Thyroid hormone levels in the cerebrospinal fluid correlate with disease severity in euthyroid patients with Alzheimer's disease

Moderation of thyroid hormones for the relationship between amyloid and tau pathology

BACKGROUND

Altered thyroid hormone levels have been associated with increased risk of Alzheimer's disease (AD) dementia and related cognitive decline. However, the neuropathological substrates underlying the link between thyroid hormones and AD dementia are not yet fully understood. We first investigated the association between serum thyroid hormone levels and in vivo AD pathologies including both beta-amyloid (Aβ) and tau deposition measured by positron emission tomography (PET). Given the well-known relationship between Aβ and tau pathology in AD, we additionally examined the moderating effects of thyroid hormone levels on the association between Aβ and tau deposition.

METHODS

This cross-sectional study was conducted as part of the Korean Brain Aging Study for Early Diagnosis and Prediction of Alzheimer's Disease (KBASE) cohort. This study included a total of 291 cognitively normal adults aged 55 to 90. All participants received comprehensive clinical assessments, measurements for serum total triiodothyronine (T3), free triiodothyronine (fT3), free thyroxine (fT4), and thyroid-stimulating hormone (TSH), and brain imaging evaluations including [11C]-Pittsburgh compound B (PiB)- PET and [18F] AV-1451 PET.

RESULTS

No associations were found between either thyroid hormones or TSH and Aβ and tau deposition on PET. However, fT4 (p = 0.002) and fT3 (p = 0.001) exhibited significant interactions with Aβ on tau deposition: The sensitivity analyses conducted after the removal of an outlier showed that the interaction effect between fT4 and Aβ deposition was not significant, whereas the interaction between fT3 and Aβ deposition remained significant. However, further subgroup analyses demonstrated a more pronounced positive relationship between Aβ and tau in both the higher fT4 and fT3 groups compared to the lower group, irrespective of outlier removal. Meanwhile, neither T3 nor TSH had any interaction with Aβ on tau deposition.

CONCLUSION

Our findings suggest that serum thyroid hormones may moderate the relationship between cerebral Aβ and tau pathology. Higher levels of serum thyroid hormones could potentially accelerate the Aβ-dependent tau deposition in the brain. Further replication studies in independent samples are needed to verify the current results.

The disturbance of thyroid-associated hormone and its receptors in brain and blood circulation existed in the early stage of mouse model of Alzheimer's disease

BACKGROUND

Studies showed that thyroid function plays an important role in the pathology of Alzheimer's disease (AD). However, changes in brain thyroid hormone and related receptors in the early stage of AD were rarely reported. The aim of this study was to explore the relationship between the early stage of AD and local thyroid hormone and its receptors in the brain.

METHODS

The animal model was established by stereotactic injection of okadaic acid (OA) into hippocampal region for the experiment, and 0.9% NS for the control. Blood sample from each mouse was collected and then the mice were sacrificed and the brain tissue was collected for detecting free triiodothyronine (FT3), free thyroid hormone (FT4), and thyroid-stimulating hormone (TSH), thyrotropin-releasing hormone (TRH) and phosphorylated tau, amyloid-β (Aβ) and thyroid hormone receptors (THRs) in the hippocampus of the mice were detected as well.

RESULTS

Enzyme-linked immunosorbent assay showed that compared with the control, FT3, FT4, TSH and TRH in brain were significantly increased in the experimental group; in the serum, FT4, TSH and TRH were increased, while FT3 had no change; western blot analysis indicated that the expression of THR α and β in the hippocampus of the experimental group was significantly higher than that of the control.

CONCLUSION

Based on the results of this study, a mouse AD model can be established successfully by injecting a small dose of OA into the hippocampus. We speculate that early AD brain and circulating thyroid dysfunction may be an early local and systemic stress repair response.

Thyroid hormone levels in Alzheimer disease: a systematic review and meta-analysis

BACKGROUND AND OBJECTIVE

Thyroid hormone (TH) disturbances are perceived to contribute to cognitive impairment and dementia. However, there is no consensus on the association between TH levels and Alzheimer Disease (AD). In this study, we aimed to compare serum and cerebrospinal fluid (CSF) TH levels in AD patients to controls by performing a meta-analysis.

