MEK-ERK1/2-dependent FLNA overexpression promotes abnormal dendritic patterning in tuberous sclerosis independent of mTOR

FLNA regulates neuronal maturation by modulating RAC1-Cofilin activity in the developing cortex

Periventricular nodular heterotopia (PNH), the most common brain malformation diagnosed in adulthood, is characterized by the presence of neuronal nodules along the ventricular walls. PNH is mainly associated with mutations in the FLNA gene - encoding an actin-binding protein - and patients often develop epilepsy. However, the molecular mechanisms underlying the neuronal failure still remain elusive. It has been hypothesized that dysfunctional cortical circuitry, rather than ectopic neurons, may explain the clinical manifestations. To address this issue, we depleted FLNA from cortical pyramidal neurons of a conditional Flnaflox/flox mice by timed in utero electroporation of Cre recombinase. We found that FLNA regulates dendritogenesis and spinogenesis thus promoting an appropriate excitatory/inhibitory inputs balance. We demonstrated that FLNA modulates RAC1 and cofilin activity through its interaction with the Rho-GTPase Activating Protein 24 (ARHGAP24). Collectively, we disclose an uncharacterized role of FLNA and provide strong support for neural circuit dysfunction being a consequence of FLNA mutations.

Focal cortical dysplasia II caused by brain somatic mutation of IRS-1 is associated with ERK signaling pathway activation

Somatic mutations have been identified in 10% to 63% of focal cortical dysplasia type II samples, primarily linked to the mTOR pathway. When the causative genetic mutations are not identified, this opens the possibility of discovering new pathogenic genes or pathways that could be contributing to the condition. In our previous study, we identified a novel candidate pathogenic somatic variant of IRS-1 c.1791dupG in the brain tissue of a child with focal cortical dysplasia type II. This study further explored the variant's role in causing type II focal cortical dysplasia through in vitro overexpression in 293T and SH-SY5Y cells and in vivo evaluation via in utero electroporation in fetal brains, assessing effects on neuronal migration, morphology, and network integrity. It was found that the mutant IRS-1 variant led to hyperactivity of p-ERK, increased cell volume, and was predominantly associated with the MAPK signaling pathway. In vivo, the IRS-1 c.1791dupG variant induced abnormal neuron migration, cytomegaly, and network hyperexcitability. Notably, the ERK inhibitor GDC-0994, rather than the mTOR inhibitor rapamycin, effectively rescued the neuronal defects. This study directly highlighted the ERK signaling pathway's role in the pathogenesis of focal cortical dysplasia II and provided a new therapeutic target for cases of focal cortical dysplasia II that are not treatable by rapamycin analogs.

A clinically relevant selective ERK-pathway inhibitor reverses core deficits in a mouse model of autism

BACKGROUND

Extracellular signal-regulated kinase (ERK/MAPK) pathway in the brain is hypothesized to be a critical convergent node in the development of autism spectrum disorder. We reasoned that selectively targeting this pathway could reverse core autism-like phenotype in animal models.

METHODS

Here we tested a clinically relevant, selective inhibitor of ERK pathway, PD325901 (Mirdametinib), in a mouse model of idiopathic autism, the BTBR mice.

FINDINGS

We report that treating juvenile mice with PD325901 reduced ERK pathway activation, dose and duration-dependently reduced core disease-modeling deficits in sociability, vocalization and repetitive behavior, and reversed abnormal EEG signals. Further analysis revealed that subchronic treatment did not affect weight gain, locomotion, or neuronal density in the brain. Parallel treatment in the C57BL/6J mice did not alter their phenotype.

INTERPRETATION

Our data indicate that selectively inhibiting ERK pathway using PD325901 is beneficial in the BTBR model, thus further support the notion that ERK pathway is critically involved in the pathophysiology of autism. These results suggest that a similar approach could be applied to animal models of syndromic autism with dysregulated ERK signaling, to further test selectively targeting ERK pathway as a new approach for treating autism.

FUNDING

This has beenwork was supported by Alberta Children's Hospital Research Foundation (JMR & NC), University of Calgary Faculty of Veterinary Medicine (NC), Kids Brain Health Network (NC), and Natural Sciences and Engineering Research Council of Canada (NC).

Direct and Indirect Effects of Filamin A on Tau Pathology in Neuronal Cells

In Alzheimer disease (AD), Tau, an axonal microtubule-associated protein, becomes hyperphosphorylated, detaches from microtubules, accumulates, and self-aggregates in the somatodendritic (SD) compartment. The accumulation of hyperphosphorylated and aggregated Tau is also seen in other neurodegenerative diseases such as frontotemporal lobar degeneration (FTLD-Tau). Previous studies reported a link between filamin A (FLNA), an actin-binding protein found in the SD compartment, and Tau pathology. In the present study, we further explored this link. We confirmed the interaction of Tau with FLNA in neuroblastoma 2a (N2a) cells. This interaction was mediated by a domain located between the 157 and 383 amino acids (a.a.) of Tau. Our results also revealed that the overexpression of FLNA resulted in an intracellular accumulation of wild-type Tau and Tau mutants (P301L, V337M, and R406W) in N2a cells. Tau phosphorylation and cleavage by caspase-3 but not its aggregation were increased upon FLNA overexpression in N2a cells. In the parietal cortex of AD brain, insoluble FLNA was increased compared to control brain, but it did not correlate with Tau pathology. Interestingly, Tau binding to microtubules and F-actin was preserved upon FLNA overexpression in N2a cells. Lastly, our results revealed that FLNA also induced the accumulation of annexin A2, a Tau interacting partner involved in its axonal localization. Collectively, our data indicated that in Tauopathies, FLNA could contribute to Tau pathology by acting on Tau and annexin A2.

Evidence of Filamin A loss of solubility at the prodromal stage of neuropathologically-defined Alzheimer's disease

Introduction

Alzheimer's disease (AD) is a multifactorial disorder diagnosed through the assessment of amyloid-beta (Aβ) and tau protein depositions. Filamin A (FLNA) could be a key partner of both Aβ and tau pathological processes and may be an important contributor to AD progression. The main aim of this study was to describe the differences in FLNA levels across clinicopathologic groups.

Methods

From parietal cortex samples of 57 individuals (19 with no cognitive impairment (NCI), 19 mild cognitively impaired (MCI) and 19 with dementia) from the Religious Orders Study (ROS), we quantified total tau, phosphorylated tau (pTau), FLNA, synaptophysin, vesicular acetylcholine transporters (VAChT) and choline acetyltransferase (ChAT) by Western blot. Aβ42 and neuritic plaques (NP) were quantified by ELISA and Bielschowsky silver impregnation, respectively. AD staging was determined using ABC method combining Thal, Braak and the CERAD staging. From this, clinicopathologic stages of AD were established by subdividing subjects with neuropathological AD between preclinical AD, prodromal AD and AD dementia (ADD). Receiver operating characteristics analyses were performed to predict AD neuropathology from FLNA quantifications.

Results

Insoluble FLNA was significantly and positively correlated with Aβ42, NP, Thal stages, ABC scores and AD clinicopathologic stages (p < 0.05 False discovery rate-corrected). No correlation of FLNA with tau measures was found. Insoluble FLNA levels were significantly higher in the prodromal AD, ADD and intermediate ABC groups. This was consistent with significantly lower levels of soluble FLNA specifically in prodromal AD. Insoluble (AUC: 0.830) and soluble FLNA levels (AUC: 0.830) as well as the ratio of soluble over insoluble FLNA (AUC: 0.852), were excellent predictors of prodromal AD among subjects with MCI from the ROS cohort.

