Inactivity-induced modulation of Hsp20 and Hsp25 content in rat hindlimb muscles

Effect of HSP25 loss on muscle contractile function and running wheel activity in young and old mice

Aging is associated with an adverse decline in muscle function, often manifesting as decreased strength and increased muscle fatigability that negatively affects the overall health of the elderly. Heat shock proteins (HSPs), a family of stress inducible proteins known to protect cells from damage, are highly induced in muscle cells following exercise, but both basal and inducible levels decline with age. Utilizing young and old mice lacking HSP25 (Hsp25^−/−) we tested the hypothesis that HSP25 is required to maintain normal muscle function and that age related decreases in HSP25 directly contribute to declining muscle function. Running wheel distances over 14 days for young Hsp25^−/− mice were significantly lower than for the corresponding Hsp25^+/+ genotype (81238 vs. 33956 AUC, respectively). While older groups both ran significantly less than young groups, in aged mice HSP25 loss did not lead to any additional decrease. Significantly lower myofibrillar (contractile) protein levels in young Hsp25^−/− vs. Hsp25^+/+ (15.7 ± 0.2 vs. 13.4 ± 0.3 mg/mg muscle) mice suggests HSP25 loss was associated with greater muscle breakdown during voluntary wheel running. In vivo, plantarflexor maximal isometric force was significantly decreased in aged vs. young mice, but the loss of HSP25 had no effect on either group. However, plantarflexor fatigability over 10 contractions was significantly higher in young Hsp25^−/− vs. Hsp25^+/+ mice (59 ± 3 vs. 49 ± 4% of initial force, respectively) but no similar effect of genotype was detected in the older groups. There was no difference in muscle caspase-3 activity between Hsp25^−/− and Hsp25^{+/+ mice}, whether young or old, but there was a significant genotype independent increase in activity with age. Overall, the results suggest that the absence of HSP25 primarily contributes to muscle fatigue resistance, rather than maximal force production, and that this effect is most evident in young compared to older mice.

Heterooligomeric complexes of human small heat shock proteins

Oligomeric association of human small heat shock proteins HspB1, HspB5, HspB6 and HspB8 was analyzed by means of size-exclusion chromatography, analytical ultracentrifugation and chemical cross-linking. Wild-type HspB1 and Cys mutants of HspB5, HspB6 and HspB8 containing a single Cys residue in position homologous to that of Cys137 of human HspB1 were able to generate heterodimers cross-linked by disulfide bond. Cross-linked heterodimers between HspB1/HspB5, HspB1/HspB6 and HspB5/HspB6 were easily produced upon mixing, whereas formation of any heterodimers with participation of HspB8 was significantly less efficient. The size of heterooligomers formed by HspB1/HspB6 and HspB5/HspB6 was different from the size of the corresponding homooligomers. Disulfide cross-linked homodimers of small heat shock proteins were unable to participate in heterooligomer formation. Thus, monomers can be involved in subunit exchange leading to heterooligomer formation and restriction of flexibility induced by disulfide cross-linking prevents subunit exchange.

Large potentials of small heat shock proteins

Modern classification of the family of human small heat shock proteins (the so-called HSPB) is presented, and the structure and properties of three members of this family are analyzed in detail. Ubiquitously expressed HSPB1 (HSP27) is involved in the control of protein folding and, when mutated, plays a significant role in the development of certain neurodegenerative disorders. HSPB1 directly or indirectly participates in the regulation of apoptosis, protects the cell against oxidative stress, and is involved in the regulation of the cytoskeleton. HSPB6 (HSP20) also possesses chaperone-like activity, is involved in regulation of smooth muscle contraction, has pronounced cardioprotective activity, and seems to participate in insulin-dependent regulation of muscle metabolism. HSPB8 (HSP22) prevents accumulation of aggregated proteins in the cell and participates in the regulation of proteolysis of unfolded proteins. HSPB8 also seems to be directly or indirectly involved in regulation of apoptosis and carcinogenesis, contributes to cardiac cell hypertrophy and survival and, when mutated, might be involved in development of neurodegenerative diseases. All small heat shock proteins play important "housekeeping" roles and regulate many vital processes; therefore, they are considered as attractive therapeutic targets.

