Neurofibromatosis-1 heterozygosity impairs CNS neuronal morphology in a cAMP/PKA/ROCK-dependent manner

Hyperactivation of MEK1 in cortical glutamatergic neurons results in projection axon deficits and aberrant motor learning

Abnormal extracellular signal-regulated kinase 1/2 (ERK1/2, encoded by Mapk3 and Mapk1, respectively) signaling is linked to multiple neurodevelopmental diseases, especially the RASopathies, which typically exhibit ERK1/2 hyperactivation in neurons and non-neuronal cells. To better understand how excitatory neuron-autonomous ERK1/2 activity regulates forebrain development, we conditionally expressed a hyperactive MEK1 (MAP2K1) mutant, MEK1S217/221E, in cortical excitatory neurons of mice. MEK1S217/221E expression led to persistent hyperactivation of ERK1/2 in cortical axons, but not in soma/nuclei. We noted reduced axonal arborization in multiple target domains in mutant mice and reduced the levels of the activity-dependent protein ARC. These changes did not lead to deficits in voluntary locomotion or accelerating rotarod performance. However, skilled motor learning in a single-pellet retrieval task was significantly diminished in these MEK1S217/221E mutants. Restriction of MEK1S217/221E expression to layer V cortical neurons recapitulated axonal outgrowth deficits but did not affect motor learning. These results suggest that cortical excitatory neuron-autonomous hyperactivation of MEK1 is sufficient to drive deficits in axon outgrowth, which coincide with reduced ARC expression, and deficits in skilled motor learning. Our data indicate that neuron-autonomous decreases in long-range axonal outgrowth may be a key aspect of neuropathogenesis in RASopathies.

Interactions between Ras and Rap signaling pathways during neurodevelopment in health and disease

The Ras family of small GTPases coordinates tissue development by modulating cell proliferation, cell-cell adhesion, and cellular morphology. Perturbations of any of these key steps alter nervous system development and are associated with neurological disorders. While the underlying causes are not known, genetic mutations in Ras and Rap GTPase signaling pathways have been identified in numerous neurodevelopmental disorders, including autism spectrum, neurofibromatosis, intellectual disability, epilepsy, and schizophrenia. Despite diverse clinical presentations, intersections between these two signaling pathways may provide a better understanding of how deviations in neurodevelopment give rise to neurological disorders. In this review, we focus on presynaptic and postsynaptic functions of Ras and Rap GTPases. We highlight various roles of these small GTPases during synapse formation and plasticity. Based on genomic analyses, we discuss how disease-related mutations in Ras and Rap signaling proteins may underlie human disorders. Finally, we discuss how recent observations have identified molecular interactions between these pathways and how these findings may provide insights into the mechanisms that underlie neurodevelopmental disorders.

RAS Signaling Gone Awry in the Skin: The Complex Role of RAS in Cutaneous Neurofibroma Pathogenesis, Emerging Biological Insights

Cutaneous neurofibromas (cNFs) are the most common tumor in people with the rasopathy neurofibromatosis type 1. They number in hundreds or even thousands throughout the body, and currently, there are no effective interventions to prevent or treat these skin tumors. To facilitate the identification of novel and effective therapies, essential studies including a more refined understanding of cNF biology and the role of RAS signaling and downstream effector pathways responsible for cNF initiation, growth, and maintenance are needed. This review highlights the current state of knowledge of RAS signaling in cNF pathogenesis and therapeutic development for cNF treatment.

Existing and Developing Preclinical Models for Neurofibromatosis Type 1-Related Cutaneous Neurofibromas

Neurofibromatosis type 1 (NF1) is caused by a nonfunctional copy of the NF1 tumor suppressor gene that predisposes patients to the development of cutaneous neurofibromas (cNFs), the skin tumor that is the hallmark of this condition. Innumerable benign cNFs, each appearing by an independent somatic inactivation of the remaining functional NF1 allele, form in nearly all patients with NF1. One of the limitations in developing a treatment for cNFs is an incomplete understanding of the underlying pathophysiology and limitations in experimental modeling. Recent advances in preclinical in vitro and in vivo modeling have substantially enhanced our understanding of cNF biology and created unprecedented opportunities for therapeutic discovery. We discuss the current state of cNF preclinical in vitro and in vivo model systems, including two- and three-dimensional cell cultures, organoids, genetically engineered mice, patient-derived xenografts, and porcine models. We highlight the models' relationship to human cNFs and how they can be used to gain insight into cNF development and therapeutic discovery.

The therapeutic potential of neurofibromin signaling pathways and binding partners

Neurofibromin controls many cell processes, such as growth, learning, and memory. If neurofibromin is not working properly, it can lead to health problems, including issues with the nervous, skeletal, and cardiovascular systems and cancer. This review examines neurofibromin's binding partners, signaling pathways and potential therapeutic targets. In addition, it summarizes the different post-translational modifications that can affect neurofibromin's interactions with other molecules. It is essential to investigate the molecular mechanisms that underlie neurofibromin variants in order to provide with functional connections between neurofibromin and its associated proteins for possible therapeutic targets based on its biological function.

Role of nerves in neurofibromatosis type 1-related nervous system tumors

BACKGROUND

Neurofibromatosis type 1 (NF1) is an autosomal dominant genetic disorder that affects nearly 1 in 3000 infants. Neurofibromin inactivation and NF1 gene mutations are involved in various aspects of neuronal function regulation, including neuronal development induction, electrophysiological activity elevation, growth factor expression, and neurotransmitter release. NF1 patients often exhibit a predisposition to tumor development, especially in the nervous system, resulting in the frequent occurrence of peripheral nerve sheath tumors and gliomas. Recent evidence suggests that nerves play a role in the development of multiple tumor types, prompting researchers to investigate the nerve as a vital component in and regulator of the initiation and progression of NF1-related nervous system tumors.

CONCLUSION

In this review, we summarize existing evidence about the specific effects of NF1 mutation on neurons and emerging research on the role of nerves in neurological tumor development, promising a new set of selective and targeted therapies for NF1-related tumors.

Functional restoration of mouse Nf1 nonsense alleles in differentiated cultured neurons

Neurofibromatosis type 1 (NF1), one of the most common autosomal dominant genetic disorders, is caused by mutations in the NF1 gene. NF1 patients have a wide variety of manifestations with a subset at high risk for the development of tumors in the central nervous system (CNS). Nonsense mutations that result in the synthesis of truncated NF1 protein (neurofibromin) are strongly associated with CNS tumors. Therapeutic nonsense suppression with small molecule drugs is a potentially powerful approach to restore the expression of genes harboring nonsense mutations. Ataluren is one such drug that has been shown to restore full-length functional protein in several models of nonsense mutation diseases, as well as in patients with nonsense mutation Duchenne muscular dystrophy. To test ataluren's potential applicability to NF1 nonsense mutations associated with CNS tumors, we generated a homozygous Nf1^R683X/R683X-3X-FLAG mouse embryonic stem (mES) cell line which recapitulates an NF1 patient nonsense mutation (c.2041 C > T; p.Arg681X). We differentiated Nf1^R683X/R683X-3X-FLAG mES cells into cortical neurons in vitro, treated the cells with ataluren, and demonstrated that ataluren can promote readthrough of the nonsense mutation at codon 683 of Nf1 mRNA in neural cells. The resulting full-length protein is able to reduce the cellular level of hyperactive phosphorylated ERK (pERK), a RAS effector normally suppressed by the NF1 protein.

