Neuroprotective effect of various cytokines on developing spinal motoneurons following axotomy

Sensory impairments and delayed regeneration of sensory axons in interleukin-6-deficient mice

Interleukin-6 (IL-6) is a multifunctional cytokine mediating inflammatory or immune reactions. Here we investigated the possible role of IL-6 in the intact or lesioned peripheral nervous system using adult IL-6 gene knockout (IL-6(-/-)) mice. Various sensory functions were tested by applying electrophysiological, morphological, biochemical, and behavioral methods. There was a 60% reduction of the compound action potential of the sensory branch of IL-6(-/-) mice as compared with the motor branch in the intact sciatic nerve. Cross sections of L5 DRG of IL-6(-/-) mice showed a shift in the relative size distribution of the neurons. The temperature sensitivity of IL-6(-/-) mice was also significantly reduced. After crush lesion of the sciatic nerve, its functional recovery was delayed in IL-6(-/-) mice as analyzed from a behavioral footprint assay. Measurements of compound action potentials 20 d after crush lesion showed that there was a very low level of recovery of the sensory but not of the motor branch of IL-6(-/-) mice. Similar results of sensory impairments were obtained with mice showing slow Wallerian degeneration (Wlds) and a delayed lesion-induced recruitment of macrophages. However, in contrast to WldS mice, in IL-6(-/-) mice we observed the characteristic lesion-induced invasion of macrophages and the upregulation of low-affinity neurotrophin receptor p75 (p75LNTR) mRNA levels identical to those of IL-6(+/+) mice. Thus, the mechanisms leading to the common sensory deficiencies were different between IL-6(-/-) and WldS mice. Altogether, the results suggest that interleukin-6 is essential to modulate sensory functions in vivo.

Molecular mechanisms regulating motor neuron development and degeneration

Motor neurons are a well-defined, although heterogeneous group of cells responsible for transmitting information from the central nervous system to the locomotor system. Spinal motor neurons are specified by soluble factors produced by structures adjacent to the primordial spinal cord, signaling through homeodomain proteins. Axonal pathfinding is regulated by cell-surface receptors that interact with extracellular ligands and once synaptic connections have formed, the survival of the somatic motor neuron is dependent on the provision of target-derived growth factors, although nontarget-derived factors, produced by either astrocytes or Schwann cells, are also potentially implicated. Somatic motor neuron degeneration leads to profound disability, and multiple pathogenetic mechanisms including aberrant growth factor signaling, abnormal neurofilament accumulation, excitotoxicity, and autoimmunity have been postulated to be responsible. Even when specific deficits have been identified, for example, mutations of the superoxide dismutase-1 gene in familial amyotrophic sclerosis and polyglutamine expansion of the androgen receptor in spinal and bulbar muscular atrophy, the mechanisms by which somatic motor neuronal degeneration occurs remain unclear. In order to treat motor system degeneration effectively, we will need to understand these mechanisms more thoroughly.

Absence of spontaneous central nervous system remyelination in class II-deficient mice infected with Theiler's virus

We previously showed that Theiler's murine encephalomyelitis virus (TMEV)-infected major histocompatibility complex (MHC) class II-deficient mice develop both demyelination and neurologic deficits, whereas MHC class I-deficient mice develop demyelination but no neurologic deficits. The absence of neurologic deficits in the class I-deficient mice was associated with preserved sodium channel densities in demyelinated lesions, a relative preservation of axons, and extensive spontaneous remyelination. In this study, we investigated whether TMEV-infected class II-deficient mice, which have an identical genetic background (C57BL/6 x 129) as the class I-deficient mice, have preserved axons and spontaneous myelin repair following chronic TMEV-infection. Both class I- and class II-deficient mice showed similar extents of demyelination of the spinal cord white matter 4 months after TMEV infection. However, the class I-deficient mice demonstrated remyelination by oligodendrocytes, whereas class II-deficient mice showed minimal if any myelin repair. Demyelinated lesions, characterized by inflammatory infiltrates in both mutants, revealed disruption of axons in class II- but not class I-deficient mice. Further characterization revealed that even though class II-deficient mice lacked TMEV-specific IgG, they had virus-specific IgM, which, however, did not neutralize TMEV in vitro. In addition, class II-deficient mice developed TMEV-specific cytotoxic T-lymphocytes in the CNS during the acute (7 days) disease, but these cytotoxic lymphocytes were not present in the chronic stage of disease, despite a high titer of infectious virus throughout the disease. We envision that the presence of demyelination, high virus titer, absence of remyelination, and axonal disruption in chronically infected class II-deficient mice contributes to the development of paralytic disease.

