The limbic system-associated membrane protein is an Ig superfamily member that mediates selective neuronal growth and axon targeting

Tlx-1 and Tlx-3 homeobox gene expression in cranial sensory ganglia and hindbrain of the chick embryo: markers of patterned connectivity

Recent evidence suggests that in vertebrates the formation of distinct neuronal cell types is controlled by specific families of homeodomain transcription factors. Furthermore, the expression domains of a number of these genes correlates with functionally integrated neuronal populations. We have isolated two members of the divergent T-cell leukemia translocation (HOX11/Tlx) homeobox gene family from chick, Tlx-1 and Tlx-3, and show that they are expressed in differentiating neurons of both the peripheral and central nervous systems. In the peripheral nervous system, Tlx-1 and Tlx-3 are expressed in overlapping domains within the placodally derived components of a number of cranial sensory ganglia. Tlx-3, unlike Tlx-1, is also expressed in neural crest-derived dorsal root and sympathetic ganglia. In the CNS, both genes are expressed in longitudinal columns of neurons at specific dorsoventral levels of the hindbrain. Each column has distinct anterior and/or posterior limits that respect inter-rhombomeric boundaries. Tlx-3 is also expressed in D2 and D3 neurons of the spinal cord. Tlx-1 and Tlx-3 expression patterns within the peripheral and central nervous systems suggest that Tlx proteins may be involved not only in the differentiation and/or survival of specific neuronal populations but also in the establishment of neuronal circuitry. Furthermore, by analogy with the LIM genes, Tlx family members potentially define sensory columns early within the developing hindbrain in a combinatorial manner.

Regulation of thalamic neurite outgrowth by the Eph ligand ephrin-A5: implications in the development of thalamocortical projections

The cerebral cortex is parcellated into different functional domains that receive distinct inputs from other cortical and subcortical regions. The molecular mechanisms underlying the specificity of connections of cortical afferents remain unclear. We report here that the Eph family tyrosine kinase receptor EphA5 and the ligand ephrin-A5 may play a key role in the exclusion of the limbic thalamic afferents from the sensorimotor cortex by mediating repulsive interactions. In situ hybridization shows that the EphA5 transcript is expressed at high levels in both cortical and subcortical limbic regions, including the frontal cortex, the subiculum, and the medial thalamic nuclei. In contrast, ephrin-A5 is transcribed abundantly in the sensorimotor cortex. Consistent with the complementary expression, the ligand inhibited dramatically the growth of neurites from neurons isolated from the medial thalamus but was permissive for the growth of neurites from lateral thalamic neurons, which is primarily nonlimbic. Similarly, the growth of neurites from Eph-A5-expressing neurons isolated from the subiculum was inhibited by ephrin-A5. Our studies suggest that the Eph family ligand ephrin-A5 serves as a general inhibitor of axonal growth from limbic neurons, which may serve to prevent innervation of inappropriate primary sensorimotor regions, thus contributing to the generation of specificity of thalamic cortical afferents.

Induction of presynaptic reexpression of an adhesion protein in lamina II after dorsal root deafferentation in adult rat spinal cord

Limbic system-associated membrane protein (LAMP), a 64-kDa membrane protein, is an axon guidance adhesion molecule expressed by neurons in limbic system-related areas of the CNS. During development, LAMP is expressed on growing axons, growth cones, and their target neurons, but in adults it is restricted to membranes of somata and dendrites. In the adult spinal cord, LAMP immunoreactivity is found only on neurons of lamina II, lamina X, and the intermediolateral cell column and its ultrastructural localization is entirely postsynaptic. We studied changes in the expression of LAMP in lamina II of adult rat spinal cord after L1-S2 dorsal rhizotomy, a procedure that partially deafferents lamina II neurons and induces axonal sprouting by spared systems in lamina II. At the light microscopic level, LAMP immunoreactivity in lamina II was decreased in density at 3, 10, and 60 days postoperatively. This decrease in immunoreactivity suggests that LAMP expression by lamina II neurons may normally be regulated by specific afferent activity. Ultrastructurally, in control lamina II and after deafferentation in both control and deafferented lamina II at 3 and 60 days postoperatively, LAMP expression was restricted to postsynaptic membranes. Ten days after deafferentation, however, when axons are actively sprouting, LAMP was expressed on both axonal and postsynaptic membranes. The reexpression of LAMP on axonal profiles after deafferentation may identify axons that undergo sprouting in response to deafferentation.

