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Harry et al. Neuroimmunol Neuroinflammation 2020;7:150-65 I http://dx.doi.org/10.20517/2347-8659.2020.07 Page 155

repair oxidative damage through glutathione- and thioredoxin-coupled pathways [147] . Glucose metabolism
influences microglial activation through an NADH-sensitive co-repressor termed C-terminal binding
+
protein (CtBP). Slowed glucose flux through glycolysis reduces NADH levels and reduce NADH:NAD
ratio [148] . In both microglia and macrophage RAW264.7 cells, glucose flux regulates iNOS expression and
+
other pro-inflammatory genes through effects on cytosolic NADH:NAD ratio and CtBP [149] .

Several glucose transporters such as GLUT1 [150] , GLUT3 [151] , and GLUT5 [152] are expressed in microglia.
Acute fluctuation of available glucose impacts microglia activity with an elevated response to LPS upon
shifting from a normal to high glucose level. Shifting from a high to normal glucose level can also
induce metabolic stress [153] . Glucose levels can influence pro-inflammatory gene transcription by several
mechanisms. One such mechanism relies on the formation of advanced glycation end-products (AGE).
These products consist of modified proteins and lipids as a result of non-enzymatic reactions with sugars.
It is known that microglia express receptors of AGE and, upon activation, pro-inflammatory signaling
pathways are stimulated [127,154] . In peripheral macrophages, it has been reported that a shift in the cell’s
energy source induced by glucose deprivation results in an altered response to a pro-inflammatory
stimulus [155-157] . Multiple studies have reported an inability of microglia to respond appropriately to LPS
under oxygen and glucose deprivation or with 2-DG inhibition of glucose metabolism [109] . However,
there is evidence that microglia are capable of functioning with alternative energy sources to adequately
respond to an inflammatory challenge. Choi et al. [158] reported an increase in mRNA and protein levels
for IL-6 in microglia after 7 h of glucose and serum free medium. Upon stimulation with LPS, glucose-
deprived microglia retained their normal ability to respond with elevations in nitrite, IL-1b, and TNFa [159] .
Primary rat microglia shifted to glucose-free medium for 1 h to LPS showed an exacerbated release of NO
within 24 h and similar elevations in TNFa and IL-1b as compared to non-glucose-deprived cells. Glucose
deprivation for 24 h prior to LPS exposure increased release of IL-1b with no deficits in NO or TNFa.
The authors suggested that microglia were able to mobilize fatty acids from intracellular lipid droplets as
an energy source. The majority of studies examining the effects of glucose deprivation have focused on
relatively short-term exposures, within 1-24 h. While these studies demonstrated that both peripheral
macrophages and microglia can shift their response to a pro-inflammatory stimulus in a selective manner,
the question remains as to whether such a response would be altered when the cells were forced to a
more prolonged shift in energy metabolism. When RAW 264.7 [Figure 1] or BV-2 [Figure 2] cells were
maintained for three days under culture conditions to force cells to rely on galactose as an alternative
energy source, the cells were able to normally respond to LPS stimulation. However, the diminished pro-
inflammatory cytokine response observed when 2-DG was used in previous studies to inhibit glycolysis
may have been related to the lower basal OXPHOS induced [160] . The differences across these studies likely
lie with the method of depleting glucose: removing glucose from the medium; the addition of 2-DG, which
in and of itself can lower basal induction of OXPHOS [160] ; or the combination of glucose deprivation with
hypoxia. In RAW cells, the morphological changes observed with LPS activation have been demonstrated
to be diminished under galactose, suggesting a requirement of glucose to facilitate cell spreading [161] . This
was not clearly observed in the current study where similar LPS-induced morphological patterns were
observed in the absence of pyruvate [Figure 1]. In BV-2 cells, a slight morphological shift was observed
with the low level of LPS stimulation with minimal induction of nitrate and an elevation in TNFa and IL-1
protein.

GLUTAMINE
Macrophages utilize glutamine at high rates to synthesize amino acids, nucleotides, NADPH, and energy
production and are dependent upon extracellular sources of the amino acid [103] . Channeling of glutamine
into the Krebs cycle is a primary route to promote succinate synthesis in macrophages. This occurs with
glutamine being used for synthesis of glutamate, GABA, and succinate, bypassing the TCA cycle [155] . This
stabilizes hypoxia-inducible factor 1-alpha (HIF-1a), an oxygen-sensitive transcription factor that allows the