Structurally, the primary cilium is an elegant piece of cellular engineering. It is anchored by the mother centriole-derived basal body, which during the G0/G1 phase or differentiation docks at the plasma membrane and nucleates a 9+0 microtubule axoneme enclosed in a specialized ciliary membrane. Assembly, maintenance and signaling competence depend on intraflagellar transport, in which IFT-A and IFT-B complexes ride along the axoneme with kinesin-2 and cytoplasmic dynein-2 motors, ferrying cargo in both directions. At the ciliary base, transition fibers and the transition zone act as a selective gate, restricting which receptors, ion channels, transporters and signaling molecules can enter. This selective compartmentalization is what allows the cilium to function as a spatially restricted signaling hub rather than a simple membrane protrusion, concentrating machinery for Hedgehog, G protein-coupled receptor, calcium, receptor tyrosine kinase, TGF-beta and Wnt pathways within a nanometer-scale compartment.

The review’s central conceptual move is to treat the cilium as plastic rather than static. Environmental cues continuously remodel its assembly, length, composition and signaling output. Glucose deprivation, for example, promotes cilium formation in cultured cells mainly through inactivation of the nutrient sensor mTORC1 rather than through autophagy, and intriguingly it increases the proportion of ciliated cells while shortening the cilia themselves, showing that assembly and morphology are regulated separately. Glutamine deprivation tells an even more dramatic story in pancreatic ductal adenocarcinoma: the stress induces a protein called melanophilin, which accumulates at the basal body and drives ciliary regrowth, and the reassembled cilia amplify phospholipase C-gamma1 signaling in a feedforward circuit that promotes metastasis. Endogenous metabolic programs matter too. Local phosphoinositide remodeling at the centrosome recruits tau tubulin kinase 2, triggering removal of CP110, a cap that blocks axoneme extension, while cholesterol accessibility within the ciliary membrane modulates Smoothened activity in the Hedgehog pathway. Conversely, activation of SREBP1-driven lipogenesis and fatty acid synthesis suppresses ciliogenesis, in part by distorting vesicular trafficking.

Mechanical forces provide a second major class of inputs. In renal epithelial cells, urinary flow bends the apical cilium and activates calcium signaling through the polycystin-1 and polycystin-2 complex, proteins whose disruption causes polycystic kidney disease. Cilia-mediated flow sensing also regulates shear-dependent apical endocytosis in proximal tubules, linking mechanotransduction to reabsorptive function. In vascular endothelium the picture is context-dependent: cilia are retained under static or disturbed-flow conditions, whereas sustained laminar shear promotes ciliary shortening or disassembly, altering the cell’s capacity for cilia-dependent nitric oxide production. In bone, cilia deflect under dynamic fluid flow and are required for osteogenic responses and flow-induced calcium deposition in osteoblasts; in cartilage, they mediate compression-induced ATP release that regulates the extracellular matrix. Even substrate stiffness matters, since rigid matrices can suppress ciliogenesis through actomyosin tension-driven nuclear deformation and keratin-associated protein upregulation.

Oxygen tension adds a third layer of regulation. The von Hippel-Lindau protein, an E3 ubiquitin ligase that degrades HIF-alpha, localizes to the cilium and basal body, where it supports microtubule stability and cilium maintenance; loss of pVHL impairs ciliogenesis in renal-derived cells, and restoring it rescues cilium formation. Under hypoxia, HIF-2alpha can interact with the transport protein IFT88 to promote ciliogenesis and influence MEK/ERK signaling. The authors are careful to note that cilia are not universal hypoxia sensors but rather context-dependent platforms connecting oxygen-responsive signaling to localized ciliary regulation. Intracellular microRNAs form a distinct regulatory class: miR-129-3p promotes ciliogenesis by targeting CP110 and modulating actin dynamics, miR-669a-5p promotes ciliary elongation, and miR-182 and miR-183 are essential for maintaining the cone photoreceptor outer segments, which are highly specialized primary cilia required for vision.

Nowhere is the metabolic role of cilia more vivid than in the pancreatic islet. Beta-cell cilia compartmentalize receptors that regulate hormone secretion: insulin stimulation recruits insulin receptor isoform A to the cilium, and disrupting ciliary integrity attenuates downstream PI3K/Akt signaling. In Bbs4-deficient mice, ciliary dysfunction delays glucose clearance and impairs insulin secretion. Cilia-localized GPCRs add further control: free fatty acid receptor 4 and prostaglandin E2 receptor 4 enhance glucose-stimulated insulin secretion through cAMP-dependent pathways, while somatostatin receptor 3, activated by delta-cell somatostatin, lowers ciliary cAMP and triggers cilia-restricted calcium signaling that drives GLI2 nuclear translocation, tying GPCR signaling to the Hedgehog axis that maintains beta-cell identity. Recent work even identified endogenous GLP-1 receptors on beta-cell cilia, with loss of cilia blunting liraglutide-induced cAMP, calcium and insulin secretion. Endothelial cilia support the islet too, facilitating VEGFR2 internalization and downstream ERK and Akt signaling needed for vascularization and glucose delivery.

