Essential Cell Processes

Four Essential Cell Processes

• There are 4 essential processes that permit tissue development which include: Cytokinesis, cell specialization, cell-cell interaction and cell movement.

Cytokinesis

• It is important for developing tissues.
• Symmetric vs. asymmetric division can give rise to a unique cell, that can carry out a unique function.
• Symmetric cell divisions proliferate and generate a tissue.

Cell Specialization

• Once cells are generated, they can become more specialized.
• Cells don't necessarily have to be fully differentiated, but potentially a progenitor cell committed to a specific tissue lineage.
• This is polarization and a bit of differentiation in how epithelial cells become polarized to support their function

Cell-Cell interaction and Cell Movement

• Cell-cell interaction is important for tissue development.
• Cell movement includes migration and chemotaxis, where a cell migrates in response to an external stimulus.

Conserved Mechanisms and Animal Body Plan

• A fertilized egg (or zygote) rapidly divides or cleaves, during which the cell does not grow and its genome is inactive and maternal protein and RNA is degraded.
• The embryo genome is activated and cells cohere to form the blastula.
• Blastula can be solid or hollow (fluid filled – rudimentary gut).
• Complex rearrangement of blastula cells into the three germ layers (ectoderm, endoderm, mesoderm) is known as gastrulation.
• These are conserved mechanisms that are used to generate basic specific patterns.
• Gastrulation is the early formation of different germ layers, different layers of early primordial tissue.
• A fertilized egg undergoes cleavage to a 16-cell stage, referred to as the blastula.
• This blastula reorganizes and gives rise to the three germ layers.
• The process of going from blastula to gastrula (that has the three germ layers) is gastrulation.
• Ectoderm gives rise to skin and the nervous system.
• Mesoderm gives rise to musculature.
• Endoderm gives rise to the internal organs.
• Germ cells arise from the blastocyst.
• A zygote moves to a blastocyst (inner cell mass that will develop into the embryo, and the outer layer, called the trophoblast, becomes the placenta in mammals).

Cell Potential

• The developmental potential of cells will become progressively restricted.
• You start with an embryonic stem cell (ESC) that can become anything, but as these cells divide, they will give rise to more specialized cells that still have the potential to differentiate into multiple types of cells (but not everything).
• Pancreatic bud cells are progenitor cells for the pancreas, capable of asymmetric division, leaving a pool of progenitor cells available to replenish the tissue over time.
• The final differentiated cell, such as the function islet cell, is terminally differentiated.
• Totipotent: Cell can become anything (ESC).
• Pluripotent: Cell can become multiple things, but not anything.

Combinatorial Control and Cell Memory

• Combinatory control and cell memory are epigenetics.
• A cell remembers what it should become based on changes in the structure of the genome that permits certain genes to open or close.
• A cell can have a memory for a specific signal that can "turn off" an area of the genome and prevent it from being pluripotent.
• With combinatorial control, the first two cells are exposed to signal A, and the first cell is also exposed to signal B, while the second cell is exposed to signal C. The combination of these signals leads to changes within the cells.
• Once a cell has responded to a signal, those changes are locked into the genome via epigenetic regulation.
• When the cell is exposed to another signal, it responds differently because of the changes that have already taken place from the first signal.

Overview of Development

• Morphogens are long-range inductive signals that exert graded effects (i.e., paracrine signaling factors).
• The four major families are: Fibroblast Growth Factor (FGF) family, Hedgehog family, Wnt family, and the TGF-β superfamily.
• The effect results at a high concentration of these signaling factors is different than the effect that results at a low concentration of these signaling factors, which is considered graded effects.

Initial Patterns are Established

• In response to a signal from B, A divides into a new group of cells “C.”
• Cell C can influence cells A and B.
• A and B differ in proximity and response to signals from C, since they're morphogens, the resulting effect will be different.
• A generates D, and B generates E and a generation of spatial patterns.
• Morphogens are long-acting and generate a gradient over a long portion of a developing organism.
• Growth factors (morphogens) secreted from one end provide different effects closer to the source, compared to the effect seen further away.
• The response is generally a change in gene expression, leading to cellular differentiation.

Mechanisms of Pattern Formation

• Initial patterns are established in small fields of cells and refined by sequential induction as the embryo grows.
• Different animals use different mechanisms to establish their primary axes of polarization
• Three axes must be established: Animal Vegetal (internal vs. external), Anterior Posterior (head vs. tail), and Dorsal Ventral (back vs. belly).
• The structure of the egg determines the axes of body development.
• There is a clear animal vegetal axis in drosophila before fertilization.
• The site of fertilization defines the DV axis.
• The nucleus's position in the egg defines the axis.
• The animal end is the pole closest to the nucleus.
• The axis runs through the nucleus.
• Fertilization leads to microtubule reorganization and localized transport of Wnt11 mRNA via Kinesin.
• Wnt11 moves to the dorsal axis and activates WNT signaling (WNT being a type of morphogen), which promotes DV structural organization.
• Different animals use different methods, and this is using a frog example (drosophila is much more complicated)
• Sperm entry defines the ventral side, making other parts of the of frog dorsal.
• Cortical rotation moves Wnt11 and Disheveled to the future dorsal side via transport on MTs.
• B-catenin accumulates dorsally, activating genes that establish the organizer.
• The organizer secretes inhibitors (Noggin, Chordin) to block BMP signaling (TGF-B), defining the dorsal axis.
• A BMP gradient is formed, specifying ventral (high BMP) and dorsal (low BMP) fates.
• The Wnt-β-catenin and BMP inhibition mechanism ensures proper D-V patterning, allowing normal development of the frog embryo.

