Protection against protein aggregation by alpha-crystallin as a mechanism of preconditioning

Effect of HSF2 silencing on NLRP3 inflammasome in THP-1 cells

AIM: To study the effect of heat shock transcription factor 2 (HSF2) silencing on NLR family, pyrin domain containing 3 (NLRP3) inflammasome in THP-1 cells.

METHODS: THP-1 cells were transfected with a lentivirial vector (LV-HSF2-RNAi) to induce HSF2 silencing. PMA was used to induce THP-1 cells to differentiate into macrophages. Then different groups of cells were stimulated with lipopolysaccharides (LPS). The mRNA and protein expression levels of NLRP3, ASC, Caspase1 and IL-1β were measured by RT-PCR and Western Blot, respectively. The level of IL-1β was measured by ELISA.

RESULTS: The protein level of HSF2 in the HSF2-siRNA group was significantly lower than those in the control group and negative siRNA group (P < 0.05). The mRNA and protein expression levels of NLRP3, ASC, Caspase1 and IL-1β in the HSF2-siRNA group were significantly higher than those in the control group and negative siRNA group (P < 0.05). IL-1β levels in cell supernants in the HSF2-siRNA group were significantly higher than those in the control group and negative siRNA group (control group: 257.010 pg/mL±26.148 pg/mL; siRNA group: 538.800 pg/mL±52.250 pg/mL; negative siRNA group: 238.231 pg/mL±29.245 pg/mL) (P < 0.05).

CONCLUSION: HSF2 silencing significantly raises the mRNA and protein expression of NLRP3 inflammasome and IL-1β in THP-1 cells.

Oxidation of an exposed methionine instigates the aggregation of glyceraldehyde-3-phosphate dehydrogenase

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a ubiquitous and abundant protein that participates in cellular energy production. GAPDH normally exists in a soluble form; however, following necrosis, GAPDH and numerous other intracellular proteins convert into an insoluble disulfide-cross-linked state via the process of "nucleocytoplasmic coagulation." Here, free radical-induced aggregation of GAPDH was studied as an in vitro model of nucleocytoplasmic coagulation. Despite the fact that disulfide cross-linking is a prominent feature of GAPDH aggregation, our data show that it is not a primary rate-determining step. To identify the true instigating event of GAPDH misfolding, we mapped the post-translational modifications that arise during its aggregation. Solvent accessibility and energy calculations of the mapped modifications within the context of the high resolution native GAPDH structure suggested that oxidation of methionine 46 may instigate aggregation. We confirmed this by mutating methionine 46 to leucine, which rendered GAPDH highly resistant to free radical-induced aggregation. Molecular dynamics simulations suggest that oxidation of methionine 46 triggers a local increase in the conformational plasticity of GAPDH that likely promotes further oxidation and eventual aggregation. Hence, methionine 46 represents a "linchpin" whereby its oxidation is a primary event permissive for the subsequent misfolding, aggregation, and disulfide cross-linking of GAPDH. A critical role for linchpin residues in nucleocytoplasmic coagulation and other forms of free radical-induced protein misfolding should now be investigated. Furthermore, because disulfide-cross-linked aggregates of GAPDH arise in many disorders and because methionine 46 is irrelevant to native GAPDH function, mutation of methionine 46 in models of disease should allow the unequivocal assessment of whether GAPDH aggregation influences disease progression.

Dynamic oligomeric properties

This chapter provides a foundation for further research into the relationship between dynamic oligomeric properties and functional diversity. The structural basis that underlies the conformational sub-states of the GAPDH oligomer is discussed. The issue of protein stability is given a thorough analysis, since it is well-established that the primary strategy for protein oligomerization is to stabilize conformation. Several factors that affect oligomerization are described, including chemical modification by synthetic reagents. The effects of native substrates and coenzymes are also discussed. The curious feature of chloride ions having a de-stabilizing effect on native GAPDH structure is described. Additionally, the role of adenine dinucleotides in tetramer-dimer equilibrium dynamics is suggested to be a major part of the physiological regulation of GAPDH structure and function. This chapter also contends that a vast amount of useful information can come from comparative analyses of diverse species, particularly regarding protein stability and subunit-subunit interaction. Lastly, the concept of domain exchange is introduced as a means of understanding the stabilization of dynamic oligomers, suggesting that inter-subunit contacts may also be a way of masking docking sites to other proteins.

Functional diversity

There is increasing evidence to support a gene economy model that is fully based on the principles of evolution in which a limited number of proteins does not necessarily reflect a finite number of biochemical processes. The concept of 'gene sharing' proposes that a single protein can have alternate functions that are typically attributed to other proteins. GAPDH appears to play this role quite well in that it exhibits more than one function. GAPDH represents the prototype for this new paradigm of protein multi-functionality. The chapter discusses the diverse functions of GAPDH among three broad categories: cell structure, gene expression and signal transduction. Protein function is curiously re-specified given the cell's unique needs. GAPDH provides the cell with the means of linking metabolic activity to various cellular processes. While interpretations may often lead to GAPDH's role in meeting focal energy demands, this chapter discusses several other very distinct GAPDH functions (i.e. membrane fusogenic properties) that are quite different from its ability to catalyze oxidative phosphorylation of the triose, glyceraldehyde 3-phosphate. It is suggested that a single protein participates in multiple processes in the structural organization of the cell, controls the transmission of genetic information (i.e. GAPDH's involvement may not be finite) and mediates intracellular signaling.

Target for diverse chemical modifications

The chapter begins with an historical perspective of GAPDH isozymes that is juxtaposed to the fact that there is only one somatic functional gene in humans that is virtually identical among the mammalian species. Over the many years of GAPDH research, dozens of labs have reported the existence of multiple forms of GAPDH, which mostly vary as a function of charge with an occasional report of truncated forms. These observations are in part due to GAPDH being a substrate for many enzymatically-controlled post-translational modifications. While target residues have been identified and predictive algorithms have implicated certain residues, this area of research appears to be in its infancy regarding GAPDH. Equally fascinating, the uniquely susceptible nature of GAPDH to non-enzymatic reactions, that typically are associated with cell stress, such as oxidation and nitration, is also discussed. Two metabolic gases, nitric oxide and hydrogen sulfide, which are enzymatically produced, appear to exert their signaling properties through non-enzymatic reaction with GAPDH. Models of cellular decline are also proposed, including the compelling hypothesis that states cell compromise occurs by the physically blocking the function of chaperonins (i.e. dual-ring multiple-subunit molecular chaperones) by the attachment of misfolded GAPDH.

GAPDH in anesthesia

Thus far, two independent laboratories have shown that inhaled anesthetics directly affect GAPDH structure and function. Additionally, it has been demonstrated that GAPDH normally regulates the function of GABA (type A) receptor. In light of these literature observations and some less direct findings, there is a discussion on the putative role of GAPDH in anesthesia. The binding site of inhaled anesthetics is described from literature reports on model proteins, such as human serum albumin and apoferritin. In addition to the expected hydrophobic residues that occupy the binding cavity, there are hydrophilic residues at or in very close proximity to the site of anesthetic binding. A putative binding site in the bacterial analog of the human GABA (type A) receptor is also described. Additionally, GAPDH may also play a role in anesthetic preconditioning, a phenomenon that confers protection of cells and tissues to future challenges by noxious stimuli. The central thesis regarding this paradigm is that inhaled anesthetics evoke an intra-molecular protein dehydration that is recognized by the cell, eliciting a very specific burst of chaperone gene expression. The chaperones that are implicated are associated with conferring protection against dehydration-induced protein aggregation.