The interaction of Hsc70 protein with fibrillar α-Synuclein and its therapeutic potential in Parkinson’s disease

Intracellular Lewy bodies (LBs) and Lewy neurites (LNs) which contain a fibrillar form of the synaptic terminal protein α‑Synuclein (α‑Syn) as their principal component are neuropathological markers of Parkinson disease (PD).¹ Although extensive research has previously shown various members of the human molecular chaperone family to hinder or promote the progression of soluble α‑Syn into fibrils (Hsp70, Hsp27, αB‑crystallin or Hsp90 respectively; for a review see refs. 2–3), no one had yet to investigate the effect of the constitutively expressed human heat shock protein, Hsc70. This was surprising as not only have Hsp70 and Hsc70 been shown to have different cellular roles and different effects on aggregated luciferase and polyQ protein but being constitutively expressed, Hsc70 is the first molecular chaperone to encounter assembling α‑Syn in a cellular context.^4,5

We showed that Hsc70 inhibits the assembly of α‑Syn into fibrils, by binding with high affinity to the soluble form of α‑Syn. We also showed that Hsc70 binds fibrillar α‑Syn, with a dissociation constant 5-fold lower than that we measured for soluble α‑Syn. Hsc70 binding to fibrils has a cytoprotective effect, as it renders α‑Syn fibrils less toxic to mammalian cultured cells.⁶

The point mutations within α‑Syn at the origin of familial early onset PD suggest that the stochastic aggregation of α‑Syn within cells is an important factor which contributes to the onset of PD.^7,8 A second recently discovered and documented factor, namely the propagation of aggregates from one cell to another accounts for the neurodegenerative patterns described, which play a critical role in PD.^9,10 Indeed, exogenous α‑Syn fibrils enter cells and recruit soluble endogenous α‑Syn, which results in synaptic dysfunction and neuronal cell death.^11,12 As our work demonstrated that Hsc70 reduces the toxicity of fibrils most likely because binding alters their physicochemical properties, there is a possibility that Hsc70 could be effective as a therapeutic agent in PD, by abolishing the cell to cell transfer of α‑Syn fibrils, as schematised in Figure 1.

Furthermore, we demonstrated that Hsc70 is capable of binding fibrillar α‑Syn in a nucleotide independent manner and in the absence of co-chaperones.⁶ This suggests that Hsc70s α‑Syn binding site, or a subset of this site may be used as a therapeutic tool to modify fibrillar α‑Syn properties and as a consequence their binding to and uptake by neurons. Thus further characterization of Hsc70 residues that interact specifically with fibrillar α‑Syn using a cross-linking and mass spectrometry approach will lead, as in the case of the GroEL apical domain,^13-15 to the design of an Hsc70 derived minichaperone.

However, there is a chance that basing a therapeutic agent on Hsc70, an endogenous protein, may evoke an autoimmune response when introduced to the body. An exogenous engineered polypeptide which has a similar if not the same effect on α‑Syn toxicity and crosses the blood-brain barrier to reach its target may be more successful as an eventual therapeutic agent – does the answer to PD lie in a bacterial or yeast homolog of Hsc70?