Umeå University's logo

Aim of Investigation: Work related illnesses are one of the main reasons for sick-leave in the western world. One of the most common is pain and discomfort in the neck-shoulder region affecting the trapezius muscle, trapezius myalgia (TM). Patients suffering from work related chronic trapezius myalgia experience muscle stiffness, weakness and tension. The biochemical mechanisms behind these sensations are not yet known. Recent proteomic screening results indicate an increased abundance of the calcium regulatory fast myosin light chains in myalgic muscle suggesting an altered contractibility in the myalgic trapezius. To characterize a general increase of Ca²⁺ in the myalgic muscle the expression level of sarco/endoplasmic reticulum Ca²⁺-ATPase, (SERCA-1) was analyzed. The aim of this study was to compare the level of Ca2⁺ regulated proteins in cleaners with trapezius myalgia (TM) and cleaners without trapezius myalgia (CON) and to verify a possible difference in phosphorylation of the contractile regulatory myosin light chains.

Methods: Two-dimensional gel electrophoresis was used to separate proteins of interest. The proteomic correlation pattern of the myosin light chain proteins and the differences between healthy and myalgic muscle were investigated using a multivariate Partial Least Square Discriminant Analysis (PLS-DA). Phosphorylations of the fast regulatory myosin light chains were analyzed using serine and tyrosin antibodies. The characterization of myosin isoforms was performed using advanced Orbitrap LC-MS/MS. The expression level of SERCA-1 was analyzed in muscle biopsies from 11 professional cleaners with TM and 11 CON using Simple western size assay (Peggy, Protein simple, CA, US)

Results: The detected systematic differences between myosin light chain proteins showed according to the PLS-DA analysis that the fast regulatory light chains were generally more abundant in the MYA group. Ten protein spots were identified as different myosin light chains isoforms according to the amino acid sequencing. Phosphorylated serine was detected in 6 of the isoforms, though there were no differences in amount of phosphorylation between spots. The expression level of Serca-1 was significantly (p = 0.017) higher in TM subjects compared to the CON.

Conclusions: There is a higher abundance of fast contracting calcium regulatory myosin light chains in muscle experiencing chronic myalgia. This increased abundance of the regulatory fast myosin light chains supports the previously presented results showing an altered contractibility of the myalgic trapezius muscle. Our results show no difference in phosphorylation between the different spots contradictory to previously published results; suggesting phosphorylation of the protein being the reason for the separation of spots on the two dimensional gels. Instead our results show the spots to have different amino acid sequences. The actual function of the detected myosin light chain proteins remains to be elucidated. The increased abundance of fast contracting regulatory light chains together with the significantly increased abundance of Serca-1 proteins in the MYA muscle supports a higher abundance of Ca²⁺ in the myalgic muscle. The increased abundance of Serca-1 might be an indication of an adaptation due to peripheral sensitization or increased neuronal signaling altering the contrability of the muscle.       

Acknowledgments: Supported by Postdoctoral Grant from Swedish Research Council for Health, Working Life and Welfare (Forte) 2013-1259 and Medical Research Council of Southeast Sweden (FORSS-159031, LIO-35923, SC-2013-00395-36).