The protein cross-linking enzyme tissue transglutaminase binds in vitro with high affinity to fibronectin via its 42-kD gelatin-binding domain. Here we report that cell surface transglutaminase mediates adhesion and spreading of cells on the 42-kD fibronectin fragment, which lacks integrin-binding motifs. Overexpression of tissue transglutaminase increases its amount on the cell surface, enhances adhesion and spreading on fibronectin and its 42-kD fragment, enlarges focal adhesions, and amplifies adhesion-dependent phosphorylation of focal adhesion kinase. These effects are specific for tissue transglutaminase and are not shared by its functional homologue, a catalytic subunit of factor XIII. Adhesive function of tissue transglutaminase does not require its cross-linking activity but depends on its stable noncovalent association with integrins. Transglutaminase interacts directly with multiple integrins of β1 and β3 subfamilies, but not with β2 integrins. Complexes of transglutaminase with integrins are formed inside the cell during biosynthesis and accumulate on the surface and in focal adhesions. Together our results demonstrate that tissue transglutaminase mediates the interaction of integrins with fibronectin, thereby acting as an integrin-associated coreceptor to promote cell adhesion and spreading.

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by an expanded stretch of CAG trinucleotide repeats that results in neuronal dysfunction and death. Here, the HD consortium reports the generation and characterization of 14 induced pluripotent stem cell (iPSC) lines from HD patients and controls. Microarray profiling revealed CAG expansion-associated gene expression patterns that distinguish patient lines from controls, and early onset versus late onset HD. Differentiated HD neural cells showed disease associated changes in electrophysiology, metabolism, cell adhesion, and ultimately cell death for lines with both medium and longer CAG repeat expansions. The longer repeat lines were however the most vulnerable to cellular stressors and BDNF withdrawal using a range of assays across consortium laboratories. The HD iPSC collection represents a unique and well-characterized resource to elucidate disease mechanisms in HD and provides a novel human stem cell platform for screening new candidate therapeutics.

Huntington's Disease (HD) is a neurodegenerative disease caused by poly-glutamine expansion in the Htt protein, resulting in Htt misfolding and cell death. Expression of the cellular protein folding and pro-survival machinery by heat shock transcription factor 1 (HSF1) ameliorates biochemical and neurobiological defects caused by protein misfolding. We report that HSF1 is degraded in cells and mice expressing mutant Htt, in medium spiny neurons derived from human HD iPSCs and in brain samples from patients with HD. Mutant Htt increases CK2α′ kinase and Fbxw7 E3 ligase levels, phosphorylating HSF1 and promoting its proteasomal degradation. An HD mouse model heterozygous for CK2α′ shows increased HSF1 and chaperone levels, maintenance of striatal excitatory synapses, clearance of Htt aggregates and preserves body mass compared with HD mice homozygous for CK2α′. These results reveal a pathway that could be modulated to prevent neuronal dysfunction and muscle wasting caused by protein misfolding in HD.

