Lysosomal Fission Gene MROH1 Governs Thyroid Hormone Levels in Mice

Thyroid hormones are the body’s master metabolic conductors, setting the pace of everything from heart rate to cholesterol metabolism. Their production is an intricate, multi-step process: follicular cells in the thyroid gland synthesize thyroglobulin, iodinate it within the follicular lumen, and then reclaim it by endocytosis before lysosomal enzymes cleave the hormone free for release into the bloodstream. Because that final liberation step depends on the lysosome, scientists have long suspected that the membrane dynamics of this organelle—its endless cycles of fusion and fission—might matter for endocrine health. A new study from the University of Tsukuba, published in Health Science Reports, now provides the first in vivo evidence that a conserved lysosomal fission factor called MROH1 is essential for maintaining normal circulating thyroid hormone levels in mammals.

MROH1, also known as HEATR7A, first drew attention from work on the humble roundworm Caenorhabditis elegans. Researchers had identified the worm protein HPO-27, a HEAT repeat protein, as a critical mediator of lysosomal membrane scission. When HPO-27 is lost, the worm’s tissues fill up with aberrant tubular lysosomal networks instead of the usual discrete vesicles. The mammalian homologue MROH1 was subsequently shown to interact with the WASH–actin machinery to regulate lysosomal fission, positioning it as a gatekeeper of lysosomal integrity. What remained entirely unknown was whether this housekeeping role had any physiological relevance for hormone-producing tissues, and no prior study had ever connected MROH1 to thyroid biology.

A crucial clue came from human gene expression databases. Transcriptomic profiling across the GTEx portal and the Human Protein Atlas classifies MROH1 as markedly tissue-enhanced in the thyroid gland relative to other organs—striking preferential expression for a core component of the lysosomal fission machinery. That observation prompted the Tsukuba team, led by Nami Ohuchi and Yoshinori Osaki under the supervision of Hitoshi Shimano, to generate a global MROH1 knockout mouse line. Sperm carrying the Mroh1(tm1a(KOMP)Wtsi) allele were obtained from the European Mouse Mutant Archive, floxed mice were created by Flpe-mediated excision of the gene trap cassette, and ubiquitous deletion of exons 5 through 7 was achieved by crossing with Ayu1-Cre transgenic mice. Cre-negative floxed littermates served as wild-type controls throughout.

The knockout strategy worked as designed: quantitative RT-PCR confirmed the complete absence of Mroh1 transcripts in the thyroids of the deficient animals. To address possible genetic compensation, the team also measured Mroh2a, a close paralog of Mroh1, and found only a non-significant upward trend in its expression. Metabolic phenotyping then produced a subtle but revealing picture. Food intake and body length were unchanged between genotypes, but the knockout mice gained weight gradually, reaching a statistically significant difference by 24 weeks of age. The weight gain occurred independently of altered skeletal growth, suggesting systemic metabolic adaptations rather than a primary effect on development.

The endocrine phenotype emerged clearly when the researchers measured serum hormones. Free thyroxine (FT4) was significantly reduced in knockout mice at both 3 and 6 months of age, while free triiodothyronine (FT3) was significantly lower by 6 months. Serum TSH showed only a non-significant increasing trend—a puzzling feature the authors openly acknowledge, since standard rodent thyroid economy would predict a compensatory TSH rise when circulating hormones fall. Whether the discrepancy reflects altered central feedback, differences in hormone metabolism, or transport effects remains unresolved and will require functional assessment of the hypothalamic–pituitary axis in future work.

Consistent with the well-known clinical association between hypothyroidism and dyslipidemia, total serum cholesterol was comparable at 3 months but significantly elevated in the knockout animals by 6 months, while triglycerides remained unchanged. Importantly, markers of liver and kidney function, blood glucose, and creatine kinase levels were all unremarkable, and histological examination of the lung, brain, liver, and skeletal muscle revealed no overt abnormalities. The thyroid, in other words, stood out as the principal site of pathological consequence—an outcome that mirrors MROH1’s preferential expression in that gland and strengthens the causal narrative.

