Activation of the stress-activated MAP kinase, p38, but not JNK in cortical motor neurons during early presymptomatic stages of amyotrophic lateral sclerosis in transgenic mice

Abstract

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder, characterized by the degeneration of upper and lower motor neurons (MNs). Central nervous system features include a loss of Betz cells and other pyramidal cells from sensorimotor cortex. The intrinsic mechanism underlying this selective motor neuron loss has not been identified. A recent in vitro study has provided evidence of a novel programmed cell death (PCD) pathway that is unique to spinal cord MNs and is exacerbated by superoxide dismutase (SOD) mutations. This PCD pathway is triggered through the Fas receptor and involves the apoptosis signal-regulating kinase 1 (ASK1), the p38 MAP kinase, and the neuronal form of nitric oxide synthase (nNOS). Previously, we found significant increases in the numbers of ventral horn MNs immunopositive for these enzymes in the spinal cords of mutant SOD transgenic (G93A) mice as early as 60 days of age, suggesting that this pathway may be active in vivo. Since the upper MNs of ALS patients and G93A mice are also known to degenerate, the purpose of the present study was to investigate the possible activation of this PCD pathway in the MNs of the sensorimotor cortex of G93A transgenic mice. Compared to non-transgenic littermates, the G93A mice showed significant increases in the numbers of MNs immunopositive for the active (phosphorylated) forms of ASK1, p38, MKK3/6 (the known activator of p38), and also active caspase-3, as early as 60 days of age. Another stress-activated protein kinase, c-Jun N-terminal kinase (JNK), commonly activated in other neurodegenerative disorders such as Alzheimer's disease, showed no increases in G93A mice at any age. These results suggest that, not only has a PCD pathway been activated in the cortical MNs, but one that may be unique to ALS. Moreover, these findings suggest that earlier diagnosis and therapeutic intervention may be possible for successful treatment of ALS. Consequently, these enzymes may provide the biochemical markers to enable earlier diagnosis of ALS and molecular targets for the development of new therapeutic compounds.