Cardiac Cell Biology Flashcards
cardiac cell biology compared with skeletal muscle
similarities: • basal lamina • striated • contractile proteins similar (but not identical) • mechanism of contraction is similar differences: • involuntary (vs. voluntary skeletal muscle, which is operated by motor nerves) • myocytes are smaller • 1-2 central nuclei (sometimes 3) (vs. skeletal muscles, which can have 100s of nuclei) • myocytes branch • ++++ vascular • ++++ mitochondria • ++++ myoglobin • ++++ lipid droplets • MB-creatine kinase • intercalated discs (nothing like this in skeletal muscle) =>cardiac muscle is aerobic!!
cardiac myocytes–when seen on LM longitudinal section
- cardiac myocytes branch (arrowheads)
- intercalated discs (arrows) are at boundaries between myocytes • cross-striations are faint, in contrast w skeletal muscle
- nuclei are central
cardiac myocytes–when seen on LM cross section
• myofibrils are discernable
• venule (V) & capillaries(C) = highly vascular
- (note RBCs in capillaries)
-no striations seen in cross section
cardiac myocyte–when seen on EMs
• sarcomeres (fundamental unit of cardiac muscle contraction) aligned as in skeletal muscle • many more mito!!! • lots of glycogen Note: • central nucleus • endothelial cell (making a capillary) • profuse mitochondria -MB-CK seen
cardiac myocyte–when seen on EM in cross section
Note:
• basal lamina (BL) v. adjacent cardiac myocytes
• 6 thin filaments per thick filament
• SR surrounds myofibrils
intercalated discs (IDs)
- sacrolemma specializations v. adjoining cells
- enable cardiac myocytes to work as a unit, as if they were in a syncytium!!
intercalated discs (IDs)–cross adjacent myocytes in “staircase fashion”
- transverse: transmits force; modified Z-band
- fascia adherens (N-cadherins) and desmosomes
- lateral: cell-cell signaling
- gap junctions (nexus) and some desmosomes
- Note: some textbooks don’t consider the lateral component as a part of the intercalated disc.
intercalated disc ultrastructure
-‘last’ I-band(s) attaching to “Z-line
-gap junctions are along this lateral border
-N-cadherin in intercellular space
• Discs cross adjacent cells in stepwise fashion.
• fascia adherens ~ zonula adherens (but doesn’t encircle the cell)
Excitation-Contraction Coupling
Excitation (electrical)
• Action Potential: depolarization –> T-tubules
• Phase 2 of AP: L-type Cav1.2–>Ca++ influx
• Ryanodine Receptors in SR:–>–>–>Ca++ (CICR=Calcuim Induced Calcium Response) (a lot more influx of Ca++ is generated from this)
Contraction (physical response to the electrical change)
• Ca++ binds troponin-C tropomyosin moves
• myosin head is activated by ATP hydrolysis & myosin head binds actin
• power stroke = contraction: myosin pulls actin into the A-band & the sarcomere shortens
when thin filaments move, sarcomeres shorten. what happens during contraction and relaxation??
During Contraction
• A-band length stays same
• I-band length shortens Relaxation
- L-type channels inactivate
- Ca++ is re-sequestered into SR via SERCA (sarco-endoplasmic reticulum Ca++ ATPase)
effect of B-adrenergic stimulation (catecholamines-i.e. norepinephrine)
increased cAMP levels–>activated protein kinases–>phosphorylation–>L-type Ca++ flux–>enhanced contractile force
increased cAMP levels–>activated protein kinases–>phosphorylation–>phospholamban (in SR)–>increased Ca++ uptake by SR–>relaxation
cardiac myocyte innervation
the myocardium has an intrinsic, rhythmic beat
vagus nerve (parasympathetic)–>ACh–>muscarinic receptors–>regulate HR (relaxation)
regional histologic differences in the heart: atria, SA, AV nodes
•atria, SA, AV nodes: smaller myocytes with fewer striations
– in atria, membrane-bound granules (G) contain atrial natriuretic factor (ANF aka ANP).
