Distinct requirements for Pot1 in limiting telomere length and maintaining chromosome stability

The fission yeast Pot1 (protection of telomeres) protein binds to the single-stranded extensions at the ends of telomeres, where its presence is critical for the maintenance of linear chromosomes. Homologs of Pot1 have been identified in a wide variety of eukaryotes, including plants, animals, and humans. We now show that Pot1 plays dual roles in telomere length regulation and chromosome end protection. Using a series of Pot1 truncation mutants, we have defined distinct areas of the protein required for chromosome stability and for limiting access to telomere ends by telomerase. We provide evidence that a large portion of Pot1, including the N-terminal DNA binding domain and amino acids close to the C terminus, is essential for its protective function. C-terminal Pot1 fragments were found to exert a dominant-negative effect by displacing endogenous Pot1 from telomeres. Reducing telomere-bound Pot1 in this manner resulted in dramatic lengthening of the telomere tract. Upon further reduction of Pot1 at telomeres, the opposite phenotype was observed: loss of telomeric DNA and chromosome end fusions. Our results demonstrate that cells must carefully regulate the amount of telomere-bound Pot1 to differentiate between allowing access to telomerase and catastrophic loss of telomeres.     

Overexpression of Spry1 in chondrocytes causes attenuated FGFR ubiquitination and sustained ERK activation resulting in chondrodysplasia

The FGF signaling pathway plays essential roles in endochondral ossification by regulating osteoblast proliferation and differentiation, chondrocyte proliferation, hypertrophy, and apoptosis. FGF signaling is controlled by the complementary action of both positive and negative regulators of the signal transduction pathway. The Spry proteins are crucial regulators of receptor tyrosine kinase-mediated MAPK signaling activity. Sprys are expressed in close proximity to FGF signaling centers and regulate FGFR-ERK-mediated organogenesis. During endochondral ossification, Spry genes are expressed in prehypertrophic and hypertrophic chondrocytes. Using a conditional transgenic approach in chondrocytes in vivo, the forced expression of Spry1 resulted in neonatal lethality with accompanying skeletal abnormalities resembling thanatophoric dysplasia II, including increased apoptosis and decreased chondrocyte proliferation in the presumptive reserve and proliferating zones. In vitro chondrocyte cultures recapitulated the inhibitory effect of Spry1 on chondrocyte proliferation. In addition, overexpression of Spry1 resulted in sustained ERK activation and increased expression of p21 and STAT1. Immunoprecipitation experiments revealed that Spry1 expression in chondrocyte cultures resulted in decreased FGFR2 ubiquitination and increased FGFR2 stability. These results suggest that constitutive expression of Spry1 in chondrocytes results in attenuated FGFR2 degradation, sustained ERK activation, and up-regulation of p21Cip and STAT1 causing dysregulated chondrocyte proliferation and terminal differentiation.     

Distribution and Numbers of Pygmies in Central African Forests

Pygmy populations occupy a vast territory extending west-to-east along the central African belt from the Congo Basin to Lake Victoria. However, their numbers and actual distribution is not known precisely. Here, we undertake this task by using locational data and population sizes for an unprecedented number of known Pygmy camps and settlements (n = 654) in five of the nine countries where currently distributed. With these data we develop spatial distribution models based on the favourability function, which distinguish areas with favourable environmental conditions from those less suitable for Pygmy presence. Highly favourable areas were significantly explained by presence of tropical forests, and by lower human pressure variables. For documented Pygmy settlements, we use the relationship between observed population sizes and predicted favourability values to estimate the total Pygmy population throughout Central Africa. We estimate that around 920,000 Pygmies (over 60% in DRC) is possible within favourable forest areas in Central Africa. We argue that fragmentation of the existing Pygmy populations, alongside pressure from extractive industries and sometimes conflict with conservation areas, endanger their future. There is an urgent need to inform policies that can mitigate against future external threats to these indigenous peoples’ culture and lifestyles.     

