The importance of controlling in vitro oxygen tension to accurately model in vivo neurophysiology

Experimental Modeling of Damaging and Protective Hypoxia of the Mammalian Brain

Currently, there is a new surge of interest in the problem of hypoxia, almost lost in recent decades. Due to the fact that the circle of competent specialists in this field has significantly narrowed, it is necessary to carry out an intensive exchange of knowledge. In order to inform a wide range of interested researchers and doctors, this review summarizes the current understanding of hypoxia, its pathogenic and adaptogenic consequences, as well as key physiological and molecular mechanisms that implement the response to hypoxia at various levels-from cellular to organismic. The review presents a modern classification of forms of hypoxia, the understanding of which is necessary for the formation of a scientifically based approach to experimental modeling of hypoxic states. An analysis of the literature covering the history and current level of hypoxia modeling in mammals and human experiments, including methods for creating moderate hypoxia used to increase the resistance of the nervous system to severe forms of hypoxia and other extreme factors, is carried out. Special attention is paid to the discussion of the features and limitations of various approaches to the creation of hypoxia, as well as the disclosure of the potential for the practical application of moderate hypoxic effects in medicine.

Characterization of Oxygen Levels in an Uninfected and Infected Human Blood-Cerebrospinal-Fluid-Barrier Model

The host–pathogen interaction during meningitis can be investigated with blood-cerebrospinal-fluid-barrier (BCSFB) cell culture models. They are commonly handled under atmospheric oxygen conditions (19–21% O₂), although the physiological oxygen conditions are significantly lower in cerebrospinal fluid (CSF) (7–8% O₂). We aimed to characterize oxygen levels in a Streptococcus (S.) suis-infected BCSFB model with transmigrating neutrophils. A BCSFB model with human choroid plexus epithelial cells growing on transwell-filters was used. The upper “blood”-compartment was infected and blood-derived neutrophils were added. S. suis and neutrophils transmigrated through the BCSFB into the “CSF”-compartment. Here, oxygen and pH values were determined with the non-invasive SensorDish^® reader. Slight orbital shaking improved the luminescence-based measurement technique for detecting free oxygen. In the non-infected BCSFB model, an oxygen value of 7% O₂ was determined. However, with S. suis and transmigrating neutrophils, the oxygen value significantly decreased to 2% O₂. The pH level decreased slightly in all groups. In conclusion, we characterized oxygen levels in the BCSFB model and demonstrated the oxygen consumption by cells and bacteria. Oxygen values in the non-infected BCSFB model are comparable to in vivo values determined in pigs in the CSF. Infection and transmigrating neutrophils decrease the oxygen value to lower values.

In vivo oxygen measurement in cerebrospinal fluid of pigs to determine physiologic and pathophysiologic oxygen values during CNS infections

During infection and inflammation, a reduced oxygen level clearly affects cellular functions. Oxygen levels during CNS infections are unknown. Here we established and evaluated an in vivo measurement system to characterize the oxygen level in parallel with bacterial numbers (CFU/mL), the cell number and pH level inside the CSF of healthy compared to Streptococcus suis-infected pigs. The animals were anesthetized over a seven-hour period with isoflurane in air/oxygen at physiologic arterial partial pressure of oxygen. Oxygen levels in CSF of anesthetized pigs were compared to euthanized pigs. The detected partial pressure of oxygen in the CSF remained constant in a range of 47-63 mmHg, independent of the infection status (bacterial or cell number). In contrast, the pH value showed a slight drop during infection, which correlated with cell and bacterial number in CSF. We present physiologic oxygen and pH values in CSF during the onset of bacterial meningitis.

The bioactivity of neuronal-derived nitric oxide in aging and neurodegeneration: Switching signaling to degeneration

The small and diffusible free radical nitric oxide (^•NO) has fascinated biological and medical scientists since it was promoted from atmospheric air pollutant to biological ubiquitous signaling molecule. Its unique physical chemical properties expand beyond its radical nature to include fast diffusion in aqueous and lipid environments and selective reactivity in a biological setting determined by bioavailability and reaction rate constants with biomolecules. In the brain, ^•NO is recognized as a key player in numerous physiological processes ranging from neurotransmission/neuromodulation to neurovascular coupling and immune response. Furthermore, changes in its bioactivity are central to the molecular pathways associated with brain aging and neurodegeneration. The understanding of ^•NO bioactivity in the brain, however, requires the knowledge of its concentration dynamics with high spatial and temporal resolution upon stimulation of its synthesis. Here we revise our current understanding of the role of neuronal-derived ^•NO in brain physiology, aging and degeneration, focused on changes in the extracellular concentration dynamics of this free radical and the regulation of bioenergetic metabolism and neurovascular coupling.

