Identification of an autoinhibitory mechanism that restricts C1 domain-mediated activation of the Rac-GAP alpha2-chimaerin

The Rac-GAP alpha2-Chimaerin Signals via CRMP2 and Stathmins in the Development of the Ocular Motor System

A precise sequence of axon guidance events is required for the development of the ocular motor system. Three cranial nerves grow toward, and connect with, six extraocular muscles in a stereotyped pattern, to control eye movements. The signaling protein alpha2-chimaerin (α2-CHN) plays a pivotal role in the formation of the ocular motor system; mutations in CHN1, encoding α2-CHN, cause the human eye movement disorder Duane Retraction Syndrome (DRS). Our research has demonstrated that the manipulation of α2-chn signaling in the zebrafish embryo leads to ocular motor axon wiring defects, although the signaling cascades regulated by α2-chn remain poorly understood. Here, we demonstrate that several cytoskeletal regulatory proteins-collapsin response mediator protein 2 (CRMP2; encoded by the gene dpysl2), stathmin1, and stathmin 2-bind to α2-CHN. dpysl2, stathmin1, and especially stathmin2 are expressed by ocular motor neurons. We find that the manipulation of dpysl2 and of stathmins in zebrafish larvae leads to defects in both the axon wiring of the ocular motor system and the optokinetic reflex, impairing horizontal eye movements. Knockdowns of these molecules in zebrafish larvae of either sex caused axon guidance phenotypes that included defasciculation and ectopic branching; in some cases, these phenotypes were reminiscent of DRS. chn1 knock-down phenotypes were rescued by the overexpression of CRMP2 and STMN1, suggesting that these proteins act in the same signaling pathway. These findings suggest that CRMP2 and stathmins signal downstream of α2-CHN to orchestrate ocular motor axon guidance and to control eye movements.SIGNIFICANCE STATEMENT The precise control of eye movements is crucial for the life of vertebrate animals, including humans. In humans, this control depends on the arrangement of nerve wiring of the ocular motor system, composed of three nerves and six muscles, a system that is conserved across vertebrate phyla. Mutations in the protein alpha2-chimaerin have previously been shown to cause eye movement disorders (squint) and axon wiring defects in humans. Our recent work has unraveled how alpha2-chimaerin coordinates axon guidance of the ocular motor system in animal models. In this article, we demonstrate key roles for the proteins CRMP2 and stathmin 1/2 in the signaling pathway orchestrated by alpha2-chimaerin, potentially giving insight into the etiology of eye movement disorders in humans.

CHN1 promotes epithelial-mesenchymal transition via the Akt/GSK-3β/Snail pathway in cervical carcinoma

Background

Metastasis and invasion are crucial in determining the mortality of cervical carcinoma (CC) patients. The epithelial–mesenchymal transition (EMT) is now a universal explanation for the mechanisms of tumor metastasis. Α-chimeric protein (α-chimaerin, CHN1) plays an important role in the regulation of signal transduction and development. However, the molecular regulatory relationships between CHN1 and CC progression in relation to EMT have not yet been identified.

Methods

The expression of CHN1 in CC tissues, adjacent tissues, and lymph node metastases from CC patients was detected by immunohistochemistry. Upregulation and knockdown of CHN1 were achieved by transfection of CC cells. The effect of CHN1 on cell proliferation was determined by CCK-8 and plate clone formation assays. Changes in migration and invasion capabilities were evaluated using scratch migration and transwell invasion assays. The effect of CHN1 overexpression and interference on xenograft tumor growth was determined by tumor weight and pathological analyses. The expression of EMT-related mRNAs was measured by qRT-PCR in transfected CC cells. EMT-related proteins and Akt/GSK-3β/Snail signaling pathway-related proteins were also evaluated by western blotting.

Results

CHN1 was overexpressed in CC tissues and was associated with lymph node metastasis and low survival in CC patients. Overexpression of CHN1 promoted cell proliferation, migration, and invasion in CC cells. In contrast, silencing of CHN1 inhibited these phenomena. Overexpression of CHN1 promoted tumor formation in an in vivo xenograft tumor mouse model, with increased tumor volumes and weights. In addition, CHN1 induced the expression of EMT-related transcription factors, accompanied by the decreased expression of epithelial markers and increased expression of mesenchymal markers. The Akt/GSK-3β/Snail signaling pathway was activated by overexpression of CHN1 in vitro, and activation of this pathway was inhibited by the signaling pathway inhibitor LY294002.

