Links between mitosis progression, cell fate, and microcephaly
Our work has used genetics, pharmacology and live imaging to discover new links between mitosis length of stem cells and cell fate. Using mice haploinsufficient for an RNA binding protein, called Magoh, we discovered a new role for mitosis length in cell fate specification. We discovered that Magoh haploinsufficient neural progenitors exhibit mitotic delay, and these progenitors directly produce more neurons instead of new progenitors. This fascinating phenotype is recapitulated using pharmacology-in vitro, ex vivo and in vivo, revealing that prolonged progenitor mitosis is sufficient to alter neural cell fates and identifying one explanation for microcephaly. Please see our studies in Neuron and Developmental Neuroscience. We have also extended these findings to the study of interneuron development. We discovered that Magoh is essential for interneuron generation and survival linked to prolonged mitosis of interneuron progenitors (Development). Our most recent study has shown that mitosis duration of Eif4a3-deficient mouse and human progenitors is also associated with altered cell fate (Development).
We continue to study the link between prolonged mitosis and altered cell fate using novel in vivo models, live imaging as well as primary cells.
The RNA binding exon junction complex and neurodevelopmental disease
Our work has demonstrated new roles for components of the RNA binding exon junction complex (EJC) in brain development. The controls many stages of the RNA life cycle, including splicing, translation, decay, and RNA localization. The EJC is composed of Magoh, Rbm8a and Eif4a3. Mutations and copy number variations in these components are associated with human disease including microcephaly, autism and intellectual disability. See this review on the EJC and this review on RNA binding proteins to learn more. Over many years we have discovered that haploinsufficiency for each of these proteins causes microcephaly and impairs neurogenesis by controlling mitosis of progenitors and survival of newborn progeny. We previously discovered that haploinsufficiency for the EJC protein, Magoh, results in microcephaly, due to defects in neural progenitor proliferation and neuronal apoptosis, as published in Nature Neuroscience and Genesis. Please also see our 2015 study on Rbm8a in The Journal of Neuroscience. In addition we have used transcriptomics and proteomics to discover common alterations in all 3 mutants, including p53 (PLoS Genetics)! Interestingly, genetic analyses also told us that a 3rd binding partner, Casc3, does not influence brain development in the same way, as reported in our 2016 study in RNA. We have collaborated with the Passo-Bueno lab to model how EIF4A3 mutations disrupt neural crest to cause Richieri-Costa-Peirera Syndrome (RCPS). Our recent study in Development used fixed and live imaging of mouse and human neural progenitors to demonstrate critical roles for Eif4a3 in mitosis duration of both species.
Our recent study defines a new non-canonical role for EIF4A3 in axon formation in vivo, by directly binding to microtubules and controlling their dynamics.
We continue to use mouse and human models to understand how the EJC controls brain development and disease and how it controls microtubules.
The translational regulator DDX3X and DDX3X syndrome
How do de novo mutations in RNA binding proteins disrupt neurodevelopment? We have been studying how an RNA binding protein called DDX3X influences brain development. De novo mutations in DDX3X underlie 1-3% of female intellectual disabilities and are linked to brain malformations including callosal agenesis. We are collaborating with Dr. Elliott Sherr, a human geneticist at UCSF, and Stephen Floor, an RNA biologist at UCSF, to understand how DDX3X missense and nonsense mutations affect cortical development and RNA metabolism. Our lab discovered that depletion of DDX3X impairs neuron number by disrupting progenitors. Further we showed that missense mutations in DDX3X cause abnormal RNA-protein granule formation in progenitors. This work was published in 2020 in Neuron. Our recent eLife study phenotypes a loss of function Ddx3x mouse model, informing new requirements for DDX3X in progenitor cell cycle, as well as using ribosome profiling to discover DDX3X translational targets in the brain.
We continue to use mouse models, cell based models, genomics and imaging to understand how DDX3X mutations impair brain development.
