Binary Neural Control of Gene Expression – A novel mechanism of gene expression (Part 3 of 3)
The binary control of gene expression in bone formation
The central global regulation of bone formation and homeostasis in vertebrates is performed by the central nervous system via the hypothalamic-pituitary-terminal endocrine glands. It is demonstrated that
“neurons in the central nervous system integrate clues from the internal and extemal milieux, such as energy homeostasis, glycemia or reproductive signals, with the regulation of bone remodeling” (Elefteriou F. 2008. Regulation of bone remodeling by the central and peripheral nervous system. Archives of Biochemistry and Biophysics 473: 231-236).
One of the important neuropeptides involved in the regulation of bone homeostasis is the hypothalamic neuropeptide Y (NPY) and one of its receptors, Y2 (Baldock, P.A. et al. 2002. Hypothalamic Y2 receptors regulate bone formation. Journal of Clinical Investigation 109: 915–921; Reid, I.R. et al. 2005. Effects of a ?-Blocker on Bone Turnover in Normal Postmenopausal Women: A Randomized Controlled Trial. The Journal of Clinical Endocrinology & Metabolism 90: 5212–5216). Neurons expressing neuropeptide Y in the hypothalamus (arcuate nucleus) also express receptors for neuropeptide Y (Y2 receptor). It is believed that this central neural production of neuropeptide Y, by stimulating expression of hypothalamic releasing hormones, activates hypothalamo-pituitary-corticotropic axis but inhibits the thyrotropic, somatotropic and gonadotropic axes. Additionally, the central neural NPY is involved in regulation of bone development by influencing the autonomic neural activity via projections from the hypothalamus to the brainstem (Figure 4).
Somatotropic axis in bone formation. The pituitary growth hormone (GH) plays a dual role in stimulating bone growth:
1. A direct IGF-1-independent role (by binding its receptor (GHR) it stimulates bone growth). GH’s direct role in bone formation is greater in the presence of IGF-1 (Wang, J., Zhou, J., Cheng, C.M., Kopchick, J. J. and
2. An indirect role (by inducing secretion of IGF-1 (insulin-like growth hormone-1) and it stimulates osteoblast proliferation.
Figure 2. Simplified diagram of the central neural control and regulation of osteogenesis
in vertebrates via the hypothalamic-pituitary-terminal endocrine glands (thyroid, gonads and adrenals) axes and via the hypothalamic-pituitary axis (prolactin). Note that the orders for activation/inactivation of the 5 signal cascades or central regulatory axes for bone homeostasis ultimately originate in the brain.
Abbreviations: ACTH, adrenocorticotropic hormone; CRH, corticotropin-releasing hormone; FSH, follicle-stimulating hormone; GH, growth hormone; GHRH, growth hormone-releasing hormone; GnRH, gonadotropin-releasing hormone; IGF-1, insulin-like growth factor-1; TRH, thyrotropin-releasing hormone; TSH, thyroid-stimulating hormone (thyrotropin); LH, luteinizing hormone (lutropin).
The adrenotropic (hypothalamic-pituitary-adrenal) axis. This branch of the central axis, via glucocorticoids, stimulates formation of osteoclasts (=bone resorption), decreases the number of osteoblasts by inhibiting their proliferation and by inducing their apoptosis (Canalis, E. and Delany, A.M. 2002. Mechanisms of Glucocorticoid Action in Bone. Annals of the New York Academy of Sciences 966: 73-81) IGF-1 production and induces IGF-1 resistance (Olney, R.C. 2009. Mechanisms of Impaired Growth: Effect of Steroids on Bone and Cartilage. Hormone Research in Pediatrics 72 Suppl. 1: 30-35). The action of all these factors results in loss of bone mass and leads to the development of oteoporosis in the patients that are under glucocorticoid treatment for considerable periods of time.
The gonadal axis is involved in bone homeostasis via sex steroids, androgens and estrogens. These steroid hormones act by binding to specific nuclear androgen and estrogen receptors (ARs and ERs). Gonadal hormones have positive effects on bone growth.
