Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 82

High
carbohydrate
levels

dALS

Fat
body 14 Ilp neurons

Serotonergic
neurons

PTTHSN

Photic
stimulus

Nucleostemin
producing neurons

ILp neuropeptides

Thoracic
ganglion

PTTH

Torso

Prothoracic
gland TS

Growth stimulation

Axon

Ecdysone

(Growth inhibition)

Fly muscles

Figure 2.11 A simplified model of the global neural regulation of muscle growth in insects
via antagonistic effects of IS and ecdysone. Left: the insulin-like peptides pathway. Right:
the PTTH-ecdysone pathway. dALS , acidlabile subunit, a fat body-derived glycoprotein; Ilp ,
insulin-like peptides; PTTH , prothoracicotropic neurohormone; PTTHSN , prothoracicotropic
hormone secreting neuron; IS , insulin signaling.

of signals known as the signal transduction pathway . Among the most important
extracellular receptors are G protein-coupled receptors (GPCRs), receptor tyrosine
kinases (RTKs), and integrins ( Figure 2.12 ).
GPCRs are a group of cell surface receptors that bind various ligands (e.g., hor-
mones and neurotransmitters) in eukaryotes. The binding of the ligand changes the
conformation of the GPCR and activation of a G protein, leading to the activation of
a second messenger pathway. RTKs are a large family of transmembrane enzymes
that serve as receptors for a number of ligands such as hormones, growth factors,
and cytokines. Upon binding a specific ligand, the receptor dimerizes and phospho-
rylates. In this phosphorylated form, the receptor activates a transduction pathway.
Integrins are transmembrane receptors that specialize in the transduction of extracel-
lular matrix (ECM) signals of fibronectin and collagen.
The activation of a signal cascade is triggered by a “perceived” difference
between a variable's actual value and its set point. The last element in the cascade
finalizes the cascade's purpose, which is the activation of a specific gene or a number
of genes, the controlled variable in engineering terminology. According to this con-
cept, the CNS interacting with the epigenetic control system of the cell determines
the level of thousands of gene products in body fluids and in cells throughout the
animal's body. The interaction of the integrated control system and the individual