Microtubule binding by dynactin is required for microtubule organization but not cargo transport

Dynactin links cytoplasmic dynein and other motors to cargo and is involved in organizing radial microtubule arrays. The largest subunit of dynactin, p150glued, binds the dynein intermediate chain and has an N-terminal microtubule-binding domain. To examine the role of microtubule binding by p150glued, we replaced the wild-type p150glued in Drosophila melanogaster S2 cells with mutant ΔN-p150 lacking residues 1–200, which is unable to bind microtubules. Cells treated with cytochalasin D were used for analysis of cargo movement along microtubules. Strikingly, although the movement of both membranous organelles and messenger ribonucleoprotein complexes by dynein and kinesin-1 requires dynactin, the substitution of full-length p150glued with ΔN-p150glued has no effect on the rate, processivity, or step size of transport. However, truncation of the microtubule-binding domain of p150glued has a dramatic effect on cell division, resulting in the generation of multipolar spindles and free microtubule-organizing centers. Thus, dynactin binding to microtubules is required for organizing spindle microtubule arrays but not cargo motility in vivo

Muscle Expression of Mutant Androgen Receptor Accounts for Systemic and Motor Neuron Disease Phenotypes in Spinal and Bulbar Muscular Atrophy

SummaryX-linked spinal and bulbar muscular atrophy (SBMA) is characterized by adult-onset muscle weakness and lower motor neuron degeneration. SBMA is caused by CAG-polyglutamine (polyQ) repeat expansions in the androgen receptor (AR) gene. Pathological findings include motor neuron loss, with polyQ-AR accumulation in intranuclear inclusions. SBMA patients exhibit myopathic features, suggesting a role for muscle in disease pathogenesis. To determine the contribution of muscle, we developed a BAC mouse model featuring a floxed first exon to permit cell-type-specific excision of human AR121Q. BAC fxAR121 mice develop systemic and neuromuscular phenotypes, including shortened survival. After validating termination of AR121 expression and full rescue with ubiquitous Cre, we crossed BAC fxAR121 mice with Human Skeletal Actin-Cre mice. Muscle-specific excision prevented weight loss, motor phenotypes, muscle pathology, and motor neuronopathy and dramatically extended survival. Our results reveal a crucial role for muscle expression of polyQ-AR in SBMA and suggest muscle-directed therapies as effective treatments

ALS-causative mutations in FUS/TLS confer gain- and loss-of-function by altered association with SMN and U1-snRNP

The RNA-binding protein FUS/TLS, mutation in which is causative of the fatal motor neuron disease ALS, is demonstrated to directly bind to the U1-snRNP and SMN complexes. ALS-causative mutations in FUS/TLS are shown to abnormally enhance their interaction with SMN and dysregulate its function, including loss of Gems and altered levels of small nuclear RNAs (snRNAs). The same mutants are found to have reduced association with U1-snRNP. Correspondingly, global RNA analysis reveals a mutant-dependent loss of splicing activity, with ALS-linked mutants failing to reverse changes caused by loss of wild-type FUS/TLS. Furthermore, a common FUS/TLS mutant-associated RNA splicing signature is identified in ALS patient fibroblasts. Taken together, these studies establish potentially converging disease mechanisms in ALS and spinal muscular atrophy, with ALS-causative mutants acquiring properties representing both gain (dysregulation of SMN) and loss (reduced RNA processing mediated by U1-snRNP) of function

Synaptic Paths to Neurodegeneration: The Emerging Role of TDP-43 and FUS in Synaptic Functions

TAR DNA-binding protein-43 KDa (TDP-43) and fused in sarcoma (FUS) as the defining pathological hallmarks for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), coupled with ALS-FTD-causing mutations in both genes, indicate that their dysfunctions damage the motor system and cognition. On the molecular level, TDP-43 and FUS participate in the biogenesis and metabolism of coding and noncoding RNAs as well as in the transport and translation of mRNAs as part of cytoplasmic mRNA-ribonucleoprotein (mRNP) granules. Intriguingly, many of the RNA targets of TDP-43 and FUS are involved in synaptic transmission and plasticity, indicating that synaptic dysfunction could be an early event contributing to motor and cognitive deficits in ALS and FTD. Furthermore, the ability of the low-complexity prion-like domains of TDP-43 and FUS to form liquid droplets suggests a potential mechanism for mRNP assembly and conversion. This review will discuss the role of TDP-43 and FUS in RNA metabolism, with an emphasis on the involvement of this process in synaptic function and neuroprotection. This will be followed by a discussion of the potential phase separation mechanism for forming RNP granules and pathological inclusions