Involvement of alpha7beta1 integrin in the conditioning-lesion effect on sensory axon regeneration

A basement membrane extract-based three-dimensional culture system promotes the neuronal differentiation of cochlear Sox10-positive glial cells in vitro

Spiral ganglion neurons (SGNs) in the mammalian cochleae are essential for the delivery of acoustic information, and damage to SGNs can lead to permanent sensorineural hearing loss as SGNs are not capable of regeneration. Cochlear glial cells (GCs) might be a potential source for SGN regeneration, but the neuronal differentiation ability of GCs is limited and its properties are not clear yet. Here, we characterized the cochlear Sox10-positive (Sox10+) GCs as a neural progenitor population and developed a basement membrane extract-based three-dimensional (BME-3D) culture system to promote its neuronal generation capacity in vitro. Firstly, the purified Sox10+ GCs, isolated from Sox10-creER/tdTomato mice via flow cytometry, were able to form neurospheres after being cultured in the traditional suspension culture system, while significantly more neurospheres were found and the expression of stem cell-related genes was upregulated in the BME-3D culture group. Next, the BME-3D culture system promoted the neuronal differentiation ability of Sox10+ GCs, as evidenced by the increased number, neurite outgrowth, area of growth cones, and synapse density as well as the promoted excitability of newly induced neurons. Notably, the BME-3D culture system also intensified the reinnervation of newly generated neurons with HCs and protected the neurospheres and derived-neurons against cisplatin-induced damage. Finally, transcriptome sequencing analysis was performed to identify the characteristics of the differentiated neurons. These findings suggest that the BME-3D culture system considerably promotes the proliferation capacity and neuronal differentiation efficiency of Sox10+ GCs in vitro, thus providing a possible strategy for the SGN regeneration study.

The Integrin Signaling Network Promotes Axon Regeneration via the Src-Ephexin-RhoA GTPase Signaling Axis

Axon regeneration is an evolutionarily conserved process essential for restoring the function of damaged neurons. In Caenorhabditis elegans hermaphrodites, initiation of axon regeneration is regulated by the RhoA GTPase-ROCK (Rho-associated coiled-coil kinase)-regulatory nonmuscle myosin light-chain phosphorylation signaling pathway. However, the upstream mechanism that activates the RhoA pathway remains unknown. Here, we show that axon injury activates TLN-1/talin via the cAMP-Epac (exchange protein directly activated by cAMP)-Rap GTPase cascade and that TLN-1 induces multiple downstream events, one of which is integrin inside-out activation, leading to the activation of the RhoA-ROCK signaling pathway. We found that the nonreceptor tyrosine kinase Src, a key mediator of integrin signaling, activates the Rho guanine nucleotide exchange factor EPHX-1/ephexin by phosphorylating the Tyr-568 residue in the autoinhibitory domain. Our results suggest that the C. elegans integrin signaling network regulates axon regeneration via the Src-RhoGEF-RhoA axis.SIGNIFICANCE STATEMENT The ability of axons to regenerate after injury is governed by cell-intrinsic regeneration pathways. We have previously demonstrated that the Caenorhabditis elegans RhoA GTPase-ROCK (Rho-associated coiled-coil kinase) pathway promotes axon regeneration by inducing MLC-4 phosphorylation. In this study, we found that axon injury activates TLN-1/talin through the cAMP-Epac (exchange protein directly activated by cAMP)-Rap GTPase cascade, leading to integrin inside-out activation, which promotes axonal regeneration by activating the RhoA signaling pathway. In this pathway, SRC-1/Src acts downstream of integrin activation and subsequently activates EPHX-1/ephexin RhoGEF by phosphorylating the Tyr-568 residue in the autoinhibitory domain. Our results suggest that the C. elegans integrin signaling network regulates axon regeneration via the Src-RhoGEF-RhoA axis.

EFA6 in Axon Regeneration, as a Microtubule Regulator and as a Guanine Nucleotide Exchange Factor

Axon regeneration after injury is a conserved biological process that involves a large number of molecular pathways, including rapid calcium influx at injury sites, retrograde injury signaling, epigenetic transition, transcriptional reprogramming, polarized transport, and cytoskeleton reorganization. Despite the numerous efforts devoted to understanding the underlying cellular and molecular mechanisms of axon regeneration, the search continues for effective target molecules for improving axon regeneration. Although there have been significant historical efforts towards characterizing pro-regenerative factors involved in axon regeneration, the pursuit of intrinsic inhibitors is relatively recent. EFA6 (exchange factor for ARF6) has been demonstrated to inhibit axon regeneration in different organisms. EFA6 inhibition could be a promising therapeutic strategy to promote axon regeneration and functional recovery after axon injury. This review summarizes the inhibitory role on axon regeneration through regulating microtubule dynamics and through affecting ARF6 (ADP-ribosylation factor 6) GTPase-mediated integrin transport.

Different epidermal growth factor receptor signaling pathways in neurons and astrocytes activated by extracellular matrix after spinal cord injury

Spinal cord injury (SCI) is a serious central nervous system (CNS) trauma that results in permanent and severe disability. The extracellular matrix (ECM) can affect the activation of extracellular signal-regulated kinase 1/2 (ERK_1/2) by interacting with the ERK integrin subunits. In this study, we built a model of SCI with glial fibrillary acidic protein-green fluorescent protein (GFAP-GFP) and thymus cell antigen 1-yellow fluorescent protein-H (Thy1-YFPH) in mice that express specific transgenes in their astrocytes or neurons. Then, we collected spinal cord neurons or astrocytes by fluorescence-activated cell sorting (FACS). In this way, we investigated the SCI-induced phosphorylation of ERK_1/2 and epidermal growth factor receptor (EGFR) in neurons and astrocytes, and we discovered that the SCI-induced EGFR signaling pathways differed between neurons and astrocytes. In the present study, we found that the Src-dependent phosphorylation of EGFR induced by SCI occurred only in neurons, not in astrocytes. This phenomenon may be due to the involvement of Thy-1, which promoted the binding between Src and EGFR in neurons after SCI. In addition, the expression of the integrin subunits after SCI differed between neurons and astrocytes. Our present study shows that the EGFR signaling pathway triggered by SCI in neurons differed from the EGFR signaling pathway triggered in astrocytes, a finding that may help to pave the way for clinical trials of therapies that inhibit EGFR signaling pathways after SCI.

An In Vitro Model for Conditioning Lesion Effect

Axons of a peripheral nerve grow faster after an axotomy if it attains a prior injury a few days earlier. This is called conditioning lesion effect (CLE) and very much valued since it may provide new insights into neuron biology and axonal regeneration. There are established in vivo experimental paradigms to study CLE, however, there is a need to have an in vitro conditioning technique where CLE occurs in a maximally controlled environment. Mouse primary sensory neurons were isolated from lumbar 4-5 dorsal root ganglia and incubated at 37 °C on a silicon-coated watch glass that prevents cell attachment. After this conditioning period they were transferred to laminin coated culture dishes. Similar cultures were set up with freshly isolated neurons from control animals and from the animals that received a sciatic nerve cut 3 days earlier. All preparations were placed on a live cell imaging microscopy providing physiological conditions and photographed for 48 h. Axonal regeneration and neuronal survival was assessed. During the conditioning incubation period neurons remained in suspended aggregates and did not grow axons. The regeneration rate of the in vitro conditioned neurons was much higher than the in vivo conditioned and control preparations during the first day of normal incubation. However, higher regeneration rates were compromised by progressive substantial neuronal death in both types of conditioned cultures but not in the control preparations. By using neutralizing antibodies, we demonstrated that activity of endogenous leukemia inhibitory factor is essential for induction of CLE in this model.

