Rescue of impaired blood-brain barrier in tuberous sclerosis complex patient derived neurovascular unit

BACKGROUND

Tuberous sclerosis complex (TSC) is a multi-system genetic disease that causes benign tumors in the brain and other vital organs. The most debilitating symptoms result from involvement of the central nervous system and lead to a multitude of severe symptoms including seizures, intellectual disability, autism, and behavioral problems. TSC is caused by heterozygous mutations of either the TSC1 or TSC2 gene and dysregulation of mTOR kinase with its multifaceted downstream signaling alterations is central to disease pathogenesis. Although the neurological sequelae of the disease are well established, little is known about how these mutations might affect cellular components and the function of the blood-brain barrier (BBB).

METHODS

We generated TSC disease-specific cell models of the BBB by leveraging human induced pluripotent stem cell and microfluidic cell culture technologies.

RESULTS

Using microphysiological systems, we demonstrate that a BBB generated from TSC2 heterozygous mutant cells shows increased permeability. This can be rescued by wild type astrocytes or by treatment with rapamycin, an mTOR kinase inhibitor.

CONCLUSION

Our results demonstrate the utility of microphysiological systems to study human neurological disorders and advance our knowledge of cell lineages contributing to TSC pathogenesis and informs future therapeutics.

Velociraptor: Cross-Platform Quantitative Search Using Hallmark Cell Features

A key challenge for single cell discovery analysis is to identify new cell types, describe them quantitatively, and seek these novel cells in new studies often using a different platform. Over the last decade, tools were developed to address identification and quantitative description of cells in human tissues and tumors. However, automated validation of populations at the single cell level has struggled due to the cytometry field's reliance on hierarchical, ordered use of features and on platform-specific rules for data processing and analysis. Here we present Velociraptor, a workflow that implements Marker Enrichment Modeling in three cross-platform modules: 1) identification of cells specific to disease states, 2) description of hallmark features for each cell and population, and 3) searching for cells matching one or more hallmark feature sets in a new dataset. A key advance is that Velociraptor registers cells between datasets, including between flow cytometry and quantitative imaging using different, overlapping feature sets. Four datasets were used to challenge Velociraptor and reveal new biological insights. Working at the individual sample level, Velociraptor tracked the abundance of clinically significant glioblastoma brain tumor cell subsets and characterized the cells that predominate in recurrent tumors as a close match for rare, negative prognostic cells originally observed in matched pre-treatment tumors. In patients with inborn errors of immunity, Velociraptor identified genotype-specific cells associated with GATA2 haploinsufficiency. Finally, in cross-platform analysis of immune cells in multiplex imaging of breast cancer, Velociraptor sought and correctly identified memory T cell subsets in tumors. Different phenotypic descriptions generated by algorithms or humans were shown to be effective as search inputs, indicating that cell identity need not be described in terms of per-feature cutoffs or strict hierarchical analyses. Velociraptor thus identifies cells based on hallmark feature sets, such as protein expression signatures, and works effectively with data from multiple sources, including suspension flow cytometry, imaging, and search text based on known or theoretical cell features.

IL-8 Instructs Macrophage Identity in Lateral Ventricle Contacting Glioblastoma

Adult IDH-wildtype glioblastoma (GBM) is a highly aggressive brain tumor with no established immunotherapy or targeted therapy. Recently, CD32+ HLA-DRhi macrophages were shown to have displaced resident microglia in GBM tumors that contact the lateral ventricle stem cell niche. Since these lateral ventricle contacting GBM tumors have especially poor outcomes, identifying the origin and role of these CD32+ macrophages is likely critical to developing successful GBM immunotherapies. Here, we identify these CD32+ cells as M_IL-8 macrophages and establish that IL-8 is sufficient and necessary for tumor cells to instruct healthy macrophages into CD32+ M_IL-8 M2 macrophages. In ex vivo experiments with conditioned medium from primary human tumor cells, inhibitory antibodies to IL-8 blocked the generation of CD32+ M_IL-8 cells. Finally, using a set of 73 GBM tumors, IL-8 protein is shown to be present in GBM tumor cells in vivo and especially common in tumors contacting the lateral ventricle. These results provide a mechanistic origin for CD32+ macrophages that predominate in the microenvironment of the most aggressive GBM tumors. IL-8 and CD32+ macrophages should now be explored as targets in combination with GBM immunotherapies, especially for patients whose tumors present with radiographic contact with the ventricular-subventricular zone stem cell niche.

Pembrolizumab alters the tumor immune landscape in a patient with dMMR glioblastoma

Congenital DNA mismatch repair defects (dMMR), such as Lynch Syndrome, predispose patients to a variety of cancers and account for approximately 1% of glioblastoma cases. While few therapeutic options exist for glioblastoma, checkpoint blockade therapy has proven effective in dMMR tumors. Here we present a case study of a male in their 30s diagnosed with dMMR glioblastoma treated with pembrolizumab who experienced a partial response to therapy. Using a multiplex IHC analysis pipeline on archived slide specimens from tumor resections at diagnosis and after therapeutic interventions, we quantified changes in the frequency and spatial distribution of key cell populations in the tumor tissue. Notably, proliferating (KI67+) macrophages and T cells increased in frequency as did other KI67+ cells within the tumor. Therapeutic intervention remodeled the cellular spatial distribution in the tumor leading to a greater frequency of macrophage/tumor cell interactions and T cell/T cell interactions, highlighting impacts of checkpoint blockade on tumor cytoarchitecture and revealing spatial patterns that may indicate advantageous immune interactions in glioma and other solid tumors treated with these agents.

Rescue of Impaired Blood-Brain Barrier in Tuberous Sclerosis Complex Patient Derived Neurovascular Unit

Tuberous sclerosis complex (TSC) is a multi-system genetic disease that causes benign tumors in the brain and other vital organs. The most debilitating symptoms result from involvement of the central nervous system and lead to a multitude of severe symptoms including seizures, intellectual disability, autism, and behavioral problems. TSC is caused by heterozygous mutations of either the TSC1 or TSC2 gene. Dysregulation of mTOR kinase with its multifaceted downstream signaling alterations is central to disease pathogenesis. Although the neurological sequelae of the disease are well established, little is known about how these mutations might affect cellular components and the function of the blood-brain barrier (BBB). We generated disease-specific cell models of the BBB by leveraging human induced pluripotent stem cell and microfluidic cell culture technologies. Using these microphysiological systems, we demonstrate that the BBB generated from TSC2 heterozygous mutant cells shows increased permeability which can be rescued by wild type astrocytes and with treatment with rapamycin, an mTOR kinase inhibitor. Our results further demonstrate the utility of microphysiological systems to study human neurological disorders and advance our knowledge of the cell lineages contributing to TSC pathogenesis.

Non-canonical functions of a mutant TSC2 protein in mitotic division

Tuberous Sclerosis Complex (TSC) is a debilitating developmental disorder characterized by a variety of clinical manifestations. TSC is caused by mutations in the TSC1 or TSC2 genes, which encode the hamartin/tuberin proteins respectively. These proteins function as a heterodimer that negatively regulates the mechanistic Target of Rapamycin Complex 1 (mTORC1). TSC research has focused on the effects of mTORC1, a critical signaling hub, on regulation of diverse cell processes including metabolism, cell growth, translation, and neurogenesis. However, non-canonical functions of TSC2 are not well studied, and the potential disease-relevant biological mechanisms of mutations affecting these functions are not well understood. We observed aberrant multipolar mitotic division, a novel phenotype, in TSC2 mutant iPSCs. The multipolar phenotype is not meaningfully affected by treatment with the inhibitor rapamycin. We further observed dominant negative activity of the mutant form of TSC2 in producing the multipolar division phenotype. These data expand the knowledge of TSC2 function and pathophysiology which will be highly relevant to future treatments for patients with TSC.

