Nuclear mechanotransduction in stem cells

In development and in homeostatic maintenance of tissues, stem cells and progenitor cells are constantly subjected to forces. These forces can lead to significant changes in gene expression and function of stem cells, mediating self-renewal, lineage specification, and even loss of function. One of the ways that has been proposed to mediate these functional changes in stem cells is nuclear mechanotransduction — the process by which forces are converted to signals in the nucleus. The purpose of this review is to discuss the means by which mechanical signals are transduced into the nucleus, through the linker of nucleoskeleton and cytoskeleton (LINC) complex and other nuclear envelope transmembrane (NET) proteins, which connect the cytoskeleton to the nucleus. We discuss how LINC/NETs confers tissue-specific mechanosensitivity to cells and further elucidate how LINC/NETs acts as a control center for nuclear mechanical signals, regulating both gene expression and chromatin organization. Throughout, we primarily focus on stem cell–specific examples, notwithstanding that this is a nascent field. We conclude by highlighting open questions and pointing the way to enhanced research efforts to understand the role nuclear mechanotransduction plays in cell fate choice.     

Impaired stria vascularis integrity upon loss of E-cadherin in basal cells

In the cochlea, sensory transduction depends on the endocochlear potential (EP) and the unique composition of the endolymph, both of which are maintained by a highly specialized epithelium at the cochlear lateral wall, the stria vascularis. The generation of the EP by the stria vascularis, in turn, relies on the insulation of an intrastrial extracellular compartment by epithelial basal cells. Despite the physiological importance of basal cells, their cellular origin and the molecular pathways that lead to their differentiation are unclear. Here, we show by genetic lineage tracing in the mouse that basal cells exclusively derive from the otic mesenchyme. Conditional deletion of E-cadherin in the otic mesenchyme and its descendants does not abrogate the transition from mesenchymal precursors to epithelial basal cells. Rather, dedifferentiation of intermediate cells, altered morphology of basal and marginal cells and hearing impairment due to decreased EP in E-cadherin mutant mice demonstrate an essential role of E-cadherin in terminal basal cell differentiation and their interaction with other strial cell types to establish and maintain the functional architecture of the stria vascularis.     

Patterning the dorsal–ventral axis of the wasp Nasonia vitripennis

Regulatory networks composed of interacting genes are responsible for pattern formation and cell type specification in a wide variety of developmental contexts. Evolution must act on these regulatory networks in order to change the proportions, distribution, and characteristics of specified cells. Thus, understanding how these networks operate in homologous systems across multiple levels of phylogenetic divergence is critical for understanding the evolution of developmental systems. Among the most thoroughly characterized regulatory networks is the dorsal–ventral patterning system of the fly Drosophila melanogaster. Due to the thorough understanding of this system, it is an ideal starting point for comparative analyses. Here we report an analysis of the DV patterning system of the wasp, Nasonia vitripennis. This wasp undergoes a mode of long germ embryogenesis that is superficially nearly identical to that of Drosophila, but one that was likely independently derived. We have found that while the expression of genes just prior to the onset of gastrulation is almost identical in Nasonia and Drosophila, both the upstream network responsible for generating this pattern, and the downstream morphogenetic movements that it sets in motion, are significantly diverged. From this we conclude that many network structures are available to evolution to achieve particular developmental ends.     

Multiple lineage-specific roles of Smad4 during neural crest development

During vertebrate development, neural crest cells are exposed to multiple extracellular cues that drive their differentiation into neural and non-neural cell lineages. Insights into the signals potentially involved in neural crest cell fate decisions in vivo have been gained by cell culture experiments that have allowed the identification of instructive growth factors promoting either proliferation of multipotent neural crest cells or acquisition of specific fates. For instance, members of the TGFβ factor family induce neurogenesis and smooth muscle cell formation at the expense of other fates in culture. In vivo, conditional ablation of various TGFβ signaling components resulted in malformations of non-neural derivatives of the neural crest, but it is unclear whether these phenotypes involved aberrant fate decisions. Moreover, it remains to be shown whether neuronal determination indeed requires TGFβ factor activity in vivo. To address these issues, we conditionally deleted Smad4 in the neural crest, thus inactivating all canonical TGFβ factor signaling. Surprisingly, neural crest cell fates were not affected in these mutants, with the exception of sensory neurogenesis in trigeminal ganglia. Rather, Smad4 regulates survival of smooth muscle and proliferation of autonomic and ENS neuronal progenitor cells. Thus, Smad signaling plays multiple, lineage-specific roles in vivo, many of which are elicited only after neural crest cell fate decision.     