Integrins promote axonal regeneration after injury of the nervous system

Integrins are cell surface receptors that form the link between extracellular matrix molecules of the cell environment and internal cell signalling and the cytoskeleton. They are involved in several processes, e.g. adhesion and migration during development and repair. This review focuses on the role of integrins in axonal regeneration. Integrins participate in spontaneous axonal regeneration in the peripheral nervous system through binding to various ligands that either inhibit or enhance their activation and signalling. Integrin biology is more complex in the central nervous system. Integrins receptors are transported into growing axons during development, but selective polarised transport of integrins limits the regenerative response in adult neurons. Manipulation of integrins and related molecules to control their activation state and localisation within axons is a promising route towards stimulating effective regeneration in the central nervous system.

Dendrite regeneration of adult Drosophila sensory neurons diminishes with aging and is inhibited by epidermal-derived matrix metalloproteinase 2

DeVault et al. show that the capacity for regeneration was present in adult neurons but diminished as the animal aged. The regenerated dendrites showed preferential alignment with the extracellular matrix, and inhibition of matrix metalloproteinase 2 led to increased dendrite regeneration.

Dendrites possess distinct structural and functional properties that enable neurons to receive information from the environment as well as other neurons. Despite their key role in neuronal function, current understanding of the ability of neurons to regenerate dendrites is lacking. This study characterizes the structural and functional capacity for dendrite regeneration in vivo in adult animals and examines the effect of neuronal maturation on dendrite regeneration. We focused on the class IV dendritic arborization (c4da) neuron of the Drosophila sensory system, which has a dendritic arbor that undergoes dramatic remodeling during the first 3 d of adult life and then maintains a relatively stable morphology thereafter. Using a laser severing paradigm, we monitored regeneration after acute and spatially restricted injury. We found that the capacity for regeneration was present in adult neurons but diminished as the animal aged. Regenerated dendrites recovered receptive function. Furthermore, we found that the regenerated dendrites show preferential alignment with the extracellular matrix (ECM). Finally, inhibition of ECM degradation by inhibition of matrix metalloproteinase 2 (Mmp2) to preserve the extracellular environment characteristics of young adults led to increased dendrite regeneration. These results demonstrate that dendrites retain regenerative potential throughout adulthood and that regenerative capacity decreases with aging.

Integrin Activation: Implications for Axon Regeneration

Integrin activation is essential for creating functional transmembrane receptors capable of inducing downstream cellular effects such as cell migration, cell spreading, neurite outgrowth and axon regeneration. Integrins are bidirectional signalling molecules that mediate their effects by 'inside-out' and 'outside-in' signalling. This review will provide a detailed overview of integrin activation focusing on intracellular activation in neurons and discussing direct implications in the regulation of neurite outgrowth and axon regeneration.

α6 and β1 Integrin Heterodimer Mediates Schwann Cell Interactions with Axons and Facilitates Axonal Regeneration after Peripheral Nerve Injury

Several isoforms of integrin subunits are expressed in Schwann cells and mediate Schwann cell interactions with axons. Here, we identify α6 and β1 integrins as heterodimeric proteins expressed in Schwann cells and define their functions in axonal regeneration. α6 and β1 integrins are induced in Schwann cells in the sciatic nerve after a crush injury, and the blocking of integrin activity by siRNA expression and by treatment with anti-integrin antibodies attenuates Schwann cell contact with cultured neurons and decreases neurite outgrowth. After nerve transection, the levels of α6 and β1 integrins in the distal nerve stump are lower than those in the corresponding nerve area after a crush injury. Schwann cells prepared from the distal nerves 7 days after transection are less supportive of neurite outgrowth in co-cultured neurons than those prepared from the nerves 7 days after a crush injury. When the transected nerves are reconnected after a delay of 1 to 2 weeks, the induced levels of α6 and β1 integrins in the reconnected distal nerves are significantly reduced compared to those in the nerves after a crush injury. These changes correlate with retarded axonal regeneration in animals that have experienced nerve transections and delayed coaptation, which implies an attenuated Schwann cell capacity to support axonal regeneration due to delayed Schwann cell contact with axons. The present data suggest that α6 and β1 integrins induced in Schwann cells after nerve injury may play a role in mediating Schwann cell interactions with axons and promote axonal regeneration.

Expression of an Activated Integrin Promotes Long-Distance Sensory Axon Regeneration in the Spinal Cord

After CNS injury, axon regeneration is blocked by an inhibitory environment consisting of the highly upregulated tenascin-C and chondroitin sulfate proteoglycans (CSPGs). Tenascin-C promotes growth of axons if they express a tenascin-binding integrin, particularly α9β1. Additionally, integrins can be inactivated by CSPGs, and this inhibition can be overcome by the presence of a β1-binding integrin activator, kindlin-1. We examined the synergistic effect of α9 integrin and kindlin-1 on sensory axon regeneration in adult rat spinal cord after dorsal root crush and adeno-associated virus transgene expression in dorsal root ganglia. After 12 weeks, axons from C6-C7 dorsal root ganglia regenerated through the tenascin-C-rich dorsal root entry zone into the dorsal column up to C1 level and above (>25 mm axon length) through a normal pathway. Animals also showed anatomical and electrophysiological evidence of reconnection to the dorsal horn and behavioral recovery in mechanical pressure, thermal pain, and ladder-walking tasks. Expression of α9 integrin or kindlin-1 alone promoted much less regeneration and recovery.

SIGNIFICANCE STATEMENT

The study demonstrates that long-distance sensory axon regeneration over a normal pathway and with sensory and sensory-motor recovery can be achieved. This was achieved by expressing an integrin that recognizes tenascin-C, one of the components of glial scar tissue, and an integrin activator. This enabled extensive long-distance (>25 mm) regeneration of both myelinated and unmyelinated sensory axons with topographically correct connections in the spinal cord. The extent of growth and recovery we have seen would probably be clinically significant. Restoration of sensation to hands, perineum, and genitalia would be a significant improvement for a spinal cord-injured patient.

