TDP-43 accumulation in inclusion body myopathy muscle suggests a common pathogenic mechanism with frontotemporal dementia

Abstract

TAR DNA binding protein-43 (TDP-43) is found in ubiquitinated inclusions (UBIs) in some frontotemporal dementias (FTD-U). One form of FTD-U, due to mutations in the valosin containing protein (VCP) gene, occurs with an inclusion body myopathy (IBMPFD). Since IBMPFD brain has TDP-43 in UBIs, we looked for TDP-43 inclusions in IBMPFD muscle. In normal muscle, TDP-43 is present in nuclei. In IBMPFD muscle, TDP-43 is additionally present as large inclusions within UBIs in muscle cytoplasm. TDP-43 inclusions were also found in 78% of sporadic inclusion body myositis (sIBM) muscles. In IBMPFD and sIBM muscle, TDP-43 migrated with an additional band on immunoblot similar to that reported in FTD-U brains. This study adds sIBM and hereditary inclusion body myopathies to the growing list of TDP-43 positive inclusion diseases.

Figure 1

A) Normal muscle immunostained with anti-TDP-43 antibody and counterstained with DAPI to allow visualization of nuclei. Figure is overlay of anti-TDP-43 and DAPI images. Arrows denote blue nuclei with TDP-43 (red dots) in scattered myonuclei. B) IBMPFD patient tissue immunostained with anti-TDP-43 (brown) and counterstained with Congo red to allow visualization of nuclei and myofibers. Arrows denote large inclusions, some of which are peripherally based. C) Overlay of anti-TDP-43 (orange) and DAPI (blue) of IBMPFD patient tissue. Note that large peripheral inclusions do not localize within nuclei (arrows). D) IBMPFD patient tissue co-immunostained with anti-TDP-43 (red) and FK2 (green). Note that TDP-43 inclusions co-localize with FK2 (ubiquitinated proteins) (arrows). E) IBMPFD patient tissue co-immunostained with anti-TDP-43 (red) and SMI-31, an antibody against phosphorylated tau epitopes (green). Note that some TDP-43 inclusions co-localize with SMI31 (closed arrows) and others do not (open arrows).

Figure 2

A) sIBM patient tissue immunostained with anti-TDP-43 (brown) and counterstained with Congo red to allow visualization of nuclei and myofibers. TDP-43 inclusions are small, in angular fibers and occasionally surround rimmed vacuoles. B) Overlay of sIBM patient biopsy immunostained with TDP-43 (red) and FK2, for ubiquitinated proteins (green). TDP-43 inclusions do not co-localize with ubiquitin. C) sIBM patient tissue co-immunostained with anti-TDP-43 (red) and anti-CD8 (green at focal sites of inflammation). Note the co-localization of TDP-43 and CD8. This is in contrast to an adjacent fiber (arrow) with sarcoplasmic TDP-43 inclusions that does not co-localize with CD8. D) Muscle tissue from sIBM, IBMPFD and normal patient biopsies were homogenized and separated by SDS-PAGE. The subsequent gel was transferred to nitrocellulose and immunoblotted with an antibody to TDP-43. Normal tissue has a discrete band at 43kDa consistent with TDP-43, while sIBM and IBMPFD have a more prominent band and also a higher migrating band. Myosin is show as a loading control. All tissues are from flash frozen biopsies except the lane noted with an * which is from autopsy muscle. All bands are from the same autoradiograph and moved for presentation purposes.