Calcitonin gene-related peptide (CGRP) in the developing mouse limb

Research Progress in Calcitonin Gene-Related Peptide and Bone Repair

Calcitonin gene-related peptide (CGRP) has 37 amino acids. Initially, CGRP had vasodilatory and nociceptive effects. As research progressed, evidence revealed that the peripheral nervous system is closely associated with bone metabolism, osteogenesis, and bone remodeling. Thus, CGRP is the bridge between the nervous system and the skeletal muscle system. CGRP can promote osteogenesis, inhibit bone resorption, promote vascular growth, and regulate the immune microenvironment. The G protein-coupled pathway is vital for its effects, while MAPK, Hippo, NF-κB, and other pathways have signal crosstalk, affecting cell proliferation and differentiation. The current review provides a detailed description of the bone repair effects of CGRP, subjected to several therapeutic studies, such as drug injection, gene editing, and novel bone repair materials.

Neurovascular coupling in bone regeneration

The mammalian skeletal system is densely innervated by both neural and vascular networks. Peripheral nerves in the skeleton include sensory and sympathetic nerves. The crosstalk between skeletal and neural tissues is critical for skeletal development and regeneration. The cellular processes of osteogenesis and angiogenesis are coupled in both physiological and pathophysiological contexts. The cellular and molecular regulation of osteogenesis and angiogenesis have yet to be fully defined. This review will provide a detailed characterization of the regulatory role of nerves and blood vessels during bone regeneration. Furthermore, given the importance of the spatial relationship between nerves and blood vessels in bone, we discuss neurovascular coupling during physiological and pathological bone formation. A better understanding of the interactions between nerves and blood vessels will inform future novel therapeutic neural and vascular targeting for clinical bone repair and regeneration.

Interaction between the nervous and skeletal systems

The skeleton is one of the largest organ systems in the body and is richly innervated by the network of nerves. Peripheral nerves in the skeleton include sensory and sympathetic nerves. Crosstalk between bones and nerves is a hot topic of current research, yet it is not well understood. In this review, we will explore the role of nerves in bone repair and remodeling, as well as summarize the molecular mechanisms by which neurotransmitters regulate osteogenic differentiation. Furthermore, we discuss the skeleton’s role as an endocrine organ that regulates the innervation and function of nerves by secreting bone-derived factors. An understanding of the interactions between nerves and bone can help to prevent and treat bone diseases caused by abnormal innervation or nerve function, develop new strategies for clinical bone regeneration, and improve patient outcomes.

Nervous System-Driven Osseointegration

Implants are essential therapeutic tools for treating bone fractures and joint replacements. Despite the in-depth study of osseointegration for more than fifty years, poor osseointegration caused by aseptic loosening remains one of the leading causes of late implant failures. Osseointegration is a highly sophisticated and spatiotemporal process in vivo involving the immune response, angiogenesis, and osteogenesis. It has been unraveled that the nervous system plays a pivotal role in skeletal health via manipulating neurotrophins, neuropeptides, and nerve cells. Herein, the research related to nervous system-driven osseointegration was systematically analyzed and reviewed, aiming to demonstrate the prominent role of neuromodulation in osseointegration. Additionally, it is indicated that the implant design considering the role of neuromodulation might be a promising way to prevent aseptic loosening.

Skeletal Stem Cells as the Developmental Origin of Cellular Niches for Hematopoietic Stem and Progenitor Cells

The skeletal system is a highly complex network of mesenchymal, hematopoietic, and vasculogenic stem cell lineages that coordinate the development and maintenance of defined microenvironments, so-called niches. Technological advancements in recent years have allowed for the dissection of crucial cell types as well as their autocrine and paracrine signals that regulate these niches during development, homeostasis, regeneration, and disease. Ingress of blood vessels and bone marrow hematopoiesis are initiated by skeletal stem cells (SSCs) and their more committed downstream lineage cell types that direct shape and form of skeletal elements. In this chapter, we focus on the role of SSCs as the developmental origin of niches for hematopoietic stem and progenitor cells. We discuss latest updates in the definition of SSCs, cellular processes establishing and maintaining niches, as well as alterations of stem cell microenvironments promoting malignancies. We conclude with an outlook on future studies that could take advantage of SSC-niche engineering as a basis for the development of new therapeutic tools to not only treat bone-related diseases but also maladies stemming from derailed niche dynamics altering hematopoietic output.

