Cytokines and cytokine-related substances regulating glial cell response to injury of the central nervous system

Associations between systemic pro-inflammatory markers, cognitive function and cognitive complaints in a population-based sample of working adults

BACKGROUND

The knowledge is limited regarding the relation between systemic inflammatory biomarkers and subjective and objective cognitive functioning in population-based samples of healthy adults across the adult age-span. Thus, the aim of this study was to study a selection of four pro-inflammatory biomarkers (IL-6, MCP-1, TNF-α, CRP) in relation to executive cognitive functioning, episodic memory and subjective cognitive complaints (SCC) in a population-based sample of 215 working adults (age 25-67).

RESULTS

Higher levels of MCP-1 were associated with poorer executive cognitive functioning, even after adjustments for demographical factors, health status/conditions, SCC and depressive symptoms. IL-6 and CRP were associated with poorer executive cognitive functioning, but these associations covaried with age especially and were not present after adjustment for demographical factors. MCP-1 was associated with poorer episodic memory, but this association also covaried with age especially and was not present after adjustment for demographical factors, and CRP was associated with episodic memory only among participants without reported health conditions. Higher MCP-1 levels were also associated with more SCC and this association covaried with depressive symptoms, while higher levels of TNF-α were associated with less SCC.

CONCLUSION

Low grade inflammatory processes in terms of higher systemic levels of pro-inflammatory biomarkers (MCP-1, IL-6 & CRP) were associated with poorer executive functioning in this sample of working adults, and MCP-1 was so after extensive adjustments. Support for associations between these biomarkers and episodic memory and SCC were more limited. Future research should address the causality of associations between low grade inflammatory processes and cognitive functioning.

The role of IL-6 and IL-1beta in painful perineural inflammatory neuritis

Inflammation along a nerve trunk (perineural inflammation), without detectable axonal damage, has been shown to induce transient pain in the organ supplied by the nerve. The aims of the present study were to study the role IL-6 and IL-1beta, in pain induced by perineural inflammation.

METHODS

IL-6 and IL-1beta secretion from rat's sciatic nerves, L-5 Dorsal Root Ganglia (DRG), and the hind paw skin, 3 and 8 days following exposure of the nerve to Complete Freund's Adjuvant (CFA), were measured using ELISA method. Hind paw tactile-allodynia, mechano-hyperalgesia, heat-allodynia and electrical detection thresholds were tested up to 8 days following the application of CFA, IL-6 or IL-1beta adjacent to the sciatic nerve trunk. Employing electrophysiological recording, saphenous nerve spontaneous activity, nerve trunk mechano-sensitivity and paw tactile detection threshold (determined by recording action potential induced by the lowest mechanical stimulus) were assessed 3 and 8 days following exposure of the nerve trunk to CFA, IL-6, or IL-1beta.

RESULTS

IL-6 and IL-1beta secretion from the nerve was significantly elevated on the 3rd day post-operation (DPO). On the 8th DPO, IL-6 levels returned to baseline while IL-1beta levels remained significantly elevated. The DRG cytokine's level was increased on the 3rd and 8th DPOs, contralateral cytokine's level was increased on the 3rd DPO. The skin IL-6 level was increased bilaterally on the 3rd DPO and returned to baseline on the 8th DPO. IL-1beta levels increased in the affected side on the 3rd and bilaterally on the 8th DPO. Direct application of IL-6 or CFA on the sciatic nerve induced significant hind paw tactile-allodynia from the 1st to 5th DPOs, reduced electrical detection threshold from the 1st to 3rd DPOs, mechano-hyperalgesia from 3rd to 5th DPOs and heat-allodynia on the 3rd DPO. Direct application of IL-1beta induced paw tactile and heat-allodynia on the 7-8th DPOs and mechano-hyperalgesia on the 5-8th DPOs. Perineural inflammation significantly increased spontaneous activity myelinated fibres 3 and 8 days following the application. Direct application of IL-6 induced elevation of spontaneous activity on the 3rd while IL-1beta on the 8th DPO. Nerve mechano-sensitivity was significantly increased on the 3rd day following exposure to CFA and IL-6 and on the 8th following CFA application. The rat's paw lowest mechanical force necessary for induction of action potential, was significantly reduced 3 days following CFA application.

