Abnormalities in the synaptic vesicle fusion machinery in Huntington’s disease

doi:10.1016/S0361-9230(01)00611-6

Brain Research Bulletin

Volume 56, Issue 2, 15 September 2001, Pages 111-117

https://doi.org/10.1016/S0361-9230(01)00611-6 Get rights and content

Abstract

We have recently described the progressive and selective loss of the presynaptic protein complexin II in brains of mice (R6/2) transgenic for the Huntington’s disease (HD) mutation. Here we have determined the expression of components of the synaptic vesicle fusion machinery in the striatum and hippocampus from post-mortem brains of HD cases and neurologically normal controls. As in the brains of R6/2 mice, complexin II was markedly depleted in the HD striatum; the depletion was compartmentally organized, with complexin II-poor regions corresponding with areas of low immunoreactivity toward the matrix marker calbindin D_28K. Decreases in the levels of the soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) protein synaptobrevin 2 and of rab3A were also seen, but none of the other proteins tested was significantly affected. In the hippocampus, levels of complexin II, synaptobrevin 2, rab3A, and also of α-SNAP, were markedly elevated in HD brains. We suggest that the observed abnormalities in the expression of proteins known to be involved in the control of neurotransmitter release, including both modulators and core components of the vesicle fusion machinery, might account for at least some of the functional abnormalities seen in HD.

Introduction

Huntington’s disease (HD) is one of a family of genetic diseases caused by an expanded CAG repeat that is translated into an expanded polyglutamine repeat [24]. In HD, the polyglutamine repeat is contained within the protein, huntingtin, the function of which is unknown. Huntingtin is found loosely attached to synaptic vesicles [5], and interacts with several proteins which may themselves associate with transport vesicles 11, 27. It has therefore been proposed that abnormal protein interactions with mutant huntingtin may influence neurotransmitter release and thereby cause neuronal dysfunction 5, 22.

The mechanism controlling neurotransmitter release is not completely understood, although a number of proteins involved in exocytotic membrane fusion have been identified. These include the synaptic vesicle membrane protein synaptobrevin and the plasma membrane proteins syntaxin and SNAP-25, which are known collectively as SNAREs (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors). The vesicle-associated (v-) SNARE synaptobrevin is known to form a tight complex with the two target membrane-associated (t-) SNAREs syntaxin and SNAP-25 [19]. Formation of this complex is believed to pull the two interacting membranes together so that they fuse, thus allowing the release of the vesicle contents [25]. In addition, a number of accessory proteins are believed to control SNARE complex assembly and disassembly [10]. These include the synaptic vesicle proteins synaptotagmin, synaptophysin and rab3A, together with the cytosolic proteins α-SNAP and complexins I and II.

At present, the roles of the accessory proteins in neurotransmission are not fully understood. The distribution of the two complexins in the brain is different 17, 26. Furthermore, although some cells, such as pyramidal cells in the CA regions of the hippocampus, express both complexins I and II [17], it has been reported that complexins I and II are expressed predominantly in inhibitory and excitatory neurones, respectively [9]. The molecular function of the complexins in neurotransmitter release is also controversial 13, 16, 17, 21. The function of α-SNAP appears to be to co-operate with N-ethylmaleimide-sensitive factor (NSF) to disassemble SNARE complexes present within the same membrane which will be produced after membrane fusion, to allow further complex formation between membranes. It is therefore required to support multiple rounds of vesicle fusion [10]. The precise role of rab3A is still unclear; however, the phenotype of rab3A knockout mice suggests that it is involved in regulating the efficiency of Ca²⁺-triggered synaptic vesicle fusion 2, 6, 7, 10.

A loss of presynaptic SNARE proteins has been described in Alzheimer’s disease [18]. These changes occurred in parallel with neurodegenerative changes and probably reflect the loss of synapses. Interestingly, however, a decrease in the expression of complexin II has also been found in the medial temporal lobes of schizophrenic patients [9], and it has been suggested that this change may contribute to the pathophysiology of this disorder. Further, we have shown recently that in the brains of mice transgenic for the HD mutation (R6/2 line [12]) there is a progressive and selective loss of complexin II [14]. This result suggests that changes in complexin II might also be important in the pathophysiology of HD itself. In this study, therefore, we looked for changes in the brains of HD patients in the levels and distribution of proteins involved in neurotransmitter release, paying particular attention to complexin II.

