Association of the Cytoskeletal GTP-binding Protein Sept4/H5 with Cytoplasmic Inclusions Found in Parkinson's Disease and Other Synucleinopathies

Abstract

␣-Synuclein-positive cytoplasmic inclusions are a pathological hallmark of several neurodegenerative disorders including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. Here we report that Sept4, a member of the septin protein family, is consistently found in these inclusions, whereas five other septins (Sept2, Sept5, Sept6, Sept7, and Sept8) are not found in these inclusions. Sept4 and ␣-synuclein can also be co-immunoprecipitated from normal human brain lysates. When co-expressed in cultured cells, FLAG-tagged Sept4 and Myc-tagged ␣-synuclein formed detergent-insoluble complex, and upon treatment with a proteasome inhibitor, they formed Lewy body-like cytoplasmic inclusions. The tagged Sept4 and ␣-synuclein synergistically accelerated cell death induced by the proteasome inhibitor, and this effect was further enhanced by expression of another Lewy body-associated protein, synphilin-1, tagged with the V5 epitope. Moreover, co-expression of the three proteins (tagged Sept4, ␣-synuclein, and synphilin-1) was sufficient to induce cell death. These data raise the possibility that Sept4 is involved in the formation of cytoplasmic inclusions as well as induction of cell death in ␣-synuclein-associated neurodegenerative disorders.

Analysis of alpha-synuclein, dopamine and parkin pathways in neuropathologically confirmed parkinsonian nigra

“…Sept4 has been ascribed a role in neuronal differentiation and axon guidance through the control of mitochondrial function [64] and importantly, causes caspase activation [21]. Interestingly, in cultured neuroblastoma and fibroblast cells, co-expression of sept4 and SNCA synergistically accelerated cell death induced by the proteasome inhibitor, lactacystin [27]. Sept4 [formerly known as bradeion and peanut-like 2 (Drosophila)] is a member of the septin gene family of nucleotide-binding proteins which were originally described in yeast as cell division cycle regulatory factors.…”

“…An immunocytochemical study of both PD and other synucleinopathies found sept4 protein in cytoplasmic inclusions [27]. In AD, sept4, in addition to septl and sept2, accumulates in neurofibrillary tangles [33].…”

Some assembly required: yeast septins provide the instruction manual

“…In addition, SNARE proteins and microtubules are reported to contribute to the localization and/or assembly of septin filaments in animal cells [23,[50][51][52]. Aberrant deposition of filamentous septin structures correlates with age-related neurodegenerative disorders such as Alzheimer's and Parkinson's disease [53,54], but whether these aggregates are a cause or a consequence of these maladies is not known.…”

The pathobiology of the septin gene family

“…Direct evidence for the binding of septins to elements of vesicle trafficking pathway also now exists, with interactions between SEPT2 and syntaxin [126], SEPT2 and GLAST [127], SEPT4 and synuclein [128], and Sept5 and syntaxin [36,37] being reported. Finally, septins appear also to have a role in platelet function, with septins 4, 5, and 8 being associated with the alpha granules of platelets [129] and platelets from the Sept5 knock-out mouse have defective platelet function [130].…”

“…SEPT5 v2 has been reported to be a parkin-binding protein and parkin can function as an E2-dependent ubiquitin ligase capable of promoting the degradation of SEPT5 [180]. SEPT5 accumulates in the brains of individuals with autosomal recessive juvenile parkinsonism [128]. Although SEPT5 is expressed in the brain, is involved in exocytosis, and has been associated with parkinsonism and Down's syndrome, it is dispensable for normal development and function, as shown by the viability and normal development of Sept5 null mice [106].…”