Thyroid hormone treatment activates protective pathways in both in vivo and in vitro models of neuronal injury

Association of aging acceleration with serum neurofilament light chain levels: Implications for the roles of modifiable aging factors

INTRODUCTION

Neurofilament light chain (NfL) is a specific biomarker of neuroaxonal damage and related neurodegenerative diseases. Aging acceleration, which reflects the impact of modifiable factors on the aging process, is increasingly recognized for its relevance. While normal aging is known to contribute substantially to neuroaxonal damage and many neurodegenerative diseases, the effects of aging acceleration warrant further investigation. This study aimed to investigate the association and causality between aging acceleration and serum NfL levels.

METHODS

We conducted a cross-sectional study involving 1695 adult participants from NHANES 2013-2014 to evaluate the association, dose-response relationship, and interaction network between aging acceleration and serum NfL levels. And we used Mendelian randomization (MR) to assess the causal effects between serum NfL levels and aging acceleration.

RESULTS

Significant positive associations were observed between aging acceleration and serum NfL levels. In linear regression, the regression coefficients were 0.016 (95 % CI: 0.011-0.021) for biological age acceleration and 0.020 (95 % CI: 0.012-0.028) for phenotypic age acceleration. In logistic regression, the odds ratios were 1.052 (95 % CI: 1.029-1.076) and 1.093 (95 % CI: 1.064-1.123), respectively. Restricted cubic spline regression identified significant positive dose-response relationships, and bidirectional MR analyses demonstrated forward causal effects.

CONCLUSION

Our study indicates that aging acceleration is significantly associated with serum NfL levels, with higher levels of aging acceleration linked to an increased risk of neuroaxonal damage. These findings provide robust evidence that aging acceleration affects the risk of neuroaxonal damage and highlight the importance of modifiable aging factors.

Role of Thyroid Hormone in Neurodegenerative Disorders of Older People

Thyroid dysfunction is associated with a number of neuropsychiatric manifestations. Cognitive decline is a common feature of hypothyroidism and clinical or subclinical hyperthyroidism. In addition, there is a significant association between thyroid hormone (TH) levels and the degree of cognitive impairment in Parkinson's disease (PD). The pathophysiology of TH-related neurodegeneration include changes in the blood-brain barrier, increased cellular stress, altered processing of β-amyloid precursor protein and the effect of TH on neuronal cell viability. The neurotoxicity of TH is partially mediated by the thyroid hormone responsive protein (THRP). This protein is 83% homologous to mouse c-Abl-interacting protein-2 (Abi2), a c-Abl-modulating protein with tumor suppressor activity. In cell cultures, increasing THRP expression either with TH treatment or exogenously through transfecting neuronal or PC 12 cells causes cell necrosis. The expression of exogenous THRP in other cells such as the colonic epithelial cell line Caco-2 and the glial cell line U251 has no effect on cell viability. The effect of THRP on cell viability is not modulated by c-Abl tyrosine kinase. The causal relationship between specific biochemical perturbations in cerebral tissue and thyroid dysfunction remains to be elucidated.

Factors and Mechanisms of Thyroid Hormone Activity in the Brain: Possible Role in Recovery and Protection

Thyroid hormones (THs) are essential in normal brain development, and cognitive and emotional functions. THs act through a cascade of events including uptake by the target cells by specific cell membrane transporters, activation or inactivation by deiodinase enzymes, and interaction with nuclear thyroid hormone receptors. Several thyroid responsive genes have been described in the developing and in the adult brain and many studies have demonstrated a systemic or local reduction in TH availability in neurologic disease and after brain injury. In this review, the main factors and mechanisms associated with the THs in the normal and damaged brain will be evaluated in different regions and cellular contexts. Furthermore, the most common animal models used to study the role of THs in brain damage and cognitive impairment will be described and the use of THs as a potential recovery strategy from neuropathological conditions will be evaluated. Finally, particular attention will be given to the link observed between TH alterations and increased risk of Alzheimer's Disease (AD), the most prevalent neurodegenerative and dementing condition worldwide.

Electroacupuncture Relieves HuR/KLF9-Mediated Inflammation to Enhance Neurological Repair after Spinal Cord Injury

Electroacupuncture (EA) is widely applied in clinical therapy for spinal cord injury (SCI). However, the associated molecular mechanism has yet to be elucidated. The current study aimed to investigate the underlying mechanism of EA in neurologic repair after SCI. First, we investigated the role of EA in the neurologic repair of the SCI rat model. The expression levels of human antigen R (HuR) and Krüppel-like factor 9 (KLF9) in spinal cord tissues were quantified after treatment. Second, we conducted bioinformatics analysis, RNA pull-down assays, RNA immunoprecipitation, and luciferase reporter gene assay to verify the binding of HuR and KLF9 mRNA for mRNA stability. Last, HuR inhibitor CMLD-2 was used to verify the enhanced effect of EA on neurologic repair after SCI via the HuR/KLF9 axis. Our data provided convincing evidence that EA facilitated the recovery of neuronal function in SCI rats by reducing apoptosis and inflammation of neurons. We found that EA significantly diminished the SCI-mediated upregulation of HuR, and HuR could bind to the 3' untranslated region of KLF9 mRNA to protect its decay. In addition, a series of in vivo experiments confirmed that CMLD-2 administration increased EA-mediated pain thresholds and motor function in SCI rats. Collectively, the present study showed that EA improved pain thresholds and motor function in SCI rats via impairment of HuR-mediated KLF9 mRNA stabilization, thus providing a better understanding of the regulatory mechanisms regarding EA-mediated neurologic repair after SCI.

Iodide intake during pregnancy and lactation stimulates KLF9, BDNF expression in offspring brain with elevated DHA, EPA metabolites

To investigate the effect of different iodide intake during pregnancy and lactation on thyroid function, docosahexaenoic acid (DHA), Eicosapentaenoic acid (EPA) metabolites, the expression of Krüppel-like factor KLF9 (KLF9), brain-derived neurotrophic factor (BDNF) in brain in offspring rats. In both male and female offspring rats, serum FT3, FT4 levels and the expression of KLF9, thyroid hormone receptors (TR)α, TRβ and BDNF in the hippocampal region and cerebellum were significantly increased in 5 times higher-than-normal pregnant iodide intake (5 HI) and 10 times higher-than-normal pregnant iodide intake (10 HI) group. The median levels of DHA metabolite (17-HDoHE) and EPA metabolites (15-HEPE, 17,18-EEQ, 9-HEPE and 14,15-DiHETE) were significantly increased in 5 HI and 10 HI group of offspring rats. Serum DHA, EPA metabolites and KLF9 as well as BDNF expression in brain might be potential iodine status biomarkers to reflect brain development in offspring.

