Gli1 Defines a Subset of Fibro-adipogenic Progenitors that Promote Skeletal Muscle Regeneration With Less Fat Accumulation

Skeletal muscle has remarkable regenerative ability after injury. Mesenchymal fibro-adipogenic progenitors (FAPs) are necessary, active participants during this repair process, but the molecular signatures of these cells and their functional relevance remain largely unexplored. Here, using a lineage tracing mouse model (Gli1-CreER Tomato), we demonstrate that Gli1 marks a small subset of muscle-resident FAPs with elevated Hedgehog (Hh) signaling. Upon notexin muscle injury, these cells preferentially and rapidly expanded within FAPs. Ablation of Gli1+ cells using a DTA mouse model drastically reduced fibroblastic colony-forming unit (CFU-F) colonies generated by muscle cells and impaired muscle repair at 28 days. Pharmacologic manipulation revealed that Gli1+ FAPs rely on Hh signaling to increase the size of regenerating myofiber. Sorted Gli1+ FAPs displayed superior clonogenicity and reduced adipogenic differentiation ability in culture compared to sorted Gli1− FAPs. In a glycerol injury model, Gli1+ FAPs were less likely to give rise to muscle adipocytes compared to other FAPs. Further cell ablation and Hh activator/inhibitor treatments demonstrated their dual actions in enhancing myogenesis and reducing adipogenesis after injury. Examining single-cell RNA-sequencing dataset of FAPs from normal mice indicated that Gli1+ FAPs with increased Hh signaling provide trophic signals to myogenic cells while restrict their own adipogenic differentiation. Collectively, our findings identified a subpopulation of FAPs that play an essential role in skeletal muscle repair. © 2021 American Society for Bone and Mineral Research (ASBMR).     

Electrochemical fabrication of conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) nanofibrils on microfabricated neural prosthetic devices

This paper describes methods for electrochemically polymerizing conducting polymer poly(3,4-dioxyethylenethiophene) (PEDOT) nanofibrils on microfabricated neural prosthetic devices from aqueous solutions containing polyacrylic acid (PAA). These fibrils have characteristic sizes ranging from 100 to 1000 nm in diameter, depending on the concentration, molecular weight of PAA and thickness of the film. The PEDOT nanofibril-coated electrodes have significantly lower electrical impedance due to their higher effective surface area. We propose a mechanism of nanofibril formation involving locally anisotropic variations in EDOT monomer transport and PEDOT film growth due to segregation of the PAA counter-ions. This deposition method provides an improvement in the electrical properties by increasing the effective surface area of the electrodes, while still maintaining the overall small electrode size. It is also opens up new reliable and reproducible strategies for the direct electrochemical polymerization of conducting polymer nanofibrils on a variety of electrodes.     

Standardized criterion to analyze and directly compare various materials and models for peripheral nerve regeneration

Progress in understanding conditions for optimal peripheral nerve regeneration has been stunted due to lack of standardization of experimental conditions and assays. In this paper we review the large database that has been generated using the Lundborg nerve chamber model and compare various theories for their ability to explain the experimental data. Data were normalized based on systematic use of the critical axon elongation, the gap length at which the probability of axon reconnection between the stumps is just 50%. Use of this criterion has led to a rank-ordering of devices or treatments and has led, in turn, to conclusions about the conditions that facilitate regeneration. Experimental configurations that have maximized facilitation of peripheral nerve regeneration are those in which the tube wall comprised degradable polymers, including collagen and certain synthetic biodegradable polymers, and was cell-permeable rather than protein-permeable. Tube fillings that showed very high regenerative activity were suspensions of Schwann cells, a solution either of acidic or basic fibroblast growth factor, insoluble ECM substrates rather than solutions or gels, polyamide filaments oriented along the tube axis and highly porous, insoluble analogs of the ECM with specific structure and controlled degradation rate. It is suggested that the data are best explained by postulating that the quality of regeneration depends on two critical processes. The first is compression of stumps and regenerating nerve by a thick myofibroblast layer that surrounds these tissues and blocks synthesis of a nerve of large diameter (pressure cuff theory). The second is synthesis of linear columns of Schwann cells that serve as tracks for axon elongation (basement membrane microtube theory). It is concluded that experimental configurations that show high regenerative activity suppress the first process while facilitating the second.     

