A kinase cascade leading to Rab11-FIP5 controls transcytosis of the polymeric immunoglobulin receptor

Yes and Lyn play a role in nuclear translocation of the epidermal growth factor receptor

The epidermal growth factor receptor (EGFR) is a central regulator of tumor progression in human cancers. Cetuximab is an anti-EGFR antibody that has been approved for use in oncology. Previously we investigated mechanisms of resistance to cetuximab using a model derived from the non-small cell lung cancer line NCI-H226. We demonstrated that cetuximab-resistant clones (Ctx(R)) had increased nuclear localization of the EGFR. This process was mediated by Src family kinases (SFKs), and nuclear EGFR had a role in resistance to cetuximab. To better understand SFK-mediated nuclear translocation of EGFR, we investigated which SFK member(s) controlled this process as well as the EGFR tyrosine residues that are involved. Analyses of mRNA and protein expression indicated upregulation of the SFK members Yes (v-Yes-1 yamaguchi sarcoma viral oncogene) and Lyn (v-yes-1 Yamaguchi sarcoma viral-related oncogene homolog) in all Ctx(R) clones. Further, immunoprecipitation analysis revealed that EGFR interacts with Yes and Lyn in Ctx(R) clones, but not in cetuximab-sensitive (Ctx(S)) parental cells. Using RNAi interference, we found that knockdown of either Yes or Lyn led to loss of EGFR translocation to the nucleus. Conversely, overexpression of Yes or Lyn in low nuclear EGFR-expressing Ctx(S) parental cells led to increased nuclear EGFR. Chromatin immunoprecipitation (ChIP) assays confirmed nuclear EGFR complexes associated with the promoter of the known EGFR target genes B-Myb and iNOS. Further, all Ctx(R) clones exhibited upregulation of B-Myb and iNOS at the mRNA and protein levels. siRNAs directed at Yes or Lyn led to decreased binding of EGFR complexes to the B-Myb and iNOS promoters based on ChIP analyses. SFKs have been shown to phosphorylate EGFR on tyrosines 845 and 1101 (Y845 and Y1101), and mutation of Y1101, but not Y845, impaired nuclear entry of the EGFR. Taken together, our findings demonstrate that Yes and Lyn phosphorylate EGFR at Y1101, which influences EGFR nuclear translocation in this model of cetuximab resistance.

c-Yes regulates cell adhesion at the apical ectoplasmic specialization-blood-testis barrier axis via its effects on protein recruitment and distribution

During spermatogenesis, extensive restructuring takes place at the cell-cell interface since developing germ cells migrate progressively from the basal to the adluminal compartment of the seminiferous epithelium. Since germ cells per se are not motile cells, their movement relies almost exclusively on the Sertoli cell. Nonetheless, extensive exchanges in signaling take place between these cells in the seminiferous epithelium. c-Yes, a nonreceptor protein tyrosine kinase belonging to the Src family kinases (SFKs) and a crucial signaling protein, was recently shown to be upregulated at the Sertoli cell-cell interface at the blood-testis barrier (BTB) at stages VIII-IX of the seminiferous epithelial cycle of spermatogenesis. It was also highly expressed at the Sertoli cell-spermatid interface known as apical ectoplasmic specialization (apical ES) at stage V to early stage VIII of the epithelial cycle during spermiogenesis. Herein, it was shown that the knockdown of c-Yes by RNAi in vitro and in vivo affected both Sertoli cell adhesion at the BTB and spermatid adhesion at the apical ES, causing a disruption of the Sertoli cell tight junction-permeability barrier function, germ cell loss from the seminiferous epithelium, and also a loss of spermatid polarity. These effects were shown to be mediated by changes in distribution and/or localization of adhesion proteins at the BTB (e.g., occludin, N-cadherin) and at the apical ES (e.g., nectin-3) and possibly the result of changes in the underlying actin filaments at the BTB and the apical ES. These findings implicate that c-Yes is a likely target of male contraceptive research.

