ProSAPiP2, a novel postsynaptic density protein that interacts with ProSAP2/Shank3

Dosage-sensitive genes in autism spectrum disorders: From neurobiology to therapy

Autism spectrum disorders (ASDs) are a group of heterogenous neurodevelopmental disorders affecting 1 in 59 children. Syndromic ASDs are commonly associated with chromosomal rearrangements or dosage imbalance involving a single gene. Many of these genes are dosage-sensitive and regulate transcription, protein homeostasis, and synaptic function in the brain. Despite vastly different molecular perturbations, syndromic ASDs share core symptoms including social dysfunction and repetitive behavior. However, each ASD subtype has a unique pathogenic mechanism and combination of comorbidities that require individual attention. We have learned a great deal about how these dosage-sensitive genes control brain development and behaviors from genetically-engineered mice. Here we describe the clinical features of eight monogenic neurodevelopmental disorders caused by dosage imbalance of four genes, as well as recent advances in using genetic mouse models to understand their pathogenic mechanisms and develop intervention strategies. We propose that applying newly developed quantitative molecular and neuroscience technologies will advance our understanding of the unique neurobiology of each disorder and enable the development of personalized therapy.

HDAC2-dependent Antipsychotic-like Effects of Chronic Treatment with the HDAC Inhibitor SAHA in Mice

Antipsychotic drugs, including both typical such as haloperidol and atypical such as clozapine, remain the current standard for schizophrenia treatment. These agents are relatively effective in treating hallucinations and delusions. However, cognitive deficits are at present essentially either persistent or exacerbated following chronic antipsychotic drug exposure. This underlines the need of new therapeutic approaches to improve cognition in treated schizophrenia patients. Our previous findings suggested that upregulation of histone deacetylase 2 (HDAC2) expression upon chronic antipsychotic treatment may lead to negative effects on cognition and cortical synaptic structure. Here we tested different phenotypes of psychosis, synaptic plasticity, cognition and antipsychotic drug action in HDAC2 conditional knockout (HDAC2-cKO) mice and controls. Conditional depletion of HDAC2 function in glutamatergic pyramidal neurons led to a protective phenotype against behavior models induced by psychedelic and dissociative drugs, such as DOI and MK801, respectively. Immunoreactivity toward synaptophysin, which labels presynaptic terminals of functional synapses, was decreased in the frontal cortex of control mice chronically treated with clozapine - an opposite effect occurred in HDAC2-cKO mice. Chronic treatment with the class I and class II HDAC inhibitor SAHA prevented via HDAC2 the disruptive effects of MK801 on recognition memory. Additionally, chronic SAHA treatment affected transcription of numerous plasticity-related genes in the frontal cortex of control mice, an effect that was not observed in HDAC2-cKO animals. Together, these findings suggest that HDAC2 may represent a novel target to improve synaptic plasticity and cognition in treated schizophrenia patients.

Actin-Dependent Alterations of Dendritic Spine Morphology in Shankopathies

Shank proteins (Shank1, Shank2, and Shank3) act as scaffolding molecules in the postsynaptic density of many excitatory neurons. Mutations in SHANK genes, in particular SHANK2 and SHANK3, lead to autism spectrum disorders (ASD) in both human and mouse models. Shank3 proteins are made of several domains-the Shank/ProSAP N-terminal (SPN) domain, ankyrin repeats, SH3 domain, PDZ domain, a proline-rich region, and the sterile alpha motif (SAM) domain. Via various binding partners of these domains, Shank3 is able to bind and interact with a wide range of proteins including modulators of small GTPases such as RICH2, a RhoGAP protein, and βPIX, a RhoGEF protein for Rac1 and Cdc42, actin binding proteins and actin modulators. Dysregulation of all isoforms of Shank proteins, but especially Shank3, leads to alterations in spine morphogenesis, shape, and activity of the synapse via altering actin dynamics. Therefore, here, we highlight the role of Shank proteins as modulators of small GTPases and, ultimately, actin dynamics, as found in multiple in vitro and in vivo models. The failure to mediate this regulatory role might present a shared mechanism in the pathophysiology of autism-associated mutations, which leads to dysregulation of spine morphogenesis and synaptic signaling.

