Centrosome Positioning Research Articles

MKS proteins regulate centrosome positioning during cell migration

Meckel-Gruber syndrome (MKS) is a severe ciliopathy, but how the underlying mutations lead to the disease is poorly understood. MKS proteins form complexes at the ciliary transition zone, and are also needed for centrosome re-orientation and docking with the plasma membrane during ciliogenesis. However, MKS-mutant cells also have an unexplained cell migration defect: cells can migrate, but the rate is much slower than usually observed. As correct centrosome orientation is vital for many migrating cells, we examined centrosome appearance and position in migrating patient fibroblasts with mutations in MKS2 and MKS3. In control cells, the centrosome and Golgi apparatus were located between the nucleus and the leading edge, but these organelles were randomly positioned in patient fibroblasts. To distinguish between failure and delay in centrosome polarisation, we assessed centrosome position during the first two hours of cell migration. We found that mutant fibroblasts failed to stably re-orient the centrosome over the time-course of the assay, suggesting this may be a contributing factor to the migration defect. Finally, we asked if a cilium is needed for correct centrosome polarisation. Wild-type fibroblasts re-oriented their centrosome in the absence of a cilium, indicating that direction sensing by the primary cilium is not needed for initial centrosome polarisation in these cells. As MKS cells are defective in centrosome positioning during migration as well as ciliogenesis, we conclude that the MKS ciliogenesis defect may not be the primary cause of disease, but a downstream consequence of the failure to correctly position the centrosome.

Centrosome positioning in vertebrate development

The centrosome, a major organizer of microtubules, has important functions in regulating cell shape, polarity, cilia formation and intracellular transport as well as the position of cellular structures, including the mitotic spindle. By means of these activities, centrosomes have important roles during animal development by regulating polarized cell behaviors, such as cell migration or neurite outgrowth, as well as mitotic spindle orientation. In recent years, the pace of discovery regarding the structure and composition of centrosomes has continuously accelerated. At the same time, functional studies have revealed the importance of centrosomes in controlling both morphogenesis and cell fate decision during tissue and organ development. Here, we review examples of centrosome and centriole positioning with a particular emphasis on vertebrate developmental systems, and discuss the roles of centrosome positioning, the cues that determine positioning and the mechanisms by which centrosomes respond to these cues. The studies reviewed here suggest that centrosome functions extend to the development of tissues and organs in vertebrates.

Centrosome movements in vivo correlate with specific neurite formation downstream of LIM homeodomain transcription factor activity

Neurons must develop complex structure to form proper connections in the nervous system. The initiation of axons in defined locations on the cell body and their extension to synaptic targets are critical steps in neuronal morphogenesis, yet the mechanisms controlling axon formation in vivo are poorly understood. The centrosome has been implicated in multiple aspects of neuronal morphogenesis; however, its function in axon development is under debate. Conflicting results from studies of centrosome function in axonogenesis suggest that its role is context dependent and underscore the importance of studying centrosome function as neurons develop in their natural environment. Using live imaging of zebrafish Rohon-Beard (RB) sensory neurons in vivo, we discovered a spatiotemporal relationship between centrosome position and the formation of RB peripheral, but not central, axons. We tested centrosome function by laser ablation and found that centrosome disruption inhibited peripheral axon outgrowth. In addition, we show that centrosome position and motility are regulated by LIM homeodomain transcription factor activity, which is specifically required for the development of RB peripheral axons. Furthermore, we show a correlation between centrosome mislocalization and ectopic axon formation in bashful (laminin alpha 1) mutants. Thus, both intrinsic transcription factor activity and extracellular cues can influence centrosome position and axon formation in vivo. This study presents the first positive association between the centrosome and axon formation in vivo and suggests that the centrosome is important for differential neurite formation in neurons with complex axonal morphologies.

