Inclusion Body Myositis (IBM) is the most common muscle disorder in patients over 50 years of age. IBM shares several features with another age-related degenerative disorder, Alzheimer's disease (AD): 1) there are both sporadic and autosomal inherited forms, 2) local inflammatory collections are characteristic but immunosuppressive therapies are only marginally effective, and 3) accumulations of AD-related proteins are a histopathological hallmark. Congo red- reactive deposits consisting of the beta-amyloid peptide (Abeta) are the most notable feature. In contrast with the extra-cellular amyloid plaques of AD, the amyloid deposits of IBM are intracellular. Several other proteins accumulate intracellularly in the vacuolated myofibers of IBM including other epitopes of beta-amyloid precursor protein, hyperphosphorylated filaments comprised of the microtubule-associated protein tau, ubiquitin, the cellular prion protein, and cyclin dependent kinase 5 (cdk5). The mechanism of muscle cell loss in IBM remains a mystery. Abeta is known to induce programmed cell death (apoptosis) of neurons in culture and in some transgenic mouse models for Alzheimer's disease.
In Aim 1, the applicants hypothesize that intracellular accumulation of Abeta in cultured skeletal muscle myotubes will induce apoptosis. Abeta deposition is provoked using an inducible promoter, a heterologous viral gene transfer system, or by direct injection. They will examine the role of the proteasome and the parkin gene in the genesis of protein accumulations.
In Aim 2, Abeta is established as a myotoxins by a thorough examination of human IBM biopsy specimens, for a correlation between Abeta accumulation and several markers of apoptosis. Mouse muscle is similarly analyzed after direct injection of an Abeta-encoding adenoviral gene construct. Their 2nd hypothesis is that deposition of intracellular Abeta adversely affects Akt kinase-dependent muscle cell survival pathways and in Aim 3 we will determine if Abeta inhibits Akt activity. Furthermore, the applicants will seek evidence that Abeta induces deleterious attempts by cells to re-enter the developmental or even the cell cycle, related to Akt, p21 Cip1/Wafl or cyclin D dysfunction. Evidence for tau hyperphosphorylation from unhindered GSK-3beta activity will be sought.
In Aim 4 the functional significance of Akt inhibition by Abeta is examined by asking whether Akt activation will protect muscle cells forced to overexpress Abeta from cell death. While some of the findings may be relevant to neuronal systems, they will provide insight into a novel mechanism of Abeta injury and the complex regulation of cell survival, replication and apoptosis from a key survival signal control point.