C-Src and c-Yes are two unlikely partners of spermatogenesis and their roles in blood-testis barrier dynamics

Src family kinases (SFKs), in particular c-Src and c-Yes, are nonreceptor protein tyrosine kinases that mediate integrin signaling at focal adhesion complex at the cell-extracellular matrix interface to regulate cell adhesion, cell cycle progression, cell survival, proliferation and differentiation, most notably in cancer cells during tumorigenesis and metastasis. Interestingly, recent studies have shown that these two proto-oncogenes are integrated components of the stem cell niche and the cell-cell actin-based anchoring junction known as ectoplasmic specialization (ES) at the: (1) Sertoli cell-spermatid interface known as apical ES and (2) Sertoli-Sertoli cell interface known as basal ES which together with tight junctions (TJ), gap junctions and desmosomes constitute the blood-testis barrier (BTB). At the stem cell niche, these SFKs regulate spermatogonial stem cell (SSC) renewal to maintain the proper population of SSC/spermatogonia for spermatogenesis. At the apical ES and the BTB, c-Src and c-Yes confer cell adhesion either by maintaining the proper phosphorylation status of integral membrane proteins at the site which in turn regulates protein-protein interactions between integral membrane proteins and their adaptors, or by facilitating androgen action on spermatogenesis via a nongenomic pathway which also modulates cell adhesion in the seminiferous epithelium. Herein, we critically evaluate recent findings in the field regarding the roles of these two unlikely partners of spermatogenesis. We also propose a hypothetical model on the mechanistic functions of c-Src and c-Yes in spermatogenesis so that functional experiments can be designed in future studies.

The role of secretory and endocytic pathways in the maintenance of cell polarity

Epithelial cells line virtually every organ cavity in the body and are important for vectorial transport through epithelial monolayers such as nutrient uptake or waste product excretion. Central to these tasks is the establishment of epithelial cell polarity. During organ development, epithelial cells set up two biochemically distinct plasma membrane domains, the apical and the basolateral domain. Targeting of correct constituents to each of these regions is essential for maintaining epithelial cell polarity. Newly synthesized transmembrane proteins destined for the basolateral or apical membrane domain are sorted into separate transport carriers either at the TGN (trans-Golgi network) or in perinuclear REs (recycling endosomes). After initial delivery, transmembrane proteins, such as nutrient receptors, frequently undergo multiple rounds of endocytosis followed by re-sorting in REs. Recent work in epithelial cells highlights the REs as a potent sorting station with different subdomains representing individual targeting zones that facilitate the correct surface delivery of transmembrane proteins.

Crag is a GEF for Rab11 required for rhodopsin trafficking and maintenance of adult photoreceptor cells

Transport of newly synthesized Rhodopsin upon light stimulation in adult Drosophila photoreceptors is mediated by a Crag/Rab11-dependent vesicular trafficking process.

Rhodopsins (Rhs) are light sensors, and Rh1 is the major Rh in the Drosophila photoreceptor rhabdomere membrane. Upon photoactivation, a fraction of Rh1 is internalized and degraded, but it remains unclear how the rhabdomeric Rh1 pool is replenished and what molecular players are involved. Here, we show that Crag, a DENN protein, is a guanine nucleotide exchange factor for Rab11 that is required for the homeostasis of Rh1 upon light exposure. The absence of Crag causes a light-induced accumulation of cytoplasmic Rh1, and loss of Crag or Rab11 leads to a similar photoreceptor degeneration in adult flies. Furthermore, the defects associated with loss of Crag can be partially rescued with a constitutive active form of Rab11. We propose that upon light stimulation, Crag is required for trafficking of Rh from the trans-Golgi network to rhabdomere membranes via a Rab11-dependent vesicular transport.

