Constitutive Ras activity induces hippocampal hypertrophy and remodeling of pyramidal neurons in synRas mice

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.

Cytoarchitectureal changes in hippocampal subregions of the NZB/W F1 mouse model of lupus

Over 50% of clinical patients affected by the systemic lupus erythematosus disease display impaired neurological cognitive functions and psychiatric disorders, a form called neuropsychiatric systemic lupus erythematosus. Hippocampus is one of the brain structures most sensitive to the cognitive deficits and psychiatric disorders related to neuropsychiatric lupus. The purpose of this study was to compare, layer by layer, neuron morphology in lupus mice model NZB/W F1 versus Wild Type mice. By a morphometric of cells identified on Nissl-stained sections, we evaluated structural alterations between NZB/W F1 and Wild Type mice in seven hippocampal subregions: Molecular dentate gyrus, Granular dentate gyrus, Polymorph dentate gyrus, Oriens layer, Pyramidal layer, Radiatum layer and Lacunosum molecular layer. By principal component analysis we distinguished healthy Wild Type from NZB/W F1 mice. In NZB/W F1 mice hippocampal cytoarchitecture, the neuronal cells resulted larger in size and more regular than those of Wild Type. In NZB/W F1, neurons were usually denser than in WT. The Pyramidal layer neurons were much denser in Wild Type than in NZB/W F1. Application of principal component analysis, allowed to distinguish NZB/W F1 lupus mice from healthy, showing as NZBW subjects presented a scattered distribution and intrasubject variability. Our results show a hypertrophy of the NZB/W F1 hippocampal neurons associated with an increase in perikaryal size within the CA1, CA2, CA3 region and the DG. These results help advance our understanding on hippocampal organization and structure in the NZB/W F1 lupus model, suggesting the hypothesis that the different subregions could be differentially affected in neuropsychiatric systemic lupus erythematosus disease. Leveraging an in-depth analysis of the morphology of neural cells in the hippocampal subregions and applying dimensionality reduction using PCA, we propose an efficient methodology to distinguish pathological NZBW mice from WT mice."

Effects of constitutively active K-Ras on axon regeneration after optic nerve injury

Visual disturbance after optic nerve injury is a serious problem. Attempts have been made to enhance the intrinsic ability of retinal ganglion cells (RGCs) to regenerate their axons, and the importance of PI3K/Akt and RAF/MEK/ERK signal activation has been suggested. Since these signals are shared with oncogenic signaling cascades, in this study, we focused on a constitutively active form of K-Ras, K-Ras^V12, to determine if overexpression of this molecule could stimulate axon regeneration. We confirmed that K-Ras^V12 phosphorylated Akt and ERK in vitro. Intravitreal delivery of AAV2-K-Ras^V12 increased the number of surviving RGCs and promoted 1.0 mm of axon regeneration one week after optic nerve injury without inducing abnormal proliferative effects in the RGCs. In addition, AAV2-K-Ras^V12 induced robust RGC axon regeneration, reaching as far as approximately 2.5 mm from the injury site, in eight weeks. Our findings suggest that AAV2-K-Ras^V12 could provide a good model for speedy and efficient analysis of the mechanism underlying axon regeneration in vivo.

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.

Ras Activity Tunes the Period and Modulates the Entrainment of the Suprachiasmatic Clock

The small GTPase Ras is a universal eukaryotic cytoplasmic membrane-anchored protein, which regulates diverse downstream signal transduction pathways that play an important role in the proper functioning of neurons. Ras activity is a central regulator of structural and functional synaptic plasticity in the adult nervous system, where it channels neuronal responses to various extracellular cues allowing the organism to adapt to complex environmental stimuli. The suprachiasmatic nucleus (SCN) is the principle pacemaker of the circadian clock, and the circadian and photic regulation of Ras activity in the SCN is an important modulator of the clockwork. We have generated transgenic mouse expressing constitutively active V12-H-Ras selectively in neurons via a synapsin I promoter (synRas mice), which serves as a suitable model to study the role of neuronal Ras signaling. Modulation of Ras activity affects ERK1,2/CREB signaling and glycogen synthase kinase-3 beta expression in the SCN, which in turn modify the photoentrainment of the clock and the fine tuning the circadian period length. The main focus of this review is to offer an overview of the function of Ras signaling in the circadian rhythm and its potential role in learning and memory consolidation.