Discussion

We observed opposite level changes between insoluble and soluble FLNA in prodromal AD. As this stage coincides with the appearance of cognitive symptoms, this may be a key event in the transition from preclinical to prodromal AD. Insoluble FLNA could be useful to identify prodromal AD among subjects with an MCI, indicating that it might be a hallmark of prodromal AD.

Tsc2 shapes olfactory bulb granule cell molecular and morphological characteristics

Tuberous Sclerosis Complex (TSC) is a neurodevelopmental disorder caused by mutations that inactivate TSC1 or TSC2. Hamartin and tuberin are encoded by TSC1 and TSC2 which form a GTPase activating protein heteromer that inhibits the Rheb GTPase from activating a growth promoting protein kinase called mammalian target of rapamycin (mTOR). Growths and lesions occur in the ventricular-subventricular zone (V-SVZ), cortex, olfactory tract, and olfactory bulbs (OB) in TSC. A leading hypothesis is that mutations in inhibitory neural progenitor cells cause brain growths in TSC. OB granule cells (GCs) are GABAergic inhibitory neurons that are generated through infancy by inhibitory progenitor cells along the V-SVZ. Removal of Tsc1 from mouse OB GCs creates cellular phenotypes seen in TSC lesions. However, the role of Tsc2 in OB GC maturation requires clarification. Here, it is demonstrated that conditional loss of Tsc2 alters GC development. A mosaic model of TSC was created by performing neonatal CRE recombinase electroporation into inhibitory V-SVZ progenitors yielded clusters of ectopic cytomegalic neurons with hyperactive mTOR complex 1 (mTORC1) in homozygous Tsc2 mutant but not heterozygous or wild type mice. Similarly, homozygous Tsc2 mutant GC morphology was altered at postnatal days 30 and 60. Tsc2 mutant GCs had hypertrophic dendritic arbors that were established by postnatal day 30. In contrast, loss of Tsc2 from mature GCs had negligible effects on mTORC1, soma size, and dendrite arborization. OB transcriptome profiling revealed a network of significantly differentially expressed genes following loss of Tsc2 during development that altered neural circuitry. These results demonstrate that Tsc2 has a critical role in regulating neural development and shapes inhibitory GC molecular and morphological characteristics.

The Coordination of mTOR Signaling and Non-Coding RNA in Regulating Epileptic Neuroinflammation

Epilepsy accounts for a significant proportion of the burden of neurological disorders. Neuroinflammation acting as the inflammatory response to epileptic seizures is characterized by aberrant regulation of inflammatory cells and molecules, and has been regarded as a key process in epilepsy where mTOR signaling serves as a pivotal modulator. Meanwhile, accumulating evidence has revealed that non-coding RNAs (ncRNAs) interfering with mTOR signaling are involved in neuroinflammation and therefore articipate in the development and progression of epilepsy. In this review, we highlight recent advances in the regulation of mTOR on neuroinflammatory cells and mediators, and feature the progresses of the interaction between ncRNAs and mTOR in epileptic neuroinflammation.

Actin-binding protein filamin-A drives tau aggregation and contributes to progressive supranuclear palsy pathology

While amyloid-β lies upstream of tau pathology in Alzheimer's disease, key drivers for other tauopathies, including progressive supranuclear palsy (PSP), are largely unknown. Various tau mutations are known to facilitate tau aggregation, but how the nonmutated tau, which most cases with PSP share, increases its propensity to aggregate in neurons and glial cells has remained elusive. Here, we identified genetic variations and protein abundance of filamin-A in the PSP brains without tau mutations. We provided in vivo biochemical evidence that increased filamin-A levels enhance the phosphorylation and insolubility of tau through interacting actin filaments. In addition, reduction of filamin-A corrected aberrant tau levels in the culture cells from PSP cases. Moreover, transgenic mice carrying human filamin-A recapitulated tau pathology in the neurons. Our data highlight that filamin-A promotes tau aggregation, providing a potential mechanism by which filamin-A contributes to PSP pathology.

Inhibition of MEK-ERK signaling reduces seizures in two mouse models of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is a monogenic disorder characterized by hyperactivation of the mTOR signaling pathway and developmental brain malformations leading to intractable epilepsy. Although treatment with the recently approved mTOR inhibitor, everolimus, results in clinically relevant seizure suppression in up to 40% of TSC patients, seizures remain uncontrolled in a large number of cases, underscoring the need to identify novel treatment targets. The MEK-ERK signaling pathway has been found to be aberrantly activated in TSC and inhibition of MEK-ERK activity independently of mTOR rescued neuronal dendrite overgrowth in mice modeling TSC neuropathology. Here, we evaluated the efficacy of MEK-ERK inhibition on seizures in two mouse models of TSC. We found that treatment with the MEK inhibitor PD0325901 (mirdametinib) significantly reduced seizure activity in both TSC mouse models. These findings support inhibiting MEK-ERK activity as a potential alternative strategy to treat seizures in TSC.

Identifying the temporal electrophysiological and molecular changes that contribute to TSC-associated epileptogenesis

Tuberous sclerosis complex (TSC), caused by heterozygous mutations in TSC1 or TSC2, frequently results in intractable epilepsy. Here, we made use of an inducible Tsc1-knockout mouse model, allowing us to study electrophysiological and molecular changes of Tsc1-induced epileptogenesis over time. We recorded from pyramidal neurons in the hippocampus and somatosensory cortex (L2/L3) and combined this with an analysis of transcriptome changes during epileptogenesis. Deletion of Tsc1 resulted in hippocampus-specific changes in excitability and adaptation, which emerged before seizure onset and progressed over time. All phenotypes were rescued after early treatment with rapamycin, an mTOR inhibitor. Later in epileptogenesis, we observed a hippocampal increase of excitation-to-inhibition ratio. These cellular changes were accompanied by dramatic transcriptional changes, especially after seizure onset. Most of these changes were rescued upon rapamycin treatment. Of the genes encoding ion channels or belonging to the Gene Ontology term action potential, 27 were differentially expressed just before seizure onset, suggesting a potential driving role in epileptogenesis. Our data highlight the complex changes driving epileptogenesis in TSC, including the changed expression of multiple ion channels. Our study emphasizes inhibition of the TSC/mTOR signaling pathway as a promising therapeutic approach to target epilepsy in patients with TSC.

ERK/MAPK signalling in the developing brain: Perturbations and consequences

The extracellular regulated kinase/microtubule-associated protein kinase (ERK/MAPK) signalling pathway transduces signals that cause an alteration in the ongoing metabolic pathways and modifies gene expression patterns; thus, influencing cellular behaviour. ERK/MAPK signalling is essential for the proper development of the nervous system from neural progenitor cells derived from the embryonic mesoderm. Several signalling molecules that regulate the well-coordinated process of neurodevelopment transduce developmental information through the ERK/MAPK signalling pathway. The ERK/MAPK is a potential novel therapeutic target in several neurodevelopmental disorders, however, despite years of study, there is still significant uncertainty about the exact mechanism by which the ERK/MAPK signalling pathway elicits specific responses in neurodevelopment. Here, we will review the evidence highlighting the role of ERK/MAPK signalling in neurodevelopment. We will also discuss the structural implication and behavioural deficits associated with perturbed ERK/MAPK signalling pathway in cortical development, whilst examining its contribution to the neuropathology of several neurodevelopmental disorders, such as Autism Spectrum Disorder, Schizophrenia, Fragile X, and Attention Deficit Hyperactive Disorder.

