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To BBB or not to BBB?
Jean-Léon Thomas, Anne Eichmann
To cite this version:
Jean-Léon Thomas, Anne Eichmann. To BBB or not to BBB?. Developmental Cell, Elsevier, 2018, 47 (6), pp.689-691. �10.1016/j.devcel.2018.11.039�. �hal-01977015�
To BBB or not to BBB?
Jean-Leon Thomas1,2 and Anne Eichmann3,4
1Université Pierre et Marie Curie Paris 06 UMRS1127, Sorbonne Université, Institut du
Cerveau et de la Moelle Epinière, Paris, France
2Department of Neurology, Yale University School of Medicine, New Haven, CT, 06511,
USA
3INSERM U970, Paris Cardiovascular Research Center, 56 Rue Leblanc, 75015 Paris,
France
4Cardiovascular Research Center and the Department of Cellular and Molecular
Physiology, Yale University School of Medicine, New Haven, Connecticut 06510-3221, USA
Abstract
In this issue of Developmental Cell, Anbalagan et al. demonstrate that pituicytes, a
subtype of astroglia, drive endothelial cell permeability in the zebrafish embryo
neurohypophysis. This occurs via secretion of Vegfa/Tgfb3 permeability factors that
promote formation of fenestrae, and the repression of anti-inflammatory retinoic acid
signaling that induces tight cell-junctions.
Blood vessels in the central nervous system (CNS) form a Blood-Brain Barrier (BBB) that controls exchange between the circulatory system and the brain. Endothelial cells forming the inner lining of BBB vessels deliver oxygen and nutriments to brain tissues to ensure neuronal function, and also protect brain tissues by limiting entry of pathogens, inflammatory cells, and neurotoxic plasma components. BBB breakdown leads to neuronal injury and is associated with neurological disease, underscoring the fundamental role of the BBB in maintaining brain health (Zhao Z, 2015). However, some vessels in the brain need to be permeable to ensure exchange of nutrients, hormones and blood-borne proteins. As examples, capillaries in the choroid plexus and circumventricular organs surrounding the brain ventricles, such as the the hypothalamic median eminence and the neurohypophysis, do not develop a tight BBB but instead have permable pores called fenestrae that facilitate exchange of fluids, nutriments, and hormones between the blood, the cerebrospinal fluid (CSF) and the brain (Wolburg, 2010). A critical question then becomes how endothelial cells are instructed whether to form a BBB or not.
In this issue of Developmental Cell, Anbalagan et al. explore this issue by focusing on the hypothalamo-neurohypophyseal (HNS) system, where the neurohormones Arginine- Vasopressin (AVP), and Oxytocin (OXT) are released from axonal terminals into the circulation to regulate water homeostasis and labor/milk let-down, respectively. They use the zebrafish model to explore the properties of a specialized glial cell type, the pituicyte, for its signaling interactions with hypophyseal vessels and its function in controling capillary permability.
In the neurohypophysis, the region studied by studied by the authors, an astroglial cell type called the pituicyte displays dynamic interactions with axonal terminals secreting OXT and AVP peptides. It had been suggested that these pituicytes could modulate neurosecretion in response to physiological demands such as parturition, lactation or deshydratation (Theodosis, 2008), but how this was achieved mechanistically remained unclear.
The development of the vascular compartment of the hypothalamo-neurohypophysal system (HNS) has been shown to be regulated by Fgf3 and Fgf10 (Liu F, 2013), and by the OXT neuropeptide itself, which regulates endothelial morphogenesis in the zebrafish HNS (Gutnick et al., 2011). In the current study, Anbalagan et al. (2018) explore whether the development and plasticity of vessels are regulated by pituicytes in the HNS. This study is the first demonstration that pituicytes promote vascular permeability in the HNS. The authors use an elegant approach with three different fluorescent markers to distinguish the glial, vascular, and neuronal components of the HNS in vivo. They labeled pituicytes with a fluorescent dipeptide derivative (b-Ala-Lys-Ne-AMCA), which is actively taken up by HNS astroglial cells but not neurons. The peptide was injected into a zebrafish line expressing two other fluorochromes in endothelial cells and hypothalamic axons, respectively. The b-Ala-Lys-Ne-AMCA-labeled cells displayed tissue organization and cytological features characteristic of pituicytes in other species, including projections that contacted the termini of HNS axons near the basal lamina of fenestrated capillaries. Interestingly, as observed in other vertebrate species, zebrafish pituicytes also exhibited heterogeneous morphologies, which may contribute to their functional diversity.
