Targeting and clustering citron to synapses

Copy Number Variations in DISC1 and DISC1-Interacting Partners in Major Mental Illness

Robust statistical, genetic and functional evidence supports a role for DISC1 in the aetiology of major mental illness. Furthermore, many of its protein-binding partners show evidence for involvement in the pathophysiology of a range of neurodevelopmental and psychiatric disorders. Copy number variants (CNVs) are suspected to play an important causal role in these disorders. In this study, CNV analysis of DISC1 and its binding partners PAFAH1B1, NDE1, NDEL1, FEZ1, MAP1A, CIT and PDE4B in Scottish and Northern Swedish population-based samples was carried out using multiplex amplicon quantification. Here, we report the finding of rare CNVs in DISC1, NDE1 (together with adjacent genes within the 16p13.11 duplication), NDEL1 (including the overlapping MYH10 gene) and CIT. Our findings provide further evidence for involvement of DISC1 and its interaction partners in neuropsychiatric disorders and also for a role of structural variants in the aetiology of these devastating diseases.

Myotonic dystrophy kinase-related Cdc42-binding kinases (MRCK), the ROCK-like effectors of Cdc42 and Rac1

Cdc42 is a member of the Rho GTPase protein family that plays key roles in local F-actin organization through a number of kinase and non-kinase effector proteins. The myotonic dystrophy kinase-related Cdc42-binding kinases (MRCKs), and the RhoA binding coiled-coil containing kinases (ROCKs) are widely expressed members of the Dystrophia myotonica protein kinase (DMPK) family. The MRCK proteins are ∼190 kDa multi-domain proteins expressed in all cells and coordinate certain acto-myosin networks. Notably MRCK is a key regulator of myosin18A and myosin IIA/B, and through phosphorylation of their common regulatory light chains (MYL9 or MLC2) to promote actin stress fiber contractility. The MRCK kinases are regulated by Cdc42, which is required for cell polarity and directional migration; MRCK links to the acto-myosin complex through interaction with a coiled-coil containing adaptor proteins LRAP35a/b. The biological activities of MRCK in model organisms such as worms and flies confirm it as a myosin II activator. In mammalian cell culture MRCK can be critical for cancer cell migration and neurite outgrowth. We review the current literatures regarding MRCK and highlight the similarities and differences between MRCK and ROCK kinases.

p140Cap regulates memory and synaptic plasticity through Src-mediated and citron-N-mediated actin reorganization

A major challenge in the neuroscience field is the identification of molecules and pathways that control synaptic plasticity and memory. Dendritic spines play a pivotal role in these processes, as the major sites of excitatory synapses in neuronal communication. Previous studies have shown that the scaffold protein p140Cap localizes into dendritic spines and that its knockdown negatively modulates spine shape in culture. However, so far, there is no information on its in vivo relevance. By using a knock-out mouse model, we here demonstrate that p140Cap is a key element for both learning and synaptic plasticity. Indeed, p140Cap(-/-) mice are impaired in object recognition test, as well as in LTP and in LTD measurements. The in vivo effects of p140Cap loss are presumably attenuated by noncell-autonomous events, since primary neurons obtained from p140Cap(-/-) mice show a strong reduction in number of mushroom spines and abnormal organization of synapse-associated F-actin. These phenotypes are most likely caused by a local reduction of the inhibitory control of RhoA and of cortactin toward the actin-depolymerizing factor cofilin. These events can be controlled by p140Cap through its capability to directly inhibit the activation of Src kinase and by its binding to the scaffold protein Citron-N. Altogether, our results provide new insight into how protein associated with dynamic microtubules may regulate spine actin organization through interaction with postsynaptic density components.

