Age-associated alterations in sympathetic neural interactions with the immune system

Regulation of Bone by Mechanical Loading, Sex Hormones, and Nerves: Integration of Such Regulatory Complexity and Implications for Bone Loss during Space Flight and Post-Menopausal Osteoporosis

During evolution, the development of bone was critical for many species to thrive and function in the boundary conditions of Earth. Furthermore, bone also became a storehouse for calcium that could be mobilized for reproductive purposes in mammals and other species. The critical nature of bone for both function and reproductive needs during evolution in the context of the boundary conditions of Earth has led to complex regulatory mechanisms that require integration for optimization of this tissue across the lifespan. Three important regulatory variables include mechanical loading, sex hormones, and innervation/neuroregulation. The importance of mechanical loading has been the target of much research as bone appears to subscribe to the "use it or lose it" paradigm. Furthermore, because of the importance of post-menopausal osteoporosis in the risk for fractures and loss of function, this aspect of bone regulation has also focused research on sex differences in bone regulation. The advent of space flight and exposure to microgravity has also led to renewed interest in this unique environment, which could not have been anticipated by evolution, to expose new insights into bone regulation. Finally, a body of evidence has also emerged indicating that the neuroregulation of bone is also central to maintaining function. However, there is still more that is needed to understand regarding how such variables are integrated across the lifespan to maintain function, particularly in a species that walks upright. This review will attempt to discuss these regulatory elements for bone integrity and propose how further study is needed to delineate the details to better understand how to improve treatments for those at risk for loss of bone integrity, such as in the post-menopausal state or during prolonged space flight.

Social Environment Ameliorates Behavioral and Immune Impairments in Tyrosine Hydroxylase Haploinsufficient Female Mice

The social environment can influence the functional capacity of nervous and immune systems, and consequently the state of health, especially in aged individuals. Adult female tyrosine hydroxylase haploinsufficient (TH-HZ) mice exhibit behavioral impairments, premature immunosenescence and oxidative- inflammatory stress. All these deteriorations are associated with a lower lifespan than wild type (WT) counterparts. The aim was to analyze whether the cohabitation with WT animals could revert or at least ameliorate the deterioration in the nervous and immune systems that female TH-HZ mice show at adult age. Female TH-HZ and WT mice at age of 3-4 weeks were divided into following groups: control TH-HZ (5 TH-HZ mice in the cage; TH-HZ100%), control WT (5 WT mice in the cage; WT100%), TH-HZ > 50% and WT < 50% (5 TH-HZ with 2 WT mice in each cage) as well as TH-HZ < 50% and WT > 50% (2 TH-HZ and 5 WT mice in each cage). At the age of 37-38 weeks, all mice were submitted to a battery of behavioral tests, evaluating sensorimotor abilities, exploratory capacities and anxiety-like behaviors. Subsequently, peritoneal leukocytes were extracted and several immune functions as well as oxidative and inflammatory stress parameters were analyzed. The results showed that the TH-HZ < 50% group had improved behavioral responses, especially anxiety-like behaviors, and the immunosenescence and oxidative stress of their peritoneal leukocytes were ameliorated. However, WT mice that cohabited with TH-HZ mice presented higher anxiety-like behaviors and deterioration in immune functions and in their inflammatory stress parameters. Thus, this social environment is capable of ameliorating the impairments associated with a haploinsufficiency of the th gene. Graphical Abstract.

Sympathetic neurotransmission in spleens from aging Brown-Norway rats subjected to reduced sympathetic tone

