The “protected” glucose transport through the astrocytic endoplasmic reticulum is too slow to serve as a quantitatively‐important highway for nutrient delivery

Abstract

A novel mechanism involving “protected glucose trafficking” through the lumen of the astrocytic endoplasmic reticulum (ER) was proposed by Müller et al. (Current Biology 28:3481, 2018) and highlighted by Pellerin (Current Biology 28:R1258, 2018) as a potential route for astrocyte‐neuron lactate shuttling. In their model, glucose is taken up from blood into astrocytic endfeet, phosphorylated, and some glucose‐6‐phosphate (Glc‐6‐P) is transported into the ER lumen where it is hydrolyzed by glucose‐6‐phosphatase‐β to produce glucose. The glucose then diffuses within the ER lumen through peripheral astrocytic processes (PAPs) to regions in close proximity to synapses where it is released to cytoplasm and metabolized to lactate and neurotransmitter precursors that can be shuttled to neurons. Experimental evidence supports aspects of this model, but essential components were not established. This commentary critically evaluates the Müller et al. study and discusses its weaknesses and intriguing aspects. Most important, the rate‐limiting step of the ER “protected highway,” transport of Glc‐6‐P into the ER, is extremely slow, on the order of 550‐3,700 times lower than glucose consumption by differentiated cultured astrocytes. Glucose diffusion through extracellular space to fuel neurons was not ruled out as an alternative mechanism. The ER glucose probes were not calibrated, and the ER luminal glucose reservoir size and flux of glucose through the ER are unknown. Effects of phosphatase knockdown are quite interesting and require further study. Glucose transport through a “protected intracellular highway” in the ER lumen is not quantitatively relevant to astrocytic glucose metabolism and is not energetically important.

Axonal Endoplasmic Reticulum Dynamics and Its Roles in Neurodegeneration

“…G6Pase-β would facilitate accumulation of glucose to the ER lumen, where it would be free to diffuse and be protected from further metabolism, until returning to the cytosol through ER glucose transporters, and being phosphorylated to G6P to enter glycolysis (Müller et al, 2018). However, it has recently been argued that glucose transport through the ER is too slow (around 550-3,700 times lower than glucose consumption) to be quantitatively relevant for nutrient delivery (Dienel, 2019), FIGURE 1 | Endoplasmic reticulum (ER) distribution in neuronal cells. (A) ER network (blue) is continuously distributed throughout the cytosol (orange) in neurons, including cell body, axon, presynaptic terminals, and most dendrites.…”

Hypothesis: A Novel Neuroprotective Role for Glucose-6-phosphatase (G6PC3) in Brain—To Maintain Energy-Dependent Functions Including Cognitive Processes

“…However, the rate-limiting step, carriermediated transport of Glc-6-P into the ER lumen, is so slow that the model cannot be energetically important. Calculated rates of Glc-6-P transport into the ER are 550-3700 times slower than glucose consumption by cultured astrocytes and considerably slower than rates of other pathways that use Glc-6-P [33]. The notion of ER luminal glucose transport from endfeet to perisynaptic processes and neurons does not withstand rigorous evaluation.…”

“…The authors' explanation for rescue was that G6PC3 knockdown prevented hydrolysis of Glc-6-P and reduced DG uptake that was restored by G6PC1. In addition to Glc-6-P transport into the ER lumen being much too slow for their model to be feasible, their DG uptake method was criticized for three reasons [33]. (i) Omission of glucose during the assay rendered the cells aglycemic so that endogenous compounds would have to be consumed to prevent energy failure and cell swelling.…”

“…All G6Pase isoforms are located in the lumen of the endoplasmic reticulum (ER) raising an interesting question as to why G6PC3 should be compartmentalized if its only or primary function is to eliminate AG6P that is produced in the cytoplasm. While this issue needs further experimental analysis, one reason could be that the rate-limiting step of Glc-6-P [33] and AG-6-P hydrolysis by G6PC3 is transport into the lumen so Glc-6-P hydrolysis is restricted. This protects the cytosolic hexose-6-phosphates from uncontrolled hydrolysis (futile cycling) and compromise of cellular glucose-dependent energetic, redox, and biosynthetic functions.…”

Computational singular perturbation analysis of brain lactate metabolism

“…There is a large body of literature advocating the validity of ANLS [6,[22][23][24][25][26][27][28][29][30][31][32], as this is supported by indirect experimental findings [4,18,[32][33][34][35][36][37][38][39][40][41][42][43][44]. The main criticism of the ANLS hypothesis stems from the NALS supporters [8,9,[45][46][47][48][49][50][51][52][53][54], and the related indirect experimental evidence [10,[13][14][15][55][56][57][58][59][60][61]…”