Macrophage Transcriptional Profile Identifies Lipid Catabolic Pathways That Can Be Therapeutically Targeted after Spinal Cord Injury.

Although infiltrating macrophages influence many pathological processes after spinal cord injury (SCI), the intrinsic molecular mechanisms that regulate their function are poorly understood. A major hurdle has been dissecting macrophage-specific functions from those in other cell types as well as understanding how their functions change over time. Therefore, we used the RiboTag method to obtain macrophage-specific mRNA directly from the injured spinal cord in mice and performed RNA sequencing to investigate their transcriptional profile. Our data show that at 7 d after SCI, macrophages are best described as foam cells, with lipid catabolism representing the main biological process, and canonical nuclear receptor pathways as their potential mediators. Genetic deletion of a lipoprotein receptor, CD36, reduces macrophage lipid content and improves lesion size and locomotor recovery. Therefore, we report the first macrophage-specific transcriptional profile after SCI and highlight the lipid catabolic pathway as an important macrophage function that can be therapeutically targeted after SCI.SIGNIFICANCE STATEMENT The intrinsic molecular mechanisms that regulate macrophage function after spinal cord injury (SCI) are poorly understood. We obtained macrophage-specific mRNA directly from the injured spinal cord and performed RNA sequencing to investigate their transcriptional profile. Our data show that at 7 d after SCI, macrophages are best described as foam cells, with lipid catabolism representing the main biological process and canonical nuclear receptor pathways as their potential mediators. Genetic deletion of a lipoprotein receptor, CD36, reduces macrophage lipid content and improves lesion size and locomotor recovery. Therefore, we report the first macrophage-specific transcriptional profile after SCI and highlight the lipid catabolic pathway as an important macrophage function that can be therapeutically targeted after SCI.

Immunoglobulin superfamily receptors: cis-interactions, intracellular adapters and alternative splicing regulate adhesion.

The immunoglobulin domain is a module found in vertebrates and invertebrates. Its ability to form linear rods when deployed in series, combined with its propensity to bind specifically to other proteins has made it ideal for building cell surface receptors and cell adhesion molecules. These features have resulted in the incorporation of immunoglobulin domains into many hundreds of cell surface molecules. Recently three major advances have been made in understanding immunoglobulin receptors. One is the recognition that their intracellular binding partners are likely to link to multiple cell surface molecules, allowing cross-talk or oligomeric complex formation. A second, but related phenomenon, is their participation in cis-interactions on the extracellular surface that regulate signaling or adhesion. The third is the dramatic ability to form dozens to thousands of different isoforms via alternative splicing. Although antibodies may have been the first example of immunoglobulin-domain-containing proteins using cis-interactions to form receptor like molecules, and the grandest instance of diversity production from limited genetic material, these are clearly old ideas in this superfamily.

IgCAMs: bidirectional signals underlying neurite growth.

Understanding how immunoglobulin superfamily cell adhesion molecules (IgCAMs) regulate nervous system development has lagged behind studies on integrins and cadherins. The recent characterization of IgCAM structures combined with cell biological studies on protein-protein interactions and membrane targeting/trafficking demonstrate that IgCAMs interact in exceedingly complex ways to regulate axonal growth and pathfinding.

Heterophilic binding of L1 on unmyelinated sensory axons mediates Schwann cell adhesion and is required for axonal survival.

This study investigated the function of the adhesion molecule L1 in unmyelinated fibers of the peripheral nervous system (PNS) by analysis of L1- deficient mice. We demonstrate that L1 is present on axons and Schwann cells of sensory unmyelinated fibers, but only on Schwann cells of sympathetic unmyelinated fibers. In L1-deficient sensory nerves, Schwann cells formed but failed to retain normal axonal ensheathment. L1-deficient mice had reduced sensory function and loss of unmyelinated axons, while sympathetic unmyelinated axons appeared normal. In nerve transplant studies, loss of axonal-L1, but not Schwann cell-L1, reproduced the L1-deficient phenotype. These data establish that heterophilic axonal-L1 interactions mediate adhesion between unmyelinated sensory axons and Schwann cells, stabilize the polarization of Schwann cell surface membranes, and mediate a trophic effect that assures axonal survival.

