Abundance and location of the small heat shock proteins HSP25 and alphaB-crystallin in rat and human heart

The small heat shock proteins Hsp20 and alphaB-crystallin in cultured cardiac myocytes: differences in cellular localization and solubilization after heat stress

Hsp20, a recently described new member of the small heat shock protein superfamily, is abundant in heart, skeletal muscle types and smooth muscle. We investigated the intracellular localization of Hsp20 in cultured rat neonatal cardiac myocytes, under normal conditions and after stress. These cellular characteristics of Hsp20 were compared with those of its closest relative, alphaB-crystallin, which is also highly expressed in heart. Like alphaB-crystallin, Hsp20 is normally located in the cytoplasm of the cardiac myocytes. After a heat stress, a subpopulation of Hsp20 migrates into the nucleus, while another part remains in the cytoplasm. In very few cells a faint sarcomeric association of Hsp20 is observed. In contrast, as previously reported, alphaB-crystallin displays a very distinct cross-striated sarcomeric staining after the heat shock, but no nuclear migration. Also at the level of Triton solubility, differences exist between the two related proteins; while alphaB-crystallin, like other small heat shock proteins, becomes insoluble upon heat stress, Hsp20 remains largely soluble. Our results indicate that Hsp20 and alphaB-crystallin, despite their structural similarities, display conspicuous functional differences.

Rat Hsp20 confers thermoresistance in a clonal survival assay, but fails to protect coexpressed luciferase in Chinese hamster ovary cells

Hsp20 is a mammalian small heat shock protein with some deviating in vitro characteristics. We now compare the in vivo cellular thermoprotective abilities of Hsp20 with those of its direct relative, alphaB-crystallin. In a clonal survival assay Chinese hamster ovary (CHO) cells stably overexpressing Hsp20 survive equally well as alphaB-crystallin-expressing cells, after a heat shock. In a transient assay, however, overexpression of Hsp20 did not result in an enhanced recovery of coexpressed firefly luciferase after heat shock, in contrast to alphaB-crystallin. This might indicate that these highly homologous stress proteins are involved in at least partially distinct protective activities in cultured cells.

Protein changes observed in pacing-induced heart failure using two-dimensional electrophoresis

Rapid ventricular pacing in dogs results in a low output cardiomyopathic state which is similar to idiopathic dilated cardiomyopathy in man. However, the pathophysiological mechanisms which cause this failure following pacing are unknown. Five dogs underwent rapid ventricular pacing. Hearts were stimulated at 245 beats per min (bpm) for four weeks and then reduced to 190 bpm to stabilize the failure. Six unoperated dogs were used as controls. This paper compares the two-dimensional gel electrophoresis (2-DE) protein patterns of left ventricular samples from the paced myocardium with the control dogs. Changes in protein expression were analyzed qualitatively and semi-quantitatively. In the paced dog samples 69 protein spots were significantly altered of which 42 were decreased and 27 were elevated. One qualitative change was observed: elongation factor Tu was present only the control hearts. Of these proteins, 20 have been identified by a combination of N-terminal protein microsequencing, peptide mass profiling by mass spectrometry, amino acid compositional analysis, and by comparison with databases of canine and human ventricular proteins. Ten of these are associated with mitochondria and energy production, including: pyruvate dehydrogenase E1 component, isocitrate dehydrogenase subunit alpha, HSP60 and HSP70, creatine kinase M and fatty acid binding protein. The cytoskeletal protein desmin was detected in reduced quantities and a spot corresponding to a fragment of desmin was increased. These results indicate that the development of heart failure in the paced dog involves alterations in mitochondrial energy production, the cytoskeleton and calcium activation.

Small heat-shock protein family: function in health and disease

In summary, several important issues were discussed at the workshop. These included the importance of oligomer size for different functions of the sHsps, the highly significant observation that the sHsps can block apoptosis; and that some mechanisms of sHsps protection involve glutathione while others may be related to microfilament stability. Further evidence was presented for the chaperone functions of sHsps, and structural studies provided additional information relating sHsps' large oligomeric structure to chaperone function. Regulation of oligomer size by phosphorylation was also a prominent topic of discussion, as was the importance of the relationship of oligomer size to differing functions of Hsp27. An interesting contrast was noted between Hsp27, whose oligomer size is regulated by phosphorylation, and alpha beta-crystallin, whose large oligomer structure is unaffected by phosphorylation. This may prove to be of physiological significance, particularly in cell types that express both of these proteins. Observations of the translocation of MAPKAP kinase 2 from the nucleus to the cytoplasm in live cells suggest that the dynamics of phosphorylation of the sHsps may be more complex than previously thought. One topic that seems to have been settled is that the binding site of at least dimers of sHsps resides in the carboxyl-terminal region of the proteins. Ever-increasing numbers of studies are reporting interesting patterns of sHsp expression, phosphorylation, and subcellular localizations of the sHsps in various cells or tissues in normal and diseased states. Unfortunately, very little is understood about what these observations may mean. It can be expected, however, that as more is learned about the function and regulation of sHsps, the relationships between sHsps and cellular response to disease will become better understood. Lastly, the discovery that specific Hsp27-binding proteins may exist should open completely new avenues of investigation into the functions of sHsps. If the observations of unique subcellular localizations of sHsps in various cells types (e.g., striated muscle and Sertoli cells) can be related to expression of specific Hsp-binding proteins, considerable advances in our understanding of sHsps should result. It was abundantly clear from results presented and from ensuing discussions in the workshop that the study of sHsps is an exceedingly dynamic area of research (FIG. 2) with an ever-expanding significance for a number of other areas of current biological research.