Receptor isoform-specific interaction of prostaglandin EP3 receptor with muskelin

Muskelin regulates actin-dependent synaptic changes and intrinsic brain activity relevant to behavioral and cognitive processes

Muskelin (Mkln1) is implicated in neuronal function, regulating plasma membrane receptor trafficking. However, its influence on intrinsic brain activity and corresponding behavioral processes remains unclear. Here we show that murine Mkln1 knockout causes non-habituating locomotor activity, increased exploratory drive, and decreased locomotor response to amphetamine. Muskelin deficiency impairs social novelty detection while promoting the retention of spatial reference memory and fear extinction recall. This is strongly mirrored in either weaker or stronger resting-state functional connectivity between critical circuits mediating locomotor exploration and cognition. We show that Mkln1 deletion alters dendrite branching and spine structure, coinciding with enhanced AMPAR-mediated synaptic transmission but selective impairment in synaptic potentiation maintenance. We identify muskelin at excitatory synapses and highlight its role in regulating dendritic spine actin stability. Our findings point to aberrant spine actin modulation and changes in glutamatergic synaptic function as critical mechanisms that contribute to the neurobehavioral phenotype arising from Mkln1 ablation.

A murine muskelin knockout induces increased exploratory drive and alters cognition and functional connectivity. These effects correlate with actin-dependent changes in dendritic branching, spine structure, and AMPAR-mediated synaptic transmission.

A Missense Variant in SLC39A4 in a Litter of Turkish Van Cats with Acrodermatitis Enteropathica

In a litter of Turkish Van cats, three out of six kittens developed severe signs of skin disease, diarrhea, and systemic signs of stunted growth at 6 weeks of age. Massive secondary infections of the skin lesions evolved. Histopathological examinations showed a mild to moderate hyperplastic epidermis, covered by a thick layer of laminar to compact, mostly parakeratotic keratin. The dermis was infiltrated with moderate amounts of lymphocytes and plasma cells. Due to the severity of the clinical signs, one affected kitten died and the other two had to be euthanized. We sequenced the genome of one affected kitten and compared the data to 54 control genomes. A search for private variants in the two candidate genes for the observed phenotype, MKLN1 and SLC39A4, revealed a single protein-changing variant, SLC39A4:c.1057G>C or p.Gly353Arg. The solute carrier family 39 member 4 gene (SLC39A4) encodes an intestinal zinc transporter required for the uptake of dietary zinc. The variant is predicted to change a highly conserved glycine residue within the first transmembrane domain, which most likely leads to a loss of function. The genotypes of the index family showed the expected co-segregation with the phenotype and the mutant allele was absent from 173 unrelated control cats. Together with the knowledge on the effects of SLC39A4 variants in other species, these data suggest SLC39A4:c.1057G>C as candidate causative genetic variant for the phenotype in the investigated kittens. In line with the human phenotype, we propose to designate this disease acrodermatitis enteropathica (AE).

MKLN1 splicing defect in dogs with lethal acrodermatitis

Lethal acrodermatitis (LAD) is a genodermatosis with monogenic autosomal recessive inheritance in Bull Terriers and Miniature Bull Terriers. The LAD phenotype is characterized by poor growth, immune deficiency, and skin lesions, especially at the paws. Utilizing a combination of genome wide association study and haplotype analysis, we mapped the LAD locus to a critical interval of ~1.11 Mb on chromosome 14. Whole genome sequencing of an LAD affected dog revealed a splice region variant in the MKLN1 gene that was not present in 191 control genomes (chr14:5,731,405T>G or MKLN1:c.400+3A>C). This variant showed perfect association in a larger combined Bull Terrier/Miniature Bull Terrier cohort of 46 cases and 294 controls. The variant was absent from 462 genetically diverse control dogs of 62 other dog breeds. RT-PCR analysis of skin RNA from an affected and a control dog demonstrated skipping of exon 4 in the MKLN1 transcripts of the LAD affected dog, which leads to a shift in the MKLN1 reading frame. MKLN1 encodes the widely expressed intracellular protein muskelin 1, for which diverse functions in cell adhesion, morphology, spreading, and intracellular transport processes are discussed. While the pathogenesis of LAD remains unclear, our data facilitate genetic testing of Bull Terriers and Miniature Bull Terriers to prevent the unintentional production of LAD affected dogs. This study may provide a starting point to further clarify the elusive physiological role of muskelin 1 in vivo.

