EGFL7 loss correlates with increased VEGF-D expression, upregulating hippocampal adult neurogenesis and improving spatial learning and memory

Neural stem cells reside in the subgranular zone, a specialized neurogenic niche of the hippocampus. Throughout adulthood, these cells give rise to neurons in the dentate gyrus, playing an important role in learning and memory. Given that these core cognitive processes are disrupted in numerous disease states, understanding the underlying mechanisms of neural stem cell proliferation in the subgranular zone is of direct practical interest. Here, we report that mature neurons, neural stem cells and neural precursor cells each secrete the neurovascular protein epidermal growth factor-like protein 7 (EGFL7) to shape this hippocampal niche. We further demonstrate that EGFL7 knock-out in a Nestin-CreERT2-based mouse model produces a pronounced upregulation of neurogenesis within the subgranular zone. RNA sequencing identified that the increased expression of the cytokine VEGF-D correlates significantly with the ablation of EGFL7. We substantiate this finding with intraventricular infusion of VEGF-D upregulating neurogenesis in vivo and further show that VEGF-D knock-out produces a downregulation of neurogenesis. Finally, behavioral studies in EGFL7 knock-out mice demonstrate greater maintenance of spatial memory and improved memory consolidation in the hippocampus by modulation of pattern separation. Taken together, our findings demonstrate that both EGFL7 and VEGF-D affect neurogenesis in the adult hippocampus, with the ablation of EGFL7 upregulating neurogenesis, increasing spatial learning and memory, and correlating with increased VEGF-D expression.

Criss-crossing autism spectrum disorder and adult neurogenesis

Autism spectrum disorder (ASD) comprises a group of multifactorial neurodevelopmental disorders primarily characterized by deficits in social interaction and repetitive behavior. Although the onset is typically in early childhood, ASD poses a lifelong challenge for both patients and caretakers. Adult neurogenesis (AN) is the process by which new functional neurons are created from neural stem cells existing in the post-natal brain. The entire event is based on a sequence of cellular processes, such as proliferation, specification of cell fate, maturation, and ultimately, synaptic integration into the existing neural circuits. Hence, AN is implicated in structural and functional brain plasticity throughout life. Accumulating evidence shows that impaired AN may underlie some of the abnormal behavioral phenotypes seen in ASD. In this review, we approach the interconnections between the molecular pathways related to AN and ASD. We also discuss existing therapeutic approaches targeting such pathways both in preclinical and clinical studies. A deeper understanding of how ASD and AN reciprocally affect one another could reveal important converging pathways leading to the emergence of psychiatric disorders.

EGFL7 enhances surface expression of integrin α5β1 to promote angiogenesis in malignant brain tumors

Glioblastoma (GBM) is a typically lethal type of brain tumor with a median survival of 15 months postdiagnosis. This negative prognosis prompted the exploration of alternative treatment options. In particular, the reliance of GBM on angiogenesis triggered the development of anti-VEGF (vascular endothelial growth factor) blocking antibodies such as bevacizumab. Although its application in human GBM only increased progression-free periods but did not improve overall survival, physicians and researchers still utilize this treatment option due to the lack of adequate alternatives. In an attempt to improve the efficacy of anti-VEGF treatment, we explored the role of the egfl7 gene in malignant glioma. We found that the encoded extracellular matrix protein epidermal growth factor-like protein 7 (EGFL7) was secreted by glioma blood vessels but not glioma cells themselves, while no major role could be assigned to the parasitic miRNAs miR-126/126*. EGFL7 expression promoted glioma growth in experimental glioma models in vivo and stimulated tumor vascularization. Mechanistically, this was mediated by an upregulation of integrin α₅β₁ on the cellular surface of endothelial cells, which enhanced fibronectin-induced angiogenic sprouting. Glioma blood vessels that formed in vivo were more mature as determined by pericyte and smooth muscle cell coverage. Furthermore, these vessels were less leaky as measured by magnetic resonance imaging of extravasating contrast agent. EGFL7-inhibition using a specific blocking antibody reduced the vascularization of experimental gliomas and increased the life span of treated animals, in particular in combination with anti-VEGF and the chemotherapeutic agent temozolomide. Data allow for the conclusion that this combinatorial regimen may serve as a novel treatment option for GBM.

