Differential bone turnover in an angulated fracture model in the rat

The role of hypertrophic chondrocytes in regulation of the cartilage-to-bone transition in fracture healing

Endochondral bone formation is an important pathway in fracture healing, involving the formation of a cartilaginous soft callus and the process of cartilage-to-bone transition. Failure or delay in the cartilage-to-bone transition causes an impaired bony union such as nonunion or delayed union. During the healing process, multiple types of cells including chondrocytes, osteoprogenitors, osteoblasts, and endothelial cells coexist in the callus, and inevitably crosstalk with each other. Hypertrophic chondrocytes located between soft cartilaginous callus and bony hard callus mediate the crosstalk regulating cell-matrix degradation, vascularization, osteoclast recruitment, and osteoblast differentiation in autocrine and paracrine manners. Furthermore, hypertrophic chondrocytes can become osteoprogenitors and osteoblasts, and directly contribute to woven bone formation. In this review, we focus on the roles of hypertrophic chondrocytes in fracture healing and dissect the intermingled crosstalk in fracture callus during the cartilage-to-bone transition.

The Effects of Calcitonin Gene-Related Peptide on Bone Homeostasis and Regeneration

PURPOSE OF REVIEW

The goals of this review are two folds: (1) to describe the recent understandings on the roles of calcitonin gene-related peptide-α (CGRP) in bone homeostasis and the underlying mechanisms of related neuronal regulation and (2) to propose innovative CGRP-modulated approaches for enhancing bone regeneration in challenging bone disorders.

RECENT FINDINGS

CGRP is predominantly produced by the densely distributed sensory neuronal fibers in bone, declining with age. Under mechanical and biochemical stimulations, CGRP releases and exerts either physiological or pathophysiological roles. CGRP at physiological level orchestrates the communications of bone cells with cells of other lineages, affecting not only osteogenesis, osteoclastogenesis, and adipogenesis but also angiogenesis, demonstrating with pronounced anabolic effect, thus is essential for maintaining bone homeostasis, with tuned nerve-vessel-bone network. In addition, its effects on immunity and cell recruitment are also crucial for bone fracture healing. Binding to the G protein-coupled receptor composited by calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein 1 (RAMP1) on cellular surface, CGRP triggers various intracellular signaling cascades involving cyclic adenosine monophosphate (cAMP) and cAMP response element-binding protein (CREB). Peaking at early stage post-fracture, CGRP promotes bone formation, displaying with larger callus. Then CGRP gradually decreases over time, allowing normal or physiological bone remodeling. By elevating CGRP at early stage, low-intensity pulsed ultrasound (LIPUS), electrical stimulation, and magnesium-based bio-mineral products may promisingly accelerate bone regeneration experimentally in medical conditions like osteoporosis, osteoporotic fracture, and spine fusion. Excess CGRP expression is commonly observed in pathological conditions including cancer metastatic lesions in bone and fracture delayed- or non-healing, resulting in persistent chronic pain. To date, these discoveries have largely been limited to animal models. Clinical applications are highly desirable. Compelling evidence show the anabolic effects of CGRP on bone in animals. However, further validation on the role of CGRP and the underlying mechanisms in human skeletons is required. It remains unclear if it is type H vessel connecting neuronal CGRP to osteogenesis, and if there is only specific rather than all osteoprogenitors responsible to CGRP. Clear priority should be put to eliminate these knowledge gaps by integrating with high-resolution 3D imaging of transparent bulk bone and single-cell RNA-sequencing. Last but not the least, given that small molecule antagonists such as BIBN4096BS can block the beneficial effects of CGRP on bone, concerns on the potential side effects of humanized CGRP-neutralizing antibodies when systemically administrated to treat migraine in clinics are arising.

Type 2 diabetes impairs tendon repair after injury in a rat model

Type 2 diabetes adversely affects the properties of native connective tissue. The underlying mechanisms, however, by which diabetes alters connective tissue metabolism, especially tendon, are poorly defined. The aim of this study was to determine the effect of type 2 diabetes on the mechanical, histological, and molecular properties of the intact and healing Achilles tendon. The right Achilles tendon was transected in 11 male diabetic Goto-Kakizaki (GK) and 10 age- and sex-matched Wistar control rats, while the left Achilles tendon was left intact. At 2 wk postinjury the intact and injured tendons were assessed by biomechanical testing and histology. The gene expression of collagen I and III, biglycan, versican, MMP-13, and MMP-3 was measured by quantitative RT-PCR, and their protein distribution was studied by immunohistochemistry. Intact tendons exhibited only small differences between the groups. In injured tendons, however, a significantly smaller transverse area and lower stiffness was found in diabetic GK compared with Wistar control rats. This correlated with impaired structural organization of collagen fibers and a reduced expression of collagen I and III in the injured tendons of the diabetic GK compared with Wistar control. Moreover, MMP-3 gene expression was downregulated in the injured diabetic GK tendons compared with injured Wistar controls. Our results indicate that in a rat model of diabetes tendon healing is impaired mainly due to altered expression of collagen and MMPs reflecting decreased degradation of matrix proteins and impaired tissue remodeling. Further our data suggest that therapeutic modulation of collagens or MMPs might be targets for new regenerative approaches in operated, injured, or maybe also degenerative tendon diseases in diabetes.

