Systemic Inflammation Leads to Changes in the Intracellular Localization of KLK6 in Oligodendrocytes in Spinal Cord White Matter.

Axonal injury and demyelination occur in demyelinating diseases, such as multiple sclerosis, and the detachment of myelin from axons precedes its degradation. Paranodes are the areas at which each layer of the myelin sheath adheres tightly to axons. The destruction of nodal and paranodal structures during inflammation is an important pathophysiology of various neurological disorders. However, the underlying pathological changes in these structures remain unclear. Kallikrein 6 (KLK6), a serine protease produced by oligodendrocytes, is involved in demyelinating diseases. In the present study, we intraperitoneally injected mice with LPS for several days and examined changes in the localization of KLK6. Transient changes in the intracellular localization of KLK6 to paranodes in the spinal cord were observed during LPS-induced systemic inflammation. However, these changes were not detected in the upper part of brain white matter. LPS-induced changes were suppressed by minocycline, suggesting the involvement of microglia. Moreover, nodal lengths were elongated in LPS-treated wild-type mice, but not in LPS-treated KLK6-KO mice. These results demonstrate the potential involvement of KLK6 in the process of demyelination.

Heterogeneous vascular permeability and alternative diffusion barrier in sensory circumventricular organs of adult mouse brain.

Fenestrated capillaries of the sensory circumventricular organs (CVOs), including the organum vasculosum of the lamina terminalis, the subfornical organ and the area postrema, lack completeness of the blood-brain barrier (BBB) to sense a variety of blood-derived molecules and to convey the information into other brain regions. We examine the vascular permeability of blood-derived molecules and the expression of tight-junction proteins in sensory CVOs. The present tracer assays revealed that blood-derived dextran 10 k (Dex10k) having a molecular weight (MW) of 10,000 remained in the perivascular space between the inner and outer basement membranes, but fluorescein isothiocyanate (FITC; MW: 389) and Dex3k (MW: 3000) diffused into the parenchyma. The vascular permeability of FITC was higher at central subdivisions than at distal subdivisions. Neither FITC nor Dex3k diffused beyond the dense network of glial fibrillar acidic protein (GFAP)-positive astrocytes/tanycytes. The expression of tight-junction proteins such as occludin, claudin-5 and zonula occludens-1 (ZO-1) was undetectable at the central subdivisions of the sensory CVOs but some was expressed at the distal subdivisions. Electron microscopic observation showed that capillaries were surrounded with numerous layers of astrocyte processes and dendrites. The expression of occludin and ZO-1 was also observed as puncta on GFAP-positive astrocytes/tanycytes of the sensory CVOs. Our study thus demonstrates the heterogeneity of vascular permeability and expression of tight-junction proteins and indicates that the outer basement membrane and dense astrocyte/tanycyte connection are possible alternative mechanisms for a diffusion barrier of blood-derived molecules, instead of the BBB.

Characterization of neural stem cells and their progeny in the sensory circumventricular organs of adult mouse.

Although evidence has accumulated that neurogenesis and gliogenesis occur in the subventricular zone (SVZ) and subgranular zone (SGZ) of adult mammalian brains, recent studies indicate the presence of neural stem cells (NSCs) in adult brains, particularly the circumventricular regions. In the present study, we aimed to determine characterization of NSCs and their progenitor cells in the sensory circumventricular organs (CVOs), including organum vasculosum of the lamina terminalis, subfornical organ, and area postrema of adult mouse. There were two types of NSCs: tanycyte-like ependymal cells and astrocyte-like cells. Astrocyte-like NSCs proliferated slowly and oligodendrocyte progenitor cells (OPCs) and neural progenitor cells (NPCs) actively divided. Molecular marker protein expression of NSCs and their progenitor cells were similar to those reported in the SVZ and SGZ, except that astrocyte-like NSCs expressed S100ß. These circumventricular NSCs possessed the capacity to give rise to oligodendrocytes and sparse numbers of neurons and astrocytes in the sensory CVOs and adjacent brain regions. The inhibition of vascular endothelial growth factor (VEGF) signaling by using a VEGF receptor-associated tyrosine kinase inhibitor AZD2171 largely suppressed basal proliferation of OPCs. A single systemic administration of lipopolysaccharide attenuated proliferation of OPCs and induced remarkable proliferation of microglia. The present study indicates that sensory circumventricular NSCs provide new neurons and glial cells in the sensory CVOs and adjacent brain regions.

