The Role of Lis1 in Adult Mammalian System

Lis1 haploinsufficiency in humans results in a “smooth brain” phenotype called lissencephaly, and also causes severe cognitive and motor impairments and epilepsy. Seizure severity and frequency typically worsens with time; patients often die within the first decade due to seizure-induced aspiration and pneumonia. Various mouse models have been used to examine the role of Lis1 during brain development, and it is clear that Lis1 regulates a microtubule motor, cytoplasmic dynein. Intriguingly, Lis1 expression remains high in adult brains indicating that it plays a role in mature systems. Indeed, our group found that Lis1 and several related proteins regulate dynein-dependent axon transport in cultured adult rat sensory neurons. Here, we hypothesize that loss of Lis1 in adult mice could lead to neurological disorders or other diseases. In order to bypass the developmental impact of Lis1 loss, we first utilized a tamoxifen inducible Cre-mediated recombination system driven by an actin promoter to knock out Lis1 in adult mice (\u3e8 weeks). We found that loss of Lis1 in adult mice caused a progressive decline and ultimately death. These phenotypes were not observed in any of a variety of control animals. Analysis of Cre activity revealed it was not uniformly stimulated in all regions of the brain, and is highest in the brainstem, where neuronal processes in regions known to control cardiorespiratory networks were activated. In non-neuronal tissues, high level of Cre activity was also detected in the hearts of tamoxifen induced Lis1 knockout mice. Secondly, to determine whether the lethal phenotype is caused by loss of Lis1 in brainstems or in hearts, we generated a cardiomyocyte-specific inducible lis1 knockout mouse model. Deletion of lis1 in hearts did not seem to induce lethality or malaise in adult mice. This strengthened the hypothesis that the death phenotype may be caused by Lis1 loss in neurons of the cardiorespiratory brainstem network. Lastly, examination of brain sections of experimental mice revealed hallmarks of chromatolysis. Cultured DRG neurons from adult tamoxifen injected experimental animals, but not controls, showed signs of axon pathology and transport defects. Together, our results indicate that Lis1 is essential for adult mammalian system, and its loss of function causes lethal phenotype in adult mice, which may be resulted from dramatic altered neuronal function. !