Vulnerability of postnatal motoneurons to axotomy: is their fate linked to their own immaturity?

Nerve injuries during early postnatal stages lead to a massive loss of motoneurons. The vulnerability of these neurons to axonal injuries contrasts with the robust survival and regenerative capability of adult ones. Since brachial plexus palsy due to obstetric accidents is the most frequent type of injury at the postnatal period, neonates can suffer a permanent motor disability of the upper extremity, with no specific treatment to reduce or even prevent this neuronal loss. Despite classical studies in the last century approached this issue, recent literature in this field is scarce and therefore, the topic has not benefited from innovative research techniques, and it is still matter of debate the mechanism that led to this neuronal loss. Thus, the aim of the present project is to further investigate the causes that induce motoneuron death when nerve injuries occur in the neonatal stage. Previous results in our lab indicate that postnatal peripheral neurons are in a transition state from development to maturity, with a decline in their intrinsic growth capacity - reminiscent of the embryonic stage- but still not mature enough to activate a robust regenerative program in response to injury, as the one observed in adults. Thus, the aim of the present project is to further investigate the role of the basal state of postnatal motoneurons to their susceptibility to axotomy. Our hypothesis is that the immaturity of these neurons, that are in a transient state from embryonic to adulthood, could be the major impediment to cope with a stressful situation such as an axonal injury. We propose that neonatal neurons could also be in a transient metabolic state, shifting from the anaerobic metabolism of embryonic neurons to the characteristic oxidative one of adult neurons, that confers neuroprotection in stressful situations. These neurons are also in a transition from an excitatory to an inhibitory GABAergic mediated effect, that confers an hyperexcitatory neuronal state in postnatal stages that could increase the fragility of postnatal motoneurons to stressors. However, since multifactorial causes can be implicated in motoneuron death, the potential contribution of microglia, that has a higher phagocytic profile in these stages, and the immaturity of Schwann cells, will also be explored. We will use an experimental model to investigate the role of the own immaturity of motoneurons and glia in the contribution of their vulnerability to axotomy. The functional impact of these factors will be explored by means of pharmacological and genetic interventions, with the final aim to elucidate which are the main actors implicated in the massive death of motoneurons when injured at early postnatal stages. In parallel, we will set up a humanized in vitro model of axotomy, by deriving human motoneurons form induced pluripotent stem cells and culturing in microfluidic chambers specifically designed to facilitate a montage closer to the in vivo setting. Finally, we will corroborate our in vivo results from the murine model in our in vitro humanized one. We expect that the findings allow us to further advance in the design of strategies to revert or minimize this important neuronal loss of neurons in the postnatal stages, to reduce the devasting functional deficits associated.