CNK2 couples NGF signal propagation to multiple regulatory cascades driving cell differentiation

The CNK-HYP scaffolding complex promotes RAF activation by enhancing KSR-MEK interaction

The RAS-MAPK pathway regulates cell proliferation, differentiation and survival, and its dysregulation is associated with cancer development. The pathway minimally comprises the small GTPase RAS and the kinases RAF, MEK and ERK. Activation of RAF by RAS is notoriously intricate and remains only partially understood. There are three RAF isoforms in mammals (ARAF, BRAF and CRAF) and two related pseudokinases (KSR1 and KSR2). RAS-mediated activation of RAF depends on an allosteric mechanism driven by the dimerization of its kinase domain. Recent work on human RAFs showed that MEK binding to KSR1 promotes KSR1-BRAF heterodimerization, which leads to the phosphorylation of free MEK molecules by BRAF. Similar findings were made with the single Drosophila RAF homolog. Here we show that the fly scaffold proteins CNK and HYP stabilize the KSR-MEK interaction, which in turn enhances RAF-KSR heterodimerization and RAF activation. The cryogenic electron microscopy structure of the minimal KSR-MEK-CNK-HYP complex reveals a ring-like arrangement of the CNK-HYP complex allowing CNK to simultaneously engage KSR and MEK, thus stabilizing the binary interaction. Together, these results illuminate how CNK contributes to RAF activation by stimulating the allosteric function of KSR and highlight the diversity of mechanisms impacting RAF dimerization as well as the regulatory potential of the KSR-MEK interaction.

WNKs regulate mouse behavior and alter central nervous system glucose uptake and insulin signaling

Certain areas of the brain involved in episodic memory and behavior, such as the hippocampus, express high levels of insulin receptors and glucose transporter-4 (GLUT4) and are responsive to insulin. Insulin and neuronal glucose metabolism improve cognitive functions and regulate mood in humans. Insulin-dependent GLUT4 trafficking has been extensively studied in muscle and adipose tissue, but little work has demonstrated either how it is controlled in insulin-responsive brain regions or its mechanistic connection to cognitive functions. In this study, we demonstrate that inhibition of WNK (With-No-lysine (K)) kinases improves learning and memory in mice. Neuronal inhibition of WNK enhances in vivo hippocampal glucose uptake. Inhibition of WNK enhances insulin signaling output and insulin-dependent GLUT4 trafficking to the plasma membrane in mice primary neuronal cultures and hippocampal slices. Therefore, we propose that the extent of neuronal WNK kinase activity has an important influence on learning, memory and anxiety-related behaviors, in part, by modulation of neuronal insulin signaling.

CNK2 promotes cancer cell motility by mediating ARF6 activation downstream of AXL signalling

Cell motility is a critical feature of invasive tumour cells that is governed by complex signal transduction events. Particularly, the underlying mechanisms that bridge extracellular stimuli to the molecular machinery driving motility remain partially understood. Here, we show that the scaffold protein CNK2 promotes cancer cell migration by coupling the pro-metastatic receptor tyrosine kinase AXL to downstream activation of ARF6 GTPase. Mechanistically, AXL signalling induces PI3K-dependent recruitment of CNK2 to the plasma membrane. In turn, CNK2 stimulates ARF6 by associating with cytohesin ARF GEFs and with a novel adaptor protein called SAMD12. ARF6-GTP then controls motile forces by coordinating the respective activation and inhibition of RAC1 and RHOA GTPases. Significantly, genetic ablation of CNK2 or SAMD12 reduces metastasis in a mouse xenograft model. Together, this work identifies CNK2 and its partner SAMD12 as key components of a novel pro-motility pathway in cancer cells, which could be targeted in metastasis.

Functions of CNKSR2 and Its Association with Neurodevelopmental Disorders

The Connector Enhancer of Kinase Suppressor of Ras-2 (CNKSR2), also known as CNK2 or MAGUIN, is a scaffolding molecule that contains functional protein binding domains: Sterile Alpha Motif (SAM) domain, Conserved Region in CNK (CRIC) domain, PSD-95/Dlg-A/ZO-1 (PDZ) domain, Pleckstrin Homology (PH) domain, and C-terminal PDZ binding motif. CNKSR2 interacts with different molecules, including RAF1, ARHGAP39, and CYTH2, and regulates the Mitogen-Activated Protein Kinase (MAPK) cascade and small GTPase signaling. CNKSR2 has been reported to control the development of dendrite and dendritic spines in primary neurons. CNKSR2 is encoded by the CNKSR2 gene located in the X chromosome. CNKSR2 is now considered as a causative gene of the Houge type of X-linked syndromic mental retardation (MRXHG), an X-linked Intellectual Disability (XLID) that exhibits delayed development, intellectual disability, early-onset seizures, language delay, attention deficit, and hyperactivity. In this review, we summarized molecular features, neuronal function, and neurodevelopmental disorder-related variations of CNKSR2.

The synaptic scaffolding protein CNKSR2 interacts with CYTH2 to mediate hippocampal granule cell development

CNKSR2 is a synaptic scaffolding molecule that is encoded by the CNKSR2 gene located on the X chromosome. Heterozygous mutations to CNKSR2 in humans are associated with intellectual disability and epileptic seizures, yet the cellular and molecular roles for CNKSR2 in nervous system development and disease remain poorly characterized. Here, we identify a molecular complex comprising CNKSR2 and the guanine nucleotide exchange factor (GEF) for ARF small GTPases, CYTH2, that is necessary for the proper development of granule neurons in the mouse hippocampus. Notably, we show that CYTH2 binding prevents proteasomal degradation of CNKSR2. Furthermore, to explore the functional significance of coexpression of CNKSR2 and CYTH2 in the soma of granule cells within the hippocampal dentate gyrus, we transduced mouse granule cell precursors in vivo with small hairpin RNAs (shRNAs) to silence CNKSR2 or CYTH2 expression. We found that such manipulations resulted in the abnormal localization of transduced cells at the boundary between the granule cell layer and the hilus. In both cases, CNKSR2-knockdown and CYTH2-knockdown cells exhibited characteristics of immature granule cells, consistent with their putative roles in neuron differentiation. Taken together, our results demonstrate that CNKSR2 and its molecular interaction partner CYTH2 are necessary for the proper development of dentate granule cells within the hippocampus through a mechanism that involves the stabilization of a complex comprising these proteins.