CONCLUSION

IL-6 and IL-1beta play an important role in pain induced by perineural inflammation. IL-6 activity is more prominent immediately following application (2-5th DPOs), while IL-1beta, activity is more significant in a later stage (5-8th DPOs).

Cytokine transport across the injured blood-spinal cord barrier

Spinal cord injury (SCI) induces dynamic changes of the blood-spinal cord barrier and even the more distant blood-brain barrier. Besides an immediate increase of paracellular permeability resulting from the direct impact of the injury, the transport systems for selective cytokines undergo regulatory changes. Since many of the transported molecules play essential roles in neuroregeneration, we propose that this altered peripheral tissue / CNS interaction benefits remodeling of the spinal cord and functional recovery after SCI. This review examines the transport of cytokines and neurotrophic factors into the spinal cord, emphasizing the upregulation of two cytokines--tumor necrosis factor alpha (TNF) and leukemia inhibitory factor (LIF)--during the course of SCI. The increased transport of TNF and LIF after SCI remains saturable and does not coincide with generalized BBB disruption, highlighting a pivotal regulatory role for the blood-spinal cord barrier.

Cognitive dysfunction in mice deficient for TNF- and its receptors

Recent evidence suggests a role for tumor necrosis factor alpha (TNF) in the functioning of the central nervous system (CNS). The aim of this work was to examine the effect of a deficiency of TNF (TNF(-/-)) and its main receptors (TNF-R1(-/-) and TNF-R2(-/-)) on cognitive function. A standardized survey on cognition-like behavior assessing learning and retention, spatial learning/memory, cognitive flexibility, and learning effectiveness was used in B6.WT and B6.TNF gene targeted mice strains (B6.wild-type, B6.TNF(-/-), B6.TNF-R1(-/-), B6.TNF-R2(-/-) mice). All studied mice strains demonstrated successful exploration and learning processes during the training phases of the tests, which made the specific cognition-like tests valid in these mice strains. In the specific cognition-like tests, the B6.TNF(-/-) mice demonstrated significantly poorer learning and retention in the novel object test compared to B6.WT, B6.TNF-R1(-/-) and B6.TNF-R2(-/-) mice. In addition, spatial learning and learning effectiveness were significantly poorer in B6.TNF(-/-) mice compared to B6.WT mice. Moreover, the moderately impaired cognitive performance with similar degrees in B6.TNF-R1(-/-) or B6.TNF-R2(-/-) mice was generally better than in TNF(-/-) mice but also poorer than in B6.WT mice. While the absence of TNF was correlated with poor cognitive functioning, the deletion of both TNF-receptors was involved in partially reduced cognitive functioning. Low-levels of TNF under non-inflammatory immune conditions appear essential for normal cognitive function. TNF displays an interesting candidate gene for cognitive function. Translational research is required to investigate associations between genetic variants of TNF and cognitive function in healthy subjects and neuropsychiatric samples.

Association between genetic variants of IL-1beta, IL-6 and TNF-alpha cytokines and cognitive performance in the elderly general population of the MEMO-study

This study is to investigate the associations between specific polymorphisms in three cytokine genes and domains of cognitive functioning in a population based study in the elderly. In a cross-sectional study of 369 community dwelling elderly subjects we examined the relationships between the polymorphisms IL-1beta-1418C-->T, IL-6-572G-->C and TNF-alpha-308G-->A and the cognitive function domains memory, processing speed and motor function using an extensive neuropsychological test battery. Linear regression models were used in the analysis and results adjusted for multiple comparisons. A significant association between the IL-1beta-1418C-->T polymorphism and memory performance was found with carriers of the T allele (dominant model) having worse memory performance than those with the C allele. In addition, a significant association between the TNF-alpha-308G-->A polymorphism and processing speed was observed, indicating better performance for heterozygous or homozygous carriers of the A allele. These results remained significant after adjustment for known confounders of cognitive function and additional Bonferroni correction for multiple comparisons. Our study provides first results on detrimental effects of the IL-1beta-1418C-->T polymorphism on memory performance and neuroprotective effects of the TNF-alpha-308G-->A polymorphism on processing speed in elderly individuals. Further research is needed to prospectively examine changes in cognitive performance in relation to cytokine genotypes.

