HSP27 and cell survival in neurones

Inducible and constitutive heat shock gene expression responds to modification of Hsp70 copy number in Drosophila melanogaster but does not compensate for loss of thermotolerance in Hsp70 null flies

Background

The heat shock protein Hsp70 promotes inducible thermotolerance in nearly every organism examined to date. Hsp70 interacts with a network of other stress-response proteins, and dissecting the relative roles of these interactions in causing thermotolerance remains difficult. Here we examine the effect of Hsp70 gene copy number modification on thermotolerance and the expression of multiple stress-response genes in Drosophila melanogaster, to determine which genes may represent mechanisms of stress tolerance independent of Hsp70.

Results

Hsp70 copy number in four strains is positively associated with Hsp70 expression and inducible thermotolerance of severe heat shock. When assayed at carefully chosen temperatures, Hsp70 null flies are almost entirely deficient in thermotolerance. In contrast to expectations, increasing Hsp70 expression levels induced by thermal pretreatment are associated with increasing levels of seven other inducible Hsps across strains. In addition, complete Hsp70 loss causes upregulation of the inducible Hsps and six constitutive stress-response genes following severe heat shocks.

Conclusion

Modification of Hsp70 copy number quantitatively and qualitatively affects the expression of multiple other stress-response genes. A positive association between absolute expression levels of Hsp70 and other Hsps after thermal pretreatment suggests novel regulatory mechanisms. Severe heat shocks induce both novel gene expression patterns and almost total mortality in the Hsp70 null strain: alteration of gene expression in this strain does not compensate for Hsp70 loss but suggests candidates for overexpression studies.

Proteasome inhibition: an early or late event in nitric oxide-induced neuronal death?

Nitric oxide (NO), ubiquitously expressed in the central nervous system, has been perceived to be a potential neuromodulator. Employing cultured murine primary cortical neurons, NO resulted in an inhibition of the ubiquitin-proteasome system (UPS) with a dose- and time-dependent decrease in cell viability. This is consistent with a previous study that reported a dysfunction of UPS with consequential apoptotic death in macrophage cell with NO treatment. However, it cannot be unclear if the drop in UPS efficiency is directly imposed on by NO. Therefore by using microarray analysis, our study revealed an early down-regulation or non-significant differential expression of genes encoding UPS proteins in NOC-18 (NO donor)-treated neurons as compared to an observed elevation of corresponding gene expression genes in lactacystin (classical proteasome inhibitor)-treated neurons (conducted earlier). Furthermore, time-course analysis of proteasome activity in NOC-18-treated neurons demonstrated a late onset of reduction. This is intriguing as it is well established that in an exclusive proteasome dysfunction-induced cell death, a compensatory feedback mechanism will be activated with an initial and concerted up-regulation of genes encoding proteins involved in UPS as seen when neurons were treated with lactacystin. Thus, it is highly suggestive that NO-triggered neuronal death takes on a different signaling cascade from that of a classical proteasome inhibitor, and that the late reduction of proteasome activity is a downstream event following the activation of apoptotic cellular signaling cascade. In intracellular condition, the proteasome is not NO preferred primary target responsible for the trigger of the cell death machinery. In conclusion, we presented novel findings that shed light into NO-induced cell death signaling cascade, which would be important in understanding the pathogenesis of neurodegenerative disorders such as Parkinson's disease.

The cellular "networking" of mammalian Hsp27 and its functions in the control of protein folding, redox state and apoptosis

Cells possess effective mechanisms to cope with chronic or acute disturbance of homeostasis. Key roles in maintaining or restoring homeostasis are played by the various heat shock or stress proteins (Hsps). Among the Hsps, the group of proteins characterized by low molecular masses (between 20 to 30 kDa) and homology to alpha-crystallin are called small stress proteins (denoted sHsps). The present chapter summarizes the actual knowledge of the protective mechanisms generated by the expression of mammalian Hsp27 (also denoted HspB1 in human) against the cytotoxicity induced by heat shock and oxidative stress. It also describes the anti-apoptotic properties of Hsp27 and their putative consequences in different pathological conditions.

