Components of a mammalian protein disaggregation/refolding machine are targeted to nuclear speckles following thermal stress in differentiated human neuronal cells

Abstract

Heat shock proteins (Hsps) are a set of highly conserved proteins involved in cellular repair and protective mechanisms. They counter protein misfolding and aggregation that are characteristic features of neurodegenerative diseases. Hsps act co-operatively in disaggregation/refolding machines that assemble at sites of protein misfolding and aggregation. Members of the DNAJ (Hsp40) family act as “holdases” that detect and bind misfolded proteins, while members of the HSPA (Hsp70) family act as “foldases” that refold proteins to biologically active states. HSPH1 (Hsp105α) is an important additional member of the mammalian disaggregation/refolding machine that acts as a disaggregase to promote the dissociation of aggregated proteins. Components of a disaggregation/refolding machine were targeted to nuclear speckles after thermal stress in differentiated human neuronal SH-SY5Y cells, namely: HSPA1A (Hsp70-1), DNAJB1 (Hsp40-1), DNAJA1 (Hsp40-4), and HSPH1 (Hsp105α). Nuclear speckles are rich in RNA splicing factors, and heat shock disrupts RNA splicing which recovers after stressful stimuli. Interestingly, constitutively expressed HSPA8 (Hsc70) was also targeted to nuclear speckles after heat shock with elements of a disaggregation/refolding machine. Hence, neurons have the potential to rapidly assemble a disaggregation/refolding machine after cellular stress using constitutively expressed Hsc70 without the time lag needed for synthesis of stress-inducible Hsp70. Constitutive Hsc70 is abundant in neurons in the mammalian brain and has been proposed to play a role in pre-protecting neurons from cellular stress.