Tissue plasminogen activator and plasminogen are critical for osmotic homeostasis by regulating vasopressin secretion

Responses of perivascular macrophages to circulating lipopolysaccharides in the subfornical organ with special reference to endotoxin tolerance

BACKGROUND

Circulating endotoxins including lipopolysaccharides (LPS) cause brain responses such as fever and decrease of food and water intake, while pre-injection of endotoxins attenuates these responses. This phenomenon is called endotoxin tolerance, but the mechanisms underlying it remain unclear. The subfornical organ (SFO) rapidly produces proinflammatory cytokines including interleukin-1β (IL-1β) in response to peripherally injected LPS, and repeated LPS injection attenuates IL-1β production in the SFO, indicating that the SFO is involved in endotoxin tolerance. The purpose of this study is to investigate features of the IL-1β source cells in the SFO of LPS-non-tolerant and LPS-tolerant mice.

METHODS

We first established the endotoxin-tolerant mouse model by injecting LPS into adult male mice (C57BL/6J). Immunohistochemistry was performed to characterize IL-1β-expressing cells, which were perivascular macrophages in the SFO. We depleted perivascular macrophages using clodronate liposomes to confirm the contribution of IL-1β production. To assess the effect of LPS pre-injection on perivascular macrophages, we transferred bone marrow-derived cells obtained from male mice (C57BL/6-Tg (CAG-EGFP)) to male recipient mice (C57BL/6N). Finally, we examined the effect of a second LPS injection on IL-1β expression in the SFO perivascular macrophages.

RESULTS

We report that perivascular macrophages but not parenchymal microglia rapidly produced the proinflammatory cytokine IL-1β in response to LPS. We found that peripherally injected LPS localized in the SFO perivascular space. Depletion of macrophages by injection of clodronate liposomes attenuated LPS-induced IL-1β expression in the SFO. When tolerance developed to LPS-induced sickness behavior in mice, the SFO perivascular macrophages ceased producing IL-1β, although bone marrow-derived perivascular macrophages increased in number in the SFO and peripherally injected LPS reached the SFO perivascular space.

CONCLUSIONS

The current data indicate that perivascular macrophages enable the SFO to produce IL-1β in response to circulating LPS and that its hyporesponsiveness may be the cause of endotoxin tolerance.

Brain Region-dependent Heterogeneity and Dose-dependent Difference in Transient Microglia Population Increase during Lipopolysaccharide-induced Inflammation

Numerous studies have reported the importance of microglial activation in various pathological conditions, whereas little attention has been given to the point for dynamics of microglial population under infection-induced inflammation. In the present study, the single systemic stimulation of 100 μg/kg lipopolysaccharide (LPS) induced robust microglial proliferation only in the circumventricular organs (CVOs) and their neighboring brain regions. More than half of microglia similarly showed proliferative activity in the CVOs and their neighboring brain regions after 1 mg/kg LPS stimulation, while this stimulation expanded microglia-proliferating brain regions including the hypothalamus, medulla oblongata, and limbic system. Microglia proliferation resulted in a transient increase of microglial density, since their density almost returned to basal levels within 3 weeks. Divided microglia survived at the same rate as non-divided ones. Proliferating microglia frequently expressed a resident microglia marker Tmem119, indicating that increase of microglia density is due to the proliferation of resident microglia. Thus, the present study demonstrates that transient increase in microglia density depends on the brain region and dose of LPS during infection-induced inflammation and could provide a new insight on microglia functions in inflammation and pathogenesis of brain diseases.