Lipid metabolism reveals the cilium’s capacity to integrate opposing signals. Cilium-dependent Hedgehog signaling suppresses the adipogenic transcriptional program, inhibiting fat cell differentiation, while lipid-raft organization at the ciliary base supports Akt-dependent adipogenesis and omega-3 fatty acid activation of ciliary FFAR4 initiates it. In muscle and brown fat, cilium-dependent Smoothened-calcium signaling activates AMPK, promoting glucose uptake and Warburg-like reprogramming. In renal epithelial cells, cilia couple extracellular cues to LKB1-AMPK signaling and mTORC1 suppression, and they sense fluid flow to promote lipophagy, mobilizing fatty acids from lipid droplets for mitochondrial beta-oxidation and ATP production. Prolonged shear stress promotes cilium-dependent mitochondrial biogenesis, increasing mitochondrial mass, DNA content, oxidative phosphorylation and ATP output through AMPK-PGC1alpha and TFEB-dependent programs, while decreased IFT88 expression with ciliary shortening is associated with impaired mitochondrial function in cisplatin-induced tubular injury.

Disease connections run deep. In the vasculature, endothelial cilia are enriched at arterial branch points where atherosclerosis preferentially develops, and loss of endothelial cilia promotes inflammatory activation and accelerates plaque progression; lipid droplet accumulation in endothelial cells sequesters palmitic acid, suppresses S-palmitoylation of ciliary proteins and impairs ciliogenesis. In reproduction, endometrial stromal cilia mediate Hedgehog-dependent epithelial-stromal communication during decidualization, and in endometriosis the cilia are shorter and decidualization responses reduced, with TGF-beta1 suppressing ciliogenesis by downregulating KIF3B. Trophoblast cilia are shortened in preeclamptic placentas, where inflammatory cytokines impair Hedgehog signaling and motility, and deleterious variants in the dynein-2 heavy chain gene DYNC2H1 have been identified in euploid recurrent pregnancy loss. In obesity, the melanocortin 4 receptor signals from hypothal neuronal cilia to suppress appetite, and MC4R-bearing cilia shorten with aging and overnutrition, reducing melanocortin sensitivity, energy expenditure and thermogenesis. Human type 2 diabetic islets show shortened beta-cell cilia and reduced beta-cell to delta-cell contacts, while classical ciliopathies such as Bardet-Biedl syndrome combine obesity with BBSome-dependent defects in mitochondrial dynamics.

Cancer adds a final, paradoxical dimension. Many tumors lose primary cilia as cells re-enter the cell cycle, yet under genotoxic or nutrient stress some cancer cells reassemble or elongate them as an adaptive survival strategy. Cisplatin-resistant pancreatic cancer cells regrow cilia through DNA damage signaling, and disrupting ciliogenesis resensitizes them; in glioblastoma, blocking ciliogenesis increases sensitivity to temozolomide and radiation. In neurodegeneration, LRRK2 mutations in Parkinson’s disease block ciliogenesis and cilia-dependent Sonic hedgehog signaling, while loss of cilia impairs clearance of damaged mitochondria in dopaminergic neurons; in Alzheimer’s models, neuronal cilia near amyloid plaques are shortened. The authors emphasize that ciliary remodeling is neither uniformly protective nor pathogenic, and that defining the cellular and environmental contexts in which it helps or harms will be essential. With live ciliary signaling reporters, tissue-specific manipulation models and spatial multi-omics now emerging, the field stands poised to move from describing these organelles to decoding, and perhaps therapeutically rewiring, the sensory-metabolic conversations they conduct.

Subject of Research: The role of primary cilia as sensory organelles and metabolic signaling hubs in physiology and disease

Article Title: Primary cilium as a sensory organelle and metabolic hub in health and diseases

Article References: Hou, H.-T., Chao, Y.-Y., Li, J.-N., Peng, I.-C., Lin, T.-C., Chiu, W.-T., Wang, C.-Y., & Tsai, S.-J. (2026). Primary cilium as a sensory organelle and metabolic hub in health and diseases. Journal of Biomedical Science, 33(1), Article 92. https://doi.org/10.1186/s12929-026-01291-6

Image Credits: AI Generated

DOI: 10.1186/s12929-026-01291-6

Keywords: primary cilium, ciliogenesis, metabolism, Hedgehog signaling, mTORC1, mechanotransduction, mitochondria, diabetes, obesity, ciliopathies, cancer, neurodegeneration

The research, led by Junrui Luo and Zhiqiang Liu at Henan Polytechnic University in Jiaozuo, China, focused on TRIM37, a member of the tripartite motif family of E3 ubiquitin ligases. These enzymes act as molecular quality-control agents, attaching small ubiquitin protein tags to specific target molecules, which then marks those targets for destruction or alters their behavior. TRIM37 had already been implicated in cancer development and in maintaining the health of centrioles, the barrel-shaped structures from which cilia are built. But its precise role in the assembly of primary cilia, a process biologists call ciliogenesis, had remained murky until now.

