Stabilization of the retinal vascular network by reciprocal feedback between blood vessels and astrocytes

Endothelial HIF2α suppresses retinal angiogenesis in neonatal mice by upregulating NOTCH signaling

Prolyl hydroxylase domain (PHD) proteins are oxygen sensors that use intracellular oxygen as a substrate to hydroxylate hypoxia-inducible factor (HIF) α proteins, routing them for polyubiquitylation and proteasomal degradation. Typically, HIFα accumulation in hypoxic or PHD-deficient tissues leads to upregulated angiogenesis. Here, we report unexpected retinal phenotypes associated with endothelial cell (EC)-specific gene targeting of Phd2 (Egln1) and Hif2alpha (Epas1). EC-specific Phd2 disruption suppressed retinal angiogenesis, despite HIFα accumulation and VEGFA upregulation. Suppressed retinal angiogenesis was observed both in development and in the oxygen-induced retinopathy (OIR) model. On the other hand, EC-specific deletion of Hif1alpha (Hif1a), Hif2alpha, or both did not affect retinal vascular morphogenesis. Strikingly, retinal angiogenesis appeared normal in mice double-deficient for endothelial PHD2 and HIF2α. In PHD2-deficient retinal vasculature, delta-like 4 (DLL4, a NOTCH ligand) and HEY2 (a NOTCH target) were upregulated by HIF2α-dependent mechanisms. Inhibition of NOTCH signaling by a chemical inhibitor or DLL4 antibody partially rescued retinal angiogenesis. Taken together, our data demonstrate that HIF2α accumulation in retinal ECs inhibits rather than stimulates retinal angiogenesis, in part by upregulating DLL4 expression and NOTCH signaling.

PFKFB3 in neovascular eye disease: unraveling mechanisms and exploring therapeutic strategies

BACKGROUND

Neovascular eye disease is characterized by pathological neovascularization, with clinical manifestations such as intraocular exudation, bleeding, and scar formation, ultimately leading to blindness in millions of individuals worldwide. Pathologic ocular angiogenesis often occurs in common fundus diseases including proliferative diabetic retinopathy (PDR), age-related macular degeneration (AMD), and retinopathy of prematurity (ROP). Anti-vascular endothelial growth factor (VEGF) targets the core pathology of ocular angiogenesis.

MAIN BODY

In recent years, therapies targeting metabolism to prevent angiogenesis have also rapidly developed, offering assistance to patients with a poor prognosis while receiving anti-VEGF therapy and reducing the side effects associated with long-term VEGF usage. Phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3), a key enzyme in targeted metabolism, has been shown to have great potential, with antiangiogenic effects and multiple protective effects in the treatment of neovascular eye disease. In this review, we summarize the mechanisms of common types of neovascular eye diseases; discuss the protective effect and potential mechanism of targeting PFKFB3, including the related inhibitors of PFKFB3; and look forward to the future exploration directions and therapeutic prospects of PFKFB3 in neovascular eye disease.

CONCLUSION

Neovascular eye disease, the most common and severely debilitating retinal disease, is largely incurable, necessitating the exploration of new treatment methods. PFKFB3 has been shown to possess various potential protective mechanisms in treating neovascular eye disease. With the development of several drugs targeting PFKFB3 and their gradual entry into clinical research, targeting PFKFB3-mediated glycolysis has emerged as a promising therapeutic approach for the future of neovascular eye disease.

Tailless and hypoxia inducible factor-2α cooperate to sustain proangiogenic states of retinal astrocytes in neonatal mice

Tailless (TLX, an orphan nuclear receptor) and hypoxia inducible factor-2α (HIF2α) are both essential for retinal astrocyte and vascular development. Tlx-/- mutation and astrocyte specific Hif2α disruption in Hif2αf/f/GFAPCre mice are known to cause defective astrocyte development and block vascular development in neonatal retinas. Here we report that TLX and HIF2α support retinal angiogenesis by cooperatively maintaining retinal astrocytes in their proangiogenic states. While Tlx+/- and Hif2αf/+/GFAPCre mice are phenotypically normal, Tlx+/-/Hif2αf/+/GFAPCre mice display precocious retinal astrocyte differentiation towards non-angiogenic states, along with significantly reduced retinal angiogenesis. In wild-type mice, TLX and HIF2α coexist in the same protein complex, suggesting a cooperative function under physiological conditions. Furthermore, astrocyte specific disruption of Phd2 (prolyl hydroxylase domain protein 2), a manipulation previously shown to cause HIF2α accumulation, did not rescue retinal angiogenesis in Tlx-/- background, which suggests functional dependence of HIF2α on TLX. Finally, the expression of fibronectin and VEGF-A is significantly reduced in retinal astrocytes of neonatal Tlx+/-/Hif2αf/+/GFAPCre mice. Overall, these data indicate that TLX and HIF2α cooperatively support retinal angiogenesis by maintaining angiogenic potential of retinal astrocytes.

Toward the next generation of vascularized human neural organoids

Thanks to progress in the development of three-dimensional (3D) culture technologies, human central nervous system (CNS) development and diseases have been gradually deciphered by using organoids derived from human embryonic stem cells (hESCs) or human induced pluripotent stem cells (hiPSCs). Selforganized neural organoids (NOs) have been used to mimic morphogenesis and functions of specific organs in vitro. Many NOs have been reproduced in vitro, such as those mimicking the human brain, retina, and spinal cord. However, NOs fail to capitulate to the maturation and complexity of in vivo neural tissues. The persistent issues with current NO cultivation protocols are inadequate oxygen supply and nutrient diffusion due to the absence of vascular networks. In vivo, the developing CNS is interpenetrated by vasculature that not only supplies oxygen and nutrients but also provides a structural template for neuronal growth. To address these deficiencies, recent studies have begun to couple NO culture with bioengineering techniques and methodologies, including genetic engineering, coculture, multidifferentiation, microfluidics and 3D bioprinting, and transplantation, which might promote NO maturation and create more functional NOs. These cutting-edge methods could generate an ever more reliable NO model in vitro for deciphering the codes of human CNS development, disease progression, and translational application. In this review, we will summarize recent technological advances in culture strategies to generate vascularized NOs (vNOs), with a special focus on cerebral- and retinal-organoid models.

More than meets the eye: The role of microglia in healthy and diseased retina

Microglia are the main resident immune cells of the nervous system and as such they are involved in multiple roles ranging from tissue homeostasis to response to insults and circuit refinement. While most knowledge about microglia comes from brain studies, some mechanisms have been confirmed for microglia cells in the retina, the light-sensing compartment of the eye responsible for initial processing of visual information. However, several key pieces of this puzzle are still unaccounted for, as the characterization of retinal microglia has long been hindered by the reduced population size within the retina as well as the previous lack of technologies enabling single-cell analyses. Accumulating evidence indicates that the same cell type may harbor a high degree of transcriptional, morphological and functional differences depending on its location within the central nervous system. Thus, studying the roles and signatures adopted specifically by microglia in the retina has become increasingly important. Here, we review the current understanding of retinal microglia cells in physiology and in disease, with particular emphasis on newly discovered mechanisms and future research directions.

