Nonapoptotic neurodegeneration in a transgenic mouse model of Huntington's disease

Abstract

Huntington's disease (HD) is a fatal inherited neurodegenerative disorder characterized by personality changes, motor impairment, and subcortical dementia. HD is one of a number of diseases caused by expression of an expanded polyglutamine repeat. We have developed several lines of mice that are transgenic for exon 1 of the HD gene containing an expanded CAG sequence. These mice exhibit a defined neurological phenotype along with neuronal changes that are pathognomonic for the disease. We have previously observed the appearance of neuronal intranuclear inclusions, but did not find evidence for neurodegeneration. In this study, we report that all lines of these mice develop a late onset neurodegeneration within the anterior cingulate cortex, dorsal striatum, and of the Purkinje neurons of the cerebellum. Dying neurons characteristically exhibit neuronal intranuclear inclusions, condensation of both the cytoplasm and nucleus, and ruffling of the plasma membrane while maintaining ultrastructural preservation of cellular organelles. These cells do not develop blebbing of the nucleus or cytoplasm, apoptotic bodies, or fragmentation of DNA. Neuronal death occurs over a period of weeks not hours. We also find degenerating cells of similar appearance within these same regions in brains of patients who had died with HD. We therefore suggest that the mechanism of neuronal cell death in both HD and a transgenic mouse model of HD is neither by apoptosis nor by necrosis.

Figure 1

Ultrastructural appearance of neurons undergoing dark cell degeneration in the striatum (A) and anterior cingulate cortex (B–D) of 17-week-old R6/2 transgenic mice. Note the juxtaposition of neurons containing NII, but without any of the characteristic features of dark cell degeneration (arrows in B and D) (magnification ×650).

Figure 2

Camera Lucida drawings of 1-μm toluidine blue-stained araldite-embedded sections through the anterior cingulate cortex showing the distribution of darkened, degenerating neurons. Sections are from R6/2 mice of 14 weeks, 15 weeks, 16 weeks, and 17 weeks.

Figure 3

Cerebellar Purkinje cells undergoing dark cell degeneration, neuronal soma (large arrows), and dendritic processes (small arrows) both show darkened features (magnification ×720).

Figure 4

Examples of degenerating neurons in the right anterior cingulate cortex from an R6/2 transgenic mouse (A–C; 17 weeks old, 141 CAG repeats) and an HD patient (D–E; 11 years old, 100 CAG repeats). NII are present within all of these neurons. (Magnifications: A–C, ×1,250; D–E, ×2,250; F, ×1,000.)