Monoclonal antibody Cat-315 detects a glycoform of receptor protein tyrosine phosphatase beta/phosphacan early in CNS development that localizes to extrasynaptic sites prior to synapse formation

Perineuronal nets (PNs) are lattice-like condensations of the extracellular matrix (ECM) that envelop synapses and decorate the surface of subsets of neurons in the CNS. Previous work has suggested that, despite the fact that PNs themselves are not visualized until later in development, some PN component molecules are expressed in the rodent CNS even before synaptogenesis. In the adult mammalian brain, monoclonal antibody Cat-315 recognizes a glycoform of aggrecan, a major component of PNs. In primary cortical cultures, a Cat-315-reactive chondroitin sulfate proteoglycan (CSPG) is also expressed on neuronal surfaces and is secreted into culture media as early as 24 h after plating. In this study, we show that in primary cortical cultures, the Cat-315 CSPG detected in early neural development is expressed in extrasynaptic sites prior to synapse formation. This suggests that ECM components in the CNS, as in the neuromuscular junction (NMJ), may prepattern neuronal surfaces prior to innervation. We further show that while the Cat-315-reactive carbohydrate decorates aggrecan in the adult, it decorates a different CSPG in the developing CNS. Using receptor protein tyrosine phosphatase beta (RPTPbeta/protein tyrosine phosphatase zeta) knock-out mice and immunoprecipitation techniques, we demonstrate here that in the developing rodent brain Cat-315 recognizes RPTPbeta isoforms. Our further examination of the Cat-315 epitope suggests that it is an O-mannose linked epitope in the HNK-1 family. The presence of the Cat-315 reactive carbohydrate on different PN components--RPTPbeta and aggrecan--at different stages of synapse development suggests a potential role for this neuron-specific carbohydrate motif in synaptogenesis.

Novel tumor-specific isoforms of BEHAB/brevican identified in human malignant gliomas

Malignant gliomas are deadly brain tumors characterized by diffuse invasion into the surrounding brain tissue. Understanding the mechanisms involved in glioma invasion could lead to new therapeutic strategies. We have previously shown that BEHAB/brevican, an extracellular matrix protein in the central nervous system, plays a role in the invasive ability of gliomas. The mechanisms that underlie BEHAB/brevican function are not yet understood, due in part to the existence of several isoforms that may have different functions. Here we describe for the first time the expression of BEHAB/brevican in human brain and characterize two novel glioma-specific isoforms, B/b(sia) and B/b(Deltag), which are generated by differential glycosylation and are absent from normal adult brain and other neuropathologies. B/b(sia) is an oversialylated isoform expressed by about half the high- and low-grade gliomas analyzed. B/b(Deltag) lacks most of the carbohydrates typically present on BEHAB/brevican and is the major up-regulated isoform of this protein in high-grade gliomas but is absent in a specific subset of low-grade, indolent oligodendrogliomas. B/b(Deltag) is detected on the extracellular surface, where it binds to the membrane by a mechanism distinct from the other BEHAB/brevican isoforms. The glioma-specific expression of B/b(Deltag), its restricted membrane localization, and its expression in all high-grade gliomas tested to date suggest that it may play a significant role in glioma progression and make it an important new potential therapeutic target. In addition, its absence from benign gliomas prompts its use as a diagnostic marker to distinguish primary brain tumors of similar histology but different pathologic course.

A novel membrane-associated glycovariant of BEHAB/brevican is up-regulated during rat brain development and in a rat model of invasive glioma

BEHAB (brain-enriched hyaluronan-binding protein)/brevican is the most abundant chondroitin sulfate proteoglycan in the extracellular matrix of the adult rat brain. BEHAB/brevican expression is up-regulated coincident with glial cell proliferation and/or motility, including during early central nervous system development and in invasive glioma. An understanding of the molecular interactions that mediate BEHAB/brevican function is still in its infancy because of the existence of several BEHAB/brevican isoforms, each of which may mediate different functions. Here, we describe a novel BEHAB/brevican isoform, B/b130, and demonstrate that it is neither the glycosylphosphatidylinositol-linked splice variant of BEHAB/brevican nor a cleavage product of the full-length protein (B/b150). B/b130 is an underglycosylated isoform of BEHAB/brevican, lacking glycosaminoglycan chains as well as most of the sugars that invest B/b150. B/b130 localizes exclusively to the particulate fraction of rat brain and associates with the cell membrane by a previously undescribed calcium-independent mechanism. In addition, B/b130 is the major isoform of BEHAB/brevican that is up-regulated in a rat model of invasive glioma and may therefore contribute to the invasive ability of glioma cells. Further understanding of BEHAB/brevican isoforms will advance our knowledge of the function of this ECM component and may help identify new potential therapeutic targets for primary brain tumors.

TUC-4b, a novel TUC family variant, regulates neurite outgrowth and associates with vesicles in the growth cone

The TUC (TOAD-64/Ulip/CRMP) proteins are homologs of UNC-33, a protein that is required for axon extension and guidance in Caenorhabditis elegans. The TUC proteins are expressed in newly born neurons in the developing nervous system and have been implicated in semaphorin signaling and neuronal polarity. Here, we identify several new variants of the TUC family, each of which is expressed during distinct periods of neural development. We cloned and characterized TUC-4b, a variant of TUC-4a that includes a unique N-terminal extension. The functional relevance of this N-terminal domain is demonstrated by the finding that overexpression of TUC-4b, but not TUC-4a, results in increased neurite length and branching. Furthermore, whereas TUC-4a is expressed throughout life, TUC-4b is expressed exclusively during embryonic development. TUC-4b is localized to SV2 (synaptic vesicle protein 2)-positive vesicles in the central domain of the growth cone, suggesting a potential role in growth cone vesicle transport. Furthermore, TUC-4b interacts with the SH3A (Src homology 3A) domain of intersectin, an endocytic-exocytic adaptor protein. Together, these data suggest that TUC-4b can regulate neurite extension and branching through a mechanism that may involve membrane transport in the growth cone.