To probe that role, the team worked with two widely used laboratory cell lines: human retinal pigment epithelial cells, known as RPE1 cells, which are a standard model for studying cilia, and HK2 cells, a human kidney cell line. Both of these cell types normally grow prominent primary cilia when they exit the active cell cycle and settle into a quiescent state. When the researchers used RNA interference to knock down TRIM37 expression, they observed a striking effect: the fraction of cells that managed to grow cilia dropped significantly, and the cilia that did form were noticeably shorter than normal. Importantly, the team verified that this defect was not simply a side effect of altered cell cycle progression, ruling out one obvious alternative explanation.

A key clue to the mechanism came from the location of the protein itself. Using fluorescence microscopy, the researchers found that TRIM37 accumulates at the basal body, the modified centriole that anchors the cilium to the cell and serves as the gateway through which building blocks are ferried into the growing organelle. This strategic positioning suggested that TRIM37 might be doing more than just housekeeping at the cilium’s foundation. It hinted that the protein could be actively managing the local environment needed for ciliary assembly, possibly by coordinating the delivery or removal of materials at the base of the structure.

That suspicion led the investigators to autophagy, the cell’s internal recycling program. During autophagy, the cell engulfs damaged proteins and organelles in double-membraned vesicles called autophagosomes, which then fuse with lysosomes, the acidic compartments where the cargo is broken down and its components are returned for reuse. Autophagy has a complicated, two-sided relationship with cilia: at moderate levels it can help clear inhibitors of ciliary assembly, but excessive or insufficient autophagic activity can disrupt cilium formation. The researchers discovered that when TRIM37 was depleted, cells accumulated LC3-II, a lipidated form of a protein that decorates autophagosomal membranes, along with p62, also known as SQSTM1, a cargo receptor that shuttles tagged material into autophagosomes. At the same time, levels of Beclin1 and WIPI2, two proteins essential for launching the autophagy program, declined, indicating that the initiation of autophagy itself was being suppressed.

To determine whether the accumulated autophagic machinery was actually working, the team deployed bafilomycin A1, a toxin that blocks the final step of autophagy by preventing the acidification of lysosomes and the fusion of autophagosomes with them. By comparing autophagic flux, the complete throughput of the recycling pipeline, in the presence and absence of this blocker, the researchers confirmed that TRIM37-deficient cells suffered from genuine flux impairment. An independent immunofluorescence assay measuring the colocalization of LC3 with LAMP2, a lysosomal membrane protein, painted the same picture: autophagosome formation was reduced, and the fusion of autophagosomes with lysosomes was defective. In other words, losing TRIM37 did not merely slow the recycling line at one checkpoint; it jammed the entire conveyor belt.

The next question was whether TRIM37’s enzymatic activity, its ability to transfer ubiquitin tags, was required for these effects. The researchers performed functional complementation experiments, re-introducing TRIM37 variants into depleted cells and testing whether the protein could rescue normal cilium growth and autophagic balance. The results were unambiguous: only a version of TRIM37 with intact E3 ubiquitin ligase activity could restore normal ciliogenesis and autophagic homeostasis. A catalytically dead variant could not. This finding elevated TRIM37 from a passive structural component to an active enzymatic regulator whose chemical function is the linchpin of the whole process.

Having established that TRIM37’s ligase activity was essential, the team searched for the critical substrate and found it in p62, the very cargo receptor that had piled up in TRIM37-depleted cells. The experiments showed that TRIM37 directly mediates the ubiquitination of SQSTM1/p62, promoting its turnover and thereby keeping autophagic activity within the proper range. When TRIM37 is absent, p62 accumulates unchecked, and the resulting imbalance disrupts both the initiation of autophagy and the maturation of autophagosomes, which in turn sabotages the carefully choreographed process of cilium construction.

The most convincing demonstration of this regulatory axis came from a double-knockdown experiment. When the researchers simultaneously depleted both TRIM37 and SQSTM1/p62, the ciliogenesis defects and autophagic abnormalities caused by losing TRIM37 alone were substantially alleviated. This genetic epistasis experiment, a classic strategy for ordering genes into a pathway, showed that p62 acts downstream of TRIM37 and that removing the problematic accumulation of p62 can compensate for the loss of its regulator. Together, the findings define a novel TRIM37–SQSTM1/p62–autophagy signaling axis that modulates mammalian ciliogenesis, adding a new branch to the molecular regulatory network that biologists have been mapping for decades.

The implications of this work extend beyond basic cell biology. TRIM37 is already known as the gene mutated in mulibrey nanism, a rare inherited growth disorder, and its involvement in centriole maintenance and tumorigenesis has made it a subject of intense interest. By connecting this ubiquitin ligase to the autophagy machinery and to ciliary assembly, the study offers a mechanistic framework that could eventually inform research into ciliopathies, kidney disease, and cancers in which autophagy and ciliary signaling go awry. It also underscores a growing theme in modern cell biology: organelle construction is not an isolated program but an integrated process that depends on the cell’s recycling systems running at exactly the right tempo. As researchers continue to untangle the connections between ubiquitination, autophagy, and cilia, the TRIM37–p62 axis identified here provides a clear example of how a single enzymatic regulator can coordinate two fundamental cellular systems to build one of the cell’s most important sensory structures.