Perivascular microglia promote blood vessel disintegration in the ischemic penumbra

The contribution of microglia to ischemic cortical stroke is of particular therapeutic interest because of the impact on the survival of brain tissue in the ischemic penumbra, a region that is potentially salvable upon a brain infarct. Whether or not tissue in the penumbra survives critically depends on blood flow and vessel perfusion. To study the role of microglia in cortical stroke and blood vessel stability, CX3CR1(+/GFP) mice were subjected to transient middle cerebral artery occlusion and then microglia were investigated using time-lapse two-photon microscopy in vivo. Soon after reperfusion, microglia became activated in the stroke penumbra and started to expand cellular protrusions towards adjacent blood vessels. All microglia in the penumbra were found associated with blood vessels within 24 h post reperfusion and partially fully engulfed them. In the same time frame blood vessels became permissive for blood serum components. Migration assays in vitro showed that blood serum proteins leaking into the tissue provided molecular cues leading to the recruitment of microglia to blood vessels and to their activation. Subsequently, these perivascular microglia started to eat up endothelial cells by phagocytosis, which caused an activation of the local endothelium and contributed to the disintegration of blood vessels with an eventual break down of the blood brain barrier. Loss-of-microglia-function studies using CX3CR1(GFP/GFP) mice displayed a decrease in stroke size and a reduction in the extravasation of contrast agent into the brain penumbra as measured by MRI. Potentially, medication directed at inhibiting microglia activation within the first day after stroke could stabilize blood vessels in the penumbra, increase blood flow, and serve as a valuable treatment for patients suffering from ischemic stroke.

EGFL7: a new player in homeostasis of the nervous system

EGFL7 drives the formation of neurons from neural stem cells. In the embryonic and adult brain this process is essential for neurogenesis and homeostasis of the nervous system. The function of adult neurogenesis is not fully understood but maybe it supports life-long learning and brain repair after injuries such as stroke. The transition of neural stem cells into mature neurons is tightly regulated. One of the essential signaling pathways governing this process is the Notch pathway, which controls metazoan development. In a recent publication, we identified a novel non-canonical Notch ligand, EGFL7, and described its impact on neural stem cells. We explored the molecular mechanisms, which this molecule affects to regulate the self-renewal capacity of neural stem cells and to promote their differentiation into neurons. In this review, we discuss the implications of our findings for adult neurogenesis and illustrate the potential of EGFL7 to serve as an agent to increase neurogenesis and the self-renewal potential of the brain

Epidermal growth factor-like domain 7 (EGFL7) modulates Notch signalling and affects neural stem cell renewal

Epidermal growth factor-like domain 7 (EGFL7) is a secreted factor implicated in cellular responses such as cell migration and blood vessel formation; however the molecular mechanisms underlying the effects of EGFL7 are largely unknown. Here we have identified transmembrane receptors of the Notch family as EGFL7-binding molecules. Secreted EGFL7 binds to a region in Notch involved in ligand-mediated receptor activation, thus acting as an antagonist of Notch signalling. Expression of EGFL7 in neural stem cells (NSCs) in vitro decreased Notch-specific signalling and consequently, reduced proliferation and self-renewal of NSCs. Such altered Notch signalling caused a shift in the differentiation pattern of cultured NSCs towards an excess of neurons and oligodendrocytes. We identified neurons as a source of EGFL7 in the brain, suggesting that brain-derived EGFL7 acts as an endogenous antagonist of Notch signalling that regulates proliferation and differentiation of subventricular zone-derived adult NSCs.

Characterization of C-terminal domains of Arabidopsis heat stress transcription factors (Hsfs) and identification of a new signature combination of plant class A Hsfs with AHA and NES motifs essential for activator function and intracellular localization

Heat stress transcription factors (Hsfs) are the major regulators of the plant heat stress (hs) response. Sequencing of the Arabidopsis genome revealed the existence of 21 open-reading frames (ORFs) encoding putative Hsfs assigned to classes A-C. Here we present results of a functional genomics approach to the Arabidopsis Hsf family focused on the analysis of their C-terminal domains (CTDs) harboring conserved modules for their function as transcription factors and their intracellular localization. Using reporter assays in tobacco protoplasts and yeast as well as glutathione-S-transferase (GST) pull-down assays, we demonstrate that short peptide motifs enriched with aromatic and large hydrophobic amino acid (aa) residues embedded in an acidic surrounding (AHA motifs) are essential for transcriptional activity of class A Hsfs. In contrast to this, class B and C Hsfs lack AHA motifs and have no activator function on their own. We also provide evidence for the function of a leucine (Leu)-rich region centered around a conserved QMGPhiL motif at the very C-terminus as a nuclear export signal (NES) of class A Hsfs. Sequence comparison indicates that the combination of a C-terminal AHA motif with the consensus sequence FWxxF/L,F/I/L as well as the adjacent NES represents a signature domain for plant class A Hsfs, which allowed to identify more than 60 new Hsfs from the expressed sequence tag (EST) database.