Mammalian telencephalic neurons express a segment-specific membrane glycoprotein, telencephalin

ICAM5 as a Novel Target for Treating Cognitive Impairment in Fragile X Syndrome

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability, resulted from the silencing of the Fmr1 gene and the subsequent loss of fragile X mental retardation protein (FMRP). Spine dysgenesis and cognitive impairment have been extensively characterized in FXS; however, the underlying mechanism remains poorly understood.

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability, resulted from the silencing of the Fmr1 gene and the subsequent loss of fragile X mental retardation protein (FMRP). Spine dysgenesis and cognitive impairment have been extensively characterized in FXS; however, the underlying mechanism remains poorly understood. As an important regulator of spine maturation, intercellular adhesion molecule 5 (ICAM5) mRNA may be one of the targets of FMRP and involved in cognitive impairment in FXS. Here we show that in Fmr1 KO male mice, ICAM5 was excessively expressed during the late developmental stage, and its expression was negatively correlated with the expression of FMRP and positively related with the morphological abnormalities of dendritic spines. While in vitro reduction of ICAM5 normalized dendritic spine abnormalities in Fmr1 KO neurons, and in vivo knockdown of ICAM5 in the dentate gyrus rescued the impaired spatial and fear memory and anxiety-like behaviors in Fmr1 KO mice, through both granule cell and mossy cell with a relative rate of 1.32 ± 0.15. Furthermore, biochemical analyses showed direct binding of FMRP with ICAM5 mRNA, to the coding sequence of ICAM5 mRNA. Together, our study suggests that ICAM5 is one of the targets of FMRP and is implicated in the molecular pathogenesis of FXS. ICAM5 could be a therapeutic target for treating cognitive impairment in FXS.

SIGNIFICANCE STATEMENT Fragile X syndrome (FXS) is characterized by dendritic spine dysgenesis and cognitive dysfunctions, while one of the FMRP latent targets, ICAM5, is well established for contributing both spine maturation and learning performance. In this study, we examined the potential link between ICAM5 mRNA and FMRP in FXS, and further investigated the molecular details and pathological consequences of ICAM5 overexpression. Our results indicate a critical role of ICAM5 in spine maturation and cognitive impairment in FXS and suggest that ICAM5 is a potential molecular target for the development of medication against FXS.

Urokinase-type plasminogen activator (uPA) promotes ezrin-mediated reorganization of the synaptic cytoskeleton in the ischemic brain

Synaptic repair in the ischemic brain is a complex process that requires reorganization of the actin cytoskeleton. Ezrin, radixin, and moesin (ERM) are a group of evolutionarily conserved proteins that link the plasma membrane to the actin cytoskeleton and act as scaffolds for signaling transduction. Urokinase-type plasminogen activator (uPA) is a serine proteinase that upon binding to the urokinase-type plasminogen activator receptor (uPAR) catalyzes the conversion of plasminogen into plasmin on the cell surface and activates intracellular signaling pathways. Early studies indicate that uPA and uPAR expression increase during the recovery phase from an ischemic stroke and that uPA binding to uPAR promotes neurorepair in the ischemic brain. The in vitro and in vivo studies presented here show that either the release of neuronal uPA or treatment with recombinant uPA induces the local synthesis of ezrin in the synapse and the recruitment of β3-integrin to the postsynaptic density (PSD) of cerebral cortical neurons by a plasminogen-independent mechanism. We found that β3-integrin has a double effect on ezrin, inducing its recruitment to the PSD via the intercellular adhesion molecule-5 (ICAM-5) and its subsequent activation by phosphorylation at Thr-567. Finally, our data indicate that by triggering the reorganization of the actin cytoskeleton in the postsynaptic terminal, active ezrin induces the recovery of dendritic spines and synapses that have been damaged by an acute ischemic stroke. In summary, our data show that uPA-uPAR binding promotes synaptic repair in the ischemic brain via ezrin-mediated reorganization of the actin cytoskeleton in the postsynaptic terminal.

Activity dependent CAM cleavage and neurotransmission

Spatially localized proteolysis represents an elegant means by which neuronal activity dependent changes in synaptic structure, and thus experience dependent learning and memory, can be achieved. In vitro and in vivo studies suggest that matrix metalloproteinase and adamalysin activity is concentrated at the cell surface, and emerging evidence suggests that increased peri-synaptic expression, release and/or activation of these proteinases occurs with enhanced excitatory neurotransmission. Synaptically expressed cell adhesion molecules (CAMs) could therefore represent important targets for neuronal activity-dependent proteolysis. Several CAM subtypes are expressed at the synapse, and their cleavage can influence the efficacy of synaptic transmission through a variety of non-mutually exclusive mechanisms. In the following review, we discuss mechanisms that regulate neuronal activity-dependent synaptic CAM shedding, including those that may be calcium dependent. We also highlight CAM targets of activity-dependent proteolysis including neuroligin and intercellular adhesion molecule-5 (ICAM-5). We include discussion focused on potential consequences of synaptic CAM shedding, with an emphasis on interactions between soluble CAM cleavage products and specific pre- and post-synaptic receptors.

