Wnt Signalling

Question 1

what are three functions of effector proteins?

Answer 1

transcription regulation
cytoskeletal regulator
metabolic enzyme

Question 2

what are the five forms of intracellular signalling?

Answer 2

juxtacrine, paracrine, autocrine, synaptic, endocrine

Question 3

what are three major classes of receptors?

Answer 3

ion-channel-couple receptors
g-protein-coupled receptors
enzyme-coupled receptors

Question 4

what are three downstream signal transduction components?

Answer 4

elements can activate or inhibit each other (often via phosphorylation- a MAPK cascade etc)
secondary messengers (cAMP, PIP3 etc)
intracellular signalling complexes (scaffolding proteins - SH2 etc)

Question 5

what is positive feedback?

Answer 5

moderate or strong (strong = all or none response)
bistable system
B enhances/strengthens A

Question 6

what is negative feedback?

Answer 6

dynamic equilibrium
B reduces A
stabilises a response

Question 7

what is a biological example of feedback?

Answer 7

pattern formation in sea snails
have activator with positive feedback on self, activates inhibitor which inhibts activator with long range diffusion
so activator determines where pigment is on the shell
also involves hormones

Question 8

how was wnt discovered?

Answer 8

in a screen for proto-oncogenes -> int1 (related to cancer)

in drosophila, int1 = wingless

Question 9

what two PTMs does wnt have?

Answer 9

glycosylation

palmitoylation (increase affinity to PAM receptor)

Question 10

how does wnt determine developmental patterning?

Answer 10

cell fate specification and axial patterning

Question 11

how is wnt implicated in disease?

Answer 11

homeostatic levels of wnt = carry out all its purposes
too little wnt = degeneration
too much wnt = cancer

Question 12

what are two proteins implicated in wnt modification and secretion?

Answer 12

porcupine

wntless

Question 13

what is porcupone?

Answer 13

multipass transmembrane o-acyltransferase
in the ER of wnt producing cells
essential for wnt palmitoylation and maturation
loss of porc leads to retention of wnt in ER and loss of signal

Question 14

what is wntless?

Answer 14

aka eveness interupted (Evi)
sorting receptor needed for wnt secretion
chaperones wnt membrane and localises to golgi, endosomes, and plasma membrane

Question 15

what is the process of wnt transport?

Answer 15

start in golgi/ER and two routes:

1. secreted on apolipoprotein and wnt loaded and transported
2. wntless and wnt trafficked to MVB vesicles which contain smaller vesicles- exosomes

Question 16

what happens when wnt signal reaches target cell?

Answer 16

binds to receptor: a seven-pass transmembrane receptor called frizzled

Question 17

what is frizzled?

Answer 17

transmembrane receptor with cystein rich domain

binds wnt

Question 18

what are two types of wnt signalling and the three subpathways?

Answer 18

canonical (wnt/beta-catenin)

non-canonical (wnt/pcp and wnt/ca2+)

Question 19

what is beta catenin?

Answer 19

involved in cell adhesion and transcription regulation

binds transcription factor TCF/LEF when wnt is present

Question 20

what happens in beta catenin pathway when wnt is not present?

Answer 20

beta catenin phosphorylated by ‘destruction complex’ leading to ubiquitination by beta-TrCP
this then targets beta-catenin for degradation by the proteosome

Question 21

what is the wnt beta catenin destruction complex made of?

Answer 21

axin - main scaffold
apc - unknown function
cki and gsk3- serion threonine kinases

Question 22

what does beta catenin pathway result in when wnt not present?

Answer 22

no target gene transcription

Question 23

what happens during beta catenin pathway when wnt is present?

Answer 23

switching on of target gene transcription
wnt binds to frizzled and axin binds LRP (mediated by dishevelled)
kinases CK1 and GSK3 phosphorylate LRP, axin and APC but not beta catenin
receptor complex at membrane is called signalosome
bet catenin is not degraded

Question 24

what is the feedback loop in wnt beta catnenin pathway?

Answer 24

tz and lrp taget genes

Question 25

what happens because of LRP signalosome endocytosis?

Answer 25

once endosome formed signalosome forms and components of destruction complex are titrated away from beta catenin
internalisation of receptor complex in endosomes and MVBs

Question 26

what are four ways wnt beta catenin signal can be inhibited?

Answer 26

dicckopf proteins - antagonise signalling by binding lrp
wnt inhibitory factor - antagonises signalling by binding to wnt
norrin and r-spondins- antagonises signal by binding to fz

Question 27

what are three aspects of directional movement?

Answer 27

orientation in developing organism
orient a particular cellular structure along a particular axis
orient tissue movement

Question 28

what is wnt pcp signalling pathway for?

Answer 28

establishment of planar cell polarity
so polarity in plane of tissue
has downstream protiens for cytoskeleton remodelling and actin remodelling

Question 29

what is an example of wnt pcp pathway?

Answer 29

axis elongation in zebrafish embryo

Question 30

how is cell polarity established?

Answer 30

signal from one side only

proximal or distal

Question 31

what is wnt ca2+ signalling pathway?

Answer 31

release of calcium from intracellular stores triggers second messenger
is g protein dependent
includes PLC and PIP3 to activate PKC and CamKII which phosphorylate target genes