Effects of growth factors and estrogen on the development of septal cholinergic neurons from the rat

Cholinergic neurons of the septum are preferentially subject to degeneration in Alzheimer's disease. There is evidence that nerve growth factor, basic fibroblast growth factor, insulin-like growth factors, and estrogen all have effects on survival of this specific population of neurons at risk. We used a bilaminar culturing method to grow embryonic septal neurons from the rat in the presence of a separate glial plane but in the absence of serum. These neurons were treated with a number of factors, and neurite development of cholinergic neurons was assessed. Basic fibroblast growth factor and estrogen altered the number of primary neurites, number of secondary neurites, and mean total neurite lengths, while none of the other factors affected these end points. This would suggest a mechanism for the effects of these factors on memory.

Identification of caveolin and caveolin-related proteins in the brain

Caveolae are 50-100 nm, nonclathrin-coated, flask-shaped plasma membrane microdomains that have been identified in most mammalian cell types, except lymphocytes and neurons. To date, multiple functions have been ascribed to caveolae, including the compartmentalization of lipid and protein components that function in transmembrane signaling events, biosynthetic transport functions, endocytosis, potocytosis, and transcytosis. Caveolin, a 21-24 kDa integral membrane protein, is the principal structural component of caveolae. We have initiated studies to examine the relationship of detergent-insoluble complexes identified in astrocytes to the caveolin-caveolae compartment detected in cells of peripheral tissues. Immunolocalization studies performed in astrocytes reveal caveolin immunoreactivity in regions that correlate well to the distribution of caveolae and caveolin determined in other cell types, and electron microscopic studies reveal multiple clusters of flask-shaped invaginations aligned along the plasma membrane. Immunoblot analyses demonstrate that detergent-insoluble complexes isolated from astrocytes are composed of caveolin-1alpha, an identification verified by Northern blot analyses and by the cloning of a cDNA using reverse transcriptase-PCR amplification from total astrocyte RNA. Using a full-length caveolin-1 probe, Northern blot analyses suggest that the expression of caveolin-1 may be regulated during brain development. Immunoblot analyses of detergent-insoluble complexes isolated from cerebral cortex and cerebellum identify two immunoreactive polypeptides with apparent molecular weight and isoelectric points appropriate for caveolin. The identification of caveolae microdomains and caveolin-1 in astrocytes and brain, as well as the apparent regulation of caveolin-1 expression during brain development, identifies a cell compartment not detected previously in brain.

Expression of basigin, a member of the immunoglobulin superfamily, in the mouse central nervous system

Basigin (Bsg) is a transmembrane glycoprotein belonging to the immunoglobulin superfamily. Chicken Bsg (HT7/neurothelin/ 5A11) is expressed in neuroblasts, but disappears from neurons after a specific stage of cytodifferentiation, and becomes restrictedly expressed in the capillary endothelium in the adult brain. We show herein by means of in situ hybridization that Bsg mRNA was expressed in neuroblasts in 13.5 day old mouse embryos. In the adult mouse, Bsg was differentially expressed in subregions of the brain. Strong Bsg expression was detected in the limbic system, including the olfactory system, hippocampal formation, septal area, amygdala, thalamic anterior nuclei, hypothalamus, mesencephalic tegmentum, entorhinal cortex, and cingulate gyrus. Bsg was also intensely expressed in the retinal neuronal layers, the Vth layer of the cerebral neocortex, Purkinje cells of the cerebellum, several nuclei of the brain stem, and the gray matter of the spinal cord. Although in situ hybridization showed a weak signal in the brain capillary endothelium, protein expression of Bsg was strong enough to be detected by immunohistochemistry. Northern blot analysis confirmed the strong expression of Bsg in the central nervous system. Taking into account that Bsg knockout mice exhibit abnormalities in behavior, but a normal blood-brain barrier function, the present findings suggest that Bsg functions actively in neuronal interactions in the central nervous system.