Substratum preferences of motor and sensory neurons in postnatal and adult rats

After peripheral nerve injuries, damaged axons can regenerate but functional recovery is limited by the specific reinnervation of targets. In this study we evaluated if motor and sensory neurites have a substrate preference for laminin and fibronectin in postnatal and adult stages. In postnatal dorsal root ganglia (DRG) explants, sensory neurons extended longer neurites on collagen matrices enriched with laminin (~50%) or fibronectin (~35%), whereas motoneurons extended longer neurites (~100%) in organotypic spinal cord slices embedded in fibronectin-enriched matrix. An increased percentage of parvalbumin-positive neurites (presumptive proprioceptive) vs. neurofilament-positive neurites was also found in DRG in fibronectin-enriched matrix. To test if the different preference of neurons for extracellular matrix components was maintained in vivo, these matrices were used to fill a chitosan guide to repair a 6-mm gap in the sciatic nerve of adult rats. However, the number of regenerating motor and sensory neurons after 1 month was similar between groups. Moreover, none of the retrotraced sensory neurons in DRG was positive for parvalbumin, suggesting that presumptive proprioceptive neurons had poor regenerative capabilities compared with other peripheral neurons. Using real-time PCR we evaluated the expression of α5β1 (receptor for fibronectin) and α7β1 integrin (receptor for laminin) in spinal cord and DRG 2 days after injury. Postnatal animals showed a higher increase of α5β1 integrin, whereas both integrins were similarly expressed in adult neurons. Therefore, we conclude that motor and sensory axons have a different substrate preference at early postnatal stages but this difference is lost in the adult.

The neuroimmunology of degeneration and regeneration in the peripheral nervous system

Peripheral nerves regenerate following injury due to the effective activation of the intrinsic growth capacity of the neurons and the formation of a permissive pathway for outgrowth due to Wallerian degeneration (WD). WD and subsequent regeneration are significantly influenced by various immune cells and the cytokines they secrete. Although macrophages have long been known to play a vital role in the degenerative process, recent work has pointed to their importance in influencing the regenerative capacity of peripheral neurons. In this review, we focus on the various immune cells, cytokines, and chemokines that make regeneration possible in the peripheral nervous system, with specific attention placed on the role macrophages play in this process.

cJun promotes CNS axon growth

A number of genes regulate regeneration of peripheral axons, but their ability to drive axon growth and regeneration in the central nervous system (CNS) remains largely untested. To address this question we overexpressed eight transcription factors and one small GTPase alone and in pairwise combinations to test whether combinatorial overexpression would have a synergistic impact on CNS neuron neurite growth. The Jun oncogene/signal transducer and activator of transcription 6 (JUN/STAT6) combination increased neurite growth in dissociated cortical neurons and in injured cortical slices. In injured cortical slices, JUN overexpression increased axon growth to a similar extent as JUN and STAT6 together. Interestingly, JUN overexpression was not associated with increased growth associated protein 43 (GAP43) or integrin alpha 7 (ITGA7) expression, though these are predicted transcriptional targets. This study demonstrates that JUN overexpression in cortical neurons stimulates axon growth, but does so independently of changes in expression of genes thought to be critical for JUNs effects on axon growth. We conclude that JUN activity underlies this CNS axonal growth response, and that it is mechanistically distinct from peripheral regeneration responses, in which increases in JUN expression coincide with increases in GAP43 expression.

A critical role for macrophages near axotomized neuronal cell bodies in stimulating nerve regeneration

Macrophages have been implicated in peripheral nerve regeneration for some time, supposedly through their involvement in Wallerian degeneration, the process by which the distal nerve degenerates after axotomy and is cleared by phagocytosis. Thus, in several studies in which macrophage accumulation in the distal nerve was reduced and Wallerian degeneration inhibited, regeneration was delayed. However, this interpretation ignores the more recent findings that macrophages also accumulate around axotomized cell bodies. The function of macrophage action at this second site has not been clear. In two mutant strains of mice, the slow Wallerian degeneration (Wld(s)) mouse and the chemokine receptor CCR2 knock-out mouse, we report that macrophage accumulation after axotomy was abolished in both the dorsal root ganglion (DRG) and the distal sciatic nerve. To measure neurite outgrowth, DRG neurons were given a conditioning lesion, and outgrowth was measured in vitro 7 d later in the absence of the distal nerve segment. The increased growth normally seen after a conditioning lesion did not occur or was reduced in Wld(s) or CCR2(-/-) mice. In the superior cervical ganglion (SCG), particularly in Wld(s) mice, macrophage accumulation was reduced but not abolished after axotomy. In SCG neurons from Wld(s) mice, the conditioning lesion response was unchanged; however, in CCR2(-/-) mice in which the effect on macrophage accumulation was greater, SCG neurite outgrowth was significantly reduced. These results indicate that macrophages affect neurite outgrowth by acting at the level of peripheral ganglia in addition to any effects they might produce by facilitation of Wallerian degeneration.

Proregenerative properties of ECM molecules

After traumatic injuries to the nervous system, regrowing axons encounter a complex microenvironment where mechanisms that promote regeneration compete with inhibitory processes. Sprouting and axonal regrowth are key components of functional recovery but are often counteracted by inhibitory molecules. This review covers extracellular matrix molecules that support neuron axonal outgrowth.

Differentiated adipose-derived stem cells act synergistically with RGD-modified surfaces to improve neurite outgrowth in a co-culture model

Peripheral nerve damage is a problem encountered after trauma and during surgery and the development of synthetic polymer conduits may offer a promising alternative to autografts. In order to improve the performance of the polymer to be used for nerve conduits, poly-ε-caprolactone (PCL) films were chemically functionalized with RGD moieties, using a chemical reaction previously developed. In vitro cultures of dissociated dorsal root ganglion (DRG) neurons provide a valid model to study different factors affecting axonal growth. In this work, DRG neurons were cultured on RGD-functionalized PCL films. Adult adipose-derived stem cells differentiated to Schwann cells (dASCs) were initially cultured on the functionalized PCL films, resulting in improved attachment and proliferation. dASCs were also co-cultured with DRG neurons on treated and untreated PCL to assess stimulation by dASCs on neurite outgrowth. Neuron response was generally poor on untreated PCL films, but long neurites were observed in the presence of dASCs or RGD moieties. A combination of the two factors enhanced even further neurite outgrowth, acting synergistically. Finally, in order to better understand the extracellular matrix (ECM)-cell interaction, a β1 integrin blocking experiment was carried out. Neurite outgrowth was not affected by the specific antibody blocking, showing that β1 integrin function can be compensated by other molecules present on the cell membrane. Copyright © 2013 John Wiley & Sons, Ltd.

Adult motor axons preferentially reinnervate predegenerated muscle nerve

Preferential motor reinnervation (PMR) is the tendency for motor axons regenerating after repair of mixed nerve to reinnervate muscle nerve and/or muscle rather than cutaneous nerve or skin. PMR may occur in response to the peripheral nerve pathway alone in juvenile rats (Brushart, 1993; Redett et al., 2005), yet the ability to identify and respond to specific pathway markers is reportedly lost in adults (Uschold et al., 2007). The experiments reported here evaluate the relative roles of pathway and end organ in the genesis of PMR in adult rats. Fresh and 2-week predegenerated femoral nerve grafts were transferred in correct or reversed alignment to replace the femoral nerves of previously unoperated Lewis rats. After 8 weeks of regeneration the motoneurons projecting through the grafts to recipient femoral cutaneous and muscle branches and their adjacent end organs were identified by retrograde labeling. Motoneuron counts were subjected to Poisson regression analysis to determine the relative roles of pathway and end organ identity in generating PMR. Transfer of fresh grafts did not result in PMR, whereas substantial PMR was observed when predegenerated grafts were used. Similarly, the pathway through which motoneurons reached the muscle had a significant impact on PMR when grafts were predegenerated, but not when they were fresh. Comparison of the relative roles of pathway and end organ in generating PMR revealed that neither could be shown to be more important than the other. These experiments demonstrate unequivocally that adult muscle nerve and cutaneous nerve differ in qualities that can be detected by regenerating adult motoneurons and that can modify their subsequent behavior. They also reveal that two weeks of Wallerian degeneration modify the environment in the graft from one that provides no modality-specific cues for motor neurons to one that actively promotes PMR.