Animals sense light through receptors called Rhodopsins. These proteins are typically localized to stacked membranes in photoreceptors. In flies, upon light exposure, Rhodopsin undergoes conformational changes and becomes active as metarhodopsin. Metarhodopsin then initiates a signaling cascade that activates the photoreceptor cell. To deactivate the light response, metarhodopsin is converted back into Rhodopsin by absorption of another photon of light. Under certain conditions, metarhodopsin cannot be converted back to Rhodopsin, and it is then endocytosed and degraded. Rhodopsin then needs to be synthesized and delivered back to the membrane stacks. Here, we show that the Calmodulin-binding protein Crag is required for the delivery of newly made Rhodopsin to the membrane stacks. Loss of Crag leads to the accumulation of Rhodopsin in the cytosol, followed by shrinkage of membrane stack volume, and, eventually, photoreceptor cell degeneration. We also show that Crag activates a target protein, Rab11, which mediates the vesicular transport of Rhodospin to the membrane. Finally, we document that the human homolog of Crag, DENND4A, is able to rescue the loss of Crag in flies, suggesting that DENND4A functions in a similar process in vertebrates.

Basolateral targeting and microtubule-dependent transcytosis of the aquaporin-2 water channel

The aquaporin-2 (AQP2) water channel relocates mainly to the apical plasma membrane of collecting duct principal cells after vasopressin (VP) stimulation. AQP2 transport to this membrane domain is assumed to be a direct route involving recycling of intracellular vesicles. However, basolateral plasma membrane expression of AQP2 is observed in vivo in principal cells. Here, we asked whether there is a transcytotic pathway of AQP2 trafficking between apical and basolateral membranes. We used MDCK cells in which AQP2 normally accumulates apically after VP exposure. In contrast, both site-specific biotinylation and immunofluorescence showed that AQP2 is strongly accumulated in the basolateral membrane, along with the endocytic protein clathrin, after a brief cold shock (4°C). This suggests that AQP2 may be constitutively targeted to basolateral membranes and then retrieved by clathrin-mediated endocytosis at physiological temperatures. Rab11 does not accumulate in basolateral membranes after cold shock, suggesting that the AQP2 in this location is not associated with Rab11-positive vesicles. After rewarming (37°C), basolateral AQP2 staining is diminished and it subsequently accumulates at the apical membrane in the presence of VP/forskolin, suggesting that transcytosis can be followed by apical insertion of AQP2. This process is inhibited by treatment with colchicine. Our data suggest that the cold shock procedure reveals the presence of microtubule-dependent AQP2 transcytosis, which represents an indirect pathway of apical AQP2 delivery in these cells. Furthermore, our data indicate that protein polarity data obtained from biotinylation assays, which require cells to be cooled to 4°C during the labeling procedure, should be interpreted with caution.

ERK1/2 regulate exocytosis through direct phosphorylation of the exocyst component Exo70

The exocyst is a multiprotein complex essential for exocytosis and plasma membrane remodeling. The assembly of the exocyst complex mediates the tethering of post-Golgi secretory vesicles to the plasma membrane prior to fusion. Elucidating the mechanisms regulating exocyst assembly is important for the understanding of exocytosis. Here we show that the exocyst component Exo70 is a direct substrate of the extracellular signal-regulated kinases 1/2 (ERK1/2). ERK1/2 phosphorylation enhances the binding of Exo70 to other exocyst components and promotes the assembly of the exocyst complex in response to epidermal growth factor (EGF) signaling. We further demonstrate that ERK1/2 regulates exocytosis, because blocking ERK1/2 signaling by a chemical inhibitor or the expression of an Exo70 mutant defective in ERK1/2 phosphorylation inhibited exocytosis. In tumor cells, blocking Exo70 phosphorylation inhibits matrix metalloproteinase secretion and invadopodia formation. ERK1/2 phosphorylation of Exo70 may thus coordinate exocytosis with other cellular events in response to growth factor signaling.

Polarized sorting and trafficking in epithelial cells

The polarized distribution of proteins and lipids at the surface membrane of epithelial cells results in the formation of an apical and a basolateral domain, which are separated by tight junctions. The generation and maintenance of epithelial polarity require elaborate mechanisms that guarantee correct sorting and vectorial delivery of cargo molecules. This dynamic process involves the interaction of sorting signals with sorting machineries and the formation of transport carriers. Here we review the recent advances in the field of polarized sorting in epithelial cells. We especially highlight the role of lipid rafts in apical sorting.

