Microglia Research in the 100th Year Since Its Discovery

In 1919, Pio del Rio-Hortega first described a class of cells residing in the brain with a tiny soma and branched architecture distinguishable from astrocytes and neurons. These cells, called microglia or mesoglia, have come to be appreciated as resident immune cells in the central nervous system (CNS). In the year 2019, we review the three most recent advances in microglia research in this 100th year since their discovery (Fig. 1). We first discuss their transcriptional diversity, which allows us to appreciate the heterogeneity of microglia across species, development, diseases, and brain regions. We also highlight recent stem cell-based approaches that allow us to study human microglia. We further review new signaling mechanisms that expand our understanding of how microglia sense synaptic changes and alter neural circuits. Ultimately, 100 years of microglial research demonstrates their incredible adaptation and plasticity, playing roles in neural development, brain homeostasis, and neurological disorders far beyond their anticipated immunological function.

Fig. 1.

Key microglial research advances in the year 2019. A Single-cell RNAseq studies show that microglial transcriptional diversity spans species, development, and brain regions in addition to noted differences in pathology. B Human stem cells can be induced to differentiate into microglia and implanted into the mouse brain. In this context, human microglia display canonical motility characteristics and have lineage (e.g. Pu.1, a transcription factor) and signature gene expression (e.g. the P2Y12 receptor and transmembrane protein TMEM119). C Novel signaling mediates microglia-neuron communication. Neuronal somatic ATP release is sensed by microglial P2Y12 receptors, forming somatic junctions between neuronal somata and microglial processes at sites with clustered potassium channels (Kv). Neuronal fractalkine (CX3CL1) signaling to the microglial receptor CX3CR1 is the basis for activity-dependent engulfment in the barrel cortex. Tonic norepinephrine (NE) from adrenergic neurons arrests microglial process surveillance through β2 receptors in awake mice while also promoting nanoscale surveillance by smaller filopodia structures.

Transcriptomic Analyses Reveal the Heterogeneity of Microglia

2019 marks a year with major advances in understanding microglial heterogeneity (Fig. 1A). Single-cell RNA sequencing (SC-RNAseq) beautifully characterizes the transcriptional diversity and conservation of microglia across millions of years of evolution [1]. Within a species, SC-RNAseq has helped to define clusters of transcriptionally distinct microglia. In mice, microglia have unique transcriptional profiles across development (particularly between the embryonic and postnatal periods) and across brain regions [2]. In addition, highly distinct microglial clusters have emerged in disease contexts, specific to either neurodegeneration or demyelination [2]. Similarly, human microglia also demonstrate clusters of transcriptional diversity between non-pathological gray and white matter, and between the healthy and tumoral portions of resected tissue [3]. These findings suggest that microglia can be stratified based on multiple axes of transcriptional diversity (Fig. 1A). Microglial heterogeneity can also serve as a critical lens through which to investigate brain diseases. Towards this end, studies of Hoxb8, one of the first markers found to differentiate microglial subtypes, demonstrate the interesting role of this subpopulation in complex behavior. The loss of Hoxb8 in microglia particularly exacerbates pathological behaviors and anxiety in females through an as-yet unknown mechanism involving circulating sex hormones [4].

The challenge for the coming decade will be the selective functional investigation of transcriptionally unique sub-populations of microglia to evaluate their role in the intact brain. To do so, we look forward to new tools that enable researchers to distinctively label microglial populations based on transcriptomics profiles, in the absence of a singular defining gene. Such an advance seems particularly necessary to understand the sub-populations of human microglia, which have the greatest transcriptional diversity [1].

Stem Cell Approaches to Study Human Microglia

2019 has also seen an explosion in stem cell-based approaches to study human microglia (Fig. 1B). Technical advances in the past few decades have culminated in our ability to derive microglia from blood monocytes [5], embryonic cell lines [6], hematopoietic stem cells [7], or human induced pluripotent stem cells [8]. These microglia-like human cells can be successfully engrafted into the developing mouse brain [6–8].

Microglia-like cells implanted into the mouse are replete with microglial signature genes [6–8], maintain markers of the myeloid lineage [7, 8], and adopt a ramified morphology [6–8] (Fig. 1B). These studies also demonstrate that human microglia display canonical motility characteristics and injury responses [7, 8]. At the same time, however, these human-derived microglia can take on a substantially different transcriptional profile in response to Alzheimer’s disease (AD) pathology from neighboring mouse microglia [6, 7]. Interestingly, microglia from patients with schizophrenia even maintain their enhanced phagocytic capacity in vitro relative to microglia-like cells from healthy controls [5]. Because stem cells retain the genetic profile of their source, stem cell techniques now enable researchers to study species-specific and patient-specific microglial responses to a uniform pathological context. Given the key role that microglia-driven inflammation plays in the amyloid-beta and tau pathology of AD [9], placing human microglia in a uniform AD model system could yield key insights into gene–environment interactions. Of interest, future studies would also be able to isolate and study human microglia from genetically high-risk and low-risk populations.

