Epigenetics Comes of Age

The mechanism determining the spatio-temporal patterns of expression of non-housekeeping genes, i.e. restriction of their expression at specific times and sites while they are suppressed in cells in all the rest of the body is one of the enigmas of modern biology. This spatio-temporal restriction of expression of genes is so vital that no organism would exist without it.  Nevertheless most biologists neglect/ignore the issue as being self-evident.

It is to be anticipated, however, that from a genecentrist view it will be “argued” that this is the result of evolution. But this is not an explanation: most things in biology are result of evolution but we can’t say we know anything about them until we know the mechanism how they work: biologists knew that cell division, chromosome duplication, DNA replication, protein biosynthesis, gene mutations, etc. have been result of evolution but they did not rest until they found the mechanisms underlying them. Similarly, the fact that the spatio-temporal expression of non-housekeeping genes is a result of evolution tells us nothing about the underlying mechanisms.  

Besides the statement that “Evolution did it”, have no hypothesis of how an organism determines the patterns of gene expression in time and space. How does a metazoan organism manage to specify the cells that have to turn on particular genes out of billions-trillions of cells it consists of, when we know that the genotype, including regulatory sequences, in all somatic cells is identical?

In 2004 (Neural Control of Development, Albanet, Dumont, N.J., pp.192-193) and 2008 (Epigenetic Principles of Evolution, Albanet, Dumont, N.J., pp. 211-212) I put forward my hypothesis on the existence of a neural mechanism of binary neural control of expression of non-housekeeping genes. What follows is a number of experimentally verified examples that demonstrate the existence and the functioning of this mechanism of regulation of the spatial and temporal expression of non-housekeeping genes in metazoans (invertebrates and vertebrates).

 Global neural control of muscle development in insects 

Muscle growth in insects depends on insulin growth factors (IGFs) that function as a single insulin-IGF system. In Drosophila, insulin-like peptides (Ilps) are produced by a group of 14 neurons (of ~ 1 million neurons in the brain of the fly) (Ito, K.  2000. Drawing a Circuit Diagram of the Fruit Fly Drosophila Brain. Biophysics (Seibutsu Butsuri) 40: 179-184) located in the pars intercerebralis in two symmetrical clusters along the midline, between the two brain hemispheres, that are generally known as insulin-producing cells (IPC). Ablation of the brain IPCs causes reduction of growth rates and leads to development of small adult flies (Géminard, C. et al. 2006. Control of Metabolism and Growth Through Insulin-Like Peptides in Drosophila. Diabetes 55 Supplement 2: S5-S8).

When the level of carbohydrates in circulation is high, fat body cells secrete a glycoprotein, dALS (acidlabile subunit) to which IPCs in the brain of the insect respond by secreting insulin-like peptides (Ilps)  (Colombani, J. et al. 2003. A Nutrient Sensor Mechanism Controls Drosophila Growth. Cell 114: 739-749; Kaplan, D.D., Zimmermann, G., Suyama, K., Meyer, T. and Scott, M.P. 2008. A nucleostemin family GTPase, NS3, acts in serotonergic neurons to regulate insulin signaling and control body size. Genes & Development 22: 1877-1893) of three types.

Via their axons, IPCs release their insulin-like neuropeptides in hemolymph, which initiate insulin-signaling cascades in cells throughout the body. Mediator of the function of Ilps is their specific insulin receptor (InR). Ilp signaling and its antagonist ecdysone converge especially in the fat body thus determining the rate of muscle growth and the period of growth, ultimately the adult size of the organism (Figure 1).

The central global control of muscle and body growth, however, includes a negative control by another neural pathway that also starts in the pars intercerebralis of the brain of the fly with the release by a small number of secretory neurons of the neuropeptide, prothoracicotropic hormone (PTTH) in response to external (photic) and internal (stretch from the growth of the body) stimuli. Via axons these neurons transport PTTH to corpora cardiaca before reaching the hemolymph. In cells of the prothoracic glands  the neurohormone binds its receptor Torso (Rewitz, K.F., Yamanaka, N., Gilbert, L.I., O’Connor, M.B.  2009. The Insect Neuropeptide PTTH Activates Receptor Tyrosine Kinase Torso to Initiate Metamorphosis. Science 326: 1403-1405) and stimulates synthesis and secretion of ecdysone. Ecdysone binds its nuclear receptor EcR that in various times is expressed in all cell types except those of the prothoracic gland. A major role ecdysone plays in the fat body where at its secretion peak impedes insulin signaling and muscle growth (Colombani, J. et al. 2005. Antagonistic Actions of Ecdysone and Insulins Determine Final Size in Drosophila. Science 310: 667-670) (Figure 1). PER/TIM clock neurons in the brain of insects determine the circadian rhythmicity of the release of PTTH in insects (Steel, C.G.H. and Vafopoulou, X. 2006.  Comparative Biochemistry and Physiology. Part A, Molecular & Integrative Physiology 144: 351-64).

