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