The research team, led by Lei Shi of the Kunming Institute of Zoology at the Chinese Academy of Sciences, exploited an unusual comparative tool: the tree shrew. This small, squirrel-like mammal is not a primate, but it sits close enough to the primate branch on the evolutionary tree to serve as an outgroup, a reference point that allows scientists to distinguish what is genuinely primate-specific from what is shared more broadly among mammals. By comparing the developing brains of humans, macaques, tree shrews and mice, the researchers could trace which genetic changes coincided with the expansion of cortical territory that defines our order.

Their first discovery was that CCNB1IP1, a gene that had little or no activity in the developing brains of other mammals, acquired expression in the primate brain. More strikingly, the level of that expression climbed progressively across the evolutionary ladder, from lower primates through monkeys to apes and finally humans. That gradient mirrored the gradient in cortical size and gyrification, the technical term for the degree of cortical folding, suggesting a quantitative relationship rather than a simple on-off switch. Statistical analysis confirmed that both the gyrification index and brain weight correlate with evolutionary divergence time across species, consistent with a gradual, cumulative process rather than a sudden leap.

Where did the increased expression come from? The answer, according to the study, lies not in the protein-coding sequence of the gene itself but in its regulatory DNA. Comparative analysis of cis-regulatory elements, the short stretches of DNA near a gene that control when and how strongly it is transcribed, revealed evolutionary changes that ratcheted up CCNB1IP1 activity. The researchers focused on two candidate regulatory regions, CRE1 and CRE2, and examined histone modifications such as H3K4me3 and H3K27ac, chemical tags on chromatin that mark active regulatory elements in human neural progenitor cells. Sequence alignments across twenty mammalian species highlighted sites that changed specifically in the human lineage, and one region, CRE1, contained sub-elements associated with the gradual expression increase during primate evolution.

To test whether the gene could actually reshape a brain, the team turned to experiment. When they overexpressed CCNB1IP1 in the embryonic mouse cortex using in utero electroporation, they saw a cascade of cellular effects: more cells entering DNA synthesis, an expanded pool of basal progenitors, and increased neuronal output. Basal progenitors are the self-amplifying stem cells of the subventricular zone, a germinal layer that is disproportionately well developed in primates and is widely regarded as the engine of cortical expansion. In the human fetal cortex, single-cell analysis showed that CCNB1IP1 is expressed most strongly in neural progenitor cells, and higher in basal progenitors than in apical progenitors at the peak of neurogenesis, precisely where extra rounds of cell division would matter most.

The mechanistic heart of the paper concerns what CCNB1IP1 actually does inside those cells. The researchers demonstrated that it represses E2F1, a transcription factor that drives the cell cycle, by targeting it for ubiquitin-mediated degradation, the cellular waste-disposal pathway that tags proteins for destruction by the proteasome. By damping E2F1, CCNB1IP1 alters the tempo of the cell cycle in neural progenitors. Slower, differently timed cycles are a long-standing theme in cortical evolution: since the classic work of Pasko Rakic and others, developmental biologists have argued that small changes in the duration and timing of progenitor cell divisions, repeated over thousands of rounds, can yield enormous differences in final neuron number. CCNB1IP1 offers a concrete molecular handle on that process.

The most dramatic experiment came next. Using CRISPR-Cas9, the team created conditional knock-in mice carrying the human CCNB1IP1 gene at the Rosa26 locus, activated in the excitatory neuronal lineage via Emx1-Cre. Ordinary mice have smooth, lissencephalic brains; their cortices lack the folds seen in humans and other gyrencephalic species. Remarkably, the knock-in mice developed recognizable cortical folding, along with an expanded cortical area, an increased proportion of upper-layer neurons, and measurable gains in cognitive performance on behavioral tests such as novel object recognition and the Morris water maze. In effect, a single regulatory change was sufficient to push a rodent brain partway toward a primate-like architecture.

The findings fit into a broader and increasingly rich picture of human brain evolution. Previous work has identified other human-specific genetic innovations that expand cortical progenitors, including the NOTCH2NL gene family, which enhances Notch signaling, and ARHGAP11B, a human-specific gene that promotes basal progenitor amplification and can even induce folding in mouse embryos. What distinguishes the new study is its emphasis on gradualism. Rather than a single human-specific invention, CCNB1IP1 shows a stepped increase in expression across the primate tree, implying that the enlargement and folding of the cortex were built incrementally, species by species, through cumulative regulatory tweaks rather than one dramatic mutation.