Conclusion

These results suggest that CHN1 promotes the development and progression of cervical carcinoma via the Akt/GSK-3β/Snail pathway by inducing EMT.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12967-021-02963-7.

Identification of a novel CHN1 p.(Phe213Val) variant in a large Han Chinese family with congenital Duane retraction syndrome

Duane retraction syndrome (DRS) is a neuromuscular dysfunction of the eyes. Although many causative genes of DRS have been identified in Europe and the United States, few reports have been published in regard to Chinese DRS. The aim of the present study was to explore the genetic defect of DRS in a Chinese family. Exome sequencing was used to identify the disease-causing gene for the two affected family members. Ophthalmic and physical examinations, as well as genetic screenings for variants in chimerin 1 (CHN1), were performed for all family members. Functional analyses of a CHN1 variant in 293T cells included a Rac-GTP activation assay, α2-chimaerin translocation assay, and co-immunoprecipitation assay. Genetic analysis revealed a NM_001822.7: c.637T > G variant in the CHN1 gene, which resulted in the substitution of a highly conserved C1 domain with valine at codon 213 (NP_001813.1: p.(Phe213Val)) (ClinVar Accession Number: SCV001335305). In-silico analysis revealed that the p.(Phe213Val) substitution affected the protein stability and connections among the amino acids of CHN1 in terms of its tertiary protein structure. Functional studies indicated that the p.(Phe213Val) substitution reduced Rac-GTP activity and enhanced membrane translocation in response to phorbol-myristoyl acetate (PMA). Together with previous studies, our present findings demonstrate that CHN1 may be an important causative gene for different ethnicities with DRS.

Ocular congenital cranial dysinnervation disorders (CCDDs): insights into axon growth and guidance

Unraveling the genetics of the paralytic strabismus syndromes known as congenital cranial dysinnervation disorders (CCDDs) is both informing physicians and their patients and broadening our understanding of development of the ocular motor system. Genetic mutations underlying ocular CCDDs alter either motor neuron specification or motor nerve development, and highlight the importance of modulations of cell signaling, cytoskeletal transport, and microtubule dynamics for axon growth and guidance. Here we review recent advances in our understanding of two CCDDs, congenital fibrosis of the extraocular muscles (CFEOM) and Duane retraction syndrome (DRS), and discuss what they have taught us about mechanisms of axon guidance and selective vulnerability. CFEOM presents with congenital ptosis and restricted eye movements, and can be caused by heterozygous missense mutations in the kinesin motor protein KIF21A or in the β-tubulin isotypes TUBB3 or TUBB2B. CFEOM-causing mutations in these genes alter protein function and result in axon growth and guidance defects. DRS presents with inability to abduct one or both eyes. It can be caused by decreased function of several transcription factors critical for abducens motor neuron identity, including MAFB, or by heterozygous missense mutations in CHN1, which encodes α2-chimaerin, a Rac-GAP GTPase that affects cytoskeletal dynamics. Examination of the orbital innervation in mice lacking Mafb has established that the stereotypical misinnervation of the lateral rectus by fibers of the oculomotor nerve in DRS is secondary to absence of the abducens nerve. Studies of a CHN1 mouse model have begun to elucidate mechanisms of selective vulnerability in the nervous system.

Mutant α2-chimaerin signals via bidirectional ephrin pathways in Duane retraction syndrome

