Genetic deletion of the glucocorticoid receptor in Cx3cr1+ myeloid cells is neuroprotective and improves motor recovery after spinal cord injury.

Glucocorticoid receptors (GRs), part of the nuclear receptor superfamily of transcription factors (TFs), are ubiquitously expressed in all cell types and regulate cellular responses to glucocorticoids (e.g., cortisol in humans; corticosterone in rodents). In myeloid cells, glucocorticoids binding to GRs can enhance or repress gene transcription, thereby imparting distinct and context-dependent functions in macrophages at sites of inflammation. In experimental models and in humans, glucocorticoids are widely used as anti-inflammatory treatments to promote recovery of function after SCI. Thus, we predicted that deleting GR in mouse myeloid lineage cells (i.e., microglia and monocyte-derived macrophages) would enhance inflammation at the site of injury and worsen functional recovery after traumatic spinal cord injury (SCI). Contrary to our prediction, the intraspinal macrophage response to a moderate (75 kdyne) spinal contusion SCI was reduced in Cx3cr1-Cre;GRf/f conditional knockout mice (with GR specifically deleted in myeloid cells). This phenotype was associated with improvements in hindlimb motor recovery, myelin sparing, axon sparing/regeneration, and microvascular protection/plasticity relative to SCI mice with normal myeloid cell GR expression. Further analysis revealed that macrophage GR deletion impaired lipid and myelin phagocytosis and foamy macrophage formation. Together, these data reveal endogenous GR signaling as a key pathway that normally inhibits mechanisms of macrophage-mediated repair after SCI.

Enhancing membrane repair increases regeneration in a sciatic injury model.

Various injuries to the neural tissues can cause irreversible damage to multiple functions of the nervous system ranging from motor control to cognitive function. The limited treatment options available for patients have led to extensive interest in studying the mechanisms of neuronal regeneration and recovery from injury. Since many neurons are terminally differentiated, by increasing cell survival following injury it may be possible to minimize the impact of these injuries and provide translational potential for treatment of neuronal diseases. While several cell types are known to survive injury through plasma membrane repair mechanisms, there has been little investigation of membrane repair in neurons and even fewer efforts to target membrane repair as a therapy in neurons. Studies from our laboratory group and others demonstrated that mitsugumin 53 (MG53), a muscle-enriched tripartite motif (TRIM) family protein also known as TRIM72, is an essential component of the cell membrane repair machinery in skeletal muscle. Interestingly, recombinant human MG53 (rhMG53) can be applied exogenously to increase membrane repair capacity both in vitro and in vivo. Increasing the membrane repair capacity of neurons could potentially minimize the death of these cells and affect the progression of various neuronal diseases. In this study we assess the therapeutic potential of rhMG53 to increase membrane repair in cultured neurons and in an in vivo mouse model of neurotrauma. We found that a robust repair response exists in various neuronal cells and that rhMG53 can increase neuronal membrane repair both in vitro and in vivo. These findings provide direct evidence of conserved membrane repair responses in neurons and that these repair mechanisms can be targeted as a potential therapeutic approach for neuronal injury.

Microglia Promote Increased Pain Behavior through Enhanced Inflammation in the Spinal Cord during Repeated Social Defeat Stress.

Clinical studies indicate that psychosocial stress contributes to adverse chronic pain outcomes in patients, but it is unclear how this is initiated or amplified by stress. Repeated social defeat (RSD) is a mouse model of psychosocial stress that activates microglia, increases neuroinflammatory signaling, and augments pain and anxiety-like behaviors. We hypothesized that activated microglia within the spinal cord facilitate increased pain sensitivity following RSD. Here we show that mechanical allodynia in male mice was increased with exposure to RSD. This stress-induced behavior corresponded with increased mRNA expression of several inflammatory genes, including IL-1ß, TNF-α, CCL2, and TLR4 in the lumbar spinal cord. While there were several adhesion and chemokine-related genes increased in the lumbar spinal cord after RSD, there was no accumulation of monocytes or neutrophils. Notably, there was evidence of microglial activation selectively within the nociceptive neurocircuitry of the dorsal horn of the lumbar cord. Elimination of microglia using the colony stimulating factor 1 receptor antagonist PLX5622 from the brain and spinal cord prevented the development of mechanical allodynia in RSD-exposed mice. Microglial elimination also attenuated RSD-induced IL-1ß, CCR2, and TLR4 mRNA expression in the lumbar spinal cord. Together, RSD-induced allodynia was associated with microglia-mediated inflammation within the dorsal horn of the lumbar spinal cord.SIGNIFICANCE STATEMENT Mounting evidence indicates that psychological stress contributes to the onset and progression of adverse nociceptive conditions. We show here that repeated social defeat stress causes increased pain sensitivity due to inflammatory signaling within the nociceptive circuits of the spinal cord. Studies here mechanistically tested the role of microglia in the development of pain by stress. Pharmacological ablation of microglia prevented stress-induced pain sensitivity. These findings demonstrate that microglia are critical mediators in the induction of pain conditions by stress. Moreover, these studies provide a proof of principle that microglia can be targeted as a therapeutic strategy to mitigate adverse pain conditions.

Salient type 1 interleukin 1 receptor expression in peripheral non-immune cells.

