Cell surface receptors transmit signals from outside the cell via signal transduction pathways. Receptors are divided into classes including ion channel-linked and enzyme-linked receptors. Enzyme-linked receptors contain intrinsic enzyme activity or associate with intracellular enzymes. Upon ligand binding, a conformational change activates the enzyme, initiating signaling cascades. Tyrosine kinase receptors have intrinsic kinase activity that phosphorylates tyrosines, creating docking sites and activating downstream pathways such as MAPK cascades. Mutations in these receptors and associated kinases can cause cancers and developmental disorders.

1. By:
Faraza Javed
Sidra Arif

2.  Cell surface receptors are specialized
integral membrane proteins that take part in
communication between cell and outside
world.
 Extracellular signaling molecules (usually
hormones, neurotransmitters, cytokines,
growth factors ) attach to receptor, triggering
changes in function of cell. This process is
called signal transduction.The binding
initiates a chemical change on intracellular
side of membrane.

3.  Based on structural and functional
similarities, membrane receptors are mainly
divided into 3 classes:
1) Ion channel-linked receptor;
2) Enzyme-linked receptor
3) G protein-coupled receptor

4.  A group of transmembrane proteins that
contain either intrinsic enzyme activity on
their intracellular domain or associate directly
with an intracellular enzyme.
 Upon ligand binding a conformational change
is transmitted via a transmembrane helix
which activates the enzyme, initiating
signaling cascades

5. 1. Ligand-binding domain
 Extracellular to allow easy access for ligands.
 Strong affinity for specific ligands - allows
different ligands that bind to same receptor
to evoke particular cellular responses.
2. Transmembrane domain
 Contains a series of hydrophobic amino
acids.
 Tethers receptor to cell membrane.

6. 3. Cytosolic "active" enzyme domain
 The intracellular domain of receptor itself is
an enzyme or interacts directly with an
enzyme.

7. 1. Receptor tyrosine kinases - phosphorylate
specific Tyrosine residues on a small set of
intracellular signaling molecules
2. Tyrosine-kinase-associated receptors -
couple to proteins that have Tyrosine kinase
activity
3. Receptor serine/threonine kinases, which
phosphorylate Serine or Threonine residues

8. 4. Histidine-kinase-associated receptors
5. Receptor tyrosine phosphatases - remove
phosphate groups from Tyrosine
6. Receptor guanylyl cyclases - catalyze
production of cGMP

9.  The predominant type of enzyme-linked
receptors is family of the tyrosine kinase
receptors, also referred to as receptor
tyrosine kinases
 These receptors have intrinsic kinase
activity encoded by intracellular
domain, also referred to as the
cytoplasmic tail

10.  1.Ligand binding to RTK monomers
results in dimer formation.
 2. Within the dimer the conformation is
changed, locking the kinase into an
active state.
 3. The kinase of one receptor then
phosphorylates a tyrosine residue
contained in the "activation lip" of the
second receptor.

11.  4.This forces the activation lip out of the
kinase active site, allowing ATP bind
and resulting in enhanced kinase activity.This
induces phosphorylation at further tyrosine
residues.
 5.Phosphotyrosine is a conserved "docking
site" for many intracellular signal
transduction proteins that contain SH2
domains .

13.  When ligand binds to extracellular domain of
growth factor receptor,a dimer is formed .
 Then tyrosine kinase domains phosphorylate
tyrosine residues.
 This phosphorylation produces binding sites
for proteins with SH2 domains.
 GRB2(Growth factor receptor-bound
protein ) is one of these proteins. GRB2,
with SOS bound to it, then binds to the
receptor complex.

14.  This causes activation of SOS.
 Son of Sevenless(SOS) refers to a set of
genes encoding guanine nucleotide release
factors for ras.
 When SOS is activated, it promote the
exchange of GDP for GTP
 When ras has GTP bound to it, it becomes
active.
 Activated ras then causes the activation of a
cellular kinase called Raf-1.