    Figure  1. Simplified  generalized mechanism of the global neural regulation of muscle growth in insects via antagonistic effects of IS and ecdysone.Abbreviations: dALS, acidlabile subunit, a fat body-derived glycoprotein in Drosophila; Ilp, insulin-like peptides; PTTH, prothoracicotropic hormone; PTTHSN, prothoracicotropic  hormone secreting neuron; IS, Insulin signaling. In this system the central nervous system exerts a global stimulatory effect via the Ilp neuropeptides and PTTH that is counterbalanced by the local inhibitory action of neuropeptides released into the prothoracic gland by axons of the neurons of the thoracic ganglions innervating the gland (Yamanaka, N. et al. 2006. Regulation of insect steroid hormone biosynthesis by innervating peptidergic neurons. Proceedings of the National Academy of Sciences USA 103: 8622-8627).

The global control of muscle growth acts via inducers (neurohormones and endocrine hormones) which  with hemolymph circulate  throughout the body and  are potentially capable of inducing expression of myogenic proteins in cells throughout the body. The  species specific restriction of expression of these genes in  muscles alone is function of  the local neural control performed by the local muscle innervation.

 Local neural control of gene expressionIn invertebrates For the first time Lawrence and Johnston (1984, 1986) observed that the patterning of segmental muscles in Drosophila is determined by motor or secretory neurons neither by myoblasts migrating to the sites of muscle development nor by the epithelial cells to which muscles attach (Lawrence. P.A. and Johnston, P. 1984. The genetic specification of pattem in a Drosophila muscle. Cell 36: 775-782; Lawrence, P.A. and Johnston P. 1986. The muscle pattern of a segment of Drosophila may be determined by neurons and not by contributing myoblasts. Cell 45: 505-513). The later conclusion drawn from experimental results was confirmed in experiments on effects of muscle denervation. Denervated muscles increased 10-fold expression of genes coding for myogenic proteins such as MyoD and MRF4 and 30-fold myogenin genes, compared to  innervated muscles (Launay, T. 2001. Expression and Neural Control of Myogenic Regulatory Factor Genes during Regeneration of  Mouse Soleus. The Journal of Histochemistry & Cytochemistry 49: 887-899). Local innervation also determines the distribution pattern of myoblasts in sites of future muscles.  

“Nerve pathways are crucially involved in the distribution of the proliferating mvoblasts as they migrate to sites of mvogenesis on the newly differentiating adult epidermis (Curie, D. and Bate, M. l991.The development of adult abdominal muscles in Drosophila: myoblasts express twist and are associated with nerves. Development 113: 91-102).

 

In Manduca sexta denervation of legs prevents proliferation and migration of myoblasts to proper locations leading to the development of muscleless legs in 26 percent of cases (Consoulas, C. and Levin, R.B. 1997. Accumulation and proliferation of adult leg muscle precursors in Manduca are dependent on innervation. Journal of Neurobiology 32: 531-553) and denervation after the development of the muscle leads to dedifferentiation of myocytes and loss of muscle fibers (Bayline, R.J. et al. 1998. Innervation regulates the metamorphic fates of larval abdominal muscles in the moth, Manduca sexta. Development, Genes and Evolution 208: 369-381).

The indirect flight muscles (IFMs) consist of DVMs (dorso-ventral muscles) and DLMs (dorsal longitudinal muscles). After denervation, DVMs and DLMs have different fates: larval DVMs degenerate during metamorphosis and fail to develop in the adult due to lack of innervation, but larval DLMs, which persist after metamorphosis are used as scaffold for the development of adult DLMs. It is observed that during metamorphosis not only motoneurons withdraw larval synapses and respecify adult nerve branches, but they also respecify adult dendritic arbors, neuronal connections and circuitries in the brain (Kent, K.S. and Levine, R.B. 1988. Neural control of leg movements in a metamorphic insect: persistence of larval leg motor neurons to innervate the adult legs of Manduca sexta. Journal of Comparative Neurology 276: 30-43). Empirical evidence has shown that:

 

“The motoneuron influences both the number of cells available for fusion, as well as potentially regulates the fusion events themselves. This in our view is an elegant mechanism for controlling muscle fiber differentiation during myogenesis, and may have evolved as a way to ensure that muscle primordial develops into muscles that meet the diverse demands placed on them by the nervous system (Fernandes, J.J. and Keshishian, H. 2005. Motoneurons regulate myoblast proliferation and patterning in Drosophila. Developmental Biology 277: 493-505).

