Lecture 5 Hh and Wnt

Shh, Desert Hh, Indian Hh

18 different Wnts

• Activating pathway, activates patched 1 which will is a negative regulator of the pathway and act on itself to limit the level of activation • Induction of patched1/patched is one of the best known targets of Hh signalling

• Negative in vertebrates and invertebrates (patched in drosophila, ptch1 in vert)

• Gli1 is induced • Gli1 cannot be proteolyzed into a repressor so it is always an activator of the Hh signal and is a “feedforward” response

The induction of patched is also seen in drosophila, but Ci is not induced.

see notes

Cyclopamine (chemical that can inhibit smoothened and block the hedgehog signal) discovered as sheep eating that plant, veratrum californicum gave birth to cyclopic lambs

Beta-catenin

• Beta-catenin is the TF in the wnt pathway and is continuously produced • It is bound by the destruction complex almost immediately • Beta catenin is phosphorylated by Gsk3 beta and CK1a • Phosphorylated beta catenin is recognised by Slimb/BetaTrcp (part of the SCF ubiquitination complex) and ubiquitinated • Ubiquitinated Beta-catenin is destroyed by the proteasome, freeing up the destruction complex

• Wnt binds to its receptors frizzled and arrow/lrp5/6 and brings them together to initiate the signal. • Dishevelled (Dsh) is recruited to the Fz receptor and phosphorylated. • This somehow leads to recruitment of the destruction complex, possibly by dsh binding axin, phosphorylation of arrow/lrp5/6 by gsk3b creating binding sites for axin and further phosphorylation events. • Importantly, it is thought that Slimb is somehow lost form this complex. • As a result Beta catenin that is produced will still bind the complex • And become phosphorylated • But is will not be ubiquitinated and degraded, but it will remain bound to the destruction complex. Thus the system clogs…newly produced beta-catenin will start accumulating • Beta-catenin will enter the nucleus to displace groucho (transcriptional reporessor) to start activating transcription

Segmentation Patterning and outgrowth of the wing at the D/V boundary

Regulation of neuronal fate and migration Studied by analysing the migration of two neuroblasts: the QRd and QLd cells

Wnt signalling in the QLd cell induces a homeobox gene mab5 which induces posterior migration.

Thus in the absence of the signal both cell move anteriorly whereas ectopic activation of wnt leads to both cells migrating posteriorly

• Differentiated enterocytes, enteroendocrine cells and goblet cells

Paneth cell

Slightly higher than the Paneth cells

Bottom of the crypt

Stem cell division Stem cells divide slowly but produce rapidly dividing transit amplifying cells thereby creating a continous flow of cells from the crypt upwards.

Wnts are expressed by the stroma below the intestinal crypts.

If you block wnt signaling it the gut it will cause the loss of stem cells in the colon crypt, indicating that Wnt signalling is required for maintenance of the stem cell compartment.

Ectopic activation of wnt can cause overproliferation probably due to overproduction of stem cells.

Loss of APC