Natural carotenoids such as astaxanthin, β,β-carotene and lutein are pigments with a high market ... more Natural carotenoids such as astaxanthin, β,β-carotene and lutein are pigments with a high market value. We studied the effects of nitrogen depletion on the carotenoid metabolism of Chlorella zofingiensis (Chlorophyta) and the subsequent treatment with diphenylamine (DPA), an inhibitor of the biosynthesis of secondary ketocarotenoids. Pigments were identified and quantified based on reversed phase ultra-high performance liquid chromatography photodiode array tandem mass spectrometry (RP-UHPLC-PDA-MSn). Nitrogen depletion (without DPA) resulted in a degradation of chlorophylls and primary carotenoids and an accumulation of astaxanthin, ketolutein, canthaxanthin, adonixanthin and β,β-carotene. The DPA treatment decreased the overall production of β,β-carotene derivatives (sum of astaxanthin, canthaxanthin, echinenone and adonixanthin); however, the production of ketolutein and degradation of primary carotenoids were not modified. This suggests that the regulatory mechanisms controlling...
ABSTRACT Natural carotenoids such as astaxanthin, β,β-carotene and lutein are pigments with a hig... more ABSTRACT Natural carotenoids such as astaxanthin, β,β-carotene and lutein are pigments with a high market value. We studied the effects of nitrogen depletion on the carotenoid metabolism of Chlorella zofingiensis (Chlorophyta) and the subsequent treatment with diphenylamine (DPA), an inhibitor of the biosynthesis of secondary ketocarotenoids. Pigments were identified and quantified based on reversed phase ultra-high performance liquid chromatography photodiode array tandem mass spectrometry (RP-UHPLC-PDA-MSn). Nitrogen depletion (without DPA) resulted in a degradation of chlorophylls and primary carotenoids and an accumulation of astaxanthin, ketolutein, canthaxanthin, adonixanthin and β,β-carotene. The DPA treatment decreased the overall production of β,β-carotene derivatives (sum of astaxanthin, canthaxanthin, echinenone and adonixanthin); however, the production of ketolutein and degradation of primary carotenoids were not modified. This suggests that the regulatory mechanisms controlling the flux towards ketolutein and primary carotenoids were not affected by the decreased levels of β,β-carotene derivatives. In addition, DPA increased production of the individual carotenoids, adonixanthin and echinenone. Insight into the regulation of microalgal carotenoid biosynthesis as demonstrated in this paper is essential when a large-scale carotenoid production process is to be optimised or a recombinant C. zofingiensis strain is to be designed with the intention of excessively producing primary or secondary carotenoids.
Influenza A viruses lacking the gene encoding the non-structural NS1 protein (delNS1) have potent... more Influenza A viruses lacking the gene encoding the non-structural NS1 protein (delNS1) have potential use as live attenuated vaccines. However, due to the lack of NS1, virus replication in cell culture is considerably reduced, prohibiting commercial vaccine production. We therefore established two stable MDCK cell lines that show inducible expression of the allele B NS1 protein. Upon induction, both cell
The effect of three different nutrient depletions (nitrogen, sulphur and magnesium) on the growth... more The effect of three different nutrient depletions (nitrogen, sulphur and magnesium) on the growth and pigment accumulation of the haptophyte Isochrysis aff. galbana (clone T-ISO) has been studied. Pigments were quantified based on RP-UHPLC-PDA-MSn analysis. All nutrient depletions led to reduced maximal biomass concentrations. Besides, all nutrient-depleted cultures accumulated 3-hydroxyechinenone. To our knowledge, this is the first time that 3-hydroxyechinenone has been found in I. aff. galbana T-ISO. Most 3-hydroxyechinenone, as well as the most echinenone and diatoxanthin, were found in the nitrogen-limited culture in which a more severe limitation resulted in higher cellular contents. Similar to accumulation of diatoxanthin, accumulation of 3-hydroxyechinenone and echinenone may be part of a global (stress) response mechanism to oversaturating light conditions.
