The genetics of isoflurane-induced developmental neurotoxicity

Abstract

Introduction Neurotoxicity induced by early developmental exposure to volatile anesthetics is a characteristic of organisms across a wide range of species, extending from the nematode C. elegans to mammals. Prevention of anesthetic-induced neurotoxicity (AIN) will rely upon an understanding of its underlying mechanisms. However, no forward genetic screens have been undertaken to identify the critical pathways affected in AIN. By characterizing such pathways, we may identify mechanisms to eliminate isoflurane induced AIN in mammals. Methods Chemotaxis in adult C. elegans after larval exposure to isoflurane was used to measure AIN. We initially compared changes in chemotaxis indices between classical mutants known to affect nervous system development adding mutants in response to data. Activation of specific genes was visualized using fluorescent markers. Animals were then treated with rapamycin or preconditioned with isoflurane to test effects on AIN. Results Forty-four mutations, as well as pharmacologic manipulations, identified two pathways, highly conserved from invertebrates to humans, that regulate AIN in C. elegans. Activation of one stress-protective pathway (DAF-2 dependent) eliminates AIN, while activation of a second stress-responsive pathway (endoplasmic reticulum (ER) associated stress) causes AIN. Pharmacologic inhibition of the mechanistic Target of Rapamycin (mTOR) blocks ER-stress and AIN. Preconditioning with isoflurane prior to larval exposure also inhibited AIN. Discussion Our data are best explained by a model in which isoflurane acutely inhibits mitochondrial function causing activation of responses that ultimately lead to ER-stress. The neurotoxic effect of isoflurane can be completely prevented by manipulations at multiple points in the pathways that control this response. Endogenous signaling pathways can be recruited to protect organisms from the neurotoxic effects of isoflurane.

Isoflurane Exposure in Juvenile Caenorhabditis elegans Causes Persistent Changes in Neuron Dynamics

“…This is in contrast to previous behavioral measurements in which daf-2 null mutants did not display changes in chemotaxis index upon developmental anesthetic exposure. 29 Thus, apoptotic cell death as well as insulin signaling, both of which have previously been implicated in the neurotoxic effects of anesthetic, appear to be independent of the subtle alterations in neuronal function measured here. Reconciliation of this discrepancy likely lies in the relative complexity of C. elegans chemotaxis, the efficient undertaking of which is dependent upon various behaviors that contribute to locomotive stochasticity and also upon neurophysiological processes, such as chemosensation.…”

“…In recent behavioral genetics studies, Morgan, Sedensky, and colleagues 10,29 identified molecular modulators of the chemotaxis deficits that follow developmental anesthetic exposure in C. elegans. Key among these are genes within the apoptosis cell death pathway, the daf-2-dependent stress-response pathway, and the mechanistic Target of Rapamycin (mTOR).…”

“…These findings are reminiscent of prior work demonstrating blockage of anesthesia-induced chemotaxis deficiency in worms via mTOR inhibition. 29 A growing body of literature further describes mTOR-dependent anesthesia-induced effects in neurodevelopmentally immature rodents. Mintz and colleagues 32,33 have demonstrated in mouse models that isoflurane anesthesia is capable of eliciting abnormal dendrite arbor and spine development, pre-and postsynaptic marker expression, and also deficiencies in spatial learning and memory.…”

“…Our findings are also consistent with the activation of neuronal stress-response pathways in response to anesthetic exposure, as supported by recent work by Morgan, Sedensky, and colleagues. 29 Indeed, whereas daf-16 facilitates a protective endoplasmic reticulum stress-response pathway, 35,36 mTOR has also been shown to interact with this same stress-response pathway and others. 37,38 Our results therefore add to a mounting body of evidence suggesting that anesthetization during critical periods of nematode development may impact normal neurodevelopment through pathologic activation of stress-response pathways and subsequently alter neuronal activity.…”

Anesthetics disrupt brain development via actions on the mTOR pathway

“…Na and co-workers found that rapamycin treatment prevented the loss of chemotaxis that results from developmental exposure of C. elegans to isoflurane. Based on this data and the results of a forward genetics screen that identified endoplasmic reticulum stress pathways as a target in anesthetic-induced neurotoxicity, that isoflurane acts on mitochondria to create reactive oxygen species that in turn activate the mTOR pathway [ 30 ]. In our recently published manuscript, we described a study in a mouse model of developmental anesthetic neurotoxicity which revealed that anesthetic effects on mTOR signaling profoundly disrupted development of newborn dentate gyrus granule cells in the hippocampus.…”