Neuroendocrinology of neurodegenerative diseases. Insights from transgenic mouse models

The supraoptic and paraventricular nuclei in healthy aging and neurodegeneration

The supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus undergo structural and functional changes over the course of healthy aging. These nuclei and their connections are also heterogeneously affected by several different neurodegenerative diseases. This chapter reviews the involvement of the SON and PVN, the hypothalamic-pituitary axes, and the peptide hormones produced in both nuclei in healthy aging and in neurodegeneration, with a focus on Alzheimer's disease (AD), frontotemporal dementia (FTD), amyotrophic lateral sclerosis, progressive supranuclear palsy, Parkinson's disease (PD), dementia with Lewy bodies (DLB), multiple system atrophy, and Huntington's disease. Although age-related changes occur in several regions of the hypothalamus, the SON and PVN are relatively preserved during aging and in many neurodegenerative disorders. With aging, these nuclei do undergo some sexually dimorphic changes including changes in size and levels of vasopressin and corticotropin-releasing hormone, likely due to age-related changes in sex hormones. In contrast, oxytocinergic cells and circulating levels of thyrotropin-releasing hormone remain stable. A relative resistance to many forms of neurodegenerative pathology is also observed, in comparison to other hypothalamic and brain regions. Mirroring the pattern observed in aging, pathologic hallmarks of AD, and some subtypes of FTD are observed in the PVN, though to a milder degree than are observed in other brain regions, while the SON is relatively spared. In contrast, the SON appears more vulnerable to alpha-synuclein pathology of DLB and PD. The consequences of these alterations may help to inform several of the physiologic changes observed in aging and neurodegenerative disease.

Neuroendocrine Disturbances in Neurodegenerative Disorders: A Scoping Review

BACKGROUND

Neurodegenerative diseases cause progressive irreversible neuronal loss that has broad downstream effects. The neuroendocrine system regulates homeostasis of circuits that control critical functions such as the stress response, metabolism, reproduction, fluid balance, and glucose control. These systems are frequently disrupted in neurodegenerative disorders yet often overlooked in clinical practice.

OBJECTIVE

This review aims to gather the available data regarding these disturbances in Alzheimer's disease, Parkinson's disease, frontotemporal dementia, amyotrophic lateral sclerosis, and Huntington's disease and also to demonstrate the volume of literature in these individual arenas.

METHODS

Using the scoping review framework, a literature search was performed in PubMed to identify relevant articles published within the past 30 years (January 1988 to November 2018). The search criteria produced a total of 2022 articles, 328 of which were identified as relevant to this review.

RESULTS

Several major themes emerged from this review. These neuroendocrine disturbances may be a precursor to the illness, a part of the primary pathophysiology, or a direct consequence of the disease or independent of it. They have the potential to further understanding of the disease, exacerbate the underlying pathology, or provide therapeutic benefit.

CONCLUSIONS

By synthesizing the data from a systems' perspective, we aim to broaden how clinicians think about these illnesses and provide care.

Anti-edema action of thyroid hormone in MCAO model of ischemic brain stroke: Possible association with AQP4 modulation

The use of neuroprotective strategies to mitigate the fatal consequences of ischemic brain stroke is a focus of robust research activity. We have previously demonstrated that thyroid hormone (T3; 3,3',5-triiodo-l-thyronine) possesses neuroprotective and anti-edema activity in pre-stroke treatment regimens when administered as a solution or as a nanoparticle formulation. In this study we have extended our evaluation of thyroid hormone use in animal models of brain stroke. We have used both transient middle cerebral artery occlusion (t-MCAO) and permanent (p-MCAO) models of ischemic brain stroke. A significant reduction of tissue infarction and a concurrent decrease in edema were observed in the t-MCAO model of brain stroke. However, no benefit of T3 was observed in p-MCAO stroke setting. Significant improvement of neurological outcomes was observed upon T3 treatment in t-MCAO mice. Further, we tested T2 (3,5-diiodo-l-thyronine) a natural deiodination metabolite of T3 in MCAO model of brain stroke. T2 potently decreased infarct size as well as edema formation. Additionally, we report here that T3 suppresses the expression of aquaporin-4 (AQP4) water channels which could be a likely mechanism of its anti-edema activity. Our studies provide evidence to stimulate clinical development of thyroid hormones for use in ischemic brain stroke.

