Self-renewing human naïve pluripotent stem cells dedifferentiate in 3D culture and form blastoids spontaneously

Human naïve pluripotent stem cells (hnPSCs) can generate integrated models of blastocysts termed blastoids upon switch to inductive medium. However, the underlying mechanisms remain obscure. Here we report that self-renewing hnPSCs spontaneously and efficiently give rise to blastoids upon three dimensional (3D) suspension culture. The spontaneous blastoids mimic early stage human blastocysts in terms of structure, size, and transcriptome characteristics and are capable of progressing to post-implantation stages. This property is conferred by the glycogen synthase kinase-3 (GSK3) signalling inhibitor IM-12 present in 5iLAF self-renewing medium. IM-12 upregulates oxidative phosphorylation-associated genes that underly the capacity of hnPSCs to generate blastoids spontaneously. Starting from day one of self-organization, hnPSCs at the boundary of all 3D aggregates dedifferentiate into E5 embryo-like intermediates. Intermediates co-express SOX2/OCT4 and GATA6 and by day 3 specify trophoblast fate, which coincides with cavity and blastoid formation. In summary, spontaneous blastoid formation results from 3D culture triggering dedifferentiation of hnPSCs into earlier embryo-like intermediates which are then competent to segregate blastocyst fates.

Generation of 3D Trophoblast Organoids from Human Naïve Pluripotent Stem Cells

The human placenta is a transient organ that functions to support the needs of the fetus throughout gestation. Trophoblasts are the major epithelial cells found within the placenta and comprise a variety of distinct cell types with specialized roles in fetal-maternal communication. Our understanding of human trophoblast development remains limited due to ethical and legal restrictions on accessing first-trimester placental tissues, as well as the inability of common animal models to replicate primate placental development. It is therefore important to advance in vitro models of human trophoblast development as a basis for studying pregnancy-associated complications and diseases. In this chapter, we describe a protocol for generating 3D trophoblast organoids from naïve human pluripotent stem cells (hPSCs). The resulting stem-cell-derived trophoblast organoids (SC-TOs) contain distinct cytotrophoblast (CTB), syncytiotrophoblast (STB), and extravillous trophoblast (EVT) cell types, which closely correspond to trophoblast identities in the human post-implantation embryo. We discuss methods for characterizing SC-TOs by immunofluorescence, flow cytometry, mRNA and microRNA expression profiling, and placental hormone secretion. Furthermore, SC-TOs can undergo differentiation into specialized 3D EVT organoids, which display robust invasion when co-cultured with human endometrial cells. Thus, the protocol described herein offers an accessible 3D model system of human placental development and trophoblast invasion.

3D-cultured blastoids model human embryogenesis from pre-implantation to early gastrulation stages

Naive human pluripotent stem cells have the remarkable ability to self-organize into blastocyst-like structures ("blastoids") that model lineage segregation in the pre-implantation embryo. However, the extent to which blastoids can recapitulate the defining features of human post-implantation development remains unexplored. Here, we report that blastoids cultured on thick three-dimensional (3D) extracellular matrices capture hallmarks of early post-implantation development, including epiblast lumenogenesis, rapid expansion and diversification of trophoblast lineages, and robust invasion of extravillous trophoblast cells by day 14. Extended blastoid culture results in the localized activation of primitive streak marker TBXT and the emergence of embryonic germ layers by day 21. We also show that the modulation of WNT signaling alters the balance between epiblast and trophoblast fates in post-implantation blastoids. This work demonstrates that 3D-cultured blastoids offer a continuous and integrated in vitro model system of human embryonic and extraembryonic development from pre-implantation to early gastrulation stages.

