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.
Peroxiredoxin 5 (Prdx5) is involved in pathophysiological regulation via the stress-induced cellular response. However, its function in the bone remains largely unknown. Here, we show that Prdx5 is involved in osteoclast and osteoblast differentiation, resulting in osteoporotic phenotypes in Prdx5 knockout (Prdx5Ko) male mice. To investigate the function of Prdx5 in the bone, osteoblasts were analyzed through immunoprecipitation (IP) and liquid chromatography combined with tandem mass spectrometry (LC–MS/MS) methods, while osteoclasts were analyzed through RNA-sequencing. Heterogeneous nuclear ribonucleoprotein K (hnRNPK) was identified as a potential binding partner of Prdx5 during osteoblast differentiation in vitro. Prdx5 acts as a negative regulator of hnRNPK-mediated osteocalcin (Bglap) expression. In addition, transcriptomic analysis revealed that in vitro differentiated osteoclasts from the bone marrow-derived macrophages of Prdx5Ko mice showed enhanced expression of several osteoclast-related genes. These findings indicate that Prdx5 might contribute to the maintenance of bone homeostasis by regulating osteoblast differentiation. This study proposes a new function of Prdx5 in bone remodeling that may be used in developing therapeutic strategies for bone diseases.
Wolfram syndrome 1 (WS1) is a rare genetic disorder caused by mutations in the WFS1 gene leading to a wide spectrum of clinical dysfunctions, among which blindness, diabetes, and neurological deficits are the most prominent. WFS1 encodes for the endoplasmic reticulum (ER) resident transmembrane protein wolframin with multiple functions in ER processes. However, the WFS1-dependent etiopathology in retinal cells is unknown. Herein, we showed that Wfs1 mutant mice developed early retinal electrophysiological impairments followed by marked visual loss. Interestingly, axons and myelin disruption in the optic nerve preceded the degeneration of the retinal ganglion cell bodies in the retina. Transcriptomics at pre-degenerative stage revealed the STAT3-dependent activation of proinflammatory glial markers with reduction of the homeostatic and pro-survival factors glutamine synthetase and BDNF. Furthermore, label-free comparative proteomics identified a significant reduction of the monocarboxylate transport isoform 1 (MCT1) and its partner basigin that are highly enriched on retinal glia and myelin-forming oligodendrocytes in optic nerve together with wolframin. Loss of MCT1 caused a failure in lactate transfer from glial to neuronal cell bodies and axons leading to a chronic hypometabolic state. Thus, this bioenergetic impairment is occurring concurrently both within the axonal regions and cell bodies of the retinal ganglion cells, selectively endangering their survival while impacting less on other retinal cells. This metabolic dysfunction occurs months before the frank RGC degeneration suggesting an extended time-window for intervening with new therapeutic strategies focused on boosting retinal and optic nerve bioenergetics in WS1.