Early Alterations in Gene Expression and Cell Morphology in a Mouse Model of Huntington's Disease

Abstract

Several mouse models for Huntington's disease (HD) have been produced to date. Based on differences in strain, promoter, construct, and number of glutamines, these models have provided a broad spectrum of neurological symptoms, ranging from simple increases in aggressiveness with no signs of neuropathology, to tremors and seizures in absence of degeneration, to neurological symptoms in the presence of gliosis and TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling) positivity, and finally to selective striatal damage associated with electrophysiological and behavioral abnormalities. We decided to analyze the morphology of striatum and hippocampus from a mouse transgenic line obtained by microinjection of exon 1 from the HD gene after introduction of a very high number of CAG repeat units. We found a massive darkening and compacting of striatal and hippocampal neurons in affected mice, associated with a lower degree of more classical apoptotic cell condensation. We then explored whether this morphology could be explained with alterations in gene expression by hybridizing normal and affected total brain RNA to a panel of 588 known mouse cDNAs. We show that some genes are significantly and consistently up-regulated and that others are down-regulated in the affected brains. Here we discuss the possible significance of these alterations in neuronal morphology and gene expression.

The R6 lines of transgenic mice: A model for screening new therapies for Huntington's disease

“…Surprisingly, in contrast to the studies reporting virtually no striatal cell death until a very late stage of the R6/2 phenotype (Iannicola et al, 2000;Turmaine et al, 2000;Yu et al, 2003), these authors were able to observe a marked increase in the number of apoptotic cells [TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling)-positive cells] in the striatum of R6/2 mice at 13 weeks of age, a striking finding that was virtually abolished by TUDCA. Furthermore, TUDCA decreased cerebral atrophy, increased striatal volume and significantly reduced the number and size of striatal NIIs.…”

“…As discussed above (Section 1), in the R6 model striatal neuronal death is very limited and most of the studies reported so far were only able to detect the occurrence of "dark cell degeneration" in a limited number of striatal neurons in very late stages of the R6/2 phenotype (Iannicola et al, 2000;Turmaine et al, 2000;Yu et al, 2003). Nevertheless, the fact that caspase-1 can cleave both mutant and wild-type huntingtin in vitro (Wellington et al, 1998) raised the question whether inhibition of this caspase could interfere with the progression of the disease.…”

“…To date, only one study reported a 25% decrease in the total number of striatal cells in 12-week-old transgenic mice (Stack et al, 2005), while another described a marked increase in the number of apoptotic cells in the striatum (Keene et al, 2002) of 13 week-old R6/2 mice. On the other hand, "dark cell degeneration" has also been described in a small number of neurons in 12- (Stack et al, 2005) and 14-week-old mice (i.e., at a very late stage in the development of the disease) (Iannicola et al, 2000;Turmaine et al, 2000;Yu et al, 2003). Moreover, a progressive loss of orexin neurons was found in the lateral hypothalamus of R6/2 mice, which was correlated with the occurrence of narcoleptic-like episodes and seemed to be related with a disintegration of the sleep-wake cycle and of the circadian rhythm in these mice (Morton et al, 2005b).…”

Does prothymosin-α act as molecular switch between apoptosis and autophagy?

“…1 Regarding neurodegeneration, it appears intriguing that PHAP can interact with the protein ataxin-1, suggesting that its proapoptotic function may contribute to the loss of Purkinje cells in spinocerebellar ataxia type 1, a polyglutamine repeat disorder. 7 Moreover, brains from mice transgenic for Huntington's disease (HD) mutation (caused by an expanded CAG repeat in exon 1 of the gene coding for the huntingtin protein) exhibit a drastic up-regulation of the ProT gene 10 ( Figure 1). Thus, paradoxically, degenerating neurons are overexpressing the caspase-inhibitory protein, ProT.…”

“…10 Degenerating neurons from HD-transgenic mice exhibit features of neurodegeneration, including peculiar nuclear and cytoplasmic abnormalities, yet fail to show signs of apoptosis such as cytoplasmic fragmentation or chromatin clumping. 10 These morphological features are suggestive of a slow or delayed neuronal death, in which the apoptotic, caspase-dependent program is suppressed. Dying neurons frequently exhibit an increase in electron-dense lysosomes and autophagic granules, indicating that the mutant huntingtin gene exacerbates autophagy.…”