Passage of immunomodulators across the blood-brain barrier

The question is considered of how and where cytokines, such as interleukin 1 (IL-1), that are released into the circulation during the host defense response, reach and interact with the central nervous system to produce fever or act as neuroimmunomodulators. Evidence is presented suggesting a role for a brain circumventricular organ (CVO) in this respect. Several interactions between a specific CVO, the organum vasculosum laminae terminalis (OVLT) and endogenous pyrogen (EP) in the production of fever are reviewed. A more general hypothesis is developed on a role for the brain CVOs in monitoring the blood concentrations of several proteins and complex polypeptides such as the circulating endocrines that are regulated via the autonomic nervous system. A proposed connection between the release of prostaglandin E (PGE) at the blood-brain interface in response to infection and the ability of the brain to maintain an immunoprivileged status in the face of exposure of its CVOs to foreign antigens is discussed.     

Making sense of it: roles of the sensory circumventricular organs in feeding and regulation of energy homeostasis

Obesity is associated with significant health risks including stroke and heart disease. The prevalence of obesity has dramatically increased over the past 20 years. Although the development of obesity is clearly related to changing lifestyles, the central nervous system plays a key role in regulation of energy balance. To develop effective strategies for treating obesity, we must gain a clearer understanding of the neuro-circuitry and signaling mechanisms involved. Toward this end, recent progress has been made in the understanding of the roles played by the sensory circumventricular organs (CVOs) of the brain. These areas lack the normal blood-brain barrier and thus act as transducers of signals between the blood, other centers in the brain, and the cerebrospinal fluid. This review focuses on the roles played by the sensory CVOs in detecting and responding to a number of signals that carry information regarding nutritional status, including cholecystokinin, amylin, ghrelin, peptide YY, pancreatic polypeptide, leptin, adiponectin, and glucose.     

Different vascular permeability between the sensory and secretory circumventricular organs of adult mouse brain

The blood-brain barrier (BBB) prevents free access of circulating molecules to the brain and maintains a specialized brain environment to protect the brain from blood-derived bioactive and toxic molecules; however, the circumventricular organs (CVOs) have fenestrated vasculature. The fenestrated vasculature in the sensory CVOs, including the organum vasculosum of lamina terminalis (OVLT), subfornical organ (SFO) and area postrema (AP), allows neurons and astrocytes to sense a variety of plasma molecules and convey their information into other brain regions and the vasculature in the secretory CVOs, including median eminence (ME) and neurohypophysis (NH), permits neuronal terminals to secrete many peptides into the blood stream. The present study showed that vascular permeability of low-molecular-mass tracers such as fluorescein isothiocyanate (FITC) and Evans Blue was higher in the secretory CVOs and kidney as compared with that in the sensory CVOs. On the other hand, vascular permeability of high-molecular-mass tracers such as FITC-labeled bovine serum albumin and Dextran 70,000 was lower in the CVOs as compared with that in the kidney. Prominent vascular permeability of low- and high-molecular-mass tracers was also observed in the arcuate nucleus. These data demonstrate that vascular permeability for low-molecular-mass molecules is higher in the secretory CVOs as compared with that in the sensory CVOs, possibly for large secretion of peptides to the blood stream. Moreover, vascular permeability for high-molecular-mass tracers in the CVOs is smaller than that of the kidney, indicating that the CVOs are not totally without a BBB.     

A candidate of organum vasculosum of the lamina terminalis with neuronal connections to neurosecretory preoptic nucleus in eels

Systemic angiotensin II (Ang II) is a dipsogen in terrestrial vertebrates and seawater teleosts. In eels, Ang II acts on the area postrema, a sensory circumventricular organ (CVO) and elicits water intake but other sensory CVOs have not yet been found in the eel forebrain. To identify sensory CVOs in the forebrain, eels were peripherally injected with Evans blue, which immediately binds to albumin, or a rabbit IgG protein. Extravasation of these proteins, which cannot cross the blood–brain barrier (BBB), was observed in the brain parenchyma of the anteroventral preoptic recess (PR) walls. Fenestrated capillaries were observed in the parenchymal margin of the ventral wall of the PR, confirming a deficit of the BBB in the eel forebrain. Immunostaining for tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT) detected neurons in the lateral region of the anterior parvocellular preoptic nucleus (PPa), which were strongly stained by BBB-impermeable N-hydroxysulfosuccinimide. In the periventricular region of the PPa, many neurons incorporated biotinylated dextran amine conjugated to fluorescein, a retrograde axonal tracer, injected into the magnocellular preoptic nucleus (PM), indicating neuronal connections from the PPa to the PM. The mammalian paraventricular and supraoptic nuclei, homologous to the teleost PM, receive principal neuronal projections from the organum vasculosum of the lamina terminalis (OVLT). These results strongly suggest that the periventricular subpopulation of the PPa, which is most likely to be a component of the OVLT, serves as a functional window of access for systemic signal molecules such as Ang II.     