The immune checkpoint molecule, VTCN1/B7-H4, guides differentiation and suppresses proinflammatory responses and MHC class I expression in an embryonic stem cell-derived model of human trophoblast

The placenta acts as a protective barrier to pathogens and other harmful substances present in the maternal circulation throughout pregnancy. Disruption of placental development can lead to complications of pregnancy such as preeclampsia, intrauterine growth retardation and preterm birth. In previous work, we have shown that expression of the immune checkpoint regulator, B7-H4/VTCN1, is increased upon differentiation of human embryonic stem cells (hESC) to an in vitro model of primitive trophoblast (TB), that VTCN1/B7-H4 is expressed in first trimester but not term human placenta and that primitive trophoblast may be uniquely susceptible to certain pathogens. Here we report on the role of VTCN1 in trophoblast lineage development and anti-viral responses and the effects of changes in these processes on major histocompatibility complex (MHC) class I expression and peripheral NK cell phenotypes.

Accessing the human trophoblast stem cell state from pluripotent and somatic cells

Trophoblasts are specialized epithelial cells that perform critical functions during blastocyst implantation and mediate maternal-fetal communication during pregnancy. However, our understanding of human trophoblast biology remains limited since access to first-trimester placental tissue is scarce, especially between the first and fourth weeks of development. Moreover, animal models inadequately recapitulate unique aspects of human placental physiology. In the mouse system, the isolation of self-renewing trophoblast stem cells has provided a valuable in vitro model system of placental development, but the derivation of analogous human trophoblast stem cells (hTSCs) has remained elusive until recently. Building on a landmark study reporting the isolation of bona fide hTSCs from blastocysts and first-trimester placental tissues in 2018, several groups have developed methods to derive hTSCs from pluripotent and somatic cell sources. Here we review the biological and molecular properties that define authentic hTSCs, the trophoblast potential of distinct pluripotent states, and methods for inducing hTSCs in somatic cells by direct reprogramming. The generation of hTSCs from pluripotent and somatic cells presents exciting opportunities to elucidate the molecular mechanisms of human placental development and the etiology of pregnancy-related diseases.

A genome-wide CRISPR-Cas9 knockout screen identifies essential and growth-restricting genes in human trophoblast stem cells

The recent derivation of human trophoblast stem cells (hTSCs) provides a scalable in vitro model system of human placental development, but the molecular regulators of hTSC identity have not been systematically explored thus far. Here, we utilize a genome-wide CRISPR-Cas9 knockout screen to comprehensively identify essential and growth-restricting genes in hTSCs. By cross-referencing our data to those from similar genetic screens performed in other cell types, as well as gene expression data from early human embryos, we define hTSC-specific and -enriched regulators. These include both well-established and previously uncharacterized trophoblast regulators, such as ARID3A, GATA2, and TEAD1 (essential), and GCM1, PTPN14, and TET2 (growth-restricting). Integrated analysis of chromatin accessibility, gene expression, and genome-wide location data reveals that the transcription factor TEAD1 regulates the expression of many trophoblast regulators in hTSCs. In the absence of TEAD1, hTSCs fail to complete faithful differentiation into extravillous trophoblast (EVT) cells and instead show a bias towards syncytiotrophoblast (STB) differentiation, thus indicating that this transcription factor safeguards the bipotent lineage potential of hTSCs. Overall, our study provides a valuable resource for dissecting the molecular regulation of human placental development and diseases.

Stem-cell-derived trophoblast organoids model human placental development and susceptibility to emerging pathogens

Trophoblast organoids derived from placental villi provide a 3D model system of human placental development, but access to first-trimester tissues is limited. Here, we report that trophoblast stem cells isolated from naive human pluripotent stem cells (hPSCs) can efficiently self-organize into 3D stem-cell-derived trophoblast organoids (SC-TOs) with a villous architecture similar to primary trophoblast organoids. Single-cell transcriptome analysis reveals the presence of distinct cytotrophoblast and syncytiotrophoblast clusters and a small cluster of extravillous trophoblasts, which closely correspond to trophoblast identities in the post-implantation embryo. These organoid cultures display clonal X chromosome inactivation patterns previously described in the human placenta. We further demonstrate that SC-TOs exhibit selective vulnerability to emerging pathogens (SARS-CoV-2 and Zika virus), which correlates with expression levels of their respective entry factors. The generation of trophoblast organoids from naive hPSCs provides an accessible 3D model system of the developing placenta and its susceptibility to emerging pathogens.