– ANF (ANP) has many functions, including vasodilation.
regional histologic differences in the heart: ‘Bundle of His’
• ‘Bundle of His’ contains ‘Purkinje’ myocytes – specialized for conduction
– endothelin –> cardiac myocyte –> Purkinje fiber
regional histologic differences in the heart: endocardium
• endocardium: single layer of endothelial cells
regional histologic differences in the heart: “cardiac” fibroblasts
• “cardiac” fibroblasts: the heart’s most abundant cell type - but, myocytes are larger & comprise most of the heart volume
chronology of a heart attack
- immediate: myocyte death–> MB-CK & cTnI
- +15 hrs: inflammation
- +2-3 days: wound healing via cardiac fibroblasts (collagen deposition; fibrosis to heal wound=good, BUT collagen does not contract like cardiofibers can and should contract)
- +2-4 days: angiogenesis (clinical enhancement via VEGF, FGF?)
- scar deposition (due to collagen cross-linking
during a heart attack, cardiomyocytes die. Inability to regenerate cardiac muscle=major clinical problem. can heart muscle regenerate?
- Dogma pre-2000: No. We received our allocation of cardiac myocytes a long time ago.
- 2000-2010: The myocardium naturally regenerates, but at a rate that is too low to restore effective function.
Can skeletal myoblasts – i.e. skeletal muscle stem cells – re-muscularize the injured heart?
This approach has been abandoned due to inefficacy and arrhythmia.
- ~3% of nuclei in skeletal muscle are skeletal myoblasts–>can regenerate skeletal muscle
- saw some improvement in function, but arrhythmias arose! makes sense…skeletal muscles do not contract in the same way as cardiac muscle
Are “cardiac” fibroblasts (CFs) endogenous CM stem
cells?
• No. Fibroblasts cannot become myocytes.
• At least 1 CF borders each CM.
• But, fibroblasts can’t regenerate the heart.
• Fibroblasts respond to injury by making a scar.
• Scars are permanent.
• Scars don’t contract.
• This is not good.
But, as of 2012, CFs may be “induced” into CMS!
Can existing myocytes mobilize to fix damaged myocardium?
– Maybe
–Adult cardiomyocyte proliferation can be induced by…
• …inhibiting p38 MAP kinase
• …pro-proliferative agents
• Phase 1 trials (safety & efficacy) are planned to evaluate neuregulin.
• Alternatively, can tumor suppressors (such as Rb, p16ink) in cardiomyocytes be inhibited to permit cell replication?
Can adult stem cells in the heart fix damaged heart?
– Obviously not; but, maybe they could be mobilized to do so.
– These adult stem cells are identified per expression of a stem cell marker termed c-Kit.
– Their existence is controversial; however, adult cardiac stem cells likely exist, in ‘niches’.
– Can they be mobilized?
– How to mobilize them?
• Treat heart with drugs and/or growth factors to mobilize & expand these stem cells in their niches.
Can c-Kit+ adult stem cells be transplanted to fix injured myocardium?
Perhaps, as per an ongoing Phase 1 clinical trial,
a. Isolate c-Kit+ cells from R. atrial appendage or R. IV. septum via catheter.
b. Expand c-Kit+ cells in culture.
c. Transplant 2 million c-Kit+ cells back into same patient (so no immune rejection).
d. Some function is restored & infarcted area is reduced, while “No harm is done”.
• These positive effects are likely mediated via ‘paracrine’ factors that enhance the heart. (Studies in animals have shown that c- Kit+ cells don’t make muscle.)
Do transplanted bone marrow cells fix injured myocardium?
• Idea: Bone marrow cells might migrate to the heart & take up residence as adult stem cells. Evidence:
– Transplant female heart into male host.
– ‘Male’ cells invade this female heart.
Can transplanted bone marrow cells fix the heart?
• Maybe.
• Harvest bone marrow cells from patient. – from bone marrow or from peripheral blood
• Transplant into same patient via catheter.
• Animal studies revealed little or no re-muscularization. – But, functional benefit occurred (via ‘paracrine’ effect?).
• Phase 1 clinical trials have been performed – ‘No harm is done.’
– Result: Modest increase in heart function, albeit transient
– Current Focus: identify the best bone marrow cell type.
• Example: mesenchymal stem cells (MSCs)
Transplantation of CMs derived from induced pluripotent cells (iPSCs)?
• iPSCs–>200 different cell types
• Generated via “reversal of aging”
– patient’sdermalfibroblasts/Tcells+3-4 transcription factors–> iPSCs + cardiomyogenic growth factors –> cardiomyocytes –> transplant into same patient
–>after the last two years, we know this is a definite possibility!
-Beating cardiomyocytes can definitely be generated from iPSCs, whereas it remains uncertain whether contractile cells can be generated from adult stem cells.
-However, inefficiency & potential tumorigenesis are still formidable problems.