Advances in Understanding of Structural Reorganization in the Hypothalamic Neurosecretory System

The hypothalamic neurosecretory system synthesizes neuropeptides in hypothalamic nuclei and releases them from axonal terminals into the circulation in the neurohypophysis (NH) and median eminence (ME). This system plays a crucial role in regulating body fluid homeostasis and social behaviors as well as reproduction, growth, metabolism, and stress responses, and activity-dependent structural reorganization has been reported. Current knowledge on dynamic structural reorganization in the NH and ME, in which the axonal terminals of neurosecretory neurons directly contact the basement membrane (BM) of a fenestrated vasculature, is discussed herein. Glial cells, pituicytes in the NH and tanycytes in the ME, engulf axonal terminals and interpose their cellular processes between axonal terminals and the BM when hormonal demands are low. Increasing demands for neurosecretion result in the retraction of the cellular processes of glial cells from axonal terminals and the BM, permitting increased neurovascular contact. The shape conversion of pituicytes and tanycytes is mediated by neurotransmitters and sex steroid hormones, respectively. The NH and ME have a rough vascular BM profile of wide perivascular spaces and specialized extension structures called "perivascular protrusions." Perivascular protrusions, the insides of which are occupied by the cellular processes of vascular mural cells pericytes, contribute to increasing neurovascular contact and, thus, the efficient diffusion of hypothalamic neuropeptides. A chronic physiological stimulation has been shown to increase perivascular protrusions via the shape conversion of pericytes and the profile of the vascular surface. Continuous angiogenesis occurs in the NH and ME of healthy normal adult rodents depending on the signaling of vascular endothelial growth factor (VEGF). The inhibition of VEGF signaling suppresses the proliferation of endothelial cells (ECs) and promotes their apoptosis, which results in decreases in the population of ECs and axonal terminals. Pituicytes and tanycytes are continuously replaced by the proliferation and differentiation of stem/progenitor cells, which may be regulated by matching those of ECs and axonal terminals. In conclusion, structural reorganization in the NH and ME is caused by the activity-dependent shape conversion of glial cells and vascular mural cells as well as the proliferation of endothelial and glial cells by angiogenesis and gliogenesis, respectively.

Functional Morphology of the Arm Spine Joint and Adjacent Structures of the Brittlestar Ophiocomina nigra (Echinodermata: Ophiuroidea)

The skeletal morphology of the arm spine joint of the brittlestar Ophiocomina nigra was examined by scanning electron microscopy and the associated epidermis, connective tissue structures, juxtaligamental system and muscle by optical and transmission electron microscopy. The behaviour of spines in living animals was observed and two experiments were conducted to establish if the spine ligament is mutable collagenous tissue: these determined (1) if animals could detach spines to which plastic tags had been attached and (2) if the extension under constant load of isolated joint preparations was affected by high potassium stimulation. The articulation normally operates as a flexible joint in which the articular surfaces are separated by compliant connective tissue. The articular surfaces comprise a reniform apposition and peg-in-socket mechanical stop, and function primarily to stabilise spines in the erect position. Erect spines can be completely immobilised, which depends on the ligament having mutable tensile properties, as was inferred from the ability of animals to detach tagged spines and the responsiveness of isolated joint preparations to high potassium. The epidermis surrounding the joint has circumferential constrictions that facilitate compression folding and unfolding when the spine is inclined. The interarticular connective tissue is an acellular meshwork of collagen fibril bundles and may serve to reduce frictional forces between the articular surfaces. The ligament consists of parallel bundles of collagen fibrils and 7-14 nm microfibrils. Its passive elastic recoil contributes to the re-erection of inclined spines. The ligament is permeated by cell processes containing large dense-core vesicles, which belong to two types of juxtaligamental cells, one of which is probably peptidergic. The spine muscle consists of obliquely striated myocytes that are linked to the skeleton by extensions of their basement membranes. Muscle contraction may serve mainly to complete the process of spine erection by ensuring close contact between the articular surfaces.