Limbic system-associated membrane protein (LAMP) induces neurite outgrowth and intracellular Ca2+ increase in primary fetal neurons

The ability of cell adhesion molecules (CAMs) to transduce cell surface signals into intracellular responses is critical for developing neurons, particularly during axonal pathfinding and targeting. It has been suggested that different CAMs can promote neuronal outgrowth via activation of common neuronal CAM-specific second-messenger pathways, although the elements involved in this cascade could differ. Limbic system-associated membrane protein (LAMP), a member of the Ig superfamily, is a molecule that promotes cell adhesion and neurite outgrowth from specific populations of fetal neurons. In the present study, we show that LAMP can induce several types of calcium (Ca2+) signals. Neurite outgrowth is promoted if fetal hippocampal neurons are grown on lamp-transfected CHO cells. This LAMP-induced outgrowth of neurons is mediated in part through activation of L-type Ca channels. Application of soluble LAMP to cultures of fetal hippocampal neurons caused a sustained (up to 60 min) elevation of intracellular Ca2+ as measured by fluo-3 fluorescence on a confocal microscope. The number of responding hippocampal neurons was initially low, but increased with age in culture and the [Ca2+]i elevation was only partially decreased by an L-type Ca(2+)-channel blocker. In contrast, at all times in culture, only a small fraction of neurons from visual cortex responded to LAMP application and only with transient elevation of cytosolic Ca2+ (< 15 min). Based on these observations, LAMP appears to function primarily through homophilic interactions and acts in part by modulating intracellular Ca2+ levels during neurite outgrowth by increasing the Ca2+ influx through L-type calcium channels, but has additional effects on intracellular Ca2+ signaling at later developmental stages.

Neuronal circuits are subdivided by differential expression of type-II classic cadherins in postnatal mouse brains

A number of type-II classic cadherin cell-cell adhesion molecules are expressed in the brain. To investigate their roles in brain morphogenesis, we selected three type-II cadherins, cadherin-6 (cad6), -8 (cad8) and -11 (cad11), and mapped their expressions in the forebrain and other restricted regions of postnatal mouse brains. In the cerebral cortex, each cortical area previously defined was delineated by a specific combinatorial expression of these cadherins. The thalamus and other subcortical regions of the forebrain were also subdivided by differential expression of the three cadherins; e.g., the medial geniculate body expressed only cad6; the ventral posterior thalamic nucleus, cad6/cad11; and the anteroventral thalamic nucleus, cad6/cad8. Likewise, in the olivocerebellar system, each subdivision of the inferior olive expressed a unique set of the three cadherins, and the cerebellar cortex had parasagittal stripes of cad8/cad11 expressions. Close analysis of these cadherin expression patterns revealed that they are correlated with neuronal connection patterns. Examples of these correlations include that cad6 delineates the auditory projection system, cad6/cad8/ cad11 are expressed by part of the Papez circuit, and cad6/cad8 are expressed by subdivisions of the olivo-nuclear circuit. Together with the recent finding that the cadherin adhesion system is localized in synaptic junctions, our findings support the notion that cadherin-mediated cell-cell adhesion plays a role in selective interneuronal connections during neural network formation.

