Genetic inactivation of ERK1 and ERK2 in chondrocytes promotes bone growth and enlarges the spinal canal

Abstract

Activating mutations in FGFR3 cause the most common forms of human dwarfism: achondroplasia and thanatophoric dysplasia. In mouse models of achondroplasia, recent studies have implicated the ERK MAPK pathway, a pathway activated by FGFR3, in creating reduced bone growth. Our recent studies have indicated that increased Fgfr3 and ERK MAPK signaling in chondrocytes also causes premature synchondrosis closure in the cranial base and vertebrae, accounting for the sometimes fatal stenosis of the foramen magnum and spinal canal in achondroplasia. Conversely, whether the decrease—or inactivation—of ERK1 and ERK2 promotes bone growth and delays synchondrosis closure remains to be investigated. In this study, we inactivated ERK2 in the chondrocytes of ERK1-null mice using the Col2a1-Cre and Col2a1-CreER transgenes. We found that the genetic inactivation of ERK1 and ERK2 in chondrocytes enhances the growth of cartilaginous skeletal elements. We also found that the postnatal inactivation of ERK1 and ERK2 in chondrocytes delays synchondrosis closure and enlarges the spinal canal. These observations make ERK1 and ERK2 an attractive target for the treatment of achondroplasia and other FGFR3-related skeletal syndromes.

Fibroblast growth factor signaling in skeletal development and disease

“…In cultured chondrocytes, FGFR3 suppression of chondrocyte proliferation required MAPK signaling and was independent of STAT signaling (Raucci et al 2004;Krejci et al 2008a). In support of this observation, mice lacking ERK1 and conditionally lacking ERK2 (Col2a1-Cre) in chondrocytes showed increased chondrocyte proliferation at late embryonic stages (Matsushita et al 2009a;Sebastian et al 2011). Some of the variability in experimental results may be due to in vitro culture conditions, the developmental stage being examined (embryonic vs. postnatal), and potential indirect effects on proliferating chondrocytes resulting from interactions with other signaling pathways in nearby cells within the groove of Ranvier, ring of LaCroix, and perichondrium.…”

The ERK MAPK Pathway in Bone and Cartilage Formation

“…We have also shown that ERK1 and ERK2 inactivation in chondrocytes promotes bone growth. We found increased length of the proximal long bones, specifically the humerus and femur, of ERK1 -/-; ERK2 flox/flox ; Col2a1-Cre embryos (Sebastian et al, 2011). These embryos also showed an increase in the width of epiphyses in the humerus and femur.…”

“…ERK1 -/-; ERK2 flox/flox ; Col2a1-Cre mutant mice died immediately after birth, likely secondary to respiratory insufficiency caused by rib cage deformity (Matsushita et al, 2009a). To circumvent the perinatal lethality, we also used the Col2a1-CreER transgene to express a tamoxifen-inducible form of Cre recombinase and examined the role of ERK1 and ERK2 in chondrocytes during postnatal growth (Nakamura et al, 2006;Sebastian et al, 2011). For the gain-of-function experiments, we generated Col2a1-MEK1 transgenic mice that express a constitutively active mutant of MEK1 in chondrocytes under the control of the regulatory sequences of Col2a1 (Murakami et al, 2004).…”

“…To test this hypothesis, we inactivated ERK2 in chondrocytes of ERK1-null mice using the Col2a1-CreER transgene (Sebastian et al, 2011). Tamoxifen injection into ERK1 -/-; ERK2 flox/flox ; Col2a1-CreER mice resulted in 60% inhibition of ERK2 expression in the epiphyseal cartilage.…”

“…We also generated a loss of function model in chondrocytes by using the Col2a1-Cre transgene (Matsushita et al, 2009a;Ovchinnikov et al, 2000). To induce postnatal inactivation of ERK1 and ERK2 in chondrocytes, we used the Col2a1-CreER transgene to express a tamoxifen-inducible form of Cre recombinase (Nakamura et al, 2006;Sebastian et al, 2011). For gain-of-function experiments, we generated Prx1-MEK1 transgenic mice that express a constitutively active mutant of MEK1 in undifferentiated mesenchymal cells under the control of a Prx1 promoter (Matsushita et al, 2009a).…”