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Our previous surgical manipulations have shown that the target ectoderm is necessary for the initial formation of one of the major cutaneous nerves in the embryonic chick limb (Honig, M.G., Camilli, S.J., Xue, Q.S., 2004. Ectoderm removal prevents cutaneous nerve formation and perturbs sensory axon growth in the chick hindlimb. Dev. Biol. 266, 27-42.). Moreover, the target ectoderm is required during a critical time period, at ~St. 24, when those axons are about to diverge from the hindlimb plexus. To elucidate the underlying mechanisms, here we examined the effects of removing the ectoderm at St. 24 on a variety of molecules expressed within the limb. We find that, while ectoderm removal is accompanied by changes in the expression of Lmx1, fibronectin, EphA7, cDermo-1, and in the complement of muscle cells, these changes do not account for the cutaneous nerve deficit. In contrast, an upregulation of PNA-binding sites and a downregulation of Bmp4 appear to be associated with this nerve deficit. Exogenous BMP4 reversed the effect of ectoderm removal on cutaneous nerve formation, but did not act as a chemoattractant. Our results suggest that BMP4, together with permissive and repulsive molecules that growing cutaneous axons encounter in the local environment and with signaling molecules, originating from and/or dependent on the ectoderm, work in concert to ensure proper cutaneous nerve formation. Keywords: Pathfinding; Sensory axons; Cutaneous nerves; Limb innervation; Axon guidance; Chick hindlimb; Skin innervation; BMPs; Chemoattraction; Ephs and ephrins

Target tissues are thought to provide important cues for growing axons, yet there is little direct evidence that they are essential for axonal pathfinding. Here we examined whether target ectoderm is necessary for the formation of cutaneous nerves, and for the normal growth and guidance of cutaneous axons as they first enter the limb plexus. To do this, we removed a patch of ectoderm from the chick hindlimb at various times during early axon outgrowth. We find there is a critical period when cutaneous nerve formation requires target ectoderm. When the ectoderm is absent during this time, axons progress into the limb more slowly and, although a few sensory axons occasionally diverge a short distance from the plexus, they do not form a discrete nerve that travels to the skin. A few days later, when the nerve pattern is mature, axons normally destined for the 'deprived' cutaneous nerve are not segregated appropriately within the plexus. Some cutaneous axons are instead misdirected along an inappropriate cutaneous nerve, while others have seemingly failed to reach their correct target, or a suitable alternative, and died. These results demonstrate that the target ectoderm is necessary for normal sensory axon growth and guidance in the hindlimb. Keywords: Pathfinding; Sensory axons; Cutaneous nerves; Limb innervation; Axon guidance; Development; Chick hindlimb: Target cues; Skin innervation

In the developing chick hindlimb, sensory axons, which grow together in bundles as they extend distally, and the motoneuron axons they encounter express the cell adhesion molecule L1. Following injection of function-blocking anti-L1 antibodies into the limb at stage 25, some sensory axons choose inappropriate peripheral nerves even though motoneuron pathfinding is unaffected. Here, to further elucidate L1's role, we assessed the effects of this perturbation using pathway tracing, immune labeling, confocal microscopy, and electron microscopy. After L1 blockade, sensory axons were still bundled and closely apposed. However, clear signs of decreased adhesion were detectable ultrastructurally. Further, sensory axons grew into the limb more slowly than normal, wandering more widely, branching more frequently, and sometimes extending along inappropriate peripheral nerves. Sensory axons that ultimately projected along different cutaneous nerves showed increased intermixing in the spinal nerves, due to errors in pathfinding and also to a decreased ability to segregate into nerve-specific fascicles. These results suggest that, in the highly complex in vivo environment, as in tissue culture, L1 stimulates axon growth and enhances fasciculation, and that these processes contribute to the orderly, timely, and specific growth of sensory axons into the limb. Key Words: pathfinding; fasciculation; L1; sensory axons; cell adhesion molecules; dorsal root ganglia; axon guidance; axon outgrowth; limb innervation; cutaneous nerve.