Subcellular localization of intercellular adhesion molecule-5 (telencephalin) in the visual cortex is not developmentally regulated in the absence of matrix metalloproteinase-9

The telencephalon-associated intercellular adhesion molecule-5 (telencephalin; ICAM-5) regulates dendritic morphology in the developing brain. In vitro studies have shown that ICAM-5 is found predominantly within dendrites and immature dendritic protrusions, with reduced expression in mushroom spines, suggesting that ICAM-5 downregulation is critical for the maturation of synaptic structures. However, developmental expression of ICAM-5 has not been explored in depth at the ultrastructural level in intact brain tissue. To investigate the ultrastructural localization of ICAM-5 with transmission electron microscopy, we performed immunoperoxidase histochemistry for ICAM-5 in mouse visual cortex at postnatal day (P)14, a period of intense synaptogenesis, and at P28, when synapses mature. We observed the expected ICAM-5 expression in dendritic protrusions and shafts at both P14 and P28. ICAM-5 expression in these dendritic protrusions decreased in prevalence with developmental age to become localized predominantly to dendritic shafts by P28. To understand better the relationship between ICAM-5 and the endopeptidase metalloproteinase-9 (MMP-9), which mediates ICAM-5 cleavage following glutamate activation during postnatal development, we also explored ICAM-5 expression in MMP-9 null animals. This analysis revealed a similar expression of ICAM-5 in dendritic elements at P14 and P28; however, an increased prevalence of ICAM-5 was noted in dendritic protrusions at P28 in the MMP-9 null animals, indicating that, in the absence of MMP-9, there is no developmental shift in ICAM-5 subcellular localization. Our ultrastructural observations shed light on possible functions mediated by ICAM-5 and their regulation by extracellular proteases.

Interactions between ICAM-5 and β1 integrins regulate neuronal synapse formation

Intercellular adhesion molecule-5 (ICAM-5) is a dendrite-specific adhesion molecule, which functions in both the immune and nervous systems. ICAM-5 is the only negative regulator that has been identified for maturation of dendritic spines so far. Shedding of the ICAM-5 ectodomain promotes spine maturation and enhances synaptic activity. However, the mechanism by which ICAM-5 regulates spine development remains poorly understood. In this study, we found that ablation of ICAM5 expression resulted in a significant increase in the formation of synaptic contacts and the frequency of miniature excitatory post-synaptic currents, an indicator of pre-synaptic release probability. Antibodies against ICAM-5 and β1 integrins altered spine maturation. Furthermore, we found that β1 integrins serve as binding partners for ICAM-5. β1 integrins were immunoprecipitated with ICAM-5 from mouse brain and the binding region in ICAM-5 was localized to the two first Ig domains. β1 integrins were juxtaposed to filopodia tips at the early stage of synaptic formation, but as synapses matured, β1 integrins covered the mushroom spines. Loss of β1 integrins from the pre-synaptic sites affected the morphology of the post-synaptic structures. ICAM-5 ectodomain cleavage decreased or increased when the interaction between ICAM-5 and β1 integrins was potentiated or weakened, respectively, using antibodies. These results suggest that the interaction between ICAM-5 and β1 integrins is important in formation of functional synapses.

Structure, Expression, and Function of ICAM-5

Cell adhesion is of utmost importance in normal development and cellular functions. ICAM-5 (intercellular adhesion molecule-5, telencephalin, TLN) is a member of the ICAM family of adhesion proteins. As a novel cell adhesion molecule, ICAM-5 shares many structural similarities with the other members of IgSF, especially the ICAM subgroup; however, ICAM-5 has several unique properties compared to the other ICAMs. With its nine extracellular Ig domains, ICAM-5 is the largest member of ICAM subgroup identified so far. Therefore, it is much more complex than the other ICAMs. The expression of ICAM-5 is confined to the telencephalic neurons of the central nervous system whereas all the other ICAM members are expressed mostly by cells in the immune and blood systems. The developmental appearance of ICAM-5 parallels the time of dendritic elongation and branching, and synapse formation in the telencephalon. As a somatodendrite-specific adhesion molecule, ICAM-5 not only participates in immune-nervous system interactions, it could also participate in neuronal activity, Dendrites' targeting signals, and cognition. It would not be surprising if future investigations reveal more binding partners and other related functions of ICAM-5.

