Role of the GH/IGF-I axis in the growth retardation of weaver mice

IGF-I is a well-established anabolic growth factor essential for growth and development. Although the role of the GH/IGF-I axis is established for normal postnatal growth, its functional state in neurodegenerative diseases is not fully characterized. The weaver mutant mouse is a commonly used model for studying hereditary cerebellar ataxia and provides an opportunity to investigate the function of IGF-I in postnatal growth following neurodegeneration. Previously, we reported that weaver mice are growth retarded and their body weights correlate with a decrease in circulating IGF-I levels. Because weaver mice have the same food intake/body weight ratios as their wild type littermates, our observation suggests that an impairment of the GH/IGF-I axis, rather than poor nutrition, likely contributes to their growth retardation. This study further investigated the etiology of reduced circulating IGF-I levels. We found that GH levels in weaver mice were reduced following acute insulin injection, but the hepatic GH receptor transduction pathway signaled normally as evidenced by increased STAT5b phosphorylation and IGF-I mRNA levels in response to acute GH administration. In addition, 2-week GH treatment induced a significant increase in body weight and circulating IGF-I levels in homozygous weaver mice but not in wild type littermates. In summary, a deficiency in the GH/IGF-I axis may be partially responsible for postnatal growth retardation in weaver mutant mice. This deficiency may occur at the level of the pituitary and/or hypothalamus and can be improved with GH administration.