CNKSR2-related neurodevelopmental and epilepsy disorder: a cohort of 13 new families and literature review indicating a predominance of loss of function pathogenic variants

BACKGROUND

Pathogenic variants in connector enhancer of kinase suppressor of Ras-2 (CNKSR2) located on the X chromosome (Xp22.12) lead to a disorder characterized by developmental delay and a characteristic seizure phenotype. To date, 20 affected males representing 13 different pathogenic variants have been published.

CASE PRESENTATION

We identified an 8-year-old male with seizures, abnormal electroencephalogram (EEG) with epileptiform abnormalities in the right hemisphere, and developmental delay with notable loss of speech following seizure onset. Additional concerns include multiple nighttime awakenings, hyperactivity, and autism spectrum disorder. Genetic testing identified a de novo pathogenic nonsense variant in CNKSR2. Through an active family support group, an additional 12 males are described, each harboring a different CNKSR2 variant. The clinical presentation and natural history consistently show early developmental delay, sleep disturbances, and seizure onset in childhood that is initially intractable but later becomes better controlled. Virtually all of the pathogenic variants are predicted to be loss of function, including genomic deletions, nonsense variants, splice site mutations, and small insertions or deletions.

CONCLUSIONS

This expanded knowledge, combined with functional studies and work with animal models currently underway, will enable a better understanding and improved ability to care for individuals with CNKSR2-related neurodevelopmental and epilepsy disorder.

Disease-associated synaptic scaffold protein CNK2 modulates PSD size and influences localisation of the regulatory kinase TNIK

Scaffold proteins are responsible for structural organisation within cells; they form complexes with other proteins to facilitate signalling pathways and catalytic reactions. The scaffold protein connector enhancer of kinase suppressor of Ras 2 (CNK2) is predominantly expressed in neural tissues and was recently implicated in X-linked intellectual disability (ID). We have investigated the role of CNK2 in neurons in order to contribute to our understanding of how CNK2 alterations might cause developmental defects, and we have elucidated a functional role for CNK2 in the molecular processes that govern morphology of the postsynaptic density (PSD). We have also identified novel CNK2 interaction partners and explored their functional interdependency with CNK2. We focussed on the novel interaction partner TRAF2- and NCK-interacting kinase TNIK, which is also associated with ID. Both CNK2 and TNIK are expressed in neuronal dendrites and concentrated in dendritic spines, and staining with synaptic markers indicates a clear postsynaptic localisation. Importantly, our data highlight that CNK2 plays a role in directing TNIK subcellular localisation, and in neurons, CNK2 participates in ensuring that this multifunctional kinase is present in the correct place at desirable levels. In summary, our data indicate that CNK2 expression is critical for modulating PSD morphology; moreover, our study highlights that CNK2 functions as a scaffold with the potential to direct the localisation of regulatory proteins within the cell. Importantly, we describe a novel link between CNK2 and the regulatory kinase TNIK, and provide evidence supporting the idea that alterations in CNK2 localisation and expression have the potential to influence the behaviour of TNIK and other important regulatory molecules in neurons.

Expanding the clinical and EEG spectrum of CNKSR2-related encephalopathy with status epilepticus during slow sleep (ESES)

OBJECTIVE

To investigate the clinical and EEG features of Encephalopathy with Status Epilepticus during slow Sleep (ESES) related to CNKSR2 pathogenic variants.

METHODS

Detailed clinical history, repeated wakefulness/overnight sleep EEGs, brain MRI were collected in five patients, including one female, with CNKSR2-related ESES.

RESULTS

Neurodevelopment in infancy was normal in two patients, delayed in three. Epilepsy onset (age range: 2-6 years) was associated with appearance or aggravation of cognitive impairment, language regression and/or behavioral disorders. Worsening of epilepsy and of cognitive/behavioral disturbances paralleled by enhancement of non-rapid eye movement (NREM) sleep-related, frontally predominant, EEG epileptic discharges [spike-wave-index (SWI): range 60-96%] was consistent with ESES. In three patients, episodes of absence status epilepticus or aggravation of atypical absences occurred, in this latter case associated with striking increment of awake SWI. Speech/oro-motor dyspraxia was diagnosed in four patients. In two patients, long-term follow-up showed epilepsy remission and persistence of mild/moderate cognitive disorders and behavioral disturbances into adulthood.

CONCLUSIONS

Novel findings of our study are occurrence also in females, normal neurodevelopment before epilepsy onset, epilepsy aggravation associated with enhanced awake SWI, mild/moderate evolution in adulthood and language disorder due to speech/oro-motor dyspraxia.

SIGNIFICANCE

Our findings expand the phenotypic spectrum of CNKSR2-related ESES.

A de novo variant in the X-linked gene CNKSR2 is associated with seizures and mild intellectual disability in a female patient

Background

Eight different deletions and point variants of the X‐chromosomal gene CNKSR2 have been reported in families with males presenting intellectual disability (ID) and epilepsy. Obligate carrier females with a frameshift variant in the N‐terminal protein coding part of CNKSR2 or with a deletion of the complete gene are not affected. Only for one C‐terminal nonsense variant, two carrier females were mildly affected by seizures without or with mild motor and language delay.