No pain, no gain? The effects of pain-promoting neuropeptides and neurotrophins on fracture healing

Neuropeptides and neurotrophins are key regulators of peripheral nociceptive nerves and contribute to the induction, sensitization, and maintenance of pain. It is now known that these peptides also regulate non-neuronal tissues, including bone. Here, we review the effects of numerous neuropeptides and neurotrophins on fracture healing. The neuropeptides calcitonin-gene related peptide (CGRP), substance P (SP), vasoactive intestinal peptide (VIP), and pituitary adenylate cyclase-activating peptide (PACAP) have varying effects on osteoclastic and osteoblastic activity. Ultimately, CGRP and SP both accelerate fracture healing, while VIP and PACAP seem to negatively impact healing. Unlike the aforementioned neuropeptides, the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) have more uniform effects. Both factors upregulate osteoblastic activity, osteoclastic activity, and, in vivo, stimulate osteogenesis to promote fracture healing. Future research will need to clarify the exact mechanism by which the neuropeptides and neurotrophins influence fracture healing. Specifically, understanding the optimal expression patterns for these proteins in the fracture healing process may lead to therapies that can maximize their bone-healing capabilities and minimize their pain-promoting effects. Finally, further examination of protein-sequestering antibodies and/or small molecule agonists and antagonists may lead to new therapies that can decrease the rate of delayed union/nonunion outcomes and fracture-associated pain.

The Role of Nerves in Skeletal Development, Adaptation, and Aging

The skeleton is well-innervated, but only recently have the functions of this complex network in bone started to become known. Although our knowledge of skeletal sensory and sympathetic innervation is incomplete, including the specific locations and subtypes of nerves in bone, we are now able to reconcile early studies utilizing denervation models with recent work dissecting the molecular signaling between bone and nerve. In total, sensory innervation functions in bone much as it does elsewhere in the body-to sense and respond to stimuli, including mechanical loading. Similarly, sympathetic nerves regulate autonomic functions related to bone, including homeostatic remodeling and vascular tone. However, more study is required to translate our current knowledge of bone-nerve crosstalk to novel therapeutic strategies that can be effectively utilized to combat skeletal diseases, disorders of low bone mass, and age-related decreases in bone quality.

Localization of CGRP and VEGF mRNAs in the mouse superior cervical ganglion during pre- and postnatal development

The neuropeptide calcitonin gene-related peptide (CGRP) mediates inflammation and head pain by influencing the functional vascular blood supply. CGRP is a well-characterized mediator of receptor-regulated neurotransmitter release. However, knowledge regarding the role of CGRP during the development of the superior cervical ganglion (SCG) is limited. In the present study, we observed the localization of CGRP and vascular endothelial growth factor (VEGFA) mRNAs during prenatal development at embryonic day 14.5 (E14.5), E17.5 and postnatal day 1 (P1) using in situ hybridization. The antisense probe for CGRP was detected by in situ hybridization at E14.5, E17.5, and P1, and the highest levels were detected at E17.5. In contrast, the antisense probe for VEGF-A was detected by in situ hybridization in gradually increasing intensity from E14.5 to P1. The differences in the expression of these two markers revealed specific characteristics related to CGRP concentration and release compared to those of VEGF-A during development. The correlation between CGRP and VEGF-A may influence functional stress and the vascular blood supply during prenatal and postnatal development.

Distribution of the neuropeptide calcitonin gene-related peptide-α of tooth germ during formation of the mouse mandible

Calcitonin gene-related peptide-α (CGRPα) is a neurotransmitter that is related to bone formation during development. However, CGRP expression is not well known to affect the formation of teeth during development. During tooth germ development, the relationships among CGRPα, calcitonin receptor-like receptor (CRLR), amelogenin (AMELX), dentin sialophosphoprotein (DSPP), osteopontin (OPN) and osteocalcin (OCN) are unclear despite various tooth and osteogenesis markers. Our real-time RT-PCR results showed that the expression levels of CGRPα mRNA gradually decreased, in contrast to the mRNA abundances of CRLR, AMELX, DSPP, OPN, and OCN, which rapidly increased from E14.5 to P1 in the mandible. In situ hybridization using an antisense probe for CGRPα mRNA showed significant localized expression levels around the tooth bud at E14.5 and epithelial cells near the dental ledge and outer and inner enamel epithelium at E17.5 compared to those at P1. The localization of the anti-CGRPα antibody reaction revealed a strong positive reaction at the surface layer of oral epithelial cells at E14.5 and oral epithelial cells of the dental lamina around the dental ledge depression in the mandible of E17.5 mice using immunohistochemical methods The different anti-CGRPα reaction revealed its important roles during tooth formation at the postnatal stage. CGRPα mRNA was also detected in the interactions of tooth germ with the formation of odontoblast and amelobast layers from dental papilla and inner enamel epithelium. CGRPα may also be related to tooth germ development. Furthermore, CGRPα is an important tooth and bone formation marker, and bone cells provide further evidence of a role in mandibular development in contrast to inflammatory systems.