Subject of Research: Regulation of primary ciliogenesis by the E3 ubiquitin ligase TRIM37 through p62 ubiquitination and autophagy control

Article Title: Ciliogenesis is regulated by TRIM37 through ubiquitinating p62 and controlling the autophagy pathway

Article References: Luo, J., Zhang, X., Zhao, C., Zhang, Y., Chen, W., Wang, Z., Li, S., & Liu, Z. (2026). Ciliogenesis is regulated by TRIM37 through ubiquitinating p62 and controlling the autophagy pathway. Cellular and Molecular Life Sciences. https://doi.org/10.1007/s00018-026-06454-0

Image Credits: AI Generated

DOI: 10.1007/s00018-026-06454-0

Keywords: TRIM37, ciliogenesis, primary cilia, autophagy, p62, SQSTM1, ubiquitination, E3 ubiquitin ligase, basal body, autophagic flux, ciliopathies, cell biology

The review, led by researchers including Peiwei Liu and supported by the National Natural Science Foundation of China, assembles evidence from mass spectrometry studies of isolated cilia in mammals and unicellular organisms to sketch the first broad map of ciliary membrane lipids. Primary cilia isolated from kidney-derived MDCK cells are enriched in cholesterol and its precursor desmosterol, apparently to reinforce membrane microdomains used for signal transduction. Motile cilia from porcine olfactory epithelium tell a different story: they lack cardiolipin, carry reduced amounts of phosphatidylethanolamine, and are enriched in sulfoglycosphingolipids, an adaptive profile thought to optimize the membrane for the mechanical stress of beating. In single-celled organisms the diversity is even more striking. The flagellar membrane of Trypanosoma brucei is rich in phosphatidylethanolamine, phosphatidylserine, ceramides, and sphingomyelin, while Chlamydomonas reinhardtii concentrates phytoceramide and ergosterol, lipids suited to photophysical processes and to ordered microdomains at the flagellar base.

Perhaps the most visually compelling finding concerns phosphoinositides, phosphorylated derivatives of phosphatidylinositol that form exquisitely organized rings within the cilium. Phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate localize to the transition zone, the molecular sieve at the ciliary base, where they occupy distinct subregions along the proximal-distal axis: PI(4,5)P2 sits closer to the basal body while PI(3,4,5)P3 occupies the distal portion. Phosphatidylinositol 4-phosphate, by contrast, is excluded from the transition zone and accumulates toward the ciliary tip. This polarized geography is not decorative. PI4P directly binds the C-terminus of the signaling protein Smoothened, driving the conformational change and phosphorylation required for Hedgehog pathway activation, one of the most conserved developmental signaling systems in animals. Cholesterol, meanwhile, can itself activate Smoothened through multiple binding domains, and its release from the receptor Patched 1 upon Hedgehog ligand binding is a central step in the cascade.

The review also confronts a stubborn technical controversy. Measurements of ciliary PI(4,5)P2 have produced conflicting results for years, with some studies detecting the lipid along the entire axoneme and others confining it strictly to the transition zone. The authors trace these discrepancies to permeabilization methods, microscopy techniques, and differences between antibodies and genetically encoded biosensors. Methanol fixation followed by confocal imaging spreads the signal along the axoneme, whereas Triton X-100 permeabilization or super-resolution STED microscopy restricts it to the transition zone. The field has not even agreed on whether low-level ciliary PI(4,5)P2 is physiological or pathological. The authors call for quantitative mass spectrometry, correlative light-electron microscopy, artificial intelligence-assisted artifact correction, and absolutely calibrated biosensors to settle the debate.

Beyond phosphoinositides, the review highlights cholesterol sequestration as a surprising regulatory mechanism. Using newly developed protein probes derived from bacterial toxins, researchers compared the ciliary membrane with the adjacent plasma membrane and found that the ratio of sphingomyelin to cholesterol is far higher in cilia, with accessible cholesterol present at extremely low levels. In other words, abundant sphingomyelin chelates cholesterol into a chemically inactive state. When sphingomyelin synthesis is inhibited, the trapped cholesterol is released and Hedgehog signaling is enhanced. This finding reframes the ciliary membrane as a buffered reservoir of sterol whose availability, not just abundance, controls signal output. Lipid rafts, the cholesterol- and sphingolipid-rich microdomains concentrated at the ciliary base, serve as platforms that gather GPI-anchored and acylated proteins, amplifying signaling, although the review notes that some ciliary proteins, such as adenylyl cyclase III and INPP5E, reach their destinations even when rafts are disrupted.

Lipids also act as molecular switches for building and dismantling the organelle itself. Phosphatidylserine on the trans-Golgi network recruits Rabin8 to initiate ciliary formation. Ceramides accumulate in an apical ceramide-enriched compartment above the basal body, and the enzyme SMPD4, which generates ceramide from sphingomyelin, is required for normal ciliary length; mutations in SMPD4 cause cilia shortening and are linked to cerebellar hypoplasia. Centrosomal PI4P must be cleared by the kinase PIPKIγ to allow recruitment of ciliogenesis factors such as TTBK2, while Golgi PI4P must be maintained by PI4KB to release Rab11a for delivery to centrioles. Distal PI(4,5)P2 controls ciliary length by regulating vesicle shedding at the tip. Even diet matters: a high-fat diet upregulates the enzyme SCD1, depleting palmitic acid needed for palmitoylation of ciliary proteins such as ARL13B and RAB8A in endothelial cells, while the same fatty acid accumulates in hypothalamic neurons, suppressing autophagy and shrinking their cilia. The microbiota-derived short-chain fatty acid butyrate, in contrast, promotes ciliogenesis in hunger-regulating neurons.

