Peripherin is not a contributing factor to motor neuron disease in a mouse model of amyotrophic lateral sclerosis caused by mutant superoxide dismutase

Abstract

Peripherin is a type III intermediate filament protein detected in axonal spheroids associated with amyotrophic lateral sclerosis (ALS). The overexpression of peripherin induces degeneration of spinal motor neurons during aging in transgenic mice and in cultured neuronal cells derived from peripherin transgenic embryos. Here, we investigated whether peripherin is a contributor of pathogenesis in mice overexpressing a mutant superoxide dismutase 1 (SOD1^G37R) gene linked to familial ALS. This was done by the generation and analysis of SOD1^G37R mice that either overexpress a peripherin transgene (G37R;TgPer mice) or lack the endogenous peripherin gene (G37R;Per−/− mice). Surprisingly, upregulation or suppression of peripherin expression had no effects on disease onset, mortality, and loss of motor neurons in SOD1^G37R mice. These results provide compelling evidence that peripherin is not a key contributor of motor neuron degeneration associated with toxicity of mutant SOD1.

Introduction

Amyotrophic lateral sclerosis (ALS) is a late-onset neurological disease characterized by the selective degeneration of upper and lower motor neurons leading ultimately to paralysis and death due to respiratory failure. ALS occurs in both sporadic (90% of cases) and familial forms, which are clinically and pathologically similar. Missense mutations in the gene encoding the free radical-scavenging metalloenzyme Cu/Zn superoxide dismutase 1 (SOD1) are responsible for approximately 20% of familial ALS (fALS) cases (Rosen et al., 1993). To date, over 90 mutations have been discovered spanning all exons of the SOD1 gene. Transgenic mice expressing various SOD1 mutants develop ALS-like phenotypes through a gain of unknown toxic properties Gurney et al 1994, Wong et al 1995, Ripps et al 1995, Tu et al 1996, Bruijn et al 1997, Bruijn et al 1998.

Peripherin is a type III neuronal intermediate filament (IF) protein of 57 kDa expressed predominantly in the peripheral nervous system. It is also detected at low levels in defined populations of central nervous system neurons, mostly in those extending to the periphery (Parysek and Goldman, 1988; Brody et al 1989, Escurat et al 1990, Gorham et al 1990. Because of its increased expression during development and following nerve injury, peripherin is believed to play a role in nerve elongation Escurat et al 1990, Gorham et al 1990, Wong and Oblinger 1990. However, the extent to which peripherin exerts its axonal growth function during development remains unclear since peripherin null mice do not show any defect in the development of large myelinated peripheral sensory and motor axons (Larivière et al., 2002). There is also evidence for an involvement of peripherin in the neuronal response to inflammation. Inflammatory cytokines, such as interleukin-6 (IL-6) and leukemia inhibitory factor (LIF), can upregulate peripherin expression through the activation of the JAK/STAT signaling pathway Djabali et al 1993, Sterneck et al 1996, Lecomte et al 1998. Also, we recently showed that peripherin expression can be induced in the brain following stab injuries and cerebral ischemia, two conditions associated with neuronal inflammation (Beaulieu et al., 2002).

Many observations have suggested a possible involvement of peripherin in the pathology of ALS. A pathological hallmark of human sporadic and fALS as well as of transgenic mice expressing ALS-linked SOD1 mutants is the presence of inclusion bodies containing peripherin, along with neurofilament (NF) proteins in the perikaryon and axon of degenerating motor neurons (reviewed in Chou 1995, Tu et al 1996; Morisson et al., 1998; Corbo and Hays 1992, Migheli et al 1993, Wong et al 2000. Moreover, overexpression of a peripherin transgene in mice resulted during aging in the selective degeneration of ventral root motor axons associated with the development of ALS-like IF inclusions in spinal motor neurons (Beaulieu et al., 1999). Finally, overexpression of peripherin in cultured embryonic neurons causes apoptotic death in synergy with tumor necrosis factor-α (TNF-α) (Robertson et al., 2001).

To further clarify the potential involvement of peripherin in ALS caused by SOD1 mutations, we generated SOD1^G37R mice that either overexpress a mouse peripherin transgene or lack the endogenous peripherin gene. We then evaluated whether these changes in peripherin gene expression affected the development of the motor neuron pathology.