The Role of Nerve Growth Factor (NGF) and Its Precursor Forms in Oral Wound Healing

Abstract

Nerve growth factor (NGF) and its different precursor forms are secreted into human saliva by salivary glands and are also produced by an array of cells in the tissues of the oral cavity. The major forms of NGF in human saliva are forms of pro-nerve growth factor (pro-NGF) and not mature NGF. The NGF receptors tropomyosin-related kinase A (TrkA) and p75 neurotrophin receptor (p75^NTR) are widely expressed on cells in the soft tissues of the human oral cavity, including keratinocytes, endothelial cells, fibroblasts and leukocytes, and in ductal and acinar cells of all types of salivary glands. In vitro models show that NGF can contribute at most stages in the oral wound healing process: restitution, cell survival, apoptosis, cellular proliferation, inflammation, angiogenesis and tissue remodeling. NGF may therefore take part in the effective wound healing in the oral cavity that occurs with little scarring. As pro-NGF forms appear to be the major form of NGF in human saliva, efforts should be made to study its function, specifically in the process of wound healing. In addition, animal and clinical studies should be initiated to examine if topical application of pro-NGF or NGF can be a therapy for chronic oral ulcerations and wounds.

Keywords: keratinocytes; oral cavity; saliva; salivary gland.

Figure 1

Western blotting of unstimulated whole saliva taken from six volunteers (lanes 1–6) and recombinant human nerve growth factor beta (NGF) (P) with antibodies reactive with (A) mature NGF or (B) pro-nerve growth factor (pro-NGF); (C) Control blot where primary antibody was substituted with normal rabbit serum. With permission from ref. [7].

Figure 2

Immunohistological expression of pro-nerve growth factor (pro-NGF) in salivary glands. (A) parotis; (B,E) submandibular gland; (C) sublingual gland; (D) labial glands. CD, collecting duct; ID, intercalated duct; MA, mucous acinus; SA, serous acinus; SD, striated duct. The brown color indicates positive staining. Counterstaining with hematoxylin (blue), except for (E) where blue, red, and green colors represent nuclei, pro-NGF, and actin, respectively. Original magnification, 20 ×. With permission from ref. [7].

Figure 3

Immunohistological expression of nerve growth factor (NGF) in salivary glands. (A) parotis. (B) submandibular gland; (C) sublingual gland; (D) labial glands. CD, collecting duct; ID, intercalated duct; MA, mucous acinus; SA, serous acinus; SD, striated duct. The brown color indicates positive staining. Counterstaining with hematoxylin (blue). Original magnification, 20 ×. With permission from ref. [7].

Figure 4

Immunohistological staining for pro-nerve growth factor (pro-NGF) (A–C) and NGF (D–F) in the epithelium of normal oral mucosa (A,D), gingiva (B,E), and skin (C,F). Original magnification, 20 ×. With permission from ref. [14].

Figure 5

Immunohistological staining for pro-nerve growth factor NGF receptor tropomyosin-related kinase A (TrkA) (A–C) and NGF receptor p75 neurotrophin receptor (p75^NTR) (D–F) in the normal epithelium from normal oral mucosa (NOM; A,D), gingiva (B,E), and skin (C,F). Original magnification, 20 ×. With permission from ref. [14].

Figure 6

Immunohistological expression of TrkA (A–D), and p75^NTR (E–H) in salivary glands; (A,E) parotis; (B,F) submandibular gland; (C,G) sublingual gland; (D,H) labial glands. CD, collecting duct; ID, intercalated duct; MA, mucous acinus; SA, serous acinus; SD, striated duct. The brown color indicates positive staining. Counterstaining with hematoxylin (blue). Original magnification, 20 ×. With permission from ref. [7].

Figure 7

Effect of NGF and NGF-neutralizing agents on the migration of human oral mucosal keratinocytes. A scratch was made in confluent cultures of human oral mucosal keratinocyte monolayers. (A) Scratch at 0 h, scale bar 200 µm for A–F; (B–F) Migration after 12 h; (B) No addition; (C) Stimulation with 50 ng/mL of recombinant human NGF. Stimulation with 50 ng/mL of NGF and treatment with 5 µg/mL of mitomycin C (MC) (D) or 5 µM cytochalasin B (CB) (E) for 4 h before scratching and during further incubation with NGF; (F) Treatment with 20 µM TrkA inhibitor AG879; (G) Quantitative evaluation of scratch closure in three independent experiments performed in triplicate. Data are given as the mean ± standard deviation. * p < 0.05, as compared with the control. Ab, antibody. With permission from ref. [14].