Ectodomain shedding of L1 adhesion molecule promotes cell migration by autocrine binding to integrins.

The L1 adhesion molecule plays an important role in axon guidance and cell migration in the nervous system. L1 is also expressed by many human carcinomas. In addition to cell surface expression, the L1 ectodomain can be released by a metalloproteinase, but the biological function of this process is unknown. Here we demonstrate that membrane-proximal cleavage of L1 can be detected in tumors and in the developing mouse brain. The shedding of L1 involved a disintegrin and metalloproteinase (ADAM)10, as transfection with dominant-negative ADAM10 completely abolishes L1 release. L1-transfected CHO cells (L1-CHO) showed enhanced haptotactic migration on fibronectin and laminin, which was blocked by antibodies to alpha v beta 5 and L1. Migration of L1-CHO cells, but not the basal migration of CHO cells, was blocked by a metalloproteinase inhibitor, indicating a role for L1 shedding in the migration process. CHO and metalloproteinase-inhibited L1-CHO cells were stimulated to migrate by soluble L1-Fc protein. The induction of migration was blocked by alpha v beta 5-specific antibodies and required Arg-Gly-Asp sites in L1. A 150-kD L1 fragment released by plasmin could also stimulate CHO cell migration. We propose that ectodomain-released L1 promotes migration by autocrine/paracrine stimulation via alpha v beta 5. This regulatory loop could be relevant for migratory processes under physiological and pathophysiological conditions.

Human neural cell adhesion molecule L1 and rat homologue NILE are ligands for integrin alpha v beta 3.

Integrin alpha v beta 3 is distinct in its capacity to recognize the sequence Arg-Gly-Asp (RGD) in many extra-cellular matrix (ECM) components. Here, we demonstrate that in addition to the recognition of ECM components, alpha v beta 3 can interact with the neural cell adhesion molecule L1-CAM; a member of the immunoglobulin superfamily (IgSF). M21 melanoma cells displayed significant Ca(++)-dependent adhesion and spreading on immunopurified rat L1 (NILE). This adhesion was found to be dependent on the expression of the alpha v-integrin subunit and could be significantly inhibited by an antibody to the alpha v beta 3 heterodimer. M21 cells also displayed some alpha v beta 3-dependent adhesion and spreading on immunopurified human L1. Ligation between this ligand and alpha v beta 3 was also observed to promote significant haptotactic cell migration. To map the site of alpha v beta 3 ligation we used recombinant L1 fragments comprising the entire extracellular domain of human L1. Significant alpha v beta 3-dependent adhesion and spreading was evident on a L1 fragment containing Ig-like domains 4, 5, and 6. Importantly, mutation of an RGD sequence present in the sixth Ig-like domain of L1 abrogated M21 cell adhesion. We conclude that alpha v beta 3-dependent recognition of human L1 is dependent on ligation of this RGD site. Despite high levels of L1 expression the M21 melanoma cells did not display significant adhesion via a homophilic L1-L1 interaction. These data suggest that M21 melanoma cells recognize and adhere to L1 through a mechanism that is primarily heterophilic and integrin dependent. Finally, we present evidence that melanoma cells can shed and deposit L1 in occluding ECM. In this regard, alpha v beta 3 may recognize L1 in a cell-cell or cell-substrate interaction.

L1-mediated axon outgrowth occurs via a homophilic binding mechanism.

The molecular mechanism by which the L1 cell adhesion molecule mediates neurite outgrowth has been examined. Purified L1 from mouse and L1 from chick brain were attached to nitrocellulose dishes. Both chick and mouse neurons were able to adhere to purified mouse L1 and chick L1. Both molecules promoted neurite extension from chick and mouse neurons. Addition of Fabs specific for chick L1 to the cultures inhibited chick neurite outgrowth on both mouse L1 and chick L1. These findings suggest that L1-like molecules support neurite outgrowth via a "homophilic" binding mechanism.