Mechanistic insights from animal models of neurofibromatosis type 1 cognitive impairment

Neurofibromatosis type 1 (NF1) is an autosomal-dominant neurogenetic disorder caused by mutations in the gene neurofibromin 1 (NF1). NF1 predisposes individuals to a variety of symptoms, including peripheral nerve tumors, brain tumors and cognitive dysfunction. Cognitive deficits can negatively impact patient quality of life, especially the social and academic development of children. The neurofibromin protein influences neural circuits via diverse cellular signaling pathways, including through RAS, cAMP and dopamine signaling. Although animal models have been useful in identifying cellular and molecular mechanisms that regulate NF1-dependent behaviors, translating these discoveries into effective treatments has proven difficult. Clinical trials measuring cognitive outcomes in patients with NF1 have mainly targeted RAS signaling but, unfortunately, resulted in limited success. In this Review, we provide an overview of the structure and function of neurofibromin, and evaluate several cellular and molecular mechanisms underlying neurofibromin-dependent cognitive function, which have recently been delineated in animal models. A better understanding of neurofibromin roles in the development and function of the nervous system will be crucial for identifying new therapeutic targets for the various cognitive domains affected by NF1.

Summary: Neurofibromin influences neural circuits through RAS, cAMP and dopamine signaling. Exploring the mechanisms underlying neurofibromin-dependent behaviors in animal models might enable future treatment of the various cognitive deficits that are associated with neurofibromatosis type 1.

A systematic review and meta-analysis of intellectual, neuropsychological, and psychoeducational functioning in neurofibromatosis type 1

Neurofibromatosis Type 1 (NF1) is a common genetic disorder frequently associated with cognitive deficits. Despite cognitive deficits being a key feature of NF1, the profile of such impairments in NF1 has been shown to be heterogeneous. Thus, we sought to quantitatively synthesize the extant literature on cognitive functioning in NF1. A random-effects meta-analysis of cross-sectional studies was carried out comparing cognitive functioning of patients with NF1 to typically developing or unaffected sibling comparison subjects of all ages. Analyses included 50 articles (Total N_NF1 = 1,522; M_Age = 15.70 years, range = 0.52-69.60), yielding 460 effect sizes. Overall moderate deficits were observed [g = -0.64, 95% CI = (-0.69, -0.60)] wherein impairments differed at the level of cognitive domain. Deficits ranged from large [general intelligence: g = -0.95, 95% CI = (-1.12, -0.79)] to small [emotion: g = -0.37, 95% CI = (-0.63, -0.11)]. Moderation analyses revealed nonsignificant contributions of age, sex, educational attainment, and parental level of education to outcomes. These results illustrate that cognitive impairments are diffuse and salient across the lifespan in NF1. Taken together, these results further demonstrate efforts should be made to evaluate and address cognitive morbidity in patients with NF1 in conjunction with existing best practices.

RAS and beyond: the many faces of the neurofibromatosis type 1 protein

Neurofibromatosis type 1 is a rare neurogenetic syndrome, characterized by pigmentary abnormalities, learning and social deficits, and a predisposition for benign and malignant tumor formation caused by germline mutations in the NF1 gene. With the cloning of the NF1 gene and the recognition that the encoded protein, neurofibromin, largely functions as a negative regulator of RAS activity, attention has mainly focused on RAS and canonical RAS effector pathway signaling relevant to disease pathogenesis and treatment. However, as neurofibromin is a large cytoplasmic protein the RAS regulatory domain of which occupies only 10% of its entire coding sequence, both canonical and non-canonical RAS pathway modulation, as well as the existence of potential non-RAS functions, are becoming apparent. In this Special article, we discuss our current understanding of neurofibromin function.

Feedback-Based Learning of Timing in Attention-Deficit/Hyperactivity Disorder and Neurofibromatosis Type 1

OBJECTIVE

Patients with Neurofibromatosis Type 1 (NF1) frequently display symptoms resembling those of Attention Deficit/Hyperactivity Disorder (ADHD). Importantly, these disorders are characterised by distinct changes in the dopaminergic system, which plays an important role in timing performance and feedback-based adjustments in timing performance. In a transdiagnostic approach, we examine how far NF1 and ADHD show distinct or comparable profiles of timing performance and feedback-based adjustments in timing.

METHOD

We examined time estimation and learning processes in healthy control children (HC), children with ADHD with predominantly inattentive symptoms and those with NF1 using a feedback-based time estimation paradigm.

RESULTS

Healthy controls consistently responded closer to the correct time window than both patient groups, were less variable in their reaction times and displayed intact learning-based adjustments across time. The patient groups did not differ from each other regarding the number of in-time responses. In ADHD patients, the performance was rather unstable across time. No performance changes could be observed in patients with NF1 across the entire task.

CONCLUSIONS

Children with ADHD and NF1 differ in feedback learning-based adjustments of time estimation processes. ADHD is characterised by behavioural fluctuations during the learning process. These are likely to be associated with inefficiencies in the dopaminergic system. NF1 is characterised by impairments of feedback learning which could be due to various neurotransmitter alterations occurring in addition to deficits in dopamine synthesis. Results show that despite the strong overlap in clinical phenotype and neuropsychological deficits between NF1 and ADHD, the underlying cognitive mechanisms are different.

Hyperactive MEK1 Signaling in Cortical GABAergic Neurons Promotes Embryonic Parvalbumin Neuron Loss and Defects in Behavioral Inhibition

Many developmental syndromes have been linked to genetic mutations that cause abnormal ERK/MAPK activity; however, the neuropathological effects of hyperactive signaling are not fully understood. Here, we examined whether hyperactivation of MEK1 modifies the development of GABAergic cortical interneurons (CINs), a heterogeneous population of inhibitory neurons necessary for cortical function. We show that GABAergic-neuron specific MEK1 hyperactivation in vivo leads to increased cleaved caspase-3 labeling in a subpopulation of immature neurons in the embryonic subpallial mantle zone. Adult mutants displayed a significant loss of parvalbumin (PV), but not somatostatin, expressing CINs and a reduction in perisomatic inhibitory synapses on excitatory neurons. Surviving mutant PV-CINs maintained a typical fast-spiking phenotype but showed signs of decreased intrinsic excitability that coincided with an increased risk of seizure-like phenotypes. In contrast to other mouse models of PV-CIN loss, we discovered a robust increase in the accumulation of perineuronal nets, an extracellular structure thought to restrict plasticity. Indeed, we found that mutants exhibited a significant impairment in the acquisition of behavioral response inhibition capacity. Overall, our data suggest PV-CIN development is particularly sensitive to hyperactive MEK1 signaling, which may underlie certain neurological deficits frequently observed in ERK/MAPK-linked syndromes.

