Lecture 15: How Do Cells Become Different?

L15 LECTURE 15: HOW DO CELLS BECOME DIFFERENT? Learning objectives: Understand that cell fate commitment involves two phases: specification and determination Understand how to distinguish the two phases of cell commitment experimentally...

L15 LECTURE 15: HOW DO CELLS BECOME DIFFERENT? Learning objectives: Understand that cell fate commitment involves two phases: specification and determination Understand how to distinguish the two phases of cell commitment experimentally Understand the “differential gene expression theory” Understand that cytoplasmic determinants can specify cell fate autonomously via asymmetric cell divisions Understand that cell fate can also be specified cell non-autonomously via cell-cell communication (or signaling) Understand induction and lateral inhibition as two different ways that allow cells to acquire different fates Understand Readings: the MBC6 Chapter 21 concept 1150-1154 of morphogen Login now to https://pollev.com/biomg1350fall24  This portion of the class: Textbook: non-ECB6 materials are posted in the Reading Assignment folder Readings: please download the new, updated schedule/reading list Smartworks: do not cover all lectures Lecture summaries: highlight all key concepts discussed in class Doina Tumbar office hours: In person: Wednesdays, 1:30-2:20 pm, 258 Biotech In person: Mondays, 4-5 PM, 258 Biotech  Recap from L14: Developmental genetics developmental genetics = the study of mutants (see HW reading) normal gene X Genotype: the set of genes that an organism carries. Phenotype: an organism’s observable characteristics normal fly fly with abnormal phenotype Possible in organisms that are amenable for genetic analysis, such as worm, fly, mouse, zebrafish -section & smartwork problems (CRISPR)  General principles of cell diversification Ectoderm Mesoderm Totipotent Endoderm Multipotent How do cells become different? Step-wise restriction of “cell fate potential” -cell intrinsic (cell autonomous) -cell extrinsic (non-cell autonomous) What makes cells different? Differential gene expression Figure 1.7 Developmental Terminally Differentiated Biology  TODAY’S TOPICS Specificatio How do What makes n and cells cells determinati become different? on different?  CE L L FATE will become a liver cell Cell fate or developmental fate describes what cells will normally develop into is a liver cell Differentiated cell cell that has become specialized (e.g. liver) usually refers to terminal state of a cell, but can also refer to intermediate states (mesoderm, endoderm, etc.) COMMITMENT Cells may have become committed to a particular fate even though they may not look differentiated The process of commitment can be divided into two phases: Specification: commitment to a particular fate, but in a labile or reversible manner Determination: irreversible commitment to a particular fate  TEST OF SPECIFICATION Isolation experiment Put the cell or tissue in isolation in a neutral environment (e.g. tissue culture). What does the cell become? 1. If it expresses a different fate than it would normally, it was NOT specified and NOT determined. 2. If it expresses its normal fate, it is specified but may or may not be determined.  I N V I TR O TEST FOR DE TE R M I N ATI ON In vitro transplantation experiment Put the cell or tissue in a new environment. What does the cell become? Specified muscle Specified muscle cell becomes cell changed to muscle despite 1. If it still expresses its neuron surrounding normal fate, then it is environment determined. 2. If not, then it is not determined. NOT DETERMINED DETERMINED  IN VIVO TEST FOR DETERMINATION Cells are DETERMINED if transplantation from their normal position to a new position in the embryo does not change their normal fate.  COM M I TM E N T TO A PA RTI CUL A R FATE I S PR OG R ESSI V E blastomere endoder pancreatic endocrine b-islet m cell bud cell pancreas cell cell Each step is irreversible => cells have memory Cell fate becomes increasingly restricted during development  TODAY’S TOPICS Specificatio How do What makes n and cells cells determinati become different? on different? I S TH E G E N OM E CH A N G I N G DUR I N G DEV E LOPM EN T? Test: nuclear transplantation EnucleatedEgg Enucleated egg  T H E F I R S T A N I M A L C LO N I N G E X P E R I M E N T Gurdon (1962) showed that the nucleus from fully differentiated tadpole skin cell can direct normal frog development! John Gurdon and Dr. Shynia Yamanaka Nobel Prize 2012 for the discovery that mature cells can be reprogrammed to become pluripotent Yamanaka factors (2006): Induced Pluripotent Stem Cells  DOLLY THE SHEEP, THE FIRST MAMMAL CLONED 1997- Ian Wilmut successfully cloned a sheep from an adult mammary cell. Dolly Differential gene expression theory The genome is constant in all somatic cells. Only a small proportion of the genome in any cell type is expressed. Different cell types express different groups of genes that confer unique cellular characteristics Unused genes that are not transcribed are not mutated or destroyed; they retain potential to be expressed. DIFFERENTIAL GENE EXPRESSION MAKES CELLS DIFFERENT Lectures 16-17  TODAY’S TOPICS Specificatio How do What makes n and cells cells determinati become different? on different?  