METHODS

We systematically searched online databases for papers comparing CSF or serum TH levels in AD patients to controls, and performed a meta-analysis on the extracted data.

RESULTS

Out of 1604 records identified, 32 studies were included. No significant difference in serum TSH (standardized mean difference (SMD): -0.03; 95% confidence interval (CI): -0.22-0.16), total T4 (SMD: 0.10; 95% CI: -0.29-0.49), and free T4 (SMD: 0.25; 95% CI: -0.16-0.69) levels were observed. However, there was significantly lower serum total T3 (SMD: -0.56; 95%CI: -0.97 to -0.15) and free T3 (SMD: -0.47; 95%CI: -0.89 to -0.05) levels in AD group compared to controls. Subgroup analyses on studies including only euthyroid patients did not show any significant difference in either of the thyroid hormone levels. Also, no significant difference in CSF total T4 and total T3/total T4 ratios but significantly lower CSF total T3 (SMD: -2.45; 95% CI: -4.89 to -0.02) in AD group were detected.

CONCLUSION

Serum total and free T3 and CSF total T3 levels are significantly lower in AD patients compared to controls. The temporality of changes in thyroid hormone levels and AD development should be illustrated by further longitudinal studies.

Triiodothyronine Treatment reverses Depression-Like Behavior in a triple-transgenic animal model of Alzheimer's Disease

Alzheimer disease's (AD) is a neurodegenerative disorder characterized by cognitive and behavioral impairment. The central nervous system is an important target of thyroid hormones (TH). An inverse association between serum triiodothyronine (T3) levels and the risk of AD symptoms and progression has been reported. We investigated the effects of T3 treatment on the depression-like behavior in male transgenic 3xTg-AD mice. Animals were divided into 2 groups treated with daily intraperitoneal injections of 20 ng/g of body weight (b.w.) L-T3 (T3 group) or saline (vehicle, control group). The experimental protocol lasted 21 days, and behavioral tests were conducted on days 18-20. At the end of the experiment, the TH profile and hippocampal gene expression were evaluated. The T3-treated group significantly increased serum T3 and decreased thyroxine (T4) levels. When compared to control hippocampal samples, the T3 group exhibited attenuated glycogen synthase kinase-3 (GSK3), metalloproteinase 10 (ADAM10), amyloid-beta precursor-protein (APP), serotonin transporter (SERT), 5HT1A receptor, monocarboxylate transporter 8 (MCT8) and bone morphogenetic protein 7 (BMP-7) gene expression, whereas augmented superoxide dismutase 2 (SOD2) and Hairless gene expression. T3-treated animals also displayed reduced immobility time in both the tail suspension and forced swim tests, and in the latter presented a higher latency time compared to the control group. Therefore, our findings suggest that in an AD mouse model, T3 supplementation promotes improvements in depression-like behavior, through the modulation of the serotonergic related genes involved in the transmission mediated by 5HT1A receptors and serotonin reuptake, and attenuated disease progression.

Mass spectrometry in the diagnosis of thyroid disease and in the study of thyroid hormone metabolism

The importance of thyroid hormones in the regulation of development, growth, and energy metabolism is well known. Over the last decades, mass spectrometry has been extensively used to investigate thyroid hormone metabolism and to discover and characterize new molecules involved in thyroid hormones production, such as thyrotropin-releasing hormone. In the earlier period, the quantification methods, usually based on gas chromatography-mass spectrometry, were complicated and time consuming. They were mainly focused on basic research, and were not suitable for clinical diagnostics on a routine basis. The development of the modern mass spectrometers, mainly coupled to liquid chromatography, enabled simpler sample preparation procedures, and the accurate quantification of thyroid hormones, of their precursors, and of their metabolites in biological fluids, tissues, and cells became feasible. Nowadays, molecules of physiological and pathological interest can be assayed also for diagnostic purposes on a routine basis, and mass spectrometry is slowly entering the clinical laboratory. This review takes stock of the advancements in the field of thyroid metabolism that were carried out with mass spectrometry, with special focus on the use of this technique for the quantification of molecules involved in thyroid diseases.

