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Zika Entry Via AXL Receptors On Neural Stem Cells - Cell SC

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  • •Single-cell analysis reveals expression and specificity of candidate Zika receptors
  • •AXL shows strong expression in human radial glia, brain capillaries, and microglia
  • •Developing human retina progenitors also show high AXL expression
  • •AXL expression is conserved in rodents and human cerebral organoid model systems
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The recent outbreak of Zika virus (ZIKV) in Brazil has been linked to substantial increases in fetal abnormalities and microcephaly. However, information about the underlying molecular and cellular mechanisms connecting viral infection to these defects remains limited. In this study we have examined the expression of receptors implicated in cell entry of several enveloped viruses including ZIKV across diverse cell types in the developing brain. Using single-cell RNA-seq and immunohistochemistry, we found that the candidate viral entry receptor AXL is highly expressed by human radial glial cells, astrocytes, endothelial cells, and microglia in developing human cortex and by progenitor cells in developing retina. We also show that AXL expression in radial glia is conserved in developing mouse and ferret cortex and in human stem cell-derived cerebral organoids, highlighting multiple experimental systems that could be applied to study mechanisms of ZIKV infectivity and effects on brain development.

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In February 2016, the World Health Organization declared the 2015 outbreak of the Zika virus (ZIKV) in Central and South America a global health emergency (Heymann et al., 2016) following a strong correlation between cases of ZIKV infection and a dramatic increase in microcephaly cases in Brazil (Oliveira Melo et al., 2016, Schuler-Faccini et al., 2016). Subsequent reports have now established the ability of ZIKV to cross the human fetal-placental barrier to infect the developing central nervous system (Calvet et al., 2016, Martines et al., 2016,Mlakar et al., 2016). The neurotropism and neurovirulence of ZIKV has been appreciated in model systems since the earliest description of the virus (Bell et al., 1971, Dick, 1952, Dick et al., 1952), but it has only recently been described in human neural stem and progenitor cells using in vitro systems (Tang et al., 2016; P.P. Garcêz, E.C. Loiola, R.M. da Costa, L.M. Higa, P. Trindade, R. Delvecchio, J.M. Nascimento, R. Brindeiro, A. Tanuri, and S.K. Rehen, 2016,PeerJ, preprint). Although pathology data is currently limited, the first imaging studies and cases with confirmed ZIKV infection in the prenatal brain showed devastating consequences, including severe microcephaly, lissencephaly, hydrocephaly, necrosis, periventricular and cortical calcification, diffuse astrogliosis, and activated microglia (Mlakar et al., 2016, Schuler-Faccini et al., 2016). The findings of massive cell death and necrosis reflect a far more destructive process than occurs in many genetic forms of microcephaly.

Primary microcephaly is thought to result from a depletion of the founder population of radial glia, the neural stem cells in developing brain, either through cell death or premature differentiation (Barkovich et al., 2012). Infrequent cases of neurodevelopmental brain malformations including microcephaly have been reported in association with viral infections, including cytomegalovirus (CMV), rubella virus, West Nile Virus, HIV, herpes simplex, and chikungunya (Ahlfors et al., 1986, Gérardin et al., 2014, Lanari et al., 2012, Nakao and Chiba, 1970,O’Leary et al., 2006, Sinha et al., 1972, Teissier et al., 2014, von der Hagen et al., 2014). Of the few viruses known to cross the placental barrier, CMV infection causes similar neurodevelopmental brain abnormalities to those caused by ZIKV (Conboy et al., 1986, Fowler et al., 1992, Teissier et al., 2014). CMV neuroinvasiveness is mediated by a variety of entry factors, including integrins and EGFR, which are highly expressed by radial glia, a neural stem cell population. Higher expression of these entry proteins determines the initial susceptible cell population (Kawasaki et al., 2015).

