17. Developmental Regulation

Developmental Regulation Many inherited diseases result from mutations in genes regulating development Treatments/therapies may be discerned by understanding regulatory mechanisms Three basic points of control: 1. Transcriptional regulation 2. Polarity within the cell 3. Extracellular signaling

Transcriptional Regulatory Cascades Transcription proteins and cofactors – activators and repressors Cis-acting elements Locus arrangement (gene order) Chromatin structure – domains, methylation, acetylation Cell polarity and signal transduction

Cis-acting transcriptional regulatory elements: Definitions Locus Control Region (LCR) 1. Opens locus chromatin domains 2. Insulates against effects of surrounding positive or negative chromatin 3. Has cell lineage-specific enhancer activity 4. Influences timing of replication and choice of origin utilized Enhancer 1.Stimulates transcription in an orientation-independent manner 2.Classical enhancers function independently of orientation and distance Promoter 1. Region of DNA at which RNA polymerase binds and initiates transcription Response elements 1.Causes a gene to respond to a regulatory transcription factor

Cis-acting transcriptional regulatory elements: Definitions (continued) Insulator 1.Creates independent functional domain without enhancement or activation function by blocking effects of surrounding positive or negative chromatin 2. Interrupts communication between a promoter and another regulatory element when placed between them Matrix attachment region (MAR) or scaffold attachment region (SAR) 1. DNA segment that may bind the nuclear scaffold 2. A DNA loop between two MARs may form an independent chromosomal domain 3. Has no enhancer activity

LCR HSs HS Insulator MAR Promoters Enhancers Response elements (Regulatory promoter)

Action of trans-acting transcription factors Transcriptional environment is dynamic and changes as development proceeds.

Control of human  -globin gene expression – a model of developmental transcription regulation Carries O 2 and CO 2 Tetramer (  2  2 ) Allosteric regulation Tetramer composition varies with age  -globin  -globin

Synthesis of hemoglobin is coordinately regulated

Pattern of globin chain synthesis during development

The human globin genes Each gene cluster is under the control of a major enhancer (HS –40) or locus control region (LCR)      -like genes on chr 16  -like genes on chr 11   GG AA LCR HS-40

 -globin chain

The Locus Control Region The LCR is DNAse-hypersensitive in cells expressing the globin genes Sensitivity to DNAse reflects a relaxed chromatin structure that allows binding of transcription factors The LCR regulates the entire gene cluster permitting it to be further regulated on a gene-by-gene basis LCR DNAse- hypersensitive sites Sites that are less-sensitive to DNAse between nucleosomes and at other regulatory regions adjacent to genes promotertranscribed region +1

The LCR and the globin genes bind common protein factors

Prototypical  -like globin gene EnhancerPromoter

A) Looping B) Tracking Models proposed for LCR-globin gene interaction

C) Facilitated tracking D) Linking

Cellular Polarity Morphogen gradient A morphogen is a protein whose local concentration (or activity) causes the surrounding region to take up a particular structure or fate. Patterning Partitioning of the cell or embryo into “zones” destined to develop into different tissues or structures.

A series of events resulting from the initial asymmetry within the egg is translated into the control of gene expression so that specific regions of the egg acquire different properties. At the start of development, gradients are established in the egg along two axes, anterior-posterior and dorsal-ventral. Anterior (head) Posterior (tail) (underneath) (top)

Regulatory cascades - concept

1.Maternal gene products, called morphogens, establish gradients in early embryogenesis. 2.Anterior-posterior development uses localized gene regulators. 3.Dorsal-ventral development uses localized receptor- ligand interactions.

Expressed during oogenesis by the mother. Act upon or within the maturing oocyte. Expressed after fertilization. Mutations in these genes alter the number or polarity of segments. Three groups of segmentation genes act sequentially to define increasingly smaller regions of the embryo. Control the identity of a segment, but do not affect the number, polarity or size of segments. Mutations in these genes cause one body part to develop the phenotype of another part.

Homeotic (Hox) Genes LabpbZenDfdScrAntp Ubx Abd-AAbd-B 3. Spatial colinearity – Hox gene order strictly corresponds to their expression domains along the body of the embryo. 1.Impose the program that determines the unique differentiation of each segment. 2.Complex loci found as gene clusters.

HoxA B1 B3 B4 B5 B6 B7 B8 B9B13 B2 HoxB C4 C5 C6 C8 C9C10C11C12C13 HoxC D1 D3 D4 D8 D9D10D11D12D13 HoxD anterior 3’ A1 A3 A4 A5 A6 A7 A9A10A11A13 A2 5’ posterior ANT-C Drosophila Mouse BX-C Chromosome lab pb Dfd Scr Ant p Ubx Abd-AAbd-B Evx1 Evx2 Hox gene clusters are conserved between species. Human and Mouse: Four gene clusters (A, B, C, D) that are organized into 13 paralogue groups.

Spatial colinearity in mouse Adapted from Hunt et al. (1991) Development Supp.: B13 B9 B8 B7 B6 B5 B4 B3 B1 B2

Comparison of spatial colinearity between fruit fly and mouse. Humans have the same number of Hox gene clusters as mice and spatial colinearity is conserved between species.

Combinations of Hox genes specify the development of the anterior-posterior axis

Temporal colinearity in mouse earliest anterior 3’ 5’ posterior latest direction of opening of chromatin in complex during gastrulation Temporal colinearity: As the body plan develops in an anterior- posterior direction during gastrulation, there is a sequential expression of the homeotic complex.

Polydactyly Diseases associated with Hox gene mutations 1.Hand-foot-genital syndrome (Hoxa deletion) 2.Synpolydactyly (Hoxd deletion) 3.Cleft palate 4.Brain abnormalities 5.Uterine abnormalities 6.Retinoic acid, which causes birth defects, affects Hox genes

Extracellular Signaling Communication of environmental stimuli to intracellular milieu, thus effecting appropriate cellular (developmental) response Four types –Endocrine –Paracrine –Neuronal –Contact-dependent