Use of a human embryonic stem cell model to discover GABRP, WFDC2, VTCN1 and ACTC1 as markers of early first trimester human trophoblast

Human placental development during early pregnancy is poorly understood. Many conceptuses are lost at this stage. It is thought that preeclampsia, intrauterine growth restriction and other placental syndromes that manifest later in pregnancy may originate early in placentation. Thus, there is a need for models of early human placental development. Treating human embryonic stem cells (hESCs) with BMP4 (bone morphogenic protein 4) plus A83-01 (ACTIVIN/NODAL signaling inhibitor) and PD173074 (fibroblast growth factor 2 or FGF2 signaling inhibitor) (BAP conditions) induces differentiation to the trophoblast lineage (hESCBAP), but it is not clear which stage of trophoblast differentiation these cells resemble. Here, comparison of the hESCBAP transcriptome to those of trophoblasts from human blastocysts, trophoblast stem cells and placentas collected in the first-third trimester of pregnancy by principal component analysis suggests that hESC after 8 days BAP treatment most resemble first trimester syncytiotrophoblasts. To further test this hypothesis, transcripts were identified that are expressed in hESCBAP but not in cultures of trophoblasts isolated from term placentas. Proteins encoded by four genes, GABRP (gamma-aminobutyric acid type A receptor subunit Pi), WFDC2 (WAP four-disulfide core domain 2), VTCN1 (V-set domain containing T-cell activation inhibitor 1) and ACTC1 (actin alpha cardiac muscle 1), immunolocalized to placentas at 4-9 weeks gestation, and their expression declined with gestational age (R2 = 0.61-0.83). None are present at term. Expression was largely localized to syncytiotrophoblast of both hESCBAP cells and placental material from early pregnancy. WFDC2, VTCN1 and ACTC1 have not previously been described in placenta. These results support the hypothesis that hESCBAP represent human trophoblast analogous to that of early first trimester and are a tool for discovery of factors important to this stage of placentation.

Valproic acid enhances the efficacy of radiation therapy by protecting normal hippocampal neurons and sensitizing malignant glioblastoma cells

Neurocognitive deficits are serious sequelae that follow cranial irradiation used to treat patients with medulloblastoma and other brain neoplasms. Cranial irradiation causes apoptosis in the subgranular zone of the hippocampus leading to cognitive deficits. Valproic acid (VPA) treatment protected hippocampal neurons from radiation-induced damage in both cell culture and animal models. Radioprotection was observed in VPA-treated neuronal cells compared to cells treated with radiation alone. This protection is specific to normal neuronal cells and did not extend to cancer cells. In fact, VPA acted as a radiosensitizer in brain cancer cells. VPA treatment induced cell cycle arrest in cancer cells but not in normal neuronal cells. The level of anti-apoptotic protein Bcl-2 was increased and the pro-apoptotic protein Bax was reduced in VPA treated normal cells. VPA inhibited the activities of histone deacetylase (HDAC) and glycogen synthase kinase-3β (GSK3β), the latter of which is only inhibited in normal cells. The combination of VPA and radiation was most effective in inhibiting tumor growth in heterotopic brain tumor models. An intracranial orthotopic glioma tumor model was used to evaluate tumor growth by using dynamic contrast-enhanced magnetic resonance (DCE MRI) and mouse survival following treatment with VPA and radiation. VPA, in combination with radiation, significantly delayed tumor growth and improved mouse survival. Overall, VPA protects normal hippocampal neurons and not cancer cells from radiation-induced cytotoxicity both in vitro and in vivo. VPA treatment has the potential for attenuating neurocognitive deficits associated with cranial irradiation while enhancing the efficiency of glioma radiotherapy.

