In this preprint, the authors address the mechanism which underlies cargo transport in neurons. Neurons are highly polarized cells that comprise of a cell body (a dendrite) and a long and thin axon. It is known that cargo can be transported in a bidirectional (anterograde and retrograde) manner in the terminal of the axon and retrograde within the axonal shaft. Interestingly, the transported cargo can be larger than the diameter of the axon. For example, human corpus callosum axons reach a diameter of ca. 0.64-0.74 µm whilst transported autophagosomes can be almost 1.5 µm, mitochondria up to 3 µm and endosomes up to 1 µm in diameter. Therefore, the authors hypothesize that there must be radial contractility in axons that allows transient expansion to transport such large cargo.
The authors combine super-resolution microscopy, electron and confocal microscopy to image fixed and live rat hippocampal neurons labeled with selected dyes, transfected with fluorescently-tagged constructs or NM-II antibodies. The authors find that:
1) The speed of cargo transportation inversely correlates with cargo size.
2) The axons undergo dynamic local deformation during the transportation of larger cargo, as the diameter is enlarged to allow the cargo to pass through and then immediately constricted afterwards.
3) This transient change in axon diameter is mediated by NM-II.
4) NM-II forms a ~ 200 nm periodic structure that is placed perpendicular to the longitudinal axon axis, similar to the F-actin ring MPS. It is a pity that the authors could not find a clear co-localization pattern between NM II and F-actin MPS. However, their further experiments seem to support that NM-II correlates with actin MPS. Thus, the authors conclude that NM-II controls the contraction of the subcortical actin MPS and this actomyosin MPS underlies the radial contractility of axons.
Co-localization of actomyosin MPS and their position relatively to the longitudinal axis of the axon.
- Question: Would the transfection with the phospho-MRLC or MRLC-GFP construct help to elucidate the colocalization issue? The authors transfected hippocampal neurons with the MRLC-GFP construct later in the manuscript.
5) Next, the authors asked whether the radial contraction of axons could play a role in cargo transport. Using blebbistatin, which affects NM-II function, for 60 mins they find that axons dilate. This is followed by an initial increase of the speed for retrograde moving large cargos, which then stalls in a back-and-forth state. Interestingly, the speed of small cargo transportation is not affected. Looking at the large cargo in detail, they also identify that overall efficacy of the cargo transport is actually reduced. This leads them into a conclusion that radial axonal contractility is required for correct long-range retrograde cargo transport and their directionality in neurons.
- Question: The explanation in this preprint as to why large cargoes initially travel quickly and then end up in a back-and-forth state after 60 min of blebbistatin treatment seems to me interesting. However, why is the large cargo not moving at least slowly after the blebbistatin treatment (axon dilatation) when dynein and microtubules are not affected within 60 mins? Is dynein-supported transportation limited to a certain cargo size?
6) Long blebbistatin treatment (60-120 mins) results in focal axon swelling (FAS) and an axonal degeneration-like state. The authors also show that transfection with the mutant MRLC-GFP construct leads to a similar axonal damage.
- Question: What do dynein and microtubules look like then in these long-time blebbistatin treated axons and in those transfected with the MRLC-GFP mutant construct? If affected, do you think that NM-II defines/regulates microtubules (axonal structure) in these neurons?
- Further questions: Coming from the planar cell polarity field, does the size spacing (~ 200 nm)/regularity of actomyosin MPS have a function in pushing large cargoes? Have you tried a transfection with polarity mutant constructs that disrupt this regular MPS distribution (or cargo transport)?
The authors conclude that CNS axons are under constitutive radial constriction/contractility, which limits their diameter and supports effectively unidirectional retrograde cargo transport (to the neuron body). In turn, this contributes to the survival and healthy function of neurons.
Why I chose it
I find it cool that it is possible to simply culture rat neurons in a dish and observe how they transport stained cargo. I like the article as it is well written and easy to understand. Although many experiments are performed here, this preprint is focused, simple and clean.
Posted on: 15th January 2019 , updated on: 16th January 2019