Autocrine Signaling

• There are different routes cells take to communicate with each other

Signaling to Themselves

• Autocrine and intracrine signaling are mechanisms where cells signal to themselves

Autocrine Signaling

• A cell produces and secretes a signaling molecule
• Examples of signaling molecules: Cytokine, growth factor, or hormone
• The signaling molecule binds to receptors on the same cell
• Binding leads to a signaling cascade
• Common in immune responses and tumor cell proliferation

Examples of Autocrine Signaling

• Macrophages release IL-1, which binds to their own receptors
• This triggers further cytokine release and amplifies the immune response
• T-helper cells (Th cells) produce IL-2
• IL-2 binds back to IL-2 receptors on the same T cell
• Binding stimulates the T cell's growth and proliferation
• Cancer cells upregulate autocrine growth and survival factors
• Immunology uses autocrine signaling to promote or inhibit cell activation and fine-tune functional responses

Purinergic Signaling

• A cell releases ATP (or other purines) and activates its own purinergic receptors
• Release influences its own function
• Chemotaxis is an example of purinergic signaling
• Neutrophils sense bacterial peptides, like fMLP, through formyl peptide receptors (FPRs)
• This triggers ATP release through Pannexin-1 channels, creating an autocrine ATP loop
• The released ATP binds to P2Y2 receptors on the neutrophil
• Binding amplifies intracellular signaling for gradient sensing
• It allows the neutrophil to detect the strongest chemotactic signal and orient itself in the correct direction

Intracrine Signaling

• The signaling molecule never leaves the cell
• The signaling molecule acts within the cytoplasm or nucleus
• Molecules that signal are typically steroid hormones
• They bind to intracellular receptors and regulate gene expression

Examples of Intracrine Signaling

• Estrogen produced in ovarian cells binds to estrogen receptors inside the same cell, influencing gene transcription
• ROS-related peptides are generated inside the cell and modulate oxidative stress responses
• These peptides influence intracellular signaling pathways like NF-κB, MAPK, and PI3K-Akt
• Affects cell survival, inflammation, and metabolism

Signaling to Neighboring Cells

• Gap junctions, adhesion junctions, and juxtacrine signaling are ways cells communicate with neighboring cells

Juxtacrine Signaling

• Involves direct cell contact mediated by membrane-bound molecules
• The Notch signaling pathway is an example of juxtacrine signaling
• Notch mutations are frequently observed in cancer
• Faulty Notch signaling is observed in diverse cancers, T-cell acute lymphoblastic leukemia, multiple sclerosis, and Tetralogy of Fallot

Notch Receptor Structure

• Notch extracellular domain: binds to ligand
• Notch intracellular domain (NICD)
• Transmembrane component: connects the extra and intra cellular domains

How Notch Signaling Activates

• Initially, all cells express both Notch receptors and ligands
• However, one cell begins expressing slightly more ligand, leading to Notch receptor activation in neighboring cells

Ligand Activation by MIB

• The ligand, such as DLL1, is activated by the Mindbomb (MIB) protein
• MIB is an E3 ubiquitin ligase that ubiquitinates the ligand
• Ubiquitination enables the ligand to bind effectively to the Notch receptor on the receiving cell

Ligand-Receptor Binding

• The activated DLL ligand on the signaling cell binds to the extracellular domain of the Notch receptor on the neighboring cell

Extracellular Domain Cleavage

• The DLL binding to Notch triggers the ADAM protease action
• ADAM protease cleaves the extracellular domain of the Notch receptor, releasing it from the membrane
• This is known as S2 cleavage

Intracellular Domain Release

• After extracellular cleavage, γ-secretase cleaves the Notch Intracellular Domain (NICD) from the remaining transmembrane portion of the receptor, releasing NICD into the cytoplasm
• This is known as S3 cleavage

NICD Binding to CSL and Mam Complex

• In the cytoplasm, the NICD binds to a protein complex containing CSL and Mam (Mastermind-like protein)
• This complex further binds to p300

Nuclear Translocation of the Complex

• The NICD-CSL-Mam complex translocates into the nucleus
• p300 acts as a histone acetylase

Chromatin Modification

• p300 acetylates histones, altering chromatin structure
• This exposes gene regulatory sites and enables transcription

Transcription of Notch Target Genes

• The exposed chromatin allows the transcription of Notch target genes
• Myc regulates cell proliferation
• p21 controls cell cycle regulation
• Cyclin D3 governs cell division

Signal Termination

• FBW7, an E3 ubiquitin ligase, degrades NICD through ubiquitination to prevent excessive signaling

