Neuroinflammation and motor neuron loss in SMA

PROJECT SUMMARY Spinal muscular atrophy (SMA), a leading genetic cause of infant mortality, is a degenerative disease characterized by loss of motor neurons in the spinal cord, skeletal muscle atrophy, and death. SMA is caused by the disruption or deletion of the survival motor neuron (SMN) gene and a substantial reduction in the associated SMN protein; however, the specific role SMN loss plays in disease pathology is still unclear. Current therapies replace and/or increase SMN levels in patients, and although this strategy is largely successful, it is surprisingly not a cure even when treatment initiates in pre-symptomatic stages. Motor neuron loss is essential for the development of SMA, but the mechanisms underlying motor neuron loss is unknown. Growing evidence from our group and others suggests that astrocytes contribute to the complex SMA phenotype and motor neuron loss. We have found that SMA astrocytes (i) exhibit altered morphology, (ii) lack growth factor production, (iii) have aberrant MAPK signaling, (iv) have increased nuclear localization of NFκB, (v) show aberrant upregulation of GATA6 expression, (vi) exhibit increased cytokine expression, (vii) differentially express and produce microRNAs, and (viii) directly induce motor neuron loss in mouse and human iPSC models. SMA patient postmortem tissues also demonstrate astrogliosis and increased cytokine expression providing important confirmation of the experimental results. SMA microglia also show altered activation states, proteolytic activity and phagocytosis, and our recent data demonstrate that astrocytes further induce microglial malfunction. Notably, astrocyte abnormalities occur very early in the disease process, prior to overt motor neuron loss and microglial malfunction leading us to conceptually advance the premise that astrocytes drive the disease- modifying neuroinflammatory cascade in SMA. Based on our extensive published and preliminary data, we hypothesize that a GATA6-mediated cytokine cascade underlies astrocyte malfunction and disease pathology via both contact dependent and independent mechanisms. Here we will leverage our extensive expertise in SMA, iPSC disease modeling, gene therapy, and surface proteomics to elucidate the molecular mechanisms causing astrocyte dysfunction and the downstream impacts on microglial function and motor neuron survival. The collaborative team is highly experienced in all aspects of the proposed experiments and is poised to make impacts on our foundational understanding of human glial-neuron interactions in health and disease. The long-term objective is to identify novel non-SMN therapeutic targets to supplement currently approved therapies, and the proposed experiments make great strides toward achieving that goal.