Adhesive and Signaling Functions of Cadherins and Catenins in Vertebrate Development

Properly regulated intercellular adhesion is critical for normal development of all metazoan organisms. Adherens junctions play an especially prominent role in development because they link the adhesive function of cadherin–catenin protein complexes to the dynamic forces of the actin cytoskeleton, which helps to orchestrate a spatially confined and very dynamic assembly of intercellular connections. Intriguingly, in addition to maintaining intercellular adhesion, cadherin–catenin proteins are linked to several major developmental signaling pathways crucial for normal morphogenesis. In this article we will highlight the key genetic studies that uncovered the role of cadherin–catenin proteins in vertebrate development and discuss the potential role of these proteins as molecular biosensors of external cellular microenvironment that may spatially confine signaling molecules and polarity cues to orchestrate cellular behavior throughout the complex process of normal morphogenesis.Development of any multicellular organism is impossible without a dynamic and properly regulated intercellular adhesion. Adhesive contacts between cells provide a physical anchoring system that is necessary to form highly organized tissues, and these contacts are essential for effective intercellular communication that ensures the homeostasis and survival of the entire organism. A number of unique developmental processes, including such early events as embryonic compaction and first cell fate specification, as well as later tissue morphogenesis and organogenesis, rely on a dynamic balance between cellular adhesion and migration. Cadherin–catenin protein complexes, which constitute the core of a specialized subtype of cellular adhesion structures termed adherens junctions (AJs), play a particularly important role during these processes. Apart from maintaining adhesive contacts at the cell–cell junctions, they are actively involved in epithelial-to-mesenchymal and mesenchymal-to-epithelial transitions, which are crucial to sustain the tissue plasticity during development. Most importantly, the components of cadherin–catenin complexes are tightly linked to several major signaling networks controlling cell division, differentiation, and apoptosis and this feature is crucial for the broad roles of the AJs throughout the vertebrate development (see Cavey and Lecuit 2009).This article will focus on the role of cadherin–catenin proteins in regulating the signaling events critical for vertebrate development. Altering the expression pattern of particular cadherin–catenin complex components in the developing embryo often leads to major developmental defects, which reflect their role in both signaling and mechanical adhesion. In this article, we will highlight crucial findings suggesting that cadherin–catenin complexes provide not only the structural integrity of the tissue, but may also serve as biosensors of the external cellular microenvironment that modulate cellular behavior and make individual cells work together to ensure the fitness of the entire organism.     

Forest response to chronic hurricane disturbance in coastal New England

Question: Hurricanes and cyclones cause a wide range of damage to coastal forests worldwide. Most of these storms are not catastrophic in ecological terms, but forest responses to storms of moderate intensities are poorly understood. In regions with a high frequency of moderate hurricanes, how does variation in disturbance intensity affect the magnitude of ecological responses? Location: Naushon Island, Massachusetts, USA. Methods: We use historical records and dendroecological methods to characterize establishment and growth of Fagus grandifolia, Quercus alba, and Quercus velutina in response to seven non‐catastrophic hurricanes of varying intensity, and a major logging event, relative to baseline conditions, over the past 150 years. Our aim was to document variation in the magnitude of responses to known disturbance events of varying intensity, and to determine whether tree growth after moderate hurricanes differs from growth during periods of no disturbance. Results: Forest harvesting in 1824‐1827 had a strong impact on forest composition and growth. Since then, the study region has been characterized by little harvesting but frequent hurricanes. However, only one of the seven storms examined caused substantial increases in growth and new establishment for the dominant species; most moderate disturbances had minimal impact on growth and regeneration dynamics. We also document highly variable responses among species to individual storms, including substantial growth decreases that may not be detected by standard analytical approaches. Conclusions: Our results caution against the use of simple metrics such as wind speed to predict forest response to specific hurricanes, and highlight the importance of individual disturbance events in controlling long‐term forest dynamics, even in regions characterized by high disturbance frequency. In addition, we show that standard approaches to reconstructing disturbance history based on increases in radial growth and pulses of tree establishment are likely to underestimate the frequency of moderate disturbances.     

Abnormal connexin43 in arrhythmogenic right ventricular cardiomyopathy caused by plakophilin-2 mutations

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a disorder of cardiomyocyte intercalated disk proteins causing sudden death. Heterozygous mutations of the desmosomal protein plakophilin-2 (PKP-2) are the commonest genetic cause of ARVC. Abnormal gap junction connexin43 expression has been reported in autosomal dominant forms of ARVC (Naxos and Carvajal disease) caused by homozygous mutations of desmosomal plakoglobin and desmoplakin. In tissue culture, suppression of PKP-2 results in decreased expression of connexin43. We sought to characterize the expression and localization of connexin43 in patients with ARVC secondary to heterozygous PKP-2 mutations. Complete PKP-2 gene sequencing of 27 ARVC patients was utilized to identify mutant genotypes. Endomyocardial biopsies of identified carriers were then assessed by immunofluorescence to visualize intercalated disk proteins. N-cadherin was targeted to highlight intercalated disks, followed by counterstaining for PKP-2 or connexin43 using confocal double immunofluorescence microscopy. Immunofluorescence was quantified using an AdobeA Photoshop protocol, and colocalization coefficients were determined. PKP-2 siRNA experiments were performed in mouse cardiomyocyte (HL1) cell culture with Western blot analysis to assess connexin43 expression following PKP-2 suppression. Missense and frameshift mutations of the PKP-2 gene were found in four patients with biopsy material available for analysis. Immunofluorescent studies showed PKP-2 localization to the intercalated disk despite mutations, but associated with decreased connexin43 expression and abnormal colocalization. PKP-2 siRNA in HL1 culture confirmed decreased connexin43 expression. Reduced connexin43 expression and localization to the intercalated disk occurs in heterozygous human PKP-2 mutations, potentially explaining the delayed conduction and propensity to develop arrhythmias seen in this disease.     