The Kvβ2 subunit of voltage-gated potassium channels is interacting with ProSAP2/Shank3 in the PSD

The postsynaptic density is an electron dense meshwork composed of a variety of molecules facilitating neuronal signal transmission. ProSAP2/Shank3 represents a crucial player at postsynaptic sites, assembling large multimeric platforms and anchoring numerous other molecules, thereby linking the functional synapse with the cytoskeleton. ProSAP2/Shank3 is also implicated in the pathogenesis of numerous diseases, including autism spectrum disorders. KvBeta2 (Kvβ2) on the other hand serves as a regulatory subunit of voltage-gated potassium channels. Kvβ2 is located at various sites in the neuron including the axon (binding to Kv1.2), the dendrites (binding to Kv4.2) and the synapse. Binding of Kvβ2 to either Kv1.2 or Kv4 modulates not only the channel conformation but directs targeting of the channel protein complex to distinct loci within the cell. Thus an interaction between ProSAP2 and Kvβ2 could have important roles at diverse cellular compartments and moreover during maturation stages. We report here on the direct protein-protein interaction of the postsynaptic density anchoring molecule ProSAP2 and the potassium channel subunit Kvβ2, initially identified in a yeast-two-hybrid-screen. Furthermore, we characterize this interaction at synapses using primary hippocampal neurons in vitro.

Modeling autism by SHANK gene mutations in mice

Shank family proteins (Shank1, Shank2, and Shank3) are synaptic scaffolding proteins that organize an extensive protein complex at the postsynaptic density (PSD) of excitatory glutamatergic synapses. Recent human genetic studies indicate that SHANK family genes (SHANK1, SHANK2, and SHANK3) are causative genes for idiopathic autism spectrum disorders (ASD). Neurobiological studies of Shank mutations in mice support a general hypothesis of synaptic dysfunction in the pathophysiology of ASD. However, the molecular diversity of SHANK family gene products, as well as the heterogeneity in human and mouse phenotypes, pose challenges to modeling human SHANK mutations. Here, we review the molecular genetics of SHANK mutations in human ASD and discuss recent findings where such mutations have been modeled in mice. Conserved features of synaptic dysfunction and corresponding behaviors in Shank mouse mutants may help dissect the pathophysiology of ASD, but also highlight divergent phenotypes that arise from different mutations in the same gene.

Calcium activated potassium channel expression during human iPS cell-derived neurogenesis

The family of calcium activated potassium channels of low and intermediate conductance, known as SK channels, consists of four members (SK1-4). These channels are widely expressed throughout the organism and involved in various cellular processes, such as the afterhyperpolarization in excitable cells but also in differentiation processes of various tissues. To date, the role of SK channels in developmental processes has been merely a marginal focus of investigation, although it is well accepted that cell differentiation and maturation affect the expression patterns of certain ion channels. Recently, several studies from our laboratory delineated the influence of SK channel expression and their respective activity on cytoskeletal reorganization in neural and pluripotent stem cells and regulation of cell fate determination toward the cardiac lineage in human and mouse pluripotent stem cells. Herein, we have now analyzed SK channel expression patterns and distribution at various stages of human induced pluripotent stem cell-derived neurogenesis particularly focusing on undifferentiated iPS cells, neural progenitors and mature neurons. All family members could be detected starting at the iPS cell level and were differentially expressed during the subsequent maturation process. Intriguingly, we found obvious discrepancies between mRNA and protein expression pointing toward a complex regulatory mechanism. Inhibition of SK channels with either apamin or clotrimazol did not have any significant effects on the speed or amount of neurogenesis in vitro. The abundance and specific regulation of SK channel expression during iPS cell differentiation indicates distinct roles of these ion channels not only for the cardiac but also for neuronal cell differentiation and in vitro neurogenesis.

Tubulin-binding cofactor B is a direct interaction partner of the dynactin subunit p150(Glued)

The dynactin p150(Glued) subunit, encoded by the gene DCTN1, is part of the dynein-dynactin motor protein complex responsible for retrograde axonal transport in motor neurons. The p150 subunit is a candidate gene for neurodegenerative diseases, in particular motor neuron and extrapyramidal diseases. Tubulin-binding cofactors are believed to be involved in tubulin biogenesis and degradation and therefore to contribute to microtubule functional diversity and regulation. A yeast-two-hybrid screen for putative interacting proteins of dynactin p150(Glued) has revealed tubulin-folding cofactor B (TBCB). We analyzed the interaction of these proteins and investigated the impact of this complex on the microtubule network in cell lines and primary hippocampal neurons in vitro. We especially concentrated on neuronal morphology and synaptogenesis. Overexpression of both proteins or depletion of TBCB alone does not alter the microtubule network and/or neuronal morphology. The demonstration of the interaction of the transport molecule dynactin and the tubulin-regulating factor TBCB is thought to have an impact on several cellular mechanisms. TBCB expression levels have been found to have only a subtle influence on the microtubule network and neuronal morphology. However, overexpression of TBCB leads to the decreased localization of p150 to the microtubule network that might result in a functional modulation of this protein complex.

