Welch K, Yuan J.
Releasing the nerve cell killers.
Nat Med. 2002 Jun;8(6):564-5.
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I found this paper quite intriguing and very much relevant to PD. I believe it provides a very plausible model that tries to address the long-standing question of the fairly selective neurodegenerative process seen in PD (i.e. dopaminergic neuronal vulnerability), and it convincingly links the metabolisms of dopamine and α-synuclein to each other and to a neurotoxic process. That the work was carried out in cultured human neurons no doubt brings this to a new level. Peter Lansbury's team has addressed this in an in-vitro system (see related news item).
If I were to be the reviewer, I would raising the following points:
1. I think there is a significant element of confusion in the terminology—not the actual data—in this article. The authors speak of a "soluble" synuclein form/complex. However, they mean "solubilized", as their regimen to study proteins in cells and tissue employs several membrane-disrupting detergents that clearly release α-synuclein and other proteins from membrane structures. This makes sense, then, with what they describe in the nigra from PD patients, where people have shown repeatedly an α-synuclein increase in detergent-requiring extracted material.
2. The data provided on the association of 14-3-3 and α-synuclein is less convincing than they write when one looks at Figure 6e. There is a significant component of non-specificity, which should be clarified. Nevertheless, there are already other reports (Ostrerova et al, 1999; Perez et al., 2002, and two neuropathological studies) that show an interaction of α-synuclein with 14-3-3.
3. The authors did not discuss that, in PD, a significantly affected area is that of Meynert's basal nucleus, a group of cholinergic neurons that use the same principal neurotransmitter, i.e. acetylcholine, as the human cortical neurons used as controls in their study.
This paper does not directly address the glycosylated species of α-synuclein that we found in human brain (see related news item). However, the work is consistent with our findings in that Xu et al. do not identify parkin associated with their higher Mr complex that contains regular, non-glycosylated α-synuclein both in cellular and brain extracts (we also don't see this.
We need to remember that many studies, such as this one by Yankner, involve in vitro systems that are not sustained by a complex network, as is the case in vivo in transgenic mice. This is also relevant to Alzheimer disease models.
Specifically, activation of caspase pathways is more easily triggered in vitro than in vivo. In general, we have a very difficult time triggering these cascades in transgenic mice. However--and this goes for humans, as well--synapses are very sensitive and vulnerable even in vivo, and apoptotic pathways that are activated in the whole cell in vitro are activated at the synaptic site in vivo. In the AβPP/synuclein-transgenic models, we observe widespread synaptic damage accompanied by focal activation of caspases (unpublished). Greg Cole and Mark Mattson call this process "synaptosis." It probably reflects more closely what happens in human brains and mice than do in vitro models.
It is interesting that the authors observed that α-synuclein is toxic to dopaminergic neurons while being rather protective for cortical neurons. Indeed, some papers have published cytotoxic effects of α-synuclein, whereas others have report on protective effects of α-synuclein against oxidative stress. As we recently reported (Hashimoto et al., 2002), such selectivity of the response against oxidative stress might reflect the interaction of α-synuclein with components of stress signaling pathways, such as JIP.
Finally, a series of biochemical experiments by J. Kim. and colleagues clearly showed that α-synuclein behaves as a high molecular weight molecule (~around 53 kD) under non-denaturing conditions, such as HPLC using size-exclusion columns. This seems to be due to the unique elongated structure of α-synuclein. Therefore, I wonder if the 54-83 kD immunoreactivity of α-synuclein, which was detectable by size-exclusive chromatography in the paper, might be due to neither α-synuclein oligomerization nor complex formation with other molecules, such as 14-3-3. More scrutiny will be required on this point.
Xu et al., present very interesting data that will improve our understanding of the relationship between α-synuclein and the degeneration seen in PD patients. Their work is in support of an interaction of dopamine and α-synuclein to mediate toxic effects specifically in dopaminergic cells of the substantia nigra pars compacta, while α-synuclein in the absence of dopamine may, in contrast, be neuroprotective. Secondly, they show that α-synuclein in these cells remains in a detergent-extractable fraction, possibly bound to 14-3-3 protein. It remains to be determined whether this is the toxic species in the human disease, or inversely, whether the formation of inclusions is protective.
Regarding the apparent similarity of human wildtype versus mutant a-synuclein proteins in their in-vitro and our in-vivo experiments: The difference between these two may be more obvious if the expression level is low. An indication of this sort is in the Xu et al. paper. In our material we think we have about a 10-fold induction above the normal levels, which may be well above where you can detect this difference (see ARF news story).
If their hypothesis is true, then overexpression of 14-3-3 protein should be helpful in reversing the apoptotic changes and, by analogy, should reduce the cell death seen in our rat model. In addition it would be worth trying whether antioxidants protect against α-synuclein toxicity in vivo.
The notion that dopamine accelerates the neurodegenerative process while treating symptoms is interesting. We are currently testing the interaction of α-synuclein and dopamine in various in-vivo models where we overexpress α-synuclein using the AAV vectors, but it is too early to give any results on that.
I appreciated reading the above discussion of Xu et al.’s paper regarding a potential role for α-synuclein, 14-3-3, and dopamine in the pathogenesis of Parkinson's disease. I agree that α-synuclein and dopamine are a toxic mix. I do, however, want to point out our paper in the April 15th Journal of Neuroscience (Perez et al., 2002), which was the first to report a function for α-synuclein that is specific to dopamine neurons. Our data show that α-synuclein, in addition to 14-3-3, is an important regulator of tyrosine hydroxylase, the rate-limiting enzyme in dopamine biosynthesis.
Our data clearly show that dopaminergic cells stably overexpressing α-synuclein are healthy, maintain high-level tyrosine hydroxylase expression, co-localize tyrosine hydroxylase with α-synuclein, and have dramatically reduced dopamine synthesis. First reported at the Society for Neuroscience meeting in 2000, our findings prompted us to speculate even then that α-synuclein likely contributes to macromolecular damage underlying the selective loss of dopamine neurons in Parkinson's disease, and that it does so by an interaction with 14-3-3 that regulates dopamine with potential toxic consequences. Elegant work by Peter Lansbury's group (see related news item) also implicates α-synuclein and dopamine in Parkinson's disease pathogenesis, if by another mechanism.
The findings of Xu et al. are intriguing. It is curious, however, that nigral neurons overexpressing α-synuclein were not protected against dopamine-related toxicity, as would be expected since a-synuclein downregulates dopamine synthesis. It will be important to elucidate the mechanism(s) by which α-synuclein affects dopaminergic cell viability. Nonetheless, ours is the first report to identify a function for α-synuclein that is specific to dopaminergic neurons."
Reply by Bruce Yankner
The paper by Perez et al. is quite interesting because it suggests that α-synuclein can downregulate tyrosine hydroxylase activity in a stably transfected dopaminergic cell line. However, this is a cell line that has been selected for stable α-synuclein overexpression. The cells probably survive because they have downregulated dopamine production. It remains to be determined whether primary neurons can do the same thing. Our report suggests that α-synuclein-induced degeneration of dopaminergic neurons is dependent on dopamine production. Thus, primary neurons may not have the same capacity for TH downregulation as a transformed cell line. Interestingly, we also have been able to select for stably transfected SH-SY5Y cell lines that overexpress α-synuclein and survive. However, during this selection process most cells die. It is only a small subpopulation that can survive and be expanded. And although these cells survive, they exhibit increased vulnerability to exogenous dopamine.