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Joshua C. Sandquist
Current focus of the lab - in a nutshell.
The actin-based motor protein called myosin-10 appears to contribute to the proper structure and function of the mitotic spindle, the cellular apparatus that separates a cell's chromosomes during mitosis. Spindle abnormalities observed upon loss of myosin-10 function are similar to spindle abnormalities observed in many cancer cells. Studying the role of myosin-10 at the mitotic spindle will hopefully (1) increase our understanding of mitosis, a process fundamental to life and (2) shed light on mechanisms of tumorigenesis. In order to examine the function of myosin-10 at the spindle my lab uses frog (Xenopus laevis) embryos and a variety of biochemical, molecular and cell biological techniques. If this sounds interesting to you please read below for more information and/or contact me (sandquis[at]grinnell[dot]edu).
Detailed description of lab interests.
All living things are comprised of cells. Despite their exalted role as "the fundamental unit of life", cells have some rather mundane concerns. For instance, many cells need to be able to move from point A to point B at one time or another in their existence and all cells must be able to physically separate their chromosomes in space during mitosis. Put another way, cells exist in a physical world and, therefore, must be able to generate force in order to move themselves or their components about as needed. Moreover, cells cannot just throw their weight around all willy-nilly. The appropriate amount of force must be generated at the correct place and time in order to produce the desired effect. The cytoskeleton – a collection of dynamic biological polymers and associated proteins – is the cellular system that enables cells to generate force in such a highly controlled fashion. My lab is broadly interested in understanding the functions and regulation of a particular family of cytoskeleton-based motor proteins – i.e. force producers – called myosins.
Myosins are actin-based motor proteins (see Figure 1). As motor proteins, myosins use the energy stored in ATP to do work. As actin-based motor proteins, myosins do work by moving, or moving along, filaments of a protein called actin. These actin filaments (a.k.a filamentous actin or F-actin) can be thought of either as cables that anchored myosins pull on or as tracks that free myosin motors walk along, pulling cargo. Current work in my lab is focused on understanding how one myosin motor, myosin-10, contributes to the structure and function of the mitotic spindle.
The mitotic spindle is the cellular apparatus charged with the task of separating the cell's duplicated chromosomes during cell division. In short, the cell division cycle can be thought of as alternating periods of growth and division (see Figure 2). During the growth period, or interphase, cells physically grow in size and duplicate their genetic material (DNA). The normal genetic content of human cells is contained within 46 discrete units of DNA called chromosomes. So, during interphase each chromosome is copied once and only once such that at the end of interphase the cell has 92 chromosomes. When the cell has grown large enough and completely duplicated its DNA it enters the division period, M phase. In early M phase a special cytoskeleton-based structure call the mitotic spindle forms (Figure 2). The mitotic spindle locates, organizes and then separates the 92 chromosomes into two identical pools of 46 chromosomes. This process is called mitosis. Following mitosis the cell divides itself in half between the two pools of DNA, creating two 'new' daughter cells with identical genetic content. This later process is referred to as cytokinesis. In the average human cell M phase (mitosis + cytokinesis) lasts approximately one hour. Thus, in approximately 60 minutes the mitotic spindle forms, finds and attaches to 92 chromosomes; the spindle accurately separates the chromosomes into two identical pools; and the cell divides in half. Amazing!
Previous work in frog embryos has shown that when myosin-10 protein is depleted from cells mitotic spindles develop an abnormal structure and cannot separate the chromosomes. That myosin-10, and actin-based motor, contributes in such a meaningful way to the mitotic spindle is both exciting and confusing. These findings are exciting, or at least interesting, because the abnormal-looking spindles observed in the absence of functional myosin-10 are similar to abnormal-looking spindles commonly seen in cancer cells. As these abnormal spindles are thought to contribute to tumorigenesis, studying how myosin-10 contributes to mitotic spindle structure and function will not only increase are understanding of an essential biological process, mitosis, but might also increase our understanding of cancer. That myosin-10 contributes to mitotic spindle structure and function is confusing because the mitotic spindle is a microtubule-based structure (microtubules are red in Figure 2) and several lines of evidence suggest that the spindle can form and function without actin filaments, leading to the question: what is the function of an actin-based motor at the spindle?
Much has yet to be learned about the mechanisms by which myosin-10 contributes to mitotic spindle structure and function and how the cell regulates these activities of myosin-10. Further, we are interested in understanding other functions of myosin-10 not described here. My lab uses several molecular, biochemical and cell biological approaches to study myosin-10 in Xenopus laevis embryos (and occasionally eggs and oocytes). A key technique is microscopy in which mitotic spindles are imaged in the epithelium of fixed or live embryos (Figure 3). Frog embryos are a great system to study mitotic spindles for several reasons, but a main one is that the experiments are conducted in a fully intact, vertebrate embryo. Thus, we are studying mitosis in the natural state, with all the normal regulatory systems in place. If you are interested in learning more about the cytoskeleton or mitosis or Xenopus please stop by my office (Noyce 1203) or email me (sandquis[at]grinnell[dot]edu).