Behavioral impairments are linked to neuroinflammation in mice with Cerebral Cavernous Malformation disease.

Background: Cerebral Cavernous Malformations (CCMs) are neurovascular abnormalities in the central nervous system (CNS) caused by loss of function mutations in KRIT1 (CCM1), CCM2, or PDCD10 (CCM3) genes. One of the most common symptoms in CCM patients is associated with motor disability, weakness, seizures, stress, and anxiety, and the extent of the symptom or symptoms may be due to the location of the lesion within the CNS or whether multiple lesions are present. Previous studies have primarily focused on understanding the pathology of CCM using animal models. However, more research has yet to explore the potential impact of CCM lesions on behavioral deficits in animal models, including effects on short-term and long-term memory, motor coordination, and function. Methods: We used the accelerating RotaRod test to assess motor and coordination deficits. We also used the open field test to assess locomotor activity and pathology-related behavior and Pavlovian fear conditioning to assess short-and long-term memory deficits. Our behavioral studies were complemented by proteomics, histology, immunofluorescence, and imaging techniques. We found that neuroinflammation is crucial in behavioral deficits in male and female mice with neurovascular CCM lesions ( Slco1c1-iCreERT2; Pdcd10 fl/fl ; Pdcd10 BECKO ). Results: Functional behavior tests in male and female Pdcd10 BECKO mice revealed that CCM lesions cause sudden motor coordination deficits associated with the manifestation of profound neuroinflammatory lesions. Our findings indicate that maturation of CCM lesions in Pdcd10 BECKO mice also experienced a significant change in short- and long-term memory compared to their littermate controls, Pdcd10 fl/fl mice. Proteomic experiments reveal that as CCM lesions mature, there is an increase in pathways associated with inflammation, coagulation, and angiogenesis, and a decrease in pathways associated with learning and plasticity. Therefore, our study shows that Pdcd10 BECKO mice display a wide range of behavioral deficits due to significant lesion formation in their central nervous system and that signaling pathways associated with neuroinflammation and learning impact behavioral outcomes. Conclusions: Our study found that CCM animal models exhibited behavioral impairments such as decreased motor coordination and amnesia. These impairments were associated with the maturation of CCM lesions that displayed a neuroinflammatory pattern.

Mild Hypoxia Accelerates Cerebral Cavernous Malformation Disease Through CX3CR1-CX3CL1 Signaling.

BACKGROUND: Heterogeneity in the severity of cerebral cavernous malformations (CCMs) disease, including brain bleedings and thrombosis that cause neurological disabilities in patients, suggests that environmental, genetic, or biological factors act as disease modifiers. Still, the underlying mechanisms are not entirely understood. Here, we report that mild hypoxia accelerates CCM disease by promoting angiogenesis, neuroinflammation, and vascular thrombosis in the brains of CCM mouse models. METHODS: We used genetic studies, RNA sequencing, spatial transcriptome, micro-computed tomography, fluorescence-activated cell sorting, multiplex immunofluorescence, coculture studies, and imaging techniques to reveal that sustained mild hypoxia via the CX3CR1-CX3CL1 (CX3C motif chemokine receptor 1/chemokine [CX3C motif] ligand 1) signaling pathway influences cell-specific neuroinflammatory interactions, contributing to heterogeneity in CCM severity. RESULTS: Histological and expression profiles of CCM neurovascular lesions (Slco1c1-iCreERT2;Pdcd10fl/fl; Pdcd10BECKO) in male and female mice found that sustained mild hypoxia (12% O2, 7 days) accelerates CCM disease. Our findings indicate that a small reduction in oxygen levels can significantly increase angiogenesis, neuroinflammation, and thrombosis in CCM disease by enhancing the interactions between endothelium, astrocytes, and immune cells. Our study indicates that the interactions between CX3CR1 and CX3CL1 are crucial in the maturation of CCM lesions and propensity to CCM immunothrombosis. In particular, this pathway regulates the recruitment and activation of microglia and other immune cells in CCM lesions, which leads to lesion growth and thrombosis. We found that human CX3CR1 variants are linked to lower lesion burden in familial CCMs, proving it is a genetic modifier in human disease and a potential marker for aggressiveness. Moreover, monoclonal blocking antibody against CX3CL1 or reducing 1 copy of the Cx3cr1 gene significantly reduces hypoxia-induced CCM immunothrombosis. CONCLUSIONS: Our study reveals that interactions between CX3CR1 and CX3CL1 can modify CCM neuropathology when lesions are accelerated by environmental hypoxia. Moreover, a hypoxic environment or hypoxia signaling caused by CCM disease influences the balance between neuroinflammation and neuroprotection mediated by CX3CR1-CX3CL1 signaling. These results establish CX3CR1 as a genetic marker for patient stratification and a potential predictor of CCM aggressiveness.

