SORLA mediates endocytic uptake of proIAPP and protects against islet amyloid deposition.

OBJECTIVE: Sorting-related receptor with type A repeats (SORLA) is a neuronal sorting receptor that prevents accumulation of amyloid-beta peptides, the main constituent of senile plaques in Alzheimer disease. Recent transcriptomic studies show that SORLA transcripts are also found in beta cells of pancreatic islets, yet the role of SORLA in islets is unknown. Based on its protective role in reducing the amyloid burden in the brain, we hypothesized that SORLA has a similar function in the pancreas via regulation of amyloid formation from islet amyloid polypeptide (IAPP). METHODS: We generated human IAPP transgenic mice lacking SORLA (hIAPP:SORLA KO) to assess the consequences of receptor deficiency for islet histopathology and function in vivo. Using both primary islet cells and cell lines, we further investigated the molecular mechanisms whereby SORLA controls the cellular metabolism and accumulation of IAPP. RESULTS: Loss of SORLA activity in hIAPP:SORLA KO resulted in a significant increase in islet amyloid deposits and associated islet cell death compared to hIAPP:SORLA WT animals. Aggravated islet amyloid deposition was observed in mice fed a normal chow diet, not requiring high-fat diet feeding typically needed to induce islet amyloidosis in mouse models. In vitro studies showed that SORLA binds to and mediates the endocytic uptake of proIAPP, but not mature IAPP, delivering the propeptide to an endolysosomal fate. CONCLUSIONS: SORLA functions as a proIAPP-specific clearance receptor, protecting against islet amyloid deposition and associated cell death caused by IAPP.

Whole exome sequencing analysis in a couple with three children who died prematurely due to carnitine-acylcarnitine translocase deficiency.

OBJECTIVE: We investigated a strategy of exome sequencing DNA from the unaffected parents and applied a set of filtering criteria to identify genes where both partners are heterozygous for a potentially pathogenic variant. CASE REPORT: We report a non-consanguineous couple who had three daughters, all spontaneous preterm birth at 36 weeks gestation and died in the first period after birth, suspected inborn errors of metabolism. Two days after birth, the first daughter presented with difficulty breathing, cyanosis and died; the second died at 33 days old; the third daughter was isolated under special care and was taken to the mother's room, developed the same symptoms and died after 5 days. Dried blood spot testing screen of 55 congenital metabolic disorders was negative. CONCLUSION: Heterogenous variant in SLC25A20 gene was found in both parents, contributing to the delineations of the neonatal phenotypes related to SLC25A20 mutation in CACTD.

Hypoketotic hypoglycemia without neuromuscular complications in patients with SLC25A32 deficiency.

Mitochondrial flavin adenine dinucleotide (FAD) transporter deficiencies are new entities recently reported to cause a neuro-myopathic phenotype. We report three patients from two unrelated families who presented primarily with hypoketotic hypoglycemia. They all had acylcarnitine profiles suggestive of multiple acyl-CoA dehydrogenase deficiency (MADD) with negative next-generation sequencing of electron-transfer flavoprotein genes (ETFA, ETFB, and ETFDH). Whole exome sequencing revealed a homozygous c.272 G > T (p.Gly91Val) variant in exon 2 of the SLC25A32 gene. The three patients shared the same variant, and they all demonstrated similar clinical and biochemical improvement with riboflavin supplementation. To date, these are the first patients to be reported with hypoketotic hypoglycemia without the neuromuscular phenotype previously reported in patients with SLC25A32 deficiency.

NPC1L1 Deficiency Suppresses Ileal Fibroblast Growth Factor 15 Expression and Increases Bile Acid Pool Size in High-Fat-Diet-Fed Mice.

Niemann-Pick C1-like 1 (NPC1L1) mediates intestinal uptake of dietary and biliary cholesterol and is the target of ezetimibe, a cholesterol absorption inhibitor used to treat hypercholesterolemia. Genetic deletion of NPC1L1 or ezetimibe treatment protects mice from high-fat diet (HFD)-induced obesity; however, the molecular mechanisms responsible for this therapeutic benefit remain unknown. A major metabolic fate of cholesterol is its conversion to bile acids. We found that NPC1L1 knockout (L1-KO) mice fed an HFD had increased energy expenditure, bile acid pool size, and fecal bile acid excretion rates. The elevated bile acid pool in the HFD-fed L1-KO mice was enriched with tauro-ß-muricholic acid. These changes in the L1-KO mice were associated with reduced ileal mRNA expression of fibroblast growth factor 15 (FGF15) and increased hepatic mRNA expression of cholesterol 7α-hydroxylase (Cyp7A1) and mitochondrial sterol 27-hydroxylase (Cyp27A1). In addition, mRNA expression of the membrane bile acid receptor Takeda G protein-coupled receptor 5 (TGR5) and type 2 iodothyronine deiodinase (Dio2) were elevated in brown adipose tissue of L1-KO mice, which is known to promote energy expenditure. Thus, altered bile acid homeostasis and signaling may play a role in protecting L1-KO mice against HFD-induced obesity.

