Acute glial activation by stab injuries does not lead to overt damage or motor neuron degeneration in the G93A mutant SOD1 rat model of amyotrophic lateral sclerosis
Introduction
Amyotrophic lateral sclerosis (ALS) is typified by progressive loss of motor neurons in the brain and spinal cord, leading to paralysis and death within 3–4 years (Cleveland and Rothstein, 2001, Boillee et al., 2006). The mean age of onset for familial ALS (FALS) is 60 years, and patients have no symptoms until shortly before the disease begins. While the majority of ALS cases are sporadic in nature, a small percentage (∼ 10%) has a familial component. Although the cause of sporadic ALS remains unclear, a clear genetic link to point mutations in the cytosolic Cu2+/Zn2+ superoxide dismutase 1 (SOD1) has been shown in a small group of familial ALS (FALS) patients (Rosen, 1993, Andersen et al., 2003). This has led to the generation of transgenic mice and rats overexpressing multiple copies of mutant SOD1 (SOD1G93A) that have many of the characteristics of both the familial and sporadic form of human disease (Gurney, 1994, Hall et al., 1998, Howland et al., 2002) . The exact etiology of ALS is still mystery, but recent observations suggest an involvement of acute trauma on the occurrence or disease progression of ALS (Chio et al., 2005, Chen et al., 2007).
Motor neuron death in ALS is a complex process and may involve multiple pathways including formation of protein aggregates, axonal transport defects, oxidative damage, mitochondrial defects, and alterations in calcium homeostasis (Julien, 2001, Rowland and Shneider, 2001). While the primary trigger for ALS remains a mystery, there is also increasing evidence that excitotoxicity due to astrocyte dysfunction and inflammatory processes from microglia activation play a crucial role in disease progression (Hall et al., 1998, Barbeito et al., 2004, Boillee et al., 2006). In rodent models of FALS expressing high levels of mutant SOD1, astrocytes are also reactive early in disease progression and are known to release factors that contribute to motor neuron death (Van Den et al., 2008). Recent studies showed that in chimeric mice with both wild type and mutant SOD1 expressing cells there is a clear protective effect when wild type astrocytes are associated with mutant SOD1 expressing motor neurons (Clement et al., 2003) suggesting a potential protective role under certain conditions. Most of the studies to date have focused on modulating the ratio of mutant SOD1 over expression and normal cells in transgenic mice and have shown that reducing the number of glia expressing the mutation can have a significant impact on motor neuron survival and disease progression (Clement et al., 2003, Beers et al., 2006).
Previous evidence points to a mechanism where the astrocytes become reactive but lose regulation of their normal functions in ALS. However, it has not been previously determined whether spinal cord injuries, which initiate acute glial activation and inflammation, could affect motor neuron survival in presymptomatic SOD1G93A rats. In this study, we hypothesized that host glial activation by experimental stab injuries in the spinal cord of presymptomatic SODG93A rats may trigger motor neuron degeneration through abnormal astrocyte responses. To address this issue, we used a model of experimental spinal cord injury at the lumbar spinal cord (L2–4) with a longitudinal stab injury using a micro-knife. This injury model has been used to study the cell biology of penetrating spinal cord injury in vivo in a context that caused no grossly detectable behavioral impairments in animals unless the division of host astrocytes was inhibited (Faulkner et al., 2004). Here we show that acute astrocyte activation by stab injury does not cause motor neuron degeneration in pre symptomatic rats carrying the SOD1G93A mutation, a rat model of familial ALS.
Section snippets
Animals
Breeder SOD1G93A rats, which had been originally generated by Howland et al. (2002), were obtained from Taconic (Hudson, NY), and colonies were developed by crossing male founders with female Sprague-Dawley rats (Suzuki et al., 2007b). Heterozygous SOD1G93A progeny were identified with polymerase chain reaction (PCR) of tail DNA with primers specific for human SOD1. They were maintained in a room with controlled illumination (lights on 0600–1800 h) and temperature (23 ± 1 °C) and given free
Spinal cord injury by a micro-knife does not change motor function in ipsilateral limb in SOD1G93A rats
To establish the lesion model, a wire micro-knife lesion parallel to the rostral–caudal axis was unilaterally performed at L2–4 in wild type rats (Fig. 1A). At 2 weeks after surgery, the animals were perfused, and the level of the knife injury in the spinal cord was observed by Nissl staining (Fig. 1B). A longitudinal stab injury did not cause severe trauma to the motor neuron pools within the ventral horn beneath the lesion (Fig. 1B) as shown in previous studies (Faulkner et al., 2004).
A
Discussion
Here we hypothesized that glial activation by a stab injury may lead to increased motor neuron loss and early onset of disease symptoms in presymptomatic SOD1G93A rats We found that while a micro-knife injury leads to significant host astrogliosis in the transgenic animals, it was not accompanied by either limb dysfunction or motor neuron degeneration This suggests that acute trauma or insertion of a needle to inject cells into the spinal cord of pre symptomatic animals has no acute effect on
Acknowledgments
This work was supported by the grant from the ALS Association, NIH/NINDS [PO1NS057778 (to C.N.S.) and R21NS06104 (to M.S.)], the University of Wisconsin Foundation, and the Les Turner ALS foundation.
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