CSPα reduces aggregates and rescues striatal dopamine release in αsynuclein transgenic mice

Background

Cysteine string protein-alpha (CSPα) and alpha-Synuclein (αSyn) are critical players in synaptic function as they regulate vesicle trafficking and neurotransmitter release. Both act as chaperones in the formation of the soluble N-ethylmaleimide sensitive fusion attachment protein receptor (SNARE) complex, an essential step in vesicle exocytosis (1). Specifically, CSPα promotes the assembly of SNAP-25 with other SNARE proteins in complex with Heat shock cognate 70kDa protein (Hsc70) (2).

Synaptic abnormalities including impaired neurotransmitter release are closely associated with neurodegenerative processes. In Parkinson’s disease (PD), these occur even before the characteristic loss of dopaminergic neurons and are thought to be mediated by intracellular αSyn aggregates, which are the main pathological hallmark of this disorder (3).

Despite its detrimental role in neurodegeneration, ablation of αSyn does not cause gross synaptic alterations (4). On the contrary, CSPα KO mice display impaired synaptic transmission and progressive neurodegeneration resulting in death after several weeks (5). Gene deletion studies provide mounting evidence for a neuroprotective function of CSPα. Strikingly, CSPα KO phenotypes can be rescued by αSyn overexpression, while additional αSyn KO exacerbates these phenotypes (6). Furthermore, mice lacking synucleins show increased CSPα levels (7).

Considering the pathological role of αSyn aggregates in synaptic dysfunction and the chaperone activity of CSPα in vesicle exocytosis, this preprint aimed to investigate their mutual impact.

Results

In their study, Calò et al. used 1-120hαsyn-transgenic mice, which express a C-terminal truncated version of αSyn to specifically induce its aggregation in neurons in the absence of endogenous αSyn (8). Importantly, these animals show relevant features also observed in PD patients such as locomotor impairment, reduced dopamine levels in the striatum and re-distribution of SNARE proteins (8, 9).

Immunoblotting revealed a reduction of CSPα protein levels in synaptosomal extracts of the striatum in 1-120hαsyn mice accompanied by decreased levels of CSPα/Hsc70 complexes.

CSPα plays an important role in vesicle trafficking and αSyn aggregation can alter exocytosis events. Thus, they examined whether CSPα can rescue aggregate-induced vesicle cycle impairment in vitro. Therefore, PC12 cells stably expressing 1-120hαSyn and showing vesicle trafficking deficits were transduced with a viral vector encoding human CSPα (9). Interestingly, this treatment completely restored vesicle cycling in 1-120hαSyn PC12 cells back to wildtype cell levels.

Subsequently, by employing a similar approach, they assessed whether these results can also be observed in vivo. Following injection of CSPα-containing viral vector into the substantia nigra of 1-120hαsyn mice, striatal dopamine release was measured by microdialysis. CSPα vector-treatment significantly increased the release of dopamine compared to control mice treated with an empty vector. Immunostaining and dStorm super-resolution microscopy of the striatum revealed that CSPα expression also directly impacts the amount of these pathological aggregates. Specifically, injection of CSPα vector reduced the number of αSyn aggregates in 1-120hαsyn mice, while monomeric αSyn species were increased.

In summary, these results indicate that αSyn aggregation adversely affects CSPα expression at the synapse, while forced CSPα expression diminished αSyn aggregates and restored impeded dopamine release.

Why I chose this preprint

Like αSyn, CSPα facilitates vesicle cycling – thus pulling on the same side of the string in synaptic function. Synaptic dysfunction is common in neurodegenerative diseases and often precedes neuronal death. Interestingly, CSPα is reduced in post mortem brains of Alzheimer’s disease (AD) and PD patients indicating a universal neuroprotective role (10). In their study, Calò et al., show a mutual impact of pathological αSyn aggregation and CSPα and provide further evidence that CSPα and other chaperones acting at the synapse are potential targets for the treatment of different neurodegenerative diseases. Moreover, this preprint highlights the importance of gaining in-depth knowledge about the function and the interconnection of proteins in a given pathway in a pathological context – such as αSyn and CSPα in synaptic dysfunction – as this can provide the basis for the development of new therapeutic strategies.

