Move over α-synuclein; your smaller brother wants in on the act. In the November 2 Nature Communications, researchers led by Makoto Hashimoto at the Tokyo Metropolitan Institute for Neuroscience, Japan, report that a β-synuclein mutation linked to dementia with Lewy bodies (DLB) causes neurodegeneration in mice—even in the absence of α-synuclein. The finding may come as a surprise, since research indicates that β-synuclein protects against toxic effects of α-synuclein, the major component of Lewy bodies. The new work suggests that β-synuclein mutants are dangerous, not because they fail to protect against α-synuclein, but because β-synuclein causes neurodegeneration in its own right. How frequently that happens in humans remains to be seen. DLB is the second most common form of dementia after Alzheimer’s disease, but the β-synuclein mutation in question, a histidine in place of a proline at position 123 (P123H), seems quite rare. “It is unclear how many families might be affected by this particular mutation,” said Albert LaSpada, University of California, San Diego. The mutation was discovered at LaSpada’s laboratory when he was at the University of Washington, Seattle. LaSpada collaborated with the Japanese group to develop the P123H β-synuclein mice.

Only a small stretch of amino acids differentiates α- and β-synucleins. The former contain the highly amyloidogenic NAC domain, which drives protein aggregation. Being NAC free, β-synuclein has less propensity for such shenanigans, and in fact, seems to dampen aggregation of α-synuclein. Hashimoto, when working with Eliezer Masliah at the University of California, San Diego, found that β-synuclein curbed pathology in α-synuclein transgenic mice and improved their motor function (see ARF related news story on Hashimoto et al., 2001). Later, LaSpada and colleagues found that β-synuclein also prevents expression of the α isoform (see Fan et al., 2006). These mouse studies hint that β-synuclein mutations might cause DLB (see Ohtake et al., 2004) because they abolish the protein’s protective effects. An alternative explanation, supported by the dominant nature of the mutations, is that a gain of function in mutant β-synucleins causes neurodegeneration. Mutant β-synuclein forms inclusions when overexpressed in neuroblastoma cells, for example (Wei et al., 2007).

The mice developed by the Hashimoto lab support the gain-of-function hypothesis. First author Masayo Fujita and colleagues generated four lines of transgenic mice that express human P123H β-synuclein under control of the Thy-1 promoter. These animals (P123H βS) formed no Lewy bodies, but by six months of age, β-synuclein had amassed in various brain regions, including the cerebral cortex and the hippocampus. Neurons in the striatum had β-synuclein-laden axonal swellings, or globules, which were positive for autophagy markers, suggesting that process is blocked or retarded in these mice. The lysosome is responsible for mopping up autophagic vesicles, but activities of the lysosomal proteases cathepsins B and D were lower in the brains of the transgenic mice compared to wild-type, which hints that it might be the last step in the autophagy process that is blocked.

The β-synuclein inclusions continued to accumulate until the animals were tested at 18 months old. Regions positive for these globules also had rampant gliosis, evident by a dramatic uptick in expression of astroglial cell markers. Behaviorally, P123H βS mice had both cognitive and motor problems. The former showed up as early as six months, when the animals performed poorly compared to controls in the Morris water maze test of learning and memory. The animals also showed odd social behavior, such as reduced sniffing, twitching, and grooming at six months. Physically, six-month-old mice were as capable as age-matched normals on the rotarod test of motor control, but they became progressively weaker at this task over the next 12 months.

The lack of Lewy bodies in these animals might suggest that the pathology was independent of α-synuclein, but as LaSpada told ARF, it is difficult to fully recapitulate human pathology in mice. Instead, proof that the larger α-synuclein was not involved came from crossing the P123H βS mice with α-synuclein knockouts. Fujita and colleagues found that the P123H βS/α-synuclein KO mice showed exactly the same pathology as the parent P123H strain, indicating that α-synuclein is not required for pathology.

Toxicity may be exacerbated by α-synuclein, however. Crossing P123H βS mice with a strain that overexpresses α-synuclein led to offspring with enhanced pathology. “If you overexpress α-synuclein you enhance pathology, but if you take α-synuclein away it does not affect β-synuclein,” said LaSpada.

Why β-synuclein is toxic in the absence of its larger isoform is not yet clear. LaSpada believes it is related to the protein’s ability to misfold. However, β-synuclein is not amyloidogenic, LaSpada pointed out, so the nature of the misfolded β-synuclein is unclear. Soluble oligomeric forms of other proteins, such as amyloid-β and α-synuclein are now widely believed to be the most toxic forms of those proteins. LaSpada suggested the same might be true for β-synuclein. “That is what we are coming to realize, that what you don’t see is what causes the toxicity,” he said.—Tom Fagan.