HSP25 can modulate myofibrillar desmin cytoskeleton following the phosphorylation at Ser15 in rat soleus muscle

The main purpose of the present study was to investigate the role(s) of 25-kDa heat shock protein (HSP25) in the regulation and integration of myofibrillar Z-disc structure during down- or upregulation of the size in rat soleus muscle fibers. Hindlimb unloading by tail suspension was performed in adult rats for 7 days, and reloading was allowed for 5 days after the termination of suspension. Interaction of HSP25 and Z-disc proteins, phosphorylation status, distribution, and complex formation of HSP25 were investigated. Non- and single-phosphorylated HSP25s were generally expressed in the cytoplasmic fraction of normal muscle. The level of total HSP25, as well as the phosphorylation ratio, did not change significantly in response to atrophy. Increased expressions of HSP25, phosphorylated at serine 15 (p-Ser15) and dual-phosphorylated form, were noted, when atrophied muscles were reloaded. Myofibrillar HSP25 was also noted in reloaded muscle. Histochemical analysis further indicated the localization of p-Ser15 in the regions with disorganization of Z-disc structure in reloaded muscle fibers. HSP25 formed a large molecular complex in the cytoplasmic fraction of normal muscle, whereas dissociation of free HSP25 with Ser15 phosphorylation was noted in reloaded muscle. The interaction of p-Ser15 with desmin and actinin was detected in Z-discs by proximity ligation assay. Strong interaction between p-Ser15 and desmin, but not actinin, was noted in the disorganized areas. These results indicated that HSP25 contributed to the desmin cytoskeletal organization following the phosphorylation at Ser15 during reloading and regrowing of soleus muscle.

Early response of heat shock proteins to functional overload of the soleus and plantaris in rats and mice

Heat shock proteins (HSPs) are important factors in the response of skeletal muscles to chronic increases or decreases in activation and loading. The purpose of this study was to compare species-, time- and muscle-dependent changes in protein expression of Hsp20, Hsp25, αB-crystallin, Hsp72 and Hsp90 in response to functional overload (FO) in rats and mice. We compared protein levels of Hsp20, Hsp25, αB-crystallin, Hsp72 and Hsp90 in soleus and plantaris in baseline conditions and following 0.5, 1, 2, 3 and 7 days (rats) or 3 and 7 days (mice) of FO. Baseline levels of all HSPs were higher in rat soleus than plantaris, whereas only baseline expression of Hsp20 was higher in mouse soleus than plantaris. Levels of Hsp72 and Hsp90 were higher in plantaris and soleus of FO than control mice and rats after 3 and 7 days of FO. Protein levels and phosphorylation of Hsp25 in mouse plantaris and soleus were higher than control levels after 3 and 7 days of FO, except for soleus at 3 days. αB-crystallin levels were higher in plantaris of FO than control mice after 3 and 7 days of FO and in FO than control rats after 7 days of FO. Heat shock protein 20 was the least responsive, increasing only in 7 day FO rat plantaris compared with control rats. Overall, the results demonstrate that levels of both large and small HSPs increase with FO, suggesting a contributory role during the compensatory hypertrophy response.

The small heat shock protein, HSPB6, in muscle function and disease

The small heat shock protein, HSPB6, is a 17-kDa protein that belongs to the small heat shock protein family. HSPB6 was identified in the mid-1990s when it was recognized as a by-product of the purification of HSPB1 and HSPB5. HSPB6 is highly and constitutively expressed in smooth, cardiac, and skeletal muscle and plays a role in muscle function. This review will focus on the physiologic and biochemical properties of HSPB6 in smooth, cardiac, and skeletal muscle; the putative mechanisms of action; and therapeutic implications.

Protective effects of exercise preconditioning on hindlimb unloading-induced atrophy of rat soleus muscle

AIM

A chronic decrease in the activation and loading levels of skeletal muscles as occurs with hindlimb unloading (HU) results in a number of detrimental changes. Several proteolytic pathways are involved with an increase in myofibrillar protein degradation associated with HU. Exercise can be used to counter this increase in proteolytic activity and, thus, may be able to protect against some of the detrimental changes associated with chronic decreased use. The purpose of the present study was to determine the potential of a single bout of preconditioning endurance exercise in attenuating the effects of 2 weeks of HU on the mass, phenotype and force-related properties of the soleus muscle in adult rats.

METHODS

Male Wistar rats were subjected to HU for 2 weeks. One half of the rats performed a single bout of treadmill exercise for 25 min immediately prior to the 2 weeks of HU.

RESULTS

Soleus mass, maximum tetanic tension, myofibrillar protein content, fatigue resistance and percentage of type I (slow) myosin heavy chain were decreased in HU rats. In addition, markers for the cathepsin, calpain, caspase and ATP-ubiquitin-proteasome proteolytic pathways were increased. The preconditioning endurance exercise bout attenuated all of the detrimental changes associated with HU, and increased HSP72 mRNA expression and protein levels.