The diverse origin of bone-forming osteoblasts

Osteoblasts are the only cells that can give rise to bones in vertebrates. Thus, one of the most important functions of these metabolically active cells is mineralized matrix production. Because osteoblasts have a limited lifespan, they must be constantly replenished by preosteoblasts, their immediate precursors. Because disruption of the regulation of bone-forming osteoblasts results in a variety of bone diseases, a better understanding of the origin of these cells by defining the mechanisms of bone development, remodeling, and regeneration is central to the development of novel therapeutic approaches. In recent years, substantial new insights into the origin of osteoblasts—largely owing to rapid technological advances in murine lineage-tracing approaches and other single-cell technologies—have been obtained. Collectively, these findings indicate that osteoblasts involved in bone formation under various physiological, pathological, and therapeutic conditions can be obtained from numerous sources. The origins of osteoblasts include, but are not limited to, chondrocytes in the growth plate, stromal cells in the bone marrow, quiescent bone-lining cells on the bone surface, and specialized fibroblasts in the craniofacial structures, such as sutures and periodontal ligaments. Because osteoblasts can be generated from local cellular sources, bones can flexibly respond to regenerative and anabolic cues. However, whether osteoblasts derived from different cellular sources have distinct functions remains to be investigated. Currently, we are at the initial stage to aptly unravel the incredible diversity of the origins of bone-forming osteoblasts. © 2021 American Society for Bone and Mineral Research (ASBMR).     

Bioactive glasses for in situ tissue regeneration

Historically the function of biomaterials has been to replace diseased or damaged tissues. Recent findings show that controlled release of the ionic dissolution products of bioactive glasses results in regeneration of tissues. The mechanism for in situ tissue regeneration involves upregulation of seven families of genes that control the osteoblast cell cycle, mitosis and differentiation. In the presence of critical concentrations of Si and Ca ions, within 48 h osteoblasts that are capable of differentiating into a mature osteocyte phenotype begin to proliferate and regenerate new bone. Osteoblasts that are not in the correct phase of the cell cycle and unable to proceed towards differentiation are switched into apoptosis by the ionic dissolution products. A controlled release of soluble Ca and Si from bioactive glass - resorbable polymer composites leads to vascularised soft tissue regeneration. Gene activation by controlled ion release provides the conceptual basis for molecular design of a third generation of biomaterials optimised for in situ tissue regeneration.     

Spinal cord regeneration induced by a voltage-gated calcium channel agonist

Abstract

Regeneration in the central nervous system (CNS) is prohibitive. This is likely due to an interplay of cellular (gene expression, growth factors) and environmental (inhibition by CNS myelin) factors. Calcium supports various intracellular functions, and multiple in vitro studies have shown a role of calcium in axonal growth. In this study, we examine the role of a calcium agonist, S(-)-Bay K 8644, in promoting or impeding CNS growth in vivo, in an effort to understand further the relationship between the voltage-gated L type calcium channel and regeneration. Using a well-established rat spinal cord model of regeneration, we have injected various doses of S(-)-Bay K 8644 (30-240 M) around the injured spinal cord. Our results demonstrate that S(-)-Bay K 8644 enhances regeneration in a dose-dependent fashion. In addition, at very specific concentrations, the same agonist has no effect on or even inhibits regeneration. We conclude that spinal regeneration is highly dependent on intracellular calcium concentration. Furthermore, depending on the dose used, the effect of calcium agonist supplementation on spinal regeneration can be supportive or inhibitory.     

Neuroregenerative and neuroprotective actions of neuroimmunophilin compounds in traumatic and inflammatory neuropathies

Abstract

FK506 (tacrolimus, Prograf^®) is an immunosuppressant drug that also has profound neuroregenerative and neuroprotective actions independent of its immunosuppressant activity. The separation of these properties has led to the development of non-immunosuppressant derivatives that retain the neurotrophic activity. This review focuses on the peripheral nerve actions of these compounds following mechanical injury (nerve crush or transection with graft repair) and in models of inflammatory neuropathies. Whereas FK506 may be indicative for the treatment of inflammatory neuropathies where its immunosuppressive action would be advantageous, non-immunosuppressant derivatives represent a new class of potential therapeutic agents for the treatment of human neurological conditions in general. Moreover, these studies have led to the discovery of a novel mechanism whereby these compounds activate intrinsic neuroregenerative and neuroprotective pathways in the neuron.     

