The biological mechanisms underlying decline in muscle power and fatigue with age are not completely understood. The contribution of alterations in the excitation-calcium release coupling in single muscle fibers was explored in this work. Single muscle fibers were voltage-clamped using the double Vaseline gap technique. The samples were obtained by needle biopsy of the vastus lateralis (quadriceps) from 9 young (25-35 years; 25.9 +/- 9.1; 5 female and 4 male) and 11 old subjects (65-75 years; 70.5 +/- 2.3; 6 f, 5 m). Data were obtained from 36 and 39 fibers from young and old subjects, respectively. Subjects included in this study had similar physical activity. Denervated and slow-twitch muscle fibers were excluded from this study. A significant reduction of maximum charge movement (Qmax) and DHP-sensitive Ca current were recorded in muscle fibers from the 65-75 group. Qmax values were 7.6 +/- 0.9 and 3.2 +/- 0.3 nC/muF for young and old muscle fibers, respectively (P < 0.01). No evidences of charge inactivation or interconversion (charge 1 to charge 2) were found. The peak Ca current was (-)4.7 +/- 0.08 and (-)2.15 +/- 0.11 muA/muF for young and old fibers, respectively (P < 0.01). The peak calcium transient studied with mag-fura-2 (400 microM) was 6.3 +/- 0.4 microM and 4.2 +/- 0.3 microM for young and old muscle fibers, respectively. Caffeine (0.5 mM) induced potentiation of the peak calcium transient in both groups. The decrease in the voltage-/Ca-dependent Ca release ratio in old fibers (0.18 +/- 0.02) compared to young fibers (0.47 +/- 0.03) (P < 0.01), was recorded in the absence of sarcoplasmic reticulum calcium depletion. These data support a significant reduction of the amount of Ca available for triggering mechanical responses in aged skeletal muscle and, the reduction of Ca release is due to DHPR-ryanodine receptor uncoupling in fast-twitch fibers. These alterations can account, at least partially for the skeletal muscle function impairment associated with aging.

Objective. Antigen-specific CD4+ T cells treated with DNA methylation inhibitors become autoreactive, suggesting a novel mechanism for autoimmunity. To test whether this mechanism might be involved in systemic lupus erythematosus (SLE), phenotypic markers for the autoreactive cells were sought.Methods. Cloned normal T cells were treated with the DNA methylation inhibitor 5-azacytidine (5-azaC) and studied for altered gene expression. T cells from patients with active SLE were then studied for a similar change in gene expression, and cells expressing the marker were tested for autoreactivity.Results. 5-azaC-treated normal T cells had increased CDlla (leukocyte function-associated antigen la) expression relative to other membrane molecules. A T cell subset with similar CDlla expression was found in patients with active SLE. This subset contained cells that spontaneously lysed autologous macrophages, with a specificity similar to that of 5-azaC-treated cells. Conclusion. The model of 5-azaC-induced autoreactivity may have relevance to SLE.Antigen-specific CD4+ T cell clones respond to autologous macrophages (Mgi) without antigen, following treatment with DNA methylation inhibitors such as 5-azacytidine (5-azaC). The response is inhibited by monoclonal antibodies (MAb) against CD3 and class I1 major histocompatibility complex (MHC) determinants, and occurs with autologous but not allogeneic Mgi (1,2), suggesting that the cells are responding to autologous class I1 MHC determinants via a pathway involving the antigedMHC receptor. Similar concentrations of 5-azaC alter gene expression in a wide range of cell types, including T cells (3-3, and the autoreactivity may be caused by changes in the expression of as-yet-unidentified genes involved in T cell activation.5-azaC-induced autoreactivity may have relevance to autoimmunity. Recent experiments have shown that murine T cells can become autoreactive following 5-azaC treatment, responding to and, intriguingly, lysing, syngeneic Mgi. Adoptive transfer of murine 5-azaC-treated cells induces an immune complex glomerulonephritis and anti-DNA antibodies in nonirradiated syngeneic mice (6). These findings suggest a novel model for systemic lupus erythematosus (SLE), in which T cell DNA hypomethylation could alter expression of crucial genes, resulting in T cell autoreactivity, and the autoreactive T cells could then produce a lupus-like illness. Reports that lupusinducing drugs inhibit T cell DNA methylation and induce autoreactivity (7) support this concept. Moreover, recent evidence indicates that T cell DNA methylation is impaired in patients with active SLE (8).

Objective. To determine if DNA methylation inhibitors make T cells autoreactive by inducing lymphocyte function-associated antigen type 1 (LFA-1) ( C D l l d CD18) overexpression.Methods. T cell clones were treated with 3 distinct DNA methylation inhibitors or were stably transfected with a CD18 cDNA in a mammalian expression vector, and the effects on LFA-1 expression and activation requirements were examined.Results. LFA-1 overexpression, caused by DNA methylation inhibitors or by transfection, correlates with the development of autoreactivity.Conclusion. LFA-1 overexpression may contribute to T cell autoreactivity.

Abbreviations: ADAR, adenosine deaminases that act on RNA; bp, base pair; IFN, interferon; IP 6 , inositol hexakisphosphate; NSAID, non-steroidal anti-inflammatory drug; PCR, polymerase chain reaction; SLE, systemic lupus erythematosus; sscDNA, single-stranded complementary DNA; SUMO, small ubiquitin modifier.