Abnormal RNA stability in amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) share key features, including accumulation of the RNA-binding protein TDP-43. TDP-43 regulates RNA homeostasis, but it remains unclear whether RNA stability is affected in these disorders. We use Bru-seq and BruChase-seq to assess genome-wide RNA stability in ALS patient-derived cells, demonstrating profound destabilization of ribosomal and mitochondrial transcripts. This pattern is recapitulated by TDP-43 overexpression, suggesting a primary role for TDP-43 in RNA destabilization, and in postmortem samples from ALS and FTD patients. Proteomics and functional studies illustrate corresponding reductions in mitochondrial components and compensatory increases in protein synthesis. Collectively, these observations suggest that TDP-43 deposition leads to targeted RNA instability in ALS and FTD, and may ultimately cause cell death by disrupting energy production and protein synthesis pathways.

Genetic defects in acetylcholine signalling promote protein degradation in muscle cells of Caenorhabditis elegans.

A myosin-lacZ fusion, expressed in 103 muscle cells of Caenorhabditis elegans, reports on how proteolysis in muscle is controlled by neural and intramuscular signals. Upon acute starvation, the fusion protein is degraded in the posterior 63 cells of the body-wall muscle, but remains stable in 32 anterior body-wall muscles and 8 vulval muscle cells. This distinction correlates with differences in the innervation of these cells. Reporter protein in the head and vulval muscles becomes labile upon genetic 'denervation' in mutants that have blocks in pre-synaptic synthesis or release of acetylcholine (ACh) or post-synaptic reception at nicotinic ACh receptors (nAChR), whereas protein in all 103 muscles is stabilized by the nicotinic agonist levamisole in the absence of ACh production. Levamisole does not stabilize muscle protein in nAChR mutants that are behaviorally resistant to levamisole. Neural inputs thus exert negative control over the proteolytic process in muscle by stimulating muscle nicotinic ACh receptors.

Transgene-coded chimeric proteins as reporters of intracellular proteolysis: starvation-induced catabolism of a lacZ fusion protein in muscle cells of Caenorhabditis elegans.

The product of an integrated transgene provides a convenient and cell-specific reporter of intracellular protein catabolism in 103 muscle cells of the nematode Caenorhabditis elegans. The transgene is an in-frame fusion of a 5'-region of the C. elegans unc-54 (muscle myosin heavy-chain) gene to the lacZ gene of Escherichia coli [Fire and Waterston (1989): EMBO J 8:3419-3428], encoding a 146-kDa fusion polypeptide that forms active beta-galactosidase tetramers. The protein is stable in vivo in well-fed animals, but upon removal of the food source it is inactivated exponentially (t1/2 = 17 h) following an initial lag of 8 h. The same rate constant (but no lag) is observed in animals starved in the presence of cycloheximide, implying that inactivation is catalyzed by pre-existing proteases. Both the 146-kDa fusion polypeptide (t1/2 = 13 h) and a major 116-kDa intermediate (t1/2 = 7 h) undergo exponential physical degradation after a lag of 8 h. Degradation is thus paradoxically faster than inactivation, and a number of characteristic immunoreactive degradation intermediates, some less than one-third the size of the parent polypeptide, are found in affinity-purified (active) protein. Some of these intermediates are conjugated to ubiquitin. We infer that the initial proteolytic cleavages occur in the cytosol, possibly by a ubiquitin-mediated proteolytic pathway and do not necessarily inactivate the fusion protein tetramer.