Delivery of endosomes to lysosomes via microautophagy in the visceral endoderm of mouse embryos

Abstract

The differentiation and patterning of murine early embryos are sustained by the visceral endoderm, an epithelial layer of polarised cells that has critical roles in multiple signalling pathways and nutrient uptake. Both nutritional and signalling functions rely upon the endocytosis of various molecules from the cell surface via the endocytic pathway. However, endocytic membrane dynamics in this embryonic tissue remain poorly understood. Here we show that the functions of rab7, a small GTP-binding protein regulating the late endocytic pathway, are essential for embryonic patterning during gastrulation. The endosomes of visceral endoderm cells are delivered via a unique microautophagy-like process to the apical vacuole, a large compartment exhibiting lysosomal characteristics. Loss of rab7 function results in severe inhibition of this endocytic pathway. our results indicate that the microautophagic process and flow of the endocytic membrane have essential roles in early embryonic development.

Critical roles of type III phosphatidylinositol phosphate kinase in murine embryonic visceral endoderm and adult intestine

“…S8), the different effect of PIPKIII deficiency could be due to different natures of the two types of cells: Endocytic organelles develop less extensively in the epiblast (16). In WT embryos, VE cells develop an extensive vesicular system that enables them to provide maternal nutrients to the epiblasts (17,18). In VE cells, the vesicles containing extracellular materials are conveyed by a retrograde transport mechanism to the apical vacuoles, which are LAMP1/2-positive endosomes positioned on the apical side of the cell (19,20).…”

CORVET and HOPS tethering complexes–coordinators of endosome and lysosome fusion

“…Mutations in Vps33B and Vps16B, which could be part of the metazoan CORVET, are further linked to diseases such as arthrogryposisrenal-dysfunction-cholestasis (ARC) syndrome, an autosomal recessive disorder, and cancer (Gissen et al, 2004;Roy et al, 2011). Several of the observed defects are likely to be associated with defective signaling through the endosome, which impairs morphogen gradients, receptor degradation and subsequently affects embryonic development or causes strong developmental defects within the entire organism (Charng et al, 1998;Wurthner et al, 2001;Felici et al, 2001;Aoyama et al, 2012;Kawamura et al, 2012;Messler et al, 2011;Wilkin et al, 2008). Data from localizing hVps39-1 and hVps41 as well as other class C proteins in mammalian cells are consistent with the observations in yeast that HOPS is involved in endosome-lysosome fusion (Caplan et al, 2001;Pols et al, 2012).…”

“…Defects in HOPS or CORVET subunits in mammalian tissues result in strong deficiencies. For instance, loss of hVps39-1 (TLP), the homologous hVps39-2 (TRAP-1), hVps41 (hVam2) or hRab7 results in embryonic lethality as early as gastrulation Kawamura et al, 2012;Messler et al, 2011) and also causes extensive developmental defects in zebrafish (Schonthaler et al, 2008). Likewise, mutants in HOPS impair infection by Ebola virus (Carette et al, 2011) and export of HIV virions (Tomita et al, 2011), affect endosomal, phagosomal and lysosomal biogenesis (Caplan et al, 2001;Poupon et al, 2003;Pols et al, 2012;Sriram et al, 2003;Swetha et al, 2011;Kinchen et al, 2008) and, subsequently, development (Wilkin et al, 2008).…”

Membrane dynamics in mammalian embryogenesis: Implication in signal regulation

“…Each subcellular compartment is specialized by distinct rab GTPases (Hutagalung and Novick, 2011), the tethering complex, and SNARE proteins (Schmitt and Jahn, 2009). For example, the early endosome compartments are enriched in rab5, the CORVET tethering complex, and syntaxin-12 for SNAREs (Gorvel et al, 1991;Tang et al, 1998;Peplowska et al, 2007), whereas the late endocytic compartments are site of the rab7, HOPS complex, and syntaxins 7 and 8 (Mullock et al, 2000;Nakamura et al, 2000;Kawamura et al, 2012;Pols et al, 2013). Meanwhile, proteins can be involved in multiple distinctive steps.…”

“…Therefore, defects in late endosome maturation may cause developmental phenotypes, as seen in mutation of PIPK(III) (Takasuga et al, 2013) or the rab7 small GTP binding protein (Kawamura et al, 2012). mVam2 is a subunit of the tethering complex functioning in the late endosomes (Brocker et al, 2012).…”