Optineurin modulates the maturation of dendritic cells to regulate autoimmunity through JAK2-STAT3 signaling.

Optineurin (OPTN) has important functions in diverse biological processes and diseases, but its effect on dendritic cell (DC) differentiation and functionality remains elusive. Here we show that OPTN is upregulated in human and mouse DC maturation, and that deletion of Optn in mice via CD11c-Cre attenuates DC maturation and impairs the priming of CD4+ T cells, thus ameliorating autoimmune symptoms such as experimental autoimmune encephalomyelitis (EAE). Mechanistically, OPTN binds to the JH1 domain of JAK2 and inhibits JAK2 dimerization and phosphorylation, thereby preventing JAK2-STAT3 interaction and inhibiting STAT3 phosphorylation to suppress downstream transcription of IL-10. Without such a negative regulation, Optn-deficient DCs eventually induce an IL-10/JAK2/STAT3/IL-10 positive feedback loop to suppress DC maturation. Finally, the natural product, Saikosaponin D, is identified as an OPTN inhibitor, effectively inhibiting the immune-stimulatory function of DCs and the disease progression of EAE in mice. Our findings thus highlight a pivotal function of OPTN for the regulation of DC functions and autoimmune disorders.

Creatine transporter deficiency impairs stress adaptation and brain energetics homeostasis.

The creatine transporter (CrT) maintains brain creatine (Cr) levels, but the effects of its deficiency on energetics adaptation under stress remain unclear. There are also no effective treatments for CrT deficiency, the second most common cause of X-linked intellectual disabilities. Herein, we examined the consequences of CrT deficiency in brain energetics and stress-adaptation responses plus the effects of intranasal Cr supplementation. We found that CrT-deficient (CrT-/y) mice harbored dendritic spine and synaptic dysgenesis. Nurtured newborn CrT-/y mice maintained baseline brain ATP levels, with a trend toward signaling imbalance between the p-AMPK/autophagy and mTOR pathways. Starvation elevated the signaling imbalance and reduced brain ATP levels in P3 CrT-/y mice. Similarly, CrT-/y neurons and P10 CrT-/y mice showed an imbalance between autophagy and mTOR signaling pathways and greater susceptibility to cerebral hypoxia-ischemia and ischemic insults. Notably, intranasal administration of Cr after cerebral ischemia increased the brain Cr/N-acetylaspartate ratio, partially averted the signaling imbalance, and reduced infarct size more potently than intraperitoneal Cr injection. These findings suggest important functions for CrT and Cr in preserving the homeostasis of brain energetics in stress conditions. Moreover, intranasal Cr supplementation may be an effective treatment for congenital CrT deficiency and acute brain injury.

SORL1 deficiency in human excitatory neurons causes APP-dependent defects in the endolysosome-autophagy network.

Dysfunction of the endolysosomal-autophagy network is emerging as an important pathogenic process in Alzheimer's disease. Mutations in the sorting receptor-encoding gene SORL1 cause autosomal-dominant Alzheimer's disease, and SORL1 variants increase risk for late-onset AD. To understand the contribution of SORL1 mutations to AD pathogenesis, we analyze the effects of a SORL1 truncating mutation on SORL1 protein levels and endolysosome function in human neurons. We find that truncating mutation results in SORL1 haploinsufficiency and enlarged endosomes in human neurons. Analysis of isogenic SORL1 wild-type, heterozygous, and homozygous null neurons demonstrates that, whereas SORL1 haploinsufficiency results in endosome dysfunction, complete loss of SORL1 leads to additional defects in lysosome function and autophagy. Neuronal endolysosomal dysfunction caused by loss of SORL1 is relieved by extracellular antisense oligonucleotide-mediated reduction of APP protein, demonstrating that PSEN1, APP, and SORL1 act in a common pathway regulating the endolysosome system, which becomes dysfunctional in AD.