Centrosome positioning and primary cilia assembly orchestrate neuronal development

Establishment of axon and dendrite polarity, migration to a desired location in the developing brain, and establishment of proper synaptic connections are essential processes during neuronal development. The cellular and molecular mechanisms that govern these processes are under intensive investigation. The function of the centrosome in neuronal development has been examined and discussed in few recent studies that underscore the fundamental role of the centrosome in brain development. Clusters of emerging studies have shown that centrosome positioning tightly regulates neuronal development, leading to the segregation of cell factors, directed neurite differentiation, neuronal migration, and synaptic integration. Furthermore, cilia, that arise from the axoneme, a modified centriole, are emerging as new regulatory modules in neuronal development in conjunction with the centrosome. In this review, we focus on summarizing and discussing recent studies on centrosome positioning during neuronal development and also highlight recent findings on the role of cilia in brain development. We further discuss shared molecular signaling pathways that might regulate both centrosome and cilia associated signaling in neuronal development. Furthermore, molecular determinants such as DISC1 and LKB1 have been recently demonstrated to be crucial regulators of various aspects of neuronal development. Strikingly, these determinants might exert their function, at least in part, via the regulation of centrosome and cilia associated signaling and serve as a link between these two signaling centers. We thus include an overview of these molecular determinants.

Cell confinement controls centrosome positioning and lumen initiation during epithelial morphogenesis

Epithelial organ morphogenesis involves sequential acquisition of apicobasal polarity by epithelial cells and development of a functional lumen. In vivo, cells perceive signals from components of the extracellular matrix (ECM), such as laminin and collagens, as well as sense physical conditions, such as matrix stiffness and cell confinement. Alteration of the mechanical properties of the ECM has been shown to promote cell migration and invasion in cancer cells, but the effects on epithelial morphogenesis have not been characterized. We analyzed the effects of cell confinement on lumen morphogenesis using a novel, micropatterned, three-dimensional (3D) Madin-Darby canine kidney cell culture method. We show that cell confinement, by controlling cell spreading, limits peripheral actin contractility and promotes centrosome positioning and lumen initiation after the first cell division. In addition, peripheral actin contractility is mediated by master kinase Par-4/LKB1 via the RhoA-Rho kinase-myosin II pathway, and inhibition of this pathway restores lumen initiation in minimally confined cells. We conclude that cell confinement controls nuclear-centrosomal orientation and lumen initiation during 3D epithelial morphogenesis.

Centrosome movements in vivo correlate with specific neurite formation downstream of LIM homeodomain transcription factor activity

Neurons must develop complex structure to form proper connections in the nervous system. The initiation of axons in defined locations on the cell body and their extension to synaptic targets are critical steps in neuronal morphogenesis, yet the mechanisms controlling axon formation in vivo are poorly understood. The centrosome has been implicated in multiple aspects of neuronal morphogenesis; however, its function in axon development is under debate. Conflicting results from studies of centrosome function in axonogenesis suggest that its role is context dependent and underscore the importance of studying centrosome function as neurons develop in their natural environment. Using live imaging of zebrafish Rohon-Beard (RB) sensory neurons in vivo, we discovered a spatiotemporal relationship between centrosome position and the formation of RB peripheral, but not central, axons. We tested centrosome function by laser ablation and found that centrosome disruption inhibited peripheral axon outgrowth. In addition, we show that centrosome position and motility are regulated by LIM homeodomain transcription factor activity, which is specifically required for the development of RB peripheral axons. Furthermore, we show a correlation between centrosome mislocalization and ectopic axon formation in bashful (laminin alpha 1) mutants. Thus, both intrinsic transcription factor activity and extracellular cues can influence centrosome position and axon formation in vivo. This study presents the first positive association between the centrosome and axon formation in vivo and suggests that the centrosome is important for differential neurite formation in neurons with complex axonal morphologies.

Regulation of dynein localization and centrosome positioning by Lis-1 and asunder during Drosophila spermatogenesis

Dynein, a microtubule motor complex, plays crucial roles in cell-cycle progression in many systems. The LIS1 accessory protein directly binds dynein, although its precise role in regulating dynein remains unclear. Mutation of human LIS1 causes lissencephaly, a developmental brain disorder. To gain insight into the in vivo functions of LIS1, we characterized a male-sterile allele of the Drosophila homolog of human LIS1. We found that centrosomes do not properly detach from the cell cortex at the onset of meiosis in most Lis-1 spermatocytes; centrosomes that do break cortical associations fail to attach to the nucleus. In Lis-1 spermatids, we observed loss of attachments between the nucleus, basal body and mitochondria. The localization pattern of LIS-1 protein throughout Drosophila spermatogenesis mirrors that of dynein. We show that dynein recruitment to the nuclear surface and spindle poles is severely reduced in Lis-1 male germ cells. We propose that Lis-1 spermatogenesis phenotypes are due to loss of dynein regulation, as we observed similar phenotypes in flies null for Tctex-1, a dynein light chain. We have previously identified asunder (asun) as another regulator of dynein localization and centrosome positioning during Drosophila spermatogenesis. We now report that Lis-1 is a strong dominant enhancer of asun and that localization of LIS-1 in male germ cells is ASUN dependent. We found that Drosophila LIS-1 and ASUN colocalize and coimmunoprecipitate from transfected cells, suggesting that they function within a common complex. We present a model in which Lis-1 and asun cooperate to regulate dynein localization and centrosome positioning during Drosophila spermatogenesis.

