Smerdu Vika
Institute of Anatomy, Faculty of Medicine, University of Ljubljana, Ljubljana, Slovenia.
J Anat. 2023 Nov;243(5):826-841. doi: 10.1111/joa.13923. Epub 2023 Jul 7.
To build on the existing data on the pattern of myosin heavy chain (MyHC) isoforms expression in the human muscle spindles, we aimed to verify whether the 'novel' MyHC-15, -2x and -2b isoforms are co-expressed with the other known isoforms in the human intrafusal fibres. Using a set of antibodies, we attempted to demonstrate nine isoforms (15, slow-tonic, 1, α, 2a, 2x, 2b, embryonic, neonatal) in different regions of intrafusal fibres in the biceps brachii and flexor digitorum profundus muscles. The reactivity of some antibodies with the extrafusal fibres was also tested in the masseter and laryngeal cricothyreoid muscles. In both upper limb muscles, the expression of slow-tonic isoform was a reliable marker for differentiating positive bag fibres from negative chain fibres. Generally, bag1 and bag2 fibres were distinguished in isoform 1 expression; the latter consistently expressed this isoform over their entire length. Although isoform 15 was not abundantly expressed in intrafusal fibres, its expression was pronounced in the extracapsular region of bag fibres. Using a 2x isoform-specific antibody, this isoform was demonstrated in the intracapsular regions of some intrafusal fibres, particularly chain fibres. To the best of our knowledge, this study is the first to demonstrate 15 and 2x isoforms in human intrafusal fibres. However, whether the labelling with an antibody specific for rat 2b isoform reflects the expression of this isoform in bag fibres and some extrafusal ones in the specialised cranial muscles requires further evaluation. The revealed pattern of isoform co-expression only partially agrees with the results of previous, more extensive studies. Nevertheless, it may be inferred that MyHC isoform expression in intrafusal fibres varies along their length, across different muscle spindles and muscles. Furthermore, the estimation of expression may also depend on the antibodies utilised, which may also react differently with intrafusal and extrafusal fibres.
为了在人类肌梭中肌球蛋白重链（MyHC）同工型表达模式的现有数据基础上进一步拓展，我们旨在验证“新型”MyHC-15、-2x和-2b同工型是否与人类梭内肌纤维中的其他已知同工型共表达。我们使用一组抗体，试图在肱二头肌和指深屈肌的梭内肌纤维不同区域中证实九种同工型（15、慢张力型、1、α、2a、2x、2b、胚胎型、新生儿型）。还在咬肌和喉环甲肌中测试了一些抗体与梭外肌纤维的反应性。在这两块上肢肌肉中，慢张力型同工型的表达是区分阳性袋状纤维和阴性链状纤维的可靠标志物。一般来说，在同工型1的表达中可以区分袋1和袋2纤维；后者在其整个长度上持续表达这种同工型。尽管同工型15在梭内肌纤维中表达不丰富，但其在袋状纤维的囊外区域表达明显。使用2x同工型特异性抗体，在一些梭内肌纤维，特别是链状纤维的囊内区域证实了这种同工型。据我们所知，本研究首次在人类梭内肌纤维中证实了15和2x同工型。然而，用大鼠2b同工型特异性抗体进行的标记是否反映了该同工型在袋状纤维以及特殊颅肌中的一些梭外肌纤维中的表达，还需要进一步评估。所揭示的同工型共表达模式仅部分与先前更广泛研究的结果一致。然而，可以推断，梭内肌纤维中MyHC同工型的表达沿其长度、在不同的肌梭和肌肉之间存在差异。此外，表达的估计可能还取决于所使用的抗体，这些抗体与梭内肌纤维和梭外肌纤维的反应也可能不同。