Asn54-linked glycan is critical for functional folding of intercellular adhesion molecule-5

Intercellular adhesion molecule-5 (ICAM-5, telencephalin) is a dendritically polarized type I membrane glycoprotein, and promotes dendritic filopodia formation. Although we have determined the N-glycan structures of ICAM-5 in a previous report, their function is unknown. Here, we produced fifteen ICAM-5 gene constructs, in which each potential N-glycosylation site was mutated, to elucidate the function of the N-glycans of ICAM-5, and observed the effects of transfection of them on a neuronal cell line, Neuro-2a (N2a). Only the N54Q mutant, which is the mutant for the most N-terminal glycosylation site, failed to induce filopodia-like protrusions in N2a cells. Immunofluorescence staining and cell surface biotinylation revealed that N54Q ICAM-5 was confined to the ER and also could not be expressed on the cell surface. This is further supported by the biochemical evidence that almost all N-glycans of N54Q ICAM-5 were digested by Endo glycosidase H and peptide:N-glycanase, indicating that almost all of them retain high-mannose-type structures in ER. In additon, it also failed to form disulfide bonds or functional protein complexes. The stable transformants of N54Q ICAM-5 showed retarded cell growth, but it was interesting that there was no apparent ER stress, because the mutant was sequentially degraded via ER associated degradation pathway by comparing the susceptibilities of the responses to various inhibitors of this pathway in wild-type and N54Q ICAM-5 transfectants. Taken together, the Asn(54)-linked glycan is necessary for normal trafficking and function of ICAM-5, but is unassociated with ER-associated degradation of it.

Genetically encoded dendritic marker sheds light on neuronal connectivity in Drosophila

In recent years, Drosophila melanogaster has emerged as a powerful model for neuronal circuit development, pathology, and function. A major impediment to these studies has been the lack of a genetically encoded, specific, universal, and phenotypically neutral marker of the somatodendritic compartment. We have developed such a marker and show that it is effective and specific in all neuronal populations tested in the peripheral and central nervous system. The marker, which we name DenMark (Dendritic Marker), is a hybrid protein of the mouse protein ICAM5/Telencephalin and the red fluorescent protein mCherry. We show that DenMark is a powerful tool for revealing novel aspects of the neuroanatomy of developing dendrites, identifying previously unknown dendritic arbors, and elucidating neuronal connectivity.

Behavioral analyses of transgenic mice harboring bipolar disorder candidate genes, IMPA1 and IMPA2

The inositol depletion hypothesis proposes the inhibition of IMPase (myo-inositol monophosphatase) by lithium, a mood stabilizer, as a mechanism of lithium's efficacy. This hypothesis predicts that the upregulation of this biochemical pathway may underlie the pathophysiology of bipolar disorder. In favor of this idea, IMPA2 encoding IMPase is a candidate susceptibility gene for bipolar disorder and that the risk-conferring single nucleotide polymorphisms enhance the promoter activity of IMPA2. However, it is yet unknown whether such upregulation has a biological role in bipolar disorder. To address this issue, we generated transgenic mice for the two IMPase genes (IMPA1 and IMPA2). The expression levels of the transgene were robust in IMPA2 Tg lines, but moderate in IMPA1 Tg lines, when compared to those of endogenous proteins. The transgenic mice behaved normally under drug-naïve conditions, and did not exhibit signs for manic change when an antidepressant amitriptyline was administrated. Interestingly, the male transgenic mice for IMPA2 exhibited a lithium-resistant phenotype in the forced swim test. The current study, as a whole, did not support a substantial role of the upregulation of IMPase in bipolar disorder, although the lithium-insensitivity trait seen in IMPA2 transgenic mice might represent some aspect relevant to the inositol depletion hypothesis.

Modulation of synaptic transmission and plasticity by cell adhesion and repulsion molecules

Adhesive and repellent molecular cues guide migrating cells and growing neurites during development. They also contribute to synaptic function, learning and memory in adulthood. Here, we review the roles of cell adhesion molecules of the immunoglobulin superfamily (Ig-CAMs) and semaphorins (some of which also contain Ig-like domains) in regulation of synaptic transmission and plasticity. Interestingly, among the seven studied Ig-CAMs, the neuronal cell adhesion molecule proved to be important for all tested forms of hippocampal plasticity, while its associated unusual glycan polysialic acid is necessary and sufficient part for synaptic plasticity only at CA3-CA1 synapses. In contrast, Thy-1 and L1 specifically regulate long-term potentiation (LTP) at synapses formed by entorhinal axons in the dentate gyrus and cornu ammonis, respectively. Contactin-1 is important for long-term depression but not for LTP at CA3-CA1 synapses. Analysis of CHL1-deficient mice illustrates that at intermediate stages of development a deficit in a cell adhesion molecule is compensated but appears as impaired LTP during early and late postnatal development. The emerging mechanisms by which adhesive Ig-CAMs contribute to synaptic plasticity involve regulation of activities of NMDA receptors and L-type Ca2+ channels, signaling via mitogen-activated protein kinase p38, changes in GABAergic inhibition and motility of synaptic elements. Regarding repellent molecules, available data for semaphorins demonstrate their activity-dependent regulation in normal and pathological conditions, synaptic localization of their receptors and their potential to elevate or inhibit synaptic transmission either directly or indirectly.

