The role of the diencephalon in the guidance of thalamocortical axons in mice

Pax6 and Pax7 in the Central Nervous System of Cladistian Fishes: A Comprehensive Expression Analysis

Among actinopterygian fishes, cladistians stand as the more basal extant species in the group, holding a key phylogenetic position close to the common ancestor of Osteichthyes. Despite the recent publication of studies regarding the neurochemical organization of their central nervous system (CNS), there is still a significant lack of genoarchitectonic data that may prove essential to fully understand the patterning of the brain of these fishes. The paired box genes Pax6 and Pax7 are known to determine several boundaries in the CNS and are indispensable, for instance, for the survival of neurons and the change from cell proliferation to cell differentiation. By means of immunohistofluorescence methods, we analyzed the expression patterns of the transcription factors Pax6 and Pax7 in the CNS of three representative species of cladistian fishes, with a particular focus on their evolutionary implications. Thus, conserved Pax6 immunoreactive cell groups were present in the olfactory bulb, subpallial areas, the prethalamus, the basal prosomere 3, the pretectum, the mesencephalic tegmentum, the cerebellum, the basal rhombencephalon, the spinal cord, and the retina. A number of exclusive features were identified, including the almost total absence of expression in the pallium, which was observed only in cladistians, and its absence in the hypothalamus, which is a primitive anamniote trait. Likewise, the Pax7 expression pattern was generally conserved, with traits like the absence of labeling in the telencephalon and the expression in the retromamillary hypothalamic domain, the basal prosomere 3, the pretectum, the optic tectum, and the alar part of the first rhombomere. Additionally, no Pax7 labeling was detected in the spinal cord, comprising a specific cladistian feature.

Semaphorin 3A on Osteoporosis: An Overreview of the Literature

Semaphorin 3A (Sema3A) is a signaling protein that has attracted increasing attention in recent years for its important role in regulating bone metabolism. In this review, we searched different databases with various combinations of keywords to analyze the effects of Sema3A on osteoporosis. Sema3A promotes bone formation and inhibits bone resorption by directly affecting the osteoblast and osteoclast or indirectly targeting the nervous system. The sympathetic nervous system may be the main link between the central nervous system and bone metabolism for Sema3A. In the peripheral nervous system, Sema3A may improve bone quality via sensory nervous innervation. In addition, estrogen is found to regulate Sema3A levels to improve bone homeostasis. Lots of Sema3A agonists have been documented to exhibit anti-osteoporotic potential in preclinical investigations. Therefore, Sema3A can be considered a novel therapeutic target for preserving bone mass, highlighting an alternative strategy for the development of anti-osteoporosis drugs.

Regulation of neuronal fate specification and connectivity of the thalamic reticular nucleus by the Ascl1-Isl1 transcriptional cascade

The thalamic reticular nucleus (TRN) is an anatomical and functional hub that modulates the flow of information between the cerebral cortex and thalamus, and its dysfunction has been linked to sensory disturbance and multiple behavioral disorders. Therefore, understanding how TRN neurons differentiate and establish connectivity is crucial to clarify the basics of TRN functions. Here, we showed that the regulatory cascade of the transcription factors Ascl1 and Isl1 promotes the fate of TRN neurons and concomitantly represses the fate of non-TRN prethalamic neurons. Furthermore, we found that this cascade is necessary for the correct development of the two main axonal connections, thalamo-cortical projections and prethalamo-thalamic projections. Notably, the disruption of prethalamo-thalamic axons can cause the pathfinding defects of thalamo-cortical axons in the thalamus. Finally, forced Isl1 expression can rescue disruption of cell fate specification and prethalamo-thalamic projections in in vitro primary cultures of Ascl1-deficient TRN neurons, indicating that Isl1 is an essential mediator of Ascl1 function in TRN development. Together, our findings provide insights into the molecular mechanisms for TRN neuron differentiation and circuit formation.

Building thalamic neuronal networks during mouse development

The thalamic nuclear complex contains excitatory projection neurons and inhibitory local neurons, the two cell types driving the main circuits in sensory nuclei. While excitatory neurons are born from progenitors that reside in the proliferative zone of the developing thalamus, inhibitory local neurons are born outside the thalamus and they migrate there during development. In addition to these cell types, which occupy most of the thalamus, there are two small thalamic regions where inhibitory neurons target extra-thalamic regions rather than neighboring neurons, the intergeniculate leaflet and the parahabenular nucleus. Like excitatory thalamic neurons, these inhibitory neurons are derived from progenitors residing in the developing thalamus. The assembly of these circuits follows fine-tuned genetic programs and it is coordinated by extrinsic factors that help the cells find their location, associate with thalamic partners, and establish connections with their corresponding extra-thalamic inputs and outputs. In this review, we bring together what is currently known about the development of the excitatory and inhibitory components of the thalamocortical sensory system, in particular focusing on the visual pathway and thalamic interneurons in mice.