Thy-1 and AvGp50 signal transduction complex in the avian nervous system: c-Fyn and G alpha i protein association and activation of signalling pathways

We have previously reported the isolation of two distinct populations of detergent resistant membrane complexes (DRMC's) from day-old chick brain (Henke et al.: J Neurosci Res 45:617-630, 1996). We now show that the glycosylphosphatidylinositol-anchored proteins Thy-1 and AvGp50 are associated in a signalling complex with c-Fyn, the heterotrimeric G alpha i subfamily members G alpha i-3, G alpha z, and G alpha o, alpha and beta tubulin, and a number of other phosphoproteins in immunocomplexes isolated from both populations of DRMC's. Activation of this signalling complex via Thy-1 monoclonal antibody incubation with chick forebrain cells, elicited a decrease in total phosphoprotein profile and tyrosine kinase activity present in DRMC fractions isolated from these cells, while AvGp50 and control antibodies had no effect. Down-regulation of the DRMC phosphoprotein profile was accompanied by an increase in the Thy-1-associated signalling complex, suggesting that activation of this complex initiates the decreases seen in overall DRMC kinase activity. This signalling complex provides the basis for GPI-anchored protein-mediated signal transduction within the unique plasma membrane domains represented by DRMC's.

Dynamic expression suggests multiple roles of the eph family receptor brain-specific kinase (Bsk) during mouse neurogenesis

The eph family ligands and receptors have been implicated in mediating topographic neuron-target interactions. We recently isolated Bsk, a new member of the eph family receptors, and showed that it is expressed primarily in the brain. To investigate the role of Bsk in the development of the nervous system, we examined the temporal and spatial patterns of Bsk expression using in situ hybridization. We report here that Bsk expression exhibits dynamic changes during embryogenesis. In early embryos, Bsk is widely transcribed in the nervous system, including the forebrain, midbrain, hindbrain and spinal cord. Bsk expression in the midbrain, hindbrain and spinal cord, however, gradually decreases while in the forebrain increases over time. By embryonic day 18, the most intense Bsk expression was found in the limbic system. High levels of the expression in the limbic system persisted throughout post-natal development and remained stable in the adult up to 24 month. The topography of Bsk expression is in the form of gradients in several regions of the brain, including the lateral septum, spinal cord, as well as the hippocampus. Selective expression was also observed in Purkinje cells. Our findings on the topography of Bsk expression provide support to potential roles of Bsk in topographic projection. Our analyses further suggest that there may be other novel functions of Bsk in early neurogenesis in addition to potential roles in topographic mapping.

Cloning and characterization of a neural cell recognition molecule on axons of the retinotectal system and spinal cord

Immunoglobulin superfamily molecules in the brain are involved in distinct aspects of nervous system histogenesis, for example neuronal migration and axonal growth. To identify novel members of this superfamily in the chick nervous system, we developed a polymerase chain reaction-based approach making use of sequence motifs of immunoglobulin-like domains. In the present study, we report the molecular cloning of three isoforms, the biochemical analysis and the immunohistochemical characterization of one of the proteins identified in this screen. This molecule has 91% sequence identity with the limbic system-associated membrane protein (LAMP) characterized in the rat and is therefore referred to as the chicken homologue of the latter (chLAMP). The molecule is a glycosylphosphatidyl-inositol-anchored 60 kDa protein with three immunoglobulin-like domains and contains 40% N-linked carbohydrate. We identify three different mRNA forms of chLAMP and show that two forms with distinct 5'-termini are differentially transcribed in neural development. In addition, we demonstrate using a fusion protein expressed in eukaryotic cells that chLAMP has homophilic binding activity. The protein was found on a subset of axons in the central and peripheral nervous system and is likely to be involved in specific cell-cell interactions in neurohistogenesis.

Mechanisms specifying area fate in cortex include cell-cycle-dependent decisions and the capacity of progenitors to express phenotype memory

Progenitor cells in the early developing cerebral cortex produce neurons destined for discrete functional areas in response to specific inductive signals. Using lineage analysis, we show that cortical progenitor cells at different fetal ages retain the memory of an area-specific inductive signal received in vivo, even though they may pass through as many as two cell cycles in the absence of the signal in culture. When exposed to inductive signals in vitro, only those progenitors that progress through at least one complete cell cycle alter their areal phenotype. Our findings suggest that induction of an areal phenotype is linealy inherited, with the phenotype specified prior to the final cell cycle.

