"Aging involves a gradual decline in physiological function throughout the organism, decreasing the ability to respond to stress and homeostatic imbalance, and increasing susceptibility to disease. One mechanism that may contribute to aging is a progressive decline in the proliferative capacity of various cell types, including adult stem cells, which impairs tissue maintenance and regenerative potential[this may be related to chromatins and histones as genes related to them correlated with height]. Declining proliferative capacity in many tissues does not begin during adult life but instead initiates much earlier[this decline in proliferative capacity may be related to growth cessation]. In embryonic and early postnatal life, many tissues show rapid replication, leading to rapid somatic growth. Subsequently, cellular mechanisms slow this robust proliferation, such that, by adulthood, most tissues have entered a quiescent state where proliferation occurs only as needed to replace dying cells[we need to break out of this quiescent state to grow taller].
We recently found evidence that the mechanism responsible for this decline in juvenile cell proliferation involves a genetic program that is common to multiple organs and includes the downregulation of multiple growth-promoting genes with age[so we need to upregulate these genes back to juvenile levels]. We hypothesized that the mechanisms that progressively restrain juvenile growth continue to progress into adult life, thus contributing to the decline in proliferative capacity associated with aging. Progression of the underlying proliferation-limiting genetic program might lead first to cellular quiescence in young adulthood, such that some cells can still proliferate when stimulated for tissue renewal, but then, during aging, continued progression of the program might further limit the proliferative capacity of cells to a level below that needed for tissue renewal. As an initial test of this hypothesis, we used expression microarray to compare changes in gene expression that occur in liver, kidney and lung of mice during adult aging with changes that occur in juvenile life, as somatic growth slows. We found that many of the changes in gene expression that occur during adult aging originate in early postnatal life, during the juvenile period of growth deceleration[so the reduction in height growth genes begins almost immediately after you're born]. Furthermore, bioinformatic analysis of these genes that showed persistent changes in expression, both during juvenile life and during adult aging, indicate that cell-cycle related genes are strongly over-represented. Thus, the findings support the hypothesis that the genetic program that slows growth in juvenile life in order to limit adult body size persists into adulthood, and may eventually hamper maintenance and repair of multiple organs and thus contribute to the aging process. This hypothesis might provide an explanation for the observation that small mammals generally undergo both aging and suppression of juvenile growth on a far shorter time scale than do large mammals. The hypothesis might also help explain why growth-inhibiting conditions such as caloric restriction, growth hormone deficiency, or insulin-like growth factor-I deficiency slow aging. We have previously shown that growth-inhibiting conditions slow the juvenile growth-limiting genetic program. This slowing of the program may then conserve proliferative capacity, and therefore slow aging.
Mechanisms limiting skeletal growth Mammalian body length is primarily determined by bone elongation, which occurs at the growth plate. These cartilaginous structures are organized into three distinct layers -- the resting zone, the proliferative zone, and the hypertrophic zone. Growth plate chondrocytes undergo sequential differentiation from the resting to the proliferative to the hypertrophic state as their spatial position shifts. Bone elongation is rapid in early life but gradually slows with age, until growth velocity eventually approaches zero in adulthood. The decline in longitudinal bone growth with age is associated with functional, structural and molecular changes in the growth plate. These senescent changes include a decline in the chondrocyte proliferation rate, overall growth plate height, proliferative and hypertrophic zone heights, column density, and extensive changes in gene expression[thus we have to change those genes back to pre-growth plate levels]. Based on prior findings, we hypothesized that growth plate senescence is not simply dependent on time per se but rather depends on chondrocyte proliferation. Thus, for example, growth plate chondrocytes may have a finite proliferative capacity, which is gradually exhausted, leading to a decline in growth rate and other senescent changes in the growth plate. To test this hypothesis, we used a tryptophan-deficient diet to suppress growth temporarily in newborn rats. Afterwards, growth in these rats was allowed to recover by switching to a replete diet. We found that structural, functional, and molecular markers of growth plate senescence were delayed by prior tryptophan deficiency, indicating that the developmental program of senescence had occurred more slowly during the period of growth inhibition. Combined with previous findings using other models of growth inhibition, the cumulative evidence support the hypothesis that delayed growth plate senescence is a consequence of growth inhibition, which in turn suggests that growth plate senescence is not simply a function of time per se but rather depends on growth.