RAS and downstream RAF-MEK and PI3K-AKT signaling in neuronal development, function and dysfunction

In postmitotic neurons, the activation of RAS family small GTPases regulates survival, growth and differentiation. Dysregulation of RAS or its major effector pathway, the cascade of RAF-, mitogen-activated and extracellular-signal regulated kinase kinases (MEK), and extracellular-signal regulated kinases (ERK) causes the RASopathies, a group of neurodevelopmental disorders whose pathogenic mechanisms are the subject of intense research. I here summarize the functions of RAS-RAF-MEK-ERK signaling in neurons in vivo, and discuss perspectives for harnessing this pathway to enable novel treatments for nervous system injury, the RASopathies, and possibly other neurological conditions.

Inhomogeneous distribution of Alzheimer pathology along the isocortical relief. Are cortical convolutions an Achilles heel of evolution?

Alzheimer's disease (AD) is neuropathologically characterized by neuritic plaques and neurofibrillary tangles. Progression of both plaques and tangles throughout the brain follows a hierarchical distribution which is defined by intrinsic cytoarchitectonic features and extrinsic connectivity patterns. What has less well been studied is how cortical convolutions influence the distribution of AD pathology. Here, the distribution of both plaques and tangles within subsulcal gyral components (fundi) to components forming their top regions at the subarachnoidal brain surface (crowns) by stereological methods in seven different cortical areas was systematically compared. Further, principle differences in cytoarchitectonic organization of cortical crowns and fundi that might provide the background for regionally selective vulnerability were attempted to identify. It was shown that both plaques and tangles were more prominent in sulcal fundi than gyri crowns. The differential distribution of pathology along convolutions corresponds to subgyral differences in the vascular network, GFAP-positive astrocytes and intracortical and subcortical connectivity. While the precise mechanisms accounting for these differences remain open, the presence of systematic inhomogeneities in the distribution of AD pathology along cortical convolutions indicates that the phylogenetic shaping of the cortex is associated with features that render the human brain vulnerable to AD pathology.

Human iPS Cell-Derived Neurons Uncover the Impact of Increased Ras Signaling in Costello Syndrome

Increasing evidence implicates abnormal Ras signaling as a major contributor in neurodevelopmental disorders, yet how such signaling causes cortical pathogenesis is unknown. We examined the consequences of aberrant Ras signaling in the developing mouse brain and uncovered several critical phenotypes, including increased production of cortical neurons and morphological deficits. To determine whether these phenotypes are recapitulated in humans, we generated induced pluripotent stem (iPS) cell lines from patients with Costello syndrome (CS), a developmental disorder caused by abnormal Ras signaling and characterized by neurodevelopmental abnormalities, such as cognitive impairment and autism. Directed differentiation toward a neuroectodermal fate revealed an extended progenitor phase and subsequent increased production of cortical neurons. Morphological analysis of mature neurons revealed significantly altered neurite length and soma size in CS patients. This study demonstrates the synergy between mouse and human models and validates the use of iPS cells as a platform to study the underlying cellular pathologies resulting from signaling deficits.

SIGNIFICANCE STATEMENT

Increasing evidence implicates Ras signaling dysfunction as a major contributor in psychiatric and neurodevelopmental disorders, such as cognitive impairment and autism, but the underlying cortical cellular pathogenesis remains unclear. This study is the first to reveal human neuronal pathogenesis resulting from abnormal Ras signaling and provides insights into how these phenotypic abnormalities likely contribute to neurodevelopmental disorders. We also demonstrate the synergy between mouse and human models, thereby validating the use of iPS cells as a platform to study underlying cellular pathologies resulting from signaling deficits. Recapitulating human cellular pathologies in vitro facilitates the future high throughput screening of potential therapeutic agents that may reverse phenotypic and behavioral deficits.