The heterogeneity of astrocytes in glaucoma

Glaucoma is a leading cause of blindness with progressive degeneration of retinal ganglion cells. Aging and increased intraocular pressure (IOP) are major risk factors. Lowering IOP does not always stop the disease progression. Alternative ways of protecting the optic nerve are intensively studied in glaucoma. Astrocytes are macroglia residing in the retina, optic nerve head (ONH), and visual brain, which keep neuronal homeostasis, regulate neuronal activities and are part of the immune responses to the retina and brain insults. In this brief review, we discuss the activation and heterogeneity of astrocytes in the retina, optic nerve head, and visual brain of glaucoma patients and animal models. We also discuss some recent transgenic and gene knockout studies using glaucoma mouse models to clarify the role of astrocytes in the pathogenesis of glaucoma. Astrocytes are heterogeneous and play crucial roles in the pathogenesis of glaucoma, especially in the process of neuroinflammation and mitochondrial dysfunction. In astrocytes, overexpression of Stat3 or knockdown of IκKβ/p65, caspase-8, and mitochondrial uncoupling proteins (Ucp2) can reduce ganglion cell loss in glaucoma mouse models. Based on these studies, therapeutic strategies targeting the heterogeneity of reactive astrocytes by enhancing their beneficial reactivity or suppressing their detrimental reactivity are alternative options for glaucoma treatment in the future.

Zinc improves neurological recovery by promoting angiogenesis via the astrocyte-mediated HIF-1α/VEGF signaling pathway in experimental stroke

BACKGROUND

Ischemic stroke is a serious cerebrovascular disease with high morbidity and disability. Zinc accumulation has been shown to play a vital role in neuronal death and blood-brain barrier damage following ischemia in acute stage. However, almost nothing is known about whether zinc is involved in neurological recovery in ischemic prolonged period. This study investigates whether zinc promotes neurological recovery through astrocytes-induced angiogenesis during ischemic repair phase.

METHODS

Sprague-Dawley rats were subjected to 2 h ischemia/14, 21, and 28 days reperfusion by middle cerebral artery occlusion, then administered ZnCl₂ (10 mg/kg) via intraperitoneally daily from 7 days to tissue collection to observe brain tissue morphology, neurological function recovery by cortical width index, Adhesive removal test, and Forelimb placing test. Angiogenesis, astrocyte activation, and HIF-1α/VEGF pathway were assessed via Western blot, immunofluorescence, and BrdU method in vivo and in vitro.

RESULTS

The results showed that zinc significantly alleviated brain atrophy and improved neurological function recovery during the cerebral ischemia repair stage. Zinc significantly increased the protein levels of HIF-1α, VEGF-A, and VEGF-R2 in astrocytes, and promoted angiogenesis during cerebral ischemia repair. In vitro and in vivo studies confirmed that zinc promoted angiogenesis via the astrocyte-mediated HIF-1α/VEGF signaling pathway.

CONCLUSIONS

Zinc significantly improves neurological function recovery during the cerebral ischemia repair stage, providing new evidence supporting zinc as a potential therapeutic target for ischemic stroke by promoting astrocyte induced angiogenesis.

Myopia Alters the Structural Organization of the Retinal Vasculature, GFAP-Positive Glia, and Ganglion Cell Layer Thickness

To describe the effect of myopic eye growth on the structure and distribution of astrocytes, vasculature, and retinal nerve fiber layer thickness, which are critical for inner retinal tissue homeostasis and survival. Astrocyte and capillary distribution, retinal nerve fiber (RNFL), and ganglion cell layer (GCL) thicknesses were assessed using immunochemistry and spectral domain optical coherence tomography on eleven retinas of juvenile common marmosets (Callithrix Jacchus), six of which were induced with lens-induced myopia (refraction, Rx: -7.01 ± 1.8D). Five untreated age-matched juvenile marmoset retinas were used as controls (Rx: -0.74 ± 0.4D). Untreated marmoset eyes grew normally, their RNFL thickened and their astrocyte numbers were associated with RNFL thickness. Marmosets with induced myopia did not show this trend and, on the contrary, had reduced astrocyte numbers, increased GFAP-immunopositive staining, thinner RNFL, lower peripheral capillary branching, and increased numbers of string vessels. The myopic changes in retinal astrocytes, vasculature, and retinal nerve fiber layer thickness suggest a reorganization of the astrocyte and vascular templates during myopia development and progression. Whether these adaptations are beneficial or harmful to the retina remains to be investigated.

Coupling of angiogenesis and odontogenesis orchestrates tooth mineralization in mice

The skeletal system consists of bones and teeth, both of which are hardened via mineralization to support daily physical activity and mastication. The precise mechanism for this process, especially how blood vessels contribute to tissue mineralization, remains incompletely understood. Here, we established an imaging technique to visualize the 3D structure of the tooth vasculature at a single-cell level. Using this technique combined with single-cell RNA sequencing, we identified a unique endothelial subtype specialized to dentinogenesis, a process of tooth mineralization, termed periodontal tip-like endothelial cells. These capillaries exhibit high angiogenic activity and plasticity under the control of odontoblasts; in turn, the capillaries trigger odontoblast maturation. Metabolomic analysis demonstrated that the capillaries perform the phosphate delivery required for dentinogenesis. Taken together, our data identified the fundamental cell-to-cell communications that orchestrate tooth formation, angiogenic–odontogenic coupling, a distinct mechanism compared to the angiogenic–osteogenic coupling in bones. This mechanism contributes to our understanding concerning the functional diversity of organotypic vasculature.

Key Role of Astrocytes in Postnatal Brain and Retinal Angiogenesis

Angiogenesis is a key process in various physiological and pathological conditions in the nervous system and in the retina during postnatal life. Although an increasing number of studies have addressed the role of endothelial cells in this event, the astrocytes contribution in angiogenesis has received less attention. This review is focused on the role of astrocytes as a scaffold and in the stabilization of the new blood vessels, through different molecules release, which can modulate the angiogenesis process in the brain and in the retina. Further, differences in the astrocytes phenotype are addressed in glioblastoma, one of the most devastating types of brain cancer, in order to provide potential targets involved in the cross signaling between endothelial cells, astrocytes and glioma cells, that mediate tumor progression and pathological angiogenesis. Given the relevance of astrocytes in angiogenesis in physiological and pathological conditions, future studies are required to better understand the interrelation between endothelial and astrocyte signaling pathways during this process.

