Zfp521, a novel zinc finger transcription factor, regulates chondrocyte differentiation at multiple steps acting downstream of parathyroid hormone (PTH)-related peptide (PTHrP)

Zfp521 was originally identified as a part of a molecular complex that regulates osteoblast differentiation given its negative effect over the transcription factor Runx2. It is preferentially expressed both in vivo and in vitro by pre-hypertrophic chondrocytes where it colocalizes with the receptor for parathyroid hormone (PTH)-related peptide (PTHrP) (PTHR1). In vitro overexpression of Zfp521 negatively regulates early and late chondrocyte differentiation processes, comparable to the negative effect of PTHrP (1-34). In fact, PTHrP (1-34) dose- and time-dependently upregulates Zfp521 expression via cAMP/PKA signaling pathway. Moreover, PTHrP signaling is at least required for the in vivo steady state expression of Zfp521, as mice lacking PTHR1 exhibits low levels of Zfp521 while high PTHR1 signaling significantly upregulates Zfp521 expression. The chondrocyte-specific Zfp521 conditional KO (cKO) mice exhibit postnatal growth retardation evident as early as 2 weeks of age. The absence of Zfp521 leads to a reduced thickness of the growth plate involving all constitutive layers. Specifically, in pre-hypertrophic chondrocytes lacking Zfp521, PTHrP failed to upregulate cyclin D1 and antagonize Runx2 generating a reduced cell proliferation and an early hypertrophy. These mutant hypertrophic chondrocytes are bigger than their respective control and functionally impaired as they secrete less extracellular matrix, which becomes also early mineralized. Zfp521 cKO growth plates have a significantly reduced hypertrophic zone in spite of an enhanced hypertrophic differentiation. This is secondary to the decreased secretory function in hypertrophic chondrocytes and an enhanced bone formation generated by an increased chondrocyte apoptosis mediated by low levels of Bcl-2 and high Caspase-3 expression and activity, and an augmented osteoclast differentiation in the chondro-osseus junction driven by an increase in the pro-osteoclastogenic RANKL/OPG ratio.