A Diffusion Barrier Between the Area Postrema and Nucleus Tractus Solitarius

The blood–brain barrier (BBB) is a structural and functional barrier that prevents free exchange of circulating substances with the brain, where the endothelial cells of microvessels are joined by tight junctions. The circumventricular organs (CVOs), by contrast, lack tight junctions and exhibit more direct communication with the circulating blood and cerebrospinal fluid. Despite many outstanding morphological studies at the electron microscopic level, there remain misconceptions that the CVOs provide direct passage of blood-borne substances to the rest of the brain. This study will show the structure of the anatomical borders of the dorsal vagal complex in the brainstem. A distinct diffusion barrier between the area postrema (AP, a CVO) and the nucleus tractus solitarius (NTS) was illustrated by immunohistochemistry at both the light and electron microscopic levels. The border zone between the AP and NTS was underlined by a continuous monolayer of columnar cells that were immunopositive for both the tight junction protein zona occludin-1 and the astrocyte marker glial fibrillary acidic protein. This observation of a diffusion barrier between the AP and NTS resolves a long-standing dispute about whether the NTS is a structural extension of the AP with a leaky BBB. Special issue article in honor of Dr. Ji-Sheng Han.     

The area postrema in hindbrain is a central player for regulation of drinking behavior in Japanese eels

It is recognized that fish will drink the surrounding water by reflex swallowing without a thirst sensation. We evaluated the role of the area postrema (AP), a sensory circumventricular organ (CVO) in the medulla oblongata, in the regulation of drinking behavior of seawater (SW) eels. The antidipsogenic effects of ghrelin and atrial natriuretic peptide and hypervolemia and hyperosmolemia (1 M sucrose or 10% NaCl) as well as the dipsogenic effects of angiotensin II and hypovolemia (hemorrhage) were profoundly diminished after AP lesion (APx) in eels compared with sham controls. However, the antidipsogenic effect of urotensin II was not influenced by APx, possibly due to the direct baroreflex inhibition on the swallowing center in eels. When ingested water was drained via an esophageal fistula, water intake increased 30-fold in sham controls but only fivefold in APx eels, suggesting a role for the AP in continuous regulation of drinking by SW eels. After transfer from freshwater to SW, APx eels responded normally with an immediate burst of drinking, but after 4 wk these animals showed a much greater increase in plasma osmolality than controls, suggesting that the AP is involved in acclimation to SW by fine tuning of the drinking rate. Taken together, the AP in the hindbrain of eels plays an integral role in SW acclimation, acting as a conduit of information from plasma for the regulation of drinking, probably without a thirst sensation. This differs from mammals in which sensory CVOs in the forebrain play pivotal roles in thirst regulation.     

The rat SCO responsiveness to prolonged water deprivation: implication of Reissner's fiber and serotonin system

The osmotic stress is a potent stimulus that can trigger several peripheral as well as central impairments. The brain is a vulnerable target of the osmotic stress and particularly circumventricular organs (CVOs) regarding their strategic localization as sensory organs of biochemical changes in the blood and cerebrospinal fluid circulations. The subcommissural organ (SCO) is a CVO which releases doubly in the CSF and blood circulation a glycoprotein called Reissner's fiber (RF) that has been associated to several functions including electrolyte and water balances. The present work was aimed on the assessment of the secretory activity of the SCO and its serotoninergic innervation following 2 weeks of total water restriction in Wistar rat. Using the immunohistochemistry of RF and serotonin (5HT), our data showed a significant overall reduction of RF immunoreactivity within both ependymal and hypendymal cells of the SCO of dehydrated rats compared to their corresponding controls, this decrease was concomitant with an enhancement of fibers 5HT immunoreactivity in the SCO as well as in the classical ependyma and in the dorsal raphe nucleus (DRN), constituting the origin of this innervation. The present findings support the possible involvement of the SCO in the response to prolonged water deprivation by decreasing its secretory materials which may result from either a direct peripheral hormonal control and/or the consequence of the enhanced 5HT innervation of the SCO.     

