Download Calcitonin

Megan
Hutcherson
March
3,
2014
A
Study
of
the
hormone
Calcitonin
Calcitonin
is
a
hypocalcemic
agent,
acting
on
both
osteoclasts
and
renal
tubules
of
the
kidney
to
reduce
calcium
levels
in
the
blood
(Pondel
1).
Calcitonin
prevents
bone
reabsorption
in
osteoclasts
by
inhibiting
osteoclast
motility
and
inducing
a
gradual
retraction
of
the
osteoclasts
in
the
body
(Masi,
Brandi
4).
These
actions
are
completed
through
the
use
of
the
second
messenger
system;
where
both
calcium
and
CAMP
amplify
and
carry
out
this
bioregulator’s
signal
to
target
molecules
in
the
cell
(Norris,
Carr
54,
55).
In
addition,
calcitonin
has
the
ability
to
interfere
with
osteoclast
differentiation
and
inhibit
mononucleated
precursors
to
form
multinucleated
cells
(Masi,
Brandi
4).
Calcitonin
is
regulated
by
calcium
levels
in
the
blood
and
Calcitonin
will
function
to
decrease
systemic
levels
of
serum
calcium
when
hypercalcemia
occurs
(Wikipedia
1).
In
addition,
calcitonin
will
be
stimulated
through
the
release
of
gastrin
and
pentagastrin
(Wikipedia
1).
Calcitonin
and
calcitonin
gene
related
peptide
are
members
of
the
same
peptide
family,
which
consists
of
6
members
including
calcitonin
gene
related
peptide
one
and
two,
calcitonin,
amylin,
adrenomedullin,
and
intermedlin
(Barwell,
et.
all
2).
Calitonin
gene
related
peptide
functions
as
a
vasodilator
throughout
the
vascular
system
and
CGRP
also
has
a
neuromodulatory
role
in
the
nervous
system
(Masi,
Brandi
4).
It
has
been
proven
that
calcitonin
not
only
has
receptors
in
osteoclasts
and
the
kidneys,
but
in
the
gastrointestinal
and
reproduction
system
as
well
(Masi,
Brandi
5,6,7
).
In
humans
calcitonin
is
produced
by
parafolicular
cells
(c
cells)
in
the
thyroid
(Norris,
Car
508).
Parathyroid
cells
originate
from
the
ultimobrachial
body
and
this
is
made
from
the
sixth
pharyngeal
pouch
(Norris,
Car
508).
The
ultimobrachial
body
will
be
incorporated
into
the
thyroid
gland
in
humans,
but
it
does
exist
as
a
separate
structure
in
some
vertebrates
(Norris,
Car
508).
Calcitonin
is
a
32
amino
acid
polypeptide
hormone
with
a
molecular
mass
of
3418
Da
(Masi,
Brandi
2).
Calcitonin
exists
in
the
shape
of
an
alpha
helix
and
it
has
one
disulfide
bridge
connecting
Cysteines
at
positions
1
and
7
to
produce
a
7
amino
acid
ring
structure
at
the
amino
terminus
(Pondel
2).
Please
see
figure
one
below
for
an
image
of
the
structure
of
calcitonin.
Calcitonin
is
amphipathic
with
hydrophobic
regions
on
LEU
4,9,12,16
and
hydrophobic
regions
on
the
opposite
side
of
this
chain,
Cys
7,
Lys
11,
Glu
15,
and
lys
18
(Pateu
1).
Please
see
figure
two
below
for
an
image
of
these
hydrophobic
regions.
The
protein
sequence
for
calcitonin
has
been
determined
in
many
species
and
it
has
been
noted
that
this
sequence
is
highly
conserved
at
the
N
terminal
loop
region,
but
there
divergence
in
the
rest
of
the
sequence
(Pondel
2.)
“Fully
processed
CT
and
CGRP
transcripts
share
sequence
identity
in
the
amino
terminal
regions
but
are
almost
entirely
different
in
the
carboxy‐terminal
regions”
(Pondel
3).
Calcitonin
gene
related
peptide
is
a
37
amino
acid
polypeptide.
Thyroid
stimulating
hormone
exhibits
a
different
structure
than
both
calcitonin
and
calcitonin
gene
related
peptide.
TSH
is
a
category
one
trophic
hormone
(including
LH
and
FSH)
and
it
is
a
heterodimeric
cysteine
knot
glycoprotein
(Szkundiski
1).
This
hormone
contains
an
alpha
and
beta
subunit
and
it
is
encoded
by
genes
that
are
located
on
chromosome
1
and
6.
The
alpha
and
beta
subunits
contain
a
central
cysteine
knot
which
is
composed
of
three
disulfide
bonds
(Szkundiski
1).