Dorsal telencephalon-specific RA-GEF-1 knockout mice develop heterotopic cortical mass and commissural fiber defect

Neural migration defects lead to various types of human malformations of cortical development including subcortical band heterotopia, which shows formation of a secondary cortical plate beneath the primary cortex and is typically caused by mutation of the DCX (doublecortin) gene. Subcortical band heterotopia is usually associated with mental retardation and epilepsy. We previously discovered RA-GEF-1 as a guanine nucleotide exchange factor (GEF) for Rap1 small GTPase. Here we have analysed its in-vivo role in formation of the adult cerebral cortex by using telencephalon-specific RA-GEF-1 conditional knockout (cKO) mice, generated by mating RA-GEF-1(flox/flox) mice with Emx1-cre knockin mice. RA-GEF-1 cKO mice showed severe defects in their brain structures including an ectopic cortical mass underlying a relatively normal cortex. The ectopic cortical mass lacked the normal six-layered lamination but preserved the subcortical connectivity as revealed by retrograde tracing. Further, RA-GEF-1 cKO mice exhibited a lower threshold for the induction of epileptic seizures. These phenotypes have a resemblance to those of human subcortical band heterotopia. In addition, the agenesis of anterior commissures, the dorsal hippocampus commissure, the corpus callosum and the enlargement of the lateral ventricles were observed in cKO mice. Our findings suggest a crucial function of RA-GEF-1 in neural migration.

An unusual allosteric mobility of the C-terminal helix of a high-affinity alphaL integrin I domain variant bound to ICAM-5

Integrins are cell surface receptors that transduce signals bidirectionally across the plasma membrane. The key event of integrin signaling is the allosteric regulation between its ligand-binding site and the C-terminal helix (alpha7) of integrin's inserted (I) domain. A significant axial movement of the alpha7 helix is associated with the open, active conformation of integrins. We describe the crystal structure of an engineered high-affinity I domain from the integrin alpha(L)beta(2) (LFA-1) alpha subunit in complex with the N-terminal two domains of ICAM-5, an adhesion molecule expressed in telencephalic neurons. The finding that the alpha7 helix swings out and inserts into a neighboring I domain in an upside-down orientation in the crystals implies an intrinsically unusual mobility of this helix. This remarkable feature allows the alpha7 helix to trigger integrin's large-scale conformational changes with little energy penalty. It serves as a mechanistic example of how a weakly bound adhesion molecule works in signaling.

Interaction between telencephalin and ERM family proteins mediates dendritic filopodia formation

Dendritic filopodia are long, thin, actin-rich, and dynamic protrusions that are thought to play a critical role as a precursor of spines during neural development. We reported previously that a telencephalon-specific cell adhesion molecule, telencephalin (TLCN) [intercellular adhesion molecule-5 (ICAM-5)], is highly expressed in dendritic filopodia, facilitates the filopodia formation, and slows spine maturation. Here we demonstrate that TLCN cytoplasmic region binds ERM (ezrin/radixin/moesin) family proteins that link membrane proteins to actin cytoskeleton. In cultured hippocampal neurons, phosphorylated active forms of ERM proteins are colocalized with TLCN in dendritic filopodia, whereas alpha-actinin, another binding partner of TLCN, is colocalized with TLCN at surface membranes of soma and dendritic shafts. Expression of constitutively active ezrin induces dendritic filopodia formation, whereas small interference RNA-mediated knockdown of ERM proteins decreases filopodia density and accelerates spine maturation. These results indicate the important role of TLCN-ERM interaction in the formation of dendritic filopodia, which leads to subsequent synaptogenesis and establishment of functional neural circuitry in the developing brain.

alpha-Actinin-dependent cytoskeletal anchorage is important for ICAM-5-mediated neuritic outgrowth

