The small heat shock proteins αB-crystallin (HSPB5) and Hsp27 (HSPB1) inhibit the intracellular aggregation of α-synuclein

Abstract

Protein homeostasis, or proteostasis, is the process of maintaining the conformational and functional integrity of the proteome. Proteostasis is preserved in the face of stress by a complex network of cellular machinery, including the small heat shock molecular chaperone proteins (sHsps), which act to inhibit the aggregation and deposition of misfolded protein intermediates. Despite this, the pathogenesis of several neurodegenerative diseases has been inextricably linked with the amyloid fibrillar aggregation and deposition of α-synuclein (α-syn). The sHsps are potent inhibitors of α-syn aggregation in vitro. However, the limited availability of a robust, cellbased model of α-syn aggregation has, thus far, restricted evaluation of sHsp efficacy in the cellular context. As such, this work sought to establish a robust model of intracellular α-syn aggregation using Neuro-2a cells. Aggregation of α-syn was found to be sensitive to inhibition of autophagy and the proteasome, resulting in a significant increase in the proportion of cells containing α-syn inclusions. This model was then used to evaluate the capacity of the sHsps, αB-c and Hsp27, to prevent α-syn aggregation in cells. To do so, we used bicistronic expression plasmids to express the sHsps. Unlike traditional fluorescent fusion constructs, these bicistronic expression plasmids enable only individual transfected cells expressing the sHsps (via expression of the fluorescent reporter) to be analysed, but without the need to tag the sHsp, which can affect its oligomeric structure and chaperone activity. Overexpression of both αB-c and Hsp27 significantly reduced the intracellular aggregation of α-syn. Thus, these findings suggest that overexpressing or boosting the activity of sHsps may be a way of preventing amyloid fibrillar aggregation of α-syn in the context of neurodegenerative disease.

Small heat shock proteins in neurodegenerative diseases

“…sHSPs were shown to counter α-synuclein aggregation in vitro and, so far only tested for HSPB5, in vivo (Bruinsma et al 2011;Duennwald et al 2012;Prabhu et al 2012;Tue et al 2012;Alderson et al 2019). Moreover, the overexpression of HSPB1 and HSPB5 reduces intracellular aggregation and counters cytotoxicity induced by α-synuclein (McLean et al 2002;Klucken et al 2004;Zourlidou et al 2004;Outeiro et al 2006;Cox and Ecroyd 2017).…”

Therapeutic potential of Hsp27 in neurological diseases

“…Studies have revealed that sHsp upregulation in disease progression of PD prevents the degeneration of neurons [40]. Overexpression of α-B crystallin and Hsp27 [41] by using bicistronic expression plasmids prevented the intracellular aggregation of α-syn. It was also suggested that the effectiveness of Hsp27 was dependent on the kinetics of α-syn aggregation.…”

Hsp27 reduces glycation‐induced toxicity and aggregation of alpha‐synuclein

“…The small heat‐shock proteins (sHsps) are a family of small molecular weight proteins that are characterized by a highly conserved domain, known as alpha‐crystallin domain 20,21 . sHsps play an important role in cellular proteostasis, 22‐25 in particular by preventing protein aggregation. They act as molecular chaperones, promoting protein folding/refolding, stabilization, and by targeting damaged proteins for degradation 24‐26 .…”