Vascular endothelial growth factor-dependent angiogenesis and dynamic vascular plasticity in the sensory circumventricular organs of adult mouse brain.

The sensory circumventricular organs (CVOs), which comprise the organum vasculosum of the lamina terminalis (OVLT), the subfornical organ (SFO) and the area postrema (AP), lack a typical blood-brain barrier (BBB) and monitor directly blood-derived information to regulate body fluid homeostasis, inflammation, feeding and vomiting. Until now, almost nothing has been documented about vascular features of the sensory CVOs except fenestration of vascular endothelial cells. We therefore examine whether continuous angiogenesis occurs in the sensory CVOs of adult mouse. The angiogenesis-inducing factor vascular endothelial growth factor-A (VEGF-A) and the VEGF-A-regulating transcription factor hypoxia-inducible factor-1α were highly expressed in neurons of the OVLT and SFO and in both neurons and astrocytes of the AP. Expression of the pericyte-regulating factor platelet-derived growth factor B was high in astrocytes of the sensory CVOs. Immunohistochemistry of bromodeoxyuridine and Ki-67, a nuclear protein that is associated with cellular proliferation, revealed active proliferation of endothelial cells. Moreover, immunohistochemistry of caspase-3 and the basement membrane marker laminin showed the presence of apoptosis and sprouting of endothelial cells, respectively. Treatment with the VEGF receptor-associated tyrosine kinase inhibitor AZD2171 significantly reduced proliferation and filopodia sprouting of endothelial cells, as well as the area and diameter of microvessels. The mitotic inhibitor cytosine-b-D-arabinofuranoside reduced proliferation of endothelial cells and the vascular permeability of blood-derived low-molecular-weight molecules without changing vascular area and microvessel diameter. Thus, our data indicate that continuous angiogenesis is dependent on VEGF signaling and responsible for the dynamic plasticity of vascular structure and permeability.

Antidepressant-induced vascular dynamics in the hippocampus of adult mouse brain.

New neurons are continuously added to hippocampal circuitry involved with spatial learning and memory throughout life. These new neurons originate from neural stem/progenitor cells (NSPCs) in the subgranular zone (SGZ) of the dentate gyrus (DG). Recent studies indicate that vascular reconstruction is closely connected with neurogenesis, but little is known about its mechanism. We have examined vascular reconstruction in the hippocampus of adult mouse brain after the administration of the antidepressant fluoxetine, a potent inducer of hippocampal neurogenesis. The immunohistochemistry of laminin and CD31 showed that filopodia of endothelial cells sprouted from existing thick microvessels and often formed a bridge between two thick microvessels. These filopodia were frequently seen at the molecular layer and dentate hilus of the DG, the stratum lacunosum-moleculare of the CA1, and the stratum oriens of the CA3. The filopodia were exclusively localized along cellular processes of astrocytes, but such intimate association was not seen with cell bodies and processes of NSPCs. The administration of fluoxetine significantly increased vascular density by enlarging the luminal size of microvessels and eliminating the filopodia of endothelial cells in the molecular layer and dentate hilus. Treatment with fluoxetine increased the number of proliferating NSPCs in the granule cell layer and dentate hilus, and that of endothelial cells in the granule cell layer. Thus, antidepressant-induced vascular dynamics in the DG are possibly attributable to the alteration of the luminal size of microvessels rather than to proliferation of endothelial cells.

Astrocytic TRPV1 ion channels detect blood-borne signals in the sensory circumventricular organs of adult mouse brains.

The circumventricular organs (CVOs), including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP) sense a variety of blood-borne molecules because they lack typical blood-brain barrier. Though a few signaling pathways are known, it is not known how endogenous ligands for transient receptor potential vanilloid receptor 1 ion channel (TRPV1) are sensed in the CVOs. In this study, we aimed to examine whether or not astrocytic TRPV1 senses directly blood-borne molecules in the OVLT, SFO, and AP of adult mice. The reverse transcription-polymerase chain reaction and Western analysis revealed the expression of TRPV1 in the CVOs. Confocal microscopic immunohistochemistry further showed that TRPV1 was localized prominently at thick cellular processes of astrocytes rather than fine cellular processes and cell bodies. TRPV1-expressing cellular processes of astrocytes surrounded the vasculature to constitute dense networks. The expression of TRPV1 was also found at neuronal dendrites but not somata in the CVOs. The intravenous administration of a TRPV1 agonist resiniferatoxin (RTX) prominently induced Fos expression at astrocytes in the OVLT, SFO, and AP and neurons in adjacent related nuclei of the median preoptic nuclei (MnPO) and nucleus of the solitary tract (Sol) of wild-type but not TRPV1-knockout mice. The intracerebroventricular infusion of RTX induced Fos expression at both astrocytes and neurons in the CVOs, MnPO, and Sol. Thus, this study demonstrates that blood-borne molecules are sensed directly by astrocytic TRPV1 of the CVOs in adult mammalians.