The clinical payoff of this basic science is substantial. In Bardet-Biedl syndrome, mutations in more than twenty genes disrupt the BBSome, a protein complex that balances lipid and protein traffic between the plasma membrane and the cilium. Loss of bbs1 in zebrafish photoreceptors causes free cholesterol to accumulate, and BBSome deficiency allows enzymes such as phospholipase D to build up inside cilia, altering their lipid composition. The authors propose a Fish Trap model in which the transition zone admits membrane proteins freely but blocks their exit, making BBSome-mediated export the only rescue route. In autosomal dominant polycystic kidney disease, cholesterol binding is essential for localizing the ion channel polycystin-2 to cilia, and the oxysterol 7β,27-dihydroxycholesterol gates the polycystin complex. Glycosphingolipids such as glucosylceramide and lactosylceramide are markedly elevated in polycystic kidneys. Zellweger syndrome, caused by peroxisome failure, starves cilia of cholesterol because peroxisomes normally ferry sterol along microtubules to the ciliary pocket, crippling Hedgehog signaling.

Phosphoinositide disorders complete the picture. Joubert syndrome, marked by the molar tooth sign on brain imaging, frequently stems from mutations in INPP5E, the enzyme that removes PI(4,5)P2 and PI(3,4,5)P3 from cilia. Without it, these lipids accumulate, disrupting the recruitment of the cargo adaptor TULP3 and derailing Hedgehog signaling. Intriguingly, different mutations in the same gene produce different diseases: catalytic-domain variants preserve ciliary targeting and cause Joubert syndrome, whereas C-terminal truncations destroy targeting but spare enzymatic activity, producing MORM syndrome with obesity and intellectual disability. Hydrolethalus syndrome, a lethal fetal ciliopathy, arises when mutant HYLS1 fails to activate PIPKIγ and clear centrosomal PI4P, simultaneously blocking ciliogenesis and Hedgehog signaling.

Therapeutically, the lipid angle is already yielding drugs. The glucosylceramide synthase inhibitor Genz-667161 alleviated obesity, retinal degeneration, and olfactory deficits in BBS mice while restoring ciliary structure and Hedgehog activity, and the related inhibitor Venglustat slowed cyst progression in polycystic kidney disease models. The approved MC4R agonist setmelanotide remains the core targeted therapy for BBS, though it requires dermatologic monitoring, and antisense oligonucleotides have restored protein expression in preclinical models of BBS16 splicing defects. A ketogenic diet improved kidney function in the KETO-ADPKD trial, and the PI3K inhibitor LY294002 partially rescued ciliary defects in zebrafish lacking inpp5e. The authors point to three frontiers: cryo-electron microscopy to resolve lipid-protein complexes at atomic resolution, deeper study of the cilia-metabolism axis linking ciliary dysfunction to obesity and dyslipidemia, and dynamic mapping of ciliary lipid rafts with spatial lipidomics and super-resolution imaging. What was once an afterthought of cilia biology, the membrane itself, is now the most promising therapeutic frontier in the field.

Subject of Research: Lipid homeostasis of the ciliary membrane and its role in cilia function and ciliopathies

Article Title: Ciliary Membrane Lipid Homeostasis in Health and Disease

Article References: Ciliary Membrane Lipid Homeostasis in Health and Disease. (n.d.). https://doi.org/10.1002/advs.77971

Image Credits: AI Generated

DOI: 10.1002/advs.77971

Keywords: cilia, ciliary membrane, lipids, cholesterol, phosphoinositides, ceramide, Hedgehog signaling, ciliopathies, Bardet-Biedl syndrome, polycystic kidney disease, Joubert syndrome, lipid rafts

Cilia and flagella are slender, microtubule-based projections that protrude from cells and serve as both propulsion engines and signaling antennae. Because cilia lack the protein-making machinery found in the main cell body, every protein destined for the ciliary membrane must be synthesized elsewhere and ferried in. That ferrying job belongs to intraflagellar transport, a process first discovered in Chlamydomonas in 1993, in which large protein assemblies called IFT trains are hauled along the microtubule tracks of the cilium by two molecular motors: kinesin-2 drives cargo toward the ciliary tip in the anterograde direction, while cytoplasmic dynein 1b hauls it back toward the base in the retrograde direction. Disruption of this two-way traffic underlies a family of human genetic disorders known as ciliopathies, including Bardet-Biedl syndrome and polycystic kidney disease.