Based on the role of neural stem cells in other forms of microcephaly, we hypothesized that human radial glia may selectively express proteins promoting ZIKV entry and infectivity during neurogenesis. In support of this hypothesis, two recent papers demonstrated the vulnerability of neural stem and progenitor cells to ZIKV using in vitro cultures derived from pluripotent stem cells (Tang et al., 2016; P.P. Garcêz, E.C. Loiola, R.M. da Costa, L.M. Higa, P. Trindade, R. Delvecchio, J.M. Nascimento, R. Brindeiro, A. Tanuri, and S.K. Rehen, 2016,PeerJ, preprint). Many surface proteins facilitate flavivirus entry into cells (Perera-Lecoin et al., 2014), but the precise mechanism remains largely unknown and additional factors may also contribute to infection. Several of these proteins are sufficient to support ZIKV entry into HEK293T cells that normally have low infectivity, including DC-SIGN (encoded by CD209), TIM1 (encoded byHAVCR1), TYRO3, and AXL. Furthermore, blocking or silencing AXL reduces infectivity in cultured fibroblasts and alveolar epithelial cells by as much as 90% (Hamel et al., 2015). Understanding the expression patterns of putative flavivirus receptors could strengthen the possible link between ZIKV infection and microcephaly and support the discovery of a mechanism of ZIKV neurovirulence.

To identify cell populations that may be particularly vulnerable to ZIKV infection, we analyzed the expression of candidate genes mediating flavivirus entry across single cells from the developing human cerebral cortex (Figure 1A). We previously classified single cells from developing cortex as astrocytes, radial glia, intermediate progenitor cells, and immature excitatory and inhibitory neurons using patterns of genome-wide gene expression (Pollen et al., 2015). To survey additional cell types, we also analyzed cells from developing cortex that express markers of microglia and endothelial cells (Table S1). Importantly, while many candidate entry receptors and attachment factors have been described, other unknown factors may mediate ZIKV entry, and we also include a global table of gene expression across single cells (Table S2). Across cell types, we found that multiple putative flavivirus entry receptor genes, including AXL and heat shock protein genes, showed a strong pattern of enrichment in radial glia cells, astrocytes, endothelial cells, and microglia, suggesting that these cell types may be particularly vulnerable to ZIKV infection (Figures 1A and 1B).

 

AXL, known to mediate ZIKV and dengue virus entry in human skin cells (Hamel et al., 2015), showed particularly high expression in radial glia (78/96 radial glia displayed expression greater than 6 log2 normalized read counts). In contrast, other candidate genes known to permit ZIKV entry showed more limited expression at this threshold including TYRO3 (7/418 cells and 5/96 radial glia) and CD209 (DC-SIGN, 0/418 cells, Figure S1). Based on these observations, we further investigated the expression pattern of AXL protein in primary human tissue samples using immunohistochemistry. At mid-neurogenesis, AXL is expressed in a highly reproducible pattern throughout the cortex, with strong expression bordering the lateral ventricle and in the outer subventricular zone (OSVZ) (Figures 2C, 2D, and S1). Closer examination revealed that staining along the ventricle resulted from specific localization of AXL to radial glia apical end-feet (Figures 2D and S1). AXL was also detected at the pial end-feet of radial glia near the meninges (Figure 2B). In recent years a second population of radial glial cells, known as outer radial glia (oRG), has been identified in the OSVZ of the developing human brain (Fietz et al., 2010, Hansen et al., 2010). We observed high levels of AXL in the cell bodies of oRG cells, accounting for the pattern of AXL labeling in the OSVZ (Figures 2C and S1). In addition, pronounced AXL immunostaining outlined brain capillaries (Figures 2A and S1), consistent with AXL expression observed in endothelial cells by single-cell analysis. We further examined AXL expression from stages of early neurogenesis (GW13.5) to term. We found that AXL expression persisted in radial glia throughout the period of neurogenesis and in capillaries and astrocytes to term but remained largely absent from SATB2-expressing neurons, even at later developmental stages (Figure S1).

