Nano-topography sensing by osteoclasts

Vitronectin regulates osteoclastogenesis and bone remodeling in a mouse model of osteoporosis

Vitronectin (VN) is an extracellular matrix protein with a crucial role in regulating bone remodeling. In this study, we aimed to investigate the effect of VN deficiency in a mouse model of osteoporosis induced by ovariectomy (OVX). The findings revealed that the absence of VN led to an increase in the activity of tartrate-resistant acid phosphatase (TRAP), a marker for osteoclasts, in the plasma of OVX-operated mice. TRAP staining further demonstrated that VN deficiency resulted in a higher number of osteoclasts within the femurs of OVX-operated mice. X-ray micro-computed tomography analysis of the femurs in OVX-operated mice indicated that VN deficiency significantly suppressed the OVX-induced increase of marrow area and total volume of bone. Additionally, we assessed structural model index (SMI) and degree of anisotropy (DA) as indices of osteoporosis. The results showed that VN deficiency effectively attenuated the OVX-induced increase in SMI and DA among OVX-operated mice. In summary, our study demonstrates the vital role of VN in regulating osteoclastogenesis and bone remodeling in the mouse model of osteoporosis.

Nanotopographical cues for regulation of macrophages and osteoclasts: emerging opportunities for osseointegration

Nanotopographical cues of bone implant surface has direct influences on various cell types during the establishment of osseointegration, a prerequisite of implant bear-loading. Given the important roles of monocyte/macrophage lineage cells in bone regeneration and remodeling, the regulation of nanotopographies on macrophages and osteoclasts has arisen considerable attentions recently. However, compared to osteoblastic cells, how nanotopographies regulate macrophages and osteoclasts has not been properly summarized. In this review, the roles and interactions of macrophages, osteoclasts and osteoblasts at different stages of bone healing is firstly presented. Then, the diversity and preparation methods of nanotopographies are summarized. Special attentions are paid to the regulation characterizations of nanotopographies on macrophages polarization and osteoclast differentiation, as well as the focal adhesion-cytoskeleton mediated mechanism. Finally, an outlook is indicated of coordinating nanotopographies, macrophages and osteoclasts to achieve better osseointegration. These comprehensive discussions may not only help to guide the optimization of bone implant surface nanostructures, but also provide an enlightenment to the osteoimmune response to external implant.

Topography-mediated immunomodulation in osseointegration; Ally or Enemy

Osteoimmunology is at full display during endosseous implant osseointegration. Bone formation, maintenance and resorption at the implant surface is a result of bidirectional and dynamic reciprocal communication between the bone and immune cells that extends beyond the well-defined osteoblast-osteoclast signaling. Implant surface topography informs adherent progenitor and immune cell function and their cross-talk to modulate the process of bone accrual. Integrating titanium surface engineering with the principles of immunology is utilized to harness the power of immune system to improve osseointegration in healthy and diseased microenvironments. This review summarizes current information regarding immune cell-titanium implant surface interactions and places these events in the context of surface-mediated immunomodulation and bone regeneration. A mechanistic approach is directed in demonstrating the central role of osteoimmunology in the process of osseointegration and exploring how regulation of immune cell function at the implant-bone interface may be used in future control of clinical therapies. The process of peri-implant bone loss is also informed by immunomodulation at the implant surface. How surface topography is exploited to prevent osteoclastogenesis is considered herein with respect to peri-implant inflammation, osteoclastic precursor-surface interactions, and the upstream/downstream effects of surface topography on immune and progenitor cell function.

Distinct Morphologies of Bone Apatite Clusters in Endochondral and Intramembranous Ossification

Bone apatite crystals grow in clusters, but the microstructure of these clusters is unknown. This study compares the structural and compositional differences between bone apatite clusters formed in intramembranous (IO) and endochondral ossification (EO). Calvaria (IO) and femurs (EO) are isolated from mice at embryonic days (E) 14.5 to 15.5 and post-natal days (P) 6 to 7, respectively. Results show that the initially formed bone apatite clusters in EO (≅1.2 µm² ) are >10 times larger than those in IO (≅0.1 µm² ), without significant changes in ion composition. In IO (E14.5 calvarium), early minerals are formed inside matrix vesicles (MVs). In contrast, in EO (P6 femur epiphysis), no MVs are observed, and chondrocyte-derived plasma membrane nanofragments (PMNFs) are the nucleation site for mineralization. Apatite cluster size difference is linked with the different nucleation sites. Moreover, an alkaline pH and slow P supply into a Ca-rich microenvironment are suggested to facilitate apatite cluster growth, as demonstrated in a biomimetic mineralization system. Together, the results reveal for the first time the distinct and exquisite microstructures of bone apatite clusters in IO and EO, and provide insightful inspirations for the design of more efficient materials for bone tissue engineering and repair.

Matrix Metalloproteinases Shape the Tumor Microenvironment in Cancer Progression

Cancer progression with uncontrolled tumor growth, local invasion, and metastasis depends largely on the proteolytic activity of numerous matrix metalloproteinases (MMPs), which affect tissue integrity, immune cell recruitment, and tissue turnover by degrading extracellular matrix (ECM) components and by releasing matrikines, cell surface-bound cytokines, growth factors, or their receptors. Among the MMPs, MMP-14 is the driving force behind extracellular matrix and tissue destruction during cancer invasion and metastasis. MMP-14 also influences both intercellular as well as cell-matrix communication by regulating the activity of many plasma membrane-anchored and extracellular proteins. Cancer cells and other cells of the tumor stroma, embedded in a common extracellular matrix, interact with their matrix by means of various adhesive structures, of which particularly invadopodia are capable to remodel the matrix through spatially and temporally finely tuned proteolysis. As a deeper understanding of the underlying functional mechanisms is beneficial for the development of new prognostic and predictive markers and for targeted therapies, this review examined the current knowledge of the interplay of the various MMPs in the cancer context on the protein, subcellular, and cellular level with a focus on MMP14.