IFT20 occupies a special place in this machinery. Among the components of the IFT-B subcomplex, it is the only one known to also localize to the Golgi apparatus in mammalian cells, positioning it at the junction where newly synthesized membrane proteins are packaged into vesicles before being routed to the cilium. Previous work in mice showed that IFT20 is required for the proper trafficking of rhodopsin to the photoreceptor outer segment, a modified cilium, hinting that the protein acts as a cargo adaptor linking vesicle transport to the IFT system. The new study, led by Alka Kumari and Suneel Kateriya, set out to test whether that role is conserved in a lower eukaryote, using Chlamydomonas as a genetically tractable model.

The team’s first line of evidence came from co-immunocytochemistry, a technique that uses fluorescently labeled antibodies to reveal where two proteins reside within the same cell. In wild-type Chlamydomonas cells, IFT20 and channelrhodopsin-1 were found to co-localize along the entire length of the flagella, suggesting that the light-gated channel travels in the company of the IFT adaptor as it moves toward its membrane destination. This spatial overlap provided the initial indication that ChR1 is not simply diffusing into the flagellar membrane but is actively escorted by the transport machinery.

To determine whether that escort depends on the motors, the researchers turned to two well-characterized mutant strains. In the fla8 mutant, which carries a defect in the kinesin-2 motor responsible for anterograde transport, the co-localization between IFT20 and ChR1 collapsed, and both proteins accumulated near the basal body, the structure at the base of the flagellum where cargo enters. Conversely, in the dhc1b-3 mutant, which disrupts the dynein motor powering retrograde transport, the proteins stagnated at the ciliary tip, unable to return. Together, these results show that the paired movement of IFT20 and channelrhodopsin-1 is strictly motor-dependent, with kinesin-2 delivering the cargo complex into the flagellum and dynein managing its turnover and recycling, a bidirectional choreography that keeps the ciliary membrane supplied and balanced.

The study then probed the role of the BBSome, a multi-protein complex named for its connection to Bardet-Biedl syndrome that cooperates with IFT trains to move signaling receptors on and off the ciliary membrane. In the bbs1 mutant strain, which lacks a core BBSome subunit, the researchers observed a striking dissociation: IFT20 remained distributed along the flagellar length and around the basal body, but channelrhodopsin-1 was restricted to the flagella alone, with its normal distribution pattern altered. This separation implies that the BBSome contributes to the coordinated trafficking of rhodopsin cargo, consistent with earlier findings in Chlamydomonas showing that the BBSome acts as an IFT cargo required for exporting specific signaling proteins from flagella, and with work in other systems showing BBS1 involvement in retrograde trafficking of ciliary GPCRs.

To build a systems-level picture, the team performed protein interaction network analysis, which placed IFT20 at the center of a hub connecting the IFT complex, BBSome subunits, and an ancillary trafficking component called CrARL11, a Chlamydomonas member of the Arf family of small GTP-binding proteins. Arf and Arf-like GTPases are well known regulators of vesicle budding and membrane identity in the secretory pathway, and in mammalian photoreceptors the Arf family member ARF4 binds the rhodopsin C-terminus to direct it toward the cilium. In the new study, CrARL11 was shown to co-localize with IFT20 in the flagella of wild-type cells, suggesting a potential physical interaction that would echo the Arf-dependent ciliary targeting mechanisms documented in animal cells and extend them into the green algal lineage.

Perhaps the most intriguing results came from the biophysical characterization of IFT20 itself. Using fluorescence spectroscopy, the researchers found that upon binding GTP, recombinant IFT20 undergoes concentration-dependent fluorescence quenching, an indication that the nucleotide induces changes in the protein’s local environment or oligomeric state. Far-ultraviolet circular dichroism spectroscopy revealed that IFT20 adopts a predominantly alpha-helical fold, with modest spectral shifts upon GTP addition. Crucially, sequence analysis showed that IFT20 lacks both a canonical GTPase switch region and the Ras-like G-domain that defines classical small GTPases. This means that whatever GTP interaction IFT20 engages in, it operates through an atypical, non-canonical mode, distinct from the switch-based molecular toggles of Arf, Rab, and other signaling GTPases. The finding raises the possibility that IFT20 senses or responds to nucleotide state through an allosteric mechanism that has so far gone unrecognized in the IFT field, and it adds IFT20 to a growing list of IFT-B proteins, such as IFT22, that display unusual nucleotide-binding properties.

Taken together, the study sketches a conserved delivery route for ciliary membrane proteins that spans more than a billion years of evolution. A light-sensing rhodopsin in a green alga and a visual rhodopsin in a mouse photoreceptor both depend on IFT20, both interface with Arf-family GTPases, and both rely on the coordinated action of kinesin and dynein motors and the BBSome. Because Chlamydomonas is optically simple, genetically malleable, and the original source of channelrhodopsins used in neuroscience laboratories worldwide, it offers an unusually clean system for dissecting these trafficking steps. The authors also note connections to their earlier work showing that bacterial-type rhodopsins reach the Chlamydomonas eyespot and flagella through IFT-mediated transport, and that other rhodopsin family members depend on IFT88 and IFT52 for their turnover, reinforcing the picture of a general IFT-dependent routing system for the alga’s photoreceptive apparatus.