 

A recent report of 29 infants with presumed ZIKV microcephaly reported that 10 (34.5%) had severe ocular abnormalities. The ocular lesions consisted of focal pigment mottling and chorioretinal atrophy that was particularly severe in the macula (de Paula Freitas et al., 2016). Therefore, we examined AXL expression in developing human retina. We dissected two human neural retina samples at GW10 and GW12 and captured single cells for mRNA sequencing. AXL was highly expressed in cells that had a stem cell gene signature (Figures 1G andS1). To confirm this finding, we immunostained tissue sections of developing retina. AXL was expressed along the outer margin of the neural retina, where it was co-expressed with SOX2, a marker of neural stem cells. In addition, AXL was highly enriched in cells of the ciliary marginal zone, adjacent to the neural retina (Figure 1G).

We next investigated the possible conservation of AXL expression across model systems that could be used to study the mechanism of ZIKV infection and pathogenicity. Public repositories of in situ hybridization data indicate AXL expression in mouse radial glia (Figure 2A). In addition, previous studies of mouse cortex reported enriched Axl expression in the apical end-feet of radial glia cells (Wang et al., 2011). We examined the expression pattern of AXL in developing ferret cortex and found that, similar to human cortex, AXL is expressed in the end-feet of radial glia cells at the ventricular edge and in oRG cells in the OSVZ (Figure 2B). Finally, we generated human iPSC-derived cerebral organoids and observed AXL expression along the lumen of neuroepithelial-like rosette regions in the organoids, which resemble the VZ of primary human cortex, and in SOX2-expressing cells away from the lumen (Figure S2). The specific expression of AXL in radial glia-like and oRG-like cells in the organoids and limited expression in neurons is consistent with observations from single-cell mRNA-seq analysis of similarly derived cerebral organoids (Figures 2C and S2). Interestingly, human cerebral organoids also contain cells that resemble early choroid plexus cells (Sakaguchi et al., 2015), and these cells strongly express AXL (Figures 2C and S2), consistent with the expression pattern in embryonic mouse (Figure 2A).

Here we report that the candidate ZIKV receptor AXL is highly enriched in radial glia, the neural stem cells of the human fetal cerebral cortex, providing a hypothesis for why these cells are particularly vulnerable to ZIKV infection and providing a candidate mechanism for ZIKV-induced microcephaly. This finding supports recent suggestions that ZIKV preferentially targets in-vitro-derived progenitor cells rather than immature neurons (Tang et al., 2016). Furthermore, we show that AXL is expressed by cortical astrocytes, blood microcapillaries, microglia, and progenitors in the neural retina and ciliary marginal zone. The latter finding could help explain how ZIKV causes ocular lesions (de Paula Freitas et al., 2016). The specificity of AXL expression in radial glia neural stem cells is also conserved in mouse and ferret cerebral cortex and in human PSC-derived cerebral organoids. We suggest that these diverse systems may support studies of ZIKV infectivity in radial glia and the downstream consequences that may mediate disease pathogenesis.

Transgenic mouse models of microcephaly mutations often show less severe phenotypes than human patients with the same mutation (Barkovich et al., 2012,Gruber et al., 2011, Lizarraga et al., 2010, Woods et al., 2005). Differences in brain development that include massively expanded OSVZ and increased diversity of cortical progenitors in the human cortex likely contribute to this difference. For example, the contribution of oRG cells to brain malformations such as microcephaly or lissencephaly is largely unknown, although this cell type becomes the predominant neural stem cell population in the developing primate and human cortex toward mid-gestation when OSVZ proliferation dramatically increases (Lukaszewicz et al., 2005, Hansen et al., 2010). Our results indicate that oRG cells express AXL at very high levels and are likely targets for ZIKV infectivity. Involvement of oRG cells, which have been linked to developmental and evolutionary cortical expansion (Hansen et al., 2010, Ostrem et al., 2014, Pollen et al., 2015), may make a significant contribution to the severe phenotype of ZIKV microcephaly and agyria.