ß1 integrin monoclonal antibody treatment ameliorates cerebral cavernous malformations.

ß1 integrins are important in blood vessel formation and function, finely tuning the adhesion of endothelial cells to each other and to the extracellular matrix. The role of integrins in the vascular disease, cerebral cavernous malformation (CCM) has yet to be explored in vivo. Endothelial loss of the gene KRIT1 leads to brain microvascular defects, resulting in debilitating and often fatal consequences. We tested administration of a monoclonal antibody that enforces the active ß1 integrin conformation, (clone 9EG7), on a murine neonatal CCM mouse model, Krit1flox/flox ;Pdgfb-iCreERT2 (Krit1ECKO ), and on KRIT1-silenced human umbilical vein endothelial cells (HUVECs). In addition, endothelial deletion of the master regulator of integrin activation, Talin 1 (Tln1), in Krit1ECKO mice was performed to assess the effect of completely blocking endothelial integrin activation on CCM. Treatment with 9EG7 reduced lesion burden in the Krit1ECKO model and was accompanied by a strong reduction in the phosphorylation of the ROCK substrate, myosin light chain (pMLC), in both retina and brain endothelial cells. Treatment of KRIT1-silenced HUVECs with 9EG7 in vitro stabilized cell-cell junctions. Overnight treatment of HUVECs with 9EG7 resulted in significantly reduced total surface expression of ß1 integrin, which was associated with reduced pMLC levels, supporting our in vivo findings. Genetic blockade of integrin activation by Tln1ECKO enhanced bleeding and did not reduce CCM lesion burden in Krit1ECKO mice. In sum, targeting ß1 integrin with an activated-specific antibody reduces acute murine CCM lesion development, which we found to be associated with suppression of endothelial ROCK activity.

Neuroinflammation Plays a Critical Role in Cerebral Cavernous Malformation Disease.

