Integrin messenger RNAs in the red nucleus after axotomy and neurotrophic administration

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.

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.

Axonal Localization of Integrins in the CNS Is Neuronal Type and Age Dependent

The regenerative ability of CNS axons decreases with age, however, this ability remains largely intact in PNS axons throughout adulthood. These differences are likely to correspond with age-related silencing of proteins necessary for axon growth and elongation. In previous studies, it has been shown that reintroduction of the α9 integrin subunit (tenascin-C receptor, α9) that is downregulated in adult CNS can improve neurite outgrowth and sensory axon regeneration after a dorsal rhizotomy or a dorsal column crush spinal cord lesion. In the current study, we demonstrate that virally expressed integrins (α9, α6, or β1 integrin) in the adult rat sensorimotor cortex and adult red nucleus are excluded from axons following neuronal transduction. Attempts to stimulate transport by inclusion of a cervical spinal injury and thus an upregulation of extracellular matrix molecules at the lesion site, or cotransduction with its binding partner, β1 integrin, did not induce integrin localization within axons. In contrast, virally expressed α9 integrin in developing rat cortex (postnatal day 5 or 10) demonstrated clear localization of integrins in cortical axons revealed by the presence of integrin in the axons of the corpus callosum and internal capsule, as well as in the neuronal cell body. Furthermore, examination of dorsal root ganglia neurons and retinal ganglion cells demonstrated integrin localization both within peripheral nerve as well as dorsal root axons and within optic nerve axons, respectively. Together, our results suggest a differential ability for in vivo axonal transport of transmembrane proteins dependent on neuronal age and subtype.

Neuronal myosin-X is upregulated after peripheral nerve injury and mediates laminin-induced growth of neurites

The successful outcome of peripheral neuronal regeneration is attributed both to the growth permissive milieu and the intrinsic ability of the neuron to initiate appropriate cellular responses such as changes in gene expression and cytoskeletal rearrangements. Even though numerous studies have shown the importance of interactions between the neuron and the extracellular matrix (ECM) in axonal outgrowth, the molecular mechanisms underlying the contact between ECM receptors and the cellular cytoskeleton remain largely unknown. Unconventional myosins constitute an important group of cytoskeletal-associated motor proteins. One member of this family is the recently described myosin-X. This protein interacts with several members of the axon growth-associated ECM receptor family of integrins and could therefore be important in neuronal outgrowth. In this study, using radioactive in situ hybridization, we found that expression of myosin-X mRNA is upregulated in adult rat sensory neurons and spinal motoneurons after peripheral nerve injury, but not after central injury. Thus, myosin-X was upregulated after injuries that can be followed by axonal regeneration. We also found that the protein is localized to neuronal growth cones and that silencing of myosin-X using RNA interference impairs the integrin-mediated growth of neurites on laminin, but has no effect on non-integrin mediated growth on N-cadherin.

β1 integrin-focal adhesion kinase (FAK) signaling modulates retinal ganglion cell (RGC) survival

Extracellular matrix (ECM) integrity in the central nervous system (CNS) is essential for neuronal homeostasis. Signals from the ECM are transmitted to neurons through integrins, a family of cell surface receptors that mediate cell attachment to ECM. We have previously established a causal link between the activation of the matrix metalloproteinase-9 (MMP-9), degradation of laminin in the ECM of retinal ganglion cells (RGCs), and RGC death in a mouse model of retinal ischemia-reperfusion injury (RIRI). Here we investigated the role of laminin-integrin signaling in RGC survival in vitro, and after ischemia in vivo. In purified primary rat RGCs, stimulation of the β1 integrin receptor with laminin, or agonist antibodies enhanced RGC survival in correlation with activation of β1 integrin’s major downstream regulator, focal adhesion kinase (FAK). Furthermore, β1 integrin binding and FAK activation were required for RGCs’ survival response to laminin. Finally, in vivo after RIRI, we observed an up-regulation of MMP-9, proteolytic degradation of laminin, decreased RGC expression of β1 integrin, FAK and Akt dephosphorylation, and reduced expression of the pro-survival molecule bcl-xL in the period preceding RGC apoptosis. RGC death was prevented, in the context of laminin degradation, by maintaining β1 integrin activation with agonist antibodies. Thus, disruption of homeostatic RGC-laminin interaction and signaling leads to cell death after retinal ischemia, and maintaining integrin activation may be a therapeutic approach to neuroprotection.

The role of well-defined patterned substrata on the regeneration of DRG neuron pathfinding and integrin expression dynamics using chondroitin sulfate proteoglycans

Injured neurons intrinsically adapt to and partially overcome inhibitory proteoglycan expression in the central nervous system by upregulating integrin expression. It remains unclear however, to what extent varying proteoglycan concentrations influence the strength of this response, how rapidly neurons adapt to proteoglycans, and how pathfinding dynamics are altered over time as integrin expression is modulated in response to proteoglycan signals. To investigate these quandaries, we created well-defined substrata in which postnatal DRG neuron pathfinding dynamics and growth cone integrin expression were interrogated as a function of proteoglycan substrata density. DRGs responded by upregulating integrin expression in a proteoglycan dose dependent fashion and exhibited robust outgrowth over all proteoglycan densities at initial time frames. However, after prolonged proteoglycan exposure, neurons exhibited decreasing velocities associated with increasing proteoglycan densities, while neurons growing on low proteoglycan levels exhibited robust outgrowth at all time points. Additionally, DRG outgrowth over proteoglycan density step boundaries, and a brief β1 integrin functional block proved that regeneration was integrin dependent and that DRGs exhibit delayed slowing and loss in persistence after even transient encounters with dense proteoglycan boundaries. These findings demonstrate the complexity of proteoglycan regulation on integrin expression and regenerative pathfinding.

