egr1

 Egr1 is upregulated  4.973 fold by LSJL.  Under normal axial loading on 20 week old mice, egr1 was downregulated which could mean that egr1 is a key reason as to why LSJL can increase chondrogenesis but axial loading cannot.  Chondrocytic cells under hydrostatic pressure had egr1 upregulated.  Egr1 was also upregulated by dynamic chondrocyte compression.  LIPUS can also upregulate EGR1.  Removing the sciatic nerve increased EGR1 levels.  One study found that Egr1 was upregulated in older rat growth plates.  Increases in HMGA2(which is linked to height and upregulated by LSJL) levels also increased EGR1 levels.  Surprisingly Egr1 was downregulated in mesodermal progenitor cell differentiation to chondrocytes.  Arachidonic acid can increase Egr1 levels.

The Immediate Early Gene Product EGR1 and Polycomb Group Proteins Interact in Epigenetic Programming during Chondrogenesis.

"Initiation of and progression through chondrogenesis is driven by changes in the cellular microenvironment. At the onset of chondrogenesis, resting mesenchymal stem cells are mobilized in vivo and a complex, step-wise chondrogenic differentiation program is initiated {our goal with LSJL is to induce chondrogenic differentiation of resting mesenchymal stem cells}. Differentiation requires coordinated transcriptomic reprogramming and increased progenitor proliferation; both processes require chromatin remodeling. The nature of early molecular responses that relay differentiation signals to chromatin is poorly understood. Immediate early genes are rapidly and transiently induced in response to differentiation stimuli in vitro {So does EGR1 cause the differentiation or is EGR1 the byproduct?}. Functional ablation of the immediate early factor EGR1 severely deregulates expression of key chondrogenic control genes at the onset of differentiation. In addition, differentiating cells accumulate DNA damage, activate a DNA damage response and undergo a cell cycle arrest and prevent differentiation associated hyper-proliferation. Failed differentiation in the absence of EGR1 affects global acetylation and terminates in overall histone hypermethylation. Polycomb associated histone H3 lysine27 trimethylation (H3K27me3) blocks chromatin access of EGR1. In addition, EGR1 ablation results in abnormal Ezh2 and Bmi1 expression. Consistent with this functional interaction, we identify a number of co-regulated targets genes in a chondrogenic gene network. EGR1 [is involved] in early chondrogenic epigenetic programming to accommodate early gene-environment interactions in chondrogenesis."

ATDC5 cells were used which are technically chondrocyte progenitor cells like those in the growth plate resting zone and not true mesenchymal stem cells.

"Progression through chondrogenesis is in part driven by interaction with a constantly changing microenvironment, which is defined by soluble growth and differentiation factors, hormones, oxygen tension, cell-cell and cell-ECM contacts"

"abnormal skeletogenesis [occurs] in PRC1 LOF[Loss of Function} mice"

"Egr1 mRNA induction in chondrogenesis is transient and precedes transcriptional upregulation of Sox9"

"Sox6 and Agc1-loci do not represent early EGR1-targets for transcriptional activation in chondrogenesis."<-Although aggrecan was upregulated by LSJL.

"Consistent with reduced chondrogenic capacity; shEgr1{silenced-Egr1} significantly reduced Col2a1 expression at the mRNA and protein levels"

"chondrogenic markers Sox9, Agc1, Col2A1 and Col10A1 showed a delayed (5–10 days) differentiation response [after Egr1 is silenced]" All of these genes were up in LSJL.

"Enchondral ossification pathway analysis for predicted EGR1 targets (green) and published PRC1 targets (blue)."

That Runx2 is an EGR1 target could explain how Egr1 could upregulate osteoblast differentiation as well.

There is evidence though that loss Egr1 can be compensated for.

"H3K27me3-decorated chromatin prevents EGR1 from accessing promoters."<-thus an H3K27me3 inhibitor may be a well to allow EGR1 to access chondrogenic promoters again.

Sox9 was not upregulated by Egr1 in H3K27me3 cells.

"EZH2 levels decrease in the context of replicative senescence"

"shEgr1 cultures do not reach super-confluence and do not form chondrogenic nodules"

EGFR ligands drive multipotential stromal cells to produce multiple growth factors and cytokines via early growth response-1. establishes a connection between MSCs and Egr1 but not a relationship between the two and chondrogenic differentiation.

Early growth response genes signaling supports strong paracrine capability of mesenchymal stem cells.

"EGF [facilitates] in vitro expansion of MSCs without altering multipotency. The molecular machinery underlying MSCs' strong paracrine capability lies downstream of EGFR signaling, and we focus on transcription factors EGR1 and EGR2. EGR1 regulates angiogenic and fibrogenic factor production in MSCs{fibrogenic factors are involved in chondrogenesis}, and an EGFR-EGR1-EGFR ligands autocrine loop is one of the underlying mechanisms supporting their strong paracrine machinery through EGR1. EGR2{also up in LSJL} appears to regulate the expression of immunomodulatory molecules."

Egr1 is expressed 3 fold higher in human MSCs than human fibroblasts.

So Egr1 likely plays a role in LSJL but unfortunately this was only established in chondrocyte progenitor cells and not MSCs directly.