Nutritional care in motor neurone disease/ amyotrophic lateral sclerosis

Patients with amyotrophic lateral sclerosis (ALS) often present changes in nutritional status. Based on weight loss and on difficulty in nutritional management, this study aims to review the different possibilities and to present guidelines concerning nutritional treatment to such patients. Diet characteristics, types of treatment and nutritional therapy indicating administration routes and discussing the details of the disease are described herein. Nutritional therapy has been a substantial therapeutic resource for ALS development.

Thyrotropin secretion patterns in health and disease

Thyroid hormones are extremely important for metabolism, development, and growth during the lifetime. The hypothalamo-pituitary-thyroid axis is precisely regulated for these purposes. Much of our knowledge of this hormonal axis is derived from experiments in animals and mutations in man. This review examines the hypothalamo-pituitary-thyroid axis particularly in relation to the regulated 24-hour serum TSH concentration profiles in physiological and pathophysiological conditions, including obesity, primary hypothyroidism, pituitary diseases, psychiatric disorders, and selected neurological diseases. Diurnal TSH rhythms can be analyzed with novel and precise techniques, eg, operator-independent deconvolution and approximate entropy. These approaches provide indirect insight in the regulatory components in pathophysiological conditions.

Neuroendocrinology-based therapy for Alzheimer's disease

The nervous system interacts directly with the endocrine system to control a plethora of central nervous system (CNS) functions. Metabolic and reproductive hormones are known to be important in the maintenance of neuronal health and their fluctuations are important for CNS aspects ranging from sleep and appetite regulation to cognitive function. This review will summarize and critically evaluate how age-related changes in sex and metabolic hormones modulate affect cognitive function and the implications of targeting the neuroendocrinological system as a therapeutic strategy in Alzheimer's disease.

Genetic ablation of luteinizing hormone receptor improves the amyloid pathology in a mouse model of Alzheimer disease

Amyloid-beta peptide (Abeta) plays an essential pathophysiologic role in Alzheimer disease, and elevation of luteinizing hormone (LH) levels during aging has been implicated in its pathogenesis. To assess the effect of LH receptor deficiency on Abeta accumulation, we generated a bigenic mouse model, APPsw(+)/Lhr(-/-), which expresses human amyloid precursor protein (APPsw) in the background of LH receptor (Lhr) knockout. Genetic ablation of Lhr resulted in a significant decrease in the number of Abeta plaques and protein content in the hippocampus and cerebral cortex in both male and female mice. Accordingly, several Abeta deposition-related neuropathologic features and functionally relevant molecules were markedly improved, including decreased astrogliosis, reductions of elevated phosphorylated tau, c-fos, alpha7-nicotinic acetylcholine receptor, and restoration of the altered neuropeptide Y receptors Y1 and Y2. Diminution of Abeta accumulation in the absence of LH receptor supports the contention that dysregulation of LH may impact the pathogenesis of Alzheimer disease. The APPsw(+)/Lhr(-/-) mouse may be a useful tool for advancing understanding of the role of LH-mediated events in Alzheimer disease and a model in which to test therapeutic interventions.

