Molecular structure and function of biological barriers

Biological barriers are indispensable for the integrity and function of many vertebrate organs. The barrier function is based on intercellular protein complexes of the plasma membrane which form paracellular diffusion barriers and separate internal and external fluid compartments, an indispensable prerequisite for every organ development and function. The review summarizes key characteristics and molecular structure of intercellular junctions (tight junctions and adherens junctions) responsible for cellular barrier formation. One of the most important such cellular barriers is the blood-brain barrier (BBB) which forms an active interface between the circulation and neural tissue. Its principal cellular components are cerebral endothelial cells, pericytes and astrocytes, whose finely tuned interactions are needed for a proper function. The review highlights the most important functions of the BBB including some novel regulatory aspects as well

Role of Pattern Recognition Receptors of the Neurovascular Unit in Inflamm-Aging

Aging is associated with chronic inflammation (inflamm-aging) partly mediated by increased levels of damage-associated molecular patterns (DAMPs) which activate pattern recognition receptors (PRRs) of the innate immune system. Furthermore, many aging-related disorders are associated with inflammation. PRRs, like Toll-like receptors (TLRs) and NOD-like receptors (NLRs) are not only expressed in cells of the innate immune system, but other cells as well, including cells of the neurovascular unit (NVU) and cerebral vasculature forming the blood-brain barrier (BBB). In this review we summarize our current knowledge about the relationship among activation of PRRs expressed by cells of the NVU/BBB, chronic inflammation and aging-related pathologies of the brain. The most important DAMP-sensing PRRs in the brain are TLR2, TLR4, NLRP1 and NLRP3, which are activated during physiological and pathological aging in microglia, neurons, astrocytes and possibly endothelial cells and pericytes

Temporary opening of the blood-brain barrier with the nitrone compound OKN-007

The blood-brain barrier (BBB) is usually impermeable to several drugs, which hampers treatment of various brain-related diseases/disorders. There have been several approaches to open the BBB, including intracarotid infusion of hyperosmotic concentrations of arabinose, mannitol, oleic or linoleic acids, or alkylglycerols, intravenous infusion of bradykinin B2, administration of a fragment of the ZO toxin from vibrio cholera, targeting specific components of the tight junctions (e.g. claudin-5) with siRNA or novel peptidomimetic drugs, or the use of ultrasound with microbubbles. We propose the use of a low molecular weight (MW), nitrone-type compound, OKN-007, which can temporarily open up the BBB for 1-2 hours. Gadolinium (Gd)-based compounds assessed ranged in MW from 546 (Gd-DTPA) to 465 kDa (beta-galactosidase Gd DOTA). We also included an albumin-based CA (albumin-Gd-DTPA-biotin) for assessment, as well as an antibody (Ab) against a neuron-specific biomarker conjugated to Gd-DOTA (anti-EphB2-Gd-DOTA). For the anti-EphB2 (goat Ab)-Gd-DOTA assessment, we utilized an anti goat Ab conjugated with horse radish peroxidase (HRP) for confirmation of the presence of the anti-EphB2-Gd-DOTA probe. In addition, a Cy5 labeled anti-EphB2 Ab was co-administered with the anti-EphB2-Gd-DOTA probe, and assessed ex vivo. This study demonstrates that OKN-007 may be able to temporarily open up the BBB to augment the delivery of various compounds ranging in MW from as small as similar to 550 to as large as similar to 470 kDa. This compound is an investigational new drugfor glioblastoma (GBM) therapy in clinical trials. The translational capability for human use to augment the delivery of non-BBB-permeable drugs is extremely high

CB2 Receptor Activation Inhibits Melanoma Cell Transmigration through the Blood-Brain Barrier

