Stepwise Biogenesis of Subpopulations of Lipid Droplets in Nitrogen Starved Phaeodactylum tricornutum Cells

Diatoms are unicellular heterokonts, living in oceans and freshwaters, exposed to frequent environmental variations. They have a sophisticated membrane compartmentalization and are bounded by a siliceous cell-wall. Formation of lipid droplets (LDs), filled with triacylglycerol (TAG), is a common response to stress. The proteome of mature-LDs from Phaeodactylum tricornutum highlighted the lack of proteins involved in early-LD formation, TAG biosynthesis or LD-to-LD connections. These features suggest that cytosolic LDs might reach a size limit. We analyzed the dynamics of LD formation in P. tricornutum (Pt1 8.6; CCAP 1055/1) during 7 days of nitrogen starvation, by monitoring TAG by mass spectrometry-based lipidomics, and LD radius using epifluorescence microscopy and pulse field gradient nuclear magnetic resonance. We confirmed that mature LDs reach a maximal size. Based on pulse field gradient nuclear magnetic resonance, we did not detect any LD-LD fusion. Three LD subpopulations were produced, each with a different maximal size, larger-sized LDs (radius 0.675 ± 0.125 µm) being generated first. Mathematical modeling showed how smaller LDs are produced once larger LDs have reached their maximum radius. In a mutant line having larger cells, the maximal size of the first LD subpopulation was higher (0.941 ± 0.169 µm), while the principle of stepwise formation of distinct LD populations was maintained. Results suggest that LD size is determined by available cytosolic space and sensing of an optimal size reached in the previous LD subpopulation. Future perspectives include the unraveling of LD-size control mechanisms upon nitrogen shortage. This study also provides novel prospects for the optimization of oleaginous microalgae for biotechnological applications.

Non-Enzymatic Synthesis of Bioactive Isoprostanoids in the Diatom Phaeodactylum following Oxidative Stress

The ecological success of diatoms requires a remarkable ability to survive many types of stress, including variations in temperature, light, salinity, and nutrient availability. On exposure to these stresses, diatoms exhibit common responses, including growth arrest, impairment of photosynthesis, production of reactive oxygen species, and accumulation of triacylglycerol (TAG). We studied the production of cyclopentane oxylipins derived from fatty acids in the diatom Phaeodactylum tricornutum in response to oxidative stress. P. tricornutum lacks the enzymatic pathway for producing cyclopentane-oxylipins, such as jasmonate, prostaglandins, or thromboxanes. In cells subjected to increasing doses of hydrogen peroxide (H₂O₂), we detected nonenzymatic production of isoprostanoids, including six phytoprostanes, three F_2t-isoprostanes, two F_3t-isoprostanes, and three F_4t-neuroprostanes, by radical peroxidation of α-linolenic, arachidonic, eicosapentaenoic, and docosahexanoic acids, respectively. H₂O₂ also triggered photosynthesis impairment and TAG accumulation. F_1t-phytoprostanes constitute the major class detected (300 pmol per 1 million cells; intracellular concentration, ∼4 µm). Only two glycerolipids, phosphatidylcholine and diacylglycerylhydroxymethyl-trimethyl-alanine, could provide all substrates for these isoprostanoids. Treatment of P. tricornutum with nine synthetic isoprostanoids produced an effect in the micromolar range, marked by the accumulation of TAG and reduced growth, without affecting photosynthesis. Therefore, the emission of H₂O₂ and free radicals upon exposure to stresses can lead to glycerolipid peroxidation and nonenzymatic synthesis of isoprostanoids, inhibiting growth and contributing to the induction of TAG accumulation via unknown processes. This characterization of nonenzymatic oxylipins in P. tricornutum opens a field of research on the study of processes controlled by isoprostanoid signaling in various physiological and environmental contexts in diatoms.

Caruncle single injection episcleral (Sub-tenon) anesthesia for cataract surgery: mepivacaine versus a lidocaine-bupivacaine mixture

We compared the quality of anesthesia provided by mepivacaine 2% or a mixture of lidocaine 2%-bupivacaine 0.5%, both with hyaluronidase, in caruncle single-injection episcleral (sub-Tenon) anesthesia. Sixty patients undergoing cataract surgery were included in this randomized, double-blinded study. The time to the onset of blockade, maximal akinesia, need for supplemental injection, and time to recovery were recorded. With mepivacaine, the time to onset was slightly shorter, and the akinesia score higher, than with the mixture. Although statistically significant, these differences are small. With mepivacaine, the time to recovery was shorter. We conclude that the reproducible short duration of the block may be an advantage in outpatient surgery.

IMPLICATIONS

We compared the classic mixture of lidocaine 2% plus bupivacaine 0.5% to mepivacaine 2% for caruncle episcleral (sub-Tenon) anesthesia for cataract surgery. Mepivacaine provided a more efficient block with a quicker onset and a quicker recovery. However, these differences were very small and were of little clinical interest.

Ophthalmic regional anesthesia: medial canthus episcleral (sub-tenon) anesthesia is more efficient than peribulbar anesthesia: A double-blind randomized study

BACKGROUND

Regional anesthesia and especially peribulbar anesthesia commonly is used for cataract surgery. Failure rates and need for reinjection remains high, however, with peribulbar anesthesia. Single-injection high-volume medial canthus episcleral (sub-Tenon's) anesthesia has proven to be an efficient and safe alternative to peribulbar anesthesia.

METHODS

The authors, in a blind study, compared the effectiveness of both techniques in 66 patients randomly assigned to episcleral anesthesia or single-injection peribulbar anesthesia. Motor blockade (akinesia) was used as the main index of anesthesia effectiveness. It was assessed using an 18-point scale (0-3 for each of the four directions of the gaze, lid opening, and lid closing, the total being from 0 = normal mobility to 18 = no movement at all). This score was compared between the groups 1, 5, 10, and 15 min after injection and at the end of the surgical procedures. Time to onset of the blockade also was compared between the two groups, as was the incidence of incomplete blockade with a need for supplemental injection and the satisfaction of the surgeon, patient, and anesthesiologist.

RESULTS

Episcleral anesthesia provided a quicker onset of anesthesia, a better akinesia score, and a lower rate of incomplete blockade necessitating reinjection (0 vs. 39%; P < 0.0001) than peribulbar anesthesia. Even after supplemental injection, peribulbar anesthesia had a lower akinesia score than did episcleral anesthesia. Peribulbar anesthesia began to wear off during surgery, whereas episcleral anesthesia did not.

CONCLUSION

Medial canthus single-injection episcleral anesthesia is a suitable alternative to peribulbar anesthesia. It provides better akinesia, with a quicker onset and more constancy in effectiveness.