Increase of interleukin-6 mRNA in the spinal cord following peripheral nerve injury in the rat: potential role of IL-6 in neuropathic pain

Interleukin-6 (IL-6) is a multifunctional cytokine whose actions include modulation of proliferation, differentiation, and maturation of hemapoietic progenitors and other cell lineages; growth regulation of certain carcinoma cell lines; and control of cellular metabolic activities. Initially described in terms of its activities in the immune system and inflammation, accumulating evidence supports an essential role of IL-6 in the development, differentiation, regeneration and degeneration of neurons in the peripheral and central nervous system. We have previously demonstrated that immunoreactive-like IL-6 protein is significantly elevated in the spinal cord in response to peripheral nerve injury that results in neuropathic pain behaviors in the rat. In the current study, our objective was to determine if the source of IL-6 protein was endogenous to the central nervous system by measuring any detectable increases in spinal IL-6 mRNA expression following established mononeuropathy procedures associated with neuropathic pain: spinal nerve cryoneurolysis (SPCN) or spinal nerve tight ligation (SPTL). Using in situ hybridization and a digoxigenin-labeled oligonucleotide, IL-6 mRNA in neurons was significantly elevated at 3 and 7 days post SPCN and 7 days post SPTL in both dorsal and ventral horns. The cellular localization of the IL-6 mRNA expression was predominately neuronal as confirmed by NeuN serial staining. For example, in the SPCN 7 day group, IL-6 mRNA cell profiles in the ipsilateral dorsal horn were significantly different from the normal group (38.7+/-12.8 vs. 4.89+/-1.6, p<0.001). These data demonstrate the central, spinal production of a proinflammatory cytokine in response to a peripheral nerve injury. In addition, these results add to the growing body of literature implicating these immune products, cytokines, as potential neuromodulators/neurotransmitters and provides further evidence for their role in the nociceptive processing which leads to chronic pain.

Long-term observation of the effect of peripheral nerve injury in neonatal and young rats

The purpose of this study was to observe functional recovery and motoneuron death after nerve transection-and-repair in neonatal versus young animals. One hundred nine Lewis rats underwent posterior tibial nerve transection-and-repair at 6 or 22 days of age. Fifty-two and fifty-seven nerves at the 6- and 22-day times were used for endpoint analysis at 1, 3, 10, and 14 months. These assessments included serial functional walking track analysis, electrophysiologic studies, muscle mass evaluation, motoneuron counts with retrograde horseradish peroxidase tracing, and histologic and morphometric nerve analysis. Walking track analysis and nerve conduction velocity indicated significantly poorer functional regeneration in the 6-day-old group than in the 22-day-old group. Muscle mass in the 6-day-old group did not recover as well as in the 22-day-old group. Motoneuron numbers stained with horseradish peroxidase were less in the 6-day-old group than in the 22-day-old group. In contrast, morphometric analysis did not reach significance. This study suggests that the same nerve injury sustained in a neonatal rat is less likely to demonstrate functional recovery than one sustained in a young rat.