Nanoporous titanium implant surface promotes osteogenesis by suppressing osteoclastogenesis via integrin β1/FAKpY397/MAPK pathway

Macrophages and osteoclasts are both derived from monocyte/macrophage lineage, which plays as the osteoclastic part of bone metabolism. Although they are regulated by bone implant surface nanoarchitecture and involved in osseointegration, the beneath mechanism has not been simultaneously analyzed in a given surface model and their communication with osteoblasts is also blurring. Here, the effect of implant surface topography on monocyte/macrophage lineage osteoclastogenesis and the subsequent effect on osteogenesis are systematically investigated. The nanoporous surface is fabricated on titanium implant by etching and anodizing to get the nanotubes structure. The early bone formation around implant is significantly accelerated by the nanoporous surface in vivo. Meanwhile, the macrophage recruitment and osteoclast formation are increased and decreased respectively. Mechanistically, the integrin mediated FAK phosphorylation and its downstream MAPK pathway (p-p38) are significantly downregulated by the nanoporous surface, which account for the inhibition of osteoclastogenesis. In addition, the nanoporous surface can alleviate the inhibition of osteoclasts on osteogenesis by changing the secretion of clastokines, and accelerate bone regeneration by macrophage cytokine profiles. In conclusion, these data indicate that physical topography of implant surface is a critical factor modulating monocyte/macrophage lineage commitment, which provides theoretical guidance and mechanism basis for promoting osseointegration by coupling the osteogenesis and osteoclastogenesis.

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Nanoporous implant inhibits osteoclastogenesis via integrin β1/FAKpY397/MAPK.

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Nanoporous implant with larger diameter inhibits osteoclastogenesis more strongly.

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Nanoporous implant increases osteogenic cytokines of macrophages/osteoclasts.

The Mechanism Switching the Osteoclast From Short to Long Duration Bone Resorption

The current models of osteoclastic bone resorption focus on immobile osteoclasts sitting on the bone surface and drilling a pit into the bone matrix. It recently appeared that many osteoclasts also enlarge their pit by moving across the bone surface while resorbing. Drilling a pit thus represents only the start of a resorption event of much larger amplitude. This prolonged resorption activity significantly contributes to pathological bone destruction, but the mechanism whereby the osteoclast engages in this process does not have an answer within the standard bone resorption models. Herein, we review observations that lead to envision how prolonged resorption is possible through simultaneous resorption and migration. According to the standard pit model, the “sealing zone” which surrounds the ruffled border (i.e., the actual resorption apparatus), “anchors” the ruffled border against the bone surface to be resorbed. Herein, we highlight that continuation of resorption demands that the sealing zone “glides” inside the cavity. Thereby, the sealing zone emerges as the structure responsible for orienting and displacing the ruffled border, e.g., directing resorption against the cavity wall. Importantly, sealing zone displacement stringently requires thorough collagen removal from the cavity wall - which renders strong cathepsin K collagenolysis indispensable for engagement of osteoclasts in cavity-enlargement. Furthermore, the sealing zone is associated with generation of new ruffled border at the leading edge, thereby allowing the ruffled border to move ahead. The sealing zone and ruffled border displacements are coordinated with the migration of the cell body, shown to be under control of lamellipodia at the leading edge and of the release of resorption products at the rear. We propose that bone resorption demands more attention to osteoclastic models integrating resorption and migration activities into just one cell phenotype.

Pathophysiological mechanisms of root resorption after dental trauma: a systematic scoping review

Background

The objective of this scoping review was to systematically explore the current knowledge of cellular and molecular processes that drive and control trauma-associated root resorption, to identify research gaps and to provide a basis for improved prevention and therapy.

Methods

Four major bibliographic databases were searched according to the research question up to February 2021 and supplemented manually. Reports on physiologic, histologic, anatomic and clinical aspects of root resorption following dental trauma were included. Duplicates were removed, the collected material was screened by title/abstract and assessed for eligibility based on the full text. Relevant aspects were extracted, organized and summarized.

Results

846 papers were identified as relevant for a qualitative summary. Consideration of pathophysiological mechanisms concerning trauma-related root resorption in the literature is sparse. Whereas some forms of resorption have been explored thoroughly, the etiology of others, particularly invasive cervical resorption, is still under debate, resulting in inadequate diagnostics and heterogeneous clinical recommendations. Effective therapies for progressive replacement resorptions have not been established. Whereas the discovery of the RANKL/RANK/OPG system is essential to our understanding of resorptive processes, many questions regarding the functional regulation of osteo-/odontoclasts remain unanswered.

Conclusions

This scoping review provides an overview of existing evidence, but also identifies knowledge gaps that need to be addressed by continued laboratory and clinical research.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-021-01510-6.

The State of the Art and Prospects for Osteoimmunomodulatory Biomaterials

The critical role of the immune system in host defense against foreign bodies and pathogens has been long recognized. With the introduction of a new field of research called osteoimmunology, the crosstalk between the immune and bone-forming cells has been studied more thoroughly, leading to the conclusion that the two systems are intimately connected through various cytokines, signaling molecules, transcription factors and receptors. The host immune reaction triggered by biomaterial implantation determines the in vivo fate of the implant, either in new bone formation or in fibrous tissue encapsulation. The traditional biomaterial design consisted in fabricating inert biomaterials capable of stimulating osteogenesis; however, inconsistencies between the in vitro and in vivo results were reported. This led to a shift in the development of biomaterials towards implants with osteoimmunomodulatory properties. By endowing the orthopedic biomaterials with favorable osteoimmunomodulatory properties, a desired immune response can be triggered in order to obtain a proper bone regeneration process. In this context, various approaches, such as the modification of chemical/structural characteristics or the incorporation of bioactive molecules, have been employed in order to modulate the crosstalk with the immune cells. The current review provides an overview of recent developments in such applied strategies.

3D Chitin Scaffolds of Marine Demosponge Origin for Biomimetic Mollusk Hemolymph-Associated Biomineralization Ex-Vivo

Structure-based tissue engineering requires large-scale 3D cell/tissue manufacture technologies, to produce biologically active scaffolds. Special attention is currently paid to naturally pre-designed scaffolds found in skeletons of marine sponges, which represent a renewable resource of biomaterials. Here, an innovative approach to the production of mineralized scaffolds of natural origin is proposed. For the first time, a method to obtain calcium carbonate deposition ex vivo, using living mollusks hemolymph and a marine-sponge-derived template, is specifically described. For this purpose, the marine sponge Aplysin aarcheri and the terrestrial snail Cornu aspersum were selected as appropriate 3D chitinous scaffold and as hemolymph donor, respectively. The formation of calcium-based phase on the surface of chitinous matrix after its immersion into hemolymph was confirmed by Alizarin Red staining. A direct role of mollusks hemocytes is proposed in the creation of fine-tuned microenvironment necessary for calcification ex vivo. The X-ray diffraction pattern of the sample showed a high CaCO3 amorphous content. Raman spectroscopy evidenced also a crystalline component, with spectra corresponding to biogenic calcite. This study resulted in the development of a new biomimetic product based on ex vivo synthetized ACC and calcite tightly bound to the surface of 3D sponge chitin structure.

