In the clinic preconditioning is typically ascribed

In the clinic, ‘preconditioning’ is typically ascribed to a transient mild stress followed by a recovery interval (Nishio et al., 2000; Stetler et al., 2014). Here we have applied this ahr inhibitor term in its broadest sense — i.e., a subtoxic cellular stress that can lead to a protective state (Stetler et al., 2014) in order to account for the proteostatic priming observed during our pre-incubation phase of cooling. This definition circumvents the need for a re-warming phase which would confound analysis of oxidative injury by inducing relative hyperthermic and hypoxic stresses (Liu et al., 1994; Lleonart, 2010; Chip et al., 2011; Neutelings et al., 2013). Hypothermic preconditioning may reconcile conflicting data describing UPR modulation in neuronal health; (1) that ER stress can elicit UPR-mediated hormesis (Mendes et al., 2009; Fouillet et al., 2012), (2) that circumventing UPR-mediated translational repression promotes long-term survival (Moreno et al., 2013), and (3) that inhibiting eIF2α phosphatases resolves ER stress (Kiskinis et al., 2014). This highlights the importance of fine-tuning the entire network, rather than adjusting a single pathway or component — such a combinatorial approach has been proposed for amyotrophic lateral sclerosis (Kiskinis et al., 2014). Whilst hypothermic preconditioning originates from the acute injury setting, impaired stress responses underlie several neurodegenerative disorders (Hetz and Mollereau, 2014) and preconditioning in general is a proposed target (Stetler et al., 2014). Cooling has recently demonstrated some benefit in an in vivo model of spastic paraplegia (Baxter et al., 2014) and Peretti et al. (2015) observed that neurodegenerative synaptic loss could be partially rescued through early cooling-induced enhancement of RBM3 expression. Conceivably, this temporal dependency might relate to hypothermia-mediated proteostatic priming, elicited prior to the build-up of a significant protein aggregate load. Whether the hypothermic preconditioning described here is linked to a cytoprotective mechanism that is synergistic with the preservation of synaptic plasticity is worthy of further investigation (Peretti et al., 2015). Ultimately, disease stage and neuronal subtype would determine whether enhanced or prophylactic preconditioning could be useful in the context of neurodegeneration (Saxena et al., 2009). In response to cooling, hCNs displayed all the hallmarks of an adaptive, preconditioning UPR response: mild ER stress and activation of all 3 ER-stress transducers, a low level of CHOP induction that was insufficient to effect apoptosis, absence of detectable levels of phospho-eIF2α, and residual expression of key ER chaperones (Rutkowski et al., 2006; Tabas and Ron, 2011). The reversibility of these effects and the period over which they would remain protective is currently unknown and is part of ongoing work. Since our cooling paradigm can be used to titrate UPR activation, it represents a simple method to address subtle but important effects dictating adaptive versus maladaptive outcomes of this cascade in any cell type. We propose that ER-hormesis is an important outcome of the cold-shock response that protects human neurons from both ER and oxidative stress. This ‘cross-tolerance’ effect (Rutkowski et al., 2006; Stetler et al., 2014) places exponential value on the molecular neurobiology of cooling, which may deliver novel therapeutic targets for an unmet need.
Author Contributions
Role of the Funding Sources
Acknowledgements This research was funded by a Wellcome Trust Integrated Training Fellowship for Veterinarians (096409/Z/11/Z to N.M.R.), an Anne Rowling Senior Clinical Research Fellowship (to P.C.), an Anne Rowling Fellowship and Wellcome Trust Intermediate Clinical Fellowship (101149/Z/13/Z to R.P.) and the Euan MacDonald centre and the Fidelity Foundation (to S.C.). We thank the following at the University of Edinburgh: MRC Edinburgh Brain & Tissue Bank and Nusrat Khan, Peault lab, MRC Centre for Regenerative Medicine (human post-mortem brain tissue); David Hay lab, MRC centre for Regenerative Medicine (Glomax luminometer); Steve West, Wellcome Trust Centre for Cell Biology (MYC primers).