BACKGROUND: Cerebral cavernous malformations (CCMs) are neurovascular lesions caused by loss of function mutations in 1 of 3 genes, including KRIT1 (CCM1), CCM2, and PDCD10 (CCM3). CCMs affect ≈1 out of 200 children and adults, and no pharmacologic therapy is available. CCM lesion count, size, and aggressiveness vary widely among patients of similar ages with the same mutation or even within members of the same family. However, what determines the transition from quiescent lesions into mature and active (aggressive) CCM lesions is unknown. METHODS: We use genetic, RNA-sequencing, histology, flow cytometry, and imaging techniques to report the interaction between CCM endothelium, astrocytes, leukocytes, microglia/macrophages, neutrophils (CCM endothelium, astrocytes, leukocytes, microglia/macrophages, neutrophils interaction) during the pathogenesis of CCMs in the brain tissue. RESULTS: Expression profile of astrocytes in adult mouse brains using translated mRNAs obtained from the purification of EGFP (enhanced green fluorescent protein)-tagged ribosomes (Aldh1l1-EGFP/Rpl10a) in the presence or absence of CCM lesions (Slco1c1-iCreERT2;Pdcd10fl/fl; Pdcd10BECKO) identifies a novel gene signature for neuroinflammatory astrocytes. CCM-induced reactive astrocytes have a neuroinflammatory capacity by expressing genes involved in angiogenesis, chemotaxis, hypoxia signaling, and inflammation. RNA-sequencing analysis on RNA isolated from brain endothelial cells in chronic Pdcd10BECKO mice (CCM endothelium), identified crucial genes involved in recruiting inflammatory cells and thrombus formation through chemotaxis and coagulation pathways. In addition, CCM endothelium was associated with increased expression of Nlrp3 and Il1b. Pharmacological inhibition of NLRP3 (NOD [nucleotide-binding oligomerization domain]-' LRR [leucine-rich repeat]- and pyrin domain-containing protein 3) significantly decreased inflammasome activity as assessed by quantification of a fluorescent indicator of caspase-1 activity (FAM-FLICA [carboxyfluorescein-fluorochrome-labeled inhibitors of caspases] caspase-1) in brain endothelial cells from Pdcd10BECKO in chronic stage. Importantly, our results support the hypothesis of the crosstalk between astrocytes and CCM endothelium that can trigger recruitment of inflammatory cells arising from brain parenchyma (microglia) and the peripheral immune system (leukocytes) into mature active CCM lesions that propagate lesion growth, immunothrombosis, and bleedings. Unexpectedly, partial or total loss of brain endothelial NF-κB (nuclear factor κB) activity (using Ikkbfl/fl mice) in chronic Pdcd10BECKO mice does not prevent lesion genesis or neuroinflammation. Instead, this resulted in a trend increase in the number of lesions and immunothrombosis, suggesting that therapeutic approaches designed to target inflammation through endothelial NF-κB inhibition may contribute to detrimental side effects. CONCLUSIONS: Our study reveals previously unknown links between neuroinflammatory astrocytes and inflamed CCM endothelium as contributors that trigger leukocyte recruitment and precipitate immunothrombosis in CCM lesions. However, therapeutic approaches targeting brain endothelial NF-κB activity may contribute to detrimental side effects.

Astrocytes propel neurovascular dysfunction during cerebral cavernous malformation lesion formation.

Cerebral cavernous malformations (CCMs) are common neurovascular lesions caused by loss-of-function mutations in 1 of 3 genes, including KRIT1 (CCM1), CCM2, and PDCD10 (CCM3), and generally regarded as an endothelial cell-autonomous disease. Here we reported that proliferative astrocytes played a critical role in CCM pathogenesis by serving as a major source of VEGF during CCM lesion formation. An increase in astrocyte VEGF synthesis is driven by endothelial nitric oxide (NO) generated as a consequence of KLF2- and KLF4-dependent elevation of eNOS in CCM endothelium. The increased brain endothelial production of NO stabilized HIF-1α in astrocytes, resulting in increased VEGF production and expression of a "hypoxic" program under normoxic conditions. We showed that the upregulation of cyclooxygenase-2 (COX-2), a direct HIF-1α target gene and a known component of the hypoxic program, contributed to the development of CCM lesions because the administration of a COX-2 inhibitor significantly prevented the progression of CCM lesions. Thus, non-cell-autonomous crosstalk between CCM endothelium and astrocytes propels vascular lesion development, and components of the hypoxic program represent potential therapeutic targets for CCMs.

Inhibition of the HEG1-KRIT1 interaction increases KLF4 and KLF2 expression in endothelial cells.

