Tissue-specific modifier alleles determine Mertk loss-of-function traits.

Knockout (KO) mouse models play critical roles in elucidating biological processes behind disease-associated or disease-resistant traits. As a presumed consequence of gene KO, mice display certain phenotypes. Based on insight into the molecular role of said gene in a biological process, it is inferred that the particular biological process causally underlies the trait. This approach has been crucial towards understanding the basis of pathological and/or advantageous traits associated with Mertk KO mice. Mertk KO mice suffer from severe, early-onset retinal degeneration. MERTK, expressed in retinal pigment epithelia, is a receptor tyrosine kinase with a critical role in phagocytosis of apoptotic cells or cellular debris. Therefore, early-onset, severe retinal degeneration was described to be a direct consequence of failed MERTK-mediated phagocytosis of photoreceptor outer segments by retinal pigment epithelia. Here, we report that the loss of Mertk alone is not sufficient for retinal degeneration. The widely used Mertk KO mouse carries multiple coincidental changes in its genome that affect the expression of a number of genes, including the Mertk paralog Tyro3. Retinal degeneration manifests only when the function of Tyro3 is concomitantly lost. Furthermore, Mertk KO mice display improved anti-tumor immunity. MERTK is expressed in macrophages. Therefore, enhanced anti-tumor immunity was inferred to result from the failure of macrophages to dispose of cancer cell corpses, resulting in a pro-inflammatory tumor microenvironment. The resistance against two syngeneic mouse tumor models observed in Mertk KO mice is not, however, phenocopied by the loss of Mertk alone. Neither Tyro3 nor macrophage phagocytosis by alternate genetic redundancy accounts for the absence of anti-tumor immunity. Collectively, our results indicate that context-dependent epistasis of independent modifier alleles determines Mertk KO traits.

FIT2 is an acyl-coenzyme A diphosphatase crucial for endoplasmic reticulum homeostasis.

The endoplasmic reticulum is a cellular hub of lipid metabolism, coordinating lipid synthesis with continuous changes in metabolic flux. Maintaining ER lipid homeostasis despite these fluctuations is crucial to cell function and viability. Here, we identify a novel mechanism that is crucial for normal ER lipid metabolism and protects the ER from dysfunction. We identify the molecular function of the evolutionarily conserved ER protein FIT2 as a fatty acyl-coenzyme A (CoA) diphosphatase that hydrolyzes fatty acyl-CoA to yield acyl 4'-phosphopantetheine. This activity of FIT2, which is predicted to be active in the ER lumen, is required in yeast and mammalian cells for maintaining ER structure, protecting against ER stress, and enabling normal lipid storage in lipid droplets. Our findings thus solve the long-standing mystery of the molecular function of FIT2 and highlight the maintenance of optimal fatty acyl-CoA levels as key to ER homeostasis.

Seipin is required for converting nascent to mature lipid droplets.

How proteins control the biogenesis of cellular lipid droplets (LDs) is poorly understood. Using Drosophila and human cells, we show here that seipin, an ER protein implicated in LD biology, mediates a discrete step in LD formation-the conversion of small, nascent LDs to larger, mature LDs. Seipin forms discrete and dynamic foci in the ER that interact with nascent LDs to enable their growth. In the absence of seipin, numerous small, nascent LDs accumulate near the ER and most often fail to grow. Those that do grow prematurely acquire lipid synthesis enzymes and undergo expansion, eventually leading to the giant LDs characteristic of seipin deficiency. Our studies identify a discrete step of LD formation, namely the conversion of nascent LDs to mature LDs, and define a molecular role for seipin in this process, most likely by acting at ER-LD contact sites to enable lipid transfer to nascent LDs.