Objective: To determine the effect of dissolved oxygen tension (DO) on the redifferentiation of d... more Objective: To determine the effect of dissolved oxygen tension (DO) on the redifferentiation of dedifferentiated adult human nasal septum chondrocytes cultured as pellets. - Design: After isolation, human nasal chondrocytes were expanded in monolayer culture, which resulted in their dedifferentiation. Dedifferentiated cells were pelleted, transferred to a bioreactor and maintained for up to 21 days at 100% DO (21% oxygen),
Objective: To determine the effect of dissolved oxygen tension (DO) on the redifferentiation of d... more Objective: To determine the effect of dissolved oxygen tension (DO) on the redifferentiation of dedifferentiated adult human nasal septum chondrocytes cultured as pellets. Design: After isolation, human nasal chondrocytes were expanded in monolayer culture, which resulted in their dedifferentiation. Dedifferentiated cells were pelleted, transferred to a bioreactor and maintained for up to 21 days at 100% DO (21% oxygen), 25%
The nutritional requirements of stem cells have not been determined; in particular, the amino aci... more The nutritional requirements of stem cells have not been determined; in particular, the amino acid metabolism of stem cells is largely unknown. In this study, we investigated the amino acid metabolism of human mesenchymal stem cells (hMSCs), with focus on two questions: Which amino acids are consumed and/or secreted by hMSCs and at what rates? To answer these questions, hMSCs
Microalgae are often considered as a promising alternative source of vegetable oils. These oils c... more Microalgae are often considered as a promising alternative source of vegetable oils. These oils can be used for food and biofuel applications. Productivities that are projected for large-scale microalgal oil production are, however, often poorly supported by scientific evidence and based on too optimistic assumptions. To facilitate the inclusion of the microalgal physiology in these projections, existing knowledge and novel scientific insights were condensed into a mechanistic model that describes photosynthesis and carbon partitioning during nitrogen starvation. The model is validated using experimental data from both wild-type and a starchless mutant of Scenedesmus obliquus. The model is subsequently used to quantify how reactor design, process design, and strain improvement can improve the oil productivity from 2.1 to up to 10.9gm(-2)day(-1). These projected productivities are used to reflect on commonly assumed oil productivities and it is concluded that the microalgal oil produ...
Preventing or delaying cell death is a challenge in mammalian cell cultures for the development a... more Preventing or delaying cell death is a challenge in mammalian cell cultures for the development and optimization of production processes for biopharmaceuticals. Cell cultures need to be maintained highly viable for extended times in order to reach maximum production yields. Moreover, programmed cell death through apoptosis is often believed to occur without being detected by classical viability measurements. In this study, we characterized cell death in PER.C6® batch and perfusion cultures using three flow cytometry techniques measuring different steps of the apoptosis cascade: DNA fragmentation, caspases activation and phosphatidylserine externalization. We showed that apoptosis is the main pathway of PER.C6® cell death in batch cultures after depletion of main carbon sources. In high cell density perfusion cultures fed at a constant specific perfusion rate, both high viability and very limited apoptosis were observed. When extending this perfusion process far beyond standard opera...
The circadian clock schedules processes in microalgae cells at suitable times in the day/night cy... more The circadian clock schedules processes in microalgae cells at suitable times in the day/night cycle. To gain knowledge about these biological time schedules, Neochloris oleoabundans was grown under constant light conditions and nitrogen limitation. Under these constant conditions, the only variable was the circadian clock. The results were compared to previous work done under nitrogen-replete conditions, in order to determine the effect of N-limitation on circadian rhythms in the cell cycle and biomass composition of N. oleoabundans. The circadian clock was not affected by nitrogen-limitation, and cell division was timed in the natural night, despite of constant light conditions. However, because of nitrogen-limitation, not the entire population was able to divide every day. Two subpopulations were observed, which divided alternately every other day. This caused oscillations in biomass yield and composition. Starch and total fatty acids (TFA) were accumulated during the day. Also, ...
Reproducing the native collagen structure and glycosaminoglycan (GAG) distribution in tissue-engi... more Reproducing the native collagen structure and glycosaminoglycan (GAG) distribution in tissue-engineered cartilage constructs is still a challenge. Articular cartilage has a specific nutrient supply and mechanical environment due to its location and function in the body. Efforts to simulate this native environment have been reported through the use of bioreactor systems. However, few of these devices take into account the existence of gradients over cartilage as a consequence of the nutrient supply by diffusion. We hypothesized that culturing chondrocytes in an environment, in which gradients of nutrients can be mimicked, would induce zonal differentiation. Indeed, we show that glucose gradients facilitating a concentration distribution as low as physiological glucose levels enhanced a zonal chondrogenic capacity similar to the one found in native cartilage. Furthermore, we found that the glucose consumption rates of cultured chondrocytes were higher under physiological glucose conce...
