Study of histopathological and molecular changes of rat kidney under simulated weightlessness and resistance training protective effect

Abstract

To explore the effects of long-term weightlessness on the renal tissue, we used the two months tail suspension model to simulate microgravity and investigated the simulated microgravity on the renal morphological damages and related molecular mechanisms. The microscopic examination of tissue structure and ultrastructure was carried out for histopathological changes of renal tissue morphology. The immunohistochemistry, real-time PCR and Western blot were performed to explore the molecular mechanisms associated the observations. Hematoxylin and eosin (HE) staining showed severe pathological kidney lesions including glomerular atrophy, degeneration and necrosis of renal tubular epithelial cells in two months tail-suspended rats. Ultrastructural studies of the renal tubular epithelial cells demonstrated that basal laminas of renal tubules were rough and incrassate with mitochondria swelling and vacuolation. Cell apoptosis in kidney monitored by the expression of Bax/Bcl-2 and caspase-3 accompanied these pathological damages caused by long-term microgravity. Analysis of the HSP70 protein expression illustrated that overexpression of HSP70 might play a crucial role in inducing those pathological damages. Glucose regulated protein 78 (GRP78), one of the endoplasmic reticulum (ER) chaperones, was up-regulated significantly in the kidney of tail suspension rat, which implied that ER-stress was associated with apoptosis. Furthermore, CHOP and caspase-12 pathways were activated in ER-stress induced apoptosis. Resistance training not only reduced kidney cell apoptosis and expression of HSP70 protein, it also can attenuate the kidney impairment imposed by weightlessness. The appropriate optimization might be needed for the long term application for space exploration.

Figure 1. Body weight and renal index under microgravity condition.

A: Dynamic changes of rats body weight in control group (CON), tail-suspended group (TS) and, tail-suspended and resistance training group (TS&RT). B: Renal index was obtained by dividing total left and right kidney weight to the body weight of euthanized rats. Data are shown as means±SD. * Significant P values<0.05.

Figure 2. Histopathological analysis of rats' kidneys paraffin sections stained with hematoxylin and eosin (HE), mallory's trichrome staining and sirius red staining.

A: Morphological changes of renal cortex: The normal structure of rats' kidney in the control group (A1). In TS group, rat's renal cortex showed glomerular atrophy, distension of the renal glomerulus capsular space (arrowhead) and interstitial edema (arrow, A2). In TS&RT group, rat's renal cortex displayed moderately glomerular atrophy (arrowhead) and interstitial congestion (arrow, A3). B: Morphological changes of renal medulla: The normal structure of rats' kidney in the control group (B1). In TS group, rat's renal medulla abundantly illustrated degeneration and necrosis of renal tubular epithelial cells (arrowhead), interstitial congestion (arrow), and protein and cellular cast in nephric tubules (B2). In TS&RT group, rat's renal medulla showed scattered degeneration and necrosis of renal tubular epithelial cells (arrowhead, B3). C&D: Mallory's trichrome (C) and Sirius Red (D) stained paraffin sections displayed different levels of proliferation of kidney fibrous tissue in CON(C1,D1), TS(C2,D2) and TS&RT groups (C3,D3)respectively. Collagen fibers were stained blue with Mallory's trichrome and red with Sirius Red respectively (arrow).

Figure 3. The transmission electron micrographs of kidney tissue in CON, TS and TS&RT groups.

A1 and B1: The basal lamina of renal tubules was intact (arrow) and there were abundance of mitochondria of spherical or elongated shape with mitochondrial cristae in the CON group (arrowhead) A2 and B2: The basal laminas of renal tubules were rough and incrassate (arrow), the proximal tubules and distal tubules had numerous dead cells (arrow). Mitochondria in electron-dense cytoplasm of the cell were swollen with thin cristae (arrowhead) in the TS group. A3 and B3: The incrassate of renal tubules basal lamina (arrow) with mitochondria swelling (arrowhead) in renal tubule epithelia of TS&RT group.

Figure 4. Immunohistochemical staining of PCNA and HSP70 proteins in rats' kidneys.

A: Histological sections of kidneys in CON, TS and TS&RT groups (A1–A3) respectively were stained with anti PCNA serum. B: Histological sections of kidneys in CON, TS and TS&RT groups (B1–B3) respectively were stained with anti HSP70 protein serum. C and D: Semi-quantitative analysis of PCNA and HSP70 proteins expression in kidneys with immunohistochemical staining. Data are shown as means±SD. *P significant p values<0.05 and **P significant p values<0.01.

Figure 5. Immunohistochemical staining and real-time PCR analysis of apoptosis.

A: Histological sections of kidneys in CON, TS and TS&RT groups (A1–A3) respectively were stained with anti Bax protein serum. B: Histological sections of kidneys in CON, TS and TS&RT groups (B1–B3) respectively were stained with anti Bcl-2 protein serum. C: Histological sections of kidneys in CON, TS and TS&RT groups (C1–C3) respectively were stained with anti caspase-3 protein serum. D: The protein ratio of Bax/Bcl-2 is obtained by dividing signal intensity of Bax positive substance and Bcl-2 positive substance. E: Semi-quantitative analysis of caspase-3 protein with immunohistochemical staining. F and G: mRNA ratio of Bax/Bcl2 and relative mRNA levels of caspase-3 in CON, TS and TS&RT rats' kidney A total of 10 samples for each group were used, and each sample was run in triplicate for real-time PCR. Data are shown as means±SD. *P significant p values<0.05 and **P significant p values<0.01.

Figure 6. Relative mRNA levels of GRP78(A) and CHOP(B), caspase-12(C), and JNK(D) in CON, TS and TS&RT rats' kidney.

A total of 10 samples for each group were used, and each sample was run in triplicate for real-time PCR. Data are shown as means±SD. *P significant p values <0.05 and **P significant p values <0.01.

Figure 7. Western blot analysis of the expression of GRP78, caspase-12, CHOP, and JNK in the kidneys of CON, TS and TS&RT groups.