Citron kinase mediates transition from constriction to abscission through its coiled-coil domain

Cytokinesis is initiated by constriction of the cleavage furrow, and completed with separation of the two daughter cells by abscission. Control of transition from constriction to abscission is therefore crucial for cytokinesis. However, the underlying mechanism is largely unknown. Here, we analyze the role of Citron kinase (Citron-K) that localizes at the cleavage furrow and the midbody, and dissect its action mechanisms during this transition. Citron-K forms a stable ring-like structure at the midbody and its depletion affects the maintenance of the intercellular bridge, resulting in fusion of two daughter cells after the cleavage furrow ingression. RNA interference (RNAi) targeting Citron-K reduced accumulation of RhoA, Anillin, and septins at the intercellular bridge in mid telophase, and impaired concentration and maintenance of KIF14 and PRC1 at the midbody in late telophase. RNAi rescue experiments revealed that these functions of Citron-K are mediated by its coiled-coil (CC) domain, and not by its kinase domain. The C-terminal part of CC contains a Rho-binding domain and a cluster-forming region and is important for concentrating Citron-K from the cleavage furrow to the midbody. The N-terminal part of CC directly binds to KIF14, and this interaction is required for timely transfer of Citron-K to the midbody after furrow ingression. We propose that the CC-domain-mediated translocation and actions of Citron-K ensure proper stabilization of the midbody structure during the transition from constriction to abscission.

p27(Kip1) controls cytokinesis via the regulation of citron kinase activation

p27(Kip1) (p27) acts as a tumor suppressor by inhibiting cyclin-cyclin-dependent kinase (cyclin-CDK) activity. However, mice expressing a form of p27 that is unable to bind or inhibit cyclin-CDK complexes (p27(CK-)) have increased incidence of tumor development as compared with wild-type and p27(-/-) mice, revealing an oncogenic role for p27. Here, we identified a phenotype of multinucleation and polyploidy in p27(CK-) mice not present in p27(-/-) animals, suggesting a role for p27 in G2/M that is independent of cyclin-CDK regulation. Further analysis revealed that p27(CK-) expression caused a cytokinesis and abscission defect in mouse embryonic fibroblasts. We identified the Rho effector citron kinase (citron-K) as a p27-interacting protein in vitro and in vivo and found that p27 and citron-K colocalized at the contractile ring and mid-body during telophase and cytokinesis. Moreover, overexpression of the minimal p27-binding domain of citron-K was sufficient to rescue the phenotype caused by p27(CK-). Conversely, expression of a mutant p27(CK-) unable to bind citron-K did not induce multinucleation. Finally, by binding to citron-K, p27 prevented the interaction of citron-K with its activator RhoA. Taken together, these data suggest a role for p27 during cytokinesis via the regulation of citron-K activity.

Induction and expression rules of synaptic plasticity in hippocampal interneurons

The knowledge that excitatory synapses on aspiny hippocampal interneurons can develop genuine forms of activity-dependent remodeling, independently from the surrounding network of principal cells, is a relatively new concept. Cumulative evidence has now unequivocally demonstrated that, despite the absence of specialized postsynaptic spines that serve as compartmentalized structure for intracellular signaling in principal cell plasticity, excitatory inputs onto interneurons can undergo forms of synaptic plasticity that are induced and expressed autonomously from principal cells. Yet, the rules for induction and expression of interneuron plasticity are much more heterogeneous than in principal neurons. Long-term plasticity in interneurons is not necessarily dependent upon postsynaptic activation of NMDA receptors nor relies on the same postsynaptic membrane potential requirements as principal cells. Plasticity in interneurons rather requires activation of Ca(2+)-permeable AMPA receptors and/or metabotropic glutamate receptors and is triggered by postsynaptic hyperpolarization. In this review we will outline these distinct features of interneuron plasticity and identify potential critical candidate molecules that might be important for sustaining long-lasting changes in synaptic strength at excitatory inputs onto interneurons. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

IFN-beta pharmacogenomics in multiple sclerosis

Multiple sclerosis (MS) is a condition of the CNS marked by inflammation and neurodegeneration. Interferon (IFN)-beta was the first, and still is the main, immunomodulatory treatment for MS. Its clinical efficacy is limited, and a proportion of patients, ranging between 20-55%, do not respond to the therapy. Identification and subsequently, implementation in the clinic of biomarkers predictive for individual therapeutic response would facilitate improved patient care in addition to ensuring a more rational provision of this therapy. In this article, we summarize the main findings from studies addressing the pharmacogenomics of clinical response to IFN-beta in MS by either whole-genome association scans, candidate gene or transcriptomics studies. Whole-genome DNA association screens have revealed a high representation of brain-specific genes, and have hinted toward both extracellular ligand-gated ion channels and type I IFNs pathway genes as important categories of genetic IFN-beta response modifiers. One hit, glypican 5 (GPC5), was recently replicated in an independent study of IFN-beta responsiveness. Recent RNA transcriptomics studies have revealed the occurrence of a pre-existing type I IFN gene-expression signature, composed of genes that are predominantly induced by type I IFNs, as a potential contributing feature of poor response to therapy. Thus, while the outlines of a complex polygenic mechanism are gradually being uncovered, the main challenges for the near future will reside in the robust validation of identified response-modifying genes as well as in the decipherment of the mechanistic relationships between these genes and clinical response to IFN-beta.