We have also found evidence that growth deceleration in non-skeletal tissues is similarly driven by growth itself, indicating that growth in many tissues is fundamentally limited by a negative feedback loop[altering this feedback loop is the post powerful ability to alter height].
Regulation of skeletal growth by Indian hedgehog (Ihh) and parathyroid hormone-related protein (PTHrP)
Indian hedgehog (Ihh) and parathyroid hormone-related protein (PTHrP) are secreted proteins that act as paracrine signals, participating in a negative feedback loop that regulates chondrocyte differentiation and proliferation. However, the role of these proteins has primarily been studied in embryonic growth cartilage. Postnatally, this region undergoes major structural and functional changes. To explore the organization of the Ihh-PTHrP system in the postnatal growth plate, we microdissected growth plates of 7-day-old rats into their constituent zones and assessed expression of genes participating in the Ihh-PTHrP feedback loop. Ihh, Ptch1, Smo, Gli1, Gli2, Gli3, and Pthr1 were expressed in regions analogous to the expression domains in embryonic growth cartilage. PTHrP, however, was expressed in resting zone cartilage, a site that differs from the embryonic source, the periarticular cells. We then used mice in which lacZ has replaced coding sequences of Gli1 and thus serves as a marker for active hedgehog signaling. At 1-, 4-, 8-, and 12-weeks of age, lacZ expression was detected in a pattern analogous to that of embryonic cartilage. The findings support the hypothesis that the embryonic Ihh-PTHrP feedback loop is maintained in the postnatal growth plate except that the source of PTHrP has shifted from its embryonic location in the periarticular cartilage to a more proximal location in the resting zone."
In his paper An extensive genetic program occurring during postnatal growth in multiple tissues, he echoes sentiments of the Yokota team that IGF-2 can be related to height growth.
He also wrote a paper related to growth plate senescence. The understanding of why senescence occurs may enable us to keep growth plates open longer.
Growth plate senescence and catch-up growth.
"Longitudinal bone growth is rapid in prenatal and early postnatal life, but then slows with age and eventually ceases. This growth deceleration is caused primarily by a decrease in chondrocyte proliferation, and is associated with other structural, functional, and molecular changes collectively termed growth plate senescence[if we alter these changes we can delay senescence]. Growth plate senescence occurs because the progenitor chondrocytes in the resting zone have a limited replicative capacity which is gradually exhausted with increasing cell division. Growth plate senescence explains the phenomenon of catch-up growth. Growth-inhibiting conditions such as glucocorticoid excess and hypothyroidism delay the program of growth plate senescence. Consequently, growth plates are less senescent after these conditions resolve and therefore grow more rapidly than is normal for age, resulting in catch-up growth."
"there is no known growth- regulating hormone whose concentration changes in a pattern that would explain the slowing of linear growth."
"the rate of longitudinal bone growth can be approximated mathematically by the number of cell divisions occurring in each proliferative column per unit time, multiplied by the height of the terminal hypertrophic chondrocyte"
"Cartilage matrix synthesis is required to expand the intercolumnar extracellular space in the direction parallel to the long axis of the bone at a rate commensurate with that of the cellular expansion within the columns."<-So cartilage matrix is what pushes the bones apart and terminal hypertrophic chondrocytes are what fill the spaces in.
"chondrocytes might have some mechanisms to count cell cycles."<-DNA Methylation.
"Prior growth history is retained in the resting zone. This zone appears to contain progenitor chondrocytes that can produce new columnar clones of proliferative and hypertrophic chondrocytes and are thought to persist, self- renewing by slow cell division, throughout the lifespan of the growth plate. If this slow replication of progenitor cells in the resting zone were to gradually deplete their replicative capacity, then their clonal progeny in the proliferative zone might also show progressively diminished proliferative activity, leading to growth deceleration"<-so you'd want the resting zone to think that it hasn't grown. Embryonic Stem Cells would be superior to MSCs.