It is demonstrated that the protective action of estrogens in skeleton is related not only to its inhibiting effect on bone resorption but also to a direct stimulating effect on osteoblast proliferation (Vanderschueren, D. & Venken, K. 2007. Hormonal regulation of periosteal bone growth. Endocrine Abstracts 14: S15.2). The role of ovarian hormones in bone homeostasis is demonstrated in experiments with ovariectomized (OV) animals. These animals show increased bone formation rate while in hypophysectomized animals no bone forms at all. And administration of estrogens in hypophysectomized+ovariectomized rats does not prevent the bone loss. The fact that the hypophysectomy in rats causes a rapid decrease in the estrogen receptors is used to explain the unability of administered estrogens to stimulate bone formation (Yeh, J.K., Chen, M.-M. and Aloia, J.F. 1996. Ovariectomy-Induced High Turnover in Cortical Bone Is Dependent on Pituitary Hormone in Rats. Bone 18: 443-450).
Ovariectomy and the resulting estrogen deficiency in animals causes osteoporosis but it is also associated with a dramatic decrease in innervation of bones suggesting that increased innervation has a role in bone loss during osteoporosis and may have a major role in bone metabolism (Burt-Pichat, B. et al. 2005. Dramatic Decrease of Innervation Density in Bone after Ovariectomy. Endocrinology 146: 503-510).
Positive effects of androgens in prevention of osteopenia (a light form of osteoporosis) in mammals demonstrate their role in increasing bone mass and androgen deficiency leads to intracortical porosis (Prakasam, G. et al. 1999. Effects of Growth Hormone and Testosterone on Cortical Bone Formation and Bone Density in Aged Orchiectomized Rats. Bone 24: 491-497). In males, stimulation of bone formation requires functioning of the somatotropic (GH-IGF-1) axis (Vanderschueren, D. & Venken, K. 2007. Hormonal regulation of periosteal bone growth. Endocrine Abstracts 14: S15.2).
is interesting to observe that leptin and bony skeleton appear together during metazoan evolution. Leptin is a protein synthesized by adipocytes that is known for its anti-orexigenic, anti-obesity and anti-osteogenic properties. Leptin acts directly on osteoblasts by stimulating their activity but it seems that the central nervous system translates the presence of leptin in blood into two opposite instructions (Hamrick, M.W. and Ferrari S.L. 2008. Leptin and the sympathetic connection of fat to bone. Osteoporosis International 19: 905-912, Bouxsein, M. L. et al. 2009. Mice Lacking ?-Adrenergic Receptors Have Increased Bone Mass but Are Not Protected from Deleterious Skeletal Effects of Ovariectomy. Endocrinology150: 144–152):
1. Instructions for stimulating bone formation via a humoral (hormonal) pathway along the GHRHà GH à IGF-1 axis succinctly described above, and
2. Instructions for inhibiting bone formation via local sympathetic innervation.
The sympathetic innervation is the mediator of leptin regulation of bone mass (loss or gain in bone mass) (Takeda, S. et al. 2002. Leptin Regulates Bone Formation via the Sympathetic Nervous System. Cell 111: 305-317).
Contrary to the earlier belief, bones are intensely innervated and by early 90es it was understood that local nerves are involved in bone growth and metabolism by releasing neuropeptides (Edoff, K., Hellman, J., Persliden, J. and Hildebrand, C. 1997. The developmental skeletal growth in the rat foot is reduced after denervation. Anatomy and Embryology 195: 531 -538) and via nerves are linked to the CNS. Osteoblasts, the bone cells, express beta2-adrenergic receptors (beta2-AR) suggesting that they are receptive to sympathetic tone. Administration of propranolol a beta adrenergic blocker stimulates increase of the body mass while administration of beta-adrenergic agonists causes bone loss (Elefteriou F. 2008. Regulation of bone remodeling by the central and peripheral nervous system. Archives of Biochemistry and Biophysics 473: 231-23633).