LIF (AM424), a promising growth factor for the treatment of ALS

Growth factors are theoretically promising agents for ALS therapy, but have been disappointing in subcutaneous delivery due to either toxicity or lack of major efficacy. Leukaemia inhibitory factor (LIF), was named after its effect on haemopoietic cells, and belongs to a group of cytokines which includes CNTF, IL-6, CT-1, OM and IL-11. All group members use the gp130 signal transducing subunit for intracellular signalling, but show differences in biological effect. In vitro and in vivo studies on axotomy and nerve crush models demonstrate a powerful effect of LIF in the survival of both motor and sensory neurones, while reducing denervation induced muscle atrophy. Its effects in muscle also include stimulating myoblast proliferation in vitro, and up-regulation after muscle injury. LIF will also stimulate muscle regeneration in vivo when applied exogenously after injury. In published studies of both axotomy induced neuronal death and in the Wobbler mouse models LIF is active at doses of 10 microg/kg delivered systemically, well below the expected maximum tolerated dose suggested by primate safety studies. LIF is expressed in low levels by spinal cord neurones with significant up-regulation when the neurones are damaged by BOAA toxin, an excitatory amino acid associated with a form of ALS. This augments other evidence suggesting LIF is a trauma factor playing a role in the injury response of adult neuronal tissue, and may be more effective than related growth factors. Taken together, the data suggests LIF is a physiologically relevant trophic factor with implications in clinical medicine as a therapy for ALS, and a human recombinant form (AM424), entered human clinical trials during 1998.

Cytokine mRNA profiles in contused spinal cord and axotomized facial nucleus suggest a beneficial role for inflammation and gliosis

We have studied temporal mRNA expression patterns for interleukin-1beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), macrophage colony stimulating factor (M-CSF), and transforming growth factor-beta1 (TGF-beta1) in two rat injury paradigms with very different cellular inflammatory reactions: contussion of the spinal cord and axotomy of the facial nerve. Our comparative analyses using semiquantitative reverse transcription polymerase chain reaction (RT-PCR) show an early and robust upregulation of IL-1beta, TNF-alpha, IL-6, and M-CSF mRNAs in spinal cord after contusion injury. Peak expression of these mRNAs was transient and returned to control levels by 24 h postinjury. In contrast, expression of IL-1beta and TNF-alpha mRNAs in the axotomized facial nucleus was minimal and delayed, and levels of M-CSF mRNA remained unaltered. Similar to injured spinal cord, the axotomized nucleus showed a dramatic and early upregulation of IL-6 mRNA, but unlike spinal cord, IL-6 mRNA levels subsided only gradually. Both injury paradigms showed gradually increasing levels of TGF-beta1 mRNA which were maximal at 7 days postinjury. RT-PCR analyses were also performed on isolated blood-borne mononuclear cells and neutrophils. The results showed that these cells contain high levels of IL-1beta and M-CSF mRNAs, moderate levels of TGF-beta and TNF-alpha mRNAs, and minimal levels of IL-6 mRNA. The RT-PCR analyses together with histological observations indicate that expression of the proinflammatory cytokines IL-1beta, TNF-alpha, and IL-6 is short-lived and self-limited after contusion injury, and that it occurs primarily within endogenous glial cells. Transient expression of these molecules likely triggers secondary events which may be beneficial to wound repair and regeneration.

Interleukin-6 (IL-6)--a molecule with both beneficial and destructive potentials

Interleukin-6 (IL-6), a member of the neuropoietic cytokine family, initially was described in terms of its activities in the immune system and during inflammation. Accumulating evidence supports an essential role of IL-6 in the development, differentiation, regeneration and degeneration of neurons in the peripheral and central nervous system. Major sites of IL-6 synthesis are neurons and glial cells. Interleukin-6 functions are mediated by a specific receptor system composed of a binding site and a signal transducer. This receptor system can be modulated by a complex of IL-6 and soluble IL-6 receptor acting as agonist. The IL-6 can exert completely opposite actions on neurons, triggering either neuronal survival after injury or causing neuronal degeneration and cell death in disorders such as Alzheimer's disease. Development of selective IL-6 agonists and antagonists, as well as the usage of soluble IL-6 receptors, offers new possibilities for the treatment of neurodegenerative disorders. Furthermore, optimized genetic mouse models, including transgenic and knockout animals, should help to define the physiological and pathophysiological role of IL-6 in the nervous system.