N-cadherin and Ng-CAM/8D9 are involved serially in the migration of newly generated neurons into the adult songbird brain.

In the adult avian forebrain, neurons continue to be produced in the subependymal zone (SZ), from which they migrate upon radial fibers. To identify ligands regulating this process, we studied N-cadherin and Ng-CAM/8D9 expression in HVC, a neurogenic region of the canary neostriatum. N-cadherin was relatively restricted to the SZ and was expressed by dividing, [3H]thymidine-labeled precursor cells. However, cellular N-cadherin was down-regulated prior to neuronal migration from the SZ. Addition of anti-N-cadherin Fab hastened neuronal migration from adult SZ explants, without influencing neuronal number. Unlike N-cadherin, Ng-CAM/8D9 was expressed by migrating neurons. Anti-8D9 Fab inhibited neuronal migration upon cultured ependymoglia, which did not express Ng-CAM/8D9. Thus, the departure of new neurons from the adult SZ may require their suppression of N-cadherin, whereas their subsequent migration and survival may depend upon neuronal expression of Ng-CAM/8D9 and its interaction with a heterophilic radial cell receptor.

Mutations in the cell adhesion molecule L1 cause mental retardation.

Recently, studies in the usually disparate fields of human genetics and developmental neurobiology have converged to reveal that some types of human mental retardation and brain malformations are due to mutations that affect the neural cell adhesion molecule L1. L1 has a very complex biology, interacting with a variety of ligands, and functioning in migration of neurons and growth of axons. Over the past few years, it has also become clear that L1 is able to influence intracellular second messengers. The identification of a number of different mutations in L1, some of which alter the extracellular portion of the molecule, and others that change only the cytoplasmic tail, confirm that L1 is a crucial player in normal brain development. The information gained from genetic analysis of human L1 is giving new insights into how L1 functions in the formation of major axon pathways, but it also raises unanticipated questions about how L1 participates in the development of cortical and ventricular systems.

Recycling of the cell adhesion molecule L1 in axonal growth cones.

The cell adhesion molecule (CAM) L1 plays crucial roles in axon growth in vitro and in the formation of major axonal tracts in vivo. It is generally thought that CAMs link extracellular immobile ligands with retrogradely moving actin filaments to transmit force that pulls the growth cone forward. However, relatively little is known about the fate of CAMs that have been translocated into the central (C)-domain of the growth cone. We have shown previously that L1 is preferentially endocytosed at the C-domain. In the present study, we further analyze the subcellular distribution of endocytic organelles containing L1 at different time points and demonstrate that internalized L1 is transported into the peripheral (P)-domain of growth cones advancing via an L1-dependent mechanism. Internalized L1 is found in vesicles positioned along microtubules, and the centrifugal transport of these L1-containing vesicles is dependent on dynamic microtubules in the P-domain. Furthermore, we show that endocytosed L1 is reinserted into the plasma membrane at the leading edge of the P-domain. Monitoring recycled L1 reveals that it moves retrogradely on the cell surface into the C-domain. In contrast, the growth cone advancing independently of L1 internalizes and recycles L1 within the C-domain. For the growth cone to advance, the leading edge needs to establish strong adhesive interactions with the substrate while attachments at the rear are released. Recycling L1 from the C-domain to the leading edge provides an effective way to create asymmetric L1-mediated adhesion and therefore would be critical for L1-based growth cone motility.

Brain studies of mouse models for neurogenetic disorders using in vivo magnetic resonance imaging (MRI).

Magnetic resonance imaging (MRI) is a technique commonly used to detect neural abnormalities in routine clinical practice. It is perhaps less well known that the technique can be adapted to measure various anatomical and physiological features of small laboratory rodents. This review focuses on the potential of the MRI technique to image the brain of (transgenic) mouse models for neurological diseases, and aims to introduce these exciting new technological developments to the non-specialist reader.

