An optogenetic approach for assessing formation of neuronal connections in a co-culture system.

Here we describe a protocol to generate a co-culture consisting of 2 different neuronal populations. Induced pluripotent stem cells (iPSCs) are reprogrammed from human fibroblasts using episomal vectors. Colonies of iPSCs can be observed 30 days after initiation of fibroblast reprogramming. Pluripotent colonies are manually picked and grown in neural induction medium to permit differentiation into neural progenitor cells (NPCs). iPSCs rapidly convert into neuroepithelial cells within 1 week and retain the capability to self-renew when maintained at a high culture density. Primary mouse NPCs are differentiated into astrocytes by exposure to a serum-containing medium for 7 days and form a monolayer upon which embryonic day 18 (E18) rat cortical neurons (transfected with channelrhodopsin-2 (ChR2)) are added. Human NPCs tagged with the fluorescent protein, tandem dimer Tomato (tdTomato), are then seeded onto the astrocyte/cortical neuron culture the following day and allowed to differentiate for 28 to 35 days. We demonstrate that this system forms synaptic connections between iPSC-derived neurons and cortical neurons, evident from an increase in the frequency of synaptic currents upon photostimulation of the cortical neurons. This co-culture system provides a novel platform for evaluating the ability of iPSC-derived neurons to create synaptic connections with other neuronal populations.

Lmx1a allows context-specific isolation of progenitors of GABAergic or dopaminergic neurons during neural differentiation of embryonic stem cells.

LIM homeobox transcription factor 1 alpha (Lmx1a) is required for the development of midbrain dopaminergic neurons, roof plate formation, and cortical hem development. We generated a reporter embryonic stem cell (ESC) line for Lmx1a and used it to track differentiation and extract neural progenitors from differentiating mouse ESCs. Lmx1a(+) cells gave rise to functional cortical upper layer GABAergic neurons or dopaminergic neurons depending on the culture conditions used for differentiation. Under chemically defined neurobasal conditions, ESC differentiation resulted in widespread and transient expression of Lmx1a, without the addition of exogenous factors such as sonic hedgehog (Shh), Wnts, and/or bone morphogenic proteins (BMPs). Under neutral conditions, Lmx1a(+) cells express genes known to be downstream of Lmx1a and cortical hem markers Wnt3a and p73. The majority of these cells did not express the ventral midbrain dopaminergic marker Foxa2 or dorsal roof plate marker BMP-2. Lmx1a(+) -Foxa2(-) cells were primed to become SatB2(+) GABAergic neurons and appeared to be resistant to dopaminergic patterning cues. PA6 coculture produced a substantial population of Lmx1a(+) progenitors that also expressed Foxa2 and on further differentiation gave rise to dopaminergic neurons at high frequency. We conclude that Lmx1a is a useful marker for the extraction of progenitors of GABAergic or dopaminergic neurons. We caution against the assumption that it indicates dopaminergic commitment during in vitro differentiation of ESCs. Indeed, in monolayer culture under neurobasal conditions, with or without the addition of Shh and fibroblast growth factor 8 (FGF8), Lmx1a(+) cells were predominantly progenitors of forebrain GABAergic neurons. We obtained dopaminergic cells in large numbers only by coculture with PA6 cells.