Neuroprotective effect of thyroid hormones on methamphetamine-induced neurotoxicity via cell surface receptors

Thyroid hormones (THs) have an essential role in normal brain development and function. Methamphetamine (MA) is a widely abused psychostimulant that induces irreversible damages to neuronal cells. In the current study, we used rat primary hippocampal neurons (PHNs) to investigate the neuroprotective effect of THs against MA neurotoxicity. PHNs were prepared from 18-day rat embryos and cell viability was assessed using MTT assay, following treatment with various concentrations of MA, T3, T4 or tetrac, an integrin αvβ3 cell surface receptor antagonist. Our results showed that 7 mM MA induced an approximately 50 % reduction in the PHNs viability. Treatment with 800 nM T3 or 8 μM T4 protected PHNs against MA toxicity, an effect which was blocked in the presence of tetrac. These findings suggest that THs protect PHNs against MA-induced cell death by the activation of integrin αvβ3 cell surface receptors. So, targeting integrin αvβ3 receptors or using THs can be considered as promising therapeutic strategies to overcome MA neurotoxicity.

Cardiovascular and Neuronal Consequences of Thyroid Hormones Alterations in the Ischemic Stroke

Ischemic stroke is one of the leading global causes of neurological morbidity and decease. Its etiology depends on multiple events such as cardiac embolism, brain capillaries occlusion and atherosclerosis, which ultimately culminate in blood flow interruption, incurring hypoxia and nutrient deprivation. Thyroid hormones (THs) are pleiotropic modulators of several metabolic pathways, and critically influence different aspects of tissues development. The brain is a key TH target tissue and both hypo- and hyperthyroidism, during embryonic and adult life, are associated with deranged neuronal formation and cognitive functions. Accordingly, increasing pieces of evidence are drawing attention on the consistent relationship between the THs status and the acute cerebral and cardiac diseases. However, the concrete contribution of THs systemic or local alteration to the pathology outcome still needs to be fully addressed. In this review, we aim to summarize the multiple influences that THs exert on the brain and heart patho-physiology, to deepen the reasons for the harmful effects of hypo- and hyperthyroidism on these organs and to provide insights on the intricate relationship between the THs variations and the pathological alterations that take place after the ischemic injury.

Is Restless Legs Syndrome De Facto Thyroid Disease?

While a primary role in the pathogenesis of restless legs syndrome (RLS) has been attributed to dysfunction of the dopaminergic system and impaired iron metabolism (particularly in the central nervous system), it has been hypothesized that an imbalance between thyroid hormones and dopaminergic activity may be the starting point for all aspects of RLS. Although this hypothesis was proposed more than a decade ago, it has not yet been verified beyond doubt. The main aim of this study is to compare the prevalence of RLS in a population of patients with the most common thyroid gland diseases with a population of individuals with a healthy thyroid gland. The study included 237 participants divided into smaller groups according to the thyroid disease concerning them. Each participant had a laboratory diagnosis, an ultrasound scan and an assessment of the fulfilment of RLS criteria according to the International Restless Legs Syndrome Study Group (IRLSSG) criteria. The results obtained were subjected to statistical analysis. RLS is significantly more common in patients with known thyroid disease; Hashimoto’s disease, among others, manifests a 2.56× higher risk of a positive diagnosis for RLS than the general population. The association of RLS with thyroid disease is notable, although it is difficult to conclude unequivocally that there is a cause-and-effect relationship between the two. Further investigation into a potentially autoimmune cause of restless legs syndrome should be considered.

Thyroid Hormone-mediated Histone Modification Protects Cortical Neurons From the Toxic Effects of Hypoxic Injury

Context

Thyroid hormone has been shown to have a protective role in neuronal injury, although the mechanisms have not been established. The cellular response to stress that promotes adaptation and survival has been shown to involve epigenetic modifications.

Objective

We hypothesized that the neuroprotective role of thyroid hormone was associated with epigenetic modifications of histone proteins. We used hypoxic neurons as a model system for hypoxia-induced brain injury.

Methods

Mouse primary cortical neurons were exposed to 0.2% oxygen for 7 hours, with or without, treatment with triiodothyronine (T3). We analyzed the expression of histone-modifying enzymes by RNA-seq and the post-translationally modified histone 3 proteins by enzyme-linked immunosorbent assay (ELISA) and Western blot.

Results

We found that methylation of H3K27, associated with inactive promoters, was highly induced in hypoxic neurons, and this histone methylation was reduced by T3 treatment. H3K4 methylation is the hallmark of active promoters. The expression of 3 (Set1db, Kmta2c, and Kmt2e) out of 6 H3K4 methyltransferases was downregulated by hypoxia and expression was restored by T3 treatment. H3K4me3 protein, measured by ELISA, was increased 76% in T3-treated hypoxic neurons compared with the levels without T3 treatment. H3K56ac plays a critical role in transcription initiation and was markedly increased in T3-treated hypoxic neurons compared with those without T3 treatment, indicating stimulation of gene transcription. Additionally, T3 treatment restored hypoxia-induced downregulation of histone acetyltransferase, Kat6a, Kat6b, and Crebbp, which function as transcription factors.

Conclusion

These findings indicate that T3 treatment mitigates hypoxia-induced histone modifications and protects neurons from hypoxia-induced injury.

Triiodothyronine attenuates neurocognitive dysfunction induced by sevoflurane in the developing brain of neonatal rats

BACKGROUND

Whilst concerns have been raised about the detrimental effects of general anaesthetics on the brain's development and function in the young, reports have indicated that thyroid hormones are able to promote neurogenesis in the developing brain. This present study aimed to investigate the effects of triiodothyronine (T3) on the neonatal rat brain, following sevoflurane exposure.

METHODS

Postnatal day 7 (P7) ratpups were treated with Triiodothyronine (T3) (1 µg/100 g body weight, i.p. injection, once/day for 3 days) after 2% sevoflurane exposure for 6 h. They were sacrificed at either P7 (immediately), P15 or P30 and their brains were harvested to assess cell death, proliferation in the hippocampus, N-methyl-D-aspartate (NMDA) receptor subunit A and B, and a post-synaptic protein (PSD-95 in the hippocampus,). Neuro-behavioral changes in other cohorts between P27 and P30 were evaluated with Morris water maze and open field tests.

RESULTS

Sevoflurane exposure caused cell death and suppressed the proliferation of astrocytes and neurons, as well as the dendritic growth of neurons in the hippocampus which were all reversed by the administration of T3. Moreover, cognitive function, including learning, memory, and adaptability to a new environment, were impaired by sevoflurane exposure, which was also negated by T3 treatment. Furthermore, sevoflurane decreased the expression of NMDA receptor subunits NR2A and NR2B, as well as PSD-95 in the hippocampus at P15 and those effects of sevoflurane were abolished by T3 administration.

CONCLUSIONS

A potential therapeutic role of T3 in protecting general anesthetic induced neuronal injury in the developing brain is likely to occur through enhancing expression of PSD-95 and the NMDA NR2A and NR2B expression.