Senescence of innate and adaptive responses and low-grade inflammation (inflammaging) hallmarks normal aging, which increases vulnerability to infectious diseases, autoimmunity and cancer. In normal aging, sympathetic dysregulation contributes to the dysregulation of innate and adaptive immunity and inflammaging. Sympathetic innervation of immune cells in secondary immune organs regulates immune responses. Previously in Fischer 344 (F344) rats, we reported an age-related increase in sympathetic tone and sympathetic dysfunction in beta-adrenergic receptor (AR) signaling of splenic lymphocytes that contributes to immune senescence, although the responsible mechanisms remains unexplored. In this study, we extend our previous findings using the much longer-lived Brown-Norway (BN) rats, whose behavior and immune response profile differ strikingly from F344 rats. Here, we investigated whether increased sympathetic nerve activity (SNA) in the aging spleen contributes to age-related sympathetic neuropathy and altered neurotransmission in splenic lymphocytes in BN rats. Fifteen-month male BN rats received 0, 0.5 or 1.5 μg/kg/day rilmenidine intraperitoneally for 90 days to lower sympathetic tone. Untreated young and age-matched rats controlled for effects of age. We found that elevated SNA in the aging BN rat spleen does not contribute significantly to sympathetic neuropathy or the aging-induced impairment of canonical β-AR signal transduction. Despite the rilmenidine-induced increase in β-AR expression, splenocyte c-AMP production was comparable with age-matched controls, thus dampening nerve activity had no effect on receptor coupling to adenylate cyclase. Understanding how aging affects neuroimmune regulation in healthy aging rodent models may eventually lead to strategies that improve health in aging populations vulnerable to immunosenescence and low-grade systemic inflammation.

Premature aging in behavior and immune functions in tyrosine hydroxylase haploinsufficient female mice. A longitudinal study

Aging is accompanied by impairment in the nervous, immune, and endocrine systems as well as in neuroimmunoendocrine communication. In this context, there is an age-related alteration of the physiological response to acute stress, which is modulated by catecholamine (CA), final products of the sympathetic-adreno-medullary axis. The involvement of CA in essential functions of the nervous system is consistent with the neuropsychological deficits found in mice with haploinsufficiency (hemizygous; HZ) of tyrosine hydroxylase (TH) enzyme (TH-HZ). However, other possible alterations in regulatory systems have not been studied in these animals. The aim of the present work was to analyze whether adult TH-HZ female mice presented the impairment of behavioral traits and immunological responses that occurs with aging and whether they had affected their mean lifespan. ICR-CD1 female TH-HZ and wild type (WT) mice were used in a longitudinal study. Behavioral tests were performed on adult and old mice in order to evaluate their sensorimotor abilities and exploratory capacity, as well as anxiety-like behaviors. At the ages of 2 ± 1, 4 ± 1, 9 ± 1, 13 ± 1 and 20 ± 1 months, peritoneal leukocytes were extracted and several immune functions were assessed (phagocytic capacity, Natural Killer (NK) cytotoxicity, and lymphoproliferative response to lipopolysaccharide (LPS) and concanavalin A (ConA)). In addition, several oxidative stress parameters (catalase, glutathione reductase and glutathione peroxidase activities, and reduced glutathione (GSH) concentrations as antioxidant compounds as well as xanthine oxidase activity, oxidized glutathione (GSSG) concentrations, and GSSG/GSH ratio as oxidants) were analyzed. As inflammatory stress parameters TNF-alpha and IL-10 concentrations, and TNF-alpha/IL-10 ratios as inflammatory/anti-inflammatory markers, were measured. Animals were maintained in standard conditions until their natural death. The results indicate that adult TH-HZ mice presented worse sensorimotor abilities and exploratory capacity than their WT littermates as well as greater anxiety-like behaviors. With regards to the immune system, adult TH-HZ animals exhibited lower values of phagocytic capacity, NK cytotoxicity, and lymphoproliferative response to LPS and ConA than WT mice. Moreover, immune cells of TH-HZ mice showed higher oxidative and inflammatory stress than those of WT animals. Although these differences between TH-HZ and WT, in general, decreased with aging, this premature immunosenescence and impairment of behavior of TH-HZ mice was accompanied by a shorter mean lifespan in comparison to WT counterparts. In conclusion, haploinsufficiency of th gene in female mice appears to provoke premature aging of the regulatory systems affecting mean lifespan.

Sympathoadrenergic modulation of hematopoiesis: a review of available evidence and of therapeutic perspectives

Innervation of the bone marrow (BM) has been described more than one century ago, however the first in vivo evidence that sympathoadrenergic fibers have a role in hematopoiesis dates back to less than 25 years ago. Evidence has since increased showing that adrenergic nerves in the BM release noradrenaline and possibly also dopamine, which act on adrenoceptors and dopaminergic receptors (DR) expressed on hematopoietic cells and affect cell survival, proliferation, migration and engraftment ability. Remarkably, dysregulation of adrenergic fibers to the BM is associated with hematopoietic disturbances and myeloproliferative disease. Several adrenergic and dopaminergic agents are already in clinical use for non-hematological indications and with a usually favorable risk-benefit profile, and are therefore potential candidates for non-conventional modulation of hematopoiesis.

