Neuroactive Steroids, Toll-like Receptors, and Neuroimmune Regulation: Insights into Their Impact on Neuropsychiatric Disorders

Abstract

Pregnane neuroactive steroids, notably allopregnanolone and pregnenolone, exhibit efficacy in mitigating inflammatory signals triggered by toll-like receptor (TLR) activation, thus attenuating the production of inflammatory factors. Clinical studies highlight their therapeutic potential, particularly in conditions like postpartum depression (PPD), where the FDA-approved compound brexanolone, an intravenous formulation of allopregnanolone, effectively suppresses TLR-mediated inflammatory pathways, predicting symptom improvement. Additionally, pregnane neurosteroids exhibit trophic and anti-inflammatory properties, stimulating the production of vital trophic proteins and anti-inflammatory factors. Androstane neuroactive steroids, including estrogens and androgens, along with dehydroepiandrosterone (DHEA), display diverse effects on TLR expression and activation. Notably, androstenediol (ADIOL), an androstane neurosteroid, emerges as a potent anti-inflammatory agent, promising for therapeutic interventions. The dysregulation of immune responses via TLR signaling alongside reduced levels of endogenous neurosteroids significantly contributes to symptom severity across various neuropsychiatric disorders. Neuroactive steroids, such as allopregnanolone, demonstrate efficacy in alleviating symptoms of various neuropsychiatric disorders and modulating neuroimmune responses, offering potential intervention avenues. This review emphasizes the significant therapeutic potential of neuroactive steroids in modulating TLR signaling pathways, particularly in addressing inflammatory processes associated with neuropsychiatric disorders. It advances our understanding of the complex interplay between neuroactive steroids and immune responses, paving the way for personalized treatment strategies tailored to individual needs and providing insights for future research aimed at unraveling the intricacies of neuropsychiatric disorders.

Keywords: allopregnanolone; cytokines; inflammation; neurosteroid; pregnenolone.

Figure 1

Neurosteroid biosynthesis and classification. Neurosteroid synthesis begins with the translocation of cholesterol into the mitochondria, where it is metabolized into pregnenolone by the cytochrome P450scc, a mitochondrial cholesterol side-chain cleavage enzyme (CYP11A1). Pregnenolone undergoes further conversions. It is transformed into progesterone by 3β-HSD 1/2 (3β-hydroxysteroid dehydrogenase). Progesterone can then be converted into 5α- or 5β-dihydroprogesterone (5α-DHP/5β-DHP) by 5α/β-reductase type I/II. 5α/β-DHP can be reduced to allopregnanolone (3α,5α-THP) or pregnanolone (3α,5β-THP) by the 3α-hydroxysteroid dehydrogenase III (3α-HSD) enzyme. Allopregnanolone can be reconverted into 5α-DHP or 5β-DHP. Progesterone can also be metabolized into 11-deoxycorticosterone (DOC) by the cytochrome P450 21-hydroxylase (CYP21A2), and further converted into 5α-dihydrodeoxycorticosterone (5α-DHDOC) or 5β-dihydrodeoxycorticosterone (5β-DHDOC) by the 5α- or 5β-reductases, respectively. 5α-DHDOC can be reduced into 3α,5α-THDOC (3α,5α-tetrahydro-deoxycorticosterone) and reconverted into 5α-DHDOC by the 3α-HSD enzyme. Likewise, 5β-DHDOC can be reduced into 3α,5β-THDOC (3α,5β-tetrahydrodeoxycorticosterone) and reconverted into 5β-DHDOC by the 3α-HSD enzyme. To form androstane steroids, pregnenolone can also be converted by the cytochrome P450-17A1 (CYP17A1) into 17OH-pregnanolone, then into dehydroepiandrosterone (DHEA). DHEA can be metabolized by 3β-HSD 1/2 into androstenedione, and then into estrone by the enzyme aromatase (CYP19A1). Estrone can be further converted into estradiol by 17β-HSD 1 (17β-hydroxysteroid dehydrogenase). Estradiol can be reconverted into estrone by 17β-HSD 2/4. The same enzyme, 17β-HSD 2/4, also converts DHEA into androstenediol (5-androstenediol, also known as androst-5-ene-3β,17β-diol; ADIOL). Androstenediol can be reconverted into DHEA by 17β-HSD 1 and further converted into testosterone by 3β-HSD 1/2. Moreover, androstenedione can be metabolized into testosterone by 17β-HSD 2 and reconverted into androstenedione by 17β-HSD 3/5. Alternatively, androstenedione can be converted to androstanedione by the 5α- or 5β-reductase enzymes and 3α-HSD to form 5α- or 5β-androsterone. Testosterone can be converted into estradiol by the aromatase (CYP19A1) or into dihydrotestosterone (5α-DHT or 5β-DHT) by the 5α- or 5β-reductase enzymes. Finally, 5α-DHT can be converted into androstanediol (3α-androstanediol also known as 5α-androstane-3α,17β-diol; 3α-diol) and reconverted into 5α-DHT by the 3α-HSD II/III enzymes. Pregnenolone and DHEA can also be converted into pregnenolone sulfate (PS) and DHEA sulfate (DHEAS) by the sulfatase enzyme and reconverted into pregnenolone and DHEA by removing the sulfate group with the sulfotransferase enzyme. Allopregnanolone, pregnanolone, 3α,5α- or 3α,5β-THDOC, 3α,5α- or 3α,5β-androsterone and 3α,5α- or 3α,5β-androstanediol all act as positive modulators of GABA_A receptors; however, the pregnane derivatives have much higher affinity and potency than the androstane derivatives.

