Endogenous recovery after brain damage: molecular mechanisms that balance neuronal life/death fate

Neuronal survival depends on multiple factors that comprise a well-fueled energy metabolism, trophic input, clearance of toxic substances, appropriate redox environment, integrity of blood-brain barrier, suppression of programmed cell death pathways and cell cycle arrest. Disturbances of brain homeostasis lead to acute or chronic alterations that might ultimately cause neuronal death with consequent impairment of neurological function. Although we understand most of these processes well when they occur independently from one another, we still lack a clear grasp of the concerted cellular and molecular mechanisms activated upon neuronal damage that intervene in protecting damaged neurons from death. In this review, we summarize a handful of endogenously activated mechanisms that balance molecular cues so as to determine whether neurons recover from injury or die. We center our discussion on mechanisms that have been identified to participate in stroke, although we consider different scenarios of chronic neurodegeneration as well. We discuss two central processes that are involved in endogenous repair and that, when not regulated, could lead to tissue damage, namely, trophic support and neuroinflammation. We emphasize the need to construct integrated models of neuronal degeneration and survival that, in the end, converge in neuronal fate after injury. Under neurodegenerative conditions, endogenously activated mechanisms balance out molecular cues that determine whether neurons contend toxicity or die. Many processes involved in endogenous repair may as well lead to tissue damage depending on the strength of stimuli. Signaling mediated by trophic factors and neuroinflammation are examples of these processes as they regulate different mechanisms that mediate neuronal demise including necrosis, apoptosis, necroptosis, pyroptosis and autophagy. In this review, we discuss recent findings on balanced regulation and their involvement in neuronal death.