"If the progenitor cells were simply to become depleted numerically, the proliferative columns might be renewed less often. This might account for the decrease in column density that occurs with senescence as well as the decrease in proliferation within each column. In fact, there is evidence that the resting zone chondrocytes do become depleted both in number and proliferative activity as senescence progresses"<-In this case all you'd have to do is keep a fresh supply of stem cells to the resting zone.
"the mechanisms limiting growth in vivo are not cell autonomous but rather require cell- cell interaction."
"Igf2, a growth- promoting gene which is downregulated almost 1,000- fold during senescence"<-Find ways to upregulate Igf2 to grow taller. "FGF, WNT, eicosanoid, p38- MAPK and vitamin D receptor signaling may be involved [in senescence]".<-an eicosanoid is a product derived from fatty acids a concept we have not previously explored.
"The stem- like cells in [the resting zone] appear to be depleted during senescence, both quantitatively and also qualitatively, in terms of their proliferative activity."<-exercise may help increase stem cell quality.
Mechanisms limiting body growth in mammals.
"An increase in the cell-cycle time indicates that a longer time is required to pass through the entire cell cycle. The growth fraction represents the number of cells that remain in the cell cycle divided by the total number of cells in an organ. Therefore, a decrease in growth fraction indicates that fewer cells remain actively dividing, whereas more cells have dropped out of the cell cycle or remain in the G0 phase."
The study mentions that IGF-1 and HGH coordinate body growth. It mentions also that IGF-1 levels raise as growth rates slow but IGFBP-3 increases as well which could counteract the increase in IGF-1.
"In humans, the 20-fold decline in linear growth rate that occurs between fetal life and preadolescence appears to be largely independent of sex steroids. However, with puberty, in both males and females, estrogen levels increase, causing linear growth acceleration (due in part to increased GH secretion)."<-because before puberty estrogen is below equilibrium.
"growth deceleration is an intrinsic property of the organ rather than the systemic environment."
Nutritrion effects the mTor pathway which effects height in two ways "Phosphorylated S6K acts on a variety of effectors to promote cell growth, cell-cycle progression, and protein synthesis. Phosphorylated 4E-BP is released from the translation initiation factor eIF4E, allowing its binding with 5′-capped mRNAs to initiate translation"
Potential growth promoting genes: "Igf2, Mdk, and Ptn{upregulated by LSJL} and transcription factors such as Ezh2, Mycn, Peg3, and Plagl1"
"CDKN1C, which inhibits cyclin E-Cdk2 but activates cyclin D-Cdk4 for cell cycle G1 phase progression"
Growth-inhibiting conditions slow growth plate senescence.
"Indian hedgehog (Ihh) [is] a secreted protein that signals through the receptor Patched 1 (Ptch1). In the absence of ligand binding, Ptch1 represses the activity of Smoothened (Smo). Upon Ihh binding to Ptch1, repression of Smo is released, allowing Smo to inhibit phosphorylation and proteolytic cleavage of the Gli family of transcription factors Gli2 and Gli3. As a result, hedgehog signaling leads to transcriptional activation of multiple genes, including Gli1 "
"In embryonic growth cartilage, Ihh is produced primarily by prehypertrophic and hypertrophic chondrocytes and stimulates parathyroid hormone-related protein (PTHrP) production by periarticular chondrocytes at the ends of long bones. PTHrP, in turn, diffuses into the underlying layer of flat, proliferating chondrocytes. In these cells, PTHrP signals through its receptor, Pthr1, to delay further differentiation along the differentiation program, keeping the cells in the proliferative state. When these cells become sufficiently distant from the source of PTHrP due to interstitial growth, they differentiate into prehypertrophic and early hypertrophic chondrocytes, postmitotic cells that express Ihh. Ihh and PTHrP thereby form a feedback loop that controls the site of hypertrophic differentiation and consequently also the length of the proliferative columns "
Gli3 expression is highest in the resting zone and epiphysis which is consistent with the hypothesis that LSJL stimulates the early stages of chondrogenesis and thus makes LSJL more attractive to form new growth plates.
