Neuroprotective Function of Cellular Prion Protein in a Mouse Model of Amyotrophic Lateral Sclerosis

Transgenic mice expressing human mutated superoxide dismutase 1 (SOD1) linked to familial forms of amyotrophic lateral sclerosis are frequently used as a disease model. We used the SOD1^G93A mouse in a cross-breeding strategy to study the function of physiological prion protein (Prp). SOD1^G93APrp−/− mice exhibited a significantly reduced life span, and an earlier onset and accelerated progression of disease, as compared with SOD1^G93APrp+/+ mice. Additionally, during disease progression, SOD1^G93APrp−/− mice showed impaired rotarod performance, lower body weight, and reduced muscle strength. Histologically, SOD1^G93APrp−/− mice showed reduced numbers of spinal cord motor neurons and extended areas occupied by large vacuoles early in the course of the disease. Analysis of spinal cord homogenates revealed no differences in SOD1 activity. Using an unbiased proteomic approach, a marked reduction of glial fibrillary acidic protein and enhanced levels of collapsing response mediator protein 2 and creatine kinase were detected in SOD1^G93APrp−/− versus SOD1^G93A mice. In the course of disease, Bcl-2 decreases, nuclear factor-κB increases, and Akt is activated, but these changes were largely unaffected by Prp expression. Exclusively in double-transgenic mice, we detected a significant increase in extracellular signal-regulated kinase 2 activation at clinical onset. We propose that Prp has a beneficial role in the SOD1^G93A amyotrophic lateral sclerosis mouse model by influencing neuronal and/or glial factors involved in antioxidative defense, rather than anti-apoptotic signaling.Amyotrophic lateral sclerosis is characterized by rapid degeneration of motor neurons in the spinal cord, brain stem, and cortical Betz cells. As a result, focal muscle wasting, weakness, and spasticity develop focally. These symptoms ultimately lead to global paralysis. Patients usually die due to respiratory failure within 3 years of symptom onset.¹The causes of ALS are diverse; 10 to 15% of cases are familial with autosomal dominant inheritance, and 20% of these are related to point mutations in the gene encoding Cu/Zn superoxide dismutase 1 (SOD1). SOD1 is a ubiquitously expressed homodimeric protein that catalyzes the reaction of O₂⁻ to O₂ and H₂O₂, which is then further metabolized by glutathione peroxidase. Mice overexpressing human mutated SOD1 (muSOD1) linked to ALS, develop disease resembling ALS in humans by a toxic gain of function.² Several properties of muSOD1 were proposed to contribute to toxic gain of function, including enhanced peroxidase activity and formation of peroxynitrite, changes in copper and zinc binding, and aggregation of the enzyme. ALS progression is accompanied by oxidative stress processes, glutamate-induced excitotoxicity, cytoskeletal abnormalities, inflammatory processes, and toxicity via extracellular muSOD1.^2,3 The apoptotic cascade is activated in the ALS model, shown by sequential activation of caspase-1 and −3.⁴ Interestingly, Bcl-2 overexpression had a neuroprotective effect and, like intrathecal administration of caspase inhibitors, led to a slowed disease progression and increased life span in these mice.^5,6 More recently, it was proposed that not only neurons themselves contribute to the neurodegenerative process in ALS but also non-neuronal cells, particularly microglia and astrocytes.^7–10 Expression of muSOD1 in non-neuronal cells was sufficient to induce cell death in nearby motor neurons lacking muSOD1.^11,12Since an early decrease of prion protein (Prp) mRNA has been described in an ALS model,¹³ loss of Prp function might contribute to the neurodegenerative process not only in prion diseases but also in ALS. A number of physiological functions have meanwhile been attributed to Prp, including antioxidative and anti-apoptotic properties and involvement in transmembrane signaling and cell adhesion.¹⁴ Additionally, as recently reported in cell culture models, Prp regulates astrocytic signaling^15,16 and thereby might also influence neuron−glia signaling relevant for ALS pathogenesis. Clarifying the role of Prp in models of neurodegeneration is of special interest, since Prp as membrane protein might become an easily accessible drug target for treatment of neurodegenerative diseases as a spin-off of the current search for antiprion drugs.¹⁷In the present study, we analyzed the function of physiological Prp in a mouse model of ALS by cross-breeding mice transgenic for human SOD1 with G93A mutation (SOD1^G93A) with Prp knockout (Prp−/−) mice. Characterization of the SOD1^G93APrp−/− mice concerning motoric properties, disease progression, and life span, paralleled by histopathological, immunochemical, and proteomic analyses, revealed that Prp has a protective role potentially by influencing mechanisms assuring neuronal survival.