Structural characterization and statistical-mechanical model of epidermal patterns

In proliferating epithelia of mammalian skin, cells of irregular polygonal-like shapes pack into complex nearly flat two-dimensional structures that are pliable to deformations. In this work, we employ various sensitive correlation functions to quantitatively characterize structural features of evolving packings of epithelial cells across length scales in mouse skin. We find that the pair statistics in direct and Fourier spaces of the cell centroids in the early stages of embryonic development show structural directional dependence, while in the late stages the patterns tend towards statistically isotropic states. We construct a minimalist four-component statistical-mechanical model involving effective isotropic pair interactions consisting of hard-core repulsion and extra short-ranged soft-core repulsion beyond the hard core, whose length scale is roughly the same as the hard core. The model parameters are optimized to match the sample pair statistics in both direct and Fourier spaces. By doing this, the parameters are biologically constrained. Our model predicts essentially the same polygonal shape distribution and size disparity of cells found in experiments as measured by Voronoi statistics. Moreover, our simulated equilibrium liquid-like configurations are able to match other nontrivial unconstrained statistics, which is a testament to the power and novelty of the model. We discuss ways in which our model might be extended so as to better understand morphogenesis (in particular the emergence of planar cell polarity), wound-healing, and disease progression processes in skin, and how it could be applied to the design of synthetic tissues

The cell biology of planar cell polarity

Planar cell polarity (PCP) refers to the coordinated alignment of cell polarity across the tissue plane. Key to the establishment of PCP is asymmetric partitioning of cortical PCP components and intercellular communication to coordinate polarity between neighboring cells. Recent progress has been made toward understanding how protein transport, endocytosis, and intercellular interactions contribute to asymmetric PCP protein localization. Additionally, the functions of gradients and mechanical forces as global cues that bias PCP orientation are beginning to be elucidated. Together, these findings are shedding light on how global cues integrate with local cell interactions to organize cellular polarity at the tissue level

The role of the dim-1 gene in muscle maintenance and stability in Caenorhabditis elegans

Unc-112 and dim-1 are a pair of interacting genes that are required for myofilament lattice assembly and maintenance in the nematode, Caenorhabdidtis elegans. The unc-112 gene encodes a novel protein localized to attachment structures that are responsible for anchoring the myofilament lattice to the muscle cell membrane and underlying body wall layers. Loss of UNC- 112 results in the failure of myofilament lattice assembly and lethality. Animals homozygous for the missense mutation unc-112 (r367), on the other hand, survive to adulthood but are paralyzed and have severely disorganized body wall muscle. Mutations in the dim-1 gene can suppress the phenotypic defects associated with unc-112 (r367). Animals homozygous for both dim-1 and unc-112 (r367) display wild type movement and have relatively well organized body wall muscle. Animals homozygous for the dim-1 mutation alone display mildly disorganized muscle, thus the dim-1 gene is required for maintaining muscle stability. The dim-1 gene encodes a 325 amino acid protein that constitutes three immunoglobulin repeats that are most similar to the intracellular muscle proteins, titin and twitchin. Immunofluorescence analysis revealed that DIM-1 is expressed in body wall muscle in a pattern reminiscent of myofilament associated proteins. Preliminary results suggest DIM-1 may associate with actin containing thin filaments. The disorganized muscle phenotype of dim-1 mutants and the localization of its gene product suggest that DIM-1 maintains the integrity of the myofilament lattice through the stabilization of thin filaments. Results presented in this thesis suggest that the suppression of unc-112 (r367) by dim-1 is indirect. First, sequence alterations for eight dim-1 alleles have been identified all of which result in the loss of the dim-1 gene product. Thus, the absence of DIM-1 results in the suppression of unc-112 (r367). Second, DIM-1 is not required for localization of UNC-112 to attachment structures and third, the DIM-1 protein is localized to myofilaments rather than attachment structures. These results indicate that the genetic interaction between dim-1 and unc-112 is not due to a direct interaction between their gene products. Rather, suppression of unc-112 appears to result from a change in the overall stability of the myofilament lattice caused by the loss of dim-1. This change may allow the altered r367 protein to maintain the integrity of the myofilament lattice.Science, Faculty ofZoology, Department ofGraduat