Astrocyte-intrinsic and -extrinsic Fat1 activities regulate astrocyte development and angiogenesis in the retina

Angiogenesis is a stepwise process leading to blood vessel formation. In the vertebrate retina, endothelial cells are guided by astrocytes migrating along the inner surface, and the two processes are coupled by a tightly regulated cross-talks between the two cell types. Here, I have investigated how the FAT1 cadherin, a regulator of tissue morphogenesis that governs tissue cross-talk, influences retinal vascular development. Late-onset Fat1 inactivation in the neural lineage in mice, by interfering with astrocyte progenitor migration polarity and maturation, delayed postnatal retinal angiogenesis, leading to persistent vascular abnormalities in adult retinas. Impaired astrocyte migration and polarity were not associated with alterations of retinal ganglion cell axonal trajectories or of the inner limiting membrane. In contrast, inducible Fat1 ablation in postnatal astrocytes was sufficient to alter their migration polarity and proliferation. Altogether, this study uncovers astrocyte-intrinsic and -extrinsic Fat1 activities that influence astrocyte migration polarity, proliferation and maturation, disruption of which impacts retinal vascular development and maintenance.

LIF, a mitogen for choroidal endothelial cells, protects the choriocapillaris: implications for prevention of geographic atrophy

In the course of our studies aiming to discover vascular bed-specific endothelial cell (EC) mitogens, we identified leukemia inhibitory factor (LIF) as a mitogen for bovine choroidal EC (BCE), although LIF has been mainly characterized as an EC growth inhibitor and an anti-angiogenic molecule. LIF stimulated growth of BCE while it inhibited, as previously reported, bovine aortic EC (BAE) growth. The JAK-STAT3 pathway mediated LIF actions in both BCE and BAE cells, but a caspase-independent proapoptotic signal mediated by cathepsins was triggered in BAE but not in BCE. LIF administration directly promoted activation of STAT3 and increased blood vessel density in mouse eyes. LIF also had protective effects on the choriocapillaris in a model of oxidative retinal injury. Analysis of available single-cell transcriptomic datasets shows strong expression of the specific LIF receptor in mouse and human choroidal EC. Our data suggest that LIF administration may be an innovative approach to prevent atrophy associated with AMD, through protection of the choriocapillaris.

Seeing stars: Development and function of retinal astrocytes

Throughout the central nervous system, astrocytes adopt precisely ordered spatial arrangements of their somata and arbors, which facilitate their many important functions. Astrocyte pattern formation is particularly important in the retina, where astrocytes serve as a template that dictates the pattern of developing retinal vasculature. Thus, if astrocyte patterning is disturbed, there are severe consequences for retinal angiogenesis and ultimately for vision - as seen in diseases such as retinopathy of prematurity. Here we discuss key steps in development of the retinal astrocyte population. We describe how fundamental developmental forces - their birth, migration, proliferation, and death - sculpt astrocytes into a template that guides angiogenesis. We further address the radical changes in the cellular and molecular composition of the astrocyte network that occur upon completion of angiogenesis, paving the way for their adult functions in support of retinal ganglion cell axons. Understanding development of retinal astrocytes may elucidate pattern formation mechanisms that are deployed broadly by other axon-associated astrocyte populations.

The Interaction Between Microglia and Macroglia in Glaucoma

Glaucoma, a neurodegenerative disease that leads to irreversible vision loss, is characterized by progressive loss of retinal ganglion cells (RGCs) and optic axons. To date, elevated intraocular pressure (IOP) has been recognized as the main phenotypic factor associated with glaucoma. However, some patients with normal IOP also have glaucomatous visual impairment and RGC loss. Unfortunately, the underlying mechanisms behind such cases remain unclear. Recent studies have suggested that retinal glia play significant roles in the initiation and progression of glaucoma. Multiple types of glial cells are activated in glaucoma. Microglia, for example, act as critical mediators that orchestrate the progression of neuroinflammation through pro-inflammatory cytokines. In contrast, macroglia (astrocytes and Müller cells) participate in retinal inflammatory responses as modulators and contribute to neuroprotection through the secretion of neurotrophic factors. Notably, research results have indicated that intricate interactions between microglia and macroglia might provide potential therapeutic targets for the prevention and treatment of glaucoma. In this review, we examine the specific roles of microglia and macroglia in open-angle glaucoma, including glaucoma in animal models, and analyze the interaction between these two cell types. In addition, we discuss potential treatment options based on the relationship between glial cells and neurons.

Environmental oxygen regulates astrocyte proliferation to guide angiogenesis during retinal development

Angiogenesis in the developing mammalian retina requires patterning cues from astrocytes. Developmental disorders of retinal vasculature, such as retinopathy of prematurity (ROP), involve arrest or mispatterning of angiogenesis. Whether these vascular pathologies involve astrocyte dysfunction remains untested. Here, we demonstrate that the major risk factor for ROP – transient neonatal exposure to excess oxygen – disrupts formation of the angiogenic astrocyte template. Exposing newborn mice to elevated oxygen (75%) suppressed astrocyte proliferation, whereas return to room air (21% oxygen) at postnatal day 4 triggered extensive proliferation, massively increasing astrocyte numbers and disturbing their spatial patterning prior to the arrival of developing vasculature. Proliferation required astrocytic HIF2α and was also stimulated by direct hypoxia (10% oxygen), suggesting that astrocyte oxygen sensing regulates the number of astrocytes produced during development. Along with astrocyte defects, return to room air also caused vascular defects reminiscent of ROP. Strikingly, these vascular phenotypes were more severe in animals that had larger numbers of excess astrocytes. Together, our findings suggest that fluctuations in environmental oxygen dysregulate molecular pathways controlling astrocyte proliferation, thereby generating excess astrocytes that interfere with retinal angiogenesis.

Highlighted Article: Oxygen regulates proliferation of immature retinal astrocytes. Perturbing this mechanism inflates astrocyte numbers, disrupts retinal angiogenesis and leads to vascular pathologies resembling retinopathy of prematurity.

EVL regulates VEGF receptor-2 internalization and signaling in developmental angiogenesis

Endothelial tip cells are essential for VEGF‐induced angiogenesis, but underlying mechanisms are elusive. The Ena/VASP protein family, consisting of EVL, VASP, and Mena, plays a pivotal role in axon guidance. Given that axonal growth cones and endothelial tip cells share many common features, from the morphological to the molecular level, we investigated the role of Ena/VASP proteins in angiogenesis. EVL and VASP, but not Mena, are expressed in endothelial cells of the postnatal mouse retina. Global deletion of EVL (but not VASP) compromises the radial sprouting of the vascular plexus in mice. Similarly, endothelial‐specific EVL deletion compromises the radial sprouting of the vascular plexus and reduces the endothelial tip cell density and filopodia formation. Gene sets involved in blood vessel development and angiogenesis are down‐regulated in EVL‐deficient P5‐retinal endothelial cells. Consistently, EVL deletion impairs VEGF‐induced endothelial cell proliferation and sprouting, and reduces the internalization and phosphorylation of VEGF receptor 2 and its downstream signaling via the MAPK/ERK pathway. Together, we show that endothelial EVL regulates sprouting angiogenesis via VEGF receptor‐2 internalization and signaling.

