In vivo modelling of human axon degeneration

Abstract

Similar mechanisms regulate axon loss in both acute injury (a process known as Wallerian degeneration) and disease, as molecular attempts to slow down degeneration of injured axons attenuate the progression of clinical pathologies in animal models of chronic disorders. Therefore, understanding the mechanisms of nerve degeneration in response to injuries allows to model how axons are lost in more complex neurodegenerative conditions. Among these diseases, the present work has focused on Down syndrome (DS), as a key feature of this condition is a striking propensity to develop early-onset Alzheimer’s disease (EOAD). Complete trisomy of chromosome 21 causes the development of amyloid plaques and neurofibrillary tangles, which are typical characteristics of AD brain pathology. Most of our knowledge of AD-like syndrome in DS comes from post-mortem human samples and studies using animal models. Despite the great importance of these works, the early events of human axon pathology are still uncovered, mainly due to experimental limitations. In vitro studies using human iPSC-derived neurons have advanced our understanding, acting as a promising platform for genetic and pharmacological manipulation in the attempt of rescuing and attenuating the pathological phenotypes. However, this in vitro system allows only a partial modelling of disease. Indeed, limitations due to a lack of more complex cell-cell interactions in monolayer neuronal cultures or inner vascularization need to be addressed when recapitulating a complex human brain condition like Down syndrome. To overcome the limitation of in vitro systems, human cortical neural progenitor cells and excitatory neurons derived from an adult individual with DS, were transplanted into the somatosensory cortex of adult mice, in which a cranial window was implanted for longitudinal multiphoton intravital imaging. Since spontaneous degeneration could not be studied systematically inside the grafts, axons were challenged with a localised, reproducible laser- mediated microlesion. This strategy has allowed for the first time to investigate the spatiotemporal dynamics of human axon degeneration in response to injury, in a multicellular and highly vascularized complex microenvironment, highlighting differences and similarities in the degenerative kinetics that occur in Down syndrome compared to healthy controls.