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

Abstract

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disorder resulting in selective neuronal loss and dysfunction in the striatum and cortex. The molecular pathways leading to the selectivity of neuronal cell death in HD are poorly understood. Proteolytic processing of full-length mutant huntingtin (Htt) and subsequent events may play an important role in the selective neuronal cell death found in this disease. Despite the identification of Htt as a substrate for caspases, it is not known which caspase(s) cleaves Htt in vivo or whether regional expression of caspases contribute to selective neuronal cells loss. Here, we evaluate whether specific caspases are involved in cell death induced by mutant Htt and if this correlates with our recent finding that Htt is cleaved in vivo at the caspase consensus site 552. We find that caspase-2 cleaves Htt selectively at amino acid 552. Further, Htt recruits caspase-2 into an apoptosome-like complex. Binding of caspase-2 to Htt is polyglutamine repeat-length dependent, and therefore may serve as a critical initiation step in HD cell death. This hypothesis is supported by the requirement of caspase-2 for the death of mouse primary striatal cells derived from HD transgenic mice expressing full-length Htt (YAC72). Expression of catalytically inactive (dominant-negative) forms of caspase-2, caspase-7, and to some extent caspase-6, reduced the cell death of YAC72 primary striatal cells, while the catalytically inactive forms of caspase-3, -8, and -9 did not. Histological analysis of post-mortem human brain tissue and YAC72 mice revealed activation of caspases and enhanced caspase-2 immunoreactivity in medium spiny neurons of the striatum and the cortical projection neurons when compared to controls. Further, upregulation of caspase-2 correlates directly with decreased levels of brain-derived neurotrophic factor in the cortex and striatum of 3-month YAC72 transgenic mice and therefore suggests that these changes are early events in HD pathogenesis. These data support the involvement of caspase-2 in the selective neuronal cell death associated with HD in the striatum and cortex.

Huntingtin Phosphorylation Sites Mapped by Mass Spectrometry

“…We have previously shown that Htt is cleaved in several places by caspase enzymes (8,9) and cleavage contributes to toxicity (10,11). Caspase enzymes that can cleave Htt include caspase-2, -3, -6, and -7, and cleavage occurs between amino acids 513 and 587 (9,11,12). Htt is cleaved in several places by calpains as well (13,14) and this may also contribute to toxicity (15).…”

The use of the R6 transgenic mouse models of Huntington’s disease in attempts to develop novel therapeutic strategies

“…Thus, they only express a short N-terminal fragment of the protein that does not include, e.g., the caspase cleavage site that has been identified around amino acid 552. 103 Consequently, treatments with drugs that inhibit proteases cannot act primarily through inhibiting proteolytic cleavage of huntingtin itself in the R6 mice. Instead, in the R6 models protease inhibitors would target, e.g., caspase activation that could occur as a downstream consequence of the toxic effects of the N-terminal fragment of mutant huntingtin, which are not yet fully understood.…”

Huntington's Disease and the Striatal Medium Spiny Neuron: Cell-Autonomous and Non-Cell-Autonomous Mechanisms of Disease

“…MSNs express high levels of TrkB, the major receptor for BDNF [62,[169][170][171][172]. Transcriptional dysregulation in human HD patients and mouse HD models includes down-regulation of BDNF in the corticostriatal projection neurons [17,60,61,173], and down-regulation of striatal TrkB [174,175]. The decrease of BDNF may represent a loss of normal htt function, as wtHtt augments transcription of the BDNF gene [60].…”