In the developing chick hindlimb, pathfinding by sensory axons is affected by their interactions with other axons entering the limb. Cell adhesion molecules (CAMs) expressed on the growing axons are likely to influence these interactions. Accordingly, to elucidate how CAMs affect sensory axon pathfinding, we injected antibodies that block the functions of NCAM, G4/L1, or N-cadherin into the hindlimb, starting at St. 25, when all motoneuron axons but only a few sensory axons had entered the plexus. In each case, the segmental pattern of projections was assessed 9-3 days later by retrogradely labeling individual peripheral nerves. With all perturbations, the gross anatomical nerve pattern developed normally; that is, some sensory axons formed cutaneous nerves while other sensory axons projected to muscles. However, the segmental pattern of sensory projections was changed when either G4/L1 or N-cadherin function was blocked in that fewer sensory axons crossed the anterior-posterior axis of the plexus. A likely reason for this effect is that anti-G4/L1 and anti-N-cadherin each decreased the amount of fasciculation and that sensory axons are less able to travel across the plexus when they are defasciculated. Anti-G4/L1 affected both cutaneous and muscle sensory projections while anti-N-cadherin affected cutaneous but not muscle sensory projections, in accord with known differences in the expression of these two CAMs on sensory and motoneuron axons. Although anti-NCAM did not appear to alter sensory projections, when polysialic acid (PSA) was enzymatically removed from NCAM, there was a marked increase in cutaneous projections from the most proximate DRG, although muscle sensory projections were unchanged. PSA removal may cause an increase in fasciculation that forces sensory axons to track along neighboring axons. Thus, without PSA, cutaneous axons project more in accord with the relative anterior-posterior positions they had as they entered the plexus. Taken together, these studies suggest that axonal fasciculation mediated by CAMs and regulated by PSA influences the ability of sensory growth cones to navigate through the plexus and project along the correct peripheral nerves.

A study of the expression of cell adhesion molecules (CAMs) on the growth cones of chick cutaneous and muscle sensory neurons through the immunofluorescent labeling of the growth cones of muscle and cutaneous sensory neurons reveals an even labeling on the growth cone, the growth cone-adjacent neurite segment and the filopodia for all CAMs studied. Sites of interneuronal contact exhibit intense labeling for axonin-1. Image analysis of labeling intensities reveals that muscle and cutaneous sensory neurons exhibit similar intensities for axonin-1 and G4/L1.

A study of the presence and specificity of attraction between sensory axons and target tissues from chick hindlimbs during early development through retrograde labeling of muscle or cutaneous sensory neurons before three-dimensional collagen gel coculturing reveals that sensory neurons are attracted by the target cells, without any specificity. Diffusible factors secreted by muscle and dermis cells stimulate the sensory axons to migrate away from the plexus toward the target cells, forming discrete peripheral nerves.

We have previously reported that mild TBI created by focal left-side cranial blast in mice produces widespread axonal injury, microglial activation, and a variety of functional deficits. We have also shown that these functional deficits are reduced by targeting microglia through their cannabinoid type-2 (CB2) receptors using 2-week daily administration of the CB2 inverse agonist SMM-189. CB2 inverse agonists stabilize the G-protein coupled CB2 receptor in an inactive conformation, leading to increased phosphorylation and nuclear translocation of the cAMP response element binding protein (CREB), and thus bias activated microglia from a pro-inflammatory M1 to a pro-healing M2 state. In the present study, we showed that SMM-189 boosts nuclear pCREB levels in microglia in several brain regions by 3 days after TBI, by using pCREB/CD68 double immunofluorescent labeling. Next, to better understand the basis of motor deficits and increased fearfulness after TBI, we used unbiased stereological methods to characterize neuronal loss in cortex, striatum, and basolateral amygdala (BLA) and assessed how neuronal loss was affected by SMM-189 treatment. Our stereological neuron counts revealed a 20% reduction in cortical and 30% reduction in striatal neurons bilaterally at 2-3 months post blast, with SMM-189 yielding about 50% rescue. Loss of BLA neurons was restricted to the blast side, with 33% of Thy1+ fear-suppressing pyramidal neurons and 47% of fear-suppressing parvalbuminergic (PARV) interneurons lost, and Thy1-negative fear-promoting pyramidal neurons not significantly affected. SMM-189 yielded 50-60% rescue of Thy1+ and PARV neuron loss in BLA. Thus, fearfulness after mild TBI may result from the loss of fear-suppressing neuron types in BLA, and SMM-189 may reduce fearfulness by their rescue. Overall, our findings indicate that SMM-189 rescues damaged neurons and thereby alleviates functional deficits resulting from TBI, apparently by selectively modulating microglia to the beneficial M2 state. CB2 inverse agonists thus represent a promising therapeutic approach for mitigating neuroinflammation and neurodegeneration. [ABSTRACT FROM AUTHOR]

