Dual Effects of Neuroprotection and Neurotoxicity by General Anesthetics on Neural Stem Cells: Role of Autophagy

General anesthetics (GAs) are widely used for various essential surgical or medical procedures. Recent studies implicate the GAs has dual effects of neuroprotection and neurotoxicity on neurogenesis with unclear mechanisms. This minireview summarizes recent studies on GAs mediated effects on neurogenesis and proposed mechanisms, with focus on autophagy regulation and intracellular calcium homeostasis. DOI : 10.14302/issn.2574-4372.jesr-16-1380 Corresponding author: Huafeng Wei, Department of Anesthesiology and Critical Care, University of Pennsylvania, 305 John Morgan Building, 3610 Hamilton Walk, Philadelphia, PA 19104, Phone: 215-662-3193, FAX: 215-349-5078, Email: weih@uphs.upenn.edu


Introduction
Each year, millions of fetuses, infants and preschool children are exposed to general anesthetics (GAs) worldwide for various essential surgical or medical procedures.2][3] Many mechanisms have been proposed to explain GAs mediated neurotoxicity, including activation of NMDA and GABA receptors 4,5 , mitochondrial damage with excessive free radicals [6][7][8] , activation of P75 growth factor 9,10 , excessive inflammation 11,12 , and disruption of intracellular Ca 2+ homeostasis [13][14][15] .Recent studies have also suggested neurogenesis impairment may play a role, since neurotoxic effects of anesthetics occur both in vitro [16][17][18][19] and in vivo [20][21][22] .As methods improve, a full mechanistic understanding of neurotoxicity becomes more realistic.For example, with the development of an in vitro neurogenesis system using hESCs, induced pluripotent stem cells (iPSCs), and neural stem cells (NSCs), investigators can now study the mechanisms underlying brain development and screen the toxic effects of various anesthetics under controlled conditions (dose, number of exposures, or developmental stage).New neuroprotective strategies to avoid the anesthetics mediated toxicity, then, can be generated through neurogenesis modeling. 23eeley et al 24 27 and animals 28,29 demonstrated that isoflurane for short exposure did not induced neuronal cell damage by itself, but significantly inhibited neurodegeneration induced by isoflurane for prolonged use.
Unfortunately, we did not examine the possible dual effects of general anesthetics on stem or neuroprogenitor cells in these studies.
Recent stem cell studies have opened up avenues for research in GA induced developmental toxicity 16,18,20 .
Accumulated data indicate that ketamine can cause neuronal damage in several major brain regions in animal models during certain periods of development 30,31 .On the other hand, ketamine at concentrations ranging from 1 to 500 mM did not cause significant toxicity in NSCs. 32[35][36] We have focused on studying the role of intracellular calcium regulation in GA mediated effects on autophagy and neurogenesis.Our previous studies clearly demonstrated that GAs, especially isoflurane, at ). Isoflurane at a low concentration (0.6%) increased proliferation of these neural progenitor cells, whereas no effect was seen with a higher clinically relevant isoflurane concentration (1.2%).In contrast, isoflurane at high concentration (2.4%) decreased proliferation. 164][45] Our findings suggest that isoflurane may affect ReNcell CX NPC survival and neurogenesis in a dual manner through differential activation of InsP 3 and/or RYR.
Propofol has become one of the most widely used intravenous GAs. 46Twaroski et al found that a high dose of propofol induced developmental toxicity. 47On the other hand, Jeffrey et al found that propofol at clinically relevant concentrations (<7.1μmol/L) increases neuronal differentiation but is not toxic to hippocampal neural precursor cells in vitro. 48Other studies showed that very low doses of propofol inhibit neuronal arborization in vitro 49 , and increase the number of neuronal spines on differentiated cells in vivo. 50 reported exposure of third-trimester fetal macaque monkeys to isoflurane in utero caused widespread apoptosis of neurons and oligodendroglia critical for myelination.They use high concentrations of isoflurane, which was adjusted by painful stimulation.The volatile anesthetic concentration was titrated according to a predefined clinical endpoint that represents an intermediate surgical plane of anesthesia, where there was no motor response and only a mild sympathetic response with an increase of 10% or less in heart rate or blood pressure.Researchers achieved this endpoint via deep nail-bed stimulation at the hand and foot [mosquito-clamp pinch].The turning points of anesthetic concentration and duration varied among different animal species and in human beings and depended on the combination of both anesthetic concentration and exposure duration.Our previous study by Li et al 25 , showed 1.3% isoflurane for 6 hours reduced apoptosis in the rat fetal brain, while our follow up study by Wang et al 26 demonstrated that 3% isoflurane for only 1 hour significantly increased neuroapoptosis in the fetal developing brains.These studies supported our view of an association between the dual effects of GA-mediated neuroprotection and neurotoxicity and anesthetic concentration and exposure duration.Our previous studies in both cell cultures Freely Available Online www.openaccesspub.org| JESR CC-license DOI : 10.14302/issn.2574-4372.jesr-16-1380Vol-1 Issue 2 Pg.no. 14 low concentrations for short exposure, provide neuroprotection by adequate activation of inositol triphosphate receptors or ryanodine receptors (InsP 3 R and RYR) on the membrane of the endoplasmic reticulum (ER).However, GAs at high concentrations for prolonged use cause neurotoxicity. 37-39Our previous study further demonstrated that isoflurane affects ReNcell CX (human neural progenitor cell (NPC) line, immortalized by retroviral transduction with the c-myc oncogene and derived from the cortical region of the human fetal brain) proliferation and differentiation via differential activation of InsP 3 R and RYR and elevation of cytosolic Ca 2+ concentration ([Ca 2+ ] c

Figure 1 .
Figure 1.Role of autophagy in anesthetic mediated dual effects of neuroprotection and neurotoxicity.General anesthetics at low concentrations for short exposure induce physiological autophagy, which in turn inhibits apoptosis and promotes neurogenesis and eventually provides neuroprotection (left side).On the other hand, general anesthetics at high concentrations for prolonged use result in impairment of autophagy, which in turn promotes apoptosis and inhibits neurogenesis and eventually causes neurotoxicity (right side).