PTEN is essential for cell migration but not for fate determination and tumourigenesis in the cerebellum

Brain patterning perturbations following PTEN loss

This review will consider the impact of compromised PTEN signaling in brain patterning. We approach understanding the contribution of PTEN to nervous system development by surveying the findings from the numerous genetic loss-of-function models that have been generated as well as other forms of PTEN inactivation. By exploring the developmental programs influenced by this central transduction molecule, we can begin to understand the molecular mechanisms that shape the developing brain. A wealth of data indicates that PTEN plays critical roles in a variety of stages during brain development. Many of them are considered here including: stem cell proliferation, fate determination, polarity, migration, process outgrowth, myelination and somatic hypertrophy. In many of these contexts, it is clear that PTEN phosphatase activity contributes to the observed effects of genetic deletion or depletion, however recent studies have also ascribed non-catalytic functions to PTEN in regulating cell function. We also explore the potential impact this alternative pool of PTEN may have on the developing brain. Together, these elements begin to form a clearer picture of how PTEN contributes to the emergence of brain structure and binds form and function in the nervous system.

Mechanisms regulating neuronal excitability and seizure development following mTOR pathway hyperactivation

The phosphatidylinositol-3-kinase/phosphatase and tensin homolog (PTEN)-mammalian target of rapamycin (mTOR) pathway regulates a variety of neuronal functions, including cell proliferation, survival, growth, and plasticity. Dysregulation of the pathway is implicated in the development of both genetic and acquired epilepsies. Indeed, several causal mutations have been identified in patients with epilepsy, the most prominent of these being mutations in PTEN and tuberous sclerosis complexes 1 and 2 (TSC1, TSC2). These genes act as negative regulators of mTOR signaling, and mutations lead to hyperactivation of the pathway. Animal models deleting PTEN, TSC1, and TSC2 consistently produce epilepsy phenotypes, demonstrating that increased mTOR signaling can provoke neuronal hyperexcitability. Given the broad range of changes induced by altered mTOR signaling, however, the mechanisms underlying seizure development in these animals remain uncertain. In transgenic mice, cell populations with hyperactive mTOR have many structural abnormalities that support recurrent circuit formation, including somatic and dendritic hypertrophy, aberrant basal dendrites, and enlargement of axon tracts. At the functional level, mTOR hyperactivation is commonly, but not always, associated with enhanced synaptic transmission and plasticity. Moreover, these populations of abnormal neurons can affect the larger network, inducing secondary changes that may explain paradoxical findings reported between cell and network functioning in different models or at different developmental time points. Here, we review the animal literature examining the link between mTOR hyperactivation and epileptogenesis, emphasizing the impact of enhanced mTOR signaling on neuronal form and function.

PTEN: A master regulator of neuronal structure, function, and plasticity

PTEN (phosphatase and tensin homolog on chromosome ten) is a dual protein/lipid phosphatase that dephosphorylates PIP3, thereby inhibiting the AKT/mTOR pathway. This inhibition ultimately decreases protein translation, cell proliferation and cell growth. In the central nervous system, inhibition of PTEN leads to increased stem cell proliferation, somatic, dendritic and axonal growth, accelerated spine maturation, diminished synaptic plasticity, and altered intrinsic excitability. In agreement with these findings, patients carrying single-copy inactivating mutations of PTEN suffer from autism, macrocephaly, mental retardation, and epilepsy.(1) (-) (9) Understanding the mechanisms through which PTEN modulates the structure, function, and plasticity of cortical networks is a major focus of study. Preventing and reversing the changes induced by loss of Pten in model animals will pave the way for treatments in humans.

Neurobiology of autism gene products: towards pathogenesis and drug targets

RATIONALE

The genetic heterogeneity of autism spectrum disorders (ASDs) is enormous, and the neurobiology of proteins encoded by genes associated with ASD is very diverse. Revealing the mechanisms on which different neurobiological pathways in ASD pathogenesis converge may lead to the identification of drug targets.

OBJECTIVE

The main objective is firstly to outline the main molecular networks and neuronal mechanisms in which ASD gene products participate and secondly to answer the question how these converge. Finally, we aim to pinpoint drug targets within these mechanisms.

METHOD

Literature review of the neurobiological properties of ASD gene products with a special focus on the developmental consequences of genetic defects and the possibility to reverse these by genetic or pharmacological interventions.

RESULTS

The regulation of activity-dependent protein synthesis appears central in the pathogenesis of ASD. Through sequential consequences for axodendritic function, neuronal disabilities arise expressed as behavioral abnormalities and autistic symptoms in ASD patients. Several known ASD gene products have their effect on this central process by affecting protein synthesis intrinsically, e.g., through enhancing the mammalian target of rapamycin (mTOR) signal transduction pathway or through impairing synaptic function in general. These are interrelated processes and can be targeted by compounds from various directions: inhibition of protein synthesis through Lovastatin, mTOR inhibition using rapamycin, or mGluR-related modulation of synaptic activity.

CONCLUSIONS

ASD gene products may all feed into a central process of translational control that is important for adequate glutamatergic regulation of dendritic properties. This process can be modulated by available compounds but may also be targeted by yet unexplored routes.

Germline disruption of Pten localization causes enhanced sex-dependent social motivation and increased glial production

PTEN Hamartoma Tumor Syndrome (PHTS) is an autosomal-dominant genetic condition underlying a subset of autism spectrum disorder (ASD) with macrocephaly. Caused by germline mutations in PTEN, PHTS also causes increased risks of multiple cancers via dysregulation of the PI3K and MAPK signaling pathways. Conditional knockout models have shown that neural Pten regulates social behavior, proliferation and cell size. Although much is known about how the intracellular localization of PTEN regulates signaling in cancer cell lines, we know little of how PTEN localization influences normal brain physiology and behavior. To address this, we generated a germline knock-in mouse model of cytoplasm-predominant Pten and characterized its behavioral and cellular phenotypes. The homozygous Pten(m3m4) mice have decreased total Pten levels including a specific drop in nuclear Pten and exhibit region-specific increases in brain weight. The Pten(m3m4) model displays sex-specific increases in social motivation, poor balance and normal recognition memory-a profile reminiscent of some individuals with high functioning ASD. The cytoplasm-predominant protein caused cellular hypertrophy limited to the soma and led to increased NG2 cell proliferation and accumulation of glia. The animals also exhibit significant astrogliosis and microglial activation, indicating a neuroinflammatory phenotype. At the signaling level, Pten(m3m4) mice show brain region-specific differences in Akt activation. These results demonstrate that differing alterations to the same autism-linked gene can cause distinct behavioral profiles. The Pten(m3m4) model is the first murine model of inappropriately elevated social motivation in the context of normal cognition and may expand the range of autism-related behaviors replicated in animal models.