Uptake of Exogenous Glutamate and Aspartate by Circumventricular Organs but Not Other Regions of Brain

Abstract: Glutamate (Glu) and aspartate (Asp) concentrations in blood and selected regions of brain were measured at sequential intervals over a 3-h period following subcutaneous administration of Glu, Asp, or Glu plus Asp (2 mg/g body wt) to 4-day-old mouse or rat pups. Marked serum elevations of the administered amino acids (peak values exceeding 200 times control levels) were detected within 1 h. In circumventricular organ (CVO) regions of brain, which are thought to have no blood-brain barriers, a sharp and steady increase in tissue concentrations of the administered amino acids (peak values 4–10 times higher than control levels) occurred during a 15–120 min interval, whereas no appreciable increases were detected in other brain regions. When 2 mg/g Glu plus 2 mg/g Asp were administered, CVO tissue concentrations of each amino acid rose to approximately the same level obtained when the individual amino acids were given. It is concluded that blood-brain barriers preventing net entry of Glu or Asp into brain proper are relatively well established by the 4th postnatal day in rodents, but that CVO brain regions lack such barriers; selective access of blood-borne Glu or Asp to CVO neurons explains why these neurons are selectively destroyed by systemic administration of these neurotoxic amino acids.     

Differential effects of intravenous hyperosmotic solutes on drinking latency and c-Fos expression in the circumventricular organs and hypothalamus of the rat

Hyperosmotic intravenous infusions of NaCl are more potent for inducing drinking and vasopressin (AVP) secretion than equally osmotic solutions of glucose or urea. The fact that all three solutes increased cerebrospinal fluid osmolality and sodium concentration led the investigators to conclude that critical sodium receptors or osmoreceptors for stimulating drinking and AVP secretion were outside the blood-brain barrier (BBB) in the circumventricular organs (CVOs). We tested an obvious prediction of this hypothesis: that all three solutes should increase c-Fos-like immunoreactivity (Fos-ir) inside the BBB, but that only NaCl should increase Fos-ir in the CVOs. We gave intravenous infusions of 3.0 Osm/l NaCl, glucose, or urea to rats for 11 or 22 min at 0.14 ml/min and perfused the rats for assay of Fos-ir at 90 min. Controls received isotonic NaCl at the same volume. Drinking latency was measured, but water was then removed. Drinking consistently occurred with short latency during hyperosmotic NaCl infusions only. Fos-ir in the forebrain CVOs, the subfornical organ, and organum vasculosum laminae terminalis was consistently elevated only by hyperosmotic NaCl. However, all three hyperosmotic solutes potently stimulated Fos-ir in the supraoptic and paraventricular nuclei of the hypothalamus inside the BBB. Hyperosmotic NaCl greatly elevated Fos-ir in the area postrema, but even glucose and urea caused moderate elevations that may be related to volume expansion rather than osmolality. The data provide strong support for the conclusion that the osmoreceptors controlling drinking are located in the CVOs.     

Sodium-level-sensitive sodium channel Na(x) is expressed in glial laminate processes in the sensory circumventricular organs

Na(x) is an atypical sodium channel that is assumed to be a descendant of the voltage-gated sodium channel family. Our recent studies on the Na(x)-gene-targeting mouse revealed that Na(x) channel is localized to the circumventricular organs (CVOs), the central loci for the salt and water homeostasis in mammals, where the Na(x) channel serves as a sodium-level sensor of the body fluid. To understand the cellular mechanism by which the information sensed by Na(x) channels is transferred to the activity of the organs, we dissected the subcellular localization of Na(x) in the present study. Double-immunostaining and immunoelectron microscopic analyses revealed that Na(x) is exclusively localized to perineuronal lamellate processes extended from ependymal cells and astrocytes in the organs. In addition, glial cells isolated from the subfornical organ, one of the CVOs, were sensitive to an increase in the extracellular sodium level, as analyzed by an ion-imaging method. These results suggest that glial cells bearing the Na(x) channel are the first to sense a physiological increase in the level of sodium in the body fluid, and they regulate the neural activity of the CVOs by enveloping neurons. Close communication between inexcitable glial cells and excitable neural cells thus appears to be the basis of the central control of the salt homeostasis.     