MAPK-pathway activity, Lrrk2 G2019S, and Parkinson's disease

The 6055G>A mutation in the leucine-rich repeat kinase 2 (LRRK2) gene results in a G2019S substitution in the mixed-lineage kinase domain of Lrrk2, causing autosomal dominant Parkinson's disease (PD). We hypothesized the mutation alters cellular mitogen-activated protein kinase (MAPK) signalling cascades, and might be detectable in tissues other than in the brain. We therefore compared total levels and activation of the signalling proteins Src, HSP27, p38 MAPK, JNK, and ERK, in extracts of leukocytes isolated from patients with PD carrying the G2019S mutation, healthy mutation carriers, patients with idiopathic PD, and healthy controls. Phosphorylation of Src, HSP27, and JNK was reduced significantly in cell extracts from patients with G2019S-associated PD compared to healthy controls. Similarly, phosphorylation was reduced significantly in Src and HSP27 in the group of healthy carriers of the mutation, as well as in patients with idiopathic PD. Significant reductions in total Src were also observed in these three groups compared to the controls. The results of this pilot project therefore indicate significant alterations in key signalling proteins in leukocytes from patients with PD, and were most pronounced in G2019S-associated PD. Changes in MAPK-signalling may thus be common to PD pathophysiology, regardless of aetiology. Such changes may also be shown in blood samples during the preclinical stage of LRRK2-associated PD, which could be particularly important for the development of neuroprotective strategies to delay onset, or slow progression of PD.

Hsp27 (HspB1) and alphaB-crystallin (HspB5) as therapeutic targets

Hsp27 and alphaB-crystallin are molecular chaperones that are constitutively expressed in several mammalian cells, particularly in pathological conditions. These proteins share functions as diverse as protection against toxicity mediated by aberrantly folded proteins or oxidative-inflammation conditions. In addition, these proteins share anti-apoptotic properties and are tumorigenic when expressed in cancer cells. This review summarizes the current knowledge about Hsp27 and alphaB-crystallin and the implications, either positive or deleterious, of these proteins in pathologies such as neurodegenerative diseases, myopathies, asthma, cataracts and cancers. Approaches towards therapeutic strategies aimed at modulating the expression and/or the activities of Hsp27 and alphaB-crystallin are presented.

Proteomic expression analysis of cardiomyocytes subjected to proteasome inhibition

We hypothesized that impaired proteasomal function affects gene expression in cardiomyocytes. To identify those genes, a proteomics-based analysis of neonatal rat cardiac myocytes treated with the proteasome inhibitor MG132 in comparison to vehicle treated control cells was performed. MG132 treatment induced reproducible changes in the protein expression profile, which was analyzed by two-dimensional difference gel electrophoresis followed by tryptic peptide mass fingerprinting for spot identification by MALDI-TOF mass spectrometry. The identified protein alterations could be grouped into three major categories: (1) induction of small heat shock proteins (HSPs) with chaperonic function, such as HSP27, alphaB-crystallin, and cardiovascular HSP, (2) altered expression of actin associated proteins, such as cofilin-1 and transgelin, and (3) induction of antioxidant proteins, such as peroxiredoxin-1, superoxide dismutase-1, and hemeoxygenase-1. Northern blotting revealed that expression was regulated at the mRNA level. Given that proteasomal activity is decreased in cardiovascular diseases, alterations in proteasome-dependent control of mRNA expression could provide a novel mechanism by which disease progression is modulated.

Stress-induced heat shock protein 27 expression and its role in dorsal root ganglion neuronal survival

Heat shock protein 27 (Hsp27), a molecular chaperone ubiquitously expressed in many cell types, has been shown to play a role in protecting neurons from cellular stresses. Unlike adult DRG neurons in vitro, neonatal DRG neurons require NGF for survival; withdrawal of NGF results in apoptosis of a majority of neonatal neurons. We hypothesized that Hsp27 contributes to the neurotrophin-independent survival of adult DRG neurons. Constitutive Hsp27 expression is higher in adult DRG neurons compared to neonates, although both upregulate Hsp27 expression after heat shock (HS). We found that increasing endogenous Hsp27 by HS in neonatal neurons was able to inhibit NGF withdrawal-induced apoptosis. Heat shock of adult and neonatal neurons also resulted in Akt activation, which could be a mechanism for the increased survival. Hsp27 siRNA treatment of adult neurons effected a decreased expression of Hsp27, which correlated with increased apoptosis in these neurons. Downregulation of Hsp27 via siRNA also blocked the HS-induced rescue of neonatal neurons after NGF withdrawal. These results indicate that physiologically induced upregulation of Hsp27 is sufficient to provide some degree of neuronal protection. Further, this induction appears to be regulated by the transcriptional activation of HSF1 as shown by HSF1 nuclear translocation and by EMSA analyses of HSF1 binding to nuclear protein.