Hypothyroidism and Diabetes-Related Dementia: Focused on Neuronal Dysfunction, Insulin Resistance, and Dyslipidemia

The incidence of dementia is steadily increasing worldwide. The risk factors for dementia are diverse, and include genetic background, environmental factors, sex differences, and vascular abnormalities. Among the subtypes of dementia, diabetes-related dementia is emerging as a complex type of dementia related to metabolic imbalance, due to the increase in the number of patients with metabolic syndrome and dementia worldwide. Thyroid hormones are considered metabolic regulatory hormones and affect various diseases, such as liver failure, obesity, and dementia. Thyroid dysregulation affects various cellular mechanisms and is linked to multiple disease pathologies. In particular, hypothyroidism is considered a critical cause for various neurological problems-such as metabolic disease, depressive symptoms, and dementia-in the central nervous system. Recent studies have demonstrated the relationship between hypothyroidism and brain insulin resistance and dyslipidemia, leading to diabetes-related dementia. Therefore, we reviewed the relationship between hypothyroidism and diabetes-related dementia, with a focus on major features of diabetes-related dementia such as insulin resistance, neuronal dysfunction, and dyslipidemia.

TREM2 is thyroid hormone regulated making the TREM2 pathway druggable with ligands for thyroid hormone receptor

Triggering receptor expressed on myeloid cells-2 (TREM2) is a cell surface receptor on macrophages and microglia that senses and responds to disease-associated signals to regulate the phenotype of these innate immune cells. The TREM2 signaling pathway has been implicated in a variety of diseases ranging from neurodegeneration in the central nervous system to metabolic disease in the periphery. Here, we report that TREM2 is a thyroid hormone-regulated gene and its expression in macrophages and microglia is stimulated by thyroid hormone and synthetic thyroid hormone agonists (thyromimetics). Our findings report the endocrine regulation of TREM2 by thyroid hormone, and provide a unique opportunity to drug the TREM2 signaling pathway with orally active small-molecule therapeutic agents.

Association between thyroid function and Alzheimer's disease: A systematic review

Alterations in metabolic parameters have been associated with an increased risk of dementia, among which thyroid function has gained great importance in Alzheimer's disease (AD) pathology in recent years. However, it remains unclear whether thyroid dysfunctions could influence and contribute to the beginning and/or progression of AD or if it results from AD. This systematic review was conducted to examine the association between thyroid hormone (TH) levels and AD. Medline, ISI Web of Science, EMBASE, Cochrane library, Scopus, Scielo, and LILACS were searched, from January 2010 to March 2020. A total of 17 articles were selected. The studies reported alterations in TH and circadian rhythm in AD patients. Behavior, cognition, cerebral blood flow, and glucose consumption were correlated with TH deficits in AD patients. Whether thyroid dysfunctions and AD have a cause-effect relationship was inconclusive, however, the literature was able to provide enough data to corroborate a relationship between TH and AD. Although further studies are needed in this field, the current systematic review provides information that could help future investigations.

The connections of Locus Coeruleus with hypothalamus: potential involvement in Alzheimer's disease

The hypothalamus and Locus Coeruleus (LC) share a variety of functions, as both of them take part in the regulation of the sleep/wake cycle and in the modulation of autonomic and homeostatic activities. Such a functional interplay takes place due to the dense and complex anatomical connections linking the two brain structures. In Alzheimer's disease (AD), the occurrence of endocrine, autonomic and sleep disturbances have been associated with the disruption of the hypothalamic network; at the same time, in this disease, the occurrence of LC degeneration is receiving growing attention for the potential roles it may have both from a pathophysiological and pathogenetic point of view. In this review, we summarize the current knowledge on the anatomical and functional connections between the LC and hypothalamus, to better understand whether the impairment of the former may be responsible for the pathological involvement of the latter, and whether the disruption of their interplay may concur to the pathophysiology of AD. Although only a few papers specifically explored this topic, intriguingly, some pre-clinical and post-mortem human studies showed that aberrant protein spreading and neuroinflammation may cause hypothalamus degeneration and that these pathological features may be linked to LC impairment. Moreover, experimental studies in rodents showed that LC plays a relevant role in modulating the hypothalamic sleep/wake cycle regulation or neuroendocrine and systemic hormones; in line with this, the degeneration of LC itself may partly explain the occurrence of hypothalamic-related symptoms in AD.

