Pathogenic variants of sphingomyelin synthase SMS2 disrupt lipid landscapes in the secretory pathway

Sphingomyelin is a dominant sphingolipid in mammalian cells. Its production in the trans-Golgi traps cholesterol synthesized in the ER to promote formation of a sphingomyelin/sterol gradient along the secretory pathway. This gradient marks a fundamental transition in physical membrane properties that help specify organelle identify and function. We previously identified mutations in sphingomyelin synthase SMS2 that cause osteoporosis and skeletal dysplasia. Here, we show that SMS2 variants linked to the most severe bone phenotypes retain full enzymatic activity but fail to leave the ER owing to a defective autonomous ER export signal. Cells harboring pathogenic SMS2 variants accumulate sphingomyelin in the ER and display a disrupted transbilayer sphingomyelin asymmetry. These aberrant sphingomyelin distributions also occur in patient-derived fibroblasts and are accompanied by imbalances in cholesterol organization, glycerophospholipid profiles, and lipid order in the secretory pathway. We postulate that pathogenic SMS2 variants undermine the capacity of osteogenic cells to uphold nonrandom lipid distributions that are critical for their bone forming activity.Peer reviewe

Sondes fluorescentes ciblées sensibles à l’environnement pour l’imagerie de cellules vivantes

Specific targeting, imaging and probing of cell plasma membranes and intracellular organelles can be addressed by rationally designed polarity-sensitive fluorescent probes. Here, a new efficient plasma membrane-targeting moiety was developed and tested in five cyanine dyes, showing excellent performance in cellular and in vivo microscopy. Next, the targeting moiety was grafted to a solvatochromic dye Prodan, yielding a plasma membrane probe with high lipid order sensitivity. Modifying a Nile Red using the moieties with varied alkyl chain lengths resulted in two solvatochromic plasma membrane probes: NR12A with high affinity to membranes for conventional microscopy, and NR4A, a low-affinity probe for PAINT super-resolution microscopy. Tethering Nile Red with organelle-targeted groups yielded an array of probes, able to sense polarity and lipid order in organelle membranes. The synthesized probes will find applications in bioimaging, cell biology, biophysics or mechanobiology.Le ciblage, l'imagerie et le sondage spécifiques des membranes plasmiques et des organites intracellulaires peuvent être faits par des sondes fluorescentes à façon sensibles à la polarité. Ici, un nouveau fragment ciblant la membrane plasmique à été développé et testé dans cinq colorants cyanines, montrant d'excellentes performances en microscopie cellulaire et in vivo. Le fragment à été greffé à un fluorophore solvatochrome Prodan, donnant une sonde de membrane plasmique avec une sensibilité élevée à l'ordre lipidique. Le rouge de Nil, greffé aux fragments avec les chaînes alkyles C12 et C4, à donné deux sondes solvatochromes à membrane plasmique : NR12A pour la microscopie conventionnelle, et NR4A pour la microscopie à super-résolution PAINT. Le rouge de Nil avec des groupes ciblant les organites à donné un éventail de sondes sensibles à la polarité et à l'ordre lipidique dans les membranes des organites. Les sondes synthétisées trouveront des applications en bioimagerie, biologie cellulaire, biophysique ou mécanobiologie

Sondes fluorescentes ciblées sensibles à l’environnement pour l’imagerie de cellules vivantes

Le ciblage, l'imagerie et le sondage spécifiques des membranes plasmiques et des organites intracellulaires peuvent être faits par des sondes fluorescentes à façon sensibles à la polarité. Ici, un nouveau fragment ciblant la membrane plasmique à été développé et testé dans cinq colorants cyanines, montrant d'excellentes performances en microscopie cellulaire et in vivo. Le fragment à été greffé à un fluorophore solvatochrome Prodan, donnant une sonde de membrane plasmique avec une sensibilité élevée à l'ordre lipidique. Le rouge de Nil, greffé aux fragments avec les chaînes alkyles C12 et C4, à donné deux sondes solvatochromes à membrane plasmique : NR12A pour la microscopie conventionnelle, et NR4A pour la microscopie à super-résolution PAINT. Le rouge de Nil avec des groupes ciblant les organites à donné un éventail de sondes sensibles à la polarité et à l'ordre lipidique dans les membranes des organites. Les sondes synthétisées trouveront des applications en bioimagerie, biologie cellulaire, biophysique ou mécanobiologie.Specific targeting, imaging and probing of cell plasma membranes and intracellular organelles can be addressed by rationally designed polarity-sensitive fluorescent probes. Here, a new efficient plasma membrane-targeting moiety was developed and tested in five cyanine dyes, showing excellent performance in cellular and in vivo microscopy. Next, the targeting moiety was grafted to a solvatochromic dye Prodan, yielding a plasma membrane probe with high lipid order sensitivity. Modifying a Nile Red using the moieties with varied alkyl chain lengths resulted in two solvatochromic plasma membrane probes: NR12A with high affinity to membranes for conventional microscopy, and NR4A, a low-affinity probe for PAINT super-resolution microscopy. Tethering Nile Red with organelle-targeted groups yielded an array of probes, able to sense polarity and lipid order in organelle membranes. The synthesized probes will find applications in bioimaging, cell biology, biophysics or mechanobiology