Reference:
Fujita M, Sugama S, Sekiyama K, Sekigawa A, Tsukui T, Nakai M, Waragai M, Takenouchi T, Takamatsu Y, Wei J, Rockenstein E, LaSpada AR, Masliah E, Inoue S, Hashimoto M. A beta-synuclein mutation linked to dementia produces neurodegeneration when expressed in mouse brain. Nature Communications 2010 Nov. 2. Abstract

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  1. No doubt, dementia with Lewy bodies (DLB) is a complex disease with many possible factors contributing to the pathogenesis. Hashimoto and colleagues show that the P123H mutation in β-synuclein linked to DLB causes neurodegeneration in a mouse model, and this appears independent of α-synuclein. In terms of DLB genetics, only two familial mutations in DLB have been reported thus far, including α-synuclein E46K (Zarranz et al., 2004) and β-synuclein P123H (Ohtake et al., 2004), as well as one mutation, V70M, in an apparently sporadic case of DLB. In this context, the P123H mouse model is very important to demonstrate that the β-synuclein P123H mutation is pathogenic in an vivo animal model.

    It is still unclear how β-synuclein causes neurodegeneration in this model, but in terms of DLB, it may interact with other molecules other than α-synuclein, such as amyloid oligomers which are also found in DLB, resulting in neuronal toxicity (as in β-synuclein crossed with APP-overexpressing mice). Therefore, this model may be useful to further explore the biology and interactions of β-synuclein, and to provide insight into the etiologies of DLB.

    View all comments by Gilbert J. Ho
  2. For a long time, β-synuclein (βS) was considered to be a neuroprotective homolog of the cytotoxic, disease-causing α-synuclein (αS). However, this recent study by Fujita et al. demonstrates that the P123H βS mutation, associated with familial cases of dementia with Lewy bodies (DLB) is pathogenic in itself, causing neuronal degeneration, and learning and memory deficits in mice. In this study, the authors nicely show that P123H βS pathology is not affected by silencing the endogenous αS expression, supporting the occurrence of a unique mechanism of toxicity for βS mutation rather than loss of a protective effect of βS. Importantly, crossing these P123H βS mice with αS overexpressing mice that model the synucleinopathies resulted in enhanced neurodegeneration, suggesting that P123H βS may cooperate with pathogenic αS and accelerate its pathogenesis.

    While the nature of the association between αS and βS is still unresolved, growing evidence suggests a physical association between the two homologous proteins. βS was shown to normally occur in hetero-oligomers with αS in wild-type mouse brain (1) and in this way control polyunsaturated fatty acid (PUFA)-enhanced αS oligomerization (1). In addition, βS was shown to directly inhibit αS aggregation and protofibril formation in vitro (2-4).

    The authors state that the mechanism underlying the conversion from neuroprotective to neurotoxic βS by the single amino acid substitution P123H is unknown. One potential approach to understanding this pathogenic conversion may rely on elucidating the nature of βS association with brain lipids. While a role for brain lipids in αS pathogenesis is consolidating, only a limited number of studies focus on βS associations with lipids. This may be due to differences in the lipid-binding domains between αS and βS, and specifically, the deletion of 11 residues in the βS sequence affecting the degree of homology to the apolipoprotein A1 in the N-terminal region of the βS protein (5). Yet, in vivo, βS associates with lipids; it is found in purified myelin, and in additional lipid-rich fractions of the mouse brains (1,6-7).

    The direct involvement of βS in pathogenesis of DLB as shown in this study calls for revisiting the definition of the synucleinopathies, taking into account a role for βS alongside αS abnormalities.

    View all comments by Ronit Sharon

References

News Citations

  1. α-Synuclein Aggregation Prevented by β-Homolog

Paper Citations

  1. . beta-Synuclein inhibits alpha-synuclein aggregation: a possible role as an anti-parkinsonian factor. Neuron. 2001 Oct 25;32(2):213-23. PubMed.
  2. . Beta-synuclein modulates alpha-synuclein neurotoxicity by reducing alpha-synuclein protein expression. Hum Mol Genet. 2006 Oct 15;15(20):3002-11. PubMed.
  3. . Beta-synuclein gene alterations in dementia with Lewy bodies. Neurology. 2004 Sep 14;63(5):805-11. PubMed.
  4. . Enhanced lysosomal pathology caused by beta-synuclein mutants linked to dementia with Lewy bodies. J Biol Chem. 2007 Sep 28;282(39):28904-14. PubMed.
  5. . A β-synuclein mutation linked to dementia produces neurodegeneration when expressed in mouse brain. Nat Commun. 2010;1:110. PubMed.

Further Reading

Papers

  1. . A β-synuclein mutation linked to dementia produces neurodegeneration when expressed in mouse brain. Nat Commun. 2010;1:110. PubMed.

Primary Papers

  1. . A β-synuclein mutation linked to dementia produces neurodegeneration when expressed in mouse brain. Nat Commun. 2010;1:110. PubMed.