Matricellular protein thrombospondins: influence on ocular angiogenesis, wound healing and immuneregulation

Abstract

Thrombospondins are a family of large multi-domain glycoproteins described as matricelluar proteins based on their ability to interact with a broad range of receptors, matrix molecules, growth factors or proteases, and to modulate array of cellular functions including intracellular signaling, proliferation and migration. Two members of the thrombospondin family, thrombospondin 1 (TSP-1) and thrombospondin 2 (TSP-2) are studied extensively to determine their structure and function. While expressed at low levels in normal adult tissues, their increased expression is seen predominantly in response to cellular perturbations. Despite structural similarities, a notable functional difference between TSP-1 and TSP-2 includes the ability of former to activate of latent TGF-β and its competitive inhibition by the latter. Both these thrombospondins are reported to play important roles in TGF-β rich ocular environment with most reports related to TSP-1. They are expressed by many ocular cell types and detectable in the aqueous and vitreous humor. TSP-1 and TSP-2 influence many cellular interactions in the eye such as angiogenesis, cell migration, wound healing, TGF-β activation and regulation of inflammatory immune responses. Together, these processes are known to contribute to the immune privilege status of the eye. Emerging roles of TSP-1 and TSP-2 in ocular functions and pathology are reviewed here.

Keywords: Angiogenesis; immuneregulation; inflammation; lymphangiogenesis; wound healing.

Figure 1. Functional domains of thrombospondins

Schematic diagram of the multi-domain structures of the members of thrombospondin family. Group A thrombospondins include TSP-1 and TSP-2 that are found in trimeric conformation, while Group B includes TSP-3, TSP-4 and TSP-5/COMP that form pentamers. (vWC = von Willebrand type C domain).

Figure 2. Increased lymphangiogenesis in TSP-1 deficient cornea

Corneal flat mounts from WT and TSP-1 deficient mice stained with fluorescence conjugated anti-CD31 (green) and anti-LYVE-1 (red) under spontaneous (A) and suture-induced inflammatory conditions (B). Morphometric analysis of vascularized areas with LYVE-1⁺⁺⁺ /CD31⁺ lymphatic vessels is presented below each set of staining. (* p < 0.05). From Cursiefen et al. J.Exp.Med. 2011, 208(5):1083

Figure 3. Regulation of lymphangiogenic factor VEGF-c expression in macrophages via CD36-TSP interaction

Binding of CD36 by thrombospondin inhibits TGF-β-induced VEGF-c expression in macrophages. In a normal cornea such regulation is likely with TSP-2:CD36 interactions that may prevent VEGF-c secretion in response to any TGF-b activated during normal wound healing processes.

Figure 4. Abberations in wound healing of TSP-1 deficient cornea

C57BL/6 mouse eyes 3 weeks (A, B) or 1-week (C, D) post-keratectomy in wild type (WT: A, C) and TSP-1-deficient mice (B, D). A) Slit lamp image of WT mouse cornea showing haze present within the circle of arrows, indicating original wound area. (B) Slit lamp image of TSP-1-deficient mouse cornea demonstrating lack of haze formation within original wound area (arrows). C) Whole mount immunolocalization of α-smooth muscle actin (SMA) within the stroma at the original wound site in WT mice. (D) TSP-1-deficient cornea demonstrating dramatic decrease in SMA. TOPRO3 was used as a counterstain to mark all cell nuclei. Bar = 50 μm.

Figure 5

Schematic representation of TSP-1 dependent regulation of immune responses

Figure 6

Overview of immunopathogenesis of autoimmune Sjögren's syndrome in TSP-1 deficient mice.