Methods

Exome sequencing was performed in one female child of a Dutch family, presenting seizures, mild ID, facial dysmorphisms, and abnormalities of the extremities. Potential causative variants were validated by Sanger sequencing. X‐chromosome‐inactivation (XCI) analysis was performed by methylation‐sensitive PCR and fragment‐length analysis of the androgen‐receptor CAG repeat polymorphism.

Results

We identified a de novo variant, c.2304G>A (p.(Trp768*)), in the C‐terminal protein coding part of the X‐chromosomal gene CNKSR2 in a female patient with seizures and mild ID. Sanger sequencing confirmed the presence of this nonsense variant. XCI analysis showed a mild skewing of X inactivation (20:80) in the blood of our patient. Our variant is the second C‐terminal–affecting CNKSR2 variant described in neurologically affected females.

Conclusion

Our results indicate that CNKSR2 nonsense variants in the C‐terminal coding part can result in ID with seizures in female variant carriers.

ProNGF, but Not NGF, Switches from Neurotrophic to Apoptotic Activity in Response to Reductions in TrkA Receptor Levels

Nerve growth factor (NGF) promotes the survival and differentiation of neurons. NGF is initially synthesized as a precursor, proNGF, which is the predominant form in the central nervous system. NGF and proNGF bind to TrkA/p75NTR to mediate cell survival and to sortilin/p75NTR to promote apoptosis. The ratio of TrkA to p75NTR affects whether proNGF and mature NGF signal cell survival or apoptosis. The purpose of this study was to determine whether the loss of TrkA influences p75NTR or sortilin expression levels, and to establish whether proNGF and mature NGF have a similar ability to switch between cell survival and cell death. We systematically altered TrkA receptor levels by priming cells with NGF, using small interfering RNA, and using the mutagenized PC12nnr5 cell line. We found that both NGF and proNGF can support cell survival in cells expressing TrkA, even in the presence of p75NTR and sortilin. However, when TrkA is reduced, proNGF signals cell death, while NGF exhibits no activity. In the absence of TrkA, proNGF-induced cell death occurs, even when p75NTR and sortilin levels are reduced. These results show that proNGF can switch between neurotrophic and apoptotic activity in response to changes in TrkA receptor levels, whereas mature NGF cannot. These results also support the model that proNGF is neurotrophic under normal circumstances, but that a loss in TrkA in the presence of p75NTR and sortilin, as occurs in neurodegenerative disease or injury, shifts proNGF, but not NGF, signalling from cell survival to cell death.

How Genetics Has Helped Piece Together the MAPK Signaling Pathway

Cells respond to changes in their environment, to developmental cues, and to pathogen aggression through the action of a complex network of proteins. These networks can be decomposed into a multitude of signaling pathways that relay signals from the microenvironment to the cellular components involved in eliciting a specific response. Perturbations in these signaling processes are at the root of multiple pathologies, the most notable of these being cancer. The study of receptor tyrosine kinase (RTK) signaling led to the first description of a mechanism whereby an extracellular signal is transmitted to the nucleus to induce a transcriptional response. Genetic studies conducted in drosophila and nematodes have provided key elements to this puzzle. Here, we briefly discuss the somewhat lesser known contribution of these multicellular organisms to our understanding of what has come to be known as the prototype of signaling pathways. We also discuss the ostensibly much larger network of regulators that has emerged from recent functional genomic investigations of RTK/RAS/ERK signaling.

Raf-interactome in tuning the complexity and diversity of Raf function

Raf kinases have been intensely studied subsequent to their discovery 30 years ago. The Ras-Raf-mitogen-activated protein kinase/extracellular signal-regulated kinase kinase-extracellular signal-regulated kinase/mitogen-activated protein kinase (Ras-Raf-MEK-ERK/MAPK) signaling pathway is at the heart of the signaling networks that control many fundamental cellular processes and Raf kinases takes centre stage in the MAPK pathway, which is now appreciated to be one of the most common sources of the oncogenic mutations in cancer. The dependency of tumors on this pathway has been clearly demonstrated by targeting its key nodes; however, blockade of the central components of the MAPK pathway may have some unexpected side effects. Over recent years, the Raf-interactome or Raf-interacting proteins have emerged as promising targets for protein-directed cancer therapy. This review focuses on the diversity of Raf-interacting proteins and discusses the mechanisms by which these proteins regulate Raf function, as well as the implications of targeting Raf-interacting proteins in the treatment of human cancer.

Absent CNKSR2 causes seizures and intellectual, attention, and language deficits

Synaptic function is central to brain function. Understanding the synapse is aided by studies of patients lacking individual synaptic proteins. Common neurological diseases are genetically complex. Their understanding is likewise simplified by studies of less common monogenic forms. We detail the disease caused by absence of the synaptic protein CNKSR2 in 8 patients ranging from 6 to 62 years old. The disease is characterized by intellectual disability, attention problems, and abrupt lifelong language loss following a brief early childhood epilepsy with continuous spike-waves in sleep. This study describes the phenotype of CNKSR2 deficiency and its involvement in systems underlying common neurological disorders.

The CNK2 scaffold interacts with vilse and modulates Rac cycling during spine morphogenesis in hippocampal neurons

Protein scaffolds play an important role in signal transduction, functioning to facilitate protein interactions and localize key pathway components to specific signaling sites. Connector enhancer of KSR-2 (CNK2) is a neuronally expressed scaffold recently implicated in nonsyndromic, X-linked intellectual disability (NS-XLID) [1-3]. NS-XLID patients have deficits in cognitive function and their neurons often exhibit dendritic spine abnormalities [4], suggesting a role for CNK2 in synaptic signaling and/or spine formation. To gain insight regarding how CNK2 might contribute to these processes, we used mass spectrometry to identify proteins that interact with the endogenous CNK2 scaffold. Here, we report that the major binding partner of CNK2 is Vilse/ARHGAP39 and that CNK2 complexes are enriched for proteins involved in Rac/Cdc42 signaling, including Rac1 itself, α-PIX and β-PIX, GIT1 and GIT2, PAK3 and PAK4, and members of the cytohesin family. Binding between CNK2 and Vilse was found to be constitutive, mediated by the WW domains of Vilse and a proline motif in CNK2. Through mutant analysis, protein depletion and rescue experiments, we identify CNK2 as a spatial modulator of Rac cycling during spine morphogenesis and find that the interaction with Vilse is critical for maintaining RacGDP/GTP levels at a balance required for spine formation.