Studies in Drosophila

• Bicoid mRNA is maternally deposited in the anterior of the egg before fertilization.
• After fertilization, bicoid protein is translated and forms a concentration gradient, highest at the anterior and gradually decreasing toward the posterior.
• This gradient determines cell fate along the A-P axis.

Bicoid as Transcriptional Factor

• Bicoid directly activates the transcription of gap genes, such as hunchback (hb), in the anterior.
• The level of Bicoid regulates target gene expression thresholds, ensuring proper segmentation.
• Higher Bicoid levels activate genes required for head and thorax formation.
• Bicoid inhibits caudal (cad) mRNA translation by binding to its 3' UTR.
• Caudal is necessary for posterior development, so Bicoid prevents it from interfering with anterior structures.
• Bicoid establishes the anterior-posterior axis in drosophila by: acting as a morphogen and regulating transcription.
• Regulating transcription, activating genes like hunchback and repressing translation of caudal to prevent posteriorization of the anterior.
• Three groups of segment genes control drosophila segmentation along the A-P axis, expression is regionally distinct, and included are: Segmentation Genes, Gap Genes, Pair-rule genes and Segment polarity genes.

Gap Genes

• Define large broad regions of the embryo.
• They're activated by maternal morphogens (e.g., Bicoid and Nanos).
• Examples include Hunchback (hb), Krüppel (Kr), Giant (Gt), Knirps (Kni).
• Mechanism: Bicoid activates hunchback in the anterior.
• Mutual repression between gap genes sharpens their expression domains (e.g., Hunchback represses Knirps).
• If mutated large sections of the embryo are missing (gaps), leading to loss of multiple consecutive segments. Gap genes are like the rough blueprint, dividing the embryo into broad sections

Pair-Rule Genes

• Divide the embryo into 7 repeating stripes, setting up future segment boundaries.
• They're activated by gap genes.
• Examples include Even-skipped (eve), Fushi tarazu (ftz).
• Mechanism: Expressed in alternating stripes, corresponding to every other segment, activating Eve and ftz in odd and even parasegments.
• Their mutation causes every other segment to be missing, leading to a zebra-stripe phenotype.
• Pair-rule genes are like stencils, marking where future segments will form.

Segment Polarity Genes

• Establish the anterior-posterior polarity within each of the 14 individual segments and are activated by pair-rule genes.
• Engrailed marks the anterior of each segment.
• Wingless marks the posterior.
• Hedgehog and Wingless signaling pathways maintain segment boundaries and polarity.
• If mutated, segments lose polarity, often leading to mirror-image duplication of structures.
• These genes are like street signs, ensuring each segment has a correct front and back.
• Initial signal transduction for segmentation occurs prior to cellularization.

Gene Hierarchy

• A gene hierarchy subdivides the drosophila embryo.
• Regulation is sequential: Bicoid- morphogen prior to cellularization, Gap Gene Products, and Pair Rule genes.
• Regulation is sequential: GAP gene products drive expression of pair-rule genes.
• Paired Rule are co-expressed with GAP genes to subdivide segments and drive segment polarity genes

Gap Genes and Segments

• GAP genes present in one segment is different from the one in the next segment, and they repress each other.
• These GAP genes drive expressions of different Pair ruled genes with each segment.

Summary of Segmentation Gene function

• Maternal genes (e.g., Bicoid) set up initial broad positional information.
• Gap genes (e.g., Hunchback, Krüppel) define broad body regions.
• Pair-rule genes (e.g., Even-skipped, Fushi tarazu) generate alternating segmental stripes.
• Segment polarity genes (e.g., Engrailed, Wingless) refine polarity within each segment.

Hierarchy cont.

• Cell polarization leads to segmentation of the embryo to yield functionally different sets of cells.

