Nitrite is the typical byproduct of nitric oxide (^•NO) autooxidation in biological systems. However, certain circumstances favor its reduction “back” to the signaling free radical, providing a non-enzymatic route for the synthesis of ^•NO. In pathophysiological conditions such as ischemia/reperfusion (I/R), where low oxygen availability limits nitric oxide synthase activity, nitrite reduction to ^•NO may allow protective modulation of mitochondrial oxidative metabolism and thus reduce the impact of I/R on brain tissue. In the current study, we used high-resolution respirometry to evaluate the effects of nitrite in an in vitro model I/R using hippocampal slices. We found that reoxygenation was accompanied by an increase in oxygen flux, a phenomenon that has been coined “oxidative burst”. The amplitude of this “oxidative burst” was decreased by nitrite in a concentration-dependent manner. These results support the notion that nitrite mediates a decrease in the hyper-reduction of the electron transport system during ischemia, decreasing the accelerated oxygen consumption that characterizes the reoxygenation phase of I/R that has been associated with an increase in oxidant production. Additionally, a pilot in vivo study in which animals received a nitrate-rich diet as a strategy to increase circulating and tissue levels of nitrite also revealed that the “oxidative burst” was decreased in the nitrate-treated animals. These results may provide mechanistic support to the observation of a protective effect of nitrite in situations of brain ischemia.

Humans are complex holobionts in which many physiological functions are ensured by the gut microbiota. The communication between the microbiota and its human host relies on immune, neural, metabolic and endocrine pathways and the derailment of this interaction can lead to gastrointestinal and systemic diseases. Here, we propose a novel form of communication between the microbiota and the host, based on the production of redox species by gut bacteria and the activation of signaling cascades in host mucosa. The biological significance of such a pathway is further highlighted by the observation that these inter-kingdom interactions are modulated by dietary nitrate, the major precursor of nitrite and NO in vivo. We demonstrate that nitrate has a positive metabolic effect in a murine model of antibiotic-induced dysbiosis by regulating cecum morphology and body weight (p<0.05). In agreement with these observations, shallow shotgun sequencing analysis showed that nitrate modulates the metabolic function of bacteria involved in the metabolism of carbohydrates, likely aiding in food digestion and substrate delivery to the host. Furthermore, we observed that the exposure to antibiotics decreases the expression of tight junction proteins in the colon and that nitrate recovers the expression of both occludin (p<0.05) and claudin-5 (p<0.01). The activation of the Nrf2/ARE pathway was also investigated by the downstream expression of detoxifying enzymes including NQO1 and GCLM/GCLC. Here, dietary nitrate emerges as a pivot regulating microbiota-host interactions through redox pathways. Nitrate modulates the function of gut microbiota during dysbiosis by enhancing bacterial metabolic performance with positive effects on host body weight and prevents the loss of tight junction proteins likely reinforcing gut barrier integrity. Given that increased epithelial permeability may lead to leaky gut syndrome, triggering local and systemic disorders, this study has the potential to transform the way Redox Biology expands from the bench to patient's bedside.   

The physicochemical properties of nitric oxide (NO) as an intercellular messenger, in particular the way it conveys information via volume signaling, translate into advantages of communication in the brain. This becomes apparent when considering neurovascular coupling (NVC), the tightly temporal and spatial functional communication between active neurons and local blood microvessels. That the brain is energetically expensive given its mass and that increased neuronal activity in a region of the brain is associated with a local increase in blood flow (CBF) has been known since the XIX century. In turn, the association between CBF dysregulation and cognitive decline has been consistently established in older adults (brain aging, neurodegenerative diseases, type II DM) and lab rodent models but the neurobiological links are poorly understood. I will discuss the notion that neuronal-derived NO is the key mediator of NVC in the hippocampus and that impairment of NVC is an early and likely causative event leading to cognitive decline. The premise is that by rescuing the functionality of NVC then cognitive enhancement should be observed. This will be experimentally supported on basis of a diet-driven redox mechanism, involving the interaction of nitrite with ascorbate released from active neurons. Data suggest that an operational NVC, allocating energy resources according to neuronal activity, is a most fundamental biochemical process that underlines biological organization to support cognition.   

Supported by project 2022.05454.PTDC (https://doi.org/10.54499/2022.05454.PTDC).