The transcriptomic response of mixed neuron-glial cell cultures to 1,25-dihydroxyvitamin d3 includes genes limiting the progression of neurodegenerative diseases

Seasonal or chronic vitamin D deficiency and/or insufficiency is highly prevalent in the human population. Receptors for 1,25-dihydroxyvitamin D3, the hormonal metabolite of vitamin D, are found throughout the brain. To provide further information on the role of this hormone on brain function, we analyzed the transcriptomic profiles of mixed neuron-glial cell cultures in response to 1,25-dihydroxyvitamin D3. 1,25-dihydroxyvitamin D3 treatment increases the mRNA levels of 27 genes by at least 1.9 fold. Among them, 17 genes were related to neurodegenerative and psychiatric diseases, or brain morphogenesis. Notably, 10 of these genes encode proteins potentially limiting the progression of Alzheimer's disease. These data provide support for a role of 1,25-dihydroxyvitamin D3 in brain disease prevention. The possible consequences of circannual or chronic vitamin D insufficiencies on a tissue with a low regenerative potential such as the brain should be considered.

Loss-of-Function CNKSR2 Mutation Is a Likely Cause of Non-Syndromic X-Linked Intellectual Disability

In a non-dysmorphic 5-year-old boy with developmental delay, well-controlled epilepsy, and microcephaly, a 234-kb deletion of Xp22.12 was detected by copy number analysis. The maternally inherited deletion removed the initial 15 of the 21 exons of the connector enhancer of KSR-2 gene called CNKSR2 or CNK2. Our finding suggests that loss of CNKSR2 is a novel cause of non-syndromic X-linked mental retardation, an assumption supported by high gene expression in the brain, localization to the post-synaptic density, and a role in RAS/MAPK-dependent signal transduction.

Raf family kinases: old dogs have learned new tricks

First identified in the early 1980s as retroviral oncogenes, the Raf proteins have been the objects of intense research. The discoveries 10 years later that the Raf family members (Raf-1, B-Raf, and A-Raf) are bona fide Ras effectors and upstream activators of the ubiquitous ERK pathway increased the interest in these proteins primarily because of the central role that this cascade plays in cancer development. The important role of Raf in cancer was corroborated in 2002 with the discovery of B-Raf genetic mutations in a large number of tumors. This led to intensified drug development efforts to target Raf signaling in cancer. This work yielded not only recent clinical successes but also surprising insights into the regulation of Raf proteins by homodimerization and heterodimerization. Surprising insights also came from the hunt for new Raf targets. Although MEK remains the only widely accepted Raf substrate, new kinase-independent roles for Raf proteins have emerged. These include the regulation of apoptosis by suppressing the activity of the proapoptotic kinases, ASK1 and MST2, and the regulation of cell motility and differentiation by controlling the activity of Rok-α. In this review, we discuss the regulation of Raf proteins and their role in cancer, with special focus on the interacting proteins that modulate Raf signaling. We also describe the new pathways controlled by Raf proteins and summarize the successes and failures in the development of efficient anticancer therapies targeting Raf. Finally, we also argue for the necessity of more systemic approaches to obtain a better understanding of how the Ras-Raf signaling network generates biological specificity.

CNK1 and other scaffolds for Akt/FoxO signaling

FoxO transcription factors mediate anti-proliferative and pro-apoptotic signals and act as tumor suppressors in cancer. Posttranslational modifications including phosphorylation and acetylation regulate FoxO activity by a cytoplasmic-nuclear shuttle mechanism. Scaffold proteins coordinating signaling pathways in time and space play a critical role in this process. CNK1 acts as a scaffold protein in several signaling pathways controlling the function of FoxO proteins. An understanding of CNK1 and other scaffolds in the FoxO signaling network will provide insights how to release the tumor suppressor function of FoxO as a possibility to block oncogenic pathways. This article is part of a Special Issue entitled: P13K-AKT-FoxO axis in cancer and aging.

The CNK1 scaffold binds cytohesins and promotes insulin pathway signaling

Protein scaffolds play an important role in signal transduction, regulating the localization of signaling components and mediating key protein interactions. Here, we report that the major binding partners of the Connector Enhancer of KSR 1 (CNK1) scaffold are members of the cytohesin family of Arf guanine nucleotide exchange factors, and that the CNK1/cytohesin interaction is critical for activation of the PI3K/AKT cascade downstream from insulin and insulin-like growth factor 1 (IGF-1) receptors. We identified a domain located in the C-terminal region of CNK1 that interacts constitutively with the coiled-coil domain of the cytohesins, and found that CNK1 facilitates the membrane recruitment of cytohesin-2 following insulin stimulation. Moreover, through protein depletion and rescue experiments, we found that the CNK1/cytohesin interaction promotes signaling from plasma membrane-bound Arf GTPases to the phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks) to generate a PIP(2)-rich microenvironment that is critical for the membrane recruitment of insulin receptor substrate 1 (IRS1) and signal transmission to the PI3K/AKT cascade. These findings identify CNK1 as a new positive regulator of insulin signaling.