Extracellular matrix-associated gene expression in adult sensory neuron populations cultured on a laminin substrate

Background

In our previous investigations of the role of the extracellular matrix (ECM) in promoting neurite growth we have observed that a permissive laminin (LN) substrate stimulates differential growth responses in subpopulations of mature dorsal root ganglion (DRG) neurons. DRG neurons expressing Trk and p75 receptors grow neurites on a LN substrate in the absence of neurotrophins, while isolectin B4-binding neurons (IB4⁺) do not display significant growth under the same conditions. We set out to determine whether there was an expression signature of the LN-induced neurite growth phenotype. Using a lectin binding protocol IB4⁺ neurons were isolated from dissociated DRG neurons, creating two groups - IB4⁺ and IB4^-. A small-scale microarray approach was employed to screen the expression of a panel of ECM-associated genes following dissociation (t=0) and after 24 hr culture on LN (t=24LN). This was followed by qRT-PCR and immunocytochemistry of selected genes.

Results

The microarray screen showed that 36 of the 144 genes on the arrays were consistently expressed by the neurons. The array analyses showed that six genes had lower expression in the IB4+ neurons compared to the IB4- cells at t=0 (CTSH, Icam1, Itgβ1, Lamb1, Plat, Spp1), and one gene was expressed at higher levels in the IB4⁺ cells (Plaur). qRT-PCR was carried out as an independent assessment of the array results. There were discrepancies between the two methods, with qRT-PCR confirming the differences in Lamb1, Plat and Plaur, and showing decreased expression of AdamTs1, FN, and Icam in the IB4⁺ cells at t=0. After 24 hr culture on LN, there were no significant differences detected by qRT-PCR between the IB4⁺ and IB4^- cells. However, both groups showed upregulation of Itgβ1 and Plaur after 24 hr on LN, the IB4⁺ group also had increased Plat, and the IB4^- cells showed decreased Lamb1, Icam1 and AdamTs1. Further, the array screen also detected a number of genes (not subjected to qRT-PCR) expressed similarly by both populations in relatively high levels but not detectably influenced by time in culture (Bsg, Cst3, Ctsb, Ctsd, Ctsl, Mmp14, Mmp19, Sparc. We carried out immunohistochemistry to confirm expression of proteins encoded by a number of these genes.

Conclusions

Our results show that 1B4⁺ and IB4^- neurons differ in the expression of several genes that are associated with responsiveness to the ECM prior to culturing (AdamTs1, FN, Icam1, Lamb1, Plat, Plaur). The data suggest that the genes expressed at higher levels in the IB4^- neurons could contribute to the initial growth response of these cells in a permissive environment and could also represent a common injury response that subsequently promotes axon regeneration. The differential expression of several extracellular matrix molecules (FN, Lamb1, Icam) may suggest that the IB4^- neurons are capable of maintaining /secreting their local extracellular environment which could aid in the regenerative process. Overall, these data provide new information on potential targets that could be manipulated to enhance axonal regeneration in the mature nervous system.

Specificity of peripheral nerve regeneration: interactions at the axon level

Peripheral nerves injuries result in paralysis, anesthesia and lack of autonomic control of the affected body areas. After injury, axons distal to the lesion are disconnected from the neuronal body and degenerate, leading to denervation of the peripheral organs. Wallerian degeneration creates a microenvironment distal to the injury site that supports axonal regrowth, while the neuron body changes in phenotype to promote axonal regeneration. The significance of axonal regeneration is to replace the degenerated distal nerve segment, and achieve reinnervation of target organs and restitution of their functions. However, axonal regeneration does not always allows for adequate functional recovery, so that after a peripheral nerve injury, patients do not recover normal motor control and fine sensibility. The lack of specificity of nerve regeneration, in terms of motor and sensory axons regrowth, pathfinding and target reinnervation, is one the main shortcomings for recovery. Key factors for successful axonal regeneration include the intrinsic changes that neurons suffer to switch their transmitter state to a pro-regenerative state and the environment that the axons find distal to the lesion site. The molecular mechanisms implicated in axonal regeneration and pathfinding after injury are complex, and take into account the cross-talk between axons and glial cells, neurotrophic factors, extracellular matrix molecules and their receptors. The aim of this review is to look at those interactions, trying to understand if some of these molecular factors are specific for motor and sensory neuron growth, and provide the basic knowledge for potential strategies to enhance and guide axonal regeneration and reinnervation of adequate target organs.

Downstream effector molecules in successful peripheral nerve regeneration

The robust axon regeneration that occurs following peripheral nerve injury is driven by transcriptional activation of the regeneration program and by the expression of a wide range of downstream effector molecules from neuropeptides and neurotrophic factors to adhesion molecules and cytoskeletal adaptor proteins. These regeneration-associated effector molecules regulate the actin-tubulin machinery of growth-cones, integrate intracellular signalling and stimulatory and inhibitory signals from the local environment and translate them into axon elongation. In addition to the neuronally derived molecules, an important transcriptional component is found in locally activated Schwann cells and macrophages, which release a number of cytokines, growth factors and neurotrophins that support neuronal survival and axonal regeneration and that might provide directional guidance cues towards appropriate peripheral targets. This review aims to provide a comprehensive up-to-date account of the transcriptional regulation and functional role of these effector molecules and of the information that they can give us with regard to the organisation of the regeneration program.

Vimentin phosphorylation by Cdc2 in Schwann cell controls axon growth via β1-integrin activation

Although preconditioning injury on the peripheral nerve induces axonal regenerative capacity in neurons, it is not known whether similar lesion effects occur in glial cells. Here we demonstrate that Schwann cells are activated by peripheral nerve preinjury and primed to mediate axon regeneration. Cdc2, which was induced from Schwann cells after sciatic nerve injury, phosphorylated vimentin almost exclusively in the distal nerve area. Phospho-vimentin-positive Schwann cells showed increased migration activity and were in close contact with process outgrowth of co-cultured neurons. Vimentin phosphorylation by Cdc2 was involved in β1-integrin activation leading to FAK phoshorylation and associated with Erk1/2 activation in Schwann cells. Neurite outgrowth of dorsal root ganglion neurons was increased by co-culture with activated Schwann cells, in which phospho-vimentin signaling was transmitted into β1-integrin activation. Then neurite outgrowth was suppressed by genetic depletion of phospho-vimentin and β1 integrin as well as inhibition of vimentin phosphorylation by Cdc2 inhibitor purvalanol A. The sciatic nerve graft harboring activated Schwann cells into the spinal cord induced Schwann cell migration beyond the graft-host barrier and facilitated regeneration of spinal axons, which was inhibited by purvalanol A pretreatment of the graft. This is the first report to our knowledge demonstrating that activation of phospho-vimentin linked to β1-integrin pathway may mediate transcellular signaling to promote axon growth.