Lethal acrodermatitis (LAD) is an autosomal recessive hereditary disease in dogs. It is characterized by poor growth, immune deficiency and characteristic skin lesions of the paws and of the face. We mapped the LAD locus to a ~1.11 Mb segment on canine chromosome 14. Whole genome sequence data of an LAD affected dog and 191 controls revealed a candidate causative variant in the MKLN1 gene, encoding muskelin 1. The identified variant, a single nucleotide substitution, MKLN1:c.400+3A>C, altered the 5’-splice site at the beginning of intron 4. We experimentally confirmed that this variant leads to complete skipping of exon 4 in the MKLN1 mRNA in skin. Various cellular functions have been postulated for muskelin 1 including roles in intracellular transport processes, cell morphology, cell spreading, and cell adhesion. Our data from dogs reveal a novel in vivo role for muskelin 1 that is related to the immune system and skin. MKLN1 thus represents a novel candidate gene for human patients with unsolved acrodermatitis and/or immune deficiency phenotypes. LAD affected dogs may serve as models to gain more insights into the function of muskelin 1.

The LisH motif of muskelin is crucial for oligomerization and governs intracellular localization

Neurons regulate the number of surface receptors by balancing the transport to and from the plasma membrane to adjust their signaling properties. The protein muskelin was recently identified as a key factor guiding the transport of α1 subunit-containing GABAA receptors. Here we present the crystal structure of muskelin, comprising its N-terminal discoidin domain and Lis1-homology (LisH) motif. The molecule crystallized as a dimer with the LisH motif exclusively mediating oligomerization. Our subsequent biochemical analyses confirmed that the LisH motif acts as a dimerization element in muskelin. Together with an intermolecular head-to-tail interaction, the LisH-dependent dimerization is required to assemble a muskelin tetramer. Intriguingly, our cellular studies revealed that the loss of this dimerization results in a complete redistribution of muskelin from the cytoplasm to the nucleus and impairs muskelin's function in GABAA receptor transport. These studies demonstrate that the LisH-dependent dimerization is a crucial factor for muskelin function.

Structure of mouse muskelin discoidin domain and biochemical characterization of its self-association

Muskelin is an intracellular kelch-repeat protein comprised of discoidin, LisH, CTLH and kelch-repeat domains. It is involved in cell adhesion and the regulation of cytoskeleton dynamics as well as being a component of a putative E3 ligase complex. Here, the first crystal structure of mouse muskelin discoidin domain (MK-DD) is reported at 1.55 Å resolution, which reveals a distorted eight-stranded β-barrel with two short α-helices at one end of the barrel. Interestingly, the N- and C-termini are not linked by the disulfide bonds found in other eukaryotic discoidin structures. A highly conserved MIND motif appears to be the determinant for MK-DD specific interaction together with the spike loops. Analysis of interdomain interaction shows that MK-DD binds the kelch-repeat domain directly and that this interaction depends on the presence of the LisH domain.

Alternative splicing of T-box transcription factor genes

T-box (TBX) transcription factors are an ancient gene family with critical roles in embryogenesis. Currently, TBX3, TBX5, and TBX20 are TBX genes defined to have multiple protein isoforms created by alternative splicing and characterized by expression and functional studies. These proteins are important for development as mutations lead to severe developmental disorders in humans and mice. Cumulative studies suggest that alternative splicing of these genes can regulate TBX activities during multiple biological processes including cardiogenesis, limb development, and cancer mechanisms. This mini-review focuses on how alternative splicing adds complexity to transcriptional regulation of target genes controlled by TBX transcription factors.

Characterization of the novel interaction between muskelin and TBX20, a critical cardiogenic transcription factor

The genetic regulation necessary for the formation of a four-chambered heart is tightly regulated by transcription factors such as TBX20, a member of the T-box (TBX) transcription factor family. TBX20 is critical for proper cardiogenesis and is expressed in the heart throughout development. Missense mutations in TBX20 have been found in patients with congenital heart defects (CHD). Characterization of modifiers of TBX20 activity will help elucidate the genetic mechanisms of heart development and CHD. A yeast two-hybrid assay screening an embryonic mouse heart cDNA library with TBX20b as bait was used to identify potential modifiers of TBX20 activity and identified an interaction with muskelin (MKLN1), a primarily cytoplasmic protein with potential roles in signal transduction machinery scaffolding and nucleocytoplasmic protein shuttling. In cellular studies, MKLN1 directly binds to the T-box DNA-binding domain of only the TBX20b isoform by its kelch repeats domain. Immunostaining of mammalian cells transfected with tagged TBX20b and MKLN1 revealed colocalization primarily in the cytoplasm. Immunohistochemistry analysis of embryonic mouse hearts reveals coexpression in the developing endocardial valvular and myocardial interventricular cells. This novel interaction between TBX20b and MKLN1 may help elucidate new regulatory mechanisms within heart development.

