On the distribution of intranuclear and cytoplasmic aggregates in the brainstem of patients with spinocerebellar ataxia type 2 and 3

Abstract

The polyglutamine (polyQ) diseases are a group of genetically and clinically heterogeneous neurodegenerative diseases, characterized by the expansion of polyQ sequences in unrelated disease proteins, which form different types of neuronal aggregates. The aim of this study was to characterize the aggregation pathology in the brainstem of spinocerebellar ataxia type 2 (SCA2) and 3 (SCA3) patients. For good recognition of neurodegeneration and rare aggregates, we employed 100 µm PEG embedded brainstem sections, which were immunostained with the 1C2 antibody, targeted at polyQ expansions, or with an antibody against p62, a reliable marker of protein aggregates. Brainstem areas were scored semiquantitatively for neurodegeneration, severity of granular cytoplasmic staining (GCS) and frequency of neuronal nuclear inclusions (NNI). SCA2 and SCA3 tissue exhibited the same aggregate types and similar staining patterns. Several brainstem areas showed statistically significant differences between disease groups, whereby SCA2 showed more severe GCS and SCA3 showed more numerous NNI. We observed a positive correlation between GCS severity and neurodegeneration in SCA2 and SCA3 and an inverse correlation between the frequency of NNI and neurodegeneration in SCA3. Although their respective disease proteins are unrelated, SCA2 and SCA3 showed the same aggregate types. Apparently, the polyQ sequence alone is sufficient as a driver of protein aggregation. This is then modified by protein context and intrinsic properties of neuronal populations. The severity of GCS was the best predictor of neurodegeneration in both disorders, while the inverse correlation of neurodegeneration and NNI in SCA3 tissue implies a protective role of these aggregates.

Keywords: neurodegeneration; p62; polyglutamine; protein aggregation disease; spinocerebellar ataxia.

Figure 1

Aggregates in SCA2 and SCA 3 patient tissue (A‐D). GCS in brainstem neurons in SCA2 (A,B) and SCA3 (C,D) patient tissue, stained with the 1C2 (A,C) and p62 (B,D) antibodies. Note the extension of GCS into proximal neurites in SCA2 (A, arrowhead). (E–H) NNI in brainstem neurons of SCA2 (E,F) and SCA3 (G,H) stained with the 1C2 (E,G) and p62 antibodies (F,H) (arrows). A,C,E,G: 1C2 staining; B,D,F,H: anti‐p62 staining; 100 μm PEG embedded sections.

Figure 2

Aggregates in the pontine reticular formation and the pontine nuclei (A–D). The oral pontine reticular formation (PNO) shows moderate GCS in SCA2 (A) and SCA3 (C), whereby the staining of SCA2 tissue is slightly more intense. NNI were rare (D, insert). (E–H) The basal pontine nuclei show moderate GCS in both SCA2 (E) and SCA3 (G). NNI were very frequent in the pontine nuclei, especially in SCA3 (H). A,C,E,G: 1C2 staining; B,D,F,H: p62 staining; 100 µm PEG embedded sections.

Figure 3

Aggregates in the superior olive and the facial nucleus (A‐D). The facial nucleus of SCA2 and SCA3 cases shows severe GCS (A,C) (arrowheads), which extends into the proximal neuronal processes in both SCA2 and SCA3. NNI were rare but present (B,D, inserts). (E–H) The superior olive shows severe GCS in SCA2 (A) and moderate staining in SCA3 (G). NNI were usually rare or completely absent (F,H). A,C,E,G: 1C2 staining; B,D,F,H: p62 staining; 100 μm PEG embedded sections.

Figure 4

Aggregates in the lateral reticular nucleus and the inferior olive (A‐D). The lateral reticular nucleus (LRT) of the medulla oblongata displays severe GCS in a SCA2 and a SCA3 case (A,C) (arrowheads), while showing almost no NNI in the SCA2 patient (B) and only a singular NNI in SCA3 (D, insert). (E–H) The inferior olive shows intense GCS in SCA2 (E) and only moderate GCS in SCA3 case (G), while discernable NNI are almost absent in SCA2 but frequently present in SCA3 (G,H). A,C,E,G: 1C2 staining; B,D,F,H: p62 staining; 100 μm PEG embedded sections.