PINK1-mediated mitophagy maintains pluripotency through optineurin.

OBJECTIVES: Dysfunction of autophagy results in accumulation of depolarized mitochondria and breakdown of self-renewal and pluripotency in ESCs. However, the regulators that control how mitochondria are degraded by autophagy for pluripotency regulation remains largely unknown. This study aims to dissect the molecular mechanisms that regulate mitochondrial homeostasis for pluripotency regulation in mouse ESCs. MATERIALS AND METHODS: Parkin+/+ and parkin-/- ESCs were established from E3.5 blastocysts of parkin+/- x parkin+/- mating mice. The pink1-/- , optn-/- and ndp52-/- ESCs were generated by CRISPR-Cas9. shRNAs were used for function loss assay of target genes. Mito-Keima, ROS and ATP detection were used to investigate the mitophagy and mitochondrial function. Western blot, Q-PCR, AP staining and teratoma formation assay were performed to evaluate the PSC stemness. RESULTS: PINK1 or OPTN depletion impairs the degradation of dysfunctional mitochondria during reprogramming, and reduces the reprogramming efficiency and quality. In ESCs, PINK1 or OPTN deficiency leads to accumulation of dysfunctional mitochondria and compromised pluripotency. The defective mitochondrial homeostasis and pluripotency in pink1-/- ESCs can be compensated by gain expression of phosphomimetic Ubiquitin (Ub-S65D) together with WT or a constitutively active phosphomimetic OPTN mutant (S187D, S476D, S517D), rather than constitutively inactive OPTN (S187A, S476A, S517A) or a Ub-binding dead OPTN mutant (D477N). CONCLUSIONS: The mitophagy receptor OPTN guards ESC mitochondrial homeostasis and pluripotency by scavenging damaged mitochondria through TBK1-activated OPTN binding of PINK1-phosphorylated Ubiquitin.

Deficiency of optineurin enhances osteoclast differentiation by attenuating the NRF2-mediated antioxidant response.

Abnormally increased resorption contributes to bone degenerative diseases such as Paget's disease of bone (PDB) through unclear mechanisms. Recently, the optineurin (OPTN) gene has been implicated in PDB, and global OPTN knockout mice (Optn-/-) were shown to exhibit increased formation of osteoclasts (osteoclastogenesis). Growing evidence, including our own, has demonstrated that intracellular reactive oxygen species (ROS) stimulated by receptor activator of nuclear factor kappa-B ligand (RANKL) can act as signaling molecules to promote osteoclastogenesis. Here, we report that OPTN interacts with nuclear factor erythroid-derived factor 2-related factor 2 (NRF2), the master regulator of the antioxidant response, defining a pathway through which RANKL-induced ROS could be regulated for osteoclastogenesis. In this study, monocytes from Optn-/- and wild-type (Optn+/+) mice were utilized to differentiate into osteoclasts, and both qRT-PCR and tartrate-resistant acid phosphatase (TRAP) staining showed that the Optn-/- monocytes exhibited enhanced osteoclastogenesis compared to the Optn+/+ cells. CellROX® staining, qRT-PCR, and Western blotting indicated that OPTN deficiency reduced the basal expression of Nrf2, inhibited the expression of NRF2-responsive antioxidants, and increased basal and RANKL-induced intracellular ROS levels, leading to enhanced osteoclastogenesis. Coimmunoprecipitation (co-IP) showed direct interaction, and immunofluorescence staining showed perinuclear colocalization of the OPTN-NRF2 granular structures during differentiation. Finally, curcumin and the other NRF2 activators attenuated the hyperactive osteoclastogenesis induced by OPTN deficiency. Collectively, our findings reveal a novel OPTN-mediated mechanism for regulating the NRF2-mediated antioxidant response in osteoclasts and extend the therapeutic potential of OPTN in the aging process resulting from ROS-triggered oxidative stress, which is associated with PDB and many other degenerative diseases.

Complete Deletion of Slc52a2 Causes Embryonic Lethality in Mice.

