Response of retinal ganglion cell axons to striped linear gradients of repellent guidance molecules.

Although molecular gradients have long been postulated to play a role in the development of topographic projections in the nervous system, relatively little is known about how axons evaluate gradients. Do growth cones respond to concentration or to slope? Do they react suddenly or gradually? Is there adaptation? In the developing retinotectal system, temporal retinal ganglion cell axons have previously been shown to avoid repellent cell-surface activities distributed in gradients across the optic tectum. We confronted temporal retinal axons with precisely formed striped linear gradients of repellent tectal membranes and of two candidate repellent molecules, ephrin-A2 and -A5. Axons entered gradient stripes independently of their slope and extended unhindered in the uphill direction until they suddenly avoided an apparent threshold concentration of repellent material that was independent of slope. This critical concentration was similar in both linear and nonlinear gradients, and hence independent of gradient shape. When gradients of identical slope were formed on different basal levels of repellent material, axons grew uphill for a fixed increment of concentration, possibly measured from the lowest point of the gradient, rather than up to a fixed absolute concentration. The speed of growth cones was not affected by repellent unstriped gradients below the critical concentration level. Similar results were found with membranes from cell lines stably transfected with either ephrin-A5 or ephrin-A2, two previously identified growth cone repellent cell-surface proteins. These data suggest that growth cones or axons can integrate guidance information over large distances, probably by a combined memory and adaptation mechanism.

Growth regulation of rat thyrocytes (FRTL-5 cells) by the secreted ectodomain of beta-amyloid precursor-like proteins.

The TSH-dependent expression of amyloid precursor-like proteins and the secretion of their ectodomain (sAPP) in rat thyroids coincide with increased rates of thyrocyte proliferation. To analyze whether the secretion of sAPP and the proliferation of thyrocytes are regulatorily linked, we employed [3H]thymidine or 5-bromo-2'-deoxyuridine assays and found that conditioned culture medium stimulated the proliferation of FRTL-5 cells depending on the content of sAPP. These observations prompted experiments with sAPP-derived peptides known to stimulate the growth of APP-deficient fibroblasts. Using autoradiography and radiochemical assays, we observed that an iodinated 19-mer sAPP peptide was bound specifically to the surface of FRTL-5 cells. Binding of this peptide was followed by a 2- to 8-fold increase in cell proliferation, which reached a plateau at 1 nM. This effect was significant only when cells were cultured in nonconfluent monolayers, and contact inhibition did not interfere. Our observations indicate that sAPP and sAPP-derived peptides increased the proportion of proliferation-competent FRTL-5 cells and suggest that sAPP may be a new member in the family of peptides involved in the growth regulation of thyrocytes.