Daily Endocrinology Research Analysis
Three papers stand out today in endocrinology and metabolism: a Nature Communications study reveals a lysosomal pathway for adipocyte lipolysis that predominates during prolonged fasting; a PNAS paper uncovers region- and cell type–specific thyroid hormone signaling in the brain driven by differential D2 ubiquitination and USP33 activity, challenging assumptions behind L‑T4 monotherapy; and a JCEM cohort demonstrates the power of long-read sequencing to resolve CYP21A1P/CYP21A2 chimeras in 21-hy
Summary
Three papers stand out today in endocrinology and metabolism: a Nature Communications study reveals a lysosomal pathway for adipocyte lipolysis that predominates during prolonged fasting; a PNAS paper uncovers region- and cell type–specific thyroid hormone signaling in the brain driven by differential D2 ubiquitination and USP33 activity, challenging assumptions behind L‑T4 monotherapy; and a JCEM cohort demonstrates the power of long-read sequencing to resolve CYP21A1P/CYP21A2 chimeras in 21-hydroxylase deficiency, refining diagnosis and genotype–phenotype correlations.
Research Themes
- Tissue- and cell-specific endocrine signaling mechanisms
- Metabolic adaptations to fasting and lipid mobilization
- Genomic diagnostics and precision endocrinology
Selected Articles
1. Non-canonical lysosomal lipolysis drives mobilization of adipose tissue energy stores with fasting.
This study identifies a lysosomal lipolysis program in adipocytes that depends on LAL (LIPA) and MiT/TFE transcription factors and predominates during prolonged fasting, whereas canonical neutral lipases (e.g., ATGL) dominate acute adrenergic responses. The mechanism was demonstrated using pharmacologic and genetic perturbations in mice and in murine/human adipocyte and adipose explants.
Impact: Revealing a dominant lysosomal pathway for lipolysis during prolonged fasting reframes a core assumption in metabolic physiology and opens new therapeutic avenues to modulate energy mobilization.
Clinical Implications: Although preclinical, targeting the MiT/TFE–LAL axis could modulate fasting responses, adiposity, and metabolic disease; biomarkers of lysosomal lipolysis may inform dietary or pharmacologic interventions.
Key Findings
- Identified a lysosomal lipolysis program in adipocytes that depends on LAL (LIPA) and MiT/TFE transcription factors.
- Demonstrated with pharmacologic/genetic tools in mice and murine/human adipocyte and adipose explant cultures.
- Proposed a model where canonical neutral lipases dominate acute fasting, while lysosomal lipolysis dominates prolonged fasting.
Methodological Strengths
- Multi-system validation across mice, murine and human adipocytes, and adipose explants
- Mechanistic dissection using both pharmacological inhibition and genetic perturbation
Limitations
- Predominantly preclinical with no human in vivo clinical outcome data
- Metabolic consequences beyond lipolysis flux (e.g., glycemia, energy expenditure) not fully characterized in humans
Future Directions: Test pharmacologic activators/inhibitors of MiT/TFE–LAL in metabolic disease models and quantify lysosomal lipolysis in humans under fasting or dietary interventions.
Physiological adaptations to fasting enable humans to survive for prolonged periods without food and involve molecular pathways that may drive life-prolonging effects of dietary restriction in model organisms. Mobilization of fatty acids and glycerol from adipocyte lipid stores by canonical neutral lipases, including the rate limiting adipose triglyceride lipase (Pnpla2/ATGL), is critical to the adaptive fasting response. Here we discovered an alternative mechanism of lipolysis in adipocytes involving a lysosomal program. We functionally tested lysosomal lipolysis with pharmacological and genetic approaches in mice and in murine and human adipocyte and adipose tissue explant culture, establishing dependency on lysosomal acid lipase (LIPA/LAL) and the microphthalmia/transcription factor E (MiT/TFE) family. Our study establishes a model whereby the canonical pathway is critical for rapid lipolytic responses to adrenergic stimuli operative in the acute stage of fasting, while the alternative lysosomal pathway dominates with prolonged fasting.
2. Variable transduction of thyroid hormone signaling in structures of the mouse brain.
Using T3-responsive reporter mice, the authors show that L‑T4 fails to uniformly restore brain T3 signaling. Tanycytes sustain D2 via USP33-mediated deubiquitination to amplify T3 signaling, while astrocytes limit D2 through ubiquitination, explaining regional heterogeneity and limitations of L‑T4 monotherapy.
Impact: Reveals cell-specific control of brain T3 production that challenges the assumption that systemic L‑T4 normalizes tissue-level TH action, providing a mechanistic basis for personalized hypothyroidism therapy.
Clinical Implications: Supports consideration of tailored strategies (e.g., combination L‑T4/L‑T3 or novel modulators of D2/USP33) and motivates development of biomarkers for tissue-specific TH action.
Key Findings
- L‑T4 did not uniformly restore T3 signaling in brain regions of TH Action Indicator mice.
- Cell type–specific responses: tanycytes sustain D2 and T3 signaling via USP33-mediated deubiquitination, whereas astrocytes limit D2 via ubiquitination.
- Intracerebroventricular T4 elicited stronger T3 signaling in mediobasal hypothalamus than cortex, highlighting regional heterogeneity.
Methodological Strengths
- Use of T3-responsive luciferase reporter mice to map in vivo TH signaling
- Mechanistic validation with USP inhibition and USP33 knockout, plus complementary astrocyte/tanycyte culture models
Limitations
- Translational relevance to human brain TH signaling requires clinical validation
- Behavioral or cognitive outcomes under different replacement strategies were not assessed
Future Directions: Test combination L‑T4/L‑T3 or D2/USP33 modulators in preclinical models and evaluate tissue-level TH biomarkers and symptom outcomes in clinical trials.
