Binary decisions refine fate decisions: Neural crest cells develop into tissues ranging from craniofacial bones to peripheral neurons. Combining single-cell RNA sequencing with spatial transcriptomics, Soldatov et al. analyzed how neural crest cells in mouse embryos decide among the various fates available to them (see the Perspective by Mayor). These multipotent cells become biased toward a given fate early on and step through a progression of binary decisions as their fate is refined. Competing fate programs coexist until increased synchronization favors one and repression disfavors the other. Science , this issue p. eaas9536 ; see also p. 937

Cardiac pacemaking generation and modulation rely on the coordinated activity of several processes. Although a wealth of evidence indicates a relevant role of the I _f (“funny,” or pacemaker) current, whose molecular constituents are the hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels and particularly HCN4, work with mice where Hcn genes were knocked out, or functionally modified, has challenged this view. However, no previous studies used a cardiac-specific promoter to induce HCN4 ablation in adult mice. We report here that, in an inducible and cardiac-specific HCN4 knockout (ciHCN4-KO) mouse model, ablation of HCN4 consistently leads to progressive development of severe bradycardia (∼50% reduction of original rate) and AV block, eventually leading to heart arrest and death in about 5 d. In vitro analysis of sinoatrial node (SAN) myocytes isolated from ciHCN4-KO mice at the mean time of death revealed a strong reduction of both the I _f current (by ∼70%) and of the spontaneous rate (by ∼60%). In agreement with functional results, immunofluorescence and Western blot analysis showed reduced expression of HCN4 protein in SAN tissue and cells. In ciHCN4-KO animals, the residual I _f was normally sensitive to β-adrenergic receptor (β-AR) modulation, and the permanence of rate response to β-AR stimulation was observed both in vivo and in vitro. Our data show that cardiac HCN4 channels are essential for normal heart impulse generation and conduction in adult mice and support the notion that dysfunctional HCN4 channels can be a direct cause of rhythm disorders. This work contributes to identifying the molecular mechanism responsible for cardiac pacemaking.

Exploiting nervous paths already traveled: The parasympathetic nervous system helps regulate the functions of many tissues and organs, including the salivary glands and the esophagus. To do so, it needs to reach throughout the body, connecting central systems to peripheral ones. Dyachuk et al. and Espinosa-Medina et al. explored how these connections are established in mice (see the Perspective by Kalcheim and Rohrer). Progenitor cells that travel along with the developing nerves can give rise to both myelinforming Schwann cells and to parasympathetic neurons. That means the interacting nerves do not have to find each other. Instead, the beginnings of the connections are laid down as the nervous system develops. Science , this issue p. 82 , p. 87 ; see also p. 32

We have identified a transcription factor specifically expressed in the developing vertebrate eye. We named this gene vax2 because of the high degree of sequence similarity to the recently described vax1. Both in the human and mouse genomes, vax2 is localized in the vicinity of the emx1 gene. This mapping assignment, together with the previously reported colocalization of Vax1 and Emx2 in mouse, indicates that the vax and the emx genes may be organized in clusters. vax2 has a remarkable expression domain confined to the ventral portion of the prospective neural retina in mouse, human, and Xenopus. The overexpression of either the frog Xvax2 or the human VAX2 in Xenopus embryos leads to an aberrant eye phenotype and, in particular, determines a ventralizing effect on the developing eye. The expression domain of the transcription factor Xpax2, normally confined to the ventral developing retina, extends to the dorsal region of the retina after overexpression of vax2. On the other hand, the expression of Xvent2, a molecular marker of the dorsal retina, is strongly reduced. Furthermore, vax2 overexpression induces a striking expansion of the optic stalk, a structure deriving from the ventralmost region of the eye vesicle. Altogether, these data indicate that vax2 plays a crucial role in eye development and, in particular, in the specification of the ventral optic vesicle.