Ras Activity Oscillates in the Mouse Suprachiasmatic Nucleus and Modulates Circadian Clock Dynamics

Circadian rhythms, generated in the mouse suprachiasmatic nucleus (SCN), are synchronized to the environmental day-night changes by photic input. The activation of the extracellular signal-regulated kinases 1 and 2 (ERK1,2) and cAMP response element-binding protein (CREB)-mediated transcription play a critical role in this photoentrainment. The small GTPase Ras is one of the major upstream regulators of the ERK1,2/CREB pathway. In contrast to the well-described role of Ras in structural and functional synaptic plasticity in the adult mouse brain, the physiological regulation of Ras by photic sensory input is yet unknown. Here, we describe for the first time a circadian rhythm of Ras activity in the mouse SCN. Using synRas transgenic mice, expressing constitutively activated V12-Ha-Ras selectively in neurons, we demonstrate that enhanced Ras activation causes shortening of the circadian period length. We found upregulated expression and decreased inhibitory phosphorylation of the circadian period length modulator, glycogen synthase kinase-3 beta (GSK3β), in the SCN of synRas mice. Conversely, downregulation of Ras activity by blocking its function with an antibody in oscillating cell cultures reduced protein levels and increased phosphorylation of GSK3β and lengthened the period of BMAL1 promoter-driven luciferase activity. Furthermore, enhanced Ras activity in synRas mice resulted in a potentiation of light-induced phase delays at early subjective night, and increased photic induction of pERK1,2/pCREB and c-Fos. In contrast, at late subjective night, photic activation of Ras/ERK1,2/CREB in synRas mice was not sufficient to stimulate c-Fos protein expression and phase advance the clock. Taken together, our results demonstrate that Ras activity fine tunes the period length and modulates photoentrainment of the circadian clock.

Ras does not contribute to the facilitation of hippocampal synaptic plasticity enabled by environmental enrichment

Environmental enrichment (EE), which mimics the wealth of sensory, motor and cognitive stimuli that arise through intense interactions with the ambient environment, results in enhanced hippocampal long-term potentiation (LTP) and spatial learning. A key molecular factor in the mediation of these changes is the brain-derived neurotrophic factor (BDNF). One of the downstream cascades that is activated by BDNF is the cascade linked to the small GTPase, Ras, that triggers mitogen-activated protein kinase (MAPK) activity and is part of the cAMP response element-binding protein (CREB) pathway that can lead to synaptic restructuring to support LTP. Here, we explored whether persistent activation of Ras in neurons further enhances LTP following EE of rodents. Immediately following weaning, transgenic mice that expressed constitutively activated neuronal Ras, or their wildtype (Wt) littermates, underwent 3weeks of constant EE. In the absence of EE, theta burst stimulation (TBS) evoked LTP in the CA1 region of transgenic mice that was not significantly different from LTP in Wts. After 3weeks of EE, hippocampal LTP was improved in Wt mice. Enriched transgenic mice showed an equivalent level of LTP to enriched Wts, but it was not significantly different from non-enriched synRas controls. Western blot analysis performed after a pull-down assay showed that non-enriched transgenic mice expressed higher Ras activity compared to non-enriched Wts. Following EE, Ras activity was reduced in transgenics to levels detected in Wts. These results show that constitutive activation of Ras does not mimic the effects of EE on LTP. In addition, EE results in an equivalent enhancement of LTP transgenics and Wts, coupled with a decrease in Ras activity to Wt levels. This suggests that permanent activation of Ras in neurons of synRas animals following EE results in an altered feedback regulation of endogenous Ras activity that is not a key factor in LTP enhancements. The maintenance of Ras within a physiological range may thus be required for the optimization of LTP in the hippocampus.

The small G protein H-Ras in the mesolimbic system is a molecular gateway to alcohol-seeking and excessive drinking behaviors

Uncontrolled consumption of alcohol is a hallmark of alcohol abuse disorders; however, the central molecular mechanisms underlying excessive alcohol consumption are still unclear. Here, we report that the GTP binding protein, H-Ras in the nucleus accumbens (NAc) plays a key role in neuroadaptations that underlie excessive alcohol-drinking behaviors. Specifically, acute (15 min) systemic administration of alcohol (2.5 g/kg) leads to the activation of H-Ras in the NAc of mice, which is observed even 24 h later. Similarly, rat operant self-administration of alcohol (20%) also results in the activation of H-Ras in the NAc. Using the same procedures, we provide evidence suggesting that the exchange factor GRF1 is upstream of H-Ras activation by alcohol. Importantly, we show that infection of mice NAc with lentivirus expressing a short hairpin RNA that targets the H-Ras gene produces a significant reduction of voluntary consumption of 20% alcohol. In contrast, knockdown of H-Ras in the NAc of mice did not alter water, quinine, and saccharin intake. Furthermore, using two-bottle choice and operant self-administration procedures, we show that inhibiting H-Ras activity by intra-NAc infusion of the farnesyltransferase inhibitor, FTI-276, produced a robust decrease of rats' alcohol drinking; however, sucrose consumption was unaltered. Finally, intra-NAc infusion of FTI-276 also resulted in an attenuation of seeking for alcohol. Together, these results position H-Ras as a central molecular mediator of alcohol's actions within the mesolimbic system and put forward the potential value of the enzyme as a novel target to treat alcohol use disorders.

