Cell signaling involves communication between cells through signaling molecules called ligands. Ligands bind to receptors on target cells and initiate cellular responses. There are several types of cell signaling depending on the location of the signaling and target cells. These include paracrine signaling between nearby cells, endocrine signaling between distant cells via the bloodstream, and autocrine signaling where cells signal themselves. The signaling molecules can be membrane-bound or secreted. Receptors are generally intracellular or cell surface proteins that receive signals and transmit them into the cell. Common signaling pathways involve G protein-coupled receptors activating intracellular secondary messengers like cyclic AMP.

1. What is cell signaling?
• All cells receive and respond to signals from their surroundings
• Communication between cells.
Hormone
Growth factor
Cytokine
Steroid
Polypeptide
Signal from where?
The ligand is the signaling molecule. ligand may be
Signaling molecules, which
are released by signal-
producing cells, reach and
transfer biological signals to
their target cells to initiate
specific cellular responses

2. CONDITION FOR SIGNALING
Receptor activated by ligand, the receptor
causes a change in the target cell in which
it is expressed
 Extracellular molecules
Protein, peptides, Hormone, cytokine
adrenaline, thyroxine .prostaglandin
 Intracellular molecules
CCa2+, carbohydrate, CAMP/CGMP
Intercellular signaling
Communication between cells
Intracellular signaling
Signaling chains within the cell

3. Intercellular signalling Controls
Metabolic fluxes
Cell division
Growth
Development
Processes sensory information
Differentiation
Cells communicate via
Messenger
substances
Gap junctions
Surface proteins
Electrical
signals

4. Hormones
As a Signal molecules
Binding with
Receptor
Activated Enzyme
Second
messenger
Transcription
Effect on
cellular
function

5. The signalling molecules produce an effect on same cell that produce it
Autocrine signalling
Autocrine signalling the cell that is
producing the messenger expresses
receptors on its surface that can respond
to that messenger Consequently, cells
releasing the message will stimulate (or
inhibit) themselves.

6. The cytokine interleukin-1 in monocytes.
When interleukin-1 is produced in
response to external stimuli, it can bind
to cell-surface receptors on the same cell
that produced it.
Activated T lymphocytes T lymphocyte
respond to antigenic stimulation by
synthesizing a growth factor that derives their
own proliferation, and there by increasing the
number of responsive T-Lymphocytes
Cancer development- Autocrine
signaling plays critical roles in cancer
activation and also in providing self-
sustaining growth signals tumors
EXAMPLES OF
AUTOCRINE SIGNALING

7. •Paracrine signaling is a form of cell-to-cell
communication in which a cell produces a signal to
induce changes in nearby cells(close proximity)
• Signaling molecules known as paracrine factors diffuse
over a relatively short distance (local action)
• Cells that produce paracrine factors secrete them into
the Immediate extracellular environment.
• Follow paracrine signaling pathway-Fibroblast family,
Hedgehog family, Wnt family, and TGF-B superfamily.
Growth Factor(FGF)
Receptors available on the cell membrane to receive the
signals, also known as Being competent
PARACRINE SIGNALING

8. • The signaling molecule act on target cells distantly
located from the Site of synthesis.
• It is a long range signaling and signal molecules is
transported by Blood streams.
The pancreas is an endocrine gland and produces the
hormone insulin, which regulates the uptake of glucose
in cells all over the body.
Hormones that function in an endocrine manner
include testosterone, progesterone and gonadotropins.
ENDOCRINE SIGNALING
EXAMPLES OF ENDOCRINE SIGNALING

9. JUXTACRINE SIGNALING
• This kind of signaling require physical contact between the cells. Also known as contact-
dependent signaling.
A membrane ligand (protein, oligosaccharide, lipid)
and a membrane protein of two adjacent cells
interact.
A communicating junction links the intracellular
compartments adjacent cells, allowing transit of
relatively small molecules.
An extracellular matrix glycoprotein and a
membrane protein interact.
3 types of juxtacrine signaling molecules.