Close interactions between sympathetic neural fibres and follicular dendritic cells network are not altered in Peyer's patches and spleen of C57BL/6 mice during the preclinical stage of 139A scrapie infection

During preclinical stage of prion diseases, secondary lymphoid organs seem to play an important role in prion amplification prior the invasion of the associated peripheral nervous system. In mice, it was shown that the relative positioning of follicular dendritic cells (FDC) and sympathetic nervous system (SNS) affects the velocity of neuroinvasion following scrapie inoculation. In this study, we checked if scrapie infection, by oral or intraperitoneal route, could influence this neuroimmune interface between FDC and tyrosine hydroxylase (TH) positive neural fibres within Peyer's patches (PP) and spleen of the C57BL/6 mouse strain. We concluded that, in vivo, scrapie 139A and ME7 strains do not modify FDC-SNS neuroimmune interface. However, age seems to alter this neuroimmune interface and thus could influence the neuroinvasion in prion pathogenesis.

Social temperament and lymph node innervation

Socially inhibited individuals show increased vulnerability to viral infections, and this has been linked to increased activity of the sympathetic nervous system (SNS). To determine whether structural alterations in SNS innervation of lymphoid tissue might contribute to these effects, we assayed the density of catecholaminergic nerve fibers in 13 lymph nodes from seven healthy adult rhesus macaques that showed stable individual differences in propensity to socially affiliate (Sociability). Tissues from Low Sociable animals showed a 2.8-fold greater density of catecholaminergic innervation relative to tissues from High Sociable animals, and this was associated with a 2.3-fold greater expression of nerve growth factor (NGF) mRNA, suggesting a molecular mechanism for observed differences. Low Sociable animals also showed alterations in lymph node expression of the immunoregulatory cytokine genes IFNG and IL4, and lower secondary IgG responses to tetanus vaccination. These findings are consistent with the hypothesis that structural differences in lymphoid tissue innervation might potentially contribute to relationships between social temperament and immunobiology.

Social stress enhances sympathetic innervation of primate lymph nodes: mechanisms and implications for viral pathogenesis

Behavioral processes regulate immune system function in part via direct sympathetic innervation of lymphoid organs, but little is known about the factors that regulate the architecture of neural fibers in lymphoid tissues. In the present study, we find that experimentally imposed social stress can enhance the density of catecholaminergic neural fibers within axillary lymph nodes from adult rhesus macaques. This effect is linked to increased transcription of the key sympathetic neurotrophin nerve growth factor and occurs predominately in extrafollicular regions of the paracortex that contain T-lymphocytes and macrophages. Functional consequences of stress-induced increases in innervation density include reduced type I interferon response to viral infection and increased replication of the simian immunodeficiency virus. These data reveal a surprising degree of behaviorally induced plasticity in the structure of lymphoid innervation and define a novel pathway by which social factors can modulate immune response and viral pathogenesis.

Age-associated changes in CD90 expression on thymocytes and in TCR-dependent stages of thymocyte maturation in male rats

To elucidate the effects of ageing on T-cell-maturation, in 3- and 18-month-old rats, we analysed the expression of: (i) CD4/CD8/TCRalphabeta and (ii) Thy-1, which is supposed to be a regulator of TCRalphabeta signalling, and thereby the thymocyte selection thresholds. Since an essential role for TCRalphabeta signalling in the development of CD4+25+T(reg)-cells was suggested, the frequency of these cells was also quantified. We demonstrated that, as for mice, early thymocyte differentiational steps within the CD4-8- double negative (DN) developmental stage are age-sensitive. Furthermore, we revealed that TCRalphabeta-dependent stages of T-cell development are affected by ageing, most likely due to an impaired expression of Thy-1 on TCRalphabeta(low) thymocytes entering selection processes. The diminished frequency of the post-selection CD4+8+ double positive (DP) cells in aged rats, together with an overrepresentation of mature single positive (SP) cells, most probably suggests more efficient differentiational transition from the DP TCRalphabeta(high) to the SP TCRalphabeta(high) developmental stage, which is followed by an increase in pre-migration proliferation of the mature SP cells. Moreover, the study indicated impaired intrathymic generation of CD4+25+T(reg)-cells in aged rats, thus providing a possible explanation for the increased frequency of autoimmune diseases in ageing.