 

In Manduca sexta the dorsal external oblique 1 muscle (DEO1) during metamorphosis is radically remodeled. The larval DEO1 muscle consists of five muscle fibers but all of them but one degenerate and are eliminated. The only surviving muscle fiber serves as Anlage for the adult DEO1 muscle. The proximate cause for this is the fact that this fiber

is the only that increases expression of the ecdysone receptor isoform EcB-l, which stimulates myoblast proliferation, while the other fibers express the apoptosis-inducing receptor EcRA. Just before ecdysis the terminal arbor of the motoneuron innervating DEO1 recedes from all but one of larval muscle fibers, the surviving one that becomes the adult DEO1.

 

“That innervation is essential for the fiber to respond to ecdysteroids is shown by the results of the denervation experiments. Denervation entirely eliminated the upregulation of EcR-Bl if performed before the upregulation occurred. If performed after the upregulation had commenced, denervation dramatically reduced the expression by 24 hr and eliminated it by 48 hr later. Thus, both exposure to ecdysteroids and local influences from the motoneuron are required for the upregulation and the maintenance of this high level of EcR-Bl expression that is associated with muscle regrowth.” (Hegstrom, C.D. Truman J.W. 1996. Synapse loss and axon retraction in response to local muscle degeneration. Journal of Neurobiology 31: 175-188).

 

In other words, the choice by the local innervation of the EcR isoform that cells express determines the response to hormonal stimulation (indeed to biologists have been puzzled by the unpredictable responses of developing muscles to changes in the level of ecdysteroids).

During metamorphosis in insects both ecdysteroids and local innervation are involved in myoblast proliferation, survival (Currie, D.A. and Bate, M. 1995. Innervation is essential for the development and differentiation of a sex-specific adult muscle in Drosophila melanogaster. Development 121: 2549 -2557) and muscle patterning (Currie, D. and Bate, M. 1991. The development of adult abdominal muscles in Drosophila myoblasts express twist and are associated with nerves. Development 113: 91-102; Currie, D.A. and Bate, M. 1995. Innervation is essential for the development and differentiation of a sexspecific adult muscle in Drosophila melanogaster. Development 121: 2549-2557) and both of them are necessary for the remodeling of leg muscles (Consoulas, C. and Levine, R.B. 1997. Accumulation and proliferation of adult leg muscle precursors in Manduca are dependent on innervation. Journal of Neurobiology 32:531-553) and abdominal muscles (Hegstrom, C.D. Truman J.W. 1996. Synapse loss and axon retraction in response to local muscle degeneration. Journal of Neurobiology 31: 175-188) during metamorphosis.

 

“Muscle remodeling is dependent on steroid hormones, the ecdysteroids, and a dialogue between motoneuron and muscle. The combination of influences is especially evident during adult muscle growth when proliferation of myoblasts depends on both ecdysteroids and innervation” (Hegstrom, C.D., Riddiford, L.M. and. Trurnan, J.W. 1998. Steroid and Neuronal Regulation of Ecdysone Receptor Expression during Metamorphosis of Muscle in the Moth, Manduca sexta. The Journal of Neuroscience 18: 1786-1794).

 

In vitro experiments with cultures of myogenic cells have also shown that both neurons and ecdysteroids stimulate their proliferation (Luedeman, R. and Levine, R.B. 1996. Neurons and ecdysteroids promote the proliferation of myogenic cells cultured from the developing adult legs of Manduca sexta. Developmental Biology 173: 5l-68). Soluble signals released from the ventral region of the neural tube could control the proliferation of myoblasts and prevent their apoptosis (Pelletier, M. et al.2006. Soluble factors from neuronal cultures induce a specific proliferation and resistance to apoptosis of cognate mouse skeletal muscle precursor cells. Neuroscience Letters 407: 20-25). An experimentally verified mechanism of the complementary functioning of the humoral (long range) and local (short range) of the neural regulation of muscle development in insects is the selective induction/suppression of the ecdysteroid receptor (EcR) isoforms (EcR-A, EcR-B1, and EcR-B2) in cell nuclei. Based on experimental evidence it is concluded that

 

“Innervation regulates the choice of EcR isoforms expressed in growing muscle. This choice may then determine the nature of the response of the muscle to changing steroid titers” (Hegstrom, C.D., Riddiford, L.M. and Truman, J.W. 1998. Steroid and Neuronal Regulation of Ecdysone Receptor Expression during Metamorphosis of Muscle in the Moth, Manduca sexta. The Journal of Neuroscience 18: 1786-1794).

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