The broader implications reach into medicine and biotechnology. Ciliopathies arise when any link in this delivery chain fails, and drugs or gene therapies aimed at restoring ciliary protein trafficking need a complete parts list of the machinery. By establishing IFT20 as a central adaptor with an unexpected nucleotide-sensitive behavior, the study adds both a component and a potential regulatory mechanism to that list. For the optogenetics community, meanwhile, the work offers a reminder that the tools of modern neuroscience were borrowed from organisms that solved the problem of targeting light-sensitive channels to ciliary membranes long ago, using a transport system whose logic we are only now beginning to read. Future experiments will need to test directly whether IFT20 and CrARL11 bind one another, define the structural basis of the atypical GTP interaction, and determine how the BBSome discriminates rhodopsin cargo from other membrane proteins competing for the same molecular trains.

Subject of Research: IFT20-mediated ciliary membrane trafficking of channelrhodopsin-1 in Chlamydomonas reinhardtii

Article Title: Intraflagellar transport-20 guides the ciliary membrane trafficking of channelrhodopsin in Chlamydomonas reinhardtii

Article References: Intraflagellar transport-20 guides the ciliary membrane trafficking of channelrhodopsin in Chlamydomonas reinhardtii. (n.d.). https://doi.org/10.1007/s11033-026-12855-y

Image Credits: AI Generated

DOI: 10.1007/s11033-026-12855-y

Keywords: IFT20, channelrhodopsin-1, Chlamydomonas reinhardtii, intraflagellar transport, cilia, membrane trafficking, kinesin-2, dynein, BBSome, Arf GTPase, optogenetics, ciliopathies

The article, led by Carsten Janke of Institut Curie and Université Paris-Saclay together with Anna Akhmanova of Utrecht University and an international consortium of sixteen colleagues from institutions spanning Europe and the United States, lays out a coordinated agenda for the microtubule field. Its authors span structural biology, biophysics, cell biology, neuroscience, parasitology and computational modelling, reflecting their central conviction that progress will come not from any single discipline but from a deliberate fusion of them. The paper is punctuated by the admission that some of the most basic questions remain unexplored, including how the properties and functions of microtubules are affected by the chemical marks known as tubulin post-translational modifications, by disease-related mutations in tubulin genes, or by variation in the microtubule lattice itself.

Microtubules are hollow tubes assembled from αβ-tubulin dimers that stack into protofilaments, typically thirteen of which align laterally to form the wall of the polymer. What makes these structures extraordinary is their behaviour: individual microtubules stochastically switch between phases of growth and rapid shrinkage, a phenomenon called dynamic instability, which was first described by Tim Mitchison and Marc Kirschner in 1984. A stabilizing cap of GTP-bound tubulin at the growing plus end controls this behaviour, and its loss triggers catastrophic depolymerization. Beyond pure polymerization, microtubules can self-repair by incorporating new tubulin along damaged stretches of lattice, a property revealed in studies showing that lattice defects actually induce self-renewal and protect the polymers from destruction by molecular motors. These discoveries have transformed the view of microtubules from static beams into dynamic, self-healing machines.

The Roadmap emphasizes that microtubule function is not determined by the polymer alone but by a dense layer of regulation encoded on the tubulin subunit itself. Enzymes of the tubulin tyrosine ligase-like family decorate the unstructured carboxy-terminal tails of tubulin with modifications such as tyrosination, detyrosination, acetylation, glutamylation and glycylation, creating what has become known as the tubulin code. Recent structural work has shown, for example, that enzymes involved in polyglutamylation recognize microtubules through a quadrivalent mechanism that links microtubule geometry to the generation of localized modification patterns, and that protofilament-specific nanopatterns of these marks tune the mechanics of beating cilia. The Roadmap argues that understanding how this chemical language is written, read and erased across cellular contexts is one of the field’s most pressing challenges, particularly because disruptions in the code are now firmly linked to neurodegeneration, ciliopathies and other disorders.

Nowhere is the clinical relevance more evident than in the nervous system. Neurons depend on microtubule tracks for the long-range transport of organelles, messenger RNAs and signalling endosomes along axons and dendrites, a process powered by kinesin and dynein motor proteins. Mutations in tubulin genes such as TUBA1A cause lissencephaly, a severe developmental brain malformation, by perturbing neuronal migration, and variants in a growing list of tubulin genes underlie a spectrum of neurodevelopmental disorders collectively known as tubulinopathies. Defective axonal transport is a shared hallmark of motor neuron diseases, and recent work has shown that excessive polyglutamylation of tubulin in the brain is sufficient to drive neurodegeneration by disrupting transport, while restoring the balance of these modifications can rescue affected neurons. The Roadmap positions a systems-level understanding of these processes as essential for translating molecular insight into therapies.

The heart and the immune system tell equally compelling stories. In beating cardiomyocytes, detyrosinated microtubules act as load-bearing elements that buckle under contraction and stiffen the cell, and suppressing this modification improves cardiac function in models of heart failure. In immune cells, microtubule networks orchestrate cell shape, migration and the directed release of lytic granules by natural killer cells, while platelets, whose tubulin repertoire is dominated by the β1-tubulin isotype, rely on precisely regulated microtubule assembly during their biogenesis; mutations in the TUBB1 gene cause congenital macrothrombocytopenia and thyroid dysgenesis. The Roadmap argues that these tissue-specific roles cannot be understood piecemeal and demand approaches that bridge the molecular properties of tubulin to the physiology of whole organs.