To understand the structural basis of the hormone deficit, the team turned to histology. At 3 months of age, thyroid morphology in the knockout mice looked essentially normal. By 6 months, however, the gland showed clear architectural remodelling: follicular area was reduced, and follicles were progressively replaced by interstitial cells and adipocyte-like structures. Quantification of the follicle area ratio showed a downward trend that narrowly missed statistical significance, but a more sensitive analysis of individual follicles—over 1,700 follicles measured across both groups—revealed a statistically significant shift towards smaller follicle sizes in the knockout mice (p = 0.0095, Kolmogorov–Smirnov test). The gland, in effect, was quietly remodelling itself at the cellular level long before gross pathology would appear.

Molecular profiling pointed to the transcriptional roots of the defect. Expression of Nkx2-1 and Foxe1, the master regulators of thyroid differentiation, was significantly reduced in the knockout thyroids, with Pax8 and Hhex trending in the same direction. Concurrently, thyroglobulin—the essential precursor of thyroid hormone synthesis—was significantly downregulated, as was Slc16a2 (Mct8), the thyroid hormone transporter. Other genes involved in hormone production, including Tshr, Slc5a5 (Nis), and Slc26a7, showed decreasing trends. This coordinated loss of lineage-defining transcription factors and their functional target genes suggests that MROH1 is required for the long-term maintenance of follicular cell identity and differentiated function, not merely for organelle housekeeping.

What the study did not find is equally telling. Given MROH1’s established role in lysosomal fission and the dependence of hormone release on lysosomal proteolysis, the team fully expected to find impaired lysosomal function. Instead, immunoblotting showed that protein levels of LAMP2, cathepsin L, and cathepsin D were unchanged, and two independent assays—a live-cell fluorogenic substrate assay in primary thyroid cells and an enzymatic activity assay in whole-tissue lysates—revealed no significant difference in cathepsin L activity. The fundamental capacity for lysosomal degradation appeared intact. The authors caution that bulk assays may simply be too blunt to detect localized defects in lysosomal membrane trafficking, and that subtle, progressive endo-lysosomal trafficking failures could still accumulate over time, as seen in the delayed thyroid pathology of lysosomal storage disease models such as cystinosis.

The broader significance is twofold. First, the work establishes MROH1 as a genetic factor for thyroid hormone homeostasis in vivo, with a phenotype distinct from the profound hypothyroidism and goitre seen in complete MCT8 or thyroglobulin knockouts—the partial downregulation of thyroid genes produces a milder, late-onset endocrine defect without gland enlargement. Second, it extends the emerging view that lysosomal membrane dynamics are not merely cellular plumbing but active participants in tissue-level physiology, with the caveat that the precise mechanistic bridge from MROH1 deficiency to transcriptional downregulation remains to be charted. Because the mice carry a congenital deletion yet show a late-onset phenotype, extrathyroidal contributions cannot be excluded. Tissue-specific knockout models, currently the next step for the Tsukuba group, should disentangle the global versus thyroid-specific roles of MROH1—and may ultimately clarify whether subtle lysosomal trafficking defects in humans contribute to mild hypothyroidism, thyroid remodelling, and the cardiovascular risk that follows from unexplained hypercholesterolemia.

Subject of Research: The role of the HEAT repeat protein MROH1 in lysosomal fission and thyroid hormone homeostasis in mice

Article Title: The HEAT Repeat Protein MROH1 Deficiency Leads to Reduced Circulating Thyroid Hormone Levels in Mice

Article References: Ohuchi, N., Osaki, Y., Nakagawa, Y., Miyamoto, T., Araki, M., Mizunoe, Y., Matsuda, T., Murayama, Y., Sugano, Y., Iwasaki, H., Matsuzaka, T., Sekiya, M., & Shimano, H. (2026). The HEAT Repeat Protein MROH1 Deficiency Leads to Reduced Circulating Thyroid Hormone Levels in Mice. Endocrinology, Diabetes & Metabolism, 9(5), Article e70348. https://doi.org/10.1002/edm2.70348

Image Credits: AI Generated

DOI: 10.1002/edm2.70348

Keywords: MROH1, thyroid hormone, lysosomal fission, hypothyroidism, hypercholesterolemia, knockout mice, Nkx2-1, Foxe1, thyroglobulin, cathepsin L, endocrine physiology, University of Tsukuba