Duane retraction syndrome (DRS) is the most common form of congenital paralytic strabismus in humans and can result from α2-chimaerin (CHN1) missense mutations. We report a knockin α2-chimaerin mouse (Chn1KI/KI) that models DRS. Whole embryo imaging of Chn1KI/KI mice revealed stalled abducens nerve growth and selective trochlear and first cervical spinal nerve guidance abnormalities. Stalled abducens nerve bundles did not reach the orbit, resulting in secondary aberrant misinnervation of the lateral rectus muscle by the oculomotor nerve. By contrast, Chn1KO/KO mice did not have DRS, and embryos displayed abducens nerve wandering distinct from the Chn1KI/KI phenotype. Murine embryos lacking EPH receptor A4 (Epha4KO/KO), which is upstream of α2-chimaerin in corticospinal neurons, exhibited similar abducens wandering that paralleled previously reported gait alterations in Chn1KO/KO and Epha4KO/KO adult mice. Findings from Chn1KI/KI Epha4KO/KO mice demonstrated that mutant α2-chimaerin and EphA4 have different genetic interactions in distinct motor neuron pools: abducens neurons use bidirectional ephrin signaling via mutant α2-chimaerin to direct growth, while cervical spinal neurons use only ephrin forward signaling, and trochlear neurons do not use ephrin signaling. These findings reveal a role for ephrin bidirectional signaling upstream of mutant α2-chimaerin in DRS, which may contribute to the selective vulnerability of abducens motor neurons in this disorder.

Protein kinase C in cancer: The top five unanswered questions

Few kinases have been studied as extensively as protein kinase C (PKC), particularly in the context of cancer. As major cellular targets for the phorbol ester tumor promoters and diacylglycerol (DAG), a second messenger generated by stimulation of membrane receptors, PKC isozymes play major roles in the control of signaling pathways associated with proliferation, migration, invasion, tumorigenesis, and metastasis. However, despite decades of research, fundamental questions remain to be answered or are the subject of intense controversy. Primary among these unresolved issues are the role of PKC isozymes as either tumor promoter or tumor suppressor kinases and the incomplete understanding on isozyme-specific substrates and effectors. The involvement of PKC isozymes in cancer progression needs to be reassessed in the context of specific oncogenic and tumor suppressing alterations. In addition, there are still major hurdles in addressing isozyme-specific function due to the limited specificity of most pharmacological PKC modulators and the lack of validated predictive biomarkers for response, which impacts the translation of these agents to the clinic. In this review we focus on key controversial issues and upcoming challenges, with the expectation that understanding the intricacies of PKC function will help fulfill the yet unsuccessful promise of targeting PKCs for cancer therapeutics.

Developmental RacGAP α2-Chimaerin Signaling Is a Determinant of the Morphological Features of Dendritic Spines in Adulthood

Morphological characteristics of dendritic spines form the basis of cognitive ability. However, molecular mechanisms involved in fine-tuning of spine morphology during development are not fully understood. Moreover, it is unclear whether, and to what extent, these developmental mechanisms determine the normal adult spine morphological features. Here, we provide evidence that α2-isoform of Rac-specific GTPase-activating protein α-chimaerin (α2-chimaerin) is involved in spine morphological refinement during late postnatal period, and furthermore show that this developmental α2-chimaerin function affects adult spine morphologies. We used a series of mice with global and conditional knock-out of α-chimaerin isoforms (α1-chimaerin and α2-chimaerin). α2-Chimaerin disruption, but not α1-chimaerin disruption, in the mouse results in an increased size (and density) of spines in the hippocampus. In contrast, overexpression of α2-chimaerin in developing hippocampal neurons induces a decrease of spine size. Disruption of α2-chimaerin suppressed EphA-mediated spine morphogenesis in cultured developing hippocampal neurons. α2-Chimaerin disruption that begins during the juvenile stage results in an increased size of spines in the hippocampus. Meanwhile, spine morphologies are unaltered when α2-chimaerin is deleted only in adulthood. Consistent with these spine morphological results, disruption of α2-chimaerin beginning in the juvenile stage led to an increase in contextual fear learning in adulthood; whereas contextual learning was recently shown to be unaffected when α2-chimaerin was deleted only in adulthood. Together, these results suggest that α2-chimaerin signaling in developmental stages contributes to determination of the morphological features of adult spines and establishment of normal cognitive ability.

SIGNIFICANCE STATEMENT

Recent studies of neurodevelopmental disorders in humans and their animal models have led to an attractive hypothesis that spine morphogenesis during development forms the basis of adult cognition. In particular, the roles of Rac and its regulators, such as Rac-specific GTPase-activating proteins (RacGAPs) and Rac guanine nucleotide exchange factors, are a topic of focus in spine morphogenesis and cognitive ability. Using a series of mice with global and conditional knock-out (KO) of RacGAP α-chimaerin isoforms (α1-chimaerin and α2-chimaerin), we provide compelling evidence demonstrating that α2-chimaerin is involved in spine morphological refinement during late postnatal development and that this developmental α2-chimaerin function affects adult spine morphologies. Furthermore, our results clearly showed that α2-chimaerin signaling during late postnatal development contributes to normal cognitive ability in adult mice.