Thyrotropic Axis and Disorders of Consciousness in Acquired Brain Injury: A Potential Intriguing Association?

PURPOSE

A potential involvement of thyrotropic axis in influencing the state of consciousness could be hypothesized. We aimed at investigating thyroid function tests as predictors of disorders of consciousness (DoC) and relating recovery in a large cohort of patients with DoC secondary to acquired brain injury (ABI).

METHODS

This retrospective, multicenter, cohort study included 151 patients with DoC following ABI, consecutively admitted for a 6-month neurorehabilitation program. Data on etiology of brain injury, evolution of DoC, disability and rehabilitation assessments, and death during rehabilitation were collected at baseline and on discharge. Thyroid function tests (serum TSH, fT4 and fT3 levels) were assessed on admission in all patients and at final discharge in 50 patients.

RESULTS

Lower baseline TSH levels and greater TSH increments (ΔTSH) after neurorehabilitation predicted a favorable change in DoC independent of age, sex, BMI, etiology of brain injury and initial DoC subtype (TSH: OR=0.712, CI 95% 0.533-0.951, p=0.01; ΔTSH: OR=2.878, CI 95% 1.147-7.223, p=0.02). On the other hand, neither fT4 nor fT3 or their variations appeared to play any role on DoC changes after 6-months inpatient neurorehabilitation. A lower magnitude of ΔfT4 acted as a strong predictor of improved functional disability level (β=0.655, p=0.002) and cognitive functions (β=-0.671, p=0.003), implying that smaller changes in fT4 were associated with higher outcomes.

CONCLUSIONS

Serum TSH levels assessed in the subacute post-ABI phase and its variation during neurorehabilitation could represent a potential biomarker of DoC evolution, while variations in fT4 levels seem to be associated with rehabilitation and cognitive functions. Further studies are needed to investigate the mechanisms underlying these associations.

RNA-seq analysis reveals potential molecular mechanisms of ZNF580/ZFP580 promoting neuronal survival and inhibiting apoptosis after Hypoxic-ischemic brain damage

Neonatal hypoxic-ischemic brain damage (HIBD) is one of the main causes of neonatal acute death and chronic nervous system impairment, but still lacks effective treatments. ZNF580/ZFP580, reported in our previous studies, may be a newly identified member of the Krüppel-like factor (KLF) family, and has anti-apoptotic effects during ischemic myocardial injury. In the present study, we showed that the expression levels of both ZFP580/ZNF580 mRNA and protein increased significantly in neonatal HIBD rats and oxygen-glucose deprivation (OGD) SH-SY5Y cell models. ZNF580 overexpression promoted neuron survival and suppressed neuron apoptosis after OGD in neuron-like SH-SY5Y cells, while interference with ZNF580 resulted in the opposite results. RNA-seq analysis identified 248 differentially-expressed genes (DEGs) between ZNF580 overexpression SH-SY5Y cells and interference-expressed SH-SY5Y cells. Gene Ontology functional enrichment analysis showed that these DEGs played significant roles in the growth, development, and regeneration of axons, DNA biosynthetic processes, DNA replication, and apoptosis. Kyoto Encyclopedia of Genes and Genomes enrichment analysis indicated that these DEGs were found in some pathways, including ferroptosis, glutamatergic synapses, protein processing in the endoplasmic reticulum, estrogen signaling pathways, the TGF-beta signaling pathway, and the longevity regulating pathway. The qRT-PCR validation results were consistent with RNA-seq results, which showed that HSPA5, IGFBP3, NTN4, and KLF9 increased in ZNF580-overexpressed SH-SY5Y cells and decreased in interference-expressed SH-SY5Y cells, when compared with normal cells. Together, the results suggested that ZNF580 targeted these genes to inhibit neuronal apoptosis.

In vitro differentiation of human bone marrow stromal cells into neural precursor cells using small molecules

BACKGROUND

Neurogenic differentiation of human marrow stromal stem cells (hMSCs) into neural precursor cells (NPCs) offers new hope in many neurological diseases. Stromal cells can be differentiated into NPCs using small molecules acting as chemical inducers. The aim of this study is to formulate an efficient, direct, fast and safe protocol to differentiate hMSCs into NPCs using different inducers: b-mercaptoethanol (BME), triiodothyronine (T3), and curcumin (CUR). NEW METHOD: hMSCs were subjected to either 1 mM BME, 0.5 µM T3, or 5 µM CUR. Neurogenic differentiation was determined by assessing the protein expression of PAX6, SOX2, DLX2, and GAP-43 with flow cytometry and immunofluorescence, along with Nissl staining of differentiated cells.

RESULTS AND COMPARISON WITH EXISTING METHOD

It was revealed that T3 and CUR are 70-80% better than BME in terms of efficiency and safety, and surprisingly BME was a good promoting factor for cell preconditioning with limited effects on neural trans-differentiation related to its toxic effects on cell viability.

CONCLUSION

Reprogramming of bone marrow stromal cells into neural cells gives hope for treating different neurological disorders. Our study shows that T3 and CUR were effective in generation of NPCs from hMSCs with preservation of cell viability. BME was a good promoting factor for cell preconditioning with limited effects on neural transdifferentiation related to its toxic effects on cell viability.

NRF2 Activation and Downstream Effects: Focus on Parkinson's Disease and Brain Angiotensin

Reactive oxygen species (ROS) are signalling molecules used to regulate cellular metabolism and homeostasis. However, excessive ROS production causes oxidative stress, one of the main mechanisms associated with the origin and progression of neurodegenerative disorders such as Parkinson's disease. NRF2 (Nuclear Factor-Erythroid 2 Like 2) is a transcription factor that orchestrates the cellular response to oxidative stress. The regulation of NRF2 signalling has been shown to be a promising strategy to modulate the progression of the neurodegeneration associated to Parkinson's disease. The NRF2 pathway has been shown to be affected in patients with this disease, and activation of NRF2 has neuroprotective effects in preclinical models, demonstrating the therapeutic potential of this pathway. In this review, we highlight recent advances regarding the regulation of NRF2, including the effect of Angiotensin II as an endogenous signalling molecule able to regulate ROS production and oxidative stress in dopaminergic neurons. The genes regulated and the downstream effects of activation, with special focus on Kruppel Like Factor 9 (KLF9) transcription factor, provide clues about the mechanisms involved in the neurodegenerative process as well as future therapeutic approaches.

T3 as predictor of mortality in any cause non-critically ill patients

BACKGROUND

Low T3 syndrome refers to a set of thyroid hormone metabolism alterations present in the disease state. A correlation between low T3 and poor clinical outcomes in the intensive care unit is more established. Nonetheless, studies on non-critically ill patients are few and controversial.

