Switching from Matrix to Cells
The sperm protein fertilin β (or ADAM 2) is crucial for binding between sperm and egg. Its counterpart on the egg is the α6β1 integrin, a surprising finding given that α6β1 is known as a laminin receptor, and integrins are primarily known for their binding to the extracellular matrix. Now Chen et al. (page ) find that distinct states of α6β1 can bind to either fertilin β or laminin. “This may be a paradigm for switching between cell–cell and cell–matrix interactions,” says senior author Judith White.
After treatment with phorbol esters or manganese ions, macrophages transfected with α6 bind more to laminin and less to sperm. Eggs treated the same way bind less to sperm or fertilin β–coated beads, but only the manganese treatment increases laminin binding. This may reflect a difference in the complement of integrin-associated proteins in macrophages and eggs.
Binding of fertilin β to eggs is also inhibited by disrupting actin structures with latrunculin, suggesting that tethered α6β1 may promote fertilin β binding, whereas laminin binding may require movement and reorganization of α6β1.
There are several circumstances in which an integrin switch could be advantageous. In the egg, a switch to laminin binding could help block fusion of more sperm (polyspermy) and promote the later α6β1-dependent outgrowth of the endoderm in implantation. In the dermis, an integrin switch could aid proliferating cells in detaching from the basement membrane and increase their adhesion to cells in the nonproliferating layers above.
Proton Gradients in Pollen Tubes
On page , Feijó et al. report the presence of proton gradients in lily pollen tubes. They suggest that the gradients escaped detection by others because high concentrations of detection dyes acted as buffers.
Growth materials are delivered near the ends of pollen tubes by cortical actin streaming. Where this streaming halts and turns backs along the tube core there is a clear zone; vesicles and other materials are then presumed to diffuse the rest of the way to the tip. Feijó et al. find that protons are ejected from the clear zone, which is alkaline, and flow into the tip, which is acidic. Protons may enter the tip through stretch-activated channels, which would not be opened on the rigid side walls of the pollen tube. A H+-ATPase probably drives proton exit. This pump may be deposited at the tip but only be active once it diffuses down the pollen tube to an area with lower calcium levels.
High rates of tip growth are correlated with a larger patch of acidity at the tip and reduced alkalinity in the clear zone. Oscillation in pH is most obvious in the clear zone and may, say the authors, provide a model to study the biological oscillation theory of morphogenesis and pattern formation first proposed by Alan Turing in the 1950s.
If high concentrations of buffer are injected into the pollen tubes, growth is halted. Protons or proton gradients may be affecting any of a number of growth processes. Acidity at the tip could promote exocytosis, and alkalinity in the clear zone may remodel actin. Alternatively, a steady state electrostatic field could drive vesicle movement to the tip or sense extracellular electrical and ionic gradients that are known to orient tip growth.
Tension Turns Off Endocytosis
Endocytosis is markedly reduced in mitosis, and on page 497, Raucher and Sheetz report that this correlates with increased membrane tension. However, correlation does not equal causation; it is also possible that another biochemical pathway triggers the inhibition of endocytosis. Previous work has shown that phosphorylation can inhibit both the invagination of coated pits and the fusion of endocytic vesicles.
To demonstrate the importance of tension, Raucher and Sheetz added the detergent deoxycholate to decrease membrane tension in mitotic cells (by intercalation into the membrane), and found that this restored interphase levels of endocytosis. High membrane tension may be physically inhibiting the deformation of the membrane to form endocytic vesicles.
Membrane tension arises because the membrane is stretched over and attached to a cytoskeleton of finite size. The level of tension may be set by changes both in lipid metabolism and the relative levels of secretion and endocytosis. Secretion is inhibited in mitosis, and this inhibition may trigger a rise in membrane tension that turns off endocytosis. High membrane tension in mitosis may help the cell to round up and inhibit membrane deformation needed for motility.
For cancer cells that are often in mitosis, endocytosis and therefore drug uptake should be drastically reduced. Taxol is often formulated in lipid mixtures because of the drug's hydrophobicity, but the formulation may also be decreasing membrane tension and increasing endocytosis.
Making Dendritic Spines
Ethell and Yamaguchi (page ) show that ectopic expression of syndecan-2 induces the formation of dendritic spines on cultured central nervous system neurons. Structural modifications of dendritic spines are thought to be central to memory formation and neural plasticity.
Syndecan-2 is a heparan sulfate proteoglycan that appears to bind to PDZ domain proteins via an intracellular EFYA motif. The EFYA motif is not required for syndecan-2 targeting to dendritic protrusions. However, it is required for conversion of those membrane protrusions into morphologically mature spines. PDZ domain proteins are known to localize signal transduction proteins and ion channels to synapses.
Syndecan-2's relevant extracellular ligand for spine formation is unknown. Ectopic expression of syndecan-2 may be short circuiting this normal signaling process as not all dendritic spines are associated with presynaptic specializations.
Cadherins are a cellular glue that maintain the polarity and structural integrity of cells. Removal of cadherins causes some motile events to fail, but this could be a secondary effect of the loss of structural integrity. On page , Niewiadomska et al. separate motility from structural integrity. They report that Drosophila E-cadherin is necessary for the movement of two cell groups, but not for the gross organization of either the cell groups or their cellular substrate.
The cell groups, border cells and centripetal cells, are both subsets of the somatic follicular cells that surround the 16 germline cells (the oocyte and 15 nurse cells) of the Drosophila follicle. Both cell groups migrate to the oocyte at the posterior end of the follicle.
In genetic mosaics, the border cells fail to penetrate or migrate between the nurse cells on their way to the oocyte when either the border cells or the nurse cells lack E-cadherin. In both cases the group of border cells still forms and the nurse cells remain adherent to one another. When only some of the border cells lack E-cadherin, the wild-type cells lead the way through the nurse cells with the mutant cells apparently dragged along behind, adhering to the leading cells via some other mechanism. Reducing but not eliminating the amount of E-cadherin in all cells slows and delays the migration of the border cells. Thus, the border cells (and the centripetal cells, which show similar requirements) apparently use homotypic E-cadherin contacts to move over the nurse cells.
By William A. Wells, 1095 Market St. #516, San Francisco, CA 94103. E-mail: firstname.lastname@example.org