Single-cell approaches define two groups of mammalian oligodendrocyte precursor cells and their evolution over developmental time

Here, we used single-cell RNA sequencing (scRNA-seq), single-cell ATAC sequencing (scATAC-seq), and single-cell spatial transcriptomics to characterize murine cortical OPCs throughout postnatal life. During development, we identified two groups of differentially localized PDGFRα+ OPCs that are transcriptionally and epigenetically distinct. One group (active, or actOPCs) is metabolically active and enriched in white matter. The second (homeostatic, or hOPCs) is less active, enriched in gray matter, and predicted to derive from actOPCs. In adulthood, these two groups are transcriptionally but not epigenetically distinct, and relative to developing OPCs are less active metabolically and have less open chromatin. When adult oligodendrogenesis is enhanced during experimentally induced remyelination, adult OPCs do not reacquire a developmental open chromatin state, and the oligodendrogenesis trajectory is distinct from that seen neonatally. These data suggest that there are two OPC groups subserving distinct postnatal functions and that neonatal and adult OPC-mediated oligodendrogenesis are fundamentally different.

Schwann Cells Are Key Regulators of Corneal Epithelial Renewal

Purpose

Corneal sensory nerves protect the cornea from injury. They are also thought to stimulate limbal stem cells (LSCs) to produce transparent epithelial cells constantly, enabling vision. In other organs, Schwann cells (SCs) associated with tissue-innervating axon terminals mediate tissue regeneration. This study defines the critical role of the corneal axon-ensheathing SCs in homeostatic and regenerative corneal epithelial cell renewal.

Methods

SC localization in the cornea was determined by in situ hybridization and immunohistochemistry with SC markers. In vivo SC visualization and/or ablation were performed in mice with inducible corneal SC-specific expression of tdTomato and/or Diphtheria toxin, respectively. The relative locations of SCs and LSCs were observed with immunohistochemical analysis of harvested genetically SC-prelabeled mouse corneas with LSC-specific antibodies. The correlation between cornea-innervating axons and the appearance of SCs was ascertained using corneal denervation in rats. To determine the limbal niche cellular composition and gene expression changes associated with innervation-dependent epithelial renewal, single-cell RNA sequencing (scRNA-seq) of dissociated healthy, de-epithelized, and denervated cornea limbi was performed.

Results

We observed limbal enrichment of corneal axon-associated myelinating and non-myelinating SCs. Induced local genetic ablation of SCs, although leaving corneal sensory innervation intact, markedly inhibited corneal epithelial renewal. scRNA-seq analysis (1) highlighted the transcriptional heterogenicity of cells populating the limbal niche, and (2) identified transcriptional changes associated with corneal innervation and during wound healing that model potential regulatory paracrine interactions between SCs and LSCs.

Conclusions

Limbal SCs are required for innervation-dependent corneal epithelial renewal.

Nail-associated mesenchymal cells contribute to and are essential for dorsal digit tip regeneration

Here, we ask why the nail base is essential for mammalian digit tip regeneration, focusing on the inductive nail mesenchyme. We identify a transcriptional signature for these cells that includes Lmx1b and show that the Lmx1b-expressing nail mesenchyme is essential for blastema formation. We use a combination of Lmx1bCreERT2-based lineage-tracing and single-cell transcriptional analyses to show that the nail mesenchyme contributes cells for two pro-regenerative mechanisms. One group of cells maintains their identity and regenerates the new nail mesenchyme. A second group contributes specifically to the dorsal blastema, loses their nail mesenchyme phenotype, acquires a blastema transcriptional state that is highly similar to blastema cells of other origins, and ultimately contributes to regeneration of the dorsal but not ventral dermis and bone. Thus, the regenerative necessity for an intact nail base is explained, at least in part, by a requirement for the inductive nail mesenchyme.

Vitamin D Receptor Activation Attenuates Hippo Pathway Effectors and Cell Survival in Metastatic Neuroblastoma

Survival for high-risk neuroblastoma remains poor. Most patients who recur, present with metastatic disease, and few targetable pathways that govern spread to distant sites are currently known. We previously developed a metastatic mouse model to select cells with enhanced ability to spread to the bone and brain and identified a signature based on differentially expressed genes, which also predicted patient survival. To discover new neuroblastoma therapies, we utilized the Connectivity Map to identify compounds that can reverse this metastatic transcriptional signature and found calcipotriol, a vitamin D3 analog, to be a compound that selectively targets cell lines with enhanced metastatic potential. Calcipotriol treatment of enhanced metastatic, but not parental, cells reduces proliferation and survival via vitamin D receptor (VDR) signaling, increases the expression of RASSF2, a negative regulator of the Hippo signaling pathway, and reduces the levels of the Hippo pathway effectors YAP and TAZ. RASSF2 is required for the effects of calcipotriol and for the reduction of levels and nuclear localization of YAP/TAZ. Migration of the enhanced metastatic cells and YAP/TAZ levels are reduced after calcipotriol treatment and YAP overexpression reduces calcipotriol sensitivity. Furthermore, metastatic cells that overexpress VDR also showed lower tumor burden in vivo.

IMPLICATIONS

This newly identified link between VDR signaling and the Hippo pathway could inform treatment strategies for metastatic neuroblastoma.

Restoration of hippocampal neural precursor function by ablation of senescent cells in the aging stem cell niche

Senescent cells are responsible, in part, for tissue decline during aging. Here, we focused on CNS neural precursor cells (NPCs) to ask if this is because senescent cells in stem cell niches impair precursor-mediated tissue maintenance. We demonstrate an aging-dependent accumulation of senescent cells, largely senescent NPCs, within the hippocampal stem cell niche coincident with declining adult neurogenesis. Pharmacological ablation of senescent cells via acute systemic administration of the senolytic drug ABT-263 (Navitoclax) caused a rapid increase in NPC proliferation and neurogenesis. Genetic ablation of senescent cells similarly activated hippocampal NPCs. This acute burst of neurogenesis had long-term effects in middle-aged mice. One month post-ABT-263, adult-born hippocampal neuron numbers increased and hippocampus-dependent spatial memory was enhanced. These data support a model where senescent niche cells negatively influence neighboring non-senescent NPCs during aging, and ablation of these senescent cells partially restores neurogenesis and hippocampus-dependent cognition.