OBJECTIVE

To evaluate the prevalence and predictive value of low T3 levels on 30-day and 6-month mortality in non-critically ill patients. Secondary outcomes evaluated the length of hospital stay, overall mortality, and hospital readmission.

DESIGN

Prospective cohort study.

METHODS

A total of 345 consecutive patients from the Internal Medicine ward of a tertiary hospital in southern Brazil were included and followed from October 2018 to April 2019 (6 months). Levels of total serum T3 were measured weekly, from admission to discharge, and correlated with 30-day and 6-month mortality.

RESULTS

Prevalence of low T3 was 36.6%. Low T3 levels were associated with higher 30-day hospital mortality (15.1% vs 4.1%, P < 0.001) and higher 6-month overall mortality (31.7% vs 13.2%, P < 0.001). Total serum T3 at admission was an independent predictor of 30-day hospital mortality.

CONCLUSION

Low T3 levels are a prevalent condition among non-critically ill patients, and this condition is associated with poor clinical outcomes in this population. Total serum T3 levels, alone or in association with other predictive scores, were demonstrated to be an easy and valuable tool for risk stratification and should be further employed in this setting.

The Role of Thyroid Hormone in Neuronal Protection

Thyroid hormone is essential for brain development and brain function in the adult. During development, thyroid hormone acts in a spatial and temporal-specific manner to regulate the expression of genes essential for normal neural cell differentiation, migration, and myelination. In the adult brain, thyroid hormone is important for maintaining normal brain function. Thyroid hormone excess, hyperthyroidism, and thyroid hormone deficiency, hypothyroidism, are associated with disordered brain function, including depression, memory loss, impaired cognitive function, irritability, and anxiety. Adequate thyroid hormone levels are required for normal brain function. Thyroid hormone acts through a cascade of signaling components: activation and inactivation by deiodinase enzymes, thyroid hormone membrane transporters, and nuclear thyroid hormone receptors. Additionally, the hypothalamic-pituitary-thyroid axis, with negative feedback of thyroid hormone on thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) secretion, regulates serum thyroid hormone levels in a narrow range. Animal and human studies have shown both systemic and local reduction in thyroid hormone availability in neurologic disease and after brain trauma. Treatment with thyroid hormone and selective thyroid hormone analogs has resulted in a reduction in injury and improved recovery. This article will describe the thyroid hormone signal transduction pathway in the brain and the role of thyroid hormone in the aging brain, neurologic diseases, and the protective role when administered after traumatic brain injury. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

iTRAQ-based proteomic profiling reveals protein alterations after traumatic brain injury and supports thyroxine as a potential treatment

Traumatic brain injury (TBI) is a primary cause of disability and death across the world. Previously, RNA analysis was widely used to study the pathophysiological mechanisms underlying TBI; however, the relatively low correlation between the transcriptome and proteome revealed that RNA transcription abundance does not reliably predict protein abundance, which led to the emergence of proteomic research. In this study, an iTRAQ proteomics approach was applied to detect protein alterations after TBI on a large scale. A total of 3937 proteins were identified, and 146 proteins were significantly changed after TBI. Moreover, 23 upregulated proteins were verified by parallel reaction monitoring (PRM), and fold changes in 16 proteins were consistent with iTRAQ outcomes. Transthyretin (Ttr) upregulation has been demonstrated at the transcriptional level, and this study further confirmed this at the protein level. After treatment with thyroxine (T4), which is transported by Ttr, the effects of T4 on neuronal histopathology and behavioral performance were determined in vivo (TBI + T4 group). Brain edema was alleviated, and the integrity of the blood brain barrier (BBB) improved. Escape latency in the Morris water maze (MWM) declined significantly compared with the group without T4 treatment. Modified neurological severity scores (mNSS) of the TBI + T4 group decreased from day 1 to day 7 post-TBI compared with the TBI + saline group. These results indicate that T4 treatment has potential to alleviate pathologic and behavioral abnormalities post-TBI. Protein alterations after T4 treatment were also detected by iTRAQ proteomics. Upregulation of proteins like Lgals3, Gfap and Apoe after TBI were reversed by T4 treatment. GO enrichment showed T4 mainly affected intermediate filament organization, cholesterol transportation and axonal regeneration. In summary, iTRAQ proteomics provides information about the impact of TBI on protein alterations and yields insight into underlying mechanisms and pathways involved in TBI and T4 treatment. Finally, Ttr and other proteins identified by iTRAQ may become potential novel treatment targets post-TBI.

Effect of sodium selenite on synaptic plasticity and neurogenesis impaired by hypothyroidism

AIM OF THE STUDY

We investigated protective effect of sodium selenite (Se) on hypothyroidism-induced impairments in, Morris water maze (MWM), long-term potentiation (LTP) and hippocampal neurogenesis male Wistar rats aged of 2 months.

MATERIALS AND METHODS

Hypothyroidism was induced by administration of propylthiouracil (Ptu, 1 mg/kg/d) solution to the rats from postnatal day 60 for 81 days with or without Se (0.5mg/kg/d). Neurogenesis was examined by Ki-67 immunohistochemical staining. Se values on plasma and hippocampus were measured with inductively coupled plasma-mass spectrometry (ICP-MS).

RESULTS

Measurement of fT3 and fT4 levels confirmed that the fT3 levels, but not fT4, in Ptu-treated rats (5435.44±816.05 fg/ml, p < 0.05) has returned to control values (8721.66±2567.68 fg/ml) by Se treatment (8661.65±711.43 fg/ml). Analysis of learning performance in water escape learning task showed that Se supplementation disappeared memory deficit in Ptu-treated rats as shown by significantly decreased time spent in the target quadrant (33.7±0.24% in control group; 26.1±0.48% in Ptu-group, p < 0.05; 33.9±0.44 in Ptu+Se group), although there was no significant difference among groups in any measurement of learning performance on the last day. Considering LTP, Se supplementation improved the deficit in synaptic plasticity in Ptu-treated rats, as shown by significant increase in the excitatory postsynaptic potential slope (% 243±31 in control group; 172±49 in Ptu-group, p < 0.05; 222±65 in Ptu+Se group) without affecting of the impairment in somatic plasticity. Se supplementation did not improve the decrease in the number of progenitor cells in the subgranular layer (SGL) of dentate gyrus (DG) of Ptu treated rats.

CONCLUSIONS

These findings suggest that selenium supplementation in hypothyroid patients may improve learning and memory disorders with different physiological mechanisms.HighlightsSe increased serum fT3 levels and hippocampus Se levels in hypothyroid rats.Se attenuated impairment of population spike-LTP in hypothyroid ratsHypothyroidism disrupts neurogenesis process in the dentate gyrus of hippocampus.Se supplementation could not increase new born cells in hypothyroid rats.