Tissue morphodynamics: Translating planar polarity cues into polarized cell behaviors

The ability of cells to collectively orient and align their behaviors is essential in multicellular organisms for unidirectional cilia beating, collective cell movements, oriented cell divisions, and asymmetric cell fate specification. The planar cell polarity pathway coordinates a vast and diverse array of collective cell behaviors by intersecting with downstream pathways that regulate cytoskeletal dynamics and intercellular signaling. How the planar polarity pathway translates directional cues to produce polarized cell behaviors is the focus of this review

Planar cell polarity: global inputs establishing cellular asymmetry

Many tissues develop coordinated patterns of cell polarity that align with respect to the tissue axes. This phenomenon refers to planar cell polarity (PCP) and is controlled by multiple conserved PCP modules. A key feature of PCP proteins is their asymmetric localization within the tissue plane, whose orientation is guided by global directional cues. Here, we highlight current models and recent findings on the role of tissue-level gradients, local organizer signals, and mechanical forces in establishing the global patterns of PCP

Trans-endocytosis of Planar Cell Polarity Complexes during Cell Division.

To coordinate epithelial architecture with proliferation, cell polarity proteins undergo extensive remodeling during cell division [1-3]. A dramatic example of polarity remodeling occurs in proliferative basal cells of mammalian epidermis whereupon cell division, transmembrane planar cell polarity (PCP) proteins are removed from the cell surface via bulk endocytosis [4]. PCP proteins form intercellular complexes, linked by Celsr1-mediated homophilic adhesion, that coordinate polarity non-autonomously between cells [5, 6]. Thus, the mitotic reorganization of PCP proteins must alter not only proteins intrinsic to the dividing cell but also their interacting partners on neighboring cells. Here, we show that intercellular Celsr1 complexes that connect dividing cells with their neighbors remain intact during mitotic internalization, resulting in an uptake of Celsr1 protein from interphase neighbors. Trans-internalized Celsr1 carries with it additional core PCP proteins, including the posteriorly enriched Fz6 and anteriorly enriched Vangl2. Cadherin-mediated homophilic adhesion is necessary for trans-endocytosis, and adhesive junctional PCP complexes appear to be destined for degradation upon internalization. Surprisingly, whereas Fz6 and Vangl2 both internalize in trans, Vangl2 proteins intrinsic to the dividing cell remain associated with the plasma membrane. Persistent Vangl2 stabilizes Celsr1 and impedes its internalization, suggesting that dissociation of Vangl2 from Celsr1 is a prerequisite for Celsr1 endocytosis. These results demonstrate an unexpected transfer of PCP complexes between neighbors and suggest that the Vangl2 population that persists at the membrane during cell division could serve as an internal cue for establishing PCP in new daughter cells

Counter-rotational cell flows drive morphological and cell fate asymmetries in mammalian hair follicles.

Organ morphogenesis is a complex process coordinated by cell specification, epithelial-mesenchymal interactions and tissue polarity. A striking example is the pattern of regularly spaced, globally aligned mammalian hair follicles, which emerges through epidermal-dermal signaling and planar polarized morphogenesis. Here, using live-imaging, we discover that developing hair follicles polarize through dramatic cell rearrangements organized in a counter-rotational pattern of cell flows. Upon hair placode induction, Shh signaling specifies a radial pattern of progenitor fates that, together with planar cell polarity, induce counter-rotational rearrangements through myosin and ROCK-dependent polarized neighbour exchanges. Importantly, these cell rearrangements also establish cell fate asymmetry by repositioning radial progenitors along the anterior-posterior axis. These movements concurrently displace associated mesenchymal cells, which then signal asymmetrically to maintain polarized cell fates. Our results demonstrate how spatial patterning and tissue polarity generate an unexpected collective cell behaviour that in turn, establishes both morphological and cell fate asymmetry