Recent findings show that cilia are sensory organelles that display specific receptors and ion channels, which transmit signals from the extracellular environment via the cilium to the cell to control tissue homeostasis and function. Agenesis of primary cilia or mislocation of ciliary signal components affects human pathologies, such as polycystic kidney disease and disorders associated with Bardet-Biedl syndrome. Primary cilia are essential for hedgehog ligand-induced signaling cascade regulating growth and patterning. Here, we show that the primary cilium in fibroblasts plays a critical role in growth control via platelet-derived growth factor receptor alpha (PDGFRalpha), which localizes to the primary cilium during growth arrest in NIH3T3 cells and primary cultures of mouse embryonic fibroblasts. Ligand-dependent activation of PDGFRalphaalpha is followed by activation of Akt and the Mek1/2-Erk1/2 pathways, with Mek1/2 being phosphorylated within the cilium and at the basal body. Fibroblasts derived from Tg737(orpk) mutants fail to form normal cilia and to upregulate the level of PDGFRalpha; PDGF-AA fails to activate PDGFRalphaalpha and the Mek1/2-Erk1/2 pathway. Signaling through PDGFRbeta, which localizes to the plasma membrane, is maintained at comparable levels in wild-type and mutant cells. We propose that ciliary PDGFRalphaalpha signaling is linked to tissue homeostasis and to mitogenic signaling pathways.

Transforming growth factor β (TGF-β) signaling is regulated by clathrin-dependent endocytosis (CDE) for the control of cellular processes during development and in tissue homeostasis. The primary cilium coordinates several signaling pathways, and the pocket surrounding the base and proximal part of the cilium is a site for CDE. We report here that TGF-β receptors localize to the ciliary tip and endocytic vesicles at the ciliary base in fibroblasts and that TGF-β stimulation increases receptor localization and activation of SMAD2/3 and ERK1/2 at the ciliary base. Inhibition of CDE reduced TGF-β-mediated signaling at the cilium, and TGF-β signaling and CDE activity are reduced at stunted primary cilia in Tg737orpk fibroblasts. Similarly, TGF-β signaling during cardiomyogenesis correlated with accumulation of TGF-β receptors and activation of SMAD2/3 at the ciliary base. Our results indicate that the primary cilium regulates TGF-β signaling and that the ciliary pocket is a compartment for CDE-dependent regulation of signal transduction.

It is becoming increasingly clear that herpesviruses can exploit the endocytic pathway to infect cells, yet several important features of this process remain poorly defined. Using herpes simplex virus-1 (HSV-1) as a model, we demonstrate that endocytosis of the virions mimic many features of phagocytosis. During entry, HSV-1 virions associated with plasma membrane protrusions followed by a phagocytosis-like uptake involving rearrangement of actin cytoskeleton and trafficking of the virions in large phagosome-like vesicles. RhoA GTPase was activated during this process and the mode of entry was cell type–specific. Clathrin-coated vesicles had no detectable role in virion trafficking as Eps15 dominant-negative mutants failed to affect HSV-1 uptake. Binding and fusion of the virion envelope with the phagosomal membrane is likely facilitated by clustering of nectin-1 (or HVEM) in phagosomes, which was observed in infected cells. Collectively, our data suggests a novel mode of uptake by which the virus can infect both professional and nonprofessional phagocytes.

Primary cilia are microtubule-based sensory organelles that coordinate signalling pathways in cell-cycle control, migration, differentiation and other cellular processes critical during development and for tissue homeostasis. Accordingly, defects in assembly or function of primary cilia lead to a plethora of developmental disorders and pathological conditions now known as ciliopathies. In this review, we summarize the current status of the role of primary cilia in coordinating receptor tyrosine kinase (RTK) signalling pathways. Further, we present potential mechanisms of signalling crosstalk and networking in the primary cilium and discuss how defects in ciliary RTK signalling are linked to human diseases and disorders.

Herpes simplex virus type-1 (HSV-1) infection of human brain cells induces changes in gene expression favorable to the propagation of the infecting agent and detrimental to the function of the host cells. We report that infection of human primary neural cells with a high phenotypic reactivator HSV-1 (17syn +) induces upregulation of a brain-enriched microRNA (miRNA)-146a that is associated with proinflammatory signaling in stressed brain cells and Alzheimer's disease. Expression of cytoplasmic phospholipase A 2 , the inducible prostaglandin synthase cyclooxygenase-2, and the neuroinflammatory cytokine interleukin-1β were each upregulated. A known miRNA-146a target in the brain, complement factor H, was downregulated. These data suggest a role for HSV-1-induced miRNA-146a in the evasion of HSV-1 from the complement system, and the activation of key elements of the arachidonic acid cascade known to contribute to Alzheimer-type neuropathological change.

