Formation of polyglutamine inclusions in a wide range of non-CNS tissues in the HdhQ150 knock-in mouse model of Huntington's disease

Abstract

Background: 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.

Methodology/principal findings: We have previously standardized the CAG repeat size and strain background of the R6/2 and HdhQ150 knock-in mouse models and found that they develop a comparable and widespread neuropathology. To determine whether HdhQ150 knock-in mice also develop peripheral inclusion pathology, homozygous Hdh(Q150/Q150) mice were perfusion fixed at 22 months of age, and tissues were processed for histology and immunohistochemistry with the anti-huntingtin antibody S830. The peripheral inclusion pathology was almost identical to that found in R6/2 mice at 12 weeks of age with minor differences in inclusion abundance.

Conclusions/significance: The highly comparable peripheral inclusion pathology that is present in both the R6/2 and HdhQ150 knock-in models of HD indicates that the presence of peripheral inclusions in R6/2 mice is not a consequence of the aberrant expression of an N-terminal huntingtin protein. It remains to be determined whether peripheral inclusions are a pathological feature of the human disease. Both mouse models carry CAG repeats that cause childhood disease in humans, and therefore, inclusion pathology may be a feature of the childhood rather than the adult forms of HD. It is important to establish the extent to which peripheral pathology causes the peripheral symptoms of HD from the perspective of a mechanistic understanding and future treatment options.

Figure 1. Nuclear inclusions in the skeletal muscle and liver of Hdh ^Q150/Q150 mice.

Nuclear inclusions in longitudinal sections of quadriceps muscle are present in Hdh ^Q150/Q150 (A) but absent from wild-type (B) mice. Nuclear inclusions are present in hepatocytes from Hdh ^Q150/Q150 (C) but not in those from wild-type (D) mice and are abundant in an Hdh ^Q150/Q150 intrahepatic bile duct (E). The insert shows a higher magnification of the epithelial cells of the bile duct. Nuclear inclusions are indicated by arrowheads. Scale bar = 20 µm.

Figure 2. Distribution of nuclear inclusions in the adrenal glands of the Hdh ^Q150/Q150 and R6/2 mice.

(A) transverse section from an Hdh ^Q150/Q150 mouse illustrating the structure of the adrenal gland. Inclusions present in the outer cortex (B) and inner cortex and medulla (C) from Hdh ^Q150/Q150 mice. Insert in (B) shows an inclusion in a fibroblast from the adrenal capsule. Inclusions are absent from the Hdh ^Q150/Q150 wild type control (D,E). A comparable distribution of inclusions is present in the R6/2 mouse (F,G) except for a greater density of inclusions in the cortex. Nuclear inclusions are indicated by arrowheads. ZG = zona glomerulosa, ZF = zona fasciculata, M = medulla. Scale bar (A) = 100 µm, scale bar (B–G) = 50 µm.

Figure 3. Pancreatic pathology in the HdhQ150 mouse model.

Nuclear inclusions were present in the pancreatic islets of Hdh ^Q150/Q150 mice (A) and absent from wild-type controls (B). Insert shows higher magnification image of a nucleus containing a single inclusion indicated by arrow. Electron micrograph showing cytoplasmic detail of β-cells in the islets from an Hdh ^Q150/Q150 mouse (C), a wild-type control (D), and an R6/2 mouse (E). The Hdh ^Q150/Q150 β-cell appears normal as compared to the control. The R6/2 β-cell is atrophied and degranulated in comparison. Arrows indicate β-granules (C–E). Pi = pancreatic islet, NI = nuclear inclusion. Scale bar (A,B) = 50 µm, scale bar (C,D) = 1 µm, scale bar (E) = 2 µm.

Figure 4. Inclusion distribution across the stomach wall in the Hdh ^Q150/Q150 mice.

(A) Transverse section through the stomach wall of an Hdh ^Q150/Q150 mouse showing structural detail. Inclusions were present in the mucus cells in the gastric gland neck (B), the basal cells of the gastric gland (C), the submucosal ganglia (D), smooth muscle (E), the myenteric ganglia (F), and the serosal ganglia (G) of Hdh ^Q150/Q150 but absent from the wild-type control. A similar distribution was found in R6/2 in comparison to the Hdh ^Q150/Q150. Nuclear inclusions are indicated by arrowheads. Scale bar (A) = 100 µm, scale bar (B–G) = 20 µm.

Figure 5. Inclusions are present in the male reproductive glandular epithelium and spleen connective tissue.

Nuclear inclusions were present in the epithelial cells of the seminal vesicles (A) and the epithelium of the coagulation gland (B) in Hdh ^Q150/Q150 but absent from wild-type mice. A similar distribution was identified in R6/2 mice although the number of affected cells was fewer in number in both cases. Nuclear inclusions were present in connective tissue cells of the spleen capsule from Hdh ^Q150/Q150 mice (C) with a similar distribution in R6/2. Nuclear inclusions are indicated by arrowheads. Scale bar = 20 µm.