A gain-of-function mutation in oma-1, a C. elegans gene required for oocyte maturation,results in delayed degradation of maternal proteins and embryonic lethality

In vertebrates, oocytes undergo maturation, arrest in metaphase II, and can then be fertilized by sperm. Fertilization initiates molecular events that lead to the activation of early embryonic development. In Caenorhabditis elegans, where no delay between oocyte maturation and fertilization is apparent, oocyte maturation and fertilization must be tightly coordinated. It is not clear what coordinates the transition from an oocyte to an embryo in C. elegans, but regulated turnover of oocyte-specific proteins contributes to the process. We describe here a gain-of-function mutation (zu405) in a gene that is essential for oocyte maturation, oma-1. In wild type animals, OMA-1 protein is expressed at a high level exclusively in oocytes and newly fertilized embryos and is degraded rapidly after the first mitotic division. The zu405 mutation results in improper degradation of the OMA-1 protein in embryos. In oma-1(zu405) embryos, the C blastomere is transformed to the EMS blastomere fate, resulting in embryonic lethality. We show that degradation of several maternally supplied cell fate determinants, including SKN-1, PIE-1, MEX-3, and MEX-5, is delayed in oma-1(zu405) mutant embryos. In wild type embryos, SKN-1 functions in EMS for EMS blastomere fate specification. A decreased level of maternal SKN-1 protein in the C blastomere relative to EMS is believed to be responsible for this cell expressing the C, instead of the EMS, fate. Delayed degradation of maternal SKN-1 protein in oma-1(zu405) embryos and resultant elevated levels in C blastomere is likely responsible for the observed C-to-EMS blastomere fate transformation. These observations suggest that oma-1, in addition to its role in oocyte maturation, contributes to early embryonic development by regulating the temporal degradation of maternal proteins in early C. elegans embryos.     

Gonadotropin-releasing hormone (GnRH) cells arise from cranial neural crest and adenohypophyseal regions of the neural plate in the zebrafish,Danio rerio

The olfactory placodes generate the primary sensory neurons of the olfactory sensory system. Additionally, the olfactory placodes have been proposed to generate a class of neuroendocrine cells containing gonadotropin-releasing hormone (GnRH). GnRH is a multifunctional decapeptide essential for the development of secondary sex characteristics in vertebrates as well as a neuromodulator within the central nervous system. Here, we show that endocrine and neuromodulatory GnRH cells arise from two separate, nonolfactory regions in the developing neural plate. Specifically, the neuromodulatory GnRH cells of the terminal nerve arise from the cranial neural crest, and the endocrine GnRH cells of the hypothalamus arise from the adenohypophyseal region of the developing anterior neural plate. Our findings are consistent with cell types generated by the adenohypophysis, a source of endocrine tissue in vertebrate animals, and by neural crest, a source of cells contributing to the cranial nerves. The adenohypophysis arises from a region of the anterior neural plate flanked by the olfactory placode fields at early stages of development, and premigratory cranial neural crest lies adjacent to the caudal edge of the olfactory placode domain [Development 127 (2000), 3645]. Thus, the GnRH cells arise from tissue closely associated with the developing olfactory placode, and their different developmental origins reflect their different functional roles in the adult animal.     

