The LIM-homeodomain transcription factors Lmx1a and Lmx1b play critical roles during the development of midbrain dopaminergic progenitors, but their functions in the adult brain remain poorly understood. We show here that sustained expression of Lmx1a and Lmx1b is required for the survival of adult midbrain dopaminergic neurons. Strikingly, inactivation of Lmx1a and Lmx1b recreates cellular features observed in Parkinson's disease. We found that Lmx1a/b control the expression of key genes involved in mitochondrial functions, and their ablation results in impaired respiratory chain activity, increased oxidative stress, and mitochondrial DNA damage. Lmx1a/b deficiency caused axonal pathology characterized by α-synuclein + inclusions, followed by a progressive loss of dopaminergic neurons. These results reveal the key role of these transcription factors beyond the early developmental stages and provide mechanistic links between mitochondrial dysfunctions, α-synuclein aggregation, and the survival of dopaminergic neurons. M idbrain dopaminergic (mDA) neurons control key functions in the mammalian brain, including voluntary movement, associative learning, and motivated behaviors. Dysfunctions of the dopaminergic (DA) system underlie a wide variety of neurological and psychiatric disorders. The progressive and rather selective degeneration of mDA neurons is one of the principal pathological features of Parkinson's disease (PD) (1). In PD, neuronal loss is accompanied by the appearance of α-synucleinenriched intraneuronal inclusions called "Lewy bodies" and "Lewy neurites." The etiologies of PD remain unsolved, but mitochondrial dysfunction emerges as a central mechanism in inherited, sporadic, and toxin-induced PD (2).Specification of the subtype identities of mDA neurons begins during embryonic development. The combinatory activation of transcription factors (TFs) and their target genes allows the progenitors to mature progressively and terminally differentiate into postmitotic neuron subtypes. Tremendous efforts have been made to describe the complex spatiotemporal expression of TFs during mDA neuronal development (see refs. 3 and 4 for reviews). After mDA neuron maturation, a large number of developmentally expressed TFs remain active throughout adulthood. Our knowledge of the functional roles of these TFs in mature neurons remains rudimentary. Accumulating evidence shows that transcription factors including the nuclear receptor related 1 protein (Nurr1), En1, Pitx3, Otx2, and Foxa2, which are recognized for their role in the early development of mDA neurons, are also required for the maintenance of phenotypic neuronal identity in the adult (5).The LIM homeodomain genes Lmx1a/b are early determinants of the fate of mDA progenitors (6), and their actions are essential at each step of DA neuronal generation (7,8). The murine Lmx1a and Lmx1b proteins are closely related and share an overall amino acid identity of 64%, with 100% identity in their homeodomain and 67% and 83% identity in each LIM domain (9). These neuron...

Summary Combinatorial action of transcription factors (TFs) with partially overlapping expression is a widespread strategy to generate novel gene-expression patterns and, thus, cellular diversity. Known mechanisms underlying combinatorial activity require co-expression of TFs within the same cell. Here, we describe the mechanism by which two TFs that are never co-expressed generate a new, intersectional expression pattern in C. elegans embryos: lineage-specific priming of a gene by a transiently expressed TF generates a unique intersection with a second TF acting on the same gene four cell divisions later; the second TF is expressed in multiple cells but only activates transcription in those where priming occurred. Early induction of active transcription is necessary and sufficient to establish a competent state, maintained by broadly expressed regulators in the absence of the initial trigger. We uncover additional cells diversified through this mechanism. Our findings define a mechanism for combinatorial TF activity with important implications for generation of cell-type diversity.

Mesodiencephalic dopamine neurons play central roles in the regulation of a wide range of brain functions, including voluntary movement and behavioral processes. These functions are served by distinct subtypes of mesodiencephalic dopamine neurons located in the substantia nigra pars compacta and the ventral tegmental area, which form the nigrostriatal, mesolimbic, and mesocortical pathways. Until now, mechanisms involved in dopaminergic circuit formation remained largely unknown. Here, we show that Lmx1a, Lmx1b, and Otx2 transcription factors control subtype-specific mesodiencephalic dopamine neurons and their appropriate axon innervation. Our results revealed that the expression of Plxnc1, an axon guidance receptor, is repressed by Lmx1a/b and enhanced by Otx2. We also found that Sema7a/Plxnc1 interactions are responsible for the segregation of nigrostriatal and mesolimbic dopaminergic pathways. These findings identify Lmx1a/b, Otx2, and Plxnc1 as determinants of dopaminergic circuit formation and should assist in engineering mesodiencephalic dopamine neurons capable of regenerating appropriate connections for cell therapy.