Thyroid Hormone and Neural Stem Cells: Repair Potential Following Brain and Spinal Cord Injury

Neurodegenerative diseases are characterized by chronic neuronal and/or glial cell loss, while traumatic injury is often accompanied by the acute loss of both. Multipotent neural stem cells (NSCs) in the adult mammalian brain spontaneously proliferate, forming neuronal and glial progenitors that migrate toward lesion sites upon injury. However, they fail to replace neurons and glial cells due to molecular inhibition and the lack of pro-regenerative cues. A major challenge in regenerative biology therefore is to unveil signaling pathways that could override molecular brakes and boost endogenous repair. In physiological conditions, thyroid hormone (TH) acts on NSC commitment in the subventricular zone, and the subgranular zone, the two largest NSC niches in mammals, including humans. Here, we discuss whether TH could have beneficial actions in various pathological contexts too, by evaluating recent data obtained in mammalian models of multiple sclerosis (MS; loss of oligodendroglial cells), Alzheimer’s disease (loss of neuronal cells), stroke and spinal cord injury (neuroglial cell loss). So far, TH has shown promising effects as a stimulator of remyelination in MS models, while its role in NSC-mediated repair in other diseases remains elusive. Disentangling the spatiotemporal aspects of the injury-driven repair response as well as the molecular and cellular mechanisms by which TH acts, could unveil new ways to further exploit its pro-regenerative potential, while TH (ant)agonists with cell type-specific action could provide safer and more target-directed approaches that translate easier to clinical settings.

Dual effects of thyroid hormone on neurons and neurogenesis in traumatic brain injury

Thyroid hormone (TH) plays a crucial role in neurodevelopment, but its function and specific mechanisms remain unclear after traumatic brain injury (TBI). Here we found that treatment with triiodothyronine (T3) ameliorated the progression of neurological deficits in mice subjected to TBI. The data showed that T3 reduced neural death and promoted the elimination of damaged mitochondria via mitophagy. However, T3 did not prevent TBI-induced cell death in phosphatase and tensin homolog (PTEN)-induced putative kinase 1 (Pink1) knockout mice suggesting the involvement of mitophagy. Moreover, we also found that T3 promoted neurogenesis via crosstalk between mature neurons and neural stem cells (NSCs) after TBI. In neuron cultures undergoing oxygen and glucose deprivation (OGD), conditioned neuron culture medium collected after T3 treatment enhanced the in vitro differentiation of NSCs into mature neurons, a process in which mitophagy was required. Taken together, these data suggested that T3 treatment could provide a therapeutic approach for TBI by preventing neuronal death via mitophagy and promoting neurogenesis via neuron–NSC crosstalk.

Thyroid Hormone Transporters

Thyroid hormone transporters at the plasma membrane govern intracellular bioavailability of thyroid hormone. Monocarboxylate transporter (MCT) 8 and MCT10, organic anion transporting polypeptide (OATP) 1C1, and SLC17A4 are currently known as transporters displaying the highest specificity toward thyroid hormones. Structure-function studies using homology modeling and mutational screens have led to better understanding of the molecular basis of thyroid hormone transport. Mutations in MCT8 and in OATP1C1 have been associated with clinical disorders. Different animal models have provided insight into the functional role of thyroid hormone transporters, in particular MCT8. Different treatment strategies for MCT8 deficiency have been explored, of which thyroid hormone analogue therapy is currently applied in patients. Future studies may reveal the identity of as-yet-undiscovered thyroid hormone transporters. Complementary studies employing animal and human models will provide further insight into the role of transporters in health and disease. (Endocrine Reviews 41: 1 - 55, 2020).

Effects of the LHPP gene polymorphism on the functional and structural changes of gray matter in major depressive disorder

Background

A single-nucleotide polymorphism (SNP) of the LHPP gene (rs35936514) has been reported to be associated with major depressive disorder (MDD) in genome-wide association studies. However, the systems-level neural effects of rs35936514 that mediate the association are unknown. We hypothesized that variations in rs35936514 would be associated with structural and functional changes in gray matter (GM) at rest in MDD patients.

Methods

A total of 50 MDD patients and 113 healthy controls (HCs) were studied. Functional connectivity (FC) was analyzed by defining the bilateral hippocampus as the seed region. Voxel-based morphometry (VBM) was performed to assess the patterns of GM volume. The subjects were further divided into two groups: a CC homozygous group (CC; 24 MDD and 56 HC) and a risk T-allele carrier group (CT/TT genotypes; 26 MDD and 57 HC). A 2×2 analysis of variance (ANOVA: diagnosis × genotype) was used to determine the interaction effects and main effect (P<0.05).

Results

Significant diagnosis × genotype interaction effects on brain morphology and FC were noted. Compared to other subgroups, the MDD patients with the T allele showed an increased hippocampal FC in the bilateral calcarine cortex and cuneus and a decreased hippocampal FC in the right dorsolateral prefrontal cortex (DLPFC), bilateral anterior cingulate cortex (ACC), and medial prefrontal cortex (MPFC), in addition to reduced GM volume in the right DLPFC, bilateral temporal cortex, and posterior cingulate cortex (PCC).

Conclusions

LHPP gene polymorphisms may affect functional and structural changes in the GM at rest and may play an important role in the pathophysiological mechanisms of MDD.

Triiodothyronine modulates neuronal plasticity mechanisms to enhance functional outcome after stroke

The development of new therapeutic approaches for stroke patients requires a detailed understanding of the mechanisms that enhance recovery of lost neurological functions. The efficacy to enhance homeostatic mechanisms during the first weeks after stroke will influence functional outcome. Thyroid hormones (TH) are essential regulators of neuronal plasticity, however, their role in recovery related mechanisms of neuronal plasticity after stroke remains unknown. This study addresses important findings of 3,5,3′-triiodo-L-thyronine (T₃) in the regulation of homeostatic mechanisms that adjust excitability – inhibition ratio in the post-ischemic brain. This is valid during the first 2 weeks after experimental stroke induced by photothrombosis (PT) and in cultured neurons subjected to an in vitro model of acute cerebral ischemia. In the human post-stroke brain, we assessed the expression pattern of TH receptors (TR) protein levels, important for mediating T₃ actions.

Our results show that T₃ modulates several plasticity mechanisms that may operate on different temporal and spatial scales as compensatory mechanisms to assure appropriate synaptic neurotransmission. We have shown in vivo that long-term administration of T₃ after PT significantly (1) enhances lost sensorimotor function; (2) increases levels of synaptotagmin 1&2 and levels of the post-synaptic GluR2 subunit in AMPA receptors in the peri-infarct area; (3) increases dendritic spine density in the peri-infarct and contralateral region and (4) decreases tonic GABAergic signaling in the peri-infarct area by a reduced number of parvalbumin⁺ / c-fos⁺ neurons and glutamic acid decarboxylase 65/67 levels. In addition, we have shown that T₃ modulates in vitro neuron membrane properties with the balance of inward glutamate ligand-gated channels currents and decreases synaptotagmin levels in conditions of deprived oxygen and glucose. Interestingly, we found increased levels of TRβ1 in the infarct core of post-mortem human stroke patients, which mediate T₃ actions. Summarizing, our data identify T₃ as a potential key therapeutic agent to enhance recovery of lost neurological functions after ischemic stroke.

