Phosphatidylinositol 3-phosphates-at the interface between cell signalling and membrane traffic.

Phosphoinositides (PIs) form a minor class of phospholipids with crucial functions in cell physiology, ranging from cell signalling and motility to a role as signposts of compartmental membrane identity. Phosphatidylinositol 3-phosphates are present at the plasma membrane and within the endolysosomal system, where they serve as key regulators of both cell signalling and of intracellular membrane traffic. Here, we provide an overview of the metabolic pathways that regulate cellular synthesis of PI 3-phosphates at distinct intracellular sites and discuss the mechanisms by which these lipids regulate cell signalling and membrane traffic. Finally, we provide a framework for how PI 3-phosphate metabolism is integrated into the cellular network.

The Connecdenn DENN domain: a GEF for Rab35 mediating cargo-specific exit from early endosomes.

The DENN domain is an evolutionarily ancient protein module. Mutations in the DENN domain cause developmental defects in plants and human diseases, yet the function of this common module is unknown. We now demonstrate that the connecdenn/DENND1A DENN domain functions as a guanine nucleotide exchange factor (GEF) for Rab35 to regulate endosomal membrane trafficking. Loss of Rab35 activity causes an enlargement of early endosomes and inhibits MHC class I recycling. Moreover, it prevents early endosomal recruitment of EHD1, a common component of tubules involved in endosomal cargo recycling. Our data reveal an enzymatic activity for a DENN domain and demonstrate that distinct Rab GTPases can recruit a common protein machinery to various sites within the endosomal network to establish cargo-selective recycling pathways.

Phosphorylation-dependent Regulation of Connecdenn/DENND1 Guanine Nucleotide Exchange Factors.

Connecdenn 1/2 are DENN (differentially expressed in normal and neoplastic cells) domain-bearing proteins that function as GEFs (guanine nucleotide exchange factors) for the small GTPase Rab35. Disruption of connecdenn/Rab35 function leads to defects in the recycling of multiple cargo proteins from endosomes with altered cell function, yet the regulation of connecdenn GEF activity is unexplored. We now demonstrate that connecdenn 1/2 are autoinhibited such that the purified, full-length proteins have significantly less Rab35 binding and GEF activity than the isolated DENN domain. Both proteins are phosphorylated with prominent phosphorylation sites between residues 500 and 600 of connecdenn 1. A large scale proteomics screen revealed that connecdenn 1 is phosphorylated at residues Ser-536 and Ser-538 in an Akt-dependent manner in response to insulin stimulation of adipocytes. Interestingly, we find that an Akt inhibitor reduces connecdenn 1 interaction with Rab35 after insulin treatment of adipocytes. Remarkably, a peptide flanking Ser-536/Ser-538 binds the DENN domain of connecdenn 1, whereas a phosphomimetic peptide does not. Moreover, connecdenn 1 interacts with 14-3-3 proteins, and this interaction is also disrupted by Akt inhibition and by mutation of Ser-536/Ser-538. We propose that Akt phosphorylation of connecdenn 1 downstream of insulin activation regulates connecdenn 1 function through an intramolecular interaction.

DENN domain proteins: regulators of Rab GTPases.

The DENN domain is a common, evolutionarily ancient, and conserved protein module, yet it has gone largely unstudied; until recently, little was known regarding its functional roles. New studies reveal that various DENN domains interact directly with members of the Rab family of small GTPases and that DENN domains function enzymatically as Rab-specific guanine nucleotide exchange factors. Thus, DENN domain proteins appear to be generalized regulators of Rab function. Study of these proteins will provide new insights into Rab-mediated membrane trafficking pathways.

The connecdenn family, Rab35 guanine nucleotide exchange factors interfacing with the clathrin machinery.

Rabs constitute the largest family of monomeric GTPases, yet for the majority of Rabs relatively little is known about their activation and recruitment to vesicle-trafficking pathways. We recently identified connecdenn (DENND1A), which contains an N-terminal DENN (differentially expressed in neoplastic versus normal cells) domain, a common and evolutionarily ancient protein module. Through its DENN domain, connecdenn functions enzymatically as a guanine-nucleotide exchange factor (GEF) for Rab35. Here we identify two additional connecdenn family members and demonstrate that all connecdenns function as Rab35 GEFs, albeit with different levels of activity. The DENN domain of connecdenn 1 and 2 binds Rab35, whereas connecdenn 3 does not, indicating that Rab35 binding and activation are separable functions. Through their highly divergent C termini, each of the connecdenns binds to clathrin and to the clathrin adaptor AP-2. Interestingly, all three connecdenns use different mechanisms to bind AP-2. Characterization of connecdenn 2 reveals binding to the beta2-ear of AP-2 on a site that overlaps with that used by the autosomal recessive hypercholesterolemia protein and betaarrestin, although the sequence used by connecdenn 2 is unique. Loss of connecdenn 2 function through small interference RNA knockdown results in an enlargement of early endosomes, similar to what is observed upon loss of Rab35 activity. Our studies reveal connecdenn DENN domains as generalized GEFs for Rab35 and identify a new AP-2-binding motif, demonstrating a complex link between the clathrin machinery and Rab35 activation.

Protein kinase N controls a lysosomal lipid switch to facilitate nutrient signalling via mTORC1.

