David Teis

Localization of the MP1-MAPK Scaffold Complex to Endosomes Is Mediated by p14 and Required for Signal Transduction

Eukaryotic cells use the extracellular signal regulated kinase (ERK) cascade to connect cell-surface receptors to intracellular targets. Although various signals are routed through the ERK pathway, cells respond accordingly to a given stimulus. To regulate proper signal transduction, scaffolds and adaptors are employed to organize specific signaling units. The scaffold protein MP1 (MEK1 partner) assembles a scaffold complex in the ERK cascade. We show that p14 functions as an adaptor protein, which is required and sufficient to localize MP1 to endosomes. Reduction of MP1 or p14 protein levels by siRNAi results in defective signal transduction. Therefore, our results suggest that the endosomal localization of the p14/MP1-MAPK scaffold complex is crucial for signal transduction.

Comm Sorts Robo to Control Axon Guidance at the Drosophila Midline

Axon growth across the Drosophila midline requires Comm to downregulate Robo, the receptor for the midline repellent Slit. We show here that comm is required in neurons, not in midline cells as previously thought, and that it is expressed specifically and transiently in commissural neurons. Comm acts as a sorting receptor for Robo, diverting it from the synthetic to the late endocytic pathway. A conserved cytoplasmic LPSY motif is required for endosomal sorting of Comm in vitro and for Comm to downregulate Robo and promote midline crossing in vivo. Axon traffic at the CNS midline is thus controlled by the intracellular trafficking of the Robo guidance receptor, which in turn depends on the precisely regulated expression of the Comm sorting receptor.

p14-MP1-MEK1 signaling regulates endosomal traffic and cellular proliferation during tissue homeostasis.

The extracellular signal-regulated kinase (ERK) cascade regulates proliferation, differentiation, and survival in multicellular organisms. Scaffold proteins regulate intracellular signaling by providing critical spatial and temporal specificity. The scaffold protein MEK1 (mitogen-activated protein kinase and ERK kinase 1) partner (MP1) is localized to late endosomes by the adaptor protein p14. Using conditional gene disruption of p14 in mice, we now demonstrate that the p14–MP1-MEK1 signaling complex regulates late endosomal traffic and cellular proliferation. This function its essential for early embryogenesis and during tissue homeostasis, as revealed by epidermis-specific deletion of p14. These findings show that endosomal p14–MP1-MEK1 signaling has a specific and essential function in vivo and, therefore, indicate that regulation of late endosomal traffic by extracellular signals is required to maintain tissue homeostasis.

A novel human primary immunodeficiency syndrome caused by deficiency of the endosomal adaptor protein p14

Lysosome-related organelles have versatile functions, including protein and lipid degradation, signal transduction and protein secretion. The molecular elucidation of rare congenital diseases affecting endosomal-lysosomal biogenesis has given insights into physiological functions of the innate and adaptive immune system. Here, we describe a previously unknown human primary immunodeficiency disorder and provide evidence that the endosomal adaptor protein p14, previously characterized as confining mitogen-activated protein kinase (MAPK) signaling to late endosomes, is crucial for the function of neutrophils, B cells, cytotoxic T cells and melanocytes. Combining genetic linkage studies and transcriptional profiling analysis, we identified a homozygous point mutation in the 3′ untranslated region (UTR) of p14 (also known as MAPBPIP), resulting in decreased protein expression. In p14-deficient cells, the distribution of late endosomes was severely perturbed, suggesting a previously unknown role for p14 in endosomal biogenesis. These findings have implications for understanding endosomal membrane dynamics, compartmentalization of cell signal cascades, and their role in immunity.

The ESCRT machinery.

Summary The endosomal sorting complexes required for transport (ESCRT) assemble into a multisubunit machinery that performs a topologically unique membrane bending and scission reaction away from the cytoplasm. This evolutionarily highly conserved process is required for the multivesicular body (MVB) pathway, cytokinesis and HIV budding. The modular setup of the machinery with five distinct ESCRT complexes (ESCRT-0, -I, -II, -III and the Vps4 complex) that have a clear division of tasks — from interaction with ubiquitinated membrane proteins to membrane deformation and abscission — allows them to be flexibly integrated into these three very different biological processes (Figure 1).

The odd couple: signal transduction and endocytosis

Cell surface receptors are used to transmit extracellular information. The activation of cell surface receptors initiates signal transduction and receptor endocytosis. Signal transduction and the endosomal transport of activated receptors require precise regulation. New concepts for the integration of endocytosis and signaling arise from recent findings that suggest bidirectional interplay of these two processes. This review discusses the following questions: (i) do activated cell surface receptors modify the endosomal system to promote internalization and endosomal traffic, and (ii) do internalized cell surface receptors use specifically localized signaling complexes to generate specific biological signals?

Coordinated binding of Vps4 to ESCRT-III drives membrane neck constriction during MVB vesicle formation

Vps4 both recycles ESCRT-III subunits and cooperates with ESCRT-III to drive distinct membrane remodeling steps that lead to efficient membrane scission during the biogenesis of multivesicular bodies.

Assembly and disassembly of the ESCRT‐III membrane scission complex

The ESCRT (endosomal sorting complex required for transport) pathway promotes the final membrane scission step at the end of cytokinesis, assists viral budding and generates multivesicular bodies (MVBs). These seemingly unrelated processes require a topologically similar membrane deformation and scission event that buds membranes/vesicles out of the cytoplasm. The topology of this budding reaction is ‘opposite’ to reactions that bud endocytic and secretory vesicles into the cytoplasm. Here we summarize recent findings that help to understand how the ESCRT machinery, in particular the ESCRT‐III complex, assembles on its target membranes, executes membrane scission and is disassembled by the AAA‐ATPase Vps4.

Apoptosis resistance of senescent human fibroblasts is correlated with the absence of nuclear IGFBP-3.

Signaling through the insulin/IGF axis plays a major role in determining the rate of aging in many species. IGF‐binding proteins (IGFBPs) modulate the IGF pathway in higher organisms. IGFBP‐3 accumulates in conditioned medium of senescent human fibroblasts, suggesting that it may contribute to the senescent phenotype. IGFBP‐3 can enhance apoptotic cell death in tumor cells due to its ability to target intracellular regulators of apoptosis, including nuclear transcription factors. Senescent fibroblasts are highly resistant to apoptosis, suggesting that IGFBP‐3 fails to induce apoptosis in this cell type; however, mechanisms of apoptosis resistance in senescent cells are poorly understood. To address this question, we studied the production and intracellular localization of IGFBP‐3 in senescent fibroblasts. Whereas IGFBP‐3 is highly overexpressed by senescent fibroblasts, IGFBP‐3 was not detectable in the nucleus of senescent fibroblasts. In tumor cells, IGFBP‐3 can be internalized by endocytosis, which is considered as a prerequisite for the intracellular functions of IGFBP‐3 and probably also for its transport to the nucleus; we show here that endocytotic uptake of IGFBP‐3 does not occur in senescent human fibroblasts. This is correlated with a generally decreased endocytotic activity of these cells, as shown with the model substrate transferrin. The data are consistent with a model where IGFBP‐3 accumulation in conditioned medium of senescent fibroblasts contributes to growth arrest of these cells, whereas the failure to endocytose IGFBP‐3 and the absence of nuclear IGFBP‐3 may contribute to the well‐established apoptosis resistance of senescent human fibroblasts.

The late endosomal p14–MP1 (LAMTOR2/3) complex regulates focal adhesion dynamics during cell migration

Late endosomes locally regulate cell migration by transporting the p14–MP1 scaffold complex to the vicinity of focal adhesions.