The postsynaptic density protein Abelson interactor protein 1 interacts with the motor protein Kinesin family member 26B in hippocampal neurons

Abelson interactor protein 1 (Abi-1) localizes to postsynaptic densities (PSDs) of excitatory synapses and was shown to be transported from the PSD to the nucleus and back depending upon synaptic activation. We employed a yeast-two-hybrid screen to search for putative transport molecules. We found Kif26B a member of the Kif family of transport proteins that has not been characterized in the central nervous system as a direct interaction partner of Abi-1. We delineated a proline-rich motif within the cargo-binding domain of Kif26B to be responsible for this protein-protein interaction. Kif26B was able to recruit Abi-1 to the microtubule network and we found that the expression of Kif26B is responsible for the localization of Abi-1 to PSDs in maturing neurons. Taken together we report that Abi-1 is a cargo of Kif26B in primary hippocampal neurons, pointing to a role of this transport molecule in the movement of Abi-1 between different cell compartments. Additionally, we provide the first detailed investigation of Kif26B and its cargo molecules in neuronal cells.

Scaffold proteins at the postsynaptic density

Scaffold proteins are abundant and essential components of the postsynaptic density (PSD). They play a major role in many synaptic functions including the trafficking, anchoring, and clustering of glutamate receptors and adhesion molecules. Moreover, they link postsynaptic receptors with their downstream signaling proteins and regulate the dynamics of cytoskeletal structures. By definition, PSD scaffold proteins do not have intrinsic enzymatic activities but are formed by modular and specific domains deputed to form large protein networks. Here, we will discuss the latest findings regarding the structure and functions of major PSD scaffold proteins. Given that scaffold proteins are central components of PSD architecture, it is not surprising that deletion or mutations in their human genes cause severe neuropsychiatric disorders including autism, mental retardation, and schizophrenia. Thus, their dynamic organization and regulation are directly correlated with the essential structure of the PSD and the normal physiology of neuronal synapses.

Heterogeneous nuclear ribonucleoprotein k interacts with Abi-1 at postsynaptic sites and modulates dendritic spine morphology

Background

Abelson-interacting protein 1 (Abi-1) plays an important role for dendritic branching and synapse formation in the central nervous system. It is localized at the postsynaptic density (PSD) and rapidly translocates to the nucleus upon synaptic stimulation. At PSDs Abi-1 is in a complex with several other proteins including WASP/WAVE or cortactin thereby regulating the actin cytoskeleton via the Arp 2/3 complex.

Principal Findings

We identified heterogeneous nuclear ribonucleoprotein K (hnRNPK), a 65 kDa ssDNA/RNA-binding-protein that is involved in multiple intracellular signaling cascades, as a binding partner of Abi-1 at postsynaptic sites. The interaction with the Abi-1 SH3 domain is mediated by the hnRNPK-interaction (KI) domain. We further show that during brain development, hnRNPK expression becomes more and more restricted to granule cells of the cerebellum and hippocampal neurons where it localizes in the cell nucleus as well as in the spine/dendritic compartment. The downregulation of hnRNPK in cultured hippocampal neurons by RNAi results in an enlarged dendritic tree and a significant increase in filopodia formation. This is accompanied by a decrease in the number of mature synapses. Both effects therefore mimic the neuronal morphology after downregulation of Abi-1 mRNA in neurons.

Conclusions

Our findings demonstrate a novel interplay between hnRNPK and Abi-1 in the nucleus and at synaptic sites and show obvious similarities regarding both protein knockdown phenotypes. This indicates that hnRNPK and Abi-1 act synergistic in a multiprotein complex that regulates the crucial balance between filopodia formation and synaptic maturation in neurons.

An inducible expression system of the calcium-activated potassium channel 4 to study the differential impact on embryonic stem cells

Rationale. The family of calcium-activated potassium channels consists of four members with varying biological functions and conductances. Besides membrane potential modulation, SK channels have been found to be involved in cardiac pacemaker cell development from ES cells and morphological shaping of neural stem cells. Objective. Distinct SK channel subtype expression in ES cells might elucidate their precise impact during cardiac development. We chose SK channel subtype 4 as a potential candidate influencing embryonic stem cell differentiation. Methods. We generated a doxycycline inducible mouse ES cell line via targeted homologous recombination of a cassette expressing a bicistronic construct encoding SK4 and a fluorophore from the murine HPRT locus. Conclusion. We characterized the mouse ES cell line iSK4-AcGFP. The cassette is readily expressed under the control of doxycycline, and the overexpression of SK4 led to an increase in cardiac and pacemaker cell differentiation thereby serving as a unique tool to characterize the cell biological variances due to specific SK channel overexpression.