Centrosomes Can Initiate a Polarity Axis from Any Position within One-Cell C. elegans Embryos

The stereotyped asymmetry of one-cell C. elegans embryos has proven to be an important model for identifying molecular determinants of cell polarity. How polarity is initiated is less well understood. Polarity establishment depends on centrosomes, which use two molecularly distinct pathways to break symmetry. In both, the centrosome's position adjacent to the cell cortex is thought to determine where polarization starts. Defects in centrosome-cortex juxtaposition correlate with defects in polarity establishment in several mutants, suggesting that these processes may be linked, but there is no direct test of this. Here we assess how centrosome position relative to the cortex affects polarity establishment. We find that centrosomes can initiate polarity from any position within the embryo volume, but centrosome-cortex proximity decreases the time required to initiate polarity. Polarization itself brings about close centrosome-cortex proximity. Prior to polarization, cytoplasmic microtubules constrain centrosome movement near the cortex, expanding the controversial role of microtubules during polarity establishment. The ability of centrosomes to induce a single polarity axis from any position within the egg emphasizes the flexible, self-organizing properties of polarization in C. elegans embryos and contrasts the common view of C. elegans development as invariant.

A Role for the Centrosome and PAR-3 in the Hand-Off of MTOC Function during Epithelial Polarization

The centrosome is the major microtubule organizing center (MTOC) in dividing cells and in many postmitotic, differentiated cells. In other cell types, however, MTOC function is reassigned from the centrosome to noncentrosomal sites. Here, we analyze how MTOC function is reassigned to the apical membrane of C. elegans intestinal cells. After the terminal intestinal cell division, the centrosomes and nuclei move near the future apical membranes, and the postmitotic centrosomes lose all, or nearly all, of their associated microtubules. We show that microtubule-nucleating proteins such as γ-tubulin and CeGrip-1 that are centrosome components in dividing cells become localized to the apical membrane, which becomes highly enriched in microtubules. Our results suggest that centrosomes are critical to specify the apical membrane as the new MTOC. First, γ-tubulin appears to redistribute directly from the migrating centrosome onto the lateral then apical membrane. Second, γ-tubulin fails to accumulate apically in wild-type cells following laser ablation of the centrosome. We show that centrosomes localize apically by first moving toward lateral foci of the conserved polarity proteins PAR-3 and PAR-6 and then move together with these foci toward the future apical surface. Embryos lacking PAR-3 fail to localize their centrosomes apically and have aberrant localization of γ-tubulin and CeGrip-1. These data suggest that PAR proteins contribute to apical polarity in part by determining centrosome position and that the reassignment of MTOC function from centrosomes to the apical membrane is associated with a physical hand-off of nucleators of microtubule assembly.

N-cadherin specifies first asymmetry in developing neurons

The precise polarization and orientation of developing neurons is essential for the correct wiring of the brain. In pyramidal excitatory neurons, polarization begins with the sprouting of opposite neurites, which later define directed migration and axo-dendritic domains. We here show that endogenous N-cadherin concentrates at one pole of the newborn neuron, from where the first neurite subsequently emerges. Ectopic N-cadherin is sufficient to favour the place of appearance of the first neurite. The Golgi and centrosome move towards this newly formed morphological pole in a second step, which is regulated by PI3K and the actin/microtubule cytoskeleton. Moreover, loss of function experiments in vivo showed that developing neurons with a non-functional N-cadherin misorient their cell axis. These results show that polarization of N-cadherin in the immediate post-mitotic stage is an early and crucial mechanism in neuronal polarity.

