The cation exchanger Letm1, circadian rhythms, and NAD(H) levels interconnect in diurnal zebrafish.

Mitochondria are fundamental for life and require balanced ion exchange to maintain proper functioning. The mitochondrial cation exchanger LETM1 sparks interest because of its pathophysiological role in seizures in the Wolf Hirschhorn Syndrome (WHS). Despite observation of sleep disorganization in epileptic WHS patients, and growing studies linking mitochondria and epilepsy to circadian rhythms, LETM1 has not been studied from the chronobiological perspective. Here we established a viable letm1 knock-out, using the diurnal vertebrate Danio rerio to study the metabolic and chronobiological consequences of letm1 deficiency. We report diurnal rhythms of Letm1 protein levels in wild-type fish. We show that mitochondrial nucleotide metabolism is deregulated in letm1-/- mutant fish, the rate-limiting enzyme of NAD+ production is up-regulated, while NAD+ and NADH pools are reduced. These changes were associated with increased expression amplitude of circadian core clock genes in letm1-/- compared with wild-type under light/dark conditions, suggesting decreased NAD(H) levels as a possible mechanism for circadian system perturbation in Letm1 deficiency. Replenishing NAD pool may ameliorate WHS-associated sleep and neurological disorders.

Establishment and maintenance of motor neuron identity via temporal modularity in terminal selector function.

Terminal selectors are transcription factors (TFs) that establish during development and maintain throughout life post-mitotic neuronal identity. We previously showed that UNC-3/Ebf, the terminal selector of C. elegans cholinergic motor neurons (MNs), acts indirectly to prevent alternative neuronal identities (Feng et al., 2020). Here, we globally identify the direct targets of UNC-3. Unexpectedly, we find that the suite of UNC-3 targets in MNs is modified across different life stages, revealing 'temporal modularity' in terminal selector function. In all larval and adult stages examined, UNC-3 is required for continuous expression of various protein classes (e.g. receptors, transporters) critical for MN function. However, only in late larvae and adults, UNC-3 is required to maintain expression of MN-specific TFs. Minimal disruption of UNC-3's temporal modularity via genome engineering affects locomotion. Another C. elegans terminal selector (UNC-30/Pitx) also exhibits temporal modularity, supporting the potential generality of this mechanism for the control of neuronal identity.

The genetic basis of diurnal preference in Drosophila melanogaster.

BACKGROUND: Most animals restrict their activity to a specific part of the day, being diurnal, nocturnal or crepuscular. The genetic basis underlying diurnal preference is largely unknown. Under laboratory conditions, Drosophila melanogaster is crepuscular, showing a bi-modal activity profile. However, a survey of strains derived from wild populations indicated that high variability among individuals exists, including flies that are nocturnal. RESULTS: Using a highly diverse population, we performed an artificial selection experiment, selecting flies with extreme diurnal or nocturnal preference. After 10 generations, we obtained highly diurnal and nocturnal strains. We used whole-genome expression analysis to identify differentially expressed genes in diurnal, nocturnal and crepuscular (control) flies. Other than one circadian clock gene (pdp1), most differentially expressed genes were associated with either clock output (pdf, to) or input (Rh3, Rh2, msn). This finding was congruent with behavioural experiments indicating that both light masking and the circadian pacemaker are involved in driving nocturnality. CONCLUSIONS: Our study demonstrates that genetic variation segregating in wild populations contributes to substantial variation in diurnal preference. We identified candidate genes associated with diurnality/nocturnality, while data emerging from our expression analysis and behavioural experiments suggest that both clock and clock-independent pathways are involved in shaping diurnal preference. The diurnal and nocturnal selection strains provide us with a unique opportunity to understand the genetic architecture of diurnal preference.

A terminal selector prevents a Hox transcriptional switch to safeguard motor neuron identity throughout life.

To become and remain functional, individual neuron types must select during development and maintain throughout life their distinct terminal identity features, such as expression of specific neurotransmitter receptors, ion channels and neuropeptides. Here, we report a molecular mechanism that enables cholinergic motor neurons (MNs) in the C. elegans ventral nerve cord to select and maintain their unique terminal identity. This mechanism relies on the dual function of the conserved terminal selector UNC-3 (Collier/Ebf). UNC-3 synergizes with LIN-39 (Scr/Dfd/Hox4-5) to directly co-activate multiple terminal identity traits specific to cholinergic MNs, but also antagonizes LIN-39's ability to activate terminal features of alternative neuronal identities. Loss of unc-3 causes a switch in the transcriptional targets of LIN-39, thereby alternative, not cholinergic MN-specific, terminal features become activated and locomotion defects occur. The strategy of a terminal selector preventing a transcriptional switch may constitute a general principle for safeguarding neuronal identity throughout life.