Reactive oxygen species, Ki-Ras, and mitochondrial superoxide dismutase cooperate in nerve growth factor-induced differentiation of PC12 cells

Nerve growth factor (NGF) induces terminal differentiation in PC12, a pheochromocytoma-derived cell line. NGF binds a specific receptor on the membrane and triggers the ERK1/2 cascade, which stimulates the transcription of neural genes. We report that NGF significantly affects mitochondrial metabolism by reducing mitochondrial-produced reactive oxygen species and stabilizing the electrochemical gradient. This is accomplished by stimulation of mitochondrial manganese superoxide dismutase (MnSOD) both transcriptionally and post-transcriptionally via Ki-Ras and ERK1/2. Activation of MnSOD is essential for completion of neuronal differentiation because 1) expression of MnSOD induces the transcription of a neuronal specific promoter and neurite outgrowth, 2) silencing of endogenous MnSOD by small interfering RNA significantly reduces transcription induced by NGF, and 3) a Ki-Ras mutant in the polylysine stretch at the COOH terminus, unable to stimulate MnSOD, fails to induce complete differentiation. Overexpression of MnSOD restores differentiation in cells expressing this mutant. ERK1/2 is also downstream of MnSOD, as a SOD mimetic drug stimulates ERK1/2 with the same kinetics of NGF and silencing of MnSOD reduces NGF-induced late ERK1/2. Long term activation of ERK1/2 by NGF requires SOD activation, low levels of hydrogen peroxide, and the integrity of the microtubular cytoskeleton. Confocal immunofluorescence shows that NGF stimulates the formation of a complex containing membrane-bound Ki-Ras, microtubules, and mitochondria. We propose that active NGF receptor induces association of mitochondria with plasma membrane. Local activation of ERK1/2 by Ki-Ras stimulates mitochondrial SOD, which reduces reactive oxygen species and produces H(2)O(2). Low and spatially restricted levels of H(2)O(2) induce and maintain long term ERK1/2 activity and ultimately differentiation of PC12 cells.

New insight on FGFR3-related chondrodysplasias molecular physiopathology revealed by human chondrocyte gene expression profiling

Endochondral ossification is the process by which the appendicular skeleton, facial bones, vertebrae and medial clavicles are formed and relies on the tight control of chondrocyte maturation. Fibroblast growth factor receptor (FGFR)3 plays a role in bone development and maintenance and belongs to a family of proteins which differ in their ligand affinities and tissue distribution. Activating mutations of the FGFR3 gene lead to craniosynostosis and multiple types of skeletal dysplasia with varying degrees of severity: thanatophoric dysplasia (TD), achondroplasia and hypochondroplasia. Despite progress in the characterization of FGFR3-mediated regulation of cartilage development, many aspects remain unclear. The aim and the novelty of our study was to examine whole gene expression differences occurring in primary human chondrocytes isolated from normal cartilage or pathological cartilage from TD-affected fetuses, using Affymetrix technology. The phenotype of the primary cells was confirmed by the high expression of chondrocytic markers. Altered expression of genes associated with many cellular processes was observed, including cell growth and proliferation, cell cycle, cell adhesion, cell motility, metabolic pathways, signal transduction, cell cycle process and cell signaling. Most of the cell cycle process genes were down-regulated and consisted of genes involved in cell cycle progression, DNA biosynthesis, spindle dynamics and cytokinesis. About eight percent of all modulated genes were found to impact extracellular matrix (ECM) structure and turnover, especially glycosaminoglycan (GAG) and proteoglycan biosynthesis and sulfation. Altogether, the gene expression analyses provide new insight into the consequences of FGFR3 mutations in cell cycle regulation, onset of pre-hypertrophic differentiation and concomitant metabolism changes. Moreover, impaired motility and ECM properties may also provide clues about growth plate disorganization. These results also suggest that many signaling pathways may be directly or indirectly altered by FGFR3 and confirm the crucial role of FGFR3 in the control of growth plate development.

Signaling threshold regulation by the Ras effector IMP

The Ras effector and E3 ligase family member IMP (impedes mitogenic signal propagation) acts as a steady-state resistor within the Raf-MEK-ERK kinase module. IMP concentrations are directly regulated by Ras, through induction of autoubiquitination, to permit productive Raf-MEK complex assembly. Inhibition of Raf-MEK pathway activation by IMP occurs through the inactivation of KSR, a scaffold/adapter protein that couples activated Raf to its substrate MEK1. The capacity of IMP to inhibit signal propagation through Raf to MEK is, in part, a consequence of disrupting KSR1 homo-oligomerization and c-Raf-B-Raf hetero-oligomerization. These observations suggest that IMP functions as a threshold modulator, controlling sensitivity of the cascade to stimulus by directly limiting the assembly of functional KSR1-dependent Raf-MEK complexes.

Ras-sensitive IMP modulation of the Raf/MEK/ERK cascade through KSR1

The E3 ubiquitin ligase IMP (impedes mitogenic signal propagation) was isolated as a novel Ras effector that negatively regulates ERK1/2 activation. Current evidence suggests that IMP limits the functional assembly of Raf/MEK complexes by inactivation of the KSR1 adaptor/scaffold protein. Interaction with Ras-GTP stimulates IMP autoubiquitination to relieve limitations on KSR function. The elevated sensitivity of IMP-depleted cells to ERK1/2 pathway activation suggests IMP acts as a signal threshold regulator by imposing reversible restrictions on the assembly of functional Raf/MEK/ERK kinase modules. These observations challenge commonly held concepts of signal transmission by Ras to the MAPK pathway and provide evidence for the role of amplitude modulation in tuning cellular responses to ERK1/2 pathway engagement. Here we describe details of the methods, including RNA interference, ubiquitin ligase assays, and protein complex analysis, that can be used to display the Ras-sensitive contribution of IMP to KSR-dependent modulation of the Raf/MEK/ERK pathway.