Neurofibromin Structure, Functions and Regulation

Neurofibromin is a large and multifunctional protein encoded by the tumor suppressor gene NF1, mutations of which cause the tumor predisposition syndrome neurofibromatosis type 1 (NF1). Over the last three decades, studies of neurofibromin structure, interacting partners, and functions have shown that it is involved in several cell signaling pathways, including the Ras/MAPK, Akt/mTOR, ROCK/LIMK/cofilin, and cAMP/PKA pathways, and regulates many fundamental cellular processes, such as proliferation and migration, cytoskeletal dynamics, neurite outgrowth, dendritic-spine density, and dopamine levels. The crystallographic structure has been resolved for two of its functional domains, GRD (GAP-related (GTPase-activating protein) domain) and SecPH, and its post-translational modifications studied, showing it to be localized to several cell compartments. These findings have been of particular interest in the identification of many therapeutic targets and in the proposal of various therapeutic strategies to treat the symptoms of NF1. In this review, we provide an overview of the literature on neurofibromin structure, function, interactions, and regulation and highlight the relationships between them.

An update on the central nervous system manifestations of neurofibromatosis type 1

Neurofibromatosis 1 (NF1) is an autosomal dominant genetic disorder that presents with variable phenotypes as a result of mutations in the neurofibromatosis type 1 (NF1) gene and subsequently, abnormal function of the protein product, neurofibromin. Patients with NF1 are at increased risk for central nervous system (CNS) manifestations including structural, functional, and neoplastic disease. The mechanisms underlying the varied manifestations of NF1 are incompletely understood, but the loss of functional neurofibromin, resulting in sustained activation of the oncoprotein RAS, is responsible for tumorigenesis throughout the body, including the CNS. Much of our understanding of NF1-related CNS manifestations is from a combination of data from animal models and natural history studies of people with NF1 and CNS disease. Data from animal models suggest the importance of both Nf1 mutations and somatic genetic alterations, such as Tp53 loss, for development of neoplasms, as well as the role of the timing of the acquisition of such alterations on the variability of CNS manifestations. A variety of non-neoplastic structural (macrocephaly, hydrocephalus, aqueductal stenosis, and vasculopathy) and functional (epilepsy, impaired cognition, attention deficits, and autism spectrum disorder) abnormalities occur with variable frequency in individuals with NF1. In addition, there is increasing evidence that similar appearing CNS neoplasms in people with and without the NF1 syndrome are due to distinct oncogenic pathways. Gliomas in people with NF1 show alterations in the RAS/MAPK pathway, generally in the absence of BRAF alterations (common to sporadic pilocytic astrocytomas) or IDH or histone H3 mutations (common to diffuse gliomas subsets). A subset of low-grade astrocytomas in these patients remain difficult to classify using standard criteria, and occasionally demonstrate morphologic features resembling subependymal giant cell astrocytomas that afflict patients with tuberous sclerosis complex ("SEGA-like astrocytomas"). There is also emerging evidence that NF1-associated high-grade astrocytomas have frequent co-existing alterations such as ATRX mutations and an alternative lengthening of telomeres (ALT) phenotype responsible for unique biologic properties. Ongoing efforts are seeking to improve diagnostic accuracy for CNS neoplasms in the setting of NF1 versus sporadic tumors. In addition, MEK inhibitors, which act on the RAS/MAPK pathway, continue to be studied as rational targets for the treatment of NF1-associated tumors, including CNS tumors.

Optical dopamine monitoring with dLight1 reveals mesolimbic phenotypes in a mouse model of neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder whose neurodevelopmental symptoms include impaired executive function, attention, and spatial learning and could be due to perturbed mesolimbic dopaminergic circuitry. However, these circuits have never been directly assayed in vivo. We employed the genetically encoded optical dopamine sensor dLight1 to monitor dopaminergic neurotransmission in the ventral striatum of NF1 mice during motivated behavior. Additionally, we developed novel systemic AAV vectors to facilitate morphological reconstruction of dopaminergic populations in cleared tissue. We found that NF1 mice exhibit reduced spontaneous dopaminergic neurotransmission that was associated with excitation/inhibition imbalance in the ventral tegmental area and abnormal neuronal morphology. NF1 mice also had more robust dopaminergic and behavioral responses to salient visual stimuli, which were independent of learning, and rescued by optogenetic inhibition of non-dopaminergic neurons in the VTA. Overall, these studies provide a first in vivo characterization of dopaminergic circuit function in the context of NF1 and reveal novel pathophysiological mechanisms.

Neurodevelopmental Aspects of RASopathies

RAS gene mutations are frequently found in one third of human cancers. Affecting approximately 1 in 1,000 newborns, germline and somatic gain-of-function mutations in the components of RAS/mitogen-activated protein kinase (RAS/MAPK) pathway has been shown to cause developmental disorders, known as RASopathies. Since RAS-MAPK pathway plays essential roles in proliferation, differentiation and migration involving developmental processes, individuals with RASopathies show abnormalities in various organ systems including central nervous system. The frequently seen neurological defects are developmental delay, macrocephaly, seizures, neurocognitive deficits, and structural malformations. Some of the defects stemmed from dysregulation of molecular and cellular processes affecting early neurodevelopmental processes. In this review, we will discuss the implications of RAS-MAPK pathway components in neurodevelopmental processes and pathogenesis of RASopathies.

The Noonan Syndrome-linked Raf1L613V mutation drives increased glial number in the mouse cortex and enhanced learning

RASopathies are a family of related syndromes caused by mutations in regulators of the RAS/Extracellular Regulated Kinase 1/2 (ERK1/2) signaling cascade that often result in neurological deficits. RASopathy mutations in upstream regulatory components, such as NF1, PTPN11/SHP2, and RAS have been well-characterized, but mutation-specific differences in the pathogenesis of nervous system abnormalities remain poorly understood, especially those involving mutations downstream of RAS. Here, we assessed cellular and behavioral phenotypes in mice expressing a Raf1^L613V gain-of-function mutation associated with the RASopathy, Noonan Syndrome. We report that Raf1^L613V/wt mutants do not exhibit a significantly altered number of excitatory or inhibitory neurons in the cortex. However, we observed a significant increase in the number of specific glial subtypes in the forebrain. The density of GFAP⁺ astrocytes was significantly increased in the adult Raf1^L613V/wt cortex and hippocampus relative to controls. OLIG2⁺ oligodendrocyte progenitor cells were also increased in number in mutant cortices, but we detected no significant change in myelination. Behavioral analyses revealed no significant changes in voluntary locomotor activity, anxiety-like behavior, or sociability. Surprisingly, Raf1^L613V/wt mice performed better than controls in select aspects of the water radial-arm maze, Morris water maze, and cued fear conditioning tasks. Overall, these data show that increased astrocyte and oligodendrocyte progenitor cell (OPC) density in the cortex coincides with enhanced cognition in Raf1^L613V/wt mutants and further highlight the distinct effects of RASopathy mutations on nervous system development and function.

The RASopathies are a large and complex family of syndromes caused by mutations in the RAS/MAPK signaling cascade with no known cure. Individuals with these syndromes often present with heart defects, craniofacial differences, and neurological abnormalities, such as developmental delay, cognitive changes, epilepsy, and an increased risk of autism. However, there is wide variation in the extent of intellectual ability between individuals. It is currently unclear how different RASopathy mutations affect brain development. Here, we describe the cellular and behavioral consequences of a mutation in a gene called Raf1 that is associated with a common RASopathy, Noonan Syndrome. We find that mice harboring a mutation in Raf1 show moderate increases in the number of two subsets of glial cells, which is also observed in a number of other RASopathy brain samples. Surprisingly, we found that Raf1 mutant mice show improved performance in several learning and memory tasks. Our work highlights potential mutation-specific changes in RASopathy brain function and helps set the framework for future personalized therapeutic approaches.

