Functional Consequences of Synapse Remodeling Following Astrocyte-Specific Regulation of Ephrin-B1 in the Adult Hippocampus.

Astrocyte-derived factors can control synapse formation and functions, making astrocytes an attractive target for regulating neuronal circuits and associated behaviors. Abnormal astrocyte-neuronal interactions are also implicated in neurodevelopmental disorders and neurodegenerative diseases associated with impaired learning and memory. However, little is known about astrocyte-mediated mechanisms that regulate learning and memory. Here, we propose astrocytic ephrin-B1 as a regulator of synaptogenesis in adult hippocampus and mouse learning behaviors. We found that astrocyte-specific ablation of ephrin-B1 in male mice triggers an increase in the density of immature dendritic spines and excitatory synaptic sites in the adult CA1 hippocampus. However, the prevalence of immature dendritic spines is associated with decreased evoked postsynaptic firing responses in CA1 pyramidal neurons, suggesting impaired maturation of these newly formed and potentially silent synapses or increased excitatory drive on the inhibitory neurons resulting in the overall decreased postsynaptic firing. Nevertheless, astrocyte-specific ephrin-B1 knock-out male mice exhibit normal acquisition of fear memory but enhanced contextual fear memory recall. In contrast, overexpression of astrocytic ephrin-B1 in the adult CA1 hippocampus leads to the loss of dendritic spines, reduced excitatory input, and impaired contextual memory retention. Our results suggest that astrocytic ephrin-B1 may compete with neuronal ephrin-B1 and mediate excitatory synapse elimination through its interactions with neuronal EphB receptors. Indeed, a deletion of neuronal EphB receptors impairs the ability of astrocytes expressing functional ephrin-B1 to engulf synaptosomes in vitro Our findings demonstrate that astrocytic ephrin-B1 regulates long-term contextual memory by restricting new synapse formation in the adult hippocampus.SIGNIFICANCE STATEMENT These studies address a gap in our knowledge of astrocyte-mediated regulation of learning and memory by unveiling a new role for ephrin-B1 in astrocytes and elucidating new mechanisms by which astrocytes regulate learning. Our studies explore the mechanisms underlying astrocyte regulation of hippocampal circuit remodeling during learning using new genetic tools that target ephrin-B signaling in astrocytes in vivo On a subcellular level, astrocytic ephrin-B1 may compete with neuronal ephrin-B1 and trigger astrocyte-mediated elimination of EphB receptor-containing synapses. Given the role EphB receptors play in neurodevelopmental disorders and neurodegenerative diseases, these findings establish a foundation for future studies of astrocyte-mediated synaptogenesis in clinically relevant conditions that can help to guide the development of clinical applications for a variety of neurological disorders.

Activated microglia enhance neurogenesis via trypsinogen secretion.

White matter neurons in multiple sclerosis brains are destroyed during demyelination and then replaced in some chronic multiple sclerosis lesions that exhibit a morphologically distinct population of activated microglia [Chang A, et al. (2008) Brain 131(Pt 9):2366-2375]. Here we investigated whether activated microglia secrete factors that promote the generation of neurons from white matter cells. Adult rat brain microglia (resting or activated with lipopolysaccharide) were isolated by flow cytometry and cocultured with neonatal rat optic nerve cells in separate but media-connected chambers. Optic nerve cells cocultured with activated microglia showed a significant increase in the number of cells of neuronal phenotype, identified by neuron-specific class III beta-tubulin (TUJ-1) labeling, compared with cultures with resting microglia. To investigate the possible source of the TUJ-1-positive cells, A2B5-positive oligodendrocyte progenitor cells and A2B5-negative cells were isolated and cocultured with resting and activated microglia. Significantly more TUJ-1-positive cells were generated from A2B5-negative cells (∼70%) than from A2B5-positive cells (~30%). Mass spectrometry analysis of microglia culture media identified protease serine 2 (PRSS2) as a factor secreted by activated, but not resting, microglia. When added to optic nerve cultures, PRSS2 significantly increased neurogenesis, whereas the serine protease inhibitor, secretory leukocyte protease inhibitor, decreased activated microglia-induced neurogenesis. Collectively our data provide evidence that activated microglia increase neurogenesis through secretion of PRSS2.

The liver and muscle secreted HFE2-protein maintains central nervous system blood vessel integrity.

