Pathways

The Krebs cycle, also known as the citric acid cycle (CAC) or tricarboxylic acid cycle (TCA cycle) occurs in the mitochondria, and it involves the oxidation of acetyl-CoA from glycolysis to form molecules of ATP, as well as NADH, which will later be used to form more ATP. Intermediates from the Krebs cycle can be used as inflammatory signals in the body, specifically in immune cells such as macrophages. Succinic acid, or its anion succinate, can leave the mitochondria and can directly inhibit the prolyl 4-hydroxylase subunit alpha-3 protein, which then allows for additional activation of the hypoxia-inducible factor 1-alpha (HF-1α). The higher levels of HF-1α enhance the expression of genes, including those for interleukin-1 beta (IL-1β). Succinic acid is also necessary for the succinylation of proteins, leading to changes in their structure and function. Another intermediate of the Krebs cycle, NAD, activates the NAD-dependent protein deacetylase sirtuin-3, which is involved in the deacetylase of proteins in the cell, regulating ATP levels and promoting mtDNA transcription when needed. Activated sirtuin-3 inhibits NACHT, LRR and PYD domains-containing protein 3, which works to activate the inflammasome, and thus the increase in NAD+ leads to anti-inflammatory actions in the body. Citric acid is another intermediate of the Krebs cycle, and it activates the production of reactive oxygen species, nitric oxide, which is the precursor for reactive nitrogen species, and prostaglandins. Prostaglandins can act as vasodilators, and as such are involved in the inflammation response. Finally, glutamine is important for immune cells to carry out their functions, and when LPS binds to the Toll-like receptor 4 (TLR4) on the cell surface, activating this response, extra L-glutamine can be transported into the cell to fill this need. The L-glutamine can then be converted to oxoglutaric acid, which is important in the Krebs cycle, leading to the effects from its intermediates on the rest of the inflammatory response.

The Krebs cycle, also known as the citric acid cycle (CAC) or tricarboxylic acid cycle (TCA cycle) occurs in the mitochondria, and it involves the oxidation of acetyl-CoA from glycolysis to form molecules of ATP, as well as NADH, which will later be used to form more ATP. Intermediates from the Krebs cycle can be used as inflammatory signals in the body, specifically in immune cells such as macrophages. Succinic acid, or its anion succinate, can leave the mitochondria and can directly inhibit the prolyl 4-hydroxylase subunit alpha-3 protein, which then allows for additional activation of the hypoxia-inducible factor 1-alpha (HF-1α). The higher levels of HF-1α enhance the expression of genes, including those for interleukin-1 beta (IL-1β). Succinic acid is also necessary for the succinylation of proteins, leading to changes in their structure and function. Another intermediate of the Krebs cycle, NAD, activates the NAD-dependent protein deacetylase sirtuin-3, which is involved in the deacetylase of proteins in the cell, regulating ATP levels and promoting mtDNA transcription when needed. Activated sirtuin-3 inhibits NACHT, LRR and PYD domains-containing protein 3, which works to activate the inflammasome, and thus the increase in NAD+ leads to anti-inflammatory actions in the body. Citric acid is another intermediate of the Krebs cycle, and it activates the production of reactive oxygen species, nitric oxide, which is the precursor for reactive nitrogen species, and prostaglandins. Prostaglandins can act as vasodilators, and as such are involved in the inflammation response. Finally, glutamine is important for immune cells to carry out their functions, and when LPS binds to the Toll-like receptor 4 (TLR4) on the cell surface, activating this response, extra L-glutamine can be transported into the cell to fill this need. The L-glutamine can then be converted to oxoglutaric acid, which is important in the Krebs cycle, leading to the effects from its intermediates on the rest of the inflammatory response.

Succinate induces calcium mobilization in an adenylyl cyclase (AC) and protein kinase A (PKA)-dependent manner. Succinate receptor 1 (SUCNR1) engagement activates phospholipase C (PLC), resulting in the cleavage of phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol trisphosphate (IP3) and diacylglycerol (DAG). These second messengers induce calcium mobilization and PKC activation, respectively, and subsequent nitric oxide (NO) and prostaglandin E2 (PGE2) production as well as p38 activation. PKC-dependent phosphorylation of extracellular signal-related kinases ERK1/2 can also drive PG production. SUCNR1 signaling might act in synergy with several inflammatory signaling cascades. PKA is known to phosphorylate and activate the p65 subunit of nuclear factor κB (NF-κB) and cAMP response element-binding protein (CREB). Furthermore, NF-κB, activating protein (AP)-1, nuclear factor of activated T cells (NFAT), and ETS domain-containing protein (Elk-1) are all downstream targets of PKC and MAPKs.

Succinate is the anionic form of succinic acid found in the body, and it is a compound generated as part of the Krebs cycle. Succinate is involved in ATP production as a part of the Krebs cycle, and can also be important in extracellular signaling when it binds to the G-protein coupled receptor GPR91, also known as the succinate receptor 1. When succinic acid binds to the succinate receptor 1 and activates it, an activating signal is sent to 1-phosphatidylinositol 4,5-bisohosphate phosphodiesterase beta-1 (PLCB1). This enzyme cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,3,4-triphosphate (IP3) and diacylglycerol (DG). IP3 then activates an IP3 sensitive calcium channel, increasing the concentration of intracellular calcium. This then activates nitric oxide synthase, producing nitric oxide in the cell. It also activates mitogen-activated protein kinase 11 (MAPK11), which in turn activates the cyclic AMP-responsive element-binding protein 1 (CREB1), which stimulates the transcription of DNA. The increased calcium levels can also activate prostaglandin G/H synthase 1 which synthesizes prostaglandins, hormone-like compounds with vasodilating effects that are present in the inflammatory response. DG also serves as an activator in this pathway, activating protein kinase C alpha type. This activates mitogen-activated protein kinases 1 and 3 (MAPK1 and MAPK3), which then lead to the production of prostaglandin E2, as well as the activation of prostaglandin G/H synthase 1. Protein kinase C alpha type can also activate the NF-kappa-B essential modulator, releasing the nuclear factor NF-kappa-B p105 subunit, which then makes its way into the nucleus, where it stimulates transcription of DNA necessary for the inflammatory response.