Alignment, segmentation and neighborhood analysis in cyclic immunohistochemistry data using CASSATT

Cyclic immunohistochemistry (cycIHC) uses sequential rounds of colorimetric immunostaining and imaging for quantitative mapping of location and number of cells of interest. Additionally, cycIHC benefits from the speed and simplicity of brightfield microscopy, making the collection of entire tissue sections and slides possible at a trivial cost compared to other high dimensional imaging modalities. However, large cycIHC datasets currently require an expert data scientist to concatenate separate open-source tools for each step of image pre-processing, registration, and segmentation, or the use of proprietary software. Here, we present a unified and user-friendly pipeline for processing, aligning, and analyzing cycIHC data - Cyclic Analysis of Single-Cell Subsets and Tissue Territories (CASSATT). CASSATT registers scanned slide images across all rounds of staining, segments individual nuclei, and measures marker expression on each detected cell. Beyond straightforward single cell data analysis outputs, CASSATT explores the spatial relationships between cell populations. By calculating the log odds of interaction frequencies between cell populations within tissues and tissue regions, this pipeline helps users identify populations of cells that interact-or do not interact-at frequencies that are greater than those occurring by chance. It also identifies specific neighborhoods of cells based on the assortment of neighboring cell types that surround each cell in the sample. The presence and location of these neighborhoods can be compared across slides or within distinct regions within a tissue. CASSATT is a fully open source workflow tool developed to process cycIHC data and will allow greater utilization of this powerful staining technique.

An immunosuppressed microenvironment distinguishes lateral ventricle-contacting glioblastomas

Radiographic contact of glioblastoma (GBM) tumors with the lateral ventricle and adjacent stem cell niche correlates with poor patient prognosis, but the cellular basis of this difference is unclear. Here, we reveal and functionally characterize distinct immune microenvironments that predominate in subtypes of GBM distinguished by proximity to the lateral ventricle. Mass cytometry analysis of IDH-wildtype human tumors identified elevated T cell checkpoint receptor expression and greater abundance of a specific CD32+CD44+HLA-DRhigh macrophage population in ventricle-contacting GBM. Multiple computational analysis approaches, phospho-specific cytometry, and focal resection of GBMs confirmed and extended these findings. Phospho-flow quantified cytokine-induced immune cell signaling in ventricle-contacting GBM revealing differential signaling between GBM subtypes. Subregion analysis within a given tumor supported initial findings and revealed intratumoral compartmentalization of T cell memory and exhaustion phenotypes within GBM subtypes. Collectively, these results characterize immunotherapeutically targetable features of macrophages and suppressed lymphocytes in glioblastomas defined by MRI-detectable lateral ventricle contact.

Jedi-1/MEGF12-mediated phagocytosis controls the pro-neurogenic properties of microglia in the ventricular-subventricular zone

Microglia are the primary phagocytes in the central nervous system and are responsible for clearing dead cells generated during development or disease. The phagocytic process shapes the phenotype of the microglia, which affects the local environment. A unique population of microglia reside in the ventricular-subventricular zone (V-SVZ) of neonatal mice, but how they influence this neurogenic niche is not well-understood. Here, we demonstrate that phagocytosis creates a pro-neurogenic microglial phenotype in the V-SVZ and that these microglia phagocytose apoptotic cells via the engulfment receptor Jedi-1. Deletion of Jedi-1 decreases apoptotic cell clearance, triggering the development of a neuroinflammatory phenotype, reminiscent of neurodegenerative and-age-associated microglia, that reduces neural precursor proliferation via elevated interleukin (IL)-1β signaling; inhibition of IL-1 receptor rescues precursor proliferation in vivo. Together, these results reveal a critical role for Jedi-1 in connecting microglial phagocytic activity to a phenotype that promotes neurogenesis in the developing V-SVZ.

Dephosphorylation of 4EBP1/2 Induces Prenatal Neural Stem Cell Quiescence

A limiting factor in the regenerative capacity of the adult brain is the abundance and proliferative ability of neural stem cells (NSCs). Adult NSCs are derived from a subpopulation of embryonic NSCs that temporarily enter quiescence during mid-gestation and remain quiescent until postnatal reactivation. Here we present evidence that the mechanistic/mammalian target of rapamycin (mTOR) pathway regulates quiescence entry in embryonic NSCs of the developing forebrain. Throughout embryogenesis, two downstream effectors of mTOR, p-4EBP1/2 T37/46 and p-S6 S240/244, were mutually exclusive in NSCs, rarely occurring in the same cell. While 4EBP1/2 was phosphorylated in stem cells undergoing mitosis at the ventricular surface, S6 was phosphorylated in more differentiated cells migrating away from the ventricle. Phosphorylation of 4EBP1/2, but not S6, was responsive to quiescence induction in cultured embryonic NSCs. Further, inhibition of p-4EBP1/2, but not p-S6, was sufficient to induce quiescence. Collectively, this work offers new insight into the regulation of quiescence entry in embryonic NSCs and, thereby, correct patterning of the adult brain. These data suggest unique biological functions of specific posttranslational modifications and indicate that the preferential inhibition of such modifications may be a useful therapeutic approach in neurodevelopmental diseases where NSC numbers, proliferation, and differentiation are altered.

Learning cell identity in immunology, neuroscience, and cancer

Suspension and imaging cytometry techniques that simultaneously measure hundreds of cellular features are powering a new era of cell biology and transforming our understanding of human tissues and tumors. However, a central challenge remains in learning the identities of unexpected or novel cell types. Cell identification rubrics that could assist trainees, whether human or machine, are not always rigorously defined, vary greatly by field, and differentially rely on cell intrinsic measurements, cell extrinsic tissue measurements, or external contextual information such as clinical outcomes. This challenge is especially acute in the context of tumors, where cells aberrantly express developmental programs that are normally time, location, or cell-type restricted. Well-established fields have contrasting practices for cell identity that have emerged from convention and convenience as much as design. For example, early immunology focused on identifying minimal sets of protein features that mark individual, functionally distinct cells. In neuroscience, features including morphology, development, and anatomical location were typical starting points for defining cell types. Both immunology and neuroscience now aim to link standardized measurements of protein or RNA to informative cell functions such as electrophysiology, connectivity, lineage potential, phospho-protein signaling, cell suppression, and tumor cell killing ability. The expansion of automated, machine-driven methods for learning cell identity has further created an urgent need for a harmonized framework for distinguishing cell identity across fields and technology platforms. Here, we compare practices in the fields of immunology and neuroscience, highlight concepts from each that might work well in the other, and propose ways to implement these ideas to study neural and immune cell interactions in brain tumors and associated model systems.

Modeling tuberous sclerosis with organoids

[Figure: see text].

Histological Studies of the Ventricular-Subventricular Zone as Neural Stem Cell and Glioma Stem Cell Niche

The neural stem cell niche of the ventricular-subventricular zone supports the persistence of stem and progenitor cells in the mature brain. This niche has many notable cytoarchitectural features that affect the activity of stem cells and may also support the survival and growth of invading tumor cells. Histochemical studies of the niche have revealed many proteins that, in combination, can help to reveal stem-like cells in the normal or cancer context, although many caveats persist in the quest to consistently identify these cells in the human brain. Here, we explore the complex relationship between the persistent proliferative capacity of the neural stem cell niche and the malignant proliferation of brain tumors, with a special focus on histochemical identification of stem cells and stem-like tumor cells and an eye toward the potential application of high-dimensional imaging approaches to the field.

