The transmembrane kinase Ire1p is required for activation of the unfolded protein response (UPR), the increase in transcription of genes encoding endoplasmic reticulum (ER) resident proteins that occurs in response to the accumulation of unfolded proteins in the ER. Ire1p spans the ER membrane (or the nuclear membrane with which the ER is continuous), with its kinase domain localized in the cytoplasm or in the nucleus. Consistent with this arrangement, it has been proposed that Ire1p senses the accumulation of unfolded proteins in the ER and transmits the signal across the membrane toward the transcription machinery, possibly by phosphorylating downstream components of the UPR pathway. Molecular genetic and biochemical studies described here suggest that, as in the case of growth factor receptors of higher eukaryotic cells, Ire1p oligomerizes in response to the accumulation of unfolded proteins in the ER and is phosphorylated in trans by other Ire1p molecules as a result of oligomerization. In addition to its kinase domain, a C-terminal tail domain of Ire1p is required for induction of the UPR. The role of the tail is probably to bind other proteins that transmit the unfolded protein signal to the nucleus.

Oligomerization and phosphorylation of the Ire1p kinase during intracellular signaling from the endoplasmic reticulum to the nucleus.

The transforming growth factor beta (TGF-beta) family of growth factors control the development and homeostasis of most tissues in metazoan organisms. Work over the past few years has led to the elucidation of a TGF-beta signal transduction network. This network involves receptor serine/threonine kinases at the cell surface and their substrates, the SMAD proteins, which move into the nucleus, where they activate target gene transcription in association with DNA-binding partners. Distinct repertoires of receptors, SMAD proteins, and DNA-binding partners seemingly underlie, in a cell-specific manner, the multifunctional nature of TGF-beta and related factors. Mutations in these pathways are the cause of various forms of human cancer and developmental disorders.

TGF-beta signal transduction.

https://www.cell.com/cell/pdf/S0092-8674(03)00432-X.pdf

Mechanisms of TGF-β Signaling from Cell Membrane to the Nucleus

Transforming growth factor-beta (TGF-beta) proteins regulate cell function, and have key roles in development and carcinogenesis The intracellular effectors of TGF-beta signalling, the Smad proteins, are activated by receptors and translocate into the nucleus, where they regulate transcription Although this pathway is inherently simple, combinatorial interactions in the heteromeric receptor and Smad complexes, receptor-interacting and Smad-interacting proteins, and cooperation with sequence-specific transcription factors allow substantial versatility and diversification of TGF-beta family responses Other signalling pathways further regulate Smad activation and function In addition, TGF-beta receptors activate Smad-independent pathways that not only regulate Smad signalling, but also allow Smad-independent TGF-beta responses

Smad-dependent and Smad-independent pathways in TGF-beta family signalling.

Biological barriers to drug transport prevent successful accumulation of nanotherapeutics specifically at diseased sites, limiting efficacious responses in disease processes ranging from cancer to inflammation. Although substantial research efforts have aimed to incorporate multiple functionalities and moieties within the overall nanoparticle design, many of these strategies fail to adequately address these barriers. Obstacles, such as nonspecific distribution and inadequate accumulation of therapeutics, remain formidable challenges to drug developers. A reimagining of conventional nanoparticles is needed to successfully negotiate these impediments to drug delivery. Site-specific delivery of therapeutics will remain a distant reality unless nanocarrier design takes into account the majority, if not all, of the biological barriers that a particle encounters upon intravenous administration. By successively addressing each of these barriers, innovative design features can be rationally incorporated that will create a new generation of nanotherapeutics, realizing a paradigmatic shift in nanoparticle-based drug delivery.

/pdf/principles-of-nanoparticle-design-for-overcoming-biological-4c2c5quqtc.pdf

Principles of nanoparticle design for overcoming biological barriers to drug delivery

Transforming growth factor-β (TGF-β) signals by contacting two distantly related transmem-brane serine/threonine kinases called receptors I and II. The role of these molecules in signalling has now been determined. TGF-β binds directly to receptor II, which is a constitutively active kinase. Bound TGF-β is then recognized by receptor I which is recruited into the complex and becomes phosphorylated by receptor II. Phosphorylation allows receptor I to propagate the signal to downstream substrates. This provides a mechanism by which a cytokine can generate the first step of a signalling cascade.

Mechanism of activation of the TGF-β receptor

TGFβ signals through a heteromeric protein kinase receptor complex

Endocytosis of cell surface receptors is an important regulatory event in signal transduction. The transforming growth factor beta (TGF-beta) superfamily signals to the Smad pathway through heteromeric Ser-Thr kinase receptors that are rapidly internalized and then downregulated in a ubiquitin-dependent manner. Here we demonstrate that TGF-beta receptors internalize into both caveolin- and EEA1-positive vesicles and reside in both lipid raft and non-raft membrane domains. Clathrin-dependent internalization into the EEA1-positive endosome, where the Smad2 anchor SARA is enriched, promotes TGF-beta signalling. In contrast, the lipid raft-caveolar internalization pathway contains the Smad7-Smurf2 bound receptor and is required for rapid receptor turnover. Thus, segregation of TGF-beta receptors into distinct endocytic compartments regulates Smad activation and receptor turnover.

Distinct endocytic pathways regulate TGF-beta receptor signalling and turnover.

The internalization of various cargo proteins and lipids from the mammalian cell surface occurs through the clathrin and lipid-raft endocytic pathways. Protein-lipid and protein-protein interactions control the targeting of signalling molecules and their partners to various specialized membrane compartments in these pathways. This functions to control the activity of signalling cascades and the termination of signalling events, and therefore has a key role in defining how a cell responds to its environment.

/pdf/clathrin-and-non-clathrin-mediated-endocytic-regulation-of-48hm6xvlvc.pdf

Jeffrey L. Wrana

Papers

Mechanism of activation of the TGF-β receptor

TGFβ signals through a heteromeric protein kinase receptor complex

Distinct endocytic pathways regulate TGF-beta receptor signalling and turnover.

Clathrin- and non-clathrin-mediated endocytic regulation of cell signalling

Betaglycan presents ligand to the TGFβ signaling receptor