"if the model suggested by this study applies to large mammals, Ihh and PTHrP would have to diffuse greater distances than in small mammals. For example in the distal femur growth plate, the height of the proliferative zone is ∼0.7 mm in a juvenile human, compared with ∼0.25 mm in the rat"
Here's Baron's lab home page.
"the mechanisms limiting growth in vivo are not cell autonomous but rather require cell- cell interaction."
"Igf2, a growth- promoting gene which is downregulated almost 1,000- fold during senescence"<-Find ways to upregulate Igf2 to grow taller. "FGF, WNT, eicosanoid, p38- MAPK and vitamin D receptor signaling may be involved [in senescence]".<-an eicosanoid is a product derived from fatty acids a concept we have not previously explored.
"The stem- like cells in [the resting zone] appear to be depleted during senescence, both quantitatively and also qualitatively, in terms of their proliferative activity."<-exercise may help increase stem cell quality.
Mechanisms limiting body growth in mammals.
"An increase in the cell-cycle time indicates that a longer time is required to pass through the entire cell cycle. The growth fraction represents the number of cells that remain in the cell cycle divided by the total number of cells in an organ. Therefore, a decrease in growth fraction indicates that fewer cells remain actively dividing, whereas more cells have dropped out of the cell cycle or remain in the G0 phase."
The study mentions that IGF-1 and HGH coordinate body growth. It mentions also that IGF-1 levels raise as growth rates slow but IGFBP-3 increases as well which could counteract the increase in IGF-1.
"In humans, the 20-fold decline in linear growth rate that occurs between fetal life and preadolescence appears to be largely independent of sex steroids. However, with puberty, in both males and females, estrogen levels increase, causing linear growth acceleration (due in part to increased GH secretion)."<-because before puberty estrogen is below equilibrium.
"growth deceleration is an intrinsic property of the organ rather than the systemic environment."
Nutritrion effects the mTor pathway which effects height in two ways "Phosphorylated S6K acts on a variety of effectors to promote cell growth, cell-cycle progression, and protein synthesis. Phosphorylated 4E-BP is released from the translation initiation factor eIF4E, allowing its binding with 5′-capped mRNAs to initiate translation"
Potential growth promoting genes: "Igf2, Mdk, and Ptn{upregulated by LSJL} and transcription factors such as Ezh2, Mycn, Peg3, and Plagl1"
"CDKN1C, which inhibits cyclin E-Cdk2 but activates cyclin D-Cdk4 for cell cycle G1 phase progression"
Growth-inhibiting conditions slow growth plate senescence.
"We used tryptophan deficiency to temporarily inhibit growth in newborn rats for 4 weeks. We then allowed the animals to recover and studied the effects on growth plate senescence. We found that structural, functional, and molecular markers of growth plate senescence were delayed by prior tryptophan deficiency, indicating that the developmental program of senescence had occurred more slowly during the period of growth inhibition."
Gene changes in senescence:
Up:
IGFBP7
HTRA1(up in LSJL)
Pycard(apoptosis)
Cdkn2a
Calca
Down:
IGF2
Rxrg
Asb4
Coordinated postnatal down-regulation of multiple growth-promoting genes: evidence for a genetic program limiting organ growth.
Coordinated postnatal down-regulation of multiple growth-promoting genes: evidence for a genetic program limiting organ growth.
"Down-regulated genes in the program showed declining histone H3K4 trimethylation with age, implicating underlying epigenetic mechanisms. To investigate the physiological processes driving the genetic program, a tryptophan-deficient diet was used to temporarily inhibit juvenile growth in newborn rats for 4 wk. Afterward, microarray analysis showed that the genetic program had been delayed, implying that it is driven by body growth itself rather than age. Taken together, the findings suggest that growth in early life induces progressive down-regulation of a large set of proliferation-stimulating genes, causing organ growth to slow and eventually cease."
" During tryptophan deficiency (0–4 wk, solid boxes, pertains to all graphs), growth was inhibited in the tryptophan-deficient group (25 mg tryptophan/100 g food) but not in the control group (280 mg tryptophan/100 g food). After 4 wk, a regular diet was given to both groups, and growth was resumed in the tryptophan-deficient group."
