Directional selectivity is formed at multiple levels by laterally offset inhibition in the rabbit retina

The excitatory and inhibitory inputs to directionally selective (DS) ganglion cells are themselves directionally selective. Directionality is achieved because excitation is reduced during null-direction movement along a GABAergic pathway. Inhibition is reduced during preferred-direction movement along a pathway that includes cholinergic synapses. Both excitation and inhibition are made directional by laterally offset inhibitory signals similar to the spatial offset of the direct inhibitory input to the DS cell dendrites. Thus, spatially offset lateral inhibition generates directionality at three different levels in the DS circuitry. We also found that for stimuli falling within the dendritic field, cholinergic input is delivered to the OFF but not the ON dendrites. Cholinergic pathways from outside the dendritic field reach both ON and OFF dendrites, but both of these pathways are normally inactivated by GABAergic synapses.     

Global-scale genetic identification of hammerhead sharks: Application to assessment of the international fin trade and law enforcement

The future status of sharks is an issue of widespread conservation concern due to declines in many species in the face of high levels of exploitation to satisfy market demands for products, especially fins. Substantial declines in the large-bodied hammerhead sharks, Sphyrna lewini, S. mokarran and S. zygaena, even in regions where some management occurs, indicate that informed conservation measures are warranted for these circumglobally distributed species. Despite the importance of assessing shark catch and trade on a species-specific basis to detect potential overexploitation of individual species, achieving this goal for hammerheads has proven elusive due to difficulties in identification of their products. Here, we present the development and application of a diagnostic, streamlined, five-primer multiplex polymerase chain reaction assay utilizing species-specific primers based on nuclear ribosomal ITS2 for the three hammerhead species throughout their global distribution. Application of this assay to investigations of the fin market confirmed the presence of hammerhead fins in the international trade. A study of the world’s largest fin market in Hong Kong revealed a high concordance between specific Chinese-name trade categories and fins from these three species (“Bai Chun” with S. lewini, “Gui Chun” with S. zygaena and “Gu Pian” with S.␣mokarran), and clear species preferences. This concordance information allows the use of market records for monitoring species-specific trends in trade and exploitation rates. The assay is also proving useful for identification of shark body parts in U.S. fisheries law-enforcement activities. Screening of morphologically identified “ S. lewini” from globally distributed areas using this assay with subsequent whole ITS2 sequencing suggests a cryptic species closely related to S. lewini occurs off the SE USA coast.     

Shigella dysenteriae ShuS promotes utilization of heme as an iron source and protects against heme toxicity

Shigella dysenteriae serotype 1, a major cause of bacillary dysentery in humans, can use heme as a source of iron. Genes for the transport of heme into the bacterial cell have been identified, but little is known about proteins that control the fate of the heme molecule after it has entered the cell. The shuS gene is located within the heme transport locus, downstream of the heme receptor gene shuA. ShuS is a heme binding protein, but its role in heme utilization is poorly understood. In this work, we report the construction of a chromosomal shuS mutant. The shuS mutant was defective in utilizing heme as an iron source. At low heme concentrations, the shuS mutant grew slowly and its growth was stimulated by either increasing the heme concentration or by providing extra copies of the heme receptor shuA on a plasmid. At intermediate heme concentrations, the growth of the shuS mutant was moderately impaired, and at high heme concentrations, shuS was required for growth on heme. The shuS mutant did not show increased sensitivity to hydrogen peroxide, even at high heme concentrations. ShuS was also required for optimal utilization of heme under microaerobic and anaerobic conditions. These data are consistent with the model in which ShuS binds heme in a soluble, nontoxic form and potentially transfers the heme from the transport proteins in the membrane to either heme-containing or heme-degrading proteins. ShuS did not appear to store heme for future use.     

ADAMTS1 proteinase is up-regulated in wounded skin and regulates migration of fibroblasts and endothelial cells

The metalloproteinase ADAMTS1 (a disintegrin and metalloproteinase with thrombospondin motifs) is induced under inflammatory conditions, and it is also a potent inhibitor of angiogenesis. Due to these properties, we speculated about the role of ADAMTS1 in cutaneous wound repair. Here we have shown up-regulation of ADAMTS1 expression in wounds of normal and particularly of healing-impaired genetically diabetic mice. Immunofluorescence staining identified macrophages as the source of ADAMTS1 in early wounds, whereas keratinocytes and fibroblasts produce this protein at later stages of wound healing. The distribution of ADAMTS1 in the normal and wounded epidermis, its regulation in cultured keratinocytes, as well as the skin phenotype of ADAMTS1 knock-out mice suggests a role of this metalloproteinase in keratinocyte differentiation. Furthermore, we provide evidence for a novel dual function of ADAMTS1 in fibroblast migration; although low concentrations of this protein stimulate fibroblast migration via its proteolytic activity, high concentrations inhibit this process because of binding to fibroblast growth factor-2 and subsequent inhibition of its promotogenic activity. Similar effects were also observed with endothelial cells. Taken together, our results suggest a role of ADAMTS1 in keratinocyte differentiation and migration of fibroblasts and endothelial cells in healing skin wounds.     