Riboflavin (vitamin B2) plays an important role in cellular growth and function. Riboflavin transporter 2 (RFVT2) is widely expressed in several tissues, especially in the brain and salivary glands, and plays an important role in the tissue disruption of riboflavin. During the last 10 years, mutations in SLC52A2 have been documented in patients with a rare neurological disorder known as Brown-Vialetto-Van Laere syndrome. However, no suitable animal model of this disease has been reported. Here, we aimed to clarify the physiological role of RFVT2 using Slc52a2-mutant mice. The appearance, body weight, and plasma riboflavin concentration of Slc52a2 heterozygous mutant (Slc52a2+/-) mice were similar to those of wild-type (WT) mice. However, intercrossing between Slc52a2+/- mice failed to generate Slc52a2 homozygous mutant (Slc52a2-/-) mice. This suggested that Slc52a2 gene deficiency results in early embryonic lethality. Our findings suggested that RFVT2 is essential for growth and development, and its deletion may influence embryonic survival.

Optineurin deletion disrupts metabotropic glutamate receptor 5-mediated regulation of ERK1/2, GSK3ß/ZBTB16, mTOR/ULK1 signaling in autophagy.

Optineurin (OPTN) is a multifunctional protein that mediates a network of cellular processes regulating membrane trafficking, inflammatory responses and autophagy. The OPTN-rich interactome includes Group I metabotropic glutamate receptors (mGluR1 and 5), members of the Gαq/11 protein receptor family. Recent evidence has shown that mGluR5, in addition to its canonical Gαq/11 protein-coupled signaling, regulates autophagic machinery via mTOR/ULK1 and GSK3ß/ZBTB16 pathways in both Alzheimer's and Huntington's disease mouse models. Despite its potential involvement, the role of OPTN in mediating mGluR5 downstream signaling cascades remains largely unknown. Here, we employed a CRISPR/Cas9 OPTN-deficient STHdhQ7/Q7 striatal cell line and global OPTN knockout mice to investigate whether Optn gene deletion alters both mGluR5 canonical and noncanonical signaling. We find that OPTN is required for mGluR5-activated Ca2+ flux and ERK1/2 signaling following receptor activation in STHdhQ7/Q7 cells and acute hippocampal slices. Deletion of OPTN impairs both GSK3ß/ZBTB16 and mTOR/ULK1 autophagic signaling in STHdhQ7/Q7 cells. Furthermore, mGluR5-dependent regulation of GSK3ß/ZBTB16 and mTOR/ULK1 autophagic signaling is impaired in hippocampal slices of OPTN knockout mice. Overall, we show that the crosstalk between OPTN and mGluR5 can have major implication on receptor signaling and therefore potentially contribute to the pathophysiology of neurodegenerative diseases.

The peptide symporter SLC15a4 is essential for the development of systemic lupus erythematosus in murine models.

Systemic Lupus Erythematosus (SLE) is a chronic autoimmune disease representing a serious unmet medical need. The disease is associated with the loss of self-tolerance and exaggerated B cell activation, resulting in autoantibody production and the formation of immune complexes that accumulate in the kidney, causing glomerulonephritis. TLR7, an important mediator of the innate immune response, drives the expression of type-1 interferon (IFN), which leads to expression of type-1 IFN induced genes and aggravates lupus pathology. Because the lysosomal peptide symporter slc15a4 is critically required for type-1 interferon production by pDC, and for certain B cell functions in response to TLR7 and TLR9 signals, we considered it as a potential target for pharmacological intervention in SLE. We deleted the slc15a4 gene in C57BL/6, NZB, and NZW mice and found that pristane-challenged slc15a4-/- mice in the C57BL/6 background and lupus prone slc15a4-/- NZB/W F1 mice were both completely protected from lupus like disease. In the NZB/W F1 model, protection persisted even when disease development was accelerated with an adenovirus encoding IFNα, emphasizing a broad role of slc15a4 in disease initiation. Our results establish a non-redundant function of slc15a4 in regulating both innate and adaptive components of the immune response in SLE pathobiology and suggest that it may be an attractive drug target.

Purinergic signaling is enhanced in the absence of UT-A1 and UT-A3.

ATP is an important paracrine regulator of renal tubular water and urea transport. The activity of P2Y2 , the predominant P2Y receptor of the medullary collecting duct, is mediated by ATP, and modulates urinary concentration. To investigate the role of purinergic signaling in the absence of urea transport in the collecting duct, we studied wild-type (WT) and UT-A1/A3 null (UT-A1/A3 KO) mice in metabolic cages to monitor urine output, and collected tissue samples for analysis. We confirmed that UT-A1/A3 KO mice are polyuric, and concurrently observed lower levels of urinary cAMP as compared to WT, despite elevated serum vasopressin (AVP) levels. Because P2Y2 inhibits AVP-stimulated transport by dampening cAMP synthesis, we suspected that, similar to other models of AVP-resistant polyuria, purinergic signaling is increased in UT-A1/A3 KO mice. In fact, we observed that both urinary ATP and purinergic-mediated prostanoid (PGE2 ) levels were elevated. Collectively, our data suggest that the reduction of medullary osmolality due to the lack of UT-A1 and UT-A3 induces an AVP-resistant polyuria that is possibly exacerbated by, or at least correlated with, enhanced purinergic signaling.