A novel NF1 frame-shift mutation c.703_704delTA in a Chinese pedigree with neurofibromatosis type 1

We analyzed the clinical features and NF1 gene mutation in a Chinese pedigree of neurofibromatosis type 1 (NF1). Three members of this family were NF1 patients presenting with different clinical phenotypes and the others were asymptomatic. Exons of NF1 were amplified by polymerase chain reaction, sequenced, compared with a reference database. One novel NF1 frame-shift mutation c.703_704delTA, which resulted in a premature stop signal at codon 720 and the synthesis of truncated, was revealed. This mutation segregated with the NF1 members is likely responsible for the pathogenesis of NF1 in the family.

Status of KRAS in iPSCs Impacts upon Self-Renewal and Differentiation Propensity

Oncogenic KRAS mutations in hematopoietic stem cells cause RAS-associated autoimmune lymphoproliferative syndrome-like disease (RALD). KRAS plays essential roles in stemness maintenance in some types of stem cells. However, its roles in pluripotent stem cells (PSCs) are poorly understood. Here, we investigated the roles of KRAS on stemness in the context of induced PSCs (iPSCs). We used KRAS mutant (G13C/WT) and wild-type isogenic (WT/WT) iPSCs from the same RALD patients, as well as wild-type (WT^ed/WT) and heterozygous knockout (Δ^ed/WT) iPSCs, both obtained by genome editing from the same G13C/WT clone. Compared with WT iPSCs, G13C/WT iPSCs displayed enforced retention of self-renewal and suppressed capacity for neuronal differentiation, while Δ^ed/WT iPSCs showed normalized cellular characteristics similar to those of isogenic WT^ed/WT cells. The KRAS-ERK pathway, but not the KRAS-PI3K pathway, was shown to govern these G13C/WT-specific phenotypes, indicating the strong impact of the KRAS-ERK signaling upon self-renewal and differentiation propensity in human iPSCs.

Mek1Y130C mice recapitulate aspects of human cardio-facio-cutaneous syndrome

The RAS/MAPK signaling pathway is one of the most investigated pathways, owing to its established role in numerous cellular processes and implication in cancer. Germline mutations in genes encoding members of the RAS/MAPK pathway also cause severe developmental syndromes collectively known as RASopathies. These syndromes share overlapping characteristics, including craniofacial dysmorphology, cardiac malformations, cutaneous abnormalities and developmental delay. Cardio-facio-cutaneous syndrome (CFC) is a rare RASopathy associated with mutations in BRAF, KRAS, MEK1 (MAP2K1) and MEK2 (MAP2K2). MEK1 and MEK2 mutations are found in ∼25% of the CFC patients and the MEK1^Y130C substitution is the most common one. However, little is known about the origins and mechanisms responsible for the development of CFC. To our knowledge, no mouse model carrying RASopathy-linked Mek1 or Mek2 gene mutations has been reported. To investigate the molecular and developmental consequences of the Mek1^Y130C mutation, we generated a mouse line carrying this mutation. Analysis of mice from a Mek1 allelic series revealed that the Mek1^Y130C allele expresses both wild-type and Y130C mutant forms of MEK1. However, despite reduced levels of MEK1 protein and the lower abundance of MEK1 Y130C protein than wild type, Mek1^Y130C mutants showed increased ERK (MAPK) protein activation in response to growth factors, supporting a role for MEK1 Y130C in hyperactivation of the RAS/MAPK pathway, leading to CFC. Mek1^Y130C mutant mice exhibited pulmonary artery stenosis, cranial dysmorphia and neurological anomalies, including increased numbers of GFAP⁺ astrocytes and Olig2⁺ oligodendrocytes in regions of the cerebral cortex. These data indicate that the Mek1^Y130C mutation recapitulates major aspects of CFC, providing a new animal model to investigate the physiopathology of this RASopathy.

This article has an associated First Person interview with the first author of the paper.

Summary: A mouse model for cardio-facio-cutaneous syndrome caused by MEK1 Y130C mutant protein reveals the role of hyperactivation of the RAS/MAPK pathway in the development of the syndrome.

Dopaminergic dysfunction in neurodevelopmental disorders: recent advances and synergistic technologies to aid basic research

Neurodevelopmental disorders (NDDs) represent a diverse group of syndromes characterized by abnormal development of the central nervous system and whose symptomatology includes cognitive, emotional, sensory, and motor impairments. The identification of causative genetic defects has allowed for creation of transgenic NDD mouse models that have revealed pathophysiological mechanisms of disease phenotypes in a neural circuit- and cell type-specific manner. Mouse models of several syndromes, including Rett syndrome, Fragile X syndrome, Angelman syndrome, Neurofibromatosis type 1, etc., exhibit abnormalities in the structure and function of dopaminergic circuitry, which regulates motivation, motor behavior, sociability, attention, and executive function. Recent advances in technologies for functional circuit mapping, including tissue clearing, viral vector-based tracing methods, and optical readouts of neural activity, have refined our knowledge of dopaminergic circuits in unperturbed states, yet these tools have not been widely applied to NDD research. Here, we will review recent findings exploring dopaminergic function in NDD models and discuss the promise of new tools to probe NDD pathophysiology in these circuits.

Fundamental Elements in Autism: From Neurogenesis and Neurite Growth to Synaptic Plasticity

Autism spectrum disorder (ASD) is a set of neurodevelopmental disorders with a high prevalence and impact on society. ASDs are characterized by deficits in both social behavior and cognitive function. There is a strong genetic basis underlying ASDs that is highly heterogeneous; however, multiple studies have highlighted the involvement of key processes, including neurogenesis, neurite growth, synaptogenesis and synaptic plasticity in the pathophysiology of neurodevelopmental disorders. In this review article, we focus on the major genes and signaling pathways implicated in ASD and discuss the cellular, molecular and functional studies that have shed light on common dysregulated pathways using in vitro, in vivo and human evidence.

Highlights

Autism spectrum disorder (ASD) has a prevalence of 1 in 68 children in the United States.

ASDs are highly heterogeneous in their genetic basis.

ASDs share common features at the cellular and molecular levels in the brain.

Most ASD genes are implicated in neurogenesis, structural maturation, synaptogenesis and function.

Response inhibition in Attention deficit disorder and neurofibromatosis type 1 - clinically similar, neurophysiologically different

There are large overlaps in cognitive deficits occurring in attention deficit disorder (ADD) and neurodevelopmental disorders like neurofibromatosis type 1 (NF1). This overlap is mostly based on clinical measures and not on in-depth analyses of neuronal mechanisms. However, the consideration of such neuronal underpinnings is crucial when aiming to integrate measures that can lead to a better understanding of the underlying mechanisms. Inhibitory control deficits, for example, are a hallmark in ADD, but it is unclear how far there are similar deficits in NF1. We thus compared adolescent ADD and NF1 patients to healthy controls in a Go/Nogo task using behavioural and neurophysiological measures. Clinical measures of ADD-symptoms were not different between ADD and NF1. Only patients with ADD showed increased Nogo errors and reductions in components reflecting response inhibition (i.e. Nogo-P3). Early perceptual processes (P1) were changed in ADD and NF1. Clinically, patients with ADD and NF1 thus show strong similarities. This is not the case in regard to underlying cognitive control processes. This shows that in-depth analyses of neurophysiological processes are needed to determine whether the overlap between ADD and NF1 is as strong as assumed and to develop appropriate treatment strategies.