L-Plastin deficiency produces increased trabecular bone due to attenuation of sealing ring formation and osteoclast dysfunction

Bone resorption requires the formation of complex, actin-rich cytoskeletal structures. During the early phase of sealing ring formation by osteoclasts, L-plastin regulates actin-bundling to form the nascent sealing zones (NSZ). Here, we show that L-plastin knockout mice produce osteoclasts that are deficient in the formation of NSZs, are hyporesorptive, and make superficial resorption pits in vitro. Transduction of TAT-fused full-length L-plastin peptide into osteoclasts from L-plastin knockout mice rescued the formation of nascent sealing zones and sealing rings in a time-dependent manner. This response was not observed with mutated full-length L-plastin (Ser-5 and -7 to Ala-5 and -7) peptide. In contrast to the observed defect in the NSZ, L-plastin deficiency did not affect podosome formation or adhesion of osteoclasts in vitro or in vivo. Histomorphometry analyses in 8- and 12-week-old female L-plastin knockout mice demonstrated a decrease in eroded perimeters and an increase in trabecular bone density, without a change in bone formation by osteoblasts. This decrease in eroded perimeters supports that osteoclast function is attenuated in L-plastin knockouts. Micro-CT analyses confirmed a marked increase in trabecular bone mass. In conclusion, female L-plastin knockout mice had increased trabecular bone density due to impaired bone resorption by osteoclasts. L-plastin could be a potential target for therapeutic interventions to treat trabecular bone loss.

[Research progress on the pathogenesis of inflammatory external root resorption]

Inflammatory external root resorption (IERR) refers to the pathological process of dissolving the hard tissue on the outer surface of the tooth root by the body's own immune system under the stimulation of various physical and chemical factors such as infection, stress, trauma and orthodontic treatment. Severe IERR can lead to endodontic and periodontal diseases, and even the loss of teeth. Therefore, understanding the etiology and the pathogenic mechanism of IERR are of importance in its prevention and treatment. This article will review the etiology and the regulation mechanisms of IERR.

Effects of hydroxyapatite surface nano/micro-structure on osteoclast formation and activity

The surface structure of calcium phosphate (CaP) ceramic plays an important role in its osteoinductivity; however, little is known about its effects on osteoclastogenesis. In this study, an intramuscular implantation model suggested a potential relationship between hydroxyapatite (HA)-induced bone formation and osteoclast appearance in the non-osseous site, which might be modulated by scaffold surface structure. Then, three dense HA discs with different grain sizes from biomimetic nanoscale (∼100 nm) to submicron scale (∼500 nm) were fabricated via distinct sintering procedures, and their impacts on osteoclastic differentiation of RAW 264.7 macrophages under RANKL stimulation were further investigated. Our results showed that compared with the ones in the submicron-scale dimension, nano-structured HA discs markedly impaired osteoclastic formation and function, as evidenced by inhibited cell fusion, reduced osteoclast size, less-defined actin ring, increased osteoclast apoptosis, suppressed expression of osteoclast specific genes and proteins, decreased TRAP-positive cells, and hampered resorption activity. This demonstrated that the surface structure of CaP ceramics has a great influence on osteoclastogenesis, which might be further related to its osteoinductive capacity. These findings might not only help us gain insight into biomolecular events during CaP-involved osteoinduction, but also offer a principle for designing orthopaedic implants with an ability of regulating both osteogenesis and osteoclastogenesis to achieve the desired performance.

Costameres, dense plaques and podosomes: the cell matrix adhesions in cardiovascular mechanosensing

The stiffness of the cardiovascular environment changes during ageing and in disease and contributes to disease incidence and progression. For instance, increased arterial stiffness can lead to atherosclerosis, while stiffening of the heart due to fibrosis can increase the chances of heart failure. Cells can sense the stiffness of the extracellular matrix through integrin adhesions and other mechanosensitive structures and in response to this initiate mechanosignalling pathways that ultimately change the cellular behaviour. Over the past decades, interest in mechanobiology has steadily increased and with this also our understanding of the molecular basis of mechanosensing and transduction. However, much of our knowledge about the mechanisms is derived from studies investigating focal adhesions in non-muscle cells, which are distinct in several regards from the cell-matrix adhesions in cardiomyocytes (costameres) or vascular smooth muscle cells (dense plaques or podosomes). Therefore, we will look here first at the evidence for mechanical sensing in the cardiovascular system, before comparing the different cytoskeletal arrangements and adhesion sites in cardiomyocytes and vascular smooth muscle cells and what is known about mechanical sensing through the various structures.

Bioinspired polydopamine and polyphenol tannic acid functionalized titanium suppress osteoclast differentiation: a facile and efficient strategy to regulate osteoclast activity at bone-implant interface

Osseointegration of metallic implants in porous osteoporotic bone remains a challenge. Surface modification of implants to reduce peri-implant osteoclastic bone resorption was explored in the study. Bioinspired polydopamine (pDOP) and polyphenol tannic acid (pTAN) are nature-derived universal coating systems that have emerged either as a sole coating or ad-layer for biomolecular conjugation on different biomaterials. The effects pDOP and pTAN on osteoclast development have not been reported before. In this study, osteoclast development was investigated on titanium (Ti) substrates coated with pDOP (Ti-pDOP) and pTAN (Ti-pTAN). The results showed that Ti-pDOP and Ti-pTAN coating reduced tartrate-resistant acid phosphatase activity and osteoclast cell number as compared with pristine Ti. Intriguingly, the reduction was higher on Ti-pTAN than on Ti-pDOP. Economical and biocompatible tannic acid serves as a superior coating in decreasing osteoclast activity when compared with that of pDOP coating and could be used to modulate osteoclast activity at bone-implant interfaces.

