The ability of skeletal muscle to withstand the stress generated by contractile activity is fundamental to maintaining its functional capacity throughout the life span. During the evolutionary process, various strategies developed in myocytes for this purpose. Resulting from a stress event, in the first instance, the acute stress response ensures the homeostasis and consequently the functionality of the cells. Repeated stress events lead to a persistent expansion of the compensation capacities or to adaptation. The specific type of adaptation is directly related to the characteristic of the induced stress. This stress, in turn, is in the physiological sense only identifiable by the specificity of the stress response. Thus, a sufficient understanding of targeted muscular adaptation is inseparably linked to a comprehensive understanding of the regulation of the molecular stress response.
A key player in the myocellular immediate protection mechanism is αB-Crystallin (CRYAB), an ATP- independent chaperone belonging to the family of small heat shock proteins. As such, the protein has a cytoprotective function by "capturing" unfolding or misfolded proteins, therefore preventing their irreversible aggregation and, in cooperation with other ATP-dependent chaperones, leading them to renaturation or autophagosomal degradation. It is known from skeletal muscle that CRYAB translocates from the cytosol to cytoskeletal structures and associates with these as a result of contraction-induced stress, thereby stabilizing and protecting against potential (further) damage. In the human skeletal muscle, the mechanical stress component seems to be the most important factor for the induction of CRYAB due to contractile activity. An essential factor for the function and thus the cytoprotective properties of CRYAB is its phosphorylation on serine 59.
The aim of the present work was to generate a comprehensive understanding of the regulation of CRYAB in response to resistance exercise-induced mechanical stress. Due to the heterogeneity of skeletal muscle tissue with respect to its fiber types (type I, IIA, IIX) and the resulting functional properties, it was also intended to generate a fiber type-specific image of CRYAB-regulation. The investigation of the regulation included the phosphorylation of CRYAB at serine 59 (pCRYABS59), the expression of total CRYAB and its translocation or association with cytoskeletal structures.
Therefore, in a first study, the regulation of CRYAB was investigated under the influence of different loading types and thus different levels of contraction-induced mechanical stress, as well as different stress volumes(73). The focus was on the acute response of CRYAB following single, non-repetitive stress. Based on these findings, the influence of chronic stress on acute CRYAB-regulation was examined. It was investigated whether the acute myocellular stress response is modified as a result of systematic repetitive stress as well as of a temporary termination of this stress. Assuming that systematically repeated stress exposure leads to an adaptation in the sense of an increased resistance to the stressor, the interest was focused on possible factors that could contribute to this potential myocellular mechanical stress resistance. For this purpose, it was determined whether the type III 5
intermediary filament desmin, which is an important structural protein involved in lateral force transmission, is upregulated in the course of repeated resistance exercise stimuli.
In summary, we could show that resistance exercise-associated stress leads to a strong increase in pCRYABS59, for which the mechanical stress component is essential. Systematic repetition of exercise leads to a successive attenuation of the pCRYABS59-response and increases the amount of desmin; however, if regular stress exposure of the muscle is terminated for a certain period of time, the amount of desmin decreases and the pCRYABS59-response increases again as a result of renewed exercise. Thus, the pCRYABS59-signal appears to follow the amount of desmin in a kind of inverse relationship. In the course of this, it seems that high mechanical stress in particular promotes the increase in desmin. Furthermore, the pCRYABS59 response and the translocation of CRYAB are fiber type-specific and depend on the type of loading applied. In detail, while pCRYABS59 increases in Type I fibers as a result of loadings with low as well as with high external resistances, an increase in Type II fibers can only be observed as a result of loadings with high external resistances. In general, CRYAB is phosphorylated due to the accumulation of a certain load volume (multiple sets). Moreover, an increased loading volume can promote the phosphorylation of CRYAB in type II fibers, even if the pure intensity (level of external resistance) per se is relatively low.
We conclude that the acute myocellular stress defense response in the form of phosphorylation of CRYAB at serine 59 appropriately indicates the influence of contraction-induced mechanical stress at the single-fiber level. Furthermore, CRYAB reflects the desensitization of myocytes to repeated mechanical stress as well as their resensitization in the absence of regular stimulation. A dynamic process of building and breaking down the cytoskeletal desmin network is used to adapt to the changing requirements of myocellular resistance to mechanical stress. On the one hand, these findings serve to understand the function of CRYAB in the context of the stress defense reaction under practically relevant in vivo conditions. On the other hand, they can help to differentiate between more and less effective types of exercise for the induction of acute stress as well as chronic adaptation of muscle fibers. In the context of applied exercise science, the knowledge of cause-effect relationships (load-adaptation) is thus expanded, which makes it possible to control training more precisely on the basis of physiological observations over time.