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Senescent neural precursor cells accumulate in the hippocampus with age

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Senescent precursor accumulation is coincident with declining adult neurogenesis

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Ablating senescent precursors increases precursor proliferation and neurogenesis

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Ablating senescent precursors improves hippocampus-dependent spatial memory

Single-Cell Profiling Shows Murine Forebrain Neural Stem Cells Reacquire a Developmental State when Activated for Adult Neurogenesis

The transitions from developing to adult quiescent and activated neural stem cells (NSCs) are not well understood. Here, we use single-cell transcriptional profiling and lineage tracing to characterize these transitions in the murine forebrain. We show that the two forebrain NSC parental populations, embryonic cortex and ganglionic eminence radial precursors (RPs), are highly similar even though they make glutamatergic versus gabaergic neurons. Both RP populations progress linearly to transition from a highly active embryonic to a dormant adult stem cell state that still shares many similarities with embryonic RPs. When adult NSCs of either embryonic origin become reactivated to make gabaergic neurons, they acquire a developing ganglionic eminence RP-like identity. Thus, transitions from embryonic RPs to adult NSCs and back to neuronal progenitors do not involve fundamental changes in cell identity, but rather reflect conversions between activated and dormant NSC states that may be determined by the niche environment.

The DEAD-box helicase DDX56 is a conserved stemness regulator in normal and cancer stem cells

Across the animal kingdom, adult tissue homeostasis is regulated by adult stem cell activity, which is commonly dysregulated in human cancers. However, identifying key regulators of stem cells in the milieu of thousands of genes dysregulated in a given cancer is challenging. Here, using a comparative genomics approach between planarian adult stem cells and patient-derived glioblastoma stem cells (GSCs), we identify and demonstrate the role of DEAD-box helicase DDX56 in regulating aspects of stemness in four stem cell systems: planarians, mouse neural stem cells, human GSCs, and a fly model of glioblastoma. In a human GSC line, DDX56 localizes to the nucleolus, and using planarians, when DDX56 is lost, stem cells dysregulate expression of ribosomal RNAs and lose nucleolar integrity prior to stem cell death. Together, a comparative genomic approach can be used to uncover conserved stemness regulators that are functional in both normal and cancer stem cells.

(H)Elping nerve growth factor: Elp1 inhibits TrkA's phosphatase to maintain retrograde signaling

Nerve growth factor (NGF) regulates many aspects of neuronal biology by retrogradely propagating signals along axons to the targets of those axons. How this occurs when axons contain a plethora of proteins that can silence those signals has long perplexed the neurotrophin field. In this issue of the JCI, Li et al. suggest an answer to this vexing problem, while exploring why the Elp1 gene that is mutated in familial dysautonomia (FD) causes peripheral neuropathy. They describe a distinctive function of Elp1 as a protein that is required to sustain NGF signaling by blocking the activity of its phosphatase that shuts off those signals. This finding helps explain the innervation deficits prominent in FD and reveals a unique role for Elp1 in the regulation of NGF-dependent TrkA activity.

The Protein Tyrosine Phosphatase Receptor Delta Regulates Developmental Neurogenesis

PTPRD is a receptor protein tyrosine phosphatase that is genetically associated with neurodevelopmental disorders. Here, we asked whether Ptprd mutations cause aberrant neural development by perturbing neurogenesis in the murine cortex. We show that loss of Ptprd causes increases in neurogenic transit-amplifying intermediate progenitor cells and cortical neurons and perturbations in neuronal localization. These effects are intrinsic to neural precursor cells since acute Ptprd knockdown causes similar perturbations. PTPRD mediates these effects by dephosphorylating receptor tyrosine kinases, including TrkB and PDGFRβ, and loss of Ptprd causes the hyperactivation of TrkB and PDGFRβ and their downstream MEK-ERK signaling pathway in neural precursor cells. Moreover, inhibition of aberrant TrkB or MEK activation rescues the increased neurogenesis caused by knockdown or homozygous loss of Ptprd. These results suggest that PTPRD regulates receptor tyrosine kinases to ensure appropriate numbers of intermediate progenitor cells and neurons, suggesting a mechanism for its genetic association with neurodevelopmental disorders.

Glioma-derived IL-33 orchestrates an inflammatory brain tumor microenvironment that accelerates glioma progression

Despite a deeper molecular understanding, human glioblastoma remains one of the most treatment refractory and fatal cancers. It is known that the presence of macrophages and microglia impact glioblastoma tumorigenesis and prevent durable response. Herein we identify the dual function cytokine IL-33 as an orchestrator of the glioblastoma microenvironment that contributes to tumorigenesis. We find that IL-33 expression in a large subset of human glioma specimens and murine models correlates with increased tumor-associated macrophages/monocytes/microglia. In addition, nuclear and secreted functions of IL-33 regulate chemokines that collectively recruit and activate circulating and resident innate immune cells creating a pro-tumorigenic environment. Conversely, loss of nuclear IL-33 cripples recruitment, dramatically suppresses glioma growth, and increases survival. Our data supports the paradigm that recruitment and activation of immune cells, when instructed appropriately, offer a therapeutic strategy that switches the focus from the cancer cell alone to one that includes the normal host environment.

Abstract PR15: Acting at a distance: Medulloblastoma-secreted ligands disrupt normal neural stem cell function

Long-term cognitive impairments are common in pediatric brain cancer survivors. While these impairments are thought to arise following radiation treatment, recent reports suggest a link to tumor-specific mechanisms. We therefore hypothesized that pediatric brain tumors, more specifically medulloblastoma (MB), can directly affect neural stem and precursor cell function in the forebrain stem cell niche—the subventricular zone (V-SVZ) —by secreting bioactive factors. Mice harboring subcutaneous flank MB tumors had fewer proliferating neural precursor cells in the V-SVZ than controls as well as decreased olfactory bulb neurogenesis and white matter oligodendrogenesis. To assess the effects of the MB secretome in the brain, concentrated conditioned media from MB cell lines (MB-CM) was injected intracerebroventricular (ICV) into mouse pups. ICV injection of MB-CM decreased neural precursor cell proliferation as well as decreased numbers of V-SVZ neurospheres in culture. MB-CM from multiple cell lines decreased V-SVZ neurosphere number and promoted astrocyte differentiation in culture. To identify the ligands contributing to the phenotype, an interaction model was developed extracting ligands from MB microarray data, networking them to receptors on NSCs. Of the predicted ligands, IL6-family cytokine expression and secretion was validated in MB cells. When added in culture, recombinant IL6, IL11, and CT1 decreased neurosphere number and ICV injection of IL11 into mouse pups decreased V-SVZ neural precursor cell proliferation. Overall, this work demonstrates that medulloblastoma secretes bioactive compounds that perturb neural stem cell function and the circuity involved in normal cognitive function.