CONCLUSION

These findings indicate that exercise preconditioning may be an effective countermeasure to the detrimental effects of chronic decreases in activation and loading levels on skeletal muscles and that an elevation in HSP72 may be one of the mechanisms associated with these responses.

Small heat shock proteins in smooth muscle

The small heat shock proteins (HSPs) HSP20, HSP27 and alphaB-crystallin are chaperone proteins that are abundantly expressed in smooth muscles are important modulators of muscle contraction, cell migration and cell survival. This review focuses on factors regulating expression of small HSPs in smooth muscle, signaling pathways that regulate macromolecular structure and the biochemical and cellular functions of small HSPs. Cellular processes regulated by small HSPs include chaperoning denatured proteins, maintaining cellular redox state and modifying filamentous actin polymerization. These processes influence smooth muscle proliferation, cell migration, cell survival, muscle contraction and synthesis of signaling proteins. Understanding functions of small heat shock proteins is relevant to mechanisms of disease in which dysfunctional smooth muscle causes symptoms, or is a target of drug therapy. One example is that secreted HSP27 may be a useful marker of inflammation during atherogenesis. Another is that phosphorylated HSP20 which relaxes smooth muscle may prove to be highly relevant to treatment of hypertension, vasospasm, asthma, premature labor and overactive bladder. Because small HSPs also modulate smooth muscle proliferation and cell migration they may prove to be targets for developing effective, novel treatments of clinical problems arising from remodeling of smooth muscle in vascular, respiratory and urogenital systems.

Time-dependent changes in caspase-3 activity and heat shock protein 25 after spinal cord transection in adult rats

Chronic reductions in muscle activation and loading are associated with decreased heat shock protein 25 (Hsp25) expression and phosphorylation (pHsp25) which, in turn, may contribute to elevated caspase-3-mediated muscle protein breakdown. Thus, the purpose of the present study was to determine whether there are any changes in Hsp25, pHsp25 and caspase-3 activity among rat muscles having different fibre type compositions and functions [soleus, adductor longus (AL), plantaris and tibialis anterior (TA)] at 0 (control), 1, 8 or 28 days after a complete spinal cord transection (ST). The Hsp25 levels were unaffected on days 1 and 8 in all muscles, except for a significant reduction on day 8 in plantaris. The Hsp25 levels were lower than control values in all muscles except TA on day 28. The pHsp25 levels were lower than control values after 8 and 28 days in plantaris and AL and after 28 days in soleus, but higher than control in TA after 8 and 28 days. Caspase-3 activity was higher in ST than control rats on day 8 in all muscles except TA. Caspase-3 activity was negatively correlated with muscle mass for all muscles. In plantaris, Hsp25 and pHsp25 were negatively correlated with caspase-3 activity and Hsp25 was correlated with muscle mass. These relationships were not observed in other muscles. Thus, the effects of ST on Hsp25 and caspase-3 are muscle specific and time dependent, factors that should be considered in developing any intervention to maintain muscle mass after a spinal cord injury.

Consumption of a moderately Zn-deficient and Zn-supplemented diet affects soluble protein expression in rat soleus muscle

Zinc deficiency negatively affects muscle function, but there are limited biochemical data identifying the cause of this reduction in function. The objective of the present study was to identify soluble proteins in rat soleus muscle that were responsive to different levels of dietary zinc. Rats (n=21) were fed diets containing three concentrations of zinc: 5, 30 and 200 ppm for 42 days. There was no difference in body weights of the rats consuming the 5-ppm zinc diet compared to the rats consuming the 30- or 200-ppm zinc diets; however, bone zinc levels were significantly decreased in the 5-ppm dietary zinc group. Individual soluble protein fractions were isolated from these muscles and the samples were prepared for two-dimensional polyacrylamide gel electrophoresis. The expression levels of four proteins were significantly depressed by dietary Zn depletion and supplementation, S-glutathiolated carbonic anhydrase, myosin light polypeptide 3, heat shock protein 20 and heart fatty acid binding protein. This is the first report that indicates that both Zn depletion and supplementation result in protein expression profiles that may negatively affect skeletal muscle function. These results indicate that there are specific signaling pathways that require proper Zn nutriture for maintaining optimal muscle function and suggest that the consumption of pharmacologic doses of Zn may be detrimental to muscle function.

