Nature | Photosynthesis improves cell anabolism in mammalian cells
Key substances for cell anabolism
Insufficient intracellular anabolism (manifested by intracellular ATP and NADPH deficiency) is a crucial factor involved in many pathological processes in the body. The anabolism of intracellular substances requires the consumption of sufficient ATP and relies on NADPH (reduced coenzyme II) to provide a key electron donor for reducing energy for anabolism.
The tricarboxylic acid (TCA) cycle is the major energy metabolic process for ATP generation in most mammalian cells. However, interventions that target the TCA cycle can not rectify the dysregulated supply of ATP in pathological conditions.?The TCA cycle involves various metabolic networks, and the delivery of a specific factor that changes its intrinsic pathway may even cause cell death. In addition, the direct provision of exogenous ATP has little effect on cellular metabolism.
NADPH can provide reducing power for synthesis reactions and redox Balance. Cellular NADPH levels are regulated through the production and utilization of several metabolic pathways (that is, the pentose phosphate pathway, fatty acid oxidation, and glutamine metabolism), and direct interventions that target these pathways may lead to cellular metabolic imbalance. Moreover, #NADPH is expensive, and an uncontrolled supply of NADPH can cause the production of cytotoxic superoxide, which in turn can result in oxidative stress.
Briefly, under pathological conditions, it is difficult to increase deficient ATP and NADPH levels to optimal concentrations. Therefore, building a controllable and independent self-supply system of ATP and NADPH is particularly important for enhancing cell anabolism.
Plant-derived natural photosynthetic system based on NTUs
This year in December, Nature published a research paper entitled?A plant-derived natural photosynthetic system for improving cell anabolism. The study developed an independent and controllable plant-derived natural photosynthetic system based on nano thylakoid units (NTUs).
Nano-encapsulating NTUs with a chondrocyte-derived membrane (CM) produced CM-NTUs which were then delivered into chondrocytes. CM-NTUs in situ increased intracellular ATP and NADPH levels and improved anabolism of degenerated chondrocytes after light exposure. They also systemically corrected energy imbalances, improved cartilage homeostasis, and prevented the pathological progression of osteoarthritis in mice. This natural photosynthetic system has successfully achieved cross-species applications, can effectively enhance cell anabolism, and has shown encouraging clinical potential in the treatment of degenerative diseases.
? NTUs produce ATP and NADPH
The authors analyzed NTUs first. Proteomics results showed that the NTUs retained all the protein components required for photosynthesis on the surface of the thylakoid membrane (Fig 1a). The gene ontology (GO) biological process analysis suggested that NTUs were able to catalyze the production of ATP from ADP after exposure to light, and catalyze the light-dependent reduction of NADP+ to NADPH (Fig 1b). To verify the above results, the authors measured the abundance of D1 and D2 proteins in isolated NTUs over time. As shown in Figure 1c, D1 and D2 proteins were completely degraded under light (within 8-16 h) and almost completely degraded under dark conditions (within 5-7 days).
Tips: D1 and D2 proteins are the core subunit proteins of PSII, an important complex for plant photosynthesis, and are capable of photosynthesis.
The authors then measured the changes in the ATP production capacity of the NTUs over time. The capacity of NTUs to produce ATP decreased significantly after 16 h of light exposure or after 7 days of storage in the dark (Fig 1d-e). Overall, the change in the capacity of NTUs to produce ATP under light and dark conditions was consistent with the change in protein degradation levels.
? How to apply #NTUs across species?
Although NTUs can produce ATP and NADPH, how to effectively avoid in vivo clearance and immune rejection of mammalian cells? The authors suggest that using specific mature cell membranes as camouflage may be an effective strategy to implant photosynthetic systems and avoid cross-species elimination.
Therefore, this study used a chondrocyte membrane (CM) to encapsulate NTUs to prepare #CM-NTUs. The results showed that these CM-NTUs could enter chondrocytes through membrane fusion, avoid lysosomal degradation and achieve rapid penetration (Fig 2).
