miR-155 Deletion in Mice Overcomes Neuron-Intrinsic and Neuron-Extrinsic Barriers to Spinal Cord Repair.

UNLABELLED: Axon regeneration after spinal cord injury (SCI) fails due to neuron-intrinsic mechanisms and extracellular barriers including inflammation. microRNA (miR)-155-5p is a small, noncoding RNA that negatively regulates mRNA translation. In macrophages, miR-155-5p is induced by inflammatory stimuli and elicits a response that could be toxic after SCI. miR-155 may also independently alter expression of genes that regulate axon growth in neurons. Here, we hypothesized that miR-155 deletion would simultaneously improve axon growth and reduce neuroinflammation after SCI by acting on both neurons and macrophages. New data show that miR-155 deletion attenuates inflammatory signaling in macrophages, reduces macrophage-mediated neuron toxicity, and increases macrophage-elicited axon growth by ∼40% relative to control conditions. In addition, miR-155 deletion increases spontaneous axon growth from neurons; adult miR-155 KO dorsal root ganglion (DRG) neurons extend 44% longer neurites than WT neurons. In vivo, miR-155 deletion augments conditioning lesion-induced intraneuronal expression of SPRR1A, a regeneration-associated gene; ∼50% more injured KO DRG neurons expressed SPRR1A versus WT neurons. After dorsal column SCI, miR-155 KO mouse spinal cord has reduced neuroinflammation and increased peripheral conditioning-lesion-enhanced axon regeneration beyond the epicenter. Finally, in a model of spinal contusion injury, miR-155 deletion improves locomotor function at postinjury times corresponding with the arrival and maximal appearance of activated intraspinal macrophages. In miR-155 KO mice, improved locomotor function is associated with smaller contusion lesions and decreased accumulation of inflammatory macrophages. Collectively, these data indicate that miR-155 is a novel therapeutic target capable of simultaneously overcoming neuron-intrinsic and neuron-extrinsic barriers to repair after SCI. SIGNIFICANCE STATEMENT: Axon regeneration after spinal cord injury (SCI) fails due to neuron-intrinsic mechanisms and extracellular barriers, including inflammation. Here, new data show that deleting microRNA-155 (miR-155) affects both mechanisms and improves repair and functional recovery after SCI. Macrophages lacking miR-155 have altered inflammatory capacity, which enhances neuron survival and axon growth of cocultured neurons. In addition, independent of macrophages, adult miR-155 KO neurons show enhanced spontaneous axon growth. Using either spinal cord dorsal column crush or contusion injury models, miR-155 deletion improves indices of repair and recovery. Therefore, miR-155 has a dual role in regulating spinal cord repair and may be a novel therapeutic target for SCI and other CNS pathologies.

Autonomic dysreflexia causes chronic immune suppression after spinal cord injury.

Autonomic dysreflexia (AD), a potentially dangerous complication of high-level spinal cord injury (SCI) characterized by exaggerated activation of spinal autonomic (sympathetic) reflexes, can cause pulmonary embolism, stroke, and, in severe cases, death. People with high-level SCI also are immune compromised, rendering them more susceptible to infectious morbidity and mortality. The mechanisms underlying postinjury immune suppression are not known. Data presented herein indicate that AD causes immune suppression. Using in vivo telemetry, we show that AD develops spontaneously in SCI mice with the frequency of dysreflexic episodes increasing as a function of time postinjury. As the frequency of AD increases, there is a corresponding increase in splenic leucopenia and immune suppression. Experimental activation of spinal sympathetic reflexes in SCI mice (e.g., via colorectal distension) elicits AD and exacerbates immune suppression via a mechanism that involves aberrant accumulation of norepinephrine and glucocorticoids. Reversal of postinjury immune suppression in SCI mice can be achieved by pharmacological inhibition of receptors for norepinephrine and glucocorticoids during the onset and progression of AD. In a human subject with C5 SCI, stimulating the micturition reflex caused AD with exaggerated catecholamine release and impaired immune function, thus confirming the relevance of the mouse data. These data implicate AD as a cause of secondary immune deficiency after SCI and reveal novel therapeutic targets for overcoming infectious complications that arise due to deficits in immune function.

Mechanisms underlying the rapid peroxisome proliferator-activated receptor-γ-mediated amyloid clearance and reversal of cognitive deficits in a murine model of Alzheimer's disease.

Alzheimer's disease is associated with a disruption of amyloid ß (Aß) homeostasis, resulting in the accumulation and subsequent deposition of Aß peptides within the brain. The peroxisome proliferator-activated receptor-γ (PPARγ) is a ligand-activated nuclear receptor that acts in a coupled metabolic cycle with Liver X Receptors (LXRs) to increase brain apolipoprotein E (apoE) levels. apoE functions to promote the proteolytic clearance of soluble forms of Aß, and we found that the synthetic PPARγ agonist, pioglitazone, stimulated Aß degradation by both microglia and astrocytes in an LXR and apoE-dependent manner. Remarkably, a brief 9 d oral treatment of APPswe/PS1Δe9 mice with pioglitazone resulted in dramatic reductions in brain levels of soluble and insoluble Aß levels which correlated with the loss of both diffuse and dense-core plaques within the cortex. The removal of preexisting amyloid deposits was associated with the appearance of abundant Aß-laden microglia and astrocytes. Pioglitazone treatment resulted in the phenotypic polarization of microglial cells from a proinflammatory M1 state, into an anti-inflammatory M2 state that was associated with enhanced phagocytosis of deposited forms of amyloid. The reduction in amyloid levels was associated with a reversal of contextual memory deficits in the drug-treated mice. These data provide a mechanistic explanation for how PPARγ activation facilitates amyloid clearance and supports the therapeutic utility of PPARγ agonists for the treatment of Alzheimer's disease.

