Postganglionic sympathetic neurons (symNs) belong to the autonomic nervous system (ANS) and have multiple important roles in responding and regulating homeostasis of the body independent of consciousness. For example, stress stimulates symNs and evokes the fight-or-flight response that leads to an increase in heart rate, blood pressure, and sweating. SymNs are affected in multiple human disorders due to genetics, toxicity/injury, or as companions to other diseases. An example of a genetic neuropathy is the childhood disorder Familial Dysautonomia (FD), where a severe dysregulation of symNs causes dysautonomic crisis, evident by sweating, blotching of the skin, vomiting attacks, hypertension, and anxiety1. An example of toxicity is chemotherapy treatment, which has been reported to have toxic side effects on autonomic neurons2. It is known that autonomic denervation and hyper-innervation can both lead to, or accompany, diseases such as Parkinson’s disease or hypertensive renal disease3,4. Thus, being able to conduct research and understand the mechanisms of symN biology and defects in the context of disease is beneficial for the search of novel and effective treatments.
Anatomy
The peripheral nervous system branches into sensory and autonomic divisions. The afferent nerves of the sensory nervous system are responsible for sensation of pain and touch, whereas the ANS is responsible for relaying information from all organs to the brain. The ANS is divided into the enteric nervous system, innervating the gastrointestinal tract, the parasympathetic nervous system, which is important for relaxation, and the sympathetic nervous system (SNS), which is important for activation/regulation of organs. The SNS adapts a two-neuron system5. Preganglionic sympathetic neural axons in the spinal cord first project to the sympathetic ganglia, where postganglionic symN cell bodies are located. These neurons then send long projections to innervate the target tissues of every organ in the body. Signals transmitted by preganglionic neurons are cholinergic, whereas postganglionic symNs are adrenergic and thus express norepinephrine (NE) as their main neurotransmitter. There are few notable exceptions of postganglionic, sympathetic neurons that are cholinergic, including the ones innervating blood vessels. Adrenergic postganglionic neurons express the enzymes tyrosine hydroxylase (TH), aromatic L-amino acid decarboxylase (AAAD), dopamine β-hydroxylase (DBH), and monoamine oxidase (MAO-A), all responsible for generating and metabolizing NE. Furthermore, they express the NE recycling transporters and/or receptors α-adrenergic receptor (ADRA2), β-adrenergic receptor (ADR2B), norepinephrine transporter (NET1), and vesicular monoamine transporter (VMAT1/2).
Development
During embryonic development symNs are derived from the neural crest (NC), which emerges between the neural tube and overlaying ectoderm6, and can differentiate into multiple cell lineages, including melanocytes, osteoblasts, adipocytes, glia, enteric neurons, sensory neurons, and autonomic neurons7. Neural crest cells (NCCs) are highly migratory cells that take several routes through the embryo. At this early stage of NC development, the cells express the markers SNAIL1/2, FOXD3, and SOX108,9,10,11. The migration route together with the axial location they adopt determines the NC subtype into which they will develop. These NC subtypes can be distinguished by their specific HOX gene expression: Cranial NCCs do not express HOX genes, vagal NCCs express HOX 1–5, trunk NCCs express HOX 6–9, and sacral NCCs express HOX 10–1112. Among them, trunk NCCs are recognized as the main source of symNs. SymN precursors express the transcription factor MASH1/ASCL113, which promotes expression of PHOX2B14 and INSM115. The GATA family of transcription factors is expressed during late sympathetic development. GATA2 and GATA3 are expressed in the symNs, which in turn activates DBH16. The transcription factor HAND2 is also important for the expression and maintenance of DBH and TH17.
HPSCs (e.g., embryonic and induced pluripotent stem cells) are a powerful tool18 to recapitulate developmental paradigms and generate symNs that can then be employed for disease modeling of various human disorders. Thus, while generating symNs from hPSCs, it is crucial to follow developmental guidelines and assess expression of appropriate markers along the differentiation process.
Previous symN protocol
Few research groups have previously reported the generation of symNs from hPSCs19,20,21. The direct comparison of these protocols to each other and ours was reviewed recently22. In 201623, we published a differentiation protocol for the generation of autonomic neurons with symN character (Figure 1A). This protocol used KSR-based medium, which was used in both the maintenance of undifferentiated hPSCs and cell differentiation. Furthermore, hPSCs were maintained on mouse embryonic fibroblasts (MEF feeder cells). We employed this protocol and PSCs from patients with FD to model the disorder23. In 2019, we described a more detailed version of this older protocol24. In summary, the neural fate was induced by dual SMAD inhibition25 to block TGF-β and BMP signaling in the first 2 days. WNT activation using CHIR99021 promoted neural progenitors to become NC cells. On day 11, cells were sorted by FACS for CD49D+ or SOX10+ populations26,23, which yielded about 40% NC generation efficiency. Thus, sorting was needed to ensure the efficiency and purity for the next steps of differentiation. The NCCs were maintained and amplified as spheroids with the combined treatment of FGF2 and CHIR. After 4 days, the NC spheroids of maintenance were plated and given BDNF, GDNF, and NGF to finish the symN maturation. Although these symNs expressed strong symN markers such as ASCL1, TH, DBH, and PHOX2A, markers for more mature symNs, including expression of the nicotinic acetylcholine receptor (CHRNA3/CHRNB4) and vesicle transporter (VMAT1/2), were low even after 70 days of differentiation. HOX genes in this protocol were not formally tested, and mature neural properties, including electrophysiological activity of the cells, were not verified.
Here, we present an optimized protocol to generate symNs (Figure 1B). HPSCs are maintained in feeder-free conditions, on vitronectin (VTN)-coated dishes, using Essential 8 (E8) media27. The formula of the differentiation media has been modified at each stage, thereby increasing the percentage of the NC population28. The symN maturation can be done on CD49D+/SOX10+ sorted or unsorted bulk NCC populations. Both show high levels of symN marker expression by day 30. Moreover, the symNs generated with this protocol are responsive to electrophysiological recording and to treatments with symN activator and inhibitor compounds.