Expression of CHI3L1 and CHIT1 in osteoarthritic rat cartilage model. A morphological study

Abstract

Osteoarthritis is a degenerative joint disease, which affects millions of people around the world. It occurs when the protective cartilage at the end of bones wears over time, leading to loss of flexibility of the joint, pain and stiffness. The cause of osteoarthritis is unknown, but its development is associated with different factors, such as metabolic, genetic, mechanical and inflammatory ones. In recent years the biological role of chitinases has been studied in relation to different inflammatory diseases and more in particular the elevated levels of human cartilage glycoprotein 39 (CHI3L1) and chitotriosidase (CHIT1) have been reported in a variety of diseases including chronic inflammation and degenerative disorders. The aim of this study was to investigate, by immunohistochemistry, the distribution of CHI3L1 and CHIT1 in osteoarthritic and normal rat articular cartilage, to discover their potential role in the development of this disease. The hypothesis was that the expression of chitinases could increase in OA disease. Immunohistochemical analysis showed that CHI3L1 and CHIT1 staining was very strong in osteoarthritic cartilage, especially in the superficial areas of the cartilage most exposed to mechanical load, while it was weak or absent in normal cartilage. These findings suggest that these two chitinases could be functionally associated with the development of osteoarthritis and could be used as markers, so in the future they could have a role in the daily clinical practice to stage the severity of the disease. However, the longer-term in vivo and in vitro studies are needed to understand the exact mechanism of these molecules, their receptors and activities on cartilage tissue.

Figure 1.

A) Kraus’ modified Mankin score between groups; results are presented as the mean ±SEM; Student’s ttest was used to evaluate the significance of the results; *P<0.01, when compared to the control group. B) Histopathology OARSI system between groups; results are presented as the mean ± SEM; Student’s ttest was used to evaluate the significance of the results; *P<0.01, when compared to the control group.

Figure 2.

A) H&E staining demonstrated absence of structural alterations in control cartilage, group 1 without anterior cruciate ligament transection (ACLT); the dashed line represents the layers (thickness) of hyaline healthy cartilage; in the superficial zone, cells are flat and small; in the middle and deep zone, cells are organized in columns; the tidemark is evident. B) H&E staining demonstrated signs of structural alterations in moderate OA cartilage (group 2 with ACLT); the dashed line represents the layers (thickness) of hyaline cartilage; the structural alterations included a reduction of cartilage thickness of the superficial and the middle zones; the tidemark is almost intact. C) H&E staining demonstrated signs of structural alterations in severe OA (group 2 with ACLT); the dashed line represents the layers (thickness) of hyaline cartilage; severe OA cartilage demonstrated deep surface clefts, disappearance of cells from the superficial zone, cloning, and a lack of cells in the intermediate and deep zone, which are not arranged in columns; the tidemark is no longer intact and the subchondral bone shows fibrillation; cartilage is completely replaced by fibrocartilaginous, scar-like tissue with fibroblast like cells.

Figure 3.

A) CHI3L1 immunohistochemistry specimen from control cartilage, group 1 without anterior cruciate ligament transection (ACLT), exhibited a weak/absent (ES=+; IS=1) immunostaining in chondrocytes from rat femoral articular cartilage. B) CHI3L1 immunohistochemistry specimen from moderate/severe OA cartilage (group 2, with ACLT) exhibited a very strong (ES=++++; IS=4) immunostaining in chondrocytes from rat femoral articular cartilage (superficial and middle zone). C) Magnification of panel B.

Figure 4.

A) Immunohistochemistry: percentage of CHI3L1 positive cells out of the total number of cells counted in control group and in treated group; results are presented as the mean ±SEM; Student’s t-test, was used to evaluate the significance of the results; *P<0.01, when compared to the control group. B) Immunohistochemistry: percentage of CHIT1 positive cells out of the total number of cells counted in control group and in treated group; results are presented as the mean ±SEM; Student’s t-test, was used to evaluate the significance of the results; *P<0.01, when compared to the control group.

Figure 5.

A) CHIT1 immunohistochemistry specimen from control cartilage, group 1 without anterior cruciate ligament transection (ACLT), exhibited a weak/absent (ES=+; IS=1) immunostaining in chondrocytes from rat femoral articular cartilage. B) CHIT1 immunohistochemistry specimen from moderate/severe OA cartilage (group 2, with ACLT) exhibited a very strong (ES=++++; IS=4) immunostaining in chondrocytes from rat femoral articular cartilage (superficial and middle zone). C) Magnification of panel B.

Figure 6.

Graphic representation of CHI3L1 and CHIT1 expression in osteoarthritis.