Elimination of microglia in mouse spinal cord alters the retrograde CNS plasticity observed following peripheral axon injury

Continual Deletion of Spinal Microglia Reforms Astrocyte Scar Favoring Axonal Regeneration

Astrocyte scar formation after spinal cord injury (SCI) efficiently limits the accurate damage but physically restricts the following axon regeneration. Lately, fine tuning scar formation is becoming a novel strategy to develop SCI treatment, yet how to leverage these opposite effects remains challenging. Here, utilizing an improved drug administration approach, we show that in a mouse model of spinal cord injury, continual deletion of microglia, especially upon scar formation, by pexidartinib decreases the amount of microglia-derived collagen I and reforms the astrocyte scar. The astrocytes become less compacted in the scar, which permits axon regeneration and extension. Although continual microglia deletion did not significantly improve the locomotive performance of the SCI mice, it did ameliorate their weight loss, possibly by improving their relevant health conditions. We thus identified a novel approach to regulate astrocyte scars for improved axon regeneration, which is indicative of the clinical treatment of SCI patients.

Activated Microglia in the Rat Spinal Cord Following Peripheral Axon Injury Promote Glial and Neuronal Plasticity Which is Necessary for Long-Term Neuronal Survival

Following the transection of peripheral sympathetic preganglionic axons comprising the cervical sympathetic trunk (CST), we observe robust glial and neuronal plasticity at 1 week post-injury in the rat spinal cord intermediolateral cell column (IML), which houses the injured parent neuronal cell bodies. This plasticity contributes to neuroprotection, as no neuronal loss in the IML is present at 16 weeks post-injury. Here, we administered the antibiotic minocycline or vehicle (VEH) daily for 1 week after CST transection to investigate the role of activated microglia in IML glial and neuronal plasticity and subsequent neuronal survival. At 1 week post-injury, minocycline treatment did not alter microglia number in the IML, but led to a dampened microglia activation state. In addition, the increases in oligodendrocyte (OL) lineage cells and activated astrocytes following injury in VEH rats were attenuated in the minocycline-treated rats. Further, the normal downregulation of choline acetyltransferase (ChAT) in the injured neurons was blunted. At 16 weeks post-injury, fewer ChAT+ neurons were present in the minocycline-treated rats, suggesting that activated microglia together with the glial and neuronal plasticity at 1 week post-injury contribute to the long-term survival of the injured neurons. These results provide evidence for beneficial crosstalk between activated microglia and neurons as well as other glial cells in the cord following peripheral axon injury, which ultimately leads to neuroprotection. The influences of microglia activation in promoting neuronal survival should be considered when developing therapies to administer minocycline for the treatment of neurological pathologies.

CSF1R inhibition by a small-molecule inhibitor is not microglia specific; affecting hematopoiesis and the function of macrophages

Colony-stimulating factor 1 receptor (CSF1R) inhibition has been proposed as a method for microglia depletion, with the assumption that it does not affect peripheral immune cells. Here, we show that CSF1R inhibition by PLX5622 indeed affects the myeloid and lymphoid compartments, causes long-term changes in bone marrow-derived macrophages by suppressing interleukin 1β, CD68, and phagocytosis but not CD208, following exposure to endotoxin, and also reduces the population of resident and interstitial macrophages of peritoneum, lung, and liver but not spleen. Thus, small-molecule CSF1R inhibition is not restricted to microglia, causing strong effects on circulating and tissue macrophages that perdure long after cessation of the treatment. Given that peripheral monocytes repopulate the central nervous system after CSF1R inhibition, these changes have practical implications for relevant experimental data.

A Review of Exercise-Induced Neuroplasticity in Ischemic Stroke: Pathology and Mechanisms

After ischemic stroke, survivors experience motor dysfunction and deterioration of memory and cognition. These symptoms are associated with the disruption of normal neuronal function, i.e., the secretion of neurotrophic factors, interhemispheric connections, and synaptic activity, and hence the disruption of the normal neural circuit. Exercise is considered an effective and feasible rehabilitation strategy for improving cognitive and motor recovery following ischemic stroke through the facilitation of neuroplasticity. In this review, our aim was to discuss the mechanisms by which exercise-induced neuroplasticity improves motor function and cognitive ability after ischemic stroke. The associated mechanisms include increases in neurotrophins, improvements in synaptic structure and function, the enhancement of interhemispheric connections, the promotion of neural regeneration, the acceleration of neural function reorganization, and the facilitation of compensation beyond the infarcted tissue. We also discuss some common exercise strategies and a novel exercise therapy, robot-assisted movement, which might be widely applied in the clinic to help stroke patients in the future.

Spinal microglia-neuron interactions in chronic pain

Current deficiency in our understanding of acute-to-chronic pain transition remains a hurdle for developing effective treatments against chronic pain. Whereas neurocentric mechanisms alone are insufficient to provide satisfactory explanation for such transition, neuro-immune crosstalk has attracted attention in recent pain research. In contrast to brain microglia, spinal microglia are activated immediately in various pain states. The fast-responsive enrichment and activation of spinal microglia among different pain conditions have highlighted the crucial role of neuroinflammation caused by microglia-neuron crosstalk in pain initiation. Recent studies have revealed spinal microglia-neuron interactions are also involved in chronic pain maintenance, albeit, with different anatomic distribution, cellular and molecular mechanisms, and biologic functions. Delineating the exact temporal discrepancies of spinal microglia distribution and functions along acute-to-chronic pain transition may provide additional mechanistic insights for drug development to prevent deterioration of acute pain into the chronic state. This narrative review summerizes the longitudinal alterations of spinal microglia-neuron interactions in the initiation of pain hypersensitivity, acute-to-chronic pain progression, and chronic pain maintenance, followed by an overview of current clinical translation of preclinical studies on spinal microglia. This review highlights the crucial role of the interaction between spinal microglia and neighboring neurons in the initiation and maintenance of pain hypersensitivity, in relation to the release of cytokines, chemokines, and neuroactive substances, as well as the modulation of synaptic plasticity. Further exploration of the uncharted functions of spinal microglia-neuron crosstalk may lead to the design of novel drugs for preventing acute-to-chronic pain transition.