Heart Muscle Differentiation

Heart Muscle DifferentiationTHE HEART• Structure and function• Anatomy of the heart

HEART DEVELOPMENT• Overview of Heart Formation• Genetic factors regulating cardiac muscle differentiation

WHAT HAPPENS WHEN IT ALL GOES WRONG?• Congenital Heart Disease

WHAT ARE THE REQUIREMENTSOF A PUMP???

1. Receiving ChambersLeft and Right

Atria

2. Delivery Chambers

Left and Right Ventricles

3. Valves to direct the flow of blood

pulmonaryValve

WHAT ARE THE REQUIREMENTSOF A PUMP???

tricuspidValve

Aortic Valve

Mitral/Bicuspid Valve

IntercalatedDisc

Nucleus

4. Strongly contractile wall to provide the force required to propel blood

WHAT ARE THE REQUIREMENTS OF A PUMP?

CARDIAC MUSCLECELLS

•Cardiomyocytes

• contractile element of cardiac muscle

• elongated cells

• centrally located nucleus

• branched cells

• separated from each other by the presence of intercalated discs

• filled with rod-like bundles of myofibrils (contractile proteins s.a. myosin and Actin)

NucleusIntercalatedDisc

THE 6 REQUIREMENTS OF A PUMP

1. Receiving chambers

2. Delivery chambers

3. Valves to direct the flow of blood

4. Strong contractile wall to propel blood

5. Vessels to deliver blood

6. Conduction system to regulate the pump

MITRAL VALVE

TRICUSPID VALVE

PULMONARY VALVE

AORTIC VALVE

RIGHTATRIUM

INTERVENTRICULARSEPTUM

AORTA

PULMONARY ARTERY

LEFT ATRIUM

LEFT VENTRICLE

RIGHT VENTRICLE

ASCENDING AORTA

LEFT ATRIUM

LEFT ANTERIOR DESCENDINGCORONARY ARTERY

SUPERIOR VENA CAVA

INFERIOR VENA CAVA

RIGHT ATRIUM

CONDUCTION SYSTEM

AV NODE

SA NODE

5. Vessels to Deliver Blood6. Conduction System

HEART FORMATION

• Gastrulation - formation of 3 germ layers

• ENDODERM• ECTODERM• MESODERM

• Heart is derived from the mesoderm

• First indication of human heart development is around day 16-19

• How do we progress from a single layer of mesoderm to the complex 3 dimensional structure of the heart???

Morphogenesis

differentiation• Genes

• Process of HeartFormation

CardiacCresent

Cardiac Progenitor

cells)

Cardiogenic Mesoderm

Endothelial (endocardial) Tubes

HEART FORMATION1. Formation of endocardial

tubes derived from mesodermal cells

2. Formation of the Primitiveheart tube

Primative/linear Heart Tube

Day 18 Day 22

FusingHearttubes

HEART FORMATION3. Primitive heart tube develops into 5 distinct regions

Day 22

FusingHearttubes

BulbusCordis

Ventricle

Sinus VenosusAtrium

TruncusArteriosus

HEART FORMATION4. Primitive heart tube Twists

Looping brings the distinct regions of the heart into the basic pattern that prefigures the adult structure

Atrium

Bulbus Cordis

Ventricle

Sinus Venosus

Truncus Arteriosus

Day 23

HEART FORMATIONThe bulbus cordis and truncus arteriosus have divided into two

vessels forming the aorta and pulmonary trunk

PulmonaryTrunk

Ventricle

Superiorvena cava

Atrium

Aorta

InferiorVena cava

Week 7 - the interatrial and interventricular septa have formedpartitioning the atria and ventricles into L and R compartments

Cardiac crescent stage

(E7.75)

Intra-embryonic coelomHeart progenitors

Linear heart tube(E8.0)

MyocardiumEndocardium

Looping heart(E10.5) Inter-ventricular septum

Trabeculae

Endocardial cushions

Conotruncal cushions

LVRV

Remodelling heart(E12.5) Inter-ventricular septumRV LV

LARAEndocardial cushions

Trabeculae

Atrial septum

Mouse Heart Formation

Neural Plate

Ectoderm

Cardiac Mesoderm

Head Mesoderm

Early identification of Cardiac Progenitorsin Mouse Embryo Day 7/7.5

E7.75

CardiacCresent

Cardiac Progenitor

cells)

Whole mouse embryo Transverse section

Heart Development in the Mouse

J.M. Icardo, 1997

AT WHAT STAGE DOES THEHEART START PUMPING?

• Human primitive heart begins contracting at day 22

• Mouse heart starts to contract around day 8

• Why does the heart start to contract so early???