Neuroendocrinal, Neurodevelopmental, and Embryotoxic Effects of Recombinant Tissue Plasminogen Activator Treatment for Pregnant Women with Acute Ischemic Stroke

Thrombolysis with recombinant tissue plasminogen activator (rTPA) was the first evidence-based treatment approved for acute stroke. Ischemic stroke is relatively uncommon in fertile women but treatment is often delayed or not given. In randomized trials, pregnancy has been an exclusion criterion for thrombolysis. Physiologic TPA has been shown to have neuroendocrine effects namely in vasopressin secretion. Important TPA effects in brain function and development include neurite outgrowth, migration of cerebellar granular neurons and promotion of long-term potentiation, among others. Until now, no neuroendocrine side-effects have been reported in pregnant women treated with rTPA. The effects of rTPA exposure in the fetus following intravenous thrombolysis in pregnant women are still poorly understood. This depends on low case frequency, short-duration of exposure and the fact that rTPA molecule is too large to pass the placenta. rTPA has a short half-life of 4–5 min, with only 10% of its concentration remaining in circulation after 20 min, which may explain its safety at therapeutically doses. Ischemic stroke during pregnancy occurs most often in the third trimester. Complication rates of rTPA in pregnant women treated for thromboembolic conditions and ischemic stroke were found to be similar when compared to non-pregnant women (7–9% mortality). In embryos of animal models so far, no indications of a teratogenic or mutagenic potential were found. Pregnancy is still considered a relative contraindication when treating acute ischemic stroke with rTPA, however, treatment risk must be balanced against the potential of maternal disability and/or death.

New aspects in fenestrated capillary and tissue dynamics in the sensory circumventricular organs of adult brains

The blood-brain barrier (BBB) generally consists of endothelial tight junction barriers that prevent the free entry of blood-derived substances, thereby maintaining the extracellular environment of the brain. However, the circumventricular organs (CVOs), which are located along the midlines of the brain ventricles, lack these endothelial barriers and have fenestrated capillaries; therefore, they have a number of essential functions, including the transduction of information between the blood circulation and brain. Previous studies have demonstrated the extensive contribution of the CVOs to body fluid and thermal homeostasis, energy balance, the chemoreception of blood-derived substances, and neuroinflammation. In this review, recent advances have been discussed in fenestrated capillary characterization and dynamic tissue reconstruction accompanied by angiogenesis and neurogliogenesis in the sensory CVOs of adult brains. The sensory CVOs, including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP), have size-selective and heterogeneous vascular permeabilities. Astrocyte-/tanycyte-like neural stem cells (NSCs) sense blood- and cerebrospinal fluid-derived information through the transient receptor potential vanilloid 1, a mechanical/osmotic receptor, Toll-like receptor 4, a lipopolysaccharide receptor, and Nax, a Na-sensing Na channel. They also express tight junction proteins and densely and tightly surround mature neurons to protect them from blood-derived neurotoxic substances, indicating that the NSCs of the CVOs perform BBB functions while maintaining the capacity to differentiate into new neurons and glial cells. In addition to neurogliogenesis, the density of fenestrated capillaries is regulated by angiogenesis, which is accompanied by the active proliferation and sprouting of endothelial cells. Vascular endothelial growth factor (VEGF) signaling may be involved in angiogenesis and neurogliogenesis, both of which affect vascular permeability. Thus, recent findings advocate novel concepts for the CVOs, which have the dynamic features of vascular and parenchymal tissues.

The use of recently developed histochemical markers for localizing neurotoxicant induced regional brain pathologies

Neuronal and vascular brain components are interrelated morphologically, physiologically and developmentally. Due to this close interrelationship, it is often difficult to understand the cause and effect relationship between neuronal vs. vascular dysfunction and pathology. This review will discuss four of the more promising recent developments for detecting vascular pathology, and will compare them with the labeling pattern seen with markers of glial and neuronal pathology; following exposure to well characterized neurotoxicants. To detect the vascular dysfunction in the brain, we recently developed a Fluoro-Turquoise gelatin conjugate (FT-gel), a fluorescent probe that helps to delineate between healthy vs. sclerotic vessels. Similarly, we have investigated the potential for Fluoro-Gold to label in vivo all the endothelial cells in the brain as they co-localize with RECA, an endothelial cell marker. We have also developed Amylo-Glo, a fluorescent tracer that can detect neurotoxic A-beta aggregates in the brain. In this article, we will discuss the potential use of these novel histochemical markers to study the neurotoxicant induced brain. We will also discuss neurovascular strategies that may offer novel therapeutic opportunities for neurodegenerative disorders.