The process of revascularization in the neonatal mouse retina following short-term blockade of vascular endothelial growth factor receptors

Misdirected vascular growth frequently occurs in the neovascular diseases in the retina. However, the mechanisms are still not fully understood. In the present study, we created capillary-free zones in the central and peripheral retinas in neonatal mice by pharmacological blockade of vascular endothelial growth factor (VEGF) signaling. Using this model, we investigated the process and mechanisms of revascularization in the central and peripheral avascular areas. After the completion of a 2-day treatment with the VEGF receptor tyrosine kinase inhibitor KRN633 on postnatal day (P) 4 and P5, revascularization started on P8 in the central avascular area where capillaries had been dropped out. The expression levels of VEGF were higher in the peripheral than in the central avascular area. However, the expansion of the vasculature in the peripheral avascular retina remained suppressed until revascularization had been completed in the central avascular area. Additionally, we found disorganized endothelial cell division, misdirected blood vessels with irregular diameters, and abnormal fibronectin networks at the border of the vascular front and the avascular retina. In the central avascular area, a slight amount of fibronectin as non-vascular component re-formed to provide a scaffold for revascularization. Mechanistic analysis revealed that higher levels of VEGF attenuated the migratory response of endothelial cells without decreasing the proliferative activity. These results suggest that the presence of concentration range of VEGF, which enhances both migration and proliferation of the endothelial cells, and the structurally normal fibronectin network contribute to determine the proper direction of angiogenesis.

Endothelial Yes-Associated Protein 1 Promotes Astrocyte Proliferation and Maturation via Cytoplasmic Leukemia Inhibitory Factor Secretion in Oxygen-Induced Retinopathy

Purpose

Purpose The role of endothelial Yes-associated protein 1 (YAP) in the pathogenesis of retinal angiogenesis and the astrocyte network in the mouse oxygen-induced retinopathy (OIR) model is unknown.

Methods

For in vivo studies, OIR was induced in conditional endothelial YAP knockout mice and their wild-type littermates. Retinal vascularization and the astrocyte network were evaluated by whole-mount fluorescence and Western blotting. In vitro experiments were performed in astrocytes cultured with human microvascular endothelial cell-1–conditioned medium to analyze the mechanisms underlying the effect of endothelial YAP on astrocytes.

Results

Endothelial YAP deletion not only impaired retinal blood vessels, but also caused a sparse and disrupted astrocyte network in response to OIR. Levels of the immature astrocyte marker (platelet-derived growth factor A) in the retina were substantially increased owing to YAP deficiency, suggesting a possible failure in astrocyte maturation, whereas retinal expression of leukemia inhibitory factor (LIF) was decreased. In vitro studies suggested that loss or overexpression of YAP resulted in elevated or decreased LIF secretion by human microvascular endothelial cell-1, respectively. Increased LIF levels in the culture medium promoted astrocyte maturation and proliferation and rescued YAP inhibition-induced astrocyte loss. Finally, activating YAP could protect against the pathology of the astrocyte network and even suppress pathologic retinal vascularization in control OIR mice, but not in endothelial YAP-deficient OIR mice.

Conclusions

Endothelial YAP regulation of LIF secretion is required for normalized astrocyte network formation in OIR, thereby providing a novel target for protecting the astrocyte network and thus benefiting retinal blood vessels.

Paracentral acute middle maculopathy and the organization of the retinal capillary plexuses

The retinal capillary vasculature serves the formidable role of supplying the metabolically active inner and middle retina. In the parafoveal region, the retinal capillary plexuses (RCP) are organized in a system of three capillary layers of varying retinal depths: the superficial capillary plexus (SCP), intermediate capillary plexus (ICP) and deep capillary plexus (DCP). While the dynamic flow through these plexuses is complex and not completely understood, current research points to a hybrid model that includes both parallel and in series components in which blood flows in a predominantly serial direction between the superficial vascular complex (SVC) and deep vascular complex (DVC). Each capillary plexus autoregulates independently, so that under most conditions the retinal vasculature supplies adequate blood flow and oxygen saturation at varying depths despite diverse environmental stressors. When the flow in the deep vascular complex (i.e. ICP and DCP) fails, an ischemic lesion referred to as Paracentral Acute Middle Maculopathy (PAMM) can be identified. PAMM is an optical coherence tomography (OCT) finding defined by the presence of a hyperreflective band at the level of the inner nuclear layer (INL) that indicates INL infarction caused by globally impaired perfusion through the retinal capillary system leading to hypoperfusion of the DVC or specifically the DCP. Patients present with an acute onset paracentral scotoma and typically experience a permanent visual defect. Lesions can be caused by a diverse set of local retinal vascular diseases and systemic disorders. PAMM is a manifestation of the retinal ischemic cascade in which the mildest forms of ischemia develop at the venular end of the DCP, i.e. perivenular PAMM, while more severe forms progress horizontally to diffusely involve the INL, and the most severe forms progress vertically to infarct the inner retina. Management is targeted toward the identification and treatment of related vasculopathic and systemic risk factors.

Bim expression modulates the pro-inflammatory phenotype of retinal astroglial cells

Apoptosis of neurovascular cells, including astroglial cells, contributes to the pathogenesis of diseases in which neurovascular disruption plays a central role. Bim is a pro-apoptotic protein that modulates not only apoptosis but also various cellular functions such as migration and extracellular matrix protein expression. Astroglial cells act as an intermediary between neural and vascular cells facilitating retinal vascular development and remodeling while maintaining normal vascular function and neuronal integrity. We previously showed that Bim deficient (Bim -/-) mice were protected from hyperoxia mediated vessel obliteration and ischemia-mediated retinal neovascularization. However, the underlying mechanisms and more specifically the role Bim expression in astroglial cells play remains elusive. Here, using retinal astroglial cells prepared from wild-type and Bim -/- mice, we determined the impact of Bim expression in retinal astroglial cell function. We showed that astroglial cells lacking Bim expression demonstrate increased VEGF expression and altered matricellular protein production including increased expression of thrombospondin-2 (TSP2), osteopontin and SPARC. Bim deficient astroglial cells also exhibited altered proliferation, migration, adhesion to various extracellular matrix proteins and increased expression of inflammatory mediators. Thus, our data emphasizes the importance of Bim expression in retinal astroglia cell autonomous regulatory mechanisms, which could influence neurovascular function.