Requirement for protein synthesis at developing synapses

Activity and protein synthesis act cooperatively to generate persistent changes in synaptic responses. This forms the basis for enduring memory in adults. Activity also shapes neural circuits developmentally, but whether protein synthesis plays a congruent function in this process is poorly understood. Here, we show that brief periods of global or local protein synthesis inhibition decrease the synaptic vesicles available for fusion and increase synapse elimination. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is a critical target; its levels are controlled by rapid turnover, and blocking its activity or knocking it down recapitulates the effects of protein synthesis inhibition. Mature presynaptic terminals show decreased sensitivity to protein synthesis inhibition, and resistance coincides with a developmental switch in regulation from CaMKII to PKA (protein kinase A). These findings demonstrate a novel mechanism regulating presynaptic activity and synapse elimination during development, and suggest that protein translation acts coordinately with activity to selectively stabilize appropriate synaptic interactions.

Genetic and systems level analysis of Drosophila sticky/citron kinase and dFmr1 mutants reveals common regulation of genetic networks

Background

In Drosophila, the genes sticky and dFmr1 have both been shown to regulate cytoskeletal dynamics and chromatin structure. These genes also genetically interact with Argonaute family microRNA regulators. Furthermore, in mammalian systems, both genes have been implicated in neuronal development. Given these genetic and functional similarities, we tested Drosophila sticky and dFmr1 for a genetic interaction and measured whole genome expression in both mutants to assess similarities in gene regulation.

Results

We found that sticky mutations can dominantly suppress a dFmr1 gain-of-function phenotype in the developing eye, while phenotypes produced by RNAi knock-down of sticky were enhanced by dFmr1 RNAi and a dFmr1 loss-of-function mutation. We also identified a large number of transcripts that were misexpressed in both mutants suggesting that sticky and dFmr1 gene products similarly regulate gene expression. By integrating gene expression data with a protein-protein interaction network, we found that mutations in sticky and dFmr1 resulted in misexpression of common gene networks, and consequently predicted additional specific phenotypes previously not known to be associated with either gene. Further phenotypic analyses validated these predictions.

Conclusion

These findings establish a functional link between two previously unrelated genes. Microarray analysis indicates that sticky and dFmr1 are both required for regulation of many developmental genes in a variety of cell types. The diversity of transcripts regulated by these two genes suggests a clear cause of the pleiotropy that sticky and dFmr1 mutants display and provides many novel, testable hypotheses about the functions of these genes. As both of these genes are implicated in the development and function of the mammalian brain, these results have relevance to human health as well as to understanding more general biological processes.

Profilin, a multi-modal regulator of neuronal plasticity

Thirty years after its initial characterization and more than 1000 publications listed in PubMed describing its properties, the small (ca 15 kDa) protein profilin continues to surprise us with new, recently discovered functions. Originally described as an actin-binding protein, profilin has now been shown to interact with more than a dozen proteins in mammalian cells. Some of the more recently described and intriguing interactions are within neurons involving a neuronal profilin family member. Profilin is now regarded as a regulator of various cellular processes such as cytoskeletal dynamics, membrane trafficking and nuclear transport. Profilin is a necessary element in key steps of neuronal differentiation and synaptic plasticity, and embodies properties postulated for a synaptic tag. These findings identify profilin as an important factor linking cellular and behavioural plasticity in neural circuits.

The RhoA-associated protein Citron-N controls dendritic spine maintenance by interacting with spine-associated Golgi compartments

Dendritic spines are highly dynamic protuberances that are thought to be crucial for learning and memory. Although it is well known that actin filaments and membrane dynamics regulate spine plasticity, how these two events are linked locally is less clear. Here, we provide evidence that Citron-N (CIT-N), a binding partner of the small GTPase RhoA, is associated with the actin filaments and Golgi compartments of dendritic spines. We also show that CIT-N is required for recruiting F-actin and Golgi membranes at spines of in vitro-grown neurons. Studies in knockout mice show that this protein is essential for the maturation of dendritic spines. We suggest that CIT-N might function as a scaffold protein in spine organization through its ability to bind to Golgi membranes and by affecting actin remodelling.