Role(s) of nucleoli and phosphorylation of ribosomal protein S6 and/or HSP27 in the regulation of muscle mass

Effects of 14 days of hindlimb unloading or synergist ablation-related overloading with or without deafferentation on the fiber cross-sectional area, myonuclear number, size, and domain, the number of nucleoli in a single myonucleus, and the levels in the phosphorylation of the ribosomal protein S6 (S6) and 27-kDa heat shock protein (HSP27) were studied in rat soleus. Hypertrophy of fibers (+24%), associated with increased nucleolar number (from 1–2 to 3–5) within a myonucleus and myonuclear domain (+27%) compared with the preexperimental level, was induced by synergist ablation. Such phenomena were associated with increased levels of phosphorylated S6 (+84%) and HSP27 (+28%). Fiber atrophy (−52%), associated with decreased number (−31%) and domain size (−28%) of myonuclei and phosphorylation of S6 (−98%) and HSP27 (−63%), and with increased myonuclear size (+19%) and ubiquitination of myosin heavy chain (+33%, P > 0.05), was observed after unloading, which inhibited the mechanical load. Responses to deafferentation, which inhibited electromyogram level (−47%), were basically similar to those caused by hindlimb unloading, although the magnitudes were minor. The deafferentation-related responses were prevented and nucleolar number was even increased (+18%) by addition of synergist ablation, even though the integrated electromyogram level was still 30% less than controls. It is suggested that the load-dependent maintenance or upregulation of the nucleolar number and/or phosphorylation of S6 and HSP27 plays the important role(s) in the regulation of muscle mass. It was also indicated that such regulation was not necessarily associated with the neural activity.

Modulation of HSP25 and TNF-alpha during the early stages of functional overload of a rat slow and fast muscle

Early events in response to abrupt increases in activation and loading with muscle functional overload (FO) are associated with increased damage and inflammation. Heat shock protein 25 (HSP25) may protect against these stressors, and its expression can be regulated by muscle loading and activation. The purpose of this study was to investigate the responses of HSP25, phosphorylated HSP25 (pHSP25), and tumor necrosis factor-alpha (TNF-alpha) during FO of the slow soleus and fast plantaris. We compared the HSP25 mRNA, HSP25 protein, pHSP25, and TNF-alpha responses in the soleus and plantaris after 0.5, 1, 2, 3, and 7 days of FO. HSP25 and pHSP25 were quantified in soluble and insoluble fractions. HSP25 mRNA increased immediately in both muscles and decreased with continued FO. However, HSP25 mRNA levels were consistently higher in the muscles of FO than control rats. In the soluble fraction, HSP25 increased in the plantaris after 2-7 days of FO with the greatest response at 3 and 7 days. The pHSP25 response to FO was greater in the plantaris than soleus at all points in the soluble fraction and at 0.5 days in the insoluble fraction. TNF-alpha levels in the plantaris, but not soleus, were higher than control at 0.5-2 days of FO. This may have contributed to the greater FO response in pHSP25 in the plantaris than soleus as TNF-alpha increased pHSP25 in C2C12 myotubes. These results suggest that the initial responses of pHSP25 and TNF-alpha to mechanical stress and inflammation associated with FO are greater in a fast than slow extensor muscle.

Changes in the rat heart proteome induced by exercise training: Increased abundance of heat shock protein hsp20

Chronic exercise training elicits adaptations in the heart that improve pump function and confer cardioprotection. To identify molecular mechanisms by which exercise training stimulates this favorable phenotype, a proteomic approach was employed to detect rat cardiac proteins that were differentially expressed or modified after exercise training. Exercise-trained rats underwent six weeks of progressive treadmill training five days/week, 0% grade, using an interval training protocol. Sedentary control rats were age- and weight-matched to the exercise-trained rats. Hearts were harvested at various times (0-72 h) after the last bout of exercise and were used to generate 2-D electrophoretic proteome maps and immunoblots. Compared with hearts of sedentary rats, 26 protein spot intensities were significantly altered in hypertrophied hearts of exercise-trained rats (p <0.05), and 12 spots appeared exclusively on gels from hearts of exercise-trained rats. Immunoblotting confirmed that chronic exercise training, but not a single bout of exercise, elicited a 2.5-fold increase in the abundance of one of the candidate proteins in the heart, a 20 kDa heat shock protein (hsp20) that persisted for at least 72 h of detraining. Thus, exercise training alters the cardiac proteome of the rat heart; the changes include a marked increase in the expression of hsp20.