CRASH syndrome: clinical spectrum of corpus callosum hypoplasia, retardation, adducted thumbs, spastic paraparesis and hydrocephalus due to mutations in one single gene, L1.

L1 is a neuronal cell adhesion molecule with important functions in the development of the nervous system. The gene encoding L1 is located near the telomere of the long arm of the X chromosome in Xq28. We review here the evidence that several X-linked mental retardation syndromes including X-linked hydrocephalus (HSAS), MASA syndrome, X-linked complicated spastic paraparesis (SP1) and X-linked corpus callosum agenesis (ACC) are all due to mutations in the L1 gene. The inter- and intrafamilial variability in families with an L1 mutation is very wide, and patients with HSAS, MASA, SP1 and ACC can be present within the same family. Therefore, we propose here to refer to this clinical syndrome with the acronym CRASH, for Corpus callosum hypoplasia, Retardation, Adducted thumbs, Spastic paraplegia and Hydrocephalus.

Afferent and efferent connections of the striate and extrastriate visual cortex of the normal and reeler mouse.

In order to analyze the role of lamination in establishing the precisely ordered connectional pattern of the neocortex, we compared the afferent and efferent connections of the visual cortical areas in normal mice with those of the mutant mouse reeler (rl). The reeler mutation causes disruption of the laminar organization of the neocortex; all classes of neurons are present but are abnormally located. The corticocortical and thalamocortical connection os visual cortical areas 17, 18a, and 18b were determined in normal and reeler mice with injections of horseradish peroxidase (HRP) or HRP conjugated with wheat germ agglutinin (HRP-WGA). The diffusion of HRP-WGA is highly restricted due to the surface binding properties of the lectin; it was particularly effective in demonstrating retinotopically ordered connections. We found that the patterns of connections made the reeler mutant are indistinguishable from normal. Cortical loci in area 17 are reciprocally connected to homotopic locations in areas 18a and 18b. Area 17 is also reciprocally connected with dorsal lateral geniculate nucleus of the thalamus and projects to the superior colliculus. Areas 18a and 18b are reciprocally connected with each other and with the lateral posterior and lateral nuclei of the thalamus, respectively. In addition, we found evidence of reciprocal connections between the lateral posterior nucleus and area 17, and between the lateral nucleus and areas 17 and 18a. The results indicate the neurons in visual cortical areas of the reeler mutant mouse are capable of forming retinotopically organized corticocortical and thalamocortical connections in a pattern similar to that found in normal animals. Thus the genetic anomaly producing incorrect neuronal positioning during development of the reeler cortex does not seriously impede the pathway and target recognition mechanisms responsible for formation of functionally appropriate cortical connections.

A monoclonal antibody that binds to cones.

A monoclonal antibody that binds to cones has been produced. This antibody, 50-1B11, binds to the outer segments of cones in rhesus monkeys. Immunohistochemical experiments indicate that 50-1B11 binds to a subset of photoreceptors, probably cones, in all vertebrate species tested thus far, including man. In vitro experiments on chicken retina indicate that the antigen is intracellular and associated with the plasma membrane, while electronmicroscopic-immunohistochemical studies demonstrate that the antigen is contained in the lamellae of the outer segments of rhesus cones.

A monoclonal antibody that binds to the surface of photoreceptors.

A monoclonal antibody (MAB), 7H11, that binds to the surface of photoreceptors has been produced. Using immunoaffinity procedures, proteins with molecular weights of 53, 48 and 42 kDa were found to bind a MAB 7H11 column, although the 48- and 42-kDa proteins may be proteolytic breakdown products of the 53 kDa protein. Electron microscopic-immunohistochemical studies show that the antigen is concentrated on the external surface of inner and outer segments of photoreceptors. It is likely that the 7H11 antigen is a component of the interphotoreceptor matrix and, therefore, may contribute to some aspect of photoreceptor-pigment epithelial interactions. However, the expression of the 7H11 antigen in cell culture suggests that it is expressed independently of direct cell-cell interactions with either pigment epithelial cells, Müller cells or other photoreceptors.

