Beyond Activation: Characterizing Microglial Functional Phenotypes.

Lier J, Streit WJ, Bechmann I

Cells 2021

1. Cells. 2021 Aug 28;10(9):2236. doi: 10.3390/cells10092236. Beyond Activation: Characterizing Microglial Functional Phenotypes. Lier J(1)(2), Streit WJ(3), Bechmann I(1). Author information: (1)Institute of Anatomy, University of Leipzig, 04109 Leipzig, Germany. (2)Department of Neurology, University of Leipzig, 04109 Leipzig, Germany. (3)Department of Neuroscience, University of Florida College of Medicine, Gainesville, FL 32611, USA. Classically, the following three morphological states of microglia have been defined: ramified, amoeboid and phagocytic. While ramified cells were long regarded as "resting", amoeboid and phagocytic microglia were viewed as "activated". In aged human brains, a fourth, morphologically novel state has been described, i.e., dystrophic microglia, which are thought to be senescent cells. Since microglia are not replenished by blood-borne mononuclear cells under physiological circumstances, they seem to have an "expiration date" limiting their capacity to phagocytose and support neurons. Identifying factors that drive microglial aging may thus be helpful to delay the onset of neurodegenerative diseases, such as Alzheimer's disease (AD). Recent progress in single-cell deep sequencing methods allowed for more refined differentiation and revealed regional-, age- and sex-dependent differences of the microglial population, and a growing number of studies demonstrate various expression profiles defining microglial subpopulations. Given the heterogeneity of pathologic states in the central nervous system, the need for accurately describing microglial morphology and expression patterns becomes increasingly important. Here, we review commonly used microglial markers and their fluctuations in expression in health and disease, with a focus on IBA1 low/negative microglia, which can be found in individuals with liver disease. DOI: 10.3390/cells10092236 PMCID: PMC8464670 PMID: 34571885 [Indexed for MEDLINE] Conflict of interest statement: The authors declare no conflict of interest.

3 Figures Extracted

Figure 1 PMC

Comparison of frequently used microglial markers in the typically described morphological phenotypes . In human tissue from the frontal cortex and hip...

Figure 2 PMC

IBA1-negative microglia. Regions seemingly devoid of microglia in the IBA1 staining, exhibit positive staining for several other microglial markers s...

Figure 3 PMC

Pronounced morphological activation of microglial cells. In a case with hypoxic brain damage after CPR, one can detect pronounced activation and high...