Ibandronate Treatment Before and After Implant Insertion Impairs Osseointegration in Aged Rats with Ovariectomy Induced Osteoporosis

Excessive decreases in bone volume (BV) and bone mineral density (BMD) can lead to osteoporosis, potentially hindering implant osseointegration. Bisphosphonates are commonly used to combat osteoporosis by slowing osteoclast-mediated resorption; however, functional osteoclasts are integral to bone remodeling and, thus, implant osseointegration, potentially contraindicating bisphosphonate use during implantation. To optimize the use of implant technologies in patients with compromised bone structure and metabolism, we need a more complete understanding of the biological response to surface design. The goal of this study was to assess the effects of osteoporosis and bisphosphonates on osseointegration of titanium (Ti) implants with microstructured surfaces, which have been shown to support osteoblast differentiation in vitro and rapid osseointegration in vivo. Forty, 8-month-old, virgin, female CD Sprague Dawley rats underwent ovariectomy (OVX) or sham (SHOVX) surgery. After 5 weeks, animals were injected subcutaneously with either the bisphosphonate (BIS), Ibandronate (25 µg/kg), or phosphate-buffered saline (PBS) every 25 days. 1 week after the initial injection, Ø2.5mm × 3.5mm microrough (SLA; grit-blasted/acid etched) implants were placed transcortically in the distal metaphysis of each femur resulting in four groups: 1) SHOVX+PBS; 2) SHOVX+BIS; 3) OVX+PBS; and 4) OVX+BIS. After 28d, qualitative properties of the bone and implant osseointegration were assessed using micro-computed tomography (microCT), calcified histomorphometry (Van Gieson's stain), and removal torque testing. microCT revealed decreased bone volume in OVX rats, which was slowed by bisphosphonate treatment. Reduced bone-to-implant contact (BIC) was evident in OVX+PBS compared to SHOVX+PBS. Although BV/TV was increased in OVX+BIS compared to OVX+PBS, bisphosphonate treatment had no effect on BIC. Removal torque testing revealed a higher maximum torque, torsional stiffness, and torsional energy in SHOVX compared to OVX with no effects due to bisphosphonate treatment. Our results show that osseointegration is decreased in osteoporotic animals. Ibandronate halts the progression of osteoporosis but does not enhance osseointegration. © 2019 The Authors. JBMR Plus Published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Myosins in Osteoclast Formation and Function

Skeletal quantity and quality are determined by processes of bone modeling and remodeling, which are undertaken by cells that build and resorb bone as they respond to mechanical, hormonal, and other external and internal signals. As the sole bone resorptive cell type, osteoclasts possess a remarkably dynamic actin cytoskeleton that drives their function in this enterprise. Actin rearrangements guide osteoclasts’ capacity for precursor fusion during differentiation, for migration across bone surfaces and sensing of their composition, and for generation of unique actin superstructures required for the resorptive process. In this regard, it is not surprising that myosins, the superfamily of actin-based motor proteins, play key roles in osteoclast physiology. This review briefly summarizes current knowledge of the osteoclast actin cytoskeleton and describes myosins’ roles in osteoclast differentiation, migration, and actin superstructure patterning.

Combinatorial Surface Roughness Effects on Osteoclastogenesis and Osteogenesis

Implant surface properties are a key factor in bone responses to metallic bone implants. In view of the emerging evidence on the important role of osteoclasts in bone regeneration, we here studied how surface roughness affects osteoclastic differentiation and to what extent these osteoclasts have stimulatory effects on osteogenic differentiation of osteoprogenitor cells. For this, we induced osteoclasts derived from RAW264.7 cell line and primary mouse macrophages on titanium surfaces with different roughness ( R_a 0.02-3.63 μm) and analyzed osteoclast behavior in terms of cell number, morphology, differentiation, and further anabolic effect on osteoblastic cells. Surfaces with different roughness induced the formation of osteoclasts with distinct phenotypes, based on total osteoclast numbers, morphology, size, cytoskeletal organization, nuclearity, and osteoclastic features. Furthermore, these different osteoclast phenotypes displayed differential anabolic effects toward the osteogenic differentiation of osteoblastic cells, for which the clastokine CTHRC1 was identified as a causative factor. Morphologically, osteoclast potency to stimulate osteogenic differentiation of osteoblastic cells was found to logarithmically correlate with the nuclei number per osteoclast. Our results demonstrate the existence of a combinatorial effect of surface roughness, osteoclastogenesis, and osteogenic differentiation. These insights open up a new dimension for designing and producing metallic implants by considering the implant roughness to locally regulate osseointegration through coupling osteoclastogenesis with osteogenesis.

Non-canonical Wnt signals regulate cytoskeletal remodeling in osteoclasts

Osteoclasts are multinucleated cells responsible for bone resorption. Osteoclasts adhere to the bone surface through integrins and polarize to form actin rings, which are formed by the assembly of podosomes. The area contained within actin rings (also called sealing zones) has an acidic pH, which causes dissolution of bone minerals including hydroxyapatite and the degradation of matrix proteins including type I collagen by the protease cathepsin K. Osteoclasts resorb bone matrices while moving on bone surfaces. Osteoclasts change their cell shapes and exhibit three modes for bone resorption: motile resorbing mode for digging trenches, static resorbing mode for digging pits, and motile non-resorbing mode. Therefore, the actin cytoskeleton is actively remodeled in osteoclasts. Recent studies have revealed that many molecules, such as Rac, Cdc42, Rho, and small GTPase regulators and effectors, are involved in actin cytoskeletal remodeling during the formation of actin rings and resorption cavities on bone slices. In this review, we introduce how these molecules and non-canonical Wnt signaling regulate the bone-resorbing activity of osteoclasts.

Proliferation of Osteoblasts on Laser-Modified Nanostructured Titanium Surfaces

Nanostructured titanium has become a useful material for biomedical applications such as dental implants. Certain surface properties (grain size, roughness, wettability) are highly expected to promote cell adhesion and osseointegration. The aim of this study was to compare the biocompatibilities of several titanium materials using human osteoblast cell line hFOB 1.19. Eight different types of specimens were examined: machined commercially pure grade 2 (cpTi2) and 4 (cpTi4) titanium, nanostructured titanium of the same grades (nTi2, nTi4), and corresponding specimens with laser-treated surfaces (cpTi2L, cpTi4L, nTi2L, nTi4L). Their surface topography was evaluated by means of scanning electron microscopy. Surface roughness was measured using a mechanical contact profilometer. Specimens with laser-treated surfaces had significantly higher surface roughness. Wettability was measured by the drop contact angle method. Nanostructured samples had significantly higher wettability. Cell proliferation after 48 hours from plating was assessed by viability and proliferation assay. The highest proliferation of osteoblasts was found in nTi4 specimens. The analysis of cell proliferation revealed a difference between machined and laser-treated specimens. The mean proliferation was lower on the laser-treated titanium materials. Although plain laser treatment increases surface roughness and wettability, it does not seem to lead to improved biocompatibility.

