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PhD offer - Sorbonne University - Biomineralization and Pigmentation in Sea Urchins H/F

This offer is available in the following languages:
Français - Anglais

Date Limite Candidature : jeudi 4 novembre 2021

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General information

Reference : UMR7574-MARALB-002
Workplace : PARIS 05
Date of publication : Thursday, October 14, 2021
Scientific Responsible name : Marie Albéric
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 February 2022
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly

Description of the thesis topic

Project description: Although the biomineralization mechanisms (i.e. the formation of minerals by organisms) in sea urchin spines have been already studied [1,2], the pigmentation phenomena controlling the different colors of the spines have been so far overlooked. The diverse colors observed in sea urchin spines are due to a family of organic molecules, the polyhydroxylated-naphthoquinone (PHNQ) (Fig. A), which are produced within the so-called red-spherule cells (RSC) (Fig. B) [3]. PHNQs have anti-bacterial properties making the red-spherules cells part of the immune system of the urchins [3]. Our preliminary results show that when sea urchin spines fracture, RSC that actively participate to the healing of the epithelium tissues are closely located to the new growing micro-spines composed of a mixture of amorphous calcium carbonate (ACC) and calcite (Fig. C, D). In addition, micrometric vesicles containing the PHNQ molecules seem to travel from the RSC to the growing mineral surface in which they are likely incorporated (Fig. D). We also observed color variations during spine's growth: micro-spines are at first transparent and then gradually color in light pink (Fig. E) whereas shades from brown to green can be found in mature crystalline spines. However, the origin of the color and its variations remains unclear.

Figs. A) Different colors of P. lividus sea urchins and PHNQ molecule structures, B) red-spherule cells at the surface of the tip of a growing spine, C) in-vivo confocal laser scanning microscopy of a calcein stained micro-spine (in green), D) focused ion beam scanning electron microscopy micrograph of a freeze substituted micro-spine in contact with a red-spherule cell and E) growing tip of a spine composed of micro-spines (CT-data).

The objective of this PhD is to improve our understanding of the mechanisms involved in the PHNQ incorporation within the growing mineral during the coupled biomineralization and pigmentation pathways in sea urchins. For this purpose, the PhD will be divided in 2 parts, which will be studied in parallel.

The first approach will be dedicated to the study of the in-vivo mechanisms leading to the PHNQs incorporation within the biominerals. Living Paracentrotus lividus sea urchins will be hosted in the aquariums of the aquatic platform of Sorbonne University. The vesicles and then the PHNQ molecules will be extracted from the red-spherule cells according to established protocols [4]. The former will be characterized by (cryo)-TEM and the later by UV-visible spectroscopy, 1H solution state NMR and HPLC/LC-MS. Moreover, pH likely plays an important role in vesicles opening and release of the PHNQs. Therefore, intra-vesicular pH measurements will be performed using pH sensitive dye that will be imaged by live-cell fluorescence microscopy [5,6] and possibly by a recent histological procedure developed for bones at the LCMCP. Live fluorescence and confocal microscopies will allow determining the relation between RSC and Ca transport, and cryo(FIB-SEM) imaging vesicles at higher magnification.

The second approach will focus on bio-mimicking the opening of naphthoquinone-loaded pH-sensitive liposomes during the precipitation of calcium carbonates. Based on established method [7-9], PHNQ-loaded pH-sensitive liposomes will be first synthesized. By varying the PHNQ composition and concentration as well as the liposome size, the effect of confinement on the color changes will be also studied. Second, liposome opening will be controlled by modifying the pH and third CaCO3 precipitation will be induced. The possibility of forming a dense 3D network of hybrid vesicles may be considered to guide CaCO3 network patterning. pH measurements, (cryo)-TEM, fluorescence and confocal microscopy as well as (cryo)-FIB-SEM will also be performed on these in-vitro systems.

Work Context

Topic: Biomineralization, Biochemistry, Biology, Pigmentation
Keywords: sea urchin spines, polyhydroxylated naphthoquinones, amorphous CaCO3, pH-sensitive liposomes
Host Institution: Sorbonne University, “Laboratoire Chimie de la Matière Condensée de Paris”, France
Supervisors/Contact: Marie Albéric and Nadine Nassif
Equipe d'accueil: SMiLES
Research webpage: https://lcmcp.upmc.fr/site/biomineralization/
Email: marie.alberic@sorbonne-universite.fr, nadine.nassif@sorbonne-universite.fr
Starting date: January 2022
Funding: ANR JCJC Grant – ColMhyBio

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