Bulk RNA-Seq analysis of the transcriptome of FACS-sorted-b-Ala-Lys-AMCA-positive cells revealed hallmarks of astroglia (S100b) and pituicytes (Crabp1a, Cyp26b1), as expected, but also of activated microglia, with enriched expression of inflammatory cytokines and permeability growth factors (Il1b, Mmp2/9/14, Vegf-a, TGFb3). The expression of these genes was further confirmed by qPCR and in situ hybridization
studies, which also showed that pituicyte-specific genes were locally restricted in the neurohypophysis during development, supporting the idea that pituicytes may contribute to establish permeable neurovascular interfaces in the HNS.
The expression of pro-inflammatory and permeability genes by b -Ala-Lys-AMCA-positive cells suggested that the glial environment may provide a latent pro-inflammatory environment that could induce permeability in hypophyseal loop vessels. The permeability of vessels can result from the opening of tight junctions between endothelial cells and/or from trans-endothelial cell transport through fenestrated endothelial cells (review Komarova YA, 2017). Anbalagan et al. (2018) found that fenestrae and tight junctions of HNS endothelial cells were regulated by two independant pituicyte-derived signals, Vegfa/Tgfb3 and Retinoic Acid (RA), respectively.
Vegfa/Tgfb3 induces the expression of Plvap, a specific marker of fenestrae. Vegf-a is upregulated in reactive astrocytes upon inflammatory conditions and disrupts the BBB, and was previously implicated in the blood-hypothalamus barrier permeability (Langlet F, 2013). However, pituicyte-derived Tgfb signaling appears to be the major regulator of neurohypophyseal vascular permeability, as shown by Tgfb3 inhibition, which reduced the extravasation of tagged vitamin D-binding protein (DBP-GFP), from the blood to the pituitary, more dramatically than the Vegfr inhibitor SU5416. Vegfa and TGF-β may thus work in concert to control HNS endothelial cell permeability.
On the other hand, RA is known to be important for BBB formation and in restoration of BBB functional integrity in inflammatory conditions (Mizee MR, 2014). Interestingly, treatment with dexamethasone, a glucocorticoid suppressor of inflammation, inhibited DBP-GFP extravasation without modification of Vegfa or Tgfb transcript expression but with a strong decrease in the expression of the pituicyte marker Cyp26b1. Cyp26b is a major brain-specific enzyme that catabolizes RA, which suggested to the authors that pituicytes may attenuate RA signaling to promote vascular permeability. Using a specific Cyp26 inhibitor, talarozole, to block RA degradation during HNS development, the authors found that DBP-GFP extravasation was decreased, while the expression of Claudin 5, a specific marker of tight junctions, was increased (Anbalagan et al. 2018). Interstingly, talarozole treatment had no effect on plvap expression, indicating that pituicyte-derived Vegf-a/Tgfb3 and RA signals can act separately to regulate endothelial expression of Plvap in fenestrae and Claudin 5 in tight junctions, thereby modulating endothelial cell permeability.
Several intriguing questions arise from this study. For example, it’s not clear whether pituicytes alone provide permeability signals or whether they act in concert with microglial cells. FACS-sorted-b-Ala-Lys-AMCA-positive cells may include contaminating microglia or other cell types that could contribute to endothelial cell permeability. Microglia have been reported to be continuously activated in the circumventricular organs of mouse brain, hence they are poised to release inflammatory mediators(Takagi S, 2017). Alternatively, as the pituicyte population is heterogenous, perhaps specific pituicyte subsets may have a microglia-like inflammatory cell type. Single cell RNA Seq analysis of FACS-sorted-b-Ala-Lys-AMCA-positive cells should help to solve these questions and may extend the range of cell type-interactions involved in vascular plasticity in the neurohypophysis. They may also help uncover how pituicytes coordinate the action of their permeability-inducing signals. Such approaches may also identify if permeability-inducing signals released by other glial cell types are the same or different from those in pituicytes, and how these differ from permeability-restricting factors produced by pericytes and other CNS parenchymal astrocytes that maintain the BBB.
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