It is also a study in methodological craft. The authors combined laser microdissection of individual cortical layers in tree shrew embryos, bulk and single-cell transcriptomics across species, comparative genomics of regulatory elements, protein interaction assays, and transgenic mouse modeling into a single coherent argument. Data from brain organoids derived from humans, chimpanzees, bonobos and gorillas reinforced the expression gradient, and postnatal expression data from public brain atlases showed that the gene’s activity persists across multiple brain regions during development. All sequencing data have been deposited in public repositories, allowing other groups to scrutinize and extend the results.

Caveats remain, as they always do. Correlation between gene expression and cortical complexity across species does not by itself prove causation for every step of primate evolution, and the knock-in mouse, while striking, models only one component of a process that surely involved many genes. CCNB1IP1 has also been implicated in other contexts, including cancer biology, where it modulates ubiquitination of proteins such as MYCN, so its functions are likely broader than neurodevelopment alone. Still, the study delivers something rare: a gene whose evolutionary upregulation tracks the growth of the primate cortex, a plausible molecular mechanism linking it to progenitor behavior, and proof that boosting it can fold a smooth brain. In the long story of how our ancestors’ minds swelled and wrinkled into their modern form, CCNB1IP1 now stands as one of the clearest characters yet identified.

Subject of Research: Evolutionary changes in CCNB1IP1 gene regulation and their role in primate cortical expansion and folding

Article Title: CCNB1IP1 regulatory evolution underlies gradual increases in cortical size and folding in primates

Article References: Hu, T., Ma, P., Kong, Y., Tan, Y., Sun, X., Wang, J., Xiang, K., Mao, B., Wu, Q., Yi, S. V., & Shi, L. (2026). CCNB1IP1 regulatory evolution underlies gradual increases in cortical size and folding in primates. Nature Genetics. https://doi.org/10.1038/s41588-026-02773-x

Image Credits: AI Generated

DOI: 10.1038/s41588-026-02773-x

Keywords: CCNB1IP1, primate evolution, cortex, cortical folding, neurogenesis, basal progenitors, cis-regulatory elements, E2F1, ubiquitin degradation, tree shrew, knock-in mice, brain evolution

Lmx1b is a LIM homeodomain transcription factor with a well-established role in development, and nowhere is that role more consequential than in the podocytes, the specialized epithelial cells that wrap around the capillaries of the glomerulus. Podocytes form the last and most selective layer of the glomerular filtration barrier, interdigitating with one another through foot processes that are bridged by a slit diaphragm — a molecular zipper that prevents proteins such as albumin from escaping into the urine. When Lmx1b function is compromised, as it is in humans with nail-patella syndrome who develop nephropathy, podocytes fail to express a normal complement of slit diaphragm and basement membrane components, and the filtration barrier leaks. Proteinuria follows, and with it a progressive scarring process that can end in focal segmental glomerulosclerosis and kidney failure.

The researchers behind the new study set out to understand precisely how a dysfunctional LIM domain destabilizes the podocyte, and whether the cell’s own protein quality control systems are part of the problem or part of the solution. Using knock-in mice in which the LIM domains of Lmx1b carry point mutations that impair their zinc-coordinating structure, the team showed that the mutant protein is not simply inert. Instead, it appears to engage the same genomic targets as the wild-type factor while failing to execute the transcriptional program properly, producing a population of podocytes that are chronically stressed, structurally disorganized, and progressively unable to maintain their elaborate cytoarchitecture. The animals develop robust albuminuria early in life, and their glomeruli show the foot process effacement and segmental scarring characteristic of human FSGS.

What happens to protein homeostasis — proteostasis — inside such a stressed podocyte is where the story takes an unexpected turn. The authors found that the mutant Lmx1b protein itself is recognized by cellular quality control machinery and shuttled to the proteasome for destruction. In trying to clear the defective transcription factor, podocytes appear to ramp up proteasomal activity and, in doing so, deplete their degradative capacity for other client proteins. The result is a kind of proteolytic bottleneck: essential regulatory proteins and structural components that depend on tightly calibrated turnover are degraded too aggressively or incompletely, and the podocyte’s already fragile cytoskeleton and slit diaphragm lose what little stability they have left. Proteostasis, in this model, is not merely a bystander to disease but an active participant in its progression.

That mechanistic picture immediately suggested a therapeutic experiment. If excessive proteasomal degradation is accelerating podocyte collapse, then temporarily inhibiting the proteasome might relieve the pressure. The team treated the knock-in mice with established proteasome inhibitors and monitored proteinuria, glomerular histology, and the molecular state of the podocytes over time. The results were striking. Treated animals showed a marked reduction in albumin excretion compared with vehicle-treated controls, and their glomeruli retained more of the normal foot process architecture. At the molecular level, the intervention rebalanced the podocyte proteome, preserving proteins that had been prematurely depleted in untreated mutants and dampening the stress responses that accompany uncontrolled protein turnover.

