Researchers have explained how a protein found in both yeast and humans facilitates the destruction of the core protein responsible for Parkinson’s disease.
An aggregate that impairs clearance
The loss of proteostasis involves the growing inability of cells’ lysosomes to clear toxic, misfolded proteins, which gradually worsens due to various processes of aging [1]. However, α-synuclein (α-syn), the protein responsible for Parkinson’s and Lewy body dementia [2], has itself been documented to impair lysosomal machinery and thus make things worse [3].
Proteins are often modified after they are created by cells, and α-syn is susceptible to being phosphorylated at serine 129. This alteration makes this protein much more prone to aggregation [4], and α-syn with this modification is commonly found in Lewy bodies [5].
Under normal circumstances, soluble α-syn is elimated through two methods. The 26S proteasome, which uses ATP energy and ubiquitin, is the primary method [6]; however, 20S requires neither to destroy unfolded α-syn [7].
Yeast and humans are surprisingly similar
In α-syn studies, mammalian cells aren’t required to understand proteosomal function; instead, these researchers previously conducted a yeast study that provided insight into α-syn’s effects on 26S [8]. At the time, they had noted that the protein Blm10, a proteosome activator with a human ortholog of PA200, was stabilized when α-syn was introduced; this protein had been previously noted to promote 20S-related protein degradation as well [9].
The researchers began this study by confirming their previous findings. Using two distinct fluorescent reporters, one that forms quickly and another that forms slowly, the researchers confirmed that Blm10 stability increases in the presence of α-syn; this stabilization is decreased when the researchers used a variant that cannot be phosphorylated at serine 129 (S129A) and is increased with a variant that is always phosphorylated there (S129D). Using a human kinase, GRK5, phosphorylated α-syn with similar results.
However, no direct interactions between Blm10 and α-syn were found, so Blm10 must have become stabilized through an indirect process instead. Further work found that this process was autophagy, the process by which cells normally consume their own components; Blm10 is normally consumed by autophagy, but autophagy is inhibited by α-syn, particularly phosphorylated α-syn.
A protein that helps destroy proteins
Further work found that Blm10 is protective against α-syn when expressed at very high levels. Yeast cells that expressed α-syn or S129A grew more quickly when this protein was substantially overexpressed, but lower levels, or even complete deletion of the protein, did not seem to have any effect on growth. Similarly, substantial Blm10 overexpression was found to remove most of the α-syn in these cells.
These findings were confirmed in human neuroglioma cells. The researchers caused these cells to express α-syn aggregates, and they caused some of the cells to express high levels of PA200 as well. The cells that expressed more PA200 had fewer α-syn aggregate inclusions than the cells that expressed less. Further work found that this was indeed due to accelerated destruction of α-syn aggregates.
Interestingly, the way that Blm10 benefited cells against α-syn was found to be dependent on its serine 129 phosphorylation. Ordinary α-syn was found to diminish the S26 pathway’s expression and could not be degraded by it, but Blm10 caused yeast cells to compensate by greatly increasing S20, which was able to destroy the unfolded protein. However, against S129A, which did not significantly impair S26, Blm10 increased S26 and did not affect S20.
The researchers also found out that α-syn harms the S20 pathway as well, inhibiting its function. However, when Blm10 was introduced into these proteasomes, it formed a protective cap that restored its activity and allowed it to destroy the α-syn. While the similar effect of PA200 appeared to be slightly less pronounced in human proteasomes, it was still significantly present.
The authors hold that their results “provide a new promising perspective, which points to novel therapeutics with potential uses against neurodegenerative diseases including PD as well as other aggregopathies.” If PA200 can be harnessed to benefit the proteasome in living human neurons, restoring cells’ ability to destroy α-syn, this could allow for an entirely new class of therapies against Parkinson’s disease and Lewy body dementia.
Literature
[1] Hipp, M. S., Kasturi, P., & Hartl, F. U. (2019). The proteostasis network and its decline in ageing. Nature reviews Molecular cell biology, 20(7), 421-435.
[2] 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.
[3] Lindersson, E., Beedholm, R., Højrup, P., Moos, T., Gai, W., Hendil, K. B., & Jensen, P. H. (2004). Proteasomal inhibition by α-synuclein filaments and oligomers. Journal of Biological Chemistry, 279(13), 12924-12934.
[4] Kleinknecht, A., Popova, B., Lázaro, D. F., Pinho, R., Valerius, O., Outeiro, T. F., & Braus, G. H. (2016). C-terminal tyrosine residue modifications modulate the protective phosphorylation of serine 129 of α-synuclein in a yeast model of Parkinson’s disease. PLoS genetics, 12(6), e1006098.
[5] Anderson, J. P., Walker, D. E., Goldstein, J. M., De Laat, R., Banducci, K., Caccavello, R. J., … & Chilcote, T. J. (2006). Phosphorylation of Ser-129 is the dominant pathological modification of α-synuclein in familial and sporadic Lewy body disease. Journal of Biological Chemistry, 281(40), 29739-29752.
[6] Bi, M., Du, X., Jiao, Q., Chen, X., & Jiang, H. (2021). Expanding the role of proteasome homeostasis in Parkinson’s disease: beyond protein breakdown. Cell death & disease, 12(2), 154.
[7] Tofaris, G. K., Layfield, R., & Spillantini, M. G. (2001). α-Synuclein metabolism and aggregation is linked to ubiquitin-independent degradation by the proteasome. FEBS letters, 509(1), 22-26.
[8] Galka, D., Ali, T. T., Bast, A., Niederleithinger, M., Gerhardt, E., Motosugi, R., … & Braus, G. H. (2024). Inhibition of 26S proteasome activity by α‐synuclein is mediated by the proteasomal chaperone Rpn14/PAAF1. Aging Cell, 23(5), e14128.
[9] Dange, T., Smith, D., Noy, T., Rommel, P. C., Jurzitza, L., Cordero, R. J., … & Schmidt, M. (2011). Blm10 protein promotes proteasomal substrate turnover by an active gating mechanism. Journal of Biological Chemistry, 286(50), 42830-42839.
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