This abstract is also being presented as Poster B64.

Citation Format: Alexander Gont, Jaclin V. Simonetta, Jenna Park, Alice R. Shan, Michael J. Borrett, Michael D. Taylor, Freda D. Miller, David R. Kaplan. Acting at a distance: Medulloblastoma-secreted ligands disrupt normal neural stem cell function [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr PR15.

Abstract B72: Dissecting the heterogeneity of metastatic neuroblastoma cells by single-cell RNA-seq

Introduction: The major cause of death in neuroblastoma (NB) is metastatic relapse, and most therapies do not specifically target metastases. We previously developed a metastatic mouse model using intracardiac injection of tagged human SK-N-AS NB cells, followed by multiple rounds of in vivo selection from bone and brain metastatic sites. Gene expression profiles identified novel genes and pathways dysregulated in metastatic NB cells that, when genetically or pharmacologically manipulated, affected metastasis. A subset of genes up- or downregulated in the enhanced metastatic cells was used to generate a metastatic gene signature that predicted patient survival. We have now asked (1) whether there is a transcriptionally distinct subpopulation of cells in the parental population that selectively metastasizes to bone and brain, and (2) if there are transcriptional differences that can account for the ability of the enhanced metastatic cells to hone to bone/bone marrow vs. brain.

Methods: We used 10X Genomics single-cell RNA sequencing (scRNA-seq) to transcriptionally profile parental cells and two in vivo-selected cell lines that showed enhanced metastatic spread to bone/bone marrow (B5) and brain (BR2). Approximately 5,000 genes and 10,000 transcripts per cell were sequenced, with the data analyzed using a unique analysis pipeline incorporating extensive data quality analysis with visualization and clustering methods using evidence-based parameter selection. Genes with high variance were used to compute principal components as inputs to visualize cells in two dimensions (t-SNE plots) in clusters.

Results: scRNA-seq data showed that the parental cells clustered separately from B5 and BR2 cells, with clusters in the parental cells that expressed genes enriched in the B5 and BR2 cells that we previously reported drove migration/invasion in vitro and/or metastasis in vivo, including sphingosine kinase 1. These results suggest that subpopulations of the parental cells with enhanced metastatic gene expression are selected in vivo for honing to metastatic sites. We identified genes enriched in B5 and BR2 cells encoding cell surface proteins, chemokines including CCL2, and transcription factors including Msx1 involved in metastasis of other cancers, that we will assess for roles in NB metastasis. While the B2 and BR5 cells were transcriptionally similar to each other, we did identify genes highly expressed in B2 subclusters that are associated with bone cancers including FGF5, and that are highly expressed in BR2 subclusters that are associated with brain cancers including RARRES2. These findings suggest that transcriptional differences between B5 and BR2 cells may influence metastasis to the bone or brain.

Conclusion: The identification of genes that are differentially expressed in the more metastatic subpopulations will contribute to our understanding of the molecular mechanisms governing metastases and identify novel targetable pathways that were not detected by profiling bulk tumor populations.

Citation Format: Alice R. Shan, Alexander Gont, David R. Kaplan, Meredith S. Irwin. Dissecting the heterogeneity of metastatic neuroblastoma cells by single-cell RNA-seq [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr B72.

Methylglyoxal couples metabolic and translational control of Notch signalling in mammalian neural stem cells

Gene regulation and metabolism are two fundamental processes that coordinate the self-renewal and differentiation of neural precursor cells (NPCs) in the developing mammalian brain. However, little is known about how metabolic signals instruct gene expression to control NPC homeostasis. Here, we show that methylglyoxal, a glycolytic intermediate metabolite, modulates Notch signalling to regulate NPC fate decision. We find that increased methylglyoxal suppresses the translation of Notch1 receptor mRNA in mouse and human NPCs, which is mediated by binding of the glycolytic enzyme GAPDH to an AU-rich region within Notch1 3ʹUTR. Interestingly, methylglyoxal inhibits the enzymatic activity of GAPDH and engages it as an RNA-binding protein to suppress Notch1 translation. Reducing GAPDH levels or restoring Notch signalling rescues methylglyoxal-induced NPC depletion and premature differentiation in the developing mouse cortex. Taken together, our data indicates that methylglyoxal couples the metabolic and translational control of Notch signalling to control NPC homeostasis.

Gene regulation and metabolism co-ordinate self-renewal and differentiation of neural precursors (NPCs) in the developing brain. Here the authors show that methylglyoxal, a glycolytic intermediate metabolite, promotes GADPH-dependent translational repression of Notch1, thereby promoting NPC differentiation.

Acquisition of a Unique Mesenchymal Precursor-like Blastema State Underlies Successful Adult Mammalian Digit Tip Regeneration

Here, we investigate the origin and nature of blastema cells that regenerate the adult murine digit tip. We show that Pdgfra-expressing mesenchymal cells in uninjured digits establish the regenerative blastema and are essential for regeneration. Single-cell profiling shows that the mesenchymal blastema cells are distinct from both uninjured digit and embryonic limb or digit Pdgfra-positive cells. This unique blastema state is environmentally determined; dermal fibroblasts transplanted into the regenerative, but not non-regenerative, digit express blastema-state genes and contribute to bone regeneration. Moreover, lineage tracing with single-cell profiling indicates that endogenous osteoblasts or osteocytes acquire a blastema mesenchymal transcriptional state and contribute to both dermis and bone regeneration. Thus, mammalian digit tip regeneration occurs via a distinct adult mechanism where the regenerative environment promotes acquisition of a blastema state that enables cells from tissues such as bone to contribute to the regeneration of other mesenchymal tissues such as the dermis.

First-in-Human Brain Imaging of [18F]TRACK, a PET tracer for Tropomyosin Receptor Kinases

The tropomyosin receptor kinase TrkA/B/C family is responsible for human neuronal growth, survival, and differentiation from early nervous system development stages onward. Downregulation of TrkA/B/C receptors characterizes numerous neurological disorders including Alzheimer's disease (AD). Abnormally expressed Trk receptors or chimeric Trk fusion proteins are also well-characterized oncogenic drivers in a variety of neurogenic and non-neurogenic human neoplasms and are currently the focus of intensive clinical research. Previously, we have described the clinical translation of a highly selective and potent carbon-11-labeled pan-Trk radioligand and the preclinical characterization of the optimized fluorine-18-labeled analogue, [¹⁸F]TRACK, for in vivo Trk positron emission tomography (PET) imaging. We describe herein central nervous system selectivity assessment and first-in-human study of [¹⁸F]TRACK.