Periodic microstructures on bioactive glass surfaces enhance osteogenic differentiation of human mesenchymal stromal cells and promote osteoclastogenesis in vitro

Bioactive glasses (BG) are known for their ability to bond to hard and soft tissues. We hypothesized that the stimulation of bone remodeling, including cellular bone forming and bone resorbing processes, can be increased by applying periodic microstructures on the glass surfaces in vitro. To test our hypothesis, two different BG (45S5 and 13-93) were microstructured in a groove-and-ridge pattern of different sizes by a novel casting process and tested in cell culture experiments using human mesenchymal stromal cells (hMSCs) and RAW 264.7 cells. The microstructures induced contact guidance of hMSCs and increased osteogenic marker gene expression of the stem cells, compared to non-structured glass surfaces as verified by ELISA and quantitative real-time PCR (qPCR) analyses. Furthermore, the structures stimulated the differentiation of RAW cells to osteoclast-like cells confirmed by TRAP gene expression and their resorption activity causing visible resorption lacunae. Our results demonstrate that periodically microstructured BG (especially 45S5) might improve the osteogenic differentiation of hMSCs and influence the activity of material resorbing cells in vitro. Hence, microstructuring of BG could enhance the remodeling process of bone substitutes critical for the formation of new bone tissue in vivo and thus be used to trigger bone remodeling kinetics in vivo. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 1965-1978, 2018.

Bone degradation machinery of osteoclasts: An HIV-1 target that contributes to bone loss

Bone deficits are frequent in HIV-1-infected patients. We report here that osteoclasts, the cells specialized in bone resorption, are infected by HIV-1 in vivo in humanized mice and ex vivo in human joint biopsies. In vitro, infection of human osteoclasts occurs at different stages of osteoclastogenesis via cell-free viruses and, more efficiently, by transfer from infected T cells. HIV-1 infection markedly enhances adhesion and osteolytic activity of human osteoclasts by modifying the structure and function of the sealing zone, the osteoclast-specific bone degradation machinery. Indeed, the sealing zone is broader due to F-actin enrichment of its basal units (i.e., the podosomes). The viral protein Nef is involved in all HIV-1-induced effects partly through the activation of Src, a regulator of podosomes and of their assembly as a sealing zone. Supporting these results, Nef-transgenic mice exhibit an increased osteoclast density and bone defects, and osteoclasts derived from these animals display high osteolytic activity. Altogether, our study evidences osteoclasts as host cells for HIV-1 and their pathological contribution to bone disorders induced by this virus, in part via Nef.

Micro- and Nanoscale Topographies on Silk Regulate Gene Expression of Human Corneal Epithelial Cells

Purpose

Corneal basement membrane has topographical features that provide biophysical cues to direct cell adherence, migration, and proliferation. In this study, we hypothesize that varying topographic pitch created on silk films can alter epithelial cell morphology, adhesion, and the genetic expression involved in cytoskeletal dynamics-related pathways.

Methods

Silicon wafers with parallel ridge widths of 2000, 1000, and 800 nm were produced and used to pattern silk films via soft lithography. Human corneal epithelial cells were cultured onto silk. After 72 hours of incubation, images were taken to study cell morphology and alignment. Cytoskeletal structures were studied by immunofluorescent staining. RNA was collected from cultured cells to perform RNA-Seq transcriptome analysis using the Illumina Hiseq 2500 sequencing system. Differentially expressed genes were identified using DNAstar Qseq then verified using quantitative real-time PCR. These genes were used to perform pathway analyses using Ingenuity Pathways Analysis.

Results

Primary human corneal epithelial cell alignment to the surface pattern was the greatest on 1000-nm features. Fluorescent microscopy of f-actin staining showed cell cytoskeleton alignment either in parallel (2000 nm) or perpendicular (1000 and 800 nm) to the long feature axis. Z-stack projection of vinculin staining indicated increased focal adhesion formation localized on the cellular basal surface. RNA-seq analysis revealed differentially expressed genes involved in actin organization, integrin signaling, and focal adhesion kinase signaling (−log (P)>5).

Conclusions

Patterned silk film substrates may serve as a scaffold and provide biophysical cues to corneal epithelial cells that change their gene expression, alter cellular adherence, morphology, and may offer a promising customizable material for use in ocular surface repair.

Surface microtopography modulates sealing zone development in osteoclasts cultured on bone

Bone homeostasis is continuously regulated by the coordinated action of bone-resorbing osteoclasts and bone-forming osteoblasts. Imbalance between these two cell populations leads to pathological bone diseases such as osteoporosis and osteopetrosis. Osteoclast functionality relies on the formation of sealing zone (SZ) rings that define the resorption lacuna. It is commonly assumed that the structure and dynamic properties of the SZ depend on the physical and chemical properties of the substrate. Considering the unique complex structure of native bone, elucidation of the relevant parameters affecting SZ formation and stability is challenging. In this study, we examined in detail the dynamic response of the SZ to the microtopography of devitalized bone surfaces, taken from the same area in cattle femur. We show that there is a significant enrichment in large and stable SZs (diameter larger than 14 µm; lifespan of hours) in cells cultured on rough bone surfaces, compared with small and fast turning over SZ rings (diameter below 7 µm; lifespan approx. 7 min) formed on smooth bone surfaces. Based on these results, we propose that the surface roughness of the physiologically relevant substrate of osteoclasts, namely bone, affects primarily the local stability of growing SZs.

β3 integrin expression is required for invadopodia-mediated ECM degradation in lung carcinoma cells

Cancer related deaths are primarily due to tumor metastasis. To facilitate their dissemination to distant sites, cancer cells develop invadopodia, actin-rich protrusions capable of degrading the surrounding extracellular matrix (ECM). We aimed to determine whether β3 integrin participates in invadopodia formed by lung carcinoma cells, based on our previous findings of specific TGF-β induction of β3 integrin dependent metastasis in animal models of lung carcinoma. In this study, we demonstrate that lung carcinoma cells form invadopodia in response to TGF-β exposure. Invadopodia formation and degradation activity is dependent on β3 integrin expression since β3 integrin deficient cells are not able to degrade gelatin-coated surfaces. Even more, transient over-expression of SRC did not restore invadopodia formation in β3 integrin deficient cells. Finally, we observed that blockade of PLC-dependent signaling leads to more intense labeling for β3 integrin in invadopodia. Our results suggest that β3 integrin function, and location, in lung cancer cells are essential for invadopodia formation, and this integrin regulates the activation of different signal pathways necessary for the invasive structure. β3 integrin has been associated with poor prognosis and increased metastasis in several carcinoma types, including lung cancer. Our findings provide new evidence to support the use of targeted therapies against this integrin to combat the onset of metastases.