Characterization of neural stem cells on electrospun poly(L-lactic acid) nanofibrous scaffold

Nanofibrous poly(L-lactic acid) (PLLA) scaffolds were fabricated by an electrospinning technique and characterized by scanning electron microscopy, mercury porosimeter, atomic force microscopy and contact-angle test. The produced PLLA fibers with diameters ranging from 150 to 350 nm were randomly orientated with interconnected pores varying from several μm to about 140 μm in-between to form a three-dimensional architecture, which resembles the natural extracellular matrix structure in human body. The in vitro cell culture study was performed and the results indicate that the nanofibrous scaffold not only supports neural stem cell (NSC) differentiation and neurites out-growth, but also promotes NSC adhesion. The favorable interaction between the NSCs and the nanofibrous scaffold may be due to the greatly improved surface roughness of the electrospun nanofibrous scaffold. As evidenced by this study, the electrospun nanofibrous scaffold is expected to play a significant role in neural tissue engineering.     

The Effect of Aligned Core–Shell Nanofibres Delivering NGF on the Promotion of Sciatic Nerve Regeneration

Recent bioengineering strategies for peripheral nerve regeneration have been focusing on the development of alternative treatments for nerve repair. In this study, we incorporated nerve growth factor (NGF) into aligned core–shell nanofibres by coaxial electrospinning, and reeled the scaffold into aligned fibrous nerve guidance conduits (NGCs) for nerve regeneration study. This aligned PLGA/NGF NGC combined physical guidance cues and biomolecular signals to closely mimic the native extracellular matrix (ECM). The effect of this aligned PLGA/NGF NGC on the promotion of nerve regeneration was evaluated in a 13-mm rat sciatic nerve defect using functional and morphological analysis. After 12 weeks implantation, the results of electrophysiological and muscle weight examination demonstrated that the functional recovery of the regenerated nerve in the PLGA/NGF NGC group was significantly better than that in the PLGA group, yet had no significant difference compared with the autograft group. The toluidine blue staining study showed that more nerve fibres were regenerated in the PLGA/NGF group, while the electron microscopy study indicated that the regenerated nerve in the PLGA/NGF group was more mature than that in the PLGA group. This study demonstrated that the aligned PLGA/NGF could greatly promote peripheral nerve regeneration and have a potential application in nerve regeneration.     

Neural Crest‐Specific TSC1 Deletion in Mice Leads to Sclerotic Craniofacial Bone Lesion

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder caused by mutations in either TSC1 or TSC2. TSC has high frequency of osseous manifestations such as sclerotic lesions in the craniofacial region. However, an animal model that replicates TSC craniofacial bone lesions has not yet been described. The roles of Tsc1 and the sequelae of Tsc1 dysfunction in bone are unknown. In this study, we generated a mouse model of TSC with a deletion of Tsc1 in neural crest‐derived (NCD) cells that recapitulated the sclerotic craniofacial bone lesions in TSC. Analysis of this mouse model demonstrated that TSC1 deletion led to enhanced mTORC1 signaling in NCD bones and the increase in bone formation is responsible for the aberrantly increased bone mass. Lineage mapping revealed that TSC1 deficient NCD cells overpopulated the NCD bones. Mechanistically, hyperproliferation of osteoprogenitors at an early postnatal stage accounts for the increased osteoblast pool. Intriguingly, early postnatal treatment with rapamycin, an mTORC1 inhibitor, can completely rescue the aberrant bone mass, but late treatment cannot. Our data suggest that enhanced mTOR signaling in NCD cells can increase bone mass through enlargement of the osteoprogenitor pool, which likely explains the sclerotic bone lesion observed in TSC patients. © 2015 American Society for Bone and Mineral Research.