Effects of Mutations in TSC Genes on Neurodevelopment and Synaptic Transmission

Mutations in TSC1 or TSC2 genes are linked to alterations in neuronal function which ultimately lead to the development of a complex neurological phenotype. Here we review current research on the effects that reduction in TSC1 or TSC2 can produce on the developing neural network. A crucial feature of the disease pathophysiology appears to be an early deviation from typical neurodevelopment, in the form of structural abnormalities. Epileptic seizures are one of the primary early manifestation of the disease in the CNS, followed by intellectual deficits and autism spectrum disorders (ASD). Research using mouse models suggests that morphological brain alterations might arise from the interaction of different cellular types, and hyperexcitability in the early postnatal period might be transient. Moreover, the increased excitation-to-inhibition ratio might represent a transient compensatory adjustment to stabilize the developing network rather than a primary factor for the development of ASD symptoms. The inhomogeneous results suggest region-specificity as well as an evolving picture of functional alterations along development. Furthermore, ASD symptoms and epilepsy might originate from different but potentially overlapping mechanisms, which can explain recent observations obtained in patients. Potential treatment is determined not only by the type of medicament, but also by the time point of treatment.

Filamin A inhibition reduces seizure activity in a mouse model of focal cortical malformations

Epilepsy treatments for patients with mechanistic target of rapamycin (mTOR) disorders, such as tuberous sclerosis complex (TSC) or focal cortical dysplasia type II (FCDII), are urgently needed. In these patients, the presence of focal cortical malformations is associated with the occurrence of lifelong epilepsy, leading to severe neurological comorbidities. Here, we show that the expression of the actin cross-linking protein filamin A (FLNA) is increased in resected cortical tissue that is responsible for seizures in patients with FCDII and in mice modeling TSC and FCDII with mutations in phosphoinositide 3-kinase (PI3K)-ras homolog enriched in brain (Rheb) pathway genes. Normalizing FLNA expression in these mice through genetic knockdown limited cell misplacement and neuronal dysmorphogenesis, two hallmarks of focal cortical malformations. In addition, Flna knockdown reduced seizure frequency independently of mTOR signaling. Treating mice with a small molecule targeting FLNA, PTI-125, before the onset of seizures alleviated neuronal abnormalities and reduced seizure frequency compared to vehicle-treated mice. In addition, the treatment was also effective when injected after seizure onset in juvenile and adult mice. These data suggest that targeting FLNA with either short hairpin RNAs or the small molecule PTI-125 might be effective in reducing seizures in patients with TSC and FCDII bearing mutations in PI3K-Rheb pathway genes.

Convergent and Divergent Mechanisms of Epileptogenesis in mTORopathies

Hyperactivation of the mechanistic target of rapamycin complex 1 (mTORC1) due to mutations in genes along the PI3K-mTOR pathway and the GATOR1 complex causes a spectrum of neurodevelopmental disorders (termed mTORopathies) associated with malformation of cortical development and intractable epilepsy. Despite these gene variants’ converging impact on mTORC1 activity, emerging findings suggest that these variants contribute to epilepsy through both mTORC1-dependent and -independent mechanisms. Here, we review the literature on in utero electroporation-based animal models of mTORopathies, which recapitulate the brain mosaic pattern of mTORC1 hyperactivity, and compare the effects of distinct PI3K-mTOR pathway and GATOR1 complex gene variants on cortical development and epilepsy. We report the outcomes on cortical pyramidal neuronal placement, morphology, and electrophysiological phenotypes, and discuss some of the converging and diverging mechanisms responsible for these alterations and their contribution to epileptogenesis. We also discuss potential therapeutic strategies for epilepsy, beyond mTORC1 inhibition with rapamycin or everolimus, that could offer personalized medicine based on the gene variant.

Small Extracellular Vesicles Control Dendritic Spine Development through Regulation of HDAC2 Signaling

The release of small extracellular vesicles (sEVs) has recently been reported, but knowledge of their function in neuron development remains limited. Using LC-MS/MS, we found that sEVs released from developing cortical neurons in vitro obtained from mice of both sexes were enriched in cytoplasm, exosome, and protein-binding and DNA/RNA-binding pathways. The latter included HDAC2, which was of particular interest, because HDAC2 regulates spine development, and populations of neurons expressing different levels of HDAC2 co-exist in vivo during the period of spine growth. Here, we found that HDAC2 levels decrease in neurons as they acquire synapses and that sEVs from HDAC2-rich neurons regulate HDAC2 signaling in HDAC2-low neurons possibly through HDAC2 transfer. This regulation led to a transcriptional decrease in HDAC2 synaptic targets and the density of excitatory synapses. These data suggest that sEVs provide inductive cell-cell signaling that coordinates the development of dendritic spines via the activation of HDAC2-dependent transcriptional programs.SIGNIFICANCE STATEMENT A role of small extracellular vesicles (sEVs; also called exosomes) in neuronal development is of particular interest, because sEVs could provide a major signaling modality between developing neurons when synapses are not fully functional or immature. However, knowledge of sEVs on neuron, and more precisely spine development, is limited. We provide several lines of evidence that sEVs released from developing cortical neurons regulate the development of dendritic spines via the regulation of HDAC2 signaling. This paracrine communication is temporally restricted during development because of the age-dependent decrease in sEV release as neurons mature and acquire spines.

The function and pathogenic mechanism of filamin A

Filamin A(FLNa) is an actin-binding protein, which participates in the formation of the cytoskeleton, anchors a variety of proteins in the cytoskeleton and regulates cell adhesion and migration. It is involved in signal transduction, cell proliferation and differentiation, pseudopodia formation, vesicle transport, tumor resistance and genetic diseases by binding with interacting proteins. In order to fully elucidate the structure, function and pathogenesis of FLNa, we summarized all substances which directly or indirectly act on FLNa so far, upstream and downstream targets which having effect on it, signaling pathways and their functions. It also recorded the expression and effect of FLNa in different diseases, including hereditary disease and tumors.

Rab27a-Dependent Paracrine Communication Controls Dendritic Spine Formation and Sensory Responses in the Barrel Cortex

Rab27a is an evolutionarily conserved small GTPase that regulates vesicle trafficking, and copy number variants of RAB27a are associated with increased risk of autism. However, the function of Rab27a on brain development is unknown. Here, we identified a form of paracrine communication that regulates spine development between distinct populations of developing cortical neurons. In the developing somatosensory cortex of mice, we show that decreasing Rab27a levels in late-born pyramidal neurons destined for layer (L) 2/3 had no cell-autonomous effect on their synaptic integration but increased excitatory synaptic transmission onto L4 neurons that receive somatosensory information. This effect resulted in an increased number of L4 neurons activated by whisker stimulation in juvenile mice. In addition, we found that Rab27a, the level of which decreases as neurons mature, regulates the release of small extracellular vesicles (sEVs) in developing neurons in vitro and decreasing Rab27a levels led to the accumulation of CD63-positive vesicular compartments in L2/3 neurons in vivo. Together, our study reveals that Rab27a-mediated paracrine communication regulates the development of synaptic connectivity, ultimately tuning responses to sensory stimulation, possibly via controlling the release of sEVs.