Expression of CGRP, vasculogenesis and osteogenesis associated mRNAs in the developing mouse mandible and tibia

The neuropeptide Calcitonin Gene-Related Peptide (CGRP) is a well-characterized neurotransmitter. However, little is known about the role of CGRP in osteogenesis and vascular genesis during the developmental formation of bone. In the present study, we assessed the abundance of CGRP mRNA and the mRNA of osteogenesis and vascular genesis markers in the foetal mouse mandible and leg bone (tibia). We also analysed the expression and localization of CGRP, osteopontin (OPN) and vascular endothelial growth factor (VEGF-A) using in situ hybridization and immunohistochemical localization in the mouse mandible and tibia at embryonic days 12.5 (E12.5), E14.5, E17.5, and postnatal day 1 (P1). CGRP was clearly detected in the mandible relative to the tibia at E14.5. Hybridization using an anti-sense probe for CGRP was not detected in the mandible at P1. Hybridization with an anti-sense probe for OPN was detected at E14.5, later in the mandible and at P1 in Meckel’s cartilage. However, OPN was only detected in the tibia at E17.5 and later. The abundance of CGRP mRNA differed between the mandible and tibia. The level of vasculogenesis markers, such as VEGF-A, was similar to that of CGRP in the mandible. The levels of VEGF-A, cluster of differentiation 31 (CD31) and lymphatic vessel endothelial hyaluronan receptor 1 (LIVE-1) differed from that of OPN in the mandible. In contrast, the levels of VEGF-A, CD31, matrix metalloproteinase-2 (MMP-2), collagen I (Col I), collagen II (Col II) and OPN mRNA differed from E12.5 to P1 (P<0.001) in the tibia. The abundance of mRNA of CGRP and bone matrix markers (Col I, Col II, and OPN) was low at P5 in the tibia. These differences in CGRP and other mRNAs may induce a different manner of ossification between the mandible and tibia. Therefore, a time lag of ossification occurs between the mandible and tibia during foetal development.

Expression of CGRP in embryonic mouse masseter muscle

Neuropeptide calcitonin gene-related peptide (CGRP) is a mediator of inflammation and head pain that influences the functional vascular blood supply. The CGRP also regulate myoblast and acetylcholine receptors on neuromuscular junctions in development. However, little is known about its appearance and location during mouse masseter muscle (MM) development. We detected the mRNA abundance of CGRP, vascular genesis markers (Vascular endothelial growth factor A (VEGF-A), PECAM (CD31), lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1)) and embryonic and adult myosin heavy chain (MyHCs) (embryonic, IIa, IIb, and IIx) using real-time RT-PCR during development from the embryonic stage to after birth (E12.5, E14.5, E17.5, E18.5, P0, P1 and P5). We also endeavored to analyze the expression and localization of CGRP in situ hybridization in the developing mouse MM during development from the embryonic stage to after birth (E12.5, E14.5, E17.5, and P1). The antisense probe for CGRP was detected by in situ hybridization at E12.5, E14.5 E17.5 and then no longer detected after birth. The CGRP, CD31, embryonic MyHC abundance levels are highest at E17.5 (p<0.001) and they show a pattern similar to that of the other markers from E12.5 to P5. PCA analysis indicates a specific relation between CGRP and embryonic MyHC, CD31, and LYVE-1 in MM development. Cluster analyses identified the following distinct clusters for mRNA abundance in the MM: cluster 1, P5; cluster 2, E12.5, E14.5, E17.5, E18.5, P0, and P1. The positive correlation between CGRP and embryonic MyHC (Pearson's r>0.65; p<0.01) was analyzed. These data suggested that CGRP may have an influence on embryonic MyHC during mouse MM development. CGRP also affects the angiogenesis markers at embryonic stages.