Differential Depth Sensing Reduces Cancer Cell Proliferation via Rho-Rac-Regulated Invadopodia

Bone, which is composed of a porous matrix, is one of the principal secondary locations for cancer. However, little is known about the effect of this porous microenvironment in regulating cancer cell proliferation. Here, we examine how the depth of the pores can transduce a mechanical signal and reduce the proliferation of noncancer breast epithelial cells (MCF-10A) and malignant breast cancer cells (MDA-MB-231 and MCF-7) using micrometer-scale topographic features. Interestingly, cells extend actin-rich protrusions, such as invadopodia, to sense the depth of the matrix pore and activate actomyosin contractility to decrease MCF-10A proliferation. However, in MDA-MB-231, depth sensing inactivates Rho-Rac-regulated actomyosin contractility and phospho-ERK signaling. Inhibiting contractility on this porous matrix using blebbistatin further reduces MDA-MB-231 proliferation. Our findings support the notion of mechanically induced dormancy through depth sensing, where invadopodia-mediated depth sensing can inhibit the proliferation of noncancer and malignant breast cancer cells through differential regulation of actomyosin contractility.

Time-lapse reveals that osteoclasts can move across the bone surface while resorbing

Bone erosion both demands that the osteoclast resorbs bone matrix and moves over the bone surface. It is widely accepted that these two activities alternate, because they are considered mutually exclusive since resorption is believed to involve an immobilizing seal to the bone surface. However, clear real-time observations are still lacking. Herein, we used specific markers and time-lapse to monitor live the spatiotemporal generation of resorption events by osteoclasts cultured on bone slices. In accordance with the current view, we found alternating episodes of resorption and migration resulting in the formation of clusters of round pits. However, very importantly, we also demonstrate that more than half of the osteoclasts moved laterally, displacing their extracellular bone-resorbing compartment over the bone surface without disassembling and reconstructing it, thereby generating long trenches. Compared to pit events, trench events show properties enabling higher aggressiveness: long duration (days), high erosion speed (two times faster) and long-distance erosion (several 100 µm). Simultaneous resorption and migration reflect a unique situation where epithelial/secretory and mesenchymal/migratory characteristics are integrated into just one cell phenotype, and deserves attention in future research.

Summary: Bone erosion requires that osteoclasts both resorb and migrate. According to common belief, these activities are mutually exclusive and alternate. Paradoxically, we show here simultaneous resorption and migration.

Protein-crystal interface mediates cell adhesion and proangiogenic secretion

The nanoscale materials properties of bone apatite crystals have been implicated in breast cancer bone metastasis and their interactions with extracellular matrix proteins are likely involved. In this study, we used geologic hydroxyapatite (HAP, Ca10(PO4)6(OH)2), closely related to bone apatite, to investigate how HAP surface chemistry and nano/microscale topography individually influence the crystal-protein interface, and how the altered protein deposition impacts subsequent breast cancer cell activities. We first utilized Förster resonance energy transfer (FRET) to assess the molecular conformation of fibronectin (Fn), a major extracellular matrix protein upregulated in cancer, when it adsorbed onto HAP facets. Our analysis reveals that both low surface charge density and nanoscale roughness of HAP facets individually contributed to molecular unfolding of Fn. We next quantified cell adhesion and secretion on Fn-coated HAP facets using MDA-MB-231 breast cancer cells. Our data show elevated proangiogenic and proinflammatory secretions associated with more unfolded Fn adsorbed onto nano-rough HAP facets with low surface charge density. These findings not only deconvolute the roles of crystal surface chemistry and topography in interfacial protein deposition but also enhance our knowledge of protein-mediated breast cancer cell interactions with apatite, which may be implicated in tumor growth and bone metastasis.

Role for Mechanotransduction in Macrophage and Dendritic Cell Immunobiology

Tissue homeostasis is not only controlled by biochemical signals but also through mechanical forces that act on cells. Yet, while it has long been known that biochemical signals have profound effects on cell biology, the importance of mechanical forces has only been recognized much more recently. The types of mechanical stress that cells experience include stretch, compression, and shear stress, which are mainly induced by the extracellular matrix, cell-cell contacts, and fluid flow. Importantly, macroscale tissue deformation through stretch or compression also affects cellular function.Immune cells such as macrophages and dendritic cells are present in almost all peripheral tissues, and monocytes populate the vasculature throughout the body. These cells are unique in the sense that they are subject to a large variety of different mechanical environments, and it is therefore not surprising that key immune effector functions are altered by mechanical stimuli. In this chapter, we describe the different types of mechanical signals that cells encounter within the body and review the current knowledge on the role of mechanical signals in regulating macrophage, monocyte, and dendritic cell function.

Microscale mechanical and mineral heterogeneity of human cortical bone governs osteoclast activity

Human cortical bone permanently remodels itself resulting in a haversian microstructure with heterogeneous mechanical and mineral properties. Remodeling is carried out by a subtle equilibrium between bone formation by osteoblasts and bone degradation by osteoclasts. The mechanisms regulating osteoclast activity were studied using easy access supports whose homogeneous microstructures differ from human bone microstructure. In the current study, we show that human osteoclasts resorb human cortical bone non-randomly with respect to this specific human bone microstructural heterogeneity. The characterization of this new resorption profile demonstrates that osteoclasts preferentially resorb particular osteons that have weak mechanical properties and mineral contents and that contain small hydroxyapatite crystals with a high carbonate content. Therefore, the influence of human bone microstructure heterogeneity on osteoclast activity could be a key parameter for osteoclast behaviour, for both in vitro and clinical studies.

Cellular and Molecular Pathways Leading to External Root Resorption

External apical root resorption during orthodontic treatment implicates specific molecular pathways that orchestrate nonphysiologic cellular activation. To date, a substantial number of in vitro and in vivo molecular, genomic, and proteomic studies have supplied data that provide new insights into root resorption. Recent mechanisms and developments reviewed here include the role of the cellular component-specifically, the balance of CD68⁺, iNOS⁺ M1- and CD68⁺, CD163⁺ M2-like macrophages associated with root resorption and root surface repair processes linked to the expression of the M1-associated proinflammatory cytokine tumor necrosis factor, inducible nitric oxide synthase, the M1 activator interferon γ, the M2 activator interleukin 4, and M2-associated anti-inflammatory interleukin 10 and arginase I. Insights into the role of mesenchymal dental pulp cells in attenuating dentin resorption in homeostasis are also reviewed. Data on recently deciphered molecular pathways are reviewed at the level of (1) clastic cell adhesion in the external apical root resorption process and the specific role of α/β integrins, osteopontin, and related extracellular matrix proteins; (2) clastic cell fusion and activation by the RANKL/RANK/OPG and ATP-P2RX7-IL1 pathways; and (3) regulatory mechanisms of root resorption repair by cementum at the proteomic and transcriptomic levels.