Liver failure causes breakdown of the Blood CNS Barrier (BCB) leading to damages of the Central-Nervous-System (CNS), however the mechanisms whereby the liver influences BCB-integrity remain elusive. One possibility is that the liver secretes an as-yet to be identified molecule(s) that circulate in the serum to directly promote BCB-integrity. To study BCB-integrity, we developed light-sheet imaging for three-dimensional analysis. We show that liver- or muscle-specific knockout of Hfe2/Rgmc induces BCB-breakdown, leading to accumulation of toxic-blood-derived fibrinogen in the brain, lower cortical neuron numbers, and behavioral deficits in mice. Soluble HFE2 competes with its homologue RGMa for binding to Neogenin, thereby blocking RGMa-induced downregulation of PDGF-B and Claudin-5 in endothelial cells, triggering BCB-disruption. HFE2 administration in female mice with experimental autoimmune encephalomyelitis, a model for multiple sclerosis, prevented paralysis and immune cell infiltration by inhibiting RGMa-mediated BCB alteration. This study has implications for the pathogenesis and potential treatment of diseases associated with BCB-dysfunction.

Corrigendum: Acute ablation of cortical pericytes leads to rapid neurovascular uncoupling.

[This corrects the article DOI: 10.3389/fncel.2020.00027.].

APOE4 accelerates advanced-stage vascular and neurodegenerative disorder in old Alzheimer's mice via cyclophilin A independently of amyloid-ß.

Apolipoprotein E4 (APOE4), the main susceptibility gene for Alzheimer's disease (AD), leads to vascular dysfunction, amyloid-ß pathology, neurodegeneration and dementia. How these different pathologies contribute to advanced-stage AD remains unclear. Using aged APOE knock-in mice crossed with 5xFAD mice, we show that, compared to APOE3, APOE4 accelerates blood-brain barrier (BBB) breakdown, loss of cerebral blood flow, neuronal loss and behavioral deficits independently of amyloid-ß. BBB breakdown was associated with activation of the cyclophilin A-matrix metalloproteinase-9 BBB-degrading pathway in pericytes. Suppression of this pathway improved BBB integrity and prevented further neuronal loss and behavioral deficits in APOE4;5FAD mice while having no effect on amyloid-ß pathology. Thus, APOE4 accelerates advanced-stage BBB breakdown and neurodegeneration in Alzheimer's mice via the cyclophilin A pathway in pericytes independently of amyloid-ß, which has implication for the pathogenesis and treatment of vascular and neurodegenerative disorder in AD.

Netrin participates in the development of retinotectal synaptic connectivity by modulating axon arborization and synapse formation in the developing brain.

Netrin has been implicated in retinal ganglion cell (RGC) axon pathfinding in a number of species. In Xenopus laevis, RGC axons reaching their target in the optic tectum can be repelled by a netrin-1 gradient in vitro, suggesting that netrin may also function in wiring events that follow successful axon pathfinding. Here, we examined the contribution of netrin to RGC axon arborization and synapse formation at the target. Time-lapse confocal microscopy imaging of individual RGC axons coexpressing GFP-synaptobrevin and DsRed in the intact Xenopus brain demonstrated a role for deleted in colorectal cancer (DCC)-mediated netrin signaling. Microinjection of netrin-1 into the tectum induced a rapid and transient increase in presynaptic site addition that resulted in higher presynaptic site density over a 24 h observation period. Moreover, netrin induced dynamic axon branching, increasing branch addition and retraction; a behavior that ultimately increased total branch number. In contrast, microinjection of DCC function-blocking antibodies prevented the increase in presynaptic site number normally observed in control axons as well as the associated increase in branch number and axon arbor growth. Dynamic analysis of axon arbors demonstrated that the effects of anti-DCC on axon morphology and presynaptic connectivity were attributable to a specific decrease in new synapse and branch additions, without affecting the stability of existing synapses and branches. Together, these results indicate that, in the absence of DCC signaling, RGC axons fail to branch and differentiate, and support a novel role for netrin in later phases of retinotectal development.

Acute Ablation of Cortical Pericytes Leads to Rapid Neurovascular Uncoupling.

Pericytes are perivascular mural cells that enwrap brain capillaries and maintain blood-brain barrier (BBB) integrity. Most studies suggest that pericytes regulate cerebral blood flow (CBF) and oxygen delivery to activated brain structures, known as neurovascular coupling. While we have previously shown that congenital loss of pericytes leads over time to aberrant hemodynamic responses, the effects of acute global pericyte loss on neurovascular coupling have not been studied. To address this, we used our recently reported inducible pericyte-specific Cre mouse line crossed to iDTR mice carrying Cre-dependent human diphtheria toxin (DT) receptor, which upon DT treatment leads to acute pericyte ablation. As expected, DT led to rapid progressive loss of pericyte coverage of cortical capillaries up to 50% at 3 days post-DT, which correlated with approximately 50% reductions in stimulus-induced CBF responses measured with laser doppler flowmetry (LDF) and/or intrinsic optical signal (IOS) imaging. Endothelial response to acetylcholine, microvascular density, and neuronal evoked membrane potential responses remained, however, unchanged, as well as arteriolar smooth muscle cell (SMC) coverage and functional responses to adenosine, as we previously reported. Together, these data suggest that neurovascular uncoupling in this model is driven by pericyte loss, but not other vascular deficits or neuronal dysfunction. These results further support the role of pericytes in CBF regulation and may have implications for neurological conditions associated with rapid pericyte loss such as hypoperfusion and stroke, as well as conditions where the exact time course of global regional pericyte loss is less clear, such as Alzheimer's disease (AD) and other neurogenerative disorders.

