Glutamate Slows Axonal Transport of Neurofilaments in Transfected Neurons

Abstract

Neurofilaments are transported through axons by slow axonal transport. Abnormal accumulations of neurofilaments are seen in several neurodegenerative diseases, and this suggests that neurofilament transport is defective. Excitotoxic mechanisms involving glutamate are believed to be part of the pathogenic process in some neurodegenerative diseases, but there is currently little evidence to link glutamate with neurofilament transport. We have used a novel technique involving transfection of the green fluorescent protein–tagged neurofilament middle chain to measure neurofilament transport in cultured neurons. Treatment of the cells with glutamate induces a slowing of neurofilament transport. Phosphorylation of the side-arm domains of neurofilaments has been associated with a slowing of neurofilament transport, and we show that glutamate causes increased phosphorylation of these domains in cell bodies. We also show that glutamate activates members of the mitogen-activated protein kinase family, and that these kinases will phosphorylate neurofilament side-arm domains. These results provide a molecular framework to link glutamate excitotoxicity with neurofilament accumulation seen in some neurodegenerative diseases.

Protective and Toxic Neuroinflammation in Amyotrophic Lateral Sclerosis

“…In patients with ALS, microgliosis occurs specifically with motor neuron injury in the motor cortex, along the corticospinal tract, and in the ventral horn of the spinal cord [24,53,[69][70][71][72] radioligands that bind microglia, has been used to image microglial activation corresponding with the location motor neuron injury in patients with ALS [72,73]. This radiographic evidence in humans correlates the with evidence in the G93A mSOD1 transgenic mouse, which demonstrates microglial activation even before motor neuron cell death [74,75].…”

“…Most importantly, many of these immune changes in the animal model have been confirmed in patients with nonhereditary ALS [24,70,99,100,131,132]. Henkel et al [133] demonstrated that in patients with rapidly progressing clinical states, an inverse correlation was seen between Treg numbers in the blood and leukocyte levels of FoxP3, a transcription factor required for Treg suppressive function [134].…”

“…In the G93A mSOD1 transgenic mouse, activation of astrocytes occurs concomitantly with a decrease in motor neurons [75,82,83]. Although astrocyte numbers increase with disease progression, astrocytes do not proliferate like microglia, but may be derived from ependymal cells lining the central canal of the spinal cord or oligodendrocyte precursor cells [24,[84][85][86][87][88].…”

“…Astrocytes can also contribute to the immune response via pattern recognition receptors, including TLRs and mannose receptors. Following activation, astrocytes can secrete neurotoxic factors and cytokines, such as C-X-X motif chemokine 10, chemokine (C-C motif) ligand 2, and IL-6, greatly affecting the local environment [24,40,53,76,80].…”

“…injurious pathways occurring peripherally at the neuromuscular junction and centrally at the cell body of the motor neuron [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Histopatholgy in the animal model reveals a disruption at the neuromuscular junction that appears first followed by a "dying back" of the axon [25].…”