AvGp50, a predominantly axonally expressed glycoprotein, is a member of the IgLON's subfamily of cell adhesion molecules (CAMs)

We have previously reported a 50 kDa glycoprotein (AvGp50) expressed specifically in the chick nervous system [Hancox, K.A., Sheppard, A.M. and Jeffrey, P.L., Characterisation of a novel glycoprotein (AVGP50) in the avian nervous system, with a monoclonal antibody, Dev. Brain Res., 70 (1992) 25-37], and we present its molecular characterization. A PCR fragment was generated following sequencing of peptide and N-terminal fragments derived from purified AvGp50. A 1.58 kb clone (pUEX762) containing the 5'-UTR, the entire coding sequence and a short 3'-UTR was then isolated from a chick embryonic day 18 forebrain library. The deduced amino acid sequence encodes a 338 amino acid peptide containing a 31 amino acid signal peptide at the N-terminal and a 19 amino acid phosphatidylinositol glycan linkage sequence at the C-terminal. The mature protein contains three C2-immunoglobulin-like domains and a glycosyl phosphatidylinositol anchor and shares significant homology to other members of the immunoglobulin superfamily, including neural cell adhesion molecule (N-CAM), myelin-associated glycoprotein (MAG) and the Drosophila protein Amalgam. AvGp50 exhibits highest sequence identity to a recently classified subgroup of the immunoglobulin superfamily (IgLONs - immunoglobulin LAMP, OBCAM and neurotrimin - classified by Pimenta et al. [Pimenta, A.F., Zhukareva, V., Barbe, M.F., Reinoso, B.S., Grimley, C., Henzel, W., Fischer, I. and Levitt, P., The limbic system-associated membrane protein is an Ig superfamily member that mediates selective neuronal growth and axon targeting, Neuron, 15 (1995) 287-297], comprising the opioid binding cell adhesion molecule (OBCAM), neurotrimin and the limbic system-associated membrane protein (LAMP) suggesting that AvGp50 is a member of this subgroup. AvGp50 is expressed predominantly on the cell surface of axons, in particular Purkinje cell and granule cell axons in the cerebellum. In cerebellar and forebrain neuronal cultures, protein expression is exclusively located at the cell surface. Despite its cell surface localization, AvGp50 does not directly influence the outgrowth of neurons from explant cultures from ED8 to ED10 chick forebrain, prompting the suggestion that AvGp50 may act in later maturational events.

Neuropathological studies of brain tissue in schizophrenia

Postmortem neuropathological investigations in the last half decade provide increasing evidence compatible with a neurodevelopmental defect in schizophrenia. Basic and clinical data support hypotheses suggesting that disturbances in neurodevelopment in schizophrenia may involve the cortical subplate and a theorized second trimester "window of vulnerability". The focus of this paper is on (1) selected methodological issues involved in the collection, analyses and preservation of human postmortem brain tissue; (2) a review of evidence showing morphological defects particularly in prefrontal cortical regions of the schizophrenic brain; and (3) potential future research directions.

A procedure for in situ hybridization combined with retrograde labeling of neurons: application to the study of cell adhesion molecule expression in Dil-labeled rat pyramidal neurons

We have devised a simple method that combines retrograde labeling of projecting neurons and in situ hybridization histochemistry to examine mRNA expression in the retrogradely labeled neurons. First, projecting neurons were retrogradely labeled in vivo by injection of the lipophilic neuronal tracer Dil. The fluorescence of the labeled neurons in the brain slices was photoconverted into stable DAB precipitate by green light illumination. The slices were cut into thinner sections and processed for detection of specific mRNA by in situ hybridization. Using this highly sensitive method, we demonstrate here that the corticospinal tract neurons in newborn rats express mRNA for the cell adhesion molecule L1. TAG-1 mRNA was not detected in these neurons. Therefore, the present method provides an important tool to study the molecular expression of projection neurons during the development of neuronal circuitry.

cDNA cloning and chromosomal localization of the human telencephalin and its distinctive interaction with lymphocyte function-associated antigen-1