Drosophila sticky/citron kinase is a regulator of cell-cycle progression, genetically interacts with Argonaute 1 and modulates epigenetic gene silencing

The sticky/citron kinase protein is a conserved regulator of cell-cycle progression from invertebrates to humans. While this kinase is essential for completion of cytokinesis, sticky/citron kinase phenotypes disrupting neurogenesis and cell differentiation suggest additional non-cell-cycle functions. However, it is not known whether these phenotypes are an indirect consequence of sticky mutant cell-cycle defects or whether they define a novel function for this kinase. We have isolated a temperature-sensitive allele of the Drosophila sticky gene and we show that sticky/citron kinase is required for histone H3-K9 methylation, HP1 localization, and heterochromatin-mediated gene silencing. sticky genetically interacts with Argonaute 1 and sticky mutants exhibit context-dependent Su(var) and E(var) activity. These observations indicate that sticky/citron kinase functions to regulate both actin-myosin-mediated cytokinesis and epigenetic gene silencing, possibly linking cell-cycle progression to heterochromatin assembly and inheritance of gene expression states.

The Down syndrome critical region protein TTC3 inhibits neuronal differentiation via RhoA and Citron kinase

The Down syndrome critical region (DSCR) on Chromosome 21 contains many genes whose duplication may lead to the major phenotypic features of Down syndrome and especially the associated mental retardation. However, the functions of DSCR genes are mostly unknown and their possible involvement in key brain developmental events still largely unexplored. In this report we show that the protein TTC3, encoded by one of the main DSCR candidate genes, physically interacts with Citron kinase (CIT-K) and Citron N (CIT-N), two effectors of the RhoA small GTPase that have previously been involved in neuronal proliferation and differentiation. More importantly, we found that TTC3 levels can strongly affect the NGF-induced differentiation of PC12 cells, by a CIT-K-dependent mechanism. Indeed, TTC3 overexpression leads to strong inhibition of neurite extension, which can be reverted by CIT-K RNAi. Conversely, TTC3 knockdown stimulates neurite extension in the same cells. Finally, we find that Rho, but not Rho kinase, is required for TTC3 differentiation-inhibiting activity. Our results suggest that the TTC3-RhoA-CIT-K pathway could be a crucial determinant of in vivo neuronal development, whose hyperactivity may result in detrimental effects on the normal differentiation program.

A thermodynamic ligand binding study of the third PDZ domain (PDZ3) from the mammalian neuronal protein PSD-95

The thermodynamic parameters associated with the binding of several series of linear peptides to the third PDZ domain (PDZ3) of the postsynaptic density 95 protein (PSD-95) have been measured using isothermal titration calorimetry (ITC). Two strategies were pursued in developing these binding ligands: (1) systematic N-terminal truncation of sequences derived from the C-terminal regions of identified PDZ3-binding proteins (CRIPT, neuroligin-1, and citron) and (2) selective mutation of specific positions within a consensus hexapeptide (KKETEV) known to bind PDZ3. Each synthetically prepared peptide was used to titrate PDZ3, which yielded the changes in Gibbs free energy (DeltaG), enthalpy (DeltaH), and entropy (TDeltaS) for the binding event. Selected peptides were subjected to additional analysis, which entailed (1) measuring the change in heat capacity (DeltaCp) upon association, to assess the character of the binding interface, and (2) constructing thermodynamic double mutant cycles, to determine the presence of cooperative effects. From the first series, the CRIPT protein proved to be the better source for higher affinity sequences. From the second series, enhanced binding was associated with peptides that closely adhered to the established motif for class I PDZ domain C-termini, X-(T/S)-X-(V/I/L), and more specifically to a narrower motif of X-T-X-V. Further, in both series a length of six residues was necessary and sufficient to capture maximal affinity. In addition, there were significant influences upon binding by modifying the abutting "X" positions. The cumulative results provide greater detail into the specific nature of ligand binding to PDZ3 and will assist in the development of selective molecular probes for the study of this and structurally homologous PDZ domains.