Overexpression of MHC class I heavy chain protein in young skeletal muscle leads to severe myositis: implications for juvenile myositis

Abstract

Folding and transport of proteins, such as major histocompatibility complex (MHC) class I, through the endoplasmic reticulum (ER) is tightly regulated in all cells, including muscle tissue, where the specialized ER sarcoplasmic reticulum is also critical to muscle fiber function. Overexpression of MHC class I protein is a common feature of many muscle pathologies including idiopathic myositis and can induce ER stress. However, there has been no comparison of the consequences of MHC overexpression in muscle at different ages. We have adapted a transgenic model of myositis induced by overexpression of MHC class I protein in skeletal muscle to investigate the effects of this protein overload on young muscle fibers, as compared with adult tissue. We find a markedly more severe disease phenotype in young mice, with rapid onset of muscle weakness and pathology. Gene expression profiling to compare the two models indicates rapid onset of ER stress in young muscle tissue but also that gene expression of key muscle structural proteins is affected more rapidly in young mice than adults after this insult. This novel model has important implications for our understanding of muscle pathology in dermatomyositis of both adults and children.

Figure 1

Early induction of self MHC class I in murine skeletal muscle leads to rapid onset of weakness and reduced survival. Kaplan-Meier plot comparing two groups of mice (HT-E, HT-L) that differ only in time of transgene activation. Doxycycline was removed at mean 21.2 days (SD ± 1.3 days) for HT-E mice (bold line, n = 35 mice), and 37.4 days (SD ± 8 days) for HT-L mice (fine line, n = 37 mice). Control HT mice were maintained on doxycycline at all times (dotted line, n = 10 mice).

Figure 2

A: H&E-stained sections of quadriceps femoris muscle from control, HT-E, and HT-L models 2 and 4 weeks after transgene expression or no transgene expression in controls, as shown. B: Quantitation of abnormality of histological findings for quadriceps femoris (top graph) and gastrocnemius (lower graph) muscles from HT-E (bold line), and HT-L mice (fine line) at 0, 2, 4, and 8 weeks after transgene expression. Data points represent mean values from groups (each group n = 4), error bars ±1 SD. C: Quadriceps femoris sections from HT-E mice stained with H&E (left), and by immunostaining for macrophages, identified by MOMA positivity (middle), and T cells (CD3, right) as shown. All panels ×200 final magnification.

Figure 3

A: Cluster dendrogram indicating relationships of gene expression overall between the nine samples analyzed by gene expression profiling. Each sample is indicated by a number and designations (early indicating HT-E mice; control; or late indicating HT-L mice). B: Heat map of the top 48 probes across all nine samples represented in log2 intensity, dark blue indicating low expression and yellow indicating high expression. The dendrogram of the nine samples above the heat map is clustered using these 48 probes only; green: HT-E samples; grey: controls; red: HT-L samples. Affymetrix probe IDs and names of the 48 probes are detailed in Table 1.

Figure 4

Immunohistological analysis of Quadriceps femoris sections for Ubiquitin complex, in HT-E (left) and HT-L (right) models at 2, 4, and 8 to 12 weeks after transgene overexpression as shown. All panels ×200 final magnification.

Figure 5

Gene expression in 3 of the muscle specific genes whose expression was altered in the HT-E model, left-Mbnl2; middle-TNFsf12a; and right-Tbc1d4. Each panel shows data points for control, HT-E (early) and HT-L (late) samples as marked, plotted as log2 intensity values.