Prostaglandin EP3 receptor superactivates adenylyl cyclase via the Gq/PLC/Ca2+ pathway in a lipid raft-dependent manner

We previously demonstrated that prostaglandin EP3 receptor augments EP2-elicited cAMP formation in COS-7 cells in a G(i/o)-insensitive manner. The purpose of our current study was to identify the signaling pathways involved in EP3-induced augmentation of receptor-stimulated cAMP formation. The enhancing effect of EP3 receptor was irrespective of the C-terminal structure of the EP3 isoform. This EP3 action was abolished by treatment with inhibitors for phospholipase C and intracellular Ca(2+)-related signaling molecules such as U73122, staurosporine, 2-APB and SK&F 96365. Indeed, an EP3 agonist stimulated IP(3) formation and intracellular Ca(2+) mobilization, which was blocked by U73122, but not by pertussis toxin. The enhancing effect by EP3 on cAMP formation was mimicked by both a Ca(2+) ionophore and the activation of a typical G(q)-coupled receptor. Moreover, EP3 was exclusively localized to the raft fraction in COS-7 cells and EP3-elicited augmentation of cAMP formation was abolished by cholesterol depletion and introduction of a dominant negative caveolin-1 mutant. These results suggest that EP3 elicits adenylyl cyclase superactivation via G(q)/phospholipase C activation and intracellular Ca(2+) mobilization in a lipid raft microdomain-dependent manner.

Novel role of the muskelin-RanBP9 complex as a nucleocytoplasmic mediator of cell morphology regulation

The evolutionarily conserved kelch-repeat protein muskelin was identified as an intracellular mediator of cell spreading. We discovered that its morphological activity is controlled by association with RanBP9/RanBPM, a protein involved in transmembrane signaling and a conserved intracellular protein complex. By subcellular fractionation, endogenous muskelin is present in both the nucleus and the cytosol. Muskelin subcellular localization is coregulated by its C terminus, which provides a cytoplasmic restraint and also controls the interaction of muskelin with RanBP9, and its atypical lissencephaly-1 homology motif, which has a nuclear localization activity which is regulated by the status of the C terminus. Transient or stable short interfering RNA-based knockdown of muskelin resulted in protrusive cell morphologies with enlarged cell perimeters. Morphology was specifically restored by complementary DNAs encoding forms of muskelin with full activity of the C terminus for cytoplasmic localization and RanBP9 binding. Knockdown of RanBP9 resulted in equivalent morphological alterations. These novel findings identify a role for muskelin-RanBP9 complex in pathways that integrate cell morphology regulation and nucleocytoplasmic communication.

Structural similarities and functional diversity of eukaryotic discoidin-like domains

The discoidin domain is a approximately 150 amino acid motif common in both eukaryotic and prokaryotic proteins. It is found in a variety of extracellular, intracellular and transmembrane multidomain proteins characterized by a considerable functional diversity, mostly involved in developmental processes. The biological role of the domain depends on its interactions with different molecules, including growth factors, phospholipids and lipids, galactose or its derivatives, and collagen. The conservation of the motif, as well as the serious physiological consequences of discoidin domain disorders underscore the importance of the fold, while the ability to accommodate such an extraordinarily broad range of ligand molecules makes it a fascinating research target. In present review we characterize the distinctive features of discoidin domains and briefly outline the biological role of this module in various eukaryotic proteins.

Neuronal expression of muskelin in the rodent central nervous system

Background

The kelch repeat protein muskelin mediates cytoskeletal responses to the extracellular matrix protein thrombospondin 1, (TSP1), that is known to promote synaptogenesis in the central nervous system (CNS). Muskelin displays intracellular localization and affects cytoskeletal organization in adherent cells. Muskelin is expressed in adult brain and has been reported to bind the Cdk5 activator p39, which also facilitates the formation of functional synapses. Since little is known about muskelin in neuronal tissues, we here analysed the tissue distribution of muskelin in rodent brain and analysed its subcellular localization using cultured neurons from multiple life stages.