Cortical Dynein Controls Microtubule Dynamics to Generate Pulling Forces that Position Microtubule Asters

Dynein at the cortex contributes to microtubule-based positioning processes such as spindle positioning during embryonic cell division and centrosome positioning during fibroblast migration. To investigate how cortical dynein interacts with microtubule ends to generate force and how this functional association impacts positioning, we have reconstituted the 'cortical' interaction between dynein and dynamic microtubule ends in an in vitro system using microfabricated barriers. We show that barrier-attached dynein captures microtubule ends, inhibits growth, and triggers microtubule catastrophes, thereby controlling microtubule length. The subsequent interaction with shrinking microtubule ends generates pulling forces up to several pN. By combining experiments in microchambers with a theoretical description of aster mechanics, we show that dynein-mediated pulling forces lead to the reliable centering of microtubule asters in simple confining geometries. Our results demonstrate the intrinsic ability of cortical microtubule-dynein interactions to regulate microtubule dynamics and drive positioning processes in living cells.

Disruption of IFT Complex A Causes Cystic Kidneys without Mitotic Spindle Misorientation

Intraflagellar transport (IFT) complexes A and B build and maintain primary cilia. In the mouse, kidney-specific or hypomorphic mutant alleles of IFT complex B genes cause polycystic kidneys, but the influence of IFT complex A proteins on renal development is not well understood. In the present study, we found that HoxB7-Cre-driven deletion of the complex A gene Ift140 from collecting ducts disrupted, but did not completely prevent, cilia assembly. Mutant kidneys developed collecting duct cysts by postnatal day 5, with rapid cystic expansion and renal dysfunction by day 15 and little remaining parenchymal tissue by day 20. In contrast to many models of polycystic kidney disease, precystic Ift140-deleted collecting ducts showed normal centrosomal positioning and no misorientation of the mitotic spindle axis, suggesting that disruption of oriented cell division is not a prerequisite to cyst formation in these kidneys. Precystic collecting ducts had an increased mitotic index, suggesting that cell proliferation may drive cyst expansion even with normal orientation of the mitotic spindle. In addition, we observed significant increases in expression of canonical Wnt pathway genes and mediators of Hedgehog and tissue fibrosis in highly cystic, but not precystic, kidneys. Taken together, these studies indicate that loss of Ift140 causes pronounced renal cystic disease and suggest that abnormalities in several different pathways may influence cyst progression.

A cellular funicular: A hydrodynamic coupling between the anterior- and posterior-directed cytoplasmic flows.

Organelles inside cells move to position themselves at the right place at the right time. A mechanism for generating active force exists for each of such directed organelle movements. In our recent study on cytoplasmic streaming in the Caenorhabditis elegans one-cell embryo, we demonstrated that an anterior-directed force generated by myosin could drive not only anterior-directed cortical flow but also posterior-directed cytoplasmic flow. This coupling of flows in opposing directions is mediated by the hydrodynamic properties of the cytoplasm. This work provided a good example of an active force generation mechanism that drives organelle movements in two opposite directions inside the cell, just as a funicular moves up and down a slope. Interestingly, the funicular-like coupling of intracellular movements is also seen in our recent studies on centrosome positioning in the C. elegans embryo and on interkinetic nuclear movement during mouse neurogenesis. Thus, funicular-like coupling may be a general strategy used repeatedly in cells. The use of the funicular-like coupling seems advantageous because it is efficient, as one active force generation mechanism can drive movements in two directions, and also because the two movements can be coordinated to have similar speeds.

Casein kinase I delta controls centrosome positioning during T cell activation

Although termed central body, the centrosome is located off-center in many polarized cells. T cell receptor (TCR) engagement by antigens induces a polarity switch in T cells. This leads to the recruitment of the centrosome to the immunological synapse (IS), a specialized cell-cell junction. Despite much recent progress, how TCR signaling triggers centrosome repositioning remains poorly understood. In this paper, we uncover a critical requirement for the centrosomal casein kinase I delta (CKIδ) in centrosome translocation to the IS. CKIδ binds and phosphorylates the microtubule plus-end-binding protein EB1. Moreover, a putative EB1-binding motif at the C terminus of CKIδ is required for centrosome translocation to the IS. We find that depletion of CKIδ in T lymphocytes and inhibition of CKI in epithelial cells reduce microtubule growth. Therefore, we propose that CKIδ-EB1 complexes contribute to the increase in microtubule growth speeds observed in polarized T cells, a mechanism that might serve to generate long-stable microtubules necessary for centrosome translocation.