Signaling through AXL suppresses the innate immune response (Rothlin et al., 2007). In dengue virus infection, AXL not only supports virus entry, but its kinase domain also enhances virus infectivity following entry (Meertens et al., 2012). If ZIKV binds AXL during entry, it may similarly activate AXL signaling and suppress the innate immune response, enabling the virus to better establish an infection and prevent viral clearance (Mlakar et al., 2016). These features suggest that a small-molecule inhibitor of AXL function may be protective against ZIKV infectivity. However, signaling through Axl normally supports neural stem cell survival, proliferation, and neurogenesis (Ji et al., 2014, Lemke and Burstyn-Cohen, 2010), and Axl also maintains the blood-brain barrier, protecting against the neurotropism of other viruses (Miner et al., 2015). Interference with normal AXL has been shown to stimulate production of inflammatory cytokines, promote microglia activation, and eventually lead to the loss of neural stem cells (Ji et al., 2013). Therefore, while blocking AXL may protect against cellular infection or viral replication, perturbation of AXL function may also have multiple adverse consequences.

We propose a testable hypothesis: after breaching the placental-fetal barrier, ZIKV reaches the developing brain by hematogenous spread or via the cerebrospinal fluid (CSF) and invades radial glia cells as the first target population with highest AXL expression, either through their processes that often make contact with blood vessels, or via their apical end-feet that make direct contact with the CSF. By preferentially destroying radial glia cells, the founder cell population that generates all cortical neurons, ZIKV can produce severe microcephaly. Future studies will be needed to test this hypothesis and particularly whether AXL expression alone determines the cellular population with enhanced neurotropism for ZIKV in the developing human brain or whether other binding factors, including genes expressed at low levels, may be involved. In addition, further studies are urgently needed to determine (1) how the virus crosses the placenta to infect fetal brain and causes generalized growth restriction (Brasil et al., 2016) and (2) whether the virus infects adult human brain, as ZIKV has recently been detected in the CSF of adults (Carteaux et al., 2016, Mécharles et al., 2016). Finally, other flaviviruses that use similar entry receptors have not been strongly associated with fetal brain abnormalities, and future work must examine potential changes in recent strains of ZIKV. The current manuscript constitutes an initial step toward the understanding of how ZIKV might cause developmental brain malformations.

Author Contributions

T.J.N. and E.D.L performed expression studies. T.J.N., A.A.P., and C.S.E. performed bioinformatic analysis. E.D.L. and M.B. generated cerebral organoids. T.J.N, A.A.P., and A.R.K. conceived of the project and wrote the manuscript with input from all authors.

Acknowledgments

We are grateful to Hanna Retallack, Shaohui Wang, Anne Leyrat, Joe Shuga, Aaron Diaz, Mercedes Paredes, Joseph LoTurco, Melanie Bedolli, Lillian Adame, Joe DeRisi, and Jeremy Reiter for helpful comments, suggestions, and technical help. A.A.P. is supported by a Damon Runyon Cancer Research Foundation postdoctoral fellowship (DRG-2166-13). This research was supported by NIH awards U01 MH105989, R37 NS35710, and R01NS075998 to A.R.K. and CIRM award GCIR-06673-A, and by gifts from Helen Ford and Bernard Osher. All work using human tissues and cells was conducted in strict observance of the protocols approved by the Human Gamete, Embryo, and Stem Cell Research Committee (institutional review board), and work involving animal tissues was performed in accordance with protocols approved by the University of California San Francisco Institutional Animal Care and Use Committee.

Accession Numbers

The accession number for the single cell sequencing data reported in this paper is dbGaP: phs000989.v2.p1.

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        Study explores mechanism linking Zika virus, birth defects

        Researchers say a protein the Zika virus uses to infect skin cells and cause a rash is also present in stem cells of the developing brain and retina of a fetus.
         
