Neurobiology of microglial action in CNS injuries: receptor-mediated signaling mechanisms and functional roles

Abstract

Microglia are the first line of immune defense against central nervous system (CNS) injuries and disorders. These highly plastic cells play dualistic roles in neuronal injury and recovery and are known for their ability to assume diverse phenotypes. A broad range of surface receptors are expressed on microglia and mediate microglial 'On' or 'Off' responses to signals from other host cells as well as invading microorganisms. The integrated actions of these receptors result in tightly regulated biological functions, including cell mobility, phagocytosis, the induction of acquired immunity, and trophic factor/inflammatory mediator release. Over the last few years, significant advances have been made toward deciphering the signaling mechanisms related to these receptors and their specific cellular functions. In this review, we describe the current state of knowledge of the surface receptors involved in microglial activation, with an emphasis on their engagement of distinct functional programs and their roles in CNS injuries. It will become evident from this review that microglial homeostasis is carefully maintained by multiple counterbalanced strategies, including, but not limited to, 'On' and 'Off' receptor signaling. Specific regulation of theses microglial receptors may be a promising therapeutic strategy against CNS injuries.

Keywords: Central nervous system injuries; Microglia; Receptor.

Figure 1. Microglial receptors

Various surface receptors act as opposing and complementary switches to maintain microglial homeostasis. “On” receptors: A large number of receptors have been identified to turn on microglial activation in response to different stimuli. “Off” receptors: Several receptors constitutively expressed on resting microglia help to maintain their quiescence. Dualistic receptors: The activation of these receptors induces both activating and inhibitory effects in microglia, allowing for a high degree of resolution in microglial responsiveness to differing stimuli. P1: P1 adenosine receptors; P2: P2 ATP receptors; RAGE: receptor for advanced glycation endproducts; TLR: toll-like receptor; TREM: triggering receptors expressed on myeloid cells.

Figure 2. TREM-2 signaling

The ITAM-containing adapter DAP12 is important for both activating and inhibitory TREM-2 signaling. A consensus ITIM sequence is embedded in the ITAM sequence of DAP12 and is called a ‘closet’ ITIM. DAP12 might be incompletely or completely phosphorylated depending on the binding affinity between the associated receptor and its ligands. While incomplete phosphorylation of DAP12 upon weak TREM-2 receptor binding can result in ITIM phosphorylation, high affinity receptor binding may result in complete ITAM phosphorylation. The phosphorylated tyrosine in the DAP12 closet ITIM may recruit inhibitory phosphatases (SHP-1 or SHIP1) to attenuate TREM-2 activation. In the absence of SHIP1, ligation of TREM-2 recruits PI3K p85 subunit to the DAP12 ITAM with the assistance of another adaptor protein, DAP10. This, in turn, results in the activation of ERK1/2, Akt,, and the guanine nucleotide exchange factor Vav3.

Figure 3. Fcγ receptors

Most FcγRs have a common α chain that contains an extracellular region with varying numbers of the immunoglobulin (Ig)-like domain. FcγRIIa has only a single α chain. FcγRI and FcγRIIIa are coupled to additional receptor subunits (two γ chains) that are involved in downstream signaling. The signaling pathways initiated by the engagement of different activating FcγRs usually begin with tyrosine phosporylation of the conserved immunoreceptor tyrosine-based activation motif (ITAM) in the cytoplasmic tail of the receptors or their associated subunits by kinases in the Src family. The subsequent recruitment of Syk-family kinases in turn permits interaction with other signaling proteins, such as PI3K, Src, and Ras, and further activates downstream signaling pathways that lead to specific biological functions. The activation of NFκB or the Ras/MEK/ERK pathway is critical for cytokine/chemokine production whereas the PI3K-Akt pathway is important for microglial phagocytosis. In contrast to the ITAM-mediated activation of microglia, the engagement of inhibitory FcγRIIb triggers immunoreceptor tyrosine-based inhibition motif (ITIM) activation through its cytoplasmic domain, resulting in the recruitment of phosphatases such as SHIP and SHP1. This attenuates the signaling cascades engaged by activating FcRs and diminishes downstream biological effects by acting in counterpoint to ITAM.

Figure 4. P2X4 receptors in tactile allodynia after spinal nerve injury

Spinal nerve injury may induce P2X4 receptor upregulation in microglia through the activation of fibronectin/integrin, Lyn kinase, PI3K/Akt, and MEK/ERK-dependent signaling pathways. Chemokine CCL2 and CCL21 are also involved in transforming quiescent microglia into the P2X4-expressing phenotype through interaction with microglial CCR2, CXCR3, and CCR7 receptors. Stimulation of the P2X4 receptor by ATP causes the SNARE-mediated release and synthesis of BDNF, which is dependent on extracellular Ca²⁺ influx and activation of p38-MAPK. The elevated BDNF in turn weakens the tonic inhibition of lamina I GABAergic interneurons and results in the development of allodynia.

Figure 5. Phosphatidylserine receptors and bridging proteins are critical for the microglial clearance of apoptotic neurons

MFG-E8 is expressed in microglia and secreted in response to soluble factors, such as CX3CL1, which is released from degenerating neurons. MFG-E8 can bind to phosphatidylserine on apoptotic neurons and simultaneously engage integrin αvβ3 on microglia. Elevated MFG-E8 expression therefore results in accelerated microglial clearance activity. Moreover, MFG-E8 enhances production of heme oxygenase-1 (HO-1) through Nrf2 activation. Another bridging protein, Gas6, bridges phosphatidylserine residues on the surface of apoptotic cells to the Axl/Mer family of tyrosine kinases (Mer, Axl, and Tyro3) on microglia and stimulates microglial phagocytosis of apoptotic neurons in a vav/Rac-dependent manner. Gas6 receptors mediate an anti-inflammatory response to apoptotic cells by inhibiting NF-κB activation.