Somatotopy of Mouse Spinothalamic Innervation and the Localization of a Noxious Stimulus Requires Deleted in Colorectal Carcinoma Expression by Phox2a Neurons

Anterolateral system (AS) neurons transmit pain signals from the spinal cord to the brain. Their morphology, anatomy, and physiological properties have been extensively characterized and suggest that specific AS neurons and their brain targets are concerned with the discriminatory aspects of noxious stimuli, such as their location or intensity, and their motivational/emotive dimension. Among the recently unraveled molecular markers of AS neurons is the developmentally expressed transcription factor Phox2a, providing us with the opportunity to selectively disrupt the embryonic wiring of AS neurons to gain insights into the logic of their adult function. As mice with a spinal-cord-specific loss of the netrin-1 receptor deleted in colorectal carcinoma (DCC) have increased AS neuron innervation of ipsilateral brain targets and defective noxious stimulus localization or topognosis, we generated mice of either sex carrying a deletion of Dcc in Phox2a neurons. Such DccPhox2a mice displayed impaired topognosis along the rostrocaudal axis but with little effect on left-right discrimination and normal aversive responses. Anatomical tracing experiments in DccPhox2a mice revealed defective targeting of cervical and lumbar AS axons within the thalamus. Furthermore, genetic labeling of AS axons revealed their expression of DCC on their arrival in the brain, at a time when many of their target neurons are being born and express Ntn1 Our experiments suggest a postcommissural crossing function for netrin-1:DCC signaling during the formation of somatotopically ordered maps and are consistent with a discriminatory function of some of the Phox2a AS neurons.SIGNIFICANCE STATEMENT How nociceptive (pain) signals are relayed from the body to the brain remains an important question relevant to our understanding of the basic physiology of pain perception. Previous studies have demonstrated that the AS is a main effector of this function. It is composed of AS neurons located in the spinal cord that receive signals from nociceptive sensory neurons that detect noxious stimuli. In this study, we generate a genetic miswiring of mouse AS neurons that results in a decreased ability to perceive the location of a painful stimulus. The precise nature of this defect sheds light on the function of different kinds of AS neurons and how pain information may be organized.

Rapid and robust directed differentiation of mouse epiblast stem cells into definitive endoderm and forebrain organoids

Directed differentiation of pluripotent stem cells (PSCs) is a powerful model system for deconstructing embryonic development. Although mice are the most advanced mammalian model system for genetic studies of embryonic development, state-of-the-art protocols for directed differentiation of mouse PSCs into defined lineages require additional steps and generates target cell types with lower purity than analogous protocols for human PSCs, limiting their application as models for mechanistic studies of development. Here, we examine the potential of mouse epiblast stem cells cultured in media containing Wnt pathway inhibitors as a starting point for directed differentiation. As a proof of concept, we focused our efforts on two specific cell/tissue types that have proven difficult to generate efficiently and reproducibly from mouse embryonic stem cells: definitive endoderm and neural organoids. We present new protocols for rapid generation of nearly pure definitive endoderm and forebrain-patterned neural organoids that model the development of prethalamic and hippocampal neurons. These differentiation models present new possibilities for combining mouse genetic tools with in vitro differentiation to characterize molecular and cellular mechanisms of embryonic development.

Gli2-Mediated Shh Signaling Is Required for Thalamocortical Projection Guidance

The thalamocortical projections are part of the most important higher level processing connections in the vertebrates and follow a highly ordered pathway from their origin in the thalamus to the cerebral cortex. Their functional complexities are not only due to an extremely elaborate axon guidance process but also due to activity-dependent mechanisms. Gli2 is an intermediary transcription factor in the Sonic hedgehog (Shh) pathway. During neural early development, Shh has an important role in dorsoventral patterning, diencephalic anteroposterior patterning, and many later developmental processes, such as axon guidance and cell migration. Using a Gli2 knockout mouse line, we have studied the role of Shh signaling mediated by Gli2 in the development of the thalamocortical projections during embryonic development. In wild-type brains, we have described the normal trajectory of the thalamocortical axons into the context of the prosomeric model. Then, we have compared it with the altered thalamocortical axons course in Gli2 homozygous embryos. The thalamocortical axons followed different trajectories and were misdirected to other territories probably due to alterations in the Robo/Slit signaling mechanism. In conclusion, the alteration of Gli2-mediated Shh signaling produces an erroneous specification of several territories related with the thalamocortical axons. This is translated into a huge modification in the pathfinding signaling mechanisms needed for the correct wiring of the thalamocortical axons.