Interleukin-6 Regulates Adult Neural Stem Cell Numbers during Normal and Abnormal Post-natal Development

Circulating systemic factors can regulate adult neural stem cell (NSC) biology, but the identity of these circulating cues is still being defined. Here, we have focused on the cytokine interleukin-6 (IL-6), since increased circulating levels of IL-6 are associated with neural pathologies such as autism and bipolar disorder. We show that IL-6 promotes proliferation of post-natal murine forebrain NSCs and that, when the IL-6 receptor is inducibly knocked out in post-natal or adult neural precursors, this causes a long-term decrease in forebrain NSCs. Moreover, a transient circulating surge of IL-6 in perinatal or adult mice causes an acute increase in neural precursor proliferation followed by long-term depletion of adult NSC pools. Thus, IL-6 signaling is both necessary and sufficient for adult NSC self-renewal, and acute perturbations in circulating IL-6, as observed in many pathological situations, have long-lasting effects on the size of adult NSC pools.

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The cytokine IL-6 promotes self-renewal of post-natal forebrain NSCs

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Inducible knockout of the IL-6 receptor causes long-term decreases in post-natal NSCs

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A transient surge of circulating IL-6 ultimately depletes adult NSC pools

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IL-6 signaling is both necessary and sufficient for adult NSC self-renewal

Small molecule epigenetic screen identifies novel EZH2 and HDAC inhibitors that target glioblastoma brain tumor-initiating cells

Glioblastoma (GBM) is the most lethal and aggressive adult brain tumor, requiring the development of efficacious therapeutics. Towards this goal, we screened five genetically distinct patient-derived brain-tumor initiating cell lines (BTIC) with a unique collection of small molecule epigenetic modulators from the Structural Genomics Consortium (SGC). We identified multiple hits that inhibited the growth of BTICs in vitro, and further evaluated the therapeutic potential of EZH2 and HDAC inhibitors due to the high relevance of these targets for GBM. We found that the novel SAM-competitive EZH2 inhibitor UNC1999 exhibited low micromolar cytotoxicity in vitro on a diverse collection of BTIC lines, synergized with dexamethasone (DEX) and suppressed tumor growth in vivo in combination with DEX. In addition, a unique brain-penetrant class I HDAC inhibitor exhibited cytotoxicity in vitro on a panel of BTIC lines and extended survival in combination with TMZ in an orthotopic BTIC model in vivo. Finally, a combination of EZH2 and HDAC inhibitors demonstrated synergy in vitro by augmenting apoptosis and increasing DNA damage. Our findings identify key epigenetic modulators in GBM that regulate BTIC growth and survival and highlight promising combination therapies.

A Translational Repression Complex in Developing Mammalian Neural Stem Cells that Regulates Neuronal Specification

The mechanisms instructing genesis of neuronal subtypes from mammalian neural precursors are not well understood. To address this issue, we have characterized the transcriptional landscape of radial glial precursors (RPs) in the embryonic murine cortex. We show that individual RPs express mRNA, but not protein, for transcriptional specifiers of both deep and superficial layer cortical neurons. Some of these mRNAs, including the superficial versus deep layer neuron transcriptional regulators Brn1 and Tle4, are translationally repressed by their association with the RNA-binding protein Pumilio2 (Pum2) and the 4E-T protein. Disruption of these repressive complexes in RPs mid-neurogenesis by knocking down 4E-T or Pum2 causes aberrant co-expression of deep layer neuron specification proteins in newborn superficial layer neurons. Thus, cortical RPs are transcriptionally primed to generate diverse types of neurons, and a Pum2/4E-T complex represses translation of some of these neuronal identity mRNAs to ensure appropriate temporal specification of daughter neurons.

RNAi screen identifies essential regulators of human brain metastasis-initiating cells

Brain metastases (BM) are the most common brain tumor in adults and are a leading cause of cancer mortality. Metastatic lesions contain subclones derived from their primary lesion, yet their functional characterization is limited by a paucity of preclinical models accurately recapitulating the metastatic cascade, emphasizing the need for a novel approach to BM and their treatment. We identified a unique subset of stem-like cells from primary human patient brain metastases, termed brain metastasis-initiating cells (BMICs). We now establish a BMIC patient-derived xenotransplantation (PDXT) model as an investigative tool to comprehensively interrogate human BM. Using both in vitro and in vivo RNA interference screens of these BMIC models, we identified SPOCK1 and TWIST2 as essential BMIC regulators. SPOCK1 in particular is a novel regulator of BMIC self-renewal, modulating tumor initiation and metastasis from the lung to the brain. A prospective cohort of primary lung cancer specimens showed that SPOCK1 was overexpressed only in patients who ultimately developed BM. Protein-protein interaction network mapping between SPOCK1 and TWIST2 identified novel pathway interactors with significant prognostic value in lung cancer patients. Of these genes, INHBA, a TGF-β ligand found mutated in lung adenocarcinoma, showed reduced expression in BMICs with knockdown of SPOCK1. In conclusion, we have developed a useful preclinical model of BM, which has served to identify novel putative BMIC regulators, presenting potential therapeutic targets that block the metastatic process, and transform a uniformly fatal systemic disease into a locally controlled and eminently more treatable one.

A neuroprotective agent that inactivates prodegenerative TrkA and preserves mitochondria

The pan-kinase inhibitor foretinib is identified as a potent suppressor of sympathetic, sensory, and motor neuron axon degeneration, acting in part by inhibiting the activity of the unliganded TrkA/nerve growth factor receptor and by preserving mitochondria in die-back and Wallerian degeneration models.

