Neuroactive steroids, which are naturally occurring steroids in the brain and elsewhere in the body, have many functions critical for life and health. These steroids decline with aging and are deficient in many neuropsychiatric conditions, such as depression. Inflammatory cell signaling in the brain is heightened in many neuropsychiatric conditions, including alcohol use disorder, depression, and post-traumatic stress. It is also seen in sepsis, epilepsy, multiple sclerosis, and Alzheimer’s disease. For the first time, scientists discovered how neuroactive steroids naturally found in the brain and bloodstream inhibit the activity of a specific kind of protein called Toll-like receptors (TLR4), which have been known to play a role in inflammation in many organs, including the brain. This UNC School of Medicine-University of Maryland collaboration, shows how the neurosteroid allopregnanolone prevents the activation of pro-inflammatory proteins important for gene regulation, as well as the creation of cytokines, which are known to be involved in many different inflammatory conditions. The researchers have proposed that treatment with these compounds may prevent uncontrolled TLR4 signaling in conditions where this signaling contributes to disease.
Recent studies showed that the neurosteroid compounds pregnenolone and allopregnanolone have therapeutic effects in depression, schizophrenia and PTSD. But until now, scientists didn’t understand how. The UNC-Maryland study suggests that inhibition of inflammatory signaling may contribute to these effects, and inhibition of TLR4 signaling may be a new target for these conditions. In collaboration with Dr. Laure Aurelian, PhD at the University of Maryland, Dr. Morrow and colleagues found that allopregnanolone inhibits TLR4 activation in macrophages, which are found in white blood cells and part of the immune system, including in the brain. The researchers found that allopregnanolone prevents TLR4 binding to MD2 proteins. These are scaffold proteins that launch a cascade signaling upon TLR4 recruitment. They then activate transcription factors that regulate the genes responsible for inflammatory responses in cells and tissues. Allopregnanolone also tamps down chemokines and cytokines, such as NFkB, HMGB1, MCP-1 and TNF-a, all of which are part of the immune system and involved in many different inflammatory diseases.
In a previous study of the team, pregnenolone was found to promotes ubiquitination and degradation of TLR2 and the TLR2/4 adaptor protein TIRAP in macrophages and microglial cells. Pregnenolone and its metabolites suppressed the secretion of cytokines TNF-α and IL-6 mediated through TLR-2 and -4 signaling. Pregnenolone was reported to induce the activation of cytoplasmic linker protein-170 (CLP-170); this protein was recently shown to promote targeted degradation of TIRAP. However, the molecular mechanism behind the anti-inflammatory and neuroprotective functions of allopregnenolone remains largely unknown. Glucocorticoids exert their immunosuppressive effect by binding to the glucocorticoid receptor. However, pregnenolone was believed to use a glucocorticoid receptor-independent mechanism, since its antagonist mifepristone did not affect the immunosuppressive property of allopregnenolone. Now that scientists have identified this inhibitory mechanism, they can create new compounds to fill this particular role of neurosteroids without unwanted side effects. In addition, researchers can now plan clinical studies to determine the best doses, formulations, and modes of administration.
Senior author A. Leslie Morrow, PhD, the John Andrews Distinguished Professor in the Departments of Psychiatry and Pharmacology at the UNC School of Medicine, precised some concepts: “Pregnenolone’s effects in the brain were less pronounced; but inhibition of peripheral inflammation protects the brain as well because systemic inflammation affects organs throughout the body indirectly. It has been very difficult to treat brain disease that involves inflammation, but allopregnanolone’s inhibition of TLR4 signaling activation in macrophages and the brain provides hope that we can develop better therapies to help millions of people suffering with these conditions”.
- Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
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