Everyone who has ever tried to radically change metabolic fluxes knows that it is often harder to... more Everyone who has ever tried to radically change metabolic fluxes knows that it is often harder to determine which enzymes have to be modified than it is to actually implement these changes. In the more traditional genetic engineering approaches ’bottle-necks’ are pinpointed using qualitative, intuitive approaches, but the alleviation of suspected ’rate-limiting’ steps has not often been successful. Here the
Natural carotenoids such as astaxanthin, β,β-carotene and lutein are pigments with a high market ... more Natural carotenoids such as astaxanthin, β,β-carotene and lutein are pigments with a high market value. We studied the effects of nitrogen depletion on the carotenoid metabolism of Chlorella zofingiensis (Chlorophyta) and the subsequent treatment with diphenylamine (DPA), an inhibitor of the biosynthesis of secondary ketocarotenoids. Pigments were identified and quantified based on reversed phase ultra-high performance liquid chromatography photodiode array tandem mass spectrometry (RP-UHPLC-PDA-MSn). Nitrogen depletion (without DPA) resulted in a degradation of chlorophylls and primary carotenoids and an accumulation of astaxanthin, ketolutein, canthaxanthin, adonixanthin and β,β-carotene. The DPA treatment decreased the overall production of β,β-carotene derivatives (sum of astaxanthin, canthaxanthin, echinenone and adonixanthin); however, the production of ketolutein and degradation of primary carotenoids were not modified. This suggests that the regulatory mechanisms controlling...
ABSTRACT Natural carotenoids such as astaxanthin, β,β-carotene and lutein are pigments with a hig... more ABSTRACT Natural carotenoids such as astaxanthin, β,β-carotene and lutein are pigments with a high market value. We studied the effects of nitrogen depletion on the carotenoid metabolism of Chlorella zofingiensis (Chlorophyta) and the subsequent treatment with diphenylamine (DPA), an inhibitor of the biosynthesis of secondary ketocarotenoids. Pigments were identified and quantified based on reversed phase ultra-high performance liquid chromatography photodiode array tandem mass spectrometry (RP-UHPLC-PDA-MSn). Nitrogen depletion (without DPA) resulted in a degradation of chlorophylls and primary carotenoids and an accumulation of astaxanthin, ketolutein, canthaxanthin, adonixanthin and β,β-carotene. The DPA treatment decreased the overall production of β,β-carotene derivatives (sum of astaxanthin, canthaxanthin, echinenone and adonixanthin); however, the production of ketolutein and degradation of primary carotenoids were not modified. This suggests that the regulatory mechanisms controlling the flux towards ketolutein and primary carotenoids were not affected by the decreased levels of β,β-carotene derivatives. In addition, DPA increased production of the individual carotenoids, adonixanthin and echinenone. Insight into the regulation of microalgal carotenoid biosynthesis as demonstrated in this paper is essential when a large-scale carotenoid production process is to be optimised or a recombinant C. zofingiensis strain is to be designed with the intention of excessively producing primary or secondary carotenoids.
Influenza A viruses lacking the gene encoding the non-structural NS1 protein (delNS1) have potent... more Influenza A viruses lacking the gene encoding the non-structural NS1 protein (delNS1) have potential use as live attenuated vaccines. However, due to the lack of NS1, virus replication in cell culture is considerably reduced, prohibiting commercial vaccine production. We therefore established two stable MDCK cell lines that show inducible expression of the allele B NS1 protein. Upon induction, both cell
The effect of three different nutrient depletions (nitrogen, sulphur and magnesium) on the growth... more The effect of three different nutrient depletions (nitrogen, sulphur and magnesium) on the growth and pigment accumulation of the haptophyte Isochrysis aff. galbana (clone T-ISO) has been studied. Pigments were quantified based on RP-UHPLC-PDA-MSn analysis. All nutrient depletions led to reduced maximal biomass concentrations. Besides, all nutrient-depleted cultures accumulated 3-hydroxyechinenone. To our knowledge, this is the first time that 3-hydroxyechinenone has been found in I. aff. galbana T-ISO. Most 3-hydroxyechinenone, as well as the most echinenone and diatoxanthin, were found in the nitrogen-limited culture in which a more severe limitation resulted in higher cellular contents. Similar to accumulation of diatoxanthin, accumulation of 3-hydroxyechinenone and echinenone may be part of a global (stress) response mechanism to oversaturating light conditions.
