FMRP regulation of mitochondria and metabolism in mammalian brain development

Neuronal activity is critical for brain development and altered activity-dependent plasticity is found in neurodevelopment disorders. RNA binding protein FMRP, encoded by the X-linked Fragile X Messenger Ribonucleoprotein 1 (FMR1) gene, has been shown to regulate activity-dependent neuronal function and plasticity. FMRP loss of function leads to fragile X Syndrome (FXS) 7-9, the most common heritable cause of intellectual disability and a top contributor to autism spectrum disorders (ASD). Despite extensive interest, the mechanisms underlying FXS and FMRP regulation, especially in humans, are not fully clear. Neurons have high energy demand and both mitochondrial content and oxidative phosphorylation increase during neuronal maturation and activation. We published the first paper to show that FMRP deficiency in mouse neurons leads to mitochondrial fragmentation and impaired mitochondrial function. To translate these exciting findings into humans, we assessed human pluripotent stem cells (hPSCs)-differentiated neurons and discovered that FMRP-deficient human neurons have not only mitochondrial fragmentation but also reduced amount of mitochondria. In addition to MFNs, human FXS neurons also have reduced PGC1α (PPARGC1A), a master regulator of mitochondrial biogenesis and energy metabolism. We and others have shown that human FXS neurons exhibit hyperexcitability in the absence of stimulation. Our preliminary study found that human FXS neurons had reduced response to neuronal activation stimulus. Using CLIP-seq we have identified RACK1 (Receptor for Activated C Kinase 1) as a direct target of FMRP in human but not mouse neurons. RACK1- defieicent human neurons phenocopy mitochondrial deficits and hyperexcitability of human FXS neurons and exogenous RACK1 rescues deficits of human FXS neurons. RACK1 is essential for both human and mouse development; however, the role of RACK1 human neuron development is unknown. We hypothesize that FMRP is required for neuronal activity-dependent mitochondrial biogenesis and energy metabolism during human neuron development and RACK1 is a key mediator of such regulation. We will investigate the impact of FMRP and RACK1 deficiency on activity-dependent mitochondrial dynamics and functions in human neurons; determine the mechanisms underlying RACK1 regulation of mitochondrial dynamics and energy metabolism in human neurons, and comprehensively characterize proteome and metabolome of FXS human neurons. The proposed work will fill a major gap of our knowledge in understanding metabolic regulation in human neuron development, which is critical for therapeutic development for FXS, ASD, and other brain disorders.