Characterization of TRPM8-expressing neurons in the adult mouse hypothalamus.

Transient receptor potential melastatin 8 (TRPM8) is a menthol receptor that detects cold temperatures and influences behaviors and autonomic functions under cold stimuli. Despite the well-documented peripheral roles of TRPM8, the evaluation of its central functions is still of great interest. The present study clarifies the nature of a subpopulation of TRPM8-expressing neurons in the adult mice. Combined in situ hybridization and immunohistochemistry revealed that TRPM8-expressing neurons are exclusively positive for glutamate decarboxylase 67 mRNA signals in the lateral septal nucleus (LS) and preoptic area (POA) but produced no positive signal for vesicular glutamate transporter 2. Double labeling immunohistochemistry showed the colocalization of TRPM8 with vesicular GABA transporter at axonal terminals. Immunohistochemistry further revealed that TRPM8-expressing neurons frequently expressed calbindin and calretinin in the LS, but not in the POA. TRPM8-expressing neurons in the POA expressed a prostaglandin E2 receptor, EP3, and neurotensin, whereas expression in the LS was minimal. These results indicate that hypothalamic TRPM8-expressing neurons are inhibitory GABAergic, while the expression profile of calcium-binding proteins, neurotensin, and EP3 differs between the POA and LS.

Neural stem cell phenotype of tanycyte-like ependymal cells in the circumventricular organs and central canal of adult mouse brain.

Tanycyte is a subtype of ependymal cells which extend long radial processes to brain parenchyma. The present study showed that tanycyte-like ependymal cells in the organum vasculosum of the lamina terminalis, subfornical organ and central canal (CC) expressed neural stem cell (NSC) marker nestin, glial fibrillar acidic protein and sex determining region Y. Proliferation of these tanycyte-like ependymal cells was promoted by continuous intracerebroventricular infusion of fibroblast growth factor-2 and epidermal growth factor. Tanycytes-like ependymal cells in the CC are able to form self-renewing neurospheres and give rise mostly to new astrocytes and oligodendrocytes. Collagenase-induced small medullary hemorrhage increased proliferation of tanycyte-like ependymal cells in the CC. These results demonstrate that these tanycyte-like ependymal cells of the adult mouse brain are NSCs and suggest that they serve as a source for providing new neuronal lineage cells upon brain damage in the medulla oblongata.

Microglia are continuously activated in the circumventricular organs of mouse brain.

Microglia are the primary resident immune cells of the brain parenchyma and transform into the amoeboid form in the "activated state" under pathological conditions from the ramified form in the "resting state" under physiologically healthy conditions. In the present study, we found that microglia in the circumventricular organs (CVOs) of adult mice displayed the amoeboid form with fewer branched cellular processes even under normal conditions; however, those in other brain regions showed the ramified form, which is characterized by well-branched and dendritic cellular processes. Moreover, microglia in the CVOs showed the strong protein expression of the M1 markers CD16/32 and CD86 and M2 markers CD206 and Ym1 without any pathological stimulation. Thus, the present results indicate that microglia in the CVOs of adult mice are morphologically and functionally activated under normal conditions, possibly due to the specialized features of the CVOs, namely, the entry of blood-derived molecules into parenchyma through fenestrated capillaries and the presence of neural stem cells.

VEGF- and PDGF-dependent proliferation of oligodendrocyte progenitor cells in the medulla oblongata after LPC-induced focal demyelination.

