Contractile Forces by Chondrocytes and Height Increase

Previously, we found that cells have a rounding force when they enter mitosis.  And, it is known that when muscles and heart cells need to generate more force they hypertrophy.  If enochondral ossification were a purely transformative process then you wouldn't grow taller at all.  Your cartilage would just turn into bone and you'd stay the same size.  Since hypertrophy is linked to force generation in muscle organs it follows too that it might be the case that hypertrophy may be important for force generation for chondrocytes.  Chondrocytes also have contractile ability which means they are capable of generating force.  If we knew how much force was required to elongate bone during endochondral ossification then we could mimic this force and generate height via other means.


Contractile forces generated by articular chondrocytes in collagen-glycosaminoglycan matrices.


"The objective of the study was to directly measure the force of contraction of adult articular chondrocytes and to examine their contractile behavior in collagen-glycosaminoglycan analogs of extracellular matrix[this is for articular rather than growth plate cartilage but force generation and contractile behavior should be similar between articular carilage and growth plate cartilage]. The contractile forces generated by passages 2 and 3 adult canine articular chondrocytes were measured using a cell force monitor. Passage 2 cells seeded in a collagen-glycosaminoglycan scaffold were capable of generating a force of 0.3 nN/cell. Chondrocytes subcultured through a third passage generated a force twice that level, paralleling the increase in the alpha-smoothmuscle actin (SMA) content of the cells. Treatment of passage 3 cells with staurosporine reduced the force of contraction by approximately one-half, reflecting the effects of this agent in reducing the SMA content of the cells and disrupting the microfilaments. These values compare with 1 nN previously reported for lapine dermal fibroblasts of passage 5-7[dermal fibroblasts are a type of skin cell that are capable of stretching and elongating], using the same apparatus. Direct live cell imaging documented the contractile behavior of the articular chondrocytes in the collagen-glycosaminoglycan matrix in the time frame in which the force was directly measured in the cell force monitor. the cells acted individually and in unison to buckle the collagen struts[so the chondrocytes could act in unison in the growth plate to make bone longer] making up the matrix. Adult articular chondrocytes are capable of generating a SMA-enabled force of contraction sufficient to model extracellular matrix molecules."

"The force of contraction recorded in the present study for canine chondrocytes is approximately twice the value previously measured for second passage adult human articular chondrocytes in the same apparatus"<-But humans are taller than dogs despite dags having larger contraction forces.  Maybe human chondrocytes are better organized or humans have more chondrocytes than dogs making up for the reduction in force generation.

Growth Factor Regulation of Smooth Muscle Actin Expression and Contraction of Human Articular Chondrocytes and Meniscal Cells in a Collagen–GAG Matrix

"human articular chondrocytes can express the gene for a contractile actin isoform, α-smooth muscle actin (SMA), in vivo. The objective of the present study was to evaluate the effects of two growth factors, transforming growth factor (TGF)-β1 and platelet-derived growth factor (PDGF)-BB, on the SMA content of these cells and their contraction of a collagen–glycosaminoglycan (GAG) analog of extracellular matrix in vitro. TGF-β1 was found to markedly increase SMA content of the cells and PDGF-BB decreased SMA expression, with both findings achieving statistical significance. A notable finding was the increased contraction of the collagen–GAG matrix induced by TGF-β1 and the decrease in contraction resulting from PDGF-BB treatment, indicating a causal relationship between expression of SMA and the contractility of the cells[So TGF-Beta1 may increase height by increasing the contractility of the cells.  However, TGF-Beta1 has not been shown to necessarily increase height only in inducing chondrogenesis(TGF-Beta1 has other effects as well) and dogs are not taller than humans despite having more SMA(a contractile actin)]. A novel cell force monitor, employed to estimate the force exerted per cell, demonstrated a higher force exerted by the TGF-β1-treated cells. The findings demonstrate that the expression of SMA by articular chondrocytes and meniscal cells and their associated contractile behavior can be regulated by selected growth factors."

BMP-2 causing Smad 1/5/8 phosphorylation was found to accelerate mineralization and terminal differentiation whereas TGF-Beta1 causing Smad 2/3 phosphorylation was found to deccelerate terminal differentiation.  Whether either gene inactivation can increase height is unknown.  ESL-1 inhibits TGF-Beta and when ESL-1 knockout mice were generated height decrease resulted.  So when TGF-Beta1 can't be inhibited height increase results.