Intrinsic mechanisms regulating axon regeneration: an integrin perspective

Adult central nervous system (CNS) axons fail to regenerate after injury because of inhibitory factors in the surrounding environment and a low intrinsic regenerative capacity. Axons in the adult peripheral nervous system have a higher regenerative capacity, due in part to the presence of certain integrins-receptors for the extracellular matrix. Integrins are critical for axon growth during the development of the nervous system but are absent from some adult CNS axons. Here, we discuss the intrinsic mechanisms that regulate axon regeneration and examine the role of integrins. As correct localization is paramount to integrin function, we further discuss the mechanisms that regulate integrin traffic toward the axonal growth cone.

The unusual response of serotonergic neurons after CNS Injury: lack of axonal dieback and enhanced sprouting within the inhibitory environment of the glial scar

Serotonergic neurons possess an enhanced ability to regenerate or sprout after many types of injury. To understand the mechanisms that underlie their unusual properties, we used a combinatorial approach comparing the behavior of serotonergic and cortical axon tips over time in the same injury environment in vivo and to growth-promoting or growth-inhibitory substrates in vitro. After a thermocoagulatory lesion in the rat frontoparietal cortex, callosal axons become dystrophic and die back. Serotonergic axons, however, persist within the lesion edge. At the third week post-injury, 5-HT+ axons sprout robustly. The lesion environment contains both growth-inhibitory chondroitin sulfate proteoglycans (CSPGs) and growth-promoting laminin. Transgenic mouse serotonergic neurons specifically labeled by enhanced yellow fluorescent protein under control of the Pet-1 promoter/enhancer or cortical neurons were cultured on low amounts of laminin with or without relatively high concentrations of the CSPG aggrecan. Serotonergic neurons extended considerably longer neurites than did cortical neurons on low laminin and exhibited a remarkably more active growth cone on low laminin plus aggrecan during time-lapse imaging than did cortical neurons. Chondroitinase ABC treatment of laminin/CSPG substrates resulted in significantly longer serotonergic but not cortical neurite lengths. This increased ability of serotonergic neurons to robustly grow on high amounts of CSPG may be partially due to significantly higher amounts of growth-associated protein-43 and/or β1 integrin than cortical neurons. Blocking β1 integrin decreased serotonergic and cortical outgrowth on laminin. Determining the mechanism by which serotonergic fibers persist and sprout after lesion could lead to therapeutic strategies for both stroke and spinal cord injury.

Transcribing the path to neurological recovery-From early signals through transcription factors to downstream effectors of successful regeneration

The peripheral nervous system is known to regenerate comparatively well and this ability is mirrored in the de novo expression or upregulation of a wide variety of molecules involved in axonal outgrowth starting with transcription factors, but also including growth-stimulating substances, guidance and cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions. Recent studies using pharmacological agents, and global as well as neuron-selective gene inactivation techniques have shed light on those endogenous molecules that play a non-redundant role in mediating regenerative axonal outgrowth in vivo. The aim of the current review is to sketch the sequence of molecular events from early sensors of injury to transcription factors to downstream effectors that cooperate in successful regeneration and functional recovery.

Laminin-121--recombinant expression and interactions with integrins

Laminin-121, previously referred as to laminin-3, was expressed recombinantly in human embryonic kidney (HEK) 293 cells by triple transfection of full-length cDNAs encoding mouse laminin α1, β2 and γ1 chains. The recombinant laminin-121 was purified using Heparin-Sepharose followed by molecular sieve chromatography and shown to be correctly folded by electron microscopy and circular dichroism (CD). The CD spectra of recombinant laminin-121 were very similar to those of laminin-111 isolated from Engelbreth-Holm-Swarm tumor (EHS-laminin) but its T(m) value was smaller than EHS-laminin and recombinant lamnin-111 suggesting that the replacement of the β chain reduced the stability of the coiled-coil structure of laminin-121. Its binding to integrins was compared with EHS-laminin, laminin-3A32 purified from murine epidermal cell line and recombinantly expressed laminins-111, -211 and -221. Laminin-121 showed the highest affinity to α6β1 and α7β1 integrins and furthermore, laminin-121 most effectively supported neurite outgrowth. Together, this suggests that the β2 laminins have higher affinity for integrins than the β1 laminins.

Extrinsic and intrinsic factors controlling axonal regeneration after spinal cord injury

Spinal cord injury is one of the most devastating conditions that affects the central nervous system. It can lead to permanent disability and there are around two million people affected worldwide. After injury, accumulation of myelin debris and formation of an inhibitory glial scar at the site of injury leads to a physical and chemical barrier that blocks axonal growth and regeneration. The mammalian central nervous system thus has a limited intrinsic ability to repair itself after injury. To improve axonal outgrowth and promote functional recovery, it is essential to identify the various intrinsic and extrinsic factors controlling regeneration and navigation of axons within the inhibitory environment of the central nervous system. Recent advances in spinal cord research have opened new avenues for the exploration of potential targets for repairing the cord and improving functional recovery after trauma. Here, we discuss some of the important key molecules that could be harnessed for repairing spinal cord injury.

Overcoming macrophage-mediated axonal dieback following CNS injury

Trauma to the adult CNS initiates multiple processes including primary and secondary axotomy, inflammation, and glial scar formation that have devastating effects on neuronal regeneration. After spinal cord injury, the infiltration of phagocytic macrophages coincides with long-distance axonal retraction from the initial site of injury, a deleterious phenomenon known as axonal dieback. We have previously shown that activated macrophages directly induce long-distance retraction of dystrophic axons in an in vitro model of the glial scar. We hypothesized that treatments that are primarily thought to increase neuronal regeneration following spinal cord injury may in fact derive a portion of their beneficial effects from inhibition of macrophage-mediated axonal retraction. We analyzed the effects of protease inhibition, substrate modification, and neuronal preconditioning on macrophage-axon interactions using our established in vitro model. General inhibition of matrix metalloproteinases and specific inhibition of MMP-9 prevented macrophage-induced axonal retraction despite significant physical interactions between the two cell types, whereas inhibition of MMP-2 had no effect. Chondroitinase ABC-mediated digestion of the aggrecan substrate also prevented macrophage-induced axonal retraction in the presence of extensive macrophage-axon interactions. The use of a conditioning lesion to stimulate intrinsic neuronal growth potential in the absence of substrate modification likewise prevented macrophage-induced axonal retraction in vitro and in vivo following spinal cord injury. These data provide valuable insight into the cellular and molecular mechanisms underlying macrophage-mediated axonal retraction and demonstrate modifications that can alleviate the detrimental effects of this unfavorable phenomenon on the postlesion CNS.

