R-spondin-2 is a Wnt agonist that regulates osteoblast activity and bone mass.

The R-spondin family of proteins are Wnt agonists, and the complete embryonic disruption of Rspo2 results in skeletal developmental defects that recapitulate the phenotype observed with Lrp5/6 deficiency. Previous work has shown that R-spondin-2 (Rspo2, RSPO2) is both highly expressed in Wnt-stimulated pre-osteoblasts and its overexpression induces osteoblast differentiation in the same cells, supporting its putative role as a positive autocrine regulator of osteoblastogenesis. However, the role of Rspo2 in regulating osteoblastogenesis and bone formation in postnatal bone has not been explored. Here we show that limb-bud progenitor cells from Rspo2 knockout mice undergo reduced mineralization during osteoblastogenesis in vitro and have a corresponding alteration in their osteogenic gene expression profile. We also generated the first Rspo2 conditional knockout (Rspo2floxed) mouse and disrupted Rspo2 expression in osteoblast-lineage cells by crossing to the Osteocalcin-Cre mouse line (Ocn-Cre + Rspo2f/f). Ocn-Cre + Rspo2f/f male and female mice at 1, 3, and 6 months were examined. Ocn-Cre + Rspo2f/f mice are decreased in overall body size compared to their control littermates and have decreased bone mass. Histomorphometric analysis of 1-month-old mice revealed a similar number of osteoblasts and mineralizing surface per bone surface with a simultaneous decrease in mineral apposition and bone formation rates. Consistent with this observation, serum osteocalcin in 3-month-old Ocn-Cre + Rspo2f/f was reduced, and bone marrow-mesenchymal stem cells from Ocn-Cre + Rspo2f/f mice undergo less mineralization in vitro. Finally, gene expression analysis and immunohistochemistry of mature bone shows reduced beta-catenin signaling in Ocn-Cre + Rspo2f/f. Overall, RSPO2 reduces osteoblastogenesis and mineralization, leading to reduced bone mass.

R-spondins: novel matricellular regulators of the skeleton.

R-spondins are a family of four matricellular proteins produced by a variety of cell-types. Structurally, R-spondins contain a TSR1 domain that retains the tryptophan structure and a modified cysteine-rich CSVCTG region. In addition, the R-spondins contain two furin repeats implicated in canonical Wnt signaling. R-spondins positively regulate canonical Wnt signaling by reducing Wnt receptor turnover and thereby increasing beta-catenin stabilization. R-spondins are prominently expressed in the developing skeleton and contribute to limb formation, particularly of the distal digit. Additionally, results suggest that R-spondins may contribute to the maintenance of adult bone mass by regulating osteoblastogenesis and bone formation.

Mesenchymal Stem Cells in Bone Regeneration.

SIGNIFICANCE: Mesenchymal stem cells (MSCs) play a key role in fracture repair by differentiating to become bone-forming osteoblasts and cartilage-forming chondrocytes. Cartilage then serves as a template for additional bone formation through the process of endochondral ossification. RECENT ADVANCES: Endogenous MSCs that contribute to healing are primarily derived from the periosteum, endosteum, and marrow cavity, but also may be contributed from the overlying muscle or through systemic circulation, depending on the type of injury. A variety of growth factor signaling pathways, including BMP, Wnt, and Notch signaling, influence MSC proliferation and differentiation. These MSCs can be therapeutically manipulated to promote differentiation. Furthermore, MSCs can be harvested, cultivated, and delivered to promote bone healing. CRITICAL ISSUES: Pharmacologically manipulating the number and differentiation capacity of endogenous MSCs is one potential therapeutic approach to improve healing; however, ideal agents to influence signaling pathways need to be developed and additional therapeutics that activate endogenous MSCs are needed. Whether isolated and purified, MSCs participate directly in the healing process or serve a bystander effect and indirectly influence healing is not well defined. FUTURE DIRECTIONS: Studies must focus on better understanding the regulation of endogenous MSCs durings fracture healing. This will reveal novel molecules and pathways to therapeutically target. Similarly, while animal models have demonstrated efficacy in the delivery of MSCs to promote healing, more research is needed to understand ideal donor cells, cultivation methods, and delivery before stem cell therapy approaches can be utilized to repair bone.

Operational plasticity enables hsp104 to disaggregate diverse amyloid and nonamyloid clients.

It is not understood how Hsp104, a hexameric AAA+ ATPase from yeast, disaggregates diverse structures, including stress-induced aggregates, prions, and α-synuclein conformers connected to Parkinson disease. Here, we establish that Hsp104 hexamers adapt different mechanisms of intersubunit collaboration to disaggregate stress-induced aggregates versus amyloid. To resolve disordered aggregates, Hsp104 subunits collaborate noncooperatively via probabilistic substrate binding and ATP hydrolysis. To disaggregate amyloid, several subunits cooperatively engage substrate and hydrolyze ATP. Importantly, Hsp104 variants with impaired intersubunit communication dissolve disordered aggregates, but not amyloid. Unexpectedly, prokaryotic ClpB subunits collaborate differently than Hsp104 and couple probabilistic substrate binding to cooperative ATP hydrolysis, which enhances disordered aggregate dissolution but sensitizes ClpB to inhibition and diminishes amyloid disaggregation. Finally, we establish that Hsp104 hexamers deploy more subunits to disaggregate Sup35 prion strains with more stable "cross-ß" cores. Thus, operational plasticity enables Hsp104 to robustly dissolve amyloid and nonamyloid clients, which impose distinct mechanical demands.

A synergistic small-molecule combination directly eradicates diverse prion strain structures.

Safely eradicating prions, amyloids and preamyloid oligomers may ameliorate several fatal neurodegenerative disorders. Yet whether small-molecule drugs can directly antagonize the entire spectrum of distinct amyloid structures or 'strains' that underlie distinct disease states is unclear. Here, we investigated this issue using the yeast prion protein Sup35. We have established how epigallocatechin-3-gallate (EGCG) blocks synthetic Sup35 prionogenesis, eliminates preformed Sup35 prions and disrupts inter- and intramolecular prion contacts. Unexpectedly, these direct activities were strain selective, altered the repertoire of accessible infectious forms and facilitated emergence of a new prion strain that configured original, EGCG-resistant intermolecular contacts. In vivo, EGCG cured and prevented induction of susceptible, but not resistant strains, and elicited switching from susceptible to resistant forms. Importantly, 4,5-bis-(4-methoxyanilino)phthalimide directly antagonized EGCG-resistant prions and synergized with EGCG to eliminate diverse Sup35 prion strains. Thus, synergistic small-molecule combinations that directly eradicate complete strain repertoires likely hold considerable therapeutic potential.