The
alpha
subunit
of
TSH
has
a
two
turn
alpha
helix
and
the
beta
subunit
is
composed
of
a
beta
hair
pin
loop
on
one
side
of
the
cysteine
knot.
TSH
has
3
asparagine
linked
carbohydrate
chains
in
total
with
two
carbohydrate
chains
in
the
alpha
subunit
and
one
located
in
the
beta
subunit
(Szkundiski
1).
The
carbohydrate
component
of
this
hormone
makes
up
15
to
30%
of
the
alpha
and
beta
subunits
weight
(Norris,
Carr
111).
The
alpha
subunit
for
LH,
FSH,
and
TSH
is
identical
and
the
beta
subunit
is
responsible
for
the
hormones
individual
properties
(Norris,
Carr
111).
Figure
1
Figure
2
Calcitonin
is
encoded
by
a
gene
named
CALC‐1
and
this
gene
is
located
on
chromosome
11
in
the
human
genome
(Masi,
Brandi
2).
CALC‐1
is
a
member
of
the
calcitonin
gene
family
and
this
gene
family
includes
CALC‐2,
CALC‐3,
and
CALC‐4
(Masi,
Brandi
2).
CALC‐1
is
the
only
gene
in
this
gene
family
which
produces
calcitonin
and
this
gene
can
also
encode
calcitonin
gene
related
peptide
one
and
two
(Masi,
Brandi
2).
Calcitonin
and
calcitonin
gene
related
peptide
production
depends
on
the
actions
of
alternative
splicing
during
post‐transcriptional
processing
(Pondel
3).
Alternative
splicing
occurs
after
RNA
is
produced
and
during
this
process
introns
are
removed
and
then
exons
are
spliced
together
to
form
mRNA
(Norris,
Carr
43).
The
production
of
calcitonin
gene
related
peptide/calcitonin
depends
on
the
inclusion
or
exclusion
of
the
exon
four
(Pondel
3).
In
parafolicular
cells
of
the
thyroid
CT/CGRP
pre‐mRNA
is
processed
to
include
exon
four
(Pondel
3).
This
is
followed
by
polyadenylation
(Synthesis
of
a
poly
A
tail
at
the
three
prime
end
of
RNA)
and
the
transcription
of
Calcitonin
(Pondel
3).
Calcitonin
gene
related
peptide
is
formed
through
the
exclusion
of
exon
four
and
the
inclusion
of
exon
five
and
six
(Pondel
3).
Exon
6
is
used
as
a
polyadenylation
site
to
produce
calcitonin
gene
related
peptide.
In
addition
to
mRNA
processing
of
CALC‐1,
the
production
of
both
these
hormones
is
regulated
through
the
use
of
transcriptional
regulatory
elements
(Pondel
4).
CALC‐1
gene’s
promoter
contains
CPG
islands
near
exon
one
and
1.5kb
after
this
exon
(Pondel
4).
CPG
islands
are
unmethylated
CPG
regions
of
DNA
that
have
a
different
chromatin
structure
from
bulk
chromatin
due
to
the
absence
of
histone
H1.
In
addition
CPG
islands
contain
hyperacetylated
histones
(H3
and
H4)
and
have
nucleosome‐free
segments
of
DNA
(Pondel
4).
The
CPG
islands
at
the
end
of
CALC‐1
gene
may
affect
a
promoter’s
availability
to
transcription
factors
(Pondel
4).
Therefore
it
is
important
to
keep
an
open
chromatin
conformation,
through
the
use
of
hypermethylated
DNA,
to
help
transcription
factors
bind
to
the
CALC1
promoter
and
activate
CT
gene
expression
(Pondel
4).
The
biosynthesis
of
Calcitonin
follows
the
same
mechanisms
involved
in
the
synthesis
of
protein
bioregulators
including
transcription,
alternative
splicing,
and
translation.
Calcitonin
is
initially
transcribed
from
CALC‐1
to
form
a
136
amino
acid
preporpeptide
(PRECT),
which
contains
a
leader
sequence
at
the
amino
terminal
region
(Pondel
3).
This
leader
sequence
is
cleaved
when
PRECT
is
transported
to
the
endoplasmic
reticulum
(Pondel
3).
After
transportation
to
the
endoplasmic
reticulum,
procalcitonin
is
produced
(PROCT)
and
transported
to
the
golgi
apparatus
via
exocytosis
(Masi,
Brandi
2).
In
the
Golgi
apparatus
PROCT
is
cleaved
by
Prohormone
Convertase
enzymes
to
produce
immature
calcitonin
(ICT)
(Masi,
Brandi
2).
