Tracing axons in the peripheral nerve using lacZ gene recombinant adenovirus and its application to regeneration of the peripheral nerve

Adenoviral gene transfer of bone morphogenetic protein-7 enhances functional recovery after sciatic nerve injury in rats

Bone morphogenetic proteins (BMPs), members of the transforming growth factor-β subfamily, function as instructive signals for neuronal lineage commitment and promote neuronal differentiation. However, the mechanism of BMP7 action in vivo after peripheral nerve injury is poorly understood. This study examines the efficacy of gene transfer of adenoviral (Ad) BMP7 on peripheral neuropathy. Transgene expression was found in both Ad-infected sciatic nerves and their respective remote neurons, indicating Ad transduction by a retrograde transport. After AdBMP7 infection to nerves, the sciatic nerves were crushed or transected. Hind limb functional behavior, including rotarod test and sciatic functional index, were conducted in rats weekly after nerve injury. Interestingly, enhanced BMP7 expression significantly improved hind limb functional recovery in AdBMP7-transduced rats when compared with AdGFP-transduced nerve-crushed or transected rats. Furthermore, AdBMP7 transduction reduced injury-induced macrophage activation, nerve demyelination and axonal degeneration. By contrast, AdBMP7 infection did not affect the hyperalgesia paw-withdrawal latency after nerve injury. We further examined the effect of AdBMP7 infection on sciatic nerve explant and Schwann cell cultures. Enhanced cell proliferation was significantly increased by AdBMP7 transduction in both cultures. Taken together, BMP7 overexpression by Ad gene transfer was beneficial in both nerves and Schwann cells on functional recovery after sciatic nerve injury in rats.

Newly‐formed axonal branches of rat sciatic neurons sprouting in the spinal cord after peripheral axotomy

A retrograde method of nerve tracing using a recombinant adenovirus was applied to experimental regeneration of peripheral nerves to study the sprouting position of the regenerating axon. This enabled us to see the entire length of the axons on a whole-mount neural specimen. The peroneal nerve was transsected and infected with this virus, and the tibial nerve was transsected and sutured in eight Wistar rats. Four to five weeks later, labelled axons appeared in the tibial nerve, some of which could be traced from the tibial nerve to the spinal cord without making a connection with other labelled fibres. Control experiments negated the possibilities of transneuronal immigration or contamination of the virus. When the peroneal and tibial nerves were double-labelled with fluorescent tracers four weeks after their transsection, double-labelled motor neurons appeared. Based on these findings, we conclude that regenerating branches do sprout in the spinal cord after axotomy.

Sprouting of sensory neurons in dorsal root ganglia after transection of peripheral nerves

Morphological reaction of sensory neurons of dorsal root ganglia after peripheral nerve transection was investigated by a nerve tracing method using E. coli lacZ (beta-galactosidase) gene recombinant adenovirus. The sciatic nerve of the rat was transected and inoculated with the gene recombinant adenovirus from the cutting end of nerve fibers. The fixation was accomplished from one to six weeks after inoculation. A whole mount specimen was observed after the reaction in a X-galactocidase substrate. Newly formed sprouting processes of dorsal root ganglion (DRG) cells appeared, all of them sprouting from the primary segment of DRG cells. Developed branches were morphologically categorized in to two types: one was the "linear type" which showed diverged branches running straightly along the major axis of the DRG; the other was the "winding type" which exhibited a random running pattern to the original axons and wound and extended in all directions in dorsal root ganglia with many branches. Many of this type encircled other cell bodies and formed a ring-like structure. There was no difference in the size of cell bodies in either type or between the ring-like structure forming the cells and those cells encircled by them.