Figure 5. Diagram of the mechanism of local sympathetic downregulation of bone mass. Left: When the level of leptin in body fluids is high (obesity) it binds its receptors ObRb of serotonergic neurons of the brainstem stimulating their electrical activity and secretion of serotonin and its binding to Htr2c receptor in specific hypothalamic neurons. This leads to decreased activity of the sympathetic system, i.e.decreased secretion of noradrenaline, thus stimulating osteoblast proliferation, with bone growth as a result. Right: When the level of leptin is low (non-obese individuals) it doesn’t bind its receptor ObRb in brainstem neurons, which consequently, secrete no serotonin leading to strengthening of the sympathetic tone (increased noradrenalin secretion). Higher sympathetic tone leads to inhibition of osteoblast proliferation and increase of osteoclast proliferation resulting in bone resorption and bone loss.
Local innervation also influences bone development, growth and repair by releasing in bones neuropeptides, such as substance P (SP), vasoactive intestinal peptide (VIP), neuropeptide Y, calcitonin gene-related peptide (CGRP), etc. secreted in bones by sensory and sympathetic nerve fibers (Liu, D., Li, H., Zhao, C.-Q. , Jiang, L.-S. and Dai, L.-Y. 2008. Changes of substance P-immunoreactive nerve fiber innervation density in the sublesional bones in young growing rats at an early stage after spinal cord injury. Osteoporosis International 19: 559-569). Sciatic neurectomy causes bone loss in cancellous and cortical bones (Zeng, Q.Q. et al. 1996. Time Responses of Cancellous and Cortical Bones to Sciatic Neurectomy in Growing Female Rats. Bone 19: 13-21).
There is also a spinal cord component of regulation of bone homeostasis. It is observed that spinal cord injury, similarly to sciatic neurectomy, causes osteopenia (thinning of bones) in rats (Jiang, S.-D., Jiang, L.-S. and Da, L.-Y. 2007. Spinal cord injury causes more damage to bone mass, bone structure, biomechanical properties and bone metabolism than sciatic neurectomy in young rats. Osteoporosis International 17: 1552-1561) and osteoporosis, i.e. loss of calcium in bones. The latter condition is related to an increase in the density of innervation that secretes neuropeptide substance P in bones (Liu, D., Li, H., Zhao, C.-Q. , Jiang, L.-S. and Dai, L.-Y. 2008. Changes of substance P-immunoreactive nerve fiber innervation density in the sublesional bones in young growing rats at an early stage after spinal cord injury. Osteoporosis International 19: 559-569).
Ovarian function
For a long time it has been thought that ovarian function is hormonally regulated by the central nervous system. But evidence that vagotomy (sectioning of vagus nerve) leads to lower estradiol levels, etc. have shown that the central regulation of the ovarian function in female rats is complemented by the regulatory activity of the ovarian plexus nerves and the superior ovarian nerve which via vagus nerve transmit information from the ovary to the CNS (hypothalamus) thus modulating ovarian response to gonadotropins via the superior ovarian nerve (Riboni, L. 2002. Effects of sympathetic denervation on follicular distribution, oestradiol and progesterone serum levels in prepubertal hemi-ovariectomized female guinea pig. Animal Reproduction Science 73: 63-71). Experiments with peripheral sympathetic denervation (Trujillo, A. and Riboni L. 2002. Effects of functional peripheral sympathetic denervation induced by guanethidine on follicular development and ovulation of the adult female guinea pig. General and Comparative Endocrinology 127: 273-278, Chávez-Genaro R, Lombide P, Domínguez R, Rosas P, Vázquez-Cuevas F. 2007. Sympathetic pharmacological denervation in ageing rats: effects on ovulatory response and follicular population. Reproduction, Fertility and Development 19: 954-960) in guinea pigs and vagotomy in prepubertal rats also support the role of the ovarian innervation in regulation of follicle development (Morales-Ledesma, L., Betanzos-Garcia R. and Dominguez-Casala, R. 2004. Unilateral vagotomy performed on prepubertal rats at puberty onset of female rat deregulates ovarian function. Archives of Medical Research 35: 279-283) and ovarian response to gonadotropins.
