A truncated version of the only insulin receptor in C. elegans has been discovered.
It would be hard to overstate the importance of a receptor called DAF-2 to our understanding of aging and longevity. Almost 30 years ago it was discovered that loss of the daf-2 gene doubles lifespan in the worm C. elegans (Kenyon et al., 1993), and a few years later it was reported that DAF-2 is the only insulin/IGF-1-like receptor in C. elegans (Kimura et al., 1997). These findings led to an explosion of research into aging and longevity, revealing an intricate insulin signaling pathway that coordinates the sensing of nutrient levels with the regulation of age-related decline. In particular, it was found that reduced insulin signaling extends lifespan and increases stress resistance in flies and mice (Clancy et al., 2001; Holzenberger et al., 2003). Moreover, mutations in some of the genes associated with this pathway were found in centenarians (Suh et al., 2008). And in worms it became clear that, in addition to longevity and age-related declines, DAF-2 is involved in the regulation of a wide range of biological processes, including development, reproduction, memory, and stress responses.
DAF-2 was originally discovered for its role in controlling the dauer stage – an alternative stage of development in which a larva goes into a type of stasis to help it survive harsh conditions (Riddle et al., 1981). A lack of DAF-2 causes C. elegans to enter dauer, as does a lack of a number of other kinases (Paradis and Ruvkun, 1998). An ongoing mystery is why C. elegans has just a single gene for an insulin receptor despite having 40 different insulin-like peptides (Pierce et al., 2001). Some of these peptides are agonists (that is, they activate the receptor) and others are antagonists (they inhibit the receptor).
Given three decades of extensive research into the insulin signaling pathway in C. elegans, it would be shocking to find new functions for DAF-2 at this point. However, in a new paper in eLife, Matthew Gill of the Scripps Research Institute and colleagues – including Bryan Martinez and Pedro Reis Rodrigues as joint first authors – report evidence for such a shock: the gene for DAF-2 can also express another, truncated isoform of this protein as a result of alternative splicing (Martinez et al., 2020). The truncated version, which is called DAF-2B, can still form dimers but, unlike the full-length version, it is not expected to be able to span the membrane: this suggests that the truncated form could be secreted.
Truncated insulin receptors that have extracellular ligand-binding domains, but lack intracellular signaling domains, have been reported in both Drosophila and mammals, and are known to sequester insulin peptides. However, in these cases the full-length receptors and the truncated receptors are expressed from separate genes. Martinez et al. found that although DAF-2B was expressed in neuronal cells, it accumulated in cells called coelomocytes (macrophage-like cells that attack invading organisms such as bacteria and viruses). These results suggest that DAF-2B can indeed be secreted, rather than being retained in the neurons in which it is expressed and spliced.
But what does this shortened form of DAF-2 do? The best-characterized functions of the insulin signaling pathway are dauer formation and lifespan regulation, so Martinez et al. used these phenotypes to study DAF-2B. They found that overexpressing DAF-2B increased dauer formation, slowed dauer exit, and increased lifespan, whereas a lack of DAF-2B had the opposite effect. Basically, the data suggest that the function of DAF-2B is essentially the opposite of the function of DAF-2.
Martinez et al. also explored the interactions between DAF-2B and insulin-like peptides that were either agonists or antagonists. Overexpression of two peptides that are agonists (DAF-28 and INS-6) reduced the dauer-promoting effects of DAF-2B. Conversely, the overexpression of a peptide that is an antagonist (INS-18) would be expected to promote dauer, but this effect was blunted when DAF-2B was also overexpressed. Additionally, the researchers found that a point mutation in the proposed insulin-binding domain resulted in a form of DAF-2B that exhibited reduced dauer formation. Together, these results suggest that DAF-2B binds and may sequester insulin-like peptides, and/or form dimers with DAF-2.
Of course, mysteries remain. Given that worms have dozens of insulin-like peptides (Pierce et al., 2001), which of these bind to DAF-2B, and under what circumstances? And if DAF-2B is secreted, why does it matter where it is expressed, unless there are highly localized interactions? Finally, the mechanism by which DAF-2B acts and its dimerization state is not entirely understood.
The discovery of the truncated version of DAF-2, and the fact that it essentially works in opposition to the full-length version, raises new questions and will change how we think about DAF-2's role in insulin signaling regulation of aging and longevity.
During formation of the mammalian placenta, trophoblasts invade the maternal decidua and remodel spiral arteries to bring maternal blood into the placenta. This process, known as endovascular invasion, is thought to involve the adoption of functional characteristics of vascular endothelial cells (ECs) by trophoblasts. The genetic and molecular basis of endovascular invasion remains poorly defined, however, and whether trophoblasts utilize specialized endothelial proteins in an analogous manner to create vascular channels remains untested. Vascular endothelial (VE-)cadherin is a homotypic adhesion protein that is expressed selectively by ECs in which it enables formation of tight vessels and regulation of EC junctions. VE-cadherin is also expressed in invasive trophoblasts and is a prime candidate for a molecular mechanism of endovascular invasion by those cells. Here, we show that VE-cadherin is required for trophoblast migration and endovascular invasion into the maternal decidua in the mouse. VE-cadherin deficiency results in loss of spiral artery remodeling that leads to decreased flow of maternal blood into the placenta, fetal growth restriction, and death. These studies identify a non-endothelial role for VE-cadherin in trophoblasts during placental development and suggest that endothelial proteins may play functionally unique roles in trophoblasts that do not simply mimic those in ECs.
To identify roles of RNA binding proteins (RBPs) in the differentiation or survival of antibody secreting plasma cells we performed a CRISPR/Cas9 knockout screen of 1213 mouse RBPs for their ability to affect proliferation and/or survival, and the abundance of differentiated CD138 + cells in vitro. We validated the binding partners CSDE1 and STRAP as well as the m6A binding protein YTHDF2 as promoting the accumulation of CD138 + cells in vitro. We validated the EIF3 subunits EIF3K and EIF3L and components of the CCR4-NOT complex as inhibitors of CD138 + cell accumulation in vitro. In chimeric mouse models YTHDF2-deficient plasma cells failed to accumulate.