Rab11 is phosphorylated by classical and novel protein kinase C isoenzymes upon sustained phorbol ester activation

BACKGROUND INFORMATION

Rab11 is a small GTPase that controls diverse intracellular trafficking pathways. However, the molecular machinery that regulates the participation of Rab11 in those different transport events is poorly understood. In resting cells, Rab11 localizes at the endocytic recycling compartment (ERC), whereas the different protein kinase C (PKC) isoforms display a cytosolic distribution.

RESULTS

Sustained phorbol ester stimulation induces the translocation of the classical PKCα and PKCβII isoenzymes to the ERC enriched in Rab11, and results in transferrin recycling inhibition. In contrast, novel PKCε and atypical PKCζ isoenzymes neither redistribute to the perinucleus nor modify transferrin recycling transport after phorbol ester stimulation. Although several Rabs have been shown to be phosphorylated, there is to date no evidence indicating Rab11 as a kinase substrate. In this report, we show that Rab11 appears phosphorylated in vivo in phorbol ester-stimulated cells. A bioinformatic analysis of Rab11 allowed us to identify several high-probability Ser/Thr kinase phosphorylation sites. Our results demonstrate that classical PKC (PKCα and PKCβII but not PKCβI) directly phosphorylate Rab11 in vitro. In addition, novel PKCε and PKCη but not PKCδ isoenzymes also phosphorylate Rab11. Mass spectrometry analysis revealed that Ser 177 is the Rab11 residue to be phosphorylated in vitro by either PKCβII or PKCε. In agreement, the phosphomimetic mutant, Rab11 S177D, retains transferrin at the ERC in the absence of phorbol-12-myristate-13-acetate stimulus.

CONCLUSIONS

This report shows for the first time that Rab11 is differentially phosphorylated by distinct PKC isoenzymes and that this post-translational modification might be a regulatory mechanism of intracellular trafficking.

Reduced immunoglobulin A transcytosis associated with immunoglobulin A nephropathy and nasopharyngeal carcinoma

Polymeric IgA (pIgA) is transcytosed by the pIgA receptor (pIgR) across mucosal epithelial cells. After transcytosis to the apical surface, the extracellular, ligand-binding portion of the pIgR is proteolytically cleaved. A missense mutation in human pIgR, A580V, is associated with IgA nephropathy and nasopharyngeal carcinoma. We report that this mutation reduces the rate of transcytosis of pIgR and pIgA, and seemingly the rate of pIgR cleavage. We propose that the defects in pIgR trafficking caused by the A580V mutation may underlie the pathogenesis of both diseases.

Regulation of the polymeric immunoglobulin receptor and IgA transport: new advances in environmental factors that stimulate pIgR expression and its role in mucosal immunity

Secretory IgA (SIgA) antibodies represent the first line of antigen-specific immune defense protecting the mucosal surfaces against environmental pathogens and antigens, and maintaining homeostasis with the commensal microbiota. The polymeric immunoglobulin receptor (pIgR) has the dual role of transporting locally produced dimeric IgA across mucosal epithelia, and serving as the precursor of secretory component, a glycoprotein that enhances the immune functions of SIgA. The complex regulation of pIgR expression and transcytosis by host and microbial factors is finely tuned to optimize the role of SIgA in mucosal immunity. Disruption of this regulatory network in disease states similar to inflammatory bowel disease can result in profound consequences for mucosal homeostasis and systemic sequelae. Future research into the function and regulation of pIgR and SIgA may offer new insights into the prevention and treatment of infectious and inflammatory diseases that originate at mucosal surfaces.