The authors are careful to frame the effect as modulation rather than rescue. Proteasome inhibition does not repair the mutant Lmx1b protein, restore its DNA binding specificity, or rewrite the developmental errors that caused the disease. What it does is buy time. By slowing the degradation of proteins the podocyte still produces correctly, the treatment allows the cell to maintain a functional filtration barrier despite an upstream genetic defect that cannot be corrected. In a disease landscape where most genetic kidney disorders have no targeted therapy at all, the demonstration that a downstream, druggable node in the cellular network can meaningfully change the disease trajectory is significant.

Proteasome inhibitors are not a new class. Bortezomib and related agents are approved therapies for multiple myeloma, where they exploit the fact that malignant plasma cells, which churn out enormous quantities of immunoglobulin, are exquisitely dependent on proteasomal degradation to survive. Their safety profile in the kidney has been scrutinized because the drugs can, paradoxically, cause proteinuric injury in some patients through direct tubular toxicity. The new study therefore does not imply that proteasome inhibitors are ready for immediate use in patients with Lmx1b-related nephropathy. Dose, timing, duration, and the specific compound all matter enormously, and the therapeutic window that exists in a mouse may be narrow or absent in a human. What the work establishes is a mechanism and a proof of principle: that proteostasis manipulation can change the course of a structural kidney disease driven by a transcription factor mutation.

The broader implications reach beyond Lmx1b. A growing list of inherited and acquired glomerular diseases — including those driven by mutations in podocyte genes such as NPHS1, NPHS2, TRPC6, and components of the cytoskeleton — involve downstream consequences that are not the mutated protein itself but the misregulation of protein turnover, ER stress, and proteasomal load. If the bottleneck mechanism identified here proves generalizable, proteostasis-targeting strategies could become a shared therapeutic axis for a family of disorders that currently have few options beyond supportive care and, eventually, dialysis or transplantation. The podocyte, once considered a terminally differentiated cell with limited capacity for self-repair, is increasingly understood as a dynamic system whose fate can be influenced by manipulating quality control pathways.

Several questions will need answers before the approach moves toward the clinic. The study’s follow-up period, while sufficient to demonstrate reduced proteinuria, does not establish whether long-term proteasome modulation slows the progression to glomerulosclerosis or preserves kidney function over the lifespan of the animals. It also remains to be seen whether the benefit depends on catching the disease early, before irreversible scarring has begun, or whether established lesions also respond. And because proteasome activity is essential in virtually every tissue, any chronic regimen will have to navigate a delicate balance between protecting the podocyte and harming other organs. Nonetheless, the fact that a single course of treatment produced measurable improvement in proteinuria — a clinical endpoint that correlates strongly with long-term renal outcomes in human glomerular disease — gives the finding real translational weight.

For the researchers, clinicians, and patients following the genetics of nephropathy, the study reframes what a LIM domain mutation actually does to the kidney. The damage is not a simple absence of a transcription factor; it is a cascade in which a defective protein disrupts gene expression, destabilizes the cell, and then drags the entire proteostasis network into a self-defeating overdrive. Interrupting that final stage of the cascade, with a drug class that already exists, turns a developmental catastrophe into a partially manageable physiological problem. It is a reminder that in complex genetic diseases, the most actionable therapeutic target is often not the broken gene itself, but the cellular systems that fail in its shadow — and that sometimes, the counterintuitive intervention of slowing down the cell’s own cleanup machinery is exactly what a stressed tissue needs to keep functioning.

Subject of Research: Proteasome inhibition as a therapy for proteinuria in mice with dysfunctional Lmx1b LIM domains

Article Title: Proteasome inhibition alleviates proteinuria in Lmx1b knock-in mice with dysfunctional LIM domains

Article References: Hermens, J., Lucke, L., Pieles, O., Maier, O., Othmen, H., Burghardt, T., Schmidt, A., Moser, M., Madej, M. G., Schwartz, U., Zaparty, M., & Witzgall, R. (2026). Proteasome inhibition alleviates proteinuria in Lmx1b knock-in mice with dysfunctional LIM domains. Nature Communications, 17(1), Article 9731. https://doi.org/10.1038/s41467-026-77485-1

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77485-1

Keywords: proteasome inhibition, proteinuria, Lmx1b, LIM domains, glomerulus, podocytes, focal segmental glomerulosclerosis, kidney disease, proteostasis, knock-in mice, genetic kidney disease, renal filtration barrier