Impairments in dendrite morphogenesis as etiology for neurodevelopmental disorders and implications for therapeutic treatments

Dendrite morphology is pivotal for neural circuitry functioning. While the causative relationship between small-scale dendrite morphological abnormalities (shape, density of dendritic spines) and neurodevelopmental disorders is well established, such relationship remains elusive for larger-scale dendrite morphological impairments (size, shape, branching pattern of dendritic trees). Here, we summarize published data on dendrite morphological irregularities in human patients and animal models for neurodevelopmental disorders, with focus on autism and schizophrenia. We next discuss high-risk genes for these disorders and their role in dendrite morphogenesis. We finally overview recent developments in therapeutic attempts and we discuss how they relate to dendrite morphology. We find that both autism and schizophrenia are accompanied by dendritic arbor morphological irregularities, and that majority of their high-risk genes regulate dendrite morphogenesis. Thus, we present a compelling argument that, along with smaller-scale morphological impairments in dendrites (spines and synapse), irregularities in larger-scale dendrite morphology (arbor shape, size) may be an important part of neurodevelopmental disorders' etiology. We suggest that this should not be ignored when developing future therapeutic treatments.

Neurofibromin Regulates Seizure Attacks in the Rat Pilocarpine-Induced Model of Epilepsy

Studies have shown that neurofibromin (NF1) restricts GABA release at inhibitory synapses and regulates dendritic spine formation, which may play an important role in temporal lobe epilepsy (TLE). NF1 expression was detected by double-label immunofluorescence, immunohistochemistry, and western blot analysis in the brains of pilocarpine-induced epilepsy model rats at 6 h, 24 h, 72 h, 7 days, 14 days, 30 days, and 60 days after kindling. NF1 was localized primarily in the nucleus and cytoplasm of neurons. NF1 protein levels significantly increased in the chronic phase (from 7 days until 60 days) in this epileptic rat model. After NF1 expression was knocked down by specific siRNA, the effects of kindling with pilocarpine were evaluated on the 7th day after kindling. The onset latencies of pilocarpine-induced seizures were elevated, and the seizure frequency and duration were reduced in these rats. Our study demonstrates that NF1 promoted seizure attacks in rats with pilocarpine-induced epilepsy.

Apaf1-deficient cortical neurons exhibit defects in axonal outgrowth

The establishment of neuronal polarity and axonal outgrowth are key processes affecting neuronal migration and synapse formation, their impairment likely leading to cognitive deficits. Here we have found that the apoptotic protease activating factor 1 (Apaf1), apart from its canonical role in apoptosis, plays an additional function in cortical neurons, where its deficiency specifically impairs axonal growth. Given the central role played by centrosomes and microtubules in the polarized extension of the axon, our data suggest that Apaf1-deletion affects axonal outgrowth through an impairment of centrosome organization. In line with this, centrosomal protein expression, as well as their centrosomal localization proved to be altered upon Apaf1-deletion. Strikingly, we also found that Apaf1-loss affects trans-Golgi components and leads to a robust activation of AMP-dependent protein kinase (AMPK), this confirming the stressful conditions induced by Apaf1-deficiency. Since AMPK hyper-phosphorylation is known to impair a proper axon elongation, our finding contributes to explain the effect of Apaf1-deficiency on axogenesis. We also discovered that the signaling pathways mediating axonal growth and involving glycogen synthase kinase-3β, liver kinase B1, and collapsing-response mediator protein-2 are altered in Apaf1-KO neurons. Overall, our results reveal a novel non-apoptotic role for Apaf1 in axonal outgrowth, suggesting that the neuronal phenotype due to Apaf1-deletion could not only be fully ascribed to apoptosis inhibition, but might also be the result of defects in axogenesis. The discovery of new molecules involved in axonal elongation has a clinical relevance since it might help to explain neurological abnormalities occurring during early brain development.

Rho kinase is required to prevent retinal axons from entering the contralateral optic nerve

To grow out to contact target neurons an axon uses its distal tip, the growth cone, as a sensor of molecular cues that help the axon make appropriate guidance decisions at a series of choice points along the journey. In the developing visual system, the axons of the output cells of the retina, the retinal ganglion cells (RGCs), cross the brain midline at the optic chiasm. Shortly after, they grow past the brain entry point of the optic nerve arising from the contralateral eye, and extend dorso-caudally through the diencephalon towards their optic tectum target. Using the developing visual system of the experimentally amenable model Xenopus laevis, we find that RGC axons are normally prevented from entering the contralateral optic nerve. This mechanism requires the activity of a Rho-associated kinase, Rock, known to function downstream of a number of receptors that recognize cues that guide axons. Pharmacological inhibition of Rock in an in vivo brain preparation causes mis-entry of many RGC axons into the contralateral optic nerve, and this defect is partially phenocopied by selective disruption of Rock signaling in RGC axons. These data implicate Rock downstream of a molecular mechanism that is critical for RGC axons to be able to ignore a domain, the optic nerve, which they previously found attractive.

Axon regeneration impediment: the role of paired immunoglobulin-like receptor B

Regenerative capacity is weak after central nervous system injury because of the absence of an enhancing microenvironment and presence of an inhibitory microenvironment for neuronal and axonal repair. In addition to the Nogo receptor (NgR), the paired immunoglobulin-like receptor B (PirB) is a recently discovered coreceptor of Nogo, myelin-associated glycoprotein, and myelin oligodendrocyte glycoprotein. Concurrent blocking of NgR and PirB almost completely eliminates the inhibitory effect of myelin-associated inhibitory molecules on axonal regeneration. PirB participates in a key pathological process of the nervous system, specifically axonal regeneration inhibition. PirB is an inhibitory receptor similar to NgR, but their effects are not identical. This study summarizes the structure, distribution, relationship with common nervous system diseases, and known mechanisms of PirB, and concludes that PirB is also distributed in cells of the immune and hematopoietic systems. Further investigations are needed to determine if immunomodulation and blood cell migration involve inhibition of axonal regeneration.

In vitro modeling of hyperpigmentation associated to neurofibromatosis type 1 using melanocytes derived from human embryonic stem cells

"Café-au-lait" macules (CALMs) and overall skin hyperpigmentation are early hallmarks of neurofibromatosis type 1 (NF1). One of the most frequent monogenic diseases, NF1 has subsequently been characterized with numerous benign Schwann cell-derived tumors. It is well established that neurofibromin, the NF1 gene product, is an antioncogene that down-regulates the RAS oncogene. In contrast, the molecular mechanisms associated with alteration of skin pigmentation have remained elusive. We have reassessed this issue by differentiating human embryonic stem cells into melanocytes. In the present study, we demonstrate that NF1 melanocytes reproduce the hyperpigmentation phenotype in vitro, and further characterize the link between loss of heterozygosity and the typical CALMs that appear over the general hyperpigmentation. Molecular mechanisms associated with these pathological phenotypes correlate with an increased activity of cAMP-mediated PKA and ERK1/2 signaling pathways, leading to overexpression of the transcription factor MITF and of the melanogenic enzymes tyrosinase and dopachrome tautomerase, all major players in melanogenesis. Finally, the hyperpigmentation phenotype can be rescued using specific inhibitors of these signaling pathways. These results open avenues for deciphering the pathological mechanisms involved in pigmentation diseases, and provide a robust assay for the development of new strategies for treating these diseases.