Future perspectives

Although the results reported here are encouraging, it is necessary to further investigate the impact of αSyn aggregation on CSPα expression and function and vice versa to assess the actual value of CSPα as treatment target. Importantly, the model used here reflects neuronal alterations in an early stage of disease before cell death occurs. In the future, it is of major interest i) to evaluate whether CSPα expression can also ultimately rescue neuronal loss and ii) to determine its effect on synaptic function in other neurodegenerative conditions in the absence of αSyn aggregation, for example in AD.

Questions to the authors

1. CSPα is known to regulate correct SNARE complex assembly by maintaining the confirmation of SNAP-25. Did CSPα treatment change SNAP-25 levels in 1-120hαsyn mice?
2. As described previously, aged 1-120hαsyn mice show impaired spontaneous locomotion (8). Does CSPα expression show beneficial effects on locomotor deficits?
3. 1-120hαsyn mice are lacking endogenous mouse α Do you think the presence of physiological αSyn affects the impact of CSPα on αSyn aggregation and neurotransmitter release?

 

References:

1. Burgoyne, R. D., & Morgan, A. (2011). Chaperoning the SNAREs: a role in preventing neurodegeneration?. Nature cell biology, 13(1), 8-9.
2. Sharma, M., Burré, J., & Südhof, T. C. (2011). CSPα promotes SNARE-complex assembly by chaperoning SNAP-25 during synaptic activity. Nature cell biology, 13(1), 30-39.
3. Spillantini, M. G., Schmidt, M. L., Lee, V. M. Y., Trojanowski, J. Q., Jakes, R., & Goedert, M. (1997). α-Synuclein in Lewy bodies. Nature, 388(6645), 839-840.
4. Abeliovich, A., Schmitz, Y., Fariñas, I., Choi-Lundberg, D., Ho, W. H., Castillo, P. E., … & Hynes, M. (2000). Mice lacking α-synuclein display functional deficits in the nigrostriatal dopamine system. Neuron, 25(1), 239-252.
5. Fernández-Chacón, R., Wölfel, M., Nishimune, H., Tabares, L., Schmitz, F., Castellano-Muñoz, M., … & Südhof, T. C. (2004). The synaptic vesicle protein CSPα prevents presynaptic degeneration. Neuron, 42(2), 237-251.
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7. Burré, J., Sharma, M., Tsetsenis, T., Buchman, V., Etherton, M. R., & Südhof, T. C. (2010). α-Synuclein promotes SNARE-complex assembly in vivo and in vitro. Science, 329(5999), 1663-1667.
8. Tofaris, G. K., Garcia, R. P., Humby, T., Lambourne, S. L., O’Connell, M., Ghetti, B., … & Spillantini, M. G. (2006). Pathological changes in dopaminergic neurons in mice transgenic for human α-synuclein (1–120): implications for Lewy body disorders. Neurosci, 26, 3942-3950.
9. Garcia-Reitböck, P., Anichtchik, O., Bellucci, A., Iovino, M., Ballini, C., Fineberg, E., … & Goedert, M. (2010). SNARE protein redistribution and synaptic failure in a transgenic mouse model of Parkinson’s disease. Brain, 133(7), 2032-2044.
10. Sharma, M., Burré, J., & Südhof, T. C. (2012). Proteasome inhibition alleviates SNARE-dependent neurodegeneration. Science translational medicine, 4(147), 147ra113-147ra113.

Tags: alpha-synuclein, cysteine string protein-alpha, neurodegeneration, synaptic dysfunction

doi: https://doi.org/10.1242/prelights.24153

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