Assessment of Jatropha curcas L. biodiesel seed cake toxicity using the zebrafish (Danio rerio) embryo toxicity (ZFET) test

Consequent to the growing demand for alternative sources of energy, the seeds from Jatropha curcas remain to be the favorite for biodiesel production. However, a significant volume of the residual organic mass (seed cake) is produced during the extraction process, which raises concerns on safe waste disposal. In the present study, we assessed the toxicity of J. curcas seed cake using the zebrafish (Danio rerio) embryotoxicity test. Within 1-h post-fertilization (hpf), the fertilized eggs were exposed to five mass concentrations of J. curcas seed cake and were followed through 24, 48, and 72 hpf. Toxicity was evaluated based on lethal endpoints induced on zebrafish embryos namely egg coagulation, non-formation of somites, and non-detachment of tail. The lowest concentration tested, 1 g/L, was not able to elicit toxicity on embryos whereas 100 % mortality (based also on lethal endpoints) was recorded at the highest concentration at 2.15 g/L. The computed LC50 for the J. curcas seed cake was 1.61 g/L. No further increase in mortality was observed in the succeeding time points (48 and 72 hpf) indicating that J. curcas seed cake exerted acute toxicity on zebrafish embryos. Sublethal endpoints (yolk sac and pericardial edema) were noted at 72 hpf in zebrafish embryos exposed to higher concentrations. The observed lethal endpoints induced on zebrafish embryos were discussed in relation to the active principles, notably, phorbol esters that have remained in the seed cake even after extraction.

Functional cross-talk between ras and rho pathways: a Ras-specific GTPase-activating protein (p120RasGAP) competitively inhibits the RhoGAP activity of deleted in liver cancer (DLC) tumor suppressor by masking the catalytic arginine finger

The three deleted in liver cancer genes (DLC1-3) encode Rho-specific GTPase-activating proteins (RhoGAPs). Their expression is frequently silenced in a variety of cancers. The RhoGAP activity, which is required for full DLC-dependent tumor suppressor activity, can be inhibited by the Src homology 3 (SH3) domain of a Ras-specific GAP (p120RasGAP). Here, we comprehensively investigated the molecular mechanism underlying cross-talk between two distinct regulators of small GTP-binding proteins using structural and biochemical methods. We demonstrate that only the SH3 domain of p120 selectively inhibits the RhoGAP activity of all three DLC isoforms as compared with a large set of other representative SH3 or RhoGAP proteins. Structural and mutational analyses provide new insights into a putative interaction mode of the p120 SH3 domain with the DLC1 RhoGAP domain that is atypical and does not follow the classical PXXP-directed interaction. Hence, p120 associates with the DLC1 RhoGAP domain by targeting the catalytic arginine finger and thus by competitively and very potently inhibiting RhoGAP activity. The novel findings of this study shed light on the molecular mechanisms underlying the DLC inhibitory effects of p120 and suggest a functional cross-talk between Ras and Rho proteins at the level of regulatory proteins.

β3-chimaerin, a novel member of the chimaerin Rac-GAP family

Chimaerins are a family of diacylglycerol- and phorbol ester-regulated GTPase activating proteins (GAPs) for the small G-protein Rac. Extensive evidence indicates that these proteins play important roles in development, axon guidance, metabolism, cell motility, and T cell activation. Four isoforms have been reported to-date, which are products of CHN1 (α1- and α2-chimaerins) and CHN2 (β1- and β2-chimaerins) genes. Although these gene products are assumed to be generated by alternative splicing, bioinformatics analysis of the CHN2 gene revealed that β1- and β2-chimaerins are the products of alternative transcription start sites (TSSs) in different promoter regions. Furthermore, we found an additional TSS in CHN2 gene that leads to a novel product, which we named β3-chimaerin. Expression profile analysis revealed predominantly low levels for the β3-chimaerin transcript, with higher expression levels in epididymis, plasma blood leucocytes, spleen, thymus, as well as various areas of the brain. In addition to the prototypical SH2, C1, and Rac-GAP domains, β3-chimaerin has a unique N-terminal domain. Studies in cells established that β3-chimaerin has Rac-GAP activity and is responsive to phorbol esters. The enhanced responsiveness of β3-chimaerin for phorbol ester-induced translocation relative to β2-chimaerin suggests differential ligand accessibility to the C1 domain.