ERK phosphorylation: spatial and temporal regulation by G protein-coupled receptors

G protein-coupled receptors (GPCRs) are a major target in the drug discovery process. One important response that results from activation of a wide range of GPCRs is activation of the ERK signalling cascade. Given the abundance of both upstream activators and downstream targets of ERK1/2, the precise spatiotemporal control of ERK1/2 phosphorylation is crucial for maintaining the specificity of the physiological outcome. ERK activity is regulated via a number of mechanisms including compartmentalisation and scaffolding proteins. These scaffolding proteins can enhance the transduction of a specific signalling pathway by targeting pathway components to particular intracellular locations or signalling complexes. Recently, a number of fluorescent indicators of ERK1/2 phosphorylation have been developed that allow the regulation of this pathway to be investigated with greater spatiotemporal resolution than was previously possible. These fluorescent probes in conjunction with those for other signalling cascades should help unravel the spatiotemporal organisation of this pathway.

IMP modulates KSR1-dependent multivalent complex formation to specify ERK1/2 pathway activation and response thresholds

The Ras effector and ubiquitin-protein isopeptide ligase family member IMP acts as a steady-state resistor within the Raf-MEK-ERK kinase module. IMP concentrations are regulated by Ras through induction of autodegradation and can modulate signal/response thresholds by directly limiting the assembly of functional KSR1-dependent Raf.MEK complexes. Here, we show that the capacity of IMP to inhibit signal propagation through Raf to MEK is a consequence of disrupting KSR1 homooligomerization and B-Raf/c-Raf hetero-oligomerization. This impairs both the recruitment of MEK to activated Raf family members and the contribution of Raf oligomers to c-Raf kinase activation. Our observations indicate that human KSR1 proteins promote assembly of multivalent Raf.MEK complexes that are required for c-Raf kinase activation and functional coupling of active kinases to downstream substrates. This property is engaged by IMP for modulation of signal amplitude.

Scaffold proteins of MAP-kinase modules

Mitogen-activated protein kinases (MAPKs) regulate critical signaling pathways involved in cell proliferation, differentiation and apoptosis. Recent studies have shown that a novel class of scaffold proteins mediates the structural and functional organization of the three-tier MAPK module. By linking the MAP3K, MAP2K and MAPK into a multienzyme complex, these MAPK-specific scaffold proteins provide an insulated physical conduit through which signals from the respective MAPK can be transmitted to the appropriate spatiotemporal cellular loci. Scaffold proteins play a determinant role in modulating the signaling strength of their cognate MAPK module by regulating the signal amplitude and duration. The scaffold proteins themselves are finely regulated resulting in dynamic intra- and inter-molecular interactions that can modulate the signaling outputs of MAPK modules. This review focuses on defining the diverse mechanisms by which these scaffold proteins interact with their respective MAPK modules and the role of such interactions in the spatiotemporal organization as well as context-specific signaling of the different MAPK modules.

KSR and CNK: two scaffolds regulating RAS-mediated RAF activation

The RAS-RAF-MEK-extracellular-regulated kinase (RAS/ERK) pathway is a major intracellular route used by metazoan cells to channel to downstream targets a diverse array of signals, including those controlling cell proliferation and survival. Recent findings suggest that the pathway is assembled by specific scaffolding proteins that in turn regulate the efficiency, the location and/or the duration of signal transmission. Here, through the angle of studies conducted in Drosophila and C. elegans, we present two such proteins, the kinase suppressor of RAS (KSR) and connector enhancer of KSR (CNK) scaffolds, and highlight their implication in a novel mechanism regulating RAS-mediated RAF activation. Based on recent findings, we discuss the possibility that KSR, a RAF-like protein, does not solely act as a scaffold, but directly induces RAF catalytic function by a kinase-independent mechanism apparently shared by RAF-like proteins.

X chromosome cDNA microarray screening identifies a functional PLP2 promoter polymorphism enriched in patients with X-linked mental retardation

X-linked Mental Retardation (XLMR) occurs in 1 in 600 males and is highly genetically heterogeneous. We used a novel human X chromosome cDNA microarray (XCA) to survey the expression profile of X-linked genes in lymphoblasts of XLMR males. Genes with altered expression verified by Northern blot and/or quantitative PCR were considered candidates. To validate this approach, we documented the expected changes of expression in samples from a patient with a known X chromosome microdeletion and from patients with multiple copies of the X chromosome. We used our XCA to survey lymphoblast RNA samples from 43 unrelated XLMR males and found 15 genes with significant (>or=1.5-fold) reduction in expression in at least one proband. Of these, subsequent analysis confirmed altered expression in 12. We followed up one, PLP2, at Xp11.23, which exhibits approximately fourfold decreased expression in two patients. Sequencing analysis in both patients revealed a promoter variant, -113C>A, that alters the core-binding site of the transcription factor ELK1. We showed that PLP2-(-113C>A) is sufficient to cause reduced expression using a luciferase reporter system and is enriched in a cohort of males with probable XLMR (14 of 239, 5.85%) as compared to normal males (9 of 577, 1.56%) (chi2=11.07, P<0.001). PLP2 is expressed abundantly in the pyramidal cells of hippocampus and granular cells of the cerebellum in the brain. We conclude that our XCA screening is an efficient strategy to identify genes that show significant changes in transcript abundance as candidate genes for XLMR.

The integration of signaling by multiprotein complexes containing Raf kinases

In vivo, eukaryotic cells are subjected simultaneously to a broad array of signals ranging from mitogens and inflammatory inputs to environmental stresses and developmental cues. The combinatorial nature of cellular signaling necessitates that a cell integrate its signal transduction pathways so as to implement rapidly and efficiently an appropriate suite of responses. Emerging evidence indicates that, over the course of evolution, cells have developed multiprotein signaling complexes, or "signalosomes" that mediate the coordinate regulation of different signaling pathways. Such molecular signal integration contrasts with the classical notion of signaling complexes assembled by scaffold proteins-entities that function to segregate specific pathways from one another. This review will focus on two signal integrating multiprotein complexes that involve Raf family kinases: the MLK3-B-Raf-Raf-1 complex and the Raf-1-Mst-2 complex.

