What makes a model system great?

Imaging actin organisation and dynamics in 3D

The actin cytoskeleton plays a critical role in cell architecture and the control of fundamental processes including cell division, migration and survival. The dynamics and organisation of F-actin have been widely studied in a breadth of cell types on classical two-dimensional (2D) surfaces. Recent advances in optical microscopy have enabled interrogation of these cytoskeletal networks in cells within three-dimensional (3D) scaffolds, tissues and in vivo. Emerging studies indicate that the dimensionality experienced by cells has a profound impact on the structure and function of the cytoskeleton, with cells in 3D environments exhibiting cytoskeletal arrangements that differ to cells in 2D environments. However, the addition of a third (and fourth, with time) dimension leads to challenges in sample preparation, imaging and analysis, necessitating additional considerations to achieve the required signal-to-noise ratio and spatial and temporal resolution. Here, we summarise the current tools for imaging actin in a 3D context and highlight examples of the importance of this in understanding cytoskeletal biology and the challenges and opportunities in this domain.

Parallel assembly of actin and tropomyosin, but not myosin II, during de novo actin filament formation in live mice

Many actin filaments in animal cells are co-polymers of actin and tropomyosin. In many cases, non-muscle myosin II associates with these co-polymers to establish a contractile network. However, the temporal relationship of these three proteins in the de novo assembly of actin filaments is not known. Intravital subcellular microscopy of secretory granule exocytosis allows the visualisation and quantification of the formation of an actin scaffold in real time, with the added advantage that it occurs in a living mammal under physiological conditions. We used this model system to investigate the de novo assembly of actin, tropomyosin Tpm3.1 (a short isoform of TPM3) and myosin IIA (the form of non-muscle myosin II with its heavy chain encoded by Myh9) on secretory granules in mouse salivary glands. Blocking actin polymerization with cytochalasin D revealed that Tpm3.1 assembly is dependent on actin assembly. We used time-lapse imaging to determine the timing of the appearance of the actin filament reporter LifeAct-RFP and of Tpm3.1-mNeonGreen on secretory granules in LifeAct-RFP transgenic, Tpm3.1-mNeonGreen and myosin IIA-GFP (GFP-tagged MYH9) knock-in mice. Our findings are consistent with the addition of tropomyosin to actin filaments shortly after the initiation of actin filament nucleation, followed by myosin IIA recruitment.

Lighting up microtubule cytoskeleton dynamics in skeletal muscle

In the past few decades, live cell microscopy techniques in combination with fluorescent tagging have provided a true explosion in our knowledge of the inner functioning of the cell. Dynamic phenomena can be observed inside living cells and the behavior of individual molecules participating in those events can be documented. However, our preference for simple or easy model systems such as cell culture, has come at a cost of chasing artifacts and missing out on understanding real biology as it happens in complex multicellular organisms. We are now entering a new era where developing meaningful, but also tractable model systems to study biological phenomenon dynamically in vivo in a mammal is not only possible; it will become the gold standard for scientific quality and translational potential.^1,2 A study by Oddoux et al. describing the dynamics of the microtubule (MT) cytoskeleton in skeletal muscle is one example that demonstrates the power of developing in vivo/ex vivo models.³ MTs have long attracted attention as targets for cancer therapeutics ⁴ and more recently as mediators of Duchene muscular dystrophy.⁵ The muscle fiber MT cytoskeleton forms an intricate rectilinear lattice beneath the sarcolemma and is essential for the structural integrity of the muscle. Cultured cells do not develop such a specialized organization of the MT cytoskeleton and our understanding of it has come from static snapshots of muscle sections.⁶ In this context, the methodology and the findings reported by Oddoux et al. are a significant step forward.