Monoclonal antibodies specific for glia in the chick nervous system.

Two monoclonal antibodies that bind to previously undescribed glial antigens are discussed. One antibody, 3A7, binds to Müller cells in the chick retina and radial glia in the optic tectum. The other, 7G4, also binds to Müller cells, However, in the tectum it binds to a small cell type in the stratum opticum. The developmental appearance of the antigens and their localization in cells in tissue culture are also presented.

The development distribution of vimentin in the chick retina.

The developmental distribution of vimentin was localized in chick retina using light and electromicroscopic immunohistochemical techniques. From embryonic day 4 (E4) to E8 vimentin was present in cells throughout the retina. By E10 vimentin immunoreactivity was restricted to Müller cells. In tissue culture of chick neural retina vimentin was found to be restricted to flat cells that are thought by others to be derived from Müller cells. Neurons in culture did not contain vimentin.

A monoclonal antibody which binds to the surface of chick brain cells and myotubes: cell selectivity and properties of the antigen.

A monoclonal antibody has been obtained which binds to the cell surface of cultured chick myotubes and retinal and tectal neurons but not fibroblasts, myoblasts and embryonic liver cells. Indirect immunocytochemistry reveals that antigen is present in all layers of the chick neural retina. The antibody therefore recognizes an antigen common to most, if not all, chick neurons. The antigen has been identified by staining SDS gels with [125I]monoclonal antibody and appears to be a polydisperse collection of polypeptides with molecular weights centered about 250 kdalton.

N-cadherin expression and function in cultured oligodendrocytes.

N-Cadherin is a major cell adhesion molecule that is expressed in the developing nervous system where it has been implicated in neural migration and axon growth. Recently, a role for N-cadherin in oligodendrocyte differentiation has been identified [23]. Oligodendrocyte precursors adhere to N-cadherin and mature rapidly to produce myelin sheets. Since this implies that oligodendrocytes express N-cadherin, we examined the expression of N-cadherin by oligodendrocytes in culture. N-Cadherin was expressed by O-2A progenitors, immature oligodendrocytes and mature oligodendrocytes, but at a lower level than in type 1 astrocytes in the same cultures. On mature oligodendrocytes, the N-cadherin was concentrated on the major processes emerging from the soma. The ability of N-cadherin and merosin to promote oligodendrocyte precursor migration was also studied. Average migration rates were significantly higher on merosin (11.2 microns/h) than on N-cadherin (5.6 microns/h). These results suggest that N-cadherin is not likely to function predominantly as a substrate that stimulates migration of O-2A progenitors, but may be more important in initiating early oligodendrocyte-axon interactions that promote the process of myelination.

Growth cone interactions with purified cell and substrate adhesion molecules visualized by interference reflection microscopy.

The migration of growth cones on substrates consisting of naturally occurring cell adhesion molecules has been extensively studied in cell culture. However, relatively little is known about how growth cones contact the substrate or how the patterns of contact change as growth cones move forward. We have examined the interactions of chick retinal ganglion cell growth cones with laminin, merosin, N-cadherin, L1 and poly-L-lysine by time-lapse interference reflection microscopy (IRM) using a laser scanning confocal microscope. In images obtained by IRM, areas of a cell that are closely apposed to the substrate appear dark whereas areas that are farther away appear light. Growth cones on Jaminin and merosin were almost uniformly light, indicating that very little of the membrane was in close contact with the substrate. Growth cones on N-cadherin had a mottled appearance with some relatively large dark gray areas. The proximal portions of filopodia often were dark, in contrast to those on laminin and merosin which were light. In addition, growth cones on N-cadherin had numerous dark gray punctate regions of close association with the substrate. Growth cones on L1 had darker regions than growth cones on other substrates and these comprised a larger fraction of their area. There also were differences in the temporal dynamics of growth cone interactions with different substrates and these differences correlated with differences in rates of growth. None of the contacts observed in growth cones were as dark or stable as focal contacts of fibroblasts.