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Associated with the Subplate Neurons of the Mammalian Cerebral Cortex (Trophic Factors/Neural Development/Cell Death/Brain/Transient Neurons) KAREN L

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Associated with the Subplate Neurons of the Mammalian Cerebral Cortex (Trophic Factors/Neural Development/Cell Death/Brain/Transient Neurons) KAREN L Proc. Natl. Acad. Sci. USA Vol. 87, pp. 187-190, January 1990 Neurobiology Nerve growth factor receptor immunoreactivity is transiently associated with the subplate neurons of the mammalian cerebral cortex (trophic factors/neural development/cell death/brain/transient neurons) KAREN L. ALLENDOERFER*, DAVID L. SHELTONt, ERIC M. SHOOTER, AND CARLA J. SHATZ Department of Neurobiology, Stanford University School of Medicine, Stanford, CA 94305 Communicated by Gunther S. Stent, October 13, 1989 ABSTRACT Nerve growth factor and its receptor (NGFR) do (5, 6). Furthermore, they receive functional synaptic are known to be present in diverse embryonic and neonatal inputs (refs. 5 and 7; E. Friauf, S. K. McConnell, and C.J.S., central nervous system tissues, including the cerebral cortex. unpublished data) and pioneer elaborate projections to cor- However, the identity of the cortical cells expressing NGFR tical and subcortical targets (5, 8). Subplate neurons are also immunoreactivity has not been established. We have used thought to interact with the many ingrowing axonal systems immunolabeling coupled with [3HJthymidine autoradiography from the thalamus and cortex that are known to wait in the to identify such cells in ferret and cat brain. Polyclonal subplate zone (2, 9, 10) before finally invading the cortical antibodies raised against a synthetic peptide corresponding to plate, at -1 week before birth (11). After the waiting period a conserved amino acid sequence of the NGFR were used for ends, subplate neurons begin a programmed cell death, which this purpose. Western (immunologic) blot analyses show that eliminates over 90% of the population, until finally only these antibodies specifically recognize NGFR and precursor 1%o remain by 3 months of age (12, 13). Here we examine proteins. In both species, NGFR immunoreactivity is primarily the possibility that nerve growth factor (NGF) plays a role in associated with the early generated and transient subplate the maintenance of subplate neurons during early cortical neuron population of the developing neocortex, as indicated by development and in their subsequent death. We studied the the following evidence: the immunoreactive cells (i) are located distribution of NGF receptor (NGFR) immunoreactivity of directly beneath the developing cortical plate, (a) frequently the subplate neurons from the time they become postmitotic have the inverted pyramid shape characteristic of subplate until after they begin to die, in view of recent reports that the neurons, and (iii) can be labeled by an injection of receptor is likely to be transiently expressed in the central [3H]thymidine on embryonic day (E) 28, a time when only nervous system and, in particular, in the telencephalon, subplate neurons are being generated. Intense NGFR immu- during embryonic and neonatal life (14-17). nostaining is seen on the cell bodies ofthese neurons as early as E30, several days after their last round ofcell division, and this immunostaining remains strong for -3 weeks. The NGFR MATERIALS AND METHODS immunoreactivity begins to decline around E52 and has dis- Eight cat brains between E35 and E60 and four ferret brains, appeared from the region altogether by E60, at which time ages postnatal days (P)2 and 10 were studied. Cat fetal tissue subplate neurons begin to die. The cellular localization and was obtained by sterile surgery on timed pregnant cats, as timing of expression suggest that the NGFR may play a role in described (1, 4). Timed pregnant sable ferrets were obtained the maintenance of subplate neurons and in the maturation of from Marshall Laboratories (North Rose, NY). As discussed the cerebral cortex. below, the pace and pattern of development of cat and ferret cortex are strikingly similar. During the development of the mammalian brain, the forma- Preparation of Antibody. The peptide YNSRPVNQTPP- tion of the adult six-layered cerebral cortex is preceded by a PEGEKLHSD (in one-letter code), corresponding to resi- period in which a special class of transient neurons is gener- dues 258-276 of the rat low-affinity NGFR (18) plus a ated (1-3). These neurons are among the first postmitotic cells terminal tyrosine, was synthesized commercially using of the telencephalon and, in higher mammals, undergo their FMOC (9-fluorenylmethoxycarbonyl) chemistry. This se- final round ofcell division well before the neurons ofthe adult quence is part of the intracellular domain of the protein and cortical plate [cat at embryonic day (E) 24-30 (1, 4); ferret at is completely conserved at the amino acid level between the E20-24 (3)]. These earliest generated cells then migrate away rat and human low-affinity NGFR (18, 19). The peptide was from the germinal zone and initially take up positions