TWO WAYS OF MAKING SISTER CELLS DIFFERENT Cell autonomous or cell intrinsic mechanism sister cells are born different Asymmetric cell division Non-cell autonomous or cell extrinsic mechanism -involves ‘cell signaling’ sister cells become different as a result of Figure 21-12 MBC6  ASYMMETRIC CELL DIVISION Cytoplasmic Cytoplasmic determinants determinants become are inherited by asymmetrically localized one daughter cell This is a cell autonomous mechanism that determines cell fate. Figure 21-12 MBC6 AN EXAMPLE OF ASYMMETRIC CELL DIVISION: THE FIRST DIVISION IN C. ELEGANS PARs MEX-5 In C. elegans, at the two-cell stage, the cells are different PIE-1  T W O WAY S O F M A K I N G S I S T E R C E L L S D I F F E R E N T Cell autonomous (cell intrinsic) Asymmetric division Cell nonautonomous (cell extrinsic) Inductive signaling Lateral inhibition Figure 21-12 MBC6  CELL SIGNALING IS A MAJOR CELL EXTRINSIC MECHANISM OF CELL DIVERSIFICATION AND ORGANISMAL DEVELOPMENT Extracellular signaling receptor molecule intracellular relay amplify integrate distribute may alter: effector gene expression metabolism cytoskeleton etc Cell division ignaling molecule could be: art of the extracellular space Growth, response: Survival ttached to another cell lowing through the circulatory system Migration Figure 16-2 ECB5 Lectures 18-19 Secretion,  TWO WAYS OF MAKING SISTER CELLS DIFFERENT Cell autonomous (cell intrinsic) Asymmetric division Cell nonautonomous (cell extrinsic) Inductive signaling Lateral inhibition Figure 21-12 MBC6  Inductive signaling Inductive signal Some inductive signals function in an all-or- none manner Some inductive signals function in a concentration- dependent manner Morphogens Cells directed to new developmental fate The blue cells are INDUCED by the dark gray cells to adopt a certain fate. Signaling molecules can function as MORPHOGEN morphogens FATE A FATE B FATE C A morphogen... is a diffusible molecule secreted from a source (signaling cell or cells) forms a graded distribution (concentration gradient: higher near the source and lower farther away) produces specific cellular responses depending on its local concentration provides a mechanism to specify fates in a reproducible pattern An example of a morphogen gradient Shh (sonic hedgehog) mRNA in the chick limb bud An example of a morphogen gradient Inappropriate expression of Shh or Shh signaling causes extra digit formation  T W O WAY S O F M A K I N G S I S T E R C E L L S D I F F E R E N T Cell autonomous Asymmetric division Cell nonautonomous Inductive signaling Lateral inhibition Cells born as equivalent cells can Figure 21-12 MBC6 adopt different fates due to cell-cell  Lateral inhibition: equivalent cells can adopt different fates in the absence of tissue asymmetries Both cells produce the same = molecule important amount of X, and inhibit the or adopting a certain cell fate production of X in their neighbor equally. A transient increase in X produced by cell 1 causes a stronger inhibition of X production by cell 2. A decrease in X produced by cell 2, allows cell 1 to make more X. Positive feedback loop in the red cell  Lateral inhibition: example Cells compete by Group of equivalent cells inhibiting neighbors Some cells predominate Lateral inhibition works between Fly two neighboring bristle pattern cells that are directly adjacent to each other  Summary: how do cells become different? asymmetric cell cell autonomous (cell division intrinsic) cell non-autonomous lateral inhibition (cell extrinsic) require cell-cell inductive signaling or cell-cell signaling communication morphogen gradients Outcome: differential gene expression Cells adopt different fates… You will go through more exercises, during active learning section 9, to gain a better understanding of the mechanisms involved in cell fate specification. Cell-cell communication in animal development is mediated by only a small number of conserved cell- cell signaling pathways, including Transforming growth factor-ß (TGFß) Wnt Hedgehog (Hh) Notch Receptor tyrosine kinase (RTKs) Yet,…there is enormous cell diversity in animals The same inductive signal can generate different responses working with other signals Sequential induction