Herpes simplex virus type 1 (HSV-1) infection is the most common cause of infectious blindness in developed countries (33,34,49,62). Following initial infection of epithelial cells, HSV establishes latency in the sensory nerve ganglia of the host (26, 58). The virus emerges sporadically from latency and causes lesions on mucosal epithelium, skin, and the cornea. Prolonged or multiple recurrent episodes of corneal infections can result in vision impairment or blindness due to the development of herpetic stromal keratitis (HSK) (8,23,28,42,44,71). The HSK condition is typically characterized by inflammation leading to scarring, thinning, and vascularization of the corneal stroma (12,15,19,24,33,34). It accounts for 20 to 48% of all recurrent ocular HSV infection cases (33).Primary infection begins with the entry of HSV into host cells. It is a complex process initiated by specific interaction of viral envelope glycoproteins and host cell surface receptors (10,18,52,(58)(59)(60). Both HSV-1 and HSV-2 use glycoproteins B and C (gB and gC, respectively) to mediate their initial attachment to cell surface heparan sulfate (HS) proteoglycans (25,53,67). Binding of herpesviruses to HS proteoglycans likely precedes a conformational change that brings viral gD to the binding domain of host cell surface gD receptors (29-31). Thereafter, a concerted action involving gD, its receptor, three additional HSV glycoproteins (gB, gH, and gL), and possibly an additional gH coreceptor triggers fusion of the viral envelope with the plasma membrane of host cells (43,45,50). Subsequently, viral capsids and tegument proteins are released into the cytoplasm of the host cell.The gD receptors include cell surface molecules derived from three structurally unrelated families. These include a member of the tumor necrosis factor receptor family (40), two members of the nectin family of receptors (21), and the product of certain 3-O-sulfated (3-OS) sulfotransferases (3-OSTs), 3-OS HS (41,55,(57)(58)(59)(60). Herpesvirus entry mediator (HVEM or TNFRSF14) principally mediates entry of HSV-1 and HSV-2 (40, 66) into human T lymphocytes and trabecular meshwork cells and is expressed in many fetal and adult human tissues, including the lung, liver, kidney, and lymphoid tissues (27,32,37,40,62). Nectin-1 and nectin-2, also known as herpesvirus entry proteins C B, respectively, belong to the immunoglobulin superfamily (7,13,21). Both nectin-1 and nectin-2 mediate entry of HSV-1 and HSV-2, but only nectin-1 mediates bovine herpesvirus type 1 entry (9,21,36,38,39,65). The HSV-1 entry-mediating activity of nectin-2 is limited to some mutant strains only (65). Nectin-1 is extensively expressed in human cells of epithelial and neuronal origin (22,48,56,58), while nectin-2 is widely expressed in many human tissues, but with only limited expression in neuronal cells and

Membrane fusion induced by herpes simplex virus (HSV) is required for both entry and cell-to-cell spread. It is mediated by the viral glycoprotein gB, gD, gH-gL and gD receptors. Although 3-O-sulfated heparan sulfate (3-OS HS) is a receptor for HSV-1 entry, the requirement for heparan sulfate in the fusion process has been ruled out. Here, it is demonstrated that cells expressing 3-OS HS, generated by D-glucosaminyl 3-O-sulfotransferase isoforms-3 and/or -5 (3-OST-3 and 3-OST-5), fused with cells expressing the four glycoproteins. The cell fusion observed exhibited similar requirements but was independent of protein receptors, HVEM or nectin-1. Additionally, removal of 3-OS HS from the cell surface by heparinase-I treatment and, in separate experiments, the presence of soluble 3-OST-3-and 3-OST-5-modified HS, significantly inhibited fusion. Taken together, these results indicate that 3-OS HS can play a crucial role in virus entry and cell fusion.

Human embryonic stem cells (hESCs) are potential therapeutic tools and models of human development. With a growing interest in primary cilia in signal transduction pathways that are crucial for embryological development and tissue differentiation and interest in mechanisms regulating human hESC differentiation, demonstrating the existence of primary cilia and the localization of signaling components in undifferentiated hESCs establishes a mechanistic basis for the regulation of hESC differentiation. Using electron microscopy (EM), immunofluorescence, and confocal microscopies, we show that primary cilia are present in three undifferentiated hESC lines. EM reveals the characteristic 9 + 0 axoneme. The number and length of cilia increase after serum starvation. Important components of the hedgehog (Hh) pathway, including smoothened, patched 1 (Ptc1), and Gli1 and 2, are present in the cilia. Stimulation of the pathway results in the concerted movement of Ptc1 out of, and smoothened into, the primary cilium as well as up-regulation of GLI1 and PTC1. These findings show that hESCs contain primary cilia associated with working Hh machinery.