L17 - Cardiogenesis Flashcards by Ellie R

Cardiovascular disease facts

Leading cause of death worldwide –31
152,000 people in the UK died from CVD last year – 416 people per day
Congenital heart diseases affect around 1% of live births
Cost to the UK economy around £19 billion per year

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What organisms have 4 chambered hearts?

Human
Mouse
Chick

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What organism has a 3 chambered heart?

Xenopus

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What organism has a 2 chambered heart?

Zebrafish

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What organism has a tubular heart?

Drosophila

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Morphological heart development in humans at 2 weeks

Cardiac precursor cells found as bilateral populations of cells around midline

Mesodermal tissues
Two endocardial tubes

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Morphological heart development in humans at 3 weeks

Cardiac precursor cells migrate to midline and fuse to form the heart tube

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Morphological heart development in humans at 4 weeks

Heart tube undergoes asymmetric bending morphogenesis - heart looping

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Morphological heart development in humans at 7 weeks

Undergoes maturation, resulting in formation of structures required for function
- Valves, septa, trabeculae

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Morphological heart development in zebrafish

20 hours - cardiac disc - formed from two groups of precursors that migrate to the midline
36 hours - heart looping begins
48 hours – maturation begins

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What are the two heart fields?

The heart is constructed from two populations of cardiac cells

Specified early during development
Exhibit distinct spatiotemporal differentiation
- First heart field (FHF)
- Second heart field (SHF)

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What does the first heart field form?

Left ventricle, left and right atria

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What does the second heart field form?

Right ventricle, left and right atria, outflow tract

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Where are cardiac cells specified from in mice?

Specified from mesodermal tissue along the primitive streak

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Where are cardiac cells specified from in fish?

Specified from mesodermal tissue along the embryonic margin

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What is specification of cardiac cells controlled by?

Combinatorial morphogen signalling

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What happens to cardiac cells once they are specified?

They migrate anteriorly to form the FHF and SHF in the adjacent mesoderm

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What signals determine where the cardiac cells migrate to?

All show non-canonical Wnt
Then either
- FGF, Wnt - SHF
- BMP, Tbx5 - FHF

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What are the 3 functional regionalisation’s that need to occur in the heart?

Chamber vs non-chamber
Atrial vs ventricular contractility
- Directional conduction to allow correct flow of blood through the heart
Different regions
- Inflow region - pacemaker/SA node
- Atrioventricular canal - valve/ AV node
- Outflow region - valves

What controls the functional regionalisation of the heart?

Complex interplay of activating and inhibitory interactions

Notch – Tbx20
- Important for specifying chamber
Bmp – Tbx2
- Important for specifying atrioventricular canal

How do Tbx20 and Tbx2 interact?

Tbx20 represses Tbx2 which represses Tbx20

What phenotypes are shown in a Tbx2 knockout?

Mice have abnormal valve morphology and Nppa expression

Expansion of chamber markers in the AVC – Tbx20

What phenotypes are shown in a Tbx20 knockout?

Mice exhibit a loss of chamber identity

Expansion of valve markers throughout heart – Tbx2

How can heart looping morphogenesis be monitored?

Linear heart tube from zebrafish removed

- Can be developed in a dish to allow us to visualise development

What controls heart looping morphogenesis?

Changes in cell shape - Cells on outer curvature – grow more and elongate - Cell on inner curvature – stay cuboidal Growth of the heart tube - SHF cells add into the heart tube as into undergoes morphogenesis Asymmetric cell movements at the poles of the heart Regional changes in ECM composition Lateralised cell signalling from the embryo - Helps the cell know which direction it should bend in

How can you visualise the SHF in heart looping morphogenesis?

Promoter of gene (myl7) used to drive GFP expression - All cardiac cells express GFP - Can visualise linear heart tube - Expressed in SHF cells in lower levels

Where do SHF cells migrate to?

Cells migrate to the two poles of the cell during morphogenesis and get incorporated

What helps promote looping morphogenesis?

Growth of the heart through SHF addition

What does mRNA ISH analysis of Islet1 show?

Expressed in the mesoderm adjacent to the heart - Expressed in the SHF - LacZ lineage tracing of Islet-1 expressing SHF cells show they end up in the heart itself

What does mRNA ISH analysis of myl7in Islet-1 mutant show?

Heart is reduced in size and is incompletely looped | - Structure of the heart is much simpler

What are the conclusions from these mRNA analysis of ISH?

SHF cells reside in the mesoderm adjacent to the heart SHF cells become incorporated in the heart If SHF cells not present looping is not completed and heart is smaller

What is heterotaxia?

L/R asymmetry abnormalities | Often have congenital heart defects

What is the situs inversus form of heterotaxia?

Approximately 1/10,000 of population | Usually asymptomatic

What is the situs ambiguous/heterotaxia form of heterotaxia?

Unknown prevalence Usually associated with significant pathology Abnormal cardiac development and function common

What are the congenital heart defects associates with heterotaxia?

``` VSD – ventral septal defect DILV – double inlet left ventricle DORV – double outlet right ventricle TGA – transposition of the great arteries PTA – persistent truncus arteriosus ASD – atrial septal defect ```

What is the importance of Nodal in organ asymmetry?

Nodal - asymmetrically expressed in left lateral plate mesoderm prior to organ formation Loss of Nodal - disrupted organ asymmetry

How is organ asymmetry generates in the mouse and zebrafish?

Through the node, nodal flow, and Nodal | In zebrafish – Spaw in Nodal homologue

The node and nodal flow method

1. Transient cup-shaped organ forms during early somitogenesis at posterior end of embryo 2. Lined with motile cilia - beat in a clockwise movement 3. Creates a directional fluid flow - elevated calcium in left side of embryo 4. Leads to asymmetric expression of nodal - Spaw propagates its expression in left lateral plate mesoderm 5. Asymmetric gene expression in organ anlage - Nodal target genes are expressed in left half of cardiac disc 6. Asymmetric movement of cells to form the heart tube - Cardiac disc undergoes rotation and involution to form tube

Loss of nodal flow in fish

Randomisation of spaw expression and directional heart displacement

Loss of nodal flow in humans

Individuals with mutation in ciliary genes which cause primary ciliary dyskinesia also exhibit heterotaxy

What happens with a loss of Nodal?

Absence of lateralised gene expression

Loss of Nodal in fish

Loss of asymmetric gene expression results in a failure of leftward displacement of the heart tube

Loss of Nodal in humans

Identification of Nodal variants in individuals with heterotaxy and isolated cardiovascular malformations

Is the heart functional during heart development?

Yes

What are the sources of mechanical force in the heart during development?

Cardiac contractility Contractility causes blood to flow through the heart - Important for spatiotemporal gene expression of forming valves - Occurs through flow-responsive genes e.g. klf2