Wunen, a Drosophila lipid phosphate phosphatase, is required for septate junction-mediated barrier function

Lipid phosphate phosphatases (LPPs) are integral membrane enzymes that regulate the levels of bioactive lipids such as sphingosine 1-phosphate and lysophosphatidic acid. The Drosophila LPPs Wunen (Wun) and Wunen-2 (Wun2) have a well-established role in regulating the survival and migration of germ cells. We now show that wun has an essential tissue-autonomous role in development of the trachea: the catalytic activity of Wun is required to maintain septate junction (SJ) paracellular barrier function, loss of which causes failure to accumulate crucial luminal components, suggesting a role for phospholipids in SJ function. We find that the integrity of the blood-brain barrier is also lost in wun mutants, indicating that loss of SJ function is not restricted to the tracheal system. Furthermore, by comparing the rescue ability of different LPP homologs we show that wun function in the trachea is distinct from its role in germ cell migration.     

Changes in pericytic expression of NG2 and PDGFRB and vascular permeability in the sensory circumventricular organs of adult mouse by osmotic stimulation

The blood–brain barrier (BBB) is a barrier that prevents free access of blood‐derived substances to the brain through the tight junctions and maintains a specialized brain environment. Circumventricular organs (CVOs) lack the typical BBB. The fenestrated vasculature of the sensory CVOs, including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO) and area postrema (AP), allows parenchyma cells to sense a variety of blood‐derived information, including osmotic ones. In the present study, we utilized immunohistochemistry to examine changes in the expression of NG2 and platelet‐derived growth factor receptor beta (PDGFRB) in the OVLT, SFO and AP of adult mice during chronic osmotic stimulation. The expression of NG2 and PDGFRB was remarkably prominent in pericytes, although these angiogenesis‐associated proteins are highly expressed at pericytes of developing immature vasculature. The chronic salt loading prominently increased the expression of NG2 in the OVLT and SFO and that of PDGFRB in the OVLT, SFO and AP. The vascular permeability of low‐molecular‐mass tracer fluorescein isothiocyanate was increased significantly by chronic salt loading in the OVLT and SFO but not AP. In conclusion, the present study demonstrates changes in pericyte expression of NG2 and PDGFRB and vascular permeability in the sensory CVOs by chronic osmotic stimulation, indicating active participation of the vascular system in osmotic homeostasis. Copyright © 2013 John Wiley & Sons, Ltd.     

Blood-brain barrier and atrial natriuretic factor

In brain, binding sites for atrial natriuretic factor (ANF) have been characterized in areas such as circumventricular organs that lack the tight capillary endothelial junctions of the blood-brain barrier and therefore are exposed to circulating peptides. Since atrial natriuretic factor acts directly on vascular endothelium and has been proposed to be actively involved in blood pressure regulation and fluid homeostasis, it is interesting to know whether ANF receptors exist on brain capillaries that constitute the blood-brain barrier and participate in the constant fluid exchange between blood and brain. The present paper reports recent evidence of the presence of ANF receptors located on the structure. It assesses the specific binding of 125I-labelled ANF on bovine brain microvessel preparations and its coupling with a guanylate cyclase system. The potential physiological role of ANF on brain microcirculation and blood-brain barrier functions is discussed.     

The G protein G alpha(13) is required for growth factor-induced cell migration

Heterotrimeric G proteins are critical cellular signal transducers. They are known to directly relay signals from seven-transmembrane G protein-coupled receptors (GPCRs) to downstream effectors. On the other hand, receptor tyrosine kinases (RTKs), a different family of membrane receptors, signal through docking sites in their carboxy-terminal tails created by autophosphorylated tyrosine residues. Here we show that a heterotrimeric G protein, G alpha(13), is essential for RTK-induced migration of mouse fibroblast and endothelial cells. G alpha(13) activity in cell migration is retained in a C-terminal mutant that is defective in GPCR coupling, suggesting that the migration function is independent of GPCR signaling. Thus, G alpha(13) appears to be a critical signal transducer for RTKs as well as GPCRs. This broader role of G alpha(13) in cell migration initiated by two types of receptors could provide a molecular basis for the vascular system defects exhibited by G alpha(13) knockout mice.     

