Developmental Biology - Conserved Mechanisms

Introduction

  • Lecture focuses on commonalities in the development of different organisms, particularly at the molecular level.
  • يتعلق الأمر بالتمييز بين جينات التدبير المنزلي (مثل الريبوسومات والأكتين) والمنظمين الرئيسيين لتشكيل الأنماط متعددة الخلايا.
  • Key regulatory molecules are conserved, including transcriptional regulators (transcription factors) and cell signaling pathways.
  • Experiment:
    • Mouse mutant cerebellum successfully rescued by a human gene.
    • Indicates conservation of function across species.
  • Similarity in genes allows insights from simpler organisms (e.g., flies) to inform understanding of human gene function.

Conserved Transcription Regulators

  • Three levels of conservation:
    • Protein level
    • DNA binding
    • Functional similarity
  • Transcription factors bind to DNA and control gene expression.
  • Conservation includes the DNA binding domain.
  • Homeodomain:
    • Conserved DNA-binding region.
    • Features a helix-turn-helix structure.
    • The third helix directly interacts with DNA.
  • DNA binding:
    • Bind to similar DNA sequences that contain the sequence "AAT".
  • Functional similarity: conserved functions in different organisms.

Hox Genes and Body Plan

  • Functions were originally identified through genetics.
  • Mutations in Hox genes lead to body part transformations.
  • Example - Antennapedia:
    • Mutation transforms antenna into a leg.
  • Flies (Diptera) have two wings and halteres (rudimentary organs).
  • These proteins are also conserved in development.

Eye Development

  • Eyeless (flies) and PAX6 (mammals) are similar regulators expressed in the eye.
  • Mutations cause eye development problems (Aniridia in humans).
  • Conclusion: PAX6 is necessary for appropriate eye development.
  • Sufficiency Question: Is PAX6 sufficient for eye development?
  • Experiment:
    • Express Eyeless in a leg disc.
    • Eye structures form on the leg.
    • Some cells are connected to the brain via neurons.
  • Conclusion: Eyeless/PAX6 seems sufficient for eye development.

Molecular Mechanisms

  • Master regulators (like PAX6) activate level one targets (genes).
  • These genes activate intermediate proteins.
  • Eventually, the eye is built, including photoreceptors and lenses.
  • Top regulators are similar and have similar DNA binding sites.
  • Intermediate level:
    • Fly makes a fly eye.
    • Mouse makes a mouse eye.
  • Bottom-level genes (interpreters) may be different, but top regulation is common.
  • Fly Eyeless in mouse cerebellum results in a mouse cerebellum.

Heart Development

  • Tinman (flies) is similar to factors in heart development.
  • Tinman mutation leads to heart tube development issues.

Cell Signaling Systems

  • Cells serve as a source of a signal.
  • The signal diffuses to affect nearby cells (paracrine signaling).
  • Cells closest to the signal receive the strongest signal.
  • Examples of conserved signaling systems:
    • Receptor Tyrosine Kinases (RTKs):
      • EGF Receptors.
      • FGF Receptor (tracheal development).
    • Wnt:
      • Stem cell maintenance in the intestine.
      • Stem-like properties in various organisms.
    • Notch Signaling:
      • Delta-Notch is used to create molecular differences in cells that are originally uniform.
      • Stochastic event results in one cell having slightly higher Delta ligand levels.
      • Surrounding cells expressing Notch receptor receive the signal, instructing them down a different developmental pathway.

Lateral Inhibition

  • Delta expressing cell inhibits nearby cells from becoming neurons.
  • Results in proper formation of the nervous system.
  • Drosophila embryo:
    • Central nervous system (packed neurons).
    • Peripheral neurons (single cells with surrounding non-neuronal cells).

Neurogenic Effect

  • Loss of Notch signaling results in the whole embryo inappropriately filled with neuronal cells.

Morphogens and Gradient Interpretation

  • Signaling center sends high levels of signal.
  • Signal diffuses, forming a gradient.
  • Cells closest to the source see the highest levels.
  • Gradient is interpreted in a stepwise fashion.
  • Morphogen: a molecule expressed as a smooth gradient but interpreted in a discrete fashion (threshold response).

Limb Development and Sonic Hedgehog (SHH)

  • Example: Chick limb development (vertebrates).
  • Limb bud develops with a signaling center in the posterior region.
  • Classic Experiment: Posterior tissue implanted in the anterior region results in mirror image duplication.
  • Sonic Hedgehog (SHH) protein is present in the normal limb bud and orchestrates specification.
  • SHH mRNA is localized in the posterior region.
  • SHH protein has a wider distribution, forming a gradient.
  • Highest SHH levels correspond to posterior digit identities (5 and 4), medium to 3, and lower to 2.

Polydactylous Cats

  • Polydactylous cats (Hemingway cats) have extra digits.
  • Due to ectopic, inappropriate expression of SHH gene in the anterior caused by a spontaneous mutation.
  • Mutation is located 1,000,000 bases away from the gene.
  • Regulatory region normally drives expression of the actin gene.
  • Human cases of polydactyly also occur through extra SHH in the anterior part.

Developmental Novelty

  • Gene duplication and divergence:
    • Duplication creates a second location for the same protein, controlling the same genes.
    • Over time, sequences diverge, accumulating mutations.
    • New function is created.
  • Combination of existing signaling pathways:
    • Mix and match previously found pathways.