A Cell Culture Model that Mimics Physiological Tissue Oxygenation Using Oxygen-permeable Membranes

Dissolved oxygen and its availability to cells in culture is an overlooked variable which can have significant consequences on experimental research outcomes, including reproducibility. Oxygen sensing pathways play key roles in cell growth and behavior and pericellular oxygen levels should be controlled when establishing in vitro models. Standard cell culture techniques do not have adequate control over pericellular oxygen levels. Slow diffusion through culture media limits the precision of oxygen delivery to cells, making it difficult to accurately reproduce in vivo-like oxygen conditions. Furthermore, different types of cells consume oxygen at varying rates and this can be affected by the density of growing cells. Here, we describe a novel in vitro system that utilizes hypoxic chambers and oxygen-permeable culture dishes to control pericellular oxygen levels and provide rapid oxygen delivery to adherent cells. This procedure is particularly relevant for protocols studying effects of rapid oxygen changes or intermittent hypoxia on cellular behavior. The system is inexpensive and easily assembled without highly specialized equipment.

Neuroprotective Role of Hypothermia in Hypoxic-ischemic Brain Injury: Combined Therapies using Estrogen

Hypoxic-ischemic brain injury is a complex network of factors, which is mainly characterized by a decrease in levels of oxygen concentration and blood flow, which lead to an inefficient supply of nutrients to the brain. Hypoxic-ischemic brain injury can be found in perinatal asphyxia and ischemic-stroke, which represent one of the main causes of mortality and morbidity in children and adults worldwide. Therefore, knowledge of underlying mechanisms triggering these insults may help establish neuroprotective treatments. Selective Estrogen Receptor Modulators and Selective Tissue Estrogenic Activity Regulators exert several neuroprotective effects, including a decrease of reactive oxygen species, maintenance of cell viability, mitochondrial survival, among others. However, these strategies represent a traditional approach of targeting a single factor of pathology without satisfactory results. Hence, combined therapies, such as the administration of therapeutic hypothermia with a complementary neuroprotective agent, constitute a promising alternative. In this sense, the present review summarizes the underlying mechanisms of hypoxic-ischemic brain injury and compiles several neuroprotective strategies, including Selective Estrogen Receptor Modulators and Selective Tissue Estrogenic Activity Regulators, which represent putative agents for combined therapies with therapeutic hypothermia.

The Differentiation of Rat Oligodendroglial Cells Is Highly Influenced by the Oxygen Tension: In Vitro Model Mimicking Physiologically Normoxic Conditions

Oligodendrocyte progenitor cells (OPCs) constitute one of the main populations of dividing cells in the central nervous system (CNS). Physiologically, OPCs give rise to mature, myelinating oligodendrocytes and confer trophic support to their neighboring cells within the nervous tissue. OPCs are known to be extremely sensitive to the influence of exogenous clues which might affect their crucial biological processes, like survival, proliferation, differentiation, and the ability to generate a myelin membrane. Alterations in their differentiation influencing their final potential for myelinogenesis are usually the leading cause of CNS dys- and demyelination, contributing to the development of leukodystrophic disorders. The evaluation of the mechanisms that cause oligodendrocytes to malfunction requires detailed studies based on designed in vitro models. Since OPCs readily respond to changes in local homeostasis, it is crucial to establish restricted culture conditions to eliminate the potential stimuli that might influence oligodendrocyte biology. Additionally, the in vitro settings should mimic the physiological conditions to enable the obtained results to be translated to future preclinical studies. Therefore, the aim of our study was to investigate OPC differentiation in physiological normoxia (5% O₂) and a restricted in vitro microenvironment. To evaluate the impact of the combined microenvironmental clues derived from other components of the nervous tissue, which are also influenced by the local oxygen concentration, the process of generating OPCs was additionally analyzed in organotypic hippocampal slices. The obtained results show that OPC differentiation, although significantly slowed down, proceeded correctly through its typical stages in the physiologically relevant conditions created in vitro. The established settings were also conducive to efficient cell proliferation, exerting also a neuroprotective effect by promoting the proliferation of neurons. In conclusion, the performed studies show how oxygen tension influences OPC proliferation, differentiation, and their ability to express myelin components, and should be taken into consideration while planning preclinical studies, e.g., to examine neurotoxic compounds or to test neuroprotective strategies.