Dynamics of spinal microglia repopulation following an acute depletion

Abstract

Our understanding on the function of microglia has been revolutionized in the recent 20 years. However, the process of maintaining microglia homeostasis has not been fully understood. In this study, we dissected the features of spinal microglia repopulation following an acute partial depletion. By injecting intrathecally Mac-1-saporin, a microglia selective immunotoxin, we ablated 50% microglia in the spinal cord of naive mice. Spinal microglia repopulated rapidly and local homeostasis was re-established within 14 days post-depletion. Mac-1-saporin treatment resulted in microglia cell proliferation and circulating monocyte infiltration. The latter is indeed part of an acute, transient inflammatory reaction that follows cell depletion, and was characterized by an increase in the expression of inflammatory molecules and by the breakdown of the blood spinal cord barrier. During this period, microglia formed cell clusters and exhibited a M1-like phenotype. MCP-1/CCR2 signaling was essential in promoting this depletion associated spinal inflammatory reaction. Interestingly, ruling out MCP-1-mediated secondary inflammation, including blocking recruitment of monocyte-derived microglia, did not affect depletion-triggered microglia repopulation. Our results also demonstrated that newly generated microglia kept their responsiveness to peripheral nerve injury and their contribution to injury-associated neuropathic pain was not significantly altered.Although neurons in the central nervous system (CNS) have limited capacity for regeneration, glial cells exhibit remarkable self-renewal potential. Aroused from yolk sac progenitors that populate the CNS during embryogenesis, microglia in adulthood has been well recognized for their capability in preserving local homeostasis. Failure to keep up microglia in their normal physiological states leads to alteration in the stability of CNS micro-environment, as microglia are not only overseers of pathological disturbances 1,2 they also have physiological roles in normal CNS function 3,4 . However, the question of how microglia strive to maintain the integrity of the cell population is intriguing and unresolved, it has drawn much attention in recent research of microglia cell biology. Several research groups have investigated microglia repopulation after depletion in the brain parenchyma using genetic and/or pharmacological approaches. The main findings have identified the CNS resident microglia as the cell population responsible for re-establishing the CNS microglia compartment. Elmore et al. 5 reported that following depletion by blocking colony-stimulating factor1 receptor (CSF1R) signaling, microglia can repopulate solely through proliferation of nestin-positive, resident cells which then differentiate into microglia. The notion that microglia repopulation relies fully on CNS resident cells is further supported by the group of Bruttger 6 where Cx3cr1CreER :iDTR system has been used to ablate microglia cells. The participation of bone marrow-derived cells in the regeneration process...

Frontline Science: Blood-circulating leukocytes fail to infiltrate the spinal cord parenchyma after spared nerve injury

“…However, it is noteworthy that even in naive animals, GFP+ cells were found in the spinal cord, which suggested that the leukocyte infiltration was independent of SNI induction (Fig. B), most likely caused by the irradiation as an experimental artifact …”

“…In agreement, we found that when we applied a lethal irradiation, there is a significant engraftment of circulating cells (GFP + ) into the spinal cord of animals, even prior to SNI induction. Furthermore, besides BBB disruption there is evidence indicating that irradiation induces CNS resident cell death, especially microglia, which could cause leukocyte migration into the spinal cord in response to several inflammatory mediators released in the tissue . In this context, the increase in the number of GFP + cells in the spinal cord after SNI is simple explained by the fact that GFP + cells that engrafted after irradiation cells start to proliferate .…”

“…In addition to the role of spinal microglial cells in the development of neuropathic pain after peripheral nerve injury, some studies suggest that myeloid cells infiltrate the spinal cord and then contribute for this process in experimental models . Nevertheless, these studies used the classical methodology to generate bone marrow (BM) chimeric animals to investigate circulating cells infiltration in the CNS, which consists of the irradiation of the animals followed by the transfer of BM cells from GFP + donor mice.…”

Blood–spinal cord barrier breakdown and pericyte deficiency in peripheral neuropathy

“…In previous studies, there was controversial evidence of infiltration of macrophages and T cells into the spinal cord and dorsal root ganglia after CCI or transection of the rat sciatic nerve . Extravasation of leukocytes (macrophages and/or T cells) occurs in the lumbar spinal cord 3–14 days after peripheral L5 nerve transection.…”

“…One of the consequences of peripheral nerve injury is the entry of T cells from the circulation into the spinal dorsal horn parenchyma as part of the microglial reaction . Whether blood monocytes infiltrate the spinal cord in neuropathy remains controversial …”