Aggrecan glycoforms contribute to the molecular heterogeneity of perineuronal nets

The perineuronal net forms the extracellular matrix of many neurons in the CNS, surrounding neuron cell bodies and proximal dendrites in a mesh-like structure with open "holes" at the sites of synaptic contacts. The perineuronal net is first detected late in development, approximately coincident with the transformation of the CNS from an environment conducive to neuronal growth and motility to one that is restrictive, suggesting a role for the perineuronal net in this developmental transition. Perineuronal nets show a great degree of molecular heterogeneity. Using monoclonal antibodies Cat-301, Cat-315, and Cat-316, we have shown previously that although all antibodies recognize chondroitin sulfate proteoglycans of similar sizes, each antibody recognizes perineuronal nets on distinct but overlapping sets of neurons in the adult cat CNS. An understanding of the heterogeneity demonstrated by these antibodies is critical to understanding the organization and function of perineuronal nets. Using aggrecan knock-out mice (cmd), we have now determined that all three antibodies recognize aggrecan. Chemical and enzymatic deglycosylation show that the differences revealed by the three antibodies arise from differential glycosylation of aggrecan. We further demonstrate that aggrecan mRNA is expressed relatively late in development and that neurons themselves are likely the predominant cellular sites of aggrecan expression. This work indicates that neurons can directly regulate the composition of their extracellular matrix by regulated synthesis and differential glycosylation of aggrecan in a cell type-specific manner. These results have important implications for the role of regulated microheterogeneity of glycosylation in the CNS.

Differential proteome analysis of replicative senescence in rat embryo fibroblasts

Normal somatic cells undergo a finite number of divisions and then cease dividing whereas cancer cells are able to proliferate indefinitely. To identify the underlying mechanisms that limit the mitotic potential, a two-dimensional differential proteome analysis of replicative senescence in serially passaged rat embryo fibroblasts was undertaken. Triplicate independent two-dimensional gels containing over 1200 spots each were run, curated, and analyzed. This revealed 49 spots whose expression was altered more than 2-fold. Of these, 42 spots yielded positive protein identification by mass spectrometry comprising a variety of cytoskeletal, heat shock, and metabolic proteins, as well as proteins involved in trafficking, differentiation, and protein synthesis, turnover, and modification. These included gelsolin, a candidate tumor suppressor for breast cancer, and alpha-glucosidase II, a member of the family of glucosidases that includes klotho; a defect in klotho expression in mice results in a syndrome that resembles human aging. Changes in expression of TUC-1, -2, -4, and -4 beta, members of the TUC family critical for neuronal differentiation, were also identified. Some of the identified changes were also shown to occur in two other models of senescence, premature senescence of REF52 cells and replicative senescence of mouse embryo fibroblasts. The majority of these candidate proteins were unrecognized previously in replicative senescence. They are now implicated in a new role.

Brain enriched hyaluronan binding (BEHAB)/brevican increases aggressiveness of CNS-1 gliomas in Lewis rats

Gliomas are the most common primary intracranial tumors. One extracellular matrix component that has been implicated in glial tumor biology is brain enriched hyaluronan binding (BEHAB)/brevican. In this study, the CNS-1 rat glioma cell line was transfected with a vector containing either a full-length BEHAB/brevican cDNA, a 5' insert encoding the NH(2)-terminal BEHAB/brevican cleavage product, or a 3' insert encoding the COOH-terminal cleavage product. As a control, CNS-1 cells were transfected with green fluorescent protein. Rats with intracranial grafts of BEHAB/brevican-transfected CNS-1 cells displayed significantly shorter survival times than did rats with CNS-green fluorescent protein intracranial grafts (P < 0.001). Histological examination showed that the BEHAB/brevican-transfected tumors were just as, if not more, aggressive than control tumors, even though the BEHAB/brevican tumors had been growing for only approximately two-thirds the time as long as control tumors. These data suggest that up-regulation and proteolytic cleavage of BEHAB/brevican increase significantly the aggressiveness of glial tumors. It will be important to investigate the effect of inhibiting cleavage of BEHAB/brevican in these cells and to determine the therapeutic potential of inhibiting BEHAB/brevican cleavage in gliomas.

Glial tumor invasion: a role for the upregulation and cleavage of BEHAB/brevican

Glial tumors, gliomas, are the most common primary intracranial tumors. Their distinct ability to invade the normal surrounding tissue makes them difficult to control and nearly impossible to completely remove surgically, and it accounts for the extraordinarily high lethality associated with gliomas. The ability of these transformed glial cells to invade the normal surrounding tissue is relatively unique in the adult CNS, which under most circumstances, is inhibitory to cell movement. The extracellular matrix (ECM) can modulate, in part, the permissiveness of a tissue to cell movement. Accordingly, the ability of gliomas to modify the ECM of the CNS may mediate the invasiveness of these cells. One ECM molecule that shows dramatic upregulation in gliomas is BEHAB (brain enriched hyaluronan binding)/brevican, a brain-specific chondroitin sulfate proteoglycan. BEHAB/brevican expression is also upregulated during periods of increased glial cell motility in development and following brain injury. Experimental evidence suggests that in glioma, in addition to upregulation of BEHAB/brevican, proteolytic processing of the full-length protein also may contribute to invasion. Here, the authors present a review of the literature on glial tumor invasion by modulation of the ECM and propose a two-step model for BEHAB/brevican's role in this process.

BEHAB/brevican: an extracellular matrix component associated with invasive glioma

The extracellular matrix (ECM) has a prominent role in many physiological processes, including organ development, wound healing, and neoplastic growth and invasion. In each of these processes, changes in the composition of the matrix can lead to increased cell movement. In this review, we discuss the role of ECM components in glioma invasion, with special emphasis on the brain-specific proteoglycan, Brain-Enriched Hyaluronan Binding (BEHAB)/brevican.

cDNA cloning, chromosomal localization, and expression analysis of human BEHAB/brevican, a brain specific proteoglycan regulated during cortical development and in glioma

BEHAB (Brain Enriched HyAluronan Binding)/brevican, a brain-specific member of the lectican family of chondroitin sulfate proteoglycans (CSPGs), may play a role in both brain development and human glioma. BEHAB/brevican has been cloned from bovine, mouse and rat. Two isoforms have been reported: a full-length isoform that is secreted into the extracellular matrix (ECM) and a shorter isoform with a sequence that predicts a glycophosphatidylinositol (GPI) anchor. Here, we report the characterization of BEHAB/brevican isoforms in human brain. First, BEHAB/brevican maps to human chromosome 1q31. Second, we report the sequence of both isoforms of human BEHAB/brevican. The deduced protein sequence of full-length, secreted human BEHAB/brevican is 89.7, 83.3 and 83.2% identical to bovine, mouse and rat homologues, respectively. Third, by RNase protection analysis (RPA) we show the developmental regulation of BEHAB/brevican isoforms in normal human cortex. The secreted isoform is highly expressed from birth through 8years of age and is downregulated by 20years of age to low levels that are maintained in the normal adult cortex. The GPI isoform is expressed at uniformly low levels throughout development. Fourth, we confirm and extend previous studies from our laboratory, here demonstrating the upregulation of BEHAB/brevican mRNA in human glioma quantitatively. RPA analysis shows that both isoforms are upregulated in glioma, showing an approximately sevenfold increase in expression over normal levels. In contrast to the developmental regulation of BEHAB/brevican, where only the secreted isoform is regulated, both isoforms are increased in parallel in human glioma. The distinct patterns of regulation of expression of the two isoforms suggest distinct mechanisms of regulation of BEHAB/brevican during development and in glioma.

