Intranasal administration of IGF-I improves behavior and Purkinje cell pathology in SCA1 mice

Abstract

Spinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disease caused by the expansion of polyglutamine repeat within ataxin-1 protein. Cerebellar Purkinje cells are the primary targets of SCA1 pathology. These cells synthesize insulin-like growth factor-I (IGF-I) and express its receptors during their entire life. The aim of present study was to determine if intranasally administered IGF-I to SCA1 transgenic mice suppresses toxic effects of ataxin-1. Two-week old SCA1 heterozygous animals were randomly divided into two treatment groups of IGF-I (30 and 60 μg IGF-I/animal) and a vehicle-treated control group. The wildtype animals served as normal controls. IGF-I or vehicle was administered at 48 h intervals for the total of 10 doses. Animals were then subjected to rotarod test, sacrificed, brains removed and processed for immunohistochemical and Western blot analysis. Radiolabeled IGF-I and bioactive TAT peptide accumulated in the brains of SCA1 mice following intranasal administration validating the use of intranasal route. SCA1 mice showed SCA1 pathology with impaired motor function and downregulation of calcium binding proteins as compared to wildtype mice. However, 30 and 60 μg IGF-I-treated animals showed improved performance on the rotarod as compared to vehicle-treated SCA1 mice with significant improvement (p < 0.05) on day 3 in 60 μg IGF-I group. The immunohistochemical data further showed partial recovery in the expression of calbindin D28k and protein kinase C-γ in Purkinje cells in IGF-I-treated SCA1 animals. Our results indicate that suppression of ataxin-1-mediated adverse effects by intranasal IGF-I treatment may be of a therapeutic value to treat SCA1.

Introduction

Several growth factors and their specific receptors are expressed in the central nervous system suggesting a role in regulation of cell growth and differentiation in the developing and mature brain. Insulin-like growth factor-I (IGF-I) and its receptors are abundant in cerebellum [1], [2], [3], [22], [29] Cerebellar Purkinje cells synthesize IGF-I and express IGF-I receptors during their entire life [3]. IGF-I serves as a promoting factor for Purkinje cell development, particularly postnatal survival and dendritic growth [9]. Further, in Purkinje cells IGF-I up-regulates the signaling pathways involving the phosphoinositide 3-kinase, mitogen-activated protein kinase, and p38 kinase [9]. An additional source of IGF-I for these cells is provided by climbing fiber afferents originating in the inferior olive nucleus. IGF-I from the inferior olive is necessary for motor learning processes probably involving Purkinje cell synaptic plasticity [3], [14], [22].

IGF-I and its related molecules are also involved in neurodegenerative processes in which IGF-I-containing pathways are compromised [8]. Our earlier studies have demonstrated that the process of cerebellar degeneration in human olivopontocerebellar atrophy and in lurcher mutant mouse is associated with altered IGF-I receptor binding and protein tyrosine phosphorylation [24]. Further, it has been shown that patients with late-onset cerebellar ataxias with degeneration in their IGF-I containing neuronal pathways also show significant changes in the peripheral IGF system [8], [21], [29]. Zhang et al. [29] demonstrated that apoptotic Purkinje cells in Purkinje cell degeneration mice contain significantly low levels of IGF-I mRNA suggesting the involvement of IGF-I pathways in the degenerative processes. Furthermore, our recent data on double mutant mice generated by mating SCA1 transgenics with IGF-1 overexpressing transgenic mice showed that the double mutants have improved motor coordination on the rotating rod with decreased Purkinje cell pathology (unpublished observations). Therefore, the present study was undertaken to determine if intranasally administered IGF-I has any beneficial effects on the pathogenesis of SCA1 in SCA1 transgenic mice.