2020 lecture 13

How does the cytoskeleton help with cell migration?

• Embryonic development

• Wound healing

• Immune response

Whats the primary component of the cytoskeleton that drives cell migration?

Actin filaments.

They polymerise at leading edge of the cell, forms protusion called “lamellipodium”, which pushes cell forward. Also form stress fibres which connect leading edge to the cell body and generate tension to pull cell forward.

What happenes during cell migration?

Actin filaments constantly reorganised and dissasembled to allow cell to change shape and direction. Process is regulated by protein networks including Rho GTPases which act as molecular switches to control actin dynamics.

Components of Rho GTPases family

Rac1 and Cdc42 are 2 family members involved in lamellipodium formation (Rac1) and Cdc42 for filopodia.

Rac1 and Cdc42 activate downstream effector proteins, such as WAVE complex and Arp2/3 complex, which promote formation and reorganization of actin fillaments and cytoskeleton.

Gastrulation

Key process in embryonic development involving formation of 3 germ layers. Wnt siganling pathway involved, activated in spatially and temporarily regulated manner (Beta-cantenin).

Why is Wnt signalling important?

Key in gastrulation. Essential for formation of the dorsal-ventral axis, which establishes the back and belly of the embryo. In vertebrates, Wnt signalling activated in dorsal side of embryo, leads to formation of organiser (a cluster of cells that induces formation of neural tube (precursor to the CNS))

What signalling is involved in gastrulation?

BMP and FGF signalling involved in specifying anterior-posterior axis, which determines head and tail of embryo, and the left-right axis which determines orientation of internal organs.

The cytoskeletons role in cell differentiation

Cytoskeleton regulates gene expression and cell fate by influencing organization and function of nucleus by providing stuctural support for it along with actin fillament microtubules.

Whats the cytoskeletons role in regulation of transcription factors?

Yes-associated protein (YAP) regulated by actin cytoskeleton and Hippo signaling pathway, controllign cell proliferation and differentiation. YAP activated in response to changes in cell shape and tension

Myocardin-related transcription factor (MRTF) activated by changes in actin dynamics and interacts with cytoskeleton to regulate gene expression and cell fate determination.

What happens with defects in a nuclear lamin?

Can cause a rare class of premature aging disorder called Progeria.

Nuclear instability leads to impared cell division, increased cell death, a diminished capacity for tissue repair, or combination of the 3.

What are Purkinije neurones?

Found in cerebellum (reigion of brain involved with motor control, balance and coordination.). During development, undergo series of changes in morphology and function regulated by cytoskeleton (microtublues).

Purkinije neurones and the cytoskeleton

cytoskeleton involved with Purkinije neurones precursor migration to cerebellar plate, the establishment of dendritic arbors and the formation of axons and synapses

• Migration regulated by reelin signalling

• Actin is important in formation of dendritic arbors

What is the Purkinje effect?

Phenomenon where in low light condition, human eye becomes sensitive to bue and green wavelengths, making these colours apprear more vibrant, while reds become darker.

• Beacuse out vision shifts from cone-based (photopic vision) to rod-based (scotopic vision) in dim light

Lissencephaly

“smooth brain”. Uncommon, genetically-associated brain Mal-developmet disorder marked by nil folds or convolutions in cerebral cortex as well as microcephaly (small head).

Involves Lis1, DCX or Reelin.

Effects of Lissencephaly

Effected children elicit developmental delays, varies between children depending on severity of brain deformity and regulation of seizures.

Shortened life expectancy due to underlying respiratory anomalies.

Mutations and lissencephaly

VLDLR and ApoER2 gene mutations (which encode gene receptors) can lead to neuronal migration defects and lissencephaly.

Mutations in reelin gene also been linked to lissencephaly, as can lead to reduced levels of reelin in the brain which can impair neuronak migration and brain development.

What are the mechanisms of Microcephaly (small head)?

Abnormal spindle-like microcephaly associated protein (ASPM) involved in mitotic spindle function including orentation or clevage plane.

What happens when ASPM (microcephaly) mutates?

Mutations are linked with most common type of autosomal recessive primary microcephaly.

Mutations can lead to abnormal spindle formation, defective chromosome segregation and altered cell cycle progression (ASPM required for symetrical division)

What are neural progenitor cells

Cells that have the ability to differenctiate into different types of neurones, play role in nervous system development.

ASPM aids neural progenitor cells divide correctly, and that genetic material is even in daughter cells.

Why is the orientation of mitotic spindles important?

• horizontal (symmetric): more Gila cells, fewer neurons (smaller brain)

• vertical (asymmetric): more neurons (larger brain)

Alpha-synuclein involvement in parkinsons

Protein of high concentration in Lewy bodies. alpha-s binds to microtubles and actin filaments, binding disrupts normal cytoskeleton function leading to cellular defects.

Alpha-synuclein used as a biomarker

Levels decrease in CSF in parkinsons patients, suggest protein accumulating in brain. Can predict disease progression.

How do we treat Alpha-synuclein?

• Reduce production

• Increase clearance

  • RNAi or ASOs target & degrade mRNA that encodes Alpha-synuclein

• immunotherapy to target Alpha-synuclein

  • use of antbodies

Tumorigenesis

solid Tumor cells and haematopoietic malignanciies can contain centrosome abnormalities. Present in early tumours, provides malignant potential information.

What do supernumerary centrosomes do?

Their presence promotes tumourigenesis by fostering errors in chromosome segregation. This instability is driver of cancer, as can lead to accumulation of genetic mutations.

Epthelial-mesenchymal transition (EMT)

EMT is biological process which epithelial cells lose polarity and cell-cell contacts but gain mesenchymal characteristics (increased motilitty and invasivness) - critical for pathological processes incudign embryonic development, wound healing, and cancer metastasis.

Cytoskeleton regulates cell shape motility and signalling.

Cytoskeleton dysfunction and EMT

Actin filaments organised, forming cortical actin network. This is disrupted during EMT, forming stress fibres.

Involved pathways:

• Rho GTPase pathway

• PI3K/Akt and MAPK pathways