Neuronal and glial expression of the adhesion molecule TAG-1 is regulated after peripheral nerve lesion or central neurodegeneration of adult nervous system

Expression of the cell adhesion molecule TAG-1 is down-regulated in adult brain, with the exception of certain areas exhibiting structural plasticity. Here, we present evidence that TAG-1 expression persists also in adult rat spinal cord and dorsal root ganglia (DRG), and can be up-regulated after injury. On Western blots of adult tissue, TAG-1 is detected as a 135-kDa band, with an additional specific 90-kDa band, not present in developing tissue. TAG-1 expression is found both in DRG neurons and in Schwann cells, particularly those associated with the peripherally projecting DRG processes. Quantitative in situ hybridization revealed that TAG-1 expression is significantly higher in small neurons that give rise to unmyelinated fibers, than in large DRG neurons. The regulation of TAG-1 was then examined in two different lesion paradigms. After a sciatic nerve lesion, TAG-1 expression is not up-regulated in DRG neurons, but decreases with time. At the lesion site, reactive Schwann cells up-regulate TAG-1, as demonstrated by both immunohistochemistry and in situ hybridization. In a second paradigm, we injected kainic acid into the spinal cord that kills neurons but spares glia and axons. TAG-1 is up-regulated in the spinal neuron-depleted area as well as in the corresponding dorsal and ventral roots, associated with both target-deprived afferent fibers and with the non-neuronal cells that invade the lesion site. These results demonstrate a local up-regulation of TAG-1 in the adult that is induced in response to injury, suggesting its involvement in axonal re-modelling, neuron-glia interactions, and glial cell migration.     

Analysis of interactions of the adhesion molecule TAG-1 and its domains with other immunoglobulin superfamily members

Cell adhesion molecules of the immunoglobulin superfamily promote cell aggregation and neurite outgrowth via homophilic and heterophilic interactions. The transient axonal glycoprotein TAG-1 induces cell aggregation through homophilic interaction of its fibronectin repeats. We investigated the domains responsible for the neurite outgrowth promoting activity of TAG-1 as well as its interactions with other cell adhesion molecules. Binding experiments with Fc-chimeric proteins revealed that TAG-1 interacts with L1, NrCAM, and F3/contactin. The membrane-associated as opposed to the soluble form of TAG-1 behaves differently in these assays. We demonstrate that both the immunoglobulin as well as the fibronectin domains promote neurite outgrowth when used as substrates. Furthermore we investigated the putative role of L1 and NrCAM as the neuronal TAG-1 receptors mediating neurite extension. DRG neurons from L1-deficient mice were found to extend neurites on TAG-1 substrates and blocking NrCAM function did not diminish the TAG-1-dependent neurite outgrowth. These results indicate that neither L1 nor NrCAM are required for TAG-1-elicited neurite outgrowth.     

Early arrest of B cell development in nude, X-linked immune-deficient mice

Mice simultaneously expressing the nude and xid mutations have a severe deficit of both mature T and B cells. We now report studies designed to determine at which point in B cell differentiation development is arrested. Nude-xid mice have normal numbers of hematopoietic colony forming units (CFU-s) but lack two early pre-B cell markers: susceptibility to transformation by Abelson murine leukemia virus (A-MuLV) and production of cytoplasmic mu (C mu) heavy chain. Thus, there is a defect in lymphocyte development prior to or early in pre-B cell differentiation but after hematopoietic stem cell formation. The monoclonal reagents DNL 1.9, 14.8 and RA3-3A1/6.1, which react with the surface protein B220 (Ly5) on pro-B, C mu- pre-B, C mu+ pre-B, and surface Ig+ B cells, revealed the presence of positive cells in nude-xid mice. The bone marrow of nude-xid mice contains more B220+ cells than C mu+ cells. Our data suggest that the developmental block in these mice occurs at the earliest identifiable step in the B lymphocyte lineage, after the appearance of B220+ C mu- pro-B cells, but before the differentiation of C mu-bearing (pre-B) cells.     

