Ultrastructural diversity of inclusions and aggregations in the lumbar spinal cord of SOD1-G93A transgenic mice

Abstract

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by selective motor neuron death. We report the characteristics of ultrastructural pathological changes of inclusions and aggregations in the neuronal axons, glial cells and ventral roots of lumbar spinal cord in SOD1-G93A transgenic mice using light and electron transmission microscope at different stages of disease. The most noteworthy is that mutant SOD1 accumulations in the cytoplasm of motor neurons precede the numerous inclusions. Inclusions manifested differently according to the specified locations. This study provided further information to the previous reports about pathological changes of ALS.

Introduction

Amyotrophic lateral sclerosis (ALS) is a progressive, adult-onset neurodegenerative disease that mainly affects motor neurons in cortex, brain stem, and spinal cord. ALS occurs in both sporadic and familial forms, which are clinically and pathologically similar. About 90–95% cases of ALS were sporadic, and the remaining 5–10% of patients inherit the disease in an autosomal dominant manner. SOD1 mutations account for approximately 20% of familial cases (Rosen et al., 1993). Currently, more than 153 different SOD1 mutations have been linked to ALS through a gain of unknown toxicity (Felbecker et al., 2010). SOD1-G93A mice showed an ALS-like phenotype and were widely used in ALS pathogenic and therapeutic researches (Gurney et al., 1994). Several hypotheses have been proposed to explain motor neuron death in ALS, including glutamate-induced excitotoxicity, oxidative stress, cytoskeletal abnormality, and protein aggregation (VanDen-Bosch et al., 2006, Cleveland & Rothstein, 2001, Julien, 2001, Julien et al., 2005, Wijesekera & Leigh, 2009).

General pathological changes have been revealed in SOD1 mutant transgenic mice, including astrogliosis, fragmentation of Golgi apparatus, SOD1 aggregation, and vacuolar degeneration (Mourelatos et al., 1996, Bruijn et al., 1998, Dal-Canto & Gurney, 1995, Stieber et al., 2000). A hallmark of ALS is the aberrant accumulation of neurofilaments in the cell body and proximal axons of motor neurons. Although the extent to which neurofilament abnormalities contribute to the pathogenesis of ALS remains unknown, emerging evidence suggests that disorganized neurofilaments can provoke degeneration and death of neurons (Lee et al., 1994, Perrone-Capano et al., 2001). It was reported that accumulation of SOD1-positive aggregates in the neuronal processes, predominantly in the axons, probably causing impairment of axonal transport, which contribute to the pathogenesis of SOD1-G93A transgenic mice (Sasaki et al., 2005). Although ALS is characterized by motor neuron degeneration, the role of non-neuronal cells has recently been recognized (Pramatarova et al., 2001, Clement et al., 2003). All of the above studies indicate that the pathological changes of ALS involved more than one organelle, one cell type or even one system. We have recently reported that sensory axons in the lumbar dorsal root and posterior funiculus are also affected as early as motor axons do in the SOD1-G93A transgenic mice (Guo et al., 2009). However, the precise mechanisms leading to ALS were still unknown.

In the current work, we extended previous pathological observations and made an intensive examination of the ultrastructural pathological features of inclusions and abnormal aggregations in neuronal axons and glial cells, as well as neurons, in the lumbar spinal cord of SOD1-G93A transgenic mice at different stages of disease. We present new evidence that SOD1 accumulation in the cytoplasm of motor neurons precede the appearance of numerous inclusions in the axons and astrocytes. The ultrastructural pathological features of inclusions and aggregations differ according to their locations, and astrocytic and axonal inclusions were predominant changes at the end stage of disease, which may contribute to the degeneration of motor neurons or be the results of motor neuron degeneration.