Abstract 846: Inhibition of lysophosphatidic acid receptor 1 radiosensitizes lung cancer cells

Despite advances in diagnostic and therapeutic modalities, lung cancer remains the leading cause of cancer-related mortality in the United States. In most cases, lung cancer presents as an unresectable mass requiring a combination of chemotherapy and radiotherapy (CRT) for treatment. A barrier to efficacy of this treatment is radioresistance. Radiation induced activation of pro-survival pathways are currently thought to play an important role in radioresistance. The enzyme cytosolic phospholipase A2 (cPLA2) has been identified as a key component in radiation induced pro-suvival pathways. Ionizing radiation activates cPLA2, which then cleaves phosphatidylcholine (PC) to yield lysophosphatidylcholine (LPC). LPC is then converted to lysophosphatidic acid (LPA) by autotaxin (ATX). LPA binds to distinct G-protein coupled receptors (GPCRs) on the plasma membrane; LPA1, LPA2 and LPA3, which are encoded by the endothelial differentiation gene family. These receptors are involved in the regulation of various aspects of cancer, including proliferation, migration and metastasis. Previously, we have shown that inhibition of cPLA2, as well as inhibition of ATX, was sufficient to enhance the sensitivity of cancer cells to radiation. However, the question remains as to whether inhibition of LPA receptor activity would have a similar effect. Thus, we studied the role of LPA receptors as mediators of radioresponse in human A549 and murine Lewis Lung Carcinoma (LLC) cell models.

Quantitative polymerase chain reaction (QPCR) analysis of lung cancer cells showed that LPA1 was highly expressed in LLC and A549, and that both cell lines had nearly undetectable levels of LPA2 and LPA3 mRNA. Hence, we used Ki16425 and VPC12249, specific inhibitors of LPA1 and LPA3, to study the effect of LPA1 inhibition on clonogenic survival, cellular proliferation and invasion after irradiation. We found that pre-treatment of LLC and A549 cells with 10µM Ki16425 or 10µM VPC12249 significantly attenuated cell proliferation and viability after irradiation. Pre-treatment of LLC cells with 10µM Ki16425 or 10µM VPC12249 also reduced clonogenic survival. Transwell cell-invasion assays showed that the combination of LPA1 inhibitors with irradiation significantly reduced LPA-induced cell invasion in A549 and LLC cells. To investigate a potential mechanism for these effects, we also determined the activation of AKT and ERK signaling pathways. Inhibition of LPA1 resulted in reduced radiation-induced phosphorylation of both AKT and ERK. Overall, our results indicate that LPA1 is a mediator of pro-survival signaling in the adaptive response to radiation, and could serve as a viable target for enhancing the efficacy of radiation therapy of lung cancer.

Note: This abstract was not presented at the meeting.

Citation Format: David Y.A. Dadey, Rowan M. Karvas, Rama Kotipatruni, Jerry Jaboin, Dennis Hallahan, Dinesh Thotala. Inhibition of lysophosphatidic acid receptor 1 radiosensitizes lung cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 846. doi:10.1158/1538-7445.AM2014-846

Abstract 3941: The Egr1 transcription factor contributes to radiation-induced apoptosis in the mouse hippocampus and intestinal crypts