Creation and validation of 3D-printed head molds for stereotaxic injections of neonatal mouse brains

BACKGROUND

An increasing number of rodent model systems use injection of DNA or viral constructs in the neonatal brain. However, approaches for reliable positioning and stereotaxic injection at this developmental stage are limited, typically relying on handheld positioning or molds that must be re-aligned for use in a given laboratory.

NEW METHOD

A complete protocol and open-source software pipeline for generating 3D-printed head molds derived from a CT scan of a neonatal mouse head cast, together with a universal adapter that can be placed on a standard stereotaxic stage.

RESULTS

A series of test injections with adenovirus encoding red fluorescent protein, or Fluorogold, were conducted using original clay molds and newly generated 3D printed molds. Several metrics were used to compare spread and localization of targeted injections.

COMPARISON WITH EXISTING METHODS

The new method of head mold generation gave comparable results to the field standard, but also allowed the rapid generation of additional copies of each head mold with standardized positioning of the head each time.

CONCLUSIONS

This 3D printing pipeline can be used to efficiently develop a series of head molds with standardized injection coordinates across multiple laboratories. More broadly, this pipeline can easily be adapted to other perinatal ages or species.

Sustained response to erlotinib and rapamycin in a patient with pediatric anaplastic oligodendroglioma

One goal of precision medicine is to identify mutations within individual tumors to design targeted treatment approaches. This report details the use of genomic testing to select a targeted therapy regimen of erlotinib and rapamycin for a pediatric anaplastic oligodendroglioma refractory to standard treatment, achieving a 33-month sustained response. Immunohistochemical analysis of total and phosphorylated protein isoforms showed abnormal signaling consistent with detected mutations, while revealing heterogeneity in per-cell activation of signaling pathways in multiple subpopulations of tumor cells throughout the course of disease. This case highlights molecular features that may be relevant to designing future targeted treatments.

Glioblastoma Distance From the Subventricular Neural Stem Cell Niche Does Not Correlate With Survival

Objective

To determine the relationship between survival and glioblastoma distance from the ventricular-subventricular neural stem cell niche (VSVZ).

Methods

502 pre-operative gadolinium-enhanced, T1-weighted MRIs with glioblastoma retrieved from an institutional dataset (n = 252) and The Cancer Imaging Atlas (n=250) were independently reviewed. The shortest distance from the tumor contrast enhancement to the nearest lateral ventricular wall, the location of the VSVZ, was measured (GBM-VSVZ_Dist). The relationship of GBM-VSVZ_Dist with the proportion of glioblastomas at each distance point and overall survival was explored with a Pearson's correlation and Cox regression model, respectively, adjusting for the well-established glioblastoma prognosticators.

Results

244/502 glioblastomas had VSVZ contact. The proportion of non-VSVZ-contacting glioblastomas correlated inversely with GBM-VSVZ_Dist (partial Pearson's correlation adjusted for tumor volume R=-0.79, p=7.11x10^-7). A fit of the Cox regression model adjusted for age at diagnosis, Karnofsky performance status score, post-operative treatment with temozolomide and/or radiotherapy, IDH1/2 mutation status, MGMT promoter methylation status, tumor volume, and extent of resection demonstrated a significantly decreased overall survival only when glioblastoma contacted the VSVZ. Overall survival did not correlate with GBM-VSVZ_Dist.

Conclusions

In the two independent cohorts analyzed, glioblastomas at diagnosis were found in close proximity or in contact with the VSVZ with a proportion that decreased linearly with GBM-VSVZ_Dist. Patient survival was only influenced by the presence or absence of a gadolinium-enhanced glioblastoma contact with the VSVZ. These results may guide analyses to test differential effectiveness of VSVZ radiation in VSVZ-contacting and non-contacting glioblastomas and/or inform patient selection criteria in clinical trials of glioblastoma radiation.

Unexpected effects of ivermectin and selamectin on inducible CreER activity in mice

BACKGROUND

Anti-parasitics are frequently used in research animal facilities to treat a multitude of common infections, with pinworms and fur mites being amongst the most common. Ivermectin and selamectin are common oral and topical treatments for these infections, respectively. Although commonly thought to be innocuous to both the research animals and any transgenic elements that the animals may carry, evidence exists that ivermectin is capable of activating the recombinase activity of at least one CreER. The goal of the current study was to determine if there was an effect of either anti-parasitic agent on the activity of CreER proteins in transgenic mice.

CASE PRESENTATION

We analyzed the offspring of transgenic mice exposed to either ivermectin or selamectin during pregnancy and nursing. Through analysis of reporter genes co-expressed with multiple, independently generated transgenic CreER drivers, we report here that ivermectin and selamectin both alter recombinase activity and thus may have unintended consequences on gene inactivation studies in mice.

CONCLUSIONS

Although the mechanisms by which ivermectin and selamectin affect CreER activity in the offspring of treated dams remain unclear, the implications are important nonetheless. Treatment of pregnant transgenic mice with these anti-parasitics has the potential to alter transgene activity in the offspring. Special considerations should be made when planning treatment of transgenic mice with either of these pharmacologics.

DEPDC5 haploinsufficiency drives increased mTORC1 signaling and abnormal morphology in human iPSC-derived cortical neurons

Mutations in the DEPDC5 gene can cause epilepsy, including forms with and without brain malformations. The goal of this study was to investigate the contribution of DEPDC5 gene dosage to the underlying neuropathology of DEPDC5-related epilepsies. We generated induced pluripotent stem cells (iPSCs) from epilepsy patients harboring heterozygous loss of function mutations in DEPDC5. Patient iPSCs displayed increases in both phosphorylation of ribosomal protein S6 and proliferation rate, consistent with elevated mTORC1 activation. In line with these findings, we observed increased soma size in patient iPSC-derived cortical neurons that was rescued with rapamycin treatment. These data indicate that human cells heterozygous for DEPDC5 loss-of-function mutations are haploinsufficient for control of mTORC1 signaling. Our findings suggest that human pathology differs from mouse models of DEPDC5-related epilepsies, which do not show consistent phenotypic differences in heterozygous neurons, and support the need for human-based models to affirm and augment the findings from animal models of DEPDC5-related epilepsy.

Unsupervised machine learning reveals risk stratifying glioblastoma tumor cells

A goal of cancer research is to reveal cell subsets linked to continuous clinical outcomes to generate new therapeutic and biomarker hypotheses. We introduce a machine learning algorithm, Risk Assessment Population IDentification (RAPID), that is unsupervised and automated, identifies phenotypically distinct cell populations, and determines whether these populations stratify patient survival. With a pilot mass cytometry dataset of 2 million cells from 28 glioblastomas, RAPID identified tumor cells whose abundance independently and continuously stratified patient survival. Statistical validation within the workflow included repeated runs of stochastic steps and cell subsampling. Biological validation used an orthogonal platform, immunohistochemistry, and a larger cohort of 73 glioblastoma patients to confirm the findings from the pilot cohort. RAPID was also validated to find known risk stratifying cells and features using published data from blood cancer. Thus, RAPID provides an automated, unsupervised approach for finding statistically and biologically significant cells using cytometry data from patient samples.