Succinate induces calcium mobilization in an adenylyl cyclase (AC) and protein kinase A (PKA)-dependent manner. Succinate receptor 1 (SUCNR1) engagement activates phospholipase C (PLC), resulting in the cleavage of phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol trisphosphate (IP3) and diacylglycerol (DAG). These second messengers induce calcium mobilization and PKC activation, respectively, and subsequent nitric oxide (NO) and prostaglandin E2 (PGE2) production as well as p38 activation. PKC-dependent phosphorylation of extracellular signal-related kinases ERK1/2 can also drive PG production. SUCNR1 signaling might act in synergy with several inflammatory signaling cascades. PKA is known to phosphorylate and activate the p65 subunit of nuclear factor κB (NF-κB) and cAMP response element-binding protein (CREB). Furthermore, NF-κB, activating protein (AP)-1, nuclear factor of activated T cells (NFAT), and ETS domain-containing protein (Elk-1) are all downstream targets of PKC and MAPKs.

Succinate induces calcium mobilization in an adenylyl cyclase (AC) and protein kinase A (PKA)-dependent manner. Succinate receptor 1 (SUCNR1) engagement activates phospholipase C (PLC), resulting in the cleavage of phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol trisphosphate (IP3) and diacylglycerol (DAG). These second messengers induce calcium mobilization and PKC activation, respectively, and subsequent nitric oxide (NO) and prostaglandin E2 (PGE2) production as well as p38 activation. PKC-dependent phosphorylation of extracellular signal-related kinases ERK1/2 can also drive PG production. SUCNR1 signaling might act in synergy with several inflammatory signaling cascades. PKA is known to phosphorylate and activate the p65 subunit of nuclear factor κB (NF-κB) and cAMP response element-binding protein (CREB). Furthermore, NF-κB, activating protein (AP)-1, nuclear factor of activated T cells (NFAT), and ETS domain-containing protein (Elk-1) are all downstream targets of PKC and MAPKs.

Succinic Semialdehyde Dehydrogenase (SSADH) deficiency is a rare autosomal recessive inherited disorder affecting the metabolism of γ-aminobutyric acid (GABA). With reduced GABA activity, oxidation of succinic semialdehyde (SSA) to succinic acid is impaired causing a build up of SSA and ultimately it’s downstream metabolite γ-hydroxybutyric acid (GHB). Symptoms of SSADH deficiency are primarily neuropsychiatric including developmental delays, hypotonia, expressive language impairment, seizures, difficulty coordinating movements (ataxia), decreased reflexes (hyporeflexia), and other behavioral issues. Patients with SSADH deficiency have elevated levels of GHB in urine, however this method is not a definitive diagnosis due to the potential volatilization of acidified urine and the use of GHB as a drug. Instead SSADH can be confirmed suing enzyme analysis in leukocytes and molecular genetic analysis of the Aldh5a1 gene at chromosome 6p22.

Succinic Semialdehyde Dehydrogenase (SSADH) deficiency is a rare autosomal recessive inherited disorder affecting the metabolism of γ-aminobutyric acid (GABA). With reduced GABA activity, oxidation of succinic semialdehyde (SSA) to succinic acid is impaired causing a build up of SSA and ultimately it’s downstream metabolite γ-hydroxybutyric acid (GHB). Symptoms of SSADH deficiency are primarily neuropsychiatric including developmental delays, hypotonia, expressive language impairment, seizures, difficulty coordinating movements (ataxia), decreased reflexes (hyporeflexia), and other behavioral issues. Patients with SSADH deficiency have elevated levels of GHB in urine, however this method is not a definitive diagnosis due to the potential volatilization of acidified urine and the use of GHB as a drug. Instead SSADH can be confirmed suing enzyme analysis in leukocytes and molecular genetic analysis of the Aldh5a1 gene at chromosome 6p22.

Succinic Semialdehyde Dehydrogenase (SSADH) deficiency is a rare autosomal recessive inherited disorder affecting the metabolism of γ-aminobutyric acid (GABA). With reduced GABA activity, oxidation of succinic semialdehyde (SSA) to succinic acid is impaired causing a build up of SSA and ultimately it’s downstream metabolite γ-hydroxybutyric acid (GHB). Symptoms of SSADH deficiency are primarily neuropsychiatric including developmental delays, hypotonia, expressive language impairment, seizures, difficulty coordinating movements (ataxia), decreased reflexes (hyporeflexia), and other behavioral issues. Patients with SSADH deficiency have elevated levels of GHB in urine, however this method is not a definitive diagnosis due to the potential volatilization of acidified urine and the use of GHB as a drug. Instead SSADH can be confirmed suing enzyme analysis in leukocytes and molecular genetic analysis of the Aldh5a1 gene at chromosome 6p22.