Adhesion proteins--an impact on skeletal myoblast differentiation.

Formation of mammalian skeletal muscle myofibers, that takes place during embryogenesis, muscle growth or regeneration, requires precise regulation of myoblast adhesion and fusion. There are few evidences showing that adhesion proteins play important role in both processes. To follow the function of these molecules in myoblast differentiation we analysed integrin alpha3, integrin beta1, ADAM12, CD9, CD81, M-cadherin, and VCAM-1 during muscle regeneration. We showed that increase in the expression of these proteins accompanies myoblast fusion and myotube formation in vivo. We also showed that during myoblast fusion in vitro integrin alpha3 associates with integrin beta1 and ADAM12, and also CD9 and CD81, but not with M-cadherin or VCAM-1. Moreover, we documented that experimental modification in the expression of integrin alpha3 lead to the modification of myoblast fusion in vitro. Underexpression of integrin alpha3 decreased myoblasts' ability to fuse. This phenomenon was not related to the modifications in the expression of other adhesion proteins, i.e. integrin beta1, CD9, CD81, ADAM12, M-cadherin, or VCAM-1. Apparently, aberrant expression only of one partner of multiprotein adhesion complexes necessary for myoblast fusion, in this case integrin alpha3, prevents its proper function. Summarizing, we demonstrated the importance of analysed adhesion proteins in myoblast fusion both in vivo and in vitro.

Regulation of muscle stem cells activation: the role of growth factors and extracellular matrix.

Vertebrate skeletal muscle is composed of organized multinucleate muscle fibers and also various subpopulations of cells localized in between. Some of them can be considered as the stem cells, however, few of them are able to follow myogenic program. First and most extensively studied so far, are the satellite cells that serve as tissue-specific precursors for muscle growth and repair. They are located between the basal membrane and the sarcolemma of adult muscle myofibers. They remain quiescent but can be activated in response to muscle damage resulted from mechanical injury, stretching, exercise, denervation, or progressing muscle dystrophy. Except the satellite cells also other stem cells could participate in muscle fibers reconstruction. Such cells as pericytes and mesangioblasts, muscle-derived stem cells, including so-called muscle side population, or CD133 expressing cells, were proved to be able to undergo myogenic differentiation in experiments involving their in vitro coculture with myoblasts or transplantation to injured skeletal muscle. In the current review, we will summarize stimuli influencing skeletal muscle stem cells activation, that is, growth factors which are secreted by muscle fibers, satellite cells, inflammatory cells, or released from basal lamina. We will also describe factors present within the skeletal muscle niche which interactions with stem cells lead to their activation, proliferation, asymmetric divisions, migration, and finally differentiation into myotubes, and then terminally differentiated myofibers.

Cell cycle regulation during proliferation and differentiation of mammalian muscle precursor cells.

Proliferation and differentiation of muscle precursor cells are intensively studied not only in the developing mouse embryo but also using models of skeletal muscle regeneration or analyzing in vitro cultured cells. These analyses allowed to show the universality of the cell cycle regulation and also uncovered tissue-specific interplay between major cell cycle regulators and factors crucial for the myogenic differentiation. Examination of the events accompanying proliferation and differentiation leading to the formation of functional skeletal muscle fibers allows understanding the molecular basis not only of myogenesis but also of skeletal muscle regeneration. This chapter presents the basis of the cell cycle regulation in proliferating and differentiating muscle precursor cells during development and after muscle injury. It focuses at major cell cycle regulators, myogenic factors, and extracellular environment impacting on the skeletal muscle.

Pax3 and Pax7 expression during myoblast differentiation in vitro and fast and slow muscle regeneration in vivo.

In this report, we focused on Pax3 and Pax7 expression in vitro during myoblast differentiation and in vivo during skeletal muscle regeneration. We showed that Pax3 and Pax7 were present in EDL (extensor digitorum longus) and Soleus muscle derived cells. These cells express in vitro a similar level of Pax3 mRNA, however, differ in the levels of mRNA encoding Pax7. Analysis of Pax3 and Pax7 proteins showed that Soleus and EDL satellite cells differ in the level of Pax3/7 proteins and also in the number of Pax3/7 positive cells. Moreover, Pax3/7 expression was restricted to undifferentiated cells, and both proteins were absent at further stages of myoblast differentiation, indicating that Pax3 and Pax7 are down-regulated during myoblast differentiation. However, we noted that the population of undifferentiated Pax3/7 positive cells was constantly present in both in vitro cultured satellite cells of EDL and Soleus. In contrast, there was no significant difference in Pax3 and Pax7 during in vivo differentiation accompanying regeneration of EDL and Soleus muscle. We demonstrated that Pax3 and Pax7, both in vitro and in vivo, participated in the differentiation and regeneration events of muscle and detected differences in the Pax7 expression pattern during in vitro differentiation of myoblasts isolated from fast and slow muscles.