Single-cell systems neuroimmunology reveals a highly immunosuppressive microenvironment in human glioblastomas contacting the ventricular stem cell niche

Glioblastomas make up more than 60% of adult primary brain tumors and carry a 15 month overall survival despite aggressive standard-of-care therapy. Recent advances in immunotherapy offer an appealing alternative that may improve outcomes for patients with glioblastoma; however, clinical trials have proven unsuccessful due in part to a lack of predictive features that may inform responsiveness to immunotherapy. We have recently shown a strong correlation between 1) immune infiltration, 2) tumor cell phenotype, and 3) patient outcome. Further, patients whose tumors demonstrate radiographic contact with the ventricular-subventricular zone (V-SVZ) have reduced survival compared to patients whose tumors do not contact the V-SVZ. We therefore hypothesized that the V-SVZ acts as a previously unappreciated immunosuppressive microenvironment within the brain that promotes tumor growth by suppressing anti-tumor immunity. Primary human glioblastomas were disaggregated into single-cell suspensions and mass cytometry (CyTOF) measured >30 parameters in thirteen immune populations infiltrating human glioblastomas. Cutting-edge machine-learning tools identified key differences in the abundance and phenotypes of T cells, B cells, NK cells, microglia, and peripheral macrophages infiltrating ventricle-contacting gliomas. Further, enriched expression of immune checkpoint receptors (PD-1, TIGIT, LAG-3, TIM3) correlated with ventricular contact and outcome. These results provide key insights into the immune microenvironment of glioblastomas and elucidate several clinically actionable immunotherapeutic targets that may be used to optimize treatment strategies for glioblastoma patients based on V-SVZ contact status.

Bcl2-Expressing Quiescent Type B Neural Stem Cells in the Ventricular-Subventricular Zone Are Resistant to Concurrent Temozolomide/X-Irradiation

The ventricular-subventricular zone (V-SVZ) of the mammalian brain is a site of adult neurogenesis. Within the V-SVZ reside type B neural stem cells (NSCs) and type A neuroblasts. The V-SVZ is also a primary site for very aggressive glioblastoma (GBM). Standard-of-care therapy for GBM consists of safe maximum resection, concurrent temozolomide (TMZ), and X-irradiation (XRT), followed by adjuvant TMZ therapy. The question of how this therapy impacts neurogenesis is not well understood and is of fundamental importance as normal tissue tolerance is a limiting factor. Here, we studied the effects of concurrent TMZ/XRT followed by adjuvant TMZ on type B stem cells and type A neuroblasts of the V-SVZ in C57BL/6 mice. We found that chemoradiation induced an apoptotic response in type A neuroblasts, as marked by cleavage of caspase 3, but not in NSCs, and that A cells within the V-SVZ were repopulated given sufficient recovery time. 53BP1 foci formation and resolution was used to assess the repair of DNA double-strand breaks. Remarkably, the repair was the same in type B and type A cells. While Bax expression was the same for type A or B cells, antiapoptotic Bcl2 and Mcl1 expression was significantly greater in NSCs. Thus, the resistance of type B NSCs to TMZ/XRT appears to be due, in part, to high basal expression of antiapoptotic proteins compared with type A cells. This preclinical research, demonstrating that murine NSCs residing in the V-SVZ are tolerant of standard chemoradiation therapy, supports a dose escalation strategy for treatment of GBM. Stem Cells 2019;37:1629-1639.

Abstract 3934: Bcl2-expressing quiescent type B neural stem cells in the SVZ are resistant to concurrent temozolomide/X-irradiation

The subventricular zone (SVZ) is the largest source of neural stem cells (NSCs) in the adult brain. Emerging research indicates that NSCs within the SVZ may be cells of origin for WHO grade IV astrocytoma (glioblastoma, GBM)1. GBM consists of multiple fractions of proliferative and/or quiescent stem-like cells that are thought to be lineally related. GBM located adjacent to the SVZ are very resistant to standard of care concurrent temozolomide (TMZ)/X-irradiation (XRT) therapy, a consequence, it is hypothesized, of their NSC origin2. An important, unanswered question is the origin of this resistance. While a significant effort has been undertaken to study proliferating cells, the origins of quiescent cell resistance are not well understood. Normal NSCs adjacent to the SVZ are mainly quiescent. We rationalized that a fundamental understanding of the response of quiescent NSCs to TMZ/XRT would be informative and aid in our understanding of GBM resistance. For 5 consecutive days cohorts of C57BL/6 mice were administered TMZ (0 or 50 mg.kg i.p.). One hr later 0 or 2 Gy was administered to the brain. Transcardial perfusion was performed on day 6 for half the mice. The remaining mice received adjuvant TMZ (100 mg/kg) or vehicle on days 19-22 and transcardial perfusion was performed on day 82. 10 µm coronal brain sections were obtained and immunostained for well characterized markers of type B NSCs (GFAP and Sox2) and type A neuroblasts (Dcx). Immunofluorescence was imaged using a Leica Aperio Versa 200 slide scanning microscope. Cell Profiler software was used to quantify type B and A cells in the SVZ in all cohorts. Proliferating type A cells were exquisitely sensitive to 5 days of concurrent TMZ/XRT treatment whereas quiescent NSCs located within 30 µm of a dorsal or dorsolateral ventricle were very resistant. NSCs in mice exposed to concurrent and adjuvant therapy were also resistant and importantly, able to repopulate type A cells to sham/control levels. 53BP1 foci formation, a surrogate for DNA DSBs, was quantified in Sox2- and Dcx-expressing cells using confocal microscopy following a single TMZ/XRT exposure. Foci formation, measured 6 min to 24 hrs after TMZ/XRT, was not statistically different between cell types (P>0.05). Because TMZ/XRT induced an apoptotic response in A but not in B cells, as marked by cleaved Caspase-3 staining, we investigated expression of Bax and Bcl2 on a per cell basis. Bax expression was not significantly different for type A or B cells (P>0.05). In contrast, type B NSCs expressed 5-fold more Bcl2 than type A neuroblasts (P< 0.001). In conclusion, we demonstrate that type A neuroblasts are sensitive to TMZ/XRT but can be repopulated by inherently resistant type B NSCs given sufficient time. The resistance of quiescent NSCs to TMZ/XRT is associated with high basal expression of anti-apoptotic proteins. 1Lee et al Nature 2018, 560:243-47; 2Smith et al J Neurooncol 2016, 128:207-16

Citation Format: Brent D. Cameron, Geri Traver, Joseph T. Roland, Daniel Dean, Levi Johnson, Kelli L. Boyd, Rebecca A. Ihrie, Jialiang Wang, Michael L. Freeman. Bcl2-expressing quiescent type B neural stem cells in the SVZ are resistant to concurrent temozolomide/X-irradiation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3934.

Location-dependent maintenance of intrinsic susceptibility to mTORC1-driven tumorigenesis

Per-cell quantification of mTORC1 signaling activity in neural stem/progenitor cells reveals differential signaling, proliferative, and tumor-forming capability between dorsal and ventral cells within a single niche.

Neural stem/progenitor cells (NSPCs) of the ventricular–subventricular zone (V-SVZ) are candidate cells of origin for many brain tumors. However, whether NSPCs in different locations within the V-SVZ differ in susceptibility to tumorigenic mutations is unknown. Here, single-cell measurements of signal transduction intermediates in the mechanistic target of rapamycin complex 1 (mTORC1) pathway reveal that ventral NSPCs have higher levels of signaling than dorsal NSPCs. These features are linked with differences in mTORC1-driven disease severity: introduction of a pathognomonic Tsc2 mutation only results in formation of tumor-like masses from the ventral V-SVZ. We propose a direct link between location-dependent intrinsic growth properties imbued by mTORC1 and predisposition to tumor development.