Although radiation therapy is a mainstay of cancer treatments, it can have deleterious effects on normal tissues, leading to poor quality of life for cancer survivors. Stem cells in normal tissues are particularly sensitive to radiation. It is believed that they are depleted after radiation, resulting in late sequelae such as low IQ, cognitive disorders, intestinal malabsorption, infertility, and skin injuries. Our laboratory has found that normal tissue stem cells express high levels of Early Growth Response 1 (Egr1) protein and mRNA. Egr1 is a zinc-finger transcription factor that initiates early signaling events in response to ionizing radiation. Radiation enhances Egr1 expression and the Egr1 protein subsequently activates the transcription of genes involved in cell death. Therefore, we hypothesized that Egr1 could contribute to the sensitivity of stem cells to ionizing radiation. Using mouse models, we investigated the effect of Egr1 status on cell death in two clinically important stem cell niches-the dentate gyrus of the hippocampus and crypts of the intestine. Egr1 null mice had significantly less cell death in the dentate gyrus and small intestinal crypts following irradiation, as compared to their wild-type littermates. To elucidate the molecular basis underlying the resistance of Egr1 null hippocampi to radiation-induced cell death, we performed immunoblotting on ex vivo protein lysates made from Egr1 wild-type and knockout hippocampi. Protein lysates from Egr1 wild-type hippocampi showed that radiation treatment induced Egr1, as well as its known targets–p53 and regulators of the mitochondrial pathway of apoptosis, Bim and Bax. In contrast, the induction of these pro-apoptotic proteins by radiation was attenuated in Egr1 null hippocampal lysates. These results suggest that Egr1 is involved in the apoptosis mode of cell death in the irradiated hippocampus. The mechanism may involve Egr1 induction by ionizing radiation, followed by direct binding of Egr1 to the promoters of p53, Bim, and Bax to allow for transcription. We further conducted studies using mouse embryonic stem cell models and showed that knockdown of Egr1 using siRNA decreased cell death after radiation treatment. This finding supports our in vivo data that the hippocampal and gastrointestinal stem cell niches are radioprotected in Egr1 null mice. Thus, this study establishes an important role for Egr1 in radiation-induced apoptosis of hippocampal and intestinal tissues. Egr1 could be a potential molecular target to minimize the normal tissue complications associated with radiation therapy.

Citation Format: Diana Y. Zhao, Keith M. Jacobs, Rowan M. Karvas, Jarrett L. Joubert, Dennis E. Hallahan, Dinesh Thotala. The Egr1 transcription factor contributes to radiation-induced apoptosis in the mouse hippocampus and intestinal crypts. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3941. doi:10.1158/1538-7445.AM2014-3941

Autotaxin Inhibition with PF-8380 Enhances the Radiosensitivity of Human and Murine Glioblastoma Cell Lines

Purpose: Glioblastoma multiforme (GBM) is an aggressive primary brain tumor that is radio-resistant and recurs despite aggressive surgery, chemo, and radiotherapy. Autotaxin (ATX) is over expressed in various cancers including GBM and is implicated in tumor progression, invasion, and angiogenesis. Using the ATX specific inhibitor, PF-8380, we studied ATX as a potential target to enhance radiosensitivity in GBM.

Methods and Materials: Mouse GL261 and Human U87-MG cells were used as GBM cell models. Clonogenic survival assays and tumor transwell invasion assays were performed using PF-8380 to evaluate role of ATX in survival and invasion. Radiation dependent activation of Akt was analyzed by immunoblotting. Tumor induced angiogenesis was studied using the dorsal skin fold model in GL261. Heterotopic mouse GL261 tumors were used to evaluate the efficacy of PF-8380 as a radiosensitizer.

Results: Pre-treatment of GL261 and U87-MG cells with 1 μM PF-8380 followed by 4 Gy irradiation resulted in decreased clonogenic survival, decreased migration (33% in GL261; P = 0.002 and 17.9% in U87-MG; P = 0.012), decreased invasion (35.6% in GL261; P = 0.0037 and 31.8% in U87-MG; P = 0.002), and attenuated radiation-induced Akt phosphorylation. In the tumor window model, inhibition of ATX abrogated radiation induced tumor neovascularization (65%; P = 0.011). In a heterotopic mouse GL261 tumors untreated mice took 11.2 days to reach a tumor volume of 7000 mm³, however combination of PF-8380 (10 mg/kg) with irradiation (five fractions of 2 Gy) took more than 32 days to reach a tumor volume of 7000 mm³.