10. TYPES OF LIGANDS
1. Membrane bound signal molecule – signal molecule remain surface of the cell and mediate
contact dependent signaling.
2. Secretory signal molecules- secreted from signaling cell and binds to receptor.
Secretory signal molecules are extracellular and
divided in 3 categories depending on distance
1
2
3
Endocrine
Paracrine
Autocrine
Depending On The Nature Of Extracellular Signal
Molecules It Is Divided In 2 More Categories
LIPOPHILIC MOLECULES
HYDROPHILIC MOLECULES

11. EXAMPLE OF SIGNAL MOLECULES THAT INTRACT WITH CELL SURFACE RECEPTOR
Epinephrine
Glucagon
Insulin
Gastrin
Secretin
Adrenocorticotropic hormone

12. EXAMPLE OF SIGNAL MOLECULES THAT INTRACT WITH CYTOSOLIC OR NUCLEAR
RECEPTOR
Steroid hormones-
Progesterone, testosterone,
cortisol, aldosterone.
Non steroid hormones-
thyroid hormone and retinoic
acid.

13. • Receptors are protein molecules inside the target cell or on its surface that receive a chemical
signal.
• Chemical signals are released by signaling cells in the form of small, or soluble molecules called
ligands.
RECEPTOR AND LIGAND
Orphan receptor
Orphan receptor is a protein that has a similar structure to other identified
receptors but whose ligand has not yet been identified.

14. Intracellular receptors
Intracellular receptors are receptor proteins found on the
inside of the cell, typically in the cytoplasm or nucleus.
The ligands of intracellular receptors are small,
hydrophobic{water hating) molecules, since they must be able to
cross the plasma membrane in order to reach their receptors.
For example, receptors for hydrophobic steroid hormones, such as
the sex hormones -an oestrogen and testosterone, thyroid
hormone, retinoid and vitamin D are intracellular.
Also contain orphan receptors.

15. Intracellular receptors
Intracellular receptor belongs to Nuclear
receptor superfamily
Nuclear receptor superfamily is a family of highly
conserved transcription factors that regulates transcription
in response to small lipophilic signal molecules.
Members of the Nuclear receptor superfamily can
both positively and Negatively regulate transcription.

16. Nuclear receptor superfamily
N-terminal activation
domain, called the A/B
region,
A central DNA binding
domain(DBD) and
A C-terminal ligand
binding domain (LBD)

17. Example Steroid hormone receptor
Location-cytosol
Homodimer receptor
In cytosol= receptor +HSP - INACTIVE
In cytosol= receptor+ligand receptor-ligand complex
• It can be homodimer-example ----steroid receptor
• It can be heterodimer- example--- retinoic acid receptor, thyroid
hormone receptor, vitamin D receptor and orphan receptor
Nuclear receptor
Thyroid hormone receptor
Heterodimer receptor
Location of receptor= nucleus

18. Cell-surface receptors
Cell-surface receptors, also known as
transmembrane receptors, are
proteins that are found attached to
the cell membrane.
These receptors bind to external
ligand molecules because ligands
do not travel across the cell
membrane.
This type of receptor spans the
plasma membrane and performs
signal transduction, in which an
extracellular signal is Converted into
an intercellular signal.
Cell-surface receptors are also called
cell-specific proteins or markers
because they are specific to
individual cell types.

19. Cytosol
• Each cell-surface receptor has three main
components:
1. An external ligand-binding domain
2. A hydrophobic membrane-spanning region, and
3. An intracellular domain inside the
cell.
Cell-surface receptors

20. 1. Dimerization
2. Conformational changes
Mechanism of working
Categories of cell-surface receptors
Ion channel-linked receptors,
G-protein-linked receptors, and
Enzyme-linked receptors.

21. Ion channel linked receptors
Also called transmitter gated ion
channel or inotropic receptors.
lon channel-linked receptors bind a
ligand and open a channel through
the membrane that allows specific
ions to pass through.
To form a channel, this type of
cell-surface receptor has an
extensive membrane-spanning
region.
When a ligand binds to the
extracellular region of the channel,
there is a conformational change in
the proteins structure

22. Example- ligand gated channel for NTMs-Acetycholine
• Acetylcholine receptor is a receptor
linked to a cation channel.
• As two molecules of acetylcholine bind
to the binding sites on a subunits, the
conformation of the receptor is altered
and the gate is opened, allowing for the
entry of many ions and small molecules.

