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Cell polarity, the asymmetric distribution of proteins and organelles, is permanently or transiently established in various cell types and plays an important role in many physiological events. EH domain-binding protein 1 like 1 (EHBP1L1) is an adaptor protein that is localized on recycling endosomes and regulates apical-directed transport in polarized epithelial cells. However, the role of EHBP1L1 in non-epithelial cells remains unknown. In this study, Ehbp1l1-/- mice showed impaired erythroblast enucleation. Further analyses showed that nuclear polarization prior to enucleation was impaired in Ehbp1l1-/- erythroblasts. It was also revealed that EHBP1L1 interactors Rab10, Bin1, and dynamin were involved in erythroblast enucleation. In addition, Ehbp1l1-/- erythrocytes exhibited stomatocytic morphology and dehydration. These defects in erythroid cells culminated in early postnatal anemic lethality in Ehbp1l1-/- mice. Moreover, we found the mislocalization of nuclei and mitochondria in the skeletal muscle cells of Ehbp1l1-/- mice, as observed in patients with centronuclear myopathy with genetic mutations in Bin1 or dynamin 2. Taken together, our findings indicate that the Rab8/10-EHBP1L1-Bin1-dynamin axis plays an important role in multiple cell polarity systems in epithelial and non-epithelial cells.

Kunii et al. show that neuron-specific SNAP23 knockout mice lack a hippocampus and cerebellum. The SNAP23–VAMP8–Syntaxin1B complex mediates the apical localization of N-cadherin, which is essential for the formation of apical junctional complexes and the polarization of radial glial cells., In the developing brain, the polarity of neural progenitor cells, termed radial glial cells (RGCs), is important for neurogenesis. Intercellular adhesions, termed apical junctional complexes (AJCs), at the apical surface between RGCs are necessary for cell polarization. However, the mechanism by which AJCs are established remains unclear. Here, we show that a SNARE complex composed of SNAP23, VAMP8, and Syntaxin1B has crucial roles in AJC formation and RGC polarization. Central nervous system (CNS)–specific ablation of SNAP23 (NcKO) results in mice with severe hypoplasia of the neocortex and no hippocampus or cerebellum. In the developing NcKO brain, RGCs lose their polarity following the disruption of AJCs and exhibit reduced proliferation, increased differentiation, and increased apoptosis. SNAP23 and its partner SNAREs, VAMP8 and Syntaxin1B, are important for the localization of an AJC protein, N-cadherin, to the apical plasma membrane of RGCs. Altogether, SNARE-mediated localization of N-cadherin is essential for AJC formation and RGC polarization during brain development.

The novel protein EHBP1L1 links Rab8 to Bin1and dynamin to regulate apical transport in epithelial cells., The highly conserved Rab guanosine triphosphatase (GTPase) Rab8 plays a role in exocytosis toward the polarized plasma membrane in eukaryotic cells. In murine Rab8-deficient small intestine cells, apical proteins are missorted into lysosomes. In this study, we identified a novel Rab8-interacting protein complex containing an EH domain–binding protein 1–like 1 (EHBP1L1), Bin1/amphiphysin II, and dynamin. Biochemical analyses showed that EHBP1L1 directly bound to GTP-loaded Rab8 and Bin1. The spatial dependency of these complexes at the endocytic recycling compartment (ERC) was demonstrated through overexpression and knockdown experiments. EHBP1L1- or Bin1-depleted or dynamin-inhibited small intestine organoids significantly accumulated apical membrane proteins but not basolateral membrane proteins in lysosomes. Furthermore, in EHBP1L1-deficient mice, small intestine cells displayed truncated and sparse microvilli, suggesting that EHBP1L1 maintains the apical plasma membrane by regulating apical transport. In summary, our data demonstrate that EHBP1L1 links Rab8 and the Bin1–dynamin complex, which generates membrane curvature and excises the vesicle at the ERC for apical transport.

Kunii et al. reveal that the SNARE protein SNAP23 plays distinct roles in the secretion of amylase in exocrine cells and of insulin in endocrine cells the pancreas and show that MF286, a novel inhibitor of SNAP23, may be a new drug candidate for diabetes., The membrane fusion of secretory granules with plasma membranes is crucial for the exocytosis of hormones and enzymes. Secretion disorders can cause various diseases such as diabetes or pancreatitis. Synaptosomal-associated protein 23 (SNAP23), a soluble N-ethyl-maleimide sensitive fusion protein attachment protein receptor (SNARE) molecule, is essential for secretory granule fusion in several cell lines. However, the in vivo functions of SNAP23 in endocrine and exocrine tissues remain unclear. In this study, we show opposing roles for SNAP23 in secretion in pancreatic exocrine and endocrine cells. The loss of SNAP23 in the exocrine and endocrine pancreas resulted in decreased and increased fusion of granules to the plasma membrane after stimulation, respectively. Furthermore, we identified a low molecular weight compound, MF286, that binds specifically to SNAP23 and promotes insulin secretion in mice. Our results demonstrate opposing roles for SNAP23 in the secretion mechanisms of the endocrine and exocrine pancreas and reveal that the SNAP23-binding compound MF286 may be a promising drug for diabetes treatment.