The myelin sheath is critical in maintaining normal functions of the adult central nervous system (CNS) and the loss of the myelin sheath results in various neurological diseases. Although remyelination is the intrinsic repair system against demyelination that new myelin sheath is formed around axons in the adult CNS, little has been reported on remyelination system in the medulla oblongata. In the present study, we showed that the proliferation of oligodendrocyte progenitor cells (OPCs) was increased in the medulla oblongata by lysophosphatidylcholine (LPC)-induced focal demyelination, but that of NSCs was not changed. The inhibition of vascular endothelial growth factor (VEGF)- and platelet-derived growth factor (PDGF)-signaling suppressed the proliferation of OPCs by LPC-induced demyelination. Thus, the present study indicates that resident OPCs contribute to focal remyelination and VEGF and PDGF signaling is required for the proliferation of OPCs in the medulla oblongata of the adult mouse.

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.

Remyelination in the medulla oblongata of adult mouse brain during experimental autoimmune encephalomyelitis.

Experimental autoimmune encephalomyelitis (EAE) is primarily used as an animal model of autoimmune demyelinating disease, multiple sclerosis. In this study, we found the proliferative rate of oligodendrocyte progenitor cells (OPCs) in the medulla elevated twofold above control levels during EAE and new generation of mature oligodendrocytes was increased as well. Although astrocytes showed hypertrophic reactive phenotype, a new generation of them was rare. Astrocyte- and tanycyte-like neural stem cells (NSCs), multipotent NSCs, did not augment their low proliferative rate. Thus, the present study demonstrates that resident OPCs derived from NSCs contribute to remyelination in the medulla oblongata in EAE mice.

TRPV1 is crucial for proinflammatory STAT3 signaling and thermoregulation-associated pathways in the brain during inflammation.

Transient receptor potential vanilloid receptor 1 (TRPV1) is a non-selective cation channel that is stimulated by heat (>43 °C), mechanical/osmotic stimuli, and low pH. The importance of TRPV1 in inflammatory responses has been demonstrated, whereas its participation in brains remains unclear. In the present study, the intracerebroventricular (icv) administration of the TRPV1 agonist resiniferatoxin (RTX) induced the activation of signal transducer and activator of transcription 3 (STAT3) in circumventricular organs (CVOs) and thermoregulation-associated brain regions with a similar patttern to the peripheral and icv administration of lipopolysaccharide (LPS). With the peripheral and icv LPS stimuli, STAT3 activation was significantly lower in Trpv1(-/-) mice than in Trpv1(+/+) mice. The icv administration of RTX induced transient hypothermia, whereas that of the TRPV1 antagonist capsazepine enhanced the magnitude and period of LPS-induced hyperthermia. These results indicate that TRPV1 is important for activating proinflammatory STAT3 signaling and thermoregulation-associated brain pathways in the brain.

Robust increase of microglia proliferation in the fornix of hippocampal axonal pathway after a single LPS stimulation.

Microglia are resident immunocompetent cells having important roles in innate immunity in the brains. In the present study, we found that a single lipopolysaccharide (LPS) administration significantly increased microglial proliferation in the fornix and dentate gyrus (DG) but not the cerebral cortex and corpus callosum of adult mice. LPS-induced microglial proliferation was especially robust at the white matter of the fornix. The density of microglia increased in the fornix and DG for roughly one week and returned to basal levels at least 20days after a single LPS administration. Consecutive LPS administration did not induce such dramatic increase of microglial proliferation in the fornix. The inhibition of vascular endothelial growth factor signaling by AZD2171 largely suppressed LPS-induced increase of microglial proliferation in the fornix. In conclusion, the present study indicates that the hippocampal neuronal system has a higher proliferative microglial capability against LPS-induced inflammatory administration compared with other brain regions.

Oligodendrogenesis in the fornix of adult mouse brain; the effect of LPS-induced inflammatory stimulation.

Evidence have been accumulated that continuous oligodendrogenesis occurs in the adult mammalian brain. The fornix, projection and commissure pathway of hippocampal neurons, carries signals from the hippocampus to other parts of the brain and has critical role in memory and learning. However, basic characterization of adult oligodendrogenesis in this brain region is not well understood. In the present study, therefore, we aimed to examine the proliferation and differentiation of oligodendrocyte progenitor cells (OPCs) and the effect of acute inflammatory stimulation on oligodendrogenesis in the fornix of adult mouse. We demonstrated the proliferation of OPCs and a new generation of mature oligodendrocytes by using bromodeoxyuridine and Ki67 immunohistochemistry. Oligodendrogenesis of adult fornix was also demonstrated by using oligodendrocyte transcription factor 2 transgenic mouse. A single systemic administration of lipopolysaccharide (LPS) attenuated proliferation of OPCs in the fornix together with reduced proliferation of hippocampal neural stem/progenitor cells. Time course analysis showed that a single administration of LPS attenuated the proliferation of OPCs during 24-48 h. On the other hand, consecutive administration of LPS did not suppress proliferation of OPCs. The treatment of LPS did not affect differentiation of OPCs into mature oligodendrocytes. Treatment of a microglia inhibitor minocycline significantly attenuated basal proliferation of OPCs under normal condition. In conclusion, the present study indicates that continuous oligodendrogenesis occurs and a single administration of LPS transiently attenuates proliferation of OPCs without changing differentiation in the fornix of the adult mouse brains.

