Mutations in INPP5E, encoding inositol polyphosphate-5-phosphatase E, link phosphatidyl inositol signaling to the ciliopathies

Phosphotidylinositol (PtdIns) signaling is tightly regulated, both spatially and temporally, by subcellularly localized PtdIns kinases and phosphatases that dynamically alter downstream signaling events 1. Joubert Syndrome (JS) characterized by a specific midbrain-hindbrain malformation (“molar tooth sign”) and variably associated retinal dystrophy, nephronophthisis, liver fibrosis and polydactyly 2, and is included in the newly emerging group of “ciliopathies”. In patients linking to JBTS1, we identified mutations in the INPP5E gene, encoding inositol polyphosphate-5-phosphatase E, which hydrolyzes the 5-phosphate of PtdIns(3,4,5)P3 and PtdIns(4,5)P2. Mutations clustered in the phosphatase domain and impaired 5-phosphatase activity, resulting in altered cellular PtdIns ratios. INPP5E localized to cilia in major organs affected in JS, and mutations promoted premature destabilization of cilia in response to stimulation. Thus, these data links PtdIns signaling to the primary cilium, a cellular structure that is becoming increasingly appreciated for its role in mediating cell signals and neuronal function.

Syndecan-1: an inhibitor of arterial smooth muscle cell growth and intimal hyperplasia

OBJECTIVE

Arterial injury induces smooth muscle cell (SMC) proliferation, migration, and intimal accumulation of cells and extracellular matrix. These processes are regulated by the administration of the glycosaminoglycans heparin and heparan sulfate, but little is known about the role of endogenous heparan sulfate proteoglycans in the vessel wall. We investigated the response to carotid injury of syndecan-1-null mice to assess the function of one of a conserved family of transmembrane heparan and chondroitin sulfate proteoglycans.

METHODS AND RESULTS

Syndecan-1-null mice developed a large neointimal lesion after injury, whereas wild-type mice made little or none. This was accompanied by a significant increase in both medial and intimal SMC replication. Cultured syndecan-1-null SMCs showed a significant increase in proliferation in response to PDGF-BB, thrombin, FGF2, EGF, and serum. In response to thrombin, PDGF-BB, and serum syndecan-1-null SMCs expressed more PDGF-B chain message than did wild-type SMCs. Downregulation of PDGF-BB or PDGFRbeta inhibited thrombin-, PDGF-BB-, and serum-induced DNA synthesis in syndecan-1-null SMCs.

CONCLUSIONS

These results suggest the possibility that syndecan-1 may limit intimal thickening in injured arteries by suppressing SMC activation through inhibition of SMC PDGF-B chain expression and PDGFRbeta activation.

Endocytosis and spatial restriction of cell signaling

Endocytosis and recycling are essential components of the wiring enabling cells to perceive extracellular signals and transduce them in a temporally and spatially controlled fashion, directly influencing not only the duration and intensity of the signaling output, but also their correct location. Here, we will discuss key experimental evidence that support how different internalization routes, the generation of diverse endomembrane platforms, and cycles of internalization and recycling ensure polarized compartmentalization of signals, regulating a number of physiological and pathologically-relevant processes in which the resolution of spatial information is vital for their execution.

Mechanisms of endocytosis

Endocytic mechanisms control the lipid and protein composition of the plasma membrane, thereby regulating how cells interact with their environments. Here, we review what is known about mammalian endocytic mechanisms, with focus on the cellular proteins that control these events. We discuss the well-studied clathrin-mediated endocytic mechanisms and dissect endocytic pathways that proceed independently of clathrin. These clathrin-independent pathways include the CLIC/GEEC endocytic pathway, arf6-dependent endocytosis, flotillin-dependent endocytosis, macropinocytosis, circular doral ruffles, phagocytosis, and trans-endocytosis. We also critically review the role of caveolae and caveolin1 in endocytosis. We highlight the roles of lipids, membrane curvature-modulating proteins, small G proteins, actin, and dynamin in endocytic pathways. We discuss the functional relevance of distinct endocytic pathways and emphasize the importance of studying these pathways to understand human disease processes.

Signaling from endosomes: location makes a difference

In all transmembrane receptor systems the kinetics of receptor trafficking upon ligand stimulation is maintained in a balance between degradative and recycling pathways in order to keep homeostasis and to strictly control receptor-mediated signaling. Endocytosis is commonly considered as an efficient mechanism of uptake and transport of membrane-associated signaling molecules leading to attenuation of ligand-induced responses. Accumulating evidence, however, shows that signaling from internalized receptors not only continues in endosomal compartments, but that there are also distinct signaling events that require endocytosis. Endocytic organelles form a dynamic network of subcellular compartments, which actively control the timing, amplitude, and specificity of signaling. In this review we provide examples in which signal transduction either requires an active endocytic machinery, or directly originates from various types of endosomes. Based on recent discoveries, we emphasize the close interdependence between signaling and endocytosis, and the physiological relevance of endocytic transport in health and disease.