Involvement of protein kinase C alpha (PKC alpha) in the early action of angiotensin II type 2 (AT2) effects on neurite outgrowth in NG108-15 cells: AT2-receptor inhibits PKC alpha and p21ras activity

The aim of the present study was to investigate whether protein kinase C (PKC) isoforms may be among the putative candidates implicated in the primary effects of the Ang II type 2 (AT2) receptor. Western blot analyses revealed the presence of PKC alpha,epsilon, iota, and zeta in NG108-15 cells. After a 3-d treatment with 3 nm Gö6976, a specific inhibitor of classical PKC isoforms, cells were characterized by the presence of one elongated process similar to that observed after treatment with Ang II or with CGP42112, a selective AT2 receptor agonist. Similar findings were observed in cells expressing a dominant-negative mutant of PKC alpha (K368A). Inhibition of PKC alpha in NG108-15 cells also decreased cell number and proliferation. In conditions of acute stimulation, Ang II induced a time-dependent and transient inhibition of PKC alpha activity, as well as a decrease in PKC alpha levels associated with the membrane. Treatment of cells with Gö6976 was also found to inhibit p21(ras) (between 1-10 min) but stimulated Rap1 activity (1-5 min) in a time-course similar to that of Ang II. Incubation of NG108-15 cells with Gö6976 (3 nm) inhibited basal p42/p44(mapk) phosphorylation, but failed to interfere with its activation by the AT(2) receptor, indicating that inhibition of PKC alpha is not directly involved in the Rap1-MEK-p42/p44(mapk) cascade. Taken together, these results indicate that PKC alpha is a primary target of the AT2 receptor. Inhibition of PKC alpha leads to a decrease in both p21(ras) activity and cell proliferation, which may facilitate AT2 receptor signaling through p42/p44(mapk), thereby leading to neurite outgrowth.

WNK1 and OSR1 regulate the Na+, K+, 2Cl- cotransporter in HeLa cells

Oxidative stress-responsive kinase (OSR) 1 and sterile20-related, proline-, alanine-rich kinase (SPAK) are Ste20p-related protein kinases that bind to the sodium, potassium, two chloride cotransporter, NKCC. Here we present evidence that the protein kinase with no lysine [K] (WNK) 1 regulates OSR1, SPAK, and NKCC activities. OSR1 exists in a complex with WNK1 in cells, is activated by recombinant WNK1 in vitro, and is phosphorylated in a WNK1-dependent manner in cells. Depletion of WNK1 from HeLa cells by using small interfering RNA reduces OSR1 kinase activity. In addition, depletion of either WNK1 or OSR1 reduces NKCC activity, indicating that WNK1 and OSR1 are both required for NKCC function. OSR1 and SPAK are likely links between WNK1 and NKCC in a pathway that contributes to volume regulation and blood pressure homeostasis in mammals.

Harnessing RNAi for analyses of Ras signaling and transformation

The Ras-regulatory network is a loosely defined composition of numerous Ras family members and effector pathways that couple to critical cell-regulatory processes. Investigators are increasingly turning to RNAi-mediated inhibition of gene expression as an effective tool to help generate authentic portraits of Ras protein function in general and to accurately characterize the contribution of Ras family members and Ras effectors to oncogenic transformation in particular. Here we provide detailed protocols for high-efficiency and high-throughput delivery of siRNAs to human cancer cell lines and primary human epithelial cells. In addition, we discuss appropriate controls and limitations for the use of RNAi to derive biologically relevant observations.

A KSR/CNK complex mediated by HYP, a novel SAM domain-containing protein, regulates RAS-dependent RAF activation in Drosophila

RAF is a critical effector of the small GTPase RAS in normal and malignant cells. Despite intense scrutiny, the mechanism regulating RAF activation remains partially understood. Here, we show that the scaffold KSR (kinase suppressor of RAS), a RAF homolog known to assemble RAF/MEK/ERK complexes, induces RAF activation in Drosophila by a mechanism mediated by its kinase-like domain, but which is independent of its scaffolding property or putative kinase activity. Interestingly, we found that KSR is recruited to RAF prior to signal activation by the RAF-binding protein CNK (connector enhancer of KSR) in association with a novel SAM (sterile alpha motif) domain-containing protein, named Hyphen (HYP). Moreover, our data suggest that the interaction of KSR to CNK/HYP stimulates the RAS-dependent RAF-activating property of KSR. Together, these findings identify a novel protein complex that controls RAF activation and suggest that KSR does not only act as a scaffold for the MAPK (mitogen-activated protein kinase) module, but may also function as a RAF activator. By analogy to catalytically impaired, but conformationally active B-RAF oncogenic mutants, we discuss the possibility that KSR represents a natural allosteric inducer of RAF catalytic function.

Coordinating ERK/MAPK signalling through scaffolds and inhibitors

The pathway from Ras through Raf and MEK (MAPK and ERK kinase) to ERK/MAPK (extracellular signal-regulated kinase/mitogen-activated protein kinase) regulates many fundamental cellular processes. Recently, a number of scaffolding proteins and endogenous inhibitors have been identified, and their important roles in regulating signalling through this pathway are now emerging. Some scaffolds augment the signal flux, but also mediate crosstalk with other pathways; certain adaptors target MEK-ERK/MAPK complexes to subcellular localizations; others provide regulated inhibition. Computational modelling indicates that, together, these modulators can determine the dynamic biological behaviour of the pathway.