The relation between thyroid dysregulation and impaired cognition/behaviour: An integrative review

Despite decades of research on the relation between thyroid diseases and cognition, the nature of this relationship remains elusive. An increasing prevalence of cognitive impairment and thyroid dysfunction has been consistently observed with ageing. Also, there appears to be an association between thyroid disorders and cognitive decline. Given the increasing global burden of dementia, elucidating the relationship between thyroid disorders as a potentially modifiable risk factor of cognitive impairment was the main goal of this review. We summarise the current literature examining the relationship between thyroid hormonal dysregulation and cognition or behaviour. We present the available imaging and pathological findings related to structural and functional brain changes related to thyroid hormonal dysregulation. We also propose potential mechanisms of interaction between thyroid hormones, autoantibodies and cognition/behaviour. Effects of gender, ethnicity and environmental factors are also briefly discussed. This review highlights the need for long-term prospective studies to capture the course of brain functional changes associated with the incidence and progression of thyroid dysregulations along with the confounding effects of non-modifiable risk factors such as gender and ethnicity. Moreover, double-blind controlled clinical trials are necessary to devise appropriate treatment plans to prevent cognitive consequences of over or undertreatment of thyroid disorders.

TREM2 is thyroid hormone regulated making the TREM2 pathway druggable with ligands for thyroid hormone receptor

Triggering receptor expressed on myeloid cells-2 (TREM2) is a cell surface receptor on macrophages and microglia that senses and responds to disease associated signals to regulate the phenotype of these innate immune cells. The TREM2 signaling pathway has been implicated in a variety of diseases ranging from neurodegeneration in the central nervous system to metabolic disease in the periphery. We report here that TREM2 is a thyroid hormone regulated gene and its expression in macrophages and microglia is stimulated by thyroid hormone. Both endogenous thyroid hormone and sobetirome, a synthetic thyroid hormone agonist drug, suppress pro-inflammatory cytokine production from myeloid cells including macrophages that have been treated with the SARS-CoV-2 spike protein which produces a strong, pro-inflammatory phenotype. Thyroid hormone agonism was also found to induce phagocytic behavior in microglia, a phenotype consistent with activation of the TREM2 pathway. The thyroid hormone antagonist NH-3 blocks the anti-inflammatory effects of thyroid hormone agonists and suppresses microglia phagocytosis. Finally, in a murine experimental autoimmune encephalomyelitis (EAE) multiple sclerosis model, treatment with Sob-AM2, a CNS-penetrating sobetirome prodrug, results in increased Trem2 expression in disease lesion resident myeloid cells which correlates with therapeutic benefit in the EAE clinical score and reduced damage to myelin. Our findings represent the first report of endocrine regulation of TREM2 and provide a unique opportunity to drug the TREM2 signaling pathway with orally active small molecule therapeutic agents.

T1AM-TAAR1 signalling protects against OGD-induced synaptic dysfunction in the entorhinal cortex

Abnormalities in thyroid hormones (TH) availability and/or metabolism have been hypothesized to contribute to Alzheimer's disease (AD) and to be a risk factor for stroke. Recently, 3-iodothyronamine (T₁AM), an endogenous amine putatively derived from TH metabolism, gained interest for its ability to promote learning and memory in the mouse. Moreover, T₁AM has been demonstrated to rescue the β-Amyloid dependent LTP impairment in the entorhinal cortex (EC), a brain area crucially involved in learning and memory and early affected during AD. In the present work, we have investigated the effect of T₁AM on ischemia-induced EC synaptic dysfunction. In EC brain slices exposed to oxygen-glucose deprivation (OGD), we demonstrated that the acute perfusion of T₁AM (5 μM) was capable of preventing ischemia-induced synaptic depression and that this protective effect was mediated by the trace amine-associated receptor 1 (TAAR1). Moreover, we demonstrated that activation of the BDNF-TrkB signalling is required for T₁AM action during ischemia. The protective effect of T₁AM was more evident when using EC slices from transgenic mutant human APP (mhAPP mice) that are more vulnerable to the effect of OGD. Our results confirm that the TH derivative T₁AM can rescue synaptic function after transient ischemia, an effect that was also observed in a Aβ-enriched environment.