        By HealthDay News   |   March 30, 2016 at 12:07 PM
         
         
        microcephaly.jpg?resize=800:600
        Baby with microcephaly. Photo: U.S. Centers for Disease Control and Prevention
        WEDNESDAY, March 30, 2016 -- Scientists say they've discovered how the Zika virus might cause severe brain and eye birth defects.

        The Zika outbreak in Brazil and other parts of Latin American and the Caribbean has coincided with a sharp increase in the number of babies born with microcephaly, which results in abnormally small heads and brains.

        There has also been a rise in other brain and eye birth defects in countries affected by the Zika outbreak. But firm evidence of a link between the virus and these birth defects has been lacking.

        In a new study, researchers at the University of California, San Francisco (UCSF), found that a protein the Zika virus uses to infect skin cells and cause a rash is also present in stem cells of the developing brain and retina of a fetus.

        The so-called AXL protein sits on the surface of cells and can provide an entry point for Zika. Learning more about the link between Zika and AXL could lead to drugs to block Zika infection, according to the researchers.

        The brain and eye birth defects occurring in areas with Zika outbreaks are "precisely the kind of damage we would expect to see from something that was destroying neural and retinal stem cells during development," said study senior author Dr. Arnold Kriegstein. He is director of UCSF's Center of Regeneration Medicine and Stem Cell Research.

        "If we can understand how Zika may be causing birth defects, we can start looking for compounds to protect pregnant women who become infected," Kriegstein said in a university news release.

        The study was published online March 30 in the journal Cell Stem Cell.

        A mosquito-borne virus, Zika has been suspected of causing thousands of cases of microcephaly in Brazil.

        While the bulk of Zika cases leading to microcephaly may occur via maternal infection during pregnancy, cases of sexual transmission from a man to his female partner have come to light, according to the U.S. Centers for Disease Control and Prevention.

        Zika infection is usually a mild illness in adults, and many cases may occur without symptoms, experts say. However, because of the risk to babies, the CDC is advising that men with known or suspected infection with Zika refrain from sex -- or only have sex with a condom -- for six months after a diagnosis.

        The agency also advises that, for couples involving a man who has traveled to or resides in an area endemic for Zika:

         

        • The couple refrain from sex, or use condoms during sex, throughout the duration of a pregnancy.
        • They refrain from sex, or use condoms during sex, for eight weeks if the man has returned from travel to a Zika-endemic area but has not shown signs of infection.
        • For couples living in a Zika-endemic area, they refrain from sex or engage in sex only with a condom for as long as active Zika transmission persists in that area.

        The latest guidelines also recommend that women who know they've been infected, or who have no symptoms but have recently been to a Zika-endemic area, or think they might have been exposed via sex, should wait at least eight weeks before trying to get pregnant.

        The CDC has also advised that all pregnant women consider postponing travel to any area where Zika virus transmission is ongoing. If a pregnant woman must travel to or live in one of these areas, she should talk to her health-care provider first and strictly follow steps to prevent mosquito bites.

        In the majority of Zika infections, symptoms included rash (97 percent of cases), fever and joint pain.

        "Zika virus disease should be considered in patients with acute onset of fever, rash, arthralgia [joint pain], or conjunctivitis [pink eye] who traveled to areas with ongoing Zika virus transmission or who had unprotected sex with someone who traveled to one of those areas and developed compatible symptoms within two weeks of returning," according to the CDC.

        First discovered in Uganda in 1947, the Zika virus wasn't thought to pose major health risks until last year, when it became clear that it posed potentially devastating threats to pregnant women.

        The Zika virus has now spread to over 38 countries and territories, most in Latin America and the Caribbean. The World Health Organization estimates there could be up to 4 million cases of Zika in the Americas in the next year.

        More information

        For more on Zika virus, visit the U.S. Centers for Disease Control and Prevention.

        To see the CDC list of sites where Zika virus is active and may pose a threat to pregnant women, click here.

        http://www.upi.com/Health_News/2016/03/30/Study-explores-mechanism-linking-Zika-virus-birth-defects/3461459353864/

         

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