Local GDNF expression mediated by lentiviral vector protects facial nerve motoneurons but not spinal motoneurons in SOD1^G93A transgenic mice

Abstract

Approximately 2% of amyotrophic lateral sclerosis (ALS) cases are associated with mutations in the cytosolic Cu/Zn superoxide dismutase 1 (SOD1) gene. Transgenic SOD1 mice constitute useful models of ALS to screen therapeutical approaches. Glial cell line-derived neurotrophic factor (GDNF) holds promises for the treatment of motoneuron disease. In the present study, GDNF expression in motoneurons of SOD1^G93A transgenic mice was assessed by facial nucleus or intraspinal injection of lentiviral vectors (LV) encoding GDNF. We show that lentiviral vectors allow the expression for at least 12 weeks of GDNF that was clearly detected in motoneurons. This robust intraspinal expression did, however, not prevent the loss of motoneurons and muscle denervation of transgenic mice. In contrast, LV-GDNF induced a significant rescue of motoneurons in the facial nucleus and prevented motoneuron atrophy. The differential effect of GDNF on facial nucleus versus spinal motoneurons suggests different vulnerability of motoneurons in ALS.

Introduction

Amyotrophic lateral sclerosis (ALS) is a lethal, paralytic disorder caused by the progressive degeneration of motoneurons in the spinal cord, brainstem and cerebral cortex. Approximately 2% of all ALS cases arise as a dominantly inherited trait named familial ALS and are associated with more than 90 different missense mutations in the gene encoding cytosolic Cu/Zn superoxide dismutase 1 (SOD1) (Rowland and Shneider, 2001). Overexpression of various SOD1 mutations in transgenic mice faithfully reproduces the symptomatology of the disease and constitutes therefore a useful model to screen therapeutical approaches for ALS. Mice carrying a human SOD1 construct with Gly⁹³→Ala (G93A) mutation develop limb weakness, loss of motoneurons, impaired axonal transport, ventral roots Wallerian degeneration and muscle denervation atrophy Gurney, 1994, Zhang et al., 1997. Glial cell line-derived neurotrophic factor (GDNF) is the most potent motoneuron factor yet identified Henderson et al., 1994, Oppenheim et al., 1995, Zurn et al., 1994. GDNF prevents motoneuron degeneration in mice and rats following sciatic or facial nerve axotomy Hottinger et al., 2000, Houenou et al., 1996, Li et al., 1995, Matheson et al., 1997, Munson and McMahon, 1997, Oppenheim et al., 1995, Yan et al., 1995. GDNF also slows down motoneuron degeneration in the pmn mouse model of progressive motoneuropathy (Sagot et al., 1996), as well as programmed motoneuron cell death during development (Oppenheim et al., 1995). In transgenic mice expressing GDNF in muscle, a hyperinnervation by motoneurons has been reported (Nguyen et al., 1998). These actions are mediated through a heterodimer receptor system composed of the c-ret protein and the GDNF-α receptor Durbec et al., 1996, Treanor et al., 1996, Trupp et al., 1996 which are expressed in adult rodent motoneurons. Expression of these receptors is up-regulated in motoneurons after nerve injury Burazin and Gundlach, 1998, Glazner et al., 1998. Moreover, motoneurons can bind, internalize and retrogradely transport GDNF from muscle in a receptor-dependent manner (Yan et al., 1995). Taken together, these observations suggest that GDNF holds promises for the treatment of motoneuron diseases. Retroviral vectors based on the human immunodeficiency virus (HIV) infect non-dividing cells including neurons (Naldini et al., 1996). Stable, long-term expression of the reporter gene β-galactosidase (β-Gal) has been achieved in rodent (Hottinger et al., 2000) and primate (Kordower et al., 1999) brain. Lentiviral vector-mediated delivery of GDNF efficiently prevents the loss of dopaminergic nigrostriatal neurons in rodent Bensadoun et al., 2000, Déglon et al., 2000 and primate models of Parkinson's disease (Kordower et al., 2000) as well as axotomy-induced facial motoneuron death in adult mice (Hottinger et al., 2000). The viral vector-mediated delivery of GDNF has been studied in SOD1 transgenic mice. Significant delay in disease progression has been reported by intramuscular injection of adenoviral vectors (Acsadi et al., 2002), adeno-associated vectors Kaspar et al., 2003, Wang et al., 2002 or an ex vivo gene delivery system in which myoblasts were retrovirally transduced with a GDNF gene (Mohajeri et al., 1999). In the present work, the effect of locally expressed GDNF was assessed in SOD1^G93A transgenic mice by direct facial nucleus or intraspinal injection of lentiviral vectors encoding GDNF. The present study was designed to evaluate the transduction efficiency of motoneuron, the cell type specificity of transgene expression and the potential therapeutic effect of GDNF applied directly to the facial nucleus or ventral spinal cord in a SOD1 transgenic mice model of ALS.