Fate of midbrain dopaminergic neurons controlled by the engrailed genes

Deficiencies in neurotransmitter-specific cell groups in the midbrain result in prominent neural disorders, including Parkinson's disease, which is caused by the loss of dopaminergic neurons of the substantia nigra. We have investigated in mice the role of the engrailed homeodomain transcription factors, En-1 and En-2, in controlling the developmental fate of midbrain dopaminergic neurons. En-1 is highly expressed by essentially all dopaminergic neurons in the substantia nigra and ventral tegmentum, whereas En-2 is highly expressed by a subset of them. These neurons are generated and differentiate their dopaminergic phenotype in En-1/En-2 double null mutants, but disappear soon thereafter. Use of an En-1/tau-LacZ knock-in mouse as an autonomous marker for these neurons indicates that they are lost, rather than that they change their neurotransmitter phenotype. A single allele of En-1 on an En-2 null background is sufficient to produce a wild type-like substantia nigra and ventral tegmentum, whereas in contrast a single allele of En-2 on an En-1 null background results in the survival of only a small proportion of these dopaminergic neurons, a finding that relates to the differential expression of En-1 and En-2. Additional findings indicate that En-1 and En-2 regulate expression of alpha-synuclein, a gene that is genetically linked to Parkinson's disease. These findings show that the engrailed genes are expressed by midbrain dopaminergic neurons from their generation to adulthood but are not required for their specification. However, the engrailed genes control the survival of midbrain dopaminergic neurons in a gene dose-dependent manner. Our findings also suggest a link between engrailed and Parkinson's disease.