Mammalian protein kinase D (PKD) isoforms have been proposed to regulate diverse biological processes, including the establishment and maintenance of neuronal polarity. To investigate the function of PKD in neuronal polarization in vivo, we generated PKD knockout (KO) mice. Here, we show that the brain, particularly the hippocampus, of both PKD1 KO and PKD2 KO mice was similar to that of control animals. Neurite length in cultured PKD1 KO and PKD2 KO hippocampal neurons was similar to that of wild-type neurons. However, hippocampal neurons deficient in both PKD1 and PKD2 genes showed a reduction in axonal elongation and an increase in the percentage of neurons with multiple axons relative to control neurons. These results reveal that whereas PKD1 and PKD2 are essential for neuronal polarity, there exists a functional redundancy between the two proteins.

Protein Kinase D (PKD) 1, 2, and 3 are members of the PKD family. PKDs influence many cellular processes, including cell polarity, structure of the Golgi, polarized transport from the Golgi to the basolateral plasma membrane, and actin polymerization. However, the role of the PKD family in cell polarity has not yet been elucidated in vivo. Here, we show that KO mice displayed similar localization of the apical and basolateral proteins, transport of VSV-G and a GPI-anchored protein, and similar localization of actin filaments. As DKO mice were embryonic lethal, we generated MEFs that lacked all PKD isoforms from the PKD1 and PKD2 double floxed mice using Cre recombinase and PKD3 siRNA. We observed a similar localization of various organelles, a similar time course in the transport of VSV-G and a GPI-anchored protein, and a similar distribution of F-actin in the PKD-null MEFs. Collectively, our results demonstrate that the complete deletion of PKDs does not affect the transport of VSV-G or a GPI-anchored protein, and the distribution of F-actin. However, simultaneous deletion of PKD1 and PKD2 affect embryonic development, demonstrating their functional redundancy during development.

The molecular mechanisms of neuronal morphology and synaptic vesicle transport have been largely elusive, and only a few of the molecules involved in these processes have been identified. Here, we developed a novel morphology-based gene trap method, which is theoretically applicable to all cell lines, to easily and rapidly identify the responsible genes. Using this method, we selected several gene-trapped clones of rat pheochromocytoma PC12 cells, which displayed abnormal morphology and distribution of synaptic vesicle-like microvesicles (SLMVs). We identified several genes responsible for the phenotypes and analyzed three genes in more detail. The first gene was BTB/POZ domain-containing protein 9 (Btbd9), which is associated with restless legs syndrome. The second gene was cytokine receptor-like factor 3 (Crlf3), whose involvement in the nervous system remains unknown. The third gene was single-stranded DNA-binding protein 3 (Ssbp3), a gene known to regulate head morphogenesis. These results suggest that Btbd9, Crlf3, and Ssbp3 regulate neuronal morphology and the biogenesis/transport of synaptic vesicles. Because our novel morphology-based gene trap method is generally applicable, this method is promising for uncovering novel genes involved in the function of interest in any cell lines.—Hashimoto, Y., Muramatsu, K., Kunii, M., Yoshimura, S., Yamada, M., Sato, T., Ishida, Y., Harada, R., Harada, A. Uncovering genes required for neuronal morphology by morphology-based gene trap screening with a revertible retrovirus vector.

The small GTP-binding protein Rab8 is known to play an essential role in intracellular transport and cilia formation. We have previously demonstrated that Rab8a is required for localising apical markers in various organisms. Rab8a has a closely related isoform, Rab8b. To determine whether Rab8b can compensate for Rab8a, we generated Rab8b-knockout mice. Though the Rab8b-knockout mice did not display an overt phenotype, the Rab8a and Rab8b double-knockout mice exhibited mislocalisation of apical markers and died earlier than the Rab8a-knockout mice. The apical markers accumulated in three intracellular patterns in the double-knockout mice. However, the localisations of basolateral/dendritic markers of the double-knockout mice were apparently normal. The morphology and the lengths of various primary/motile cilia and the frequency of ciliated cells appeared to be identical between the control and double-knockout mice. However, an additional knockdown of Rab10 in the double-knockout cells greatly reduced the percentage of ciliated cells. Our results highlight the compensatory effect of Rab8a and Rab8b in apical transport and the complexity of the apical transport process. In addition, neither Rab8a nor Rab8b are required for basolateral/dendritic transport. In the meantime, the simultaneous loss of Rab8a and Rab8b has little effect on ciliogenesis, while the additional loss of Rab10 greatly affects ciliogenesis. (203 words)