Intercellular adhesion molecule-5 (ICAM-5, telencephalin) is a dendrite-expressed membrane glycoprotein of telencephalic neurons in the mammalian brain. By deletion of the cytoplasmic and membrane-spanning domains of ICAM-5, we observed that the membrane distribution of ICAM-5 was determined by the cytoplasmic portion. Therefore we have characterized the intracellular associations of ICAM-5 by using a bacterially expressed glutathione S-transferase (GST) fusion protein encompassing the cytoplasmic part of ICAM-5. One of the main proteins in the neuronal cell line Paju that bound to the ICAM-5 cytodomain was alpha-actinin. ICAM-5 expressed in transfected Paju cells was found in alpha-actinin immunoprecipitates, and ICAM-5 colocalized with alpha-actinin both in Paju cells and in dendritic filopodia and spines of primary hippocampal neurons. We were also able to coprecipitate alpha-actinin from rat brain homogenate. Binding to alpha-actinin appeared to be mediated mainly through the N-terminal region of the ICAM-5 cytodomain, as the ICAM-5(857-861) cytoplasmic peptide (KKGEY) mediated efficient binding to alpha-actinin. Surface plasmon resonance analysis showed that the turnover of the interaction was rapid. In a mutant cell line, Paju-ICAM-5-KK/AA, the distribution was altered, which implies the importance of the lysines in the interaction. Furthermore, we found that the ICAM-5/alpha-actinin interaction is involved in neuritic outgrowth and the ICAM-5(857-861) cytoplasmic peptide induced morphological changes in Paju-ICAM-5 cells. In summary, these results show that the interaction between ICAM-5 and alpha-actinin is mediated through binding of positively charged amino acids near the transmembrane domain of ICAM-5, and this interaction may play an important role in neuronal differentiation.

Telencephalin slows spine maturation

Dendritic filopodia are highly dynamic structures, and morphological maturation from dendritic filopodia to spines is intimately associated with the stabilization and strengthening of synapses during development. Here, we report that telencephalin (TLCN), a cell adhesion molecule belonging to the Ig superfamily, is a negative regulator of spine maturation. Using cultured hippocampal neurons, we examined detailed localization and functions of TLCN in spine development and synaptogenesis. At early stages of synaptogenesis, TLCN immunoreactivity gradually increased and was present in dendritic shafts and filopodia. At later stages, TLCN tended to be excluded from mature spine synapses in which PSD-95 (postsynaptic density-95) clusters were apposed to presynaptic synaptophysin clusters. To elucidate the function of TLCN in spine maturation, we analyzed the dendrite morphology of TLCN-overexpressing and TLCN-deficient neurons. Overexpression of TLCN caused a dramatic increase in the density of dendritic filopodia and a concomitant decrease in the density of spines. Conversely, TLCN-deficient mice showed a decreased density of filopodia and an acceleration of spine maturation in vitro as well as in vivo. These results demonstrate that TLCN normally slows spine maturation by promoting the filopodia formation and negatively regulating the filopodia-to-spine transition. In addition, we found that spine heads of mature neurons were wider in TLCN-deficient mice compared with wild-type mice. Thus, the preservation of immature synapses by TLCN may be an essential step for refinement of functional neural circuits in the telencephalon, that take charge of higher brain functions such as learning, memory, and emotion.

A novel phenylalanine-based targeting signal directs telencephalin to neuronal dendrites

Neurons sort out a variety of functional molecules to appropriate subcellular destinations. Telencephalin (TLCN; intercellular adhesion molecule-5) is a cell adhesion molecule specifically localized to somatodendritic membranes in the telencephalic neurons. Here, we established a new in vivo strategy to analyze neuronal sorting mechanisms by ectopic expression of molecules of interest in the cerebellar Purkinje cells of transgenic mice. By using this system, we identified a novel dendritic targeting determinant in the cytoplasmic tail region of TLCN. A full-length TLCN ectopically expressed in the Purkinje cells was localized exclusively to dendrites but not to axons. In contrast, a deletion of cytoplasmic C-terminal 12 amino acids (residues 901-912) or a point mutation of Phe905 to Ala abrogated the dendrite-specific targeting with appearance of the truncated and point-mutated TLCN in both axons and dendrites. Furthermore, an addition of the C-terminal 17 amino acids (residues 896-912) of TLCN to an unrelated molecule (CD8) was sufficient for its specific targeting to dendrites in several types of neurons. Because the C-terminal region of TLCN does not contain any canonical dendritic targeting sequences such as the tyrosine-based motif or the dileucine motif, this study suggests a novel mechanism of protein trafficking to the dendritic compartment of neurons.

ICAM-3, a ligand for DC-SIGN, was duplicated from ICAM-1 in mammalian evolution, but was lost in the rodent genome

ICAM-3 is a DC-SIGN ligand that is constitutively expressed on resting leukocytes, and is thus an important molecule for the first immune response. But, ICAM-3 has not been isolated form rodents. Thus, we compare the ICAM gene clusters in human, dog, mouse, and rat. ICAM-1, -4, -5 and -3 are located close to one another on the same chromosome and show genomic synteny in human and dog. Almost the same ICAM gene clusters were found in rodent genome, but only the ICAM-3 was not present. A phylogenetic tree plotting the cDNAs of human, dog, mouse, rat, and bovine suggested that ICAM-3 was made from a duplication of ICAM-1. Thus, ICAM-3 arose from ICAM-1 in the mammalian evolution, but was lost in the rodent's genome. Our study suggests the different immune response in the rodents in comparison with other mammals.