Beyond the message: advantages of snapshot proteomics with single-cell mass cytometry in solid tumors

Single-cell technologies that can quantify features of individual cells within a tumor are critical for treatment strategies aiming to target cancer cells while sparing or activating beneficial cells. Given that key players in protein networks are often the primary targets of precision oncology strategies, it is imperative to transcend the nucleic acid message and read cellular actions in human solid tumors. Here, we review the advantages of multiplex, single-cell mass cytometry in tissue and solid tumor investigations. Mass cytometry can quantitatively probe nearly any cellular feature or target. In discussing the ability of mass cytometry to reveal and characterize a broad spectrum of cell types, identify rare cells, and study functional behavior through protein signaling networks in millions of individual cells from a tumor, this review surveys publications of scientific advances in solid tumor biology made with the aid of mass cytometry. Advances discussed include functional identification of rare tumor and tumor-infiltrating immune cells and dissection of cellular mechanisms of immunotherapy in solid tumors and the periphery. The review concludes by highlighting ways to incorporate single-cell mass cytometry in solid tumor precision oncology efforts and rapidly developing cytometry techniques for quantifying cell location and sequenced nucleic acids.

Heterogeneity of Neural Stem Cells in the Ventricular-Subventricular Zone

In this chapter, heterogeneity is explored in the context of the ventricular-subventricular zone, the largest stem cell niche in the mammalian brain. This niche generates up to 10,000 new neurons daily in adult mice and extends over a large spatial area with dorso-ventral and medio-lateral subdivisions. The stem cells of the ventricular-subventricular zone can be subdivided by their anatomical position and transcriptional profile, and the stem cell lineage can also be further subdivided into stages of pre- and post-natal quiescence and activation. Beyond the stem cells proper, additional differences exist in their interactions with other cellular constituents of the niche, including neurons, vasculature, and cerebrospinal fluid. These variations in stem cell potential and local interactions are discussed, as well as unanswered questions within this system.

A Chimeric Egfr Protein Reporter Mouse Reveals Egfr Localization and Trafficking In Vivo

EGF receptor (EGFR) is a critical signaling node throughout life. However, it has not been possible to directly visualize endogenous Egfr in mice. Using CRISPR/Cas9 genome editing, we appended a fluorescent reporter to the C terminus of the Egfr. Homozygous reporter mice appear normal and EGFR signaling is intact in vitro and in vivo. We detect distinct patterns of Egfr expression in progenitor and differentiated compartments in embryonic and adult mice. Systemic delivery of EGF or amphiregulin results in markedly different patterns of Egfr internalization and trafficking in hepatocytes. In the normal intestine, Egfr localizes to the crypt rather than villus compartment, expression is higher in adjacent epithelium than in intestinal tumors, and following colonic injury expression appears in distinct cell populations in the stroma. This reporter, under control of its endogenous regulatory elements, enables in vivo monitoring of the dynamics of Egfr localization and trafficking in normal and disease states.

Space Invaders: Brain Tumor Exploitation of the Stem Cell Niche

Increasing evidence indicates that the adult neurogenic niche of the ventricular-subventricular zone (V-SVZ), beyond serving as a potential site of origin, affects the outcome of malignant brain cancers. Glioma contact with this niche predicts worse prognosis, suggesting a supportive role for the V-SVZ environment in tumor initiation or progression. In this review, we describe unique components of the V-SVZ that may permit or promote tumor growth within the region. Cell-cell interactions, soluble factors, and extracellular matrix composition are discussed, and the role of the niche in future therapies is explored. The purpose of this review is to highlight niche intrinsic factors that may promote or support malignant cell growth and maintenance, and point out how we might leverage these features to improve patient outcome.

Abstract 364: Mass cytometry of human glioblastoma characterizes more than 99 percent of cells and reveals intratumoral cell subsets defined by contrasting signaling network profiles

Background: Glioblastoma (GBM) remains largely incurable despite intense study of resected tissue. Prior studies have revealed GBM cell subsets (Patel et al., Science 2014) and have implicated subset emergence as a potential mechanism of poor outcome in other cancer types. Signaling in rare cells or a mix of cell subsets may enable therapy resistance and recurrence of GBM. For example, STAT3 RNA expression has been previously shown to correlate with poor outcome in GBM (Jahani-Asl et al., Nat Neurosci 2016 and TCGA). The complexity of GBM, combined with the interconnectedness between cancer and host cells in the microenvironment, means that a single cell biology approach is needed to comprehensively characterize patient biopsy cells and determine how protein expression, signaling, and functional capabilities impact treatment response.

Methods: We developed a novel mass cytometry approach to characterize human GBM that identified ~90-95% of tumor cells (Leelatian & Doxie et al., Cytometry B 2016). Here, we applied this approach using a newly created 35-antibody mass cytometry panel focused on basal phospho-protein signaling. The published panel of 16 identity proteins included SOX2, CD44, Nestin, PDGFRα, S100B, and NCAM. This panel was augmented to measure 10 additional proteins and 9 phospho-proteins including p-STAT3, p-EGFR, and p-NFκB. Signaling measurements were chosen to match prior single cell studies of signaling networks that stratified clinical outcomes in blood cancers (Irish et al., Cell 2004; PNAS 2010, Levine et al., Cell 2015). Between 10,000 and 250,000 viable cells were characterized for each tumor (N = 7). Tumors were collected with informed consent and in accord with the Declaration of Helsinki.

Results: This new 35-antibody mass cytometry panel positively identified >99% of GBM cells. Subsets of GBM cells displayed protein expression that matched previously observed transcriptional molecular subclasses (Verhaak et al., Cancer Cell 2010 and TCGA). Strikingly, this panel revealed novel GBM cell subsets defined by contrasting basal signaling profiles. An inverse correlation was observed between baseline STAT3 phosphorylation and the abundance of CD45+ leukocytes. Additionally, similar signaling patterns were seen in cells that expressed proteins associated with distinct functions, such as proliferation and migration.

Conclusions: The correlation between low STAT3 signaling and high immune cell abundance provides evidence for the idea that an intimate relationship exists between immune cells and GBM tumor growth and survival. Moreover, single cell analysis may reveal biomarkers of treatment response and allow prediction of clinical outcomes. The abnormal signaling mechanisms observed here in some GBM cell subsets should be studied further as potential targets for novel cancer-selective combination therapies.

Citation Format: Nalin Leelatian, Justine Sinnaeve, Bret C. Mobley, Akshitkumar M. Mistry, Daniel Liu, Kyle D. Weaver, Reid C. Thompson, Lola B. Chambless, Rebecca A. Ihrie, Jonathan M. Irish. Mass cytometry of human glioblastoma characterizes more than 99 percent of cells and reveals intratumoral cell subsets defined by contrasting signaling network profiles [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 364. doi:10.1158/1538-7445.AM2017-364

Preparing Viable Single Cells from Human Tissue and Tumors for Cytomic Analysis

Mass cytometry is a single-cell biology technique that samples >500 cells per second, measures >35 features per cell, and is sensitive across a dynamic range of >104 relative intensity units per feature. This combination of technical assets has powered a series of recent cytomic studies where investigators used mass cytometry to measure protein and phospho-protein expression in millions of cells, characterize rare cell types in healthy and diseased tissues, and reveal novel, unexpected cells. However, these advances largely occurred in studies of blood, lymphoid tissues, and bone marrow, since the cells in these tissues are readily obtained in single-cell suspensions. This unit establishes a primer for single-cell analysis of solid tumors and tissues, and has been tested with mass cytometry. The cells obtained from these protocols can be fixed for study, cryopreserved for long-term storage, or perturbed ex vivo to dissect responses to stimuli and inhibitors. © 2017 by John Wiley & Sons, Inc.