Interleukin 1 is a pleiotropic cytokine that mediates diverse functions through its receptor, type I interleukin 1 receptor (IL-1R1). Most previous studies have focused on the expression and function of IL-1R1 in immune cells. Here we performed a comprehensive mapping of IL-1R1 distribution in multiple peripheral tissues using our IL-1R1 reporter (IL-1R1GR/GR) mice. This method yielded the highest sensitivity of in situ detection of IL-1R1 mRNA and protein. Besides validating previously reported IL-1R1 expression in the endocrine tissues including pituitary and pancreas, our results refuted previously reported exclusive IL-1R1 expression in neurons of the spinal cord dorsal horn and dorsal root ganglia (DRG). Instead, IL-1R1 expression was detected in endothelial cells within DRG, spinal cord, pancreas, colon, muscles and many immune organs. In addition, gp38+ fibroblastic reticular cells (FRCs), rather than tissue macrophages or other immune cells, were found to express high levels of IL-1R1 in colon and many immune organs. A functional test of spleen FRCs showed that they responded rapidly to systemic IL-1ß stimulation in vivo. Taken together, this study provides a rigorous re-examination of IL-1R1 expression in peripheral tissues and reveals tissue FRCs as a previously unappreciated novel high IL-1R1-expressing cell type in peripheral IL-1 signaling.

The Effect of Glucocorticoid and Glucocorticoid Receptor Interactions on Brain, Spinal Cord, and Glial Cell Plasticity.

Stress, injury, and disease trigger glucocorticoid (GC) elevation. Elevated GCs bind to the ubiquitously expressed glucocorticoid receptor (GR). While GRs are in every cell in the nervous system, the expression level varies, suggesting that diverse cell types react differently to GR activation. Stress/GCs induce structural plasticity in neurons, Schwann cells, microglia, oligodendrocytes, and astrocytes as well as affect neurotransmission by changing the release and reuptake of glutamate. While general nervous system plasticity is essential for adaptation and learning and memory, stress-induced plasticity is often maladaptive and contributes to neuropsychiatric disorders and neuropathic pain. In this brief review, we describe the evidence that stress/GCs activate GR to promote cell type-specific changes in cellular plasticity throughout the nervous system.

Stress Increases Peripheral Axon Growth and Regeneration through Glucocorticoid Receptor-Dependent Transcriptional Programs.

Stress and glucocorticoid (GC) release are common behavioral and hormonal responses to injury or disease. In the brain, stress/GCs can alter neuron structure and function leading to cognitive impairment. Stress and GCs also exacerbate pain, but whether a corresponding change occurs in structural plasticity of sensory neurons is unknown. Here, we show that in female mice (Mus musculus) basal GC receptor (Nr3c1, also known as GR) expression in dorsal root ganglion (DRG) sensory neurons is 15-fold higher than in neurons in canonical stress-responsive brain regions (M. musculus). In response to stress or GCs, adult DRG neurite growth increases through mechanisms involving GR-dependent gene transcription. In vivo, prior exposure to an acute systemic stress increases peripheral nerve regeneration. These data have broad clinical implications and highlight the importance of stress and GCs as novel behavioral and circulating modifiers of neuronal plasticity.

Glucocorticoids and nervous system plasticity.

Glucocorticoid and glucocorticoid receptor (GC/GR) interactions alter numerous aspects of neuronal function. These consequences (e.g., anti-inflammatory vs. pro-inflammatory) can vary depending on the duration of GC exposure or central nervous system (CNS) injury model. In this review we discuss how GC/GR interactions impact neuronal recovery after a central or peripheral nerve injury and discuss how GC exposure duration can produce divergent CNS neuronal growth responses. Finally we consider how new findings on gender specific immune cell responses after a nerve injury could intersect with GC/GR interactions to impact pain processing.

Intravenous multipotent adult progenitor cell treatment decreases inflammation leading to functional recovery following spinal cord injury.

Following spinal cord injury (SCI), immune-mediated secondary processes exacerbate the extent of permanent neurological deficits. We investigated the capacity of adult bone marrow-derived stem cells, which exhibit immunomodulatory properties, to alter inflammation and promote recovery following SCI. In vitro, we show that human multipotent adult progenitor cells (MAPCs) have the ability to modulate macrophage activation, and prior exposure to MAPC secreted factors can reduce macrophage-mediated axonal dieback of dystrophic axons. Using a contusion model of SCI, we found that intravenous delivery of MAPCs one day, but not immediately, after SCI significantly improves urinary and locomotor recovery, which was associated with marked spinal cord tissue sparing. Intravenous MAPCs altered the immune response in the spinal cord and periphery, however biodistribution studies revealed that no MAPCs were found in the cord and instead preferentially homed to the spleen. Our results demonstrate that MAPCs exert their primary effects in the periphery and provide strong support for the use of these cells in acute human contusive SCI.

Modulation of the proteoglycan receptor PTPσ promotes recovery after spinal cord injury.

Contusive spinal cord injury leads to a variety of disabilities owing to limited neuronal regeneration and functional plasticity. It is well established that an upregulation of glial-derived chondroitin sulphate proteoglycans (CSPGs) within the glial scar and perineuronal net creates a barrier to axonal regrowth and sprouting. Protein tyrosine phosphatase σ (PTPσ), along with its sister phosphatase leukocyte common antigen-related (LAR) and the nogo receptors 1 and 3 (NgR), have recently been identified as receptors for the inhibitory glycosylated side chains of CSPGs. Here we find in rats that PTPσ has a critical role in converting growth cones into a dystrophic state by tightly stabilizing them within CSPG-rich substrates. We generated a membrane-permeable peptide mimetic of the PTPσ wedge domain that binds to PTPσ and relieves CSPG-mediated inhibition. Systemic delivery of this peptide over weeks restored substantial serotonergic innervation to the spinal cord below the level of injury and facilitated functional recovery of both locomotor and urinary systems. Our results add a new layer of understanding to the critical role of PTPσ in mediating the growth-inhibited state of neurons due to CSPGs within the injured adult spinal cord.