imme- crosslinked to porcine thyroglobulin by using 1-ethyl- diately beneath the pial surface. However, as the neurons of 3-(3-dimethylaminopropyl)-carbodiimide. One milligram of the future cortical plate migrate out and assume their final thyroglobulin conjugate (corresponding to 100 ,.g of peptide) locations, the early generated neuronal population is split into was mixed with 100 pug of free peptide, emulsified with 3 vol two parts, the subplate, which is located directly beneath the of Freund's complete adjuvant, and injected into 10 intra- developing cortical plate, and the marginal zone, which is muscular sites of four female New Zealand White rabbits. located directly above it (4). The animals were reinjected subcutaneously with the same Subplate neurons mature rapidly and express immunoreac- amount of antigen in Freund's incomplete adjuvant at tivity not only to neurotransmitters but also to microtubule- associated protein 2 (MAP2) long before the cortical neurons Abbreviations: NGF, nerve growth factor; NGFR, NGF receptor; E, embryonic day; P, postnatal day; MAP2, microtubule-associated protein 2. The publication costs of this article were defrayed in part by page charge *To whom reprint requests should be addressed. payment. This article must therefore be hereby marked "advertisement" tPresent address: Genentech, Inc., 460 Point San Bruno, South San in accordance with 18 U.S.C. §1734 solely to indicate this fact. Francisco, CA 94080. 187 Downloaded by guest on September 28, 2021 188 Neurobiology: Allendoerfer et al. Proc. Natl. Acad. Sci. USA 87 (1990) monthly intervals. Rabbits were bled 2 weeks after each RESULTS reinjection, and antipeptide antibodies were affinity-purified with a column of peptide crosslinked via glutaraldehyde to To investigate the presence and distribution of NGFR in the cerebral cortex during development, we immunostained sec- bovine serum albumin bound to Affi-Gel (Bio-Rad) and eluted tions from fetal cat brain at ages between E30 and E60 and with glycine hydrochloride, pH 2.5. Before use, antibodies neonatal ferret brain at postnatal day (P) 2 and P10 (see Table thus obtained were adsorbed with a similar column contain- 1). Many previous studies have demonstrated that these two ing a NGFR-unrelated peptide bound to bovine serum albu- species have a remarkably similar pace of cortical develyp- min. All ofthe experiments in this study were carried out with ment during fetal life, even though the ferret is born at 41 the antibodies obtained from a single bleeding of one rabbit. gestational days (3, 20), whereas the cat is born at 65 days (1) Western (Immunologic) Blots. Telencephalon tissue (in- (e.g., a ferret at P2 is developmentally equivalent to a cat at cluding cortical plate and subplate; excluding hippocampus) E43). After the ferret is born, its development speeds up in was dissected immediately after euthanasia of animals of the relation to that of the cat, which remains in utero, so that by appropriate age, homogenized in 10.8% sucrose, and centri- P10 a ferret cortex resembles that of an E60 cat. fuged at 3000 x g for 5 min. The pellet was dissolved in 0.1 Specificity and Cross-Reactivity of the Antibody with the M phosphate-buffered saline with 0.5% Nonidet P-40 (Sig- NGFR in Developing Carnivore Telencephalon. The Western ma), incubated on ice for 10 min, and microcentrifuged. The blots of Fig. 1 show that the anti-NGFR antibody recognizes optical density at 280 gm was read to estimate the protein a doublet with molecular masses of 80 kDa and 71 kDa in content of the Nonidet P-40 soluble fraction, and -200 ,ug of proteins extracted from a rat cell line [PC12 (21)] and a mouse protein was applied per lane to an SDS/polyacrylamide gel cell line transfected with rat genomic DNA [PCNA15 (18)], (7.5%) for electrophoresis at 30 V. The protein was trans- both of which are known to express the rat NGFR in large ferred to nitrocellulose, and the blot was incubated with quantities. The 80-kDa band corresponds, closely to the anti-NGFR antibody, iodinated by using lodogen (Pierce) to molecular mass measured for the rat low-affinity. NGFR by a specific 125I activity of 6.5 x 108 cpm/Ag. other means (22). The 71-kDa band corresponds to a meta- Immunohistochemistry. Brains were prepared for immuno- bolic precursor of the NGFR (D.L.S. and E.M.S., unpub- histochemistry by transcardial perfusion of the cat fetuses or lished data). Immunoreactivity corresponding to precursor ferret neonates with 4% (wt/vol) paraformaldehyde in 0.1 M and receptor bands can also be seen in the ferret and cat sodium phosphate buffer followed by 6-12 hr postfixation in protein extract lanes (Fig. la), indicating that a specific the same solution. Tissue was equilibrated in 20% (wt/vol) molecule similar or identical to the NGFR is present in ferret sucrose, and coronal cryostat sections (20 Am) were cut and and cat telencephalon at early developmental ages. The mounted on gelatin/albumin-coated slides. Sections were antibody binding to the pair of bands in all instances can be incubated in 1:1000 anti-MAP2 serum (5, 6) (provided by R.
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