Low free triiodothyronineis predicts worsen neurological outcome of patients with acute ischemic stroke: a retrospective study with bioinformatics analysis

Backgroud

Patients with acute ischemic stroke (AIS) often experience low serum free triiodothyronine (FT3), but the association of low FT3 with stroke severity, subtype and prognosis has not yet been thoroughly studied, and the molecular events underlying these clinical observation were also unclear.

Methods

We retrospectively collected 221 cases of AIS and 182 non-AIS cases with detailed clinical data from our department. FT3 concentrations were measured on admission to predict functional outcome within 3 months using multivariable models adjusted for other risk factors. Receiver operating characteristic (ROC) curves were calculated to define the best cutoff value of FT3 of stroke severity, subtypes and neurological outcome. Gene set enrichment, pathway mapping and network analyses of deferentially expressed genes (DEGs) were performed.

Results

FT3 was significantly decreased in AIS patients with National Institutes of Health Stroke Scale (NIHSS) > 3 and 3-months modified Rankin Scale (mRS) > 2. The cut-off value of FT3 for NIHSS on admission was 4.30 pmol/L. Also, FT3 level was significantly lower in large artery atherosclerosis (LAA) group and cardioembolism (CE) group than that in small vessel occlusion (SVO). FT3 value served as an independent predictor for neurological outcomes for which the cut-off value of FT3 was 4.38 pmol/l. Gene ontology (GO) analysis showed that the biological function of DEGs was mainly enriched in multicellur organism, neuron differentiation and cellular response to hypoxia. The cellular components were involved in extracelluar region, exosome and matrix, and the molecular functions were transcriptional activator activity, DNA binding and nuclear hormone receptor binding. Signal pathways analysis was indicative of neuroactive ligand-receptor interaction, thyroid hormone signaling pathway, and protein digestion and absorption these DEGs were involved in. Six related gene were identified as hubs from the protein-protein interaction (PPI) networks. Three modules were selected from PPI, of which MMP4, ADRA2C and EIF3E were recognized as the seed genes.

Conclusions

Low FT3 value on admission was associated with stroke severity, subtype and prognosis. In addition, DEGs identified from bioinformatics analysis are likely to be candidates for elucidating clinical outcomes with low FT3, and provide us with therapeutic targets for improving stroke prognosis.

Thyroid Hormones in the Brain and Their Impact in Recovery Mechanisms After Stroke

Thyroid hormones are of fundamental importance for brain development and essential factors to warrant brain functions throughout life. Their actions are mediated by binding to specific intracellular and membranous receptors regulating genomic and non-genomic mechanisms in neurons and populations of glial cells, respectively. Among others, mechanisms include the regulation of neuronal plasticity processes, stimulation of angiogenesis and neurogenesis as well modulating the dynamics of cytoskeletal elements and intracellular transport processes. These mechanisms overlap with those that have been identified to enhance recovery of lost neurological functions during the first weeks and months after ischemic stroke. Stimulation of thyroid hormone signaling in the postischemic brain might be a promising therapeutic strategy to foster endogenous mechanisms of repair. Several studies have pointed to a significant association between thyroid hormones and outcome after stroke. With this review, we will provide an overview on functions of thyroid hormones in the healthy brain and summarize their mechanisms of action in the developing and adult brain. Also, we compile the major thyroid-modulated molecular pathways in the pathophysiology of ischemic stroke that can enhance recovery, highlighting thyroid hormones as a potential target for therapeutic intervention.

Thyroid Hormone Protects Primary Cortical Neurons Exposed to Hypoxia by Reducing DNA Methylation and Apoptosis

Traumatic brain injury (TBI) is associated with disruption of cerebral blood flow leading to localized brain hypoxia. Thyroid hormone (TH) treatment, administered shortly after injury, has been shown to promote neural protection in rodent TBI models. The mechanism of TH protection, however, is not established. We used mouse primary cortical neurons to investigate the effectiveness and possible pathways of T3-promoted cell survival after exposure to hypoxic injury. Cultured primary cortical neurons were exposed to hypoxia (0.2% oxygen) for 7 hours with or without T3 (5 nM). T3 treatment enhanced DNA 5-hydroxymethylcytosine levels and attenuated the hypoxia-induced increase in DNA 5-methylcytosine (5-mc). In the presence of T3, mRNA expression of Tet family genes was increased and DNA methyltransferase (Dnmt) 3a and Dnmt3b were downregulated, compared with conditions in the absence of T3. These T3-induced changes decreased hypoxia-induced DNA de novo methylation, which reduced hypoxia-induced neuronal damage and apoptosis. We used RNA sequencing to characterize T3-regulated genes in cortical neurons under hypoxic conditions and identified 22 genes that were upregulated and 15 genes that were downregulated. Krüppel-like factor 9 (KLF9), a multifunctional transcription factor that plays a key role in central nervous system development, was highly upregulated by T3 treatment in hypoxic conditions. Knockdown of the KLF9 gene resulted in early apoptosis and abolished the beneficial role of T3 in neuronal survival. KLF9 mediates, in part, the neuronal protective role of T3. T3 treatment reduces hypoxic damage, although pathways that reduce DNA methylation and apoptosis remain to be elucidated.

Thyroid Hormone Signalling: From the Dawn of Life to the Bedside

Thyroid hormone (TH) signalling is a key modulator of fundamental biological processes that has been evolutionarily conserved in both vertebrate and invertebrate species. TH may have initially emerged as a nutrient signal to convey environmental information to organisms to induce morpho-anatomical changes that could maximise the exploitation of environmental resources, and eventually integrated into the machinery of gene regulation and energy production to become a key regulator of development and metabolism. As such, TH signalling is particularly sensitive to environmental stimuli, and its alterations result in fundamental changes in homeostasis and physiology. Stressful stimuli of various origins lead to changes in the TH-TH receptor (TR) axis in different adult mammalian organs that are associated with phenotypical changes in terminally differentiated cells, the reactivation of foetal development programmes, structural remodelling and pathological growth. Here, we discuss the evolution of TH signalling, review evolutionarily conserved functions of THs in essential biological processes, such as metamorphosis and perinatal development, and analyse the role of TH signalling in the phenotypical and morphological changes that occur after injury, repair and regeneration in adult mammalian organs. Finally, we examine the potential of TH treatment as a therapeutic strategy for improving organ structure and functions following injury.