Deficient Endoplasmic Reticulum Acetyl-CoA Import in Pancreatic Acinar Cells Leads to Chronic Pancreatitis.

BACKGROUND & AIMS: Maintaining endoplasmic reticulum (ER) proteostasis is essential for pancreatic acinar cell function. Under conditions of severe ER stress, activation of pathogenic unfolded protein response pathways plays a central role in the development and progression of pancreatitis. Less is known, however, of the consequence of perturbing ER-associated post-translational protein modifications on pancreatic outcomes. Here, we examined the role of the ER acetyl-CoA transporter AT-1 on pancreatic homeostasis. METHODS: We used an AT-1S113R/+ hypomorphic mouse model, and generated an inducible, acinar-specific, AT-1 knockout mouse model, and performed histologic and biochemical analyses to probe the effect of AT-1 loss on acinar cell physiology. RESULTS: We found that AT-1 expression is down-regulated significantly during both acute and chronic pancreatitis. Furthermore, acinar-specific deletion of AT-1 in acinar cells induces chronic ER stress marked by activation of both the spliced x-box binding protein 1 and protein kinase R-like ER kinase pathways, leading to spontaneous mild/moderate chronic pancreatitis evidenced by accumulation of intracellular trypsin, immune cell infiltration, and fibrosis. Induction of acute-on-chronic pancreatitis in the AT-1 model led to acinar cell loss and glad atrophy. CONCLUSIONS: These results indicate a key role for AT-1 in pancreatic acinar cell homeostasis, the unfolded protein response, and that perturbations in AT-1 function leads to pancreatic disease.

Oxidative phosphorylation in creatine transporter deficiency.

X-linked creatine transporter deficiency (CTD) is one of the three types of cerebral creatine deficiency disorders. CTD arises from pathogenic variants in the X-linked gene SLC6A8. We report the first phosphorus (31 P) MRS study of patients with CTD, where both phosphocreatine and total creatine concentrations were found to be markedly reduced. Despite the diminished role of creatine and phosphocreatine in oxidative phosphorylation in CTD, we found no elevation of lactate or lowered pH, indicating that the brain energy supply still largely relied on oxidative metabolism. Our results suggest that mitochondrial function is a potential therapeutic target for CTD.

Optineurin modulates ER stress-induced signaling pathways and cell death.

We have investigated the physiological role of the autophagy receptor Optineurin/Optn in endoplasmic reticulum (ER) stress response using cellular and animal models. In comparison to their normal counterparts, Optn-deficient mouse embryonic fibroblasts showed significantly higher cell death and caspase-3 activation upon treatment with tunicamycin and thapsigargin, inducers of ER stress. The transcript levels of some of the genes regulated by the IRE1-XBP1 and PERK-ATF4 pathways were upregulated in Optn-deficient cells, in comparison with normal cells, upon treatment with tunicamycin, and also in the brain cortex and liver of tunicamycin treated Optn-deficient mice. Also, the basal levels of IRE1α and PERK were higher in Optn-deficient cells. These results suggest that Optn modulates ER stress-induced signaling pathways and provides protection from ER stress-induced cell death.

Effect of riboflavin deficiency on development of the cerebral cortex in Slc52a3 knockout mice.

Riboflavin transporter 3 (RFVT3), encoded by the SLC52A3 gene, is important for riboflavin homeostasis in the small intestine, kidney, and placenta. Our previous study demonstrated that Slc52a3 knockout (Slc52a3-/-) mice exhibited neonatal lethality and metabolic disorder due to riboflavin deficiency. Here, we investigated the influence of Slc52a3 gene disruption on brain development using Slc52a3-/- embryos. Slc52a3-/- mice at postnatal day 0 showed hypoplasia of the brain and reduced thickness of cortical layers. At embryonic day 13.5, the formation of Tuj1+ neurons and Tbr2+ intermediate neural progenitors was significantly decreased; no significant difference was observed in the total number and proliferative rate of Pax6+ radial glia. Importantly, the hypoplastic phenotype was rescued upon riboflavin supplementation. Thus, it can be concluded that RFVT3 contributes to riboflavin homeostasis in embryos and that riboflavin itself is required during embryonic development of the cerebral cortex in mice.