The transmembrane protein heart of glass1 (HEG1) directly binds to and recruits Krev interaction trapped protein 1 (KRIT1) to endothelial junctions to form the HEG1-KRIT1 protein complex that establishes and maintains junctional integrity. Genetic inactivation or knockdown of endothelial HEG1 or KRIT1 leads to the upregulation of transcription factors Krüppel-like factors 4 and 2 (KLF4 and KLF2), which are implicated in endothelial vascular homeostasis; however, the effect of acute inhibition of the HEG1-KRIT1 interaction remains incompletely understood. Here, we report a high-throughput screening assay and molecular design of a small-molecule HEG1-KRIT1 inhibitor to uncover acute changes in signaling pathways downstream of the HEG1-KRIT1 protein complex disruption. The small-molecule HEG1-KRIT1 inhibitor 2 (HKi2) was demonstrated to be a bona fide inhibitor of the interaction between HEG1 and KRIT1 proteins, by competing orthosterically with HEG1 through covalent reversible interactions with the FERM (4.1, ezrin, radixin, and moesin) domain of KRIT1. The crystal structure of HKi2 bound to KRIT1 FERM revealed that it occupies the same binding pocket on KRIT1 as the HEG1 cytoplasmic tail. In human endothelial cells (ECs), acute inhibition of the HEG1-KRIT1 interaction by HKi2 increased KLF4 and KLF2 mRNA and protein levels, whereas a structurally similar inactive compound failed to do so. In zebrafish, HKi2 induced expression of klf2a in arterial and venous endothelium. Furthermore, genome-wide RNA transcriptome analysis of HKi2-treated ECs under static conditions revealed that, in addition to elevating KLF4 and KLF2 expression, inhibition of the HEG1-KRIT1 interaction mimics many of the transcriptional effects of laminar blood flow. Furthermore, HKi2-treated ECs also triggered Akt signaling in a phosphoinositide 3-kinase (PI3K)-dependent manner, as blocking PI3K activity blunted the Akt phosphorylation induced by HKi2. Finally, using an in vitro colocalization assay, we show that HKi6, an improved derivative of HKi2 with higher affinity for KRIT1, significantly impedes recruitment of KRIT1 to mitochondria-localized HEG1 in CHO cells, indicating a direct inhibition of the HEG1-KRIT1 interaction. Thus, our results demonstrate that early events of the acute inhibition of HEG1-KRIT1 interaction with HKi small-molecule inhibitors lead to: (i) elevated KLF4 and KLF2 gene expression; and (ii) increased Akt phosphorylation. Thus, HKi's provide new pharmacologic tools to study acute inhibition of the HEG1-KRIT1 protein complex and may provide insights to dissect early signaling events that regulate vascular homeostasis.

Distinct integrin activation pathways for effector and regulatory T cell trafficking and function.

Integrin activation mediates lymphocyte trafficking and immune functions. Conventional T cell (Tconv cell) integrin activation requires Rap1-interacting adaptor molecule (RIAM). Here, we report that Apbb1ip-/- (RIAM-null) mice are protected from spontaneous colitis due to IL-10 deficiency, a model of inflammatory bowel disease (IBD). Protection is ascribable to reduced accumulation and homing of Tconv cells in gut-associated lymphoid tissue (GALT). Surprisingly, there are abundant RIAM-null regulatory T cells (T reg cells) in the GALT. RIAM-null T reg cells exhibit normal homing to GALT and lymph nodes due to preserved activation of integrins αLß2, α4ß1, and α4ß7. Similar to Tconv cells, T reg cell integrin activation and immune function require Rap1; however, lamellipodin (Raph1), a RIAM paralogue, compensates for RIAM deficiency. Thus, in contrast to Tconv cells, RIAM is dispensable for T reg cell integrin activation and suppressive function. In consequence, inhibition of RIAM can inhibit spontaneous Tconv cell-mediated autoimmune colitis while preserving T reg cell trafficking and function.

Isolation and Purification of Mouse Brain Endothelial Cells to Study Cerebral Cavernous Malformation Disease.

We describe a method to purify primary brain microvascular endothelial cells (BMEC) from mice bearing floxed alleles of Krit1 (Krit1fl/fl) or Pdcd10 (Pdcd10fl/fl) and an endothelial-specific tamoxifen-regulated Cre recombinase (Pdgfb-iCreERT2), and used these to delete Krit1 or Pdcd10 genes in a time-controlled manner. These BMEC culture models contain a high degree of purity and have been used to identify the major molecular processes involved in loss of Krit1/Pdcd10-induced altered brain endothelial phenotype and function. In addition, these in vitro models of cerebral cavernous malformations (CCMs) enable molecular, biochemical, and pharmacological studies that have contributed significantly to understand the pathogenesis of CCMs. The findings using this in vitro CCMs model have been validated in mouse CCM models and observed in human CCMs. In this chapter, we summarize procedures for isolation and purification of BMEC from transgenic mice, as well as our experience to genetically inactivate CCM genes in the brain endothelium.