Fil-lamin-ing in the Gap in Cortical Dysplasia

[Box: see text]

Ectopic HCN4 expression drives mTOR-dependent epilepsy in mice

The causative link between focal cortical malformations (FCMs) and epilepsy is well accepted, especially among patients with focal cortical dysplasia type II (FCDII) and tuberous sclerosis complex (TSC). However, the mechanisms underlying seizures remain unclear. Using a mouse model of TSC- and FCDII-associated FCM, we showed that FCM neurons were responsible for seizure activity via their unexpected abnormal expression of the hyperpolarization-activated cyclic nucleotide-gated potassium channel isoform 4 (HCN4), which is normally not present in cortical pyramidal neurons after birth. Increasing intracellular cAMP concentrations, which preferentially affects HCN4 gating relative to the other isoforms, drove repetitive firing of FCM neurons but not control pyramidal neurons. Ectopic HCN4 expression was dependent on the mechanistic target of rapamycin (mTOR), preceded the onset of seizures, and was also found in diseased neurons in tissue resected from patients with TSC and FCDII. Last, blocking HCN4 channel activity in FCM neurons prevented epilepsy in the mouse model. These findings suggest that HCN4 play a main role in seizure and identify a cAMP-dependent seizure mechanism in TSC and FCDII. Furthermore, the unique expression of HCN4 exclusively in FCM neurons suggests that gene therapy targeting HCN4 might be effective in reducing seizures in FCDII or TSC.

The amino acid transporter Slc7a5 regulates the mTOR pathway and is required for granule cell development

Pathogenic mutations in the solute carrier family 7 member 5 (SLC7A5) gene, which encodes an amino acid transporter cause microcephaly and seizures, yet the mechanisms responsible for these phenotypes are unclear. Models have demonstrated that Slc7a5 deletion is embryonic lethal and that these embryos lack a fully formed telencephalon. This phenotype is similar to that of mammalian target of rapamycin (mTOR) protein kinase deletion or mTOR inhibition. Notably, in many cells, Slc7a5 import of amino acids is required to maintain mTOR activity. Slc7a5 is present within neurogenic regions during embryogenesis, is found in cultured neurons and can modulate neuronal electrophysiological properties. However, Slc7a5 is also highly expressed within endothelial cells of the blood-brain barrier where removal in conditional mice leads to severe behavioral defects and non-cell autonomous changes in neurons. Therefore, the extent that neural Slc7a5 is required for development is unclear. Here, subventricular zone neural stem cells that generate olfactory bulb granule cell neurons were electroporated with SLC7A5 or Slc7a5 short hairpin RNA encoding plasmids. Although early phases of neural development were unaltered, Slc7a5 knockdown effected late phases of GC dendrite maturation and survival. Slc7a5 knockdown also decreased mTOR pathway activity. Ras homolog enriched in brain, an mTOR activator, rescued the effect of Slc7a5 knockdown on mTOR pathway activity and dendrite arbors. The data presented here demonstrate that Slc7a5 is required for GC mTOR pathway activity, maturation and survival, which may help explain why Slc7a5 mutations prevent normal brain development and function.

Whole-brain multimodal MRI phenotyping of periventricular nodular heterotopia

OBJECTIVE

To test the hypothesis that in periventricular nodular heterotopia (PVNH) structure and function of cortical areas overlying the heterotopic gray matter are preferentially affected.

METHODS

We studied a group of 40 patients with PVNH and normal-appearing cortex and compared their quantitative MRI markers of brain development, structure, and function to those of 43 age- and sex-matched healthy controls. Inspired by models of neocortical development suggesting that neuronal migration follows a curvilinear path to preserve topologic correspondence between the outer ventricular zone and the cortical surface, we computationally defined the overlying cortex using the Laplace equation and generated synthetic streamlines that link the ventricles, where nodules are located, and the neocortex.

RESULTS

We found multilobar cortical thickening encompassing prefrontal, latero-basal temporal, and temporoparietal cortices largely corresponding with the PVNH group-averaged map of the overlying cortex, the latter colocalized with areas of abnormal function, as defined by resting-state fMRI. Patients also presented hippocampal functional hyperconnectivity and malrotation, the latter positively correlating with neocortical maldevelopment indexed by increased folding complexity of the parahippocampus. In clusters of thickness and curvature findings, there were no significant differences between unilateral and bilateral PVNH; contrasting brain-wide metrics between cohorts was also unrevealing. There was no relationship between imaging markers and disease duration except for positive correlation with functional anomalies.

CONCLUSION

Our quantitative image analysis demonstrates widespread structural and functional alterations in PVNH with differential interaction with the overlying cortex and the hippocampus. Right hemispheric predominance may be explained by an early insult, likely genetically determined, on brain morphogenesis.

The Neurodevelopmental Pathogenesis of Tuberous Sclerosis Complex (TSC)

Tuberous sclerosis complex (TSC) is a model disorder for understanding brain development because the genes that cause TSC are known, many downstream molecular pathways have been identified, and the resulting perturbations of cellular events are established. TSC, therefore, provides an intellectual framework to understand the molecular and biochemical pathways that orchestrate normal brain development. The TSC1 and TSC2 genes encode Hamartin and Tuberin which form a GTPase activating protein (GAP) complex. Inactivating mutations in TSC genes (TSC1/TSC2) cause sustained Ras homologue enriched in brain (RHEB) activation of the mammalian isoform of the target of rapamycin complex 1 (mTORC1). TOR is a protein kinase that regulates cell size in many organisms throughout nature. mTORC1 inhibits catabolic processes including autophagy and activates anabolic processes including mRNA translation. mTORC1 regulation is achieved through two main upstream mechanisms. The first mechanism is regulation by growth factor signaling. The second mechanism is regulation by amino acids. Gene mutations that cause too much or too little mTORC1 activity lead to a spectrum of neuroanatomical changes ranging from altered brain size (micro and macrocephaly) to cortical malformations to Type I neoplasias. Because somatic mutations often underlie these changes, the timing, and location of mutation results in focal brain malformations. These mutations, therefore, provide gain-of-function and loss-of-function changes that are a powerful tool to assess the events that have gone awry during development and to determine their functional physiological consequences. Knowledge about the TSC-mTORC1 pathway has allowed scientists to predict which upstream and downstream mutations should cause commensurate neuroanatomical changes. Indeed, many of these predictions have now been clinically validated. A description of clinical imaging and histochemical findings is provided in relation to laboratory models of TSC that will allow the reader to appreciate how human pathology can provide an understanding of the fundamental mechanisms of development.

Neuroligin 3 Regulates Dendritic Outgrowth by Modulating Akt/mTOR Signaling

Neuroligins (NLs) are a group of postsynaptic cell adhesion molecules that function in synaptogenesis and synaptic transmission. Genetic defects in neuroligin 3 (NL3), a member of the NL protein family, are associated with autism. Studies in rodents have revealed that mutations of NL3 gene lead to increased growth and complexity in dendrites in the central nervous system. However, the detailed mechanism is still unclear. In our study, we found that deficiency of NL3 led to morphological changes of the pyramidal neurons in layer II/III somatosensory cortex in mice, including enlarged somata, elongated dendritic length, and increased dendritic complexity. Knockdown of NL3 in cultured rat neurons upregulated Akt/mTOR signaling, resulting in both increased protein synthesis and dendritic growth. Treating neurons with either rapamycin to inhibit the mTOR or LY294002 to inhibit the PI3K/Akt activity rescued the morphological abnormalities resulting from either NL3 knockdown or knockout (KO). In addition, we found that the hyperactivated Akt/mTOR signaling associated with NL3 defects was mediated by a reduction in phosphatase and tensin (PTEN) expression, and that MAGI-2, a scaffold protein, interacted with both NL3 and PTEN and could be a linker between NL3 and Akt/mTOR signaling pathway. In conclusion, our results suggest that NL3 regulates neuronal morphology, especially dendritic outgrowth, by modulating the PTEN/Akt/mTOR signaling pathway, probably via MAGI-2. Thereby, this study provides a new link between NL3 and neuronal morphology.

