Ubiquitin-positive protein inclusions are a hallmark of frontotemporal dementias and other neurodegenerative diseases. In some cases, the aggregating proteins are known, such as the tau protein in frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) and Alzheimer disease, or α-synuclein in Parkinson disease. But in many diseases, the makeup of the clumps of protein dotting the nucleus, cytoplasm or neurites is a mystery.
In a paper out today in Science, Virginia Lee and coworkers at University of Pennsylvania School of Medicine in Philadelphia report a positive ID of a new offender, responsible for inclusions in two different neurodegenerative diseases. The protein, TDP-43, is cleaved, hyperphosphorylated, and ubiquitinated in inclusion bodies in both frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U) and amyotrophic lateral sclerosis (ALS). The results suggest that these two diseases may have a common pathological basis with TDP-43 at its root.
To identify their suspect, the first authors Manuela Neumann, Deepak Sampathu, and Linda Kwong made monoclonal antibodies to inclusion proteins. With the antibodies, they defined three subtypes of FTLD-U, each of which showed a disease-specific protein which reacted to a different monoclonal antibody after gel electrophoresis. By mass spectrometry analysis, they identified the protein recognized by two of the monoclonal antibodies as the TDP-43 protein. Subsequently, they showed that a polyclonal antibody raised against TDP-43 reacted with the inclusions in all three types of FTLD-U, and also inclusions in brain tissue from two different kindreds of FTDP-17U with progranulin mutations (see ARF related news story). In contrast, TDP-43-containing inclusions were not detected in either control or Alzheimer disease brains.
The TDP-43 protein in inclusions was both phosphorylated and ubiquitinated. In disease tissues, extracted protein appeared as fragments, with some full-length and high-molecular-weight aggregates as well. A product of the TARDP gene on human chromosome 1, TDP-43 is a highly conserved, ubiquitously expressed nuclear protein. It contains RNA-recognition motifs and may function in splicing. Pathological accumulation is accompanied by a movement of the protein into the cytoplasm, which may interfere with its nuclear functions.
Clinical and pathological evidence points to ALS being a different manifestation of the same neurodegenerative process as FTLD-U, and this idea was strengthened by the investigators’ finding that inclusions in ALS also contained TDP-43. All inclusions in motor neurons were stained with the TDP-43 antibody, as were formations in the hippocampus and cortex, from both sporadic and familial ALS cases.
“The identification of TDP-43 as the major component of UBIs [ubiquitin positive inclusions] specific to sporadic and familial FTLD-U as well as sporadic ALS resolves a long-standing enigma concerning the nature of the ubiquitinated disease proteins in these disorders. Thus, these diseases may represent a spectrum of disorders that share similar pathological mechanisms, culminating in the progressive degeneration of different selectively vulnerable neurons.” The identification of TDP-43 should lead to new insights into these diseases.—Pat McCaffrey