Neuron-derived VEGF contributes to cortical and hippocampal development independently of VEGFR1/2-mediated neurotrophism

Vascular endothelial growth factor (VEGF) is a potent mitogen critical for angiogenesis and organogenesis. Deletion or inhibition of VEGF during development not only profoundly suppresses vascular outgrowth, but significantly affects the development and function of various organs. In the brain, VEGF is thought to not only promote vascular growth, but also directly act on neurons as a neurotrophic factor by activating VEGF receptors. In the present study, we demonstrated that deletion of VEGF using hGfap-Cre line, which recombines genes specifically in cortical and hippocampal neurons, severely impaired brain organization and vascularization of these regions. The mutant mice had motor deficits, with lethality around the time of weaning. Multiple reporter lines indicated that VEGF was highly expressed in neurons, but that its cognate receptors, VEGFR1 and 2 were exclusive to endothelial cells in the brain. In accordance, mice lacking neuronal VEGFR1 and VEGFR2 did not exhibit neuronal deformities or lethality. Taken together, our data suggest that neuron-derived VEGF contributes to cortical and hippocampal development likely through angiogenesis independently of direct neurotrophic effects mediated by VEGFR1 and 2.

Glia of the human retina

The human retina contains three types of glial cells: microglia and two types of macroglia, astrocytes and Müller cells. Macroglia provide homeostatic and metabolic support to photoreceptors and neurons required for neuronal activity. The fovea, the site of the sharpest vision which is astrocyte- and microglia-free, contains two populations of Müller glia: cells which form the Müller cell cone in the foveola and z-shaped Müller cells of the foveal walls. Both populations are characterized by morphological and functional differences. Müller cells of the foveola do not support the activity of photoreceptors and neurons, but provide the structural stability of the foveal tissue and improve the light transmission through the tissue to the photoreceptors. This article gives overviews of the glia of the human retina and the structure and function of both Müller cell types in the fovea, and describes the contributions of astrocytes and Müller cells to the ontogenetic development of the fovea.

Large-scale death of retinal astrocytes during normal development is non-apoptotic and implemented by microglia

Naturally occurring cell death is a fundamental developmental mechanism for regulating cell numbers and sculpting developing organs. This is particularly true in the nervous system, where large numbers of neurons and oligodendrocytes are eliminated via apoptosis during normal development. Given the profound impact of death upon these two major cell populations, it is surprising that developmental death of another major cell type—the astrocyte—has rarely been studied. It is presently unclear whether astrocytes are subject to significant developmental death, and if so, how it occurs. Here, we address these questions using mouse retinal astrocytes as our model system. We show that the total number of retinal astrocytes declines by over 3-fold during a death period spanning postnatal days 5–14. Surprisingly, these astrocytes do not die by apoptosis, the canonical mechanism underlying the vast majority of developmental cell death. Instead, we find that microglia engulf astrocytes during the death period to promote their developmental removal. Genetic ablation of microglia inhibits astrocyte death, leading to a larger astrocyte population size at the end of the death period. However, astrocyte death is not completely blocked in the absence of microglia, apparently due to the ability of astrocytes to engulf each other. Nevertheless, mice lacking microglia showed significant anatomical changes to the retinal astrocyte network, with functional consequences for the astrocyte-associated vasculature leading to retinal hemorrhage. These results establish a novel modality for naturally occurring cell death and demonstrate its importance for the formation and integrity of the retinal gliovascular network.

A study of the neonatal mouse retina shows that developmental cell death of retinal astrocytes does not occur by apoptosis but is instead mediated by microglia, which kill and engulf astrocytes to effect their developmental removal.

Roles of HIFs and VEGF in angiogenesis in the retina and brain

Vascular development in the mammalian retina is a paradigm for CNS vascular development in general, and its study is revealing fundamental mechanisms that explain the efficacy of antiangiogenic therapies in retinal vascular disease. During development of the mammalian retina, hypoxic astrocytes are hypothesized to secrete VEGF, which attracts growing endothelial cells as they migrate radially from the optic disc. However, published tests of this model using astrocyte-specific deletion of Vegf in the developing mouse retina appear to contradict this theory. Here, we report that selectively eliminating Vegf in neonatal retinal astrocytes with a Gfap-Cre line that recombines with approximately 100% efficiency had no effect on proliferation or radial migration of astrocytes, but completely blocked radial migration of endothelial cells, strongly supporting the hypoxic astrocyte model. Using additional Cre driver lines, we found evidence for essential and partially redundant actions of retina-derived (paracrine) and astrocyte-derived (autocrine) VEGF in controlling astrocyte proliferation and migration. We also extended previous studies by showing that HIF-1α in retinal neurons and HIF-2α in Müller glia play distinct roles in retinal vascular development and disease, adding to a growing body of data that point to the specialization of these 2 hypoxia-sensing transcription factors.

Developmental vascular pruning in neonatal mouse retinas is programmed by the astrocytic oxygen-sensing mechanism

Vascular pruning is crucial for normal development, but its underlying mechanisms are poorly understood. Here, we report that retinal vascular pruning is controlled by the oxygen-sensing mechanism in local astrocytes. Oxygen sensing is mediated by prolyl hydroxylase domain proteins (PHDs), which use O₂ as a substrate to hydroxylate specific prolyl residues on hypoxia inducible factor (HIF)-α proteins, labeling them for polyubiquitylation and proteasomal degradation. In neonatal mice, astrocytic PHD2 deficiency led to elevated HIF-2α protein levels, expanded retinal astrocyte population and defective vascular pruning. Although astrocytic VEGF-A was also increased, anti-VEGF failed to rescue vascular pruning. However, stimulation of retinal astrocytic growth by intravitreal delivery of PDGF-A was sufficient to block retinal vascular pruning in wild-type mice. We propose that in normal development, oxygen from nascent retinal vasculature triggers PHD2-dependent HIF-2α degradation in nearby astrocytic precursors, thus limiting their further growth by driving them to differentiate into non-proliferative mature astrocytes. The physiological limit of retinal capillary density may be set by astrocytes available to support their survival, with excess capillaries destined for regression.This article has an associated 'The people behind the papers' interview.