Functional tissue engineering of chondral and osteochondral constructs

Due to the prevalence of osteoarthritis (OA) and damage to articular cartilage, coupled with the poor intrinsic healing capacity of this avascular connective tissue, there is a great demand for an articular cartilage substitute. As the bearing material of diarthrodial joints, articular cartilage has remarkable functional properties that have been difficult to reproduce in tissue-engineered constructs. We have previously demonstrated that by using a functional tissue engineering approach that incorporates mechanical loading into the long-term culture environment, one can enhance the development of mechanical properties in chondrocyte-seeded agarose constructs. As these gel constructs begin to achieve material properties similar to that of the native tissue, however, new challenges arise, including integration of the construct with the underlying native bone. To address this issue, we have developed a technique for producing gel constructs integrated into an underlying bony substrate. These osteochondral constructs develop cartilage-like extracellular matrix and material properties over time in free swelling culture. In this study, as a preliminary to loading such osteochondral constructs, finite element modeling (FEM) was used to predict the spatial and temporal stress, strain, and fluid flow fields within constructs subjected to dynamic deformational loading. The results of these models suggest that while chondral ("gel alone") constructs see a largely homogenous field of mechanical signals, osteochondral ("gel bone") constructs see a largely inhomogeneous distribution of mechanical signals. Such inhomogeneity in the mechanical environment may aid in the development of inhomogeneity in the engineered osteochondral constructs. Together with experimental observations, we anticipate that such modeling efforts will provide direction for our efforts aimed at the optimization of applied physical forces for the functional tissue engineering of an osteochondral articular cartilage substitute.     

Divergence of function in the thioredoxin fold suprafamily: evidence for evolution of peroxiredoxins from a thioredoxin-like ancestor

The thioredoxin fold is found in proteins that serve a wide variety of functions. Among these are peroxiredoxins, which catalyze the reduction of hydrogen peroxide and alkyl peroxides. Although the common structural fold shared by thioredoxins and peroxiredoxins suggests the possibility that they have evolved from a common progenitor, it has been difficult to examine this hypothesis in depth because pairwise sequence identities between proteins in these two superfamilies are statistically insignificant. Using the Shotgun program, we have found that sequences of reductases involved in maturation of cytochromes in certain bacteria bridge the sequences of thioredoxins and peroxiredoxins. Analysis of motifs found in a divergent set of thioredoxins, cytochrome maturation proteins, and peroxiredoxins provides further support for an evolutionary relationship between these proteins. Within the conserved motifs are specific residues that are characteristic of individual protein classes, and therefore are likely to be involved in the specific functions of those classes. We have used this information, in combination with existing structural and functional information, to gain new insight into the structure-function relationships in these proteins and to construct a model for the emergence of peroxiredoxins from a thioredoxin-like ancestor.     

The DinI protein stabilizes RecA protein filaments

When DinI is present at concentrations that are stoichiometric with those of RecA or somewhat greater, DinI has a substantial stabilizing effect on RecA filaments bound to DNA. Exchange of RecA between free and bound forms was almost entirely suppressed, and highly stable filaments were documented with several different experimental methods. DinI-mediated stabilization did not affect RecA-mediated ATP hydrolysis and LexA co-protease activities. Initiation of DNA strand exchange was affected in a DNA structure-dependent manner, whereas ongoing strand exchange was not affected. Destabilization of RecA filaments occurred as reported in earlier work but only when DinI protein was present at very high concentrations, generally superstoichiometric, relative to the RecA protein concentration. DinI did not facilitate RecA filament formation but stabilized the filaments only after they were formed. The interaction between the RecA protein and DinI was modulated by the C terminus of RecA. We discuss these results in the context of a new hypothesis for the role of DinI in the regulation of recombination and the SOS response.     

Actin and microtubules in neurite initiation: are MAPs the missing link?

During neurite initiation microtubules align to form a tight bundle and actin filaments reorganize to produce a growth cone. The mechanisms that underlie these highly coordinated cytoskeletal rearrangements are not yet fully understood. Recently, various levels of coordination between the actin- and microtubule-based cytoskeletons have been observed during cellular migration and morphogenesis, processes that share some similarities to neurite initiation. Direct, physical association between both cytoskeletons has been suggested, because microtubules often preferentially grow along actin bundles and transiently target actin-rich adhesion complexes. We propose that such physical association might be involved in force-based interactions and spatial organization of the two networks during neurite initiation as well. In addition, many signaling cascades that affect actin filaments are also involved in the regulation of microtubule dynamics, and vice versa. Although several candidates for mediating these effects have been identified in non-neuronal cells, the general mechanism is still poorly understood. In neurons certain plakins and neuron-specific microtubule associated proteins (MAPs), like MAP1B and MAP2, which can bind to both microtubules and F-actin, are promising candidates to play key roles in the specific cytoskeletal rearrangements controlling the transition from an undifferentiated state to neurite-bearing morphology. Here we review the effects of MAPs on microtubules and actin, as well as the coordination of both cytoskeletons during neurite initiation.