Altered thyroid hormone profile in patients with Alzheimer's disease

BACKGROUND

Epidemiological studies have linked higher levels of thyroid hormones (THs) to increased risk of Alzheimer's disease (AD), whereas in advanced AD, THs have been unchanged or even decreased. In early AD dementia, little is known whether THs are related to AD neuropathology or brain morphology.

METHODS

This was a cross-sectional study of 36 euthyroid AD patients and 34 healthy controls recruited at a single memory clinic. Levels of THs were measured in serum and cerebrospinal fluid (CSF). In addition, we determined AD biomarkers (amyloid-β_1-42, total tau and phosphorylated tau) in CSF and hippocampal and amygdalar volumes using magnetic resonance imaging.

RESULTS

Serum free thyroxine (FT4) levels were elevated, whereas serum free triiodothyronine (FT3)/FT4 and total T3 (TT3)/total T4 (TT4) ratios were decreased, in AD patients compared to controls. In addition, serum TT4 was marginally higher in AD (p = 0.05 vs. the controls). Other TH levels in serum as well as CSF concentrations of THs were similar in both groups, and there were no correlations between THs and CSF AD biomarkers. However, serum FT3 correlated positively with left amygdalar volume in AD patients and serum TT3 correlated positively with left and right hippocampal volume in controls.

CONCLUSIONS

Thyroid hormones were moderately altered in mild AD dementia with increased serum FT4, and in addition, the reduced T3/T4 ratios may suggest decreased peripheral conversion of T4 to T3. Furthermore, serum T3 levels were related to brain structures involved in AD development.

Modulation of glutamate levels and Na+,K+-ATPase activity contributes to the chrysin memory recovery in hypothyroidism mice

Abnormalities in the thyroid hormones, like in hypothyroidism, are closely related to dementia and Alzheimer's disease demonstrating the main symptom of these disorders: memory deficit. In this study we evaluated the effect of chrysin on deficit spatial and aversive memories and the contribution of glutamatergic, cholinergic pathways and Na⁺, K⁺-ATPase activity on hippocampus and prefrontal cortex in hypothyroid adult female mice C57BL/6. Hypothyroidism was induced by the continuous exposure to 0.1% methimazole (MTZ) in drinking water for 31 days. The exposure to MTZ was associated to low plasma levels of thyroid hormones (TH) compared to the control group on the 32nd. Subsequently, euthyroid and MTZ-induced hypothyroid mice received (intragastrically) either vehicle or chrysin (20 mg/kg) once a day for 28 consecutive days. After treatments mice performed the following behavioral assessments: open-field test (OFT), morris water maze (MWM) and passive avoidance test. Additionally, plasma TH levels were measured again, as well as glutamate levels, Na⁺,K⁺-ATPase and acetylcholinesterase (AChE) activities were analyzed in the hippocampus and prefrontal cortex of mice. Mice with hypothyroidism showed a deficit of spatial and aversive memory and chrysin treatment reversed these deficits. It also reduced the levels of glutamate and decreased Na⁺,K⁺-ATPase activity in both cerebral structures in the hypothyroid mice compared with the euthyroid ones, with the exception of glutamate in the hippocampus, which was a partial reversal. AChE activity was not altered by treatments. Together, our results demonstrate that chrysin normalized hippocampal glutamate levels and Na⁺,K⁺-ATPase activity, which could be involved in the reversal of memory deficit.

