Caspase Cleavage of Mutant Huntingtin Precedes Neurodegeneration in Huntington's Disease

Abstract

Huntington's disease (HD) results from polyglutamine expansion in huntingtin (htt), a protein with several consensus caspase cleavage sites. Despite the identification of htt fragments in the brain, it has not been shown conclusively that htt is cleaved by caspases in vivo. Furthermore, no study has addressed when htt cleavage occurs with respect to the onset of neurodegeneration. Using antibodies that detect only caspase-cleaved htt, we demonstrate that htt is cleaved in vivo specifically at the caspase consensus site at amino acid 552. We detect caspase-cleaved htt in control human brain as well as in HD brains with early grade neuropathology, including one homozygote. Cleaved htt is also seen in wild-type and HD transgenic mouse brains before the onset of neurodegeneration. These results suggest that caspase cleavage of htt may be a normal physiological event. However, in HD, cleavage of mutant htt would release N-terminal fragments with the potential for increased toxicity and accumulation caused by the presence of the expanded polyglutamine tract. Furthermore, htt fragments were detected most abundantly in cortical projection neurons, suggesting that accumulation of expanded htt fragments in these neurons may lead to corticostriatal dysfunction as an early event in the pathogenesis of HD.

Specific caspase interactions and amplification are involved in selective neuronal vulnerability in Huntington's disease

“…Our results demonstrate that Htt is cleaved by caspase-2 only at the 552 site when probed with antibodies that detect caspasecleaved Htt fragments (neospecific antibodies to cleavage sites at 513 and 552) (Figure 1d). 31 These results indicate that, in vitro, caspase-2 cleaves Htt at amino acid 552 and correlates with our recent results demonstrating that Htt is also cleaved in vivo at the D552 caspase consensus site in pyramidal cortical neurons of YAC72 mice and HD brains. 31 However, these results do not necessarily explain how this cleavage event is initiated in vivo.…”

“…BDNF levels are directed regulated by wild-type Htt and are reduced in HD transgenic mice and HD patient brains. 33 We find that caspase-2 cleaves Htt selectively at amino acid 552, which is the Htt caspase cleavage product detected in vivo both in early-grade HD postmortem tissue and in YAC transgenic mice expression mutant Htt, 31 and further that Htt directly interacts with specific caspases to form a cell-death complex. We found that caspase-2, -6, and -7 recruit Htt in an apoptosome-like complex.…”

“…Evidence for region-specific cleavage of Htt in vivo comes from studies of YAC transgenic mice expressing mutant full-length huntingtin (YAC72). In this model, N-terminal fragments of Htt are clearly found in the medium spiny neurons of the striatum 30 and cortex, 31 but not in other regions of the brain unaffected in HD. These results suggest that cleavage of Htt by caspases and other proteases such as calpains 32 may produce toxic fragments.…”

“…36 However, in vivo, we found that Htt is specifically cleaved at a caspase consensus site at amino acid 552 in pyramidal cortical neurons, while the caspase consensus site at amino acid 513 does not appear to be utilized. 31 Since caspase-3 cleaves at both D513 and D552, we further evaluated the specificity of caspases in cleaving Htt. Here, we show that Htt is also cleaved by caspase-2, -7, and -8 ( Figure 1b).…”

Mice overexpressing human caspase 3 appear phenotypically normal but exhibit increased apoptosis and larger lesion volumes in response to transient focal cerebral ischaemia

“…10 The decreased apoptosis observed in the brain suggested a critical role for caspase 3 in the development of the central nervous system. Caspase 3 has also been implicated in cell death following a number of neurodegenerative insults including global ischaemia, 12,13 focal ischaemia, [14][15][16][17][18][19] transient ischaemia of the spinal cord, 20 neonatal cerebral hypoxia-ischaemia, [21][22][23] traumatic brain injury, [24][25][26] Alzheimer's disease, [27][28][29] Huntington's disease, [30][31][32] and Parkinson's disease. [33][34][35][36][37] It was therefore hypothesized that overexpression of caspase 3 may sensitize the animal to normal (physiological) apoptotic processes that occur during development and natural ageing and confer particular susceptibility to neurodegenerative insults.…”