Astrocytic TLR4 expression and LPS-induced nuclear translocation of STAT3 in the sensory circumventricular organs of adult mouse brain.

The sensory circumventricular organs (CVOs) comprise the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP) and lack the blood-brain barrier. The expression of Toll-like receptor 4 (TLR4) was seen at astrocytes throughout the sensory CVOs and at microglia in the AP and solitary nucleus around the central canal. The peripheral and central administration of lipopolysaccharide induced a similar pattern of nuclear translocation of STAT3. A microglia inhibitor minocycline largely suppressed lipopolysaccharide-induced astrocytic nuclear translocation of STAT3 in the OVLT and AP, but its effect was less in the SFO.

VEGF-dependent and PDGF-dependent dynamic neurovascular reconstruction in the neurohypophysis of adult mice.

Hypothalamo-neurohypophysial system (HNS) releases arginine vasopressin (AVP) and oxytocin (OXT) from axonal terminals of the neurohypophysis (NH) into blood circulation for controlling body fluid homeostasis and lactation. Chronic osmotic and suckling stimulations have been shown to cause neurovascular and neuroglial reconstruction in the NH of adult mammals and no study has been reported for vascular dynamics. The aim of this study was to elucidate the occurrence of continuous angiogenesis and growth factor-dependent neurovascular reconstruction in the NH of adult mice. Active proliferation of endothelial cells and oligodendrocyte progenitor cells (OPCs) was observed using the immunohistochemistry of bromodeoxyuridine and Ki-67. Vascular endothelial growth factor A (VEGFA) and VEGF receptor 2 (VEGFR2 (KDR)) were highly expressed at pituicytes and endothelial cells respectively. Moreover, prominent expression of platelet-derived growth factor B (PDGFB) and PDGF receptor beta was observed at OXT-containing axonal terminals and pericytes respectively. Administration of the selective tyrosine kinase inhibitor AZD2171 for VEGFRs and STI571 for PDGFRs significantly decreased proliferation of endothelial cells and OPCs. Moreover, AZD2171 treatment decreased vascular density by facilitating apoptosis of endothelial cells and the withdrawal of its treatment led to remarkable rebound proliferation of endothelial cells, so that vascular density rapidly returned to normal levels. AZD2171 decreased the density of both AVP- and OXT-containing axonal terminals, whereas STI571 selectively decreased the density of AVP-containing ones. Thus, this study demonstrates that the signaling pathways of VEGF and PDGF are crucial mediators for determining proliferation of endothelial cells and OPCs and the density of AVP- and OXT-containing axonal terminals in the HNS.

Behavioral analysis of relaxin-3 deficient mice.

Relaxin-3 is a neuropeptide belonging to the relaxin/insulin superfamily. Studies using rodents have revealed that relaxin-3 is predominantly expressed in neurons in the nucleus incertus of the pons, projecting axons to forebrain regions including the hypothalamus. There is evidence that relaxin-3 is involved in several functions, including food intake and stress responses. We generated relaxin-3 gene knockout (KO) mice and examined them using a battery of behavioral tests of sensory/motor functions and emotion-related behaviors. Relaxin-3 KO mice exhibited normal growth and appearance. There was no difference in bodyweight among genotypes in both normal and high fat diet feeding. In addition, there were no significant differences between wild-type and KO mice in social interaction, depression-like behavior, and short memory test. However, in the elevated plus maze test, KO mice exhibited a robust increase in the tendency to enter open arms, although they exhibited normal performance in a light/dark transition test and showed no difference from wild-type mice in the open field test. Taken together, these results indicate that relaxin-3 KO mice exhibit mild anxiolytic characteristics relative to wild-type mice, suggesting that this peptide is involved in anxiety-related behavior.