Peripheral sensory axon growth: from receptor binding to cellular signaling

Regeneration following axonal injury of the adult peripheral sensory nervous system is heavily influenced by factors located in a neuron's extracellular environment. These factors include neurotrophins, such as Nerve Growth Factor (NGF) and the extracellular matrix, such as laminin. The presence of these molecules in the peripheral nervous system (PNS) is a major contributing factor for the dichotomy between regenerative capacities of central vs. peripheral neurons. Although PNS neurons are capable of spontaneous regeneration, this response is critically dependent on many different factors including the type, location and severity of the injury. In this article, we will focus on the plasticity of adult dorsal root ganglion (DRG) sensory neurons and how trophic factors and the extracellular environment stimulate the activation of intracellular signaling cascades that promote axonal growth in adult dorsal root ganglion neurons.

Dystrophin glycoprotein complex-associated Gbetagamma subunits activate phosphatidylinositol-3-kinase/Akt signaling in skeletal muscle in a laminin-dependent manner

Previously, we showed that laminin-binding to the dystrophin glycoprotein complex (DGC) of skeletal muscle causes a heterotrimeric G-protein (Galphabetagamma) to bind, changing the activation state of the Gsalpha subunit. Others have shown that laminin-binding to the DGC also leads to Akt activation. Gbetagamma, released when Gsalpha is activated, is known to bind phosphatidylinositol-3-kinase (PI3K), which activates Akt in other cells. Here, we investigate whether muscle Akt activation results from Gbetagamma, using immunoprecipitation and immunoblotting, and purified Gbetagamma. In the presence of laminin, PI3K-binding to the DGC increases and Akt becomes phosphorylated and activated (pAkt), and glycogen synthase kinase is phosphorylated. Antibodies, which specifically block laminin-binding to alpha-dystroglycan, prevent PI3K-binding to the DGC. Purified bovine brain Gbetagamma also caused PI3K and Akt activation. These results show that DGC-Gbetagamma is binding PI3K and activating pAkt in a laminin-dependent manner. Mdx mice, which have greatly diminished amounts of DGC proteins, display elevated pAkt signaling and increased expression of integrin beta1 compared to normal muscle. This integrin binds laminin, Gbetagamma, and PI3K. Collectively, these suggest that PI3K is an important target for the Gbetagamma, which normally binds to DGC syntrophin, and activates PI3K/Akt signaling. Disruption of the DGC in mdx mouse is causing dis-regulation of the laminin-DGC-Gbetagamma-PI3K-Akt signaling and is likely to be important to the pathogenesis of muscular dystrophy. Upregulating integrin beta1 expression and activating the PI3K/Akt pathway in muscular dystrophy may partially compensate for the loss of the DGC. The results suggest new therapeutic approaches to muscle disease.

Conditional deletion of the Itgb4 integrin gene in Schwann cells leads to delayed peripheral nerve regeneration

Several different integrins participate in the complex interactions that promote repair of the peripheral nervous system. The role of the integrin alpha6beta4 in peripheral nerve regeneration was investigated in mice by cre-mediated deletion of the Itgb4 (beta4) gene in Schwann cells. After a crush lesion of the sciatic nerve, the recovery of motor, but not that of sensory, nerve function in beta4(-/-) mice was delayed. Immunostaining of neurofilament-200 showed that there also is a significant reduction in the number of newly outgrowing nerve sprouts in beta4(-/-) mice. Morphometric quantitative measurements revealed that fewer axons are myelinated in the nonlesioned beta4(-/-) nerves. After a sciatic nerve crush lesion, beta4(-/-) mice did not only have fewer myelinated axons compared with lesioned wild-type nerve, but their axons also showed a higher g-ratio and a thinner myelin sheath, pointing at reduced myelination. This study revealed that the beta4 protein remains expressed in the early stages of peripheral regeneration, albeit at levels lower than those before the lesion was inflicted, and showed that laminin deposition is not altered in the absence of beta4. These results together demonstrate that integrin alpha6beta4 plays an essential role in axonal regeneration and subsequent myelination.

Enhancement of axonal regeneration by in vitro conditioning and its inhibition by cyclopentenone prostaglandins

Axonal regeneration is enhanced by the prior ;conditioning' of peripheral nerve lesions. Here we show that Xenopus dorsal root ganglia (DRG) with attached peripheral nerves (PN-DRG) can be conditioned in vitro, thereafter showing enhanced neurotrophin-induced axonal growth similar to preparations conditioned by axotomy in vivo. Actinomycin D inhibits axonal outgrowth from freshly dissected PN-DRG, but not from conditioned preparations. Synthesis of mRNAs that encode proteins necessary for axonal elongation might therefore occur during the conditioning period, a suggestion that was confirmed by oligonucleotide microarray analysis. Culturing PN-DRG in a compartmentalized system showed that inhibition of protein synthesis (but not RNA synthesis) in the distal nerve impaired the conditioning response, suggesting that changes in gene expression in cultured DRG depend on the synthesis and retrograde transport of protein(s) in peripheral nerves. The culture system was also used to demonstrate retrograde axonal transport of several proteins, including thioredoxin (Trx). Cyclopentenone prostaglandins, which react with Trx, blocked the in vitro conditioning effect, whereas inhibition of other signalling pathways thought to be involved in axonal regeneration did not. This suggests that Trx and/or other targets of these electrophilic prostaglandins regulate axonal regeneration. Consistent with this hypothesis, morpholino-induced suppression of Trx expression in dissociated DRG neurons was associated with reduced neurite outgrowth.

Integrin-laminin interactions controlling neurite outgrowth from adult DRG neurons in vitro

A prerequisite for axon regeneration is the interaction between the growth cone and the extracellular matrix (ECM). Laminins are prominent constituents of ECM throughout the body, known to support axon growth in vitro and in vivo. The regenerative capacity of adult neurons is greatly diminished compared to embryonic or early postnatal neurons. Since most lesions in the nervous system occur in the adult, we have examined neurite outgrowth from adult mouse DRG neurons on four laminin isoforms (laminin-1/LM-111, laminin-2/LM-211, laminin-8/LM-411 and laminin-10/LM-511) in vitro. The growth on laminin-1 and -10 was trophic factor-independent and superior to the one on laminin-2 and -8, where growth was very poor in the absence of neurotrophins. Among other ECM proteins, laminins were by far the most active molecules. Using function-blocking antibodies to laminin-binding integrins, we identified non-overlapping functions of integrins alpha3beta1, alpha7beta1 and alpha6beta1 on different laminin isoforms, in that alpha3beta1 and alpha7beta1 integrins appeared to be specific receptors for both laminin-1 and-2, whereas integrin alpha6beta1 was a receptor for laminin-8 and-10. Lastly, by use of immunohistochemistry, expression of subunits of laminin-1, -2, -8 and -10 in sensory organs in the human epidermis could be demonstrated, supporting an important role for these laminins in relation to primary sensory axons.