Results

Our data show that muskelin transcripts and polypeptides are expressed throughout the central nervous system with significantly high levels in hippocampus and cerebellum, a finding that resembles the tissue distribution of p39. At the subcellular level, muskelin is found in the soma, in neurite projections and the nucleus with a punctate distribution in both axons and dendrites. Immunostaining and synaptosome preparations identify partial localization of muskelin at synaptic sites. Differential centrifugation further reveals muskelin in membrane-enriched, rather than cytosolic fractions.

Conclusion

Our results suggest that muskelin represents a multifunctional protein associated with membranes and/or large protein complexes in most neurons of the central nervous system. These data are in conclusion with distinct roles of muskelin's functional interaction partners.

Regulation of post-translational modifications of muskelin by protein kinase C

Muskelin is a member of the kelch-repeat superfamily of proteins, identified as an intracellular protein involved in cell spreading responses to thombospondin-1. Muskelin is expressed by many adult tissues and has an evolutionarily conserved, multidomain architecture consisting of an amino-terminal discoidin-like domain, a central alpha-helical region and six kelch-repeats that are predicted to form a beta-propeller structure. We previous demonstrated that muskelin molecules undergo head-to-tail association, however the physiological, post-translational regulation of muskelin is not well understood. Here, we have examined the expression of muskelin during mouse embryonic development and report widespread expression that includes muscle tissues, multiple epithelia and the brain. In cultured skeletal myoblasts and vascular smooth muscle cells, muskelin exists as a complex set of isoelectric variants. Five potential sites for phosphorylation by protein kinase C (PKC), are conserved between vertebrate and Drosophila muskelins, therefore we examined the hypothesis that muskelin is regulated post-translationally by PKC activity. We demonstrate that PKC activation or inhibition regulates the profile of endogenous muskelin isoelectric variants and that muskelin is a substrate for PKCalphain vitro. Wild-type GFP-muskelin and a panel of alanine point mutations were used to test the sensitivity of self-association to PKC activation. Mutation of two of the sites, S324 and T515, partially inhibited the ability of muskelin to self-associate in cells and inhibited responsiveness to activated PKC. Interestingly, both sites are predicted to lie in surface-exposed loops on the same side of the beta-propeller, implicating a common binding interface.

G-Protein-coupled receptor-protein interactions: basis for new concepts on receptor structure and function

1. G-Protein-coupled receptors (GPCRs) constitute a large family of cell surface proteins. Their primary function is to transmit extracellular stimuli to intracellular signals. It is estimated that the human genome contains more than 1000 genes that code for proteins of the GPCR structure. These receptors also comprise the most important class of therapeutic drug targets. 2. The mechanism of GPCR signalling was initially envisioned as involving coupling to the heterotrimeric G-proteins only. However, recent developments in the field suggest that such a simplistic model cannot be sustained any longer. The emerging view is that a wide range of accessory proteins are involved in the regulation of every aspect of GPCR activity. 3. G-Protein-coupled receptor-interacting proteins are implicated in the regulation of several aspects of GPCR biology, including receptor targeting to the respective sites of action, receptor anchoring, signalling and receptor desensitization. In some cases (e.g. receptor activity modifying proteins), they may contribute to the receptor structure and form a part of the ligand-binding domain. 4. These findings have contributed to new concepts of cellular organization in which modular protein-protein interactions provide a network through which signalling pathways are assembled and controlled.

Differential expression of E prostanoid receptors in murine and human non-melanoma skin cancer

Enhanced prostaglandin production via upregulated cyclooxygenase-2 (COX-2) expression is a likely contributing factor in ultraviolet B (UVB)-induced non-melanoma skin cancer (NMSC), which consists primarily of squamous cell carcinoma (SCC) and basal cell carcinoma (BCC). The four E prostanoid (EP) receptors, designated EP1 through EP4, are known to bind prostaglandin E2 (PGE2), the major prostaglandin present in the skin. We used murine models of UVB-induced SCC and BCC, as well as human NMSC from sun-exposed sites, to investigate the expression of EP receptors during UVB-induced tumorigenesis. We observed that UVB-induced murine SCC are associated with markedly altered expression patterns of the EP receptors when compared with non-irradiated skin. In contrast, expression of all EP receptors was largely absent in UVB-induced murine BCC. We also observed expression of all four EP receptors in human SCC, with altered expression of their mRNA levels as compared with adjacent tumor-free skin. Consistent with our murine studies, no EP receptor expression was detected in human BCC, and their mRNA expression levels showed no change from the adjacent non-tumor-bearing skin. These data suggest that altered EP receptor expression may play a differential role in the development of UVB-induced SCC and BCC in murine and human skin.

