Control and regulation of the cellular responses to cold shock: the responses in yeast and mammalian systems

Abstract

Although the cold-shock response has now been studied in a number of different organisms for several decades, it is only in the last few years that we have begun to understand the molecular mechanisms that govern adaptation to cold stress. Notably, all organisms from prokaryotes to plants and higher eukaryotes respond to cold shock in a comparatively similar manner. The general response of cells to cold stress is the elite and rapid overexpression of a small group of proteins, the so-called CSPs (cold-shock proteins). The most well characterized CSP is CspA, the major CSP expressed in Escherichia coli upon temperature downshift. More recently, a number of reports have shown that exposing yeast or mammalian cells to sub-physiological temperatures (<30 or <37 degrees C respectively) invokes a co-ordinated cellular response involving modulation of transcription, translation, metabolism, the cell cycle and the cell cytoskeleton. In the present review, we summarize the regulation and role of cold-shock genes and proteins in the adaptive response upon decreased temperature with particular reference to yeast and in vitro cultured mammalian cells. Finally, we present an integrated model for the co-ordinated responses required to maintain the viability and integrity of mammalian cells upon mild hypothermic cold shock.

Figure 1. Predicted amino acid sequence alignment of the N- and C-terminal regions of the CSPs Cirp (CIRBP) and Rbm3 (RBM3) in mouse

The two proteins are highly similar and belong to the glycine-rich RNA-binding protein family, which are characterized by a consensus sequence RNA-binding domain or eukaryotic RNA-recognition motif (RRM) at the N-terminus and a glycine-rich domain at the C-terminus.

Figure 2. Predicted amino acid sequence alignment of Cirp from various species

The sequence data for each species were obtained from the NCBI (National Center for Biotechnology Information) protein database and the appropriate accession numbers are shown in parentheses. The predicted amino acid sequence across all species shown is virtually entirely conserved.

Figure 3. Proposed model for the co-ordinated cellular responses in mammalian cells upon exposure to mild hypothermia cold-shock (25–35 °C)

In the model, cold shock results in global transcription efficiency being decreased (designated by ↓ and X, top of nuclear compartment); however, both CSPs and other unidentified proteins bind to cis-elements (designated E) of target genes, up-regulating their transcription and resulting in increased hnRNA levels. Alternative promoters (designated P1 and P2) may also be activated in target genes at decreased temperature, leading to different hnRNA transcripts at lower temperatures. Alternative splicing (middle of nuclear compartment) also results in cold-specific mRNAs which may have unique 5′- and/or 3′-UTRs and/or changes in the coding regions. Cold-specific mRNAs may then bind CSPs in the nucleus before transport to the cytoplasm, stabilizing the mRNA, or be transported directly to the cytoplasm without CSP chaperones. In the cytoplasm, global mRNA translation is decreased (↓) via increased phosphorylation of IFs (IF-P) and the binding of specific, but currently unknown, proteins to ribosomes, preventing mRNA translation. Phosphorylation of eIF2α also results in the formation of mRNA stress granules, consisting of mRNAs that are required for recovery from cold shock upon rewarming. Central to translational control in this model is the interaction between CSPs, cold-specific mRNAs and the cell cytoskeleton. In the model, CSPs can ‘grab’ mRNAs as they emerge from the nucleus, or in the cytoplasm, and via interactions with the cell cytoskeleton (depicted as a ‘net’) form a ‘work bench’ for translation at local sites whereby ribosomes may be recruited. In this way, CSPs link transcription and translation via interactions with target mRNAs and the cytoskeleton. Furthermore, we predict that some cold-specific mRNAs, whose translation is required for cold-shock adaptation, contain IRES sequences, allowing the direct recruitment of ribosomes to the mRNA, circumventing the compromised cap-dependent initiation under cold-shock conditions. CSPs may also bind and stabilize their own or other mRNAs while undefined proteins (coloured green) also bind the 3′-UTR, all of which play a role in stabilizing cold-specific mRNAs at decreased temperatures. We also predict that trans-acting proteins bind secondary-structure elements in cold-shock mRNAs, stabilizing further such mRNAs and/or allowing more efficient scanning of the mRNA by ribosomes. All of these mRNA translation control mechanisms lead to an increase in the levels of CSPs and other key proteins required during cold-shock adaptation. The resulting newly synthesized CSPs (e.g. Cirp) can bind further mRNAs, stabilizing them, or be transported to the nucleus whereby they may act as transcription factors. Alternatively, CSPs may interact with specific miRNAs, preventing miRNAs from interacting with their target mRNAs and degrading them. The net effect is a decrease in mRNA degradation (↓) and thus an increase in mRNA levels. For further details see text.