Cell Signaling III and Stem Cell Lineages

Multi-Domain Adapter Proteins and RTK Phosphorylation Specificity\n\n* Receptor Tyrosine Kinase (RTK) Activation: Upon hormone or growth factor binding, two RTK monomers dimerize, leading to transphosphorylation. This creates multiple phosphorylated tyrosine residues on the cytosolic domains.\n* Phosphotyrosine Recognition Domains: Two primary protein domains bind to phosphorylated tyrosines, each with specific requirements for surrounding amino acids:\n * SH2 Domain (Sarc Homology 2): Binding specificity is determined by the amino acids located on the $C-terminal$ side of the bound tyrosine.\n * PTB Domain (Phosphotyrosine Binding): Binding specificity is derived from amino acids on the $N-terminal$ side of the bound tyrosine.\n* IRS-1 (Insulin Receptor Substrate 1): This is a specialized multi-docking adapter protein. It uses a PTB domain to bind the RTK and carries multiple internal phosphorylated amino acids. These act as additional docking sites, drastically increasing the number of signaling proteins an RTK can recruit and activate.\n\n# The RAS-MAP Kinase Pathway Strategy\n\n* RAS Activation Mechanism: RAS is a monomeric GTPase bound to the membrane by a lipid anchor. In its inactive state, it is bound to $GDP$. Activation involves several steps:\n 1. FGF Binding: Fibroblast Growth Factor (FGF) binds to its receptor (FGFR), causing dimerization and transphosphorylation.\n 2. GRB2 Adapter Protein: GRB2 contains one SH2 domain (binds p-Tyr on the receptor) and two SH3 domains (bind proline-rich regions). It acts as a bridge.\n 3. SOS (Son of Sevenless): SOS is a $GEF$ (Guanine nucleotide Exchange Factor) for RAS. It is recruited to the receptor via GRB2 and promotes the exchange of $GDP$ for $GTP$ on RAS.\n 4. RAS-GTP: Active RAS-GTP subsequently triggers the MAP kinase cascade.\n* MAP Kinase Definition: MAPK stands for Mitogen-Activated Protein Kinase, where a mitogen is a factor that stimulates cell growth.\n* Kinase Cascade Hierarchy:\n * MAPKKK (RAF): A serine/threonine kinase activated by RAS-GTP.\n * MAPKK (MEK): A dual-specificity kinase that phosphorylates both serine/threonine and tyrosine residues.\n * MAPK (ERK): The downstream kinase that phosphorylates various transcription factors.\n\n# Regulation and Structural Activation of the Kinase Cascade\n\n* RAF (MAPKKK) Regulation: Inactive RAF is held in the cytosol by an inhibitory protein called $14-3-3$, which binds at two phosphorylated sites (one on the $N-terminal$ regulatory domain and one on the $C-terminal$ kinase domain). RAS-GTP binding causes a conformational change that releases $14-3-3$ and allows phosphorylation by other kinases like $SRC$. This results in active RAF releasing from the membrane after $GTP$ hydrolysis by RAS.\n* Activation Lip Dynamics: Kinases like MEK and MAPK (ERK) contain an activation lip. Phosphorylation of this lip causes a major conformational change (approximately a 90^\\circ shift) that opens the catalytic site.\n * ERK Specificity: Phosphorylated by MEK at two specific residues: $Tyr-185$ and $Thr-183$.\n * RAF Exception: While RAF has an activation lip that is phosphorylated, it does not undergo the same dramatic conformational change seen in MEK or MAPK.\n\n# Regulation of Gene Expression by MAP Kinases\n\n* Nuclear Translocation: Activated MAPK (ERK) forms a dimer and moves into the nucleus to regulate gene expression via direct and indirect mechanisms.\n* Direct Pathway: MAPK dimer phosphorylates the transcription factor $TCF$. Phosphorylated $TCF$ binds to the $SRE$ (Serum Response Element) in the promoter of the $c-Fos$ gene.\n* Indirect Pathway: MAPK phosphorylates $p90RSK$, which then translocates to the nucleus to phosphorylate $SRF$ (Serum Response Factor). A dimer of phosphorylated $SRF$ binds to the same $SRE$ element.\n* Early Response Genes: Genes like $c-Fos$ are induced immediately after growth factor stimulation. The $c-Fos$ protein associates with $c-Jun$ to form the $AP1$ (Activator Protein 1) transcription factor, which drives the expression of cell cycle progression genes (e.g., transitioning from $G_1$ to $S$ phase).\n\n# Scaffold Proteins and Pathway Isolation\n\n* Function: Large scaffold proteins bind multiple kinases in a specific sequence to increase signaling efficiency and prevent crosstalk (interference) between different pathways.\n* Yeast Examples:\n * Mating Pathway: Uses the $Ste5$ scaffold, which binds $Ste11$ (MAPKKK), $Ste7$ (MAPKK), and $Fus3$ (MAPK).\n * Osmoregulation Pathway: Uses the $Pbs2$ scaffold, which binds $Ste11$ and the MAPK $Hog1$. $Pbs2$ itself has MEK activity.