Expression of tissue transglutaminase (transglutaminase II, tTG) was shown to increase drastically during monocyte differentiation into macrophages; however, its role in monocytic cells remains largely unknown. This study describes a novel function of cell surface tTG as an adhesion and migration receptor for fibronectin (Fn). Two structurally related transglutaminases, tTG and the A subunit of factor XIII (FXIIIA), are expressed on the surface of monocytic cells, whereas only surface tTG is associated with multiple integrins of the ␤ 1 and ␤ 3 subfamilies. Both surface levels of tTG and the amounts of integrinbound tTG are sharply up-regulated during the conversion of monocytes into macrophages. In contrast, a reduction in biosynthesis and surface expression of FXIIIA accompanies monocyte differentiation. Cell surface tTG is colocalized with IntroductionMonocytic cells are involved in a variety of immune and inflammatory processes. A number of proinflammatory cytokines can trigger extravasation of monocytes and stimulate their invasion into inflamed tissues where these cells play a key role in a local immune response. [1][2][3][4][5] Several subsets of adhesion molecules on the surface of monocytes are involved in different stages of this multistep process. This involves rolling of monocytic cells along vascular endothelium, arrest and initial adhesion to endothelium and subsequent strong adhesion, and spreading and transmigration of monocytes across the endothelial monolayer as well as invasion into underlying tissues. [6][7][8] As adhesion receptors, integrins participate in multiple aspects of monocyte adhesion and transmigration across the endothelial monolayer. The ␣ 4 ␤ 1 -integrin expressed on monocytic cells is involved in the arrest and initial adhesion of these cells to endothelium by binding to vascular cell adhesion molecule-1 (VCAM-1). 9 In addition, binding of ␣ 4 ␤ 1 -and ␣ 4 ␤ 7 -integrins to the alternatively spliced connecting segment-1 (CS-1) domain of fibronectin (Fn) contributes to monocyte-endothelial interactions. [10][11][12][13][14][15] The integrins of the ␤ 2 subfamily, ␣ L ␤ 2 and ␣ M ␤ 2 , are also present on monocytes 16 and because of interaction with intercellular adhesion molecule-1 (ICAM-1) mediate tight adhesion to the endothelial cells. 7,17,18 Finally, the ␣ V ␤ 3 -integrin promotes transmigration of monocytes through endothelial monolayers, likely by down-regulation of adhesive function of ␤ 2 -integrins. 8 Tissue transglutaminase (tTG) is a member of transglutaminase family of enzymes that covalently cross-link proteins in a Ca ϩϩ -dependent manner. 19 In addition, tTG has a guanosine triphosphatase (GTPase) activity 20 and is involved in intracellular signaling via agonist-mediated interactions with ␣ 1B -and ␣ 1D -adrenergic receptors 21 and downstream effectors such as phospholipase C␦ 1 . 22 tTG localizes mainly in the cytoplasm, yet some amounts of the enzyme are present on the cell surface and in the extracellular matrix (ECM). tTG is able to bind and cross-link sev...

Cell invasion requires cooperation between adhesion receptors and matrix metalloproteinases (MMPs). Remodeling of the extracellular matrix (ECM)1 is critical for cancer cell invasion and tumorigenesis (1-5). Membrane type matrix metalloproteinases (MT-MMPs) localized to the invasive front of highly motile cancer cells (6, 7) were shown to be directly involved in matrix breakdown (8 -13). A cooperation involving MT-MMPs and cell adhesion receptors is likely to be essential to migrating cells (3, 14 -16). So far, six members of the MT-MMP subfamily have been identified and partially characterized (2, 17-22). MT1-, MT2-, and MT3-MMP strongly contribute to tumor cell invasion (12). Recent studies demonstrated a functional significance and a direct role of MT1-, MT2-, and MT3-MMP in cell locomotion on laminin-5 (11) and three-dimensional collagen type I lattice (8, 9, 12). In addition, MT-MMPs contribute indirectly to cell invasion by activating soluble secretory MMP-2 (23) and MMP-13 (24), which further cleave multiple matrix substrates (2, 5, 25-29).Integrin adhesion receptors dynamically regulate cell-matrix interactions by the binding to matrix proteins and inside-out signaling (30,31). This allows cells to discriminate any subtle alteration of the environment and to adjust cell locomotion accordingly. Direct interactions with multiple transmembrane and cell surface proteins (32) including integrin-associated protein-50 (33), TM4SF proteins (tetraspanins) (34) and tTG (35) further attenuate adhesive and signaling efficiency of integrins.Cell surface tTG (protein-glutamine ␥-glutamyltransferase, EC 2.3.2.13) promotes integrin-dependent adhesion and spreading of cells. By both direct associations with multiple ␤ 1 and ␤ 3 integrins and the binding with Fn, tTG independently mediates the interactions of integrins with Fn (35). The high affinity binding of tTG with Fn specifically involves the 42 kDa gelatin-binding domain of the Fn molecule, which consists of modules I 6 II 1,2 I 7-9 (36). The enhancement of integrin-mediated adhesion and spreading of cells on Fn is independent from the enzymatic activity of surface tTG (35). Intriguingly, reduced expression of tTG has been linked to aggressiveness and high metastatic potential of tumors, whereas overexpression of tTG in fibrosarcomas inhibited primary tumor growth (37, 38). Proteolysis of tTG at the normal tissue/tumor boundary was observed in invasive tumors (38).Here, we report that depending on the structure of the ECM, MT-MMPs are capable of both positively and negatively regulating locomotion of cancer cells. Matrix-dependent proteolysis of surface tTG by MT1-MMP occurs on tumor cells of a diverse tissue origin, thereby representing a general phenomenon and a novel MT-MMP function. Our data suggest an existence of an unexpected link between tumor cell locomotion, the ECM and membrane-anchored MMPs. Regulatory proteolysis of cell surface adhesion proteins by the adjacent MT-MMP molecules is likely to play a significant functional role in cancer cell invasion.

