Elucidating Olfactory Mechanisms of PTSD Vulnerability and Trauma Resilience

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ABSTRACT Faced with severe psychological trauma, some people develop post-traumatic stress disorder (PTSD), but most people don't. Current understanding of the biological mechanisms underlying resilience from, and vulnerability to PTSD remains limited. Neuroscience suggests that the olfactory bulb (OB), a key structure in odor processing, may also be involved in mechanisms of traumatic stress. In animals, chronic stress reduces OB size; while OB ablation results in stress-enhanced startle reflex, amygdala reactivity, structural reorganization of limbic structures and autonomic dysregulation. Furthermore, OB lesion causes hyperexcitability of medial amygdaloid neurons through NMDA-based mechanisms. However, OB morphometry has not been adequately studied in the development of stress disorders in humans. Our recently published study of adults who suffered sexual and/or physical abuse during childhood (N=16), revealed that OB volumes of trauma-exposed PTSD subjects (T1P1) were substantially reduced compared to OB volumes of trauma-exposed subjects who did not develop PTSD (T1P0). Additionally, while OB volumes of T1P0 and non- trauma exposed healthy control (HC) groups were statistically similar, those of T1P1 were significantly reduced compared to HC. Furthermore, preliminary findings from our ongoing longitudinal study of T1P1 and T1P0 adults with childhood sexual trauma show that reduced OB size is associated with physiological indices of amygdala disinhibition 6 months later. Given recent strong preclinical evidence of bidirectional relationships in molecular events between olfactory neurons (ON) and OB, and our discovery that immunohistochemical markers of ON survival in patient-derived olfactory tissues are predictive of their OB volumes, we compared expression patterns in ON of T1P1 and T1P0 and found differentially elevated levels of Growth arrest specific 5 (GAS5) in olfactory cells derived noninvasively from T1P0. GAS5 is a long noncoding RNA (lncRNA) that mimics corticotropin response elements in the promoter regions of genes that respond to glucocorticoids. By binding to these regions, GAS5 competitively blocks the transcriptional effects of glucocorticoids on these genes and protect the tissues from atrophy. Although a mechanistic hypothesis of GAS5 is compelling from our preliminary data, the National Institute of Mental Health is moving from funding candidate gene approaches to unbiased omic approaches. As a result, we propose a 2-year prospective R21 study on a larger sample of non- smoking subjects exposed to childhood sexual abuse (N=60, 60% females) and 20 healthy controls, to: (1) validate differences in OB and other olfactory regions in T1P0 (N=30) and T1P1 (N=30), matched on duration of assault and years since last assault; (2) quantify the relationship between OB morphometry and dimensional measures of stress and resilience, including electrodermal responses to aversive visual stimuli on all subjects at baseline (2.1) and the modulatory effects of baseline OB morphometry on future stress responses on all subjects 6 months later (2.2); and (3) explore molecular mechanisms underlying the relationship between OB structure and PTSD vulnerability through unbiased (i.e. genome-wide) RNA-based epigenomic processes and through in vitro morphologic studies (including cortisol treatments) of olfactory cells derived non-invasively from their nasal brushings. The latter is needed to generate preliminary epigenome and mechanistic data for a large-scale R01 study. Accomplishment of these aims could impact the field by introducing a novel olfactory mechanism of trauma vulnerability/resilience and by introducing a solid scientific premise for direct targeting of the olfactory structures in interventions for chronic stress disorders.