Christina F. Vogelaar

Alpha9 integrin promotes neurite outgrowth on tenascin-C and enhances sensory axon regeneration

Damaged CNS axons are prevented from regenerating by an environment containing many inhibitory factors. They also lack an integrin that interacts with tenascin-C, the main extracellular matrix glycoprotein of the CNS, which is upregulated after injury. The α9β1 integrin heterodimer is a receptor for the nonalternatively spliced region of tenascin-C, but the α9 subunit is absent in adult neurons. In this study, we show that PC12 cells and adult rat dorsal root ganglion (DRG) neurons do not extend neurites on tenascin-C. However, after forced expression of α9 integrin, extensive neurite outgrowth from PC12 cells and adult rat DRG neurons occurs. Moreover, both DRG neurons and PC12 cells secrete tenascin-C, enabling α9-transfected cells to grow axons on tissue culture plastic. Using adeno-associated viruses to express α9 integrin in vivo in DRGs, we examined axonal regeneration after cervical dorsal rhizotomy or dorsal column crush in the adult rat. After rhizotomy, significantly more dorsal root axons regrew into the dorsal root entry zone at 6 weeks after injury in α9 integrin-expressing animals than in green fluorescent protein (GFP) controls. Similarly, after a dorsal column crush injury, there was significantly more axonal growth into the lesion site compared with GFP controls at 6 weeks after injury. Behavioral analysis after spinal cord injury revealed that both experimental and control groups had an increased withdrawal latency in response to mechanical stimulation when compared with sham controls; however, in response to heat stimulation, normal withdrawal latencies returned after α9 integrin treatment but remained elevated in control groups.

Axonal mRNAs: Characterisation and role in the growth and regeneration of dorsal root ganglion axons and growth cones

We have developed a compartmentalised culture model for the purification of axonal mRNA from embryonic, neonatal and adult rat dorsal root ganglia. This mRNA was used un-amplified for RT-qPCR. We assayed for the presence of axonal mRNAs encoding molecules known to be involved in axon growth and guidance. mRNAs for beta-actin, beta-tubulin, and several molecules involved in the control of actin dynamics and signalling during axon growth were found, but mRNAs for microtubule-associated proteins, integrins and cell surface adhesion molecules were absent. Quantification of beta-actin mRNA by means of qPCR showed that the transcript is present at the same level in embryonic, newborn and adult axons. Using the photoconvertible reporter Kaede we showed that there is local translation of beta-actin in axons, the rate being increased by axotomy. Knock down of beta-actin mRNA by RNAi inhibited the regeneration of new axon growth cones after in vitro axotomy, indicating that local translation of actin-related molecules is important for successful axon regeneration.

Cortical Gene Expression in Spinal Cord Injury and Repair: Insight into the Functional Complexity of the Neural Regeneration Program

Traumatic spinal cord injury (SCI) results in the formation of a fibrous scar acting as a growth barrier for regenerating axons at the lesion site. We have previously shown (Klapka et al., 2005) that transient suppression of the inhibitory lesion scar in rat spinal cord leads to long distance axon regeneration, retrograde rescue of axotomized cortical motoneurons, and improvement of locomotor function. Here we applied a systemic approach to investigate for the first time specific and dynamic alterations in the cortical gene expression profile following both thoracic SCI and regeneration-promoting anti-scarring treatment (AST). In order to monitor cortical gene expression we carried out microarray analyses using total RNA isolated from layer V/VI of rat sensorimotor cortex at 1–60 days post-operation (dpo). We demonstrate that cortical neurons respond to injury by massive changes in gene expression, starting as early as 1 dpo. AST, in turn, results in profound modifications of the lesion-induced expression profile. The treatment attenuates SCI-triggered transcriptional changes of genes related to inhibition of axon growth and impairment of cell survival, while upregulating the expression of genes associated with axon outgrowth, cell protection, and neural development. Thus, AST not only modifies the local environment impeding spinal cord regeneration by reduction of fibrous scarring in the injured spinal cord, but, in addition, strikingly changes the intrinsic capacity of cortical pyramidal neurons toward enhanced cell maintenance and axonal regeneration.

