Abstract
Prominent features of HD neuropathology are the intranuclear and cytoplasmic inclusions of huntingtin and striatal and cortical neuronal cell death. Recently, synaptic defects have been reported on HD-related studies, including impairment of neurotransmitter release and alterations of synaptic components. However, the definite characteristics of synapse dysfunction and the underlying mechanisms remain largely unknown. We studied the gene expression levels and patterns of a number of proteins forming the cytoskeletal matrix of the presynaptic active zones in HD transgenic mice (R6/1), in hippocampal neuronal cultures overexpressing mutant huntingtin and in postmortem brain tissues of HD patients. To investigate the interactions between huntingtin and active proteins, we performed confocal microscopic imaging and immunoprecipitation in mouse and HEK 293 cell line models. The mRNA and protein levels of Bassoon were reduced in mouse and cell culture models of HD and in brain tissues of patients with HD. Moreover, a striking re-distribution of a complex of proteins including Bassoon, Piccolo and Munc 13–1 from the cytoplasm and synapses into intranuclear huntingtin aggregates with loss of active zone proteins and dendritic spines. This re-localization was age-dependent and coincided with the formation of huntingtin aggregates. Using co-immunoprecipitation, we demonstrated that huntingtin interacts with Bassoon, and that this interaction is likely mediated by a third linking protein. Three structural proteins involved in neurotransmitter release in the presynaptic active zones of neurons are altered in expression and that the proteins are redistributed from their normal functional site into mutant huntingtin aggregates.
Highlights
Huntington’s disease (HD) is a fatal autosomal dominant neurodegenerative disorder, caused by a CAG-triplet repeat expansion in the gene IT15 encoding the protein huntingtin [9]
We have previously shown that the Soluble NSF Attachment Protein Receptor (SNARE) proteins and Rabphilin 3A, both of which are involved in exocytosis and transmitter release, are affected in the R6/1 mouse model and in brain tissue from HD patients [39, 40]
Expression of cytoskeletal matrix of the active zone (CAZ) and CAZ associated proteins in the R6/ 1 mouse model We have previously studied the expression of a number of proteins involved in exocytosis in animal models of HD [39] and in samples from HD patients and control subjects [40]
Summary
Huntington’s disease (HD) is a fatal autosomal dominant neurodegenerative disorder, caused by a CAG-triplet repeat expansion in the gene IT15 encoding the protein huntingtin [9]. The R6/1 mice show the first overt motor symptoms at an age of 16 weeks and decline. Synaptic alterations, especially diminished exocytosis and compromised transmitter release, were observed in other HD models [3, 12, 13, 33, 35, 48]. In ex vivo preparations of the skeletal muscle of R6/2 mice, no clear denervation was exhibited, a reduction in the size of synaptic vesicle pool was observed in R6/2 mice compared to the WT mice. This reduction was caused by impairment of vesicle mobilization in neuromuscular junctions [15]
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