Two different monoclonal antibodies, characterized initially as binding synaptic terminal regions of rat brain, bind a 65,000-dalton protein, which is exposed on the outer surface of brain synaptic vesicles . Immunocytochemical experiments at the electron microscope level demonstrate that these antibodies bind the vesicles in many different types of nerve terminals. The antibodies have been used successfully to purify synaptic vesicles from crude brain homogenates by immunoprecipitation onto the surface of polyacrylamide beads. The profiles of the structures precipitated by these beads are almost exclusively vesicular, confirming the vesicle-specificity of the antibodies . In SDS gels, the antibodies bind a single protein of 65,000 daltons. The two antibodies are not identical, but compete for binding sites on this protein .Immune competition experiments also demonstrate that the antigenic components on the 65,000-dalton protein are widely distributed in neuronal and neural secretory tissues. Detectable antigen is not found in uninnervated tissue-blood cells and extrajunctional muscle . Low levels are found in nonneural secretory tissues ; it is not certain whether this reflects the presence of low amounts of the antigen on all the exocytotic vesicles in these tissues or whether the antigen is found only in neuronal fibers within these tissues . The molecular weight and at least two antigenic determinants of the 65,000-dalton protein are highly conserved throughout vertebrate phylogeny . The two antibodies recognize a 65,000-dalton protein present in shark, amphibia, birds, and mammals. The highly conserved nature of the determinants on this protein and their specific localization on secretory vesicles of many different types suggest that this protein may be essential for the normal function of neuronal secretory vesicles .

Migrating Schwann cells in developing or regenerating peripheral nerves are known to express dramatically increased levels of nerve growth factor (NGF) and the low-affinity NGF receptor (LNGFR). Schwann cells do not express detectable ppl40", the NGF-activated receptor tyrosine kinase which is essential for neuronal responses to NGF. The temporal correlation observed in Schwann cells between migration and the enhanced expression of NGF and LNGFR suggests that NGF and LNGFR may promote Schwann cell migration. To test this possibility, we examined the effects of NGF on Schwann cell migration on cryostat sections of biologically relevant NGF-poor and NGF-rich substratesnormal or denervated peripheral (sciatic) nerve, untreated or pretreated with NGF. Results show that Schwann cells migrate more rapidly on denervated than on normal sciatic nerve. Antibodies to NGF or to LNGFR strongly, but incompletely, inhibit enhanced migration on denervated nerves. Pretreatment of denervated nerve sections with NGF increases further the rate of Schwann cell migration. The same antibodies to NGF or to LNGFR abolish this response. These results suggest that one function of the elevated levels of NGF known to be present in embryonic and regenerating peripheral nerves is to promote the migration of Schwann cells. In contrast to neurons, where ppl4O"" appears to be the functionally critical NGF receptor, NGF responses in Schwann cells depend on LNGFR.Schwann cell migration plays a fundamental role in the normal development and successful regeneration of peripheral nerves (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11) Despite the functional significance of Schwann cell migration, the molecules responsible for this cellular phenomenon remain poorly characterized. Schwann cell migration in vivo occurs on either of two substrates: (i) axonal surfaces or (ii) Schwann cell basement membranes and other extracellular matrix components (1-9, 21). It is likely that Schwann cells use different substrates at different times, and there are, therefore, a number of candidate molecules that may control Schwann cell migration. Two of these candidate molecules are nerve growth factor (NGF) and its low-affinity receptor (LNGFR). During the development and regeneration of a peripheral nerve, Schwann cells have been shown to be a rich source of both NGF and LNGFR (25)(26)(27)(28)(29)(30)(31)(32)(33) (8,9). Increased NGF and LNGFR expression is maintained until intimate axon-Schwann cell contact is established.Abbreviations: NGF, nerve growth factor; LNGFR, low-affinity NGF receptor; FBS, fetal bovine serum; DRG, dorsal root ganglion.§To whom reprint requests should be addressed. 2795The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

No abstract