An autism-associated variant of Epac2 reveals a role for Ras/Epac2 signaling in controlling basal dendrite maintenance in mice

The architecture of dendritic arbors determines circuit connectivity, receptive fields, and computational properties of neurons, and dendritic structure is impaired in several psychiatric disorders. While apical and basal dendritic compartments of pyramidal neurons are functionally specialized and differentially regulated, little is known about mechanisms that selectively maintain basal dendrites. Here we identified a role for the Ras/Epac2 pathway in maintaining basal dendrite complexity of cortical neurons. Epac2 is a guanine nucleotide exchange factor (GEF) for the Ras-like small GTPase Rap, and it is highly enriched in the adult mouse brain. We found that in vivo Epac2 knockdown in layer 2/3 cortical neurons via in utero electroporation reduced basal dendritic architecture, and that Epac2 knockdown in mature cortical neurons in vitro mimicked this effect. Overexpression of an Epac2 rare coding variant, found in human subjects diagnosed with autism, also impaired basal dendritic morphology. This mutation disrupted Epac2's interaction with Ras, and inhibition of Ras selectively interfered with basal dendrite maintenance. Finally, we observed that components of the Ras/Epac2/Rap pathway exhibited differential abundance in the basal versus apical dendritic compartments. These findings define a role for Epac2 in enabling crosstalk between Ras and Rap signaling in maintaining basal dendrite complexity, and exemplify how rare coding variants, in addition to their disease relevance, can provide insight into cellular mechanisms relevant for brain connectivity.

Activity-induced dendrite and dendritic spine development in human amyloid precursor protein transgenic mice

The amyloid precursor protein is essential for proper neuronal function but an imbalance in processing or metabolism or its overexpression lead to severe malfunction of the brain. The present study focused on dendritic morphology of hippocampal neurons in mice overexpressing the wild-type human amyloid precursor protein (hAPP). In addition, we examined whether enhanced physical activity may affect hAPP-related morphological changes. Overexpression of hAPP resulted in significant enlargement of dendrites, especially within the basal dendritic field but had no effect on spine density. Enhanced physical activity only moderately potentiated hAPP induced changes in dendritic size. Physical activity dependent increases in spine density were, however, augmented by hAPP overexpression. The results suggest that enhanced levels of wild-type hAPP do not result in degenerative changes of neuronal morphology, but rather promote dendritic growth.

Small G protein signaling in neuronal plasticity and memory formation: the specific role of ras family proteins

Small G proteins are an extensive family of proteins that bind and hydrolyze GTP. They are ubiquitous inside cells, regulating a wide range of cellular processes. Recently, many studies have examined the role of small G proteins, particularly the Ras family of G proteins, in memory formation. Once thought to be primarily involved in the transduction of a variety of extracellular signals during development, it is now clear that Ras family proteins also play critical roles in molecular processing underlying neuronal and behavioral plasticity. We here review a number of recent studies that explore how the signaling of Ras family proteins contributes to memory formation. Understanding these signaling processes is of fundamental importance both from a basic scientific perspective, with the goal of providing mechanistic insights into a critical aspect of cognitive behavior, and from a clinical perspective, with the goal of providing effective therapies for a range of disorders involving cognitive impairments.

Deprivation-induced dendritic shrinkage might be oppositely affected by the expression of wild-type and mutated human amyloid precursor protein

The physiological role of the amyloid precursor protein (APP) and its proteolytic fragments in the brain is associated with neuronal survival, neurite outgrowth, synaptic formation, and neuronal plasticity. However, malregulation of APP processing leads to disordered balance of fragments, which may results in opposite, degenerative neuronal effects. In the present study, we analyzed in vivo effects of the expression of wild-type or mutated human APP on afferent deprivation-induced changes of dendritic morphology. After vibrissectomy, expression of wild-type human APP prevented diameter shrinkage of dendritic segments as well as dendritic rarefaction of apical arbors. In contrast, mutant human APP expression exacerbated degenerative changes of deprived barrel neurons. Degradation of apical arbors was especially pronounced. Results demonstrate for the first time opposite effects of the expression of wild-type and mutated human APP on deprivation-induced dendritic restructuring in vivo.

