Abundant expression and cytoplasmic aggregations of alpha1A voltage-dependent calcium channel protein associated with neurodegeneration in spinocerebellar ataxia type 6

Abstract

Spinocerebellar ataxia type 6 (SCA6) is one of the eight neurodegenerative diseases caused by a tri-nucleotide (CAG) repeat expansion coding polyglutamine (CAG repeat/polyglutamine diseases) and is characterized by late onset autosomal dominant cerebellar ataxia and predominant loss of cerebellar Purkinje cells. Although the causative, small and stable CAG repeat expansion for this disease has been identified in the [alpha]1A voltage-dependent calcium channel gene (CACNA1A), the mechanism which leads to predominant Purkinje cell degeneration is totally unknown. In this study, we show that the calcium channel mRNA/protein containing the CAG repeat/polyglutamine tract is most intensely expressed in Purkinje cells of human brains. In SCA6 brains, numerous oval or rod-shaped aggregates were seen exclusively in the cytoplasm of Purkinje cells. These cytoplasmic inclusions were not ubiquitinated, which contrasts with the neuronal intra-nuclear inclusions of other CAG repeat/polyglutamine diseases. In cultured cells, formation of perinuclear aggregates of the channel protein and apoptotic cell death were seen when transfected with full-length CACNA1A coding an expanded polyglutamine tract. The present study indicates that the mechanism of neurodegeneration in SCA6 is associated with cytoplasmic aggregations of the [alpha]1A calcium channel protein caused by a small CAG repeat/polyglutamine expansion in CACNA1A.

Spinocerebellar ataxia type 6 knockin mice develop a progressive neuronal dysfunction with age-dependent accumulation of mutant Ca V 2.1 channels

“…It is interesting, however, that the relative abundance of MPI transcripts increased as the CAG repeat length increased. This finding is in good agreement with the observation that the cerebellar transcripts containing 28 CAG repeats, obtained from the cerebellar cortex of an SCA6 patient, had a higher relative abundance of the GGCAG-containing variant compared with those containing 13 CAG repeats (18). It is not clear how CAG repeat length affects splicing, but we propose that the CAG repeat itself or its flanking sequence may act as an exonic cis-element contributing to correct splice-site identification.…”

“…5B). These results suggest that aged Sca6 84Q/84Q mice developed neuronal inclusions, which were reminiscent of those seen in human SCA6 PCs (18,19), without apparent neurodegenerative changes.…”

“…Thus, our observations that the mutant Ca V 2.1 channel did not lose its function and that its aggregation occurred in Sca6 84Q cerebella in both an age-and dosagedependent manner support the notion that SCA6 is caused by a toxic gain-of-function mechanism. At least two forms of Ca V 2.1 accumulation (surface and cytoplasmic) have been implicated in the SCA6 pathogenesis (11,18,19). Although the Ab used here showed cytoplasmic mutant Ca V 2.1 aggregation in cerebellar PCs of Sca6 84Q/84Q mice, further study using mice with larger expanded alleles will be necessary to determine whether the mutant Ca V 2.1 also accumulates on the cellular surface or in the presynaptic terminals over time and ultimately changes the Ca 2ϩ homeostasis in PCs of aged mutant mice.…”

CaV2.1 voltage activated calcium channels and synaptic transmission in familial hemiplegic migraine pathogenesis

“…Disorders not associated with FHM are episodic ataxia type 2 (Zafeiriou et al 2009, Cuenca-Leon et al 2009, Cricchi et al 2007, Jen et al 2004, 2008, Ophoff et al 1996, progressive ataxia (Yue et al 1997), spinocerebellar ataxia type 6 (Tsou et al , Restituito et al 2000, Ishikawa et al 1999, Zhuchenko et al 1997) and absence (Imbrici et al 2004) and generalized epilepsy (Haan et al 2005, Jouvenceau et al 2001). In addition FHM1 mutations were also found in family members who had only "normal" no-paretic migraine but no FHM.…”

Development of Purkinje cell degeneration in a knockin mouse model reveals lysosomal involvement in the pathogenesis of SCA6

“…Pathologically, the formation of ubiquitinated intranuclear neuronal inclusions (NIs) is a common feature in the majority of polyQ diseases (8). However, mutant Ca v 2.1 channels form inclusions in the cytoplasm of SCA6 PCs that typically lack ubiquitin immunoreactivity (9).…”