Neuroendocrine disturbances in Huntington's disease

BACKGROUND

Huntington's disease (HD) is a severe inherited neurodegenerative disorder characterized, in addition to neurological impairment, by weight loss suggesting endocrine disturbances. The aims of this study were to look for neuroendocrine disturbances in patients with Huntington's disease (HD) and to determine the relationship with weight loss seen in HD METHODS AND FINDING: We compared plasma levels of hormones from the five pituitary axes in 219 patients with genetically documented HD and in 71 sex- and age-matched controls. Relationships between hormone levels and disease severity, including weight-loss severity, were evaluated. Growth hormone (GH) and standard deviation score of insulin-like growth factor 1 (SDS IGF-1) were significantly higher in patients than in controls (0.25 (0.01-5.89) vs. 0.15 (0.005-4.89) ng/ml, p = 0.013 and 0.16+/-1.02 vs. 0.06+/-0.91, p = 0.039; respectively). Cortisol was higher (p = 0.002) in patients (399.14+/-160.5 nmol/L vs. 279.8+/-130.1 nmol/L), whereas no differences were found for other hormone axes. In patients, elevations in GH and IGF-1 and decreases in thyroid-stimulating hormone, free triiodothyronine and testosterone (in men) were associated with severity of impairments (Independence scale, Functional score, Total Functional Capacity, Total Motor score, Behavioral score). Only GH was independently associated with body mass index (beta = -0.26, p = 0.001).

CONCLUSION

Our data suggest that the thyrotropic and in men gonadotropic axes are altered in HD according to the severity of the disease. The somatotropic axis is overactive even in patients with early disease, and could be related to the weight loss seen in HD patients.

Weight loss in neurodegenerative disorders

Unintended weight loss frequently complicates the course of many neurodegenerative disorders and can contribute substantially to both morbidity and mortality. This will be illustrated here by reviewing the characteristics of unintended weight loss in the three major neurodegenerative disorders: Alzheimer's disease, Parkinson's disease and Huntington's disease. A common denominator of weight loss in these neurodegenerative disorders is its typically complex pathophysiology. Timely recognition of the underlying pathophysiological process is of crucial importance, since a tailored treatment of weight loss can considerably improve the quality of life. This treatment is, primarily, comprised of a number of methods of increasing energy intake. Moreover, there are indications for defects in the systemic energy homeostasis and gastrointestinal function, which may also serve as therapeutic targets. However, the clinical merits of such interventions have yet to be demonstrated.

Growth hormone secretion is impaired in amyotrophic lateral sclerosis

OBJECTIVE

ALS is the most common motor neurone disorder in human adults. Scanty data on endocrine abnormalities have been reported. The aim of the present study was to investigate the GH-IGF-I axis in ALS patients.

PATIENTS

Twenty-two ALS patients (12 men, 10 women), mean age 61 years, and 25 normal age- and sex-matched subjects. No patient was under riluzole therapy.

MEASUREMENTS

Patients and controls underwent a GHRH plus arginine test. IGF-I was determined at baseline. A complete evaluation of pituitary function was also performed.

RESULTS

Mean (+/- SD) basal GH levels were significantly reduced compared with normal controls (0.2 +/- 0.3 vs 1.6 +/- 1.8 ng/ml, P < 0.01), as well as peak GH concentrations after GHRH + arginine administration (12.6 +/- 8.9 vs 39.9 +/- 18.7 ng/ml, P < 0.001). Six (27%) patients showed a normal GH response to stimulus; 7 (32%) patients displayed a moderate GH deficiency; in 9 (40%) patients GH response was markedly deficient. IGF-I levels were normal in the majority of patients (mean +/- SD: 143.6 +/- 63.8 ng/ml). No significant correlation was observed between peak GH concentrations and age, BMI, disease duration, severity or clinical form. A higher incidence of GH deficiency was observed in male compared to female patients (83%vs 60%), with a peak GH response in males significantly lower than in females (8.9 +/- 6.6 vs 17 +/- 9.6 ng/ml, P = 0.03). Eighteen patients repeated the test after 5 months and similar results were obtained.

CONCLUSIONS

The present data indicate a reduction of GH secretion in ALS patients.

