Expression of the Neural Recognition Molecule L1 by Cultured Neural Cells is influenced by K+ and the Glutamate Receptor Agonist NMDA

The role of cell adhesion molecules for navigating axons: density matters

Cell adhesion molecules of the immunoglobulin-super-family (IgSF-CAMs) do not only have a physical effect, mediating merely attachment between cell surfaces. For navigating axons, IgSF-CAMs also exert an instructive impact: Upon activation, they elicit intracellular signalling cascades in the tip of the axon, the growth cone, which regulate in a spatio-temporally concerted action both speed and direction of the axon. Density and distribution of IgSF-CAMs in the growth cone plasma membrane play important roles for the activation of IgSF-CAMs, their clustering, and the adhesive forces they acquire, as well as for the local restriction and effective propagation of their intracellular signals.

Cell adhesion molecule DM-GRASP presented as nanopatterns to neurons regulates attachment and neurite growth

Adhesion and neurite formation of neurons and neuroblastoma cells critically depends on the lateral spacing of the cell adhesion molecule DM-GRASP offered as nanostructured substrate.

BDNF/TrkB signaling regulates HNK-1 carbohydrate expression in regenerating motor nerves and promotes functional recovery after peripheral nerve repair

Functional recovery after peripheral nerve injury is often poor despite high regenerative capacity of peripheral neurons. In search for novel treatments, brief electrical stimulation of the acutely lesioned nerve has recently been identified as a clinically feasible approach increasing precision of axonal regrowth. The effects of this stimulation appear to be mediated by BDNF and its receptor, TrkB, but the down-stream effectors are unknown. A potential candidate is the HNK-1 carbohydrate known to be selectively reexpressed in motor but not sensory nerve branches of the mouse femoral nerve and to enhance growth of motor but not sensory axons in vitro. Here, we show that short-term low-frequency electrical stimulation (1 h, 20 Hz) of the lesioned and surgically repaired femoral nerve in wild-type mice causes a motor nerve-specific enhancement of HNK-1 expression correlating with previously reported acceleration of muscle reinnervation. Such enhanced HNK-1 expression was not observed after electrical stimulation in heterozygous BDNF or TrkB-deficient mice. Accordingly, the degree of proper reinnervation of the quadriceps muscle, as indicated by retrograde labeling of motoneurons, was reduced in TrkB+/- mice compared to wild-type littermates. Also, recovery of quadriceps muscle function, evaluated by a novel single-frame motion analysis approach, and axonal regrowth into the distal nerve stump, assessed morphologically, were considerably delayed in TrkB+/- mice. These findings indicate that BDNF/TrkB signaling is important for functional recovery after nerve repair and suggest that up-regulation of the HNK-1 glycan is linked to this phenomenon.

Dephosphorylation and internalization of cell adhesion molecule L1 induced by theta burst stimulation in rat hippocampus

The neural cell adhesion molecule L1 may participate in initiating and maintaining synaptic changes during learning in the hippocampus. One prominent form of synaptic change in the hippocampus is long-term potentiation (LTP) that occurs following specific patterns of synaptic activity. We present evidence that Y1176 of the YRSL motif within L1 cytoplasmic domain is dephosphorylated in LTP-induced hippocampus. The dephosphorylated L1 is associated with AP-2 and AP180 that are required for clathrin-mediated internalization of L1. These data suggest that clathrin-mediated recycling of L1 at presynaptic sites is enhanced by certain kinds of neural activity, and that maintenance of LTP-induced synaptic changes is regulated by L1 recycling.

Stress, cognitive impairment and cell adhesion molecules

Stress has profound effects on brain structure and function, but the underlying mechanisms are still poorly understood. Recent studies imply that neuronal cell adhesion molecules of the immunoglobulin superfamily--NCAM and L1--are important mediators of the effects of stress on the brain. Chronic stress regimes that lead to hippocampal atrophy and spatial-learning impairment in rodents simultaneously induce a pattern of changes in cell adhesion molecule expression that fits with a role for these molecules in stress-induced neuronal damage and neuroprotective mechanisms. These findings highlight cell adhesion molecules as potential therapeutic targets to treat stress-related cognitive disturbances.

