Motor Neurons Are Selectively Vulnerable to AMPA/Kainate Receptor-Mediated InjuryIn Vitro

Abstract

The nonphosphorylated neurofilament marker SMI-32 stains motor neurons in spinal cord slices and stains a subset of cultured spinal neurons neurons"], which have a morphology consistent with motor neurons identified in vitro: large cell body, long axon, and extensive dendritic arborization. They are found preferentially in ventral spinal cord cultures, providing further evidence that large SMI-32(ϩ) neurons are indeed motor neurons, and SMI-32 staining often colocalizes with established motor neuron markers (including acetylcholine, calcitonin gene-related peptide, and peripherin). Additionally, choline acetyltransferase activity (a frequently used index of the motor neuron population) and peripherin(ϩ) neurons share with large SMI-32(ϩ) neurons an unusual vulnerability to AMPA /kainate receptor-mediated injury. Kainateinduced loss of these motor neuron markers is Ca 2ϩ -dependent, which supports a critical role of Ca 2ϩ ions in this injury. Raising extracellular Ca 2ϩ exacerbates injury, whereas removal of extracellular Ca 2ϩ is protective. A basis for this vulnerability is provided by the observation that most peripherin(ϩ) neurons, like large SMI-32(ϩ) neurons, are subject to kainate-stimulated Co 2ϩ uptake, a histochemical stain that identifies neurons possessing Ca 2ϩ -permeable AMPA /kainate receptor-gated channels. Finally, of possibly greater relevance to the slow motor neuronal degeneration in diseases, both large SMI-32(ϩ) neurons and peripherin(ϩ) neurons are selectively damaged by prolonged (24 hr) low-level exposures to kainate (10 M) or to the glutamate reuptake blocker L-transpyrrolidine-2,4-dicarboxylic acid (100 M). During these lowlevel kainate exposures, large SMI-32(ϩ) neurons showed higher intracellular Ca 2ϩ concentrations than most spinal neurons, suggesting that Ca 2ϩ ions are also important in this more slowly evolving injury.

A role for calpain-dependent cleavage of TDP-43 in amyotrophic lateral sclerosis pathology

“…In addition, mislocalization of TDP-43 from the nucleus to the cytoplasm is observed in the cortical neurons of normal elderly people 43 and motor neurons of aged mice 44 . As a pathogenic role for abnormal Ca 2 þ signalling has been proposed in these conditions 36,45 , it is likely that activation of Ca 2 þ -dependent calpain has a role in the TDP-43 pathology observed in these conditions as well. In addition to Ca 2 þ -permeable AMPA receptors, there are multiple sources of intracellular Ca 2 þ , including Ca 2 þ -permeable receptors and channels, as well as Ca 2 þ storage vesicles such as the mitochondria and the endoplasmic reticulum 46 , and Ca 2 þ sources responsible for activation of calpains may possibly be different among these different conditions.…”

Intrathecal infusion of a Ca2+-permeable AMPA channel blocker slows loss of both motor neurons and of the astrocyte glutamate transporter, GLT-1 in a mutant SOD1 rat model of ALS

“…In initial studies we examined pathological indices in lumbar spinal cord at late presymptomatic ages (95-100 days), in comparison to both age matched controls (littermates lacking the transgene) and "end stage" transgenic animals (defined as onset of symptomatic gait impediment). MNs were labeled with Nissl stain, and immunocytochemically using an antibody to non-phosphorylated nerurofilament epitopes, SMI-32, which strongly labels MNs as well as their neuritic processes (Carriedo et al, 1996). In the WTs, virtually all MNs appeared healthy.…”

“…First, MNs are unusually sensitive to injury mediated through AMPA/kainate type glutamate receptors (Carriedo et al, 1995;Carriedo et al, 1996;Hugon et al, 1989;, and blockers of these receptors protect MNs from injury caused by prolonged blockade of glutamate uptake (Carriedo et al, 1996;Rothstein et al, 1993).…”

“…Likely contributing to this vulnerability, MNs possess substantial numbers of unusual Ca 2+ permeable AMPA type glutamate receptor channels (Ca-AMPA channels) (Carriedo et al, 1995;Carriedo et al, 1996;Van Den Bosch et al, 2000;Vandenberghe et al, 2000). Currently, the best animal models of ALS are provided by rodents harboring mutant forms of the enzyme Cu, Zn superoxide dismutase (SOD1), which are associated with familial ALS in humans.…”

Potentiation of excitotoxicity in HIV-1-associated Dementia and the significance of glutaminase

“…Presence of an edited GluR2, as is the case in an overwhelming majority of cells expressing AMPA, renders the heteromeric receptor mostly impermeable to calcium (80,81). Calcium-impermeable AMPA receptors can still cause excitotoxicity by allowing sodium influx to slightly depolarize the cell membrane, leading to the subsequent activation of NMDA receptors (83,84), as has been demonstrated by multiple investigators (84)(85)(86)(87)(88).…”