Calcium- mobilizing Receptors

OlehEsthi Candra

• Kadar Ca2+ intraseluller dipengaruhi oleh: Hidrolisis Phosphatydilinositol 4,5-biphosphat (PIP2)→ inositol 1,4,5-triphosphate (IP3) dan 1,2- diacylglycerol (DG)- DG memicu pelepasan: as. Arachidonat (prekusor prostaglandin),

tromboksan,dan leukotrien- Bukti eksperimen menunjukkan bahwa IP3 dan DG secara sinergis

memvasilitasi transduksi sinyal pada sistem reseptor yang berhubungan dengan mobilisasi Ca2+

- IP3 berperan dalam pelepasan Ca2+ dan DG dari intraseluller seperti RE dan DG yg mengaktifkan phospholipid sensitive kinase c

- Pada berbagai macam tipe sel, IP3-Ca2+ calmodulin dan sistem DG berinteraksi secara signifikan untuk menghasilkan respon fisiologis seperti metabolisme, sekresi, kontraksi otot, dan pertumbuhan sel.

a. Agonist- stimulated Turnover of PI Ach + eserine meningkatkan pembentukan Pi menjadi PA dan PI dan efek stimulasi dari neurotransmitter diblok oleh cholinergic muscarinic antagonist, atropin (tp bukan oleh d-tubocurarine), a cholinergic nicotinic antagonist.Aksi agonist pada metabolisme PI dengan cara stimulasi pemevahannya bukan sintesis PIPPI yang langsung berperan dalam Ca2+ mobilizing receptors (bukan PI)

B. Agonist stimulated breakdown of PPI

• Ach menstimulasi pemecahan PIP2 yg terjadi dalam waktu yg singkat

• Pemecahan PIP2 mengaktifkan muscarinic cholinergik

• Agonist-stimulated PIP2 breakdown dan agonist yg memicu kontraksi otot yg diinervasi saraf simpatik= diatur oleh tipe reseptor yang sama

Distribusi Polyphosphoinositides

• Phosphoinositide level pada otak tikus dan otot polos iris pada kelinci terdiri atas 3 dan 4,9% dari total phospolipid dari otak tikus dan otot iris.

• PPI terkumpul di cytoplasmic leaflet pada membran plasma→butuh diteliti lagi

• PPI byk ditemukan pada selubung myelin dan membran eritrosit

Pathways of phosphoinositide synthesis and degradation

• The phosphatidylinositol (PI) includes inositol 1-phosphate bound via its phosphate group to 1-stearoyl,2-arachidoyl diacylglycerol, prevalent in mammal cells favouring exposure of the inositol ring and its interaction with PIBMs.The 7 known PIs in eukaryotes are PI4P,PI5P,PI3P,PI45P2, PI35P2, PI34P2 and PI345P3. Each PI is indicated according to colour codes. Blue and arrows indicate routes of PI phosphorylation and dephosphorylation, respectively. Unlike phosphoinositides, the soluble inositol phosphates (IPs) can be phosphorylated in all of the six positions,giving rise to more than 60 soluble species.This is because other IP-specific enzymes are present in the cell as well as the kinases/phosphatases acting on the phosphoinositides and IPs.The PIs can be hydrolysed by PLC to generate inositol 1,4,5-trisphosphate and diacylglycerol from PI45P2; by PLA2 to LPIs; PLA/lysophospholipases (LPLA1) to form the GPIs;and by PLD to form phosphatidic acid. PLC acts preferentially on PI45P2, whereas the other phospholipases may act on the different PIs (for simplicity in the figure,all the phospholipases are shown acting only on PI45P2).

Enzim2 pada siklus Polyphosphoinositide

• Phosphoinositide kinase• Phospholipase C (PPI phosphodiesterase)• Myo-inositol polyphosphates

phosphomonoesterase

Regulation of Calcium Ion in The cell

• Calcium mengatur bermacam- macam fungsi sel, terutama pada sel yang berfungsi untuk kontraksi dan sekresi.

• ER merupakan sumber Ca yang dilepaskan ke sitosol dengan kondisi tertentu yang menstimulasi.

• Tingginya konsentrasi Ca2+ dalam sel bersifat toksik,untuk kadarnya di atur oleh mekanisme;

- Ca2+ dependent plasma membrane ATPase pump - Mekanisme pertukaran Na+/Ca2+ plasma membran - Pompa Ca2+ mitokondria - ATP-dependent pompa Ca2+ pada RE

Hormones that act throughphospholipid breakdown

• Epinephrine and norepinephrine (α-adrenergic receptors)

• Acetylcholine (muscarinic receptor)• TNFα

• PIP2 breakdown• Second messenger system for Gq family of G proteins• Gq activates phospholipase C (PLC)• Phospholipase C (PLC) is membrane bound enzyme• PLC β breaks phoshatidylinositol 4,5 bisphosphate (PIP2)

to IP3 and DAG• Both IP3 and DAG are second messengers• IP3 increases intracellular calcium levels via the release

from intracellular stores• DAG activates protein kinase C (PKC)

IP3 induces the release of Ca2+ fromthe ER

IP3 reseptor

• Responds to IP3 by release of Ca2+ from endoplasmic reticulum• Responds rapidly• Local feedback loop• Positive for low calcium• Negative for high• This gives a self-limited Ca2+ pulses

The effect of IP3 is to stimulate Ca++ release from intracellular stores in the SER and mitochondria. The released Ca++ has a variety of effects

one of which is the stimulation of protein kinase C.

Calcium as a second messenger

• Ca2+ versatile second messenger• All eukaryotes use Ca2+ signaling• Regulates many processes• Synaptic transmission• Levels controlled by release and removal• Two forms of Ca2+ release channels• IP3 receptors• Ryanodine receptors• Removal by pumps and calcium sequestering proteins

Targets• Direct effects• Cell motility• Contraction of muscle cells• Secretion• Activation of regulatory enzymes• Ca2+ activated K+ channels• Effects through calmodulin• Activation of phosphorylase kinase• Activation of cAMP phosphodiesterase• CaM kinase II

Calcium acts as a second messengerin guard cell closure

Lipid derived second messengers forintercellular signaling

• Eicosanoids• Arachidonic acid derivatives• Prostaglandins• Thromboxanes• Leukotrienes• Lipoxins

Ca+2 systems• Calcium is a very important regulator of cell function. It is involved in the control

of a large number of motile/fusion process and can act as a second messenger as we have seen already (IP3/DAG system). A variety of cellular proteins act to regulate intracellular Ca+2 levels to include gated Ca+2 channels and Ca+2 Pumps.

• We have already encountered several gated Ca+2 channels. These include the voltage-gated Ca+2 channel in the synaptic bulb, which allows for Ca+2 entry which in turn leads to vesicle fusion and neurotransmitter release into the synaptic cleft, and the ligand-gated Ca+2 channels of the SER and mitochondria which recognize IP3. In addition there are a variety of 'external' ligand-gated channels which respond to neurotransmitters, and a special channel, the ryanodine channel, which is triggered by Ca+2. The significance of the ryanodine channel is that it allows small changes in Ca+2 concentration to be magnified by release on 'large' intracellular stores.

• There are two main types of Ca+2 pumps. Those that utilize a Na+ gradient to move Ca+2 and those that use ATP directly.