Segment-specific prevention of pioneer neuron apoptosis by cell-autonomous, postmitotic Hox gene activity

In vertebrates, neurons often undergo apoptosis after differentiating and extending their axons. By contrast, in the developing nervous system of invertebrate embryos apoptosis typically occurs soon after cells are generated. Here, we show that the Drosophila dMP2 and MP1 pioneer neurons undergo segment-specific apoptosis at late embryonic stages, long after they have extended their axons and have performed their pioneering role in guiding follower axons. This segmental specificity is achieved by differential expression of the Hox gene Abdominal B, which in posterior segments prevents pioneer neuron death postmitotically and cell-autonomously by repressing the RHG-motif cell death activators reaper and grim. Our results identify the first clear case of a cell-autonomous and anti-apoptotic role for a Hox gene in vivo. In addition, they provide a novel mechanism linking Hox positional information to differences in neuronal architecture along the anteroposterior axis by the selective elimination of mature neurons.     

Segment-specific generation of Drosophila Capability neuropeptide neurons by multi-faceted Hox cues

In the Drosophila ventral nerve cord, the three pairs of Capability neuropeptide-expressing Va neurons are exclusively found in the second, third and fourth abdominal segments (A2–A4). To address the underlying mechanisms behind such segment-specific cell specification, we followed the developmental specification of these neurons. We find that Va neurons are initially generated in all ventral nerve cord segments and progress along a common differentiation path. However, their terminal differentiation only manifests itself in A2–A4, due to two distinct mechanisms: segment-specific programmed cell death (PCD) in posterior segments, and differentiation to an alternative identity in segments anterior to A2. Genetic analyses reveal that the Hox homeotic genes are involved in the segment-specific appearance of Va neurons. In posterior segments, the Hox gene Abdominal-B exerts a pro-apoptotic role on Va neurons, which involves the function of several RHG genes. Strikingly, this role of Abd-B is completely opposite to its role in the segment-specific apoptosis of other classes of neuropeptide neurons, the dMP2 and MP1 neurons, where Abd-B acts in an anti-apoptotic manner. In segments A2–A4 we find that abdominal A is important for the terminal differentiation of Va cell fate. In the A1 segment, Ultrabithorax acts to specify an alternate Va neuron fate. In contrast, in thoracic segments, Antennapedia suppresses the Va cell fate. Thus, Hox genes act in a multi-faceted manner to control the segment-specific appearance of the Va neuropeptide neurons in the ventral nerve cord.     

Retinoid receptor signaling in postmitotic motor neurons regulates rostrocaudal positional identity and axonal projection pattern

The identity of motor neurons diverges markedly at different rostrocaudal levels of the spinal cord, but the signals that specify their fate remain poorly defined. We show that retinoid receptor activation in newly generated spinal motor neurons has a crucial role in specifying motor neuron columnar subtypes. Blockade of retinoid receptor signaling in brachial motor neurons inhibits lateral motor column differentiation and converts many of these neurons to thoracic columnar subtypes. Conversely, expression of a constitutively active retinoid receptor derivative impairs the differentiation of thoracic motor neuron columnar subtypes. These findings provide evidence for a regionally restricted role for retinoid signaling in the postmitotic specification of motor neuron columnar identity.     

Directed differentiation of neural cells to hypothalamic dopaminergic neurons

Hypothalamic neurons play a key role in homeostasis, yet little is known about their differentiation. Here, we demonstrate that Shh and Bmp7 from the adjacent prechordal mesoderm govern hypothalamic neural fate, their sequential action controlling hypothalamic dopaminergic neuron generation in a Six3-dependent manner. Our data suggest a temporal distinction in the requirement for the two signals. Shh acts early to specify dopaminergic neurotransmitter phenotype. Subsequently, Bmp7 acts on cells that are ventralised by Shh, establishing aspects of hypothalamic regional identity in late-differentiating/postmitotic cells. The concerted actions of Shh and Bmp7 can direct mouse embryonic stem cell-derived neural progenitor cells to a hypothalamic dopaminergic fate ex vivo.     

Parkin is a component of an SCF-like ubiquitin ligase complex and protects postmitotic neurons from kainate excitotoxicity

Mutations in parkin, which encodes a RING domain protein associated with ubiquitin ligase activity, lead to autosomal recessive Parkinson's disease characterized by midbrain dopamine neuron loss. Here we show that parkin functions in a multiprotein ubiquitin ligase complex that includes the F-box/WD repeat protein hSel-10 and Cullin-1. HSel-10 serves to target the parkin ubiquitin ligase activity to cyclin E, an hSel-10-interacting protein previously implicated in the regulation of neuronal apoptosis. Consistent with the notion that cyclin E is a substrate of the parkin ubiquitin ligase complex, parkin deficiency potentiates the accumulation of cyclin E in cultured postmitotic neurons exposed to the glutamatergic excitotoxin kainate and promotes their apoptosis. Furthermore, parkin overexpression attenuates the accumulation of cyclin E in toxin-treated primary neurons, including midbrain dopamine neurons, and protects them from apoptosis.