Signaling Over Short Distances

• Paracrine signaling occurs between cells that are close but not touching
• Signaling involves molecules secreted by one cell diffusing to nearby cells
• Sonic Hedgehog (SHH) and Wnt signaling are examples of paracrine signaling

Main Pathways Involved

• Fibroblast growth factor pathway (RTK)
• Tumor growth B factor pathway (RTK)
• Wnt pathway
• Hedgehog pathway

Wnt and SHH

• Cells secrete Wnt proteins and Shh
• These diffuse through the extracellular space and bind to Frizzled or Patched receptors on neighboring cells

Sonic Hedgehog (SHH) Signaling

• The signaling Molecule is Hedgehog protein (Hh)
• The receptors are PTCH1 and PTCH2 in mammals
• An Important role is played in embryonic development, adult tissue maintenance, renewal and regeneration, survival and proliferation to cell fate specification and differentiation

Inactive Hedgehog Pathway (No Hh Ligand) → Gene Inhibition

• Gene transcription is actively repressed when Hedgehog (Hh) is absent

Steps when Hh is absent

• Patched (PTCH) receptor inhibits Smoothened signaling protein (SMO), preventing its activation
• Smoothened cannot enter the plasma membrane and stays in the cytosol within an endosome
• GLI proteins remain bound to SUFU (Suppressor of Fused) because Smoothened is inactive, which keeps them in a dormant state
• Since GLI is inactive, CKI, PKA, and GSK3β phosphorylate GLI proteins, marking them for partial degradation
• Proteasomes cleave phosphorylated GLI proteins, converting them into GLI2/3R (repressor forms)
• GLI2/3R translocates into the nucleus and inhibits the transcription of Hedgehog target genes

Active Hedgehog Pathway (Hh Ligand Present) → Gene Activation

• Signaling is activated when Sonic Hedgehog (SHH) binds to Patched

Steps when SHH binds to Patched

• SHH binds to Patched, facilitated by co-receptors like CDO, BOC, GAS1, and LRP2
• Patched no longer inhibits Smoothened (SMO), allowing it to become active
• Smoothened is phosphorylated by CKI (Casein Kinase I) and Gprk2 (G-protein receptor kinase 2) to activate
• Smoothened then moves to the plasma membrane, where it activates downstream components
• Smoothened associates with EVC (Ellis-van Creveld protein complex)
• KIF7 aids in the movement of SUFU proteins and GLI proteins
• GLI2/3 proteins dissociate from SUFU and are not phosphorylated by kinases, meaning they are not processed by the proteasome and do not get converted in their repressor forms
• GLI2/3 translocates into the nucleus and activates target gene expression, leading to cell proliferation, differentiation, or survival

Wnt Signaling (Canonical Path)

• Works in both paracrine and autocrine ways
• Core mechanism revolves around β-catenin stability and gene activation
• Can be seen in somite formation and the development of intestinal villi

Inactive Wnt Pathway (No Wnt Signal) → β-Catenin Degradation

• β-catenin is constantly degraded in the cytoplasm to prevent unwanted gene activation
• A destruction complex (Axin, CKI, APC, Dvl, βTrCP, and GSK3) binds and phosphorylates β-catenin, marking it for ubiquitination and proteasomal degradation
• Since β-catenin is absent, the transcription factor TCF remains inhibited by Groucho, preventing Wnt target gene activation

Active Wnt Pathway (Wnt Present) → β-Catenin Accumulates & Gene Activation

• Wnt binds to Frizzled receptor, activating it
• Frizzled phosphorylates the co-receptor LRP (LDL-receptor-related protein)
• Phosphorylated LRP attracts the destruction complex, moving it to the cell membrane
• Dvl binds to LRP and gets activated
• Activated Dvl inhibits the destruction complex, preventing β-catenin degradation
• β-catenin accumulates in the cytoplasm and translocates to the nucleus
• β-catenin displaces Groucho from TCF and binds to TCF
• TCF activates Wnt target genes, leading to growth and proliferation

Signaling Over Long Distances

• Neurocrine (synaptic) and endocrine signaling occurs over long distances

Synaptic (Neurocrine) Signaling – Fast & Targeted

• Electrical impulses travel along the neuron's axon (up to 100 m/sec)
• The impulse reaching the axon terminal triggers neurotransmitter release
• Neurotransmitters diffuse across the synapse (<100 nm gap)
• Target cell receptors receive the signal in <1 msec, leading to a fast response