Integrins mediating bone signal transduction

Integrin-mediated adhesions play critical roles in diverse cell functions. Integrins offers a platform on which mechanical stimuli, cytoskeletal organization, biochemical signals can concentrate. Mechanical stimuli transmitted by integrins influence the cytoskeleton, in turn, the cytoskeleton influences cell adhesion via integrins, then cell adhesion results in a series of signal transduction cascades. In skeleton, integrins also have a key role for bone resoption by osteoclasts and reformation by osteoblasts. In present review, the proteins involved in integrin signal transduction and integrin signal transduction pathways were discussed, mainly on the basic mechanisms of integrin signaling and the roles of integrins in bone signal transduction, which may give insight into new therapeutic agents to all kinds of skeletal diseases and new strategies for bone tissue engineering.

An engineered lamellar bone mimicking full-scale hierarchical architecture for bone regeneration

Lamellar bone,compactly and ingeniously organized in the hierarchical pattern with 6 ordered scales,is the structural motif of mature bone.Each hierarchical scale exerts an essential role in determining physiological behavior and osteogenic bioactivity of bone.Engineering lamellar bone with full-scale hierarchy remains a longstanding challenge.Herein,using bioskiving and mineralization,we attempt to engineer compact constructs resembling full-scale hierarchy of lamellar bone.Through systematically investigating the effect of mineralization on physicochemical properties and bioactivities of multi-sheeted collagen matrix fabricated by bioskiving,the hierarchical mimicry and hierarchy-property relationship are elucidated.With prolongation of mineralization,hierarchical mimicry and osteogenic bioactivity of constructs are performed in a bidirectional manner,i.e.first rising and then descending,which is supposed to be related with transformation of mineralization mechanism from nonclassical to classical crystallization.Construct mineralized 9 days can accurately mimic each hierarchical scale and efficiently promote osteogenesis.Bioinformatic analysis further reveals that this construct potently activates integrinα5-PI3K/AKT signaling pathway through mechanical and biophysical cues,and thereby repairing critical-sized bone defect.The present study provides a bioinspired strategy for completely resembling complex hierarchy of compact mineralized tissue,and offers a critical research model for in-depth studying the structure-function relationship of bone.

The structure of Rap1 in complex with RIAM reveals specificity determinants and recruitment mechanism

The small GTPase Rap1 induces integrin activation via an inside-out signaling pathway mediated by the Rapl-interacting adaptor mol- ecule （RIAM）. Blocking this pathway may suppress tumor metastasis and other diseases that are related to hyperactive integrins. However, the molecular basis for the specific recognition of RIAM by Rap1 remains largely unknown. Herein we present the crystal structure of an active, GTP-bound GTPase domain of Rap1 in complex with the Ras association （RA）-pleckstrin homology （PH） structural module of RIAM at 1.65 A. The structure reveals that the recognition of RIAM by Rap1 is governed by side-chain interactions. Several side chains are critical in determining specificity of this recognition, particularly the Lys31 residue in Rap1 that is oppositely charged compared with the Glu31/Asp31 residue in other Ras GTPases. Lys31 forms a salt bridge with RIAM residue Glu212, making it the key specificity determinant of the interaction. We also show that disruption of these interactions results in reduction of Rapl：RIAM association, leadingto a loss of co-clustering and cell adhesion. Our findings elucidate the molecular mechanism by which RIAM med- iates Rapl-induced integrin activation. The crystal structure also offers new insight into the structural basis for the specific recruitment of RA-PH module-containing effector proteins by their smaU GTPase partners.

The conformational states of talin autoinhibition complex and its activation under forces

Talin is an integrin-binding protein located at focal adhesion site and serves as both an adapter and a force transmitter. Its integrin binding activity is regulated by the intramolecular autoinhibition interaction between its F3 and RS domains. Here, we used atomic force microscopy to measure the strength of talin autoinhibition complex. Our results suggest that the lifetime of talin autoinhibition complex shows weak catch bond behavior and does not change significantly at smaller forces, while it drops rapidly at larger forces(>10 p N). Moreover, besides the complex conformation revealed by crystal structure, our molecular dynamics(MD) simulations indicate the possible existence of another stable conformation. Further analysis indicates that forces may regulate the equilibrium of the two stable binding states and result in the non-exponential force dependence of the binding lifetime. Our findings reveal a negative regulation mechanism on talin activation and provide a new point of view on the function of talin in focal adhesion.