Molecular mechanism of microtubule-dependent torque-based chemotaxis in dendritic cells

A 2025 preprint (Jakuszeit et al., bioRxiv 10.1101/2025.11.21.688683) showed that two immune cell types use fundamentally different chemotactic strategies: dendritic cells steer toward chemokine gradients in 3D collagen via a deterministic, microtubule-dependent 'effective torque' on their orientation, while neutrophils rely on modulating angular noise and speed. Low-dose nocodazole abolishes the dendritic-cell torque without stopping migration, and this loss is not a contractility artifact, but the molecular machinery coupling microtubules (likely their dynamic plus-ends in protrusions) to the torque was left unidentified, with microtubule–actin cross-linkers, septins, and GEF-H1-dependent signaling proposed but untested candidates. The question remains open: the adjacent GEF-H1/RhoA literature addresses contractility and polarity but not the torque, which the source showed is contractility-independent. The problem is to identify, by direct perturbation, the molecular mechanism that generates the gradient-aligned effective torque in dendritic cells and to determine what sets the balance between torque-based and noise-based chemotactic bias (the coordinates of the kappa–beta phase space defined by the source's Fokker–Planck framework).

Background

Directed migration in gradients of soluble chemicals (chemotaxis) is central to immunity: neutrophils, the short-lived first-responder phagocytes, swarm into infected tissue along gradients of chemokines (small secreted chemoattractant proteins), while dendritic cells (DCs, the antigen-presenting cells that prime T cells) must travel from peripheral tissue to lymphatic vessels and lymph nodes to initiate the adaptive immune response. In dense interstitial tissue both cell types crawl amoeboidly — by adhesion-independent shape change driven by protrusions — through collagen matrix, and DCs advance by extending several competing protrusions ('branches') of which one is stabilized and sets the new direction of travel (Lämmermann et al. 2009; Kopf et al. 2020).

How a gradient biases cell orientation can be decomposed quantitatively. The orientation angle $\theta(t)$ of a migrating cell relative to the gradient obeys a stochastic rotational dynamics, described at the population level by a Fokker–Planck equation (the evolution equation for the probability density $P(\theta,t)$ with a drift term describing deterministic angular velocity and a diffusion term describing orientational fluctuations). Jakuszeit et al. (2025) applied this decomposition to single-cell trajectories in microfabricated collagen-gel chemotaxis chambers and found two distinct strategies. Neutrophils bias migration mainly by increasing speed when oriented up-gradient and by reducing angular noise when aligned with the gradient, modeled as an angular diffusion coefficient $D_r(\theta)=D_{r0}(1-\beta\cos\theta)^2$, where $\beta$ is a phenomenological parameter measuring the relative amplitude of noise modulation. DCs in 3D collagen instead display a deterministic 'effective torque': their median angular velocity is approximately linear in $\theta$ (rotational drift $-k\theta$), actively turning the cell toward the gradient; the torque is reported normalized as $\kappa=k/(2D_{r0})$, that is, non-dimensionalized by the baseline angular noise. In the phase space spanned by $\kappa$ and $\beta$, the source marks the contour $\Psi=0.5$ of equal strategy strength, where $\Psi$ is the ratio between the directionality (chemotactic drift) predicted if the cell used the torque strategy exclusively and the sum of the directionalities predicted by the torque-only and the biased-noise-only strategies; a condition on one side of this line is predicted to rely more strongly on biased angular noise, a condition on the other side more strongly on the torque. All measured conditions stay close to this transition, although they fall on different sides of it, and the source reads the resulting positive correlation between $\beta$ and $\kappa$ as indicating that the underlying cellular processes are related. Modeling of self-propelled particles ('active particles') shows the two strategies have opposite population-level outcomes: torque-driven motion minimizes dispersion (accurate arrival at a fixed target, matching the DC's task of reaching a lymphatic vessel), whereas noise-biased migration spreads the population (matching the neutrophil's task of covering heterogeneous infected tissue).

Pharmacologically, the DC torque in collagen depends on microtubules, the stiff cytoskeletal polymers whose dynamically growing plus-ends probe the cell periphery and are enriched in protrusion tips. Nocodazole, a microtubule-destabilizing drug, reduces the torque in a dose-dependent way down to neutrophil levels without abolishing migration, and the effect persists when the RhoA/ROCK contractility axis is inhibited (RhoA is a small GTPase that drives actomyosin contraction through its effector kinase ROCK, i.e. Rho-kinase; microtubule disassembly normally activates this axis by releasing GEF-H1, a RhoA guanine-nucleotide-exchange factor sequestered on microtubules; Chang et al. 2008), showing the torque is not mediated by contractility. Imaging indicates microtubules bias which branch is stabilized: protrusions aligned with the gradient are preferentially kept, and this bias diminishes after nocodazole. The molecular coupling itself is unknown. Candidate mechanisms named by the source are: microtubule–actin cross-linkers operating at growing microtubule ends (Henty-Ridilla et al. 2016; Alkemade et al. 2022); septins, GTP-binding cytoskeletal scaffold filaments that couple microtubules and actin (Nakos et al. 2022); and GEF-H1-dependent signaling (Kashyap et al. 2019), which the source still names as a possible player after that control without specifying which of its effector branches would be involved, the contractility branch being the one excluded by the ROCK-control experiment. Related work shows microtubules control branch coherence in amoeboid DC migration (Kopf et al. 2020) and mediate navigation of confined cells in other systems via compression-dependent reinforcement (Ju et al. 2024).

Problem Statement

Identify, by direct experimental test, the molecular mechanism by which microtubules — most plausibly their dynamic plus-ends enriched in protrusions — generate the effective chemotactic torque in dendritic cells migrating in 3D collagen matrices under chemokine gradients: which molecular machinery (microtubule–actin cross-linkers, septins, GEF-H1-dependent signaling, microtubule plus-end dynamics, or an as-yet-unproposed component) is causally required for the deterministic, gradient-aligned bias in the stabilization of competing protrusions (branches) and in cell reorientation, with cell shape, speed, and contractility changes controlled for? A complete answer to this first question must name the molecular machinery and give the perturbation-based evidence — the discriminating perturbations, with cell shape, speed, and contractility controlled for — that would establish its causal requirement. And determine what cellular or environmental factors set the relative contributions of deterministic torque and stochastic noise modulation in a given cell type or condition — that is, what are the main factors, internal cellular or external environmental, that control the deterministic and stochastic part of the chemotactic response and hence govern a cell's or condition's coordinates in the phase space spanned by the normalized torque $\kappa$ and the noise-bias amplitude $\beta$. A complete answer to this second question must name the specific factors — internal cellular or external environmental — that are shown, or testably predicted, to shift a condition's coordinates along the $\kappa$–$\beta$ balance, together with the evidence that would establish each; factors that do not meet this bar need not be enumerated. A complete answer must address both questions posed above. As motivating context for this second question, the source reports that the conditions it measured stay close to the $\Psi=0.5$ line of equal strategy strength — the contour on which the torque-only and the biased-noise-only strategies would produce equally strong chemotactic directionality — while falling on different sides of it, and it reads the positive correlation between $\beta$ and $\kappa$ as indicating that the underlying cellular processes are related. The source's further-reaching questions — whether these strategies generalize to other immune cell types and to in vivo contexts, and what sensing mechanisms distinguish full-gradient detection (required for torque) from projection-based detection (sufficient for noise bias) — are noted as extensions beyond this problem rather than folded into it.

References

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