Brain-enriched hyaluronan binding (BEHAB)/brevican cleavage in a glioma cell line is mediated by a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) family member

Brain-enriched hyaluronan binding (BEHAB)/brevican is a brain-specific extracellular matrix protein containing a cleavage site between Glu(395)-Ser(396), which bears remarkable homology to the "aggrecanase" site in the cartilage proteoglycan aggrecan. Expression of BEHAB/brevican is dramatically increased in human gliomas, notoriously invasive tumors. Recently, we showed that the rat 9L gliosarcoma cell line, which does not express BEHAB/brevican and forms non-invasive tumors when grown as intracranial grafts, can form invasive tumors when transfected with a 5' cDNA fragment of BEHAB/brevican, but not when transfected with the full-length cDNA. In marked contrast, the highly invasive CNS-1 glioma cell line expresses and cleaves BEHAB/brevican protein when grown as an intracranial graft. These results suggest that both synthesis and cleavage of BEHAB/brevican protein may play a role in the invasiveness of gliomas. We report here, using an antibody developed to the neoepitope created by BEHAB/brevican cleavage at the Glu(395)-Ser(396) site, that the CNS-1 cells are able to cleave the protein in vitro. We characterized the CNS-1-derived cleavage activity by assaying its ability to cleave BEHAB/brevican proteoglycan, and determined that the enzyme is a constitutively expressed, secreted activity. Using a variety of protease inhibitors, reverse transcriptase-polymerase chain reaction, and specific antibodies, we determined that this activity is likely to be a member of the ADAMTS family of metalloproteinases, specifically ADAMTS4. These results suggest a novel function for ADAMTS family members in BEHAB/brevican cleavage and glioma and indicate that inhibition of ADAMTS in glioma may provide a novel therapeutic strategy.

A family of proteins implicated in axon guidance and outgrowth

Rapid progress in the identification and characterization of axon guidance molecules and their receptors has left the field poised to explore the intracellular mechanisms by which signals are transduced into growth cone responses. The TUC (TOAD/Ulip/CRMP) family of proteins has emerged as a strong candidate for a role in growth cone signaling. The TUC family members reach their highest expression levels in all neurons during their peak periods of axonal growth and are strongly down-regulated afterward. When axonal regrowth in the adult is triggered by axotomy, TUC-4 is reexpressed during the period of regrowth. Mutations in unc-33, a homologous nematode gene, lead to severe axon guidance errors in all neurons. Furthermore, the TUC family is required for the growth cone-collapsing activity of collapsin-1. An important role for the TUC family is also suggested by its high degree of interspecies amino acid sequence identity, with the rat TUC-2 protein showing 98% identity with its chick ortholog and 89% identity with its Xenopus ortholog. Information gained from the study of the TUC family will be of key importance in understanding how growth cones find their targets.

Intracranial injury acutely induces the expression of the secreted isoform of the CNS-specific hyaluronan-binding protein BEHAB/brevican

Hyaluronan (HA) plays an important role in tissue reorganization in response to injury. The mechanisms by which HA participates in these processes are likely to include HA-binding proteins. Previously, we reported the cloning and initial characterization of a central nervous system (CNS)-specific HA-binding protein, BEHAB (brain enriched hyaluronan binding), which was independently cloned in another laboratory and named brevican. BEHAB/brevican mRNA is expressed in the ventricular zone coincident with the initial proliferation and migration of glial cells and in surgical samples of human glioma, where glial-derived cells proliferate and migrate. To determine whether BEHAB/brevican is also expressed during the cellular proliferation and migration associated with CNS injury, we have examined BEHAB/brevican expression during reactive gliosis. BEHAB/brevican occurs as secreted and cell-surface, glycosylphosphatidylinositol (GPI)-anchored, isoforms. The secreted, but not the GPI-anchored, isoform is up-regulated in response to a stab wound to the adult rat brain. The temporal regulation and spatial distribution of BEHAB/brevican expression parallel the gliotic response and the expression of the intermediate filament protein nestin. The up-regulation of BEHAB/brevican in response to CNS injury suggests a role for this extracellular matrix molecule in reactive gliosis. Glial process extension, a central element in the glial response to injury, may require the reexpression of both cytoskeletal and matrix elements that are normally expressed during the glial motility seen in the immature brain.

BEHAB/brevican: a brain-specific lectican implicated in gliomas and glial cell motility

Several recent findings have advanced our understanding of the composition and function of the brain extracellular matrix (ECM). BEHAB/brevican, a recently identified CNS-specific proteoglycan, is a component of the brain ECM and is upregulated during glial cell motility. It is expressed at high levels during development, in response to injury, and in primary brain tumors. Cleavage of the BEHAB/brevican protein may increase invasion of tumor cells.

Expression of a cleaved brain-specific extracellular matrix protein mediates glioma cell invasion In vivo

Malignant gliomas (primary brain tumors) aggressively invade the surrounding normal brain. This invasive ability is not demonstrated by brain metastases of nonglial cancers. The brain-specific, brain-enriched hyaluronan binding (BEHAB)/brevican gene, which encodes an extracellular hyaluronan-binding protein, is consistently expressed by human glioma and is not expressed by tumors of nonglial origin (Jaworski et al., 1996). BEHAB/brevican can be cleaved into an N-terminal fragment that contains a hyaluronan-binding domain (HABD) and a C-terminal fragment (Yamada et al., 1995). Here, using antisera to peptides in the predicted N-terminal and C-terminal proteolytic fragments, we demonstrate that the BEHAB/brevican protein is cleaved in invasive human and rodent gliomas. A role for this protein in glioma cell invasion was tested by transfecting a noninvasive cell line with the BEHAB/brevican gene. The noninvasive 9L glioma cell was transfected with either full-length BEHAB/brevican or the HABD and tested for invasion in in vitro and in vivo invasion assays. Although both constructs increased invasion in vitro, only the HABD increased invasion by tumors growing in vivo. Experimental intracranial tumors from full-length transfectants showed no increase in invasion over control tumors, whereas tumors from HABD transfectants showed a marked potentiation of tumor invasion, producing new tumor foci at sites distant from the main tumor mass. This work demonstrates a role for a brain-specific extracellular matrix protein in glioma invasion, opening new therapeutic avenues for a uniformly fatal disease.