Axon degeneration is an early event and pathological in neurodegenerative conditions and nerve injuries. To discover agents that suppress neuronal death and axonal degeneration, we performed drug screens on primary rodent neurons and identified the pan-kinase inhibitor foretinib, which potently rescued sympathetic, sensory, and motor wt and SOD1 mutant neurons from trophic factor withdrawal-induced degeneration. By using primary sympathetic neurons grown in mass cultures and Campenot chambers, we show that foretinib protected neurons by suppressing both known degenerative pathways and a new pathway involving unliganded TrkA and transcriptional regulation of the proapoptotic BH3 family members BimEL, Harakiri,and Puma, culminating in preservation of mitochondria in the degenerative setting. Foretinib delayed chemotherapy-induced and Wallerian axonal degeneration in culture by preventing axotomy-induced local energy deficit and preserving mitochondria, and peripheral Wallerian degeneration in vivo. These findings identify a new axon degeneration pathway and a potentially clinically useful therapeutic drug.

Migrating Interneurons Secrete Fractalkine to Promote Oligodendrocyte Formation in the Developing Mammalian Brain

During development, newborn interneurons migrate throughout the embryonic brain. Here, we provide evidence that these interneurons act in a paracrine fashion to regulate developmental oligodendrocyte formation. Specifically, we show that medial ganglionic eminence (MGE) interneurons secrete factors that promote genesis of oligodendrocytes from glially biased cortical precursors in culture. Moreover, when MGE interneurons are genetically ablated in vivo prior to their migration, this causes a deficit in cortical oligodendrogenesis. Modeling of the interneuron-precursor paracrine interaction using transcriptome data identifies the cytokine fractalkine as responsible for the pro-oligodendrocyte effect in culture. This paracrine interaction is important in vivo, since knockdown of the fractalkine receptor CX3CR1 in embryonic cortical precursors, or constitutive knockout of CX3CR1, causes decreased numbers of oligodendrocyte progenitor cells (OPCs) and oligodendrocytes in the postnatal cortex. Thus, in addition to their role in regulating neuronal excitability, interneurons act in a paracrine fashion to promote the developmental genesis of oligodendrocytes.

A Metastatic Mouse Model Identifies Genes That Regulate Neuroblastoma Metastasis

Metastatic relapse is the major cause of death in pediatric neuroblastoma, where there remains a lack of therapies to target this stage of disease. To understand the molecular mechanisms mediating neuroblastoma metastasis, we developed a mouse model using intracardiac injection and in vivo selection to isolate malignant cell subpopulations with a higher propensity for metastasis to bone and the central nervous system. Gene expression profiling revealed primary and metastatic cells as two distinct cell populations defined by differential expression of 412 genes and of multiple pathways, including CADM1, SPHK1, and YAP/TAZ, whose expression independently predicted survival. In the metastatic subpopulations, a gene signature was defined (MET-75) that predicted survival of neuroblastoma patients with metastatic disease. Mechanistic investigations demonstrated causal roles for CADM1, SPHK1, and YAP/TAZ in mediating metastatic phenotypes in vitro and in vivo Notably, pharmacologic targeting of SPHK1 or YAP/TAZ was sufficient to inhibit neuroblastoma metastasis in vivo Overall, we identify gene expression signatures and candidate therapeutics that could improve the treatment of metastatic neuroblastoma. Cancer Res; 77(3); 696-706. ©2017 AACR.

Conditional ablation of p63 indicates that it is essential for embryonic development of the central nervous system

p63 is a member of the p53 family that regulates the survival of neural precursors in the adult brain. However, the relative importance of p63 in the developing brain is still unclear, since embryonic p63^−/− mice display no apparent deficits in neural development. Here, we have used a more definitive conditional knockout mouse approach to address this issue, crossing p63^fl/fl mice to mice carrying a nestin-CreERT2 transgene that drives inducible recombination in neural precursors following tamoxifen treatment. Inducible ablation of p63 following tamoxifen treatment of mice on embryonic day 12 resulted in highly perturbed forebrain morphology including a thinner cortex and enlarged lateral ventricles 3 d later. While the normal cortical layers were still present following acute p63 ablation, cortical precursors and neurons were both reduced in number due to widespread cellular apoptosis. This apoptosis was cell-autonomous, since it also occurred when p63 was inducibly ablated in primary cultured cortical precursors. Finally, we demonstrate increased expression of the mRNA encoding another p53 family member, ΔNp73, in cortical precursors of p63^−/− but not tamoxifen-treated p63^fl/fl;R26YFP^fl/fl;nestin-CreERT2^+/Ø embryos. Since ΔNp73 promotes cell survival, then this compensatory increase likely explains the lack of an embryonic brain phenotype in p63^−/− mice. Thus, p63 plays a key prosurvival role in the developing mammalian brain.

Proneurogenic Ligands Defined by Modeling Developing Cortex Growth Factor Communication Networks

The neural stem cell decision to self-renew or differentiate is tightly regulated by its microenvironment. Here, we have asked about this microenvironment, focusing on growth factors in the embryonic cortex at a time when it is largely comprised of neural precursor cells (NPCs) and newborn neurons. We show that cortical NPCs secrete factors that promote their maintenance, while cortical neurons secrete factors that promote differentiation. To define factors important for these activities, we used transcriptome profiling to identify ligands produced by NPCs and neurons, cell-surface mass spectrometry to identify receptors on these cells, and computational modeling to integrate these data. The resultant model predicts a complex growth factor environment with multiple autocrine and paracrine interactions. We tested this communication model, focusing on neurogenesis, and identified IFNγ, Neurturin (Nrtn), and glial-derived neurotrophic factor (GDNF) as ligands with unexpected roles in promoting neurogenic differentiation of NPCs in vivo.

Disulfiram when Combined with Copper Enhances the Therapeutic Effects of Temozolomide for the Treatment of Glioblastoma

PURPOSE

Glioblastoma is one of the most lethal cancers in humans, and with existing therapy, survival remains at 14.6 months. Current barriers to successful treatment include their infiltrative behavior, extensive tumor heterogeneity, and the presence of a stem-like population of cells, termed brain tumor-initiating cells (BTIC) that confer resistance to conventional therapies.

EXPERIMENTAL DESIGN

To develop therapeutic strategies that target BTICs, we focused on a repurposing approach that explored already-marketed (clinically approved) drugs for therapeutic potential against patient-derived BTICs that encompass the genetic and phenotypic heterogeneity of glioblastoma observed clinically.