Role of the PTEN signaling pathway in autism spectrum disorder

Autism is an etiologically heterogeneous group of neurodevelopmental disorders, diagnosed mostly by the clinical behavioral phenotypes. The concept that the tumor-related gene PTEN plays a critical role in autism spectrum disorder has emerged over the last decade. In this review, we focus on the essential role of the PTEN signaling pathway in neuronal differentiation and the formation of neural circuitry, as well as genetic mouse models with Pten manipulations. Particularly, accumulated data suggest that the effect of PTEN on neural stem-cell development contributes significantly to the pathophysiology of autism spectrum disorders.

Nonautonomous regulation of neuronal migration by insulin signaling, DAF-16/FOXO, and PAK-1

Neuronal migration is essential for nervous system development in all organisms and is regulated in the nematode, C. elegans, by signaling pathways that are conserved in humans. Here, we demonstrate that the insulin/IGF-1-PI3K signaling pathway modulates the activity of the DAF-16/FOXO transcription factor to regulate the anterior migrations of the hermaphrodite-specific neurons (HSNs) during embryogenesis of C. elegans. When signaling is reduced, DAF-16 is activated and promotes migration; conversely, when signaling is enhanced, DAF-16 is inactivated, and migration is inhibited. We show that DAF-16 acts nonautonomously in the hypodermis to promote HSN migration. Furthermore, we identify PAK-1, a p21-activated kinase, as a downstream mediator of insulin/IGF-1-DAF-16 signaling in the nonautonomous control of HSN migration. Because a FOXO-Pak1 pathway was recently shown to regulate mammalian neuronal polarity, our findings indicate that the roles of FOXO and Pak1 in neuronal migration are most likely conserved from C. elegans to higher organisms.

Comparative expression analysis reveals lineage relationships between human and murine gliomas and a dominance of glial signatures during tumor propagation in vitro

Brain tumors are thought to originate from stem/progenitor cell populations that acquire specific genetic mutations. Although current preclinical models have relevance to human pathogenesis, most do not recapitulate the histogenesis of the human disease. Recently, a large series of human gliomas and medulloblastomas were analyzed for genetic signatures of prognosis and therapeutic response. Using a mouse model system that generates three distinct types of intrinsic brain tumors, we correlated RNA and protein expression levels with human brain tumors. A combination of genetic mutations and cellular environment during tumor propagation defined the incidence and phenotype of intrinsic murine tumors. Importantly, in vitro passage of cancer stem cells uniformly promoted a glial expression profile in culture and in brain tumors. Gene expression profiling revealed that experimental gliomas corresponded to distinct subclasses of human glioblastoma, whereas experimental supratentorial primitive neuroectodermal tumors (sPNET) correspond to atypical teratoid/rhabdoid tumor (AT/RT), a rare childhood tumor.

Analysis of tumor heterogeneity and cancer gene networks using deep sequencing of MMTV-induced mouse mammary tumors

Cancer develops through a multistep process in which normal cells progress to malignant tumors via the evolution of their genomes as a result of the acquisition of mutations in cancer driver genes. The number, identity and mode of action of cancer driver genes, and how they contribute to tumor evolution is largely unknown. This study deployed the Mouse Mammary Tumor Virus (MMTV) as an insertional mutagen to find both the driver genes and the networks in which they function. Using deep insertion site sequencing we identified around 31000 retroviral integration sites in 604 MMTV-induced mammary tumors from mice with mammary gland-specific deletion of Trp53, Pten heterozygous knockout mice, or wildtype strains. We identified 18 known common integration sites (CISs) and 12 previously unknown CISs marking new candidate cancer genes. Members of the Wnt, Fgf, Fgfr, Rspo and Pdgfr gene families were commonly mutated in a mutually exclusive fashion. The sequence data we generated yielded also information on the clonality of insertions in individual tumors, allowing us to develop a data-driven model of MMTV-induced tumor development. Insertional mutations near Wnt and Fgf genes mark the earliest "initiating" events in MMTV induced tumorigenesis, whereas Fgfr genes are targeted later during tumor progression. Our data shows that insertional mutagenesis can be used to discover the mutational networks, the timing of mutations, and the genes that initiate and drive tumor evolution.

Targeting the PI3K/AKT/mTOR signaling pathway in glioblastoma: novel therapeutic agents and advances in understanding

Glioblastoma multiforme (GBM) is a grade IV astrocytoma with a median survival of 12 months despite current multi-modal treatment options. GBM is distinguished clinicopathologically into primary and secondary subtypes. Mutations of phosphatase and tensin homolog, and subsequent upregulation of the downstream protein kinase B/mammalian target of rapamycin (mTOR) signaling pathway, are commonly seen in primary GBM and less predominantly in secondary GBM. While investigations into targeted treatments of mTOR have been attempted, feedback regulation within the mTOR signaling pathway may account for therapeutic resistance. Currently, rapamycin analogs, dual-targeted mTOR complex 1 and 2 agents as well as dual mTOR and phosphatidylinositol-3 kinase-targeted agents are being investigated experimentally and in clinical trials. This review will discuss the experimental potential of these agents in the treatment of GBM and their current stage in the GBM drug pipeline. Knowledge obtained from the application of these agents can help in understanding the pathogenesis of GBM as well as delineating subsequent treatment strategies.

