Put together with the additional observation that Sept4 protein levels are decreased in substantia nigra dopaminergic neurons from cases of sporadic PD, these results place Sept4 at the center of PD pathology. The authors write that Sept4 may be a “dual susceptibility factor” for PD, where loss of the protein might both diminish dopaminergic signaling and enhance α-synuclein toxicity.

Sept4 is one of a large family of scaffolding proteins, cellular organizers that bring together proteins into functional complexes. Another septin family member was implicated in PD when the Sept5/CDCrel-1 protein proved to be a target for parkin (see ARF related news story).

Previously Ihara and colleagues showed that Sept4, but not other septins, was associated with α-synuclein in Lewy bodies from patients with PD and Lewy body dementia (Ihara et al., 2003). The researchers also found Sept4 in neurofibrillary tangles (Kinoshita et al., 1998). Their new work shows that Sept4 is depleted in the striatum from five cases of sporadic PD. The loss mirrors the decrease in dopamine transporter (DAT), and suggests that Sept4 deficiency is a common pathological change in PD. In healthy brain, Sept4 localized to striatal presynaptic dopaminergic terminals, where it appeared in conjunction with DAT and α-synuclein.

To pin down the physiological role of Sept4, the researchers made knockout mice. The mice appeared mostly normal, but on closer examination revealed evidence of dopaminergic deficiencies. Dopaminergic synapses in the striatum of knockout mice were structurally intact, but lacked important players in dopamine production, including tyrosine hydroxylase and DAT. Other proteins in the presynaptic exocytic pathway were reduced in the mice, suggesting that they, too, might normally associate with Sept4. From their extensive characterization of the mice, the researchers concluded that Sept4 is a key component of the presynaptic dopamine metabolism machinery, where it most likely serves a scaffolding function.

In addition, Sept4 suppressed α-synuclein-mediated neurodegeneration. The researchers showed this by crossing the knockout mice with animals carrying a pathogenic α-synuclein transgene (A53T mutant). Removing Sept4 in these mice led to worse symptoms and shorter survival compared to transgenic parents. The Sept4-/- A53T transgenic mice still had α-synuclein aggregates, but the aggregates differed from those in Sept4-expressing mice. The neurons contained more aggregated α-synuclein, and more pathologically phosphorylated protein. In-vitro studies established that a direct interaction between the two proteins protected α-synuclein from aggregation and phosphorylation.

These results suggest a model where Sept4 at first serves to protect neurons from α-synuclein aggregation, but ends up sacrificing itself to the cause. As α-synuclein accumulates and proceeds to Lewy body formation, the authors posit, Sept4 becomes depleted from presynaptic terminals, leading to impaired DA transmission and enhanced α-synuclein toxicity. “Pathological α-synuclein/Sept4 interaction in Lewy bodies and dissociation of their physiological interaction at presynaptic terminals may constitute a vicious cycle that further disrupts the homeostasis of DA neurons in PD,” they write.

The model implies that restoring Sept4 function in synapses could spare DA neurons and offer a new therapeutic opportunity, a possibility Brian Spencer, Leslie Crews, and Eliezer Masliah of University of California at San Diego discuss in a preview accompanying the paper.

In another report on α-synuclein toxicity, Michael Kramer and Walter Schulz-Schaeffer at the University of Goettingen, Germany, come from another angle to suggest a protective function for the large aggregates in dementia with Lewy bodies (DLB). While Lewy bodies are the most striking inclusions in DLB, their location and number correlate poorly with dementia. In a paper published in the Journal of Neuroscience, the authors present data that abundant small, presynaptic α-synuclein aggregates occur in DLB, and may be linked with a loss of dendritic spines, an effect that could account for the cognitive symptoms of DLB.

The researchers developed techniques to distinguish small synuclein aggregates from native protein by protease treatment of tissue sections. In brains from DLB patients, they see Lewy bodies, but also heavy deposits of aggregated (protease-resistant) α-synuclein throughout the cortex and subcortical grey matter. Sucrose gradient fractionation of brain homogenates from frozen human DLB brain showed that Lewy bodies formed a minor peak, while most of the α-synuclein was in smaller aggregates. The smallest of these were enclosed in synaptosomes. Going back to brain, they saw presynaptic α-synuclein accumulation at sites of dendritic spine retractions and loss of the spine protein drebin. These results suggest that synaptic effects of α-synuclein could be causing dementia symptoms.

The data further supports the idea that Lewy body formation protects neurons from the pathogenic effects of α-synuclein. But if the small aggregates detected by Kramer and Schulz-Schaeffer contain Sept4 on its way out of synapses, the situation may be a bit more complicated. Perhaps when it comes to protection, DA neurons might find Lewy bodies to be both their best friend, and worst enemy.—Pat McCaffrey.

Reference:
Ihara M, Yamasaki N, Hagiwara A, Tanigaki A, Kitano A, Hikawa R, Tomimoto H, Noda M, Takanashi M, Mori H, Hattori N, Miyakawa T, Kinoshita M. Sept4, a Component of Presynaptic Scaffold and Lewy Bodies, Is Required for the Suppression of alpha-synuclein Neurotoxicity. Neuron. 2007 Feb 15;53(4):519-33. Abstract

Spencer B, Crews L, Masliah E. Climbing the scaffolds of Parkinson's disease pathogenesis. Neuron. 2007 Feb 15;53(4):469-70. Abstract

Kramer ML, Schulz-Schaeffer WJ. Presynaptic alpha-synuclein aggregates, not Lewy bodies, cause neurodegeneration in dementia with Lewy bodies. J Neurosci. 2007 Feb 7;27(6):1405-10. Abstract