15.  Raf-1 kinase then phosphorylates another
cellular kinase called MEK
 This cause activation of MEK.
 Activated MEK then phosphorylates another
protein kinase called MAPK (Mitogen-
activated protein kinase) causing its
activation.
 This series of phosphylating activations is
called a kinase cascade.
 It results in amplification of signal

16.  Among the final targets of the kinase cascade
are transcriptions factors(fos and jun ).
Phosphorylation of these proteins causes
them to become active and bind to the DNA,
causing changes in gene transcription

17.  MAP kinase pathways regulate a wide range
of physiological responses, including cell
proliferation, apoptosis, cell differentiation,
and tissue development.
 MAP kinase pathways are regulated by Ras
proteins, which are found to be mutated and
constitutively active in approximately 30% of
all human cancers.

18.  The majority of mutations in ras decrease the
intrinsic rate of GTP hydrolysis by ras and
make the molecule significantly less sensitive
to GTP hydrolysis.
 Thus, the outcome is a molecule that is
predominantly GTP bound and therefore
constitutively active.

19.  It is now essentially independent of growth
factor stimulation and continues to activate
downstream pathways in the absence of any
stimulation.
 Oncogenic ras is capable of transforming
immortalized fibroblasts or epithelial cells

20.  Raf protein kinase are known to regulate
proliferation, differentiation.
 Mutated Raf-1 is constitutively active and
possesses in vitro transforming potential
 The potential for Raf-1 to play a broad role in
tumorigenesis is evidenced by its ability to
become activated by either PKC or the
antiapoptotic protein Bcl-2 in a Ras-
independent manner

21.  Importantly, raf mutations have been
identified in a range of human tumors.
 Therefore, the collective evidence suggests
that Raf-1 is a viable anticancer drug target
 The first drug licensed to inhibit raf kinase is
sorafenib.
 Other Raf inhibitors, encorafenib, dabrafenib,
vemurafenib

22.  The fibroblast growth factor family
constitutes one of the most important groups
of paracrine factors that act during
development.
 They are responsible for determining certain
cells to become mesoderm, for the production
of blood vessels, for limb outgrowth, and for
the growth and differentiation of numerous
cell types.

23.  Fibroblast growth factor binding to their
receptors causes the receptors to dimerize,
and this results in the activation of their
protein tyrosine kinases.
 These kinases phosphorylate each other and
initiate downstream signaling . There are
three components of this signal.
 The main signal involves the activation of
the ras protein and the MAP kinase cascade

24.  In addition, the activated receptor stimulates
phospholipase C to split PIP2 into IP3 and
DAG.
 A third signal involves the phosphorylation of
the Stat1 transcription factor and its
subsequent translocation into the nucleus

25.  Mutations in FGFR1 can cause the Pfeiffer
syndrome, a malformation syndrome
characterized by limb defects and by the
premature fusion of the cranial sutures
(craniosynostosis) that results in abnormal
skull and facial shape
 There is also strong evidence from
sequencing studies of candidate genes
involved in clefting that mutations in the
FGFR1 gene may be associated in the
pathogenesis of cleft lip and/or palate

26.  On a molecular level, mutations that affect
FGFR2 are associated with marked changes
in osteoblast proliferation and differentiation.
 Alteration in FGFR2 signalling is thought to
underlie the craniosynostosis syndromes

27.  There are two mechanisms of altered FGFR2
signalling.
 The first is associated with constitutive
activation of FGFR, where the FGFR2
receptor is always signalling, regardless of
the amount of FGF ligand.
 This mechanism is found in patients with
Pfeiffer syndrome.

28.  The second, which is associated with Apert
syndrome(malformations of the skull,
face, hands and feet) is a loss of specificity
of the FGFR2 isoform, resulting in the
receptor binding to FGFs that it does not
normally bind

29.  Mediate their activity by causing the addition
of a phosphate group to particular tyrosines
on certain proteins within a cell.
 Insulin binds to the tyrosine kinase receptor,
located on the cell membranes of the target
cells on skeletal muscle tissue, fat tissue, and
liver.
 The "substrate" proteins that are
phosphorylated by the Insulin Receptor
include a protein called "IRS-1" for "insulin
receptor substrate 1

30.  IRS-1 binding and phosphorylation
eventually leads to an increase in the high
affinity glucose transporter (Glut4) molecules
on the outer membrane of insulin-responsive
tissues, including muscle cells and adipose
tissue, and therefore to an increase in the
uptake of glucose from blood into these
tissues

31.  In other words, the glucose transporter Glut4
is transported from cellular vesicles to the
cell surface, where it then can mediate the
transport of glucose into the cell.