Apart from generating microglia from human stem cells, progress has also been made in generating neurons from adult microglia. Finding ways to reprogram microglia, with their relatively strong capacity to regenerate, could be an early step in combating neuronal loss. Towards this end, progress has been made in reprogramming adult microglia, even in vivo, using the transcription factor NeuroD1 [10]. Reprogrammed cells take on a neuronal transcriptional and epigenetic profile, but future work is needed to determine whether these cells properly integrate into circuitry.

Novel Signaling Regulates Bidirectional Microglial-Neuron Communication

Key studies in 2019 have expanded our knowledge of which microglial receptors mediate critical functions such as synaptic engulfment and microenvironmental surveillance (Fig. 1C). For example, fractalkine ligand (CX3CL1) signaling to the microglial fractalkine receptor (CX3CR1) has recently been shown to be a key regulator of synapse elimination in the barrel cortex [11]. As suggested by RNAseq and validated by genetic and pharmacological approaches, neuronal cleavage of the soluble CX3CL1 by the metalloprotease ADAM10 signals to CX3CR1 to begin an active engulfment process.

In addition, new studies have also expanded our understanding of microglial β2 receptors in network interactions (Fig. 1C). Two studies have recently revealed that the β2 adrenergic receptor plays an important role in regulating microglial process surveillance in awake mice [12, 13]. These studies showed that tonic norepinephrine (NE) levels in the awake mouse attenuate the extent to which microglial processes survey their surroundings. In tandem, NE promotes tiny filopodia at the ends of these arrested microglial processes to search the local milieu, promoting a “nanoscale” level of surveillance dependent on cAMP [14]. At the systems level, approaches that dampen local or global NE levels (such as anesthesia) permit microglial process outgrowth and increase microglia-neuron contact time [12, 13]. An early implication of this interaction is that prolonged NE-β2 signaling can negatively impact certain forms of visual plasticity [13]. In contrast to prolonged NE-β2 signaling, a key future question is whether a reduction in NE-β2 signaling during sleep creates a more permissive environment for microglia to alter the synaptic landscape, contributing to plasticity.

By 2019, the role of microglia in plasticity has been well established through immunological signaling mechanisms during development [15] and pathology (e.g. hypoxia) [16]. However, the role of microglia in learning and plasticity is not merely restricted to development and immunological signaling. Recent work also demonstrates that microglia can influence motor learning or the maintenance of long-term potentiation through their release of brain-derived neurotrophic factor [17, 18].

Finally, one of the last discoveries of 2019 expands our understanding of the well-known P2Y12 receptor. Previously, P2Y12 has been recognized for its role in mediating microglial process interactions with neuronal synapses [19]. In addition, neuronal somata have recently been described for their inclusion of unique microdomains, which recruit microglial processes. These mitochondria-rich sites generate activity-dependent release of ATP, which recruits microglial processes through the P2Y12 receptor [20]. Given the role of microglial P2Y12 signaling in neuroprotection during stroke and acute seizures [19, 20], it is possible that somatic as well as synaptic microglial contacts are both critical for how microglia exert P2Y12-dependent effects. Future studies are necessary to understand exactly how microglial P2Y12 signaling dampens neuronal activity and increases neuroprotection through each interaction site. Altogether, research in 2019 has greatly expanded our knowledge of the microglial signaling repertoire and its impact on the neuronal landscape. The mechanisms identified highlight both immunological and neuronal signaling axes that inform and influence neuroimmune function.

Conclusion

The 100th anniversary of microglial research has made great strides in understanding microglial transcriptional diversity and their circuit functions. While microglia are often referred to as innate immune cells of the CNS, one of the most unanticipated discoveries of the past 100 years is the ability for microglia to sense and influence neuronal networks as part of normal development. These discoveries suggest that microglia are an integral part of neuronal circuitry. Moving forward, transcriptomics will continue to provide the road map for understanding microglial response profiles in the healthy and diseased brains. A necessary next step is to begin to use this wealth of knowledge to identify rationale candidates (receptors and signals) for detailed functional studies. Ultimately, combining the strengths of transcriptional and functional approaches is necessary to identify key mechanisms mediating normal and pathological interactions. As we come to better understand their circuit function, we anticipate the promising development of microglia-specific therapeutics for treating neurological disorders in the decades to come.