Sondes fluorescentes ciblées sensibles à l’environnement pour l’imagerie de cellules vivantes

Le ciblage, l'imagerie et le sondage spécifiques des membranes plasmiques et des organites intracellulaires peuvent être faits par des sondes fluorescentes à façon sensibles à la polarité. Ici, un nouveau fragment ciblant la membrane plasmique à été développé et testé dans cinq colorants cyanines, montrant d'excellentes performances en microscopie cellulaire et in vivo. Le fragment à été greffé à un fluorophore solvatochrome Prodan, donnant une sonde de membrane plasmique avec une sensibilité élevée à l'ordre lipidique. Le rouge de Nil, greffé aux fragments avec les chaînes alkyles C12 et C4, à donné deux sondes solvatochromes à membrane plasmique : NR12A pour la microscopie conventionnelle, et NR4A pour la microscopie à super-résolution PAINT. Le rouge de Nil avec des groupes ciblant les organites à donné un éventail de sondes sensibles à la polarité et à l'ordre lipidique dans les membranes des organites. Les sondes synthétisées trouveront des applications en bioimagerie, biologie cellulaire, biophysique ou mécanobiologie.Specific targeting, imaging and probing of cell plasma membranes and intracellular organelles can be addressed by rationally designed polarity-sensitive fluorescent probes. Here, a new efficient plasma membrane-targeting moiety was developed and tested in five cyanine dyes, showing excellent performance in cellular and in vivo microscopy. Next, the targeting moiety was grafted to a solvatochromic dye Prodan, yielding a plasma membrane probe with high lipid order sensitivity. Modifying a Nile Red using the moieties with varied alkyl chain lengths resulted in two solvatochromic plasma membrane probes: NR12A with high affinity to membranes for conventional microscopy, and NR4A, a low-affinity probe for PAINT super-resolution microscopy. Tethering Nile Red with organelle-targeted groups yielded an array of probes, able to sense polarity and lipid order in organelle membranes. The synthesized probes will find applications in bioimaging, cell biology, biophysics or mechanobiology

Dissecting the mechanisms of environment sensitivity of smart probes for quantitative assessment of membrane properties

The plasma membrane, as a highly complex cell organelle, serves as a crucial platform for a multitude of cellular processes. Its collective biophysical properties are largely determined by the structural diversity of the different lipid species it accommodates. Therefore, a detailed investigation of biophysical properties of the plasma membrane is of utmost importance for a comprehensive understanding of biological processes occurring therein. During the past two decades, several environment-sensitive probes have been developed and become popular tools to investigate membrane properties. Although these probes are assumed to report on membrane order in similar ways, their individual mechanisms remain to be elucidated. In this study, using model membrane systems, we characterized the probes Pro12A, NR12S and NR12A in depth and examined their sensitivity to parameters with potential biological implications, such as the degree of lipid saturation, double bond position and configuration (cis versus trans), phospholipid headgroup and cholesterol content. Applying spectral imaging together with atomistic molecular dynamics simulations and time-dependent fluorescent shift analyses, we unravelled individual sensitivities of these probes to different biophysical properties, their distinct localizations and specific relaxation processes in membranes. Overall, Pro12A, NR12S and NR12A serve together as a toolbox with a wide range of applications allowing to select the most appropriate probe for each specific research question.Peer reviewe

Dissecting the mechanisms of environment sensitivity of smart probes for quantitative assessment of membrane properties

The plasma membrane, as a highly complex cell organelle, serves as a crucial platform for a multitude of cellular processes. Its collective biophysical properties are largely determined by the structural diversity of the different lipid species it accommodates. Therefore, a detailed investigation of biophysical properties of the plasma membrane is of utmost importance for a comprehensive understanding of biological processes occurring therein. During the past two decades, several environment-sensitive probes have been developed and become popular tools to investigate membrane properties. Although these probes are assumed to report on membrane order in similar ways, their individual mechanisms remain to be elucidated. In this study, using model membrane systems, we characterized the probes Pro12A, NR12S and NR12A in depth and examined their sensitivity to parameters with potential biological implications, such as the degree of lipid saturation, double bond position and configuration (cis versus trans), phospholipid headgroup and cholesterol content. Applying spectral imaging together with atomistic molecular dynamics simulations and time-dependent fluorescent shift analyses, we unravelled individual sensitivities of these probes to different biophysical properties, their distinct localizations and specific relaxation processes in membranes. Overall, Pro12A, NR12S and NR12A serve together as a toolbox with a wide range of applications allowing to select the most appropriate probe for each specific research question.Peer reviewe