Inclusion body myositis (IBM) is the most common cause of primary myopathy in individuals aged 50 years and over, and is pathologically characterized by protein aggregates of p62 and mislocalized cytoplasmic TDP-43, as well as mitochondrial abnormalities in affected muscle fibers. Our recent studies have shown the accumulation of TDP-43 in mitochondria in neurons from patients with amyotrophic lateral sclerosis (ALS) and frontotemporal degeneration (FTD), and revealed mitochondria as critical mediators of TDP-43 neurotoxicity. In this study, we investigated the association between mitochondria and TDP-43 in biopsied skeletal muscle samples from IBM patients. We found that IBM pathological markers TDP-43, phosphorylated TDP-43, and p62 all coexisted with intensively stained key subunits of mitochondrial oxidative phosphorylation complexes I-V in the same skeletal muscle fibers of patients with IBM. Further immunoblot analysis showed increased levels of TDP-43, truncated TDP-43, phosphorylated TDP-43, and p62, but decreased levels of key subunits of mitochondrial oxidative phosphorylation complexes I and III in IBM patients compared to aged matched control subjects. This is the first demonstration of the close association of TDP-43 accumulation with mitochondria in degenerating muscle fibers in IBM and this association may contribute to the development of mitochondrial dysfunction and pathological protein aggregates.

Skeletal muscles undergo atrophy in response to diseases and aging. Here we report that mitofusin 2 (Mfn2) acts as a dominant suppressor of neuromuscular synaptic loss to preserve skeletal muscles. Mfn2 is reduced in spinal cords of transgenic SOD1G93A and aged mice. Through preserving neuromuscular synapses, increasing neuronal Mfn2 prevents skeletal muscle wasting in both SOD1G93A and aged mice, whereas deletion of neuronal Mfn2 produces neuromuscular synaptic dysfunction and skeletal muscle atrophy. Neuromuscular synaptic loss after sciatic nerve transection can also be alleviated by Mfn2. Mfn2 coexists with calpastatin largely in mitochondria-associated membranes (MAMs) to regulate its axonal transport. Genetic inactivation of calpastatin abolishes Mfn2-mediated protection of neuromuscular synapses. Our results suggest that, as a potential key component of a novel and heretofore unrecognized mechanism of cytoplasmic protein transport, Mfn2 may play a general role in preserving neuromuscular synapses and serve as a common therapeutic target for skeletal muscle atrophy.

Tau neurofibrillary tangles are a hallmark of Alzheimer's disease neuropathological change. However, it remains largely unclear how distinctive Alzheimer's disease tau seeds (i.e. 3R/4R) correlate with histological indicators of tau accumulation. Furthermore, AD tau co-pathology is thought to influence features and progression of other neurodegenerative diseases including Lewy body disease; yet measurements of different types of tau seeds in the setting of such diseases is an unmet need. Here, we use tau real-time quaking-induced conversion (RT-QuIC) assays to selectively quantitate 3R/4R tau seeds in the frontal lobe which accumulates histologically identifiable tau pathology at late disease stages of AD neuropathologic change. Seed quantitation across a spectrum of neurodegenerative disease cases and controls indicated tau seeding activity can be detected well before accompanying histopathological indication of tau deposits, and even prior to the earliest evidence of Alzheimer's-related tau accumulation anywhere in the brain. In later stages of AD, 3R/4R tau RT-QuIC measures correlated with immunohistochemical tau burden. In addition, Alzheimer's tau seeds occur in the vast majority of cases evaluated here inclusive of primary synucleinopathies, frontotemporal lobar degeneration and even controls albeit at multi-log lower levels than Alzheimer's cases. α-synuclein seeding activity confirmed synucleinopathy cases and further indicated the co-occurrence of α-synuclein seeds in some Alzheimer's disease and primary tauopathy cases. Our analysis indicates that 3R/4R tau seeds in the mid-frontal lobe correlate with the overall Braak stage and Alzheimer's disease neuropathologic change, supporting the quantitative predictive value of tau RT-QuIC assays. Our data also indicate 3R/4R tau seeds are elevated in females compared to males at high (≥ IV) Braak stages. This study suggests 3R/4R tau seeds are widespread even prior to the earliest stages of Alzheimer's disease changes, including in normal, and even young individuals, with prevalence across multiple neurodegenerative diseases to further define disease subtypes.