Schwann Cell Expressed Nogo-B Modulates Axonal Branching of Adult Sensory Neurons Through the Nogo-B Receptor NgBR.

Christoph Eckharter,Irmgard Fischer,Steffen Kistner,Kristian Pfaller,Barbara Fogli,Nina Junker,Rüdiger Schweigreiter,Gerhard Wiche,Lars Klimaschewski,Lilli Winter,Binhai Zheng

doi:10.3389/fncel.2015.00454

Abstract

In contrast to the central nervous system (CNS) nerve fibers do regenerate in the peripheral nervous system (PNS) although in a clinically unsatisfying manner. A major problem is excessive sprouting of regenerating axons which results in aberrant reinnervation of target tissue and impaired functional recovery. In the CNS, the reticulon protein Nogo-A has been identified as a prominent oligodendrocyte expressed inhibitor of long-distance growth of regenerating axons. We show here that the related isoform Nogo-B is abundantly expressed in Schwann cells in the PNS. Other than Nogo-A in oligodendrocytes, Nogo-B does not localize to the myelin sheath but is detected in the ER and the plasma membrane of Schwann cells. Adult sensory neurons that are cultured on nogo-a/b deficient Schwann cells form significantly fewer axonal branches vs. those on wildtype Schwann cells, while their maximal axonal extension is unaffected. We demonstrate that this effect of Nogo-B on neuronal morphology is restricted to undifferentiated Schwann cells and is mediated by direct physical contact between these two cell types. Moreover, we show that blocking the Nogo-B specific receptor NgBR, which we find expressed on sensory neurons and to interact with Schwann cell expressed Nogo-B, produces the same branching phenotype as observed after deletion of Nogo-B. These data provide evidence for a novel function of the nogo gene that is implemented by the Nogo-B isoform. The remarkably specific effects of Nogo-B/NgBR on axonal branching, while leaving axonal extension unaffected, are of potential clinical relevance in the context of excessive axonal sprouting after peripheral nerve injury.Main PointsNogo-B is prominently expressed in Schwann cells and localizes to the ER and plasma membrane. It distributes to the external cytoplasmic compartment of Schwann cells in vivo, but is absent from the myelin sheath.Genetic deletion of Nogo-B in Schwann cells reduces axonal branching, but not long-distance growth, of co-cultured adult sensory neurons.Schwann cell expressed Nogo-B interacts with neuronal NgBR. Blockade of NgBR mimics the loss-of-nogo branching phenotype.

Highlights

Injury to peripheral nerves elicits a series of cellular and molecular transformations distal to the site of injury called Wallerian degeneration, which paves the way for regenerative outgrowth of lesioned nerve fibers (Chen et al, 2007; Faroni et al, 2015)
In a co-culture of Schwann cells and adult sensory neurons, we demonstrate that blocking the Nogo-B/Nogo-B receptor (NgBR) signaling axis reduces the number of axonal branches but does not compromise axonal elongation
We have shown previously that Nogo-B is abundantly expressed in the peripheral nervous system (PNS) and that it gets proteolytically cleaved after inflicting a nerve lesion in a mouse model (Schweigreiter et al, 2007)

Summary

Introduction

Injury to peripheral nerves elicits a series of cellular and molecular transformations distal to the site of injury called Wallerian degeneration, which paves the way for regenerative outgrowth of lesioned nerve fibers (Chen et al, 2007; Faroni et al, 2015). Within hours after receiving a nerve lesion they start to dedifferentiate by detaching from the axons and retracting the myelin sheath They phagocytose axonal and myelin debris and chemotactically recruit blood-borne macrophages for assistance. Each transected axon can sprout up to 25 branches from the proximal stump, which innervate multiple target sites rather than the single original site (Mackinnon et al, 1991). This aberrant reinnervation affects motor, sensory and sympathetic fibers alike and severely compromises functional recovery due to asynchronized motor function and sensory deficits (Hendry et al, 1986; Sumner, 1990; de Ruiter et al, 2008; Allodi et al, 2012). In a co-culture of Schwann cells and adult sensory neurons, we demonstrate that blocking the Nogo-B/NgBR signaling axis reduces the number of axonal branches but does not compromise axonal elongation

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Conclusion