Sodium channel in isolated human brain macrophages (microglia)

Abstract

Human brain macrophages (microglia) have been isolated from mixed brain cell cultures initiated from explants of neurosurgical adult human tissue in one step according to a method developed for rat microglia. Cells were characterized enzyme-histochemically (NDPase) in mixed and immunocytochemically (anti-CD 14) in mixed and isolated cultures. Purified cells were used to investigate in more detail membrane currents by the patch clamp technique. In 14 cells microdialyzed with a standard, K(+)-containing intracellular solution there was no indication for a hyperpolarization-induced K(+)-inward current characteristic for newborn rat microglia. However, in 12 cells depolarizing pulses initiated a rapidly inactivating inward current which was followed by an outward current (in 4 cells). The outward current appeared to be carried by K+, since it was absent in another 18 cells, recorded by micropipettes containing Cs+ instead of K+ as the main intracellular cation. The depolarization-induced inward current persisted under these conditions. This current was inhibited by tetrodotoxin (5 microM) and by substitution of Na+ by choline in the bath solution. It is suggested that this Na(+)-current is specifically expressed in macrophages derived from adult brain.

Physiology of Microglia

“…Voltage-gated Na ϩ currents were expressed only in 20% of cultured rat microglial cells; the cells with ramified morphology had a higher incidence of I Na detection (471). In contrast, in human microglial cells, cultured from explants obtained during surgery on patients suffering from brain tumors, the majority of cells (30 of 32 recorded) demonstrated TTX-sensitive I Na (658). Incidentally, coculturing microglial cells with astrocytes, which promotes ramified morphology of the former, significantly increased the proportion of cells expressing I Na (812).…”

“…] ing K ϩ currents dominate the membrane conductance of cultured microglial cells (442). The resting membrane potential of microglia cultured from rodents or humans was about Ϫ50 mV (442,658). Thus the cultured cells have a membrane current pattern different from microglial cells in slices from normal brain tissue.…”

“…264). Differences were described for human brain macrophages (microglia) isolated from explants of neurosurgical adult human tissue in that some of these cells expressed Na ϩ currents (658).…”

“…In microglial cells in situ and in the majority of studies in cultured microglia, rapid inward currents typical for voltage-gated sodium channels were not observed. Sodium currents (I Na ) were described in cultured microglial cells prepared from rat (471,812) and human (658) brains (see Table 2). Sodium currents in cultured microglial cells were similar to a typical neuronal Na ϩ channels; they were highly sensitive to TTX (complete inhibition at 2-5 M), had a very rapid kinetics, and had characteristic voltage dependence (threshold approximately Ϫ40 mV and peak ϳ0 mV).…”

“…2) Schmidtmayer (812) placed blood monocytes or spleen macrophages onto an astrocyte monolayer and termed these cells microglia. 3) In the study on human microglial cells, the cells were cultured from explants obtained during surgery on patients suffering from brain tumors (658). This cell population therefore could be potentially contaminated with blood monocytes.…”