Pericytes: Intrinsic Transportation Engineers of the CNS Microcirculation

Pericytes in the brain are candidate regulators of microcirculatory blood flow because they are strategically positioned along the microvasculature, contain contractile proteins, respond rapidly to neuronal activation, and synchronize microvascular dynamics and neurovascular coupling within the capillary network. Analyses of mice with defects in pericyte generation demonstrate that pericytes are necessary for the formation of the blood-brain barrier, development of the glymphatic system, immune homeostasis, and white matter function. The development, identity, specialization, and progeny of different subtypes of pericytes, however, remain unclear. Pericytes perform brain-wide ‘transportation engineering’ functions in the capillary network, instructing, integrating, and coordinating signals within the cellular communicome in the neurovascular unit to efficiently distribute oxygen and nutrients (‘goods and services’) throughout the microvasculature (‘transportation grid’). In this review, we identify emerging challenges in pericyte biology and shed light on potential pericyte-targeted therapeutic strategies.

Multispectral intravital microscopy for simultaneous bright-field and fluorescence imaging of the microvasculature

Intravital video microscopy permits the observation of microcirculatory blood flow. This often requires fluorescent probes to visualize structures and dynamic processes that cannot be observed with conventional bright-field microscopy. Conventional light microscopes do not allow for simultaneous bright-field and fluorescent imaging. Moreover, in conventional microscopes, only one type of fluorescent label can be observed. This study introduces multispectral intravital video microscopy, which combines bright-field and fluorescence microscopy in a standard light microscope. The technique enables simultaneous real-time observation of fluorescently-labeled structures in relation to their direct physical surroundings. The advancement provides context for the orientation, movement, and function of labeled structures in the microcirculation.

The use of pulse pressure variation for predicting impairment of microcirculatory blood flow

Dynamic parameters of preload have been widely recommended to guide fluid therapy based on the principle of fluid responsiveness and with regard to cardiac output. An equally important aspect is however to also avoid volume-overload. This accounts particularly when capillary leakage is present and volume-overload will promote impairment of microcirculatory blood flow. The aim of this study was to evaluate, whether an impairment of intestinal microcirculation caused by volume-load potentially can be predicted using pulse pressure variation in an experimental model of ischemia/reperfusion injury. The study was designed as a prospective explorative large animal pilot study. The study was performed in 8 anesthetized domestic pigs (German landrace). Ischemia/reperfusion was induced during aortic surgery. 6 h after ischemia/reperfusion-injury measurements were performed during 4 consecutive volume-loading-steps, each consisting of 6 ml kg−1 bodyweight−1. Mean microcirculatory blood flow (mean Flux) of the ileum was measured using direct laser-speckle-contrast-imaging. Receiver operating characteristic analysis was performed to determine the ability of pulse pressure variation to predict a decrease in microcirculation. A reduction of ≥ 10% mean Flux was considered a relevant decrease. After ischemia–reperfusion, volume-loading-steps led to a significant increase of cardiac output as well as mean arterial pressure, while pulse pressure variation and mean Flux were significantly reduced (Pairwise comparison ischemia/reperfusion-injury vs. volume loading step no. 4): cardiac output (l min−1) 1.68 (1.02–2.35) versus 2.84 (2.15–3.53), p = 0.002, mean arterial pressure (mmHg) 29.89 (21.65–38.12) versus 52.34 (43.55–61.14), p < 0.001, pulse pressure variation (%) 24.84 (17.45–32.22) versus 9.59 (1.68–17.49), p = 0.004, mean Flux (p.u.) 414.95 (295.18–534.72) versus 327.21 (206.95–447.48), p = 0.006. Receiver operating characteristic analysis revealed an area under the curve of 0.88 (CI 95% 0.73–1.00; p value < 0.001) for pulse pressure variation for predicting a decrease of microcirculatory blood flow. The results of our study show that pulse pressure variation does have the potential to predict decreases of intestinal microcirculatory blood flow due to volume-load after ischemia/reperfusion-injury. This should encourage further translational research and might help to prevent microcirculatory impairment due to excessive fluid resuscitation and to guide fluid therapy in the future.

