenzie IR, Neumann M, Baborie A, Sampathu DM, Du Plessis D, Jaros E, Perry RH, Trojanowski JQ, Mann DM, Lee VM. A harmonized classification system for FTLD-TDP pathology. Acta Neuropathol. 2011 Jul;122(1):111-3. Epub 2011 Jun 5 PubMed.
- Nonaka T, Masuda-Suzukake M, Arai T, Hasegawa Y, Akatsu H, Obi T, Yoshida M, Murayama S, Mann DM, Akiyama H, Hasegawa M. Prion-like properties of pathological TDP-43 aggregates from diseased brains. Cell Rep. 2013 Jul 11;4(1):124-34. PubMed.
- Jucker M, Walker LC. Self-propagation of pathogenic protein aggregates in neurodegenerative diseases. Nature. 2013 Sep 5;501(7465):45-51. PubMed.
- Mougenot AL, Nicot S, Bencsik A, Morignat E, Verchère J, Lakhdar L, Legastelois S, Baron T. Prion-like acceleration of a synucleinopathy in a transgenic mouse model. Neurobiol Aging. 2011 Aug 1; PubMed.
- Mioshi E, Hsieh S, Savage S, Hornberger M, Hodges JR. Clinical staging and disease progression in frontotemporal dementia. Neurology. 2010 May 18;74(20):1591-7. PubMed.
- Polymenidou M, Cleveland DW. The seeds of neurodegeneration: prion-like spreading in ALS. Cell. 2011 Oct 28;147(3):498-508. PubMed.
- Kanouchi T, Ohkubo T, Yokota T. Can regional spreading of amyotrophic lateral sclerosis motor symptoms be explained by prion-like propagation?. J Neurol Neurosurg Psychiatry. 2012 Jul;83(7):739-45. PubMed.
- Josephs KA, Murray ME, Whitwell JL, Parisi JE, Petrucelli L, Jack CR, Petersen RC, Dickson DW. Staging TDP-43 pathology in Alzheimer's disease. Acta Neuropathol. 2014 Mar;127(3):441-50. Epub 2013 Nov 16 PubMed.
- Brettschneider J, Del Tredici K, Irwin DJ, Grossman M, Robinson JL, Toledo JB, Fang L, Van Deerlin VM, Ludolph AC, Lee VM, Braak H, Trojanowski JQ. Sequential distribution of pTDP-43 pathology in behavioral variant frontotemporal dementia (bvFTD). Acta Neuropathol. 2014 Mar;127(3):423-39. Epub 2014 Jan 10 PubMed.