Mechanistic target of rapamycin (mTOR) kinase functions in two multiprotein complexes: lysosomal mTOR complex 1 (mTORC1) and mTORC2 at the plasma membrane. mTORC1 modulates the cell response to growth factors and nutrients by increasing protein synthesis and cell growth, and repressing the autophagy-lysosomal pathway1-4; however, dysfunction in mTORC1 is implicated in various diseases3,5,6. mTORC1 activity is regulated by phosphoinositide lipids7-10. Class I phosphatidylinositol-3-kinase (PI3K)-mediated production of phosphatidylinositol-3,4,5-trisphosphate6,11 at the plasma membrane stimulates mTORC1 signalling, while local synthesis of phosphatidylinositol-3,4-bisphosphate by starvation-induced recruitment of class II PI3K-ß (PI3KC2-ß) to lysosomes represses mTORC1 activity12. How the localization and activity of PI3KC2-ß are regulated by mitogens is unknown. We demonstrate that protein kinase N (PKN) facilitates mTORC1 signalling by repressing PI3KC2-ß-mediated phosphatidylinositol-3,4-bisphosphate synthesis downstream of mTORC2. Active PKN2 phosphorylates PI3KC2-ß to trigger PI3KC2-ß complex formation with inhibitory 14-3-3 proteins. Conversely, loss of PKN2 or inactivation of its target phosphorylation site in PI3KC2-ß represses nutrient signalling via mTORC1. These results uncover a mechanism that couples mTORC2-dependent activation of PKN2 to the regulation of mTORC1-mediated nutrient signalling by local lipid signals.

ATP-binding cassette transporters ABCA1, ABCA7, and ABCG1 in mouse spermatozoa.

Mammalian spermatozoa lose plasma membrane cholesterol during their maturation in the epididymis and during their capacitation in the female reproductive tract. While acceptors such as high-density lipoproteins (HDL) and apolipoproteins A-I (apoA-I) and J have been found in male and female reproductive tracts, transporters that mediate cholesterol efflux from plasma membranes of spermatozoa to such acceptors have not yet been defined. Candidate transporters are members of the ATP-binding cassette (ABC) transporter superfamily including ABCA1, ABCA7, ABCG1 and ABCG4, which have all been implicated in the transport of sterols and phospholipids to apolipoproteins and HDL. Here we show that mouse spermatozoa in the seminiferous tubules and epididymis express ABCA1, ABCA7 and ABCG1, but not ABCG4. Moreover, we show that ABCA1, ABCA7, and ABCG1 antibodies decrease cholesterol efflux from spermatozoa to lipid acceptors apoA-I and albumin and inhibit in vitro fertilization.

mTORC1 activity repression by late endosomal phosphatidylinositol 3,4-bisphosphate.

Nutrient sensing by mechanistic target of rapamycin complex 1 (mTORC1) on lysosomes and late endosomes (LyLEs) regulates cell growth. Many factors stimulate mTORC1 activity, including the production of phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] by class I phosphatidylinositol 3-kinases (PI3Ks) at the plasma membrane. We investigated mechanisms that repress mTORC1 under conditions of growth factor deprivation. We identified phosphatidylinositol 3,4-bisphosphate [PI(3,4)P2], synthesized by class II PI3K ß (PI3KC2ß) at LyLEs, as a negative regulator of mTORC1, whereas loss of PI3KC2ß hyperactivated mTORC1. Growth factor deprivation induced the association of PI3KC2ß with the Raptor subunit of mTORC1. Local PI(3,4)P2 synthesis triggered repression of mTORC1 activity through association of Raptor with inhibitory 14-3-3 proteins. These results unravel an unexpected function for local PI(3,4)P2 production in shutting off mTORC1.

The role of EHD proteins at the neuronal synapse.

Eps15 homology domain (EHD) proteins are conserved adenosine triphosphatases that are involved in membrane remodeling. EHD family members are structurally similar to the guanosine triphosphatase (GTPase) dynamin, and both are essential for the fission step of clathrin-mediated endocytosis. This Journal Club highlights a recent study by Jakobsson et al. that reports the unexpected finding that, rather than having a redundant function, EHD can regulate dynamin activity. Dynamin helices assemble around the neck of budding endocytic vesicles; as dynamin helices lengthen, the neck of the growing bud may become so long that GTP hydrolysis is no longer sufficient to promote fission. EHD increases the efficiency of dynamin-induced fission by restricting the length of dynamin helices. Furthermore, EHD is able to bind both dynamin and amphiphysin. Therefore, we propose a model whereby amphiphysin recruits both EHD and dynamin in neurons to regulate clathrin-dependent synaptic vesicle endocytosis.

Connecdenn 3/DENND1C binds actin linking Rab35 activation to the actin cytoskeleton.

The small GTPase Rab35 regulates endosomal membrane trafficking but also recruits effectors that modulate actin assembly and organization. Differentially expressed in normal and neoplastic cells (DENN)-domain proteins are a newly identified class of Rab guanine-nucleotide exchange factors (GEFs) that are grouped into eight families, each activating a common Rab. The members of one family, connecdenn 1-3/DENND1A-C, are all GEFs for Rab35. Why Rab35 requires multiple GEFs is unknown. We demonstrate that connecdenn 3 uses a unique C-terminal motif, a feature not found in connecdenn 1 or 2, to directly bind actin. This interaction couples Rab35 activation to the actin cytoskeleton, resulting in dramatic changes in cell shape, notably the formation of protrusive membrane extensions. These alterations are specific to Rab35 activated by connecdenn 3 and require both the actin-binding motif and N-terminal DENN domain, which harbors the GEF activity. It was previously demonstrated that activated Rab35 recruits the actin-bundling protein fascin to actin, but the relevant GEF for this activity was unknown. We demonstrate that connecdenn 3 and Rab35 colocalize with fascin and actin filaments, suggesting that connecdenn 3 is the relevant GEF. Thus, whereas connecdenn 1 and 2 activate Rab35 for endosomal trafficking, connecdenn 3 uniquely activates Rab35 for its role in actin regulation.