3K3A-activated protein C blocks amyloidogenic BACE1 pathway and improves functional outcome in mice.

3K3A-activated protein C (APC), a cell-signaling analogue of endogenous blood serine protease APC, exerts vasculoprotective, neuroprotective, and anti-inflammatory activities in rodent models of stroke, brain injury, and neurodegenerative disorders. 3K3A-APC is currently in development as a neuroprotectant in patients with ischemic stroke. Here, we report that 3K3A-APC inhibits BACE1 amyloidogenic pathway in a mouse model of Alzheimer's disease (AD). We show that a 4-mo daily treatment of 3-mo-old 5XFAD mice with murine recombinant 3K3A-APC (100 µg/kg/d i.p.) prevents development of parenchymal and cerebrovascular amyloid-ß (Aß) deposits by 40-50%, which is mediated through NFκB-dependent transcriptional inhibition of BACE1, resulting in blockade of Aß generation in neurons overexpressing human Aß-precursor protein. Consistent with reduced Aß deposition, 3K3A-APC normalized hippocampus-dependent behavioral deficits and cerebral blood flow responses, improved cerebrovascular integrity, and diminished neuroinflammatory responses. Our data suggest that 3K3A-APC holds potential as an effective anti-Aß prevention therapy for early-stage AD.

Pericyte loss leads to circulatory failure and pleiotrophin depletion causing neuron loss.

Pericytes are positioned between brain capillary endothelial cells, astrocytes and neurons. They degenerate in multiple neurological disorders. However, their role in the pathogenesis of these disorders remains debatable. Here we generate an inducible pericyte-specific Cre line and cross pericyte-specific Cre mice with iDTR mice carrying Cre-dependent human diphtheria toxin receptor. After pericyte ablation with diphtheria toxin, mice showed acute blood-brain barrier breakdown, severe loss of blood flow, and a rapid neuron loss that was associated with loss of pericyte-derived pleiotrophin (PTN), a neurotrophic growth factor. Intracerebroventricular PTN infusions prevented neuron loss in pericyte-ablated mice despite persistent circulatory changes. Silencing of pericyte-derived Ptn rendered neurons vulnerable to ischemic and excitotoxic injury. Our data demonstrate a rapid neurodegeneration cascade that links pericyte loss to acute circulatory collapse and loss of PTN neurotrophic support. These findings may have implications for the pathogenesis and treatment of neurological disorders that are associated with pericyte loss and/or neurovascular dysfunction.

Pericyte degeneration causes white matter dysfunction in the mouse central nervous system.

Diffuse white-matter disease associated with small-vessel disease and dementia is prevalent in the elderly. The biological mechanisms, however, remain elusive. Using pericyte-deficient mice, magnetic resonance imaging, viral-based tract-tracing, and behavior and tissue analysis, we found that pericyte degeneration disrupted white-matter microcirculation, resulting in an accumulation of toxic blood-derived fibrin(ogen) deposits and blood-flow reductions, which triggered a loss of myelin, axons and oligodendrocytes. This disrupted brain circuits, leading to white-matter functional deficits before neuronal loss occurs. Fibrinogen and fibrin fibrils initiated autophagy-dependent cell death in oligodendrocyte and pericyte cultures, whereas pharmacological and genetic manipulations of systemic fibrinogen levels in pericyte-deficient, but not control mice, influenced the degree of white-matter fibrin(ogen) deposition, pericyte degeneration, vascular pathology and white-matter changes. Thus, our data indicate that pericytes control white-matter structure and function, which has implications for the pathogenesis and treatment of human white-matter disease associated with small-vessel disease.

Early post-hatching sex differences in cell proliferation and survival in the quail telencephalic ventricular zone and intermediate medial mesopallium.