Tsc1-mTORC1 signaling controls striatal dopamine release and cognitive flexibility

Tuberous Sclerosis Complex (TSC) is a neurodevelopmental disorder caused by mutations in TSC1 or TSC2, which encode proteins that negatively regulate mTOR complex 1 (mTORC1). TSC is associated with significant cognitive, psychiatric, and behavioral problems, collectively termed TSC-Associated Neuropsychiatric Disorders (TAND), and the cell types responsible for these manifestations are largely unknown. Here we use cell type-specific Tsc1 deletion to test whether dopamine neurons, which modulate cognitive, motivational, and affective behaviors, are involved in TAND. We show that loss of Tsc1 and constitutive activation of mTORC1 in dopamine neurons causes somatodendritic hypertrophy, reduces intrinsic excitability, alters axon terminal structure, and impairs striatal dopamine release. These perturbations lead to a selective deficit in cognitive flexibility, preventable by genetic reduction of the mTOR-binding protein Raptor. Our results establish a critical role for Tsc1-mTORC1 signaling in setting the functional properties of dopamine neurons, and indicate that dopaminergic dysfunction may contribute to cognitive inflexibility in TSC.

Hypervascularization in mTOR-dependent focal and global cortical malformations displays differential rapamycin sensitivity

OBJECTIVES

Patients with mammalian target of rapamycin (mTOR)-dependent malformations of cortical development (MCDs) associated with seizures display hyperperfusion and increased vessel density of the dysmorphic cortical tissue. Some studies have suggested that the vascular defect occurred independently of seizures. Here, we further examined whether hypervascularization occurs in animal models of global and focal MCD with and without seizures, and whether it is sensitive to the mTOR blocker, rapamycin, that is approved for epilepsy treatment in tuberous sclerosis complex.

METHODS

We used two experimental models of mTOR-dependent MCD consisting of conditional transgenic mice containing Tsc1^null cells in the forebrain generating a global malformation associated with seizures and of wild-type mice containing a focal malformation in the somatosensory cortex generated by in utero electroporation (IUE) that does not lead to seizures. Alterations in blood vessels and the effects of a 2-week-long rapamycin treatment on these phenotypes were assessed in juvenile mice.

RESULTS

Blood vessels in both the focal and global MCDs of postnatal day 14 mice displayed significant increase in vessel density, branching index, total vessel length, and decreased tissue lacunarity. In addition, rapamycin treatment (0.5 mg/kg, every 2 days) partially rescued vessel abnormalities in the focal MCD model, but it did not ameliorate the vessel abnormalities in the global MCD model that required higher rapamycin dosage for a partial rescue.

SIGNIFICANCE

Here, we identified hypervascularization in mTOR-dependent MCD in the absence of seizures in young mice, suggesting that increased angiogenesis occurs during development in parallel to alterations in corticogenesis. In addition, a predictive functional outcome is that dysplastic neurons forming MCD will have better access to oxygen and metabolic supplies via their closer proximity to blood vessels. Finally, the difference in rapamycin sensitivity between a focal and global MCD suggest that rapamycin treatment will need to be titrated to match the type of MCD.

Genetic Etiologies, Diagnosis, and Treatment of Tuberous Sclerosis Complex

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder that affects multiple organ systems due to an inactivating variant in either TSC1 or TSC2, resulting in the hyperactivation of the mechanistic target of rapamycin (mTOR) pathway. Dysregulated mTOR signaling results in increased cell growth and proliferation. Clinically, TSC patients exhibit great phenotypic variability, but the neurologic and neuropsychiatric manifestations of the disease have the greatest morbidity and mortality. TSC-associated epilepsy occurs in nearly all patients and is often difficult to treat because it is refractory to multiple antiseizure medications. The advent of mTOR inhibitors offers great promise in the treatment of TSC-associated epilepsy and other neurodevelopmental manifestations of the disease; however, the optimal timing of therapeutic intervention is not yet fully understood.

ANXA2Tyr23 and FLNASer2152 phosphorylation associate with poor prognosis in hepatic carcinoma revealed by quantitative phosphoproteomics analysis

Hepatoma is one of the most common malignant tumors, and most patients have very poor prognosis. Early prediction and intervention of the hepatoma recurrence/metastasis are the most effective way to improve the patients' clinical outcomes. Here, we used isobaric tags for relative and absolute quantitation (iTRAQ) based quantitative phospho-proteomics approach to identify biomarkers associated with hepatoma recurrence/metastasis in hepatoma cell lines with increasing metastasis ability. In total, 75 phosphorylated peptides corresponding to 60 phosphoproteins were significantly dysregulated and the participated biological processes of these phosphoproteins were tightly associated with tumor metastasis. Further signaling pathway analysis revealed that key signaling pathways which play crucial roles in cancer metastasis have been significantly over activated in the highly metastatic cells. Furthermore, the phosphorylation of FLNA^Ser2152 and ANXA2^Tyr23 were validated to be significantly up regulated in the high-metastatic cells comparing with the low-metastatic cells. By further investigation the clinical significance of the phosphorylation of FLNA^Ser2152 and ANXA2^Tyr23 in large-scale clinical samples, revealed that the over phosphorylation of FLNA^Ser2152 and ANXA2^Tyr23 were associated with poor prognosis and might be potential prognostic biomarkers for the primary hepatoma. When FLNA^Ser2152 combined with ANXA2^Tyr23, it had a better prognostic value for both OS and TTR.

Role of mTOR Complexes in Neurogenesis

Dysregulation of neural stem cells (NSCs) is associated with several neurodevelopmental disorders, including epilepsy and autism spectrum disorder. The mammalian target of rapamycin (mTOR) integrates the intracellular signals to control cell growth, nutrient metabolism, and protein translation. mTOR regulates many functions in the development of the brain, such as proliferation, differentiation, migration, and dendrite formation. In addition, mTOR is important in synaptic formation and plasticity. Abnormalities in mTOR activity is linked with severe deficits in nervous system development, including tumors, autism, and seizures. Dissecting the wide-ranging roles of mTOR activity during critical periods in development will greatly expand our understanding of neurogenesis.

mTOR Signaling and Neural Stem Cells: The Tuberous Sclerosis Complex Model

The mechanistic target of rapamycin (mTOR), a serine-threonine kinase, plays a pivotal role in regulating cell growth and proliferation. Notably, a great deal of evidence indicates that mTOR signaling is also crucial in controlling proliferation and differentiation of several stem cell compartments. Consequently, dysregulation of the mTOR pathway is often associated with a variety of disease, such as cancer and metabolic and genetic disorders. For instance, hyperactivation of mTORC1 in neural stem cells (NSCs) is associated with the insurgence of neurological manifestation characterizing tuberous sclerosis complex (TSC). In this review, we survey the recent contributions of TSC physiopathology studies to understand the role of mTOR signaling in both neurogenesis and tumorigenesis and discuss how these new insights can contribute to developing new therapeutic strategies for neurological diseases and cancer.