Polarized targeting of IgLON cell adhesion molecule OBCAM to dendrites in cultured neurons

Opioid-binding cell adhesion molecule (OBCAM) belongs to the immunoglobulin superfamily CAMs and shows a dendritically polarized distribution in hypothalamic magnocellular neurons. In the present study, the cellular localization of OBCAM was monitored in cultured cortical and hippocampal neurons to examine its polarized distribution. Double labeling immunofluorescence microscopy after fixation showed only faint OBCAM immunoreactivity in the neuronal somata during the early stages of culture, whereas the immunoreactivity was strong in MAP2-positive somata and dendrites of fully polarized neurons after longer culture. Moreover, the immunoreactivity for OBCAM showed a punctate pattern in the dendrites similar to the immunostaining pattern of synapsin I. High resolution revealed close apposition with only a partial overlap of synapsin I and OBCAM immunoreactivities, suggesting the synaptic localization of OBCAM to the dendrites. When the fully polarized neurons were reacted with anti-OBCAM antibody before fixation, OBCAM immunoreactivity became stronger on the dendritic surface than the somatic surface. Extracellular immunoreactivity was eliminated with phosphatidylinositol-specific phospholipase C and this immunoreactivity resisted extraction with the nonionic detergent Triton X-100 at 4 degrees C, indicating that OBCAM is attached to the rafts via a glycosylphosphatidyl inositol anchor. These results indicate that OBCAM is efficiently targeted to the dendritic surface of fully polarized cortical and hippocampal neurons. OBCAM is, hence, concluded to be a dendrite-associated CAM in cortical and hippocampal neurons as in hypothalamic magnocellular neurons.

Expression of specific glycoconjugates in both primary and secondary olfactory pathways in BALB/C mice

Binding of cell surface carbohydrates to their receptors specifically promotes axon growth and synaptogenesis in select regions of the developing nervous system. In some cases these interactions depend upon cell-cell adhesion mediated by the same glycoconjugates present on the surface of apposing cells or their processes. We have previously shown that the plant lectin Dolichos biflorus agglutinin (DBA) binds to a subpopulation of mouse primary olfactory neurons whose axons selectively fasciculate prior to terminating in the olfactory bulb. In the present study, we investigated whether these glycoconjugates were also expressed by postsynaptic olfactory neurons specifically within the olfactory pathway. We show here for the first time that DBA ligands were expressed both by a subset of primary olfactory neurons as well as by the postsynaptic mitral/tufted cells in BALB/C mice. These glycoconjugates were first detected on mitral/tufted cell axons during the early postnatal period, at a time when there is considerable synaptogenesis and synaptic remodelling in the primary olfactory cortex. This is one of the few examples of the selective expression of molecules in contiguous axon tracts in the mammalian nervous system. These results suggest that glycoconjugates recognized by DBA may have a specific role in the formation and maintenance of neural connections within a select functional pathway in the brain.

Intercellular adhesion molecule-5 induces dendritic outgrowth by homophilic adhesion

Intercellular adhesion molecule-5 (ICAM-5) is a dendritically polarized membrane glycoprotein in telencephalic neurons, which shows heterophilic binding to leukocyte beta(2)-integrins. Here, we show that the human ICAM-5 protein interacts in a homophilic manner through the binding of the immunoglobulin domain 1 to domains 4-5. Surface coated ICAM-5-Fc promoted dendritic outgrowth and arborization of ICAM- 5-expressing hippocampal neurons. During dendritogenesis in developing rat brain, ICAM-5 was in monomer form, whereas in mature neurons it migrated as a high molecular weight complex. The findings indicate that its homophilic binding activity was regulated by nonmonomer/monomer transition. Thus, ICAM-5 displays two types of adhesion activity, homophilic binding between neurons and heterophilic binding between neurons and leukocytes.

Neuronal adhesion molecule telencephalin induces rapid cell spreading of microglia

Telencephalin (TLCN) is a neuronal surface glycoprotein whose expression is restricted to the telencephalon, the most rostral segment of the brain. TLCN binds to lymphocyte function-associated antigen-1 (LFA-1) integrin. In the central nervous system, LFA-1 is selectively and constitutively expressed by microglia, suggesting that TLCN/LFA-1 binding may mediate cell-cell interactions between telencephalic neurons and microglia. In the present study, we investigated the effects of recombinant TLCN protein on the morphology of microglia. TLCN induced an intensive spreading of lamellipodia, causing a rapid change in microglial morphology. In contrast, TLCN induced no significant change in morphology of neuroblastoma and fibroblasts. Furthermore, the TLCN-induced spreading of microglia was accompanied by a clustering of LFA-1 on cell surface membrane. These results provide evidence that TLCN binding to the surface of microglia transduces signals into microglia that mediate or accelerate cell spreading and LFA-1 redistribution, implying that neuronal TLCN may control the state and/or function of microglia in both physiological and pathological conditions.

