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THY-1, the smallest member of the immunoglobulin superfamily1, is a major cell-surface component expressed by several tissues2. The protein1carbohydrate3 and gene4 structures of this molecule are known, yet its function is not. It is highly expressed in nervous tissue5, where it appears on virtually all neurons after the cessation of axonal growth6. Here we show that expression of Thy-1 by a neural cell line inhibits neurite outgrowth on mature astrocytes, but not on other cellular substrata which include Schwann cells and embryonic glia. This inhibition of neurite extension on astrocytes can be reversed by low concentrations (nanomolar) of soluble Thy-1. If a similar interaction between neuronal Thy-1 and astrocytes occurs in vivo, it could stabilize neuronal connec-tions and suppress axonal regrowth after injury in the astrocyte-rich areas of adult central nervous system.

Dissociated rat hippocampal neurons were cultured on horizontal cryostat sections from neonatal and adult rat brain and their growth patterns visualized by brightfield and interference contrast microscopy. Cells adhered to the sections as individuals or in small clusters and grew extensive neurites. Neurites grew over all areas of neonatal sections without apparent selectivity. For adult sections, however, neurites grew almost exclusively on areas of grey matter: there was no neurite growth on areas of white matter, irrespective of the location of that white matter within the brain. The transition from the neonatal to the adult pattern of growth occurred for sections from animals aged between 14 and 21 days.

Publisher Summary Chemical signals appear to play important roles in development of neural tissue. For example, they may function to spatially and temporally coordinate the differentiation of specific cell groups, to provide tropic signaling and guidance, and to mediate cell-cell communication and feedback. Moreover, because of their potential to function extracellularly, chemical signals could work either in restricted local environments, or, more diffusely, over long distances. Within the last 30 years, a number of macromolecular factors have been identified that can influence the development and differentiation of the nervous system. NGF is presently the most useful model available for studying the actions of chemical factors that regulate neural development. The aim of this chapter is to explore the role of the genome in the action of nerve growth factor.

Nerve growth factor (NGF) has a variety of actions on its physiological targets, sympathetic and sensory neurons (Levi-Montalcini and Angeletti, 1968; Green and Shooter, 1980; Thoenen and Barde, 1980). Among the most striking of these actions is the promotion of neurite outgrowth. This chapter will focus on the mechanisms by which NGF causes neuritis to be initiated and maintained.

The electron spin resonance spectrum of rabbit muscle d -glyceraldehyde-3-phosphate dehydrogenase spin-labelled with 4-(2-iodoacetamido)-2,2,6,6-tetramethylpiperidinooxyl has two components. One component is due to a spin label highly immobilized on the enzyme surface and the other to a nitroxyl group able to tumble rapidly. The spin-labelled enzyme is inactive. Selective modification of the active site cysteine residue (149) and determinations of total sulphydryl content implicate this residue as the site of the immobile spin-label. The mobile spin label is attached to another sulphydryl group. Crystallographic studies on the human muscle enzyme (Watson, H.C., Duee, E. and Mercer, W.D. (1972) Nat. New Biol., 240, 130) have located a binding site for samarium ion in the active centre. Addition of the paramagnetic gadolinium ion to spin-labelled enzyme reduces the intensity of both the spin label signals (by 72% for the mobile and by 11% for the immobile component). This indicates that the metal ion site ( K d = 0.7 mM ) is close to both types of spin label. Measurements of the effect of gadolinium-protein binding on the relaxation rate of solvent water protons enable the enzyme-bound spin label-metal ion distances to be tentatively estimated as 15 A.

Nerve growth factor protein (NGF) was found to rapidly promote the activation of tyrosine hydroxylase in cultured rat PC 12 pheochromocytoma cells. PC 12 cultures were exposed to NGF for periods of less than 1 h and the soluble contents of homogenates prepared from the cells were assayed for tyrosine hydroxylase activity. Under these conditions, the specific enzymatic activity was increased by 60 ± 10% (n = 13) in comparison with that in untreated sister cultures. The increase was half maximal by 2–5 min of exposure and at NGF concentrations of about 10 ng/ml (0.36 nM). Antiserum against NGF blocked the effect. Tyrosine hydroxylase activity could also be rapidly increased by NGF in cultures of PC12 cells that had been treated with the factor for several weeks in order to produce a neuron-like phenotype. This was achieved by withdrawing NGF for about 4 h and then readding it for 30 min. The NGF-induced increase of tyrosine hydroxylase activity in PC12 cultures was not affected by inhibition of protein synthesis and therefore appeared to be due to activation of the enzyme. Kinetic experiments revealed that NGF brought about no change in the apparent Km of the enzyme for tyrosine or for co-factor (6-methyltetrahydropteridine), but that it did significantly increase the apparent maximum specific activity of the enzyme. These observations suggest that NGF (perhaps released by target organs) could promote a rapid and local enhancement of noradrenergic transmission in the sympathetic nervous system.