"The declining expression of 8 genes (Ezh2, Gpc3, Mdk, Meis1, Mest, Mycn, Peg3, and Plagl1) with age was first corroborated by real-time PCR"<-Not tested in bone but may affect bones.
"of the age-down-regulated genes that have a reported knockout phenotype, >1/3 showed a decrease in body size without any detected underlying disease, implying that these age-down-regulated genes in the program promote somatic growth"
"Somatic growth deceleration results from a multiorgan postnatal genetic program, primarily involving down-regulation of a large set of growth-promoting genes. Because this growth-limiting genetic program is occurring simultaneously in multiple tissues, the decline in growth rate of various organs occurs in a concerted fashion, which serves to maintain body proportions."<-So this heart, liver, and lung data may have implications on the bone.
"This growth-limiting program depends not simply on age but on somatic growth itself and may be orchestrated by epigenetic mechanisms including declining H3K4me3."
Organization of the Indian hedgehog--parathyroid hormone-related protein system in the postnatal growth plate.
Organization of the Indian hedgehog--parathyroid hormone-related protein system in the postnatal growth plate.
"In embryonic growth cartilage, Indian hedgehog (Ihh) and parathyroid hormone-related protein (PTHrP) participate in a negative feedback loop that regulates chondrocyte differentiation. Postnatally, this region undergoes major structural and functional changes. To explore the organization of the Ihh–PTHrP system in postnatal growth plate, we microdissected growth plates of 7-day-old rats into their constituent zones and assessed expression of genes participating in the h–PTHrP feedback loop. Ihh, Patched 1, Smoothened, Gli1, Gli2, Gli3{up in LSJL}, and Pthr1 were expressed in regions analogous to the expression domains in embryonic growth cartilage. However, PTHrP was expressed in resting zone cartilage, a site that differs from the embryonic source, the periarticular cells. We then used mice in which lacZ has replaced coding sequences of Gli1 and thus serves as a marker for active hedgehog signaling. At 1, 4, 8, and 12 weeks of age, lacZ expression was detected in a pattern analogous to that of embryonic cartilage. The findings support the hypothesis that the embryonic Ihh–PTHrP feedback loop is maintained in the postnatal growth plate except that the source of PTHrP has shifted to a more proximal location in the resting zone."
"Indian hedgehog (Ihh) [is] a secreted protein that signals through the receptor Patched 1 (Ptch1). In the absence of ligand binding, Ptch1 represses the activity of Smoothened (Smo). Upon Ihh binding to Ptch1, repression of Smo is released, allowing Smo to inhibit phosphorylation and proteolytic cleavage of the Gli family of transcription factors Gli2 and Gli3. As a result, hedgehog signaling leads to transcriptional activation of multiple genes, including Gli1 "
"In embryonic growth cartilage, Ihh is produced primarily by prehypertrophic and hypertrophic chondrocytes and stimulates parathyroid hormone-related protein (PTHrP) production by periarticular chondrocytes at the ends of long bones. PTHrP, in turn, diffuses into the underlying layer of flat, proliferating chondrocytes. In these cells, PTHrP signals through its receptor, Pthr1, to delay further differentiation along the differentiation program, keeping the cells in the proliferative state. When these cells become sufficiently distant from the source of PTHrP due to interstitial growth, they differentiate into prehypertrophic and early hypertrophic chondrocytes, postmitotic cells that express Ihh. Ihh and PTHrP thereby form a feedback loop that controls the site of hypertrophic differentiation and consequently also the length of the proliferative columns "
Gli3 expression is highest in the resting zone and epiphysis which is consistent with the hypothesis that LSJL stimulates the early stages of chondrogenesis and thus makes LSJL more attractive to form new growth plates.
"if the model suggested by this study applies to large mammals, Ihh and PTHrP would have to diffuse greater distances than in small mammals. For example in the distal femur growth plate, the height of the proliferative zone is ∼0.7 mm in a juvenile human, compared with ∼0.25 mm in the rat"
Here's Baron's lab home page.
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