Neurons produce a neuronal cell surface-associated chondroitin sulfate proteoglycan

Monoclonal antibody Cat-315 recognizes a chondroitin sulfate proteoglycan (CSPG) expressed on the surface of subsets of neurons in many areas of the mammalian CNS (). The cell type-specific expression exhibited by the Cat-315 CSPG and other perineuronal net CSPGs imparts a distinct molecular surface identity to a neuron (Celio and Blumcke, 1994; Lander et al., 1997). The cell type(s) producing these surface-associated proteins and yielding this cellular diversity has remained in question. The expression of the Cat-315 CSPG in primary rat cortical cultures has permitted an examination of the cellular source of the Cat-315 antigen, as well as a determination of its spatial relationship to the neuronal surface. Live-cell labeling of primary neuronal cultures demonstrates that the Cat-315 CSPG is on the extracellular surface of neurons. Furthermore, extraction experiments demonstrate that the Cat-315 CSPG lacks a transmembrane domain and that the entire molecule is extracellular and, therefore, can be considered a constituent of brain extracellular matrix. Several lines of evidence indicate that neurons with cell surface staining produce the Cat-315 CSPG. First, neurons with cell surface staining also show intracellular Cat-315 immunoreactivity. Second, beta-xyloside or monensin, reagents that inhibit the synthesis and transport of CSPGs, increase intracellular Cat-315 immunoreactivity within neurons that express cell surface Cat-315 immunoreactivity. Third, double labeling with Cat-315 and a polyclonal antibody for the Golgi complex demonstrates a precise colocalization of the intracellular Cat-315 immunoreactivity with the Golgi. Together, these observations demonstrate that neurons contribute to the extracellular matrix of brain and that the Cat-315 CSPG is produced by the neurons that carry Cat-315 cell surface immunoreactivity.

Glial cells assemble hyaluronan-based pericellular matrices in vitro

The extracellular matrix (ECM) of the brain contains hyaluronan and proteoglycans, as does the ECM of cartilage. Aggrecan, the major proteoglycan of cartilage, forms large aggregates with hyaluronan, which then associate with the chondrocyte cell surface through an interaction with surface hyaluronan binding proteins. In culture, chondrocytes elaborate hyaluronan-proteoglycan aggregates, which form large hydrated pericellular matrices (PCMs) that can be visualized by a particle exclusion assay (Knudson and Toole: Dev Biol 112:308, 1985). It has recently been demonstrated that embryonic glial cells can also elaborate PCMs in culture (Deyst and Toole: Dev Brain Res 28:351, 1995). We demonstrate here that different classes of glial cells elaborate different types of endogenous PCMs in culture. Less differentiated glial cells, as evidenced by their immunoreactivity for nestin, elaborate larger endogenously produced PCMs than differentiated astrocytes, as defined by immunoreactivity for GFAP. This in vitro result may be a reflection of the larger volume of extracellular space present in the embryonic than in the mature brain. We show further that glial cells can incorporate cartilage aggrecan into their PCMs, and that both endogenous and aggrecan-supplemented glial PCMs are dependent on hyaluronan. In contrast, primary neurons from newborn (P0) and P1 rat cortex neither express endogenous matrices nor can assemble exogenous hyaluronan/aggrecan aggregates into PCMs. These results suggest that immature neurons may not have the ability to assemble hyaluronan-based PCMs, and they raise the possibility that neural proteoglycans associate with neuronal surfaces through a mechanism that may not directly involve hyaluronan.

A family of activity-dependent neuronal cell-surface chondroitin sulfate proteoglycans in cat visual cortex

Monoclonal antibody Cat-301 recognizes a chondroitin sulfate proteoglycan (CSPG) expressed on the extracellular surface of cell bodies and proximal dendrites of specific subsets of neurons in many areas of the mammalian CNS, including the cat visual cortex. The Cat-301 CSPG is first detected at the close of the critical period in development, a period during which the pattern of neuronal activity determines the mature synaptic circuitry and neuronal phenotype. In the cat visual cortex, dark-rearing from birth prolongs the duration of the critical period and attenuates the expression of the Cat-301 antigen, implicating the Cat-301 CSPG in the cellular mechanisms that terminate the period of synaptic plasticity. Because the Cat-301 antigen is expressed on only a limited subset of neurons, we have further examined the molecular heterogeneity among neuronal cell-surface CSPGs and have asked (1) whether other neuronal subsets carry distinct CSPGs and (2) whether the activity-dependent expression of the Cat-301 CSPG is a property generalizable to related cell-surface CSPGs. Here, we report two new monoclonal antibodies, Cat-315 and Cat-316, which together with Cat-301 define a family of at least seven related yet distinct CSPGs. These three antibodies define nonidentical subsets of neurons in the cat visual cortex. The expression of normal levels of these CSPGs is reduced by dark-rearing. Together, these data show that the family of cell-surface CSPGs is molecularly diverse, that different sets of neurons express distinct complements of cell-surface antigens, and that the regulation of CSPG expression by activity may be a general feature of neuronal cell-surface CSPGs.

Cues intrinsic to the spinal cord determine the pattern and timing of primary afferent growth

We have used organotypic cultures of embryonic rat spinal cord and dorsal root ganglia (DRG) to study the development of central projections of primary sensory afferent axons that express calcitonin gene-related peptide (CGRP). In vivo, small- and medium-diameter CGRP-positive primary afferents terminate in laminae I, II, and V of the spinal cord and do not enter the ventral horn. A similar pattern of CGRP-positive axonal projections was observed in spinal cord slices of Day 16 embryos (E16) maintained in culture for 6 days. Both intact and dissociated DRG neurons showed the same pattern of central arborization, indicating that complex intercellular interactions between DRG neurons are not required for laminar specific targeting. Furthermore, targeting to the dorsal horn and avoidance of the ventral horn was observed in isolated dorsal and ventral hemicords, suggesting that separate mechanisms mediate the avoidance of CGRP-positive axons from the ventral horn and the elaboration of the afferent arbors within the dorsal horn. CGRP-positive afferents can grow into the dorsal horn only during a brief time window. Cultures of age-matched (isochronic) DRG and spinal cord from E14, E16, and E18 animals showed the characteristic pattern of CGRP-positive axon arborization, while cultures from E20 and neonatal animals did not. Heterochronic cultures indicate that it is the age of the spinal cord, and not the age of the DRG, that determines the ability of the CGRP-positive afferents to arborize within the dorsal horn. Together these results demonstrate that cues intrinsic to the spinal cord can direct sensory projections to appropriate locations in the spinal cord.