During parenchymal brain metastasis formation tumor cells need to migrate through cerebral endothelial cells, which form the morphological basis of the blood-brain barrier (BBB). The mechanisms of extravasation of tumor cells are highly uncharacterized, but in some aspects recapitulate the diapedesis of leukocytes. Extravasation of leukocytes through the BBB is decreased by the activation of type 2 cannabinoid receptors (CB2); therefore, in the present study we sought to investigate the role of CB2 receptors in the interaction of melanoma cells with the brain endothelium. First, we identified the presence of CB1, CB2(A), GPR18 (transcriptional variant 1) and GPR55 receptors in brain endothelial cells, while melanoma cells expressed CB1, CB2(A), GPR18 (transcriptional variants 1 and 2), GPR55 and GPR119. We observed that activation of CB2 receptors with JWH-133 reduced the adhesion of melanoma cells to the layer of brain endothelial cells. JWH-133 decreased the transendothelial migration rate of melanoma cells as well. Our results suggest that changes induced in endothelial cells are critical in the mediation of the effect of CB2 agonists. Our data identify CB2 as a potential target in reducing the number of brain metastastes originating from melanoma

Obesity-induced cognitive impairment in older adults: a microvascular perspective

Over two-thirds of individuals aged 65 and older are obese or overweight in the United States. Epidemiological data show an association between the degree of adiposity and cognitive dysfunction in the elderly. In this review, the pathophysiological roles of microvascular mechanisms, including impaired endothelial function and neurovascular coupling responses, microvascular rarefaction, and blood-brain barrier disruption in the genesis of cognitive impairment in geriatric obesity are considered. The potential contribution of adipose-derived factors and fundamental cellular and molecular mechanisms of senescence to exacerbated obesity-induced cerebromicrovascular impairment and cognitive decline in aging are discussed

Cerebral pericytes and endothelial cells communicate through inflammasome-dependent signals

By upregulation of cell adhesion molecules and secretion of proinflammatory cytokines, cells of the neurovascular unit, including pericytes and endothelial cells, actively participate in neuroinflammatory reactions. As previously shown, both cell types can activate inflammasomes, cerebral endothelial cells (CECs) through the canonical pathway, while pericytes only through the noncanonical pathway. Using complex in vitro models, we demonstrate here that the noncanonical inflammasome pathway can be induced in CECs as well, leading to a further increase in the secretion of active interleukin-1β over that observed in response to activation of the canonical pathway. In parallel, a more pronounced disruption of tight junctions takes place. We also show that CECs respond to inflammatory stimuli coming from both the apical/blood and the basolateral/brain directions. As a result, CECs can detect factors secreted by pericytes in which the noncanonical inflammasome pathway is activated and respond with inflammatory activation and impairment of the barrier properties. In addition, upon sensing inflammatory signals, CECs release inflammatory factors toward both the blood and the brain sides. Consequently, CECs activate pericytes by upregulating their expression of NLRP3 (NOD-, LRR-, and pyrin domain-containing protein 3), an inflammasome-forming pattern recognition receptor. In conclusion, cerebral pericytes and endothelial cells mutually activate each other in inflammation

Upregulation of Nucleotide-Binding Oligomerization Domain-, LRR- and Pyrin Domain-Containing Protein 3 in Motoneurons Following Peripheral Nerve Injury in Mice

Neuronal injuries are accompanied by release and accumulation of damage-associated molecules, which in turn may contribute to activation of the immune system. Since a wide range of danger signals (including endogenous ones) are detected by the nucleotide-binding oligomerization domain-, LRR- and pyrin domain-containing protein 3 (NLRP3) pattern recognition receptor, we hypothesized that NLRP3 may become activated in response to motor neuron injury. Here we show that peripheral injury of the oculomotor and the hypoglossal nerves results in upregulation of NLRP3 in corresponding motor nuclei in the brainstem of mice. Although basal expression of NLRP3 was observed in microglia, astroglia and neurons as well, its upregulation and co-localization with apoptosis-associated speck-like protein containing a caspase activation and recruitment domain, suggesting inflammasome activation, was only detected in neurons. Consequently, increased production of active pro-inflammatory cytokines interleukin-1 beta and interleukin-18 were detected after hypoglossal nerve axotomy. Injury-sensitive hypoglossal neurons responded with a more pronounced NLRP3 upregulation than injury-resistant motor neurons of the oculomotor nucleus. We further demonstrated that the mitochondrial protector diazoxide was able to reduce NLRP3 upregulation in a post-operative treatment paradigm. Our results indicate that NLRP3 is activated in motoneurons following acute nerve injury. Blockade of NLRP3 activation might contribute to the previously observed anti-inflammatory and neuroprotective effects of diazoxide