Pico Caroni

Muscle LIM protein, a novel essential regulator of myogenesis, promotes myogenic differentiation

Muscle LIM protein (MLP) is a novel positive regulator of myogenesis. Its expression and that of its Drosophila homolog DMLP1 are enriched in striated muscle and coincide with myogenic differentiation. In the absence of MLP, induced C2 cells express myogenin but fail to exit from the cell cycle and to differentiate. Over-expression of MLP in C2 myoblasts potentiates myogenic differentiation and reduces its sensitivity to TGF beta. Like MLP, single LIM domain deletion mutants of MLP and nonmuscle LIM-only proteins promote myogenic differentiation. In 3T3 fibroblasts, the same LIM proteins prevent phorbol ester-induced inhibition of DNA replication. These results establish MLP as an essential promoter of myogenesis and suggest that LIM-only proteins act via similar mechanisms to regulate aspects of cell differentiation.

Actin cytoskeleton regulation through modulation of PI(4,5)P2 rafts

The phosphoinositide lipid PI(4,5)P2 is now established as a key cofactor in signaling to the actin cytoskeleton and in vesicle trafficking. PI(4,5)P2 accumulates at membrane rafts and promotes local co‐recruitment and activation of specific signaling components at the cell membrane. PI(4,5)P2 rafts may thus be platforms for local regulation of morphogenetic activity at the cell membrane. Raft PI(4,5)P2 is regulated by lipid kinases (PI5‐kinases) and lipid phosphatases (e.g. synaptojanin). In addition, GAP43‐like proteins have recently emerged as a group of PI(4,5)P2 raft‐modulating proteins. These locally abundant proteins accumulate at inner leaflet plasmalemmal rafts where they bind to and co‐distribute with PI(4,5)P2, and promote actin cytoskeleton accumulation and dynamics. In keeping with their proposed role as positive modulators of PI(4,5)P2 raft function, GAP43‐like proteins confer competence for regulated morphogenetic activity on cells that express them. Their function has been investigated extensively in the nervous system, where their expression promotes neurite outgrowth, anatomical plasticity and nerve regeneration. Extrinsic signals and intrinsic factors may thus converge to modulate PI(4,5)P2 rafts, upstream of regulated activity at the cell surface.

Spatial and temporal control of signaling through lipid rafts

Sphingolipid- and cholesterol-dependent microdomains (rafts) order proteins at biological membranes and have been implicated in most signaling processes at the cell surface, but the principles and mechanisms through which lipid rafts influence signaling are not well understood. Recent studies have revealed how lipid rafts are rapidly redistributed and assembled locally in response to extracellular signals, and how components of raft-based signaling domains undergo rapid and regulated rearrangements influencing signal quality, duration, and strength. These findings highlight the exquisitely dynamic properties of signaling domains based on lipid rafts, and suggest that processes of raft trafficking and assembly take central roles in mediating spatial and temporal control of signaling.

Cholesterol and lipid microdomains stabilize the postsynapse at the neuromuscular junction

Stabilization and maturation of synapses are important for development and function of the nervous system. Previous studies have implicated cholesterol‐rich lipid microdomains in synapse stabilization, but the underlying mechanisms remain unclear. We found that cholesterol stabilizes clusters of synaptic acetylcholine receptors (AChRs) in denervated muscle in vivo and in nerve–muscle explants. In paralyzed muscles, cholesterol triggered maturation of nerve sprout‐induced AChR clusters into pretzel shape. Cholesterol treatment also rescued a specific defect in AChR cluster stability in cultured src−/−;fyn−/− myotubes. Postsynaptic proteins including AChRs, rapsyn, MuSK and Src‐family kinases were strongly enriched in lipid microdomains prepared from wild‐type myotubes. Microdomain disruption by cholesterol‐sequestering methyl‐β‐cyclodextrin disassembled AChR clusters and decreased AChR–rapsyn interaction and AChR phosphorylation. Amounts of microdomains and enrichment of postsynaptic proteins into microdomains were decreased in src−/−;fyn−/− myotubes but rescued by cholesterol treatment. These data provide evidence that cholesterol‐rich lipid microdomains and SFKs act in a dual mechanism in stabilizing the postsynapse: SFKs enhance microdomain‐association of postsynaptic components, whereas microdomains provide the environment for SFKs to maintain interactions and phosphorylation of these components.

A Circuit Mechanism for Neurodegeneration

How deficiency in SMN1 selectively affects motoneurons in spinal muscular atrophy is poorly understood. Here, Imlach et al. and Lotti et al. show that aberrant splicing of Stasimon in cholinergic sensory neurons and interneurons leads to motoneuron degeneration, suggesting that altered circuit function may underlie the disorder.

Modeling Neuronal Vulnerability in ALS

Using computational models of motor neuron ion fluxes, firing properties, and energy requirements, Lexa0Masson etxa0al. (2014) reveal how local imbalances in energy homeostasis may self-amplify and contribute to neurodegeneration in ALS.