Achieving CNS axon regeneration by manipulating convergent neuro-immune signaling.

After central nervous system (CNS) trauma, axons have a low capacity for regeneration. Regeneration failure is associated with a muted regenerative response of the neuron itself, combined with a growth-inhibitory and cytotoxic post-injury environment. After spinal cord injury (SCI), resident and infiltrating immune cells (especially microglia/macrophages) contribute significantly to the growth-refractory milieu near the lesion. By targeting both the regenerative potential of the axon and the cytotoxic phenotype of microglia/macrophages, we may be able to improve CNS repair after SCI. In this review, we discuss molecules shown to impact CNS repair by affecting both immune cells and neurons. Specifically, we provide examples of pattern recognition receptors, integrins, cytokines/chemokines, nuclear receptors and galectins that could improve CNS repair. In many cases, signaling by these molecules is complex and may have contradictory effects on recovery depending on the cell types involved or the model studied. Despite this caveat, deciphering convergent signaling pathways on immune cells (which affect axon growth indirectly) and neurons (direct effects on axon growth) could improve repair and recovery after SCI. Future studies must continue to consider how regenerative therapies targeting neurons impact other cells in the pathological CNS. By identifying molecules that simultaneously improve axon regenerative capacity and drive the protective, growth-promoting phenotype of immune cells, we may discover SCI therapies that act synergistically to improve CNS repair and functional recovery.

PPAR agonists as therapeutics for CNS trauma and neurological diseases.

Traumatic injury or disease of the spinal cord and brain elicits multiple cellular and biochemical reactions that together cause or are associated with neuropathology. Specifically, injury or disease elicits acute infiltration and activation of immune cells, death of neurons and glia, mitochondrial dysfunction, and the secretion of substrates that inhibit axon regeneration. In some diseases, inflammation is chronic or non-resolving. Ligands that target PPARs (peroxisome proliferator-activated receptors), a group of ligand-activated transcription factors, are promising therapeutics for neurologic disease and CNS injury because their activation affects many, if not all, of these interrelated pathologic mechanisms. PPAR activation can simultaneously weaken or reprogram the immune response, stimulate metabolic and mitochondrial function, promote axon growth and induce progenitor cells to differentiate into myelinating oligodendrocytes. PPAR activation has beneficial effects in many pre-clinical models of neurodegenerative diseases and CNS injury; however, the mechanisms through which PPARs exert these effects have yet to be fully elucidated. In this review we discuss current literature supporting the role of PPAR activation as a therapeutic target for treating traumatic injury and degenerative diseases of the CNS.

Nuclear receptors as therapeutic targets for Alzheimer's disease.

INTRODUCTION: Alzheimer's disease (AD) is characterized by the accumulation and extensive deposition of amyloid ß (Aß) in the parenchyma of the brain. This accumulation of amyloid is associated with perturbations in synaptic function, impairments in energy metabolism and induction of a chronic inflammatory response which acts to promote neuronal loss and cognitive impairment. AREAS COVERED: Currently, there are no drugs that target the underlying mechanisms of AD. Here, we propose a class of nuclear receptors as novel and promising new therapeutic targets for AD. This review summarizes the literature on nuclear receptors and their effects on AD-related pathophysiology. EXPERT OPINION: Nuclear receptors are attractive targets for the treatment of AD due to their ability to facilitate degradation of Aß, affect microglial activation and suppress the inflammatory milieu of the brain. Liver X receptor agonists have proven difficult to move into clinical trials as long-term treatment results in hepatic steatosis. It is our view that PPAR-γ activation remains a promising avenue for the treatment for AD; however, the poor BBB permeability of the currently available agonists and the negative outcome of the Phase III clinical trials are likely to diminish interest in pursuing this target.

Microglia and inflammation in Alzheimer's disease.

One hundred and fifty years have elapsed since the original discovery of the microglial cell by Virchow. While this cell type has been well studied, the role of microglia in the pathology of many central nervous system diseases still remains enigmatic. It is widely accepted that microglial-mediated inflammation contributes to the progression of Alzheimer's disease (AD); however, the precise mechanisms through which these cells contribute to AD-related inflammation remains to be elucidated. In the AD brain, microglial cells are found in close association with amyloid beta (Abeta) deposits. Histological examination of AD brains as well as cell culture studies have shown that the interaction of microglia with fibrillar Abeta leads to their phenotypic activation. The conversion of these cells into a classically 'activated' phenotype results in production of chemokines, neurotoxic cytokines and reactive oxygen and nitrogen species that are deleterious to the CNS. However, microglia also exert a neuroprotective role through their ability to phagocytose Abeta particles and clear soluble forms of Abeta. These cells have been documented to play integral roles in tissue repair and inflammation, and in recent years it has been appreciated that this cell type is capable of facilitating a more complex response to pathogens by changing their activation status. A variety of new findings indicate that their role in the central nervous system is far more complex than previously appreciated. In this review we discuss the role of microglia in the normal brain and their phenotypic heterogeneity and how this may play a role in AD-related pathophysiology. We touch on what is known about their ability to recognize and clear Abeta peptides as well as more controversial topics, including various activation states of microglia and the ability of peripheral macrophages or monocytes to infiltrate the brain.