Coordinated activation of the Rac-GAP β2-chimaerin by an atypical proline-rich domain and diacylglycerol

Chimaerins, a family of GTPase activating proteins (GAPs) for the small G-protein Rac, have been implicated in development, neuritogenesis, and cancer. These Rac-GAPs are regulated by the lipid second messenger diacylglycerol (DAG) generated by tyrosine-kinases such as the epidermal growth factor receptor (EGFR). Here we identify an atypical Pro-rich motif in chimaerins that binds to the adaptor protein Nck1. Unlike most Nck1 partners, chimaerins bind to the third SH3 domain of Nck1. This association is mediated by electrostatic interactions of basic residues within the Pro-rich motif with acidic clusters in the SH3 domain. EGF promotes the binding of β2-chimaerin to Nck1 in the cell periphery in a DAG-dependent manner. Moreover, β2-chimaerin translocation to the plasma membrane and its peripheral association with Rac1 requires Nck1. Our studies underscore a coordinated mechanism for β2-chimaerin activation that involves lipid interactions via the C1 domain and protein-protein interactions via the N-terminal Pro-rich region.

α2-Chimaerin regulates a key axon guidance transition during development of the oculomotor projection

The ocular motor system consists of three nerves which innervate six muscles to control eye movements. In humans, defective development of this system leads to eye movement disorders, such as Duane Retraction Syndrome, which can result from mutations in the α2-chimaerin signaling molecule. We have used the zebrafish to model the role of α2-chimaerin during development of the ocular motor system. We first mapped ocular motor spatiotemporal development, which occurs between 24 and 72 h postfertilization (hpf), with the oculomotor nerve following an invariant sequence of growth and branching to its muscle targets. We identified 52 hpf as a key axon guidance "transition," when oculomotor axons reach the orbit and select their muscle targets. Live imaging and quantitation showed that, at 52 hpf, axons undergo a switch in behavior, with striking changes in the dynamics of filopodia. We tested the role of α2-chimaerin in this guidance process and found that axons expressing gain-of-function α2-chimaerin isoforms failed to undergo the 52 hpf transition in filopodial dynamics, leading to axon stalling. α2-chimaerin loss of function led to ecotopic and misguided branching and hypoplasia of oculomotor axons; embryos had defective eye movements as measured by the optokinetic reflex. Manipulation of chimaerin signaling in oculomotor neurons in vitro led to changes in microtubule stability. These findings demonstrate that a correct level of α2-chimaerin signaling is required for key oculomotor axon guidance decisions, and provide a zebrafish model for Duane Retraction Syndrome.

Small G proteins and their regulators in cellular signalling

Small molecular weight GTPases (small G proteins) are essential in the transduction of signals from different plasma membrane receptors. Due to their endogenous GTP-hydrolyzing activity, these proteins function as time-dependent biological switches controlling diverse cellular functions including cell shape and migration, cell proliferation, gene transcription, vesicular transport and membrane-trafficking. This review focuses on endocrine diseases linked to small G proteins. We provide examples for the regulation of the activity of small G proteins by various mechanisms such as posttranslational modifications, guanine nucleotide exchange factors (GEFs), GTPase activating proteins (GAPs) or guanine nucleotide dissociation inhibitors (GDIs). Finally we summarize endocrine diseases where small G proteins or their regulatory proteins have been revealed as the cause.