Talin-1 is the principal platelet Rap1 effector of integrin activation.

Ras-related protein 1 (Rap1) is a major convergence point of the platelet-signaling pathways that result in talin-1 binding to the integrin ß cytoplasmic domain and consequent integrin activation, platelet aggregation, and effective hemostasis. The nature of the connection between Rap1 and talin-1 in integrin activation is an important remaining gap in our understanding of this process. Previous work identified a low-affinity Rap1-binding site in the talin-1 F0 domain that makes a small contribution to integrin activation in platelets. We recently identified an additional Rap1-binding site in the talin-1 F1 domain that makes a greater contribution than F0 in model systems. Here we generated mice bearing point mutations, which block Rap1 binding without affecting talin-1 expression, in either the talin-1 F1 domain (R118E) alone, which were viable, or in both the F0 and F1 domains (R35E,R118E), which were embryonic lethal. Loss of the Rap1-talin-1 F1 interaction in platelets markedly decreases talin-1-mediated activation of platelet ß1- and ß3-integrins. Integrin activation and platelet aggregation in mice whose platelets express only talin-1(R35E, R118E) are even more impaired, resembling the defect seen in platelets lacking both Rap1a and Rap1b. Although Rap1 is important in thrombopoiesis, platelet secretion, and surface exposure of phosphatidylserine, loss of the Rap1-talin-1 interaction in talin-1(R35E, R118E) platelets had little effect on these processes. These findings show that talin-1 is the principal direct effector of Rap1 GTPases that regulates platelet integrin activation in hemostasis.

ß7 Integrin Inhibition Can Increase Intestinal Inflammation by Impairing Homing of CD25hiFoxP3+ Regulatory T Cells.

BACKGROUND & AIMS: Integrin α4ß7 mediates lymphocyte trafficking to the gut and gut-associated lymphoid tissues, a process critical for recruitment of effector lymphocytes from the circulation to the gut mucosa in inflammatory bowel disease (IBD) and murine models of intestinal inflammation. Antibody blockade of ß7 integrins generally is efficacious in IBD; however, some patients fail to respond, and a few patients can experience exacerbations. This study examined the effects of loss of ß7 integrin function in murine models of IBD. METHODS: In a mouse IBD model caused by lack of interleukin 10, a cytokine important in CD25hiFoxP3+ regulatory T cell (Treg) function, genetic deletion of ß7 integrin or antibody blockade of α4ß7-mucosal addressin cell adhesion molecule-1 interaction paradoxically exacerbated colitis. RESULTS: Loss of ß7 impaired the capacity of Tregs homing to the gut and therefore suppress intestinal inflammation in an adoptive T-cell transfer model; however, the intrinsic suppressive function of ß7-deficient Tregs remained intact, indicating that the ß7 deficiency selectively impacts gut homing. Deletion of ß7 integrin did not worsen colitis in an acute dextran sodium sulfate model in which Treg number and function were normal. CONCLUSIONS: In Integrin subunit beta (Itgb)7-/-Il10-/- mice, loss of ß7-dependent Treg homing to gut-associated lymphoid tissues combined with loss of intrinsic Treg function exacerbated intestinal inflammation. These results suggest that IBD patients with reduced CD25hiFoxP3+ Treg numbers or function or lack of interleukin 10 could be at risk for failure of α4ß7 blocking therapy.

Rap1 binding and a lipid-dependent helix in talin F1 domain promote integrin activation in tandem.