Paradigms of Dynamic Control of Thyroid Hormone Signaling

Thyroid hormone (TH) molecules enter cells via membrane transporters and, depending on the cell type, can be activated (i.e., T4 to T3 conversion) or inactivated (i.e., T3 to 3,3'-diiodo-l-thyronine or T4 to reverse T3 conversion). These reactions are catalyzed by the deiodinases. The biologically active hormone, T3, eventually binds to intracellular TH receptors (TRs), TRα and TRβ, and initiate TH signaling, that is, regulation of target genes and other metabolic pathways. At least three families of transmembrane transporters, MCT, OATP, and LAT, facilitate the entry of TH into cells, which follow the gradient of free hormone between the extracellular fluid and the cytoplasm. Inactivation or marked downregulation of TH transporters can dampen TH signaling. At the same time, dynamic modifications in the expression or activity of TRs and transcriptional coregulators can affect positively or negatively the intensity of TH signaling. However, the deiodinases are the element that provides greatest amplitude in dynamic control of TH signaling. Cells that express the activating deiodinase DIO2 can rapidly enhance TH signaling due to intracellular buildup of T3. In contrast, TH signaling is dampened in cells that express the inactivating deiodinase DIO3. This explains how THs can regulate pathways in development, metabolism, and growth, despite rather stable levels in the circulation. As a consequence, TH signaling is unique for each cell (tissue or organ), depending on circulating TH levels and on the exclusive blend of transporters, deiodinases, and TRs present in each cell. In this review we explore the key mechanisms underlying customization of TH signaling during development, in health and in disease states.

Association of LHPP genetic variation (rs35936514) with structural and functional connectivity of hippocampal-corticolimbic neural circuitry

A single nucleotide polymorphism at the LHPP gene (rs35936514) has been reported to be associated with major depressive disorder (MDD) in genome-wide association studies. We conducted a neuroimaging analysis to explore whether and which brain neural systems are affected by LHPP variation. Since LHPP variants seem to be associated with the hippocampus, we assessed the relationship between rs35936514 variation and structural-functional connectivity within a hippocampal-corticolimbic neural system implicated in MDD. A total of 122 Chinese subjects were divided into a CC homozygous group (CC genotype, n = 60) and a T allele-carrier group (CT/TT genotypes, n = 62). All subjects participated in resting-state functional magnetic resonance imaging (rs-fMRI) and diffusion tensor imaging (DTI) scans. Structural and functional connectivity data analyses were then performed. Compared to the CC group, the T allele-carrier group showed significantly higher fractional anisotropy (FA) values in the fornix as well as increased functional connectivity from the hippocampus to the rostral part of the anterior cingulate cortex (rACC). Moreover, a significant negative correlation between fornix FA value and hippocampus-rACC functional connectivity was identified (P < 0.05). These findings suggest that there is a relationship between rs35936514 variation and both structural and functional hippocampal-corticolimbic neural system involvement in MDD. LHPP may play an important role in the neuropathophysiology of MDD.

Angiotensin II induces oxidative stress and upregulates neuroprotective signaling from the NRF2 and KLF9 pathway in dopaminergic cells

Nuclear factor-E2-related factor 2 (NRF2) is a transcription factor that activates the antioxidant cellular defense in response to oxidative stress, leading to neuroprotective effects in Parkinson's disease (PD) models. We have previously shown that Angiotensin II (AngII) induces an increase in reactive oxygen species (ROS) via AngII receptor type 1 and NADPH oxidase (NOX), which may activate the NRF2 pathway. However, controversial data suggest that AngII induces a decrease in NRF2 signaling leading to an increase in oxidative stress. We analyzed the effect of AngII and the dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA) in culture and in vivo, and examined the effects on the expression of NRF2-related genes. Treatment of neuronal cell lines Mes23.5, N27 and SH-SY5Y with AngII, 6-OHDA or a combination of both increased ROS production and reduced cell viability. Simultaneously, these treatments induced an increase in expression in the NRF2-regulated genes heme oxygenase 1 (Hmox1), NAD(P)H quinone dehydrogenase 1 (Nqo1) and Kruppel like factor 9 (Klf9). Moreover, overexpression of KLF9 transcription factor caused a reduction in the production of ROS induced by treatment with AngII or 6-OHDA and improved the survival of these neuronal cells. Rats treated with AngII, 6-OHDA or a combination of both also showed an increased expression of NRF2 related genes and KLF9. In conclusion, our data indicate that AngII induces a damaging effect in neuronal cells, but also acts as a signaling molecule to activate NRF2 and KLF9 neuroprotective pathways in cellular and animal models of PD.

T3 level may be a helpful marker to predict disease prognosis of acute central nervous system viral infections

AIM OF THE STUDY

Acute central nervous system viral infections are progressive and inflammatory diseases with inflammatory cells infiltrating into the central nervous system (CNS), and thyroid hormone (TH) level is associated with the oxidative and antioxidant status. Variations in oxidative stress and antioxidant status are related to the pathogenesis of inflammatory diseases. Our study aimed to investigate the possible correlation between viral infections in CNS and TH levels of thyroid stimulating hormone (TSH), free thyroxine (fT4) and free triiodothyronine (fT3).

MATERIALS AND METHODS

We measured serum concentrations of TSH, fT4, and fT3 in 206 individuals, including 59 viral meningitis (VM) patients, 60 viral encephalitis (VE) patients, and 87 healthy controls.

RESULTS

Our findings showed that VE and VM patients had lower levels of fT3 and higher levels of fT4 compared with healthy controls, whether male or female. Moreover, levels of TSH and fT3 in patients with viral infections in CNS were inversely correlated with disease prognosis measured by the Glasgow Outcome Scale.

CONCLUSIONS

Variations in TH level may represent the oxidative status and are surrogate biomarkers for disease prognosis of acute central nervous system viral infections.

Single cell molecular alterations reveal target cells and pathways of concussive brain injury

The complex neuropathology of traumatic brain injury (TBI) is difficult to dissect, given the convoluted cytoarchitecture of affected brain regions such as the hippocampus. Hippocampal dysfunction during TBI results in cognitive decline that may escalate to other neurological disorders, the molecular basis of which is hidden in the genomic programs of individual cells. Using the unbiased single cell sequencing method Drop-seq, we report that concussive TBI affects previously undefined cell populations, in addition to classical hippocampal cell types. TBI also impacts cell type-specific genes and pathways and alters gene co-expression across cell types, suggesting hidden pathogenic mechanisms and therapeutic target pathways. Modulating the thyroid hormone pathway as informed by the T4 transporter transthyretin Ttr mitigates TBI-associated genomic and behavioral abnormalities. Thus, single cell genomics provides unique information about how TBI impacts diverse hippocampal cell types, adding new insights into the pathogenic pathways amenable to therapeutics in TBI and related disorders.

