Research Article: Formation of Polyglutamine Inclusions in a Wide Range of Non-CNS Tissues in the HdhQ150 Knock-In Mouse Model of Huntington’s Disease

Date Published: November 30, 2009

Publisher: Public Library of Science

Author(s): Hilary Moffitt, Graham D. McPhail, Ben Woodman, Carl Hobbs, Gillian P. Bates, Hitoshi Okazawa.

Abstract: Huntington’s disease (HD) is an inherited progressive neurodegenerative disorder caused by a CAG repeat expansion in the ubiquitously expressed HD gene resulting in an abnormally long polyglutamine repeat in the huntingtin protein. Polyglutamine inclusions are a hallmark of the neuropathology of HD. We have previously shown that inclusion pathology is also present in the peripheral tissues of the R6/2 mouse model of HD which expresses a small N-terminal fragment of mutant huntingtin. To determine whether this peripheral pathology is a consequence of the aberrant expression of this N-terminal fragment, we extend this analysis to the genetically precise knock-in mouse model of HD, HdhQ150, which expresses mutant mouse huntingtin.

Partial Text: Huntington’s disease (HD) is an autosomal dominant late-onset progressive neurodegenerative disorder with a mean age of onset of 40 years. Symptoms include motor disorders, psychiatric disturbances, cognitive decline and weight loss. Disease duration is 15 –20 years and there are no effective disease modifying treatments [1]. The HD mutation is an expanded CAG repeat in the HD gene that is translated into a polyglutamine (polyQ) repeat in the huntingtin (Htt) protein [2]. Unaffected individuals have (CAG)6–35 repeats, whilst disease causing alleles of (CAG)40 and above are fully penetrant [3], [4]. Age of symptom onset can range from early childhood to extreme old age with repeats of (CAG)75 and above invariably causing the childhood form of the disease [4]. Neuropathologically, the disease is characterized by global brain atrophy [5], [6], neuronal cell loss in the striatum, cortex and other brain regions and the deposition of nuclear and cytoplasmic polyQ aggregates [7], [8].

In order to investigate the distribution of inclusions in non-CNS tissue, 22 month old HdhQ150/Q150 mice and littermate controls were perfusion fixed and organs and tissue were removed processed and wax embedded. A 12 week old R6/2 mouse plus wild type littermate was processed alongside for direct comparison. The 5 µm tissue sections were stained with haematoxylin and eosin (H&E) for tissue identification and immunostained with the S830 antibody for the identification of inclusions. Tissues and cell types in which inclusions were detected are summarised in Table 1 and examples are illustrated in Figure 1, Figure 2, Figure 3, Figure 4, and Figure 5. Inclusions were only detected in cell nuclei and in the majority of nuclei there was only one inclusion, but in some cell types: skeletal muscle fibres, liver hepatocytes and connective tissue fibroblasts, there were multiple nuclear inclusions, as in the R6/2 model. We noted considerable variation in the number of nuclear inclusions in the tissues examined between individual HdhQ150/Q150 mice but not in the overall distribution of inclusions.

Although HD is a neurodegenerative disorder, the HD gene is ubiquitously expressed, and whilst it is understandable that HD research is predominantly focussed on the central nervous system (CNS), evidence is accumulating to suggest that some HD symptoms may be caused by a peripheral pathology. The mechanistic role that polyQ inclusions play in the pathogenesis of HD remains the subject of much debate, however, their presence is considered to be indicative of pathology. In this study, we have shown that the distribution of nuclear inclusions in the peripheral tissues of the R6/2 and HdhQ150 models is almost identical at end-stage disease and therefore, the peripheral pathology of these two HD mouse models is highly comparable. Peripheral pathology was identified in skeletal muscle, heart, pancreas, adrenal gland, liver, kidney, the gastrointestinal (GI) tract, brown fat, male reproductive organs and connective tissue. In most cases, the cell types that are affected are, like neurons, terminally differentiated. Additional factors that might influence the propensity to form inclusions in a particular cell type could include expression level of the R6/2 transcript or Hdh gene, Htt proteolysis and protein folding and clearance networks. Organ atrophy was also a feature of both models with the testes exhibiting the most dramatic weight loss followed by skeletal muscle in both cases.



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