Multitarget intervention of Fasudil in the neuroprotection of dopaminergic neurons in MPTP-mouse model of Parkinson's disease

Recent studies have demonstrated that activation of the Rho-associated kinase (ROCK) pathway participates in the dopaminergic neuron degeneration and possibly in Parkinson's disease (PD). In the current study, we tried to observe the therapeutic potential of ROCK inhibitor Fasudil against dopaminergic neuron injury in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-mouse model of PD, and explore possible molecular mechanisms by enzyme-linked immunosorbent assay (ELISA), western blot and immunofluorescent assays. The results showed that MPTP-PD mice presented motor deficits, dopaminergic neuron loss, activation of inflammatory response and oxidative stress as well as ROCK and glycogen synthase kinase 3β (GSK-3β) signaling pathways. The administration of Fasudil exhibited neuroprotective effects against the dopaminergic neurons and improved the motor function recovery in the MPTP-PD mice, accompanied by the suppression of inflammatory responses (IL-1β, TNF-α, NF-κB-p65 and TLR-2), and oxidative stress (iNOS and gp91Phox), which might be associated with the inhibition of ROCK and GSK-3β activity. Simultaneously, the administration of Fasudil resulted in the shift from inflammatory M1 to anti-inflammatory/neurorepair M2 microglia. Additionally, Fasudil intervention enhanced the expression of anti-oxidative factors such as NF-E2-related factor 2 (Nrf2), Hmox as well as neurotrophic factor including GDNF. Our observations defined the neuroprotective effects of Fasudil in MPTP-PD mice, and we found a series of novel effector molecules and pathways for explaining the neuroprotective effects against dopaminergic neurons. However, a lot of investigations are warranted to further elucidate the crosstalk among Fasudil, oxidative stress, inflammatory response, GDNF and ROCK/NF-kB/Nrf2 pathways in the therapeutic potential of PD.

Metabolic consequences of interleukin-6 challenge in developing neurons and astroglia

Background

Maternal immune activation and subsequent interleukin-6 (IL-6) induction disrupt normal brain development and predispose the offspring to developing autism and schizophrenia. While several proteins have been identified as having some link to these developmental disorders, their prevalence is still small and their causative role, if any, is not well understood. However, understanding the metabolic consequences of environmental predisposing factors could shed light on disorders such as autism and schizophrenia.

Methods

To gain a better understanding of the metabolic consequences of IL-6 exposure on developing central nervous system (CNS) cells, we separately exposed developing neuron and astroglia cultures to IL-6 for 2 hours while collecting effluent from our gravity-fed microfluidic chambers. By coupling microfluidic technologies to ultra-performance liquid chromatography-ion mobility-mass spectrometry (UPLC-IM-MS), we were able to characterize the metabolic response of these CNS cells to a narrow window of IL-6 exposure.

Results

Our results revealed that 1) the use of this technology, due to its superb media volume:cell volume ratio, is ideally suited for analysis of cell-type-specific exometabolome signatures; 2) developing neurons have low secretory activity at baseline, while astroglia show strong metabolic activity; 3) both neurons and astroglia respond to IL-6 exposure in a cell type-specific fashion; 4) the astroglial response to IL-6 stimulation is predominantly characterized by increased levels of metabolites, while neurons mostly depress their metabolic activity; and 5) disturbances in glycerophospholipid metabolism and tryptophan/kynurenine metabolite secretion are two putative mechanisms by which IL-6 affects the developing nervous system.

Conclusions

Our findings are potentially critical for understanding the mechanism by which IL-6 disrupts brain function, and they provide information about the molecular cascade that links maternal immune activation to developmental brain disorders.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-014-0183-6) contains supplementary material, which is available to authorized users.

Cross-talk between neural and immune receptors provides a potential mechanism of homeostatic regulation in the gut mucosa

The relationship between elements of the immune system and the nervous system in the presence of bacteria has been addressed recently. In particular, the sensory vanilloid receptor 1 (transient receptor potential cation channel subfamily V member 1 (TRPV1)) and the neuropeptide calcitonin gene-related peptide (CGRP) have been found to modulate cytokine response to lipopolysaccharide (LPS) independently of adaptive immunity. In this review we discuss mucosal homeostasis in the gastrointestinal tract where bacterial concentration is high. We propose that the Gram-negative bacterial receptor Toll-like receptor 4 (TLR4) can activate TRPV1 via intracellular signaling, and thereby induce the subsequent release of anti-inflammatory CGRP to maintain mucosal homeostasis.

Exendin-4 alleviates angiotensin II-induced senescence in vascular smooth muscle cells by inhibiting Rac1 activation via a cAMP/PKA-dependent pathway

Vascular aging has been implicated in the progression of diabetes and age-related cardiovascular disorders. Glucagon-like peptide-1 (GLP-1) is an incretin hormone capable of cytoprotective actions in addition to its glucose-lowering effect. The present study was undertaken to examine whether Exendin-4, a specific ligand for the GLP-1 receptor, could prevent angiotensin (ANG) II-induced premature senescence in vascular smooth muscle cells (VSMCs) and to determine the underlying mechanism involved. Senescence-associated β-galactosidase (SA β-gal) assay showed that ANG II induced premature senescence of VSMCs. Pretreatment with Exendin-4 significantly attenuated ANG II-induced generation of H2O2 and the subsequent VSMC senescence. These effects were, however, reversed in the presence of exendin fragment 9-39, a GLP-1 receptor antagonist, or PKI14-22. Moreover, a marked increase in the levels of p53 and p21 induced by ANG II was blunted by the treatment with Exendin-4. Nevertheless, Exendin-4 failed to decrease ANG II-induced expression of NAD(P)H oxidase 1 (Nox1), NAD(P)H oxidase 4 (Nox4), p22(phox), or p47(phox) in VSMCs. Mechanistically, Exendin-4 blocked ANG II-induced Rac1 activation through the cAMP/PKA signaling cascade. Specifically, NSC23766, a Rac1 inhibitor, abrogated the suppressive effects of Exendin-4 on ANG II-induced premature senescence and H2O2 generation, respectively. Thus Exendin-4 confers resistance to ANG II-induced superoxide anion generation from NAD(P)H oxidase and the resultant VSMC senescence by inhibiting Rac1 activation via a cAMP/PKA-dependent pathway. These findings demonstrate that GLP-1 as well as its analogs (GLP-1-related reagents) may hold therapeutic potential in the treatment of diabetes with cardiovascular disease.

Modulation of cAMP and ras signaling pathways improves distinct behavioral deficits in a zebrafish model of neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is a common autosomal-dominant disorder associated with attention deficits and learning disabilities. The primary known function of neurofibromin, encoded by the NF1 gene, is to downregulate Ras activity. We show that nf1-deficient zebrafish exhibit learning and memory deficits and that acute pharmacological inhibition of downstream targets of Ras (MAPK and PI3K) restores memory consolidation and recall but not learning. Conversely, acute pharmacological enhancement of cAMP signaling restores learning but not memory. Our data provide compelling evidence that neurofibromin regulates learning and memory by distinct molecular pathways in vertebrates and that deficits produced by genetic loss of function are reversible. These findings support the investigation of cAMP signaling enhancers as a companion therapy to Ras inhibition in the treatment of cognitive dysfunction in NF1.

