No abstract

Robust axonal growth is required during development to establish neuronal connectivity. However, stable fibre patterns are necessary to maintain adult mammalian central nervous system (CNS) function. After adult CNS injury, factors that maintain axonal stability limit the recovery of function. Extracellular molecules play an important role in preserving the stability of the adult CNS axons and in restricting recovery from pathological damage. Adult axonal growth inhibitors include a group of proteins on the oligodendrocyte, Nogo-A, myelin-associated glycoprotein, oligodendrocyte-myelin glycoprotein and ephrin-B3, which interact with axonal receptors, such as NgR1 and EphA4. Extracellular proteoglycans containing chondroitin sulphates also inhibit axonal sprouting in the adult CNS, particularly at the sites of astroglial scar formation. Therapeutic perturbations of these extracellular axonal growth inhibitors and their receptors or signalling mechanisms provide a degree of axonal sprouting and regeneration in the adult CNS. After CNS injury, such interventions support a partial return of neurological function.

In schizophrenia, genetic predisposition has been linked to chromosome 22q11 and myelin-specific genes are misexpressed in schizophrenia. Nogo-66 receptor 1 (NGR or RTN4R) has been considered to be a 22q11 candidate gene for schizophrenia susceptibility because it encodes an axonal protein that mediates myelin inhibition of axonal sprouting. Confirming previous studies, we found that variation at the NGR locus is associated with schizophrenia in a Caucasian case-control analysis, and this association is not attributed to population stratification. Within a limited set of schizophrenia-derived DNA samples, we identified several rare NGR nonconservative coding sequence variants. Neuronal cultures demonstrate that four different schizophrenia-derived NgR1 variants fail to transduce myelin signals into axon inhibition, and function as dominant negatives to disrupt endogenous NgR1. This provides the first evidence that certain disease-derived human NgR1 variants are dysfunctional proteins in vitro. Mice lacking NgR1 protein exhibit reduced working memory function, consistent with a potential endophenotype of schizophrenia. For a restricted subset of individuals diagnosed with schizophrenia, the expression of dysfunctional NGR variants may contribute to increased disease risk.

Abstract-We investigated roles for homocellular (endothelium or smooth muscle) and heterocellular (myoendothelial) conduction pathways along hamster cheek pouch arterioles in vivo (nϭ64; diameter, 33Ϯ1 m). Endothelium-dependent and -independent vasoactive agents were delivered from micropipettes (0.5 or 1 second pulse) onto an arteriole while observing diameter changes at defined distances along the vessel. Acetylcholine (ACh) produced maximal diameter (63Ϯ1 m) locally and vasodilation conducted rapidly (Ϸ10 m response at 2 mm, Ͻ1 second). Responses to bradykinin (BK) were similar, whereas sodium nitroprusside produced maximal dilation locally without conduction. KCl evoked biphasic conduction of vasoconstriction and vasodilation, whereas phenylephrine (PE) produced conducted vasoconstriction. Disrupting the integrity of endothelium as a conduction pathway using focal light-dye treatment (LDT) abolished conducted vasodilation to BK and to KCl but not to ACh. Disruption of smooth muscle integrity with LDT abolished conducted vasoconstriction with no effect on conducted vasodilation. After LDT of respective cell layers at sites 1 mm apart, vasodilation to ACh conducted past disrupted smooth muscle or disrupted endothelium, but not beyond both sites in series. The loss of conduction after selective LDT indicates a lack of effective myoendothelial coupling along the arteriolar wall. During NO synthase inhibition (L-NA, 100 mol/L), conducted vasodilation was abolished to BK and to KCl yet remained intact to ACh. However, after LDT of smooth muscle, L-NA inhibited conduction to ACh by 60%. Thus, conduction of vasodilation entails a wave of NO release along arteriolar endothelium that is masked when smooth muscle provides a parallel conduction pathway.

The GTP-binding protein RhoA regulates microfilament dynamics in many cell types and mediates the inhibition of axonal regeneration by myelin and chondroitin sulfate proteoglycans. Unlike most other nonsteroidal anti-inflammatory drugs, ibuprofen suppresses basal RhoA activity (Zhou et al., 2003). A recent report suggested that ibuprofen promotes corticospinal axon regeneration after spinal cord injury (Fu et al., 2007). Here, we confirm that ibuprofen reduces ligand-induced Rho signaling and myelin-induced inhibition of neurite outgrowth in vitro. Following 4 weeks of subcutaneous administration of ibuprofen, beginning 3 days after spinal cord contusion, animals recovered walking function to a greater degree, with twice as many rats achieving a hind limb weight-bearing status. We examined the relative role of tissue sparing, axonal sprouting, and axonal regeneration in the action of ibuprofen. Histologically, ibuprofen-treated animals display an increase in spared tissue without an alteration in astrocytic or microglial reaction. Ibuprofen increases axonal sprouting from serotonergic raphespinal axons, and from rostral corticospinal fibers in the injured spinal cord, but does not permit caudal corticospinal regeneration after spinal contusion. Treatment of mice with complete spinal cord transection demonstrates long-distance raphespinal axon regeneration in the presence of ibuprofen. Thus, administration of ibuprofen improves the recovery of rats from a clinically relevant spinal cord trauma by protecting tissue, stimulating axonal sprouting, and allowing a minor degree of raphespinal regeneration.

Axon growth after spinal injury is thought to be limited in part by myelin-derived proteins that act via the Nogo-66 Receptor (NgR). To test this hypothesis, we sought to study recovery from spinal cord injury (SCI) after inhibiting NgR transgenically with a soluble function-blocking NgR fragment. Glial fibrillary acidic protein (gfap) gene regulatory elements were used to generate mice that secrete NgR(310)ecto from astrocytes. After mid-thoracic dorsal over-hemisection injury, gfap∷ngr(310)ecto mice exhibit enhanced raphespinal and corticospinal axonal sprouting into the lumbar spinal cord. Recovery of locomotion is improved in the gfap∷ngr(310)ecto mice. These data indicate that the NgR ligands, Nogo-66, MAG, and OMgp, play a role in limiting axonal growth in the injured adult CNS and that NgR(310)ecto might provide a therapeutic means to promote recovery from SCI.

No abstract

We tested the hypothesis that the cytosolic free calcium concentration in endothelial cells is under the influence of the smooth muscle cells in the coronary circulation. In the left descending branch of porcine coronary arteries, cytosolic free calcium concentration ([Ca(2+)](i)) was estimated by determining the fluorescence ratio of two calcium probes, fluo 4 and fura red, in smooth muscle and endothelial cells using confocal microscopy. Acetylcholine and potassium, which act directly on smooth muscle cells to increase [Ca(2+)](i), were found to indirectly elevate [Ca(2+)](i) in endothelial cells; in primary cultures of endothelial cells, neither stimulus affected [Ca(2+)](i), yet substance P increased the fluorescence ratio twofold. In response to acetylcholine and potassium, isometric tension developed by arterial strips with intact endothelium was attenuated by up to 22% (P < 0.05) compared with strips without endothelium. These findings suggest that stimuli that increase smooth muscle [Ca(2+)](i) can indirectly influence endothelial cell function in porcine coronary arteries. Such a pathway for negative feedback can moderate vasoconstriction and diminish the potential for vasospasm in the coronary circulation.