Objective: To determine the effect of dissolved oxygen tension (DO) on the redifferentiation of d... more Objective: To determine the effect of dissolved oxygen tension (DO) on the redifferentiation of dedifferentiated adult human nasal septum chondrocytes cultured as pellets. - Design: After isolation, human nasal chondrocytes were expanded in monolayer culture, which resulted in their dedifferentiation. Dedifferentiated cells were pelleted, transferred to a bioreactor and maintained for up to 21 days at 100% DO (21% oxygen),
Objective: To determine the effect of dissolved oxygen tension (DO) on the redifferentiation of d... more Objective: To determine the effect of dissolved oxygen tension (DO) on the redifferentiation of dedifferentiated adult human nasal septum chondrocytes cultured as pellets. Design: After isolation, human nasal chondrocytes were expanded in monolayer culture, which resulted in their dedifferentiation. Dedifferentiated cells were pelleted, transferred to a bioreactor and maintained for up to 21 days at 100% DO (21% oxygen), 25%
The nutritional requirements of stem cells have not been determined; in particular, the amino aci... more The nutritional requirements of stem cells have not been determined; in particular, the amino acid metabolism of stem cells is largely unknown. In this study, we investigated the amino acid metabolism of human mesenchymal stem cells (hMSCs), with focus on two questions: Which amino acids are consumed and/or secreted by hMSCs and at what rates? To answer these questions, hMSCs
Microalgae are often considered as a promising alternative source of vegetable oils. These oils c... more Microalgae are often considered as a promising alternative source of vegetable oils. These oils can be used for food and biofuel applications. Productivities that are projected for large-scale microalgal oil production are, however, often poorly supported by scientific evidence and based on too optimistic assumptions. To facilitate the inclusion of the microalgal physiology in these projections, existing knowledge and novel scientific insights were condensed into a mechanistic model that describes photosynthesis and carbon partitioning during nitrogen starvation. The model is validated using experimental data from both wild-type and a starchless mutant of Scenedesmus obliquus. The model is subsequently used to quantify how reactor design, process design, and strain improvement can improve the oil productivity from 2.1 to up to 10.9gm(-2)day(-1). These projected productivities are used to reflect on commonly assumed oil productivities and it is concluded that the microalgal oil produ...
Preventing or delaying cell death is a challenge in mammalian cell cultures for the development a... more Preventing or delaying cell death is a challenge in mammalian cell cultures for the development and optimization of production processes for biopharmaceuticals. Cell cultures need to be maintained highly viable for extended times in order to reach maximum production yields. Moreover, programmed cell death through apoptosis is often believed to occur without being detected by classical viability measurements. In this study, we characterized cell death in PER.C6® batch and perfusion cultures using three flow cytometry techniques measuring different steps of the apoptosis cascade: DNA fragmentation, caspases activation and phosphatidylserine externalization. We showed that apoptosis is the main pathway of PER.C6® cell death in batch cultures after depletion of main carbon sources. In high cell density perfusion cultures fed at a constant specific perfusion rate, both high viability and very limited apoptosis were observed. When extending this perfusion process far beyond standard opera...
The circadian clock schedules processes in microalgae cells at suitable times in the day/night cy... more The circadian clock schedules processes in microalgae cells at suitable times in the day/night cycle. To gain knowledge about these biological time schedules, Neochloris oleoabundans was grown under constant light conditions and nitrogen limitation. Under these constant conditions, the only variable was the circadian clock. The results were compared to previous work done under nitrogen-replete conditions, in order to determine the effect of N-limitation on circadian rhythms in the cell cycle and biomass composition of N. oleoabundans. The circadian clock was not affected by nitrogen-limitation, and cell division was timed in the natural night, despite of constant light conditions. However, because of nitrogen-limitation, not the entire population was able to divide every day. Two subpopulations were observed, which divided alternately every other day. This caused oscillations in biomass yield and composition. Starch and total fatty acids (TFA) were accumulated during the day. Also, ...
Reproducing the native collagen structure and glycosaminoglycan (GAG) distribution in tissue-engi... more Reproducing the native collagen structure and glycosaminoglycan (GAG) distribution in tissue-engineered cartilage constructs is still a challenge. Articular cartilage has a specific nutrient supply and mechanical environment due to its location and function in the body. Efforts to simulate this native environment have been reported through the use of bioreactor systems. However, few of these devices take into account the existence of gradients over cartilage as a consequence of the nutrient supply by diffusion. We hypothesized that culturing chondrocytes in an environment, in which gradients of nutrients can be mimicked, would induce zonal differentiation. Indeed, we show that glucose gradients facilitating a concentration distribution as low as physiological glucose levels enhanced a zonal chondrogenic capacity similar to the one found in native cartilage. Furthermore, we found that the glucose consumption rates of cultured chondrocytes were higher under physiological glucose conce...
Everyone who has ever tried to radically change metabolic fluxes knows that it is often harder to... more Everyone who has ever tried to radically change metabolic fluxes knows that it is often harder to determine which enzymes have to be modified than it is to actually implement these changes. In the more traditional genetic engineering approaches ’bottle-necks’ are pinpointed using qualitative, intuitive approaches, but the alleviation of suspected ’rate-limiting’ steps has not often been successful. Here the
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Papers by Dirk Martens