The very rapid induction of filopodia in insect cells

Biological lipid nanotubes and their potential role in evolution

The membrane of cells and organelles are highly deformable fluid interfaces, and can take on a multitude of shapes. One distinctive and particularly interesting property of biological membranes is their ability to from long and uniform nanotubes. These nanoconduits are surprisingly omnipresent in all domains of life, from archaea, bacteria, to plants and mammals. Some of these tubes have been known for a century, while others were only recently discovered. Their designations are different in different branches of biology, e.g. they are called stromule in plants and tunneling nanotubes in mammals. The mechanical transformation of flat membranes to tubes involves typically a combination of membrane anchoring and external forces, leading to a pulling action that results in very rapid membrane nanotube formation - micrometer long tubes can form in a matter of seconds. Their radius is set by a mechanical balance of tension and bending forces. There also exists a large class of membrane nanotubes that form due to curvature inducing molecules. It seems plausible that nanotube formation and functionality in plants and animals may have been inherited from their bacterial ancestors during endosymbiotic evolution. Here we attempt to connect observations of nanotubes in different branches of biology, and outline their similarities and differences with the aim of providing a perspective on their joint functions and evolutionary origin.

Cytonemes, Their Formation, Regulation, and Roles in Signaling and Communication in Tumorigenesis

Increasing evidence during the past two decades shows that cells interconnect and communicate through cytonemes. These cytoskeleton-driven extensions of specialized membrane territories are involved in cell–cell signaling in development, patterning, and differentiation, but also in the maintenance of adult tissue homeostasis, tissue regeneration, and cancer. Brain tumor cells in glioblastoma extend ultralong membrane protrusions (named tumor microtubes, TMs), which contribute to invasion, proliferation, radioresistance, and tumor progression. Here we review the mechanisms underlying cytoneme formation, regulation, and their roles in cell signaling and communication in epithelial cells and other cell types. Furthermore, we discuss the recent discovery of glial cytonemes in the Drosophila glial cells that alter Wingless (Wg)/Frizzled (Fz) signaling between glia and neurons. Research on cytoneme formation, maintenance, and cell signaling mechanisms will help to better understand not only physiological developmental processes and tissue homeostasis but also cancer progression.

Cytonemes and the dispersion of morphogens

Filopodia are cellular protrusions that have been implicated in many types of mechanosensory activities. Morphogens are signaling proteins that regulate the patterned development of embryos and tissues. Both have long histories that date to the beginnings of cell and developmental biology in the early 20th century, but recent findings tie specialized filopodia called cytonemes to morphogen movement and morphogen signaling. This review explores the conceptual and experimental background for a model of paracrine signaling in which the exchange of morphogens between cells is directed to sites where cytonemes directly link cells that produce morphogens to cells that receive and respond to them.

Cytonemes as specialized signaling filopodia

Development creates a vast array of forms and patterns with elegant economy, using a small vocabulary of pattern-generating proteins such as BMPs, FGFs and Hh in similar ways in many different contexts. Despite much theoretical and experimental work, the signaling mechanisms that disperse these morphogen signaling proteins remain controversial. Here, we review the conceptual background and evidence that establishes a fundamental and essential role for cytonemes as specialized filopodia that transport signaling proteins between signaling cells. This evidence suggests that cytoneme-mediated signaling is a dispersal mechanism that delivers signaling proteins directly at sites of cell-cell contact.

Morphogen transport

The graded distribution of morphogens underlies many of the tissue patterns that form during development. How morphogens disperse from a localized source and how gradients in the target tissue form has been under debate for decades. Recent imaging studies and biophysical measurements have provided evidence for various morphogen transport models ranging from passive mechanisms, such as free or hindered extracellular diffusion, to cell-based dispersal by transcytosis or cytonemes. Here, we analyze these transport models using the morphogens Nodal, fibroblast growth factor and Decapentaplegic as case studies. We propose that most of the available data support the idea that morphogen gradients form by diffusion that is hindered by tortuosity and binding to extracellular molecules.