We have isolated cDNA encoding human telencephalin (TLN), a brain segment-specific neuronal adhesion molecule. Human TLN comprises an NH2-terminal signal peptide, an extracellular region with nine Ig-like domains, a single transmembrane region, and a COOH-terminal cytoplasmic tail. The NH2-terminal five Ig-like domains of TLN were closely related to those of intercellular adhesion molecules (ICAMs)-1 and -3. The TLN gene was mapped to the human chromosome 19p13.2, where the ICAM-1, -3, and -4 (LW) genes are located. Furthermore, we observed lymphocyte function-associated antigen-1 (LFA-1)-mediated adhesion of HL-60 cells on recombinant TLN protein, as well as on ICAM-1. However, the interaction of TLN with LFA-1 on HL-60 cells was divalent cation-independent and phorbol 12-myristate 13-acetate stimulation-independent. We conclude that TLN is a unique neuronal member of ICAM subgroup of the Ig superfamily and propose a novel type of interaction between the Ig superfamily molecule and integrin, which does not require the activation of integrin. TLN on the surface of telencephalic neurons may be a target molecule in the brain for LFA-1-expressing microglia and leukocytes in physiological or pathological conditions.

The expression pattern of the transcription factor Phox2 delineates synaptic pathways of the autonomic nervous system

Many transcription factors, and most prominently among them, homeodomain proteins, are expressed in specific groups of cells in the developing nervous system in patterns that suggest their involvement in neural fate determination. How various aspects of neural identity are controlled by such transcription factors, or sets of them, is still mostly unknown. It has been shown previously that Phox2 is such a homeodomain protein, expressed exclusively in differentiated groups of neurons or their precursors, and that its expression correlated with that of the noradrenaline synthesis enzyme dopamine-beta-hydroxylase. Here we confirm this striking correlation at the single-cell level with the use of an anti-Phox2 antibody. Moreover, we uncover a second, nonmutually exclusive correlative clue to the Phox2 expression pattern: a high proportion of Phox2-expressing cells are involved in, or located in areas involved in, synaptic circuits, i.e., that of the medullary control reflexes of autonomic functions. This suggests that Phox2 could be involved in the establishment of these circuits.

NCAM is essential for axonal growth and fasciculation in the hippocampus

The neural cell adhesion molecule (NCAM), probably the best characterized and most abundant cell adhesion molecule on neurons, is thought to be a major regulator of axonal growth and pathfinding. Here we present a detailed analysis of these processes in mice deficient for all NCAM isoforms, generated by gene targeting. The hippocampal mossy fiber tract shows prominent expression of polysialylated NCAM and the generation of new axonal projections throughout life. Focusing on this important intrahippocampal connection, we demonstrate that in the absence of NCAM, fasciculation and pathfinding of these axons are strongly affected. In addition we show alterations in the distribution of mossy fiber terminals. The phenotype is more severe in adult than in young animals, suggesting an essential role for NCAM in the maintenance of plasticity in the mature nervous system.

The alpha 1 subunit of soluble guanylyl cyclase is expressed prenatally in the rat brain

The mRNA encoding the alpha 1 subunit of soluble guanylyl cyclase (alpha 1sGC) was identified in a different-display screening for genes spatially and temporally regulated during the development of fetal rat brain. The initially isolated fragment of the 3' untranslated region was used for in situ hybridization and to produce full-length cDNA clones by hybridization screening of cDNA libraries and by RACE (rapid amplification of cDNA ends), respectively. In situ hybridization analysis that alpha 1sGC was absent at embryonic day 12 (E12), but by E14-E15, the forebrain exhibited dense expression in the developing striatum, medial cerebral wall containing the presumptive hippocampus, cerebellar neuroepithelium, and roof plate. Weaker expression was observed in the septum, epithalamus, ventral thalamus, pineal gland and retina. This pattern is largely maintained and refined at E18, with additional expression domains in the olfactory tubercle, nucleus accumbens, zona incerta and neocortex. During early postnatal development, the adult pattern is expressed, as previously reported. The unexpected, early expression of alpha 1sGC, in conjunction with the known absence of its heterodimeric partner, the beta subunit of sGC, from the developing rodent brain during fetal ages raises potentially novel functional roles of the alpha 1 subunit during ontogeny, and might imply the existence of an alternative beta subunit specific for the prenatal brain.