Exogenous 3-Iodothyronamine Rescues the Entorhinal Cortex from β-Amyloid Toxicity

Background: A novel form of thyroid hormone (TH) signaling is represented by 3-iodothyronamine (T₁AM), an endogenous TH derivative that interacts with specific molecular targets, including trace amine-associated receptor 1 (TAAR₁), and induces pro-learning and anti-amnestic effects in mice. Dysregulation of TH signaling has long been hypothesized to play a role in Alzheimer's disease (AD). In the present investigation, we explored the neuroprotective role of T₁AM in beta amyloid (Aβ)-induced synaptic and behavioral impairment, focusing on the entorhinal cortex (EC), an area that is affected early by AD pathology. Methods: Field potentials were evoked in EC layer II, and long-term potentiation (LTP) was elicited by high frequency stimulation (HFS). T₁AM (5 μM) and/or Aβ(1-42) (200 nM), were administered for 10 minutes, starting 5 minutes before HFS. Selective TAAR₁ agonist RO5166017 (250 nM) and TAAR₁ antagonist EPPTB (5 nM) were also used. The electrophysiological experiments were repeated in EC-slices taken from a mouse model of AD (mutant human amyloid precursor protein [mhAPP], J20 line). We also assessed the in vivo effects of T₁AM on EC-dependent associative memory deficits, which were detected in mhAPP mice by behavioral evaluations based on the novel-object recognition paradigm. TAAR₁ expression was determined by Western blot, whereas T₁AM and its metabolite 3-iodothyroacetic acid (TA₁) were assayed by high-performance liquid chromatography coupled to mass spectrometry. Results: We demonstrate the presence of endogenous T₁AM and TAAR₁ in the EC of wild-type and mhAPP mice. Exposure to Aβ(1-42) inhibited LTP, and T₁AM perfusion (at a concentration of 5 μM, leading to an actual concentration in the perfusion buffer ranging from 44 to 298 nM) restored it, whereas equimolar amounts of 3,5,3'-triiodo-L-thyronine (T₃) and TA₁ were ineffective. The response to T₁AM was abolished by the TAAR₁ antagonist EPPTB, whereas it was mimicked by the TAAR₁ agonist RO5166017. In the EC of APPJ20 mice, LTP could not be elicited, but it was rescued by T₁AM. The intra-cerebro-ventricular administration of T₁AM (0.89 μg/kg) also restored recognition memory that was impaired in mhAPP mice. Conclusions: Our results suggest that T₁AM and TAAR₁ are part of an endogenous system that can be modulated to prevent synaptic and behavioral deficits associated with Aβ-related toxicity.

The effects of thyroid hormones on memory impairment and Alzheimer's disease

Thyroid hormones (THs) have a wide and important range of effects within the central nervous system beginning from fetal life and continuing throughout the adult life. Thyroid disorders are one of the major causes of cognitive impairment including Alzheimer's disease (AD). Several studies in recent years have indicated an association between hypothyroidism or hyperthyroidism and AD. Despite available evidence for this association, it remains unclear whether thyroid dysfunction results from or contributes to the progression of AD. This review discusses the role of THs in learning and memory and summarizes the studies that have linked thyroid function and AD. Eventually, we elaborate how THs may be effective in treating AD by putting forward potential mechanisms.

Low serum concentration of free triiodothyronine (FT3) is associated with increased risk of Alzheimer's disease

BACKGROUND

In epidemiological studies, thyroid hormones (THs) have been associated with the risk of dementia. However, little is known of the relation between THs and risk of Alzheimer's disease (AD) or vascular dementia (VaD) in a memory clinic population.

METHODS

In a mono-center study, serum concentrations of thyroid-stimulating hormone (TSH), free thyroxine (FT4), and free triiodothyronine (FT3) were assessed in 302 patients. All patients had subjective or objective mild cognitive impairment and none received treatment with THs. Cox proportional hazards regression analyses was used to determine whether THs at baseline were associated with the risk of conversion to all-cause dementia, AD or VaD.

RESULTS

During the follow-up (mean 2.8 years), 82 (28%) of the patients progressed to dementia [AD, n = 55 (18%) and VaD, n = 17 (6%)]. Serum concentrations of TSH, FT4, and FT3 did not associate with all-cause dementia or VaD. Higher serum FT3 was associated with lower risk of conversion to AD [hazard ratio (HR) = 054; 95% confidence interval (CI): 0.32-0.92 per 1 pmol/L increase]. Furthermore, patients in the lowest serum FT3 quartile had a twofold increased risk of AD compared to those in the highest quartile (HR = 2.63; 95% CI: 1.06-6.47). These associations remained after adjustment for multiple covariates.