Section snippets

Human tissue collection

The human brain tissue used in these studies was from the New Zealand Neurological Foundation Human Brain Bank in the Department of Anatomy, University of Auckland, New Zealand, and the collection procedures were approved by the University of Auckland Human Subjects Ethics Committee. All neurologically normal subjects had previously been in good health with no known history of neurological disease or drug treatment and all had died suddenly without receiving any form of medical treatment. For

Changes in levels of SNAREs and associated proteins in HD brains

We determined the expression of the SNAREs and associated proteins in synaptosomal fractions of the striatum and hippocampus from brains of all 12 HD patients and 5 neurologically normal subjects. The striatum was chosen because it is the primary region in which neurodegeneration occurs in HD. The hippocampus was chosen because it is important for cognitive function, and although neurodegeneration in the hippocampus is not extensive, cognitive impairments are pronounced, particularly in the

Discussion

In this study, we have identified region-specific changes in the levels of complexin II, synaptobrevin 2 and rab3A in the brains of HD patients, with decreased expression in the striatum and increased expression in the hippocampus. Notably, of the three SNARE proteins, only the v-SNARE synaptobrevin 2 was affected significantly in the HD cases; levels of the t-SNAREs syntaxin and SNAP-25 did not change. This result indicates that the depletion of synaptobrevin 2 (and also of complexin II and

Acknowledgements

We thank R. Jahn, M. Takahashi, H. McMahon and P. C. Emson for antibodies, W. Leavens and J. Bullock for technical assistance and R. Hart for photography. This work was funded by grants from the Hereditary Diseases Foundation (to A.J.M.), the Health Research Council of New Zealand and the New Zealand Neurological Foundation (to R.L.M.F.) and the Wellcome Trust (to J.M.E.)

References (27)

E.R. Chapman et al.
A novel function for the second C2 domain of synaptotagminCa²⁺-triggered dimerization
J. Biol. Chem.
(1996)
S.W. Davies et al.
Formation of neuronal intranuclear inclusions underlies the neurological dysfunction of mice transgenic for the HD mutation
Cell
(1997)
M. DiFiglia et al.
Huntingtin is a cytoplasmic protein associated with vesicles in human and rat brain neurons
Neuron
(1995)
P.J. Harrison et al.
Preferential involvement of excitatory neurons in medial temporal lobe in schizophrenia
Lancet
(1998)
L. Mangiarini et al.
Exon 1 of the HD gene with an expanded CAG repeat is sufficient to cause a progressive neurological phenotype in transgenic mice
Cell
(1996)
H.T. McMahon et al.
ComplexinsCytosolic proteins that regulate SNAP receptor function
Cell
(1995)
K. Reim et al.
Complexins regulate a late step in Ca²⁺-dependent neurotransmitter release
Cell
(2001)
S. Shimohama et al.
Differential involvement of synaptic vesicle and presynaptic plasma membrane proteins in Alzheimer’s disease
Biochem. Biophys. Res. Commun.
(1997)
H. Tokumaru et al.
SNARE complex oligomerization by synaphin/complexin is essential for synaptic vesicle exocytosis
Cell
(2001)
J. Velier et al.
Wild-type and mutant huntingtins function in vesicle trafficking in the secretory and endocytic pathways
Exp. Neurol.
(1998)

T. Weber et al.

SNAREpinsMinimal machinery for membrane fusion

Cell

(1998)

M. Yamada et al.

Immunohistochemical distribution of the two isoforms of synaphin/complexin involved in neurotransmitter releaseLocalization at the distinct central nervous system regions and synaptic types

Neuroscience

(1999)

U. Zechner et al.

Characterization of the mouse Src homology domain gene Sh3d2c on chr 7 demonstrates coexpression with huntingtin in the brain and identifies the processed pseudogene Sh3d2c-ps1 on chr 2

Genomics

(1998)

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Abnormalities in the synaptic vesicle fusion machinery in Huntington’s disease

Abstract

Introduction

Section snippets

Human tissue collection

Changes in levels of SNAREs and associated proteins in HD brains

Discussion

Acknowledgements

J. Biol. Chem.

Cell

Neuron

Lancet

Cell

Cell

Cell

Biochem. Biophys. Res. Commun.

Cell

Exp. Neurol.

Cell

Neuroscience

Genomics