Because this protein is also present in oligodendrocytes, demyelination can appear in the CNS.Schwann cells create the myelin sheath, by wrapping its plasma membrane around the axon.Neurons, Schwann cells, and fibroblasts work together to create a functional nerve. Schwann cells and neurons exchange molecular signals by gap junctions that regulate survival and differentiation.Demyelinating Schwann cells causes abnormal axon structure and function. They may cause axon degeneration, or they may simply cause axons to malfunction.The myelin sheath allows nerve cells to conduct signals faster. When the myelin sheath is damaged, nerve signals are slower, and this can be measured by a common neurological test, electromyography. When the axon is damaged, though, this results in a reduced compound muscle action potential.GARS1-Related Axonal Neuropathy
CMT2 types are typically referred to as axonal neuropathies due to the axonal degeneration observed. CMT2 types are a result of damage to the nerve axons rather than damage to the myelin sheath (as is the case with CMT1). Damaged axons cause slowed transmission of signals to the muscles and brain, causing symptoms including muscle atrophy, weakness, decreased sensitivity, and foot deformity. Symptoms of CMT2 types typically appear between the ages of 5 and 25. CMT2D is one of 31 Charcot-Marie-Tooth type 2 forms 1 and is only diagnosed if sensory deficits (such as loss of sensation due to the degradation of sensory axons) are observed along with motor deficits; otherwise, distal hereditary motor neuropathy type V is diagnosed. It is unknown why sensory involvement is so varied between GARS1 neuropathy patients.
Glycyl-tRNA is integral to protein translation and attaches glycine to its cognate RNA.Many different mutations have been found in CMT2D patients and it remains unclear how mutations in GARS1 cause CMT2D. However, it is thought that mutant glycyl-tRNA synthetase (GlyRS) interferes with transmembrane receptors, causing motor disease and that mutations in the gene could disrupt the ability of GlyRS to interact with its cognate RNA, disrupting protein production. The GARS1 mutations present in CMT2D cause a deficient amount of glycyl-tRNA in cells, preventing the elongation phase of protein synthesis. Because elongation is a key step in protein production, ribosomes are unable to continue protein synthesis at glycine sites. GARS1 mutations also stall initiation of translation. Glycine addition failure causes a stress response that further stalls protein production, preventing initiation of translation. By stalling elongation and initiation of translation, CMT2D mutations in the GARS1 gene cause translational repression, meaning that overall translation is inhibited.GARS1-associated axonal neuropathy is progressive, meaning that it worsens over time. Unknown mechanisms are thought to cause the chronic neurodegeneration resulting from the aberrant GlyRS; however, one theory for disease is VEGF-deficiency. Mutant GlysRS interferes with neuronal transmembrane receptors, including neuropilin 1 (Nrp1) and vascular endothelial growth factor (VEGF), causing neuropathy. GARS-CMT2D mutations alter GlyRS and allow it to bind to the Nrp1 receptor, interfering with the normal binding of Nrp1 to VEGF.