Development of telencephalin in the human cerebrum

Telencephalin (TLN) is a 130kDa, type 1 integral membrane glycoprotein of the immunoglobulin superfamily found in the mammalian central nervous system. TLN shows a molecular structure resembling intercellular adhesion molecules-1 and -3, and binds to the CD11a/CD18 leukocyte integrin. TLN was localized to neuronal dendrites in the telencephalic gray matter: cerebral cortex and basal ganglia. We studied immunohistochemically the expression of TLN in the developing human brain. In the hippocampus, TLN immunoreactivity appeared at 29 gestational weeks (GW), intensified subsequently, and persisted into adulthood. In the temporal cortex, labeling was weak and restricted to the cytoplasm of pyramidal neurons from 35 to 39 GW, but thereafter became diffuse and intense in the cortical layers, especially the molecular layer, by 5 months of postnatal age. The development of TLN was late compared to synaptophysin and microtubule-associated protein 2, suggesting its involvement in the functional maturation of neuronal dendrites and synapses.

Mapping of the ICAM-5 (telencephalin) gene, a neuronal member of the ICAM family, to a location between ICAM-1 and ICAM-3 on human chromosome 19p13.2

Intercellular adhesion molecule 5 (ICAM-5, telencephalin) is a cell adhesion molecule expressed on neurons in the most rostral segment of the mammalian brain, the telencephalon. Antibody studies in rodents and rabbits have demonstrated expression of this molecule on the cell body and dendrites of these neurons. We have examined the expression pattern in human brain by Northern blot analysis of 16 human brain segments. This analysis has confirmed the unique expression pattern of this ICAM in human. In addition, we report the mapping of the human ICAM-5 gene to an 80-kb region on chromosome 19p13.2 that also contains ICAM-1 and ICAM-3.

The intercellular adhesion molecule (ICAM) family of proteins. New members and novel functions

Macromolecular adhesive associations between cells are important for transmitting spatial and temporal information that is critical for immune system function. One such group of proteins, the intercellular adhesion molecules (ICAMs), has grown as newly identified members are revealed. In addition, the functions of the ICAMs, in general, have begun to be better understood, including intracellular signaling events. This information has led to the design of novel therapeutic agents that may prove effective in a variety of disease states.

Dendrite-associated cell adhesion molecule, telencephalin, promotes neurite outgrowth in mouse embryo

Telencephalin (TLCN) is a dendrite-associated cell adhesion molecule expressed by neurons within the telencephalon. It belongs to the intercellular adhesion molecule subgroup of the immunoglobulin superfamily. To examine a neurite outgrowth-promoting activity, neurons dissociated from mouse embryos were cultured on the substrate of recombinant mouse TLCN protein. Hippocampal neurons extended multiple neurites on TLCN. The neurite outgrowth on TLCN was suppressed by an anti-TLCN antibody. Non-telencephalic neurons also extended neurites on TLCN. These results demonstrate a neurite outgrowth-promoting activity of TLCN and suggest that both telencephalic and non-telencephalic neurons express TLCN counter-receptor(s) which is coupled to the neurite outgrowth.

Genomic organization and chromosomal localization of the mouse telencephalin gene, a neuronal member of the ICAM family

Telencephalin is a cell adhesion molecule belonging to the immunoglobulin (Ig) superfamily, whose expression is restricted to subsets of neurons in the telencephalon, the most rostral segment of brain. Of all the Ig superfamily molecules so far identified, the structure of telencephalin is most closely related to those of intercellular adhesion molecules (ICAMs)-1 and -3. Here we report the cloning, characterization, and chromosomal localization of the mouse telencephalin gene (Tlcn). The Tlcn gene spanned about 6.3 kb and consisted of 11 exons. A signal peptide and individual nine Ig-like domains of telencephalin were encoded by a single exon, while the transmembrane and cytoplasmic regions were fused in a same exon. The primer extension technique was used to establish that the transcription initiation sites were located 92-95 bp upstream from the ATG start codon. DNA sequencing of the 5'-flanking region revealed the presence of a strong initiator element for TATA-less genes, two CAAT boxes, and numerous potential transcription factor binding sites including four E-box and two N-box sequences. Interspecific backcross analysis demonstrated that the Tlcn gene was mapped in the proximal region of mouse chromosome 9 in close vicinity to the Icam-1 gene, suggesting that Tlcn and Icam-1 are derived from a common ancestral gene by gene duplication.

Reduction of telencephalin immunoreactivity in the brain of patients with Alzheimer's disease

Telencephalin (TLN) is a cell adhesion molecule expressed in the telencephalon of the mammalian central nervous system. We have investigated immunohistochemically the expression of TLN in human brain tissue from control subjects and patients with Alzheimer's disease (AD). In control brain, neuropil of the gray matter was stained diffusely with the anti-TLN antibody. TLN immunoreactivity was markedly decreased in the brain of AD patients, particularly in the hippocampal formation.