Oxygen Homeostasis and Mitochondrial Function in COVID-19 and ARDS Patients: The Ultimate Vital Signs

The leading cause of death from the COVID-19 is the development of Pneumonia and Acute Respiratory Distress Syndrome-ARDS. Advanced physiological monitoring of COVID -19 patients in real time is a missing tool that avoid the optimization of better diagnosis and evaluating the efficacy of the treatment given. As of today, the monitoring of the systemic vital signs provides important information regarding the respiratory and cardiovascular systems including the pulse oximetry that provide data on hemoglobin oxygenation in the macro circulation. Our hypothesis is that the pathophysiology of COVID-19 and ARDS patients includes severe changes in the microcirculatory hemodynamics and cellular disturbances in Tissue and cellular Oxygen Homeostasis. Therefore, we postulate that real time monitoring of mitochondrial NADH redox state and microcirculatory blood flow, volume and hemoglobin oxygenation is the missing information that will affect dramatically the outcome of COVID-19 and ARDS patients. During the last 2 decades we studied the mechanism of blood flow redistribution activated in animal models as well as in patients exposed to total body negative oxygen balance. This mechanism is activated by the sympathetic pathway. This effect is not equal in all organs of the body, namely, in the most vital organs - brain, heart, and adrenal glands oxygen supply is preserved while in the less vital organs (visceral and peripheral organs) hypo perfusion and negative oxygen balance is recorded. In order to evaluate the tissue oxygen homeostasis, we developed a new concept named - LifenLight Score (LLS)TM based on the monitoring of four physiological parameters measured in real time from one of the less vital organs in the body. Our developed device is monitoring mitochondrial function by measuring the NADH auto fluorescence and microcirculatory blood flow, tissue reflectance and hemoglobin oxygenation. In animal model we monitored simultaneously the brain and the small intestine. In patients we used a 3-way Foley catheter introduced to the bladder via the urethra. We found that monitoring the less vital organ could serve as an early warning signal to the development of negative oxygen balance in the body as well as indicate of a recovery process in the improvement of the oxygen balance homeostasis. In conclusion, we hypothesize that using our new monitoring system will be able to detect deterioration process related to hypoxia in COVID-19 and ARDS patients, as well as to monitor improvement in tissue oxygen balance due to various treatments such as exposure to hyperoxia.

Essential Role of Hemoglobin βCys93 in Cardiovascular Physiology

The supply of oxygen to tissues is controlled by microcirculatory blood flow. One of the more surprising discoveries in cardiovascular physiology is the critical dependence of microcirculatory blood flow on a single conserved cysteine within the β-subunit (βCys93) of hemoglobin (Hb). βCys93 is the primary site of Hb S-nitrosylation [i.e., S-nitrosothiol (SNO) formation to produce S-nitrosohemoglobin (SNO-Hb)]. Notably, S-nitrosylation of βCys93 by NO is favored in the oxygenated conformation of Hb, and deoxygenated Hb releases SNO from βCys93. Since SNOs are vasodilatory, this mechanism provides a physiological basis for how tissue hypoxia increases microcirculatory blood flow (hypoxic autoregulation of blood flow). Mice expressing βCys93A mutant Hb (C93A) have been applied to understand the role of βCys93, and RBCs more generally, in cardiovascular physiology. Notably, C93A mice are unable to effect hypoxic autoregulation of blood flow and exhibit widespread tissue hypoxia. Moreover, reactive hyperemia (augmentation of blood flow following transient ischemia) is markedly impaired. C93A mice display multiple compensations to preserve RBC vasodilation and overcome tissue hypoxia, including shifting SNOs to other thiols on adult and fetal Hbs and elsewhere in RBCs, and growing new blood vessels. However, compensatory vasodilation in C93A mice is uncoupled from hypoxic control, both peripherally (e.g., predisposing to ischemic injury) and centrally (e.g., impairing hypoxic drive to breathe). Altogether, physiological studies utilizing C93A mice are confirming the allosterically controlled role of SNO-Hb in microvascular blood flow, uncovering essential roles for RBC-mediated vasodilation in cardiovascular physiology and revealing new roles for RBCs in cardiovascular disease.