Induction and phosphorylation of the small heat shock proteins HspB1/Hsp25 and HspB5/αB-crystallin in the rat retina upon optic nerve injury

Introduction: Small heat-shock proteins (sHSPs) generally participate in cellular protein quality-control mechanisms. They are abundant in cardiomyocytes where they bind under diverse stress conditions preferentially to myofilament proteins. The functional role of this association is unclear. Hypothesis: HSP27 and αB-crystallin bind to human cardiac titin spring elements and exert a protective effect on cardiomyocyte passive stiffness under stress conditions. Methods: Binding of sHSPs to recombinant human titin constructs was characterized by GST-pulldown assay and confirmed by immunofluorescence staining of human donor and failing (DCM) cardiomyocytes. Sedimentation-velocity centrifugation, photometric and chromatographic methods were used to test for titin aggregation and protection from it by sHSPs. Passive force was measured in isolated human cardiomyocytes or single myofibrils, in search for a possible protective effect of sHSPs on mechanical function. Results: HSP27 interacted with distinct domains of the human titin-spring region in vitro and in cardiomyocytes, and independent of the presence of actin filaments in the sarcomeres. The binding sites on the elastic titin segment resemble those for αB-crystallin and include proximal Ig-domains, the N2-B and N2-A regions, but not the PEVK-domain. In-vitro assays revealed a monomeric organization of these titin-spring elements; however, unfolded N2-A domain (mainly composed of Ig-domains) aggregated in pH-6.6 buffer but not in normal-pH buffer, whereas αB-crystallin protected from this effect. The intrinsically disordered N2-Bus titin domain did not aggregate. Single skinned human cardiomyocytes showed greatly increased passive stiffness when pre-stretched under acidic stress (pH 6.6), but αB-crystallin or HSP27 corrected the stiffening. In failing patient heart tissue, both HSP27 and αB-crystallin frequently associated with the elastic I-band region, in contrast to a cytosolic and Z-disk location of these sHSPs in donor heart. Conclusions: In cardiomyocytes sHSPs bind to mechanically active titin domains under stress conditions such as intracellular acidosis ( e.g. , ischemia), protecting the titin springs from aggregation and helping reverse diastolic stiffening.

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Small heat shock proteins translocate to the cytoskeleton in human skeletal muscle following eccentric exercise independently of phosphorylation

Journal of Applied Physiology ◽

10.1152/japplphysiol.01026.2013 ◽

2014 ◽

Vol 116 (11) ◽

pp. 1463-1472 ◽

Cited By ~ 18

Author(s):

Noni T. Frankenberg ◽

Graham D. Lamb ◽

Kristian Overgaard ◽

Robyn M. Murphy ◽

Kristian Vissing

Keyword(s):

Heat Shock ◽

Heat Shock Proteins ◽

Eccentric Exercise ◽

Vastus Lateralis ◽

Small Heat Shock Proteins ◽

Control Group ◽

Vastus Lateralis Muscle ◽

Partial Redistribution ◽

Αb Crystallin ◽

Shock Proteins

Small heat shock proteins (sHSPs) are a subgroup of the highly conserved family of HSPs that are stress inducible and confer resistance to cellular stress and injury. This study aimed to quantitatively examine whether type of contraction (concentric or eccentric) affects sHSPs, HSP27 and αB-crystallin, localization, and phosphorylation in human muscle. Vastus lateralis muscle biopsies from 11 healthy male volunteers were obtained pre- and 3 h, 24 h, and 7 days following concentric (CONC), eccentric (ECC1), and repeated bout eccentric (ECC2) exercise. No changes were apparent in a control group ( n = 5) who performed no exercise. Eccentric exercise induced muscle damage, as evidenced by increased muscle force loss, perceived muscle soreness, and elevated plasma creatine kinase and myoglobin levels. Total HSP27 and αB-crystallin amounts did not change following any type of exercise. Following eccentric exercise (ECC1 and ECC2) phosphorylation of HSP27 at serine 15 (pHSP27-Ser15) was increased approximately 3- to 6-fold at 3 h, and pαB-crystallin-Ser59 increased ∼10-fold at 3 h. Prior to exercise most of the sHSP and psHSP pools were present in the cytosolic compartment. Eccentric exercise resulted in partial redistribution of HSP27 (∼23%) from the cytosol to the cytoskeletal fraction (∼28% for pHSP27-Ser15 and ∼7% for pHSP27-Ser82), with subsequent full reversal within 24 h. αB-crystallin also showed partial redistribution from the cytosolic to cytoskeletal fraction (∼18% of total) 3 h post-ECC1, but not after ECC2. There was no redistribution or phosphorylation of sHSPs with CONC. Eccentric exercise results in increased sHSP phosphorylation and translocation to the cytoskeletal fraction, but the sHSP translocation is not dependent on their phosphorylation.

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