The heparin binding domain of von Willebrand factor binds to growth factors and promotes angiogenesis in wound healing

During wound healing, the distribution, availability, and signaling of growth factors (GFs) are orchestrated by their binding to extracellular matrix components in the wound microenvironment. Extracellular matrix proteins have been shown to modulate angiogenesis and promote wound healing through GF binding. The hemostatic protein von Willebrand factor (VWF) released by endothelial cells (ECs) in plasma and in the subendothelial matrix has been shown to regulate angiogenesis; this function is relevant to patients in whom VWF deficiency or dysfunction is associated with vascular malformations. Here, we show that VWF deficiency in mice causes delayed wound healing accompanied by decreased angiogenesis and decreased amounts of angiogenic GFs in the wound. We show that in vitro VWF binds to several GFs, including vascular endothelial growth factor-A (VEGF-A) isoforms and platelet-derived growth factor-BB (PDGF-BB), mainly through the heparin-binding domain (HBD) within the VWF A1 domain. VWF also binds to VEGF-A and fibroblast growth factor-2 (FGF-2) in human plasma and colocalizes with VEGF-A in ECs. Incorporation of the VWF A1 HBD into fibrin matrices enables sequestration and slow release of incorporated GFs. In vivo, VWF A1 HBD-functionalized fibrin matrices increased angiogenesis and GF retention in VWF-deficient mice. Treatment of chronic skin wounds in diabetic mice with VEGF-A165 and PDGF-BB incorporated within VWF A1 HBD-functionalized fibrin matrices accelerated wound healing, with increased angiogenesis and smooth muscle cell proliferation. Therefore, the VWF A1 HBD can function as a GF reservoir, leading to effective angiogenesis and tissue regeneration.

Optical imaging and modulation of neurovascular responses

The cerebral microvasculature consists of pial vascular networks, parenchymal descending arterioles, ascending venules and parenchymal capillaries. This vascular compartmentalization is vital to precisely deliver blood to balance continuously varying neural demands in multiple brain regions. Optical imaging techniques have facilitated the investigation of dynamic spatial and temporal properties of microvascular functions in real time. Their combination with transgenic animal models encoding specific genetic targets have further strengthened the importance of optical methods for neurovascular research by allowing for the modulation and monitoring of neuro vascular function. Image analysis methods with three-dimensional reconstruction are also helping to understand the complexity of microscopic observations. Here, we review the compartmentalized cerebral microvascular responses to global perturbations as well as regional changes in response to neural activity to highlight the differences in vascular action sites. In addition, microvascular responses elicited by optical modulation of different cell-type targets are summarized with emphasis on variable spatiotemporal dynamics of microvascular responses. Finally, long-term changes in microvascular compartmentalization are discussed to help understand potential relationships between CBF disturbances and the development of neurodegenerative diseases and cognitive decline.

Angiopoietin-4-dependent venous maturation and fluid drainage in the peripheral retina

The maintenance of fluid homeostasis is necessary for function of the neural retina; however, little is known about the significance of potential fluid management mechanisms. Here, we investigated angiopoietin-4 (Angpt4, also known as Ang3), a poorly characterized ligand for endothelial receptor tyrosine kinase Tie2, in mouse retina model. By using genetic reporter, fate mapping, and in situ hybridization, we found Angpt4 expression in a specific sub-population of astrocytes at the site where venous morphogenesis occurs and that lower oxygen tension, which distinguishes peripheral and venous locations, enhances Angpt4 expression. Correlating with its spatiotemporal expression, deletion of Angpt4 resulted in defective venous development causing impaired venous drainage and defects in neuronal cells. In vitro characterization of angiopoietin-4 proteins revealed both ligand-specific and redundant functions among the angiopoietins. Our study identifies Angpt4 as the first growth factor for venous-specific development and its importance in venous remodeling, retinal fluid clearance and neuronal function.

Dll4-Notch1 signaling but not VEGF-A is essential for hyperoxia induced vessel regression in retina

It is well recognized that decreased vascular endothelial growth factor A (VEGF-A) mRNA plays an important role in retinal vessel regression induced by hyperoxia. However, this concept has been challenged by increasing new evidence. Furthermore, VEGF-A strongly enhances Dll4 expression and inhibition of Dll4-Notch signaling leads to excessive sprouting angiogenesis. Recently, it is shown that inactivation of Dll4-Notch1 signaling reduce hyperoxia induced vessel regression. It is unknown whether sprouting angiogenesis contributes to the protective effect or not and further investigations are needed. Moreover, the expression of Dll4 or Notch1 activation in the regressing plexus remains elucidated. To determine the role of VEGF-A and Dll4-Notch1 signaling in hyperoxia induced vascular regression in the retina, we used mice at postnatal day 5 (P5) - P7. Hyperoxia induced massive vascular regression in the central plexus but not in the angiogenic plexus and had no effect on sprouting angiogenesis. Immunostaining showed that VEGF-A was significantly repressed in the angiogenic front region after hyperoxia exposure but not detectable in the central area of both normoxia and hyperoxia treated retinas. In contrast, Notch ligand Delta-like 4 (Dll4) and Notch1 intracellular domain (N1-ICD) expression were inhibited in the regressing capillaries of central retina but comparable in the angiogenic plexus after high oxygen treatment. Moreover, administration of Dll4 neutralizing antibody or γ-Secretase inhibitor DAPT significantly aggravated vessel regression induced by short-time hyperoxia administration. Our data show that repressed Dll4-Notch1 signaling pathway but not downregulation of VEGF-A expression are responsible for hyperoxia induced pervasive vessel regression.

Deletion of p75NTR prevents vaso-obliteration and retinal neovascularization via activation of Trk- A receptor in ischemic retinopathy model

Ischemic retinopathy is characterized by ischemia followed by retinal neovascularization (RNV) resulting in visual impairment. Given the role of neuron-secreted growth factors in regulating angiogenesis, we examined how genetic deletion of the neurotrophin receptor; p75NTR can overcome retinal ischemia using oxygen-induced retinopathy (OIR) mouse model. Wildtype (WT) or p75NTR-/- mice pups were subjected to hyperoxia (70% O2, p7-p12) then returned to normal air (relative hypoxia, p12-p17). Vascular alterations were assessed at p12 and p17 time-points. Deletion of p75NTR prevented hyperoxia-associated central vascular cell death (p12) and hypoxia-associated RNV and enhanced central vascular repair (p17). Decreased expression of apoptotic markers; preserved Akt survival signal decreased proNGF were also observed at p12. During hypoxia, deletion of p75NTR maintained VEGF and VEGFR2 activation and restored NGF/proNGF and BDNF/proBDNF levels. Deletion of p75NTR coincided with significant increases in expression and activation of NGF survival receptor, TrkA at basal and hyperoxic condition. Pharmacological inhibition of TrkA using compound K-252a (0.5 μg 1 μl-1/eye) resulted in 2-fold increase in pathological RNV and 1.34-fold increase in central vascular cell death in p75NTR-/- pups. In conclusion, deletion of p75NTR protected against retinal ischemia and prevented RNV, in part, through restoring neurotrophic support and activating TrkA receptor.