Rat embryonic fibroblasts improve reprogramming of human keratinocytes into induced pluripotent stem cells

Patient-specific human induced pluripotent stem (hiPS) cells not only provide a promising tool for cellular disease models in general, but also open up the opportunity to establish cell-type-specific systems for personalized medicine. One of the crucial prerequisites for these strategies, however, is a fast and efficient reprogramming strategy from easy accessible somatic cell populations. Keratinocytes from plucked human hair had been introduced as a superior cell source for reprogramming purposes compared with the widely used skin fibroblasts. The starting cell population is, however, limited and thereby further optimization in terms of time, efficiency, and quality is inevitable. Here we show that rat embryonic fibroblasts (REFs) should replace mouse embryonic fibroblasts as feeder cells in the reprogramming process. REFs enable a significantly more efficient reprogramming procedure as shown by colony number and total amount of SSEA4-positive cells. We successfully produced keratinocyte-derived hiPS (k-hiPS) cells from various donors. The arising k-hiPS cells display the hallmarks of pluripotency such as expression of stem cell markers and differentiation into all 3 germ layers. The increased reprogramming efficiency using REFs as a feeder layer occurred independent of the proliferation rate in the parental keratinocytes and acts, at least in part, in a non-cell autonomous way by secreting factors known to facilitate pluripotency such as Tgfb1, Inhba and Grem1. Hence, we provide an easy to use and highly efficient reprogramming system that could be very useful for a broad application to generate human iPS cells.

The gray area between synapse structure and function-Gray's synapse types I and II revisited

On the basis of ultrastructural parameters, the concept was formulated that asymmetric Type I and symmetric Type II synapses are excitatory and inhibitory, respectively. This "functional Gray synapses concept" received strong support from the demonstration of the excitatory neurotransmitter glutamate in Type I synapses and of the inhibitory neurotransmitter γ-aminobutyric acid in Type II synapses, and is still frequently used in modern literature. However, morphological and functional evidence has accumulated that the concept is less tenable. Typical features of synapses like shape and size of presynaptic vesicles and synaptic cleft and presence of a postsynaptic density (PsD) do not always fit the postulated (excitatory/inhibitory) function of Gray's synapses. Furthermore, synapse function depends on postsynaptic receptors and associated signal transduction mechanisms rather than on presynaptic morphology and neurotransmitter type. Moreover, the notion that many synapses are difficult to classify as either asymmetric or symmetric has cast doubt on the assumption that the presence of a PsD is a sign of excitatory synaptic transmission. In view of the morphological similarities of the PsD in asymmetric synapses with membrane junctional structures such as the zonula adherens and the desmosome, asymmetric synapses may play a role as links between the postsynaptic and presynaptic membrane, thus ensuring long-term maintenance of interneuronal communication. Symmetric synapses, on the other hand, might be sites of transient communication as takes place during development, learning, memory formation, and pathogenesis of brain disorders. Confirmation of this idea might help to return the functional Gray synapse concept its central place in neuroscience.

An SK3 channel/nWASP/Abi-1 complex is involved in early neurogenesis

Background

The stabilization or regulated reorganization of the actin cytoskeleton is essential for cellular structure and function. Recently, we could show that the activation of the SK3-channel that represents the predominant SK-channel in neural stem cells, leads to a rapid local outgrowth of long filopodial processes. This observation indicates that the rearrangement of the actin based cytoskeleton via membrane bound SK3-channels might selectively be controlled in defined micro compartments of the cell.

Principal Findings

We found two important proteins for cytoskeletal rearrangement, the Abelson interacting protein 1, Abi-1 and the neural Wiskott Aldrich Syndrome Protein, nWASP, to be in complex with SK3- channels in neural stem cells (NSCs). Moreover, this interaction is also found in spines and postsynaptic compartments of developing primary hippocampal neurons and regulates neurite outgrowth during early phases of differentiation. Overexpression of the proteins or pharmacological activation of SK3 channels induces obvious structural changes in NSCs and hippocampal neurons. In both neuronal cell systems SK3 channels and nWASP act synergistic by strongly inducing filopodial outgrowth while Abi-1 behaves antagonistic to its interaction partners.

Conclusions

Our results give good evidence for a functional interplay of a trimeric complex that transforms incoming signals via SK3-channel activation into the local rearrangement of the cytoskeleton in early steps of neuronal differentiation involving nWASP and Abi-1 actin binding proteins.