Theory of branching morphogenesis by local interactions and global guidance

Branching morphogenesis governs the formation of many organs such as lung, kidney, and the neurovascular system. Many studies have explored system-specific molecular and cellular regulatory mechanisms, as well as self-organizing rules underlying branching morphogenesis. However, in addition to local cues, branched tissue growth can also be influenced by global guidance. Here, we develop a theoretical framework for a stochastic self-organized branching process in the presence of external cues. Combining analytical theory with numerical simulations, we predict differential signatures of global vs. local regulatory mechanisms on the branching pattern, such as angle distributions, domain size, and space-filling efficiency. We find that branch alignment follows a generic scaling law determined by the strength of global guidance, while local interactions influence the tissue density but not its overall territory. Finally, using zebrafish innervation as a model system, we test these key features of the model experimentally. Our work thus provides quantitative predictions to disentangle the role of different types of cues in shaping branched structures across scales

PRDM12 Is Required for Initiation of the Nociceptive Neuron Lineage during Neurogenesis

Summary: The sensation of pain is essential for the preservation of the functional integrity of the body. However, the key molecular regulators necessary for the initiation of the development of pain-sensing neurons have remained largely unknown. Here, we report that, in mice, inactivation of the transcriptional regulator PRDM12, which is essential for pain perception in humans, results in a complete absence of the nociceptive lineage, while proprioceptive and touch-sensitive neurons remain. Mechanistically, our data reveal that PRDM12 is required for initiation of neurogenesis and activation of a cascade of downstream pro-neuronal transcription factors, including NEUROD1, BRN3A, and ISL1, in the nociceptive lineage while it represses alternative fates other than nociceptors in progenitor cells. Our results thus demonstrate that PRDM12 is necessary for the generation of the entire lineage of pain-initiating neurons. : The sensation of pain, temperature, and itch by neurons of the nociceptive lineage is essential for animal survival. Bartesaghi et al. report that the transcriptional regulator PRDM12 is indispensable in neural crest cells (NCCs) for the initiation of the sensory neuronal differentiation program that generates the entire nociceptive lineage. Keywords: neurogenesis, pain, nociceptive neurons, Prdm12, neural crest cell

Schwann Cell Precursors Generate the Majority of Chromaffin Cells in Zuckerkandl Organ and Some Sympathetic Neurons in Paraganglia

In humans, neurosecretory chromaffin cells control a number of important bodily functions, including those related to stress response. Chromaffin cells appear as a distinct cell type at the beginning of midgestation and are the main cellular source of adrenalin and noradrenalin released into the blood stream. In mammals, two different chromaffin organs emerge at a close distance to each other, the adrenal gland and Zuckerkandl organ (ZO). These two structures are found in close proximity to the kidneys and dorsal aorta, in a region where paraganglioma, pheochromocytoma and neuroblastoma originate in the majority of clinical cases. Recent studies showed that the chromaffin cells comprising the adrenal medulla are largely derived from nerve-associated multipotent Schwann cell precursors (SCPs) arriving at the adrenal anlage with the preganglionic nerve fibers, whereas the migratory neural crest cells provide only minor contribution. However, the embryonic origin of the ZO, which differs from the adrenal medulla in a number of aspects, has not been studied in detail. The ZO is composed of chromaffin cells in direct contact with the dorsal aorta and the intraperitoneal cavity and disappears through an autophagy-mediated mechanism after birth. In contrast, the adrenal medulla remains throughout the entire life and furthermore, is covered by the adrenal cortex. Using a combination of lineage tracing strategies with nerve- and cell type-specific ablations, we reveal that the ZO is largely SCP-derived and forms in synchrony with progressively increasing innervation. Moreover, the ZO develops hand-in-hand with the adjacent sympathetic ganglia that coalesce around the dorsal aorta. Finally, we were able to provide evidence for a SCP-contribution to a small but significant proportion of sympathetic neurons of the posterior paraganglia. Thus, this cellular source complements the neural crest, which acts as a main source of sympathetic neurons. Our discovery of a nerve-dependent origin of chromaffin cells and some sympathoblasts may help to understand the origin of pheochromocytoma, paraganglioma and neuroblastoma, all of which are currently thought to be derived from the neural crest or committed sympathoadrenal precursors

Activation of protein kinase CbetaI constitutes a new neurotrophic pathway for deafferented spiral ganglion neurons

In mammals, degeneration of peripheral auditory neurons constitutes one of the main causes of sensorineural hearing loss. Unfortunately, to date, pharmacological interventions aimed at counteracting this condition have not presented complete effectiveness in protecting the integrity of cochlear neural elements. In this context, the protein kinase C (PKC) family of enzymes are important signalling molecules that play a role in preventing neurodegeneration after nervous system injury. The present study demonstrates, for the first time, that the PKC signalling pathway is directly neurotrophic to axotomised spiral ganglion neurons (SGNs). We found that PKC beta I was strictly expressed by postnatal and adult SGNs both in situ and in vitro. In cultures of SGNs, we observed that activators of PKC, such as phorbol esters and bryostatin 1, induced neuronal survival and neurite regrowth in a manner dependent on the activation of PKC beta I. The neuroprotective effects of PKC activators were suppressed by pre-treatment with LY294002 (a PI3K inhibitor) and with U0126 (a MEK inhibitor), indicating that PKC activators promote the survival and neurite outgrowth of SGNs by both PI3K/Akt and MEK/ERK-dependent mechanisms. In addition, whereas combining the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) was shown to provide only an additive effect on SGN survival, the interaction between PKC and neurotrophin signalling gave rise to a synergistic increase in SGN survival. Taken together, the data indicate that PKC beta I activation represents a key factor for the protection of the integrity of neural elements in the cochlea

Role of ephrin-A5 in the survival and connectivity of spiral ganglion neurons.

