Rod microglia: elongation, alignment, and coupling to form trains across the somatosensory cortex after experimental diffuse brain injury

Abstract

BackgroundSince their discovery, the morphology of microglia has been interpreted to mirror their function, with ramified microglia constantly surveying the micro-environment and rapidly activating when changes occur. In 1899, Franz Nissl discovered what we now recognize as a distinct microglial activation state, microglial rod cells (Stäbchenzellen), which he observed adjacent to neurons. These rod-shaped microglia are typically found in human autopsy cases of paralysis of the insane, a disease of the pre-penicillin era, and best known today from HIV-1-infected brains. Microglial rod cells have been implicated in cortical ‘synaptic stripping’ but their exact role has remained unclear. This is due at least in part to a scarcity of experimental models. Now we have noted these rod microglia after experimental diffuse brain injury in brain regions that have an associated sensory sensitivity. Here, we describe the time course, location, and surrounding architecture associated with rod microglia following experimental diffuse traumatic brain injury (TBI).MethodsRats were subjected to a moderate midline fluid percussion injury (mFPI), which resulted in transient suppression of their righting reflex (6 to 10 min). Multiple immunohistochemistry protocols targeting microglia with Iba1 and other known microglia markers were undertaken to identify the morphological activation of microglia. Additionally, labeling with Iba1 and cell markers for neurons and astrocytes identified the architecture that surrounds these rod cells.ResultsWe identified an abundance of Iba1-positive microglia with rod morphology in the primary sensory barrel fields (S1BF). Although present for at least 4 weeks post mFPI, they developed over the first week, peaking at 7 days post-injury. In the absence of contusion, Iba1-positive microglia appear to elongate with their processes extending from the apical and basal ends. These cells then abut one another and lay adjacent to cytoarchitecture of dendrites and axons, with no alignment with astrocytes and oligodendrocytes. Iba1-positive rod microglial cells differentially express other known markers for reactive microglia including OX-6 and CD68.ConclusionDiffuse traumatic brain injury induces a distinct rod microglia morphology, unique phenotype, and novel association between cells; these observations entice further investigation for impact on neurological outcome.

Temporal and Spatial Changes in the Pattern of Iba1 and CD68 Staining in the Rat Brain Following Severe Traumatic Brain Injury

“…Rod-shapedIba1 + cells were most evident 2 weeks post-severe TBI, which contrasts a previous report of maximal rod cells at 7 days after TBI [17]. Our model delivered a severe TBI via LFPI, while Ziebell et al [17] used a moderate midline TBI model.…”

“…The lack of co-labeling with SMI-71 indicated that rod-shaped Iba1 + cells do not align with nearby neurovasculature (Figure 2). Ziebell et al [17] and Taylor et al [28] suggested that rod-shapedIba1 + cells align with cortical axons. Indeed, stained axons (with NF312 antibody) and dendrites (with anti-MAP2 antibody), showed an undeniable alignment with rod-shapedIba1 + cells (Figure 2).…”

“…For example, rod microglia remain poorly understood even though they have been observed in damaged cortical tissue for over a century since Franz Nissl described them in 1899 [16]. These rod-shaped cells have been found in an array of progressive neuropathologies in both animal models and humans [16] [17]. A better understanding of the nuances of microglia activation could lead to the development of novel therapeutics.…”

Reduced bone mass accrual in mouse model of repetitive mild traumatic brain injury

“…The increased or "reactivated" astrocytes, in a process called astrocytosis, play a role in secondary injury following neurotrauma [42], and astrocytosis could be an early sign of chronic traumatic encephalopathy [27]. Serum GFAP levels predict the clinical outcome of mTBI in children [38]. Consistent with previous studies using immunochemical staining and immunoblot analysis [27,34], our real-time RT-PCR studies showed an increased expression of GFAP in the impacted compared with control brain, suggesting that secondary damage might have been underway.…”

“…This justifies our use of real-time RT-PCR to assess the reaction of the whole brain rather than the reaction of localized areas. We used two markers: IBA1 for microglia [38] and GFAP for astrocytes [39]. IBA1 is a gene that is upregulated upon the activation of microglia due to inflammation.…”