Functional recovery after the repair of transected cervical roots in the chronic stage of injury

The treatment of root injury is typically performed at the more chronic stages post injury, by which time a substantial number of neurons have died. Therefore, before being applied in the clinical setting, a treatment strategy for these lesions should prove to be as effective in the chronic stages of injury as it is in the acute stage. In this study, we simulated the most severe clinical scenarios to establish an optimal time window for repair at a chronic stage. The sixth to eighth cervical roots on the left side of female SD rats were cut at their junction with the spinal cord. One or three weeks later, the wound was reopened and these roots were repaired with intercostal nerve grafts, with subsequent application of aFGF and fibrin glue. In the control group, the wound was closed after re-exploration without further repair procedures. Sensory and motor functions were measured after the surgery. Spinal cord morphology, neuron survival, and nerve fiber regeneration were traced by CTB-HRP. Results showed that both the sensory and motor functions had significant recovery in the 1-week repair group, but not in the 3-week repair group. By CTB-HRP tracing, we found that the architecture of the spinal cords was relatively preserved in the 1-week repair group, while those of the control group showed significant atrophic change. There were regenerating nerve fibers in the dorsal horn and more motor neuron survival in the 1-week repair group compared to that of the 3-week group. It was concluded that treating transected cervical roots at a chronic stage with microsurgical nerve grafting and application of aFGF and fibrin glue can lead to significant functional recovery, as long as the repair is done before too many neurons die.

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.

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.

The influence of pre-surgical delay on functional outcome after reconstruction of brachial plexus injuries

It has been proposed that delayed surgery after traumatic brachial plexus injury may adversely affect functional outcome. In this study the influence of pre-surgical delay on the outcome of brachial plexus reconstruction was examined retrospectively. All patients who underwent surgery for traumatic brachial plexus injury in the Leeds Plastic and Reconstructive Surgery unit (UK), between 1987 and 2002, were identified. Of the 110 patients identified, 27 had nerve grafting to the upper trunk to restore shoulder and biceps muscle function. Postoperative functional outcome was evaluated in this subgroup of patients. The 27 patients were divided into three groups: surgery <2 weeks (n=10), 2 weeks to 2 months (n=10) and >2 months (n=7) following injury. The efficacy of nerve grafting was correlated to pre- and postoperative biceps strength, which was assessed using the British Medical Research Council (MRC) Motor Grading Scale. In all patients the preoperative elbow grade was M0. The results showed that in the <2 weeks, 2 weeks-2 months and >2 months delay groups, the mean postoperative elbow MRC grades were 4.2+/-SD 1.0, 3.8+/-SD 0.8 and 1.1+/-SD 1.7, respectively. Functionally better results were obtained with early surgery. When surgery was delayed beyond 2 months there was no significant difference between mean pre- and postoperative elbow grades. We therefore believe that early exploration and reconstruction of adult traumatic brachial plexus injuries minimises the pernicious adverse effects of delay attributable to recent findings of the neurobiological effects of axonal damage.

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.

The effects of delayed nerve repair on neuronal survival and axonal regeneration after seventh cervical spinal nerve axotomy in adult rats

It has been proposed clinically that delayed surgery after traumatic brachial plexus injury may adversely affect functional outcome. In the present experimental study the neuroprotective and growth-promoting effects of early and delayed nerve grafting following proximal seventh cervical spinal nerve (C7) axotomy were examined. The ventral branch of C7 spinal nerve was transected and axons projecting out of the proximal nerve stump were labelled with Fast Blue (FB). At the same time, the biceps brachii muscle was denervated by transecting the musculocutaneous nerve at its origin. Neuronal survival and muscle atrophy were then assessed at 1, 4, 8 and 16 weeks after permanent axotomy. In the experimental groups, a peripheral nerve graft was interposed between the transected C7 spinal nerve and the distal stump of the musculocutaneous nerve at 1 week [early nerve repair (ENR)] or 8 weeks [delayed nerve repair (DNR)] after axotomy. Sixteen weeks after nerve repair had been performed, a second tracer Fluoro-Ruby (FR) was applied distal to the graft to assess the efficacy of axonal regeneration. Counts of FB-labelled neurons revealed that axotomy did not induce any significant cell loss at 4 weeks, but 15% of motoneurons and 32% of sensory neurons died at 8 weeks after injury. At 16 weeks, the amount of cell loss in spinal cord and dorsal root ganglion (DRG) reached 29 and 50%, respectively. Both ENR and DNR prevented retrograde degeneration of spinal motoneurons and counteracted muscle atrophy, but failed to rescue sensory neurons. Due to substantial cell loss at 8 weeks, the number of FR-labelled neurons after DNR was significantly lower when compared to ENR. However, the proportion of regenerating neurons among surviving motoneurons and DRG neurons remained relatively constant indicating that neurons retained their regenerative capacity after prolonged axotomy. The results demonstrate that DNR could protect spinal motoneurons and reduce muscle atrophy, but had little effect on sensory DRG neurons. However, the efficacy of neuroprotection and axonal regeneration will be significantly affected by the amount of cell loss already presented at the time of nerve repair.