Conditional deletion of pten leads to defects in nerve innervation and neuronal survival in inner ear development

All cellular phenomena and developmental events, including inner ear development, are modulated through harmonized signaling networks. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a tumor suppressor, is a major signaling component involved in cross talk with key regulators of development; i.e., Wnt, Notch, and bone morphogenetic proteins. Although Pten function has been studied in various systems, its role in inner ear development is poorly understood. Here, we used inner ear-specific Pten conditional knockout mice and examined the characteristics of the inner ear. In a detailed analysis of the phenotype, reduced cochlear turning and widened epithelia were observed. Phalloidin staining of sensory epithelium revealed that hair cell patterns were disturbed; i.e., additional rows of hair cells were discovered. The neural abnormality revealed a reduction in and disorganization of nerve fibers, including apoptosis at the neural precursor stage. Pten deficiency induced increased phosphorylation of Akt at Ser473. The elevation of inhibitory glycogen synthase kinase 3β Ser9 phosphorylation (pGSK3β) was sustained until the neuronal differentiation stage at embryonic day 14.5, instead of pGSK3β downregulation. This is the first report on the influence of Pten/Akt/GSK3β signaling on the development of spiral ganglia. These results suggest that Pten is required for the maintenance of neuroblast number, neural precursors, and differentiation in the inner ear.

Bergmann glia function in granule cell migration during cerebellum development

Granule cell migration influences the laminar structure of the cerebellum and thereby affects cerebellum function. Bergmann glia are derived from radial glial cells and aid in granule cell radial migration by providing a scaffold for migration and by mediating interactions between Bergmann glia and granule cells. In this review, we summarize Bergmann glia characteristics and the mechanisms underlying the effect of Bergmann glia on the radial migration of granule neurons in the cerebellum. Furthermore, we will focus our discussion on the important factors involved in glia-mediated radial migration so that we may elucidate the possible mechanistic pathways used by Bergmann glia to influence granule cell migration during cerebellum development.

Autism spectrum disorder and the cerebellum

The cerebellum has been long known for its importance in motor learning and coordination. Recently, anatomical, clinical, and neuroimaging studies strongly suggest that the cerebellum supports cognitive functions, including language and executive functions, as well as affective regulation. Furthermore, the cerebellum has emerged as one of the key brain regions affected in autism. Here, we discuss our current understanding of the role of the cerebellum in autism, including evidence from genetic, molecular, clinical, behavioral, and neuroimaging studies. Cerebellar findings in autism suggest developmental differences at multiple levels of neural structure and function, indicating that the cerebellum is an important player in the complex neural underpinnings of autism spectrum disorder, with behavioral implications beyond the motor domain.

Pten deletion causes mTorc1-dependent ectopic neuroblast differentiation without causing uniform migration defects

Neuronal precursors, generated throughout life in the subventricular zone, migrate through the rostral migratory stream to the olfactory bulb where they differentiate into interneurons. We found that the PI3K-Akt-mTorc1 pathway is selectively inactivated in migrating neuroblasts in the subventricular zone and rostral migratory stream, and activated when these cells reach the olfactory bulb. Postnatal deletion of Pten caused aberrant activation of the PI3K-Akt-mTorc1 pathway and an enlarged subventricular zone and rostral migratory stream. This expansion was caused by premature termination of migration and differentiation of neuroblasts and was rescued by inhibition of mTorc1. This phenotype is reminiscent of lamination defects caused by Pten deletion in developing brain that were previously described as defective migration. However, live imaging in acute slices showed that Pten deletion did not cause a uniform defect in the mechanics of directional neuroblast migration. Instead, a subpopulation of Pten-null neuroblasts showed minimal movement and altered morphology associated with differentiation, whereas the remainder showed unimpeded directional migration towards the olfactory bulb. Therefore, migration defects of Pten-null neurons might be secondary to ectopic differentiation.

Cell-type specific roles for PTEN in establishing a functional retinal architecture

Background

The retina has a unique three-dimensional architecture, the precise organization of which allows for complete sampling of the visual field. Along the radial or apicobasal axis, retinal neurons and their dendritic and axonal arbors are segregated into layers, while perpendicular to this axis, in the tangential plane, four of the six neuronal types form patterned cellular arrays, or mosaics. Currently, the molecular cues that control retinal cell positioning are not well-understood, especially those that operate in the tangential plane. Here we investigated the role of the PTEN phosphatase in establishing a functional retinal architecture.

Methodology/Principal Findings

In the developing retina, PTEN was localized preferentially to ganglion, amacrine and horizontal cells, whose somata are distributed in mosaic patterns in the tangential plane. Generation of a retina-specific Pten knock-out resulted in retinal ganglion, amacrine and horizontal cell hypertrophy, and expansion of the inner plexiform layer. The spacing of Pten mutant mosaic populations was also aberrant, as were the arborization and fasciculation patterns of their processes, displaying cell type-specific defects in the radial and tangential dimensions. Irregular oscillatory potentials were also observed in Pten mutant electroretinograms, indicative of asynchronous amacrine cell firing. Furthermore, while Pten mutant RGC axons targeted appropriate brain regions, optokinetic spatial acuity was reduced in Pten mutant animals. Finally, while some features of the Pten mutant retina appeared similar to those reported in Dscam-mutant mice, PTEN expression and activity were normal in the absence of Dscam.

Conclusions/Significance

We conclude that Pten regulates somal positioning and neurite arborization patterns of a subset of retinal cells that form mosaics, likely functioning independently of Dscam, at least during the embryonic period. Our findings thus reveal an unexpected level of cellular specificity for the multi-purpose phosphatase, and identify Pten as an integral component of a novel cell positioning pathway in the retina.

Protein phosphatase magnesium dependent 1A (PPM1A) plays a role in the differentiation and survival processes of nerve cells

The serine/threonine phosphatase type 2C (PPM1A) has a broad range of substrates, and its role in regulating stress response is well established. We have investigated the involvement of PPM1A in the survival and differentiation processes of PC6-3 cells, a subclone of the PC12 cell line. This cell line can differentiate into neuron like cells upon exposure to nerve growth factor (NGF). Overexpression of PPM1A in naive PC6-3 cells caused cell cycle arrest at the G2/M phase followed by apoptosis. Interestingly, PPM1A overexpression did not affect fully differentiated cells. Using PPM1A overexpressing cells and PPM1A knockdown cells, we show that this phosphatase affects NGF signaling in PC6-3 cells and is engaged in neurite outgrowth. In addition, the ablation of PPM1A interferes with NGF-induced growth arrest during differentiation of PC6-3 cells.