Multiple paternity and kinship in the gray fox (Urocyon cinereoargenteus)

Despite mounting evidence that extra-pair copulations (EPCs) are common in the Canidae, no studies have examined the most basal member of this family, the gray fox (Urocyon cinereoargenteus). In this study we explored the possibility that gray foxes may be socially, but not genetically, monogamous. Multiple paternity was confirmed in one litter and suspected in three others (n=7), thus 14.3–57.1% of all litters had more than one father. In this high-density population, multiple paternity may be one strategy to reduce inbreeding, although only one pair was significantly related to each other (r=0.36). Mother–daughter pairs were more common than father–son pairs. These results coupled with a previous study showing a female-biased sex ratio suggest male-biased dispersal and the potential for helper females.     

Characterization of microsatellite loci from the Malagasy endemic,Tina striata Radlk. (Sapindaceae)

We isolated and characterized 12 polymorphic microsatellite loci in the Malagasy endemic, Tina striata Radlk. (Sapindaceae). The number of alleles per locus ranged from 2 to 6 (N = 28 individuals). Polymorphic information content ranged from 0.132 to 0.767 and observed heterozygosity from 0.154 to 0.800. These markers will be valuable in estimating outcrossing rates and genetic variation in populations of T. striata, and aid in the conservation of populations of T. striata and other closely related species in the Malagasy Sapindaceae.     

Development and characterization of twelve polymorphic microsatellite loci in the threatened Red Hills salamander, Phaeognathus hubrichti

We isolated and characterized 12 microsatellite loci from the endangered Red Hills salamander, Phaeognathus hubrichti. Loci were screened in 24 individuals of P. hubrichti. The number of alleles per locus ranged from 2 to 16, observed heterozygosity ranged from 0.130 to 0.750, and the probability of identity values ranged from 0.012 to 0.778. These new loci provide tools for examining the population genetics of this federally threatened salamander.     

Norbornyllactone-substituted xanthines as adenosine A(1) receptor antagonists

During the search for second-generation adenosine A(1) receptor antagonist alternatives to the clinical candidate 8-(3-oxa-tricyclo[3.2.1.0(2,4)]oct-6-yl)-1,3-dipropyl-3,7-dihydro-purine-2,6-dione (BG9719), we developed a series of novel xanthines substituted with norbornyl-lactones that possessed high binding affinities for adenosine A(1) receptors and in vivo activity.     

Morphological and genetic predictors of parental care in the North American barn swallow Hirundo rustica erythrogaster

Sexually dimorphic traits often signal the fitness benefits an individual can provide to potential mates. In species with altricial young, these signals may also predict the level of parental care an individual is expected to provide to shared offspring. In this study, we tested three hypotheses that traditionally relate sexually dimorphic traits to parental care in two populations of North American barn swallows Hirundo rustica erythrogaster. The good parent hypothesis predicts a positive relationship between an individual's ornamentation and his or her care whereas the differential allocation (more care given by individuals when paired to high quality mates) and reproductive compensation (more care given by individuals when paired to low quality mates) hypotheses predict that an individual's level of parental investment is relative to the quality of their mate. Male and female North American barn swallows have colorful ventral feathers and elongated tail streamers, but there is evidence that ventral color, not tail streamer length, predicts measures of seasonal reproductive success. Accounting for the positive correlation between within‐pair feeding rates and other potentially confounding variables in all of our models, we found no support for the good parent hypothesis because in both males and females, traits shown to be under sexual selection did not predict feeding rates in either sex. However, our data reveal that male coloration, and not streamer length, predicted a female's provisioning rate to shared offspring (females fed more when paired with darker individuals) in two separate populations, supporting the differential allocation, but not the reproductive compensation hypothesis. Because genetic traits have also been shown to affect parental investment, we evaluated this variable as well and found that a male's paternity did not have significant effects on either male or female feeding rates. Overall, our results suggest that females do not pair with darker males in order to gain direct benefits in terms of his expected levels of parental care to shared offspring, but do themselves invest greater levels of care when paired to darker males. Further, our results are consistent with previous studies which suggest that ventral feather color, not streamer length, is a target of sexual selection in North American populations of barn swallow because females invested more in their offspring when paired to darker mates.