HDLG Associated Protein 1 (DLGAP1) Supports Mpl-Driven Cell Proliferation and Differentiation Through Its Centrosomal Function,

Abstract 3402The Mpl proto-oncogene functions as a basic factor in megakaryocytic development and platelet production as well as contributing to hematopoietic stem cell (HSC) homeostasis and self-renewal. Several mutations in Mpl have been shown to be associated with the myeloproliferative neoplasms. We used retroviral insertional mutagenesis to screen for factors that might give a proliferative and/or survival advantage for cells dependent on Mpl signaling. We developed a retroviral construct expressing a dimerizable form of Mpl that can interrupt genes through insertion, and can activate adjacent genes with its long terminal repeats. We transduced the human leukemia cell line K562 with this construct and blocked the endogenous transforming BCR-ABL kinase using Imatinib. Cells dependent on Mpl signaling were selected by the addition of the dimerizing drug, AP20187. In the absence of Mpl signaling the cells underwent erythroid differentiation and died. Cells that acquired a proliferative advantage and were dependent on Mpl function were expanded. Cloning of retroviral integration sites (RIS) from the selected populations allowed the identification of recurrent RIS that co-localized in the genome. We have identified 668 RIS from 36 independent transductions with a presumed ∼ 1.9 × 106 independent insertions in the initial non-selected, cell population. Among these, 203 RIS represented independent insertion events. Three independent RIS were located in the fourth intron of the Discs large homolog-associated protein 1 (DLGAP1). This protein is a member of the Discs-large/Scribble/Lethal Giant Larvae pathway. DLGAP1, in cooperation with DLG1 and CDC42, has recently been shown to control centrosome positioning and cell polarity in astrocytes (Manneville J-B et al. 2010, JBC, v.191, no 3, pp. 585–598). We investigated the role of DLGAP1 in MPL signaling.Overexpression of full length DLGAP1 had minimal effect on plain K562 cells, but it significantly slowed down the proliferation of K562 cells switched to Mpl signaling. The identified insertion sites would predict the increased expression of an amino-end truncated isoform. When this truncated form was overexpressed in K562 cells and Mpl dependent K562 cells, the cells proliferated at an increased rate. Immunofluorescent studies revealed that the full length DLGAP1 colocalized with major centrosomal markers including gamma-tubulin, PCM1, and APC in K562, HEL, UT7/TPO and Mo7e cell lines. We did not observe colocalization with centrosomes when the truncated isoform of DLGAP1 was overexpressed. Immunoflourescent microscopy revealed that endogenous DLGAP1 in myeloid cell lines was localized in the centriolar satellites, a cellular structure essential for centrosome integrity and function. The immunoflourescent pattern using an anti DLGAP1 antibody showed cell cycle dependent assembly and disassembly. Protein sequence analyses of DLGAP1 indicated consensus sites for presumptive Jak2 phosphorylation as well as consensus sites for SRC tyrosine kinases. To test the possible influence of the two types of kinases on DLGAP1 we treated K562, HEL and UT7/TPO cell lines with an inhibitor of Jak2 (AG490) and independently with an inhibitor of SRC (SU6656). In each case the centrosomal staining of DLGAP1 was diminished with minimal effect on the cytoplasmic fraction of DLGAP1. Interestingly, treatment with SU6656 of the K562 cells switched to Mpl signaling and overexpressing full length GFP-DLGAP1 fusion resulted in a significant increase in the number of polyploid cells. The changes correlated with upregulation of the megakaryocytic marker CD41a and downregulation of the erythroid marker Glycophorin A as assessed by FACS analyses. As centrosomal aberrations are frequent markers of the myeloproliferative neoplasms, we intend to study the role of DLGAP1 in these bone marrow disorders. Disclosures:No relevant conflicts of interest to declare.

Robust Spindle Alignment in Drosophila Neuroblasts by Ultrasensitive Activation of Pins

Cellular signaling pathways exhibit complex response profiles with features such as thresholds and steep activation (i.e., ultrasensitivity). In a reconstituted mitotic spindle orientation pathway, activation of Drosophila Pins (LGN in mammals) by Gαi is ultrasensitive (apparent Hill coefficient of 3.1), such that Pins recruitment of the microtubule binding protein Mud (NuMA) occurs over a very narrow Gαi concentration range. Ultrasensitivity is required for Pins function in neuroblasts as a nonultrasensitive Pins mutant fails to robustly couple spindle position to cell polarity. Pins contains three Gαi binding GoLoco domains (GLs); Gαi binding to GL3 activates Pins, whereas GLs 1 and 2 shape the response profile. Although cooperative binding is one mechanism for generating ultrasensitivity, we find GLs 1 and 2 act as "decoys" that compete against activation at GL3. Many signaling proteins contain multiple protein interaction domains, and the decoy mechanism may be a common method for generating ultrasensitivity in regulatory pathways.