Pharmacological Suppression of CNS Scarring by Deferoxamine Reduces Lesion Volume and Increases Regeneration in an In Vitro Model for Astroglial-Fibrotic Scarring and in Rat Spinal Cord Injury In Vivo

Lesion-induced scarring is a major impediment for regeneration of injured axons in the central nervous system (CNS). The collagen-rich glial-fibrous scar contains numerous axon growth inhibitory factors forming a regeneration-barrier for axons. We demonstrated previously that the combination of the iron chelator 2,2’-bipyridine-5,5’-decarboxylic acid (BPY-DCA) and 8-Br-cyclic AMP (cAMP) inhibits scar formation and collagen deposition, leading to enhanced axon regeneration and partial functional recovery after spinal cord injury. While BPY-DCA is not a clinical drug, the clinically approved iron chelator deferoxamine mesylate (DFO) may be a suitable alternative for anti-scarring treatment (AST). In order to prove the scar-suppressing efficacy of DFO we modified a recently published in vitro model for CNS scarring. The model comprises a co-culture system of cerebral astrocytes and meningeal fibroblasts, which form scar-like clusters when stimulated with transforming growth factor-β (TGF-β). We studied the mechanisms of TGF-β-induced CNS scarring and compared the efficiency of different putative pharmacological scar-reducing treatments, including BPY-DCA, DFO and cAMP as well as combinations thereof. We observed modulation of TGF-β-induced scarring at the level of fibroblast proliferation and contraction as well as specific changes in the expression of extracellular matrix molecules and axon growth inhibitory proteins. The individual and combinatorial pharmacological treatments had distinct effects on the cellular and molecular aspects of in vitro scarring. DFO could be identified as a putative anti-scarring treatment for CNS trauma. We subsequently validated this by local application of DFO to a dorsal hemisection in the rat thoracic spinal cord. DFO treatment led to significant reduction of scarring, slightly increased regeneration of corticospinal tract as well as ascending CGRP-positive axons and moderately improved locomotion. We conclude that the in vitro model for CNS scarring is suitable for efficient pre-screening and identification of putative scar-suppressing agents prior to in vivo application and validation, thus saving costs, time and laboratory animals.

Fast direct neuronal signaling via the IL-4 receptor as therapeutic target in neuroinflammation

Targeting neuronal interleukin-4 signaling is beneficial for clinical progression and axon pathology in different neuroinflammation models. IL-4 empowers axons Multiple sclerosis (MS) is a neuroinflammatory disorder, and current therapies focus on altering immune activity to reduce symptoms. Vogelaar and colleagues tested the ability of intrathecally applied IL-4, a cytokine typically associated with T helper type 2 responses, to treat established disease in several experimental autoimmune encephalomyelitis (EAE) models. IL-4 treatment led to reduced clinical scores, improved locomotor activity, and diminished axon damage. Somewhat surprisingly, the beneficial effects of IL-4 did not depend on T cell modulation in the chronic disease phase. The receptor for IL-4 was observed in postmortem brain histology of several MS patients, and they demonstrated that IL-4 could act directly on neurons in vitro. They also showed benefits of intranasal IL-4 administration in one of the EAE models, which could be a promising avenue to pursue in the clinic. Ongoing axonal degeneration is thought to underlie disability in chronic neuroinflammation, such as multiple sclerosis (MS), especially during its progressive phase. Upon inflammatory attack, axons undergo pathological swelling, which can be reversible. Because we had evidence for beneficial effects of T helper 2 lymphocytes in experimental neurotrauma and discovered interleukin-4 receptor (IL-4R) expressed on axons in MS lesions, we aimed at unraveling the effects of IL-4 on neuroinflammatory axon injury. We demonstrate that intrathecal IL-4 treatment during the chronic phase of several experimental autoimmune encephalomyelitis models reversed disease progression without affecting inflammation. Amelioration of disability was abrogated upon neuronal deletion of IL-4R. We discovered direct neuronal signaling via the IRS1-PI3K-PKC pathway underlying cytoskeletal remodeling and axonal repair. Nasal IL-4 application, suitable for clinical translation, was equally effective in improving clinical outcome. Targeting neuronal IL-4 signaling may offer new therapeutic strategies to halt disability progression in MS and possibly also neurodegenerative conditions.