Differential regulation of cadherins and catenins during axonal reorganization in the adult rat CNS

The cadherin family consists of several homophilic adhesion molecules that, together with their intracellular binding partners the catenins, are known to mediate axonal navigation, target recognition, and synapse formation during development. Here, we have examined the potential role of these molecules in axonal sprouting induced in the adult brain. Over a period of 3 to 60 days, an episode of pilocarpine-induced status epilepticus (SE) led to sprouting of hippocampal mossy fibers both into the CA3 pyramidal cell layer and the inner molecular layer of the dentate gyrus (DG). We found focal up-regulation of N-cadherin, beta-catenin, and alpha-catenin immunoreactivity within segments of the CA3 pyramidal cell layer with pronounced neuron loss that was associated with the development of mossy fiber sprouting. In contrast, expression of these 3 molecules was unaltered in the DG molecular layer despite mossy fiber sprouting in this area. The levels of E-cadherin immunoreactivity were altered prior to the detection of mossy fiber sprouting, with a general reduction in the neuropil and increased expression in CA1/CA3 pyramidal cell somata. Our results imply that members of the cadherin/catenin families undergo specific spatiotemporal patterns of regulation, which may be important in axon target recognition and synapse formation during lesion-induced sprouting.

Upregulation of the transport system for TNFalpha at the blood-brain barrier

The transport system for the cytokine tumor necrosis factor-alpha (TNFalpha) at the blood-brain barrier (BBB) enables an enhanced yet saturable entry of TNFalpha from blood to the CNS. This review focuses on the selective upregulation of the transport system for TNFalpha at the BBB that is specific for type of pathology, region, and time. The upregulation is reflected by increased CNS tissue uptake of radiolabeled TNFalpha after iv injection in mice and by inhibition of this increase with excess non-radiolabeled TNFalpha. (1) Spinal cord injury (SCI): upregulation of TNFalpha uptake after thoracic transection is seen in the delayed phase of BBB disruption at the lumbar spinal cord. Thoracic SCI by compression, however, has a longer lasting impact on TNFalpha transport that involves thoracic and lumbar spinal cord, in contrast to the upregulation confined to the lumbar region in lumbar SCI by compression. Regardless, the uptake of TNFalpha by spinal cord does not parallel BBB disruption as measured by the leakage of radiolabeled albumin. (2) Experimental autoimmune encephalomyelitis (EAE): the increase in the differential permeability to TNFalpha is seen in all CNS regions (brain and cervical, thoracic, and lumbar spinal cord) and has a distinct time course and reversibility. Exogenous TNFalpha has biphasic effects in modulating functional scores. The BBB, a dynamically regulated barrier, is actively involved in disease processes.

STAT3 and NFkappaB activation precedes glial reactivity in the excitotoxically injured young cortex but not in the corresponding distal thalamic nuclei

In this study we evaluated the activation of the cytokine and growth factor responsive transcription factors signal transducer and activator of transcription 3 (STAT3) and nuclear factor kappa B (NFkappaB) after different grades of neural damage in the immature rat brain using double immunocytochemical techniques and electron microscopy. Following neocortical N-methyl-D-aspartate induced excitotoxic cell death, both these transcription factors are mainly activated in astrocytes, although microglia, endothelial cells, and neurons show transient activation at specific times and locations. Interestingly, activation of both transcription factors is only observed in cortical areas affected by severe tissue damage, neuronal degeneration, and blood-brain barrier (BBB) disruption. In contrast, the milder glial response occurring in the distal thalamus is not preceded by immunocytochemically detectable STAT3 and NFkappaB activation, although microglial response, astroglial hypertrophy, and glial fibrillary acidic protein (GFAP) overexpression do occur. In the cortex, astrocytes show STAT3 and NFkappaB activation already at 2 to 4 hours post-lesion, preceding cell hypertrophy and GFAP upregulation, and being maintained in the long-term formed glial scar. STAT3 and NFkappaB activation in microglial cells is protracted and observed at 10 to 24 hours post-lesion. The early activation of both transcription factors in astroglial cells could contribute to the changes in gene expression leading to astrogliosis and the release of signalling molecules which may contribute to the subsequent activation of these transcription factors in microglial cells.