Phospholipase C-beta1 is present in the botrysome, an intermediate compartment-like organelle, and Is regulated by visual experience in cat visual cortex

Monoclonal antibody Cat-307 identifies a 165 kDa neuronal protein expressed in the cat visual cortex during the period of sensitivity to alterations in visual experience (). Dark-rearing, which prolongs the sensitive period, also prolongs the expression of the Cat-307 protein. The Cat-307 protein localizes to an organelle, here called the botrysome (from the Greek botrys, cluster of grapes), that is located between the endoplasmic reticulum (ER) and Golgi apparatus. The botrysome is composed of small ring-shaped profiles with electron-dense coats. The size and morphology of the rings and their coats are similar to those described for ER to Golgi transport vesicles. Biochemically, the Cat-307 protein cofractionates with microsomes and partitions with subunits of the coatomer proteins that coat ER-to-Golgi transport vesicles. Partial amino acid sequencing reveals that the Cat-307 protein is phospholipase C-beta1, the G-protein-dependent phosphodiesterase that hydrolyses phosphatidylinositol 4,5 biphosphate into inositol 1,4,5 triphosphate and diacylglycerol after the stimulation of a variety of neurotransmitter receptors at the cell surface. These results suggest a role for phospholipase C-beta1 and the botrysome in developmental plasticity and provide a possible link between receptor activation at the cell surface and protein transport during neuronal development.

Identification and characterization of novel developmentally regulated proteins in rat spinal cord

We previously used 2-dimensional (2-D) gel electrophoresis to identify novel proteins that may be involved in the genesis of the mammalian nervous system [1]. Several novel proteins that were up- or down-regulated coincident with neurogenesis and neuronal migration in rat neocortex were identified. To further investigate the expression of some of these developmentally regulated proteins during a comparable period in spinal cord development, 2-D electrophoresis is used to study their regulation in the crude membrane and soluble fractions of spinal cord at embryonic day 12 (E12) and embryonic day 21 (E21). This analysis indicates that 7 of the proteins that exhibited large changes in their synthesis in cerebral cortex between embryonic day 14 (E14) and embryonic day 21 (E21) demonstrate similar up- or down-regulation during spinal cord neurogenesis. However, two proteins are restricted in their expression or developmental regulation. One of these, 667-800, appears cortex-specific, while the up-regulation of protein SC.1 appears to be spinal cord specific. Several of these proteins also appear to be enriched in both the cortex and spinal cord relative to non-neural tissues (117, 162, 182, 310 [TOAD-64], 667-800) and may be neural specific. To further characterize its expression, one of these neural-specific, up-regulated proteins, TOAD-64 (protein 310) [2-4], is studied throughout embryonic and postnatal spinal cord development using peptide-specific polyclonal antibodies. As suggested by the 2-D gel analysis and the previously reported expression pattern in cerebral cortex [3], TOAD-64 is transiently expressed in postmitotic spinal cord neurons early in their development and sharply down-regulated after the second postnatal week. In the adult spinal cord, TOAD-64 expression is remarkably restricted to a subset of primary afferents to the spinal cord. This expression pattern, coupled with its recently discovered homology to two proteins implicated in axon pathfinding in the chick and nematode [5,3], suggests that TOAD-64 may have a fundamental role in axon pathfinding.

BEHAB (brain enriched hyaluronan binding) is expressed in surgical samples of glioma and in intracranial grafts of invasive glioma cell lines

Malignant gliomas aggressively invade the surrounding normal brain, whereas brain metastases of nonglial tumors do not. The invasive behavior of gliomas may be mediated by tissue- or tumor-specific extracellular proteins. mRNA for the brain-specific extracellular brain enriched hyaluronan-binding protein (BEHAB) is not detectable in normal adult human cortex or in any nonglioma tumor examined. BEHAB is consistently expressed in surgical samples of glioma (n = 27). Glioma cell lines maintained under standard cell culture conditions or grown as s.c. tumors do not express BEHAB. When grown as intracranial grafts, glioma cell lines that invade the brain express BEHAB, whereas noninvasive cell lines do not. BEHAB is a unique and selective marker for glioma and may play a role in tumor invasion.

The divergent homeobox gene PBX1 is expressed in the postnatal subventricular zone and interneurons of the olfactory bulb

In the mammalian brain, an important phase of neurogenesis occurs postnatally in the subventricular zone (SVZ). This region consists of a heterogeneous population of cells, some mitotically active, others postmitotic. A subset of mitotically active SVZ precursor cells gives rise to a population of neurons that migrates over a long distance to their final destination, the olfactory bulb. Other SVZ precursor cells continue to proliferate or undergo cell death. The combination of genes that regulates proliferation and cell fate determination of SVZ precursor cells remains to be identified. We have used the rat homolog of the human homeobox gene PBX1 in Northern analysis and in situ hybridization studies to determine the temporal and regional localization of PBX1 expression during embryonic and postnatal rat brain development. PBX1 is expressed embryonically in the telencephalon. In addition, it is expressed at high levels postnatally in the SVZ, in the migratory pathway to the olfactory bulb, and in the layers of the olfactory bulb that are the targets of these migratory neurons. Combining in situ hybridization for PBX1 with immunostaining for markers of cell proliferation (PCNA), postmitotic neurons (class III beta-tubulin), and glia (GFAP), we show that SVZ proliferating cells and their neuronal progeny express rat PBX1 mRNA, whereas glial cells do not express detectable levels of PBX1. The expression of PBX1 in SVZ precursor cells and postmitotic neurons suggests a role for PBX1 in the generation of olfactory bulb interneurons and in mammalian neurogenesis.

The role of polysialic acid and other carbohydrate polymers in neural structural plasticity

Polysialic acid (PSA) fulfills several criteria for a molecule involved in structural plasticity, including expression in regions capable of plasticity, re-expression in structures undergoing synaptic rearrangement in the adult, downregulation following innervation, and regulation by activity. In addition, removal of PSA reduces the capacity for structural plasticity. PSA may be paradigmatic for other large polymeric carbohydrates, such as glycosaminoglycans and proteoglycans, which also are highly charged and can be extensively hydrated. These carbohydrates may affect structural plasticity by altering cell-cell and/or cell-matrix interactions by increasing intermolecular spacing through hydration.