Decreased survival in glioblastomas is specific to contact with the ventricular-subventricular zone, not subgranular zone or corpus callosum

The clinical effect of radiographic contact of glioblastoma (GBM) with neurogenic zones (NZ)-the ventricular-subventricular (VSVZ) and subgranular (SGZ) zones-and the corpus callosum (CC) remains unclear and, in the case of the SGZ, unexplored. We investigated (1) if GBM contact with a NZ correlates with decreased survival; (2) if so, whether this effect is associated with a specific NZ; and (3) if radiographic contact with or invasion of the CC by GBM is associated with decreased survival. We retrospectively identified 207 adult patients who underwent cytoreductive surgery for GBM followed by chemotherapy and/or radiation. Age, preoperative Karnofsky performance status score (KPS), and extent of resection were recorded. Preoperative MRIs were blindly analyzed to calculate tumor volume and assess its contact with VSVZ, SGZ, CC, and cortex. Overall (OS) and progression free (PFS) survivals were calculated and analyzed with multivariate Cox analyses. Among the 207 patients, 111 had GBM contacting VSVZ (VSVZ+GBMs), 23 had SGZ+GBMs, 52 had CC+GBMs, and 164 had cortex+GBMs. VSVZ+, SGZ+, and CC+ GBMs were significantly larger in size relative to their respective non-contacting controls. Multivariate Cox survival analyses revealed GBM contact with the VSVZ, but not SGZ, CC, or cortex, as an independent predictor of lower OS, PFS, and early recurrence. We hypothesize that the VSVZ niche has unique properties that contribute to GBM pathobiology in adults.

Single cell analysis of human tissues and solid tumors with mass cytometry

BACKGROUND

Mass cytometry measures 36 or more markers per cell and is an appealing platform for comprehensive phenotyping of cells in human tissue and tumor biopsies. While tissue disaggregation and fluorescence cytometry protocols were pioneered decades ago, it is not known whether established protocols will be effective for mass cytometry and maintain cancer and stromal cell diversity.

METHODS

Tissue preparation techniques were systematically compared for gliomas and melanomas, patient derived xenografts of small cell lung cancer, and tonsil tissue as a control. Enzymes assessed included DNase, HyQTase, TrypLE, collagenase (Col) II, Col IV, Col V, and Col XI. Fluorescence and mass cytometry were used to track cell subset abundance following different enzyme combinations and treatment times.

RESULTS

Mechanical disaggregation paired with enzymatic dissociation by Col II, Col IV, Col V, or Col XI plus DNase for 1 h produced the highest yield of viable cells per gram of tissue. Longer dissociation times led to increasing cell death and disproportionate loss of cell subsets. Key markers for establishing cell identity included CD45, CD3, CD4, CD8, CD19, CD64, HLA-DR, CD11c, CD56, CD44, GFAP, S100B, SOX2, nestin, vimentin, cytokeratin, and CD31. Mass and fluorescence cytometry identified comparable frequencies of cancer cell subsets, leukocytes, and endothelial cells in glioma (R = 0.97), and tonsil (R = 0.98).

CONCLUSIONS

This investigation establishes standard procedures for preparing viable single cell suspensions that preserve the cellular diversity of human tissue microenvironments. © 2016 International Clinical Cytometry Society.

Neural stem cell heterogeneity through time and space in the ventricular-subventricular zone

BACKGROUND

The origin and classification of neural stem cells (NSCs) has been a subject of intense investigation for the past two decades. Efforts to categorize NSCs based on their location, function and expression have established that these cells are a heterogeneous pool in both the embryonic and adult brain. The discovery and additional characterization of adult NSCs has introduced the possibility of using these cells as a source for neuronal and glial replacement following injury or disease. To understand how one could manipulate NSC developmental programs for therapeutic use, additional work is needed to elucidate how NSCs are programmed and how signals during development are interpreted to determine cell fate.

OBJECTIVE

This review describes the identification, classification and characterization of NSCs within the large neurogenic niche of the ventricular-subventricular zone (V-SVZ).

METHODS

A literature search was conducted using Pubmed including the keywords "ventricular-subventricular zone," "neural stem cell," "heterogeneity," "identity" and/or "single cell" to find relevant manuscripts to include within the review. A special focus was placed on more recent findings using single-cell level analyses on neural stem cells within their niche(s).

RESULTS

This review discusses over 20 research articles detailing findings on V-SVZ NSC heterogeneity, over 25 articles describing fate determinants of NSCs, and focuses on 8 recent publications using distinct single-cell analyses of neural stem cells including flow cytometry and RNA-seq. Additionally, over 60 manuscripts highlighting the markers expressed on cells within the NSC lineage are included in a chart divided by cell type.

CONCLUSIONS

Investigation of NSC heterogeneity and fate decisions is ongoing. Thus far, much research has been conducted in mice however, findings in human and other mammalian species are also discussed here. Implications of NSC heterogeneity established in the embryo for the properties of NSCs in the adult brain are explored, including how these cells may be redirected after injury or genetic manipulation.

Head of the Class: OLIG2 and Glioblastoma Phenotype

Genomic mapping has driven the classification of glioblastoma into distinct molecular subclasses, but mechanisms that regulate tumor subclass phenotypes are only now emerging. In this issue of Cancer Cell, Lu et al. describe a phenotypic switch from PDGFRA-enriched "proneural" to EGFR-enriched "classical" features in glioblastoma upon ablation of Olig2.

Ube3a imprinting impairs circadian robustness in Angelman syndrome models

BACKGROUND

The paternal allele of Ube3a is silenced by imprinting in neurons, and Angelman syndrome (AS) is a disorder arising from a deletion or mutation of the maternal Ube3a allele, which thereby eliminates Ube3a neuronal expression. Sleep disorders such as short sleep duration and increased sleep onset latency are very common in AS.

RESULTS

We found a unique link between neuronal imprinting of Ube3a and circadian rhythms in two mouse models of AS, including enfeebled circadian activity behavior and slowed molecular rhythms in ex vivo brain tissues. As a consequence of compromised circadian behavior, metabolic homeostasis is also disrupted in AS mice. Unsilencing the paternal Ube3a allele restores functional circadian periodicity in neurons deficient in maternal Ube3a but does not affect periodicity in peripheral tissues that are not imprinted for uniparental Ube3a expression. The ubiquitin ligase encoded by Ube3a interacts with the central clock components BMAL1 and BMAL2. Moreover, inactivation of Ube3a expression elevates BMAL1 levels in brain regions that control circadian behavior of AS-model mice, indicating an important role for Ube3a in modulating BMAL1 turnover.

CONCLUSIONS

Ube3a expression constitutes a direct mechanistic connection between symptoms of a human neurological disorder and the central circadian clock mechanism. The lengthened circadian period leads to delayed phase, which could explain the short sleep duration and increased sleep onset latency of AS subjects. Moreover, we report the pharmacological rescue of an AS phenotype, in this case, altered circadian period. These findings reveal potential treatments for sleep disorders in AS patients.

Controlling COR competence: BCL-6 regulates neurogenesis and tumor suppression

In this issue of Cancer Cell, Tiberi and colleagues describe a tumor-suppressor role for BCL6. In the cerebellum, BCL6 is required for the transition from proliferative precursor cell to a more differentiated immature neuron through repressing the expression of Hedgehog effectors, thus controlling a pathway that is aberrantly activated in medulloblastoma.