Cerebellar granule cell differentiation in mutant and X-irradiated rodents revealed by the neural adhesion molecule TAG-1

In the external granular layer of the cerebellum, the granule cell precursors express the transient axonal glycoprotein TAG-1, a molecule involved in adhesion and neurite outgrowth. Granule cells express TAG-1 transiently, just as they extend neurites before migrating over the radial glia. The present study aims to investigate whether the expression pattern of TAG-1 is altered when granule cells develop abnormally. We studied in vivo models in which Purkinje and/or granule cell defects occur during postnatal development. These include the cerebellar mutant mice staggerer and lurcher as well as rats irradiated during postnatal development. Neither alterations in Purkinje cell differentiation nor the related granule cell loss in the mouse mutants impairs the ability of the surviving granule cell precursors to express TAG-1. Also, early granule cell loss in the X-irradiated rats do not disturb the TAG-1 expression phase in the patches of surviving granule cell precursors. Ectopic granule cells found in the adult cerebellum of X-irradiated rats do not bear the molecule, although they are located in the most superficial part of the molecular layer, occupied by the immunopositive cells a few days earlier. Thus, TAG-1 marks a very precise stage of granule cell differentiation, and the inward migration process itself is not required for the cessation of the expression. We postulate that TAG-1 may be involved in local differentiation steps restricted to the deep external granular layer such as parallel migratory routes or synchrony of axonal growth. © 1996 Wiley-Liss, Inc.     

Impairment of learning and memory in TAG-1 deficient mice associated with shorter CNS internodes and disrupted juxtaparanodes

The cell adhesion molecule TAG-1 is expressed by neurons and glial cells and plays a role in axon outgrowth, migration and fasciculation during development. TAG-1 is also required for the clustering of Kv1.1/1.2 potassium channels and Caspr2 at the juxtaparanodes of myelinated fibers. Behavioral examination of TAG-1 deficient mice (Tag-1(-/-)) showed cognitive impairments in the Morris water maze and novel object recognition tests, reduced spontaneous motor activity, abnormal gait coordination and increased response latency to noxious stimulation. Investigation at the molecular level revealed impaired juxtaparanodal clustering of Caspr2 and Kv1.1/1.2 in the hippocampus, entorhinal cortex, cerebellum and olfactory bulb, with diffusion into the internode. Caspr2 and Kv1.1 levels were reduced in the cerebellum and olfactory bulb. Moreover, Tag-1(-/-) mice had shorter internodes in the cerebral and cerebellar white matter. The detected molecular alterations may account for the behavioural deficits and hyperexcitability in these animals.     

The IgLON protein Lachesin is required for the blood-brain barrier in Drosophila

In the mammalian peripheral nervous system, nerve insulation depends on the integrity of paranodal junctions between axons and their ensheathing glia. Ultrastructurally, these junctions are similar to the septate junctions (SJ) of invertebrates. In Drosophila, SJ are found in epithelia and in the glia that form the blood-brain barrier (BBB). Drosophila NeurexinIV and Gliotactin, two components of SJ, play an important role in nerve ensheathment and insulation. Here, we report that Drosophila Lachesin (Lac), another SJ component, is also required for a functional BBB. In the developing nervous system, Lac is expressed in a dynamic pattern by surface glia and a subset of neurons. Ultrastructural analysis of Lac mutant embryos shows poorly developed SJ in surface glia and epithelia where Lac is expressed. Mutant embryos undergo a phase of hyperactivity, with unpatterned muscle contractions, and subsequently become paralyzed and fail to hatch. We propose that this phenotype reflects a failure in BBB function.     

Localization of myomodulin-like immunoreactivity in the central nervous system and peripheral tissues of Aplysia californica.

The distribution of myomodulin-like peptides in the nervous system of Aplysia californica was examined by using immunocytochemical techniques. Neurons and cell clusters containing immunoreactive material were located in each of the major central ganglia. Myomodulin-like immunoreactivity was also present in fibers in each of the connectives between the ganglia and in peripheral nerves. Varicosities containing immunoreactive material were located on specific regions of peripheral tissues associated with the feeding, digestive, cardiovascular, and reproductive systems. Double-labeling experiments were used to demonstrate myomodulin-like immunoreactivity in two identified neurons, the motor neuron B16 in the buccal ganglion and the widely acting interneuron L10 in the abdominal ganglion. Structures in the eye and cerebral ganglion that may correspond to the optic circadian pacemaker system were also stained. The central and peripheral distribution of myomodulin-like immunoreactivity indicates that this family of neuropeptides is present in specific efferent, afferent, and interneuronal elements that participate in a diversity of neural circuits in Aplysia.