RESULTS

Using a high-throughput in vitro drug screen, we found that montelukast, clioquinol, and disulfiram (DSF) were cytotoxic against a large panel of patient-derived BTICs. Of these compounds, disulfiram, an off-patent drug previously used to treat alcoholism, in the presence of a copper supplement, showed low nanomolar efficacy in BTICs including those resistant to temozolomide and the highly infiltrative quiescent stem-like population. Low dose DSF-Cu significantly augmented temozolomide activity in vitro, and importantly, prolonged in vivo survival in patient-derived BTIC models established from both newly diagnosed and recurrent tumors. Moreover, we found that in addition to acting as a potent proteasome inhibitor, DSF-Cu functionally impairs DNA repair pathways and enhances the effects of DNA alkylating agents and radiation. These observations suggest that DSF-Cu inhibits proteasome activity and augments the therapeutic effects of DNA-damaging agents (temozolomide and radiation).

CONCLUSIONS

DSF-Cu should be considered as an adjuvant therapy for the treatment of patients with glioblastoma in both newly diagnosed and recurrent settings. Clin Cancer Res; 22(15); 3860-75. ©2016 AACR.

Identification of Drugs that Regulate Dermal Stem Cells and Enhance Skin Repair

Here, we asked whether we could identify pharmacological agents that enhance endogenous stem cell function to promote skin repair, focusing on skin-derived precursors (SKPs), a dermal precursor cell population. Libraries of compounds already used in humans were screened for their ability to enhance the self-renewal of human and rodent SKPs. We identified and validated five such compounds, and showed that two of them, alprostadil and trimebutine maleate, enhanced the repair of full thickness skin wounds in middle-aged mice. Moreover, SKPs isolated from drug-treated skin displayed long-term increases in self-renewal when cultured in basal growth medium without drugs. Both alprostadil and trimebutine maleate likely mediated increases in SKP self-renewal by moderate hyperactivation of the MEK-ERK pathway. These findings identify candidates for potential clinical use in human skin repair, and provide support for the idea that pharmacological activation of endogenous tissue precursors represents a viable therapeutic strategy.

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Small-molecule screens identify compounds that enhance SKP self-renewal

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Alprostadil and trimebutine maleate both increase SKP self-renewal

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Both compounds likely act by promoting activation of the MEK-ERK pathway

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Both compounds activated dermal precursors in vivo to enhance wound healing

Metformin Acts on Two Different Molecular Pathways to Enhance Adult Neural Precursor Proliferation/Self-Renewal and Differentiation

The recruitment of endogenous adult neural stem cells for brain repair is a promising regenerative therapeutic strategy. This strategy involves stimulation of multiple stages of adult neural stem cell development, including proliferation, self-renewal, and differentiation. Currently, there is a lack of a single therapeutic approach that can act on these multiple stages of adult neural stem cell development to enhance neural regeneration. Here we show that metformin, an FDA-approved diabetes drug, promotes proliferation, self-renewal, and differentiation of adult neural precursors (NPCs). Specifically, we show that metformin enhances adult NPC proliferation and self-renewal dependent upon the p53 family member and transcription factor TAp73, while it promotes neuronal differentiation of these cells by activating the AMPK-aPKC-CBP pathway. Thus, metformin represents an optimal candidate neuro-regenerative agent that is capable of not only expanding the adult NPC population but also subsequently driving them toward neuronal differentiation by activating two distinct molecular pathways.

Axon diameter and axonal transport: In vivo and in vitro effects of androgens

Testosterone is a sex hormone involved in brain maturation via multiple molecular mechanisms. Previous human studies described age-related changes in the overall volume and structural properties of white matter during male puberty. Based on this work, we have proposed that testosterone may induce a radial growth of the axon and, possibly, modulate axonal transport. In order to determine whether this is the case we have used two different experimental approaches. With electron microscopy, we have evaluated sex differences in the structural properties of axons in the corpus callosum (splenium) of young rats, and tested consequences of castration carried out after weaning. Then we examined in vitro the effect of the non-aromatizable androgen Mibolerone on the structure and bidirectional transport of wheat-germ agglutinin vesicles in the axons of cultured sympathetic neurons. With electron microscopy, we found robust sex differences in axonal diameter (males>females) and g ratio (males>females). Removal of endogenous testosterone by castration was associated with lower axon diameter and lower g ratio in castrated (vs. intact) males. In vitro, Mibolerone influenced the axonal transport in a time- and dose-dependent manner, and increased the axon caliber as compared with vehicle-treated neurons. These findings are consistent with the role of testosterone in shaping the axon by regulating its radial growth, as predicted by the initial human studies.

Anti-tumor activity of the beta-adrenergic receptor antagonist propranolol in neuroblastoma

Neuroblastoma (NB) is a pediatric tumor of the sympathetic nervous system, which is often associated with elevated catecholamines. More than half of patients with metastatic NB relapse and survival is extremely poor with current therapies. In a high-throughput screen of FDA-approved drugs we identified anti-NB activity for the nonselective β-adrenergic receptor antagonist propranolol hydrochloride.

Propranolol inhibited growth of a panel of fifteen NB cell lines irrespective of MYCN status, and treatment induced apoptosis and decreased proliferation. Activity was dependent on inhibition of the β2, and not β1, adrenergic receptor, and treatment resulted in activation of p53 and p73 signaling in vitro. The majority of NB cell lines and primary tumors express β2 adrenergic receptor and higher mRNA levels correlate with improved patient survival, but expression levels did not correlate with in vitro sensitivity to propranolol. Furthermore, propranolol is synergistic with the topoisomerase I inhibitor SN-38 and propranolol inhibits growth of NB xenografts in vivo at doses similar to those used to treat infants with hemangiomas and hypertension. Taken together, our results suggest that propranolol has activity against NB and thus should be considered in combination treatments for patients with relapsed and refractory NB.

An eIF4E1/4E-T complex determines the genesis of neurons from precursors by translationally repressing a proneurogenic transcription program

Here, we have addressed the mechanisms that determine genesis of the correct numbers of neurons during development, focusing on the embryonic cortex. We identify in neural precursors a repressive complex involving eIF4E1 and its binding partner 4E-T that coordinately represses translation of proteins that determine neurogenesis. This eIF4E1/4E-T complex is present in granules with the processing body proteins Lsm1 and Rck, and disruption of this complex causes premature and enhanced neurogenesis and neural precursor depletion. Analysis of the 4E-T complex shows that it is highly enriched in mRNAs encoding transcription factors and differentiation-related proteins. These include the proneurogenic bHLH mRNAs, which colocalize with 4E-T in granules and whose protein products are aberrantly upregulated following knockdown of eIF4E, 4E-T, or processing body proteins. Thus, neural precursors are transcriptionally primed to generate neurons, but an eIF4E/4E-T complex sequesters and represses translation of proneurogenic proteins to determine appropriate neurogenesis.