Using the neurofibromatosis tumor predisposition syndromes to understand normal nervous system development

Development is a tightly regulated process that involves stem cell self-renewal, differentiation, cell-to-cell communication, apoptosis, and blood vessel formation. These coordinated processes ensure that tissues maintain a size and architecture that is appropriate for normal tissue function. As such, tumors arise when cells acquire genetic mutations that allow them to escape the normal growth constraints. In this regard, the study of tumor predisposition syndromes affords a unique platform to better understand normal development and the process by which normal cells transform into cancers. Herein, we review the processes governing normal brain development, discuss how brain cancer represents a disruption of these normal processes, and highlight insights into both normal development and cancer made possible by the study of tumor predisposition syndromes.

Cognitive and behavioral problems in children with neurofibromatosis type 1: challenges and future directions

Cognitive and behavioral disorders affect nearly 80% of all children with the neurofibromatosis type 1 inherited cancer syndrome, and are among the most significant clinical manifestations for patients and their families. One of the barriers to successful therapeutic intervention is the wide spectrum of clinical phenotypic expression, ranging from visuospatial learning problems to social perceptual deficits (autism). Leveraging numerous small-animal models of neurofibromatosis type 1, several promising targets have been identified to treat the learning, attention, and autism spectrum phenotypes in this at-risk population. In this review, we provide an up-to-date summary of our current understanding of these disorders in NF1, and propose future research directions aimed at designing more effective therapeutic approaches and clinical trials.

Neuronal NF1/RAS regulation of cyclic AMP requires atypical PKC activation

Neurofibromatosis type 1 (NF1) is a common neurodevelopmental disorder in which affected individuals are prone to learning, attention and behavioral problems. Previous studies in mice and flies have yielded conflicting results regarding the specific effector pathways responsible for NF1 protein (neurofibromin) regulation of neuronal function, with both cyclic AMP (cAMP)- and RAS-dependent mechanisms described. Herein, we leverage a combination of induced pluripotent stem cell-derived NF1 patient neural progenitor cells and Nf1 genetically engineered mice to establish, for the first time, that neurofibromin regulation of cAMP requires RAS activation in human and mouse neurons. However, instead of involving RAS-mediated MEK/AKT signaling, RAS regulation of cAMP homeostasis operates through the activation of atypical protein kinase C zeta, leading to GRK2-driven Gαs inactivation. These findings reveal a novel mechanism by which RAS can regulate cAMP levels in the mammalian brain.

Neurofibromatosis type 1 alternative splicing is a key regulator of Ras signaling in neurons

Neurofibromatosis type I (Nf1) is a GTPase-activating protein (GAP) that inactivates the oncoprotein Ras and plays important roles in nervous system development and learning. Alternative exon 23a falls within the Nf1 GAP domain coding sequence and is tightly regulated in favor of skipping in neurons; however, its biological function is not fully understood. Here we generated mouse embryonic stem (ES) cells with a constitutive endogenous Nf1 exon 23a inclusion, termed Nf1 23aIN/23aIN cells, by mutating the splicing signals surrounding the exon to better match consensus sequences. We also made Nf1 23aΔ/23aΔ cells lacking the exon. Active Ras levels are high in wild-type (WT) and Nf1 23aIN/23aIN ES cells, where the Nf1 exon 23a inclusion level is high, and low in Nf1 23aΔ/23aΔ cells. Upon neuronal differentiation, active Ras levels are high in Nf1 23aIN/23aIN cells, where the exon inclusion level remains high, but Ras activation is low in the other two genotypes, where the exon is skipped. Signaling downstream of Ras is significantly elevated in Nf1 23aIN/23aIN neurons. These results suggest that exon 23a suppresses the Ras-GAP activity of Nf1. Therefore, regulation of Nf1 exon 23a inclusion serves as a mechanism for providing appropriate levels of Ras signaling and may be important in modulating Ras-related neuronal functions.

Sex Is a major determinant of neuronal dysfunction in neurofibromatosis type 1

OBJECTIVE

Children with neurofibromatosis-1 (NF1) are at risk for developing numerous nervous system abnormalities, including cognitive problems and brain tumors (optic pathway glioma). Currently, there are few prognostic factors that predict clinical manifestations or outcomes in patients, even in families with an identical NF1 gene mutation. In this study, we leveraged Nf1 genetically engineered mice (GEM) to define the potential role of sex as a clinically relevant modifier of NF1-associated neuronal dysfunction.

METHODS

Deidentified clinical data were analyzed to determine the impact of sex on optic glioma-associated visual decline in children with NF1. In addition, Nf1 GEM were employed as experimental platforms to investigate sexually dimorphic differences in learning/memory, visual acuity, retinal ganglion cell (RGC) death, and Nf1 protein (neurofibromin)-regulated signaling pathway function (Ras activity, cyclic adenosine monophosphate [cAMP], and dopamine levels).

RESULTS

Female patients with NF1-associated optic glioma were twice as likely to undergo brain magnetic resonance imaging for visual symptoms and 3× more likely to require treatment for visual decline than their male counterparts. As such, only female Nf1 GEM exhibited a decrement in optic glioma-associated visual acuity, shorter RGC axons, and attenuated cAMP levels. In contrast, only male Nf1 GEM showed spatial learning/memory deficits, increased Ras activity, and reduced dopamine levels.

INTERPRETATION

Collectively, these observations establish sex as a major prognostic factor underlying neuronal dysfunction in NF1, and suggest that sex should be considered when interpreting future preclinical and clinical study results.

Glucagon-like peptide-1 protects against cardiac microvascular injury in diabetes via a cAMP/PKA/Rho-dependent mechanism

Impaired cardiac microvascular function contributes to cardiovascular complications in diabetes. Glucagon-like peptide-1 (GLP-1) exhibits potential cardioprotective properties in addition to its glucose-lowering effect. This study was designed to evaluate the impact of GLP-1 on cardiac microvascular injury in diabetes and the underlying mechanism involved. Experimental diabetes was induced using streptozotocin in rats. Cohorts of diabetic rats received a 12-week treatment of vildagliptin (dipeptidyl peptidase-4 inhibitor) or exenatide (GLP-1 analog). Experimental diabetes attenuated cardiac function, glucose uptake, and microvascular barrier function, which were significantly improved by vildagliptin or exenatide treatment. Cardiac microvascular endothelial cells (CMECs) were isolated and cultured in normal or high glucose medium with or without GLP-1. GLP-1 decreased high-glucose-induced reactive oxygen species production and apoptotic index, as well as the levels of NADPH oxidase such as p47(phox) and gp91(phox). Furthermore, cAMP/PKA (cAMP-dependent protein kinase activity) was increased and Rho-expression was decreased in high-glucose-induced CMECs after GLP-1 treatment. In conclusion, GLP-1 could protect the cardiac microvessels against oxidative stress, apoptosis, and the resultant microvascular barrier dysfunction in diabetes, which may contribute to the improvement of cardiac function and cardiac glucose metabolism in diabetes. The protective effects of GLP-1 are dependent on downstream inhibition of Rho through a cAMP/PKA-mediated pathway.