The scaffold protein CNK1 interacts with the angiotensin II type 2 receptor

The scaffold protein CNK1 mediates proliferative as well as antiproliferative responses including differentiation and apoptosis. The angiotensin II type 2 (AT2) receptor belongs to the class of G protein-coupled receptors and also promotes antiproliferative effects. Here we report that CNK1 binds through the sterile alpha motif (SAM) and the conserved region in CNK (CRIC) to the AT2 receptor. The exchange of a conserved leucine residue with arginine in the CRIC domain increases the binding affinity of CNK1 to the AT2 receptor. The insertion of a negatively charged amino acid stretch into the linker region between the N- and the C-terminal part of CNK1 strengthens the interaction between CNK1 and the AT2 receptor in a Ras-regulated manner. The biological significance of the interaction was supported by coprecipitation of CNK1 and the AT2 receptor in mouse heart extracts. Thus, CNK1 may play a role in the AT2 receptor-mediated signaling pathways.

SIGNALING NETWORKS IN LIVING CELLS

Recent advances in cell signaling research suggest that multiple sets of signal transducing molecules are preorganized and sequestered in distinct compartments within the cell. These compartments are assembled and maintained by specific cellular machinery. The molecular ecology within a compartment creates an environment that favors the efficient and accurate integration of signaling information arriving from humoral, mechanical, and nutritional sources. The functional organization of these compartments suggests they are the location of signaling networks that naturally organize into hierarchical interconnected sets of molecules through their participation in different classes of interacting units. An important goal is to determine the contribution of the compartment to the function of these networks in living cells.

The love-hate relationship between Ras and Notch

The Ras and Notch signaling pathways are used over and over again during development to control many different biological processes. Frequently, these two signaling pathways intersect to influence common processes, but sometimes they cooperate and sometimes they antagonize each other. The Caenorhabditis elegans vulva and the Drosophila eye are two classic paradigms for understanding how Ras and Notch affect cell fates, and how the two pathways work together to control biological pattern. Recent advances in these systems reveal some of the mechanisms by which Ras and Notch can interact. Similar types of interactions in mammals may be important for determining whether and how alterations in Ras or Notch lead to cancer.

Caenorhabditis elegans CNK-1 promotes Raf activation but is not essential for Ras/Raf signaling

Connector enhancer of Ksr (CNK) is a conserved multidomain protein essential for Ras signaling in Drosophila melanogaster and thought to be involved in Raf kinase activation. However, the precise role of CNK in Ras signaling is not known, and mammalian CNKs are proposed to have distinct functions. Caenorhabditis elegans has a single CNK homologue, cnk-1. Here, we describe the role of cnk-1 in C. elegans Ras signaling and its requirements for LIN-45 Raf activation. We find that cnk-1 positively regulates multiple Ras signaling events during development, but, unlike Drosophila CNK, cnk-1 does not appear to be essential for signaling. cnk-1 mutants appear to be normal but show cell-type-specific genetic interactions with mutations in two other Ras pathway scaffolds/adaptors ksr-1 and sur-8. Genetic epistasis using various activated LIN-45 Raf transgenes shows that CNK-1 promotes LIN-45 Raf activation at a step between the dephosphorylation of inhibitory sites in the regulatory domain and activating phosphorylation in the kinase domain. Our data are consistent with a model in which CNK promotes Raf phosphorylation/activation through membrane localization, oligomerization, or association with an activating kinase.

CNK1 is a scaffold protein that regulates Src-mediated Raf-1 activation

Raf-1 is a regulator of cellular proliferation, differentiation, and apoptosis. Activation of the Raf-1 kinase activity is tightly regulated and involves targeting to the membrane by Ras and phosphorylation by various kinases, including the tyrosine kinase Src. Here we demonstrate that the connector enhancer of Ksr1, CNK1, mediates Src-dependent tyrosine phosphorylation and activation of Raf-1. CNK1 binds preactivated Raf-1 and activated Src and forms a trimeric complex. CNK1 regulates the activation of Raf-1 by Src in a concentration-dependent manner typical for a scaffold protein. Down-regulation of endogenously expressed CNK1 by small inhibitory RNA interferes with Src-dependent activation of ERK. Thus, CNK1 allows cross-talk between Src and Raf-1 and is essential for the full activation of Raf-1.

Src42 binding activity regulates Drosophila RAF by a novel CNK-dependent derepression mechanism

Connector enhancer of KSR (CNK), an essential component of Drosophila receptor tyrosine kinase/mitogen-activated protein kinase pathways, regulates oppositely RAF function. This bimodal property depends on the N-terminal region of CNK, which integrates RAS activity to stimulate RAF and a bipartite element, called the RAF-inhibitory region (RIR), which binds and inhibits RAF catalytic activity. Here, we show that the repressive effect of the RIR is counteracted by the ability of Src42 to associate, in an RTK-dependent manner, with a conserved region located immediately C-terminal to the RIR. Strikingly, we found that several cnk loss-of-function alleles have mutations clustered in this area and provide evidence that these mutations impair Src42 binding. Surprisingly, the derepressing effect of Src42 does not appear to involve its catalytic function, but critically depends on the ability of its SH3 and SH2 domains to associate with CNK. Together, these findings suggest that the integration of RTK-induced RAS and Src42 signals by CNK as a two-component input is essential for RAF activation in Drosophila.

Emerging Role for ERK as a Key Regulator of Neuronal Apoptosis

Extracellular signal-regulated kinases (ERKs) are traditionally viewed as a survival factor in the mitogen-activated protein kinase (MAPK) family. On the other hand, some recent reports have suggested that ERK can also be responsible for neuronal cell death in various neurodegeneration models. In-depth studies on the action of ERK in apoptosis, however, have not been done. A recent study has revealed that ERK is a key apoptotic factor in potassium deprivation-induced neuronal cell death by showing that ERK inhibitors protect neurons from low potassium conditions, whereas constitutively activated ERK activates cell death. Most important, this study shows how ERK can promote neuronal cell death by causing plasma membrane and DNA damage that is independent of caspase-3 activity. Further studies on the mechanism of ERK in neuronal cell death will shed light on the possibility of using ERK as a therapeutic target in treating neurodegeneration.