Peroxisome Proliferator-Activated Receptor-γ Knockdown Impairs Bone Morphogenetic Protein-2-Induced Critical-Size Bone Defect Repair

Collagen Modulates the Biological Characteristics of WJ-MSCs in Basal and Osteoinduced Conditions

Transcriptomic analysis revealed mesenchymal stem/stromal cells (MSCs) from various origins exhibited distinct gene and protein expression profiles dictating their biological properties. Although collagen type 1 (COL) has been widely studied in bone marrow MSCs, its role in regulating cell fate of Wharton jelly- (WJ-) MSCs is not well understood. In this study, we investigated the effects of collagen on the characteristics of WJ-MSCs associated with proliferation, surface markers, adhesion, migration, self-renewal, and differentiation capabilities through gene expression studies. The isolated WJ-MSCs expressed positive surface markers but not negative markers. Gene expression profiles showed that COL not only maintained the pluripotency, self-renewal, and immunophenotype of WJ-MSCs but also primed cells toward lineage differentiations by upregulating BMP2 and TGFB1 genes. Upon osteoinduction, WJ-MSC-COL underwent osteogenesis by switching on the transcription of BMP6/7 and TGFB3 followed by activation of downstream target genes such as INS, IGF1, RUNX2, and VEGFR2 through p38 signalling. This molecular event was also accompanied by hypomethylation at the OCT4 promoter and increase of H3K9 acetylation. In conclusion, COL provides a conducive cellular environment in priming WJ-MSCs that undergo a lineage specification upon receiving an appropriate signal from extrinsic factor. These findings would contribute to better control of fate determination of MSCs for therapeutic applications related to bone disease.

Genetic and pharmacologic suppression of PPARγ enhances NELL-1-stimulated bone regeneration

Recent investigations into mechanisms behind the development of osteoporosis suggest that suppressing PPARγ-mediated adipogenesis can improve bone formation and bone mineral density. In this study, we investigated a co-treatment strategy to enhance bone formation by combining NELL-1, an osteogenic molecule that has been extensively studied for its potential use as a therapeutic for osteoporosis, with two methods of PPARγ suppression. First, we suppressed PPARγ genetically using lentiviral PPARγ-shRNA in immunocompromised mice for a proof of concept. Second, we used a PPARγ antagonist to suppress PPARγ pharmacologically in immunocompetent senile osteopenic mice for clinical transability. We found that the co-treatment strategy significantly increased bone formation, increased the proliferation stage cell population, decreased late apoptosis of primary mouse BMSCs, and increased osteogenic marker mRNA levels in comparison to the single agent treatment groups. The addition of PPARγ suppression to NELL-1 therapy enhanced NELL-1's effects on bone formation by upregulating anabolic processes without altering NELL-1's inhibitory effects on osteoclastic and adipogenic activities. Our findings suggest that combining PPARγ suppression with therapeutic NELL-1 may be a viable method that can be further developed as a novel strategy to reverse bone loss and decrease marrow adiposity in age-related osteoporosis.

Advances in the Fabrication of Scaffold and 3D Printing of Biomimetic Bone Graft

The need for bone grafts is tremendous, and that leads to the use of autograft, allograft, and bone graft substitutes. The biology of the bone is quite complex regarding cellular composition and architecture, hence developing a mineralized connective tissue graft is challenging. Traditionally used bone graft substitutes including metals, biomaterial coated metals and biodegradable scaffolds, suffer from persistent limitations. With the advent and rise of additive manufacturing technologies, the future of repairing bone trauma and defects seems to be optimistic. 3D printing has significant advantages, the foremost of all being faster manipulation of various biocompatible materials and live cells or tissues into the complex natural geometries necessary to mimic and stimulate cellular bone growth. The advent of new-generation bioprinters working with high-precision, micro-dispensing and direct digital manufacturing is aiding in ground-breaking organ and tissue printing, including the bone. The future bone replacement for patients holds excellent promise as scientists are moving closer to the generation of better 3D printed bio-bone grafts that will be safer and more effective. This review aims to summarize the advances in scaffold fabrication techniques, emphasizing 3D printing of biomimetic bone grafts.

The neuropeptide calcitonin gene-related peptide alpha is essential for bone healing

Background

Impaired fracture healing represents an ongoing clinical challenge, as treatment options remain limited. Calcitonin gene-related peptide (CGRP), a neuropeptide targeted by emerging anti-migraine drugs, is also expressed in sensory nerve fibres innervating bone tissue.

Method

Bone healing following a femoral osteotomy stabilized with an external fixator was analysed over 21 days in αCGRP-deficient and WT mice. Bone regeneration was evaluated by serum analysis, µCT analysis, histomorphometry and genome-wide expression analysis. Bone-marrow-derived osteoblasts and osteoclasts, as well as the CGRP antagonist olcegepant were employed for mechanistic studies.