Inhibition and Termination of Physiological Responses by GTPase Activating Proteins

Physiological processes are strictly organized in space and time. However, in cell physiology research, more attention is given to the question of space rather than to time. To function as a signal, environmental changes must be restricted in time; they need not only be initiated but also terminated. In this review, we concentrate on the role of one specific protein family involved in biological signal termination. GTPase activating proteins (GAPs) accelerate the endogenously low GTP hydrolysis rate of monomeric guanine nucleotide-binding proteins (GNBPs), limiting thereby their prevalence in the active, GTP-bound form. We discuss cases where defective or excessive GAP activity of specific proteins causes significant alteration in the function of the nervous, endocrine, and hemopoietic systems, or contributes to development of infections and tumors. Biochemical and genetic data as well as observations from human pathology support the notion that GAPs represent vital elements in the spatiotemporal fine tuning of physiological processes.

ARHGAP25, a novel Rac GTPase-activating protein, regulates phagocytosis in human neutrophilic granulocytes

Members of the Rac/Rho family of small GTPases play an essential role in phagocytic cells in organization of the actin cytoskeleton and production of toxic oxygen compounds. GTPase-activating proteins (GAPs) decrease the amount of the GTP-bound active form of small GTPases, and contribute to the control of biologic signals. The number of potential Rac/RhoGAPs largely exceeds the number of Rac/Rho GTPases and the expression profile, and their specific role in different cell types is largely unknown. In this study, we report for the first time the properties of full-length ARHGAP25 protein, and show that it is specifically expressed in hematopoietic cells, and acts as a RacGAP both in vitro and in vivo. By silencing and overexpressing the protein in neutrophil model cell lines (PLB-985 and CosPhoxFcγR, respectively) and in primary macrophages, we demonstrate that ARHGAP25 is a negative regulator of phagocytosis acting probably via modulation of the actin cytoskeleton.

Shaping up the membrane: diacylglycerol coordinates spatial orientation of signaling

Diacylglycerol signals by binding and activating C1 domain-containing proteins expressed principally in neuronal and immune tissues. This restricted expression profile suggests that diacylglycerol-regulated signals are particularly relevant in cell-cell communication processes in which active endocytosis and exocytosis take place. Not surprisingly, various experimental approaches have demonstrated a crucial role for diacylglycerol effectors and metabolizing enzymes in the control of immune responses, neuron communication and phagocytosis. Current research delineates a scenario in which coordinated decoding of diacylglycerol signals is translated into complex biological responses such as neuronal plasticity, T cell development or cytolytic killing. Diacylglycerol functions reach maximal diversity in these highly specialized systems in which signal intensity directly regulates distinct biological outcomes. This review brings together the most recent studies, emphasizing the contribution of compartmentalized DAG metabolism to orientated signaling events.

Two novel CHN1 mutations in 2 families with Duane retraction syndrome

OBJECTIVE

To determine the genetic cause of Duane retraction syndrome (DRS) in 2 families segregating DRS as a dominant trait.

METHODS

Members of 2 unrelated pedigrees were enrolled in a genetic study. Linkage analysis was performed on the CHN1 locus. Probands and family members were screened for CHN1 mutations.

RESULTS

The 6 affected individuals in the 2 pedigrees have DRS. Both pedigrees are consistent with linkage to the locus. Sequence analysis revealed 2 novel heterozygous missense CHN1 mutations, c.422C>T and c.754C>T, predicted to result in α2-chimaerin amino acid substitutions P141L and P252S, respectively.

CONCLUSIONS

Genetic analysis of 2 pedigrees revealed 2 novel DRS mutations, bringing the number of DRS pedigrees known to harbor CHN1 from 7 to 9. Both mutations alter residues that participate in intramolecular interactions that stabilize the inactive, closed conformation of α2-chimaerin and, thus, are predicted to result in its hyperactivation. Moreover, amino acid residue P252 was previously reported to be altered to a different residue in a previously reported DRS pedigree; thus, this is the first report of 2 CHN1 mutations altering the same residue, further supporting a gain-of-function etiology.

CLINICAL RELEVANCE

Members of families segregating DRS as an autosomal dominant trait should be screened for mutations in the CHN1 gene, enhancing genetic counseling and permitting earlier diagnosis.

Protein kinase C regulation: C1 meets C-tail

In a recent issue of Cell, Leonard et al. (2011) describe the structure of PKCβII, an AGC kinase, revealing an unanticipated intramolecular autoinhibitory interaction between its C-terminal tail and the diacylglycerol and phorbol ester binding site of its C1b domain.

