Deregulations of miR-1 and its target Multiplexin promote dilated cardiomyopathy associated with myotonic dystrophy type 1.

Myotonic dystrophy type 1 (DM1) is the most common muscular dystrophy in adults. It is caused by the excessive expansion of noncoding CTG repeats, which when transcribed affects the functions of RNA-binding factors with adverse effects on alternative splicing, processing, and stability of a large set of muscular and cardiac transcripts. Among these effects, inefficient processing and down-regulation of muscle- and heart-specific miRNA, miR-1, have been reported in DM1 patients, but the impact of reduced miR-1 on DM1 pathogenesis has been unknown. Here, we use Drosophila DM1 models to explore the role of miR-1 in cardiac dysfunction in DM1. We show that miR-1 down-regulation in the heart leads to dilated cardiomyopathy (DCM), a DM1-associated phenotype. We combined in silico screening for miR-1 targets with transcriptional profiling of DM1 cardiac cells to identify miR-1 target genes with potential roles in DCM. We identify Multiplexin (Mp) as a new cardiac miR-1 target involved in DM1. Mp encodes a collagen protein involved in cardiac tube formation in Drosophila. Mp and its human ortholog Col15A1 are both highly enriched in cardiac cells of DCM-developing DM1 flies and in heart samples from DM1 patients with DCM, respectively. When overexpressed in the heart, Mp induces DCM, whereas its attenuation rescues the DCM phenotype of aged DM1 flies. Reduced levels of miR-1 and consecutive up-regulation of its target Mp/Col15A1 might be critical in DM1-associated DCM.

Discoidin domain receptor regulates ensheathment, survival and caliber of peripheral axons.

Most invertebrate axons and small-caliber axons in mammalian peripheral nerves are unmyelinated but still ensheathed by glia. Here, we use Drosophila wrapping glia to study the development and function of non-myelinating axon ensheathment, which is poorly understood. Selective ablation of these glia from peripheral nerves severely impaired larval locomotor behavior. In an in vivo RNA interference screen to identify glial genes required for axon ensheathment, we identified the conserved receptor tyrosine kinase Discoidin domain receptor (Ddr). In larval peripheral nerves, loss of Ddr resulted in severely reduced ensheathment of axons and reduced axon caliber, and we found a strong dominant genetic interaction between Ddr and the type XV/XVIII collagen Multiplexin (Mp), suggesting that Ddr functions as a collagen receptor to drive axon wrapping. In adult nerves, loss of Ddr decreased long-term survival of sensory neurons and significantly reduced axon caliber without overtly affecting ensheathment. Our data establish essential roles for non-myelinating glia in nerve development, maintenance and function, and identify Ddr as a key regulator of axon-glia interactions during ensheathment and establishment of axon caliber.

Eater cooperates with Multiplexin to drive the formation of hematopoietic compartments.

Blood development in multicellular organisms relies on specific tissue microenvironments that nurture hematopoietic precursors and promote their self-renewal, proliferation, and differentiation. The mechanisms driving blood cell homing and their interactions with hematopoietic microenvironments remain poorly understood. Here, we use the Drosophila melanogaster model to reveal a pivotal role for basement membrane composition in the formation of hematopoietic compartments. We demonstrate that by modulating extracellular matrix components, the fly blood cells known as hemocytes can be relocated to tissue surfaces where they function similarly to their natural hematopoietic environment. We establish that the Collagen XV/XVIII ortholog Multiplexin in the tissue-basement membranes and the phagocytosis receptor Eater on the hemocytes physically interact and are necessary and sufficient to induce immune cell-tissue association. These results highlight the cooperation of Multiplexin and Eater as an integral part of a homing mechanism that specifies and maintains hematopoietic sites in Drosophila.

Epigenetic Signaling in Glia Controls Presynaptic Homeostatic Plasticity.

Epigenetic gene regulation shapes neuronal fate in the embryonic nervous system. Post-embryonically, epigenetic signaling within neurons has been associated with impaired learning, autism, ataxia, and schizophrenia. Epigenetic factors are also enriched in glial cells. However, little is known about epigenetic signaling in glia and nothing is known about the intersection of glial epigenetic signaling and presynaptic homeostatic plasticity. During a screen for genes involved in presynaptic homeostatic synaptic plasticity, we identified an essential role for the histone acetyltransferase and deubiquitinase SAGA complex in peripheral glia. We present evidence that the SAGA complex is necessary for homeostatic plasticity, demonstrating involvement of four new genes in homeostatic plasticity. This is also evidence that glia participate in presynaptic homeostatic plasticity, invoking previously unexplored intercellular, homeostatic signaling at a tripartite synapse. We show, mechanistically, SAGA signaling regulates the composition of and signaling from the extracellular matrix during homeostatic plasticity.

Exploring the roles of MACIT and multiplexin collagens in stem cells and cancer.