Molecular Characteristics in MRI-Classified Group 1 Glioblastoma Multiforme

Glioblastoma multiforme (GBM) is a clinically and pathologically heterogeneous brain tumor. Previous studies of transcriptional profiling have revealed biologically relevant GBM subtypes associated with specific mutations and dysregulated pathways. Here, we applied a modified proteome to uncover abnormal protein expression profile in a MRI-classified group I GBM (GBM1), which has a spatial relationship with one of the adult neural stem cell niches, subventricular zone (SVZ). Most importantly, we identified molecular characteristics in this type of GBM that include up-regulation of metabolic enzymes, ribosomal proteins, and heat shock proteins. As GBM1 often recurs at great distances from the initial lesion, the rewiring of metabolism, and ribosomal biogenesis may facilitate cancer cells’ growth and survival during tumor progression. The intimate contact between GBM1 and the SVZ raises the possibility that tumor cells in GBM1 may be most related to SVZ cells. In support of this notion, we found that markers representing SVZ cells are highly expressed in GBM1. Emerged findings from our study provide a specific protein expression profile in GBM1 and offer better prediction or therapeutic implication for this multifocal GBM.

Corridors of migrating neurons in the human brain and their decline during infancy

The subventricular zone (SVZ) of many adult non-human mammals generates large numbers of new neurons destined for the olfactory bulb (OB)^1–6. Along the walls of the lateral ventricles, immature neuronal progeny migrate in tangentially-oriented chains that coalesce into a rostral migratory stream (RMS) connecting the SVZ to the OB. The adult human SVZ, in contrast, contains a hypocellular gap layer separating the ependymal lining from a periventricular ribbon of astrocytes⁷. Some of these SVZ astrocytes can function as neural stem cells in vitro, but their function in vivo remains controversial. An initial report finds few SVZ proliferating cells and rare migrating immature neurons in the RMS of adult humans⁷. In contrast, a subsequent study indicates robust proliferation and migration in the human SVZ and RMS^8,9. Here, we find that the infant human SVZ and RMS contain an extensive corridor of migrating immature neurons before 18 months of age, but, contrary to previous reports⁸, this germinal activity subsides in older children and is nearly extinct by adulthood. Surprisingly, during this limited window of neurogenesis, not all new neurons in the human SVZ are destined for the OB – we describe a major migratory pathway that targets the prefrontal cortex in humans. Together, these findings reveal robust streams of tangentially migrating immature neurons in human early postnatal SVZ and cortex. These pathways represent potential targets of neurological injuries affecting neonates.

Persistent sonic hedgehog signaling in adult brain determines neural stem cell positional identity

Neural stem cells (NSCs) persist in the subventricular zone (SVZ) of the adult brain. Location within this germinal region determines the type of neuronal progeny NSCs generate, but the mechanism of adult NSC positional specification remains unknown. We show that sonic hedgehog (Shh) signaling, resulting in high gli1 levels, occurs in the ventral SVZ and is associated with the genesis of specific neuronal progeny. Shh is selectively produced by a small group of ventral forebrain neurons. Ablation of Shh decreases production of ventrally derived neuron types, while ectopic activation of this pathway in dorsal NSCs respecifies their progeny to deep granule interneurons and calbindin-positive periglomerular cells. These results show that Shh is necessary and sufficient for the specification of adult ventral NSCs.

Lake-front property: a unique germinal niche by the lateral ventricles of the adult brain

New neurons and glial cells are generated in an extensive germinal niche adjacent to the walls of the lateral ventricles in the adult brain. The primary progenitors (B1 cells) have astroglial characteristics but retain important neuroepithelial properties. Recent work shows how B1 cells contact all major compartments of this niche. They share the "shoreline" on the ventricles with ependymal cells, forming a unique adult ventricular zone (VZ). In the subventricular zone (SVZ), B1 cells contact transit amplifying (type C) cells, chains of young neurons (A cells), and blood vessels. How signals from these compartments influence the behavior of B1 or C cells remains largely unknown, but recent work highlights growth factors, neurotransmitters, morphogens, and the extracellular matrix as key regulators of this niche. The integration of emerging molecular and anatomical clues forecasts an exciting new understanding of how the germ of youth is actively maintained in the adult brain.

PERP regulates enamel formation via effects on cell-cell adhesion and gene expression

Little is known about the role of cell-cell adhesion in the development of mineralized tissues. Here we report that PERP, a tetraspan membrane protein essential for epithelial integrity, regulates enamel formation. PERP is necessary for proper cell attachment and gene expression during tooth development, and its expression is controlled by P63, a master regulator of stratified epithelial development. During enamel formation, PERP is localized to the interface between the enamel-producing ameloblasts and the stratum intermedium (SI), a layer of cells subjacent to the ameloblasts. Perp-null mice display dramatic enamel defects, which are caused, in part, by the detachment of ameloblasts from the SI. Microarray analysis comparing gene expression in teeth of wild-type and Perp-null mice identified several differentially expressed genes during enamel formation. Analysis of these genes in ameloblast-derived LS8 cells upon knockdown of PERP confirmed the role for PERP in the regulation of gene expression. Together, our data show that PERP is necessary for the integrity of the ameloblast-SI interface and that a lack of Perp causes downregulation of genes that are required for proper enamel formation.

Abstract 4186: Development of a novel, genetically faithful model of pediatric glioblastoma

Gliomas are among the most lethal human cancers, with high-grade tumors exhibiting a dismal 5-year survival rate. Mutations and amplifications of receptor tyrosine kinases such as PDGFR and EGFR, which drive Ras-MAPK pathway activity, are among the hallmark mutations of adult gliomas. However, pediatric glioblastomas often do not exhibit the oncogenic mutations found in adult tumors, suggesting that these tumors may have distinct mechanistic origins. The unique genetic characteristics of pediatric brain tumors may also affect their response to conventional treatment. Therefore, there is a need to generate mouse models that faithfully recapitulate the genetics of pediatric glioblastomas to identify potential avenues for improving treatment of these tumors. Recent genetic profiling of pediatric brain tumors has found activating mutations of the BRaf kinase in higher grade tumors but not low-grade juvenile pilocytic astrocytomas. By contrast, BRaf is not frequently altered in adult gliomas, suggesting that cells in the developing brain may be particularly susceptible to this mechanism of Ras pathway activation. In addition, BRaf mutation is closely linked to the loss of the tumor suppressor Ink4a/Arf, indicating a possible requirement for loss of this tumor suppressor in the development of high-grade gliomas.

Here, we describe the generation of a model system allowing expression of oncogenic BRaf in spatially and temporally defined populations of neural stem cells. In BRaf CA/+; R26YFP animals, Cre-mediated recombination induces expression of the oncogenic V600E mutant form of Braf from the endogenous gene locus and simultaneously labels these cells via expression of YFP. By stereotaxically injecting Cre-expressing adenovirus into the neonatal or adult brain in these mice, we can induce oncogene expression in specific populations of neural progenitors and track its effects on infected cells.

Data from multiple tumor types suggests that the activation of Ras-MAPK signaling via BRaf mutation is tightly linked to loss of the Ink4a/Arf tumor suppressor locus. We are also using viral introduction of Cre to activate oncogenic BRaf when Ink4a/Arf expression is absent. The phenotype of these mice will be compared to human pediatric gliomas to determine if this system provides an accurate model of the human disease. Such a model would provide a platform for testing targeted therapies against BRaf - a critical step in designing new treatments for pediatric glioblastoma.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4186.