The formation of plasma membrane reticular systems in the oenocytes of an insect

Plasma membrane reticular systems (RSs) are infolds of the plasma membrane found in cells of several insect tissues that are not transporting epithelia. They form a subsurface reticular lymph space that may be involved in the loading and unloading of hemolymph carrier molecules. The development of a new RS during the fifth larval stadium has been studied in the oenocytes of Calpodes ethlius by scanning electron microscopy. The RS forms by the extension and progressive apical fusion of cell processes leaving a reticular lymph space below. Reticular system formation occurs in a front moving over the cell surface. The RS made in the 4th stadium persists through the moult to the 5th stage but diminishes for the next 3 days. A new intermoult RS then forms very quickly. Its time of formation follows the commitment ecdysteroid peak rather than the beginning of secretion by the wax glands. This new 5th stage RS is maintained during the period of intermoult synthesis, after which it declines and is nearly absent by the time of pupation.

Signaling reaches to new dimensions in Drosophila imaginal discs

Finding that peripodial cells in wing and eye imaginal discs are essential for the growth and patterning of the separate layer of disc cells now opens the study of interacting cell layers to the powerful developmental genetic techniques with which the Drosophila system is blessed. We can anticipate that future work will identify how such interactions contribute to patterning and how the mechanisms and processes that are involved are conserved in vertebrates. We can also look forward to contributions that this work will make to understand-ing the role of interconnecting cell extensions in such signaling processes. In this minireview, we have noted numerous types of signaling cells in which cellular extensions have been observed. At present, neither the functional nor structural relationship of these related structures is known. It is certainly tempting to suggest that these structures are conduits for signals or that they function as sensors. There is, as yet, no direct experimental evidence for such roles.

Filopodia: fickle fingers of cell fate?

Epithelial cells often produce extensions, known variously as filopodia, cell feet or cytonemes, which can extend across many cell diameters to directly contact non-adjacent cells. Do they function in morphogenesis, cell-cell signaling or both?.

Cytonemes: cellular processes that project to the principal signaling center in Drosophila imaginal discs

Wing imaginal disc cells in Drosophila develop by using information received from a signaling center associated with the anterior/posterior compartment border. We show here that disc cells have thin, actin-based extensions (cytonemes) that project to this signaling center. Cytonemes can be induced when cells from the lateral flanks of a wing disc are cultured next to cells from the A/P border or next to a source of fibroblast growth factor. Mouse limb bud cells also grow projections during a brief culture period, indicating that cytonemes are an attribute of both vertebrate and invertebrate cells. We suggest that cytonemes may be responsible for some forms of long-range cell-cell communication.

Caterpillars have evolved lungs for hemocyte gas exchange

Since insect blood usually lacks oxygen-carrying pigments it has always been assumed that respiratory needs are met by diffusion in the gas-filled lumen of their tracheal systems. Outside air enters the tracheal system through segmentally arranged spiracles, diffuses along tubes of cuticle secreted by tracheal epithelia and then to tissues through tracheoles, thin walled cuticle tubes that penetrate between cells. The only recognized exceptions have been blood cells (hemocytes), which are not tracheated because they float in the hemolymph. In caterpillars, anoxia has an effect on the structure of the hemocytes and causes them to be released from tissues and to accumulate on thin walled tracheal tufts near the 8th (last) pair of abdominal spiracles. Residence in the tufts restores normal structure. Hemocytes also adhere to thin-walled tracheae in the tokus compartment at the tip of the abdomen. The specialized tracheal system of the 8th segment and tokus may therefore be a lung for hemocytes, a novel concept in insect physiology. Thus, although as a rule insect tracheae go to tissues, this work shows that hemocytes go to tracheae.

Cleaning insect oocytes by dissection and enzyme treatment

Vitellogenic oocytes from the cockroach Blattella germanica were dissected free of the surrounding follicle cell layer. The continued presence of a resting potential indicated that the defolliculated oocytes remained viable. In other experiments, lysozyme was injected into last nymphal instars; scanning electron micrographs showed that the bacteroids normally found between the follicle cell epithelium and the oocyte membrane were removed by this treatment. Oocytes from injected animals were known to be viable because they developed into living progeny. To assess the mode of action of lysozyme, defolliculated oocytes were exposed in vitro to lysozyme and then a hypotonic rinse; scanning electron micrographs showed that these oocytes were largely free of bacteroids.