Tip: Osteoarthritis is a common degenerative disease in which joint cartilage is destroyed due to an imbalance of energy metabolism in chondrocytes. Pathological chondrocytes exhibit ATP and NADPH depletion, as well as increased production of reactive oxygen species (ROS) and proteins associated with extracellular matrix (ECM) degradation. Current biological treatments for osteoarthritis cannot systematically correct the metabolic imbalance of damaged and degenerated chondrocytes.
? CM-NTUs improve cell anabolism
The research team first co-incubated IL-1β-treated chondrocytes (which induce a metabolic disturbance in mouse chondrocytes) with CM-NTUs in illuminated and non-illuminated cells to track changes in cellular ATP and NADPH over time. Then, adjusted the light intensity, the irradiation time of the light, and the encapsulated ferredoxin (FDX) concentration in the CM-NTUs to optimize the experimental conditions.
The results showed that CM-NTUs exposed to red light (80 μmol photons m-2 s-1) irradiation for 30 min and with 25 μM encapsulated FDX (diluted to about 1.2 μM after delivery into cells) were the best experimental conditions, which were selected for subsequent experiments. Under these conditions, CM-NTUs restored intracellular ATP and NADPH levels close to those noted in control chondrocytes (Fig 3a-c).
Tip: As a common electron carrier, ferredoxin participates in important metabolic processes such as respiration, photosynthesis, and fermentation.
Measurement of changes in ATP and NADPH levels over time in illuminated and non-illuminated cells to clarify the functional lifetime of the NTUs in cells (Fig 4).
ATP and NADPH levels in the illuminated cells gradually increased, peaked at 1-2 h, and then plateaued owing to the depletion of intracellular ADP and NADP+?pools. After 8 h, the ATP and NADPH levels began to decrease. By 32 h, ATP and NADPH levels were similar to those observed in non-illuminated cells. In non-illuminated cells, CM-NTUs had no effect on cellular ATP levels.
The findings also showed that intracellular ROS levels were decreased in degenerative chondrocytes containing NTUs and irradiated with light.
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To explore the effect of NTUs on other degenerative diseases, the study used various membrane-coated NTUs and cultured them with corresponding cells (muscle satellite cells, nucleus pulposus cells, human umbilical vein endothelial cells).
The results showed that ATP and NADPH concentrations after light exposure were increased 3.17-3.78 and 1.37-1.40 fold than those of unirradiated ATP and NADPH, respectively (Fig 5). That is, after light exposure, both ATP and NADPH levels in the above cells were enhanced. In other words, NTUs encapsulating mature mammalian membranes enhanced cell anabolism (not limited to chondrocytes) after exposure to light.
In a nutshell, NTUs can effectively improve cell anabolism through the natural photosynthetic system.
To comprehensively determine the changes in cell anabolism, the study performed transcriptomics analysis of chondrocytes exposed to light. The authors compared gene expression patterns between the IL-1β plus CM-NTU group and the IL-1β group. The results showed that the IL-1β plus CM-NTU group exhibited upregulated expression of genes involved in the TCA cycle and oxidative phosphorylation and downregulated expression of genes involved in glycolysis and ECM degradation. (Fig. 6a).
In other words, CM-NTU reprogrammed cell metabolism and systematically corrected imbalances in energy (glycolysis, TCA cycle, and oxidative phosphorylation) and substance (collagen and glycosaminoglycan) metabolism in degenerated chondrocytes (Fig. 6b).
Finally, the study investigated whether intra-articular injection of the CM-NTUs and subsequent exposure to light irradiation can inhibit the progression of osteoarthritis induced by anterior cruciate ligament transection (ACLT) surgery in mice.
The results showed that CM-NTUs increased ATP and NADPH levels in chondrocytes in situ after light exposure, resulting in increased collagen (Col II) and aggrecan content, and effectively inhibited subchondral bone formation at 12 weeks postoperatively (Fig. 7). In short, CM-NTUs can promote cartilage homeostasis and prevent the development of osteoarthritis in animals.
Summary
The study constructed a completely natural photosynthesis system that can independently facilitate the supply of key energy and metabolic carriers in cells based on exposure to light. Most importantly, using a membrane-coating strategy, they demonstrated the feasibility and applicability of cross-species transplantation of a plant-derived natural photosynthetic system, which lays a solid foundation for the treatment of degenerative diseases.
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