Cells in the astroglial lineage are neural stem cells

A common assumption of classical neuroscience was that neurons and glial cells were derived from separate pools of progenitor cells and that, once development was completed, no new neurons were produced. The subsequent disproving of the "no new neuron" dogma suggested that ongoing adult neurogenesis was supported by a population of multipotent neural stem cells. Two germinal regions within the adult mammalian brain were shown to contain neural progenitor cells: the subventricular zone (SVZ) along the walls of the lateral ventricles, and the subgranular zone (SGZ) within the dentate gyrus of the hippocampus. Surprisingly, when the primary progenitors (stem cells) of the new neurons in these regions were identified, they exhibited structural and biological markers of astrocytes. The architecture of these germinal regions and the pattern of division of neural stem cells have raised fundamental questions about the mechanism of adult neurogenesis. This review describes studies on the origin of adult neural stem cells, the features distinguishing them from astrocytes in non-germinal regions, and the control mechanisms of the proliferation and differentiation of these cells. Astrocytic adult neural stem cells are part of a developmental lineage extending from the neuroepithelium to radial glia to germinal astrocytes. Adult neural stem cells appear to be strongly influenced by their local microenvironment, while also contributing significantly to the architecture of these germinal zones. However, environment alone does not seem to be sufficient to induce non-germinal astrocytes to behave as neural stem cells. Although emerging evidence suggests that significant heterogeneity exists within populations of germinal zone astrocytes, the way that these differences are encoded remains unclear. The further characterization of these cells should eventually provide a body of knowledge central to the understanding of brain development and disease.

The requirement for perp in postnatal viability and epithelial integrity reflects an intrinsic role in stratified epithelia

Mice lacking the desmosome protein Perp exhibit blistering in their stratified epithelia and display postnatal lethality. However, it is unclear if these phenotypes are strictly related to Perp function in stratified epithelia, as Perp expression is not restricted to these tissues during embryogenesis, and certain desmosomal blistering diseases such as pemphigus vulgaris and pemphigus foliaceus have non-cell-intrinsic bases. Furthermore, we show here that Perp is expressed in the heart, raising the possibility that defects in heart function could account for lethality in the Perp-deficient mice. To determine conclusively if Perp function in stratified epithelia is crucial for postnatal survival and epithelial adhesion, we specifically ablated Perp in stratified epithelia by breeding conditional Perp knockout mice to keratin 5 (K5)-Cre transgenic mice. We found that the majority of mice lacking Perp in stratified epithelia die within 10 days after birth, accompanied by blistering and hyperproliferation in the epithelia, similar to the constitutive Perp null mice. Together, these findings indicate that Perp's requirement for both viability and epithelial integrity reflects a role in the stratified epithelial compartment.

Mice lacking the p53/p63 target gene Perp are resistant to papilloma development

Perp is a target of the p53 tumor suppressor involved in the DNA damage-induced apoptosis pathway. In addition, Perp is a target of the p53-related transcription factor p63 during skin development, where it participates in cell-cell adhesion mediated through desmosomes. Here we test the role of Perp in tumorigenesis in a two-step skin carcinogenesis model system. We find that mice lacking Perp in the skin are resistant to papilloma development, displaying fewer and smaller papillomas than wild-type mice. Proliferation levels, apoptotic indices and differentiation patterns are similar in the skin of treated Perp-deficient and wild-type mice. Instead, impaired adhesion through aberrant desmosome assembly may explain the diminished tumor development in the absence of Perp. These studies indicate that in certain contexts, Perp is required for efficient carcinogenesis and suggest a role for intact cell-cell adhesion in supporting tumor development in these settings.

A New Perp in the Lineup: Linking p63 and Desmosomal Adhesion

The process of stratified epithelial development depends upon a transcriptional program directed by the p53-related transcription factor p63. p63 is required for the commitment of the ectoderm to stratification and for the completion of terminal differentiation in stratified epithelia, and mutations in p63 have been identified in multiple developmental disorders affecting ectoderm-derived tissues. Recent work from our laboratory has determined that the p53 target gene Perp is required for the integrity of the stratified epithelia specified by p63, and that expression of Perp in these structures depends on the presence of p63. In these tissues, Perp is a critical component of the desmosome, a cell-cell adhesion complex whose constituents are frequently mutated in human diseases affecting the skin and hair. Perp's position downstream of p63 and p53, as well as its essential role in normal desmosome function, suggest that it, like other adhesion proteins, may be a target for mutation in human blistering diseases or cancer.

Perp Is a p63-Regulated Gene Essential for Epithelial Integrity

p63 is a master regulator of stratified epithelial development that is both necessary and sufficient for specifying this multifaceted program. We show here that Perp, a tetraspan membrane protein originally identified as an apoptosis-associated target of the p53 tumor suppressor, is the first direct target of p63 clearly involved in mediating this developmental program in vivo. During embryogenesis, Perp is expressed in an epithelial pattern, and its expression depends on p63. Perp-/- mice die postnatally, with dramatic blistering in stratified epithelia symptomatic of compromised adhesion. Perp localizes specifically to desmosomes, adhesion junctions important for tissue integrity, and numerous structural defects in desmosomes are observed in Perp-deficient skin, suggesting a role for Perp in promoting the stable assembly of desmosomal adhesive complexes. These findings demonstrate that Perp is a key effector in the p63 developmental program, playing an essential role in an adhesion subprogram central to epithelial integrity and homeostasis.

Developmental context determines latency of MYC-induced tumorigenesis

One of the enigmas in tumor biology is that different types of cancers are prevalent in different age groups. One possible explanation is that the ability of a specific oncogene to cause tumorigenesis in a particular cell type depends on epigenetic parameters such as the developmental context. To address this hypothesis, we have used the tetracycline regulatory system to generate transgenic mice in which the expression of a c-MYC human transgene can be conditionally regulated in murine hepatocytes. MYC's ability to induce tumorigenesis was dependent upon developmental context. In embryonic and neonatal mice, MYC overexpression in the liver induced marked cell proliferation and immediate onset of neoplasia. In contrast, in adult mice MYC overexpression induced cell growth and DNA replication without mitotic cell division, and mice succumbed to neoplasia only after a prolonged latency. In adult hepatocytes, MYC activation failed to induce cell division, which was at least in part mediated through the activation of p53. Surprisingly, apoptosis is not a barrier to MYC inducing tumorigenesis. The ability of oncogenes to induce tumorigenesis may be generally restrained by developmentally specific mechanisms. Adult somatic cells have evolved mechanisms to prevent individual oncogenes from initiating cellular growth, DNA replication, and mitotic cellular division alone, thereby preventing any single genetic event from inducing tumorigenesis.

A transgenic mouse model demonstrates that developmental context may be the reason why the spectrum of tumors differs in children and adults

Perp-etrating p53-dependent apoptosis

The induction of apoptosis is a fundamental mechanism by which the p53 transcriptional activator protein suppresses tumor development. Recently, the roles of several p53 target genes in mediating the p53 apoptotic response have been queried through loss-of-function analysis with knockout mouse models. These studies have demonstrated that the p53 targets Noxa, Puma, and Perp play cell type-specific roles in p53-mediated apoptosis. Perp, a tetraspan protein localizing to the plasma membrane, rather than to mitochondria, is a novel type of p53 effector that may stimulate apoptosis through a different mechanism from the BH3-containing proteins Noxa, Puma, and Bax.