Associate Professor (senior researcher)
1990 BS/MS, Universidad de Buenos Aires, Argentina
1995 PhD, Universidad de Buenos Aires, Argentina
Motor Neuron Death in Amyotrophic Lateral Sclerosis
Amyotrophic lateral sclerosis (ALS) is the more common motor neuron disease. ALS is an always fatal neurodegenerative disease characterized by the death of motor neurons in the brain stem and spinal cord, as well as pyramidal neurons in the motor cortex. The progressive death of motor neurons in the spinal cord and brain stem leads to weakness and eventually death due to respiratory failure. In spite of significant advances in the understanding of genetic causes of the disease, the pathogenesis of ALS remains largely unknown and highly controversial. Riluzole and more recently edaravone are the only two FDA approved drugs for the treatment of ALS. Riluzole provides small but consistent protection for ALS patients and animal models of the disease. The protective effect of riluzole was originally attributed to its anti-glutamate effects. More recently, the protective effect has been attributed to the prevention of motor neuron membrane hyper-excitability. However, the mechanism by which riluzole is protective in ALS remains unknown. We reported that riluzole has no direct effects on cultured motor neurons, but its protection was mediate by enhancing astrocyte production of trophic factors. In addition, this stimulating effect of riluzole was short living. Long-term incubation of astrocytes and Schwann cells with riluzole results in an abolishment of the enhancement. Oral administration of riluzole to mice also results in short- term stimulation of trophic factor production, while long-term administration of the drug decreases the production of trophic factors to control or even below control levels. The goal of our research is to determine whether the discontinuous treatment with riluzole might provide a better therapeutic approach for ALS. We are currently investigating the effects of riluzole on astrocytes and motor neurons derived from human inducible pluripotent stem cells (iPSC) generated from healthy (control) and ALS patients. This research is performed in collaboration with Dr. Brain Kaspar and Kathrin Meyer (Nationwide Children Hospital, Columbus, Ohio), and David Hendrix (Oregon State University).
Peroxynitrite is a strong biological oxidant formed by the diffusion-limited reaction of nitric oxide and superoxide. The half-life of peroxynitrite in biological, relevant conditions is less than 1 second. Peroxynitrite can oxidize almost every component of the cell, either by direct reaction with thiol groups or by indirect reactions mediated by the products of peroxynitrite decomposition or by secondary products of other peroxynitrite faster reactions. Nitrated tyrosine residues in proteins are a foot-print left by peroxynitrite. Peroxynitrite stimulates apoptosis in a number of different cellular models. Incubation of PC12 cells with peroxynitrite results in the induction of both necrosis and apoptosis. Peroxynitrite-induced apoptosis in PC12 cells is mediated by nitration of tyrosine residues in proteins, leading to the simultaneous activation and inactivation of intracellular signaling pathways (Fig. 1).
Figure 1. Peroxynitrite-induced apoptosis in PC12 cells
Motor neuron death in ALS patients and animal models of the disease is also preceded by the nitration of tyrosine residues. We found that the death of motor neurons in culture stimulated by either trophic factor deprivation, the activation of the Fas death receptor, or mutations of the enzyme superoxide dismutase associated with ALS is also preceded by increased tyrosine nitration (Fig. 2).
Figure 2. Role of Peroxynitrite in motor neuron apoptosis
The chaperone heat shock protein 90 (Hsp90) was identified by mass spectrometric analysis of nitrated proteins as a critical target of peroxynitrite nitration. The intracellular delivery of nitrated Hsp90 in one of the 24 tyrosine residues is sufficient to stimulates apoptosis in motor neurons and PC12 cells. Nitrated Hsp90 activates the purine receptor P2X7, which in turn activates the apoptotic cascade (Fig. 2). The goal of our investigations is to determine the interactions of Hsp90 and nitrated Hsp90 with the P2X7 receptor complex. In addition, we are investigating whether the nitration of Hsp90 is enough to stimulate apoptosis in PC12 cells and motor neurons derived from human iPSCs by mutating the critical residues to phenylalanine using CRISPR/Cas9.