Upregulation of tumor necrosis factor alpha transport across the blood-brain barrier after acute compressive spinal cord injury

Tumor necrosis factor alpha (TNF) is a cytokine that is involved in the inflammatory process after CNS injury and is implicated in neuroregeneration. A saturable transport system for TNF located at the blood-brain barrier (BBB) is responsible for the limited entry of TNF from blood to the CNS in normal mice. After partial disruption of the BBB by compression of the lumbar spinal cord, permeability to TNF was increased not only in the lumbar spinal cord but also in brain and distal spinal cord segments, where the BBB remained intact. The increase in the entry of TNF to the CNS followed a biphasic temporal pattern, with a first peak immediately after injury and a second peak starting on day 3; these changes lasted longer than the mere disruption of the BBB. The increased entry of TNF was abolished by addition of excess unlabeled TNF, showing that the transport system for TNF remained saturable after spinal cord injury (SCI) and providing evidence that the enhanced entry of TNF could not be explained by diffusion or leakage. This study adds strong support for our concept that the saturable transport system for TNF across the BBB can be upregulated in the diseased state, and it suggests that the BBB is actively involved in the modulation of the processes of degeneration and regeneration after SCI.

Accumulation of passively transferred primed T cells independently of their antigen specificity following central nervous system trauma

The central nervous system (CNS) enjoys a unique relationship with the immune system. Under non-pathological conditions, T cells move through the CNS but do not accumulate there. CNS trauma has been shown to trigger a response to CNS self-antigens such as myelin basic protein (MBP). Here, we examined whether the injured CNS tissue undergoes changes that permit T cell accumulation. We found that injury to CNS white matter, such as the optic nerve, led to a transiently increased accumulation of T cells (between days 3 and 21). In Lewis rats with unilaterally injured optic nerves, systemic administration of passively transferred T cells recognizing either self-antigen (MBP) or non-self-antigen (ovalbumin) resulted in accumulation of the T cells in injured optic nerve, irrespective of their antigenic specificity. The effect of the T cells on the damaged nerve, the lack of selectivity in T cell accumulation and the mechanism underlying non-selective accumulation are discussed.

Neurite outgrowth through lesions of neonatal opossum spinal cord in culture

The aim of these experiments was to analyze neurite outgrowth during regeneration of opossum spinal cord isolated from Monodelfis domestica and maintained in culture for 3-5 days. Lesions were made by crushing with forceps. In isolated spinal cords of animals aged 3 days, neurites entered the crush and grew along the basal lamina of the pia mater. Growth cones with pleiomorphic appearance containing vesicles, mitochondria and microtubules were abundant in the marginal zone, as were synaptoid contacts with active zones facing basal lamina. In preparations from animals aged 11-12 days, the lesion site was disrupted and contained only degenerating axons, debris and vesicles. Axons and growth cones entered the edge of the lesion but did not extend into it. Lesions in young animals extended over distances of more than 1 mm and contained no radial glia. The damaged area in older preparations was restricted to the crush site with normal astrocytes, oligodendrocytes and neurons immediately adjacent to the lesion. Thus, similar crushes produced more extensive damage in younger spinal cords that were capable of regeneration than in older cords that were not. Dorsal root ganglion fibers labeled with carbocyanine dye (DiI) were observed by video imaging as they grew through lesions. Individual growth cones examined subsequently by electron microscopy had grown again along pial basal lamina. After 5 days in culture dorsal root stimulation gave rise to discharges in ventral roots beyond the lesion indicating that synaptic connections were formed by growing fibers.