The molecular biology of axon guidance

Neuronal growth cones navigate over long distances along specific pathways to find their correct targets. The mechanisms and molecules that direct this pathfinding are the topics of this review. Growth cones appear to be guided by at least four different mechanisms: contact attraction, chemoattraction, contact repulsion, and chemorepulsion. Evidence is accumulating that these mechanisms act simultaneously and in a coordinated manner to direct pathfinding and that they are mediated by mechanistically and evolutionarily conserved ligand-receptor systems.

Expression of the mRNAs encoding the limbic system-associated membrane protein (LAMP): II. Fetal rat brain

The limbic system-associated membrane protein (LAMP) is a 64-68 kDa neuronal surface glycoprotein expressed in cortical and subcortical regions of the limbic system of the adult and developing rat central nervous system (CNS). LAMP is a member of the immunoglobulin superfamily of cell adhesion molecules with three Ig domains and is highly conserved between rat and human. In this study, the temporal and spatial pattern of lamp gene expression during fetal rat development was analyzed by using Northern blot analysis and in situ hybridization. In Northern blot analysis, two lamp mRNA transcripts, 1.6 kb and 8.0 kb, identical in size to those present in the adult rat nervous system, were detected in developing neural tissue. In situ hybridization analysis showed close correlation, though not identity, between the expression of lamp mRNAs and the distribution of LAMP in limbic regions of the developing rat CNS, indicative of a more complex regulation of gene expression than was previously thought to be the case. The expression of lamp mRNAs is first detected on about embryonic day (E) 13. The hybridization signal is not seen in the proliferative ventricular zone at any level of the neuraxis, indicating that lamp is expressed in postmitotic neurons. In the cerebral cortex, lamp mRNAs are expressed in limbic cortical regions, such as the perirhinal cortex, prefrontal cortex, and cingulate cortex. In the hippocampus, the hybridization signal is observed in Ammon's horn by E18. The neostriatum, amygdaloid complex, and most hypothalamic areas express lamp mRNAs from early stages (E13-E14) in a pattern consistent with the onset of neurogenesis. The emerging patterns of lamp expression at the outset are similar to those seen in adult hypothalamus and dorsal thalamus. Although the hybridization signal is observed in some nonlimbic areas, including midbrain and hindbrain structures, intense labeling is evident in more classic limbic regions. The high levels of expression of lamp in limbic regions, beginning in early developmental stages, combined with the results of previous functional in vitro and in vivo studies, support a role for LAMP as a recognition molecule involved in the formation of limbic connections.

Expression of the mRNAs encoding the limbic system-associated membrane protein (LAMP): I. Adult rat brain

The search for molecular markers common to neural structures that are functionally related has become an attractive strategy for neurobiologists interested in identifying mechanisms involved in the formation of patterned connections. One such molecule is the limbic system-associated membrane protein (LAMP), a 64-68 kDa glycoprotein that is expressed in the soma and dendrites of subpopulations of adult neurons in the brain that are functionally associated with classic limbic structures. Such patterned molecular specificity is established prenatally; LAMP is detected during development on the surface of neurons, axonal membranes and pathfinding growth cones. This molecule has now been cloned (lamp) and has been shown to be highly conserved in rat and human. It is a new immunoglobulin superfamily member that has three Ig domains and a glycosyl-phosphatidylinositol (GPI) anchor to the cell membrane. In this study, the distribution of the lamp transcript in the adult rat brain was determined by using in situ hybridization. Generally, the distribution of lamp corresponds well with that of the LAMP protein. Within the cerebral cortex, the transcript is more abundant in areas that are associated with learning/memory and viscerosensory tasks. It is less abundant in somatic sensory and motor areas. The lamp transcript is also ubiquitous in the basal forebrain, amygdala, and preopticohypothalamic areas. In short, the lamp transcript is expressed heavily in areas of the forebrain and diencephalon that have been classically considered limbic and sparsely or moderately in nonlimbic midbrain and hindbrain regions. Correlative analysis of the connectivity patterns of the regions that express greater amounts of the transcript is consistent with a stronger limbic-associated function relative to the regions expressing less lamp. These quantitative differences may be significant in determining the function of LAMP in the adult brain.