32.  Insulin binds to its receptor which in turn,
starts many protein activation cascades.
These include
1)translocation of Glut-4 transporter to the
plasma membrane and influx of glucose
2)glycogen synthesis
3)Stimulates protein synthesis and inhibits
proteolysis
4)Fatty acid synthesis

33.  Many cell-surface receptors depend on tyrosine
phosphorylation for their activity and yet lack an
obvious tyrosine kinase domain.
 These receptors act through cytoplasmic
tyrosine kinases, which are associated with
receptors and phosphorylate various target
proteins when the receptors bind their ligand.
 The receptors thus function in much same way
as receptor tyrosine kinases, except that their
kinase domain is encoded by a separate gene and
is noncovalently associated with receptor
polypeptide chain.

34.  Many cytokines, lymphokines, and growth
factors signal through receptor tyrosine
kinases that are associated with and activate
Janus kinases (JAKs)
 Ligand-induced dimerization of the receptor
induces the tyrosine phosphorylation of the
associated JAKs, which, in turn,
phosphorylates tyrosine residues on the
cytoplasmic tail of the receptor.

35.  These phosphorylated tyrosines serve as
docking sites for SH 2 domain of the
STAT(signal transducer and activator of
transcription) protein, and JAK catalyzes the
tyrosine phosphorylation of the receptor-
bound STAT,
 Followed by translocation of the STAT dimer
to the nucleus.
 STAT dimers bind to specific DNA response
elements in the promoter region of target
genes to activate gene expression.

37.  IL-2 receptors are expressed on T cells ,B
cells,natural killer cells, monocytes,
macrophages.
 IL-2 stimulation induces activation of Janus
family tyrosine kinases JAK1 and
JAK3,These kinases induce tyrosine
phosphorylation of STATs (signal transducers
and activators of transcription) and various
other downstream targets

38.  The downstream signaling pathways
activated by IL-2 also involves mitogen-
activated protein kinase and phosphoinositide
3-kinase signaling modules, leading to both
mitogenic and anti-apoptotic signals.
 IL-2R signaling activates PI3K which
catalyses phosphorylation of inositol
phosphates.
 These act as second messengers and recruit
molecules such as Akt kinase to the cell
membrane

39.  Akt kinase is further activated by
phosphorylation and subsequently positively
or negatively regulates the activity of
downstream targets like PKB
 Proapoptotic proteins which can be
phosphorylated and inhibited by PKB include
BAD (BCL2 Antagonist of Cell Death) and
forkhead family of transcription factors
which regulate the genes responsible for
determining whether activated T-Cells
survive, proliferate, or die.

41. Differentiation of T lymphocyte
 The interaction of IL-2 with the IL-2 receptor
induces proliferation and differentiation of a
number of T lymphocyte subsets, and
stimulates a cytokine cascade that includes
various interleukins, interferons and tumour
necrosis factors. Antitumour effects of IL-2
appear to be mediated by its effects on natural
killer, lymphokine-activated killer (LAK) and
other cytotoxic cells.

42. Type 1 diabetes
 IL-2 is necessary during T cell development
in the thymus for maturation of regulatory T
cells (T-regs).
 After exiting from the thymus, T-Regs
function to prevent other T cells from
recognizing and reacting against self
antigens, which could result in autoimmunity.
 T-Regs do so by preventing the responding
cells from producing IL-2.

43.  Type 1 diabetes results from progressive
immune-mediated destruction of pancreatic
β-cells and associated metabolic dysfunction.
 Combined genetic and immunological studies
now highlight deficiencies in both
interleukin-2 (IL-2) receptor and its
downstream signaling pathway as a central
defect in the pathogenesis of type 1 diabetes

44.  Acts as both a pro-inflammatory and an anti-
inflammatory cytokine.IL-6 is secreted by T
cells and macrophages to stimulate immune
response.
 IL-6's role as an anti-inflammatory cytokine
is mediated through its inhibitory effects on
TNF-alpha and IL-1, and activation of IL-1ra
and IL-10.

45.  IL-6R is expressed on some cells, including
immunocompetent cells and hepatocytes
 IL-6 signals through activation of JAK
kinases and activation of ras-mediated
signaling

46.  Activated JAK kinases,STAT3 also
upregulates the transcription of genes
encoding the SOCS3 (Suppressor of
cytokine signaling 3 proteins)—
intracellular negative-feedback factors that
inhibit the JAK–STAT pathway.