Regional specification during embryogenesis in Rhynchonelliform brachiopods

Fate maps have been constructed for embryos of Hemithiris and Terebratulina, representatives of two orders in the class Rhynchonellata that have been separated since the middle Ordovician. These fate maps are identical. The animal region of the egg forms the ectodermal covering of the apical lobe and the vegetal region is the site of gastrulation; the vegetal region forms the ectoderm of the ventral and posterior regions of the larva, endoderm and mesoderm. The cells that make up the animal region shift 90 degrees with respect to the vegetal pole during gastrulation. The timing and mode of regional specification in these two species are also identical. In each case, the animal region of the unfertilized egg has the intrinsic ability to form apical lobe ectoderm, while the vegetal region has the ability to form a normal larva. During embryogenesis, the vegetal region interacts with the animal region to suppress apical tuft differentiation in the apical lobe and to promote mantle lobe ectodermal differentiation, while the ability of the vegetal half to regulate by forming apical lobe structures is lost. The plane of bilateral symmetry of the larva begins to be set up between the late blastula and early gastrula stage. The fate maps and the processes of regional specification are compared in the four subphyla that make up the Brachiopoda and used to test a developmental model that provides an explanation for the variety of different body plans generated during the Cambrian.     

Extracellular matrix dynamics during vertebrate axis formation

The first evidence for the dynamics of in vivo extracellular matrix (ECM) pattern formation during embryogenesis is presented below. Fibrillin 2 filaments were tracked for 12 h throughout the avian intraembryonic mesoderm using automated light microscopy and algorithms of our design. The data show that these ECM filaments have a reproducible morphogenic destiny that is characterized by directed transport. Fibrillin 2 particles initially deposited in the segmental plate mesoderm are translocated along an unexpected trajectory where they eventually polymerize into an intricate scaffold of cables parallel to the anterior-posterior axis. The cables coalesce near the midline before the appearance of the next-formed somite. Moreover, the ECM filaments define global tissue movements with high precision because the filaments act as passive motion tracers. Quantification of individual and collective filament "behaviors" establish fate maps, trajectories, and velocities. These data reveal a caudally propagating traveling wave pattern in the morphogenetic movements of early axis formation. We conjecture that within vertebrate embryos, long-range mechanical tension fields are coupled to both large-scale patterning and local organization of the ECM. Thus, physical forces or stress fields are essential requirements for executing an emergent developmental pattern-in this case, paraxial fibrillin cable assembly.     

Segregation of lens and olfactory precursors from a common territory: cell sorting and reciprocity of Dlx5 and Pax6 expression

Cranial placodes are focal regions of columnar epithelium next to the neural tube that contribute to sensory ganglia and organs in the vertebrate head, including the olfactory epithelium and the crystalline lens of the eye. Using focal dye labelling within the presumptive placode domain, we show that lens and nasal precursors arise from a common territory surrounding the anterior neural plate. They then segregate over time and converge to their final positions in discrete placodes by apparently directed movements. Since these events closely parallel the separation of eye and antennal primordia (containing olfactory sensory cells) from a common imaginal disc in Drosophila, we investigated whether the vertebrate homologues of Distalless (Dll) and Eyeless (Ey), which determine antennal and eye identity in the fly, play a role in segregation of lens and nasal precursors in the chick. Dlx5 and Pax6 are initially co-expressed by future lens and olfactory cells. As soon as presumptive lens cells acquire columnar morphology all Dlx family members are down-regulated in the placode, while Pax6 is lost in the olfactory region. Lens precursor cells that express ectopic Dlx5 never acquire lens-specific gene expression and are excluded from the lens placode to cluster in the head ectoderm. These results suggest that the loss of Dlx5 is required for cells to adopt a lens fate and that the balance of Pax6 and Dlx expression regulates cell sorting into appropriate placodal domains.     

Modeling Dissolution of High Explosive Formulations

A water dissolution model for solid-phase compounds in soil has been developed that is a variation of previous such models. The formulation for this model is presented along with comparison to previous formulations. The model is applied to experiments reported in the literature involving water dissolution of high explosive (HE) compounds and compared with those reported experimental results. This dissolution model is used in the TREECS? contaminant fate modeling system that was developed for predicting surface water and groundwater contaminant concentrations resulting from solid-phase contaminant particles deposited in soil. The dissolution model performed well against measured results for a single component HE (TNT), but input adjustments, primarily for initial particle size and solubility, were required for good agreement for multi-component HE formulations. These adjustments are presented.