Mechanotransduction pulls the strings of matrix degradation at invadosome

Degradation of the extracellular matrix is a critical step of tumor cell invasion. Both protease-dependent and -independent mechanisms have been described as alternate processes in cancer cell motility. Interestingly, some effectors of protease-dependent degradation are focalized at invadosomes and are directly coupled with contractile and adhesive machineries composed of multiple mechanosensitive proteins. This review presents recent findings in protease-dependent mechanisms elucidating the ways the force affects extracellular matrix degradation by targeting protease expression and activity at invadosome. The aim is to highlight mechanosensing and mechanotransduction processes to direct the degradative activity at invadosomes, with the focus on membrane tension, proteases and mechanosensitive ion channels.

Approximating bone ECM: Crosslinking directs individual and coupled osteoblast/osteoclast behavior

Osteoblast and osteoclast communication (i.e. osteocoupling) is an intricate process, in which the biophysical profile of bone ECM is an aggregate product of their activities. While the effect of microenvironmental cues on osteoblast and osteoclast maturation has been resolved into individual variables (e.g. stiffness or topography), a single cue can be limited with regards to reflecting the full biophysical scope of natural bone ECM. Additionally, the natural modulation of bone ECM, which involves collagenous fibril and elastin crosslinking via lysyl oxidase, has yet to be reflected in current synthetic platforms. Here, we move beyond traditional substrates and use cell-derived ECM to examine individual and coupled osteoblast and osteoclast behavior on a physiological platform. Specifically, preosteoblast-derived ECM is crosslinked with genipin, a biocompatible crosslinker, to emulate physiological lysyl oxidase-mediated ECM crosslinking. We demonstrate that different concentrations of genipin yield changes to ECM density, stiffness, and roughness while retaining biocompatibility. By approximating various bone ECM profiles, we examine how individual and coupled osteoblast and osteoclast behavior are affected. Ultimately, we demonstrate an increase in osteoblast and osteoclast differentiation on compact and loose ECM, respectively, and identify ECM crosslinking density as an underlying force in osteocoupling behavior.

Study of Osteoclast Adhesion to Cortical Bone Surfaces: A Correlative Microscopy Approach for Concomitant Imaging of Cellular Dynamics and Surface Modifications

Bone remodeling relies on the coordinated functioning of osteoblasts, bone-forming cells, and osteoclasts, bone-resorbing cells. The effects of specific chemical and physical bone features on the osteoclast adhesive apparatus, the sealing zone ring, and their relation to resorption functionality are still not well-understood. We designed and implemented a correlative imaging method that enables monitoring of the same area of bone surface by time-lapse light microscopy, electron microscopy, and atomic force microscopy before, during, and after exposure to osteoclasts. We show that sealing zone rings preferentially develop around surface protrusions, with lateral dimensions of several micrometers, and ∼1 μm height. Direct overlay of sealing zone rings onto resorption pits on the bone surface shows that the rings adapt to pit morphology. The correlative procedure presented here is noninvasive and performed under ambient conditions, without the need for sample labeling. It can potentially be applied to study various aspects of cell-matrix interactions.

In vitro model of bone to facilitate measurement of adhesion forces and super-resolution imaging of osteoclasts

To elucidate processes in the osteoclastic bone resorption, visualise resorption and related actin reorganisation, a combination of imaging technologies and an applicable in vitro model is needed. Nanosized bone powder from matching species is deposited on any biocompatible surface in order to form a thin, translucent, smooth and elastic representation of injured bone. Osteoclasts cultured on the layer expressed matching morphology to ones cultured on sawed cortical bone slices. Resorption pits were easily identified by reflectance microscopy. The coating allowed actin structures on the bone interface to be visualised with super-resolution microscopy along with a detailed interlinked actin networks and actin branching in conjunction with V-ATPase, dynamin and Arp2/3 at actin patches. Furthermore, we measured the timescale of an adaptive osteoclast adhesion to bone by force spectroscopy experiments on live osteoclasts with bone-coated AFM cantilevers. Utilising the in vitro model and the advanced imaging technologies we localised immunofluorescence signals in respect to bone with high precision and detected resorption at its early stages. Put together, our data supports a cyclic model for resorption in human osteoclasts.

The Crosstalk between Osteoclasts and Osteoblasts Is Dependent upon the Composition and Structure of Biphasic Calcium Phosphates

Biphasic calcium phosphates (BCPs), consisting of hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP), exhibit good biocompatibility and osteoconductivity, maintaining a balance between resorption of the biomaterial and formation of new bone. We tested whether the chemical composition and/or the microstructure of BCPs affect osteoclasts (OCs) differentiation and/or their ability to crosstalk with osteoblasts (OBs). To this aim, OCs were cultured on BCPs with HA content of 5, 20 or 60% and their differentiation and activity were assessed. We found that OC differentiation is partially impaired by increased HA content, but not by the presence of micropores within BCP scaffolds, as indicated by TRAP staining and gene profile expression. We then investigated whether the biomaterial-induced changes in OC differentiation also affect their ability to crosstalk with OBs and regulate OB function. We found that BCPs with low percentage of HA favored the expression of positive coupling factors, including sphingosine-kinase 1 (SPHK1) and collagen triple helix repeat containing 1 (Cthrc1). In turn, the increase of these secreted coupling factors promotes OB differentiation and function. All together our studies suggest that the chemical composition of biomaterials affects not only the differentiation and activity of OCs but also their potential to locally regulate bone formation.

Spatiotemporal organization and mechanosensory function of podosomes

Podosomes are small, circular adhesions formed by cells such as osteoclasts, macrophages, dendritic cells, and endothelial cells. They comprise a protrusive actin core module and an adhesive ring module composed of integrins and cytoskeletal adaptor proteins such as vinculin and talin. Furthermore, podosomes are associated with an actin network and often organize into large clusters. Recent results from our laboratory and others have shed new light on podosome structure and dynamics, suggesting a revision of the classical "core-ring" model. Also, these studies demonstrate that the adhesive and protrusive module are functionally linked by the actin network likely facilitating mechanotransduction as well as providing feedback between these two modules. In this commentary, we briefly summarize these recent advances with respect to the knowledge on podosome structure and discuss force distribution mechanisms within podosomes and their emerging role in mechanotransduction.