A Global Analysis of the Relationship between Farmed Seaweed Production and Herbivorous Fish Catch

Globally, farmed seaweed production is expanding rapidly in shallow marine habitats. While seaweed farming provides vital income to millions of artisanal farmers, it can negatively impact shallow coral reef and seagrass habitats. However, seaweed farming may also potentially provide food subsidies for herbivorous reef fish such as the Siganidae, a valuable target family, resulting in increased catch. Comparisons of reef fish landings across the central Philippines revealed that the catch of siganids was positively correlated to farmed seaweed production whilst negatively correlated to total reef fish catch over the same period of time. We tested the generality of this pattern by analysing seaweed production, siganid catch, and reef fish catch for six major seaweed-producing countries in the tropics. We hypothesized that increased seaweed production would correspond with increased catch of siganids but not other reef fish species. Analysis of the global data showed a positive correlation between farmed seaweeds and siganids in Southeast Asia (Indonesia, Malaysia, and the Philippines) but not Africa (Tanzania and Zanzibar), or the Western Pacific (Fiji). In Southeast Asia, siganid catch increased disproportionately faster with seaweed production than did reef fish catch. Low continuity, sporadic production and smaller volumes of seaweed farming may explain the differences.     

Epithelial laminin alpha5 is necessary for distal epithelial cell maturation, VEGF production, and alveolization in the developing murine lung

Laminin alpha5 is prominent in the basement membrane of alveolar walls, airways, and pleura in developing and adult lung. Targeted deletion of laminin alpha5 in mice causes developmental defects in multiple organs, but embryonic lethality has precluded examination of the latter stages of lung development. To identify roles for laminin alpha5 in lung development, we have generated an inducible lung epithelial cell-specific Lama5 null (SP-CLama5(fl/-)) mouse through use of the Cre/loxP system, the human surfactant protein C promoter, and the reverse tetracycline transactivator. SP-CLama5(fl/-) embryos exposed to doxycycline from E6.5 died a few hours after birth. Compared to control littermates, SP-CLama5(fl/-) lungs had dilated, enlarged distal airspaces, but basement membrane ultrastructure was preserved. Distal epithelial cell differentiation was perturbed, with a marked reduction of alveolar type II cells and a virtual absence of type I cells. Cell proliferation was reduced and apoptosis was increased. Capillary density was diminished, and this was associated with a decrease in total lung VEGF production. Overall, these findings indicate that epithelial laminin alpha5, independent of its structural function, is necessary for murine lung development, and suggest a role for laminin alpha5 in signaling pathways that promote alveolar epithelial cell differentiation and VEGF expression.     

Stn1 is critical for telomere maintenance and long‐term viability of somatic human cells

Disruption of telomere maintenance pathways leads to accelerated entry into cellular senescence, a stable proliferative arrest that promotes aging‐associated disorders in some mammals. The budding yeast CST complex, comprising Cdc13, Stn1, and Ctc1, is critical for telomere replication, length regulation, and end protection. Although mammalian homologues of CST have been identified recently, their role and function for telomere maintenance in normal somatic human cells are still incompletely understood. Here, we characterize the function of human Stn1 in cultured human fibroblasts and demonstrate its critical role in telomere replication, length regulation, and function. In the absence of high telomerase activity, shRNA‐mediated knockdown of hStn1 resulted in aberrant and fragile telomeric structures, stochastic telomere attrition, increased telomere erosion rates, telomere dysfunction, and consequently accelerated entry into cellular senescence. Oxidative stress augmented the defects caused by Stn1 knockdown leading to almost immediate cessation of cell proliferation. In contrast, overexpression of hTERT suppressed some of the defects caused by hStn1 knockdown suggesting that telomerase can partially compensate for hStn1 loss. Our findings reveal a critical role for human Stn1 in telomere length maintenance and function, supporting the model that efficient replication of telomeric repeats is critical for long‐term viability of normal somatic mammalian cells.