Whole exome sequencing reveals inherited and de novo variants in autism spectrum disorder: a trio study from Saudi families

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder with genetic and clinical heterogeneity. The interplay of de novo and inherited rare variants has been suspected in the development of ASD. Here, we applied whole exome sequencing (WES) on 19 trios from singleton Saudi families with ASD. We developed an analysis pipeline that allows capturing both de novo and inherited rare variants predicted to be deleterious. A total of 47 unique rare variants were detected in 17 trios including 38 which are newly discovered. The majority were either autosomal recessive or X-linked. Our pipeline uncovered variants in 15 ASD-candidate genes, including 5 (GLT8D1, HTATSF1, OR6C65, ITIH6 and DDX26B) that have not been reported in any human condition. The remaining variants occurred in genes formerly associated with ASD or other neurological disorders. Examples include SUMF1, KDM5B and MXRA5 (Known-ASD genes), PRODH2 and KCTD21 (implicated in schizophrenia), as well as USP9X and SMS (implicated in intellectual disability). Consistent with expectation and previous studies, most of the genes implicated herein are enriched for biological processes pertaining to neuronal function. Our findings underscore the private and heterogeneous nature of the genetic architecture of ASD even in a population with high consanguinity rates.

Dendritic overgrowth and elevated ERK signaling during neonatal development in a mouse model of autism

Autism spectrum disorder (hereafter referred to as “ASD”) is a heterogeneous neurodevelopmental condition characterized by impaired social communication and interactions, and restricted, repetitive activities or interests. Alterations in network connectivity and memory function are frequently observed in autism patients, often involving the hippocampus. However, specific changes during early brain development leading to disrupted functioning remain largely unclear. Here, we investigated the development of dendritic arbor of hippocampal CA1 pyramidal neurons in the BTBR T+tf/J (BTBR) mouse model of autism. BTBR mice display the defining behavioural features of autism, and also exhibit impaired learning and memory. We found that compared to control C57BL/6J (B6) animals, the lengths of both apical and basal dendrites were significantly greater in neonatal BTBR animals. Further, basal dendrites in the BTBR mice had higher branching complexity. In contrast, cross-sectional area of the soma was unchanged. In addition, we observed a similar density of CA1 pyramidal neurons and thickness of the neuronal layer between the two strains. Thus, there was a specific, compartmentalized overgrowth of dendrites during early development in the BTBR animals. Biochemical analysis further showed that the extracellular signal-regulated kinases (ERK) pathway was up-regulated in the hippocampus of neonatal BTBR animals. Since dendritic structure is critical for information integration and relay, our data suggest that altered development of dendrites could potentially contribute to impaired hippocampal function and behavior observed in the BTBR model, and that this might be related to increased activation of the ERK pathway.

Filamin, a synaptic organizer in Drosophila, determines glutamate receptor composition and membrane growth

Filamin is a scaffolding protein that functions in many cells as an actin-crosslinker. FLN90, an isoform of the Drosophila ortholog Filamin/cheerio that lacks the actin-binding domain, is here shown to govern the growth of postsynaptic membrane folds and the composition of glutamate receptor clusters at the larval neuromuscular junction. Genetic and biochemical analyses revealed that FLN90 is present surrounding synaptic boutons. FLN90 is required in the muscle for localization of the kinase dPak and, downstream of dPak, for localization of the GTPase Ral and the exocyst complex to this region. Consequently, Filamin is needed for growth of the subsynaptic reticulum. In addition, in the absence of filamin, type-A glutamate receptor subunits are lacking at the postsynapse, while type-B subunits cluster correctly. Receptor composition is dependent on dPak, but independent of the Ral pathway. Thus two major aspects of synapse formation, morphological plasticity and subtype-specific receptor clustering, require postsynaptic Filamin.

DOI:http://dx.doi.org/10.7554/eLife.19991.001

New Insights into Protein Hydroxylation and Its Important Role in Human Diseases

Protein hydroxylation is a post-translational modification catalyzed by 2-oxoglutarate-dependent dioxygenases. The hydroxylation modification can take place on various amino acids, including but not limited to proline, lysine, asparagine, aspartate and histidine. A classical example of this modification is hypoxia inducible factor alpha (HIF-α) prolyl hydroxylation, which affects HIF-α protein stability via the Von-Hippel Lindau (VHL) tumor suppressor pathway, a Cullin 2-based E3 ligase adaptor protein frequently mutated in kidney cancer. In addition to protein stability regulation, protein hydroxylation may influence other post-translational modifications or the kinase activity of the modified protein (such as Akt and DYRK1A/B). In other cases, protein hydroxylation may alter protein-protein interaction and its downstream signaling events in vivo (such as OTUB1, MAPK6 and eEF2K). In this review, we highlight the recently identified protein hydroxylation targets and their pathophysiological roles, especially in cancer settings. Better understanding of protein hydroxylation will help identify novel therapeutic targets and their regulation mechanisms to foster development of more effective treatment strategies for various human cancers.

The mTOR signalling cascade: paving new roads to cure neurological disease

Defining the multiple roles of the mechanistic (formerly 'mammalian') target of rapamycin (mTOR) signalling pathway in neurological diseases has been an exciting and rapidly evolving story of bench-to-bedside translational research that has spanned gene mutation discovery, functional experimental validation of mutations, pharmacological pathway manipulation, and clinical trials. Alterations in the dual contributions of mTOR - regulation of cell growth and proliferation, as well as autophagy and cell death - have been found in developmental brain malformations, epilepsy, autism and intellectual disability, hypoxic-ischaemic and traumatic brain injuries, brain tumours, and neurodegenerative disorders. mTOR integrates a variety of cues, such as growth factor levels, oxygen levels, and nutrient and energy availability, to regulate protein synthesis and cell growth. In line with the positioning of mTOR as a pivotal cell signalling node, altered mTOR activation has been associated with a group of phenotypically diverse neurological disorders. To understand how altered mTOR signalling leads to such divergent phenotypes, we need insight into the differential effects of enhanced or diminished mTOR activation, the developmental context of these changes, and the cell type affected by altered signalling. A particularly exciting feature of the tale of mTOR discovery is that pharmacological mTOR inhibitors have shown clinical benefits in some neurological disorders, such as tuberous sclerosis complex, and are being considered for clinical trials in epilepsy, autism, dementia, traumatic brain injury, and stroke.

Tsc1 haploinsufficiency is sufficient to increase dendritic patterning and Filamin A levels

Most individuals with tuberous sclerosis complex (TSC) are born with a mutant allele of either TSC1 or TSC2 and a mosaic of psychological and cognitive defects. Tsc1 loss of heterozygosity contributes to severe dendritic abnormalities that are rescued by normalizing the levels of the actin-cross linking protein, Filamin A (FLNA). However, it is unclear whether dendrites and FLNA levels are abnormal in an heterozygote Tsc1 condition. Here, we examined dendritic morphology and FLNA levels in the olfactory bulb of Tsc1 wild type and heterozygote mice. Using in vivo neonatal electroporation to label newborn neurons followed by sholl analysis, we found that Tsc1 haploinsufficiency is associated with increased dendritic complexity and total dendritic length as well as increased FLNA levels. Since reducing FLNA levels has been shown to decrease Tsc1(+/-) dendritic complexity, these data suggest that increased FLNA levels in Tsc1(+/-) mice contribute to abnormal dendritic patterning in the Tsc1 heterozygote condition of individuals with TSC.