A specific interaction between muskelin and the cyclin-dependent kinase 5 activator p39 promotes peripheral localization of muskelin

Previous studies implicate cyclin-dependent kinase 5 in cell adhesion and migration of epithelial cells of the cornea and lens. To explore molecular interactions underlying these functions, we performed yeast two-hybrid screening of an embryonic rat lens library for proteins that interact with cyclin-dependent kinase 5 and its regulators, p35 and p39. This screen identified a specific interaction between p39 and muskelin, an intracellular protein known to affect cytoskeletal organization in adherent cells. Immunohistochemistry detected muskelin in the developing lens and in other tissues, including brain and muscle. Glutathione S-transferase pull-down experiments and co-immunoprecipitations confirmed the specificity of the p39-muskelin interaction. Deletion analysis of p39 showed that muskelin binds to the p39 C terminus, which contains a short insertion (amino acids 329-366) absent from p35. Similar analysis of muskelin mapped the interaction with p39 to the fifth kelch repeat. Co-expression of p39 and muskelin in COS1 cells or lens epithelial cells altered the intracellular localization of muskelin, recruiting it to the cell periphery. These findings demonstrate a novel interaction between muskelin and the cyclin-dependent kinase 5 activator p39 and suggest that p39 may regulate the subcellular localization of muskelin.

Membrane trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) modulate diverse physiological and behavioral signaling pathways by virtue of changes in receptor activation and inactivation states. Functional changes in receptor properties include dynamic interactions with regulatory molecules and trafficking to various cellular compartments at various stages of the life cycle of a GPCR. This review focuses on trafficking of GPCRs to the cell surface, stabilization there, and agonist-regulated turnover. GPCR interactions with a variety of newly revealed partners also are reviewed with the intention of provoking further analysis of the relevance of these interactions in GPCR trafficking, signaling, or both. The disease consequences of mislocalization of GPCRs also are described.

The 'magic tail' of G protein-coupled receptors: an anchorage for functional protein networks

All cell types express a great variety of G protein-coupled receptors (GPCRs) that are coupled to only a limited set of G proteins. This disposition favors cross-talk between transduction pathways. However, GPCRs are organized into functional units. They promote specificity and thus avoid unsuitable cross-talk. New methodologies (mostly yeast two-hybrid screens and proteomics) have been used to discover more than 50 GPCR-associated proteins that are involved in building these units. In addition, these protein networks participate in the trafficking, targeting, signaling, fine-tuning and allosteric regulation of GPCRs. To date, proteins that interact with the GPCR C-terminus are the most abundant and are the focus of this review.

Regulation of G protein-coupled receptor signaling by scaffold proteins

The actions of many hormones and neurotransmitters are mediated through stimulation of G protein-coupled receptors. A primary mechanism by which these receptors exert effects inside the cell is by association with heterotrimeric G proteins, which can activate a wide variety of cellular enzymes and ion channels. G protein-coupled receptors can also interact with a number of cytoplasmic scaffold proteins, which can link the receptors to various signaling intermediates and intracellular effectors. The multicomponent nature of G protein-coupled receptor signaling pathways makes them ideally suited for regulation by scaffold proteins. This review focuses on several specific examples of G protein-coupled receptor-associated scaffolds and the roles they may play in organizing receptor-initiated signaling pathways in the cardiovascular system and other tissues.

Protein-protein interactions at G-protein-coupled receptors

The basic module of signal transduction that involves G-protein-coupled receptors is usually portrayed as comprising a receptor, a heterotrimeric G protein and an effector. It is now well established that regulated interactions between receptors and arrestins, and between G proteins and regulators of G-protein signalling alter the effectiveness and kinetics of information transfer. However, more recent studies have begun to identify a host of other proteins that interact selectively with individual receptors at both the intracellular and extracellular face of the membrane. Although the functional relevance of many of these interactions is only beginning to be understood, current information indicates that these interactions might determine receptor properties, such as cellular compartmentalization or signal selection, and can promote protein scaffolding into complexes that integrate function.