TOAD-64, a gene expressed early in neuronal differentiation in the rat, is related to unc-33, a C. elegans gene involved in axon outgrowth

Using two-dimensional gel electrophoresis we previously identified membrane-associated proteins that are upregulated over the course of neurogenesis. One of these, TOAD-64 (Turned On After Division, 64 kDa), is expressed immediately after neuronal birth and is dramatically downregulated in the adult. The gene encoding TOAD-64 has now been cloned, and its sequence shows homology to the unc-33 gene from C. elegans, mutations in which lead to aberrations in axon outgrowth. Northern and in situ hybridization show that TOAD-64 mRNA is enriched in the nervous system and is developmentally regulated in parallel with the protein. The expression of the TOAD-64 protein and gene coincident with initial neuronal differentiation and the downregulation when the majority of axon growth is complete suggests a role in axon elaboration. Three additional lines of evidence support this possibility: TOAD-64 is upregulated following neuronal induction of P19 and PC12 cells; the protein is found in lamellipodia and filopodia of growth cones; and axotomy of the sciatic nerve induces reexpression. While the sequence of TOAD-64 lacks a signal sequence and therefore is likely to encode a cytoplasmic protein, biochemical experiments demonstrate that the protein is tightly, but noncovalently, associated with membranes. The data presented here suggest that TOAD-64 could be a central element in the machinery underlying axonal outgrowth and pathfinding, perhaps playing a role in the signal transduction processes that permit growing axons to choose correct routes and targets.

Expression of PTPH1, a rat protein tyrosine phosphatase, is restricted to the derivatives of a specific diencephalic segment

Studies to date have identified only a few proteins that are expressed in a segment-specific manner within the mammalian brain. Here we report that a nonreceptor protein tyrosine phosphatase, PTPH1, is selectively expressed in the adult thalamus. Expression of PTPH1 mRNA is detected in most, but not all, thalamic nuclei. Nuclei that are derived embryonically from the dorsal thalamus and project to the neocortex express this gene, whereas those derived from the ventral thalamus do not. PTPH1 mRNA expression is also restricted to the dorsal thalamus during development and, thus, can serve as a specific marker for the dorsal thalamic nuclei. Since the subcellular localization of PTPH1 protein is not known, its functional role is not clear. However, the restriction of its expression to the thalamic nuclei that have thalamocortical connections suggests that PTPH1 may play a role in the maintenance of these connections or in determining the physiological properties of thalamic relay nuclei.

Early postmitotic neurons transiently express TOAD-64, a neural specific protein

To identify proteins involved in the early development of the mammalian cerebral cortex, we previously used two-dimensional gels to compare proteins synthesized at different stages in corticogenesis in the embryonic rat at embryonic day 14 (E14), E17, and E21. During this period, the cortex develops from a morphologically homogeneous population of proliferative precursor cells into a complex structure containing a diverse array of terminally differentiated neurons. Several proteins are up-regulated coincident with the generation of postmitotic neurons. Here we describe the purification, partial amino acid sequencing, and characterization of one of these proteins, TOAD-64 (Turned On After Division; 64 kDa), using polyclonal antisera to two synthetic peptides from the protein. This analysis reveals that TOAD-64 is a 64,000 Da protein that increases in abundance over the period of corticogenesis and then subsequently decreases to very low levels in the adult. The protein is neural specific and is expressed by postmitotic neurons as they begin their migration out of the ventricular zone into the developing cortical plate. It is expressed in advance of most other neuronal proteins. Progenitor cells do not express TOAD-64. Therefore, this protein is a marker for postmitotic cells that have made a commitment to a neuronal phenotype. The extremely early expression, the relative abundance in newly born neurons, as well as the restriction in expression to the period of initial neuronal differentiation suggest that TOAD-64 may be a key structural protein for early neuronal function.

Identification of nuclear proteins that are developmentally regulated in embryonic rat brain

To identify nuclear proteins that might play a role in the acquisition of neuronal phenotype, two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) was used to analyze nuclear proteins expressed over the course of embryonic rat brain development. Metabolically labeled rat brain nuclear proteins from embryonic day 14 (E14) were compared with proteins from embryonic day 20 (E20). Over this period, the rat brain develops from a collection of relatively homogeneous precursor cells into a complex structure containing many different classes of neurons. Computer-assisted analysis of 2D-PAGE fluorograms identified 11 proteins that show increases in their rate of synthesis between E14 and E20. Twenty proteins that consistently appear at E20 are not detectable on fluorograms of E14 nuclear proteins, even after long exposures, and thus may be considered to appear de novo. Fifty-eight proteins show consistent down-regulation between E14 and E20, and of these, 19 are not detectable on fluorograms of E20 nuclear proteins. The electrophoretic properties of many of these proteins suggest that they are previously unreported, developmentally regulated nuclear proteins. Some of the developmentally regulated, brain-enriched nuclear proteins identified here may play a role in regulating the expression of neural genes important for cellular differentiation in the mammalian CNS.

Expression and distribution of phosphorylated MAP1B in growing axons of cultured hippocampal neurons

Microtubule associated proteins (MAPs) interact with tubulin to modulate neurite stability and growth during development. The phosphorylated form of one of these MAPs, MAP1B (MAP1B-P) is hypothesized to be of particular importance for the regulation of neurite outgrowth. To investigate the mechanisms by which MAP1B and MAP1B-P contribute to this regulation, we used a new antibody against an isoform of MAP1B-P to determine its pattern of expression during neuronal development in vitro. We examined cultured hippocampal neurons because these provide a well-established system to evaluate the development of axons and dendrites. MAP1B, MAP1B-P and MAP2 colocalized to the cell bodies and minor processes during the first 24 hours of culture, but MAP1B-P also extended well into the growth cones. As neurite outgrowth and differentiation proceeded, MAP1B and MAP1B-P became localized to the cell bodies and axons, and MAP2 to the cell bodies and dendrites. After 3 days, MAP1B-P declined in the cell body and was segregated to the distal axon; MAP1B remained in the cell body, but was also concentrated in the distal axon. Over 5-9 days in culture, MAP1B-P levels decreased and became undetectable; MAP1B levels decreased later (19-23 days). MAP2 levels, however, remained high through the entire culture period in cell bodies and dendrites. These results are consistent with the hypothesis that MAP1B-P plays an important role in the initiation and elongation of axons by regulating the dynamics of microtubules near the growth cone: MAP1B-P expression is greatest during the period of active neurite extension, is particularly prominent in growth cones where axon outgrowth is most active, and decreases along with the decline in active axon extension.