The Oxygen Sensor PHD2 Controls Dendritic Spines and Synapses via Modification of Filamin A

Neuronal function is highly sensitive to changes in oxygen levels, but how hypoxia affects dendritic spine formation and synaptogenesis is unknown. Here we report that hypoxia, chemical inhibition of the oxygen-sensing prolyl hydroxylase domain proteins (PHDs), and silencing of Phd2 induce immature filopodium-like dendritic protrusions, promote spine regression, reduce synaptic density, and decrease the frequency of spontaneous action potentials independently of HIF signaling. We identified the actin cross-linker filamin A (FLNA) as a target of PHD2 mediating these effects. In normoxia, PHD2 hydroxylates the proline residues P2309 and P2316 in FLNA, leading to von Hippel-Lindau (VHL)-mediated ubiquitination and proteasomal degradation. In hypoxia, PHD2 inactivation rapidly upregulates FLNA protein levels because of blockage of its proteasomal degradation. FLNA upregulation induces more immature spines, whereas Flna silencing rescues the immature spine phenotype induced by PHD2 inhibition.

•

The oxygen sensor PHD2 is present in dendritic spines

•

PHD2 inhibition by hypoxia reduces spine maturation, synaptic density, and activity

•

Through hydroxylation, PHD2 targets filamin A for proteasomal degradation

•

Filamin A stabilization promotes dendritic spine remodeling

Layer specific and general requirements for ERK/MAPK signaling in the developing neocortex

Aberrant signaling through the Raf/MEK/ERK (ERK/MAPK) pathway causes pathology in a family of neurodevelopmental disorders known as 'RASopathies' and is implicated in autism pathogenesis. Here, we have determined the functions of ERK/MAPK signaling in developing neocortical excitatory neurons. Our data reveal a critical requirement for ERK/MAPK signaling in the morphological development and survival of large Ctip2⁺ neurons in layer 5. Loss of Map2k1/2 (Mek1/2) led to deficits in corticospinal tract formation and subsequent corticospinal neuron apoptosis. ERK/MAPK hyperactivation also led to reduced corticospinal axon elongation, but was associated with enhanced arborization. ERK/MAPK signaling was dispensable for axonal outgrowth of layer 2/3 callosal neurons. However, Map2k1/2 deletion led to reduced expression of Arc and enhanced intrinsic excitability in both layers 2/3 and 5, in addition to imbalanced synaptic excitation and inhibition. These data demonstrate selective requirements for ERK/MAPK signaling in layer 5 circuit development and general effects on cortical pyramidal neuron excitability.

DOI:http://dx.doi.org/10.7554/eLife.11123.001

In the nervous system, cells called neurons form networks that relay information in the form of electrical signals around the brain and the rest of the body. Typically, an electrical signal travels from branch-like structures at one end of the cell, through the cell body and then along a long fiber called an axon to reach junctions with another neurons.

The connections between neurons start to form as the nervous system develops in the embryo, and any errors or delays in this process can cause severe neurological disorders and intellectual disabilities. For example, genetic mutations affecting a communication system within cells known as the ERK/MAPK pathway can lead to a family of syndromes called the “RASopathies”. Abnormalities in this pathway may also contribute to certain types of autism. However, it is not clear how alterations to the ERK/MAPK pathway cause these conditions.

Xing et al. investigated whether ERK/MAPK signaling regulates the formation of connections between neurons and the activity of neurons in mouse brains. The experiments showed that the growth of axons that extend from an area of the brain called the cerebral cortex towards the spinal cord are particularly sensitive to changes in the level of signaling through the ERK/MAPK pathway. On the other hand, inhibiting the pathway has relatively little effect on the growth of axons within the cerebral cortex. Further experiments showed that many neurons in the cerebral cortex require the ERK/MAPK pathway to activate genes that alter neuronal activity and the strength of the connections between neurons.

Xing et al.’s findings suggest that defects in the connections between the cerebral cortex and different regions of the nervous system may contribute to the symptoms observed in patients with conditions linked to alterations in ERK/MAPK activity. Future studies will focus on understanding the molecular mechanisms by which ERK/MAPK pathway influences the organization and activity of neuron circuits during the development of the nervous system.

DOI:http://dx.doi.org/10.7554/eLife.11123.002

Hypoxia-inducible factor-1a contributes to dendritic overgrowth in tuberous sclerosis

Expression of hypoxia-inducible factor 1a (HIF1a) is increased under several pathological conditions such as hyperactive mechanistic target of rapamycin complex 1 (mTORC1) in tuberous sclerosis complex (TSC). Hyperactive mTORC1 and the resulting increased dendritic complexity of neurons are shared molecular and cellular alterations in several neurological disorders associated with cognitive disabilities. Despite some evidence that HIF1a contributes to dendritic overgrowth in vitro, it remains unknown whether increased HIF1a in TSC neurons could contribute to their increased dendritic complexity. To address this use in vivo, we generated TSC neurons by deleting Tsc1 in newborn olfactory bulb (OB) neurons of conditional Tsc1 transgenic mice using neonatal electroporation. In addition to their increased dendritic complexity, Tsc1(null) neurons have been reported to display increased Hif1a mRNA level and HIF1a transcriptional activity. We found that Tsc1(null)-dependent dendritic overgrowth was prevented by knocking down HIF1a or expressing a dominant negative HIF1a. In addition, overexpressing HIF1a in wild-type developing neurons resulted in increased dendritic complexity in vivo. These data highlight that an increase in HIF1a levels contributes to abnormal dendritic patterning in developing neurons under normal conditions and hyperactive mTORC1 conditions as in TSC.

Complex Neurological Phenotype in Mutant Mice Lacking Tsc2 in Excitatory Neurons of the Developing Forebrain(123)

Mutations in the TSC1 and TSC2 genes cause tuberous sclerosis complex (TSC), a genetic disease often associated with epilepsy, intellectual disability, and autism, and characterized by the presence of anatomical malformations in the brain as well as tumors in other organs. The TSC1 and TSC2 proteins form a complex that inhibits mammalian target of rapamycin complex 1 (mTORC1) signaling. Previous animal studies demonstrated that Tsc1 or Tsc2 loss of function in the developing brain affects the intrinsic development of neural progenitor cells, neurons, or glia. However, the interplay between different cellular elements during brain development was not previously investigated. In this study, we generated a novel mutant mouse line (NEX-Tsc2) in which the Tsc2 gene is deleted specifically in postmitotic excitatory neurons of the developing forebrain. Homozygous mutant mice failed to thrive and died prematurely, whereas heterozygous mice appeared normal. Mutant mice exhibited distinct neuroanatomical abnormalities, including malpositioning of selected neuronal populations, neuronal hypertrophy, and cortical astrogliosis. Intrinsic neuronal defects correlated with increased mTORC1 signaling, whereas astrogliosis did not result from altered intrinsic signaling, since these cells were not directly affected by the gene knockout strategy. All neuronal and non-neuronal abnormalities were suppressed by continuous postnatal treatment with the mTORC1 inhibitor RAD001. The data suggest that the loss of Tsc2 and mTORC1 signaling activation in excitatory neurons not only disrupts their intrinsic development, but also disrupts the development of cortical astrocytes, likely through the mTORC1-dependent expression of abnormal signaling proteins. This work thus provides new insights into cell-autonomous and non-cell-autonomous functions of Tsc2 in brain development.

