A genome-wide genetic screen uncovers novel determinants of human pigmentation

The skin color is one of the most diverse human traits and is determined by the quantity, type and distribution of melanin. Here, we leverage light scattering properties of melanin to conduct a genome-wide CRISPR-Cas9 screen for novel regulators of melanogenesis. We identify functionally diverse genes converging on melanosome biogenesis, endosomal transport and transcriptional/posttranscriptional gene regulation, most of which represent novel associations with pigmentation. A survey of transcriptomes from diversely pigmented individuals reveals that the majority of genes discovered in our screen are upregulated in dark skin melanocytes, in agreement with their melanin-promoting function and potential contribution to skin color variation. This association is further buttressed by the significant skin color heritability enrichment in the vicinity of these genes. Taken together, our study presents a novel approach to assay pigmentation and uncovers a plethora of melanogenesis regulators, with broad implications for human variation, cell biology and medicine.

CROP: A Retromer-PROPPIN complex mediating membrane fission in the endo-lysosomal system

Endo-lysosomal compartments exchange proteins by fusing, fissioning, and through endosomal transport carriers. Thereby, they sort many plasma membrane receptors and transporters and control cellular signaling and metabolism. How the membrane fission events are catalyzed is poorly understood. Here, we identify the novel CROP complex as a factor acting at this step. CROP joins members of two protein families: the peripheral subunits of retromer, a coat forming endosomal transport carriers, and membrane inserting PROPPINs. Integration into CROP potentiates the membrane fission activity of the PROPPIN Atg18 on synthetic liposomes and confers strong preference for binding PI(3,5)P2, a phosphoinositide required for membrane fission activity. Disrupting CROP blocks fragmentation of lysosome-like yeast vacuoles in vivo. CROP-deficient mammalian endosomes accumulate micrometer-long tubules and fail to export cargo, suggesting that carriers attempt to form but cannot separate from these organelles. PROPPINs compete for retromer binding with the SNX proteins, which recruit retromer to the membrane during the formation of endosomal carriers. Transition from retromer-SNX complexes to retromer-PROPPIN complexes might hence switch retromer activities from cargo capture to membrane fission.

Donnai–Barrow syndrome in nephrology practice

Donnai–Barrow syndrome is a multi-system disorder characterized by a variable combination of congenital anomalies, progressive myopia, sensorineural hearing loss, intellectual disability and renal disease. The article describes clinical cases of children with different phenotypes of the syndrome, including different renal disorders. One patient had isolated low-molecular-weight proteinuria, another patient suffered from proteinuria, hypercalciuria, nephrocalcinosis. Disruption of megaline-mediated endocytosis, retrograde endosomal transport of ligands, mitochondrial dysfunction, stress of the endoplasmic reticulum can lead to a different spectrum and various degrees of severity of tubular dysfunction in Donnai-Barrow syndrome. A variety of clinical manifestations of the disease can lead to a low diagnosis of Donnai-Barrow syndrome and inadequate patient management.

The kinesin motor Klp98A mediates apical to basal Wg transport

ABSTRACTDevelopment and tissue homeostasis rely on the tight regulation of morphogen secretion. In the Drosophila wing imaginal disc epithelium, Wg secretion for long-range signal transduction occurs after apical Wg entry into the endosomal system, followed by secretory endosomal transport. Although Wg release appears to occur from the apical and basal cell sides, its exact post-endocytic fate and the functional relevance of polarized endosomal Wg trafficking are poorly understood. Here, we identify the kinesin-3 family member Klp98A as the master regulator of intracellular Wg transport after apical endocytosis. In the absence of Klp98A, functional mature endosomes accumulate in the apical cytosol, and endosome transport to the basal cytosol is perturbed. Despite the resulting Wg mislocalization, Wg signal transduction occurs normally. We conclude that transcytosis-independent routes for Wg trafficking exist and demonstrate that Wg can be recycled apically via Rab4-recycling endosomes in the absence of Klp98A.

Protrudin-mediated ER–endosome contact sites promote MT1-MMP exocytosis and cell invasion

Cancer cells break tissue barriers by use of small actin-rich membrane protrusions called invadopodia. Complete invadopodia maturation depends on protrusion outgrowth and the targeted delivery of the matrix metalloproteinase MT1-MMP via endosomal transport by mechanisms that are not known. Here, we show that the ER protein Protrudin orchestrates invadopodia maturation and function. Protrudin formed contact sites with MT1-MMP–positive endosomes that contained the RAB7-binding Kinesin-1 adaptor FYCO1, and depletion of RAB7, FYCO1, or Protrudin inhibited MT1-MMP–dependent extracellular matrix degradation and cancer cell invasion by preventing anterograde translocation and exocytosis of MT1-MMP. Moreover, when endosome translocation or exocytosis was inhibited by depletion of Protrudin or Synaptotagmin VII, respectively, invadopodia were unable to expand and elongate. Conversely, when Protrudin was overexpressed, noncancerous cells developed prominent invadopodia-like protrusions and showed increased matrix degradation and invasion. Thus, Protrudin-mediated ER–endosome contact sites promote cell invasion by facilitating translocation of MT1-MMP–laden endosomes to the plasma membrane, enabling both invadopodia outgrowth and MT1-MMP exocytosis.

“TMEM16K is an interorganelle regulator of endosomal sorting”

Communication between organelles is essential for their cellular homeostasis. Neurodegeneration reflects the declining ability of neurons to maintain cellular homeostasis over a lifetime1. The endolysosomal pathway plays a prominent role in this process by regulating protein and lipid sorting and degradation2. Here, we report that TMEM16K, an endoplasmic reticulum lipid scramblase3 causative for spinocerebellar ataxia (SCAR10), is an interorganelle regulator of the endolysosomal pathway. We identify endosomal transport as a major functional cluster of TMEM16K in proximity biotinylation proteomics analyses. TMEM16K forms contact sites with endosomes, reconstituting split-GFP with small GTPase RAB7. Our study further implicates TMEM16K lipid scrambling activity in endosomal sorting at these sites. Loss of TMEM16K function led to impaired endosomal retrograde transport and neuromuscular function, one of the symptoms of SCAR10. Thus, TMEM16K-containing ER-endosome contact sites represent clinically relevant platforms for regulating endosomal sorting.