The CNS-specific hyaluronan-binding protein BEHAB is expressed in ventricular zones coincident with gliogenesis

Hyaluronan (HA) is a ubiquitous component of extracellular matrices, and in several systems it plays a central role in regulating cellular proliferation and differentiation. Cell, or tissue,-specific functions of HA are likely to be mediated by cell, or tissue,-specific HA-binding proteins. We previously hyaluronan-binding protein from rat and cat (Jaworski et al., 1994). In view of the potential role of HA in neural differentiation, we examined the expression of BEHAB during late embryonic and early postnatal development of the rat. BEHAB is expressed at very high levels in ventricular zones throughout the neuraxis. Expression is first detected at embryonic day 15 (E15) in the spinal cord, and is detected at progressively more rostral levels at later ages. BEHAB expression, like other features of neural development, follows both caudal-to-rostral and ventral-to-dorsal gradients. The timing of BEHAB expression parallels the timing of the generation of glial cells. In all areas of the CNS examined, BEHAB expression begins after the peak of neurogenesis and coincident with gliogenesis. The regulation of proliferation and differentiation by HA in other tissues, together with the expression of BEHAB in zones of mitotic activity coincident with the generation of glia, suggests that the extracellular matrix protein encoded by BEHAB could play a role in the generation or differentiation of CNS glia.

Seven protein tyrosine phosphatases are differentially expressed in the developing rat brain

Regulation of protein function through tyrosine phosphorylation is critical in the control of many developmental processes, such as cellular proliferation and differentiation. Growing evidence suggests that tyrosine phosphorylation also regulates key events in neural development. Although a large body of data has demonstrated that protein tyrosine kinases play an important role in neural development, much less is known about their counterparts, protein tyrosine phosphatases (PTPases). Using polymerase chain reaction (PCR) with degenerate primers and a neonatal rat cortex cDNA library, we have identified seven PTPases expressed in the developing rat brain. Four of these are transmembrane PTPases: LAR, LRP, RPTP gamma, and CPTP1. Three are nonreceptor PTPases: PTP-1, P19-PTP, and SHP. Northern hybridization analysis demonstrates that only CPTP1 is preferentially expressed in neural tissues, whereas the others are found abundantly in nonneural tissues as well as in the brain. Within the embryonic and early postnatal brain, the seven PTPases have overlapping, yet unique, distributions. For example, LAR mRNA is highly expressed by both proliferating and postmitotic cells in the cerebral cortex at embryonic day 17 and in all layers of the cortex at postnatal day 4. In contrast, RPTP gamma mRNA is expressed by postmitotic neurons in the embryo and predominantly by neurons in the superficial layers of the postnatal cortex. Several of the PTPases examined here are expressed at very high levels in the embryonic cortical plate and postnatal neocortex, including the subplate and subventricular zone. The spatial and temporal regulation of PTPase gene expression suggests that these PTPases have important roles in signal transduction during early neuronal differentiation and neural development.

Identification of proteins downregulated during the postnatal development of the cat visual cortex

To identify proteins that play a role in the development of the mammalian visual cortex, we have used an immunosuppression and rapid immunization strategy to generate monoclonal antibodies to antigens that are present in area 17 of the cat during the peak of cortical plasticity but are downregulated near the end of the plastic period. We report here the immunohistochemical and immunobiochemical characterization of six monoclonal antibodies that identify antigens preferentially expressed in the cat visual cortex at 5 weeks of age. Monoclonal antibodies Cat-305 and Cat-306 detect three immunoreactive elements that are not present at birth but are present at 5 weeks. The majority of immunoreactivity is associated with a population of cells in the white matter that are absent at 15 weeks of age. At both 5 and 15 weeks, a very small number of neurons show intense immunoreactivity throughout all processes, resembling that achieved with a Golgi stain. In addition, a diffuse band of immunoreactivity in layer IV is largely restricted to cortical areas 17 and 18. Cat-307 recognizes a 150 kDa soluble protein present in small cytoplasmic inclusions. These cytoplasmic "dot" are present in all layers, but are most prominent in layer V. Cat-307 immunoreactivity is present at birth and is completely downregulated by 15 weeks. Cat-104 and Cat-105 recognize a 200 kDa insoluble protein present at birth and at 5 weeks, but markedly downregulated by 15 weeks. At birth, the white matter, subplate, and layer I are most densely labeled, while at 5 weeks labeling is densest in layers II, III, and V. Cat-402 recognizes a number of high-molecular-weight antigens that are differentially expressed at 5 and 15 weeks of age. Stained non-neuronal cells that resemble protoplasmic astrocytes are present in all layers at both 5 and 15 weeks. At 5 weeks, but not at birth or 15 weeks, darkly immunoreactive radial processes are observed that run through the full depth of the cortex. We show here that immunoreactivity for several different monoclonal antibodies is detected selectively during the period of maximal developmental plasticity. The results demonstrate that the cat visual cortex at 5 weeks of age is molecularly distinct from the cortex at 15 weeks.

BEHAB, a new member of the proteoglycan tandem repeat family of hyaluronan-binding proteins that is restricted to the brain

Hyaluronan (HA) is a ubiquitous component of the extracellular matrix of all tissues. In the mammalian central nervous system (CNS) HA is present throughout development and into adulthood. While the functions of HA are likely to be mediated by HA-binding proteins, no cell or tissue specific HA-binding proteins have been reported. In an effort to characterize the composition of the extracellular matrix of the CNS, we sought to identify neural HA-binding proteins. We report here the isolation and characterization of a cDNA with a high degree of sequence homology to members of the proteoglycan tandem repeat (PTR) family of HA-binding proteins. Unlike other HA-binding proteins, the expression of this cDNA is restricted to the CNS. We propose the name BEHAB, Brain Enriched HyAluronan Binding protein, for this gene. The expression of BEHAB mRNA is developmentally regulated; expression is first detected in the late embryonic period and peaks during the first two postnatal weeks. In the embryo, BEHAB is expressed at highest levels in mitotically active cells. The sequence of BEHAB has long stretches of identity between rat and cat, suggesting that the encoded protein is functionally important. The size and sequence of BEHAB are consistent with the possibility that it could serve a function like link protein, stabilizing interactions between HA and brain proteoglycans. These observations suggest that existence of other tissue specific HA-binding proteins.

