Trends in Cell Biology
ReviewChaperoning osteogenesis: new protein-folding disease paradigms
Section snippets
Sans chaperones: osteogenesis choreography gone astray
Bone consists of flexible type I collagen fibers and hard hydroxyappatite mineral, making bone material strong yet not brittle. Bone formation (osteogenesis) involves an intricate choreography of specialized cells (Figure 1). Osteoblasts deposit collagen fibers and promote mineralization, osteoclasts resorb older bone and osteocytes act as mechano-sensors that are embedded in bone. These cells interact with each other and are regulated by outside signals from cytokines, hormones and the
Procollagen folding: descending or climbing a mountain
To understand how ER proteins chaperone collagen, we begin with a discussion of general folding principles. About 50 years ago, Anfinsen and co-workers suggested that (i) all information necessary for folding a protein is contained within its amino acid sequence, and (ii) the native state, in which the protein performs its normal functions, is the most thermodynamically favorable conformation [24]. However, further studies revealed that the native state might be less favorable than one or
Chaperoning dilemma: encouraging success versus discouraging failure
The unique uphill track of procollagen folding requires a distinct chaperoning approach (Figure 2f). Preferential ligand (here chaperone) binding always reduces the free energy of the substrate. General ER chaperones stabilize unfolded protein chains at the cost of reducing the free energy benefits of the native state. The procollagen native state is unfavorable to begin with; therefore, preferential binding of general ER chaperones to unfolded chains would make procollagen folding even more
Collagen prolyl-4-hydroxylase (C-P4H)
C-P4H is probably the best studied of these chaperones [40]. It is an α2β2 tetramer of two identical α subunits and two identical β subunits. The β subunit functions also as the independent ER chaperone protein disulfide isomerase (PDI). C-P4H catalyzes 4-hydroxylation of proline, increasing the stability of the triple helix [41] and making triple helix folding less unfavorable.
HSP47
HSP47 is an ATP-independent heat shock protein and its expression pattern indicates that it is a collagen-specific rather than general ER chaperone 34, 42. Interaction with collagen-mimetic peptides suggests binding of more than 25 HSP47 molecules to a single folded type I collagen triple helix [20] and no significant binding to unfolded chains [21]. Calculation based on measured binding constants [20] showed that preferential binding of multiple HSP47 molecules should make the triple helical
FKBP65
FKBP65 is an ER-resident protein with general chaperone and peptidylprolyl isomerase functions [44]. Accumulation of aggregated procollagen in the ER of FKBP65-deficient cells [12] indicates that FKBP65 is important for procollagen folding or trafficking. In vitro studies suggest that FKBP65 might stabilize the triple helix like HSP47 (Figure 3) and catalyze proline peptide bond isomerization [44], thus accelerating procollagen folding. However, no overhydroxylation or overglycosylation of
CRTAP, P3H1 and CYPB
CRTAP, P3H1 and CYPB form a stable, three-protein complex in the ER [47]. Similar to FKBP65, this complex has general chaperone and peptidylprolyl isomerase activity [48] as well as essential, but unclear triple helix chaperone function(s). CRTAP and P3H1 (but not CYPB [11]) are required for 3-hydroxylation of Pro986 in the α1(I) chain [49]. Pro986 3-hydroxylation does not affect triple helix stability significantly [50], but it might be required as a chaperone binding site for triple helix
When chaperones fail: unfolded protein response
In general, cells respond to the accumulation of partially unfolded polypeptide chains in the ER because exposed hydrophobic regions of these chains bind and sequester BiP and GRP94. Subsequent release of BiP and GRP94 from the ER membrane receptors PERK, ATF6 or IRE1 triggers signaling known as the unfolded protein response (UPR) 58, 59, which downregulates protein synthesis, upregulates the synthesis of BiP and other chaperones, and targets misfolded proteins for ER-associated degradation
The cost of failure: understanding procollagen folding diseases
Osteoblast malfunction is a common feature of OI and OI-like bone pathology caused by: (i) procollagen mutations 56, 68, 69; (ii) deficiency in procollagen chaperones [6]; (iii) deficiency in ER stress transducers unrelated to collagen 65, 70, 71; (iv) deficiency in osteoblast-specific transcription factor SP7 (osterix) [72]; and (v) deficiency in the low-density lipoprotein receptor-related protein LRP5, which might affect osteoblast activity via blood serotonin [73] or by disrupting Wnt
Concluding remarks
The discussion of protein folding and ER stress response to misfolding has been historically shaped by studies of stable, well-structured, globular proteins with a central hydrophobic core. Nonspecific aggregation of hydrophobic moieties has been viewed as the primary folding obstacle. The discussion of chaperones has mostly been limited to preventing such aggregation and the ER stress response has often been equated with UPR.
These paradigms are certainly valid, but they do not fully describe
Acknowledgements
This work was funded by the Intramural Research Program, NICHD, NIH. We thank Lynn S. Felts for critical reading of the manuscript.
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