Rap1 GTPases bind effectors, such as RIAM, to enable talin1 to induce integrin activation. In addition, Rap1 binds directly to the talin1 F0 domain (F0); however, this interaction makes a limited contribution to integrin activation in CHO cells or platelets. Here, we show that talin1 F1 domain (F1) contains a previously undetected Rap1-binding site of similar affinity to that in F0. A structure-guided point mutant (R118E) in F1, which blocks Rap1 binding, abolishes the capacity of Rap1 to potentiate talin1-induced integrin activation. The capacity of F1 to mediate Rap1-dependent integrin activation depends on a unique loop in F1 that has a propensity to form a helix upon binding to membrane lipids. Basic membrane-facing residues of this helix are critical, as charge-reversal mutations led to dramatic suppression of talin1-dependent activation. Thus, a novel Rap1-binding site and a transient lipid-dependent helix in F1 work in tandem to enable a direct Rap1-talin1 interaction to cause integrin activation.

Cerebral cavernous malformations form an anticoagulant vascular domain in humans and mice.

Cerebral cavernous malformations (CCMs) are common brain vascular dysplasias that are prone to acute and chronic hemorrhage with significant clinical sequelae. The pathogenesis of recurrent bleeding in CCM is incompletely understood. Here, we show that central nervous system hemorrhage in CCMs is associated with locally elevated expression of the anticoagulant endothelial receptors thrombomodulin (TM) and endothelial protein C receptor (EPCR). TM levels are increased in human CCM lesions, as well as in the plasma of patients with CCMs. In mice, endothelial-specific genetic inactivation of Krit1 (Krit1 ECKO ) or Pdcd10 (Pdcd10 ECKO ), which cause CCM formation, results in increased levels of vascular TM and EPCR, as well as in enhanced generation of activated protein C (APC) on endothelial cells. Increased TM expression is due to upregulation of transcription factors KLF2 and KLF4 consequent to the loss of KRIT1 or PDCD10. Increased TM expression contributes to CCM hemorrhage, because genetic inactivation of 1 or 2 copies of the Thbd gene decreases brain hemorrhage in Pdcd10 ECKO mice. Moreover, administration of blocking antibodies against TM and EPCR significantly reduced CCM hemorrhage in Pdcd10 ECKO mice. Thus, a local increase in the endothelial cofactors that generate anticoagulant APC can contribute to bleeding in CCMs, and plasma soluble TM may represent a biomarker for hemorrhagic risk in CCMs.

Phenotypic characterization of murine models of cerebral cavernous malformations.

Cerebral cavernous malformations (CCMs) are clusters of dilated capillaries that affect around 0.5% of the population. CCMs exist in two forms, sporadic and familial. Mutations in three documented genes, KRIT1(CCM1), CCM2, and PDCD10(CCM3), cause the autosomal dominant form of the disease, and somatic mutations in these same genes underlie lesion development in the brain. Murine models with constitutive or induced loss of respective genes have been applied to study disease pathobiology and therapeutic manipulations. We aimed to analyze the phenotypic characteristic of two main groups of models, the chronic heterozygous models with sensitizers promoting genetic instability, and the acute neonatal induced homozygous knockout model. Acute model mice harbored a higher lesion burden than chronic models, more localized in the hindbrain, and largely lacking iron deposition and inflammatory cell infiltrate. The chronic model mice showed a lower lesion burden localized throughout the brain, with significantly greater perilesional iron deposition, immune B- and T-cell infiltration, and less frequent junctional protein immunopositive endothelial cells. Lesional endothelial cells in both models expressed similar phosphorylated myosin light chain immunopositivity indicating Rho-associated protein kinase activity. These data suggest that acute models are better suited to study the initial formation of the lesion, while the chronic models better reflect lesion maturation, hemorrhage, and inflammatory response, relevant pathobiologic features of the human disease.

Thrombospondin1 (TSP1) replacement prevents cerebral cavernous malformations.

