Molecular gradients along the proximal-distal axis of embryonic insect legs: possible guidance cues of pioneer axon growth

Molecular gradients of bioinertness reveal a mechanistic difference between mammalian cell adhesion and bacterial biofilm formation

Chemical gradients play an important role in guiding the activities of both eukaryotic and prokaryotic cells. Here, we used molecularly well-defined chemical gradients formed by self-assembled monolayers (SAMs) on gold films to reveal that mammalian cell adhesion and bacterial biofilm formation respond differently to a gradient of surface chemistry that resists cell attachment. Gradient self-assembled monolayers (SAMs) consisting of two mixed alkanethiols were fabricated by differential exposure of the gold film to one alkanethiol, followed by soaking in another alkanethiol solution. A gradient in bioinertness that resisted cell attachment was created on SAMs from a gradient in the surface density of HS(CH2)11(OCH2CH2)3OH, backfilled with either HS(CH2)11OH or HS(CH2)11CH3. Measurements of the amounts of mammalian cells and bacterial biofilms on these gradient surfaces reveal that, for mammalian cells, a critical density of adhesion ligands from absorbed proteins on surfaces exists for supporting maximum adhesion and proliferation, whereas for the bacterium Escherichia coli , the amount of biofilm formed on surfaces increased linearly with the surface density of adhesive groups (methyl or hydroxyl groups) in different media. These results are consistent with mammalian cell adhesion requiring an anchorage via specific molecular recognitions and suggest that biofilms can form by immobilization of bacteria via nonspecific interaction between bacteria and surfaces.

Gradients and growth cone guidance of grasshopper neurons

The generation of a functional nervous system is dependent on precise pathfinding of axons during development. This pathfinding is directed by the distribution of local and long-range guidance cues, the latter of which are believed to be distributed in gradients. Gradients of guidance cues have been associated with growth cone function for over a hundred years. However, little is known about the mechanisms used by growth cones to respond to these gradients, in part owing to the lack of identifiable gradients in vivo. In the developing grasshopper limb, two gradients of the semaphorin Sema-2a are necessary for correct neuronal pathfinding in vivo. The gradients are found in regions where growth cones make critical steering decisions. Observations of different growth cone behaviors associated with these gradients have provided some insights into how growth cones respond to them. Growth cones appear to respond more faithfully to changes in concentration, rather than absolute levels, of Sema-2a expression, whereas the absolute levels may regulate growth cone size.

A paradoxical gradient of a basal lamina-associated repellent is essential for pathfinding by the Ti1 pioneer axons in cockroach embryos

Perturbations of in situ axon growth with proteolytic enzymes and monoclonal antibodies were used to determine the role of gradient guidance cues in the formation of the Ti1 pioneer axon trajectory in cultured cockroach embryos. Treatment with enzymes that degrade the basal lamina indicated that this substrate contains both an elastase-sensitive proximal directing cue and a collagenase-sensitive distal directing cue. The latter is shown to be a repellent of axon growth and is identical to the PROD-2 antigen that is distributed in a gradient along the proximal-distal axis of the leg with high levels in proximal regions. This means that throughout the course of their growth the axons extend in the direction of increasing levels of repellent. At a critical decision point in the trajectory the axons change both the direction of growth and the substrate to which their growth cones adhere. PROD-2 plays an essential role in both of these processes.

The in vivo regulation of pioneer axon growth by FGF-2 and heparan sulfate proteoglycans in cultured embryos of the cockroach

Antibody perturbation experiments on cultured cockroach embryos demonstrated that a localized source of an FGF-2-like immunoreactive molecule in the head is required for the proper growth of pioneer axons in the leg. The study of axon growth in various fragments of cultured embryos and in the presence of various conditioned media showed that FGF-2 is needed to counteract the effects of an inhibitor of axon growth produced in the body trunk of the embryo. Endogenous heparan sulfate proteoglycans mediate these effects of FGF-2 on axon growth. The results of experiments with FGF-2 and/or body trunk axon growth inhibitor added to the culture medium indicate that more globally and uniformly distributed molecules may play as important a role in axon guidance as the more spatially restricted guidance cues. The results are interpreted in terms of a model that is consistent with a role for the FGF-2 receptor in axon growth.

Mesodermal guidance of pioneer axon growth

Pioneer axons in insect legs are experimentally accessible model systems for the molecular identification and cellular localization of guidance cues regulating the path of axon growth. A detailed study of the Fe2 pioneer axons in the legs of the cockroach was performed to examine the diversity of guidance mechanisms. A detailed microscopic analysis of the axons at various points in their trajectory indicates that the Fe2 axons grow on a mesodermal substratum which contains the cues guiding their growth along a stereotyped path. An identified pair of muscle pioneer cells (MPC) are likely to play an important role in enabling the Fe2 growth cones to respond to mesodermal guidance cues. The addition of heparan sulfate, heparitinase, and phosphatidylinositol-specific phospholipase C to the medium perturbs the in situ path of growth of the Fe2 axons and the location of the MPC in cultured embryos. This indicates a role for heparan sulfate proteoglycans and glycosylphosphatidylinositol-anchored proteins in axon guidance. When these results are compared to those of similar experiments performed on the well-characterized Ti1 axons, they indicate significant differences in the mechanisms that are used for axon guidance. The Fe2 neurons are a good model for elucidating the mechanisms used to guide axon growth on nonmuscle mesodermal substrates often encountered in the periphery of vertebrate embryos.