Conclusion: Inhibition of ATX by PF-8380 led to decreased invasion and enhanced radiosensitization of GBM cells. Radiation-induced activation of Akt was abrogated by inhibition of ATX. Furthermore, inhibition of ATX led to diminished tumor vascularity and delayed tumor growth. These results suggest that inhibition of ATX may ameliorate GBM response to radiotherapy.

Abstract 2136: Inhibition of autotaxin enhances radiosensitivity in human and murine glioblastoma cell lines

Glioblastoma multiforme (GBM), a grade IV astrocytoma, is a very aggressive and prevalent primary brain tumor with a median survival rate of 12-15 months. Though the standard treatment relies on combinations of chemotherapy, radiation therapy and surgical resection, GBM remains radio-resistant with a high recurrence rate after resection. Understanding the molecular mechanisms involved in the therapeutic response could lead to the identification of novel targets to improve therapy. Radiation can trigger the activation of cytosolic phospholipase A2 (cPLA2), leading to production of lipid second-messengers such as lysophosphatidylcholine (LPC). Autotaxin (ATX), also known as lysophospholipase D (LysoPLD), is a secreted enzyme that catalyzes the production of lysophosphatidic acid (LPA) in the tumor microenvironment by cleaving the head-group of LPC. Specific G-protein coupled receptors (GPCRs) mediate the autocrine and paracrine effects of LPA such as migration, angiogenesis and proliferation in cancer. ATX is highly expressed in glioblastoma cells and is known to contribute to its invasive properties. We studied ATX as a potential target to enhance radiosensitivity in GBM by targeting ATX with PF-8380, a specific inhibitor developed by Pfizer Inc., and genetically by using shRNA. Pre-treatment with 1μM PF-8380 followed by irradiation with 4Gy resulted in decreased clonogenic survival, decreased migration and decreased invasion in mouse glioblastoma GL-261and human glioblastoma U87-MG cells. We confirmed these results by knocking down ATX with shRNA in GL-261 and U87-MG cells. Inhibition of ATX by PF-8380, or its knockdown by shRNA, lead to attenuation of Akt phosphorylation in GL-261 and U87-MG cells.

Inhibition or knockdown of ATX resulted in significantly reduced radiation-induced neovascularization as assayed with GL-261 tumor vascular window model. The effect of ATX inhibition by PF-8380 in-vivo was studied using a heterotopic mouse GL-261 tumor model. Tumor bearing mice were treated with DMSO or 1 mg/kg PF-8380 followed by irradiation with 1.8Gy for 5 consecutive days. Tumor growth delay was analyzed using a non-parametric (Kruskal-Wallis) test to determine the amount of days to reach a tumor volume of 0.7 cm3 and ANOVA to determine the difference between treatments. Tumors treated with DMSO or PF-8380 took 11.2 and 12.2 days respectively to reach a volume of 0.7 cm3. Tumors treated with radiation alone took 23.3 days while the combination of PF-8380 with radiation significantly delayed tumor growth to 32 days.

In this study, we found that inhibition of pro-survival cellular signaling pathways, migration, invasion and radiation-induced neovascularization can be achieved by targeting ATX. Thus, ATX could serve as a novel and effective molecular target for the development of radiosensitizers of GBM, and may provide a new approach to treating patients with this lethal form of brain cancer.