GENETIC CONTROL OF HEART MUSCLECELL DIFFERENTIATION

• Genetic dissection of heart (dorsal vessel) formation inDrosophila has led to the identification of a number ofgenes implicated in heart determination

tinman

Dhand

bagp

ipe

Dmef2

dpp

twist

What does tinman do?

• homeobox gene

• identified in 1989 by Kim and Nirenberg

• localised to the dorsal mesoderm

• later stages to heart precursors and mesoderm

• Does tinman play a role in heart formation?

• Disco - identify cardial cells of the dorsal vessel

• visceral mesoderm expression in wt embryos

• tinman k/o - no heart or visceral mesoderm formation

• tinman expression is crucial for heart formation in drosophila

Does tinman play a role inheart development?

TRANSCRIPTIONAL CASCADE FOR CARDIOGENESIS IN DROSOPHILA

TWIST

Ventralmesoderm

Dorsalmesoderm

Heart Precursors

tinmanDpp Wg

tinman

D-mef2

Contractile protein genes

Cells committedto a cardiogenic fate

Activation oftranscription

D-mef2

• twist is essential for mesoderm formation

• 6 different genes related to tinmanwere isolated from divergentspecies

Nkx2-3Nkx2-5 (1993 Kamuro and Izumo,

Harvey 1996)Nkx2-6Nkx2-7Nkx2-8Nkx2-9

• evolutionary conservation

• Does Nkx2-5 have a similar function to tinman?

E7.75

Localisation of Nkx2.5 positive cells in thedeveloping mouse embryo using Lac Z Expression

CardiacCresent

Cardiac Progenitor

cells)

• Nkx2-5 expressed in the mesoderm

• later stages it is only expressed in the heart

• panel E shows no expression of Nkx2-5 in the lungs

• k/o Nkx2.5 there is no effecton early heart formation

• looping/twisting morphogenesis isaffected

Peri

-car

dial

Mus

cle

VM

Expression of Cardiac NK2-class Homeobox genes

tinman

Nkx2-5

Drosophila

Mouse

Chick

Zebrafish

Frog Nkx2-5

GENETIC CONTROL OF HEART MUSCLECELL DIFFERENTIATION

• Nkx2 class hoemobox genes are expressed during gastrulation in the lateral plate mesoderm (mouse, frog, avian and fish embryos)

• critical determinants of cardiac development

• Studies in Drosophila have shown tinman expression to be essential for heart development

• Absence of Nkx2-5 in the mouse doesn’t prevent theformation of the heart tube but blocks loopingand septum formation

CONSERVATION OF GENETIC PATHWAYS IN HEART DEVELOPMENT

CONSERVATION OF THE GENETIC PATHWAYS IN HEART DEVELOPMENT

Congenital Heart Disease• Defects present from birth

• affect <1 % of all children

• Morphogenesis of the heart has many stages

• Ample opportunity for something to go wrong

• Not surprising that abnormalities occur, perhapsmore surprising that the occurrence of abnormality is so infrequent.

Pulsatingtube

Twisting/rotatingheart

Septa Formation 4 Chamber Organ

Congenital Heart Disease

• What kind of abnormalities occur????

• Incomplete septa formation (holes between chambers)• Incorrect connection between chambers and vessels• valves that don’t function properly

• Atrial septal defect• ventricular septal defect• Tetralogy of Fallot

Atrial Septal Defect

• incomplete closure between 2 upper chambers• Blood returning from lungs flows through the hole• More blood flows through R side of heart• pulmonary hypertension• in most cases surgery can rectify this problem

• incomplete closure between 2 ventricles• Initially blood flows from R to L• heart dilates, pressure increases resulting in

pulmonary hypertension due to increased workload• in most cases spontaneous closure occurs•If needed surgery can rectify this problem

Ventricular Septal Defect

1. Ventricular septal defect2. Pulmonary stenosis (narrowing of pulmonary artery)3. Hypertrophy (thickening of the RV wall)4. Overriding aortaDecreased blood flow to the lungs and mixing of blood

Tetralogy of Fallot

• Recent investigations have mapped a mutation causing atrial septal defects to the NKX2-5 gene(Schott et al 1998)

• Decreased capacity of this transcription factor tobind DNA

• implicates Nkx2-5 in atrial septation and conduction system development

Nkx2-5 and Human Cardiac defects

More recent reviews

Buckingham et al 2005 Building The Mammalian Heart From Two Sources Of Myocardial Cells Nature reviews –genetics 6:826-835

Srivastava 2006 Making or Breaking the Heart: From

Lineage Determination to Morphogenesis Cell 126:1037-1048

NKX2-5 in ventricular muscle – Srivastava Nature 2004