Structure and development of larval antennae in embryos ofLytta viridanaLeConte (Coleoptera: Meloidae)

At time of hatch (252–264 h at 25 ± 0.5 °C), each antenna in Lytta viridana has three flagellomeres, three extrinsic muscles, and 25 sensilla of five different types, including a large composite sensillum of 19 sensory units on flagellomere II. Each antenna evaginates from epidermis on either side the stomodaeum beginning at 16% of embryogenesis. At 21%, a cell near its apex divides into two pioneer neurons that move into its lumen and project their axons to the brain by 29%. Sensillar stem cells begin to emerge at 23%, those of the appendix within a large embryonic placode and, from 26 to 48%, divide asymmetrically to generate the neurons and accessory cells of each sensillum. Sensillar axonogenesis begins at 34%, the first axons contact the brain at 35%, and antennal glomeruli begin to form within the deutocerebra at 57%. At 35%, the trichogen cell of each sensillum begins to grow out and larval cuticle is deposited about these, beginning at 57%. Upon withdrawal of trichogen cytoplasm from within the appendix at 81%, the dendrites of each sensory unit grow into it and begin to branch. Functional aspects are addressed and the observations compared with the limited information available on embryos of other insects.

Rearrangement of epithelial cell types in an insect wing monolayer is accompanied by differential expression of a cell surface protein

The distribution of adhesive molecules on surfaces of cells represents covert information for specifying positions of cells within a tissue. In insect wing epithelia where cells are arranged in two monolayers separated by an extracellular space, these adhesive molecules are found on basal and lateral surfaces of cells. Protein 3B11 is one surface protein whose expression changes in concert with movement and alignment of cells in wing monolayers of Manduca as well as with migration of tracheoles between the two monolayers of the wing. As epithelial cells segregate into periodic, transverse rows of alternating cell types (scale cells and generalized epithelial cells), the expression of 3B11 changes from a uniform distribution throughout the epithelial monolayer to a distribution correlated with a cell's final position and phenotype. Initially protein 3B11 is uniformly expressed on nonadherent surfaces of cells, but with the inception of cell rearrangement, differential expression of 3B11 on basolateral surfaces of cells--both adherent and nonadherent surfaces--becomes a function of epithelial cell type. At the completion of the cell movements associated with segregation of cell types, 3B11 is once again uniformly expressed throughout the wing epithelium. Also, as the upper and lower epithelial monolayers interact at their basal surfaces during adult development, 3B11 is expressed at the interface between the two epithelial monolayers and presumably functions in the nonspecific interaction between these monolayers. Examining the expression patterns of this protein as well as other adhesion molecules in wing epithelia should reveal general rules about either the simplicity or the complexity of the molecular prepatterns that orchestrate overt tissue patterns in epithelial monolayers.

Expression of epithelial alkaline phosphatase in segmentally iterated bands during grasshopper limb morphogenesis

Although the study of rostral-caudal segmentation of the insect body has been a rich source of information about embryonic pattern formation, relatively little is known of the process of proximal-distal segmentation of insect appendages. Here we demonstrate that during the period of limb segmentation, five segmentally iterated, sharply demarcated bands of cell surface alkaline phosphatase activity are expressed in embryonic grasshopper limbs. These bands span each intersegmental boundary in the limb as well as one boundary within the tarsus. Within appendages, expression is restricted to epithelial cells, where activity is present on both apical and basolateral surfaces. This epithelial alkaline phosphatase remains active at neutral pH, is insensitive to levamisole inhibition, and is strongly inhibited by nucleoside monophosphates. Treatment of embryos with phosphatidylinositol-specific phospholipase C releases almost all visible chromogenic activity, indicating that the epithelial alkaline phosphatase is anchored to the plasma membrane by glycosyl-phosphatidylinositol. When material released by phosphatidylinositol-specific phospholipase C is separated on native polyacrylamide gels, a single broad band of enzymatic activity is detected following incubation with substrate. A polyclonal antiserum raised against a 55 × 10(3) M(r) alkaline phosphatase from shrimp recognizes a single band of 56 × 10(3) M(r) on immunoblots of grasshopper membrane proteins. The spatially restricted expression of epithelial alkaline phosphatase suggests that it may be involved in epithelial cell rearrangements or shape changes associated with limb segmentation and morphogenesis. It also may contribute to definition of axon routes in the limb, since pioneer afferent growth cones turn at, and migrate along, the edge of one alkaline phosphatase-expressing epithelial domain.