KRIT1 mutations are the most common cause of cerebral cavernous malformation (CCM). Acute Krit1 gene inactivation in mouse brain microvascular endothelial cells (BMECs) changes expression of multiple genes involved in vascular development. These changes include suppression of Thbs1, which encodes thrombospondin1 (TSP1) and has been ascribed to KLF2- and KLF4-mediated repression of Thbs1 In vitro reconstitution of TSP1 with either full-length TSP1 or 3TSR, an anti-angiogenic TSP1 fragment, suppresses heightened vascular endothelial growth factor signaling and preserves BMEC tight junctions. Furthermore, administration of 3TSR prevents the development of lesions in a mouse model of CCM1 (Krit1ECKO ) as judged by histology and quantitative micro-computed tomography. Conversely, reduced TSP1 expression contributes to the pathogenesis of CCM, because inactivation of one or two copies of Thbs1 exacerbated CCM formation. Thus, loss of Krit1 function disables an angiogenic checkpoint to enable CCM formation. These results suggest that 3TSR, or other angiogenesis inhibitors, can be repurposed for TSP1 replacement therapy for CCMs.

MicroRNA-155 contributes to shear-resistant leukocyte adhesion to human brain endothelium in vitro.

BACKGROUND: Increased leukocyte adhesion to brain endothelial cells forming the blood-brain barrier (BBB) precedes extravasation into the central nervous system (CNS) in neuroinflammatory diseases such as multiple sclerosis (MS). Previously, we reported that microRNA-155 (miR-155) is up-regulated in MS and by inflammatory cytokines in human brain endothelium, with consequent modulation of endothelial paracellular permeability. Here, we investigated the role of endothelial miR-155 in leukocyte adhesion to the human cerebral microvascular endothelial cell line, hCMEC/D3, under shear forces mimicking blood flow in vivo. RESULTS: Using a gain- and loss-of-function approach, we show that miR-155 up-regulation increases leukocyte firm adhesion of both monocyte and T cells to hCMEC/D3 cells. Inhibition of endogenous endothelial miR-155 reduced monocytic and T cell firm adhesion to naïve and cytokines-induced human brain endothelium. Furthermore, this effect is partially associated with modulation of the endothelial cell adhesion molecules VCAM1 and ICAM1 by miR-155. CONCLUSIONS: Our results suggest that endothelial miR-155 contribute to the regulation of leukocyte adhesion at the inflamed BBB. Taken together with previous observations, brain endothelial miR-155 may constitute a potential molecular target for treatment of neuroinflammation diseases.

Regulation of brain endothelial barrier function by microRNAs in health and neuroinflammation.

Brain endothelial cells constitute the major cellular element of the highly specialized blood-brain barrier (BBB) and thereby contribute to CNS homeostasis by restricting entry of circulating leukocytes and blood-borne molecules into the CNS. Therefore, compromised function of brain endothelial cells has serious consequences for BBB integrity. This has been associated with early events in the pathogenesis of several disorders that affect the CNS, such as multiple sclerosis, HIV-associated neurologic disorder, and stroke. Recent studies demonstrate that brain endothelial microRNAs play critical roles in the regulation of BBB function under normal and neuroinflammatory conditions. This review will focus on emerging evidence that indicates that brain endothelial microRNAs regulate barrier function and orchestrate various phases of the neuroinflammatory response, including endothelial activation in response to cytokines as well as restoration of inflamed endothelium into a quiescent state. In particular, we discuss novel microRNA regulatory mechanisms and their contribution to cellular interactions at the neurovascular unit that influence the overall function of the BBB in health and during neuroinflammation.-Lopez-Ramirez, M. A., Reijerkerk, A., de Vries, H. E., Romero, I. A. Regulation of brain endothelial barrier function by microRNAs in health and neuroinflammation.

A Dicer-miR-107 Interaction Regulates Biogenesis of Specific miRNAs Crucial for Neurogenesis.