23. G-protein-coupled receptors
It is a G-protein linked receptor
G-protein-coupled receptors bind a ligand and activate a membrane protein called a G-
protein.
The activated G-protein then interacts with an enzyme in the membrane
Before the ligand binds, the inactive G-protein can bind to a site on a specific receptor. Once the G-protein
binds to the receptor, the G-protein changes shape, becomes active, and splits into two different subunits.
One or both of these subunits may be able to activate other proteins as a result.

24. G- Protein Coupled Receptor

25. Enzyme-linked receptors
Enzyme-linked receptors are cell-surface receptors with
intracellular domains that are associated with an enzyme.
In some cases, the intracellular domain of the receptor itself is
an enzyme.
Other enzyme-linked receptors have a small intracellular
domain that interacts directly with an enzyme.
When a ligand binds to the extracellular domain, a signal is
transferred through the membrane, activating the enzyme.
Activation of the enzyme sets off a chain of events within the cell
that eventually leads to a response

26. G-PROTEIN LINKED RECEPTOR
What is GPCR?
What is G
Protein and its
mode of action?
What is GTP? What is GTPase?
What is
GEF(Guanine
exchange factor)?
What is
stimulatory G-
protein?
What is
secondary
messenger?

27. What is GPCR?
G-Protein coupled receptor(GPCR) or G-
Protein linked receptor.
Have seven transmembrane domain, so
it is also called serpentine Extracellular
receptor.
Largest family of cell surface receptor.
Multiple single polypeptide.
GPCR present is all eukaryotes
GPCR are the largest group of cell
surface receptor that transmit signal
inside the cell with the action of
guanine nucleotide binding protein
called G-protein.

28. What is G Protein?
• G proteins, also known as guanine nucleotide-binding proteins, are a family of
proteins that act as molecular switches inside cells, and are involved in
transmitting signals from outside a cell to its interior.
• G proteins belong to the larger group of enzymes called GTPases.
What is GTPase?
GTPases are large family a of hydrolase enzymes that can bind and hydrolyze
guanosine Triphosphate (GTP)
What is GTP?
It also has the role of a source of energy or an activator of substrates in metabolic reactions, like
that of ATP. GTP is essential to signal transduction, with G-proteins. It is converted to guanosine
diphosphate (GDP) through the action of GTPases.

29. Mode of Action
The inactive form of
GTPase (GDP) are
activated by a class of
protein called guanosine
nucleotide exchange factor
(GEFs)

30. GPCR has receptors for
1. NTMS
2. Neuropeptides
3. Peptide hormone
4. Immune cells
5. Disease factors
6. Adrenaline (epinephrine)
7. Smell
8. Light
9. Taste

31. What is stimulatory G- Protein
When G- protein involved in activation of enzyme like effector, Adenylate cyclase, is
called stimulatory G-protein

33. What is Primary messenger?
Ligand are termed as primary/first messengers. First messengers are
extracellular factors, often hormones, Growth Factor, neurotransmitters,
Peptides etc.
What is secondary messenger?
Second messengers are intracellular signaling molecules released by the cell in
response to exposure to extracellular signaling molecules – the first messengers.
They triggers intracellular signal transduction cascades.

34. Removal or degradation of second messenger terminate the
cellular response
Four classes of second messengers
Cyclic nucleotides
Membrane lipid derivatives
Ca2+
Nitric oxide/carbon monoxide
Examples of second
messenger molecules include
cyclic AMP, cyclic GMP,
inositol trisphosphate (IP3),
diacylglycerol, and calcium

35. Which of the following is NOT a second messenger?
1. Cyclic GMP
2. Diacylglycerol
3. Inositol triphosphate
4. Phosphatidyl inositol

36. G-protein coupled receptors (GPCR) consist of three protein subunits a. B and y. In unstimulated
state, a subunit is GDP bound and GPCR is inactive. When GPCR gets activated, it acts like
guanine nucleotide exchange (GEF) factor and induces a-subunit to release its bound GDP
allowing GTP to bind in its place. In order to regulate G protein activity by regulating GDP/GTP
concentration, a subunit acts as
GTPase
GDP kinase
CGMP-specific phosphodiesterase
CAMP-specific phosphodiesterase