Research Programmes
Cell & Developmental Biology
Microtubule organization
Jens Lüders
Group Leader
Office Tel : +34 93 40 20203
Lab Tel : +34 93 402 02 01
e-mail : jens.luders
irbbarcelona.org
Background
If you are a highly motivated student or young researcher and interested in joining our research group we encourage you to contact us at jens.ludersirbbarcelona.org.
Microtubules are long tubular polymers composed of two proteins, α- and β-tubulin, and are components of the eukaryotic cytoskeleton. Many cellular processes depend on microtubules, making microtubules indispensable for cell proliferation as well as differentiation.
Animal development requires microtubules at all stages. Even before fertilization microtubules play a role - they assemble the spindles during meiotic cell divisions leading to the production of egg and sperm, and provide movement to sperm by building the flagellum. After fertilization they are required for mitotic spindle assembly and chromosome segregation to promote cell proliferation. When cells differentiate microtubules establish cell polarity and changes in cell morphology, they participate in the communication between cells by interacting with signaling molecules, and they are involved in cell migration during tissue formation.
Fig.1: Microtubules (red) are organized into a radial array in interphase (left) and form a bipolar spindle in mitosis (right). DNA is shown in blue.
Microtubules accomplish such diverse tasks by providing tracks for the transport of molecules, vesicles and organelles, by generating mechanical force with the help of motor proteins, and by assembling into highly organized arrays that provide shape and stability. In proliferating cells microtubule arrangements undergo cell cycle-dependent reorganization to form a radial array in interphase and a bipolar spindle in mitosis (Fig.1). In differentiated cells, microtubules form a large number of different arrays, which serve the special needs and functions of each cell type (Fig.2).
Fig.2: Differentiated cell types with specialized microtubule arrays (red) organized by MTOCs (green). Nuclei are shown in blue.
Assembly of ordered microtubule arrays involves microtubule organizing centers (MTOCs), which nucleate microtubule polymerization and control attachment and release of microtubules. A well known MTOC in animal cells is the centrosome. The centrosome has been studied since its discovery by Theodor Boveri in the late nineteenth century, but our knowledge of its biology is still very limited. Centrosome abnormalities are common in tumor cells and have been implicated in genomic instability and cancer. In addition, centrosomes play roles in cellular differentiation, such as asymmetric stem cell divisions and axon growth in neurons. Several genetic diseases are associated with centrosome defects. Examples are lissencephaly, a neurodevelopmental disorder caused by mutations in genes that encode centrosome-associated proteins and affect the migration of neurons, and microcephaly, a disorder of neurogenic mitosis with reduced fetal brain growth, which involves gene defects affecting several different centrosome- and spindle-associated proteins. To gain insight into the molecular mechanisms of such diseases, we need to identify and characterize the proteins critical for centrosome function and microtubule organization.
Fig.3: The animal centrosome, composed of a pair of centrioles (mother centriole with distal and subdistal appendages, daughter centriole) surrounded by PCM. γ-TuRCs in the PCM nucleate microtubules.
The centrosome consists of a pair of centrioles surrounded by a proteinateous matrix known as pericentriolar material (PCM), and was recently shown to be composed of more than hundred proteins (Fig. 3). Contrary to initial models, the centrioles, barrel-shaped cylinders oriented perpendicular to each other and composed of precisely arranged microtubule triplets, are not essential for microtubule organization. This function requires components of the PCM such as γ-tubulin, another member of the tubulin superfamiliy. Unlike α- and β-tubulin, γ-tubulin is not incorporated into the microtubule polymer. It is part of a large protein complex, the γ-tubulin ring complex (γ-TuRC), which nucleates microtubule polymerization.
γ-TuRCs are not only found at the centrosome; they are also present in the surrounding cytoplasm, and in mitosis they associate with spindle microtubules (Fig. 4), indicating a centrosome-independent function in spindle assembly. In addition, γ-TuRCs are found at non-centrosomal MTOCs. During the differentiation of skeletal muscle, for example, myoblasts fuse to form multi-nucleated myotubes, in which γ-tubulin relocalizes to non-centrosomal MTOCs at the nuclear envelopes (Fig. 2). Very little is known about the role of γ-tubulin and other centrosome proteins at such non-centrosomal sites.
Fig.4: In mitosis γ-tubulin staining (green) reveals a diffuse cytoplasmic and a strong centrosomal localization at the spindle poles. In addition, γ-tubulin is associated with spindle microtubules. Chromosomes are shown in blue.
Research Interests
How microtubules are organized into highly ordered arrays of various sizes and shapes and how remodeling of these arrays is accomplished are fundamental questions in cell biology. Our lab addresses these questions by studying the molecules that determine where and when microtubules are made. We would like to understand at a molecular level how the γTuRC and other proteins promote microtubule nucleation in dividing as well as in differentiated cells, and how these centrosomal and non-centrosomal pathways are regulated. We also analyze the molecular requirements for building different MTOCs including the interaction of the γTuRC with these structures, which is critical for MTOC function. As model systems we combine tissue cell culture with the reconstitution of cellular pathways in vitro using extract prepared from eggs of the frog Xenopus laevis and purified proteins.
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