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.

Telencephalin: a neuronal area code molecule?

Cell adhesion molecules (CAMs) with expression restricted to specific developmental and structural units of the brain and/or selective neuronal types would play critical roles in the formation of functional neuronal networks. In this article, we summarize recent progress in knowledge on a brain segment-specific CAM, telencephalin (TLN). TLN has the following characteristic properties. (1) TLN is a neuronal glycoprotein whose expression is restricted within telencephalon, the most rostal segment of the brain. (2) TLN is localized to the soma-dendritic membrane of subsets of telencephalic neurons, but not to the axonal membrane. (3) Abrupt appearance of TLN around birth parallels the timing of dendritic development and synapse formation in the telencephalon. (4) TLN belongs to the immunoglobulin superfamily and its structure is most closely related to intercellular adhesion molecules (ICAMs)-1 and -3. These findings suggest that TLN is the first example of dendrite-associated cell adhesion molecules (DenCAMs) and that TLN may be involved in the brain segmental organization, cell-cell interactions during dendritic development, and maintenance of functional neuronal networks. We discuss the possibility that TLN is an area code-like address signal that is displayed selectively by telencephalic neurons and is decoded by specific subsets of growing axons to make proper synaptic connections.

An ICAM-related neuronal glycoprotein, telencephalin, with brain segment-specific expression

Telencephalin (TLN) is a 130 kd glycoprotein expressed exclusively in neurons of the telencephalon, the most rostral brain segment. In the neurons, TLN is localized to soma-dendritic membrane but not to axonal membrane. In this study, we have cloned cDNA encoding rabbit and mouse TLN. The cDNA-derived primary structure of TLN predicts an integral membrane protein with nine tandem immunoglobulin-like domains in an extra-cellular region, a transmembrane domain, and a short cytoplasmic tail. The distal eight immunoglobulin-like domains of TLN show highest homology with the immunoglobulin-like domains of intracellular adhesion molecules (ICAMs) 1, 2, and 3/R. The structural similarity of TLN with ICAMs provides a new and strong link between immunoglobulin superfamily molecules in the nervous and immune systems. TLN is an example of a dendrite-associated cell adhesion molecule involved in the brain's segmental organization, cell-cell interactions during dendritic development, and maintenance of functional neuronal networks.

Ultrastructural localization of telencephalin, a telencephalon-specific membrane glycoprotein, in rabbit olfactory bulb

Localization of a telencephalon-specific glycoprotein, telencephalin (TCLN), in the olfactory bulb of the rabbit was studied with an electron microscope. Anti-TCLN antisera appeared to stain plasma membrane, Golgi apparatus and multivesicular bodies of granule cells which are local circuit interneurons in the bulb. Principal neurons, mitral and tufted cells, were not immunoreactive. No glial cells showed immunoreactivity. Thus, expression of telencephalin is specific not only to the telencephalic segment of the brain, but also to the neuronal types.

Immunoglobulin superfamily molecules in the nervous system

Among the various types of membrane molecules involved in cell-cell interactions in the nervous system, we have focused in this review upon membrane proteins belonging to the immunoglobulin superfamily (IgSF). IgSF molecules are distinctive in that: (1) a large percentage of known neural adhesion molecules belongs to the IgSF; (2) they are homologous in structure (Ig domain), yet exhibit large variation of function in cell-cell interactions. The structure of IgSF molecules is briefly summarized in Section II, and each member of the IgSF which has been found in the nervous system is reviewed in Section III. In Section IV, we have discussed possible properties of yet-unknown nervous system IgSF molecules, on the assumption that nervous system IgSF molecules thus far discovered comprise only a small portion of those existing. Discussion is based upon an analogy with the immune system and upon knowledge of cell-cell interactions in the development of the nervous system. Our principal aims in this review are to summarize knowledge of neural IgSF molecules and to discuss the possibility that some IgSF molecules may encode in their structures instructions for recognizing, or for being recognized by, target neural cells. Further growth of knowledge of IgSF molecules may yield insights into the patterns of cell-cell interactions underlying the formation of neuronal circuits during development.

Variations by layers and developmental changes in expression of telencephalin in the visual cortex of cat

The expression of telencephalin in visual cortex of cat and monkey was studied immunohistochemically. In adult cats and monkeys, immunoreactivity to a polyclonal antibody raised against telencephalin was especially low in layer IV, which receives massive afferent input from the thalamus. In kitten visual cortex, the antibody bound both layer IV and other cortical layers during the most sensitive period for ocular dominance plasticity. Outside the sensitive period, the staining of layer IV was selectively reduced. These findings suggest that the expression of telencephalin is developmentally regulated during the early period and may play a role in regulating plasticity during the sensitive period.