S(+)-ketamine up-regulates neuronal regeneration associated proteins following glutamate injury in cultured rat hippocampal neurons

In previous studies, racemic ketamine improved neurological outcome after experimental brain injury and S(+)-ketamine demonstrated neuroprotective effects in neurons after damage in vitro. We compared the expression of regeneration-associated proteins in rat hippocampal neurons after glutamate injury and treatment with S(+)-ketamine versus racemic ketamine. Following an 8 minute exposure to 100 microM glutamate, neurons were maintained untreated or in the presence of S(+)-ketamine or racemic ketamine (10(-4) M, 10(-5) M, 10(-6) M) for one week. Growth-associated protein-43 (GAP-43) and synaptosomal-associated protein-25 (SNAP-25) was analyzed by Western Blotting, the mitochondrial transmembrane potential (MTP) by fluorescence imaging, and [3H]2-deoxy-D-glucose ([3H]2-DG) uptake by scintillation spectrometry. Seven days after exposure, GAP-43 decreased to 15% and SNAP-25 to 30% in the glutamate-injured, untreated neurons. The MTP declined to 50% and [3H]2-DG to 30%. Both S(+)-ketamine and racemic ketamine at 10(-4) M and 10(-5) M minimized the decline in MTP, almost maintaining it at control value. Additionally, S(+)-ketamine and racemic ketamine decreased the reduction in [3H]2-DG. S(+)-ketamine at 10(-4) M and 10(-5) M and racemic ketamine at 10(-4) M reduced the decline in SNAP-25 to 60% of controls (P < .05). However, S(+)-ketamine at 10(-4) M and 10(-5) M only reversed the decrease in GAP-43 to 50% and 40% of controls, respectively (P < .05). We conclude that the synthesis of a growth-associated protein related to plasticity and repair in the adult nervous system is increased by S(+)-ketamine but is not increased by racemic ketamine.

Functional cross-excitation between afferent A- and C-neurons in dorsal root ganglia

Electrophysiological recordings were made in vitro from primary afferent neurons with unmyelinated axons (C-neurons) in excised rat dorsal root ganglia. Spike activity triggered in neurons with myelinated axons (A-neurons) by stimulation of the peripheral nerve or the dorsal root produced a transient depolarization in passive neighboring C-neurons that share the same ganglion. About 90% of neurons sampled responded with this "cross-depolarization". Cross-depolarization was associated with functional excitation as indicated by an increase in firing probability in response to previously subthreshold intracellular test pulses. Furthermore, it yielded a net increase of the input resistance of the affected C-neurons. We suggest that functional coupling among DRG neurons could serve a metabolic role, providing a functionally relevant feedback signal useful for controlling the excitability of nociceptive sensory endings. In addition, the results provide a novel mechanism whereby afferent nociceptors could be stimulated by activity in low-threshold mechanoreceptors, particularly in the event of nerve injury. Hence, the coupling between afferent A- and C-neurons in dorsal root ganglia provides a novel candidate mechanism for neuropathic pain.

Unexplained peculiarities of the dorsal root ganglion

The cell soma of primary afferent neurons in the dorsal root ganglion (DRG) is assigned by classical neurophysiology the role of a metabolic depot, charged with supporting the peripheral sensory ending, the conducting axon, and the central synaptic terminals. However, certain peculiarities of DRG morphology and physiology do not sit well with this being its only role. For example, why are DRG cell somata electrically excitable, why are some able to fire repetitively on sustained depolarization, and why does the DRG lack a blood-nerve barrier? Consideration of these and related questions leads to several intriguing hypotheses: (1) Electrical excitability of the soma may be required to insure the reliable propagation of impulses past the DRG T-junction and into the spinal cord. (2) Invasion of the afferent spike into the cell soma may provide an essential feedback signal necessary for the cell soma to regulate the excitability of the sensory ending. 3) The subpopulation of DRG neurons that have repetitive firing capability may be responsible for generating the background sensation that we feel as our body schema. Moreover, these neurons may be chemical sensors that provide essential information about our body's internal milieu.

