Corticospinal neurons respond differentially to neurotrophins and myelin-associated glycoprotein in vitro

Elucidating the mechanisms that regulate the survival and outgrowth of corticospinal tract (CST) neurons and other CNS tracts will be a key component in developing novel approaches for the treatment of central nervous system (CNS) disorders, including stroke, spinal cord injury (SCI), and motor neuron disease (MND). However, the in vivo complexities of these diseases make a systematic evaluation of potential therapeutics that directly affect corticospinal regeneration or survival very challenging. Here, we use Thy1.2 transgenic mice expressing yellow fluorescent protein (YFP) in postnatal day 8 (P8) corticospinal neurons, as a source of CST neurons that have already established synapses in the spinal cord, to assess factors that influence neurite outgrowth and survival of axotomized CST neurons. After culture, YFP-positive corticospinal neurons represent an enriched neuronal population over other glia and interneurons, survive, and extend processes over time. YFP-positive CST neurons also continue to express the corticospinal markers CTIP2 and Otx1. CST neurons display different degrees of axon extension, dendritic branch length and elaboration, and neurite elongation in response to neurotrophin-3 and ciliary neurotrophic factor, and an inhibitory outgrowth response when cultured on myelin-associated glycoprotein. Some CST neurons are lost with extended culture, which provides a baseline from which we can also assess factors that enhance CST neuron survival. This assay thus allows us to assess independent aspects of CST axonal and dendritic outgrowth kinetics, which allows for the rapid and sensitive investigation of new therapies to address corticospinal neuron outgrowth in the context of CNS injury and neurodegenerative disorders.