Hypothalamic-endocrine aspects in Huntington's disease

Huntington's disease (HD) is a hereditary and fatal disorder caused by an expanded CAG triplet repeat in the HD gene, resulting in a mutant form of the protein huntingtin. Wild-type and mutant huntingtin are expressed in most tissues of the body but the normal function of huntingtin is not fully known. In HD, the neuropathology is characterized by intranuclear and cytoplasmic inclusions of huntingtin aggregates, and cell death primarily in striatum and cerebral cortex. However, hypothalamic atrophy occurs at early stages of HD with loss of orexin- and somatostatin-containing cell populations. Several symptoms of HD such as sleep disturbances, alterations in circadian rhythm, and weight loss may be due to hypothalamic dysfunction. Endocrine changes including increased cortisol levels, reduced testosterone levels and increased prevalence of diabetes are found in HD patients. In HD mice, alterations in the hypothalamic-pituitary-adrenal axis occurs as well as pancreatic beta-cell and adipocyte dysfunction. Increasing evidence points towards important pathology of the hypothalamus and the endocrine system in HD. As many neuroendocrine factors are secreted into the cerebrospinal fluid, blood and urine, it is possible that their levels may reflect the disease state in the central nervous system. Investigating neuroendocrine changes in HD opens up the possibility of finding biomarkers to evaluate future therapies for HD, as well as of identifying novel targets for therapeutic interventions.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Progressive alterations in the hypothalamic-pituitary-adrenal axis in the R6/2 transgenic mouse model of Huntington's disease

Huntington's disease (HD) is characterized by a triad of motor, psychiatric and cognitive symptoms. Although many of these symptoms are likely to be related to central nervous system pathology, others may be due to changes in peripheral tissues. The R6/2 mouse, a transgenic model of HD expressing exon 1 of the human HD gene, develops progressive alterations in the hypothalamic-pituitary-adrenal axis, reminiscent of a Cushing-like syndrome. We observed muscular atrophy, reduced bone mineral density, abdominal fat accumulation and insulin resistance in the mice. All these changes could be consequences of increased glucocorticoid levels. Indeed, hypertrophy of the adrenal cortex and a progressive increase in serum and urine corticosterone levels were found in R6/2 mice. In addition, the intermediate pituitary lobe was markedly enlarged and circulating adreno-corticotrophic hormone (ACTH) increased. Under normal conditions dopamine represses the ACTH expression. In the R6/2 mice, however, the expression of pituitary dopamine D2 receptors was reduced by half, possibly explaining the increase in ACTH. Urinary samples from 82 HD patients and 68 control subjects were analysed for cortisol: in accord with the observations in the R6/2 mice, urinary cortisol increased in parallel with disease progression. This progressive increase in cortisol may contribute to the clinical symptoms, such as muscular wasting, mood changes and some of the cognitive deficits that occur in HD.

Plasma aminopeptidase activities in rats after left and right intrastriatal administration of 6-hydroxydopamine

Asymmetries in the neuroendocrine system extend from central structures to paired endocrine glands and their innervation. In addition to the well-known asymmetry in the function of brain dopamine, there are also asymmetries in the peripheral response to experimental hemi-parkinsonism, performed by means of lesions of the nigrostriatal system with 6-hydroxydopamine (6-OHDA) injections into the left or right hemisphere. Therefore, it is speculated that the neuroendocrine system would also be asymmetrically affected in experimental hemi-parkinsonism. Aminopeptidases (AP) play a major role in the control of peptide concentration at both central and peripheral levels in tissues and blood, thus reflecting the functional status of their endogenous substrates. Therefore, to evaluate the peripheral response of hemi-parkinsonism, we have performed a comprehensive study of plasma AP activities after lesions of the nigrostriatal system with 6-OHDA administered into either left or right striatum of adult male rats. Saline was injected into control groups. AlaAP, CysAP, AspAP and GluAP activities were determined in plasma, using specific arylamides as substrates. Plasma AlaAP activity increased 3-fold (p < 0.001) whereas AspAP activity decreased by 30% (p < 0.05) after lesion of the right hemisphere. In contrast, CysAP and GluAP activities increased significantly after lesion of the left hemisphere by 200 and 50%, respectively (p < 0.05). The main discovery of the present results demonstrates that experimental hemi-parkinsonism affects differentially the plasma AP activities depending on the hemisphere in which the lesion is performed. This suggests that the circulating hormones, susceptible to be hydrolyzed by these enzymatic activities, are also modified.