Preclinical Remodeling of Human Prostate Cancer through the PTEN/AKT Pathway

Knowledge gained from the identification of genetic and epigenetic alterations that contribute to the progression of prostate cancer in humans is now being implemented in the development of functionally relevant translational models. GEM (genetically modified mouse) models are being developed to incorporate the same molecular defects associated with human prostate cancer. Haploinsufficiency is common in prostate cancer and homozygous loss of PTEN is strongly correlated with advanced disease. In this paper, we discuss the evolution of the PTEN knockout mouse and the cooperation between PTEN and other genetic alterations in tumor development and progression. Additionally, we will outline key points that make these models key players in the development of personalized medicine, as potential tools for target and biomarker development and validation as well as models for drug discovery.

The PTEN and Myotubularin phosphoinositide 3-phosphatases: linking lipid signalling to human disease

Two classes of lipid phosphatases selectively dephosphorylate the 3 position of the inositol ring of phosphoinositide signaling molecules: the PTEN and the Myotubularin families. PTEN dephosphorylates PtdIns(3,4,5)P(3), acting in direct opposition to the Class I PI3K enzymes in the regulation of cell growth, proliferation and polarity and is an important tumor suppressor. Although there are several PTEN-related proteins encoded by the human genome, none of these appear to fulfill the same functions. In contrast, the Myotubularins dephosphorylate both PtdIns(3)P and PtdIns(3,5)P(2), making them antagonists of the Class II and Class III PI 3-kinases and regulators of membrane traffic. Both phosphatase groups were originally identified through their causal mutation in human disease. Mutations in specific myotubularins result in myotubular myopathy and Charcot-Marie-Tooth peripheral neuropathy; and loss of PTEN function through mutation and other mechanisms is evident in as many as a third of all human tumors. This chapter will discuss these two classes of phosphatases, covering what is known about their biochemistry, their functions at the cellular and whole body level and their influence on human health.

PTEN: A molecular target for neurodegenerative disorders

PTEN (phosphatase and tensin homologue deleted in chromosome 10) was first identified as a candidate tumour suppressor gene located on chromosome 10q23. It is considered as one of the most frequently mutated genes in human malignancies. Emerging evidence shows that the biological function of PTEN extends beyond its tumour suppressor activity. In the central nervous system PTEN is a crucial regulator of neuronal development, neuronal survival, axonal regeneration and synaptic plasticity. Furthermore, PTEN has been linked to the pathogenesis of neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis. Recently increased attention has been focused on PTEN as a potential target for the treatment of brain injury and neurodegeneration. In this review we discuss the essential functions of PTEN in the central nervous system and its involvement in neurodegeneration.

Drosophila melanogaster as a model system for human brain cancers

Glioblastomas (GBM), the most common primary brain tumors, infiltrate the brain, grow rapidly, and are refractory to current therapies. Signature genetic lesions in glioblastomas include mutation of the epidermal growth factor receptor tyrosine kinase (EGFR) receptor tyrosine kinase and activating mutations in components of the PI-3 kinase (PI3K) pathway. Despite years of study, how these pathways specifically regulate glial pathogenesis is unclear. To address the genetic and cellular origins of this disease, a novel Drosophila GBM model has been developed in which glial progenitor cells give rise to proliferative and invasive neoplastic cells that create transplantable tumors in response to constitutive co-activation of the EGFR-Ras and PI3K pathways. Standing with a rich literature demonstrating the direct relevance of Drosophila to studies on human cancer, neurological disease, and neurodevelopment, this model represents a robust cell-type specific Drosophila neurological disease model in which malignant cells are created by mutations in genetic pathways thought to be driving forces in a homologous human disease. Using lineage analysis and cell-type specific markers, neoplastic glial cells were found to originate from committed glial progenitor cells, rather than from multipotent neuroblasts. Genetic analyses demonstrated that EGFR-Ras and PI3K induce fly glial neoplasia through activation of a combinatorial genetic network composed, in part, of other genetic pathways also commonly mutated in human glioblastomas. In the future, large-scale forward genetic screens with this model may reveal new insights into the origins and treatments of human glioblastoma.

Cooperativity within and among Pten, p53, and Rb pathways induces high-grade astrocytoma in adult brain

Mutations in the PTEN, TP53, and RB1 pathways are obligate events in the pathogenesis of human glioblastomas. We induced various combinations of deletions in these tumor suppressors in astrocytes and neural precursors in mature mice, resulting in astrocytomas ranging from grade III to grade IV (glioblastoma). There was selection for mutation of multiple genes within a pathway, shown by somatic amplifications of genes in the PI3K or Rb pathway in tumors in which Pten or Rb deletion was an initiating event. Despite multiple mutations within PI3K and Rb pathways, elevated Mapk activation was not consistent. Gene expression profiling revealed striking similarities to subclasses of human diffuse astrocytoma. Astrocytomas were found within and outside of proliferative niches in the adult brain.

Tau, prions and Aβ: the triad of neurodegeneration

This article highlights the features that connect prion diseases with other cerebral amyloidoses and how these relate to neurodegeneration, with focus on tau phosphorylation. It also discusses similarities between prion disease and Alzheimer's disease: mechanisms of amyloid formation, neurotoxicity, pathways involved in triggering tau phosphorylation, links to cell cycle pathways and neuronal apoptosis. We review previous evidence of prion diseases triggering hyperphosphorylation of tau, and complement these findings with cases from our collection of genetic, sporadic and transmitted forms of prion diseases. This includes the novel finding that tau phosphorylation consistently occurs in sporadic CJD, in the absence of amyloid plaques.

