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

The finding adds a striking new dimension to a long-running discussion in cell biology about how organelle membranes are mended. Over the past decade, researchers have identified several repair pathways that respond to lysosomal damage, many of them involving the protein galectin-3, which binds exposed sugars on the inner surface of the ruptured lysosome, and the lipid PI4P, produced by phosphatidylinositol 4-kinases. ESCRT machinery, a membrane-scission apparatus best known for its roles in cytokinesis and viral budding, is then recruited to seal small holes in the lysosomal limit membrane. The new study demonstrates that this repair landscape is more complex than previously appreciated, because it also draws on lipid signaling that originates on the endoplasmic reticulum, an organelle physically distinct from the wounded lysosome.

The endoplasmic reticulum, or ER, is the cell’s largest membrane network, an interconnected system of tubules and sheets that reaches nearly every corner of the cytoplasm. It is the site where lipids are synthesized and where calcium is stored, and it maintains intimate contact sites with endosomes and lysosomes. Those contacts allow the exchange of lipids and ions and coordinate processes such as organelle fission and autophagy. The notion that the ER participates in lysosomal repair fits naturally into this picture of close cross-talk, but the new work identifies a specific molecular mechanism: a spatially defined pool of PI3P that appears on ER membranes in response to lysosomal injury.

Phosphatidylinositol phosphates, or phosphoinositides, are minor lipid components of cellular membranes that act as positional labels, telling proteins where in the cell they should act. Different phosphoinositides decorate different compartments: PI4P marks the Golgi apparatus and late endosomes, PI4,5P2 marks the plasma membrane, and PI3P is characteristic of early endosomes and, notably, of autophagic structures. The key enzyme that generates PI3P for autophagy is PIK3C3, also known as VPS34, a phosphatidylinositol 3-kinase that is activated during starvation to drive the growth of autophagosomes. Because DFCP1 was already known as an autophagy-associated protein that binds PI3P and decorates nascent autophagosome precursors, the authors’ discovery that it operates in lysosomal repair connects two processes, autophagy and membrane repair, that were largely studied in isolation.

Using cell biological and imaging approaches, Radulovic and colleagues observed that when lysosomes are damaged, PI3P accumulates on discrete ER microdomains rather than being distributed uniformly across the reticular network. These PI3P-positive ER zones then serve as docking platforms for DFCP1, whose recruitment depends on its PI3P-binding activity. In cells lacking PIK3C3, the ER pool of PI3P is not formed, DFCP1 fails to be recruited to damaged lysosomes, and the repair of lysosomal membranes is compromised. Conversely, manipulations that promote PI3P formation support DFCP1 recruitment and improve repair outcomes. The experiments trace a clear causal chain from lysosomal injury, through ER-localized lipid signaling, to the assembly of a repair-competent structure at the wounded organelle.

The functional consequences of this pathway are significant for the health of the cell. Unrepaired lysosomes lose their acidic lumen, release hydrolases into the cytosol, and can ultimately rupture, a process that triggers inflammatory signaling and, in severe cases, a form of programmed cell death called lysosomal cell death. By ensuring that damaged lysosomes are rapidly resealed, the PI3P-DFCP1 axis helps preserve organelle integrity and prevents the leakage of degradative enzymes. The study also places DFCP1 in a new functional context: rather than acting only as an autophagy marker, it emerges as an active participant in membrane homeostasis, an ATPase whose enzymatic activity and lipid binding are harnessed for the physical task of restoring membrane continuity.

The discovery also raises intriguing mechanistic questions that the field will now pursue. How is PIK3C3 activated on ER microdomains specifically after lysosomal damage, and what upstream signal conveys the news of a rupture from the lysosome to the ER? Existing repair pathways appear to be organized in parallel modules, with galectins, PI4P, and ESCRT acting at different stages of the response, and it will be important to determine how the ER-derived PI3P-DFCP1 pathway is integrated with them. One possibility is that DFCP1 facilitates the recruitment or function of ESCRT complexes; another is that it contributes lipid or membrane resources from ER-lysosome contact sites to the repair process. The physical proximity of the ER to endolysosomal organelles makes both scenarios plausible and testable.

Beyond its cell biological interest, the work has potential implications for human disease. Lysosomal dysfunction is a hallmark of numerous disorders, including lysosomal storage diseases, many common neurodegenerative conditions such as Parkinson’s and Alzheimer’s disease, and disorders of autophagy. Pathogenic agents, from silica crystals to cholesterol crystals to certain bacteria, damage lysosomes as part of their life cycle or disease mechanism. If the PI3P-DFCP1 repair pathway proves to be conserved and essential in human tissues, it may represent a point of vulnerability or a therapeutic target: boosting the pathway could strengthen cells against lysosomal stress, whereas pathogens or cancer cells might be sensitized to lysosome-directed therapies by disabling it.

For researchers who have followed the lysosome repair field, the study is a reminder that organelle quality control is a whole-cell endeavor, coordinated among compartments that communicate through lipids, proteins, and physical contacts. The ER, often treated in textbooks as a passive factory for proteins and lipids, now appears to be an active sentinel that monitors and supports the integrity of its neighboring organelles. As imaging technologies and lipidomics methods continue to improve, more such inter-organellar rescue pathways are likely to come to light, and DFCP1-containing ER microdomains may prove to be just the first example of a membrane network acting as a first responder for the cell’s damaged endomembrane system.

Subject of Research: ER-localized PI3P signaling and DFCP1 recruitment in lysosomal membrane repair

Article Title: DFCP1-containing ER microdomains mediate lysosomal membrane repair

Article References: Radulovic, M., Pust, S., Kournoutis, A., Chen, D., Giner, M. I., Liang, Q., Phuyal, S., Böddeker, T. J., McCarron, K., Herrmann, E., Rose, K., Schultz, S. W., Brech, A., Hurley, J. H., Bussi, C., Bonet-Ponce, L., Gutierrez, M. G., Raiborg, C., & Stenmark, H. (2026). DFCP1-containing ER microdomains mediate lysosomal membrane repair. Nature Cell Biology. https://doi.org/10.1038/s41556-026-02062-z

Image Credits: AI Generated

DOI: 10.1038/s41556-026-02062-z

Keywords: lysosome, membrane repair, DFCP1, PI3P, PIK3C3, endoplasmic reticulum, autophagy, phosphoinositides, organelle crosstalk, cell biology, ESCRT, lysosomal damage