A Microfluidic Model of AQP4 Polarization Dynamics and Fluid Transport in the Healthy and Inflamed Human Brain: The First Step Towards Glymphatics‐on‐a‐Chip

Abstract

Dysfunction of the aquaporin‐4 (AQP4)‐dependent glymphatic waste clearance pathway has recently been implicated in the pathogenesis of several neurodegenerative diseases. However, it is difficult to unravel the causative relationship between glymphatic dysfunction, AQP4 depolarization, protein aggregation, and inflammation in neurodegeneration using animal models alone. There is currently a clear, unmet need for in vitro models of the brain's waterscape, and the first steps towards a bona fide “glymphatics‐on‐a‐chip” are taken in the present study. It is demonstrated that chronic exposure to lipopolysaccharide (LPS), amyloid‐β(1‐42) oligomers, and an AQP4 inhibitor impairs the drainage of fluid and amyloid‐β(1‐40) tracer in a gliovascular unit (GVU)‐on‐a‐chip model containing human astrocytes and brain microvascular endothelial cells. The LPS‐induced drainage impairment is partially retained following cell lysis, indicating that neuroinflammation induces parallel changes in cell‐dependent and matrisome‐dependent fluid transport pathways in GVU‐on‐a‐chip. Additionally, AQP4 depolarization is observed following LPS treatment, suggesting that LPS‐induced drainage impairments on‐chip may be driven in part by changes in AQP4‐dependent fluid dynamics.

A human initial lymphatic chip reveals distinct mechanisms of primary lymphatic valve dysfunction in acute and chronic inflammation

“…68 Additionally, the nanospheres utilized were drained into the LEC channel over the course of 16 hours, which we have found through past studies to be the optimum time for fluid equilibrium to occur across the microfluidic chip in similar systems. 30 Imaging nanosphere migration would require 16 hours of live microscopy imaging, which cannot be guaranteed to preserve the same conditions (CO 2 and humidity) of cell incubators and cannot provide luminal flow along the vessel axis that is required for proper LEC phenotype and vessel formation in vitro . 48 Thus, the data presented here represents the total drainage of nanospheres after equilibrium is reached, measured using the fluorescence intensity of nanospheres accumulated on each side of the chip.…”

“…Microfluidic devices were fabricated using a soft lithography method as we performed previously. [28][29][30][31] Briefly, the human lymphatic vessel (LV) chip was composed of a polydimethylsiloxane (PDMS) housing for two channels. The PDMS (Sylgard 184, Dow-Corning, Midland, MI) was formed by mixing with a curing agent from the Sylgard PDMS kit at a 10 : 1 ratio of base to curing agent and poured into silicon master molds for overnight curing at 80 °C.…”

“…To achieve this, we engineered an organotypic model of LVs. 30,31 Briefly, our PDMS (polydimethylsiloxane)based LV-on-chip is composed of two hollow cylindrical channels, which are completely embedded into a 3D collagen 1 matrix (Fig. 1B).…”

Solute transport in the brain tissue: what are the key biophysical parameters tying in vivo and in vitro studies together?

“…Using a 4 : 1 : 1 mixture of collagen type 1, Matrigel and hyaluronic acid, a very recent glymphatics-on-a-chip was proposed. 93 The chip consisted in two parallel endothelial vessels passing through one common chamber containing the astrocyte seeded hydrogel. Transport of an amyloid-β solution from one vessel to the other was observed but no control over the hydrostatic pressure gradient between the vessels was incorporated in the design.…”

Editorial: Cerebral microdialysis

“…These have been used in research protocols to examine proteins such as cytokines, chemokines, and growth factors in the extracellular space, in addition to monitoring small molecules. The interaction of proteins in normal and abnormal situations ranging from TBI to Alzheimer's disease may be explored by analysis of proteins in the glymphatic fluid (13,14). As most proteins are >20 kDa in size, the growing use of 100-kDa microdialysis catheters has great promise for future studies.…”