Divergent Induction of Branched-Chain Aminotransferases and Phosphorylation of Branched Chain Keto-Acid Dehydrogenase Is a Potential Mechanism Coupling Branched-Chain Keto-Acid-Mediated-Astrocyte Activation to Branched-Chain Amino Acid Depletion-Mediated Cognitive Deficit after Traumatic Brain Injury

Deficient branched-chain amino acids (BCAAs) are implicated in cognitive dysfunction after traumatic brain injury (TBI). The mechanism remains unknown. BCAAs are catabolized by neuron-specific cytosolic and astrocyte-specific mitochondrial branched-chain aminotransferases (BCATc, BCATm) to generate glutamate and branched-chain keto-acids (BCKAs) that are metabolized by the mitochondrial branched-chain keto-acid dehydrogenase (BCKD) whose activity is regulated by its phosphorylation state. BCKD phosphorylation by BCKD kinase (BCKDK) inactivates BCKD and cause neurocognitive dysfunction, whereas dephosphorylation by specific phosphatase restores BCKD activity. Real-time polymerase chain reaction showed rapidly and significantly decreased BCATc messenger RNA (mRNA) levels, but significantly increased BCATm mRNA level post-CCI (controlled cortical impact). BCKD and BCKDK mRNA decreased significantly immediately after CCI-induced TBI (CCI) in the rat. Phosphorylated BCKD proteins (pBCKD) increased significantly in the ipsilateral-CCI hemisphere. Immunohistochemistry revealed significantly increased pBCKD proteins in ipsilateral astrocytes post-CCI. BCKD protein expression is higher in primarily cultured cortical neurons than in astrocytes, whereas pBCKD protein level is higher in astrocytes than in cortical neurons. Transforming growth factor beta treatment (10 μg/mL for 48 h) significantly increased pBCKD protein expression in astrocytes, whereas glutamate treatment (25 μM for 24 h) significantly decreased pBCKD protein in neurons. Because increased pBCKD would lead to increased BCKA accumulation, BCKA-mediated astrocyte activation, cell death, and cognitive dysfunction as found in maple syrup urine disease; thus, TBI may potentially induce cognitive deficit through diverting BCAA from glutamate production in neurons to BCKA production in astrocytes through the pBCKD-dependent mechanism.

Thyroid hormone receptor α1 as a novel therapeutic target for tissue repair

Analogies between the damaged tissue and developing organ indicate that a regulatory network that drives embryonic organ development may control aspects of tissue repair. In this regard, there is a growing body of experimental and clinical evidence showing that TH may be critical for recovery after injury. Especially TRα1 has been reported to play an essential role in cell proliferation and differentiation and thus in the process of repair/regeneration in the heart and other tissues. Patients after myocardial infarction, stroke or therapeutic interventions [such as PCI for coronary artery disease (CAD)] with lower TH levels appear to have increased morbidity and mortality. Accordingly, TH treatment in clinical settings of ischemia/reperfusion such as by-pass surgery seems to be cardioprotective against ischemic injury. Furthermore, TH therapy of donors is shown to result in organ preservation and increased numbers of donors and improved post-transplantation graft survival. TH and thyroid analogs may prove novel therapeutic agents for tissue repair.

Traumatic Brain Injury: At the Crossroads of Neuropathology and Common Metabolic Endocrinopathies

Building on the seminal work by Geoffrey Harris in the 1970s, the neuroendocrinology field, having undergone spectacular growth, has endeavored to understand the mechanisms of hormonal connectivity between the brain and the rest of the body. Given the fundamental role of the brain in the orchestration of endocrine processes through interactions among neurohormones, it is thus not surprising that the structural and/or functional alterations following traumatic brain injury (TBI) can lead to endocrine changes affecting the whole organism. Taking into account that systemic hormones also act on the brain, modifying its structure and biochemistry, and can acutely and chronically affect several neurophysiological endpoints, the question is to what extent preexisting endocrine dysfunction may set the stage for an adverse outcome after TBI. In this review, we provide an overview of some aspects of three common metabolic endocrinopathies, e.g., diabetes mellitus, obesity, and thyroid dysfunction, and how these could be triggered by TBI. In addition, we discuss how the complex endocrine networks are woven into the responses to sudden changes after TBI, as well as some of the potential mechanisms that, separately or synergistically, can influence outcomes after TBI.

Thyroid hormone and the brain: Mechanisms of action in development and role in protection and promotion of recovery after brain injury

Thyroid hormone (TH) is essential for normal brain development and may also promote recovery and neuronal regeneration after brain injury. TH acts predominantly through the nuclear receptors, TH receptor alpha (THRA) and beta (THRB). Additional factors that impact TH action in the brain include metabolism, activation of thyroxine (T4) to triiodothyronine (T3) by the enzyme 5'-deiodinase Type 2 (Dio2), inactivation by the enzyme 5-deiodinase Type 3 (Dio3) to reverse T3 (rT3), which occurs in glial cells, and uptake by the Mct8 transporter in neurons. Traumatic brain injury (TBI) is associated with inflammation, metabolic alterations and neural death. In clinical studies, central hypothyroidism, due to hypothalamic and pituitary dysfunction, has been found in some individuals after brain injury. TH has been shown, in animal models, to be protective for the damage incurred from brain injury and may have a role to limit injury and promote recovery. Although clinical trials have not yet been reported, findings from in vitro and in vivo models inform potential treatment strategies utilizing TH for protection and promotion of recovery after brain injury.

Maternal hormonal milieu influence on fetal brain development

An adverse maternal hormonal environment during pregnancy can be associated with abnormal brain growth. Subtle changes in fetal brain development have been observed even for maternal hormone levels within the currently accepted physiologic ranges. In this review, we provide an update of the research data on maternal hormonal impact on fetal neurodevelopment, giving particular emphasis to thyroid hormones and glucocorticoids. Thyroid hormones are required for normal brain development. Despite serum TSH appearing to be the most accurate indicator of thyroid function in pregnancy, maternal serum free T4 levels in the first trimester of pregnancy are the major determinant of postnatal psychomotor development. Even a transient period of maternal hypothyroxinemia at the beginning of neurogenesis can confer a higher risk of expressive language and nonverbal cognitive delays in offspring. Nevertheless, most recent clinical guidelines advocate for targeted high-risk case finding during first trimester of pregnancy despite universal thyroid function screening. Corticosteroids are determinant in suppressing cell proliferation and stimulating terminal differentiation, a fundamental switch for the maturation of fetal organs. Not surprisingly, intrauterine exposure to stress or high levels of glucocorticoids, endogenous or synthetic, has a molecular and structural impact on brain development and appears to impair cognition and increase anxiety and reactivity to stress. Limbic regions, such as hippocampus and amygdala, are particularly sensitive. Repeated doses of prenatal corticosteroids seem to have short-term benefits of less respiratory distress and fewer serious health problems in offspring. Nevertheless, neurodevelopmental growth in later childhood and adulthood needs further clarification. Future studies should address the relevance of monitoring the level of thyroid hormones and corticosteroids during pregnancy in the risk stratification for impaired postnatal neurodevelopment.