Biology:Heat shock

In biochemistry, heat shock is the effect of subjecting a cell to a temperature that is greater than the optimal temperature range of function of the cell. Heat shock refers to cellular exposure to rapid stress changes such as temperature, toxins, oxidative stress, heavy metals, and pathogenic infections. Specifically temperature induced heat shock, even of a few degrees, has the potential to disrupt proper protein folding. As a result, proteins can fold incorrectly, or become entangled which can result in nonspecific aggregation. Other cellular damages induced by heat shock include cytoskeletal rearrangement, changes in organelle localization, decrease in ATP production, unsafe drop in cellular pH levels, decreased translation of proteins, and changes in RNA splicing. Introduction of heat shock to cells elicits the molecular response, the heat shock response (HSR), which repairs damages caused by stressors such as protein misfolding and protein aggregation. Heat shock proteins induced by the HSR can help prevent protein aggregation that can lead to common neurodegenerative diseases such as Alzheimer's, Huntington's, or Parkinson's Disease.

Heat shock response

The cellular response to heat shock damage, the heat shock response, includes the transcriptional up-regulation of genes encoding heat shock proteins (HSPs) as part of the cell's internal repair mechanism. The effects of stressors such as temperature changes and toxins are counteracted by these HSPs, that upon activation respond to heat, cold and oxygen deprivation by activating several cascade pathways. HSPs are also present in cells under perfectly normal conditions, but an elevation in stress levels for the cell promotes an increase in their production levels by activating heat-shock genes at levels higher than normal. Some HSPs, called chaperones, also have increased production levels when the cell faces various stress factors. Chaperone's functions includes making sure that the cell’s proteins are properly folded in the correct conformation and they ensure this by facilitating protein folding using their substrate binding domain. An example of chaperons are the HSP70 (heat shock protein) chaperones. For example, HSPs help new or misfolded proteins to fold into their correct three-dimensional conformations, which is essential for their function. They also shuttle proteins from one compartment to another inside the cell, and target old or terminally misfolded proteins to proteases for degradation. Heat shock proteins are also believed to play a role in the presentation of pieces of proteins (or peptides) on the cell surface to help the immune system recognize diseased cells. 5 major families of HSPs are recognized: the Hsp70 (DnaK) family, the chaperonins (GroEL and Hsp60), the Hsp90 family, the Hsp100 (Clp) family and the small HSP (sHSP) family. Other proteins such as, protein disulfide isomerase and calnexin/calreticulin, have chaperone functions and assist protein folding in the Endoplasmic Reticulum.

The up-regulation of HSPs during heat shock is generally controlled by a single transcription factor; in eukaryotes this regulation is performed by heat shock factor (HSF), while σ³² is the heat shock sigma factor in Escherichia coli. Under normal conditions HSF1 resides as a monomer, but when stress induces protein damage HSF1 is activated to trimerize. This trimer of HSF1 localizes to the nucleus, and here binds to the heat shock element in the promoter sequence of heat shock genes.

Inducing heat shock

In fish that survive at 0 °C, heat shock can be induced with temperatures as low as 5 °C, whereas thermophilic bacteria that proliferate at 50 °C will not express heat shock proteins until temperatures reach approximately 60 °C. The process of heat shocking can be done in a CO₂ incubator, O₂ incubator, or a hot water bath.

See also

• Bacterial stress response
• HSF1
• Chaperone
• Heat Shock Proteins