Integrin signaling is integral to regeneration

The inability of the adult injured mammalian spinal cord to successfully regenerate is not well understood. Studies suggest that both extrinsic and intrinsic factors contribute to regeneration failure. In this review, we focus on intrinsic factors that impact regeneration, in particular integrin receptors and their downstream signaling pathways. We discuss studies that address the impact of integrins and integrin signaling pathways on growth cone guidance and motility and how lessons learned from these studies apply to spinal cord regeneration in vivo.

Effects of collagen 1, fibronectin, laminin and hyaluronic acid concentration in multi-component gels on neurite extension

Recovery after peripheral nerve injury remains a significant challenge. Extracellular matrix proteins and hydrogels of extracellular matrix components have been shown to improve regeneration in peripheral nerve entubulation models, especially over long distances. The chemical properties, ligand identity and density, and mechanical properties of the hydrogel can affect neurite extension. However, the importance of combinatorial effects between different components in co-gels of several extracellular matrix components is unclear. In this study, we investigated neurite extension from explanted dorsal root ganglia cultured within co-gels made from laminin, fibronectin, collagen 1 and hyaluronic acid. Laminin had a strong, dose-dependent effect on both neurite length and outgrowth. Fibronectin was slightly, but generally not significantly, inhibitory to neurite extension. The concentration of collagen 1 and hyaluronic acid did not have significant effects on neurite extension. The combinatorial effects among the four components were additive rather than synergistic. A co-gel made with 1.5 mg/ml collagen 1 and 1.5 mg/ml laminin was optimum in this study, resulting in an average neurite length of 1532 +/- 91 microm versus 976 +/- 32 microm for controls, and an increase in overall volume outgrowth (reflecting neurite length and branching) of 85.9+/-9.3% over controls. This co-gel provides a mechanically stable scaffold with high ligand density and biochemical affinity. The results of this study support the use of co-gels of laminin and collagen 1 for promoting regeneration in peripheral nerve injuries and suggest that interactions among hydrogel components are not significant.

Impaired axonal regeneration by isolectin B4-binding dorsal root ganglion neurons in vitro

The subpopulation of dorsal root ganglion (DRG) neurons recognized by Griffonia simplicifolia isolectin B4 (IB4) differ from other neurons by expressing receptors for glial cell line-derived neurotrophic factor (GDNF) rather than neurotrophins. Additionally, IB4-labeled neurons do not express the laminin receptor, alpha7-integrin (Gardiner et al., 2005), necessary for optimal axonal regeneration in the peripheral nervous system. In cultures of dissociated DRG neurons of adult mice on laminin, robust spontaneous neurite outgrowth from IB4-negative neurons occurs and is strongly enhanced by previous axotomy. In contrast, IB4-labeled neurons show little neurite outgrowth and do not express GAP 43, even after axotomy or culture with GDNF. Moreover, growth of their axons through collagen gels is impaired compared with other DRG neurons. To determine whether the sparse neurite outgrowth of IB4-labeled neurons is attributable to lack of integrin expression, DRG cultures were infected with a herpes simplex 1 vector encoding alpha7-integrin, but its forced expression failed to promote neurite outgrowth in either IB4-labeled or other DRG neurons or in cultured adult retinal ganglion cells. Forced coexpression of both alpha7-integrin and GAP 43 also failed to promote neurite outgrowth in IB4-labeled neurons. In addition, cultured sciatic nerve segments were found to release much lower levels of GDNF, demonstrated by ELISA, than nerve growth factor. These findings together with their impaired intrinsic axonal regeneration capacity may contribute to the known vulnerability of the IB4-labeled population of DRG neurons to peripheral nerve injury.

The making of successful axonal regeneration: Genes, molecules and signal transduction pathways

Unlike its central counterpart, the peripheral nervous system is well known for its comparatively good potential for regeneration following nerve fiber injury. This ability is mirrored by the de novo expression or upregulation of a wide variety of molecules including transcription factors, growth-stimulating substances, cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions, that promote neurite outgrowth in cultured neurons. However, their role in vivo is less known. Recent studies using neutralizing antibodies, gene inactivation and overexpression techniques have started to shed light on those endogenous molecules that play a key role in axonal outgrowth and the process of successful functional repair in the injured nervous system. The aim of the current review is to provide a summary on this rapidly growing field and the experimental techniques used to define the specific effects of candidate signaling molecules on axonal regeneration in vivo.

Studies on integrins in the nervous system

Integrins are of interest to neuroscientists because they and many of their ligands are widely expressed in the nervous system and have been shown to have diverse roles in neural development and function (Clegg et al., 2003; Li and Pleasure, 2005; Pinkstaff et al., 1998, 1999; Reichardt and Tomaselli, 1991; Schmid et al., 2005). Integrins have also been implicated in control of pathogenesis in several neurodegenerative diseases, brain tumor pathogenesis, and the aftermath of brain and peripheral nervous system injury (Condic, 2001; Ekstrom et al., 2003; Kloss et al., 1999; Verdier and Penke, 2004; Wallquist et al., 2004). Using integrin antagonists as therapeutic agents in a variety of neurological diseases is of great interest at present (Blackmore and Letourneau, 2006; Mattern et al., 2005; Polman et al., 2006; Wang et al., 2006). In this chapter, we describe methods used in our laboratory to characterize neuronal responses to extracellular matrix proteins, and procedures for assessing integrin roles in neuronal cell attachment and differentiation.

Laminin and growth factor receptor activation stimulates differential growth responses in subpopulations of adult DRG neurons

Neurons in the adult rat dorsal root ganglion (DRG) can be classified into at least three separate subpopulations based on morphologic and phenotypic differences. In this study we have focused on the growth response of these specific subpopulations in vitro with respect to laminin (LN) and growth factor receptor activation. Using a cell selection approach we show that LN-induced neurite growth occurs in the absence of added trophic factors only in heavy-chain neurofilament-positive and calcitonin gene-related peptide-positive DRG neurons [nerve growth factor (NGF)-responsive population]. In contrast, LN alone is not sufficient to stimulate significant neurite growth from lectin Griffonia simplicifolia IB4-positive neurons (IB4+ve), although it is still required to elicit a growth response from these cells in the presence of glial-derived neurotrophic factor (GDNF, e.g. neurite growth occurred only when cells were plated on LN in the presence of GDNF). By using chemical inhibitors we demonstrate that only the phosphatidylinositol 3 kinase (PI 3-K)/Akt pathway is required for neurite growth from the NGF-responsive cell population. However, both the PI 3-K/Akt and MEK/mitogen-activated protein kinase signaling pathways are required for neurite growth from the IB4+ve cell population. Thus, we have identified specific signaling events and environmental requirements associated with neurite growth for different subpopulations of adult DRG neurons, pointing to potential therapeutic targets while identifying an inability for any one treatment alone to repair peripheral nerve damage.