Progesterone regulates the phosphorylation of protein phosphatases in the brain

Previous studies have shown that progesterone modulates the activity of different kinases and the phosphorylation of Tau in the brain. These actions of progesterone may be involved in the hormonal regulation of neuronal differentiation, neuronal function, and neuroprotection. However, the action of progesterone on protein phosphatases in the nervous system has not been explored previously. In this study we have assessed the effect of the administration of progesterone to adult ovariectomized rats on protein phosphatase 2A (PP2A) and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) in the hypothalamus, the hippocampus, and the cerebellum. Total levels of PP2A, the state of methylation of PP2A, and total levels of PTEN were unaffected by the hormone in the three brain regions studied. In contrast, progesterone significantly increased the levels of PP2A phosphorylated in tyrosine 307 in the hippocampus and the cerebellum and significantly decreased the levels of PTEN phosphorylated in serine 380 in the hypothalamus and in the hippocampus compared with control values. Estradiol priming blocked the effect of progesterone on PP2A phosphorylation in the hippocampus and on PTEN phosphorylation in the hypothalamus and the hippocampus. In contrast, the action of progesterone on PP2A phosphorylation in the cerebellum was not modified by estradiol priming. These findings suggest that the regulation of the phosphorylation of PP2A and PTEN may be involved in the effects of progesterone on the phosphorylation of Tau and on the activity of phophoinositide-3 kinase and mitogen-activated protein kinase in the brain.

Mouse models of inherited cancer syndromes

Animal models of cancer have been instrumental in understanding the progression and therapy of hereditary cancer syndromes. The ability to alter the genome of an individual mouse cell in both constitutive and inducible approaches has led to many novel insights into their human counterparts. In this review, knockout mouse models of inherited human cancer syndromes are presented and insights from the study of these models are highlighted.

Somatic p16(INK4a) loss accelerates melanomagenesis

Loss of p16^INK4a–RB and ARF–p53 tumor suppressor pathways, as well as activation of RAS–RAF signaling, is seen in a majority of human melanomas. Although heterozygous germline mutations of p16^INK4a are associated with familial melanoma, most melanomas result from somatic genetic events: often p16^INK4a loss and N-RAS or B-RAF mutational activation, with a minority possessing alternative genetic alterations such as activating mutations in K-RAS and/or p53 inactivation. To generate a murine model of melanoma featuring some of these somatic genetic events, we engineered a novel conditional p16^INK4a-null allele and combined this allele with a melanocyte-specific, inducible CRE recombinase strain, a conditional p53-null allele and a loxP-stop-loxP activatable oncogenic K-Ras allele. We found potent synergy between melanocyte-specific activation of K-Ras and loss of p16^INK4a and/or p53 in melanomagenesis. Mice harboring melanocyte-specific activated K-Ras and loss of p16^INK4a and/or p53 developed invasive, unpigmented and nonmetastatic melanomas with short latency and high penetrance. In addition, the capacity of these somatic genetic events to rapidly induce melanomas in adult mice suggests that melanocytes remain susceptible to transformation throughout adulthood.

Genetic determinants at the interface of cancer and neurodegenerative disease

It has been hypothesized that oncogenesis and neurodegeneration may share common mechanistic foundations. Recent evidence now reveals a number of genes in which alteration leads to either carcinogenesis or neurodegeneration, depending on cellular context. Pathways that have emerged as having critical roles in both cancer and neurodegenerative disease include those involving genes such as PARK2, ATM, PTEN, PTPRD, and mTOR. A number of mechanisms have been implicated, and commonly affected cellular processes include cell cycle regulation, DNA repair, and response to oxidative stress. For example, we have recently shown that the E3 ubiquitin ligase PARK2 is mutated or deleted in many different human malignancies and helps drive loss on chromosome 6q25.2-27, a genomic region frequently deleted in cancers. Mutation in PARK2 is also the most common cause of juvenile Parkinson's disease. Mutations in PARK2 result in an upregulation of its substrate cyclin E, resulting in dysregulated entry into the cell cycle. In neurons, this process results in cell death, but in cycling cells, the result is a growth advantage. Thus, depending on whether the cell affected is a dividing cell or a post-mitotic neuron, responses to these alterations may differ, ultimately leading to varying disease phenotypes. Here, we review the substantial data implicating specific genes in both cancer and neurodegenerative disease.

Rapid and robust transgenic high-grade glioma mouse models for therapy intervention studies

PURPOSE

To develop a transgenic mouse model of glioma that can be conveniently used for testing therapy intervention strategies. High-grade glioma is a devastating and uniformly fatal disease for which better therapy is urgently needed. Typical for high-grade glioma is that glioma cells infiltrate extensively into surrounding pivotal brain structures, thereby rendering current treatments largely ineffective. Evaluation of novel therapies requires the availability of appropriate glioma mouse models.

EXPERIMENTAL DESIGN

High-grade gliomas were induced by stereotactic intracranial injection of lentiviral GFAP-Cre or CMV-Cre vectors into compound LoxP-conditional mice, resulting in K-Ras(v12) expression and loss of p16(Ink4a)/p19(Arf) with or without concomitant loss of p53 or Pten.

RESULTS

Tumors reproduced many of the features that are characteristic for human high-grade gliomas, including invasiveness and blood-brain barrier functionality. Especially, CMV-Cre injection into p53;Ink4a/Arf;K-Ras(v12) mice resulted in high-grade glioma with a short tumor latency (2-3 weeks) and full penetrance. Early detection and follow-up was accomplished by noninvasive bioluminescence imaging, and the practical utility for therapy intervention was shown in a study with temozolomide.

CONCLUSION

We have developed a realistic high-grade glioma model that can be used with almost the same convenience as traditional xenograft models, thus allowing its implementation at the forefront of preclinical evaluation of new treatments.

Faithfull modeling of PTEN loss driven diseases in the mouse

A decade of work has indisputably defined PTEN as a pivotal player in human health and disease. Above all, PTEN has been identified as one of the most commonly lost or mutated tumor suppressor genes in human cancers. For this reason, the generation of a multitude of mouse models has been an invaluable strategy to dissect the function and consequences-of-loss of this essential, evolutionary conserved lipid phosphatase in tumor initiation and progression.In this chapter, we will summarize the mouse models that have allowed us to faithfully recapitulate features of human cancers and to highlight the network of connections between the PTEN signaling cascade and other oncogenic or tumor suppressive pathways.Notably, PTEN represents one of the most extensively modeled genes involved in human cancer and exemplifies the strength of genetic mouse modeling as an approach to gain information aimed to improve our understanding of and ability to alleviate human disease.