The LIM protein Ajuba is required for ciliogenesis and left-right axis determination in medaka

Cilia are microtubule-based organelles that are present on the surfaces of almost all vertebrate cells. Most cilia function as sensory or molecular transport structures. Malfunctions of cilia have been implicated in several diseases of human development. The assembly of cilia is initiated by the centriole (or basal body), and several centrosomal proteins are involved in this process. The mammalian LIM protein Ajuba is a well-studied centrosomal protein that regulates cell division but its role in ciliogenesis is unknown. In this study, we isolated the medaka homolog of Ajuba and showed that Ajuba localizes to basal bodies of cilia in growth-arrested cells. Knockdown of Ajuba resulted in randomized left-right organ asymmetries and altered expression of early genes responsible for left-right body axis determination. At the cellular level, we found that Ajuba function was essential for ciliogenesis in the cells lining Kupffer’s vesicle; it is these cells that induce the asymmetric fluid flow required for left-right axis determination. Taken together, our findings identify a novel role for Ajuba in the regulation of vertebrate ciliogenesis and left-right axis determination.     

Isolation and characterization of strains CVO and FWKO B, two novel nitrate-reducing, sulfide-oxidizing bacteria isolated from oil field brine

Bacterial strains CVO and FWKO B were isolated from produced brine at the Coleville oil field in Saskatchewan, Canada. Both strains are obligate chemolithotrophs, with hydrogen, formate, and sulfide serving as the only known energy sources for FWKO B, whereas sulfide and elemental sulfur are the only known electron donors for CVO. Neither strain uses thiosulfate as an energy source. Both strains are microaerophiles (1% O(2)). In addition, CVO grows by denitrification of nitrate or nitrite whereas FWKO B reduces nitrate only to nitrite. Elemental sulfur is the sole product of sulfide oxidation by FWKO B, while CVO produces either elemental sulfur or sulfate, depending on the initial concentration of sulfide. Both strains are capable of growth under strictly autotrophic conditions, but CVO uses acetate as well as CO(2) as its sole carbon source. Neither strain reduces sulfate; however, FWKO B reduces sulfur and displays chemolithoautotrophic growth in the presence of elemental sulfur, hydrogen, and CO(2). Both strains grow at temperatures between 5 and 40 degrees C. CVO is capable of growth at NaCl concentrations as high as 7%. The present 16s rRNA analysis suggests that both strains are members of the epsilon subdivision of the division Proteobacteria, with CVO most closely related to Thiomicrospira denitrifcans and FWKO B most closely related to members of the genus Arcobacter. The isolation of these two novel chemolithotrophic sulfur bacteria from oil field brine suggests the presence of a subterranean sulfur cycle driven entirely by hydrogen, carbon dioxide, and nitrate.     

A novel device of bacterial signal transducers.

The osmoregulatory expression of ompC and ompF in Escherichia coli is mediated by a pair of bacterial signal transduction proteins, EnvZ (sensory kinase) and OmpR (response regulator). We isolated previously multicopy suppressors which can complement a defect in the phosphotransfer signal transduction caused by an envZ deletion mutation. Among such suppressors, arcB and barA are of particular interest because these gene products are unique in the sense that they contain both an autophosphorylated histidine site (or transmitter module) and a phospho-accepting aspartate site (or receiver module) in their primary amino acid sequences. Here we report that ArcB and BarA possess in the C-terminal region a phosphorylated histidine site which has never been noticed, in addition to the authentic one identified previously. This newly identified histidine in ArcB and BarA was demonstrated to play a crucial role in the observed multicopy suppression. Furthermore, it was demonstrated in vivo and in vitro for ArcB that the C-terminal domain containing the histidine can function as an alternative phosphodonor (or transmitter). This novel type of sensory kinase was therefore revealed to contain two independent phosphodonor sites, together with a phospho-accepting site. These findings suggest that this unique feature of ArcB and BarA, in terms of the signaling modules, make it possible for these sensory kinases to function as dual-signaling transducers.     

Mechanisms of cold pain

Avoidance of cold pain is an important survival mechanism. Intriguingly, whilst cooling can cause numbness, damage sensing mechanisms still seem to operate at low temperatures, and pain can be perceived from cooled damaged tissue. Recent studies have identified two cold-activated transient receptor potential (TRP) channels present in sensory neurons as transducers of cold stimuli. TRPM8 seems to mediate responses to cooling whilst TRPA1 is activated, possibly indirectly, by more extreme cold conditions. The existence of cold-responsive neurons that do not express these channels suggests that other transducers of cold stimuli remain to be discovered. Subsequent action potential electrogenesis and probably propagation from sensory neurons innervating cold tissues depends upon the presence of Na(v)1.8, the sole voltage-gated sodium channel that fails to inactivate at low temperatures. This may explain the remarkable specificity of Na(v)1.8 expression in nociceptive neurons, where it plays an important role in pain pathways.