How to assemble contractile actomyosin bundles in cells?

Gergana Gateva

    Research output: ThesisDoctoral Thesis

    Abstract

    The ability of cells to migrate, adhere, and contract is an essential feature, governing basically every process in the human body. These include fundamental functions such as development, muscle contraction, immune responses, wound healing, and a plethora of diseases including cancer metastasis, vascular diseases, and cardiomyopathies. Contractile actomyosin bundles are the central cellular structures, which enable the cell to perform all these processes, and thus it is crucial to understand how such structures are assembled in cells. Assembly of actomyosin bundles both in non-muscle and in muscle cells is regulated by a multitude of proteins, acting synchronously in a fascinating complexity. Many aspects of this process have remained mysterious, because of the sheer number of the involved actin-binding proteins, the intricate signalling pathways, and the complexity of the underlying interactions. This thesis aimed at understanding of some of the fundamental processes behind the assembly of contractile structures in non-muscle cells (human osteosarcoma cells U2OS), and in heart muscle cells (primary rat cardiomyocytes). Previous studies revealed that contractile actomyosin bundles in U2OS cells, called ventral stress fibers, are generated from a network of precursors, named dorsal stress fibers and transverse arcs. This work revealed that the tropomyosin family proteins (Tpm) play central role in stress fiber assembly. Our studies on U2OS cells indicate that four functionally distinct tropomyosins orchestrate stress fiber assembly. Tpms 1.6/1.7, Tpm 2.1, and Tpms 3.1/ 3.2 stabilize specific stress fiber regions. In contrast, Tpm 4.2 regulates the recruitment of myosin II to diaphanous (Dia) 2 formin-nucleated actin filament population in stress fiber precursors. Our in vitro experiments revealed that these tropomyosin isoforms possess an intrinsic ability to segregate to different actin filaments and bind F-actin with distinct dynamics. Our results revealed that these tropomyosins determine some of the biochemical properties of actin filaments, such as their ability to associate with non-muscle myosin II and actin depolymerizing factor (ADF)/cofilin. Additionally, our studies demonstrated that the interaction between palladin and vasodilator-stimulated phosphoprotein (VASP) promotes dorsal stress fiber assembly. In contrast, the tension generated by transverse arcs inhibits this process. Tension also controls the length of ventral stress fibers and inhibits ADF/cofilin-mediated stress fiber disassembly. Furthermore, our studies in primary rat cardiomyocytes revealed that ADF/cofilins are also crucial for the length regulation and proper function of muscle sarcomeres. Collectively these studies reveal important biochemical and mechanobiological principles that regulate the assembly of contractile actin bundles in non-muscle and in muscle cells.
    Original languageEnglish
    Awarding Institution
    • University of Helsinki
    Supervisors/Advisors
    • Lappalainen, Pekka, Supervisor
    Award date17 Nov 2017
    Place of PublicationHelsinki
    Publisher
    Print ISBNs978-951-51-3733-3
    Electronic ISBNs978-951-51-3734-0
    Publication statusPublished - 17 Nov 2017
    MoE publication typeG5 Doctoral dissertation (article)

    Fields of Science

    • 1182 Biochemistry, cell and molecular biology
    • 1184 Genetics, developmental biology, physiology

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