A new mouse model with gain of iron regulatory protein 1 function

Abstract

Disorders of iron metabolism account for some of the most common human diseases, such as anemias and hemochromatosis. To maintain physiological iron balance, homeostatic mechanisms are normally in place both at the systemic and the cellular level. Cellular iron homeostasis is secured by Iron Regulatory Proteins (IRP) −1 and −2 through their binding to cis-regulatory iron-responsive elements (IRE) in target mRNAs encoding proteins with key functions in iron metabolism. In turn, the IRE−binding activity of the two IRPs is feedback regulated by the cellular labile iron pool. Mouse models with IRP deficiency have contributed valuable insights into the in vivo roles of the IRP/IRE system as well as mammalian iron biology. However, the physiological consequences of gain of IRP function have so far remained unexplored. To investigate the importance of adequate IRP expression in vivo, we have generated a mouse model allowing conditional gain of IRP function using Cre/Lox technology. This new line expresses a flag-tagged IRP1 mutant (IRP1*), which escapes iron-mediated regulation, being constitutively active in its IRE binding form. Systemic expression of the IRP1* transgene from the Rosa26 locus yields viable animals with gain of IRE-binding activity in all organs that were analyzed. IRP1* alters the expression of IRP target genes and is accompanied by abnormal body iron distribution. Furthermore, mice display macrocytic erythropenia with decreased hematocrit and hemoglobin levels. Flow cytometric analysis of bone marrow-derived erythroid precursors also revealed impaired erythroid differentiation. Under standard laboratory conditions, broad spectrum phenotyping of IRP1* mice revealed that gain of IRP1 function does not affect their general health status. Yet, when challenged, mice displayed mildly altered motor coordination and reduced endurance as well as impaired neuromuscular transmission, suggesting a potential role of appropriate IRP activity in maintenance of neuromuscular function. Ex vivo, iron challenged bone marrow-derived macrophages showed that IRP1* expression alters the cellular response to fluctuations in iron levels. However, when IRP1* mice were pharmacologically administered with iron or crossed with a mouse model of chronic iron overload, IRP1* expression did not produce any overt abnormalities in the animal response to the iron challenges. In conclusion, this work describes the first model of gain of IRP1 function in a mammalian organism. This new mouse model further highlights the importance of appropriate IRP regulation in central organs of iron metabolism as well as for general physiology. Moreover, it opens novel avenues for study of diseases associated with abnormally high IRP activity, such as Parkinson’s disease or Friedreich ataxia.