Combinations of genetic mutations in the adult neural stem cell compartment determine brain tumour phenotypes

It has been suggested that intrinsic brain tumours originate from a neural stem/progenitor cell population in the subventricular zone of the post-natal brain. However, the influence of the initial genetic mutation on the phenotype as well as the contribution of mature astrocytes to the formation of brain tumours is still not understood. We deleted Rb/p53, Rb/p53/PTEN or PTEN/p53 in adult subventricular stem cells; in ectopically neurografted stem cells; in mature parenchymal astrocytes and in transplanted astrocytes. We found that only stem cells, but not astrocytes, gave rise to brain tumours, independent of their location. This suggests a cell autonomous mechanism that enables stem cells to generate brain tumours, whereas mature astrocytes do not form brain tumours in adults. Recombination of PTEN/p53 gave rise to gliomas whereas deletion of Rb/p53 or Rb/p53/PTEN generated primitive neuroectodermal tumours (PNET), indicating an important role of an initial Rb loss in driving the PNET phenotype. Our study underlines an important role of stem cells and the relevance of initial genetic mutations in the pathogenesis and phenotype of brain tumours.

Phosphoinositide-dependent kinase 1 controls migration and malignant transformation but not cell growth and proliferation in PTEN-null lymphocytes

In normal T cell progenitors, phosphoinositide-dependent kinase l (PDK1)–mediated phosphorylation and activation of protein kinase B (PKB) is essential for the phosphorylation and inactivation of Foxo family transcription factors, and also controls T cell growth and proliferation. The current study has characterized the role of PDK1 in the pathology caused by deletion of the tumor suppressor phosphatase and tensin homologue deleted on chromosome 10 (PTEN). PDK1 is shown to be essential for lymphomagenesis caused by deletion of PTEN in T cell progenitors. However, PTEN deletion bypasses the normal PDK1-controlled signaling pathways that determine thymocyte growth and proliferation. PDK1 does have important functions in PTEN-null thymocytes, notably to control the PKB–Foxo signaling axis and to direct the repertoire of adhesion and chemokine receptors expressed by PTEN-null T cells. The results thus provide two novel insights concerning pathological signaling caused by PTEN loss in lymphocytes. First, PTEN deletion bypasses the normal PDK1-controlled metabolic checkpoints that determine cell growth and proliferation. Second, PDK1 determines the cohort of chemokine and adhesion receptors expressed by PTEN-null cells, thereby controlling their migratory capacity.

The insulin-like growth factor (IGF) receptor type 1 (IGF1R) as an essential component of the signalling network regulating neurogenesis

The insulin-like growth factor receptor type 1 (IGF1R) signalling pathway is activated in the mammalian nervous system from early developmental stages. Its major effect on developing neural cells is to promote their growth and survival. This pathway can integrate its action with signalling pathways of growth and morphogenetic factors that induce cell fate specification and selective expansion of specified neural cell subsets. This suggests that during developmental and adult neurogenesis cellular responses to many signalling factors, including ligands of Notch, sonic hedgehog, fibroblast growth factor family members, ligands of the epidermal growth factor receptor, bone morphogenetic proteins and Wingless and Int-1, may be modified by co-activation of the IGF1R. Modulation of cell migration is another possible role that IGF1R activation may play in neurogenesis. Here, I briefly overview neurogenesis and discuss a role for IGF1R-mediated signalling in the developing and mature nervous system with emphasis on crosstalk between the signalling pathways of the IGF1R and other factors regulating neural cell development and migration. Studies on neural as well as on non-neural cells are highlighted because it may be interesting to test in neurogenic paradigms some of the models based on the information obtained in studies on non-neural cell types.

Cell type specificity of PI3K signaling in Pdk1- and Pten-deficient brains

Loss of PTEN causes unregulated activation of downstream components of phosphatidylinositol 3-kinase (PI3K) signaling, including PDK1, and disrupts normal nervous system development and homeostasis. We tested the contribution of Pdk1 to the abnormalities induced by Pten deletion in the brain. Conditional deletion of Pdk1 caused microcephaly. Combined deletion of Pdk1 and Pten rescued hypertrophy, but not migration defects of Pten-deficient neurons. Pdk1 inactivation induced strikingly different effects on the regulation of phosphorylated Akt in glia versus neurons. Our results show Pdk1-dependent and Pdk1-independent abnormalities in Pten-deficient brains, and demonstrate cell type specific differences in feedback regulation of the ubiquitous PI3K pathway.

Accumulating progenitor cells in the luminal epithelial cell layer are candidate tumor initiating cells in a Pten knockout mouse prostate cancer model

The PSA-Cre;Pten-loxP/loxP mouse prostate cancer model displays clearly defined stages of hyperplasia and cancer. Here, the initial stages of hyperplasia development are studied. Immunohistochemical staining showed that accumulated pAkt⁺ hyperplastic cells overexpress luminal epithelial cell marker CK8, and progenitor cell markers CK19 and Sca-1, but not basal epithelial cell markers. By expression profiling we identified novel hyperplastic cell markers, including Tacstd2 and Clu. Further we showed that at young age prostates of targeted Pten knockout mice contained in the luminal epithelial cell layer single pAkt⁺ cells, which overexpressed CK8, Sca-1, Tacstd2 and Clu; basal epithelial cells were always pAkt⁻. Importantly, in the luminal epithelial cell layer of normal prostates we detected rare Clu⁺Tacstd2⁺Sca-1⁺ progenitor cells. These novel cells are candidate tumor initiating cells in Pten knockout mice. Remarkably, all luminal epithelial cells in the proximal region of normal prostates were Clu⁺Tacstd2⁺Sca-1⁺. However, in PSA-Cre;Pten-loxP/loxP mice, the proximal prostate does not contain hyperplastic foci. Small hyperplastic foci in prostates of PSA-Cre;Pten-loxP/+ mice found at old age, showed complete Pten inactivation and a progenitor marker profile. Finally, we present a novel model of prostate development and renewal, including lineage-specific luminal epithelial progenitor cells. It is proposed that Pten deficiency induces a shift in the balance of differentiation to proliferation in these cells.

