Mouse Brain Study Uncovers NDN and IDE as Candidate Fragile X Biomarkers

Researchers scanning the brains of fragile X syndrome model mice have identified two proteins that rise sharply in a key motor and cognitive brain region, offering the field something it has long lacked: measurable molecular markers that could track the disease in preclinical drug studies. The study, published in Frontiers in Neuroscience, combined RNA sequencing of the caudate putamen with protein-level validation across several brain regions in seven-week-old Fmr1-knockout mice, the standard animal model for fragile X syndrome, a monogenic cause of autism spectrum disorder.

The team, led by Manuela Prokesch of Scantox Neuro GmbH in Austria, sequenced RNA from the caudate putamen, also known as the dorsal striatum, of five male Fmr1-KO mice and five male C57BL/6J wild-type controls. Out of 24,298 expressed genes, differential expression analysis flagged 50 genes as significantly dysregulated: 13 upregulated and 37 downregulated, with an adjusted p-value threshold of 0.10. The strongest downregulated gene was Fmr1 itself, a result the authors describe as an internal positive control confirming the model worked as expected.

Two genes stood out at the top of the upregulated list: necdin (Ndn) and insulin degrading enzyme (Ide). Quantitative PCR confirmed that both were expressed at roughly twofold higher levels in the caudate putamen of the knockout mice compared with controls, with p-values below 0.001. The researchers then moved from transcripts to proteins, using immunofluorescent labeling to measure where and how strongly the NDN and IDE proteins appear in the brain.

The protein data matched the sequencing results. NDN immunoreactive area and object size were significantly increased in the caudate putamen of the knockout mice, with p-values of 0.004 and 0.006 respectively. The signal extended beyond the striatum: NDN area and object size were also elevated in the hippocampus, though less strongly, and object size increased in the brainstem. Notably, the density and intensity of NDN-positive objects did not change, suggesting the alteration reflects more NDN per labeled structure rather than more NDN-producing cells.

To find out which cell types carry the extra NDN, the team ran a quadruple labeling experiment marking NDN alongside tyrosine hydroxylase for catecholaminergic neurons, parvalbumin for GABAergic interneurons, and choline acetyltransferase for cholinergic neurons. NDN overlapped only with cholinergic markers, and only partially. In the caudate putamen, the proportion of NDN-positive cells that were also cholinergic dropped in the knockout mice, while the proportion of cholinergic cells that were NDN-positive rose. The authors note that NDN expression in cholinergic neurons had not been described before, and that the identity of the remaining NDN-positive cells remains an open question.

The IDE findings connected the transcriptomic data to a measurable blood-based readout. Because IDE protein is central to insulin signaling, the researchers measured plasma insulin with an ELISA and found it significantly reduced in the Fmr1-KO mice, at a p-value of 0.05. The authors are careful about causation: upregulated Ide coincided with lower insulin, suggesting FMRP may influence insulin signaling, but further work is needed to validate a causal relationship. They point to earlier findings of reduced insulin in another fragile X mouse model and, intriguingly, in serum from fragile X patients, making altered insulin metabolism a potentially translational pathology with an easily accessible readout.

Beyond the two headline candidates, the study mapped broader pathway-level changes. Gene Ontology enrichment pointed to myelin sheath biology, and gene set enrichment analysis highlighted tumor necrosis factor-alpha signaling via NF-kappa B, an inflammatory pathway. WikiPathways analysis corroborated the myelin findings and flagged calcium regulation, the Nrf2 oxidative stress pathway, and other inflammation-related programs. Yet when the team checked myelin basic protein directly, the mRNA downregulation did not carry through to the protein: MBP levels were unchanged across the caudate putamen, cortex, hippocampus, and corpus callosum, suggesting the protein remains stable despite the transcript signal.

The study also documented neuroinflammation markers. GFAP-positive object size, an indicator of astrocyte activation, increased in the cortex and hippocampus, and Iba1 labeling revealed significantly increased immunoreactive area and object density of activated microglia in the caudate putamen. The authors note this contrasts with at least one prior study reporting no microglial activation or even reduced Iba1-positive cell numbers in young knockout mice, underscoring that the role of glial activation in fragile X remains unsettled.

The authors are unusually candid about the limits of their work. With five mice per group, the sample sits at the edge of recommended sizes for RNA sequencing, and they explicitly frame Ndn and Ide as hypothesis-generating candidates rather than definitive biomarkers, calling for larger, properly powered cohorts. The seven-week age point captures only a very early stage, so the changes may be age-specific. Molecular differences cannot yet be linked to specific behaviors, and because each animal’s left hemisphere was used for RNA work and the right for histology, potential functional asymmetries were not addressed. Clinical translation of NDN also faces a structural barrier: the protein acts intracellularly, with no current evidence it is secreted into blood or cerebrospinal fluid, whereas IDE has been measured in the cerebrospinal fluid of both mice and humans.

What makes the findings compelling is their position in the translational pipeline. Fragile X syndrome arises when more than 200 CGG repeats in the FMR1 gene silence production of FMRP, a regulator that touches up to four percent of mammalian brain mRNAs. Existing molecular markers, from metabotropic glutamate receptors to GABA targets, have not produced convincing results in patients. NDN carries particular biological weight: excess levels of it are implicated in chromosome 15q11-q13 duplication, one of the most common cytogenetic abnormalities in autism, and it drives spine formation and hyperexcitability in cortical neurons. Whether disease-modulating drugs can shift NDN and IDE expression, and whether doing so alleviates symptoms, is the question the authors say future research must now answer.

Subject of Research: Candidate protein biomarkers NDN and IDE identified in the brains of fragile X syndrome model mice

Article Title: Gene expression analysis of Fmr1-KO mouse caudate putamen reveals new preclinical biomarkers for fragile X syndrome

Article References: Sharghi, S., Flunkert, S., Daurer, M., Breznik, L., Chagnaud, B. P., & Prokesch, M. (2026). Gene expression analysis of Fmr1-KO mouse caudate putamen reveals new preclinical biomarkers for fragile X syndrome. Frontiers in Neuroscience, 20, Article 1833413. https://doi.org/10.3389/fnins.2026.1833413

Image Credits: AI Generated

DOI: 10.3389/fnins.2026.1833413

Keywords: fragile X syndrome, Fmr1-KO mice, NDN, IDE, RNA sequencing, caudate putamen, biomarkers, autism spectrum disorder, insulin signaling, neuroinflammation, preclinical research, cholinergic neurons