Different dendrite and dendritic spine alterations in basal and apical arbors in mutant human amyloid precursor protein transgenic mice

The extracellular deposition of amyloid-beta peptide (Abeta) in brain parenchyma is one of the characteristic features of Alzheimer's disease and is suggested to induce reactive and degenerative changes in neuronal cell bodies, axons and dendritic processes. In particular, within and in close proximity to amyloid plaques, distinctive morphological alterations have been observed, including changes in neurite trajectory and decreases in dendritic diameter and in spine density. Apart from these plaque-associated focal aberrations, little is known regarding modifications of the global dendritic morphology including the detailed and comparative quantitative analysis of apical and basal arbors. The objective of the present study was to investigate the effects of amyloid plaque deposition and elevated soluble Abeta on neuronal morphology in mutant human amyloid precursor protein (hAPP) transgenic mice (line Tg2576; [K. Hsiao, P. Chapman, S. Nilsen, C. Eckman, Y. Harigaya, S. Younkin, F. Yang, G. Cole, Correlative memory deficits, Abeta elevation, and amyloid plaques in transgenic mice, Science 274 (1996) 99-102]). Retrogradelly labeled callosal-projecting pyramidal cells in the primary somatosensory cortex were three-dimensionally analyzed. Although basal dendrites remained unaffected, analysis of apical trees revealed a number of unambiguous morphological changes. Thus, in TG2576 mice, the apical arbors were shortened in total length and less branched. Furthermore, the diameter of proximal dendritic segments was increased whereas that of distal segments was reduced. Analysis of spine numbers and distribution on basal and apical trees demonstrated a significant reduction in spine densities along the whole course of dendrites. The findings suggest that Abeta-related pathology induces morphological aberrations in basal and apical arbors to different degrees which are unrelated to direct plaque-associated changes.

Functional assay of ARNO and ARF6 in neurite elongation and branching

During development of the nervous system, neurite outgrowth is necessary for the formation of connections between nerve cells. Neurons are highly polarized cells that send out distinct processes, axons, and dendrites; however, the molecular regulation of the differential growth of these processes remains incompletely understood. Primary cultures of rat hippocampal neurons have been used to study many aspects of neuronal cell biology, including neurite extension, establishment of polarity, biogenesis of synapses, and membrane trafficking. After attachment to the substrate, hippocampal neurons begin sending out multiple processes by approximately 12 h after plating. The axonal process is derived from one of these processes, and is evident after 48 h in culture. Complete polarity of axons and dendrites is established after 7 days in culture. The establishment of these cultures and the ability to transfect them with potential regulatory genes allows the researcher to dissect out the pathways relevant to neurite extension. To study the role of small GTPases in neurite extension and branching, we describe methods for culture of hippocampal neurons, for transfection of these cells, and assessment of neurite extension and branching.

The expression of wild-type human amyloid precursor protein affects the dendritic phenotype of neocortical pyramidal neurons in transgenic mice

The current study addresses the morphoregulatory effects of human amyloid precursor protein expression on neocortical pyramidal cells in vivo. For this purpose, a transgenic mouse line was used that expresses wild-type human amyloid precursor protein (APP) at levels similar to endogenous mouse APP. This strain does not develop Alzheimer's disease-related pathology which allowed to study effects of APP or APP cleavage products but excluded the influence of amyloid deposits. Commissural projecting pyramidal neurons of layers II/III within the primary somatosensory cortex were retrogradely labelled by injection of biotinylated dextran amine into the corpus callosum. In transgenic mice, computer-aided morphometric analysis revealed an increase in the surface area of proximal and intermediate basal dendritic segments resulting from an enlarged diameter. On the other hand, the length of the same segments was reduced. Both basal and apical dendrites were characterized by a higher dendritic density within the proximal and intermediate fields. Although the total spatial extension of basal and apical dendrites remained unchanged, a moderate withdrawal of arbors is suggested. The results implicate a physiological function for APP in regulatory mechanisms of neuronal morphogenesis.