Neurons and glia in the peripheral nervous system : interactions in health and disease

Abstract

The peripheral nervous system (PNS) is composed of nerves and ganglia that connect the brain and the spinal cord (the central nervous system, CNS) to the outside world. The nerves contain the extensions (axons) of both sensory and motor neurons, which allow us to detect and respond to different stimuli. For example, nociceptive neurons can detect pain and alpha-motoneurons can control muscle contraction and movement. In addition to neurons, nerves also contain glial or support cells called Schwann cells (SC), which are responsible for the structural organization of nerves, myelination of large caliber axons and engulfment of small caliber nerve fibers. Myelin is crucial for the fast transmission of action potentials and acts as a protective layer around axons. Schwann cells are also able to provide trophic and metabolic support to neurons, and their dysfunction can drive axonal pathology in the absence of changes to myelin itself. Neurons and glia have been well studied in the CNS, but there has been an increased interest in the PNS at the level of development and long-term maintenance. Our contributions to this field are tri-partite:

In Study I we questioned if monocarboxylate transporters MCT1 and MCT4 expressed by SC contribute to metabolic support of peripheral neurons. We employed Cre-Lox technology to conditionally delete the genes Slc16a1 and Slc16a3, coding respectively for MCT1 and MCT4, in SC. We found that both glial MCTs are largely dispensable for the development and myelination of nerves but MCT1 contributes to long term maintenance of neuromuscular innervation. Absence of MCT1 in SC induced transcriptional changes in spinal cord motoneurons, which may reflect reduced metabolic support and may be an early sign of neuronal pathology. These results suggest that MCT1 expressed by SC contributes to the support of motoneurons, but the relatively mild phenotype observed indicates that the remaining MCTs expressed by SC and neurons may play an important part as well.

In Study II we focused on the development of sensory neurons and investigated the function of PRDM12, an epigenetic regulator that can cause congenital insensitivity to pain when mutated in humans. We established constitutive knockout (KO) mice lacking the expression of Prdm12 and discovered that absence of Prdm12 results in a complete loss of the TRKA+ subpopulation in the dorsal root ganglion (DRG), which gives rise to nociceptors. This is due in part to reduced proliferation of SOX10+ precursor cells. Overexpression of Prdm12 in chicken embryos was not sufficient to induce a nociceptor fate but it did prevent the differentiation of alternative sensory neuron subtypes. Finally, we found that while the number of Ngn1+ and Ngn2+ cells was unchanged in the Prdm12 KO at embryonic day E10.5, at E12.5 the expression of Ngn1 was greatly reduced, suggesting Prdm12 is necessary to maintain its expression. We conclude that Prdm12 is necessary to maintain the expression of Ngn1 in the developing mouse DRG, it modulates the proliferation of SOX0+ progenitors and restricts the differentiation potential of sensory neurons to the nociceptive TRKA-expressing fate.

In Study III we evaluated the effect of overexpressing a constitutively active form of NFATc4 in myelinating glia, in vivo. NFATc4 is transcription factor regulated by the phospholipase Cγ/calcium/calcineurin signaling pathway. Inhibition of this pathway in neural crest cells leads to impaired SC development and reduced expression of Krox20, the master regulatory gene of myelin gene expression. We hypothesized that overexpression of an active (phosphorylation-resistant) form of NFATc4 in myelinating glia may lead to hypermyelination, but instead the transgenic mice developed a pronounced neuropathy phenotype. This was characterized by hind-limb clasping and reduced motor nerve conduction velocity. We found that at birth, the sciatic nerves of these mice were developmentally delayed, presenting a larger area occupied by unsorted bundles of axons and a complete absence of axons undergoing myelination. RNA sequencing of newborn sciatic nerves showed a dramatic alteration of the transcriptional landscape, allowing us to conclude that the timing and intensity of NFATc4 activation are crucial for proper development of peripheral nerves.

Additionally, Paper IV is a mini-review of the topic “Metabolic Interaction Between Schwann Cells and Axons Under Physiological and Disease Conditions”. In it we review recent studies on the role of SC in the metabolic support of axons and compare them to the current knowledge of the same function in CNS glia. We propose that metabolic support of axons may be the primary function of axon-ensheathing glia, as can be seen in lampreys and fruit flies, with myelination becoming an evolutionarily advantageous feature of higher vertebrates. Then we present different mouse models of SC metabolic dysfunction, focusing on the myelin-independent features of the phenotypes, and finally, we suggest that SC-axon metabolic interactions may pose an interesting target to treat peripheral neuropathic disorders such as diabetic neuropathy and Charcot-Marie-Tooth disease.