Neural cultures derived from Huntington’s disease (HD) patient-derived induced pluripotent stem cells were used for ‘omics’ analyses to identify mechanisms underlying neurodegeneration. RNA-seq analysis identified genes in glutamate and GABA signaling, axonal guidance and calcium influx whose expression was decreased in HD cultures. One-third of gene changes were in pathways regulating neuronal development and maturation. When mapped to stages of mouse striatal development, the profiles aligned with earlier embryonic stages of neuronal differentiation. We observed a strong correlation between HD-related histone marks, gene expression and unique peak profiles associated with dysregulated genes, suggesting a coordinated epigenetic program. Treatment with isoxazole-9, which targets key dysregulated pathways, led to amelioration of expanded polyglutamine repeat-associated phenotypes in neural cells and of cognitive impairment and synaptic pathology in HD model R6/2 mice. These data suggest that mutant huntingtin impairs neurodevelopmental pathways that could disrupt synaptic homeostasis and increase vulnerability to the pathologic consequence of expanded polyglutamine repeats over time.

Huntington's disease (HD) is a fatal neurodegenerative disease, caused by expansion of polyglutamine repeats in the Huntingtin gene, with longer expansions leading to earlier ages of onset. The HD iPSC Consortium has recently reported a new in vitro model of HD based on the generation of induced pluripotent stem cells (iPSCs) from HD patients and controls. The current study has furthered the disease in a dish model of HD by generating new non-integrating HD and control iPSC lines. Both HD and control iPSC lines can be efficiently differentiated into neurons/glia; however, the HD-derived cells maintained a significantly greater number of nestin-expressing neural progenitor cells compared with control cells. This cell population showed enhanced vulnerability to brain-derived neurotrophic factor (BDNF) withdrawal in the juvenile-onset HD (JHD) lines, which appeared to be CAG repeat-dependent and mediated by the loss of signaling from the TrkB receptor. It was postulated that this increased death following BDNF withdrawal may be due to glutamate toxicity, as the N-methyl-d-aspartate (NMDA) receptor subunit NR2B was up-regulated in the cultures. Indeed, blocking glutamate signaling, not just through the NMDA but also mGlu and AMPA/Kainate receptors, completely reversed the cell death phenotype. This study suggests that the pathogenesis of JHD may involve in part a population of 'persistent' neural progenitors that are selectively vulnerable to BDNF withdrawal. Similar results were seen in adult hippocampal-derived neural progenitors isolated from the BACHD model mouse. Together, these results provide important insight into HD mechanisms at early developmental time points, which may suggest novel approaches to HD therapeutics.

Huntington’s disease (HD) is a progressive neurodegenerative disorder caused by a CAG-polyglutamine repeat expansion in the huntingtin (htt) gene. We found that peroxisome proliferator-activated receptor delta (PPARδ) interacts with htt and that mutant htt represses PPARδ-mediated transactivation. Increased PPARδ transactivation ameliorated mitochondrial dysfunction and improved cell survival of HD neurons. Expression of dominant-negative PPARδ in CNS was sufficient to induce motor dysfunction, neurodegeneration, mitochondrial abnormalities, and transcriptional alterations that recapitulated HD-like phenotypes. Expression of dominant-negative PPARδ specifically in the striatum of medium spiny neurons in mice yielded HD-like motor phenotypes, accompanied by striatal neuron loss. In mouse models of HD, pharmacologic activation of PPAR δ, using the agonist KD3010, improved motor function, reduced neurodegeneration, and increased survival. PPAR δ activation also reduced htt-induced neurotoxicity in vitro and in medium spiny-like neurons generated from human HD stem cells, indicating that PPAR δ activation may be beneficial in individuals with HD and related disorders.