Agent-based computational model of retinal angiogenesis simulates microvascular network morphology as a function of pericyte coverage

OBJECTIVE

Define a role for perivascular cells during developmental retinal angiogenesis in the context of EC Notch1-DLL4 signaling at the multicellular network level.

METHODS

The retinal vasculature is highly sensitive to growth factor-mediated intercellular signaling. Although EC signaling has been explored in detail, it remains unclear how PC function to modulate these signals that lead to a diverse set of vascular network patterns in health and disease. We have developed an ABM of retinal angiogenesis that incorporates both ECs and PCs to investigate the formation of vascular network patterns as a function of pericyte coverage. We use our model to test the hypothesis that PC modulate Notch1-DLL4 signaling in endothelial cell-endothelial cell interactions.

RESULTS

Agent-based model (ABM) simulations that include PCs more accurately predict experimentally observed vascular network morphologies than simulations that lack PCs, suggesting that PCs may influence sprouting behaviors through physical blockade of endothelial intercellular connections.

CONCLUSIONS

This study supports a role for PCs as a physical buffer to signal propagation during vascular network formation-a barrier that may be important for generating healthy microvascular network patterns.

Neurovascular Communication during CNS Development

A precise communication between the nervous and the vascular systems is crucial for proper formation and function of the central nervous system (CNS). Interestingly, this communication does not only occur by neural cells regulating the growth and properties of the vasculature, but new studies show that blood vessels actively control different neurodevelopmental processes. Here, we review the current knowledge on how neurons in particular influence growing blood vessels during CNS development and on how vessels participate in shaping the neural compartment. We also review the identified molecular mechanisms of this bidirectional communication.

Spatial and temporal recruitment of the neurovascular unit during development of the mouse blood-retinal barrier

The inner blood-retinal barrier (BRB) is made up by the neurovascular unit, consisting of endothelial cells, pericytes and glial cells. The BRB maintains homeostasis of the neural retina, but in pathological eye conditions the neurovascular unit is often disrupted, causing BRB loss. Here, we investigated in detail temporal and spatial recruitment of the neurovascular unit in the neonatal mouse retina from postnatal day (P)3 to P25 employing immunohistochemical staining of vascular endothelium (isolectin B4), pericytes (α-SMA and NG2) and astrocytes (GFAP). In addition, we investigated gene expression of polarized astrocytic end-feet markers aquaporin-4 and laminin α2 chain with qPCR. We observed GFAP-positive cells migrating ahead of the retinal vasculature during the first postnatal week, suggesting that the retinal vasculature follows an astrocytic meshwork. From P9 onwards, astrocytes acquired a mature phenotype, with a more stellate shape and increased expression of aquaporin-4. NG2-positive cells and tip cells co-localized at P5 and invaded the retina together as a vascular sprouting front. In summary, these data suggest that recruitment of the cell types of the neurovascular unit is a prerequisite for proper retinal vascularization and BRB formation.

Pharmacologic management of diabetic retinopathy

Diabetic retinopathy (DR) is a leading cause of vision loss in working-age populations, primarily attributable to retinal vascular hyperpermeability, hypoperfusion, and neoangiogenesis. In the past decade, laser photocoagulation and surgical interventions to treat DR have been replaced by topical administrations of anti-vascular endothelial growth factor drugs and corticosteroids. Although these drugs have revolutionized clinical management of DR, their limited efficacy and adverse effects have raised an increasing demand for new drug development. Meanwhile, mouse retinas have been prevalently employed as an experimental model system for angiogenic research, which has greatly contributed to the understanding of general principles in vascular biology. Therefore, clinical ophthalmology and basic research have complimentarily accumulated invaluable information for DR drug discovery. This review highlights the current pharmacologic management of DR, the utility of experimental mouse retinal models, and the perspectives on new drugs targeting the angioepoitin-Tie2 signals.

Retinal Angiogenesis Regulates Astrocytic Differentiation in Neonatal Mouse Retinas by Oxygen Dependent Mechanisms

In mice, retinal vascular and astrocyte networks begin to develop at birth, expanding radially from the optic nerve head (ONH) towards the retinal periphery. The retinal vasculature grows towards the periphery ahead of differentiated astrocytes, but behind astrocytic progenitor cells (APCs) and immature astrocytes. Endothelial cell specific Vegfr-2 disruption in newborn mice not only blocked retinal vascular development but also suppressed astrocytic differentiation, reducing the abundance of differentiated astrocytes while causing the accumulation of precursors. By contrast, retinal astrocytic differentiation was accelerated by the exposure of wild-type newborn mice to hyperoxia for 24 hours, or by APC specific deficiency in hypoxia inducible factor (HIF)−2α, an oxygen labile transcription factor. These findings reveal a novel function of the retinal vasculature, and imply that in normal neonatal mice, oxygen from the retinal circulation may promote astrocytic differentiation, in part by triggering oxygen dependent HIF-2α degradation in astrocytic precursors.

Sox2 regulates astrocytic and vascular development in the retina

Sox2 is a transcriptional regulator that is highly expressed in retinal astrocytes, yet its function in these cells has not previously been examined. To understand its role, we conditionally deleted Sox2 from the population of astrocytes and examined the consequences on retinal development. We found that Sox2 deletion does not alter the migration of astrocytes, but it impairs their maturation, evidenced by the delayed upregulation of glial fibrillary acidic protein (GFAP) across the retina. The centro-peripheral gradient of angiogenesis is also delayed in Sox2-CKO retinas. In the mature retina, we observed lasting abnormalities in the astrocytic population evidenced by the sporadic loss of GFAP immunoreactivity in the peripheral retina as well as by the aberrant extension of processes into the inner retina. Blood vessels in the adult retina are also under-developed and show a decrease in the frequency of branch points and in total vessel length. The developmental relationship between maturing astrocytes and angiogenesis suggests a causal relationship between the astrocytic loss of Sox2 and the vascular architecture in maturity. We suggest that the delay in astrocytic maturation and vascular invasion may render the retina hypoxic, thereby causing the abnormalities we observe in adulthood. These studies uncover a novel role for Sox2 in the development of retinal astrocytes and indicate that its removal can lead to lasting changes to retinal homeostasis.

Retinal vasculature development in health and disease

Development of the retinal vasculature is based on highly coordinated signalling between different cell types of the retina, integrating internal metabolic requirements with external influences such as the supply of oxygen and nutrients. The developing mouse retinal vasculature is a useful model system to study these interactions because it is experimentally accessible for intra ocular injections and genetic manipulations, can be easily imaged and develops in a similar fashion to that of humans. Research using this model has provided insights about general principles of angiogenesis as well as pathologies that affect the developing retinal vasculature. In this review, we discuss recent advances in our understanding of the molecular and cellular mechanisms that govern the interactions between neurons, glial and vascular cells in the developing retina. This includes a review of mechanisms that shape the retinal vasculature, such as sprouting angiogenesis, vascular network remodelling and vessel maturation. We also explore how the disruption of these processes in mice can lead to pathology - such as oxygen induced retinopathy - and how this translates to human retinopathy of prematurity.

