Lentiviral Vector-Mediated Gradients of GDNF in the Injured Peripheral Nerve: Effects on Nerve Coil Formation, Schwann Cell Maturation and Myelination

Abstract

Although the peripheral nerve is capable of regeneration, only a small minority of patients regain normal function after surgical reconstruction of a major peripheral nerve lesion, resulting in a severe and lasting negative impact on the quality of life. Glial cell-line derived neurotrophic factor (GDNF) has potent survival- and outgrowth-promoting effects on motoneurons, but locally elevated levels of GDNF cause trapping of regenerating axons and the formation of nerve coils. This phenomenon has been called the “candy store” effect. In this study we created gradients of GDNF in the sciatic nerve after a ventral root avulsion. This approach also allowed us to study the effect of increasing concentrations of GDNF on Schwann cell proliferation and morphology in the injured peripheral nerve. We demonstrate that lentiviral vectors can be used to create a 4 cm long GDNF gradient in the intact and lesioned rat sciatic nerve. Nerve coils were formed throughout the gradient and the number and size of the nerve coils increased with increasing GDNF levels in the nerve. In the nerve coils, Schwann cell density is increased, their morphology is disrupted and myelination of axons is severely impaired. The total number of regenerated and surviving motoneurons is not enhanced after the distal application of a GDNF gradient, but increased sprouting does result in higher number of motor axon in the distal segment of the sciatic nerve. These results show that lentiviral vector mediated overexpression of GDNF exerts multiple effects on both Schwann cells and axons and that nerve coil formation already occurs at relatively low concentrations of exogenous GDNF. Controlled expression of GDNF, by using a viral vector with regulatable GDNF expression, may be required to avoid motor axon trapping and to prevent the effects on Schwann cell proliferation and myelination.

Spatiotemporally limited BDNF and GDNF overexpression rescues motoneurons destined to die and induces elongative axon growth

“…Moreover, the number of Schwann cells was increased, but the myelination of the axons was severely impaired. The total number of regenerated and surviving motoneurons was not enhanced (Eggers et al 2013). …”

“…Recent efforts to establish a gradient of neurotrophins along the injured sciatic nerve to force axons to grow along the whole length of the nerve have proven unsuccessful. The likely reason for this could be that even the lowest dose of viral constructs applied prohibited axonal growth and myelination of the coil-forming axons (Eggers et al 2013).…”

Electrical Stimulation to Enhance Axon Regeneration After Peripheral Nerve Injuries in Animal Models and Humans

“…The biological modulation of the neurotrophic factors is difficult to replicate and problems of excess administration have been described, as, for example, the administration of excess doses of GDNF using retroviral vectors in rats where the findings of axonal coils was attributed to too much GDNF [119,120] .…”

Grafted murine induced pluripotent stem cells prevent death of injured rat motoneurons otherwise destined to die

“…Reimplantation of the avulsed ventral root is the primary requirement for regeneration of the motor axons, but in a typical situation few motoneurons are able to survive and reinnervate peripheral targets without additional neuroprotective support (Carlstedt et al, 1990;Carlstedt, 2008;Nogradi and Vrbova, 1996). These neuroprotective strategies involve the use of neurotrophic factors, Na + channel blockers/glutamate release inhibitors and native or genetically enginereed stem cells (Blits et al, 2004;Eggers et al, 2008;Eggers et al, 2013;Nogradi and Vrbova, 2001;Novikov et al, 1997;Wu et al, 2003).…”

“…However, only Na + channel blockers/glutamate release inhibitors, such as riluzole have proved effective from the aspects of inducing both motoneuron survival and functional reinnervation, while neurotrophic factors and stem/progenitor cell therapies have failed to induce the axonal outgrowth of otherwise surviving motoneuron populations (Blits et al, 2004;Eggers et al, 2013;Pinter et al, 2010). The use of embryonic stem cells is otherwise complicated by ethical concerns.…”