Hedgehog Signalling

Question 1

what is hh signalling important for?

Answer 1

developmental patterning and also implicated in development and disease

Question 2

what is hh?

Answer 2

acts as typical morophogen
in drosophila- wing patterning
in vertebrates - limb buds etc

Question 3

what is morphogen signalling?

Answer 3

sending cells (secrete signal) and gradient forms based on uptake of signal by receiving cells

Question 4

how is hh expressed in drosophila?

Answer 4

in wing expressed in posterior compartment, meaning gradient produced across anterior comparment

Question 5

what are three other variations of Hh?

Answer 5

sonic hedghog
indian hedghog
desert hedghog

Question 6

what are PTMs of Hh?

Answer 6

palitation on n terminal (for signalling function and signal uptake)
cholsterol addition to c terminal (for signal spreading and multimerisation, as well as ptc and smo interactions)

Question 7

what is the hh pathwya process?

Answer 7

hh translated and transcribed, is in the ER, precursor protein and proteolytic cleavage occurs where hedge and hog domains degraded by the proteosome (based on cholesterol and HH-C)
traffick to membrane from release

Question 8

what are the four key mechanisms for Hh release from the plasma membrane?

Answer 8

transmembrane molecule dispatched and secreted scube2 bind cholesterol moiety of hh
spontaneous soluble multimers
heparan suphate proteoglycan (dally) recruits liphorin apopliproteins to form lipoprotein particles
exovesicles from MVBs

Question 9

what is transendocytosis of Hh in the producing cells?

Answer 9

apically secreted hh is endocytosed by producing cells and transendocytosed to basal side of the cells

Question 10

what are the two models of apical/basal gradient?

Answer 10

1. gradient is forming on apical side of the tissue

2. yes apical secretion , but gradient actually forms on basal side of tissue

Question 11

what is hh pathway when hh not present?

Answer 11

hedge not bound so ptc represses smoothened
blocking of smoothened trafficking to membrane leads to degradation
hh signalling complexes phosphorylate ci via kinases (cos2 main scaffold and targets ci to proteosome)

Question 12

what is hh pathway when hh present?

Answer 12

binds to pedges and leads to smoothened which is transported to membrane
pedged endocytosed and degraded then upon recruitment to membrane, events occur at complex is different when associated with cytoplasm
smo relocates to membrane and leads to associated with hh signalling complex and theres phosphorylation of cos2 and sufu which releases unphosphorylated ci

Question 13

describe signallign transduction in vertebrates

Answer 13

at primary cilium and is microtubule dependent

hh binding leads to increased cilia dwell time, dissociation of sufu/gli complex

Question 14

what are three ways a morphogen signal can be transferred?

Answer 14

diffusion
relay
protrusions

Question 15

how can we distinguish between diffusion morphogen transport and protrusion transport?

Answer 15

cytoneme presence

Question 16

what are cytonemes?

Answer 16

filopodia like structures that orient toward source of morphogen

Question 17

how does cytoskeletal regulatoin control filopodia formation?

Answer 17

small rho-gtpases regulate cytoskeleyon and these are molecular switches
whogdp to rho gtp
gef and gap

Question 18

how are filopodia formed?

Answer 18

cdc42 recruits proteins to site of protrusion

there are proteins for parallel actin bundles, actin polymerisation and actin membrane interactions

Question 19

what is the process of actin polymerisation?

Answer 19

nucleation, elongation, branching nucleation, enhanced elongation

Question 20

which regions can cytonemes be found in hh signalling?

Answer 20

LECs and histoblasts

Question 21

what is the relationship between cytoneme formation and hh activity gradients?

Answer 21

cytoneme formation correlates with establishment of hh activity gradient in space and time

Question 22

how is hh actually transported?

Answer 22

in exovesicles which are created in MVBs via endocytotic pathway and cd63 is marker for exosomes

Question 23

how does morphogen signal reach target cells?

Answer 23

developing air sac interacts with neighbouring cells to expand in the correct direction
cell-cell interactions are crucial for this and it is a split-gfp system (GRASP)

Question 24

what are two other signalling processes that cytonemes are implicated in?

Answer 24

notch and wnt

Question 25

what is signal transport via relay?

Answer 25

c elegans blastomere example

polarity in embryos