Dicer controls the biogenesis of microRNAs (miRNAs) and is essential for neurogenesis. Recent reports show that the levels and substrate selectivity of DICER result in the preferential biogenesis of specific miRNAs in vitro. However, how dicer expression levels and miRNA biogenesis are regulated in vivo and how this affects neurogenesis is incompletely understood. Here we show that during zebrafish hindbrain development dicer expression levels are controlled by miR-107 to tune the biogenesis of specific miRNAs, such as miR-9, whose levels regulate neurogenesis. Loss of miR-107 function stabilizes dicer levels and miR-9 biogenesis across the ventricular hindbrain zone, resulting in an increase of both proliferating progenitors and postmitotic neurons. miR-9 ectopic accumulation in differentiating neuronal cells recapitulated the excessive neurogenesis phenotype. We propose that miR-107 modulation of dicer levels in differentiating neuronal cells is required to maintain the homeostatic levels of specific miRNAs, whose precise accumulation is essential for neurogenesis.

MicroRNA-155 negatively affects blood-brain barrier function during neuroinflammation.

Blood-brain barrier (BBB) dysfunction is a hallmark of neurological conditions such as multiple sclerosis (MS) and stroke. However, the molecular mechanisms underlying neurovascular dysfunction during BBB breakdown remain elusive. MicroRNAs (miRNAs) have recently emerged as key regulators of pathogenic responses, although their role in central nervous system (CNS) microvascular disorders is largely unknown. We have identified miR-155 as a critical miRNA in neuroinflammation at the BBB. miR-155 is expressed at the neurovascular unit of individuals with MS and of mice with experimental autoimmune encephalomyelitis (EAE). In mice, loss of miR-155 reduced CNS extravasation of systemic tracers, both in EAE and in an acute systemic inflammation model induced by lipopolysaccharide. In cultured human brain endothelium, miR-155 was strongly and rapidly upregulated by inflammatory cytokines. miR-155 up-regulation mimicked cytokine-induced alterations in junctional organization and permeability, whereas inhibition of endogenous miR-155 partially prevented a cytokine-induced increase in permeability. Furthermore, miR-155 modulated brain endothelial barrier function by targeting not only cell-cell complex molecules such as annexin-2 and claudin-1, but also focal adhesion components such as DOCK-1 and syntenin-1. We propose that brain endothelial miR-155 is a negative regulator of BBB function that may constitute a novel therapeutic target for CNS neuroinflammatory disorders.

Role of miRNAs and epigenetics in neural stem cell fate determination.

The regulation of gene expression that determines stem cell fate determination is tightly controlled by both epigenetic and posttranscriptional mechanisms. Indeed, small non-coding RNAs such as microRNAs (miRNAs) are able to regulate neural stem cell fate by targeting chromatin-remodeling pathways. Here, we aim to summarize the latest findings regarding the feedback network of epigenetics and miRNAs during embryonic and adult neurogenesis.

Cytokine-induced changes in the gene expression profile of a human cerebral microvascular endothelial cell-line, hCMEC/D3.

BACKGROUND: The human cerebral microvascular endothelial cell line, hCMEC/D3, has been used extensively to model the blood-brain barrier (BBB) in vitro. Recently, we reported that cytokine-treatment induced loss of brain endothelial barrier properties. In this study, we further determined the gene expression pattern of hCMEC/D3 cells in response to activation with TNFα and IFNγ. FINDINGS: Using a microarray approach, we observed that expression of genes involved in the control of barrier permeability, including inter-brain endothelial junctions (e.g. claudin-5, MARVELD-2), integrin-focal adhesions complexes (e.g. integrin ß1, ELMO-1) and transporter systems (e.g. ABCB1, SLC2A1), are altered by pro-inflammatory cytokines. CONCLUSIONS: Our study shows that previously-described cytokine-induced changes in the pattern of gene expression of endothelium are reproduced in hCMEC/D3 cells, suggesting that this model is suitable to study inflammation at the BBB, while at the same time it has provided insights into novel key molecular processes that are altered in brain endothelium during neuroinflammation, such as modulation of cell-to-matrix contacts.