Technological innovation sits at the centre of the proposed strategy. Cryo-electron microscopy and cryo-electron tomography now reveal microtubules and their associated proteins at near-atomic and in situ resolution, while expansion microscopy and ultrastructure expansion microscopy allow researchers to visualize centrioles, cilia and mitotic spindles in whole cells and tissues with nanometre precision. In vitro reconstitution, a tradition stretching back to the pioneering work of Tim Mitchison and Marc Kirschner and continued in landmark experiments showing that purified microtubules and motors can self-organize into asters and vortices, remains indispensable for dissecting minimal systems. These experimental approaches are increasingly paired with computational and physical models that simulate spindle assembly, microtubule network organization and chromosome movement, and the Roadmap calls for tighter coupling between modelling and experiment, including the development of virtual cells powered by artificial intelligence.

Recombinant tubulin technology is highlighted as a particularly transformative advance. For decades, biochemists were limited to native tubulin purified from brain tissue, a mixture of isotypes and modifications that obscured cause and effect. The ability to produce functional human tubulin dimers in defined isotypes, with controlled post-translational modifications, has now enabled researchers to show directly that different isotypes confer distinct dynamic properties, that detyrosination tunes microtubule stability through selective recruitment of associated factors, and that systematic mutagenesis can map the functional landscape of disease-linked variants. Combined with deep mutational scanning and artificial-intelligence-driven phenotyping, the authors argue, these tools will allow the effects of every clinically observed tubulin mutation to be predicted and tested, a goal that seemed out of reach only a few years ago.

The Roadmap also looks beyond animal cells, drawing attention to the diversity of microtubule arrays across eukaryotes, from the cortical arrays that guide plant cell morphogenesis to the subpellicular arrays of trypanosomes and the specialized mitotic machinery of parasites. Comparative studies of parasites such as Plasmodium have revealed that adaptations in tubulin sequence generate distinct microtubule architectures, mechanics and drug susceptibilities, opening avenues for species-selective therapeutics. The authors contend that this evolutionary breadth is not a curiosity but a resource: organisms that build microtubules with unusual lattices, modifications or assembly mechanisms offer natural experiments that can illuminate principles hidden in familiar model systems.

Ultimately, the Roadmap is an invitation to think bigger. Its authors conclude that the microtubule cytoskeleton will continue to inspire scientists for decades precisely because so many fundamental questions remain open: how microtubule arrays are organized and diversified in different cell types, how the tubulin code is orchestrated across space and time, how mechanical forces reshape and stabilize microtubule lattices in living cells, and how all of this integrates into the physiology of tissues and organisms in health, ageing and disease. By bridging the atomic structure of the tubulin dimer with the behaviour of mitotic spindles, migrating neurons and beating hearts, the field aims to transform its fragmented molecular knowledge into a coherent systems-level picture, one that could reshape how disorders ranging from neurodegeneration and cancer to ciliopathies and heart failure are understood and treated.

The historical depth of the field is worth appreciating. Microtubules were first visualized in the late nineteenth and early twentieth centuries, but their protein building block was only identified in the late 1960s, when colchicine-binding assays led to the isolation of what was soon named tubulin, and amino-acid analysis of sperm flagella confirmed it as the universal subunit of the polymer. The discovery of dynamic instability two decades later established that these polymers are fundamentally nonequilibrium structures, a insight that continues to shape how spindle assembly and chromosome movement are understood today.

Equally important is the cast of regulatory proteins that act on microtubules. Microtubule-associated proteins such as tau can condense on the lattice in regulated phases, altering how motors and severing enzymes engage the polymer, while enzymes that cut microtubules generate new seeds and reshape networks. Proteins like doublecortin, which is mutated in human brain malformations, illustrate how even lattice geometry itself can be read selectively, recognizing only specific protofilament numbers. These layered interactions, spanning motors, maps, severing enzymes and modifying enzymes, form the mechanistic vocabulary that any systems-level account of the cytoskeleton will ultimately need to integrate.

Subject of Research: Systems-level integration of microtubule cytoskeleton structure, regulation and function in physiology and disease

Article Title: Towards a systems-level view of the microtubule cytoskeleton and its functions in physiology and disease

Article References: Janke, C., Akhmanova, A., Bartolini, F., Bodakuntla, S., Del Bene, F., Hamel, V., Mitchison, T. J., Müller-Reichert, T., Nédélec, F., Nogales, E., Pigino, G., Roll-Mecak, A., Schiavo, G., Surrey, T., & Lansky, Z. (2026). Towards a systems-level view of the microtubule cytoskeleton and its functions in physiology and disease. Nature Reviews Molecular Cell Biology. https://doi.org/10.1038/s41580-026-01011-w

Image Credits: AI Generated

DOI: 10.1038/s41580-026-01011-w

Keywords: microtubules, tubulin code, post-translational modifications, dynamic instability, axonal transport, tubulinopathies, ciliopathies, neurodegeneration, mitotic spindle, cryo-electron tomography, in vitro reconstitution, systems biology