Akt activation synergizes with Trp53 loss in oral epithelium to produce a novel mouse model for head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC) is a common human neoplasia with poor prognosis and survival that frequently displays Akt overactivation. Here we show that mice displaying constitutive Akt activity (myrAkt) in combination with Trp53 loss in stratified epithelia develop oral cavity tumors that phenocopy human HNSCC. The myrAkt mice develop oral lesions, making it a possible model of human oral dysplasia. The malignant conversion of these lesions, which is hampered due to the induction of premature senescence, is achieved by the subsequent ablation of Trp53 gene in the same cells in vivo. Importantly, mouse oral tumors can be followed by in vivo imaging, show metastatic spreading to regional lymph nodes, and display activation of nuclear factor-kappaB and signal transducer and activator of transcription-3 pathways and decreased transforming growth factor-beta type II receptor expression, thus resembling human counterparts. In addition, malignant conversion is associated with increased number of putative tumor stem cells. These data identify activation of Akt and p53 loss as a major mechanism of oral tumorigenesis in vivo and suggest that blocking these signaling pathways could have therapeutic implications for the management of HNSCC.

PTEN and the PI3-kinase pathway in cancer

PI3-kinase and PTEN are major positive and negative regulators, respectively, of the PI3-kinase pathway, which regulates growth, survival, and proliferation. These key signaling components are two of the most frequently mutated proteins in human cancers, resulting in unregulated activation of PI3K signaling and providing irrefutable genetic evidence of the central role of this pathway in tumorigenesis. PTEN regulates PI3K signaling by dephosphorylating the lipid signaling intermediate PIP(3), but PTEN may have additional phosphatase-independent activities, as well as other functions in the nucleus. In this review, we highlight current work showing cancer-relevant complexities in the regulation of PTEN and PI3K activity, potential novel functions for PTEN, and feedback regulation within the pathway. The significance and complexity of PI3K signaling make it an important but challenging therapeutic target for cancer.

Analysis of transcriptional profiles and functional clustering of global cerebellar gene expression in PCD3J mice

The Purkinje cell degeneration (PCD) mutant mouse is characterized by a degeneration of cerebellar Purkinje cells and progressive ataxia. To identify the molecular mechanisms that lead to the death of Purkinje neurons in PCD mice, we used Affymetrix microarray technology to compare cerebellar gene expression profiles in pcd3J mutant mice 14 days of age (prior to Purkinje cell loss) to unaffected littermates. Microarray analysis, Ingenuity Pathway Analysis (IPA) and expression analysis systematic explorer (EASE) software were used to identify biological and molecular pathways implicated in the progression of Purkinje cell degeneration. IPA analysis indicated that mutant pcd3J mice showed dysregulation of specific processes that may lead to Purkinje cell death, including several molecules known to control neuronal apoptosis such as Bad, CDK5 and PTEN. These findings demonstrate the usefulness of these powerful microarray analysis tools and have important implications for understanding the mechanisms of selective neuronal death and for developing therapeutic strategies to treat neurodegenerative disorders.

The roles of PTEN in development, physiology and tumorigenesis in mouse models: a tissue-by-tissue survey

In 1997, PTEN (phosphatase and tensin homologue deleted on chromosome 10, 10q23.3) was identified as an important tumor suppressor gene that is inactivated in a wide variety of human cancers. Ever since, PTEN's function has been extensively studied, and huge progress has been made in understanding PTEN's role in normal physiology and disease. In this review, we will systematically summarize the important data that have been gained from gene inactivation studies in mice and will put these data into physiological context using a tissue-by-tissue approach. We will cover mice exhibiting complete and constitutive inactivation of Pten as well as a large number of strains in which Pten has been conditionally deleted in specific tissues. We hope to highlight not only the tumor suppressive function of Pten but also its roles in embryogenesis and in the maintenance of the normal physiological functions of many organ systems.

PTEN functions to 'prioritize' chemotactic cues and prevent 'distraction' in migrating neutrophils

Neutrophils encounter and 'prioritize' many chemoattractants in their pursuit of bacteria. Here we tested the possibility that the phosphatase PTEN is responsible for the prioritization of chemoattractants. Neutrophils induced chemotaxis by two separate pathways, the phosphatidylinositol-3-OH kinase (PI(3)K) phosphatase and tensin homolog (PTEN) pathway, and the p38 mitogen-activated protein kinase pathway, with the p38 pathway dominating over the PI(3)K pathway. Pten(-/-) neutrophils could not prioritize chemoattractants and were 'distracted' by chemokines when moving toward bacterial chemoattractants. In opposing gradients, PTEN became distributed throughout the cell circumference, which inhibited all PI(3)K activity, thus permitting 'preferential' migration toward bacterial products via phospholipase A(2) and p38. Such prioritization was defective in Pten(-/-) neutrophils, which resulted in defective bacterial clearance in vivo. Our data identify a PTEN-dependent mechanism in neutrophils to prioritize, 'triage' and integrate responses to multiple chemotactic cues.

PTEN in brain tumors

The tumor suppressor PTEN dephosphorylates phospholipids generated through the activity of PI3K. PTEN thus antagonizes PI3K activity and regulates a multitude of cellular processes such as angiogenesis, motility, invasiveness, survival and proliferation, all of which can initiate and sustain the malignant phenotype. Although PTEN's lipid phosphatase activity is key to its tumor suppressive functions, it also dephosphorylates protein substrates and interacts with other key regulatory molecules, salient among them the tumor suppressor p53. Given the critical roles of PTEN in cellular homeostasis, it is not surprising that both PTEN expression levels and PTEN protein activities are tightly controlled by a complex conglomeration of molecules that regulate post-translational modifications, subcellular localization, transcriptional activation and transcriptional repression. As one of the most commonly altered molecules in human disease, PTEN plays an important role in a myriad of signaling cascades, and plays a central role in normal brain development and brain tumor pathogenesis. As such it influences prognosis of human cancer, predicts response to therapy, constitutes the lynchpin of genetic syndromes, and may underlie neurocognitive abnormalities such as autism spectrum disorders and Alzheimer's disease. Thus, targeting PTEN and its signaling affiliates sows the seeds for combating not only cancer but also neurocognitive disorders.