TLX activates MMP-2, promotes self-renewal of tumor spheres in neuroblastoma and correlates with poor patient survival

Nuclear orphan receptor TLX (Drosophilatailless homolog) is essential for the maintenance of neural stem/progenitor cell self-renewal, but its role in neuroblastoma (NB) is not well understood. Here, we show that TLX is essential for the formation of tumor spheres in three different NB cell lines, when grown in neural stem cell media. We demonstrate that the knock down of TLX in IMR-32 cells diminishes its tumor sphere-forming capacity. In tumor spheres, TLX is coexpressed with the neural progenitor markers Nestin, CD133 and Oct-4. In addition, TLX is coexpressed with the migratory neural progenitor markers CD15 and matrix metalloproteinase-2 (MMP-2) in xenografts of primary NB cells from patients. Subsequently, we show the effect of TLX on the proliferative, invasive and migratory properties of IMR-32 cells. We attribute this to the recruitment of TLX to both MMP-2 and Oct-4 gene promoters, which resulted in the respective gene activation. In support of our findings, we found that TLX expression was high in NB patient tissues when compared with normal peripheral nervous system tissues. Further, the Kaplan–Meier estimator indicated a negative correlation between TLX expression and survival in 88 NB patients. Therefore, our results point at TLX being a crucial player in progression of NB, by promoting self-renewal of NB tumor-initiating cells and altering their migratory and invasive properties.

To Die or Not to Die: Neurons and p63

One of the fundamental questions in neurobiology is how mammalian neurons survive for an organism's lifetime in the face of normal ongoing "wear and tear" that, in the case of neurons in the peripheral nervous system, even includes physical damage. Elucidating the mechanisms that control neuronal survival is of importance not only for our understanding of normal development of neuronal circuitry, but also to devise treatments for pathological situations such as traumatic injury, or neurodegenerative conditions. In this review, we will cover the emerging evidence that p63 plays an essential role in regulating neuronal life and death decisions in the nervous system working in concert with its two other family members, p53 and p73.

Strong performance on the Progressive Evaluation of Competency fellowship final examination predicts American Board of Pathology Certification

INTRODUCTION

The Progressive Evaluation of Competency (PEC) program was developed to help cytopathology fellowship directors evaluate the progress of fellows before program completion. There are no data on how PEC examination results compare with American Board of Pathology (ABP) certification status.

MATERIALS AND METHODS

PEC final examination results from the 2011/2012 academic year were compared with performance on the ABP cytopathology examination. The total and section PEC scores were compared with ABP scaled written and practical scores, and individuals who failed the certification examination were analyzed in detail.

RESULTS

Of the 103 fellows who took the final PEC examination in spring of 2012, 88 took the ABP examination, and 79 became certified. The total and the fine-needle aspiration scores on the PEC exam were positively and significantly correlated with performance on both ABP exam sections. Every fellow who scored in the upper one-third on the PEC exam became certified. The failing candidates scored significantly lower in both total scores and the fine-needle aspiration section of the PEC exam.

CONCLUSIONS

The PEC final examination performance is positively correlated with ABP certification status, and fellows who score in the top one-third of the PEC examination become certified. These findings can help provide guidance to both fellowship directors and fellows on competency and readiness for board certification.

The Snail transcription factor regulates the numbers of neural precursor cells and newborn neurons throughout mammalian life

The Snail transcription factor regulates diverse aspects of stem cell biology in organisms ranging from Drosophila to mammals. Here we have asked whether it regulates the biology of neural precursor cells (NPCs) in the forebrain of postnatal and adult mice, taking advantage of a mouse containing a floxed Snail allele (Snail^fl/fl mice). We show that when Snail is inducibly ablated in the embryonic cortex, this has long-term consequences for cortical organization. In particular, when Snail^fl/fl mice are crossed to Nestin-cre mice that express Cre recombinase in embryonic neural precursors, this causes inducible ablation of Snail expression throughout the postnatal cortex. This loss of Snail causes a decrease in proliferation of neonatal cortical neural precursors and mislocalization and misspecification of cortical neurons. Moreover, these precursor phenotypes persist into adulthood. Adult neural precursor cell proliferation is decreased in the forebrain subventricular zone and in the hippocampal dentate gyrus, and this is coincident with a decrease in the number of adult-born olfactory and hippocampal neurons. Thus, Snail is a key regulator of the numbers of neural precursors and newborn neurons throughout life.

Direct genesis of functional rodent and human schwann cells from skin mesenchymal precursors

Recent reports of directed reprogramming have raised questions about the stability of cell lineages. Here, we have addressed this issue, focusing upon skin-derived precursors (SKPs), a dermally derived precursor cell. We show by lineage tracing that murine SKPs from dorsal skin originate from mesenchymal and not neural crest-derived cells. These mesenchymally derived SKPs can, without genetic manipulation, generate functional Schwann cells, a neural crest cell type, and are highly similar at the transcriptional level to Schwann cells isolated from the peripheral nerve. This is not a mouse-specific phenomenon, since human SKPs that are highly similar at the transcriptome level can be made from neural crest-derived facial and mesodermally derived foreskin dermis and the foreskin SKPs can make myelinating Schwann cells. Thus, nonneural crest-derived mesenchymal precursors can differentiate into bona fide peripheral glia in the absence of genetic manipulation, suggesting that developmentally defined lineage boundaries are more flexible than widely thought.

A phosphoproteomics approach to identify candidate kinase inhibitor pathway targets in lymphoma-like primary cell lines

Mass spectrometry-based technologies are increasingly utilized in drug discovery. Phosphoproteomics in particular has allowed for the efficient surveying of phosphotyrosine signaling pathways involved in various diseases states, most prominently in cancer. We describe a phosphotyrosine-based proteomics screening approach to identify signaling pathways and tyrosine kinase inhibitor targets in highly tumorigenic human lymphoma-like primary cells. We identified several receptor tyrosine kinase pathways and validated SRC family kinases (SFKs) as potential drug targets for targeted selection of small molecule inhibitors. BMS-354825 (dasatinib) and SKI-606 (bosutinib), second and third generation clinical SFK/ABL inhibitors, were found to be potent cytotoxic agents against tumorigenic cells with low toxicity to normal pediatric stem cells. Both SFK inhibitors reduced ERK1/2 and AKT phosphorylation and induced apoptosis. This study supports the adaptation of high-end mass spectrometry techniques for the efficient identification of candidate tyrosine kinases as novel therapeutic targets in primary cancer cell lines.