Opposite effects on NCAM expression in the rat frontal cortex induced by acute vs. chronic corticosterone treatments

The temporal pattern of exposure to glucocorticoids has been reported to be a critical factor in determining the outcome of glucocorticoid actions at the brain. In this work, the effects of different regimes of subcutaneous corticosterone administration (acute-single injection-vs. chronic-daily injection for 21 days) on the expression of the neural cell adhesion molecule (NCAM) were evaluated in different rat brain regions (CA1-CA4, dentate gyrus, frontal cortex, striatum, and hypothalamus). The treatments were selected according to previous studies in which we showed biphasic effects of corticosterone on memory formation, with acute corticosterone effects being facilitating and chronic effects being deleterious. In addition, the chronic treatment was shown by others to result in structural alterations at the hippocampus. NCAM was evaluated given its cell-cell recognition and adhesion properties, and the involvement on synaptic stabilisation subserving long-term memory formation. The results showed a biphasic modulation of NCAM levels at the frontal cortex, with acute corticosterone resulting in enhanced NCAM levels at 8 h and 24 h posttraining, and the chronic treatment decreasing its expression. None of the other brain areas examined showed significant changes in NCAM expression with corticosterone treatments, except for the hypothalamus that showed reduced NCAM levels after the chronic corticosterone regime. These results support the view that NCAM regulation at the frontal cortex might be a mechanism by which corticosterone treatments influence memory formation. They also highlight the hypothalamus as a brain area particularly sensitive to NCAM regulation by prolonged exposure to elevated glucocorticoids.

Effects of ion channel activity on development of dorsal root ganglion neurons

Studies of mouse dorsal root ganglion neurons in vitro demonstrate that ion channel function and regulation can influence a wide range of developmental processes. The work suggests that much as exposure to different trophic factors, the pattern of impulse activity a neuron experiences can have significant structural and functional effects during development. Studies concerning effects of ion channel activity on growth cone motility, axon fasciculation, synaptic plasticity, myelination, and intracellular signaling pathways regulating gene expression are presented in the context of changes in endogenous firing patterns during development.

Increased flexibility and selectivity in spatial learning of transgenic mice ectopically expressing the neural cell adhesion molecule L1 in astrocytes

The expression of the neural cell adhesion molecule L1 is altered by neuronal activity and promotes neurite outgrowth in vitro. To study the effects of L1 on learning and synaptic plasticity, transgenic mice have been created which express L1 ectopically in glial fibrillary acidic protein (GFAP) expressing astrocytes. Ninety mice, including GFAP-L1-transgenic mice from two genetic backgrounds and their littermates, were tested for swimming navigation learning in the Morris water maze according to a standardized protocol. While learning the position of an invisible target platform and also relearning its position after relocation, GFAP-L1-transgenic mice spent a greater fraction of their swim time in the target quadrant. Moreover, they showed a more rapid improvement of escape performance during the first day of training. Factor analysis revealed that this difference in swimming pattern could not be explained by non-cognitive factors. Factor analysis also revealed that, during a probe trial, the GFAP-L1-transgenic mice spent comparatively less time in the old target quadrant than predicted by the increased searching they had shown during acquisition learning. Hence, ectopic expression of L1 by astrocytes in mice appears to be linked to a factor which increases behavioural flexibility and selectivity while learning and relearning, but concomitantly may lead to a relative reduction of spatial retention.

Activity-dependent regulation of N-cadherin in DRG neurons: differential regulation of N-cadherin, NCAM, and L1 by distinct patterns of action potentials

Cell adhesion molecule (CAM) expression is highly regulated during nervous system development to control cell migration, neurite outgrowth, fasciculation, and synaptogenesis. Using electrical stimulation of mouse dorsal root ganglion (DRG) neurons in cell culture, this work shows that N-cadherin expression is regulated by neuronal firing, and that expression of different CAMs is regulated by distinct patterns of neural impulses. N-cadherin was down-regulated by 0.1 or 1 Hz stimulation, but NCAM mRNA and protein levels were not altered by stimulation. L1 was down-regulated by 0.1 Hz stimulation, but not by 0.3 Hz, 1 Hz, or pulsed stimulation. N-cadherin expression was lowered with faster kinetics than L1 (1 vs. 5 days), and L1 mRNA returned to higher levels after terminating the stimulus. The RSLE splice variant of L1 was not regulated by action potential stimulation, and activity-dependent influences on L1 expression were blocked by target-derived influences. The results are consistent with changes in firing pattern accompanying DRG development and suggest that functional activity can influence distinct developmental processes by regulating the relative abundance of different CAMs.