Astrocytes follow ganglion cell axons to establish an angiogenic template during retinal development

Immature astrocytes and blood vessels enter the developing mammalian retina at the optic nerve head and migrate peripherally to colonize the entire retinal nerve fiber layer (RNFL). Retinal vascularization is arrested in retinopathy of prematurity (ROP), a major cause of bilateral blindness in children. Despite their importance in normal development and ROP, the factors that control vascularization of the retina remain poorly understood. Because astrocytes form a reticular network that appears to provide a substrate for migrating endothelial cells, they have long been proposed to guide angiogenesis. However, whether astrocytes do in fact impose a spatial pattern on developing vessels remains unclear, and how astrocytes themselves are guided is unknown. Here we explore the cellular mechanisms that ensure complete retinal coverage by astrocytes and blood vessels in mouse. We find that migrating astrocytes associate closely with the axons of retinal ganglion cells (RGCs), their neighbors in the RNFL. Analysis of Robo1; Robo2 mutants, in which RGC axon guidance is disrupted, and Math5 (Atoh7) mutants, which lack RGCs, reveals that RGCs provide directional information to migrating astrocytes that sets them on a centrifugal trajectory. Without this guidance, astrocytes exhibit polarization defects, fail to colonize the peripheral retina, and display abnormal fine-scale spatial patterning. Furthermore, using cell type-specific chemical-genetic tools to selectively ablate astrocytes, we show that the astrocyte template is required for angiogenesis and vessel patterning. Our results are consistent with a model whereby RGC axons guide formation of an astrocytic network that subsequently directs vessel development.

YAP/TAZ-CDC42 signaling regulates vascular tip cell migration

Angiogenesis and vascular remodeling are essential for the establishment of vascular networks during organogenesis. Here we show that the Hippo signaling pathway effectors YAP and TAZ are required, in a gene dosage-dependent manner, for the proliferation and migration of vascular endothelial cells (ECs) during retinal angiogenesis. Intriguingly, nuclear translocation of YAP and TAZ induced by Lats1/2-deletion blocked endothelial migration and phenocopied Yap/Taz-deficient mutants. Furthermore, overexpression of a cytoplasmic form of YAP (YAPS127D) partially rescued the migration defects caused by loss of YAP and TAZ function. Finally, we found that cytoplasmic YAP positively regulated the activity of the small GTPase CDC42, deletion of which caused severe defects in endothelial migration. These findings uncover a previously unrecognized role of cytoplasmic YAP/TAZ in promoting cell migration by activating CDC42 and provide insight into how Hippo signaling in ECs regulates angiogenesis.

Study protocol: safety and efficacy of propranolol 0.2% eye drops in newborns with a precocious stage of retinopathy of prematurity (DROP-ROP-0.2%): a multicenter, open-label, single arm, phase II trial

BACKGROUND

Retinopathy of prematurity (ROP) still represents one of the leading causes of visual impairment in childhood. Systemic propranolol has proven to be effective in reducing ROP progression in preterm newborns, although safety was not sufficiently guaranteed. On the contrary, topical treatment with propranolol eye micro-drops at a concentration of 0.1% had an optimal safety profile in preterm newborns with ROP, but was not sufficiently effective in reducing the disease progression if administered at an advanced stage (during stage 2). The aim of the present protocol is to evaluate the safety and efficacy of propranolol 0.2% eye micro-drops in preterm newborns at a more precocious stage of ROP (stage 1).

METHODS

A multicenter, open-label, phase II, clinical trial, planned according to the Simon optimal two-stage design, will be performed to analyze the safety and efficacy of propranolol 0.2% eye micro-drops in preterm newborns with stage 1 ROP. Preterm newborns with a gestational age of 23-32 weeks, with a stage 1 ROP will receive propranolol 0.2% eye micro-drops treatment until retinal vascularization has been completed, but for no longer than 90 days. Hemodynamic and respiratory parameters will be continuously monitored. Blood samplings checking metabolic, renal and liver functions, as well as electrocardiogram and echocardiogram, will be periodically performed to investigate treatment safety. Additionally, propranolol plasma levels will be measured at the steady state, on the 10th day of treatment. To assess the efficacy of topical treatment, the ROP progression from stage 1 ROP to stage 2 or 3 with plus will be evaluated by serial ophthalmologic examinations.

DISCUSSION

Propranolol eye micro-drops could represent an ideal strategy in counteracting ROP, because it is definitely safer than oral administration, inexpensive and an easily affordable treatment. Establishing the optimal dosage and treatment schedule is to date a crucial issue.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier NCT02504944, registered on July 19, 2015, updated July 12, 2016. EudraCT Number 2014-005472-29.

Vascularisation of the central nervous system

The developing central nervous system (CNS) is vascularised through the angiogenic invasion of blood vessels from a perineural vascular plexus, followed by continued sprouting and remodelling until a hierarchical vascular network is formed. Remarkably, vascularisation occurs without perturbing the intricate architecture of the neurogenic niches or the emerging neural networks. We discuss the mouse hindbrain, forebrain and retina as widely used models to study developmental angiogenesis in the mammalian CNS and provide an overview of key cellular and molecular mechanisms regulating the vascularisation of these organs.

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CNS vascularisation is initiated during embryonic development.

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CNS vascularisation is studied in the mouse forebrain, hindbrain and retina models.

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Neuroglial cells interact with endothelial cells to promote angiogenesis.

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Neuroglial cells produce growth factors and matrix cues to pattern vessels.

Control of cerebrovascular patterning by neural activity during postnatal development

The brain represents only a small portion of the body mass and yet consumes almost a quarter of the available energy, and has a limited ability to store energy. The brain is therefore highly dependent on oxygen and nutrient supply from the blood circulation, which makes it vulnerable to vascular pathologies. Key vascular determinants will ensure proper brain maturation and function: the establishment of vascular networks, the formation of the blood-brain barrier, and the regulation of blood flow. Recent evidence suggests that the phenomenon of neurovascular coupling, during which increased neural activity normally leads to increased blood flow, is not functional until few weeks after birth, implying that the developing brain must rely on alternative mechanisms to adequately couple blood supply to increasing energy demands. This review will focus on these alternative mechanisms, which have been partly elucidated recently via the demonstration that neural activity influences the maturation of cerebrovascular networks. We also propose possible mechanisms underlying activity-induced vascular plasticity.