Figure 2

The schematic model illustrates the inhibitory effects of allopregnanolone (3α,5α-THP) on toll-like receptor (TLR) inflammatory signaling pathways. Black line arrows represent TLR inflammatory pathways, while red X-shapes or arrows indicate the impacts of 3α,5α-THP. 3α,5α-THP inhibits the binding of TLR4 with both myeloid differentiation protein 2 (MD2) and myeloid differentiation primary response 88 (MyD88), as well as the α2 subunit protein of the gamma-aminobutyric acid A receptors. The binding of TLR2 and TLR7 with MyD88 is also inhibited by 3α,5α-THP (indicated by red X-shapes), preventing TLR pathway activation/initiation. Additionally, 3α,5α-THP promotes the degradation of TLR4 adapter toll/interleukin-1 receptor domain-containing adapter protein (TIRAP) (indicated by red arrows). Consequently, these events lead to decreased levels of tumor necrosis factor receptor-associated factor 6 (TRAF6) and decreased activation of transforming growth factor beta-activated kinase 1 (TAK1), as evidenced by its decreased phosphorylation (indicated by a red arrow). The inhibition of TAK1 activation suppresses the activation of nuclear factor kappa-B (NF-κB) and mitogen-activated protein kinases (MAPK), such as extracellular signal-regulated kinases 1/2 (ERK1/2), thereby inhibiting the activation of various transcription factors including cAMP response element-binding protein (CREB), signal transducer and activator of transcription 1 (STAT1) and activating transcription factor 2 (ATF2). Inhibition of TLR7/MyD88 signaling also involves the inhibition of interferon regulatory factor 7 (IRF7) activation. These events collectively result in the decline in inflammatory mediators including monocyte chemoattractant protein-1 (MCP-1), tumor necrosis factor alpha (TNF-α), interleukin 6 (IL-6), interleukin 1 beta (IL-1β), and high mobility group box 1 protein (HMGB1). The schematic figure was created with BioRender.com.

Figure 3

The schematic model illustrates how allopregnanolone (3α,5α-THP) induces endosomal toll/interleukin-1 receptor domain-containing adapter-inducing interferon-beta (TRIF)-dependent toll-like receptor 4 (TLR4) anti-inflammatory signaling, leading to elevated levels of interleukin-10 (IL-10). 3α,5α-THP facilitates the transition of TLR4 from the toll/interleukin-1 receptor (TIR) domain-containing adapter protein (TIRAP)-myeloid differentiation primary response 88 (MyD88)-associated plasma membrane complex to an endosomal TRIF-related adapter molecule (TRAM)-TRIF complex, initiating the activation of the endosomal anti-inflammatory TLR4-TRIF signal and subsequent IL-10 production. Mechanistically, 3α,5α-THP upregulates the p110δ isoform of phosphoinositide 3-kinase (PI3K), promoting the degradation of TIRAP and the release of TLR4 from the TIRAP-MyD88-associated plasma membrane complex, facilitating TLR4 translocation to endosomes. Additionally, 3α,5α-THP facilitates TRIF accumulation in endosomes. The release of TLR4 from the TIRAP-MyD88-associated plasma membrane complex may also result from direct 3α,5α-THP-induced inhibition of the binding between TLR4 and MyD88, and TLR4 and myeloid differentiation factor 2 (MD2). Furthermore, both the inhibition of binding and TIRAP degradation suppress inflammatory TLR4 pathway components, cytokines, and chemokines. 3α,5α-THP activates the anti-inflammatory endosomal TLR4-TRIF pathway by triggering an increase in phosphorylated TRAM, a specific marker for TLR4-TRIF pathway activation. The model also incorporates 3α,5α-THP-induced enhanced presence of transcription factor specificity protein 1 (SP1), leading to increased IL-10 production. Additionally, 3α,5α-THP upregulates brain-derived neurotrophic factor (BDNF) levels, potentially amplifying IL-10 production, and release. Furthermore, 3α,5α-THP stimulates the accumulation of endosomal Ras-related protein Rab7 (Rab7), which may significantly impact the equilibrium between pro-inflammatory and anti-inflammatory TLR4 signaling pathways. In the figure, an increase in protein levels is indicated by a green up-arrow, protein degradation by a blue X-shape, and inhibition of protein–protein binding by a red X-shape. The schematic figure was created with BioRender.com.