Chronic enhancement of the intrinsic growth capacity of sensory neurons combined with the degradation of inhibitory proteoglycans allows functional regeneration of sensory axons through the dorsal root entry zone in the mammalian spinal cord

Peripherally conditioned sensory neurons have an increased capacity to regenerate their central processes. However, even conditioned axons struggle in the presence of a hostile CNS environment. We hypothesized that combining an aggressive conditioning strategy with modification of inhibitory reactive astroglial-associated extracellular matrix could enhance regeneration. We screened potential treatments using a model of the dorsal root entry zone (DREZ). In this assay, a gradient of inhibitory chondroitin sulfate proteoglycans (CSPGs) stimulates formation of dystrophic end bulbs on adult sensory axons, which mimics regeneration failure in vivo. Combining inflammation-induced preconditioning of dorsal root ganglia in vivo before harvest, with chondroitinase ABC (ChABC) digestion of proteoglycans in vitro allows for significant regeneration across a once potently inhibitory substrate. We then assessed regeneration through the DREZ after root crush in adult rats receiving the combination treatment, ChABC, or zymosan pretreatment alone or no treatment. Regeneration was never observed in untreated animals, and only minimal regeneration occurred in the ChABC- and zymosan-alone groups. However, remarkable regeneration was observed in a majority of animals that received the combination treatment. Regenerated fibers established functional synapses, as demonstrated electrophysiologically by the presence of an H-reflex. Two different postlesion treatment paradigms in which the timing of both zymosan and ChABC administration were varied after injury were ineffective in promoting regeneration. Therefore, zymosan pretreatment, but not posttreatment, of the sensory ganglia, combined with ChABC modification of CSPGs, resulted in robust and functional regeneration of sensory axons through the DREZ after root injury.

Evidences that β1 integrin and Rac1 are involved in the overriding effect of laminin on myelin-associated glycoprotein inhibitory activity on neuronal cells

During neurite elongation, migrating growth cones encounter both permissive and inhibitory substrates, such as laminin and MAG (myelin-associated glycoprotein), respectively. Here, we demonstrated on two neuronal cell lines (PC12 and N1E-115), that laminin and collagen hampered, in a dose-dependent manner, MAG inhibitory activity on several integrin functions, i.e., neurite growth, cell adhesion and cell spreading. Using a function blocking antibody, in PC12 cells, we showed that alpha1beta1 integrin is required in these phenomena. In parallel, we observed that MAG perturbs actin dynamics and lamellipodia formation during early steps of cell spreading. This seemed to be independent of RhoA activation, but dependent of Rac-1 inhibition by MAG. Laminin overrode MAG activity on actin and prevented MAG inhibition NGF-induced Rac1 activation. In conclusion, we evidenced antagonistic signaling between MAG receptors and beta1 integrins, in which Rac-1 may have a central function.

Involvement of alpha4 integrins in maintenance of cardiac sympathetic axons

Sympathetic neurons extend and maintain axons that innervate the myocardium, and proper innervation is important for cardiac function. However, the molecular basis for axon outgrowth and maintenance is not well understood. We have shown previously that the integrin alpha4beta1 is expressed on developing axons, and the alpha4 function is important for the development of innervation in vivo [Wingerd, K.L., Goodman, N.L., Tresser, J.W., Smail, M.M., Leu, S.T., Rohan, S.J., Pring, J.L., Jackson, D.Y., and Clegg, D.O., 2002. Alpha 4 integrins and vascular cell adhesion molecule-1 play a role in sympathetic innervation of the heart. J. Neurosci. 22,10772-10780]. Here we examine the function of alpha4beta1 integrins in the maintenance of cardiac sympathetic innervation in vitro and in vivo, and investigate integrin expression and function after myocardial infarction and in hypertensive rats. On substrates of vascular cell adhesion molecule-1 (VCAM-1), alpha4beta1 was required for both initial outgrowth and maintenance of neurites in vitro. On fibronectin substrates, initial outgrowth requires only alpha4 integrins, but maintenance requires both alpha4 integrins and RGD-dependent integrins. In vivo, in adult Long Evans rats, inhibition of alpha4 integrins resulted in decreased maintenance of sympathetic fibers innervating the apex of the heart. However, alpha4 integrins were not detected on most sympathetic axons that sprout after myocardial infarction, and alpha4 function was not required for sprouting. Spontaneously hypertensive rats (SHR) have increased numbers of cardiac sympathetic fibers compared to the parental Wistar strain, but many of these lack alpha4 expression, and alpha4 function is not required for maintenance of these fibers in the heart. These results suggest that developing sympathetic axons and sprouting sympathetic axons use different mechanisms of outgrowth, and that maintenance of cardiac sympathetic innervation involves alpha4 integrins in some rat strains.

A conditioning lesion enhances sympathetic neurite outgrowth

Axonal regeneration can be influenced by a conditioning lesion (an axonal injury made prior to a second test lesion). Previously, sympathetic neurons in vivo were shown to respond to a conditioning lesion with decreased neurite outgrowth, in contrast to the enhanced outgrowth observed in all other peripheral neurons examined. The present experiments tested the effects of a conditioning lesion on neurite outgrowth in vitro from the superior cervical ganglion (SCG) and the impact of several factors on that response. Ganglia axotomized 1 week earlier and then explanted in Matrigel or collagen gel responded with a significant increase in neurite extension compared to sham-operated ganglia. A distal axotomy produced by unilateral removal of the salivary glands (sialectomy) caused an increase in neurite outgrowth similar to that of a proximal axotomy. These conditioning lesions induced both an increase in the rate of elongation, and, in the case of the proximally axotomized SCG, a shorter initial delay of outgrowth. The enhanced outgrowth following sialectomy was specific to the nerve containing the majority of axons projecting to the salivary glands, suggesting that the conditioning lesion effect is restricted to previously injured neurons. Deletion of the gene for leukemia inhibitory factor (LIF), a gene induced by axotomy, did not abolish the conditioning lesion effect in SCG explants or dissociated cell cultures. In conclusion, sympathetic neurons are capable of responding to a conditioning lesion with increased neurite outgrowth. The hypothesis that the neuronal cell body response to axotomy plays an important role in the conditioning lesion response is discussed.

Alpha7 integrin mediates neurite outgrowth of distinct populations of adult sensory neurons

The successful regeneration of peripheral branches of sensory neurons following injury is attributed to the presence of neurotrophins and interaction of regenerating axons with the extracellular matrix. Here, we show that the laminin receptor, alpha7beta1 integrin is a crucial mediator of neurite outgrowth from distinct populations of sensory neurons. Following sciatic nerve crush, alpha7 integrin is expressed by medium-large diameter, NF200-immunoreactive (IR), and medium diameter, CGRP-IR, neurons, but very few small diameter non-peptidergic neurons. The functional significance of alpha7 integrin expression following injury was addressed using dissociated adult rat and mouse sensory neurons. By using function-blocking antibodies and neurons isolated from alpha7 integrin null mice, we demonstrate that NGF- and NT-3-stimulated neurite outgrowth is reduced in the absence of alpha7 integrin signaling. In contrast, GDNF-stimulated neurite outgrowth is less dependent on alpha7 integrin. These results define an essential interaction between alpha7 integrin and laminin for mediating neurite outgrowth of subpopulations of injured adult sensory neurons.