Table 1.

Guide to Selected Differentiation Factors Implicated in the Development of Mesencephalic Dopaminergic Neurons

Protein	Abbreviations	Function	Floor/Base Plate Expression	Mid/Hindbrain Expression
Sonic hedgehog	SHH	Secreted morphogen	√
Fibroblast growth factor 8	FGF8	Secreted morphogen		√
Wingless homologue 1	WNT1	Secreted morphogen	√	√
Engrailed 1 and 2	EN1, EN2	Transcription factors	√	√
LIM homeoboxa transcription factor 1 alpha, beta	LMX1a, b	Transcription factors	√	√
Paired-like homeodomain1 transcription factor 3	PITX3	Transcription factor	√
Orthodenticle homolog 2	OTX2	Transcription factor	√	√
Paired box transcription factors 2 and 5	PAX2, PAX5	Transcription factors		√
Gastrulation brain homeobox 2	GBX2	Transcription factor		√
Nuclear receptor subfamily 4	NR4a2/NURR1	Nuclear orphan receptor	√
MicroRNA 133bb	mir-133b	MicroRNA	mDA expression

^aThe homeobox is a conserved sequence of 180 DNA base pairs that codes for a segment of 60 amino acids that will bind to specific DNA regions. Transcription factors that include this sequence of amino acids are called homeodomain proteins, and the 60 amino acid segment is referred to as the “homeodomain.” The homeodomain was first recognized in proteins that regulate the formation of body parts in fruit flies. Mutations in a homeobox genes could transform one body segment into a different body part. A classic example is a mutation in the antennapedia gene that converts antennae on the head of a fruit fly into legs.⁴⁸

^bMicroRNAs are a recently discovered class of RNA molecules that appear to play a critical role in the regulation of gene expression.⁴⁹ mDA, mesoencephalic dopamine.