The recycling endosome and its role in neurological disorders

The recycling endosome (RE) is an organelle in the endocytic pathway where plasma membranes (proteins and lipids) internalized by endocytosis are processed back to the cell surface for reuse. Endocytic recycling is the primary way for the cell to maintain constituents of the plasma membrane (Griffiths et al., 1989), i.e., to maintain the abundance of receptors and transporters on cell surfaces. Membrane traffic through the RE is crucial for several key cellular processes including cytokinesis and cell migration. In polarized cells, including neurons, the RE is vital for the generation and maintenance of the polarity of the plasma membrane. Many RE dependent cargo molecules are known to be important for neuronal function and there is evidence that improper function of key proteins in RE-associated pathways may contribute to the pathogenesis of neurological disorders, including Huntington's disease. The function of the RE in neurons is poorly understood. Therefore, there is need to understand how membrane dynamics in RE-associated pathways are affected or participate in the development or progression of neurological diseases. This review summarizes advances in understanding endocytic recycling associated with the RE, challenges in elucidating molecular mechanisms underlying RE function, and evidence for RE dysfunction in neurological disorders.

Enhancement of polymeric immunoglobulin receptor transcytosis by biparatopic VHH

The polymeric immunoglobulin receptor (pIgR) ensures the transport of dimeric immunoglobulin A (dIgA) and pentameric immunoglobulin M (pIgM) across epithelia to the mucosal layer of for example the intestines and the lungs via transcytosis. Per day the human pIgR mediates the excretion of 2 to 5 grams of dIgA into the mucosa of luminal organs. This system could prove useful for therapies aiming at excretion of compounds into the mucosa. Here we investigated the use of the variable domain of camelid derived heavy chain only antibodies, also known as VHHs or Nanobodies®, targeting the human pIgR, as a transport system across epithelial cells. We show that VHHs directed against the human pIgR are able to bind the receptor with high affinity (∼1 nM) and that they compete with the natural ligand, dIgA. In a transcytosis assay both native and phage-bound VHH were only able to get across polarized MDCK cells that express the human pIgR gene in a basolateral to apical fashion. Indicating that the VHHs are able to translocate across epithelia and to take along large particles of cargo. Furthermore, by making multivalent VHHs we were able to enhance the transport of the compounds both in a MDCK-hpIgR and Caco-2 cell system, probably by inducing receptor clustering. These results show that VHHs can be used as a carrier system to exploit the human pIgR transcytotic system and that multivalent compounds are able to significantly enhance the transport across epithelial monolayers.

Extracellular movement of signaling molecules

Extracellular signaling molecules have crucial roles in development and homeostasis, and their incorrect deployment can lead to developmental defects and disease states. Signaling molecules are released from sending cells, travel to target cells, and act over length scales of several orders of magnitude, from morphogen-mediated patterning of small developmental fields to hormonal signaling throughout the organism. We discuss how signals are modified and assembled for transport, which routes they take to reach their targets, and how their range is affected by mobility and stability.

IKKepsilon: a kinase at the intersection of signaling and membrane traffic

The ability to coordinate membrane trafficking events in space and time is of fundamental importance in biology. Polarized trafficking is essential for processes as diverse as cytokinesis, regulated endocytosis, cell motility, and morphogenesis, yet the interface between cellular signaling systems and the trafficking machinery remains ill defined. Using Drosophila mechanosensory bristle growth as a paradigm for polarized trafficking, a study has uncovered a role for localized activation of the inhibitor of nuclear factor κB kinase, subtype ε (IKKε) at the tip of these bristles, where it regulates the trafficking of recycling endosomal vesicles into and out of the bristle tip. IKKε phosphorylates the Rab11 effector Nuf (which is encoded by nuclear fallout), and in doing so coordinates the interaction between Nuf and the motor protein dynein, which regulates the directionality of membrane traffic by a "motor-switching" mechanism. Similar results in mammalian systems suggest that IKKε-dependent phosphorylation of Rab11 effectors is an evolutionarily conserved mechanism by which cells may regulate polarized growth, and studies of vesicle trafficking in cytokinesis support the concept of motor switching as a generic mechanism to modulate the distribution of endosomes. Phosphorylation of other Rab11 effectors also modulates polarized trafficking in other experimental systems, which hints that similar mechanisms may be widely used to control the directionality of membrane traffic.