Neuropeptide effects on rat chondrocytes and perichondrial cells in vitro

This study examines if cultured chondrocytes and perichondrial cells change the level of cAMP and/or cGMP in response to application of the neuropeptide calcitonin gene-related peptide (CGRP). Cells collected from the knee region of 4-8 days old rat pups were cultured in vitro. Cultures were exposed to 10(-10)-10(-6) M CGRP during 10 minutes. The levels of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) in the cultures and in controls were determined by radioimmunoassay. The results show that application of CGRP causes a distinctly increased level of cAMP, that was absent when CGRP was applied together with the CGRP(1) receptor antagonist. The level of cGMP was not obviously altered. Hence, it is possible that terminals of primary sensory neurones present in developing cartilage influence chondrocytes and perichondrial cells via local release of CGRP.

Calcitonin gene-related peptide acts as a mitogen for human Gin-1 gingival fibroblasts by activating the MAP kinase signalling pathway

In many peripheral tissues, calcitonin gene-related peptide (CGRP) is released from peptidergic sensory nerve fibres and acts like a growth factor during tissue development and regeneration. However, the ability of CGRP to influence gingival tissue has not been studied. To address this question, we have now examined the effects of CGRP on the proliferation of human gingival fibroblasts (Gin-1) in vitro. Gin-1 cells have approximately 3100 specific CGRP-binding sites with a Kd of 38.6 pM on their surface. Treatment with CGRP (0.1-100 nM) significantly stimulated cell proliferation in a dose-dependent manner, with maximal effects at 1-10 nM CGRP after 2 d. As one early cellular response to CGRP, p44-MAPK protein (also known as the extracellular signal response kinase [ERK]) was tyrosine- and threonine-phosphorylated within 2 min, and this phosphorylation was sustained for at least 1 h. The dose-response curve of MAPK activation was very similar to that observed for CGRP's stimulation of cell proliferation. In addition, CGRP's activation of MAPK stimulated its ability to phosphorylate the Elk-1 transcription factor. When cells were pretreated with PD98059, a selective inhibitor of MAPK kinase (also known as MEK), CGRP not only failed to induce phosphorylation of MAPK but also failed to stimulate Gin-1 cell proliferation. Our present data indicate that CGRP rapidly activates the MAPK signalling pathway, an effect which consequently stimulates the proliferation of gingival fibroblasts. Our data demonstrate specific cellular responses to CGRP by gingival fibroblasts and support the possibility that CGRP acts as a targeted local factor in the regulation of development, generation and/or regeneration of gingival tissues.

Testes exhibit elevated expression of calcitonin gene-related peptide receptor component protein

Calcitonin gene-related peptide (CGRP) receptor component protein (RCP) is a novel protein that modulates CGRP responsiveness in a variety of cell types. Using probes based on the isolation of CGRP-RCP complementary DNA (cDNA) from a guinea pig organ of Corti cDNA library, we cloned human (h) and mouse (m) CGRP-RCP cDNAs, both of which encode 148-residue proteins that at the amino acid levels are approximately 88% identical to each other and to the 146-residue guinea pig CGRP-RCP. Northern blot analysis confirmed the presence of CGRP-RCP messenger RNA in all of the human and mouse tissues tested. In these human tissues, hCGRP-RCP messenger RNA (major band at approximately 3.1 kb, minor band at approximately 7.5 kb) was most prevalent in the testis. In the mouse, the highest abundance of CGRP-RCP RNA was clearly in the testis (major band at approximately 1.6 kb, minor band at approximately 1.1 kb). Based on this tissue distribution of RNA, we sought to identify the cells in the murine testis that contained CGRP-RCP protein. Numerous antisera generated against hCGRP-RCP, including one to recombinant hCGRP-RCP, exhibited strong immunoreactivity localized to the head region of spermatozoa. No CGRP-RCP immunoreactivity was observed in other cells at less mature stages of sperm maturation, in Sertoli or interstitial (Leydig) cells, or in human spermatozoa. Murine epididymal (mature) spermatozoa exhibited CGRP-RCP immunoreactivity identical to that of testicular spermatozoa. Spermatozoa that underwent an experimentally induced acrosome reaction (acrosomal discharge) lost their CGRP-RCP immunoreactivity. Therefore, it appears that CGRP-RCP is associated with the acrosome, suggesting that it may play an important role in reproduction.