Book Notes: Signaling Part 5

• pathways involving gene regulatory proteins (who activate gene expression) depend on regulated proteolysis to control them
• Notch, Wnt, and Hedgehog are all these pathways, and they are especially involved in animal development
• Notch is famous for affecting neural cell development
• lateral inhibition: if one cell develops into a neuron, it signals to its neighbors to NOT being nerve cells and instead become epidermal
• this occurs because the developing neuron displays the Delta signal protein on its surface
• neighbors all have Notch receptors that bind Delta and receive the signal to develop into epidermis
• Notch signaling can also be used for promoting neighbors to become the same cell type in other cases
• when Notch binds Delta, a membrane-bound protease cleaves Notch's tail
• the tail zooms off into the nucleus and activates a set of genes by binding and converting a transcriptional repressor into an activator
• Notch is actually cleaved 3 times in its lifetime: (1) during synthesis, it is cleaved by Golgi to form heterodimer, (2) during ligand-binding, when the extracellular head is cleave off, and (3) after ligand-binding, when the cytoplasmic tail is cleaved off
• cleavage is irreversible, meaning Notch is a one-time use receptor
• Notch and Delta are glycoproteins, and their specific match-up depends on how their sugar sidechains are modified (glycosylation is important!)
• Wnt is famous for its versatility: it plays a role in catenins (i.e. jxns), cell polarity, etc, etc
• Wnt has 3 well-known pathways:
1. Wnt/beta-catenin pathway: Wnt affects the gene regulatory protein beta-catenin
2. planar polarity pathway: coordinate polarization in plane of epithelium
3. Wnt/Ca2+ pathway: stimulates increase of calcium level
• Wnt is a secreted molecule that binds to Frizzled receptors
• Wnt-bound Frizzled recruits Dishevelled, which then relays the signal down one of the 3 pathways
• for the beta-catenin pathway, Wnt binds to both Frizzled and an LRP (LDL-receptor-related protein)
• any beta-catenins not involved in jxn-forming is bound by degradation complex, which keeps it away from the nucleus and promotes its degradation
• 2 kinases (GSK3 and CKIgamma) phosphorylate C tail of LRP, which inactivates axin (part of the degradation complex), so free beta-catenin accumulates and migrates to the nucleus
• without Wnt, the set of genes controlled by Wnt are silent
• LEF1/TCF proteins bound to Groucho (a co-repressor)
• when Wnt activates, beta-catenin comes in and displaces Groucho as the co-partner
• Beta-catenin serves as a co-ACTIVATOR for the genes
• c-Myc is one of the proteins expressed by Wnt signaling, and it stimulates growth and proliferation (potential cancer!!!)
• Hedgehog is similar to Wnt: is secreted, modified by lipids, trigger switch from repression to activation, and leads to growth/proliferation response
• 3 transmembrane proteins mediate responses to the Hedgehog proteins:
• Patched: multipass with some of it in intracellular vesicles, and some on the cell surface to bind Hedgehog
• Smoothened: 7pass transmembrane
• iHog: 4-5 immunoglobulin domains and 2-3 fibronectin domains on cell surface, serve as co-receptors with Patched
• Patched normally sequesters Smoothened 
• Hedgehog binding to iHog partnered with Patched inhibits Patched by causing endocytosis and degradation of Patched
• result is that Smoothened is free and phosphorylated and continues the signaling pathway
• Cubitus interruptus (Ci): gene regulatory protein that is a target of Hedgehog
• in normal conditions, Ci is proteolysed into smaller proteins that hang in the nucleus as repressors
• this proteolysis is dependent on phosphorylation by PKA and GSK3 and CK1  (all Ser/Thr kinases)
• when Hedgehog binds, Smoothened runs free and recruits Costal2, a scaffold protein, and Fused, another Ser/Thr kinase
•  Costal2 normally holds the proteolytic kinases together to process Ci, but binding to Smoothened blocks this ability
• uncleaved Ci also goes to the nucleus, but acts as an activator
• uncleaved Ci can also express the Patched gene = more Patched inhibits further Hedgehog signaling

pg. 946 - 952