Occurrence of substance P in bone repair under different load comparison of straight and angulated fracture in rat tibia

Substance P (SP) has been shown in vitro to stimulate both formation and resorption of bone. This seemingly contradictory observation could be explained by in vivo variations in skeletal loading and rate of bone turnover, features which may be explored during different phases of fracture healing. In 50 SD rats, the right tibia was fractured and fixed with an intramedullary pin in straight alignment and in anterior angulation resulting in a convex and concave side under different load. Fracture repair was assessed by radiography, histology, and semi-quantitative immunohistochemistry of SP nerve fiber occurrence at days 7, 21, 35, 56, and 84 post-fracture. During regeneration, days 7-35, abundant SP-nerve ingrowth was observed in the fracture callus reaching a side-symmetrical peak at day 21 in straight fractures. In angulated fractures, the SP peak was also observed at day 21 on the concave loaded side, but not until day 35 on the convex unloaded side. Each SP-peak coincided with cortical bridging. During remodeling, days 35-84, a side-symmetrical disappearance of SP-positive fibers was seen in straight fractures. The same pattern was seen on the concave loaded side of angulated fractures. However, on the convex unloaded side, where resorption now took place, SP-fibers remained until the end of the experiment. Our study suggests that neuronal SP during bone regeneration has a stimulatory role on bone formation, while during remodeling increased SP fiber density in unloaded areas may be related to bone resorption.

Neuropeptide Y innervation during fracture healing and remodeling. A study of angulated tibial fractures in the rat

BACKGROUND AND PURPOSE

Autonomic neuropeptide Y (NPY) is involved in local bone remodeling via the central nervous system. However, the role of peripheral neuronal NPY in fracture healing is not known. We investigated the relationship between bone healing and side-specific occurrence of NPY in angular and straight fractures.

METHODS

Tibial fractures in Sprague-Dawley rats were fixed with intramedullary pins in straight alignment and anterior angulation. The samples were analyzed by radiography, histology, and immunohistochemistry (IHC) between 3 and 56 days postfracture.

RESULTS

In the angular fractures, radiography and histology showed a 3.5-fold increase in callus thickness on the concave side compared to the convex side at day 21, whereas a 0.2-fold reduction in callus thickness was seen on the convex side between days 21 and 56. IHC showed regenerating NPY fibers in the callus and woven bone in both fractures at day 7. In angular fractures, a 5-fold increase in NPY fibers was observed on the concave side compared to the convex side at 7 days, whereas a 6-fold increase in NPY fibers was seen on the convex side between 21 and 56 days; only a 0.1-fold increase in NPY fibers was seen on the concave side during the same time period. In straight fractures, similar bony and neuronal changes were observed on both sides.

INTERPRETATION

The increase in NPY innervation on the convex side appears to correlate with the loss of callus thickness on the same side in angular fractures. Our results highlight the probable function of the peripheral NPY system in local bone remodeling.

Temporal and spatial CGRP innervation in recombinant human bone morphogenetic protein induced spinal fusion in rabbits

STUDY DESIGN

Animal experiment using a rabbit posterolateral intertransverse process fusion model.

OBJECTIVE

To explore the temporal and spatial distribution of sensory nerve fibers expressing calcitonin-gene related peptide (CGRP) during spinal fusion induced by recombinant human bone morphogenetic protein-4 and the role of the CGRP innervation in ectopic bone formation and remodeling.

SUMMARY OF BACKGROUND DATA

Sensory neuropeptide CGRP involved in local bone turnover has been evidenced but its underlying mechanism is poorly understood. Knowledge in the CGRP innervation in ectopic bone induced by bone morphogenetic proteins can help us to understand its role in bone turnover.

METHODS

Twenty-seven New Zealand white rabbits underwent single level posterolateral intertransverse process fusion of the lumbar vertebrae with implantation of porous poly-d,l-lactic acid blocks loaded with 1.25 microg recombinant human bone morphogenetic protein-4 solution. Animals were killed and the operated lumbar vertebrae were harvested for histomorphological evaluation at 3 days (n = 3), 1 week (n = 6), 3 weeks (n = 6), 7 weeks (n = 6), and 12 weeks (n = 6) following surgery, respectively.

RESULTS

New cartilage presented at 1 week postimplantation adjacent to the implant, reached a peak volume at week 3 followed by a drop till week 12 after its ossification. Trabeculae-like woven bone structure presented at week 3. CGRP-positive nerve fibers regenerated already at 3 days postimplantation, reached its peak density at week 3. The CGRP-positive fibers presented both in fibrous tissues adjacent to proliferating cartilages and in bone marrow of newly formed trabecular bone.

CONCLUSIONS

The observed spatial and temporal regeneration of CGRP-positive nerve fibers in ectopic bone formation suggested CGRP innervation is associated with ectopic osteogenesis.

Site-specific CGRP innervation coincides with bone formation during fracture healing and modeling: A study in rat angulated tibia

Sensory neuropeptide involved in local bone turnover is known, but poorly understood. In the present study, we analyze the occurrence of neuronal CGRP during healing and modeling of straight and angular tibial fractures in 74 rats. Bone healing and modeling was assessed by radiography and reinnervation by semi-quantitative immunohistochemistry method at fracture site between 1-12 weeks postfracture. The regenerating nerve fibers containing CGRP were observed in fracture callus as well as in close proximity to chondrocytes, with woven bone in both fractures already at week 1. Notably, it located predominantly on the concave side of angulated fracture in the manner of sprouting into bone from weeks 3 to 5 postfracture. In both fractures, fracture calluses peaked radiographically at week 3 postfracture. In angulated fracture, a reduction of 11% in callus thickness on convex side and an increase of 365% on concave side were noted from weeks 3 to 12. A 27-fold increase in total neuronal CGRP in straight fracture and 38-fold increases in angular fracture compared to intact bone was observed at week 3. In both types of fracture, neuronal CGRP was greater on the concave side than the convex; this difference was more pronounced in the angulated fracture. CGRP immunoreactivity clearly coincides with amount of new bone formation especially on the concave side of angulated fracture. The combined results suggest that fracture evokes an intense, localized in-growth of new nerve fibers containing CGRP, which may prove to be a prerequisite of fracture healing and modeling.