Pathways

The nuclear entry of clock gene products is required to establish the negative feed back loop- a key step in proper circadian rhythm. Heterodimerization of clock proteins PER and CRY is required for translocation to the nucleus. Nuclear entry of PER can be regulated by mammalian casein kinase I (CK1). Once in the nucleus, PER and CRY proteins appear to differentially inhibit the transactivation by CLOCK/BMAL1. Proteins like REV-ERBa and DEC also regulate the transcriptional activation by CLOCK/BMAL1. In addition, CRY, PER, and BMAL1-CLOCK play bidirectional roles in transcription resulting in interactivating feedback loops. The expression of BMAL1 and CLOCK can be upregulated by CRY and PER. Such loops are believed to be important in the stability and persistence of circadian rhythm.

Cytokinins (CK) are a class of plant growth substances (phytohormones) that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence. Zeatin is an adenine-type cytokinin . The synthesis of cis-type cytokinins is carried out in the cytosol through prenylation of tRNA molecules and dimethylallyl diphosphate precursors provided through the mevalonate pathway. Hormonal homeostasis is thought to be maintained by glucosylation that inactivates cytokinins. N-glucosylation is irreversible and is hypothesized to be involved with detoxification. Only the enzymes that catalyze the first and last steps of this pathway have been elucidated (all predicted enzymes are coloured orange in the image). First, tRNA dimethylallyltransferase transfers a dimethylallyl group onto the adenine at position 37 in tRNAs that have uridine-starting codons. It requires a magnesium ion as a cofactor. Second, the predicted enzyme cis-Hydroxy-prenyl-tRNA catalyzes a reaction whereby N6-dimethylallyladenosine37 in tRNA is converted into cis-hydroxy-prenyl-tRNA. Third, the predicted enzyme cZRMP synthase catalyzes a reaction whereby cis-hydroxy-prenyl-tRNA is converted into cis-zeatin riboside monophosphate (cZRMP). Fourth, the predicted enzyme cis-Zeatin riboside synthase catalyzes a reaction whereby cis-zeatin riboside monophosphate (cZRMP) is converted into cis-zeatin riboside. Fifth, the predicted enzyme cis-Zeatin synthase catalyzes whereby cis-zeatin riboside is converted into cis-zeatin. Sixth, cytokinin UDP glycosyltransferase catalyzes the two different reactions whereby cis-zeatin is converted into either cis-zeatin-9-N-glucoside or cis-zeatin-7-N-glucoside.

Cytokinins (CK) are a class of plant growth substances (phytohormones) that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence. Zeatin is an adenine-type cytokinin . The synthesis of cis-type cytokinins is carried out in the cytosol through prenylation of tRNA molecules and dimethylallyl diphosphate precursors provided through the mevalonate pathway. Hormonal homeostasis is thought to be maintained by glucosylation that inactivates cytokinins. O-glucosylation is reversible and is hypothesized to be involved with transport, storage, and protection against zeatin oxidases. Only the enzymes that catalyze the first and last steps of this pathway have been elucidated (all predicted enzymes are coloured orange in the image). First, tRNA dimethylallyltransferase transfers a dimethylallyl group onto the adenine at position 37 in tRNAs that have uridine-starting codons. It requires a magnesium ion as a cofactor. Second, the predicted enzyme cis-Hydroxy-prenyl-tRNA catalyzes a reaction whereby N6-dimethylallyladenosine37 in tRNA is converted into cis-hydroxy-prenyl-tRNA. Third, the predicted enzyme cZRMP synthase catalyzes a reaction whereby cis-hydroxy-prenyl-tRNA is converted into cis-zeatin riboside monophosphate (cZRMP). Fourth, the predicted enzyme cis-Zeatin riboside synthase catalyzes a reaction whereby cis-zeatin riboside monophosphate (cZRMP) is converted into cis-zeatin riboside. Fifth, the predicted enzyme cis-Zeatin synthase catalyzes whereby cis-zeatin riboside is converted into cis-zeatin. Sixth, UDP glucose:cytokinin glycosyltransferase catalyzes a reaction whereby cis-zeatin is converted into cis-zeatin-O-glucoside.

Cisplatin is a chemotherapeutic alkylating agent that causes intrastand crosslinks that is more difficult to repair than regular DNA. Cisplatin diffuses into the cell either through the membrane or through metal transporter like CTR1. In the cytosol, the low concentration of chloride relative to the extracellular space causes hydrolysis of cisplatin into its aquo complex which goes into the nucleus and binds to DNA bases. It preferentially binds to guanine but can also bind to adenine bases. It's used for treated testicular cancer, ovarian cancer and other solid tumors and it targets all parts of the cell cycle (cell-cycle non specific). Most patients will eventually develop resistance to cisplatin though as cancer cells will decrease uptake or pump it out of the cell through exocytotic vesicles. Patients will has have increased production of cellular thiols like GSH which inactivates the aquated cisplatin by forming a complex with it. Cisplatin is administered intravenously usually to the site of the tumor. Cisplatin can cause nephrotoxicity due to ROS generation as well as nausea and vomiting.

Citalopram is a selective serotonin reuptake inhibitor that exerts antidepressive effects by selectively inhibiting serotonin reuptake in the brain. It does so by competing for the same binding site as serotonin on the the sodium-dependent serotonin transporter (SLC6A4). This increases the concentrations of serotonin in the synaptic cleft and reverses the state of low concentration seen in depression. Higher concentration of serotonin has also been shown to have long-term neuromodulatory effects. Binding of serotonin to certain serotonin receptors activate adenylate cyclase, which produces cAMP. cAMP activates protein kinase A which activates cAMP-responsive binding protein 1 (CREB-1). CREB-1 enters the nucleus and affects transcription of brain-derived neurotrophic factor (BDNF). BDNF subsequently stimulates neurogenesis, which may contribute to the long-term reversal of depression.

Citalopram is a selective serotonin reuptake inhibitor used to manage depression, anxiety. Off-label indications include but are not limited to: treatment of sexual dysfunction, post-stroke behavioral changes, ethanol abuse, obsessive-compulsive disorder (OCD) in children, and diabetic neuropathy. Citalopram binds selectively to the sodium dependent serotonin transporter and blocks the recycling of serotonin from the synapse. As serotonin accumulates it restores the serotonergic function of the 5-hydroxytryptamine 1A receptor leading to decreased anxiety and depressive moods. Citalopram is usually taken orally. The onset of action for depression is about 2 to 4 weeks. However, the full response may take up 8 to 12 weeks.