Gain-of-function mutations of ARHGAP31, a Cdc42/Rac1 GTPase regulator, cause syndromic cutis aplasia and limb anomalies

Regulation of cell proliferation and motility is essential for normal development. The Rho family of GTPases plays a critical role in the control of cell polarity and migration by effecting the cytoskeleton, membrane trafficking, and cell adhesion. We investigated a recognized developmental disorder, Adams-Oliver syndrome (AOS), characterized by the combination of aplasia cutis congenita (ACC) and terminal transverse limb defects (TTLD). Through a genome-wide linkage analysis, we detected a locus for autosomal-dominant ACC-TTLD on 3q generating a maximum LOD score of 4.93 at marker rs1464311. Candidate-gene- and exome-based sequencing led to the identification of independent premature truncating mutations in the terminal exon of the Rho GTPase-activating protein 31 gene, ARHGAP31, which encodes a Cdc42/Rac1 regulatory protein. Mutant transcripts are stable and increase ARHGAP31 activity in vitro through a gain-of-function mechanism. Constitutively active ARHGAP31 mutations result in a loss of available active Cdc42 and consequently disrupt actin cytoskeletal structures. Arhgap31 expression in the mouse is substantially restricted to the terminal limb buds and craniofacial processes during early development; these locations closely mirror the sites of impaired organogenesis that characterize this syndrome. These data identify the requirement for regulated Cdc42 and/or Rac1 signaling processes during early human development.

p23/Tmp21 associates with protein kinase Cdelta (PKCdelta) and modulates its apoptotic function

There is emerging evidence that C1 domains, motifs originally identified in PKC isozymes and responsible for binding of phorbol esters and diacylglycerol, interact with the Golgi/endoplasmic reticulum protein p23 (Tmp21). In this study, we investigated whether PKCδ, a kinase widely implicated in apoptosis and inhibition of cell cycle progression, associates with p23 and determined the potential functional implications of this interaction. Using a yeast two-hybrid approach, we found that the PKCδ C1b domain associates with p23 and identified two key residues (Asp(245) and Met(266)) implicated in this interaction. Interestingly, silencing p23 from LNCaP prostate cancer cells using RNAi markedly enhanced PKCδ-dependent apoptosis and activation of PKCδ downstream effectors ROCK and JNK by phorbol 12-myristate 13-acetate. Moreover, translocation of PKCδ to the plasma membrane by phorbol 12-myristate 13-acetate was enhanced in p23-depleted LNCaP cells. Notably, a PKCδ mutant that failed to interact with p23 triggered a strong apoptotic response when expressed in LNCaP cells. In summary, our data compellingly support the concept that C1 domains have dual roles both in lipid and protein associations and provide strong evidence that p23 acts as an anchoring protein that retains PKCδ at the perinuclear region, thus limiting the availability of this kinase for activation in response to stimuli.

Rac GTPase-activating protein (Rac GAP) α1-Chimaerin undergoes proteasomal degradation and is stabilized by diacylglycerol signaling in neurons

α1-Chimaerin is a neuron-specific member of the Rho GTPase-activating protein family that selectively inactivates the small GTPase Rac. It is known to regulate the structure of dendrites and dendritic spines. We describe here that under basal conditions α1-chimaerin becomes polyubiquitinated and undergoes rapid proteasomal degradation. This degradation is partly dependent on the N-terminal region that is unique to this isoform. Mimicking diacylglycerol (DAG) signaling with a phorbol ester stabilizes endogenous α1-chimaerin against degradation and causes accumulation of the protein. The stabilization requires phorbol ester binding via the C1 domain of the protein and is independent of PKC activity. In addition, overexpression of a constitutively active Rac1 mutant is sufficient to cause an accumulation of α1-chimaerin through a phospholipase C-dependent mechanism, showing that endogenous DAG signaling can also stabilize the protein. These results suggest that signaling via DAG may regulate the abundance of α1-chimaerin under physiological conditions, providing a new model for understanding how its activity could be controlled.

