Background & Aims:Intestinal dysmotility is a component of many neurodegenerative disorders, including some characterized by neuronal intranuclear inclusions. PrP-SCA7-92Q transgenic mice phenocopy many aspects of the human polyglutamine neurodegenerative disorder spinocerebellar ataxia type 7 (SCA7). The enteric neuropathology of PrP-SCA7-92Q mice was investigated after observing that they develop signs of intestinal pseudo-obstruction. Methods:Gastrointestinal transit of radio-opaque pellets through presymptomatic and symptomatic PrP-SCA7-92Q mice and nontransgenic littermates was compared. Gross, microscopic, and ultrastructural studies were conducted, along with histologic and whole mount immunohistochemistry, to identify intranuclear inclusions and quantify subsets of enteric neurons. Immunoblot analysis was performed to confirm selective loss of particular neuronal populations. Results: A subset of cholinergic enteric ganglion cells in PrP-SCA7-92Q mice harbor nuclear inclusions composed of transgene-derived ataxin-7, which contains a pathogenic polyglutamine expansion. These animals die between 15 and 20 weeks of age with intestinal distension and enterocolitis. Signs of disease are preceded by selective loss of nitric oxide synthase-positive neurons (which lack nuclear inclusions), loss of nerve fibers in the myenteric nerve plexus, and delayed gastrointestinal transit. Cholinergic neurons, including those with inclusions, are spared. Conclusions: PrP-SCA7-92Q mice may be useful models for human intestinal pseudoobstruction, particularly visceral neuropathies with neuronal intranuclear inclusions. Loss of inclusion-free inhibitory neurons supports the hypothesis that inclusions may be neuroprotective or coincidental, as opposed to harbingers of neuron death. Because enteric neuropathology in PrP-SCA7-92Q animals is easily missed by routine histopathology, quantitative immunohistochemical approaches may be required to recognize analogous forms of human enteric neuropathy.
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Mice
The PrP-SCA7-92Q transgenic mice were created as described elsewhere13 and received standard laboratory chow ad libitum. Animals were handled in accordance with guidelines established by the National Institutes of Health and our institutional animal care and use committee. To harvest tissues, mice were asphyxiated with carbon dioxide.
Mapping the Transgene Insertion Site by Fluorescence In Situ Hybridization
Cells were teased out of the spleen from male PrP-SCA7-92Q transgene-positive mice, pelleted, and cultured in RPMI with 10% fetal bovine serum and 40 μg/mL
Results
To model the retinal and cerebellar degeneration of SCA7, a human ATAXIN-7 complementary DNA that encodes a mutant form of ataxin-7 with a pathogenic 92 glutamine (Q)-long expansion (≤35 glutamines is normal) was inserted into the murine prion promoter (PrP) expression construct.13 The PrP promoter directs expression to neurons and a subset of glial cells of the central and peripheral nervous systems.18 Four independent lines of PrP-SCA7-92Q transgenic mice were produced by micronuclear
Discussion
PrP-SCA7-92Q mice show line-to-line phenotypic variability, but a consistent finding in at least 3 lines is premature death that is associated with intestinal dilatation and enteric neuronal intranuclear inclusions. Our detailed investigation of the 6529 line suggests that distension and death are the consequences of progressive loss of enteric neurons and delayed intestinal transit. In these respects, PrP-SCA7-92Q mice may be a useful model for humans with visceral neuropathies, intranuclear
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Supported by UW NIEHS-sponsored Center for Ecogenetics and Environmental Health (NIEHS P30ES07033); CHRMC Research Administration Award (to R.P.K.); and NIH grant R01 EY14061 (to A.R.L.).
Conflicts of interest: No conflicts of interest exist.
