In this report, we have presented proof-of-principle for a novel and potentially effective therapeutic strategy for human lissencephaly, using our Lis1-deficient mice that are a good model of this disorder. Therapeutic strategies for lissencephaly are a daunting consideration for several reasons. First, given the nature of lissencephaly, one would have to treat all neurons throughout development. Second, LIS1 mutations in humans are de novo, so that detection of the disorder at an early enough time point to allow effective therapy is difficult. In spite of these difficulties, there are some advantages to considering the treatment of lissencephaly that results from LIS1 haploinsufficiency. First, LIS1 protein is present and can potentially be manipulated, since individuals display heterozygous, not complete loss of LIS1. Second, there are dosage dependent effects of LIS1, so any augmentation of LIS1 protein levels will likely have a beneficial effect. Third, a great deal is known about the pathogenesis and mechanism of action of LIS1 and its pathway, so the effects of any therapeutic modality can be assessed directly with quantitative measures in vivo and in vitro.
We based this therapeutic strategy on our recent observations that LIS1 is degraded after anterograde transport to the nerve terminals in a calpain dependent fashion. Inhibition of calpain resulted in the augmentation of LIS1 protein, which led to the rescue of aberrant distribution of cytoplasmic dynein from Lis1-deficient mice. We further demonstrated that inhibition of calpain rescued neuronal migration from granule neurons from the Lis1 mutants. Most importantly, we demonstrated that daily ALLN administration in utero was partially effective in improving the defective migration phenotypes in vivo in the Lis1-deficient mice, which was associated with improvement in motor function.
Recently, it was shown that increased LIS1 expression affects human and mouse brain development32
. In our case, inhibition of calpain activity results in normalization close to the wild type levels rather than accumulation of LIS1 in excess. We believe that the restoration of more normal LIS1 levels was one reason that calpain inhibition resulted in phenotypic improvement of Lis1+/−
mice. We cannot rule out the possibility that other effects of calpain may also play some roles in the observed phenotypic rescue, including suppression of spectrin/neurofilaments/MT breakdown, cleavage of p35, a Cdk5 activator important for neuronal migration33–35
, prevention of degradation of other proteins included in the Lis1/Ndel1/Dynein complex and/or acetylated tubulin or FAK complex18,20
, which will be the subjects of further investigation.
Several problems remain and must be overcome if this promising avenue of therapy can be eventually tested in human lissencephaly, including further refinement of the use of calpain inhibitors for the effective inhibition of LIS1 degradation as well as the safe and effective delivery of such drugs for clinically effective treatment of human lissencephaly. In spite of the challenges, our work provides a potential avenue to consider therapeutic strategies for severe, early brain developmental defects such as lissencephaly due to LIS1 mutations, as well as any other disorder that results from haploinsufficiency.