Citation Format: David Y.A. Dadey, Sandeep R. Bhave, Daniel J. Ferraro, Rowan M. Karvas, Dennis E. Hallahan, Dinesh Thotala. Inhibition of autotaxin enhances radiosensitivity in human and murine glioblastoma cell lines. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2136. doi:10.1158/1538-7445.AM2013-2136

Abstract 422: Early growth response 1 is required for radiation-induced apoptosis in normal tissues

Although radiation therapy is a mainstay of cancer treatments, it can have deleterious effects on normal tissues, leading to poor quality of life for cancer survivors. Radiation can cause depletion of stem cells in normal tissues that can result in late sequelae, including poor learning, cognitive disorders, intestinal malabsorption, infertility, and skin injuries. Our laboratory has shown that low doses of radiation can induce apoptosis selectively in undifferentiated normal stem cells compared to adjacent differentiated cells in multiple niches including the brain and intestine. To elucidate the molecular basis underlying stem cell sensitivity to radiation, we performed discovery research to detect genomic differences between stem cells and differentiated cells. Among the candidates, the Early growth response 1 (Egr1) gene was expressed at high levels in stem cells, but not in isogenic differentiated cells. Egr1 is a zinc-finger transcription factor that is rapidly induced in response to radiation and activates the transcription of pro-apoptotic genes. Since cancer survivors can suffer from cognitive difficulties and intestinal malabsorption, we investigated the role of Egr1 in neuronal and intestinal cell death following irradiation. Knockdown of Egr1 using shRNA increased clonogenic survival in irradiated HT-22 hippocampal neuronal cells and IEC-6 intestinal epithelial cells. This increased radioprotection was largely due to reduced apoptosis in Egr1 knockdown cells. Additionally, suppression of Egr1 promoter binding by mithramycin A and chromomycin A3 also conferred radioprotection to HT-22 and IEC-6 cells. Mouse models further supported Egr1’s role in radiation-induced apoptosis. Egr1 null mice had significantly fewer apoptotic cells in the hippocampus and small intestinal crypts following irradiation, as compared to their wild-type littermates. Finally, we examined mechanisms by which Egr1 regulates radiation-induced apoptosis. Irradiation of HT-22 and IEC-6 cells led to a G2/M cell cycle arrest, which was abrogated by the knockdown of Egr1. Also, knockdown of Egr1 by shRNA and inhibition of Egr1 promoter binding both attenuated radiation induction of the pro-apoptotic factors p53, Bim, and Bax, and upregulated the anti-apoptotic protein Bcl-2. Thus, this study establishes a crucial role for Egr1 in the radiation-induced apoptosis of hippocampal and intestinal tissues. Egr1 could be a potential molecular target to minimize the normal tissue complications associated with radiation therapy.

Citation Format: Yi D. Zhao, Keith M. Jacobs, Rowan M. Karvas, Dennis E. Hallahan, Dinesh Thotala. Early growth response 1 is required for radiation-induced apoptosis in normal tissues. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 422. doi:10.1158/1538-7445.AM2013-422

Eph/ephrin interactions modulate muscle satellite cell motility and patterning

During development and regeneration, directed migration of cells, including neural crest cells, endothelial cells, axonal growth cones and many types of adult stem cells, to specific areas distant from their origin is necessary for their function. We have recently shown that adult skeletal muscle stem cells (satellite cells), once activated by isolation or injury, are a highly motile population with the potential to respond to multiple guidance cues, based on their expression of classical guidance receptors. We show here that, in vivo, differentiated and regenerating myofibers dynamically express a subset of ephrin guidance ligands, as well as Eph receptors. This expression has previously only been examined in the context of muscle-nerve interactions; however, we propose that it might also play a role in satellite cell-mediated muscle repair. Therefore, we investigated whether Eph-ephrin signaling would produce changes in satellite cell directional motility. Using a classical ephrin 'stripe' assay, we found that satellite cells respond to a subset of ephrins with repulsive behavior in vitro; patterning of differentiating myotubes is also parallel to ephrin stripes. This behavior can be replicated in a heterologous in vivo system, the hindbrain of the developing quail, in which neural crest cells are directed in streams to the branchial arches and to the forelimb of the developing quail, where presumptive limb myoblasts emigrate from the somite. We hypothesize that guidance signaling might impact multiple steps in muscle regeneration, including escape from the niche, directed migration to sites of injury, cell-cell interactions among satellite cell progeny, and differentiation and patterning of regenerated muscle.