Most of us will have already used cooling as pain relief or to prevent swelling after an injury. What we are less likely to have experienced is therapeutic hypothermia (also called targeted temperature management), a lowering of the body temperature to 32-34°C, which is used in medicine to prevent brain damage after cardiac arrest.
Although the benefit of cooling is undisputed, the mechanisms involved in therapeutic hypothermia are not well understood.
Interestingly, although cooling protects brain tissue, it also induces a decrease in protein manufacture inside the brain cells. A recently published paper by Bastide et al. (2017) sheds some light on this apparent paradox and provides information about the underlying mechanisms involved in the neuroprotective effect of therapeutic hypothermia.
Read more about cold shock proteins being neuroprotective
It’s all in the translation
The group of researchers led by Professor Anne Willis from the Medical Research Council’s Toxicology Unit, based at the University of Leicester, UK, postulated that neuroprotective proteins must escape the cold-induced downregulation of translation and are produced at a similar or higher rate than in normal conditions.
Translation is classically divided into 3 steps: 1, initiation, where the ribosome and mRNA assemble as a complex; 2, elongation, where the ribosome travels along the mRNA and converts it into a protein and finally; 3, termination, where the ribosome and mRNA dissociate, ending translation.
Cooling decreases protein synthesis by inhibiting the elongation and the initiation steps.
Professor Willis said, "Elongation control being the major driver of this process was somewhat unexpected."
A combined approach
In this work, combining molecular biology techniques with computational modeling, the authors identified a group of mRNAs coding for neuronal proteins which could potentially escape the repression of translation. Amongst those candidates, the expression of RTN3, a member of the reticulon family of highly conserved genes, was higher under cool-induced or drug-induced block of translation.
RTN3 has previously been shown to be neuroprotective (Teng and Tang, 2013). In particular, RTN3 binds to and inhibits BACE, an enzyme involved in the formation of β-amyloid, which forms the amyloid plaques in Alzheimer’s disease (He et al., 2004, 2006; Murayama et al., 2006; Shi et al., 2014).
"We think that RTN3’s role in BACE regulation is perhaps the most important in terms of neuroprotection and that is something that we will explore in the future." added Professor Willis.
How is the RTN3 level of protein increased upon cooling?
The study shows that RTN3 mRNA binds RBM3, an mRNA binding protein which the authors previously showed is involved in therapeutic hypothermia (Peretti et al., 2015). RBM3 was capable of controlling the amount of RTN3 produced in cells but also in vivo in the mouse brain.
Brains of mice subjected to hypothermia showed an increase in RBM3 and consequently RTN3 while a global decrease of protein synthesis (40%) was observed.
Moreover, therapeutic hypothermia improved neuronal survival and increased the lifespan of a mouse with prion disease by increasing RBM3 and RTN3.
Even in the absence of cooling, overexpression of RTN3 in the brain of the prion mice provided neuroprotective effects. Altogether this work provides insight into the cellular mechanisms involved in the neuroprotective effect of therapeutic hypothermia but more importantly it provides a new pathway to explore in the search for therapies against neurodegeneration.
"This protective pathway could be used to provide novel treatments for Alzheimer’s Disease. The data suggest that early intervention would be essential though, with little effects observed in late stage disease." finished Professor Willis.
Bastide, A., Peretti, D., Knight, J., Grosso, S., Spriggs, R., Pichon, X., Sbarrato, T., Roobol, A., Roobol, J., Vito, D., Bushell, M., von der Haar, T., Smales, C., Mallucci, G. and Willis, A. (2017). RTN3 Is a Novel Cold-Induced Protein and Mediates Neuroprotective Effects of RBM3. Current Biology, 27(5), pp.638-650.
He, W., Hu, X., Shi, Q., Zhou, X., Lu, Y., Fisher, C., and Yan, R. (2006). Mapping of interaction domains mediating binding between BACE1 and RTN/Nogo proteins. J Mol Biol 363, 625-634.
He, W., Lu, Y., Qahwash, I., Hu, X.Y., Chang, A., and Yan, R. (2004). Reticulon family members modulate BACE1 activity and amyloid-beta peptide generation. Nat Med 10, 959-965.
Murayama, K.S., Kametani, F., Saito, S., Kume, H., Akiyama, H., and Araki, W. (2006). Reticulons RTN3 and RTN4-B/C interact with BACE1 and inhibit its ability to produce amyloid beta-protein. Eur J Neurosci 24, 1237-1244.
Peretti, D., Bastide, A., Radford, H., Verity, N., Molloy, C., Martin, M.G., Moreno, J.A., Steinert, J.R., Smith, T., Dinsdale, D., Willis, A.E., and Mallucci, G.R. (2015). RBM3 mediates structural plasticity and protective effects of cooling in neurodegeneration. Nature 518, 236-239.
Shi, Q., Ge, Y., Sharoar, M.G., He, W., Xiang, R., Zhang, Z., Hu, X., and Yan, R. (2014). Impact of RTN3 deficiency on expression of BACE1 and amyloid deposition. J Neurosci 34, 13954-13962.
Teng, F.Y., and Tang, B.L. (2013). Nogo/RTN4 isoforms and RTN3 expression protect SH-SY5Y cells against multiple death insults. Mol Cell Biochem 384, 7-19.