Presenilins regulate the cellular level of the tumor suppressor PTEN

Alzheimer's Disease (AD) is characterized by amyloid plaques consisting of beta-amyloid (Abeta) peptides and neurofibrillary tangles consisting of hyperphosphorylated tau protein. Abeta is proteolytically derived from its precursor protein through cleavages by beta-secretase and gamma-secretase complex comprising presenilins (PS, PS1/PS2), nicastrin, APH-1 and PEN-2. PS1 is also known to activate the PI3K/Akt cell survival pathway in a gamma-secretase-independent manner. The tumor suppressor PTEN, which antagonizes the PI3K/Akt pathway, has increasingly been recognized to play a key role in neural functions and its level found reduced in AD brains. Here, we demonstrate that the protein level of PTEN is dramatically reduced in cultured cells and embryonic tissues deficient in PS, and in the cortical neurons of PS1/PS2 conditional double knockout mice. Restoration of PS in PS-deficient cells reverses the reduction of PTEN. Regulation of PTEN by PS is independent of the PS/gamma-secretase activity since impaired gamma-secretase by the gamma-secretase inhibitor treatment or due to nicastrin deficiency has little effect on the protein level of PTEN. Our data suggest an important role for PS in signaling pathways involving PI3K/Akt and PTEN that are crucial for physiological functions and the pathogenesis of multiple diseases.

Phosphatase and tensin homolog, deleted on chromosome 10 deficiency in brain causes defects in synaptic structure, transmission and plasticity, and myelination abnormalities

The phosphatidylinositol 3-kinase (PI3K) signaling pathway modulates growth, proliferation and cell survival in diverse tissue types and plays specialized roles in the nervous system including influences on neuronal polarity, dendritic branching and synaptic plasticity. The tumor-suppressor phosphatase with tensin homology (PTEN) is the central negative regulator of the PI3K pathway. Germline PTEN mutations result in cancer predisposition, macrocephaly and benign hamartomas in many tissues, including Lhermitte-Duclos disease, a cerebellar growth disorder. Neurological abnormalities including autism, seizures and ataxia have been observed in association with inherited PTEN mutation with variable penetrance. It remains unclear how loss of PTEN activity contributes to neurological dysfunction. To explore the effects of Pten deficiency on neuronal structure and function, we analyzed several ultra-structural features of Pten-deficient neurons in Pten conditional knockout mice. Using Golgi stain to visualize full neuronal morphology, we observed that increased size of nuclei and somata in Pten-deficient neurons was accompanied by enlarged caliber of neuronal projections and increased dendritic spine density. Electron microscopic evaluation revealed enlarged abnormal synaptic structures in the cerebral cortex and cerebellum. Severe myelination defects included thickening and unraveling of the myelin sheath surrounding hypertrophic axons in the corpus callosum. Defects in myelination of axons of normal caliber were observed in the cerebellum, suggesting intrinsic abnormalities in Pten-deficient oligodendrocytes. We did not observe these abnormalities in wild-type or conditional Pten heterozygous mice. Moreover, conditional deletion of Pten drastically weakened synaptic transmission and synaptic plasticity at excitatory synapses between CA3 and CA1 pyramidal neurons in the hippocampus. These data suggest that Pten is involved in mechanisms that control development of neuronal and synaptic structures and subsequently synaptic function.

Function of PTEN during the formation and maintenance of neuronal circuits in the brain

PTEN (phosphatase and tensin homologue deleted on chromosome 10) is a tumor suppressor that can inhibit proliferation and migration and controls apoptosis in a number of cell types, mainly through inhibition of the phosphoinositide 3-kinase (PI3K) signaling pathway. Patients carrying inactivating mutations of PTEN show a prevalence to develop tumors that can coincide with neurological defects such as mental retardation, ataxia and seizures. A number of in vitro and in vivo studies were instrumental in uncovering a direct correlation between deregulated PI3K/PTEN signaling and changes in neuronal morphogenesis, which is likely to have profound bearings upon the pathogenesis of neurological symptoms. This review outlines recent work on the function of PTEN during vertebrate brain development and the current understanding of the signaling pathways downstream of PTEN that control neuronal connectivity in the brain.

T-cell lymphomas in T-cell-specific Pten-deficient mice originate in the thymus

Phosphatase and tensin homolog deleted on chromosome 10 (Pten) is a tumor suppressor protein whose loss of lipid phosphatase activity is associated with lymphomagenesis. We made use of the Cre-loxP system to delete Pten expression in Lck- or CD4-expressing T-lineage cells. Mice initially showed modest thymic hyperplasia and subsequently developed expanding and infiltrating T-cell lymphomas, leading to a premature death within 5 to 23 weeks. Frequently, all thymocyte and peripheral T-cell populations displayed phenotypes characteristic for immature developing thymocyte precursors and shared elevated levels of clonally rearranged T-cell receptor (TCR) beta chains. In concert, CD2, CD5, CD3epsilon and CD44, proteins associated with increased expression and signaling capacity of both the immature pre-TCR and the mature alphabetaTCR, were more abundantly expressed, reflecting a constitutive state of activation. Although most T-cell lymphomas had acquired the capability to infiltrate the periphery, not all populations left the thymus and expanded clonally exclusively in the thymus. In line with this, only transplantation of thymocytes with infiltrating capacity gave rise to T-cell lymphoma in immunodeficient recipients. These results indicate that T-cell-specific Pten deletion during various stages of thymocyte development gives rise to clonally expanding T-cell lymphomas that frequently infiltrate the periphery, but originate in the thymus.