Embryonic Alcohol Exposure Disrupts the Ubiquitin–Proteasome System
Key findings
- Massachusetts General Hospital researchers used a zebrafish model of embryonic alcohol exposure to explore whether disruptions to the ubiquitin-proteasome system contribute to the development of fetal alcohol spectrum disorders
- Exposure of zebrafish larvae to ethanol led to abnormal expression of 20S, 19S, and 11S proteasome components, and increased ubiquitylated protein levels during development
- Genetic knockout of proteasome components produced numerous tissue-specific developmental defects, including craniofacial, endoderm, and nervous system abnormalities, that resembled phenotypes of fetal alcohol spectrum disorders
- Pharmacologic proteasome inhibition potentiated the effects of embryonic alcohol exposure and caused a higher prevalence of severe defects
- Zebrafish genetics and physiology are highly conserved in humans, so the findings may lead to strategies for treating ethanol-induced tissue injury
Ethanol is directly teratogenic, but its oxidative metabolism by the mother and fetus is also thought to contribute to fetal alcohol spectrum disorders (FASDs). In the end stage of mammalian ethanol metabolism, acetaldehyde (the primary metabolite of ethanol) and toxic secondary products form adducts with proteins, and these protein adducts can affect protein homeostasis.
Subscribe to the latest updates from Digestive Health Advances in Motion
Wolfram Goessling, MD, PhD, chief of the Division of Gastroenterology, Jules L. Dienstag, M.D. and Betty and Newell Hale endowed chair in Gastroenterology at Massachusetts General Hospital, and Robert H. Ebert professor of Medicine at Harvard Medical School; Olivia Weeks, PhD, formerly of the Division of Genetics at Brigham and Women's Hospital, and colleagues speculated that disruptions to the ubiquitin-proteasome system (UPS) significantly contribute to the development of FASD-related phenotypes. Their experiments with an established zebrafish model of embryonic alcohol exposure confirmed the hypothesis, as they report in JCI Insight.
Background
A normal role of the UPS is to degrade short-lived regulatory proteins from within cells and remove damaged or misfolded proteins. During these processes:
- Substrates are targeted for degradation by the 26S proteasome, which consists of a catalytically active 20S core and a 19S regulatory cap
- The 19S regulator, which contains ATPase subunits, facilitates ATP-dependent binding, deubiquitylation, and unfolding of substrates
- Substrates are then translocated to the 20S core, where they are degraded into small peptide fragments
- However, in certain circumstances, several components of the 20S proteasome are replaced to form an "immunoproteasome," which can combine with an alternative regulator known as the 11S, instead of or in addition to the 19S, to produce MHC class I ligands
- Alternatively, the 20S immunoproteasome may regulate protein homeostasis in the context of oxidative stress, which can result from ethanol exposure
The roles of specific proteasome components during development have been poorly understood, however, especially as they relate to tissue-specific effects.
Methods
In this study, zebrafish larvae were exposed to 0%–1.0% ethanol (a range physiologically relevant for women with chronic alcoholism) beginning after completion of gastrulation and continuing until five days after fertilization, when they were assumed to have developed mature organs. Some fish were then transferred to normal water for longitudinal studies.
Results
The key findings were the following:
- Embryonic alcohol exposure altered gene expression of the 26S proteasome subunits and 11S regulator, leading to higher ubiquitylated protein levels during development
- Those proteasome components were upregulated 48 hours after the fish were withdrawn from ethanol, probably as a compensatory mechanism to manage increased demand, suggesting that the effects on protein homeostasis persist
- Ethanol and acetaldehyde decreased proteasomal peptidase activity in a cell type–specific manner
- Disrupting proteasome activity by knocking out 20S subunit psmb1 and 19S ATPase psmc6 led to numerous tissue-specific developmental defects resembling FASD phenotypes, including growth restriction, neurodevelopmental defects, craniofacial malformation, and failed maturation of the liver and pancreas
- Administration of bortezomib and another proteasome inhibitor increased the incidence of cartilage abnormalities, causing even greater neuronal apoptosis, and further reduced liver and pancreas size
Laying the Groundwork for Therapy
Zebrafish genetics and physiology are highly conserved in humans. These findings may explain tissue-specific effects of ethanol during human fetal development and suggest strategies for treating ethanol-induced tissue injury.
view original journal article Subscription may be required
Learn about the Division of Gastroenterology
Refer a patient to the Division of Gastroenterology