Previous studies indicated that avian telencephalic areas related to learned behavior, such as song perception and production, are sexually dimorphic. Our study focused on the eventual occurrence of dimorphism in the intermediate medial mesopallium, an area associated with learning in non-singing birds. During early post-hatching life (days 1 and 5) cell proliferation and survival of newborn cells were studied by means of 5-bromo-2-deoxy-uridine immunocytochemistry. Programmed cell death (apoptosis) was investigated at post-hatching day 10. The ventricular zone, intermediate medial part of mesopallium and lateral septal area was analyzed using stereological methods for cell counts. Short-term experiments revealed significantly higher numbers of newborn cells in male ventricular zone of mesopallium compared to female at post-hatching day 1. Long-term survival until post-hatching day 20 showed significantly higher numbers of labeled cells in the male compared to female intermediate medial part of mesopallium, which is the final destination of migrating cells born in the overlying ventricular zone. The vast majority of these early post-hatching newborn cells residing in the intermediate medial part of mesopallium expressed a neuronal phenotype. In addition to neurogenesis, higher numbers of apoptotic figures were found in the male intermediate medial part of mesopallium at post-hatching day 10, suggesting that cell death plays a role in the control of telencephalic regional cell density in males. Our findings indicate that sex-specific mechanisms possibly stimulate increased cell genesis and survival, as well as the counteracting event of increased apoptotic cell death that characterized the male intermediate medial part of mesopallium.

Presenilin 1 promotes trypsin-induced neuroprotection via the PAR2/ERK signaling pathway. Effects of presenilin 1 FAD mutations.

Mutants of presenilin 1 (PS1) increase neuronal cell death causing autosomal-dominant familial Alzheimer's disease (FAD). Recent literature shows that treatment of neuronal cultures with low concentrations of trypsin, a member of the serine family of proteases, protects neurons from toxic insults by binding to the proteinase-activated receptor 2 and stimulating survival kinase extracellular signal-regulated kinase (ERK 1/2). Other studies show that PS1 is necessary for the neuroprotective activity of specific neurotrophic factors, such as brain-derived neurotrophic factor, against excitotoxicity and oxidative stress. Here, we show that treatment of mouse cortical neuronal cultures with trypsin activates ERK1/2 and protects neurons against glutamate excitoxicity. The trypsin-dependent ERK activation and neuroprotection requires both alleles of PS1 because neither PS1 knockout nor PS1 hemizygous neuronal cultures can use exogenous trypsin to activate ERK1/2 or increase neuronal survival. The protective effect of PS1 does not depend on its γ-secretase activity because inhibitors of γ-secretase have no effect on trypsin-mediated neuroprotection. Importantly, cortical neuronal cultures either heterozygous or homozygous for PS1 FAD mutants are unable to use trypsin to activate ERK1/2 and rescue neurons from excitotoxicity, indicating that FAD mutants inhibit trypsin-dependent neuroprotection in an autosomal-dominant manner. Furthermore, our data support the theory that PS FAD mutants increase neurodegeneration by inhibiting the ability of neurons to use cellular factors as protective agents against toxic insults.

Astrocytic Ephrin-B1 Regulates Synapse Remodeling Following Traumatic Brain Injury.

Traumatic brain injury (TBI) can result in tissue alterations distant from the site of the initial injury, which can trigger pathological changes within hippocampal circuits and are thought to contribute to long-term cognitive and neuropsychological impairments. However, our understanding of secondary injury mechanisms is limited. Astrocytes play an important role in brain repair after injury and astrocyte-mediated mechanisms that are implicated in synapse development are likely important in injury-induced synapse remodeling. Our studies suggest a new role of ephrin-B1, which is known to regulate synapse development in neurons, in astrocyte-mediated synapse remodeling following TBI. Indeed, we observed a transient upregulation of ephrin-B1 immunoreactivity in hippocampal astrocytes following moderate controlled cortical impact model of TBI. The upregulation of ephrin-B1 levels in hippocampal astrocytes coincided with a decline in the number of vGlut1-positive glutamatergic input to CA1 neurons at 3 days post injury even in the absence of hippocampal neuron loss. In contrast, tamoxifen-induced ablation of ephrin-B1 from adult astrocytes in ephrin-B1(loxP/y)ERT2-Cre(GFAP) mice accelerated the recovery of vGlut1-positive glutamatergic input to CA1 neurons after TBI. Finally, our studies suggest that astrocytic ephrin-B1 may play an active role in injury-induced synapse remodeling through the activation of STAT3-mediated signaling in astrocytes. TBI-induced upregulation of STAT3 phosphorylation within the hippocampus was suppressed by astrocyte-specific ablation of ephrin-B1 in vivo, whereas the activation of ephrin-B1 in astrocytes triggered an increase in STAT3 phosphorylation in vitro. Thus, regulation of ephrin-B1 signaling in astrocytes may provide new therapeutic opportunities to aid functional recovery after TBI.