The Stress-Induced Atf3-Gelsolin Cascade Underlies Dendritic Spine Deficits in Neuronal Models of Tuberous Sclerosis Complex

Hyperactivation of the mechanistic target of rapamycin (mTOR) kinase, as a result of loss-of-function mutations in tuberous sclerosis complex 1 (TSC1) or TSC2 genes, causes protein synthesis dysregulation, increased cell size, and aberrant neuronal connectivity. Dysregulated synthesis of synaptic proteins has been implicated in the pathophysiology of autism spectrum disorder (ASD) associated with TSC and fragile X syndrome. However, cell type-specific translational profiles in these disease models remain to be investigated. Here, we used high-fidelity and unbiased Translating Ribosome Affinity Purification (TRAP) methodology to purify ribosome-associated mRNAs and identified translational alterations in a rat neuronal culture model of TSC. We find that expression of many stress and/or activity-dependent proteins is highly induced while some synaptic proteins are repressed. Importantly, transcripts for the activating transcription factor-3 (Atf3) and mitochondrial uncoupling protein-2 (Ucp2) are highly induced in Tsc2-deficient neurons, as well as in a neuron-specific Tsc1 conditional knock-out mouse model, and show differential responses to the mTOR inhibitor rapamycin. Gelsolin, a known target of Atf3 transcriptional activity, is also upregulated. shRNA-mediated block of Atf3 induction suppresses expression of gelsolin, an actin-severing protein, and rescues spine deficits found in Tsc2-deficient neurons. Together, our data demonstrate that a cell-autonomous program consisting of a stress-induced Atf3-gelsolin cascade affects the change in dendritic spine morphology following mTOR hyperactivation. This previously unidentified molecular cascade could be a therapeutic target for treating mTORopathies.

SIGNIFICANCE STATEMENT

Tuberous sclerosis complex (TSC) is a genetic disease associated with epilepsy and autism. Dysregulated protein synthesis has been implicated as a cause of this disease. However, cell type-specific translational profiles that are aberrant in this disease are unknown. Here we show that expression of many stress and/or activity-dependent proteins is highly induced while some synaptic proteins are repressed in neurons missing the Tsc2 gene expression. Identification of genes whose translation is abnormal in TSC may provide insights to previously unidentified therapeutic targets.

Activation of extracellular regulated kinase and mechanistic target of rapamycin pathway in focal cortical dysplasia

Neuropathology of resected brain tissue has revealed an association of focal cortical dysplasia (FCD) with drug-resistant epilepsy (DRE). Recent studies have shown that the mechanistic target of rapamycin (mTOR) pathway is hyperactivated in FCD as evidenced by increased phosphorylation of the ribosomal protein S6 (S6) at serine 240/244 (S(240/244) ), a downstream target of mTOR. Moreover, extracellular regulated kinase (ERK) has been shown to phosphorylate S6 at serine 235/236 (S(235/236) ) and tuberous sclerosis complex 2 (TSC2) at serine 664 (S(664) ) leading to hyperactive mTOR signaling. We evaluated ERK phosphorylation of S6 and TSC2 in two types of FCD (FCD I and FCD II) as a candidate mechanism contributing to mTOR pathway dysregulation. Tissue samples from patients with tuberous sclerosis (TS) served as a positive control. Immunostaining for phospho-S6 (pS6(240/244) and pS6(235/236) ), phospho-ERK (pERK), and phospho-TSC2 (pTSC2) was performed on resected brain tissue with FCD and TS. We found increased pS6(240/244) and pS6(235/236) staining in FCD I, FCD II and TS compared to normal-appearing tissue, while pERK and pTSC2 staining was increased only in FCD IIb and TS tissue. Our results suggest that both the ERK and mTOR pathways are dysregulated in FCD and TS; however, the signaling alterations are different for FCD I as compared to FCD II and TS.

Activating the translational repressor 4E-BP or reducing S6K-GSK3β activity prevents accelerated axon growth induced by hyperactive mTOR in vivo

Abnormal axonal connectivity and hyperactive mTOR complex 1 (mTORC1) are shared features of several neurological disorders. Hyperactive mTORC1 alters axon length and polarity of hippocampal neurons in vitro, but the impact of hyperactive mTORC1 on axon growth in vivo and the mechanisms underlying those effects remain unclear. Using in utero electroporation during corticogenesis, we show that increasing mTORC1 activity accelerates axon growth without multiple axon formation. This was prevented by counteracting mTORC1 signaling through p70S6Ks (S6K1/2) or eukaryotic initiation factor 4E-binding protein (4E-BP1/2), which both regulate translation. In addition to regulating translational targets, S6K1 indirectly signals through GSK3β, a regulator of axogenesis. Although blocking GSK3β activity did not alter axon growth under physiological conditions in vivo, blocking it using a dominant-negative mutant or lithium chloride prevented mTORC1-induced accelerated axon growth. These data reveal the contribution of translational and non-translational downstream effectors such as GSK3β to abnormal axon growth in neurodevelopmental mTORopathies and open new therapeutic options for restoring long-range connectivity.

Significance of filamin A in mTORC2 function in glioblastoma

Background

Glioblastoma multiforme (GBM) is one of the most highly metastatic cancers. GBM has been associated with a high level of the mechanistic target of rapamycin complex 2 (mTORC2) activity. We aimed to observe roles of mTORC2 in GBM cells especially on actin cytoskeleton reorganization, cell migration and invasion, and further determine new important players involved in the regulation of these cellular processes.

Methods

To further investigate the significance of mTORC2 in GBM, we treated GBM cells with PP242, an ATP-competitive inhibitor of mTOR, and used RICTOR siRNA to knock down mTORC2 activity. Effects on actin cytoskeleton, focal adhesion, migration, and invasion of GBM cells were examined. To gain insight into molecular basis of the mTORC2 effects on cellular cytoskeletal arrangement and motility/invasion, we affinity purified mTORC2 from GBM cells and identified proteins of interest by mass spectrometry. Characterization of the protein of interest was performed.

Results

In addition to the inhibition of mTORC2 activity, we demonstrated significant alteration of actin distribution as revealed by the use of phalloidin staining. Furthermore, vinculin staining was altered which suggests changes in focal adhesion. Inhibition of cell migration and invasion was observed with PP242. Two major proteins that are associated with this mTORC2 multiprotein complex were found. Mass spectrometry identified one of them as Filamin A (FLNA). Association of FLNA with RICTOR but not mTOR was demonstrated. Moreover, in vitro, purified mTORC2 can phosphorylate FLNA likewise its known substrate, AKT. In GBM cells, colocalization of FLNA with RICTOR was observed, and the overall amounts of FLNA protein as well as phosphorylated FLNA are high. Upon treatments of RICTOR siRNA or PP242, phosphorylated FLNA levels at the regulatory residue (Ser2152) decreased. This treatment also disrupted colocalization of Actin filaments and FLNA.

Conclusions

Our results support FLNA as a new downstream effector of mTORC2 controlling GBM cell motility. This new mTORC2-FLNA signaling pathway plays important roles in motility and invasion of glioblastoma cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-015-0396-z) contains supplementary material, which is available to authorized users.