Neural cell adhesion molecules in activity-dependent development and synaptic plasticity

Cell adhesion molecules (CAMs) have a vital role in forming connections between neurons during embryonic development. Increasing evidence suggests that CAMs also participate in activity-dependent plasticity during development and synaptic plasticity in adults. Neural impulses of appropriate patterns can regulate expression of specific CAMs in mouse neurons from dorsal-root ganglia, alter cell-cell adhesion and produce structural reorganization of axon terminals in culture. Synaptic plasticity in Aplysia, learning in chick and long-term potentiation in rat hippocampus are accompanied by changes in CAM expression. Long-term potentiation can be blocked by disrupting CAM function in rat hippocampus, and learning deficits result from antibody blockade of CAMs in chicks and in transgenic mice lacking specific CAMs. Cell adhesion molecules might produce these effects by controlling several cellular processes, including cell adhesion, cytoskeletal structure and intracellular signaling.

Induction of myelination in the central nervous system by electrical activity

The oligodendrocyte is the myelin-forming cell in the central nervous system. Despite the close interaction between axons and oligodendrocytes, there is little evidence that neurons influence myelinogenesis. On the contrary, newly differentiated oligodendrocytes, which mature in culture in the total absence of neurons, synthesize the myelin-specific constituents of oligodendrocytes differentiated in vivo and even form myelin-like figures. Neuronal electrical activity may be required, however, for the appropriate formation of the myelin sheath. To investigate the role of electrical activity on myelin formation, we have used highly specific neurotoxins, which can either block (tetrodotoxin) or increase (alpha-scorpion toxin) the firing of neurons. We show that myelination can be inhibited by blocking the action potential of neighboring axons or enhanced by increasing their electrical activity, clearly linking neuronal electrical activity to myelinogenesis.

Effects of NMDA receptor blockade in the developing rat somatosensory cortex on the expression of the glia-derived extracellular matrix glycoprotein tenascin-C

The patterning of synaptic connections during development is thought to be influenced by the correlation of neuronal impulse activity. N-methyl-D-aspartate (NMDA) receptors have been implicated in the reorganization of thalamocortical afferents in the visual system. The topographic mapping of the periphery of sensory systems onto the somatosensory cortex in the whisker-barrel field of rodents has served as another important paradigm in the study of extrinsic influences on synaptic rearrangements. In a search for the molecular cues that may contribute to synaptic plasticity, we have investigated the distribution of the glia-derived extracellular matrix glycoprotein tenascin-C, which is highly expressed during the formation of the barrel field map around birth and delineates the boundaries between barrel fields after segregation of afferent inputs. Here we show that systemic and local application of NMDA receptor antagonists at postnatal day 2 inhibited the down-regulation of tenascin mRNA and protein by postnatal day 6 and prevented the appearance of tenascin-positive barrel field boundaries. Furthermore, barrels were not distinguishable by Nissl staining, and segregation of thalamocortical afferents as monitored by anterograde Dil tracing and acetylcholinesterase histochemistry was not complete. These observations indicate that expression of tenascin-C and segregation of afferent inputs are modified by NMDA receptor-dependent neuronal activity.

Neural recognition molecules in disease and regeneration

The genetic analysis of inherited human diseases of the nervous system and the characterization of transgenic mice deficient in neural recognition molecules is opening up a new dimension in understanding the cellular and molecular mechanisms underlying neuro-developmental and -degenerative diseases, as well as in delineating the functions of recognition molecules in cell-cell interactions. Progress in identifying recognition molecules that inhibit neurite outgrowth and further characterization of the mechanisms that promote neurite outgrowth are shedding more light on the processes of regeneration in the mature nervous system. In the adult, recognition functions are fine-tuned by glycan moeities associated with neural recognition molecules, and successful neurite outgrowth is likely to depend on the delicate balance between growth-promoting and inhibitory recognition cues.