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Chromaffin cells release transmitters from populations of granules to which synaptotagmin-1 and synaptotagmin-7 are selectively sorted. Rao et al. characterize the functional properties of these granules and show that synaptotagmin-7 confers fast kinetics and high efficacy to the exocytotic event.

Müller glia in the zebrafish retina respond to retinal damage by re-entering the cell cycle, which generates large numbers of retinal progenitors that ultimately replace the lost neurons. In this study we compared the regenerative outcomes of adult zebrafish exposed to one round of phototoxic treatment with adult zebrafish exposed to six consecutive rounds of phototoxic treatment. We observed that Müller glia continued to re-enter the cell cycle to produce clusters of retinal progenitors in zebrafish exposed to multiple rounds of phototoxic light. Some abnormalities were noted, however. First, we found that retinas exposed to multiple rounds of damage exhibited a greater loss of photoreceptors at 36 hours of light damage than retinas that were exposed to their first round of light damage. In addition, we found that Müller glia appeared to have an increase in the acute gliotic response in retinas exposed to multiple rounds of light treatment. This was evidenced by cellular hypertrophy, changes in GFAP cellular localization, and transient increases in stat3 and gfap expression. Finally, following the sixth round of phototoxic lesion, we observed a significant increase in mis-localized HuC/D-positive amacrine and ganglion cells in the inner plexiform layer and outer retina, and a decreased number of regenerated blue cone photoreceptors. These data add to recent findings that retinal regeneration in adult zebrafish occurs concomitant with Müller glia reactivity and can result in the generation of aberrant neurons. These data are also the first to demonstrate that Müller glia appear to modify their phenotype in response to multiple rounds of phototoxic lesion, exhibiting an increase in acute gliosis while maintaining a remarkable capacity for long-term regeneration of photoreceptors.

Transmembrane domains will sometimes contain conserved ionizable residues which are essential for protein function and regulation. This work aims to examine the effects of single Arg(R) residues within a highly dynamic transmembrane peptide helix. We have modified the dynamic transmembrane GW 4,20 ALP23 (acetyl-GGAW 4 (AL) 7 AW 20 AGA-[ethanol] amide) peptide to incorporate an Arg residue near the center of the peptide at position 12 or 14. Peptide orientation and dynamics were analyzed by means of solid-state NMR spectroscopy to monitor specific 2 H-labeled Ala residues. GW 4,20 ALP23 adopts a tilted orientation within lipid bilayers and exhibits moderate to high dynamic averaging of NMR observables, such as the 2 H quadrupolar splittings or 15 N-1 H dipolar couplings, due to competition between the interfacial Trp(W) residues on opposing helical faces. Here we examine how such extensive peptide dynamics are impacted by the introduction of central Arg residues. R14 holds significant influence over the peptide's behavior and restricts the peptide to low dynamics and a tilt that decreases as lipid thickness increases, all at the cost of partial C-terminal helix unwinding. Alternatively, R12 causes the host peptide to adopt an interfacial surface-bound orientation in DOPC bilayers. Interestingly, multistate behavior is exhibited by a single residue, Ala-9. Furthermore, numerous N-terminal quadrupolar splittings generated by 2 H-labeled residues do not fit to the same quadrupolar wave plot as the remaining C-terminal residues. Therefore, both results potentially implicate distortion of the R12 helix. The partial helix unwinding/distortion behavior observed in both the R12 and R14 peptides could be due to the competition of the terminal Trp residues attempting to compensate for the Arg's dominance over the peptide's orientation. We thus surmise that while the arginine residues are prominent factors controlling the helix dynamics, the influence of the competing tryptophan residues cannot be ignored.

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