Increased NGF proforms in aged sympathetic neurons and their targets

Target-derived neurotrophins such as nerve growth factor (NGF) and neurotrophin-3 (NT-3) regulate sympathetic neuron survival. Here, NGF and NT-3 protein and transcript were examined in sympathetic neurons and targets in order to determine their role in age-related neuronal atrophy. One obvious alteration was a dramatic increase (up to 50-fold) in NGF protein forms, corresponding to proNGF-B, in the superior cervical ganglion (SCG) and targets where sympathetic innervation shows atrophy. In the iris, where sympathetic innervation is protected into old age, proNGF-B was decreased. Alterations in NGF transcript paralleled changes in NGF protein, albeit to a lesser degree. Though significantly increased in aged SCG, NT-3 protein, found primarily as the 'mature' form, showed only minor changes in most tissues, though NT-3 mRNA generally was decreased. In contrast, both NT-3 transcript and NT-3 precursors were increased in iris. The dramatic increases in proNGF, together with minimal changes in NT-3, suggest that alterations in NGF regulation may contribute to the loss of sympathetic innervation observed in many aged peripheral targets.

Age-dependent changes in the nervous and endocrine control of the thymus

The immune system, especially the thymus, undergoes age-related modifications leading to structural and functional changes in the lymphoid organs and immunocompetent cells. Nevertheless, the consequences of thymic involution in the peripheral pool of T-cells are still a matter of controversy. The control of the thymic function is very complex and involves intrathymic signals, the autonomic nervous system, and the endocrine system. Both thymocytes and thymic stromal cells express receptors for a wide range of hormones, as well as for neurotransmitters and neuropeptides, thus affecting thymocytes maturation. This review summarizes the age-dependent variations in the extrathymic components of the thymic microenvironment, i.e., vegetative nerves and hormones, and the possible effects of those changes in the immune function.

Neuroendocrine immune interactions in health and disease

The purpose of this paper is to review ways in which the neurohormonal system can interact with the immune system and to outline the main mechanisms which are involved in this interaction. Experimental as well as clinical evidence is presented to support the existence of a close interaction and bi-directional communication between the central nervous and immune systems. The role of major endocrine mechanisms and hormones is discussed. The evidences from experimental work to support the roles of the nervous system with neurotransmitters, the endocrine system with hormones, and the immune system with cytokines are presented. Aging, depression and cancer have a high degree of co-association and share mechanisms which result in cellular immune deficiency. Hormone therapy, zinc replenishment, antidepressants, immunomodulators like MDP act on these pathways to upregulate and improve cellular immunity. The authors believe that the central nervous system (CNS)-immune interaction is an important new frontier to be considered for new combination therapy in diseases with cellular immune deficiency such as cancer particularly in the aged with depression.

Sympathetic nerve destruction in spleen in murine AIDS

In susceptible strains of mice, the LP-BM5 mixture of murine retroviruses induces the fatal immunodeficiency disease known as murine acquired immunodeficiency syndrome (murine AIDS or MAIDS). We have previously reported that murine AIDS produces a profound depletion of splenic norepinephrine (NE). Here, we demonstrate that NE depletion is limited to the spleen, a major site affected by LP-BM5 infection. NE depletion in the spleen is first observed at two weeks following LP-BM5 inoculation, concurrent with the onset of splenomegaly, and continues through 12 weeks post-infection. Neuroanatomical studies revealed that the reduction in NE is due to destruction of splenic sympathetic nerve fibers. Administration of the NE reuptake blocker desipramine did not prevent LP-BM5-induced NE depletion, suggesting that destruction is not caused by excess release and reuptake of NE. Elucidating the mechanism of MAIDS-induced sympathetic nerve destruction may provide insight into autonomic and peripheral neuropathies reported in people with AIDS.