Phosphatidylinositol Biphosphate

Pathway

Phosphatidylinositol Biphosphate

Pathway

Ghimire, PritiSalumbre, RenzSurquia, Joseph

Ghimire, PritiSalumbre, RenzSurquia, Joseph

Cellular and Molecular BiologyCellular and Molecular Biology

Inositol Phospholipids

Discovered by Hokin et al in the 50s

Also known as phosphatidylinositol (PI)

First hypothesized as a key player in hormone action

Accepted as a second messenger after 30 years of study

Inositol Phospholipids

It is the substrate for a large number of enzymes which are involved in cell signaling because it can be phosphorylated by a variety of kinases on the hydroxyl groups 3, 4 and 5 on the inositol ring in seven different combinations.

Different IPs involved in Cell Signaling

Phosphatidyl Inositol Biphosphate

PIP2

Located at the inner half of the lipid bilayer

A precursor in the production of inositol trisphosphate (IP3 or InsP3) and

diacylglycerol (DAG)

It is hydrolyzed by phospholipase C (PLC) to produce such secondary messengers

Only activated when the stimulatory G protein responded a certain stimulatory signal.

Typical PIP2 Pathway

The Role of Ca++ as Intracellular Messenger

Calcium IonsSignificant Role in various cellular activities

muscle contraction

cell division

secretion

fertilization

synaptic transmission

metabolism

transcription

cell movement

cell death

The Role of Ca++ as Intracellular Messenger

ExtracellularMessage

CellSurface

high Ca++Concentration

The Role of Ca++ as Intracellular Messenger

Calcium ion concentration regulated by the activity of calcium pumps and ion channels located within the membranes that surround the compartment

Calcium concentration of a resting cell is maintained at low levels

Whereas, within the lumen or in the extracellular space, calcium concentration is higher than that of the cytosol

Abnormal elevation of calcium ions can occur in brain cells and lead to stroke and eventually death

IP3 and Voltage-Gated Ca++ Channels

Extracellular messenger molecule + Extracellular messenger molecule + GPCRGPCR

Activation of Phospholipase C-β Activation of Phospholipase C-β

phosphoinositide PIPphosphoinositide PIP22 is split is split

IPIP33 molecule released molecule released

Calcium channels in the ER openCalcium channels in the ER open

increase in cytosolic [Caincrease in cytosolic [Ca++++]]

IP3 and Voltage-Gated Ca++ Channels

RTK activate members of the phospholipase C-ϒ subfamily

possess SH2 domain that allows them to bind to activated and phosphorylated RTK

4 PLC -carry out the same reaction producing IP3 and linking cell surface receptors to an increase in cytoplasmic calcium ions

PLC-ϒ -activated by calcium ions

PLC-ε -activated by Ras-GTP

IP3 and Voltage-Gated Ca++ Channels

nerve impulsenerve impulse

depolarization of plasma membranedepolarization of plasma membrane

opening of voltage-gated channelsopening of voltage-gated channels

influx of Cainflux of Ca++++ ions ions

Visualizing Ca++

Understanding the role of Ca++ ions was acheived through the development of indicator molecules that emit light in the presence of free calcium

Fluorescence microscopy and computerized-imaging techniques

Calcium-mediated responses are readily identifiable

Types of Ca++ ion channels in the ER membrane

Two main types

IP3 receptors

Ryanodine receptors (RyRs)

bind toxic plant alkaloid ryanodine

Ryanodine Receptors

Found in excitable cells

In skeletal and cardiac muscle cells, they mediate the rise in Ca++ levels after an arrival of APs

Can be opened by many agents

Calcium-Induced Calcium Release (CICR)

Calcium Wave Induced by Fertilizing Sperm

Propagated wave of calcium release spreads through the entire cytoplasmic compartment

Induced by the sperms contact with the egg’s plasma membrane

Sudden rise in calcium concentration following fertilization triggers a number of events

activation of cyclin-dependent kinases

Calcium waves are transient

ions are rapidly pumped out of the cytosol and back into the ER or extracellular space

Ca++ Binding ProteinsCalcium affects a number of different types of cellular effectors

Calcium ions can activate or inhibit enzymes and transport system

Calcium can also change the ionic permeability of membranes

Calcium can also induce membrane fusion or alter cytoskeletal structure and function

Ca++ Binding Proteins

Calcium-Binding Proteins help calcium ions achieve its various functions

Calmodulin

Best studied example

Universal

Same amino acid sequence from one end of the eukaryotic spectrum to the other

A molecule contains 4 binding sites for calcium

No sufficient affinity for calcium ions to bind in a nonstimulated cell

Calcium-Calmodulin Complex

May bind to a protein kinase, a cyclic nucleotide phosphodiesterase, ion channel, or the calcium-transport system of the plasma membrane

Also stimulate gene transcriptions through activation of various protein kinases that phosphorylate transcription factors

Signal Transducers and Signal Transducers and Activators of Transcription Activators of Transcription

(STATs)(STATs)STAT proteins comprise a family of transcription STAT proteins comprise a family of transcription factors that become activated by tyrosine kinases factors that become activated by tyrosine kinases in the cytoplasm and then migrate to the nucleus in the cytoplasm and then migrate to the nucleus where they directly regulate gene expression.where they directly regulate gene expression.

Structure-Function Relationships in STAT Structure-Function Relationships in STAT ProteinsProteins

Fig 1. — Generic structure of a STAT protein illustrating common functional domain elements shared by STAT family members. The sites of tyrosine (Y) and serine (S) phosphorylation are shown. SH2 = Src-homology 2 domain, N = amino terminus, C = carboxyl terminus.

- a generalized diagram depicting the location of important a generalized diagram depicting the location of important structural motifs common to most STAT family members. structural motifs common to most STAT family members.

- Each STAT molecule contains an Src-homology 2 (SH2) domain Each STAT molecule contains an Src-homology 2 (SH2) domain

- Monomeric, inactive STAT proteins associate with each other to Monomeric, inactive STAT proteins associate with each other to form active dimers through a key phosphotyrosine (pY) residue, form active dimers through a key phosphotyrosine (pY) residue, which binds to the SH2 domain of another STAT monomer which binds to the SH2 domain of another STAT monomer

- Activating event in STAT signaling is tyrosine phosphorylationActivating event in STAT signaling is tyrosine phosphorylation

- N-terminal portion: DNA-binding domainN-terminal portion: DNA-binding domain

- C-terminal portion: transactivation domain -serine residueC-terminal portion: transactivation domain -serine residue

Role of STATs in Normal Signal Role of STATs in Normal Signal

TransductionTransduction

Fig 2. — Signal transduction pathways leading to STAT activation. Stimulation with growth factors or cytokines at the cell surface results in receptor activation and subsequent tyrosine phosphorylation of STATs. Phosphorylation of STATs induces dimerization and translocation to the nucleus, where STAT dimers bind to specific STAT response elements and directly regulate gene expression. In contrast to normal signaling, oncogenic PTKs constitutively activate STATs, leading to deregulated expression of STAT-dependent genes. In some cases, but not all, JAK family tyrosine kinases are known to have a

role in STAT activation.