ICT
will
then
undergo
additional
protolithic
cleavage
in
secretory
vesicles,
of
the
Golgi
apparatus,
to
produce
mature
calcitonin
(MCT)
(Masi,
Brandi
2).
Secretory
vesicles
will
then
fuse
with
a
cells
plasma
membrane
and
calcitonin
will
be
stored
in
the
cytoplasm
till
use.
Chromogranin
B
may
act
as
a
helper
protein
to
aid
with
trans‐golgi
sorting
during
biosynthesis
(Masi,
Brandi
2).
Please
see
figure
three
bellow
for
a
description
of
Calcitonin’s
biosynthetic
pathway.
Figure
3
In
order
for
Calcitonin
to
reduce
bone
reabsorption
and
increase
calcium
excretion
from
the
kidneys
it
needs
to
be
able
to
bind
to
the
Calcitonin
receptor.
The
calcitonin
receptor
is
a
G
Protein
coupled
receptor
and
these
receptors
are
often
called
seven
transmembrane
receptors
since
they
pass
through
the
plasma
membrane
seven
times
(Wikapedia1).
Calcitonin
is
able
to
bind
to
at
least
two
signal
transduction
pathways
including
the
CAMP
signal
transduction
pathway
and
phospholipase
C
(PLC)
enzyme
pathway
(
Masi,
Brandi
5).
The
CAMP
signal
transduction
pathway
utilizes
a
G
protein
called
Gs
and
this
G
protein
consists
of
a
beta,
alpha
and
lambda
subunit
(Norris,
Carr
54).
The
alpha
subunit
of
the
GS
G
protein
will
work
with
the
enzyme
adenylyl
cyclase
to
produce
the
second
messenger
CAMP
(Norris,
Carr
54).
When
this
G
coupled
receptor
lacks
a
ligand
(in
this
case
calcitonin)
GDP
will
be
bound
to
the
GS
G
protein
and
the
enzyme
adenylyl
cyclase
is
inactive
(Norris,
Carr
54).
However,
when
a
ligand
binds
to
the
G
protein
coupled
receptor
GDP
will
be
exchanged
for
GTP
and
the
new
GTP
will
bind
to
the
alpha
subunit
of
the
G
protein
(Norris,
Carr
54).
The
alpha
subunit
can
then
dissociate
from
both
lambda
and
beta
subunits
and
interact
with
adenylyl
cyclase
to
produce
CAMP.
To
turn
off
this
receptor
the
alpha
subunit
will
convert
GTP
back
to
GDP
in
order
to
decrease
the
activity
of
adenylyl
cyclase
and
therefore
the
production
of
CAMP
(Norris,
Carr
54).
The
alpha
subunit
will
then
recombine
with
both
Lambda
and
beta
subunits
to
wait
for
the
next
ligand
to
bind
to
the
G
protein
Coupled
receptor
(Norris,
Carr
5).
It
is
important
to
note
that
in
this
signal
transduction
pathway
CAMP
act
as
a
second
messenger
to
amplify
and
carry
the
bioregulators
signal
into
the
cell.
The
Phospholipase
C
signal
transduction
pathway
is
completed
through
the
use
of
a
G
protein
named
GQ.
During
this
signal
transduction
pathway
phosphatidylinositol
biphosphate
(PIP2)
is
cleaved,
leading
to
the
production
of
Inositol
triphosphate
(IP3)
and
diacylglycerol
(DAG)
(Norris,
Carr
55).
These
molecules
will
act
as
second
messengers
in
this
pathway.
IP3
will
cause
calcium
to
be
released
from
the
endoplasmic
reticulum
and
this
calcium
can
also
act
as
a
second
messenger
(Masi,
Brandi
5).
DAG
and
calcium
will
turn
on
a
protein
kinase
C
to
activate
enzymes
in
the
cytoplasm
of
the
cell
(Norris,
Carr
55).
Both
calcitonin
gene
related
peptide
and
calcitonin
receptors
are
a
part
of
the
15
human
family
B
(or
Secretin‐like)
GPCRs.
This
subfamily
will
form
complexes
with
receptor
modifying
(RAMP
1,2,
3)
and
“RAMP
association
with
the
CTR
or
with
CLR
generates
multiple
distinct
receptor
phenotypes
with
different
specificities
for
the
CT
peptide
family”
(Barwell
2).
Both
calcitonin
and
calcitonin
gene
related
peptide
can
use
the
CAMP
or
phospholipase
C
signal
transduction
pathways.
RAMP’s
interact
with
calcitonin
and
calcitonin
gene
related
peptide
receptors
to
change
their
selectivity
to
certain
ligands
and
therefore
their
function
in
the
cell.