Electrical activity in the neuromuscular unit can influence the molecular development of motor neurons

During the first few weeks of postnatal life spinal motor neurons develop electrophysiological, morphological, and molecular features that are characteristic of adult motor neurons. To understand how the acquisition of the mature neuronal phenotype is regulated, we have examined the expression of the motor neuron cell surface proteoglycan recognized by monoclonal antibody Cat-301 in the hamster. Previously we found that Cat-301 immunoreactivity is not present on motor neurons at birth and that by the end of the second postnatal week all motor neurons are Cat-301-positive. Surgical and pharmacological lesion studies have shown that the onset of Cat-301 expression depends upon input from both large-diameter primary afferents and from supraspinal afferents. Once the Cat-301 proteoglycan is expressed on motor neurons, its continued expression is independent of these inputs. These studies suggested that motor neuron maturation depends upon the coordination of several afferent inputs during the first postnatal weeks of life. Our previous studies could not address whether segmental and descending afferents (i) provide a chemical signal (such as a trophic factor) or (ii) confer a pattern of neuronal activity upon motor neurons that then results in the expression of the Cat-301 proteoglycan. The present experiments examine the role of electrical activity in motor neuron maturation. In normal animals, all sciatic motor neurons are Cat-301-positive by Postnatal Day 19 (P19). Chronic application of the sodium channel blocker, tetrodotoxin (TTX), to the sciatic nerve in neonatal animals reduces the percentage of Cat-301-positive motor neurons found at P21 by one-third. This reduction is not due to a nonspecific inhibition of all protein synthesis, because the expression of two other motor neuron antigens proceeds normally in TTX-treated neonates. Blockade of neuromuscular transmission in neonates by Botulinus toxin A also reduces the percentage of Cat-301-positive motor neurons. Cat-301 expression is not tied simply to neuronal activity, because chronic application of TTX to the sciatic nerve, or Botulinus toxin A to muscles, in the adult does not reduce Cat-301 expression. These findings indicate that electrical activity generated within the neuromuscular unit in early postnatal life can influence the acquisition of mature molecular properties by motor neurons.

Protein tyrosine phosphatases expressed in the developing rat brain

Previous studies of the developing nervous system have shown that cell-cell and cell-matrix interactions are involved in a variety of processes such as the proliferation, migration, and differentiation of neurons. While many cell-surface molecules have been identified, the signal transduction mechanisms through which they modify cellular responses are poorly understood. Recent studies have described a new and large family of enzymes, protein tyrosine phosphatases (PTPases), that may play a key role in transduction of cell surface events. Opposing the actions of protein tyrosine kinases (PTKs), PTPases can determine the state of tyrosine phosphorylation of a protein and regulate its function. Within the family of PTPases, two subgroups have been characterized: low-molecular-weight cytoplasmic (nonreceptor) PTPases and high-molecular-weight transmembrane (receptor) PTPases. Many receptor PTPases have fibronectin type III and/or Ig-like domains in their extracellular domains, suggesting that they have dual functions: cell adhesion and signal transduction. Such molecules may play a role in cellular recognition events that mediate the accurate assembly of the nervous system. Using polymerase chain reaction with degenerate primers and a neonatal rat cortex cDNA library, we have identified a number of putative PTPase domains expressed in brain. Three are characterized here. These three sequences are most abundantly expressed in the developing cortex and so are named cortex-enriched protein tyrosine phosphatases (CPTPs) 1, 2, and 3. CPTP1 and CPTP3 show sequence homology to receptor PTPases and detect multiple high-molecular-weight mRNAs that are expressed preferentially in the developing CNS. Analysis of a longer cDNA indicates that CPTP1 and CPTP3 are the first and second phosphatase domains of a single receptor PTPase. CPTP2 identifies a single, smaller mRNA species with sequence homology to nonreceptor PTPases. Within the CNS, mRNAs detected by all three CPTPs are expressed at highest levels during prenatal and early postnatal days and are downregulated in the adult. In situ hybridization demonstrates that the CPTPs are expressed by progenitor cells and developing neurons. The spatial and temporal regulation of CPTPs suggests that they may play a role in neuronal development.

Expression of the mas proto-oncogene in the rat hippocampal formation is regulated by neuronal activity

The mas proto-oncogene encodes a seven membrane-spanning G-protein-coupled receptor which is activated by angiotensins. In the postnatal and adult rat, mas mRNA is specifically expressed at high levels in hippocampal neurons. We report here using in situ hybridization and RNase protection that brief seizure episodes lead to a significant and transient increase in mas mRNA in the hippocampus. Increased levels of mas transcripts were detected 2, 4, and 6 h following seizure. By 24 h post seizure, baseline levels were detected. The presumed subsequent increase of the mas receptor protein may contribute to anatomical and physiological plasticity that is associated with intense activation of hippocampal pathways.

Activity-dependent structural changes during neuronal development

Activity during the early postnatal period can have a pronounced effect on the structure of neurons in the central nervous system. Recent studies in the cat visual system and in the vertebrate and invertebrate neuromuscular system, have provided new insights into the cellular and molecular features of this process.

N-methyl-D-aspartate receptors are transiently expressed in the developing spinal cord ventral horn

Quantitative receptor autoradiography was used to map the distribution of N-methyl-D-aspartate (NMDA) receptors in the developing rat spinal cord. Three different specific ligands, which label partially overlapping subpopulations of NMDA receptors, were used: an agonist (L-[3H]glutamate), a noncompetitive antagonist ([3H]MK-801), and a competitive antagonist ([3H]CGP-39653). In the adult, NMDA receptors labeled with all three ligands are restricted to the substantia gelatinosa in the spinal dorsal horn. In marked distinction, at postnatal day 7 NMDA receptors labeled with L-[3H]glutamate and [3H]MK-801 are present throughout the spinal gray matter. NMDA receptors in the neonatal spinal ventral horn have a higher affinity for L-[3H]glutamate than those in the adult substantia gelatinosa. Over the second and third postnatal weeks, NMDA receptors are lost from all areas of the spinal gray matter except for the substantia gelatinosa. Neonatal NMDA receptors identified with [3H]CGP-39653 are restricted to the substantia gelatinosa. These results show that the immature ventral horn contains a subpopulation of NMDA receptors and raise the possibility that motor neurons transiently express NMDA receptors in early postnatal life. Ventral horn NMDA receptors may be a component of the mechanisms by which the mature phenotype of motor neurons is acquired through activity-dependent processes. The loss of NMDA receptors over the course of development may play a role in limiting the period of motor neuron plasticity.