\n\n# PI3 Kinase and the Phosphoinositide Pathway\n\n* Phospholipid Synthesis: PI (Phosphatidylinositol) is phosphorylated by $PI4\\text{ kinase}$ to form PIP, and then by $PIP5\\text{ kinase}$ to form $PIP_2$ (PI-4,5-bisphosphate).\n* PI3 Kinase (PI3K): Recruited to RTKs via an SH2 domain. It phosphorylates the 3-position of PIP or $PIP_2$ to create $PI(3,4)P_2$ or $PI(3,4,5)P_3$.\n* PKB (AKT) Activation: \n * Inactive PKB is recruited from the cytosol to the membrane via its $PH\\text{ domain}$ (Pleckstrin Homology), which binds to the 3-phosphate of the newly formed phospholipids.\n * Once at the membrane, PKB is phosphorylated by $PDK1$ (on the activation lip) and $PDK2$ (on the catalytic domain) to become fully active.\n* Biological Importance: This is a pro-survival pathway. PKB inactivates proteins that induce apoptosis (programmed cell death).\n* PTEN Phosphatase: PTEN acts as a tumor suppressor by removing the 3-phosphate from the phospholipids, turning off the pathway. Inactivation or deletion of $PTEN$ is one of the most common mutations in human cancers, leading to constitutive PKB activation and enhanced tumor survival.\n\n# TGF-$\beta$ and Wnt Signaling Pathways\n\n* TGF-$\beta$ Signaling: Unlike RTKs, TGF-$\beta$ receptors are serine/threonine kinases. \n * Mechanism: TGF-$\beta$ binds to types $RIII$ and $RII$. $RII$ phosphorylates $RI$, which then phosphorylates the $Smad2/3$ transcription factor. \n * Translocation: Phosphorylated $Smad2/3$ binds $Smad4$ and $Importin\\, \\beta$ (via a nuclear localization signal) to enter the nucleus. There, they complex with $TFE3$ to activate genes for cell differentiation.\n* Wnt Pathway (Protein Dissociation): In the absence of Wnt, $\\beta\\text{-catenin}$ is targeted for degradation by a complex containing $Axin$, $APC$, $GSK3$, and $CK1$. $TrCP$ (ubiquitin ligase) polyubiquitinates it for proteasomal destruction.\n * Active State: Wnt binds $Frizzled$ and the $LRP$ co-receptor. This recruits $Dishevelled$ and causes the degradation complex to fall apart. Stable $\\beta\\text{-catenin}$ enters the nucleus, displaces the repressor $Groucho$, and acts as a co-activator for $TCF$ to turn on gene expression.\n\n# Principles of Cell Lineages and Stem Cells\n\n* Cell Lineage: The series of cell divisions from a progenitor to a specialized cell. As cells move down a lineage, their potential becomes more restricted. Fully differentiated cells permanently exit the cell cycle.\n* Stem Cell Properties: Undifferentiated cells capable of self-renewal (reproducing indefinitely) and generating specialized daughter cells.\n* Potency Levels:\n * Totipotent: Can generate all cell types, including extraembryonic tissues (found in embryos up to the $8\\text{-cell}$ stage).\n * Pluripotent: Can generate all embryonic cell types but not extraembryonic tissues (e.g., Embryonic Stem Cells from the Inner Cell Mass of a blastocyst).\n * Multipotent: Can generate multiple related cell types (e.g., Hematopoietic Stem Cells).\n* iPS Cells (Induced Pluripotent Stem Cells): Created by transfecting adult fibroblasts with Yamanaka factors (transcription factors expressed in ESCs). This avoids ethical issues but carries risks of insertional mutagenesis and tumor formation.\n\n# Germ Layers and Tissue Specification\n\n* Gastrulation: Process where three germ layers are formed:\n * Ectoderm: Forms the skin and nervous system (and insect cuticle).\n * Mesoderm: Forms the skeleton, muscles, heart, blood, and kidneys.\n * Endoderm: Forms the gut, liver, and lungs.\n* Hematopoietic Stem Cells (HSC): Found in bone marrow niche. These multipotent cells divide asymmetrically to produce:\n * Lymphoid Lineage: Gives rise to $B$ cells (antibodies) and $T$ cells.\n * Myeloid Lineage: Gives rise to all other blood cells (erythrocytes, monocytes, granulocytes, megakaryocytes).\n * Regulation: Cytokines control the specific differentiation paths.\n\n# Intestinal Epithelium and Asymmetric Division\n\n* Villus Development: Intestinal stem cells are located at the base. They produce cells that move through a proliferation zone and into a differentiation zone to form the villus. Movement can be tracked using radioactive thymidine in pulse-chase experiments or genetic pulse-labeling with $\\beta\\text{-galactosidase}$.\n* Asymmetric Cell Division: Driven by the asymmetric distribution of internal determinants (mRNA/proteins) or external signals from the stem cell niche.\n* Neural Generation: In the ectoderm, a neuroblast divides asymmetrically to produce another neuroblast and a Ganglion Mother Cell (GMC). The GMC further divides into neurons and glia. Proteins like $Miranda$ are used as markers to visualize these distinct daughter cells during neurogenesis.