Regulation of HSP25 expression and phosphorylation in functionally overloaded rat plantaris and soleus muscles

Functional overload (FO) is a powerful inducer of muscle hypertrophy and both oxidative and mechanical stress in muscle fibers. Heat shock protein 25 (HSP25) may protect against both of these stressors, and its expression can be regulated by changes in muscle loading and activation. The primary purpose of the present study was to test the hypothesis that chronic FO increases HSP25 expression and phosphorylation (pHSP25) in hypertrophying rat hindlimb muscle. HSP25 and pHSP25 levels were quantified in soluble and insoluble fractions of the soleus and plantaris to determine whether 3 or 7 days of FO increase translocation of HSP25 and/or pHSP25 to the insoluble fraction. p38 protein and phosphorylation (p-p38) was measured to determine its association with changes in pHSP25. HSP25 mRNA showed time-dependent increases in both the soleus and plantaris with FO. Three or seven days of FO increased HSP25 and pHSP25 in the soluble fraction in both muscles, with a greater response in the plantaris. In the insoluble fraction, HSP25 was increased after 3 or 7 days in both muscles, whereas pHSP25 was only increased in the 7-day plantaris. p38 and p-p38 increased in the plantaris at both time points. In the soleus, p-p38 only increased after 7 days. These results show that FO is associated with changes in HSP25 expression and phosphorylation and suggest its role in the remodeling that occurs during muscle hypertrophy. Increases in HSP25 in the insoluble fraction suggest that it may help to stabilize actin and/or other cytoskeletal proteins during the stress of muscle remodeling.

Effects of innervation state on Hsp25 content and phosphorylation in inactive rat plantaris muscles

AIM

Previous reports suggest a role for neuromuscular activity levels and/or connectivity in modulating Hsp25 expression and phosphorylation (pHsp25) in skeletal muscles. However, pHsp25 has only been studied in denervated muscles and/or muscles exposed to high levels of residual neuromuscular activity. Spinal cord isolation (SI) provides a model in which the muscle is exposed to nearly complete inactivity with maintenance of the nerve-muscle connection. To parcel out the roles of innervation state and activity-independent neural factors, we compared Hsp25 and pHsp25 in the plantaris of control (Con), SI, and denervated (Den, inactivity without neural connectivity) rats.

METHODS

Hsp25 and pHsp25 protein levels (soluble and insoluble fractions) were measured with Western blot analysis after 1, 3, 8, 14, or 28 days of SI or Den. pHsp25 was normalized to non-pHsp25 at each time point.

RESULTS

Hsp25 was unchanged (days 1, 3 and 14) or increased (days 8 and 28) in the soluble fraction, and decreased (day 1) or increased (days 3, 8 and 14) in the insoluble fraction in Den compared with Con rats. pHsp25 was reduced after 1 and 28 days of Den, but near control levels on days 3, 8, and 14 in the soluble fraction. In the insoluble fraction, pHsp25 levels were lower in Den than Con rats on all days. In both fractions, Hsp25 was lower in SI than Con rats. pHsp25 levels were lower in the soluble fraction and higher in the insoluble fraction in SI than Con rats.

CONCLUSION

These results suggest that an intact innervation, even in the absence of muscle activation and/or loading, is critical for Hsp25 phosphorylation in the insoluble fraction. However, the time-dependent decrease in Hsp25 with SI suggests a role for minimal levels of muscle activation and/or loading in maintaining Hsp25 expression during sustained inactivity.

Structure, properties, and probable physiological role of small heat shock protein with molecular mass 20 kD (Hsp20, HspB6)

This review is devoted to critical analysis of data concerning the structure and functions of small heat shock proteins with apparent molecular mass 20 kD (Hsp20). We describe the structure of Hsp20, its phosphorylation by different protein kinases, interaction of Hsp20 with other small heat shock proteins, and chaperone activity of Hsp20. The distribution of Hsp20 in different animal tissues and the factors affecting expression of Hsp20 are also described. Data on the possible involvement of Hsp20 in regulation of platelet aggregation and glucose transport are presented and analyzed. Special attention is paid to literature data describing probable regulatory effect of Hsp20 on contraction of smooth muscle. Two hypotheses postulating direct effect of Hsp20 on actomyosin interaction or its effect on cytoskeleton are compared and analyzed. The most recent data on the effect of Hsp20 on apoptosis and contractile activity of cardiomyocytes are also presented.

Heat shock proteins and neuromuscular disease

The heat shock proteins are families of proteins with known activities that include chaperoning nascent peptides within the cell and cytoprotection. Most work on the nervous system has related to the role of heat shock proteins in neuroprotection from either hypoxic-ischemic or traumatic injury. The role of these proteins during normal physiological activity and injury is still under investigation. Heat shock proteins in neuromuscular disease have been investigated to some extent but were largely neglected until recently. The goal of this review is to summarize the evidence linking heat shock proteins with neuromuscular disease and to provide some insight into the roles or functions of these proteins in disease states.