Gene-based treatment of motor neuron diseases

Motor neuron diseases (MND), such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA), are progressive neurodegenerative diseases that share the common characteristic of upper and/or lower motor neuron degeneration. Therapeutic strategies for MND are designed to confer neuroprotection, using trophic factors, anti-apoptotic proteins, as well as antioxidants and anti-excitotoxicity agents. Although a large number of therapeutic clinical trials have been attempted, none has been shown satisfactory for MND at this time. A variety of strategies have emerged for motor neuron gene transfer. Application of these approaches has yielded therapeutic results in cell culture and animal models, including the SOD1 models of ALS. In this study we describe the gene-based treatment of MND in general, examining the potential viral vector candidates, gene delivery strategies, and main therapeutic approaches currently attempted. Finally, we discuss future directions and potential strategies for more effective motor neuron gene delivery and clinical translation.

Pathway sampling by regenerating peripheral axons

A century ago, Ramon y Cajal described the generalized response of regenerating peripheral axons to their environment. By using mice that express fluorescent proteins in their axons, we are now able to quantify the response of individual axons to nerve transection and repair. Sciatic nerves from nonexpressing mice were grafted into those expressing a yellow variant of green fluorescent protein, then examined at 5, 7, or 10 days after repair. Regeneration was found to be a staggered process, with only 25% of axons crossing the repair in the first week. In the setting of Wallerian degeneration, this stagger will expose growth cones to an evolving menu of molecular cues upon which to base pathway decisions. Many axons arborize, allowing them to interact simultaneously with several pathways. Arborization could serve as the anatomical substrate for specificity generation through collateral pruning. Axons often travel laterally across the face of the distal stump before choosing a pathway. As a result, the average unbranched axon has access to over 100 distal Schwann cell tubes. This extensive access, however, does not ensure correct matching of axon and end organ, suggesting that pathway choice is made on the basis of factors other than end organ identity. These observations explain the failure of refined surgical techniques to restore normal function after nerve injury. The apparent wandering of axons across the repair defies surgical control and mandates a biological approach to reuniting severed axons with appropriate distal pathways.

Tracing axons of peripheral nerves in rats: a potential technique to study the equine recurrent laryngeal nerve

To study the fascicular anatomy of peripheral nerves, three different groups of retrograde axonal tracers were evaluated: fluorophores, horseradish peroxidase conjugated to subunit B of cholera toxin (CT-HRP), and adeno-associated virus (AAV). The hindlimb nerves in rats served as a model to identify the most efficient tracer in regard to labeling axons within peripheral nerves. The rat's tibial and common peroneal nerves were injected with the different tracers and the sciatic nerve was subsequently examined for evidence of labeled axons. The CT-HRP clearly provided the best results in this rat model. Subsequently, CT-HRP was injected into the recurrent laryngeal nerve (RLN) of two horses in order to identify the location and distribution pattern of the RLN axons within the course of the cervical vagus nerve trunk. No labeling could be observed in either of the two horses.

Quantitative and qualitative analysis of Wallerian degeneration using restricted axonal labelling in YFP-H mice

We investigated the usefulness of YFP-H transgenic mice [Neuron 28 (2000) 41] which express yellow fluorescent protein (YFP) in a restricted subset of neurons to study Wallerian degeneration in the PNS. Quantification of YFP positive axons and myelin basic protein (MBP) immunocytochemistry revealed that YFP was randomly distributed to approximately 3% of myelinated motor and sensory fibres. Axotomy-induced Wallerian degeneration appeared as fragmentation of fluorescent signals in individual YFP positive axons with a morphology and timing similar to Wallerian degeneration observed by more traditional methods. In YFP-H transgenic mice co-expressing a high dosage of WldS, a chimeric gene that protects from Wallerian degeneration [Nat Neurosci. 4 (2001) 1199], axonal fragmentation in distal tibial nerves after sciatic nerve axotomy was approximately 10 times delayed. Considerable retardations of Wallerian degeneration using the same transgenic expression system were also observed in cultures of nerve explants, enabling in vitro real-time imaging of axonal fragmentation. Remarkably, single YFP-labelled axons could be traced in peripheral nerves for unusually long distances of up to 2.9 cm exploiting confocal fluorescence imaging. Altogether transgenic YFP-H mice prove to be a valuable tool to study mechanisms of Wallerian degeneration in vivo and in vitro.