Mechanisms of Pattern Formation

• A hierarchy of gene regulatory interactions subdivides the Drosophila embryo
• Post cellularization, segment polarity genes encoding the signal transduction proteins required for WNT and Hedgehog signaling are activated,
• They're Synthesized and released in different segmental bands and drive another's expression.
• They Work through effectors genes (such as engrailed) in neighboring cell to further pattern development

Mammalian v Drosophila WNT production

• Essentially one cell will be making and secreting WNT, interacting with the neighboring cell through engrailed and then engrailed will signal for the synthesis and release of Hedgehog.
• Hedgehog be then released from that cell and act back on cell, to promote WNT release and continue the cycle.
• WNT helps establish a pattern, yet patterns in the embryo still need to be locked in after these short lived signals.
• Egg-Polarity, Gap, and Pair-Rule genes create a transient pattern that is remembered by Hox Genes.
• Initial patterning is unstable and needs to become stable in order for the organism to become viable

HOX Function

• Controlled by WNT and HH
• Stabilize paterning
• Expression is regionally distinct and simultaneous.
• One (gene) will be more dominant over the others in each region.
• It defines and preserves the difference between one segment and the next.
• Wnt signaling plays a role in activating and maintaining Hox gene expression, especially in posterior development.
• Wnt ligands activate β-catenin signaling, which binds to transcription factors regulating Hox genes; Higher Wnt activity → posterior Hox gene activation.
• Wnt is like a light dimmer, controlling how much posterior identity cells receive
• Hedgehog (Hh) signaling helps refine Hox gene expression and establish segment boundaries. Hh signaling activates specific transcription factors, which influence Hox gene expression and is more prevalent in Drosophila.
• In Drosophila, Hh signaling from the posterior compartment helps maintain Hox gene expression in anterior segments. Hh is like a fine-tuner, adjusting Hox expression for precise segment and limb identity

HOX genes

• Permanently pattern the A-P axis and are identified through resultant HOX mutations.
• Mutations cause parts of the body to Transform structures appropriate for other places.
• Transformation can only occur between structures of similar general types, for example changing one limb to a different limb, or switching one segment into another.
• Hox proteins give each segment its individuality.
• 8 HOX genes work through 2 complexes or as two functional halves of one master complex known as the HOX complex. Some variant of the complex is present in all animals. Includes the Bithorax Complex for Abdominal and thoracic segments) and Antennapedia Complex for Thoracic and head
• ANT-C ensures that head and thoracic segments develop correctly while BX-C ensures the thorax and abdomen develop properly.
• Local expression of HOX genes gives regional position information, that represents an intrinsic signal of the cells location.

HOX genes Function

• They are not known for their DNA binding domains
• Loss of HOX genes = cells are all alike since HOX genes give each segment its individuality.
• Hox genes affect their segments through the transcriptional co-activators with which they interact.
• Affect cell differentiation and control genes needed for cell-cell signaling, cell polarity, and cytoskeletal dynamics.
• HOX genes show a correlation: Their order along the chromasome corresponds with area of activity along the AP axis.
• Genes are expressed according to their order in the HOX complex, with more posterior genes supressing more anterior.
• Can auto-activate and promote own permanent expression.

Epigenetics

• Genes work through epigenetic regulation
• When expressed, AbdB activates expression in the surrounding genes through epigenetic changes
• Chromatin structure is changed in the area of the genes allowing for transcription through epigenetic processes

Polycomb and Trithorax

• Proteins enable the Hox complexes to maintain a permanent record of positional information.
  • Its regulation is via epigenetic mechanisms and chromatin remodeling complexes.
  • Polycomb - keeps chromatin closed in regions where HOX genes are not expressed, but does function via the blocking of polycomb action so tht the most possited are dominant.
• Both involve DNA methylation and histone acetylation (epigenetic regulation).
• Mutation in the ESC gene blocks polycomb activity (the silencing function), causing open chromatin structure and expression of all HOX genes, resulting in all segments resembling the most posterior.

Vertebrate Embryo

• Inductive interactions subdivide the vertebrate embryo
• Competiton occurs between secreted factors among the vertebrate embryo and TGFb family patters the A/V axis. High activity near the V-pole stimulates endoderm formation. -Includes two proteins Nodal and Lefty -The TGFb family includes Nodal (TGFb activator) and Lefty (SMAD inhibitor) -Lefty active at Animal pole becomes ectoderm cells. -Nodal active more at vegetal pole becomes endoderm cell.
• With TGFb signaling, there emerges a pattern of the Axis with the enddoerm at the posterior and ectoderm and the anterior

Differentiation Signalling

• A gradient of inhibition of TGFb family members and sub patterns the DV axis while patterning the AV axis.
• There are two proteins Noggin and Chordin are TGF antagonists – SMAD inhibition where; +BMPs are activators
• Secretions run through the embryo on a dorsal to ventral gradient, and: +Noggin and Chordin Block BMP on dorsal side Resulting in: +High BMP = epidermal tissue +Low BMP= neural tissue THE ABOVE IS A SUB-PATTERN WHERE BOTH NEURAL AND EPIDERMAL CELLS RESULT FROM ECTODERM

Lateral Inhibition

• Notch-mediated lateral inhibition refines cellular spacing patterns

• Notch helps guide cells from a progenitor as they differentiate into specific somatic cells in sensory organs With stem cells: -Active Notch = stay progenitor -Inactive Notch = differentiate

• All cells initially express Notch receptor and the delta ligand, thus, Activation of Notch between neighboring cells by delta triggers inhibition, signaling them stay a mother cell.