An
association
with
RAMP
one
creates
the
calcitonin
gene
related
peptide
receptor
while
adding
RAMP
one,
two,
or
three
causes
the
calcitonin
receptor
to
take
on
the
function
an
Amylin
receptor
(Barwell
et.
all
2).
Amylin
is
a
37
amino
acid
peptide
that
functions
to
inhibit
insulin
secretion,
glucose
transport
to
skeletal
muscles,
gluconeogenesis,
and
gastric
emptying
(Masi,
Brandi
1).
Amylin
also
contains
some
capabilities
to
modulate
calcium
excretion
by
decreasing
calcium
reabsorption
(Masi,
Brandi
1).
The
only
reason
for
differentiation
between
calcitonin
and
calcitonin
gene
related
peptide
is
the
inclusion
of
RAMP
one
into
the
calcitonin
gene
related
peptides
receptor
(Barwell
et.
all
2).
The
calcitonin
receptor
does
not
exhibit
a
RAMP
protein
when
it
is
preforming
the
actions
of
interfering
with
regulation
calcium
and
phosphate
metabolism.
Both
receptors
contain
a
large
N
Terminal
Extracellular
Domain
(ECD),
3
extracellular
loops
(ECL
1,2,3),
7
transmembrane
domains,
3
intracellular
loops
(ICl
1,2,3)
and
an
intracellular
C
terminus
(Barwell
et.
all
3).
“The
concept
that
most
simply
encapsulates
the
mode
of
ligand
binding
and
activation
of
family
B
GPCR
is
known
as
the
two
domain
model”
(Barwell,
et.
all
3).
In
this
model
the
C
terminus
of
a
ligand
is
captured
by
the
ECD,
brought
to
the
ECL
and
upper
transmembrane
domain
of
the
receptor
where
the
receptor
will
undergo
a
conformational
change
(Barwell,
et.
all
3).
This
will
allow
the
receptor
to
be
turned
on
and
second
messenger
production
to
carry
the
bioregulators
signal.
The
three
intracellular
loops
and
receptor
C
terminus
are
responsible
for
interacting
with
the
GQ
or
GS
protein
to
carry
out
the
signal
transduction
pathway
(Barwell,
et.
all
3).
The
ECL
and
the
upper
transmembrane
domain
referred
as
the
juxtamembrane
region
and
the
ECD
is
the
site
where
the
ligand
will
bind
to
its
receptor.
“The
family
B
GPCR
ECDs
all
share
the
same
overall
fold
consisting
of
two
antiparallel
β‐sheets
and
an
N‐
terminal
α‐helix
that
is
stabilized
by
three
disulfide
bonds”
(Barwell,
et.
all
3).
Peptides
that
bind
to
the
ECD
will
adopt
a
helical
conformation
and
have
stabilizing
hydrophobic
and
electrostatic
interactions
with
its
receptor
(Barwell,
et.
all
3).
A
diagram
of
the
calcium
gene
related
peptide
receptor
can
be
seen
in
figure
4
and
this
receptor
structure
is
very
similar
to
Calcitonin’s
receptor.
Figure
4
The
process
of
metabolizing
calcitonin
in
the
body
is
due
to
the
existence
of
peptidase
enzymes
in
the
blood.
Peptidase
enzymes
will
attack
one
end
of
the
polypeptide
hormone
and
remove
its
amino
acids
one
at
a
time
to
decrease
the
biological
effects
of
this
hormone
(Norris,
Carr
59).
The
entire
hormone
will
eventually
be
lost
through
this
process
and
the
peptide
hormone
is
no
longer
able
to
bind
to
its
receptor
(Norris,
Carr
59)
“CT
has
a
short
biological
half‐life
(five
minutes)
related
to
the
presence
of
peptidases
in
the
blood
and
the
fact
that
all
fragments
of
CT
are
inactive”
(Norris,
Carr
509).
References
Barwell,
J.,
et.all.
(2012).
Calcitonin
and
Calcitonin
Receptor‐like
Receptors:
Common
Themes
with
Family
B
GPCRs.
US
National
Library
of
Medicine
&
National
Institutes
of
Health,
51‐65.
Carr,
J.,
&
Norris,
D.
(2013).
Vertebrate
Endocrinology.
Walthman,
MA:
Elsevier
Inc.
Masi,
L.,
&
Brandi,
M.
(2007).
Calcitonin
and
Calcitonin
Receptors.
US
National
Library
of
Medicine
&
National
Institutes
of
Health,
117‐122.
Pondel,
M.
(2000).
Calcitonin
and
Calcitonin
Receptors:
Bone
and
Beyond.
US
National
Library
of
Medicine
&
National
Institutes
of
Health,
405‐422.
Szkudinski,
M.
Thyroid
Stimulating
Hormone
and
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