Potent Targeting of the STAT3 Protein in Brain Cancer Stem Cells: A Promising Route for Treating Glioblastoma

The STAT3 gene is abnormally active in glioblastoma (GBM) and is a critically important mediator of tumor growth and therapeutic resistance in GBM. Thus, for poorly treated brain cancers such as gliomas, astrocytomas, and glioblastomas, which harbor constitutively activated STAT3, a STAT3-targeting therapeutic will be of significant importance. Herein, we report a most potent, small molecule, nonphosphorylated STAT3 inhibitor, 31 (SH-4-54) that strongly binds to STAT3 protein (K D = 300 nM). Inhibitor 31 potently kills glioblastoma brain cancer stem cells (BTSCs) and effectively suppresses STAT3 phosphorylation and its downstream transcriptional targets at low nM concentrations. Moreover, in vivo, 31 exhibited blood-brain barrier permeability, potently controlled glioma tumor growth, and inhibited pSTAT3 in vivo. This work, for the first time, demonstrates the power of STAT3 inhibitors for the treatment of BTSCs and validates the therapeutic efficacy of a STAT3 inhibitor for GBM clinical application.

Sox2-mediated regulation of adult neural crest precursors and skin repair

Nerve-derived neural crest cells are essential for regeneration in certain animals, such as newts. Here, we asked whether they play a similar role during mammalian tissue repair, focusing on Sox2-positive neural crest precursors in skin. In adult skin, Sox2 was expressed in nerve-terminal-associated neural crest precursor cells (NCPCs) around the hair follicle bulge, and following injury was induced in nerve-derived cells, likely dedifferentiated Schwann cell precursors. At later times postinjury, Sox2-positive cells were scattered throughout the regenerating dermis, and lineage tracing showed that these were all neural-crest-derived NCPCs. These Sox2-positive NCPCs were functionally important, since acute deletion of Sox2 prior to injury caused a decrease of NCPCs in the wound and aberrant skin repair. These data demonstrate that Sox2 regulates skin repair, likely by controlling NCPCs, and raise the possibility that nerve-derived NCPCs may play a general role in mammalian tissue repair.

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Sox2 regulates murine skin repair

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Sox2 regulates the neural crest precursor response to tissue injury

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Sox2 identifies a nerve terminal-associated neural crest precursor in hair follicles

A novel neurofibromin (NF1) interaction with the leucine-rich pentatricopeptide repeat motif-containing protein links neurofibromatosis type 1 and the French Canadian variant of Leigh's syndrome in a common molecular complex

Loss-of-function mutations and deletions in the neurofibromin tumor suppressor gene (NF1) cause neurofibromatosis type 1 (NF-1), the most common inherited syndrome of the nervous system in humans, with a birth incidence of 1:3,000. The most visible features of NF-1 are the neoplastic manifestations caused by the loss of Ras-GTPase-activating protein (Ras-GAP) activity mediated through the GAP-related domain (GRD) of neurofibromin (NF1), the protein encoded by NF1. However, the syndrome is also characterized by cognitive dysfunction and a number of developmental abnormalities. The molecular etiology of many of these nonneoplastic phenotypes remains unknown. Here we show that the tubulin-binding domain (TBD) of NF1 is a binding partner of the leucine-rich pentatricopeptide repeat motif-containing (LRPPRC) protein. These two proteins complex with Kinesin 5B, hnRNP A2, Staufen1, and Myelin Basic Protein (MBP) mRNA, likely in RNA granules. This interaction is of interest in that it links NF-1 with Leigh's syndrome, French Canadian variant (LSFC), an autosomal recessive neurodegenerative disorder that arises from mutations in the LRPPRC gene. Our findings provide clues to how loss or mutation of NF1 and LRPPRC may contribute to the manifestations of NF-1 and LSFC.

Metformin activates an atypical PKC-CBP pathway to promote neurogenesis and enhance spatial memory formation

VIDEO ABSTRACT

Although endogenous recruitment of adult neural stem cells has been proposed as a therapeutic strategy, clinical approaches for achieving this are lacking. Here, we show that metformin, a widely used drug, promotes neurogenesis and enhances spatial memory formation. Specifically, we show that an atypical PKC-CBP pathway is essential for the normal genesis of neurons from neural precursors and that metformin activates this pathway to promote rodent and human neurogenesis in culture. Metformin also enhances neurogenesis in the adult mouse brain in a CBP-dependent fashion, and in so doing enhances spatial reversal learning in the water maze. Thus, metformin, by activating an aPKC-CBP pathway, recruits neural stem cells and enhances neural function, thereby providing a candidate pharmacological approach for nervous system therapy.

Mobilizing endogenous stem cells for repair and regeneration: are we there yet?

Harnessing endogenous repair mechanisms to promote tissue regeneration in situations in which it does not normally occur has long been a goal in biomedical science. Recent advances in tissue stem cells indicate that this goal may now be achievable. Here we consider both the promise and the hurdles we still have to overcome.

p73 haploinsufficiency causes tau hyperphosphorylation and tau kinase dysregulation in mouse models of aging and Alzheimer's disease

Haploinsufficiency for the p53 family member p73 causes behavioral and neuroanatomical correlates of neurodegeneration in aging mice, including the appearance of aberrant phospho-tau-positive aggregates. Here, we show that these aggregates and tau hyperphosphorylation, as well as a generalized dysregulation of the tau kinases GSK3β, c-Abl, and Cdk5, occur in the brains of aged p73+/- mice. To investigate whether p73 haploinsufficiency therefore represents a general risk factor for tau hyperphosphorylation during neurodegeneration, we crossed the p73+/- mice with 2 mouse models of neurodegeneration, TgCRND8+/Ø mice that express human mutant amyloid precursor protein, and Pin1-/- mice. We show that haploinsufficiency for p73 leads to the early appearance of phospho-tau-positive aggregates, tau hyperphosphorylation, and activation of GSK3β, c-Abl, and Cdk5 in the brains of both of these mouse models. Moreover, p73+/-;TgCRND8+/Ø mice display a shortened lifespan relative to TgCRND8+/Ø mice that are wild type for p73. Thus, p73 is required to protect the murine brain from tau hyperphosphorylation during aging and degeneration.