We have developed a focal blast model of closed-head mild traumatic brain injury (TBI) in mice. As true for individuals that have experienced mild TBI, mice subjected to 50-60 psi blast show motor, visual and emotional deficits, diffuse axonal injury and microglial activation, but no overt neuron loss. Because microglial activation can worsen brain damage after a concussive event and because microglia can be modulated by their cannabinoid type 2 receptors (CB2), we evaluated the effectiveness of the novel CB2 receptor inverse agonist SMM-189 in altering microglial activation and mitigating deficits after mild TBI. In vitro analysis indicated that SMM-189 converted human microglia from the pro-inflammatory M1 phenotype to the pro-healing M2 phenotype. Studies in mice showed that daily administration of SMM-189 for two weeks beginning shortly after blast greatly reduced the motor, visual, and emotional deficits otherwise evident after 50-60 psi blasts, and prevented brain injury that may contribute to these deficits. Our results suggest that treatment with the CB2 inverse agonist SMM-189 after a mild TBI event can reduce its adverse consequences by beneficially modulating microglial activation. These findings recommend further evaluation of CB2 inverse agonists as a novel therapeutic approach for treating mild TBI. [ABSTRACT FROM AUTHOR]

Axonal interactions, which are mediated by cell adhesion molecules (CAMs) as well as other types of membrane proteins, are important for sensory axon pathfinding in the developing chick hindlimb. We have previously shown that injection of antibodies that block the function of either G4/L1 or N-cadherin into the limb, starting when the first sensory axons reach the plexus, alters the segmental pattern of projections along cutaneous nerves. Specific removal of polysialic acid from NCAM using the enzyme endoneuraminidase N (Endo N) also resulted in significant changes in cutaneous projection patterns, while injection of antibodies against NCAM itself had no obvious effect (M. G. Honig and U.S. Rutishauser, 1996, Dev. Biol. 175, 325-337). To help understand the cellular basis for these findings, we developed a tissue culture system in which the axons from dorsal root ganglion explants grow within defined laminin lanes and examined whether the same treatments increased or decreased a growth cone's tendency to be closely associated with neighboring axons. After 2 days in culture, images of the cultures were recorded, antibodies or Endo N was added, and images of the same fields were recaptured an hour later. To quantify the results, growth cones located in defined regions of the laminin lanes were classified, before and after the perturbation, as 'free' (i.e., growing primarily on the laminin substratum), 'fasciculated' (i.e., growing tightly along other neurites), or 'intermediate' (i.e., growing both on the laminin substratum and in contact with other neurites). We found that anti-G4/L1 and anti-N-cadherin, but not anti-NCAM, caused an increase in defasciculated growth cones, whereas Endo N resulted in an increase in fasciculated growth cones. These changes in fasciculation are consistent with the changes in cutaneous projections seen in our previous in ovo perturbations. The results from these tissue culture experiments thus provide strong support for the idea that one mechanism by which CAMs affect sensory axon pathfinding in vivo is by regulating the affinity of sensory growth cones for neighboring axons, which in turn can modulate the growth cone's ability to navigate through the surrounding environment. Key Words: pathfinding; cell adhesion molecules; fasciculation.