Structure/function relationships of axon-associated adhesion receptors of the immunoglobulin superfamily

Evidence is accumulating that axonal members of the Ig superfamily (IgSF) interact in a complex manner with other axonal Ig-like proteins and with proteins of the extracellular matrix. Studies investigating the structure/function relationships of these proteins have highlighted the importance of Ig-like domains near the amino terminus (N-proximal) as both necessary and sufficient for homophilic and heterophilic binding. Although efforts have been made in the past year to correlate the structure and neurite-outgrowth-promoting ability of axonal IgSF members, this work is still at an early stage.

cDNA cloning and structural analysis of the human limbic-system-associated membrane protein (LAMP)

The limbic-system-associated membrane protein (LAMP) is a 64-68-kDa neuronal surface glycoprotein distributed in cortical and subcortical regions of the limbic system. The human LAMP gene was cloned by RT-PCR using human cerebral cortex mRNA and oligodeoxyribonucleotide (oligo) primers derived from the rat lamp cDNA sequence. The human and rat LAMP cDNAs showed 94% identity at the nucleotide (nt) level, and the encoded 338-amino-acid (aa) polypeptides shared 99% sequence identity. All the important features of LAMP were conserved: (i) the deduced aa sequence reflecting a glycosyl-phosphatidylinositol (GPI)-anchor, (ii) eight putative N-linked glycosylation sites, and (iii) conserved pairs of Cys forming three internal repeats characteristic of the immunoglobulin superfamily (IgSF). Northern blot analysis indicated the presence of two mRNA transcripts in the human brain of a size identical to those identified in adult rat brain. These data indicate that LAMP is a highly conserved new member of the IgSF which, together with the opioid-binding cell adhesion molecule (OBCAM) and neurotrimin, comprises a new subfamily that has been designated as IgLONs. With a unique distribution in limbic structures, LAMP may play an important role in limbic system development and function, as suggested by previous in vitro and in vivo functional studies.

Limbic system-associated membrane protein (LAMP) in primate amygdala and hippocampus

The distribution of the limbic system-associated membrane protein in the amygdaloid complex and hippocampal formation of cynomolgus monkeys (Macaca fascicularis) was studied with immunohistochemical procedures. A highly complex and heterogeneous staining pattern is encountered in the macaque amygdala. The basal, lateral, and accessory basal nuclei display the most intense immunostaining with local heterogeneities. The lateral division of the central nucleus also stains intensely, whereas the medial division of the central nucleus and the medial nucleus are more weakly stained. The dorsal division of the bed nucleus-amygdala continuum (extended amygdala) is strongly immunoreactive. The hippocampus displays the strongest immunoreactivity encountered so far in the primate brain. The intensity of the immunostaining is highest in the cornu Ammonis (Ammon's horn; CA1-CA3 fields) and gradually decreases toward the dentate gyrus or the subicular area. In the hippocampus proper, the stratum radiatum, the pyramidal layer, the stratum oriens, and the alveus all display intense immunoreactivity. The immunostaining is much less prominent in the dentate gyrus, whose granule cell layer is completely devoid of labeling. In the subicular area, there is a lateromedial decreasing gradient in immunostaining intensity, the subiculum being moderately stained and the parasubiculum weakly stained. These results reveal that the limbic system-associated membrane protein labels structures that form the core of the limbic system in primates. Within each of these structures, however, the labeling is highly heterogeneous and appears to be confined to specific functional domains.