In mammals, degeneration of peripheral auditory neurons constitutes one of the prime causes of the sensorineural hearing loss. We believe that studying the developing auditory portion of the inner ear can provide important insights to mechanisms that would regulate regeneration after injury. In that context, our work has focused on a family of proteins, ephrins and Eph receptors, since they have been shown to have a dynamic influence in regulating developmental functions including axonal growth and guidance, synaptic formation and functions and neurogenesis. We have shown by in situ hybridization that ephrin-A5 is expressed in a small proportion of the spiral ganglion neurons, at different stages of the cochlear development. Most of them are located on the edge of the spiral ganglion. We demonstrated that the amount of neurons falls of more than 50% in P5 ephrin-A5 knockout mice as compared to wild type. Moreover, the numbers of TUNEL and active caspase-3 positive cells in the spiral ganglion are significantly increased at P0 when ephrin-A5 is deleted. Finally, we established that lack of ephrin-A5 leads to a defective inner hair cell innervation. Taken together, these results suggest a role for ephrin-A5 in regulating survival of auditory neurons and establishment of connectivity to sensory hair cells during development

A role for the canonical nuclear factor-κB pathway in coupling neurotrophin-induced differential survival of developing spiral ganglion neurons

Neurotrophins are key players of neural development by controlling cell death programs. However, the signaling pathways that mediate their selective responses in different populations of neurons remain unclear. In the mammalian cochlea, sensory neurons differentiate perinatally into type I and type II population both expressing TrkB and TrkC, which bind respectively brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3). How these two neuronal populations respond differentially to these two neurotrophins remains unknown. Here, we report in rat the segregation of the NFκB subunit p65 specifically within the type II population postnatally. Using dissociated cultures of embryonic and postnatal spiral ganglion neurons, we observed a specific requirement of NFκB for BDNF- but not NT3-dependent neuronal survival during a particular postnatal time window that corresponds to a period of neuronal cell death and hair cell innervation refinement in the developing cochlea. Consistently, postnatal p65 knockout mice showed a specific decreased number in type II spiral ganglion neurons. Taken together, these results identify NFκB as a type II neuron-specific factor that participates in the selective survival effects of BDNF and NT3 signaling on developing spiral ganglion neurons

Role of ephrin-A5 forward signaling in the survival of post-natal auditory neurons

In mammals, degeneration of peripheral auditory neurons constitutes one of the prime causes of the sensorineural hearing loss. We believe that studying the developing auditory portion of the inner ear can provide important insights to mechanisms that would regulate regeneration after injury. In that context, our work has focused on a family of proteins, ephrins and Eph receptors, since they have been shown to have a dynamic influence in regulating developmental functions including axonal growth and guidance, synaptic formation and functions and neurogenesis. We have shown that ephrin-A5, EphA4 and EphA7 are expressed in the cell bodies of the spiral ganglion neurons, at different stages of the cochlear development. Ephrin-A5 also seems to be present in the hair cells of the organ of Corti. On the one hand, we demonstrated that activation of the signaling pathway by complexed ephrin-A5 in spiral ganglion explants reduced neuronal survival. On the other hand, an increase of neuronal survival was observed when blocking this mechanism of transduction by monomeric ephrin-A5. Taken together, these results suggest a role for ephrin/Eph signaling in regulating neuronal population generated in the spiral ganglion, especially during the post-natal period of the cochlear innervation

Neuronal heterogeneity and stereotyped connectivity in the auditory afferent system

Spiral ganglion neurons (SGNs) of the cochlea receive input from hair cells and project to the auditory brainstem. Here, the authors perform single-cell RNA sequencing to identify four SGN subclasses and characterize their molecular profile, electrophysiological properties and connectivity

Role of ephrin-A5 in the survival and connectivity of spiral ganglion neurons

In mammals, degeneration of peripheral auditory neurons constitutes one of the prime causes of the sensorineural hearing loss. We believe that studying the developing auditory portion of the inner ear can provide important insights to mechanisms that would regulate regeneration after injury. In that context, our work has focused on a family of proteins, ephrins and Eph receptors, since they have been shown to have a dynamic influence in regulating developmental functions including axonal growth and guidance, synaptic formation and functions and neurogenesis. We have shown that ephrin-A5, EphA4 and EphA7 are expressed in the cell bodies of the spiral ganglion neurons, at different stages of the cochlear development. Ephrin-A5 also seems to be present in the hair cells of the organ of Corti. On the one hand, we demonstrated that activation of the signaling pathway by complexed ephrin-A5 in spiral ganglion explants reduced neuronal survival. On the other hand, an increase of neuronal survival was observed when blocking this mechanism of transduction by monomeric ephrin-A5. Taken together, these results suggest a role for ephrin/Eph signaling in regulating neuronal population generated in the spiral ganglion, especially during the post-natal period of the cochlear innervation