Modeling cognitive dysfunction in neurofibromatosis-1

Cognitive dysfunction, including significant impairments in learning, behavior, and attention, is found in over 10% of children in the general population. However, in the common inherited cancer predisposition syndrome, neurofibromatosis type 1 (NF1), the prevalence of these cognitive deficits approaches 70%. As a monogenic disorder, NF1 provides a unique genetic tool to identify and dissect mechanistically the molecular and cellular bases underlying cognitive dysfunction. In this review, we discuss Nf1 fly and mouse systems that mimic many of the cognitive abnormalities seen in children with NF1. Further, we describe discoveries from these models that have uncovered defects in the regulation of Ras activity, cAMP generation, and dopamine homeostasis as key mechanisms important for cognitive dysfunction in children with NF1.

Neurofibromatosis type 1: modeling CNS dysfunction

Neurofibromatosis type 1 (NF1) is the most common monogenic disorder in which individuals manifest CNS abnormalities. Affected individuals develop glial neoplasms (optic gliomas, malignant astrocytomas) and neuronal dysfunction (learning disabilities, attention deficits). Nf1 genetically engineered mouse models have revealed the molecular and cellular underpinnings of gliomagenesis, attention deficit, and learning problems with relevance to basic neurobiology. Using NF1 as a model system, these studies have revealed critical roles for the NF1 gene in non-neoplastic cells in the tumor microenvironment, the importance of brain region heterogeneity, novel mechanisms of glial growth regulation, the neurochemical bases for attention deficit and learning abnormalities, and new insights into neural stem cell function. Here we review recent studies, presented at a symposium at the 2012 Society for Neuroscience annual meeting, that highlight unexpected cell biology insights into RAS and cAMP pathway effects on neural progenitor signaling, neuronal function, and oligodendrocyte lineage differentiation.

Loss of NF1 expression in human endothelial cells promotes autonomous proliferation and altered vascular morphogenesis

Neurofibromatosis is a well known familial tumor syndrome, however these patients also suffer from a number of vascular anomalies. The loss of NFl from the endothelium is embryonically lethal in mouse developmental models, however little is known regarding the molecular regulation by NF1 in endothelium. We investigated the consequences of losing NF1 expression on the function of endothelial cells using shRNA. The loss of NF1 was sufficient to elevate levels of active Ras under non-stimulated conditions. These elevations in Ras activity were associated with activation of downstream signaling including activation of ERK, AKT and mTOR. Cells knocked down in NF1 expression exhibited no cellular senescence. Rather, they demonstrated augmented proliferation and autonomous entry into the cell cycle. These proliferative changes were accompanied by enhanced expression of cyclin D, phosphorylation of p27^KIP, and decreases in total p27^KIP levels, even under growth factor free conditions. In addition, NF1-deficient cells failed to undergo normal branching morphogenesis in a co-culture assay, instead forming planar islands with few tubules and branches. We find the changes induced by the loss of NF1 could be mitigated by co-expression of the GAP-related domain of NF1 implicating Ras regulation in these effects. Using doxycycline-inducible shRNA, targeting NF1, we find that the morphogenic changes are reversible. Similarly, in fully differentiated and stable vascular-like structures, the silencing of NF1 results in the appearance of abnormal vascular structures. Finally, the proliferative changes and the abnormal vascular morphogenesis are normalized by low-dose rapamycin treatment. These data provide a detailed analysis of the molecular and functional consequences of NF1 loss in human endothelial cells. These insights may provide new approaches to therapeutically addressing vascular abnormalities in these patients while underscoring a critical role for normal Ras regulation in maintaining the health and function of the vasculature.

Nf1 RasGAP inhibition of LIMK2 mediates a new cross-talk between Ras and Rho pathways

BACKGROUND

Ras GTPases mediate numerous biological processes through their ability to cycle between an inactive GDP-bound form and an active GTP-bound form. Guanine nucleotide exchange factors (GEFs) favor the formation of the active Ras-GTP, whereas GTPase activating proteins (GAPs) promote the formation of inactive Ras-GDP. Numerous studies have established complex signaling cross-talks between Ras GTPases and other members of the superfamily of small GTPases. GEFs were thought to play a major role in these cross-talks. However, recently GAPs were also shown to play crucial roles in these processes. Among RasGAPs, Nf1 is of special interest. Nf1 is responsible for the genetic disease Neurofibromatosis type I, and recent data strongly suggest that this RasGAP connects different signaling pathways.

METHODOLOGY/PRINCIPAL FINDINGS

In order to know if the RasGAP Nf1 might play a role in connecting Ras GTPases to other small GTPase pathways, we systematically looked for new partners of Nf1, by performing a yeast two-hybrid screening on its SecPH domain. LIMK2, a major kinase of the Rho/ROCK/LIMK2/cofilin pathway, was identified in this screening. We confirmed this interaction by co-immunoprecipitation experiments, and further characterized it. We also demonstrated its specificity: the close related homolog of LIMK2, LIMK1, does not interact with the SecPH domain of Nf1. We then showed that SecPH partially inhibits the kinase activity of LIMK2 on cofilin. Our results furthermore suggest a precise mechanism for this inhibition: in fact, SecPH would specifically prevent LIMK2 activation by ROCK, its upstream regulator.

CONCLUSIONS/SIGNIFICANCE

Although previous data had already connected Nf1 to actin cytoskeleton dynamics, our study provides for the first time possible detailed molecular requirements of this involvement. Nf1/LIMK2 interaction and inhibition allows to directly connect neurofibromatosis type I to actin cytoskeleton remodeling, and provides evidence that the RasGAP Nf1 mediates a new cross-talk between Ras and Rho signaling pathways within the superfamily of small GTPases.

The Learning Disabilities Network (LeaDNet): using neurofibromatosis type 1 (NF1) as a paradigm for translational research

Learning disabilities and other cognitive disorders represent one of the most important unmet medical needs and a significant source of lifelong disability. To accelerate progress in this area, an international consortium of researchers and clinicians, the Learning Disabilities Network (LeaDNet), was established in 2006. Initially, LeaDNet focused on neurofibromatosis type 1 (NF1), a common single gene disorder with a frequency of 1:3,000. Although NF1 is best recognized as an inherited tumor predisposition syndrome, learning, cognitive, and neurobehavioral deficits account for significant morbidity in this condition and can have a profound impact on the quality of life of affected individuals. Recently, there have been groundbreaking advances in our understanding of the molecular, cellular, and neural systems underpinnings of NF1-associated learning deficits in animal models, which precipitated clinical trials using a molecularly targeted treatment for these deficits. However, much remains to be learned about the spectrum of cognitive, neurological, and psychiatric phenotypes associated with the NF1 clinical syndrome. In addition, there is a pressing need to accelerate the identification of specific clinical targets and treatments for these phenotypes. The successes with NF1 have allowed LeaDNet investigators to broaden their initial focus to other genetic disorders characterized by learning disabilities and cognitive deficits including other RASopathies (caused by changes in the Ras signaling pathway). The ultimate mission of LeaDNet is to leverage an international translational consortium of clinicians and neuroscientists to integrate bench-to-bedside knowledge across a broad range of cognitive genetic disorders, with the goal of accelerating the development of rational and biologically based treatments.