A predominantly glial origin of axonal ribosomes after nerve injury

Axonal mRNA transport and local protein synthesis are crucial for peripheral axon regeneration. To date, it remains unclear how ribosomes localize to axons. They may be co‐transported with mRNAs or, as suggested by recent studies, transferred from Schwann cells (SC). Here, we generated transgenic “RiboTracker” mice expressing tdTomato‐tagged ribosomal protein L4 in specific cell types when crossed with Cre lines. Two neuronal RiboTracker‐Cre lines displayed extremely low levels of axonal L4‐tdTomato‐positive ribosomes. In contrast, two glial RiboTracker‐Cre lines revealed tagged ribosomes in sciatic nerve (SN) axons with increasing amounts after injury. Furthermore, non‐RiboTracker dorsal root ganglia co‐cultured with L4‐tdTomato‐expressing SCs displayed tagged ribosomes in axons. These data provide unequivocal evidence that SN axons receive ribosomes from SCs upon injury and indicate that glial cells are the main source of axonal ribosomes.

Experimental Spinal Cord Injury Models in Rodents: Anatomical Correlations and Assessment of Motor Recovery

Human traumatic spinal cord injury (SCI) causes disruption of descending motor and ascending sensory tracts, which leads to severe disturbances in motor functions. To date, no standard therapy for the regeneration of severed spinal cord axons in humans exists. Experimental SCI in rodents is essential for the development of new treatment strat‐ egies and for understanding the underlying mechanisms leading to motor recovery. Here, we provide an overview of the main rodent models and techniques available for the investigation of neuronal regeneration and motor recovery after experimental SCI.

Extrinsic and intrinsic mechanisms of axon regeneration: the need for spinal cord injury treatment strategies to address both

Spinal cord injury (SCI) causes disturbances in motor and sensory functions leading to paralysis, the severity of which depends on the spinal level of the injury. Traumatic lesions of spinal cord axon projection tracts are untreatable in human patients, although numerous research groups worldwide are studying putative treatment strategies. Both extrinsic factors in the environment of the axons as well as intrinsic factors in the neurons themselves play important roles in the regeneration process (Chew et al., 2012). The peripheral nervous system (PNS) provides a good example where the extrinsic and intrinsic factors play optimally together to allow regeneration. Schwann cells dedifferentiate and form new endoneurial tubes for the axons to grow through. Together with macrophages they clear the debris and produce growth factors and cytokines that positively stimulate the neurons. In parallel, the neurons intrinsically react to the injury by activating a regeneration-associated gene expression program. Most PNS axons produce a new growth cone and start growing within 3 hours (Bradke et al., 2012), eventually reinnervating their targets. In contrast, the projection neurons in the central nervous system (CNS) do not spontaneously activate regeneration-associated genes (RAGs) (van Kesteren et al., 2011). The axons first die back several hundreds of micrometers, tend to make retraction bulbs rather than growth cones, and seem unable to navigate in the correct direction (Bradke et al., 2012). Those CNS axons that do regenerate encounter a highly inhibitory scar that further blocks their growth (Fawcett et al., 2012). So, in the CNS both intrinsic and extrinsic mechanisms negatively influence regeneration. This is further corroborated by the observation that some spinal cord axons are able to regenerate through a peripheral nerve graft (van Kesteren et al., 2011) indicating again that the PNS environment is favorable to growth. However, the majority of injured neurons in the spinal cord do not regenerate spontaneously, so that peripheral nerve grafts still need to be combined with treatments such as cAMP, increasing the intrinsic regeneration capacity (Bunge, 2008). In this paper, I will address the extrinsic and intrinsic regeneration mechanisms with respect to treatments for SCI.

Maladaptive cortical hyperactivity upon recovery from experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) patients exhibit neuropsychological symptoms in early disease despite the immune attack occurring predominantly in white matter and spinal cord. It is unclear why neurodegeneration may start early in the disease and is prominent in later stages. We assessed cortical microcircuit activity by employing spiking-specific two-photon Ca2+ imaging in proteolipid protein-immunized relapsing-remitting SJL/J mice in vivo. We identified the emergence of hyperactive cortical neurons in remission only, independent of direct immune-mediated damage and paralleled by elevated anxiety. High levels of neuronal activity were accompanied by increased caspase-3 expression. Cortical TNFα expression was mainly increased by excitatory neurons in remission; blockade with intraventricular infliximab restored AMPA spontaneous excitatory postsynaptic current frequencies, completely recovered normal neuronal network activity patterns and alleviated elevated anxiety. This suggests a dysregulation of cortical networks attempting to achieve functional compensation by synaptic plasticity mechanisms, indicating a link between immune attack and early start of neurodegeneration.The authors report TNFα-dependent hyperactivity in cortical microcircuits during remission in a mouse model of multiple sclerosis, a maladaptive response to the immune attack with behavioral changes.