The extracellular matrix (ECM) is ubiquitously involved in neoplastic transformation, tumour growth and metastatic dissemination, and the interplay between tumour and stromal cells and the ECM is now considered crucial for the formation of a tumour-supporting microenvironment. The 28 different collagens (Col) form a major ECM protein family and display extraordinary functional diversity in tissue homeostasis as well as in pathological conditions, with functions ranging from structural support for tissues to regulatory binding activities and storage of biologically active cryptic domains releasable through ECM proteolysis. Two subfamilies of collagens, namely the plasma membrane-associated collagens with interrupted triple-helices (MACITs, including ColXIII, ColXXIII and ColXXV) and the basement membrane-associated collagens with multiple triple-helix domains with interruptions (multiplexins, including ColXV and ColXVIII), have highly interesting regulatory functions in tissue and organ development, as well as in various diseases, including cancer. An increasing, albeit yet sparse, data suggest that these collagens play crucial roles in conveying regulatory signals from the extracellular space to cells. We summarize here the current knowledge about MACITs and multiplexins as regulators of stemness and oncogenic processes, as well as their roles in influencing cell fate decisions in healthy and cancerous tissues. In addition, we present a bioinformatic analysis of the impacts of MACITs and multiplexins transcript levels on the prognosis of patients representing a wide array of malignant diseases, to aid future diagnostic and therapeutic efforts.

Collagen XV, a multifaceted multiplexin present across tissues and species.

Type XV collagen is a non-fibrillar collagen that is associated with basement membranes and belongs to the multiplexin subset of the collagen superfamily. Collagen XV was initially studied because of its sequence homology with collagen XVIII/endostatin whose anti-angiogenic and anti-tumorigenic properties were subjects of wide interest in the past years. But during the last fifteen years, collagen XV has gained growing attention with increasing number of studies that have attributed new functions to this widely distributed collagen/proteoglycan hybrid molecule. Despite the cumulative evidence of its functional pleiotropy and its evolutionary conserved function, no review compiling the current state of the art about collagen XV is currently available. Here, we thus provide the first comprehensive view of the knowledge gathered so far on the molecular structure, tissue distribution and functions of collagen XV in development, tissue homeostasis and disease with an evolutionary perspective. We hope that our review will open new roads for promising research on collagen XV in the coming years.

Collagen XVIII in tissue homeostasis and dysregulation - Lessons learned from model organisms and human patients.

Collagen XVIII is a ubiquitous basement membrane (BM) proteoglycan produced in three tissue-specific isoforms that differ in their N-terminal non-collagenous sequences, but share collagenous and C-terminal non-collagenous domains. The collagenous domain provides flexibility to the large collagen XVIII molecules on account of multiple interruptions in collagenous sequences. Each isoform has a complex multi-domain structure that endows it with an ability to perform various biological functions. The long isoform contains a frizzled-like (Fz) domain with Wnt-inhibiting activity and a unique domain of unknown function (DUF959), which is also present in the medium isoform. All three isoforms share an N-terminal laminin-G-like/thrombospondin-1 sequence whose specific functions still remain unconfirmed. The proteoglycan nature of the isoforms further increases the functional diversity of collagen XVIII. An anti-angiogenic domain termed endostatin resides in the C-terminus of collagen XVIII and is proteolytically cleaved from the parental molecule during the BM breakdown for example in the process of tumour progression. Recombinant endostatin can efficiently reduce tumour angiogenesis and growth in experimental models by inhibiting endothelial cell migration and proliferation or by inducing their death, but its efficacy against human cancers is still a subject of debate. Mutations in the COL18A1 gene result in Knobloch syndrome, a genetic disorder characterised mainly by severe eye defects and encephalocele and, occasionally, other symptoms. Studies with gene-modified mice have elucidated some aspects of this rare disease, highlighting in particular the importance of collagen XVIII in the development of the eye. Research with model organisms have also helped in determining other structural and biological functions of collagen XVIII, such as its requirement in the maintenance of BM integrity and its emerging roles in regulating cell survival, stem or progenitor cell maintenance and differentiation and inflammation. In this review, we summarise current knowledge on the properties and endogenous functions of collagen XVIII in normal situations and tissue dysregulation. When data is available, we discuss the functions of the distinct isoforms and their specific domains.

Matricellular proteins in development: perspectives from the Drosophila heart.

The Drosophila model represents an attractive system in which to study the functional contribution of specific genes to organ development. Within the embryo, the heart tube serves as an informative developmental paradigm to analyze functional aspects of matricellular proteins. Here, we describe two essential extracellular matricellular proteins, Multiplexin (Mp) and Lonely heart (Loh). Each of these proteins contributes to the development and morphogenesis of the heart tube by regulating the activity/localization of essential extracellular proteins. Mp, which is secreted by heart cardioblasts and is specifically distributed in the lumen of the heart tube, binds to the signaling protein Slit, and facilitates its local signaling at the heart's luminal domain. Loh is an ADAMTS-like protein, which serves as an adapter protein to Pericardin (a collagen-like protein), promoting its specific localization at the abluminal domain of the heart tube. We also introduce the Drosophila orthologues of matricellular proteins present in mammals, including Thrombospondin, and SPARC, and discuss a possible role for Teneurins (Ten-A and Ten-M) in the heart. Understanding the role of these proteins provides a novel developmental perspective into the functional contribution of matricellular proteins to organ development.