CONCLUSIONS

In a memory clinic population, there was an inverse, linear association between serum FT3 and risk of AD whereas THs did not associate with all-cause dementia or VaD. Further studies are needed to determine the underlying mechanisms as well as the clinical significance of these findings.

A decade of thyroidology

Significant scientific progress has been achieved in the past decade in thyroidology driven by scholarly enquiry, unmet patient needs, and investment by the pharmaceutical and diagnostics industry. In this review, nine publications have been selected for their impact in pushing the frontiers of knowledge and understanding. They include new perspectives in the diagnosis, pathophysiology, epidemiology and management of thyroid cancer, understanding of thyroid hormone physiology, and new treatments for Graves' orbitopathy.

Thyroid Hormone Enhances Neurite Outgrowth in Neuroscreen 1 Cells

Objectives

Alzheimer's disease (AD) is a neurodegenerative disorder that affects millions of individuals. Moreover, hypothyroidism has been identified as one of the risk factors that may contribute to the development of AD. Here, we investigated whether there was a correlation among expression levels of proteins involved in the formation of AD lesions, neurite outgrowth, and thyroid hormone levels.

Methods

Cells were grown in media supplemented with different levels of 3,5,3'-triiodothyronine (T3) and then processed for neurite outgrowth and to prepare RNA samples. RNA samples were analysed using quantitative real-time PCR. Protein levels were measured using in cell-Western blotting analysis.

Results

By using neurite outgrowth studies, it was demonstrated that T3 treatment enhanced neurite outgrowth in NS-1 cells in a time- and dose-dependent manner. Quantitative real-time PCR studies further confirmed that NS-1 cells expressed substantial levels of TRα and significantly less TRβ, either of which could be responsible for the T3-dependent effects on neurite outgrowth. Although the overall tau protein expression was not affected in response to T3 treatment, the splicing of tau exon 10 was impacted in the direction of producing more tau molecules that excluded the exon (tau 3R).

Conclusion

The results of this study are critical not only to understand the probable link between hypothyroidism and AD but also in providing the basis for future prevention and treatment of AD in hypothyroid patients.

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.

Thyronamines and Derivatives: Physiological Relevance, Pharmacological Actions, and Future Research Directions

Thyronamines (3-T₁AM, T₀AM) are endogenous compounds probably derived from L-thyroxine or its intermediate metabolites. Combined activities of intestinal deiodinases and ornithine decarboxylase generate 3-T₁AM in vitro. Alternatively, 3-T₁AM might be formed by the thyroid gland and secreted into the blood. 3-T₁AM and T₀AM concentrations have been determined by liquid chromatography-tandem mass spectrometry analysis (LC-MS/MS) in tissues, serum, and cell lines. However, large variations of 3-T₁AM concentrations in human serum were reported by LC-MS/MS compared with a monoclonal antibody-based immunoassay. These differences might be caused by strong binding of the highly hydrophobic 3-T₁AM to apolipoprotein B100. Pharmacological administration of 3-T₁AM results in dose-dependent reversible effects on body temperature, cardiac function, energy metabolism, and neurological functions. The physiological relevance of these actions is unclear, but may occur at tissue concentrations close to the estimated endogenous concentrations of 3-T₁AM or its metabolites T₀AM or thyroacetic acid (TA₁). A number of putative receptors, binding sites, and cellular target molecules mediating actions of the multi-target ligand 3-T₁AM have been proposed. Among those are members of the trace amine associated receptor family, the adrenergic receptor ADRα2a, and the thermosensitive transient receptor potential melastatin 8 channel. Preclinical studies employing various animal experimental models are in progress, and more stable receptor-selective agonistic and antagonistic analogues of 3-T₁AM are now available for testing. The potent endogenous thyroid hormone-derived biogenic amine 3-T₁AM exerts marked cryogenic, metabolic, cardiac and central actions and represents a valuable lead compound linking endocrine, metabolic, and neuroscience research to advance development of new drugs.