The interplay of hypoxic and mental stress: Implications for anxiety and depressive disorders

Adequate oxygen supply is essential for the human brain to meet its high energy demands. Therefore, elaborate molecular and systemic mechanism are in place to enable adaptation to low oxygen availability. Anxiety and depressive disorders are characterized by alterations in brain oxygen metabolism and of its components, such as mitochondria or hypoxia inducible factor (HIF)-pathways. Conversely, sensitivity and tolerance to hypoxia may depend on parameters of mental stress and the severity of anxiety and depressive disorders. Here we discuss relevant mechanisms of adaptations to hypoxia, as well as their involvement in mental stress and the etiopathogenesis of anxiety and depressive disorders. We suggest that mechanisms of adaptations to hypoxia (including metabolic responses, inflammation, and the activation of chemosensitive brain regions) modulate and are modulated by stress-related pathways and associated psychiatric diseases. While severe chronic hypoxia or dysfunctional hypoxia adaptations can contribute to the pathogenesis of anxiety and depressive disorders, harnessing controlled responses to hypoxia to increase cellular and psychological resilience emerges as a novel treatment strategy for these diseases.

Neuroprotective action of PHD inhibitors is predominantly HIF-1-independent: An Editorial for 'Sex differences in neonatal mouse brain injury after hypoxia-ischemia and adaptaquin treatment' on page 759

Hypoxia-inducible factor (HIF-1) as the primary factor mediating gene-dependent cellular responses to hypoxia represents an attractive target for the therapeutic interventions. The current Editorial comments on an as yet underestimated facet of HIF-1-related research. The activity of HIF-1 is being regulated by the availability of its α-subunit HIF-1α, which undergoes quick degradation. The process of degradation is initiated by prolyl 4-hydroxylase (PHD). PHD is an oxygen-dependent enzyme and therefore is inactivated in hypoxia, in turn resulting in HIF-1α stabilization, its dimerization with HIF-1β subunit thereby producing the transcriptionally active factor. It has been suggested that pharmacological inhibition of PHD activity might give the same results. Indeed, a large body of evidence on beneficial effects of PHD inhibitors has been accumulated in multiple laboratory and clinical trials. In addition to them, a paper by Li and colleagues published in this issue of Journal of Neurochemistry also reports that inhibition of PHD by adaptaquin reduces hypoxic-ischemic brain injury in a neonatal mouse model. When dissecting the underlying molecular mechanisms, Li and colleagues surprisingly found that the observed effects appear to be independent of HIF-1. These findings draw attention back to the question about possible HIF-1 effects independent of PHD inhibitors, which has been raised several years ago but has not received sufficient attention so far, and is being discussed in this Editorial. One of the possible mechanisms might be ascribed to the ferroptosis pathway affected by PHD inhibitors but this question needs further careful studies, as well as clarification of other mechanisms possibly involved. Even if they represent a prospective therapeutic strategy, the lack of current knowledge about endogenous targets of PHD inhibitors, except for PHD, calls for a careful and balanced approach toward their clinical use.

Neuroendocrine drivers of risk and resilience: The influence of metabolism & mitochondria

The manifestation of risk versus resilience has been considered from varying perspectives including genetics, epigenetics, early life experiences, and type and intensity of the challenge with which the organism is faced. Although all of these factors are central to determining risk and resilience, the current review focuses on what may be a final common pathway: metabolism. When an organism is faced with a perturbation to the environment, whether internal or external, appropriate energy allocation is essential to resolving the divergence from equilibrium. This review examines the potential role of metabolism in the manifestation of stress-induced neural compromise. In addition, this review details the current state of knowledge on neuroendocrine factors which are poised to set the tone of the metabolic response to a systemic challenge. The goal is to provide an essential framework for understanding stress in a metabolic context and appreciation for key neuroendocrine signals.