Cell based advanced therapeutic medicinal products for bone repair: Keep it simple?

The development of cell based advanced therapeutic medicinal products (ATMPs) for bone repair has been expected to revolutionize the health care system for the clinical treatment of bone defects. Despite this great promise, the clinical outcomes of the few cell based ATMPs that have been translated into clinical treatments have been far from impressive. In part, the clinical outcomes have been hampered because of the simplicity of the first wave of products. In response the field has set-out and amassed a plethora of complexities to alleviate the simplicity induced limitations. Many of these potential second wave products have remained "stuck" in the development pipeline. This is due to a number of reasons including the lack of a regulatory framework that has been evolving in the last years and the shortage of enabling technologies for industrial manufacturing to deal with these novel complexities. In this review, we reflect on the current ATMPs and give special attention to novel approaches that are able to provide complexity to ATMPs in a straightforward manner. Moreover, we discuss the potential tools able to produce or predict 'goldilocks' ATMPs, which are neither too simple nor too complex.

Drug delivery and cell interaction of adhesive poly(ethyleneimine)/sulfated polysaccharide complex particle films

Herein, the authors report and review polyelectrolyte complex (PEC) nanoparticles (NPs) loaded with zoledronate (ZOL) and simvastatin and their effects on bone cells. PEC NPs are intended for modification of bone substitute materials. For characterization, they can be solution casted on germanium (Ge) substrates serving as analytically accessible model substrate. PEC NPs were generated by mixing poly(ethyleneimine) (PEI) either with linear cellulose sulfate (CS) or with branched dextransulfate (DS). Four important requirements for drug loaded PEC NPs and their films are addressed herein, which are the colloidal stability of PEC dispersions (1), interfacial stability (2), cytocompatibility (3), and retarded drug release (4). Dynamic light scattering measurements (DLS) showed that both PEI/CS and PEI/DS PEC NP were obtained with hydrodynamic radii in the range of 35-170 nm and were colloidally stable up to several months. Transmission FTIR spectroscopy evidenced that films of both systems were stable in contact to the release medium up to several days. ZOL-loaded PEI/CS nanoparticles, which were immobilized on an osteoblast-derived extracellular matrix, reduced significantly the resorption and the metabolic activity of human monocyte-derived osteoclasts. FTIR spectroscopy at cast PEC/drug films at Ge substrates revealed retarded drug releases in comparison to the pure drug films.

3D surface topology guides stem cell adhesion and differentiation

Polymerized high internal phase emulsion (polyHIPE) foams are extremely versatile materials for investigating cell-substrate interactions in vitro. Foam morphologies can be controlled by polymerization conditions to result in either open or closed pore structures with different levels of connectivity, consequently enabling the comparison between 2D and 3D matrices using the same substrate with identical surface chemistry conditions. Additionally, here we achieve the control of pore surface topology (i.e. how different ligands are clustered together) using amphiphilic block copolymers as emulsion stabilizers. We demonstrate that adhesion of human mesenchymal progenitor (hES-MP) cells cultured on polyHIPE foams is dependent on foam surface topology and chemistry but is independent of porosity and interconnectivity. We also demonstrate that the interconnectivity, architecture and surface topology of the foams has an effect on the osteogenic differentiation potential of hES-MP cells. Together these data demonstrate that the adhesive heterogeneity of a 3D scaffold could regulate not only mesenchymal stem cell attachment but also cell behavior in the absence of soluble growth factors.

Comparison of osteoclastogenesis and resorption activity of human osteoclasts on tissue culture polystyrene and on natural extracellular bone matrix in 2D and 3D

Bone homeostasis is maintained by osteoblasts (bone formation) and osteoclasts (bone resorption). While there have been numerous studies investigating mesenchymal stem cells and their potential to differentiate into osteoblasts as well as their interaction with different bone substitute materials, there is only limited knowledge concerning in vitro generated osteoclasts. Due to the increasing development of degradable bone-grafting materials and the need of sophisticated in vitro test methods, it is essential to gain deeper insight into the process of osteoclastogenesis and the resorption functionality of human osteoclasts. Therefore, we focused on the comparison of osteoclastogenesis and resorption activity on tissue culture polystyrene (TCPS) and bovine extracellular bone matrices (BMs). Cortical bone slices were used as two-dimensional (2D) substrates, whereas a thermally treated cancellous bone matrix was used for three-dimensional (3D) experiments. We isolated primary human monocytes and induced osteoclastogenesis by medium supplementation. Subsequently, the expression of the vitronectin receptor (αVβ3) and cathepsin K as well as the characteristic actin formation on TCPS and the two BMs were examined. The cell area of human osteoclasts was analyzed on TCPS and on BMs, whereas significantly larger osteoclasts could be detected on BMs. Additionally, we compared the diameter of the sealing zones with the measured diameter of the resorption pits on the BMs and revealed similar diameters of the sealing zones and the resorption pits. We conclude that using TCPS as culture substrate does not affect the expression of osteoclast-specific markers. The analysis of resorption activity can successfully be conducted on cortical as well as on cancellous bone matrices. For new in vitro test systems concerning bone resorption, we suggest the establishment of a 2D assay for high throughput screening of new degradable bone substitute materials with osteoclasts.

Evolving insights in cell-matrix interactions: elucidating how non-soluble properties of the extracellular niche direct stem cell fate

The role of soluble messengers in directing cellular behaviours has been recognized for decades. However, many cellular processes, including adhesion, migration and stem cell differentiation, are also governed by chemical and physical interactions with non-soluble components of the extracellular matrix (ECM). Among other effects, a cell's perception of nanoscale features such as substrate topography and ligand presentation, and its ability to deform the matrix via the generation of cytoskeletal tension play fundamental roles in these cellular processes. As a result, many biomaterials-based tissue engineering and regenerative medicine strategies aim to harness the cell's perception of substrate stiffness and nanoscale features to direct particular behaviours. However, since cell-ECM interactions vary considerably between two-dimensional (2-D) and three-dimensional (3-D) models, understanding their influence over normal and pathological cell responses in 3-D systems that better mimic the in vivo microenvironment is essential to translate such insights efficiently into medical therapies. This review summarizes the key findings in these areas and discusses how insights from 2-D biomaterials are being used to examine cellular behaviours in more complex 3-D hydrogel systems, in which not only matrix stiffness, but also degradability, plays an important role, and in which defining the nanoscale ligand presentation presents an additional challenge.