L-thyroxine (L-T4) monotherapy is the standard treatment for hypothyroidism, administered daily to normalize TSH levels. Once absorbed, T4 is converted to T3 to alleviate most symptoms. However, this treatment abnormally elevates plasma T4 levels in over 50% of patients. Using L-T4-treated Thyroid Hormone (TH) Action Indicator mice, which express a T3-regulated luciferase (Luc) reporter, we examined whether these T4 elevations disrupt TH signaling. Hypothyroid mice exhibited reduced Luc expression across brain regions, and L-T4 treatment failed to restore T3 signaling uniformly. There was also variability in the activity of type 2 deiodinase (D2), the enzyme that generates most brain T3. Intracerebroventricular T4 administration achieved higher elevation of Luc expression in the mediobasal hypothalamus compared to the cortex, and studies on cultured cortical astrocytes and hypothalamic tanycytes revealed cell-type-specific responses to T4. In tanycytes, exposure to T4 sustained D2 activity, leading to progressive T3 signaling, whereas in astrocytes, T4 exposure triggered a drop in D2 activity, limiting T3 production through a ubiquitin-dependent, self-limiting mechanism. The sustained D2 activity in tanycytes was linked to rapid deubiquitination by USP33, as confirmed using a ubiquitin-specific protease (USP) pan-inhibitor and USP33 knockout mice. In conclusion, the brain's response to L-T4 treatment is heterogeneous, influenced by cell-specific regulation of D2-mediated T3 production. While cortical astrocytes exhibit limited T3 signaling due to D2 ubiquitination, tanycytes coexpressing USP33 amplify T3 signaling by rescuing ubiquitinated D2 from proteasomal degradation. These findings provide mechanistic insights into the limitations of L-T4 therapy and highlight the need for tailored approaches to managing hypothyroidism.
3. Chimeric CYP21A1P/CYP21A2 Genes in 21-Hydroxylase Deficiency Detected by Long-Read Sequencing and Phenotypes Correlation.
In 869 patients with 21‑OHD, long-read sequencing identified 10 CYP21A1P/CYP21A2 chimera types (including a novel CH‑10), with CH‑1 being most common. Haplotype analysis suggested no single founder effect, and in vitro assays confirmed very low residual activity for novel variants, supporting strong genotype–phenotype correlations.
Impact: Demonstrates clinical utility of long-read sequencing to resolve complex CYP21A2/CYP21A1P rearrangements, expanding the variant spectrum and strengthening genotype–phenotype links in CAH.
Clinical Implications: Supports integrating long-read sequencing into diagnostic workflows for CAH to improve detection of chimeras/deletions, inform genetic counseling, and guide individualized management.
Key Findings
- Identified 10 CYP21A1P/CYP21A2 chimera types across 119 alleles using long-read sequencing, including a novel CH‑10.
- CH‑1 was the most prevalent chimera (50.4%). Haplotype analysis revealed diverse haplotypes, arguing against a founder effect.
- In vitro assays showed very low residual activity for novel variants (e.g., p.L100P, p.L301V), aligning with simple virilizing phenotypes; genotype–phenotype consistency was ~79–84%.
Methodological Strengths
- Large single-center cohort with comprehensive long-read sequencing and long-range PCR
- Combined haplotype analysis and in vitro functional assays to validate novel variants
Limitations
- Retrospective single-center design may limit generalizability
- Clinical outcome data beyond genotype–phenotype correlation were limited
Future Directions: Broaden implementation of long-read sequencing across diverse populations and link genetic architecture with treatment response and long-term outcomes.
CONTEXT: 21-Hydroxylase deficiency (21-OHD) is caused by pathogenic variants in CYP21A2. High homology between CYP21A2 and its pseudogene CYP21A1P causes mismatches, leading to deletions and CYP21A1P/CYP21A2 chimeras. OBJECTIVE: To detect chimeric CYP21A1P/CYP21A2 in 21-OHD patients using long-read sequencing (LRS) and analyze genotype-phenotype correlations. METHODS: From 2015 to 2023, 869 21-OHD patients were enrolled at Peking Union Medical College Hospital, with 113 identified harboring CYP21A2 large deletion. Long-range PCR and LRS were used to identify the types of CYP21A1P/CYP21A2 chimeric. Haplotype analysis explored founder effects, and in vitro assays assessed the functional impact of novel mutations. Clinical data were retrospectively collected and patients were classified into 4 groups based on genotypes and residual enzyme activity to study genotype-phenotype correlations. RESULTS: Ten types of chimeric CYP21A1P/CYP21A2 genes were identified across 119 alleles, including a novel type, CH-10. The most common, CH-1, accounted for 50.4% of all types. Haplotype analysis of 24 SNPs within CYP21A1P/CYP21A2 CH-1 revealed 25 haplotypes, with haplotype 11 being the most prevalent. Variants p.L100P and p.L301V of CYP21A2 showed enzyme activities of 1.36 ± 0.44% or 1.63 ± 0.19% for 17-hydroxyprogesterone to 11-deoxycortisol, and 1.36 ± 0.58% or 3.99 ± 1.09% for progesterone to 11-deoxycorticosterone, respectively, linked to the simple virilizing type. Genotype-phenotype consistency rates were 78.6% to 84% across the 4 groups. CONCLUSION: LRS is a comprehensive genetic testing method for 21-OHD patients, effectively detecting both CYP21A2 gene variants and CYP21A1P/CYP21A2 chimeric gene types. This study expands the CYP21A2 variant spectrum by identifying a novel chimera. Haplotype analysis revealed diverse haplotypes for each chimeric gene type, suggesting the absence of a common founder effect. The strong genotype-phenotype correlation aids genetic counseling and supports personalized treatment.