A novel cross-talk in diacylglycerol signaling: the Rac-GAP beta2-chimaerin is negatively regulated by protein kinase Cdelta-mediated phosphorylation

Although the family of chimaerin Rac-GAPs has recently gained significant attention for their involvement in development, cancer, and neuritogenesis, little is known about their molecular regulation. Chimaerins are activated by the lipid second messenger diacylglycerol via their C1 domain upon activation of tyrosine kinase receptors, thereby restricting the magnitude of Rac signaling in a receptor-regulated manner. Here we identified a novel regulatory mechanism for beta2-chimaerin via phosphorylation. Epidermal growth factor or the phorbol ester phorbol 12-myristate 13-acetate caused rapid phosphorylation of beta2-chimaerin on Ser(169) located in the SH2-C1 domain linker region via protein kinase Cdelta, which retained beta2-chimaerin in the cytosol and prevented its C1 domain-mediated translocation to membranes. Furthermore, despite the fact that Ser(169) phosphorylation did not alter intrinsic Rac-GAP activity in vitro, a non-phosphorylatable beta2-chimaerin mutant was highly sensitive to translocation, and displayed enhanced association with activated Rac, enhanced Rac-GAP activity, and anti-migratory properties when expressed in cells. Our results not only revealed a novel regulatory mechanism that facilitates Rac activation, but also identified a novel mechanism of cross-talk between diacylglycerol receptors that restricts beta2-chimaerin relocalization and activation.

Biochemical characterization of hyperactive beta2-chimaerin mutants revealed an enhanced exposure of C1 and Rac-GAP domains

Recent studies established that the Rac-GAP beta2-chimaerin plays important roles in development, neuritogenesis, and cancer progression. A unique feature of beta2-chimaerin is that it can be activated by phorbol esters and the lipid second messenger diacylglycerol (DAG), which bind with high affinity to its C1 domain and promote beta2-chimaerin translocation to membranes, leading to the inactivation of the small G-protein Rac. Crystallographic evidence and cellular studies suggest that beta2-chimaerin remains in an inactive conformation in the cytosol with the C1 domain inaccessible to ligands. We developed a series of beta2-chimaerin point mutants in which intramolecular contacts that occlude the C1 domain have been disrupted. These mutants showed enhanced translocation in response to phorbol 12-myristate 13-acetate (PMA) in cells. Binding assays using [(3)H]phorbol 12,13-dibutyrate ([(3)H]PDBu) revealed that internal contact mutants have a reduced acidic phospholipid requirement for phorbol ester binding. Moreover, disruption of intramolecular contacts enhances binding of beta2-chimaerin to acidic phospholipid vesicles and confers enhanced Rac-GAP activity in vitro. These studies suggest that beta2-chimaerin must undergo a conformational rearrangement in order to expose its lipid binding sites and become activated.

Beta2-chimaerin binds to EphA receptors and regulates cell migration

Ephrins and Eph receptors have key roles in regulation of cell migration during development. We found that the RacGAP beta2-chimaerin (chimerin) bound to EphA2 and EphA4 and inactivated Rac1 in response to ephrinA1 stimulation. EphA4 bound to beta2-chimaerin through its kinase domain and promoted binding of Rac1 to beta2-chimaerin. In addition, knockdown of endogenous beta2-chimaerin blocked ephrinA1-induced suppression of cell migration. These results suggest that beta2-chimaerin is activated by EphA receptors and mediates the EphA receptor-dependent regulation of cell migration.

Lipid binding domains: more than simple lipid effectors

The spatial and temporal regulation of lipid molecules in cell membranes is a hallmark of cellular signaling and membrane trafficking events. Lipid-mediated targeting provides for strict control and versatility, because cell membranes harbor a large number of lipid molecules with variation in head group and acyl chain structures. Signaling and trafficking proteins contain a large number of modular domains that exhibit specific lipid binding properties and play a critical role in their localization and function. Nearly 20 years of research including structural, computational, biochemical and biophysical studies have demonstrated how these lipid-binding domains recognize their target lipid and achieve subcellular localization. The integration of this individual lipid-binding domain data in the context of the full-length proteins, macromolecular signaling complexes, and the lipidome is only beginning to be unraveled and represents a target of therapeutic development. This review brings together recent findings and classical concepts to concisely summarize the lipid-binding domain field while illustrating where the field is headed and how the gaps may be filled in with new technologies.