Findings

WT mice with a femoral fracture display increased CGRP serum levels. αCGRP mRNA expression after skeletal injury is exclusively induced in callus tissue, but not in other organs. On protein level, CGRP and its receptor, calcitonin receptor-like receptor (CRLR) complexing with RAMP1, are differentially expressed in the callus during bone regeneration. On the other hand, αCGRP-deficient mice display profoundly impaired bone regeneration characterised by a striking reduction in the number of bone-forming osteoblasts and a high rate of incomplete callus bridging and non-union. As assessed by genome-wide expression analysis, CGRP induces the expression of specific genes linked to ossification, bone remodeling and adipogenesis. This suggests that CGRP receptor-dependent PPARγ signaling plays a central role in fracture healing.

Interpretation

This study demonstrates an essential role of αCGRP in orchestrating callus formation and identifies CGRP receptor agonism as a potential approach to stimulate bone regeneration. Moreover, as novel agents blocking CGRP or its receptor CRLR are currently introduced clinically for the treatment of migraine disorders, their potential negative impact on bone regeneration warrants clinical investigation.

Funding

This work was funded by grants from the Else-Kröner-Fresenius-Stiftung (EKFS), the Deutsche Forschungsgemeinschaft (DFG), and the Berlin Institute of Health (BIH).

Immunosuppressive Property of MSCs Mediated by Cell Surface Receptors

In the past decade, mesenchymal stem cells (MSCs) tend to exhibit inherent tropism for refractory inflammatory diseases and engineered MSCs have appeared on the market as therapeutic agents. Recently, engineered MSCs target to cell surface molecules on immune cells has been a new strategy to improve MSC applications. In this review, we discuss the roles of multiple receptors (ICAM-1, Gal-9, PD-L1, TIGIT, CD200, and CXCR4) in the process of MSCs' immunosuppressive properties. Furthermore, we discuss the principles and strategies for developing receptor-regulated MSCs and their mechanisms of action and the challenges of using MSCs as immunosuppressive therapies.

Cumulative inactivation of Nell-1 in Wnt1 expressing cell lineages results in craniofacial skeletal hypoplasia and postnatal hydrocephalus

Upregulation of Nell-1 has been associated with craniosynostosis (CS) in humans, and validated in a mouse transgenic Nell-1 overexpression model. Global Nell-1 inactivation in mice by N-ethyl-N-nitrosourea (ENU) mutagenesis results in neonatal lethality with skeletal abnormalities including cleidocranial dysplasia (CCD)-like calvarial bone defects. This study further defines the role of Nell-1 in craniofacial skeletogenesis by investigating specific inactivation of Nell-1 in Wnt1 expressing cell lineages due to the importance of cranial neural crest cells (CNCCs) in craniofacial tissue development. Nell-1^flox/flox; Wnt1-Cre (Nell-1^Wnt1 KO) mice were generated for comprehensive analysis, while the relevant reporter mice were created for CNCC lineage tracing. Nell-1^Wnt1 KO mice were born alive, but revealed significant frontonasal and mandibular bone defects with complete penetrance. Immunostaining demonstrated that the affected craniofacial bones exhibited decreased osteogenic and Wnt/β-catenin markers (Osteocalcin and active-β-catenin). Nell-1-deficient CNCCs demonstrated a significant reduction in cell proliferation and osteogenic differentiation. Active-β-catenin levels were significantly low in Nell-1-deficient CNCCs, but were rescued along with osteogenic capacity to a level close to that of wild-type (WT) cells via exogenous Nell-1 protein. Surprisingly, 5.4% of young adult Nell-1^Wnt1 KO mice developed hydrocephalus with premature ossification of the intrasphenoidal synchondrosis and widened frontal, sagittal, and coronal sutures. Furthermore, the epithelial cells of the choroid plexus and ependymal cells exhibited degenerative changes with misplaced expression of their respective markers, transthyretin and vimentin, as well as dysregulated Pit-2 expression in hydrocephalic Nell-1^Wnt1 KO mice. Nell-1^Wnt1 KO embryos at E9.5, 14.5, 17.5, and newborn mice did not exhibit hydrocephalic phenotypes grossly and/or histologically. Collectively, Nell-1 is a pivotal modulator of CNCCs that is essential for normal development and growth of the cranial vault and base, and mandibles partially via activating the Wnt/β-catenin pathway. Nell-1 may also be critically involved in regulating cerebrospinal fluid homeostasis and in the pathogenesis of postnatal hydrocephalus.