48.  Constitutive Activation of Jaks/Stats
in Cancer
 In contrast to normal cells, in which Stat
tyrosine phosphorylation occurs transiently, it
has been determined that Stats 1, 3, and 5 are
persistently tyrosine phosphorylated in most
malignancies (particularly Stat3).

49.  The mechanisms by which Stat3 is
persistently or constitutively tyrosine
phosphorylated in cancers include
 increased production of cytokines and
cytokine receptors
 a decrease in the expression of the SOCS
proteins
 loss of tyrosine phosphatases.

50. Rheumatoid arthritis
 IL-6 induces B-cell differentiation and has
been shown to induce B-cell antibody
production.
 Neutrophil migration from blood to tissue is a
characteristic feature of inflammation

51.  Upon entry, activated neutrophils release
proteolytic enzymes and reactive oxygen
intermediates resulting in tissue destruction
and joint damage in RA.
 Neutrophils express membrane-bound IL-6R
and are activated by IL-6.
 A humanized anti-IL-6R monoclonal
antibody, tocilizumab (TCZ) is approved for
arthritis

52.  Erythropoietins(Epo) is a hormone required
for erythropoiesis.
 Epo exerts its function through the EpoR
(Epo Receptor).
 Following binding of Epo to its receptor, the
receptor forms dimers and undergoes
phosphorylation by physically associating
and interacting with the tyrosine kinase JAK2
(Janus Kinase-2).

53.  Once phosphorylated, these tyrosine residues
allow the recruitment and activation of a
number of downstream adaptors and effectors
including STAT5 (Signal Transducers and
Activators of Transcription factor-5), PI3K
(Phosphoinositide-3 Kinase)

54. Polycythemia
 Polycythemia is clinical disorder
characterized by an absolute increase in red
blood cell mass and is associated with high
serum EPO and intact EPO-R signaling.
 The extracellular domain of the EPO-R binds
circulating EPO,and the intracellular domain
controls signal transduction and has
regulatory region that acts as a brake on red
cell production.

55.  Some cases of polycythemia were found to be
associated with EPO-R mutations that were
characterized by mutations with consequent
truncations of regulatory region of the EPO-
R.
 This region includes Tyr 429, which in its
phosphorylated form is required for binding
of SHP1, negative regulator of Tyr
phosphorylation and signal transduction
 Loss of EPO-R SHP1 binding site prevents
dephosphorylation of JAK2 and activation of
STAT5, leading to hypersensitivity to EPO

56.  Receptor tyrosine phosphatases (RTP) are
cell-surface proteins that dephosphorylate
intracellular targets to activate signalling
cascades.
 An important protein tyrosine phosphatase
is CD45 protein which is found on surface
of all white blood cells and has an essential
role in activation of both T and B
lymphocytes by foreign antigens

57.  Key to ability of TCR to deliver intracellular
signals is its interactions with protein tyrosine
kinases
 CD45 activity is at very early steps of TCR
signaling, indicating that CD45 is required
for functional coupling of the TCR and its
PTKs

58.  Tenascin-C which is ligand of receptor protein-
tyrosine phosphatase, expressed on the surface
of certain glial cells in the mammalian brain.
 It binds to a receptor protein (called contactin)
on developing nerve cells indicates that their
specific functional roles in stem cell
neurobiology,which will be important for the
therapeutic application of this new technology in
facilitating nervous tissue repair and
regeneration

59.  The findings show possibility that some
receptor tyrosine kinases and receptor
tyrosine phosphatases may collaborate when
they bind their respective cell-surface-bound
ligands—with the kinases adding more
phosphates and the phosphatase removing
fewer—to maximally stimulate tyrosine
phosphorylation of selected intracellular
signaling proteins

60.  John C, Torben J.Text book of receptor
pharmacology(2nd edition)
 pharmacologycorner.com/animation-showing-
the-jak-stat-signalling-pathway-mechanism/
 www.ncbi.nlm.nih.gov/books/NBK26822/
 www.inkling.com/read/marks-medical-
biochemistry-lieberman-marks-4th/chapter-
11/iii--plasma-membrane-receptors