Hedgehog and Wingless stabilize but do not induce cell fate during Drosophila dorsal embryonic epidermal patterning

A fundamental concept in development is that secreted molecules such as Wingless (Wg) and Hedgehog (Hh) generate pattern by inducing cell fate. By following markers of cellular identity posterior to the Wg- and Hh-expressing cells in the Drosophila dorsal embryonic epidermis, we provide evidence that neither Wg nor Hh specifies the identity of the cell types they pattern. Rather, they maintain pre-existing cellular identities that are otherwise unstable and progress stepwise towards a default fate. Wg and Hh therefore generate pattern by inhibiting specific switches in cell identity, showing that the specification and the patterning of a given cell are uncoupled. Sequential binary decisions without induction of cell identity give rise to both the groove cells and their posterior neighbors. The combination of independent progression of cell identity and arrest of progression by signals facilitates accurate patterning of an extremely plastic developing epidermis.     

Plasma Catechols in Familial Dysautonomia: A Long-term Follow-up Study

This study tested whether familial dysautonomia (FD) involves progressive loss of noradrenergic nerves. Plasma levels of catechols, including dihydroxyphenylglycol (DHPG), norepinephrine (NE), dopamine (DA), and DOPA, were measured in 7 adult patients with FD and 50 healthy control subjects. FD patients were re-tested after a mean follow-up period of 13 years. Compared to controls, FD patients had low plasma levels of DHPG (P < 0.001), high DOPA and DA levels (P = 0.01, P = 0.0002), and high NE:DHPG (P < 0.0001), DA:NE (P = 0.0003), and DOPA:DHPG (P < 0.0001) ratios. At follow-up there were no changes in plasma levels of individual catechols; however, there were further increases in DOPA:DHPG ratios (mean 24 ± 7%, P = 0.01). In FD, plasma catechol profiles are sufficiently stable, at least over a decade, to be used as a biomarker of disease involvement. An increasing DOPA:DHPG ratio suggests slight but consistent, progressive loss of noradrenergic neurons.     

Total Internal Reflection Fluorescence Microscopy in Cell Biology

Key events in cellular trafficking occur at the cell surface, and it is desirable to visualize these events without interference from other regions deeper within. This review describes a microscopy technique based on total internal reflection fluorescence which is well suited for optical sectioning at cell-substrate regions with an unusually thin region of fluorescence excitation. The technique has many other applications as well, most notably for studying biochemical kinetics and single biomolecule dynamics at surfaces. A brief summary of these applications is provided, followed by presentations of the physical basis for the technique and the various ways to implement total internal reflection fluorescence in a standard fluorescence microscope.     

Cytoskeletal protein binding kinetics at planar phospholipid membranes.

It has been hypothesized that nonspecific reversible binding of cytoskeletal proteins to lipids in cells may guide their binding to integral membrane anchor proteins. In a model system, we measured desorption rates k(off) (off-rates) of the erythrocyte cytoskeletal proteins spectrin and protein 4.1 labeled with carboxyfluorescein (CF), at two different compositions of planar phospholipid membranes (supported on glass), using the total internal reflection/fluorescence recovery after photobleaching (TIR/FRAP) technique. The lipid membranes consisted of either pure phosphatidylcholine (PC) or a 3:1 mixture of PC with phosphatidylserine (PS). In general, the off-rates were not single exponentials and were fit to a combination of fast, slow, and irreversible fractions, reported both separately and as a weighted average. By a variation of TIR/FRAP, we also measured equilibrium affinities (the ratio of surface-bound to bulk protein concentration) and thereby calculated on-rates, k(on). The average off-rate of CF-4.1 from PC/PS (approximately 0.008/s) is much slower than that from pure PC (approximately 1.7/s). Despite the consequent increase in equilibrium affinity at PC/PS, the on-rate at PC/PS is also substantially decreased (by a factor of 40) relative to that at pure PC. The simultaneous presence of (unlabeled) spectrin tends to substantially decrease the on-rate (and the affinity) of CF-4.1 at both membrane types. Similar experiments for CF-spectrin alone showed much less sensitivity to membrane type and generally faster off-rates than those exhibited by CF-4.1. However, when mixed with (unlabeled) 4.1, both the on-rate and off-rate of CF-spectrin decreased drastically at PC/PS (but not PC), leading to a somewhat increased affinity. Clearly, changes in affinity often involve countervailing changes in both on-rates and off-rates. In many of these studies, the effect of varying ionic strength and bulk concentrations was examined; it appears that the binding is an electrostatic attraction and is far from saturation at the concentrations employed. These results and the techniques implemented carry general implications for understanding the functional role of nonspecific protein binding to cellular lipid membranes.