The Significant Role of Centrosomes in Stem Cell Division and Differentiation

The role of centrosomes in stem cell division has recently been highlighted and further ascribes important functions to centrosomes in stem cell maintenance, cellular differentiation, and development. Advanced cell and molecular studies coupled with immunofluorescence, electron microscopy, and live cell imaging of specific centrosome proteins have contributed greatly to our knowledge of centrosome composition, structure, and dynamics and have uncovered new insights into mechanisms of centrosome functions in asymmetric cell division. The establishment of asymmetry and differential positioning of mother and daughter centrosomes during stem cell mitosis is important for allowing one cell to maintain stem cell characteristics while the sibling cell undergoes differentiation. Another key role for centrosomes has been revealed in primary cilia of embryonic stem cells that play significant roles in cellular signaling and are therefore critically important for stem cell decisions. Studies of signaling through primary cilia may contribute important information that may aid in the production of specific cells that are suitable for tissue repair and regeneration in the field of regenerative medicine.

Inhibition of myosin II triggers morphological transition and increased nuclear motility

We investigate the effect of myosin II inhibition on cell shape and nuclear motility in cultures of mouse radial glia-like neural progenitor and rat glioma C6 cells. Instead of reducing nucleokinesis, the myosin II inhibitor blebbistatin provokes an elongated bipolar morphology and increased nuclear motility in both cell types. When myosin II is active, time-resolved traction force measurements indicate a pulling force between the leading edge and the nucleus of C6 cells. In the absence of myosin II activity, traction forces during nucleokinesis are diminished below the sensitivity threshold of our assay. By visualizing the centrosome position in C6 cells with GFP-centrin, we show that in the presence or absence of myosin II activity, the nucleus tends to overtake or lag behind the centrosome, respectively. We interpret these findings with the help of a simple viscoelastic model of the cytoskeleton consisting active contractile and passive compressed elements.

Protein farnesylation inhibitors cause donut-shaped cell nuclei attributable to a centrosome separation defect

Despite the success of protein farnesyltransferase inhibitors (FTIs) in the treatment of certain malignancies, their mode of action is incompletely understood. Dissecting the molecular pathways affected by FTIs is important, particularly because this group of drugs is now being tested for the treatment of Hutchinson-Gilford progeria syndrome. In the current study, we show that FTI treatment causes a centrosome separation defect, leading to the formation of donut-shaped nuclei in nontransformed cell lines, tumor cell lines, and tissues of FTI-treated mice. Donut-shaped nuclei arise during chromatin decondensation in late mitosis; subsequently, cells with donut-shaped nuclei exhibit defects in karyokinesis, develop aneuploidy, and are often binucleated. Binucleated cells proliferate slowly. We identified lamin B1 and proteasome-mediated degradation of pericentrin as critical components in FTI-induced "donut formation" and binucleation. Reducing pericentrin expression or ectopic expression of nonfarnesylated lamin B1 was sufficient to elicit donut formation and binucleated cells, whereas blocking proteasomal degradation eliminated FTI-induced donut formation. Our studies have uncovered an important role of FTIs on centrosome separation and define pericentrin as a (indirect) target of FTIs affecting centrosome position and bipolar spindle formation, likely explaining some of the anticancer effects of these drugs.

Systems biomechanics of centrosome positioning

Positioning of centrosomes within cells determines the directionality of cell division, as well as directionality of cellular activities in the interphase. This brief review focuses on similarities (and differences) of centrosome positioning during early divisions in the Caenorhabditis embryo and during the interaction of T lymphocytes with other cells in the course of immune response. In the study of the two phenomena, a synergy of experimentation and numerical mechanical analysis has recently been achieved. The picture that emerges from these studies is one in which simple physical forces under the constraints of the basic cell structure lead to complex, "life-like" mechanical behavior. This behavior includes instability of equilibria, irreversibility of structural transitions, and multidimensional, multiperiodic oscillations. This new picture of cell mechanics may form an interesting paradigm for future research.