According to the study "Bone Morphogenetic Protein-2-Induced Signaling and Osteogenesis Is Regulated by Cell Shape, RhoA/ROCK, and Cytoskeletal Tension", "BMP triggered rapid and sustained RhoA/Rho-associated protein kinase (ROCK) activity and contractile tension only in spread cells".  Now chondrocyte growth plate cells tend not to be spread whereas osteoblast cells tend to be spread.  So this indicates possibly that BMP doesn't generate contractile force in unspread cells(like chondrocytes).  "BMP-induced osteogenesis is progressively antagonized with decreased cell spreading".  So BMP is more likely to induce chondrogenesis when the cells are closer perhaps this is linked to the contractile forces.

So it's still unclear which is better for height growth TGF-Beta1 and BMP-2.  LSJL upregulates TGF-Beta1 and BMP-2.  If BMP-2 can't induce contractile forces in unspread cells then that may be a reason why TGF-Beta1 and BMP-2 would have different effects on adult height.  If BMP-2 does increase adult height more than TGF-Beta1 then that would be an indication that chondrocyte cell forces are not the driving force behind bone lengthening.  BMP-2 driven chondrocytes being less able to generate contractile forces than TGF-Beta1 chondrocytes might result in less efficiently being able to model the matrix thus more time to proliferate before hypertrophy.  BMP-2 chondrocytes have less type X collagen(hyptrophic collagen) and more Type II collagen.  Since BMP-2 chondrocytes can't generate any force from chondrocyte hypertrophy they have to generate it all by mitotic cell rounding thus resulting in more chondrocyte proliferation.

So, contractile forces may still be the force behind bone lengthening but with BMP-2 chondrocytes it's primarily chondrocyte proliferation driven whereas with TGF-Beta1 chondrocytes it's chondrocyte proliferation driven.

Smad6/Smurf1 overexpression in cartilage delays chondrocyte hypertrophy and causes dwarfism with osteopenia.

"Biochemical experiments have shown that Smad6 and Smad ubiquitin regulatory factor 1 (Smurf1) block the signal transduction of bone morphogenetic proteins (BMPs). However, their in vivo functions are largely unknown. Here, we generated transgenic mice overexpressing Smad6 in chondrocytes. Smad6 transgenic mice showed postnatal dwarfism with osteopenia and inhibition of Smad1/5/8 phosphorylation in chondrocytes. "

LSJL alteration of either Smurf1 or 2 is predicted and downregulates Smad1.

"Smad ubiquitin regulatory factor 1 (Smurf1) and Smurf2 induce the ubiquitination and degradation of Smad1 and Smad5"<-We need to know the proportion of how much LSJL upregulates Smad1 relative to the Smurf's.

"Smurf1 and Smurf2 interact with nuclear Smad7 and induce the nuclear export of Smad7. Smurf–Smad7 complexes then associate with type I receptor for TGF-β and enhance its turnover "<-LSJL had no effect on Smad7 but LSJL did upregulate TGF-Beta Receptor I.

"Smurf1 binds to BMP type I receptors via Smad6 and Smad7, and that it induces the ubiquitination and degradation of these receptors. Thus, Smad6 and Smurf1 cooperatively down-regulate BMP signals by degradation of R-Smads as well as BMP receptors."<-LSJL induces BMP RII rather than BMP RI which is consistent with the elevated Smurf levels.

"Exogenous rhBMP2 added to Smurf1 transgenic cartilage in organ cultures of metatarsal rudiments at 15.0 d.p.c. caused proliferative cartilage outgrowth and hypertrophic center formation as in normal mice. "<-So maybe the Smurf1 upregulation doesn't inhibit the height growth inducing elements of LSJL.

"Smad6 appears to block BMP signaling, whereas Smad7 blocks that of both TGF-β and BMP"  rhBMP2 is what phosphorylates Smad1/5/8.

So BMP-2 may be better for height growth than TGF-Beta1 and this may be linked to BMP-2 chondrocytes lower contractile ability which results in slower terminal differentiation and mineralization.