Antioxidant Amelioration of Riboflavin Transporter Deficiency in Motoneurons Derived from Patient-Specific Induced Pluripotent Stem Cells.

Mitochondrial dysfunction is a key element in the pathogenesis of neurodegenerative disorders, such as riboflavin transporter deficiency (RTD). This is a rare, childhood-onset disease characterized by motoneuron degeneration and caused by mutations in SLC52A2 and SLC52A3, encoding riboflavin (RF) transporters (RFVT2 and RFVT3, respectively), resulting in muscle weakness, ponto-bulbar paralysis and sensorineural deafness. Based on previous findings, which document the contribution of oxidative stress in RTD pathogenesis, we tested possible beneficial effects of several antioxidants (Vitamin C, Idebenone, Coenzyme Q10 and EPI-743, either alone or in combination with RF) on the morphology and function of neurons derived from induced pluripotent stem cells (iPSCs) from two RTD patients. To identify possible improvement of the neuronal morphotype, neurite length was measured by confocal microscopy after ß-III tubulin immunofluorescent staining. Neuronal function was evaluated by determining superoxide anion generation by MitoSOX assay and intracellular calcium (Ca2+) levels, using the Fluo-4 probe. Among the antioxidants tested, EPI-743 restored the redox status, improved neurite length and ameliorated intracellular calcium influx into RTD motoneurons. In conclusion, we suggest that antioxidant supplementation may have a role in RTD treatment.

Respiratory pathology in the Optn-/- mouse model of Amyotrophic Lateral Sclerosis.

Amyotrophic Lateral Sclerosis (ALS) is a devastating neurodegenerative disorder that results in death due to respiratory failure. Many genetic defects are associated with ALS; one such defect is a mutation in the gene encoding optineurin (OPTN). Using an optineurin null mouse (Optn-/-), we sought to characterize the impact of optineurin deficiency on respiratory neurodegeneration. Respiratory function was assessed at 6 and 12 mo of age using whole body plethysmography at baseline during normoxia (FiO2: 0.21; N2 balance) and during a respiratory challenge with hypoxia and hypercapnia (FiCO2: 0.07, FiO2: 0.10; N2 balance). Histological analyses to assess motor neuron viability and respiratory nerve integrity were performed in the medulla, cervical spinal cord, hypoglossal nerve, and phrenic nerve. Minute ventilation, peak inspiratory flow, and peak expiratory flow are significantly reduced during a respiratory challenge in 6 mo Optn-/-mice. By 12 mo, tidal volume is also significantly reduced in Optn-/- mice. Furthermore, 12mo Optn-/- mice exhibit hypoglossal motor neuron loss, phrenic and hypoglossal dysmyelination, and accumulated mitochondria in the hypoglossal nerve axons. Overall, these data indicate that Optn-/- mice display neurodegenerative respiratory dysfunction and are a useful model to study the impact of novel therapies on respiratory function for optineurin-deficient ALS patients.

Lack of Flvcr2 impairs brain angiogenesis without affecting the blood-brain barrier.

Fowler syndrome is a rare autosomal recessive brain vascular disorder caused by mutation in FLVCR2 in humans. The disease occurs during a critical period of brain vascular development, is characterized by glomeruloid vasculopathy and hydrocephalus, and is almost invariably prenatally fatal. Here, we sought to gain insights into the process of brain vascularization and the pathogenesis of Fowler syndrome by inactivating Flvcr2 in mice. We showed that Flvcr2 was necessary for angiogenic sprouting in the brain, but surprisingly dispensable for maintaining the blood-brain barrier. Endothelial cells lacking Flvcr2 had altered expression of angiogenic factors, failed to adopt tip cell properties, and displayed reduced sprouting, leading to vascular malformations similar to those seen in humans with Fowler syndrome. Brain hypovascularization was associated with hypoxia and tissue infarction, ultimately causing hydrocephalus and death of mutant animals. Strikingly, despite severe vascular anomalies and brain tissue infarction, the blood-brain barrier was maintained in Flvcr2 mutant mice. Our Fowler syndrome model therefore defined the pathobiology of this disease and provided new insights into brain angiogenesis by showing uncoupling of vessel morphogenesis and blood-brain barrier formation.