Folliculogenesis, i.e. the growth and development of follicles in ovary depends on gonadotropic signals from the pituitary follicle-stimulating hormone (FSH), which in turn is induced by gonadotropin-releasing hormone (GnRH) released by hypothalamic neurons in response to chemical output generated by processing of various external and internal stimuli in different brain regions. However, experimental evidence in female rats has shown that this central effect of FSH on folliculogenesis would not be possible without the local control performed by ovarian innervation. Release of neurotransmitters norepinephrine (NE) and/or VIP by sympathetic nerve endings stimulates granulosa cells to express the FSH receptor (the mediator of the gonadotropic functions of the pituitary FSH). The local innervation only “targets a circumscribed subpopulation of ovarian cells.” (Mayerhof, A., Dissen, G.A., Costa, M.F. and Ojeda, S.R. 1997. A role for neurotransmitters in early follicular development: induction of functional follicle-stimulating hormone receptors in newly formed follicles of the rat ovary. Endocrinology 138: 3320-3329), i.e. granulosa cells in the vicinity of developing follicles. FSH stimulates granulosa cells that through aromatase transform androgens into estradiol making thus possible development of primordial follicles through successive stages of folliculogenesis until the development of Graafian follicle and ovulation. This is confirmed by the fact that experiments of neonatal sympathectomy result in stunted folliculogenesis, reduced production of estradiol and delayed ovulation (Lara, H.E., McDonald, J.K. and Ojeda, S.R. 1990. Involvement of nerve growth factor in female sexual development. Endocrinology 126: 364–375, Riboni,L, Escamilla, C., Chavira, R. and Dominguez, R. 1998. Effects of peripheral sympathetic denervation induced by guanethidine administration on the mechanisms regulating puberty in the female guinea pig. Journal of Endocrinology156: 91-98).
Allatostatins are neuropeptides released by neurons of the brain and ganglions of insects with inhibiting effect on production of juvenile hormone by the endocrine glands, corpora allata. The nervous system exerts the inhibitory effect on juvenile hormone (JH) production not only hormonally, via hemolymph, but also via the innervation of corpora allata where nerve endings of the NCA I (nervi corporis allati I) and NCA II (nervi corporis allati II) (Stay, B., Fairbairn, S. and Yu C.G. 1996. Role of allatostatins in the regulation of juvenile hormone synthesis. Archives of Insect Biochemistry and Physiology 32: 287-297; Kou, R. and Chen, S.J. 2000. Allatotropic and nervous control of corpora allata in the adult male loreyi leafworm, Mythimna loreyi (Lepidoptera: Noctuidae). Physiological Entomology 25: 273-278). Similarly, the synthesis and secretion of JH by corpora allata in Manduca sexta is induced not only via hemolymph by brain neuropeptides, allatotropins, but also by nerves innervating corpora allata that release dopamine in these glands (Granger, N.A. et al., 1996. Dopaminergic control of corpora allataactivity in the larval tobacco hornworm, Manduca sexta. Archives of Insect Biochemistry and Physiology 32: 449-466).
Expression of receptors for angiotensin II
Sympathetic innervation is found to determine the density of angiotensin II receptors in the heart of rats (Hunt, R.A., Ciuffo, G.M., Saavedra, J.M. and. Tucker, D.C. 1995. Sympathetic Innervation Modulates the Expression of Angiotensin II Receptors in Embryonic Rat Heart Grafted.in oculo. Journal of Molecular and Cellular Cardiology 27:2445-2452) via the action of norepinephrine and dopamine on alpha-1 adrenergic receptors (Sumners C, Watkins LL, Raizada MK. 1986. Alpha 1-adrenergic receptor-mediated downregulation of angiotensin II receptors in neuronal cultures. Journal of Neurochemistry 47: 1117-1126).
Besides the global neural of the reproductive activity in male vertebrates, via the hypothalamic-pituitary-testicular axis, the hypothalamus has another neural pathway that through the spinal cord plays a critical role in the function of Leydig cells and this is the reason why injuries of the spinal cord cause deterioration of spermatogenesis in rats (Lee, S., Miselis, R., and River, C. 2002. Anatomical and Functional Evidence for a Neural Hypothalamic-Testicular Pathway that is Independent of the Pituitary. Endocrinology 143: 4447-4454).