Henrietta Nielsen at the Mayo Clinic in Jacksonville, Florida, wrote to Alzforum that the study is well executed and intriguing. “These novel findings position α-synuclein as a key player in AD pathophysiology, in the absence of Lewy pathology, and may offer important implications to future drug discovery trials,” she wrote (see full comment below). However, other researchers in the field noted that there are many twofold differences reported in the literature, so it is hard to know how significant this finding is.

Though the mechanism behind the cognitive effect is unknown, another paper in the same issue of the journal sheds some light on the subject. Researchers led by Subhojit Roy at the University of California, La Jolla, report that doubling α-synuclein in neuronal cultures slows the movement of vesicles between presynaptic boutons and shrinks the size of neurotransmitter pools available for release. These changes would effectively weaken synaptic transmission.

Synuclein is the primary constituent of the Lewy bodies found in all Parkinson’s disease brains and almost half of AD brains. Despite this, evidence is growing that the soluble form of the protein causes the most problems in PD (see, e.g., ARF related news story; ARF news story). Lesné and colleagues wondered what role soluble α-synuclein might play in AD.

To examine this, first author Megan Larson used brain tissue from the inferior temporal gyrus of 84 participants in the Religious Order Study, a longitudinal project that correlates clinical and cognitive changes with brain pathology. This gyrus is distinct from the nearby hippocampus and entorhinal cortex, which exhibit some of the earliest signs of AD. Sylvain noted they chose the temporal gyrus because it develops characteristic AD pathology, including plaque burden, reduced glucose use, and thinning gray matter. Larson and colleagues extracted and measured soluble α-synuclein from a variety of cellular and extracellular compartments, finding that the intracellular protein was about twofold higher in people with AD regardless of the presence of Lewy bodies. High amounts of soluble α-synuclein correlated strongly with low test scores on several measures of cognitive ability (episodic, semantic, and working memory, perceptual speed, visuospatial ability, and global cognition). In fact, regression models showed that the protein was a better predictor of cognitive impairment than were levels of soluble Aβ, total tau, or phosphorylated tau from the same brain region. This implies that α-synuclein could contribute to cognitive problems, the authors note. Strengthening this idea, seven-month-old TgI2.2 mice that overexpress human wild-type α-synuclein (see Lee et al., 2002) showed memory problems in the Barnes circular maze that were comparable to the deficits of similarly aged J20 AD mice that overexpress mutant human APP.

The authors wanted to know what causes α-synuclein to rise in AD brains. To investigate this, they turned to mouse models, crossing Tg2576 APP mutant mice with Tg4510 mice that express mutant human tau. Although soluble α-synuclein does not rise with age in the parent strains, eight-month-old offspring showed almost a twofold increase over three-month-old levels. This implies that Aβ and tau pathology work together to upregulate α-synuclein, the authors suggest. Other work has shown that Aβ, tau, and α-synuclein cooperate to speed each other’s aggregation (see ARF related news story).

Another unanswered question is how α-synuclein might compromise cognition. Earlier work by Roy’s group and others tied excessive levels of the protein to presynaptic deficits. In particular, neurons from mice with elevated α-synuclein poorly recycled neurotransmitter pools, and showed a dearth of some presynaptic proteins such as synapsin (see ARF related news story; ARF news story). Therefore, Lesné and colleagues examined their human brain samples for presynaptic problems. Just as in the animal models, they found less synapsin in brains with more α-synuclein. Speaking with Alzforum, Lesné pointed out that Aβ and tau mainly harm the postsynapse, whereas α-synuclein appears to attack the presynapse. Its dampening of presynaptic activity may leave neurons unable to compensate for the initial insult to the postsynapse, Lesné speculated, creating a two-pronged attack on synaptic transmission.

In future work, Lesné plans to examine what form of soluble α-synuclein damages synapses—monomers, dimers, higher-order oligomers? Some work in the field suggests that α-synuclein natively forms a tetramer in the brain, although the findings remain controversial (see ARF related news story; ARF news story). Lesné will also look at CSF levels of the protein in people with AD to see if these correlate with brain levels and could be used as a biomarker.

Nielsen noted that previous work documented less CSF α-synuclein in people with synucleinopathies such as Parkinson’s disease and dementia with Lewy bodies, but more in AD, suggesting the protein might be a useful biomarker for distinguishing between these diseases. In addition, her lab reported that CSF tau and α-synuclein are highly correlated both in AD and PD, fitting with the authors’ model that tau helps to upregulate α-synuclein.

In his paper, Roy further elucidated α-synuclein’s effect on presynapses. First author David Scott made primary hippocampal cultures from wild-type mice, α-synuclein-null mice, and mice that overexpressed α-synuclein. An automated imaging algorithm allowed the researchers to visualize thousands of presynaptic boutons per experiment. As in their previous results, they found that neurons that overexpressed the protein had smaller recycling pools at presynapses than did wild-type cells. Unexpectedly, however, α-synuclein knockout cells showed a converse effect, with larger recycling pools than controls. “The data suggest that synuclein fine-tunes the size of the recycling pool and, consequently, regulates neurotransmitter release,” Roy told Alzforum.

Recycling pools can be replenished not just by synaptic reuptake, but also by trafficking of vesicles between nearby synapses. To look at this “superpool” trafficking, the authors loaded synapses with fluorescent dyes, then photobleached one bouton and measured how quickly unbleached dye from other boutons moved in. The more α-synuclein a bouton contained, the more slowly it recovered, the authors found. Boutons from knockout animals showed the fastest recovery of all. The results suggest that soluble α-synuclein inhibits vesicle mobility, the authors note. In agreement with this, experiments in yeast and other lower-order animals have shown that α-synuclein interferes with vesicle movement between the endoplasmic reticulum and Golgi (see ARF related news story on Outeiro and Lindquist, 2003; ARF related news story). The findings imply that the superpool trafficking deficit underlies shrunken recycling pools, but the current data cannot yet prove that directly, Roy cautioned.

Commenting on the Roy paper, Lesné noted, “Studies such as this one are really important, because we don’t know enough about the intrinsic function of α-synuclein, especially its soluble forms.”—Madolyn Bowman Rogers.

References:
Larson ME, Sherman MA, Greimel S, Kuskowski M, Schneider JA, Bennett DA, Lesné SE. Soluble α-synuclein is a novel modulator of Alzheimer’s disease pathophysiology. J Neurosci. 2012 Jul 25;32(30):10253-66. Abstract

Scott D, Roy S. α-Synuclein inhibits intersynaptic vesicle mobility and maintains recycling-pool homeostasis. J Neurosci. 2012 Jul 25;32(30):10129-35. Abstract