Epithelial cells supply Sonic Hedgehog to the perinatal dentate gyrus via transport by platelets

A subpopulation of astrocyte progenitors defined by Sonic hedgehog signaling

BACKGROUND

The molecular signaling pathway, Sonic hedgehog (Shh), is critical for the proper development of the central nervous system. The requirement for Shh signaling in neuronal and oligodendrocyte development in the developing embryo are well established. However, Shh activity is found in discrete subpopulations of astrocytes in the postnatal and adult brain. Whether Shh signaling plays a role in astrocyte development is not well understood.

METHODS

Here, we use a genetic inducible fate mapping approach to mark and follow a population of glial progenitor cells expressing the Shh target gene, Gli1, in the neonatal and postnatal brain.

RESULTS

In the neonatal brain, Gli1-expressing cells are found in the dorsolateral corner of the subventricular zone (SVZ), a germinal zone harboring astrocyte progenitor cells. Our data show that these cells give rise to half of the cortical astrocyte population, demonstrating their substantial contribution to the cellular composition of the cortex. Further, these data suggest that the cortex harbors astrocytes from different lineages. Gli1 lineage astrocytes are distributed across all cortical layers, positioning them for broad influence over cortical circuits. Finally, we show that Shh activity recurs in mature astrocytes in a lineage-independent manner, suggesting cell-type dependent roles of the pathway in driving astrocyte development and function.

CONCLUSION

These data identify a novel role for Shh signaling in cortical astrocyte development and support a growing body of evidence pointing to astrocyte heterogeneity.

Functions of Wnt and Hedgehog-containing extracellular vesicles in development and disease

Secreted morphogens play a major role in the intercellular communication necessary for animal development. It was initially thought that, in order to organize tissue morphogenesis and control cell fate and proliferation, morphogens diffused freely in the extracellular space. This view has since changed following the discovery that morphogens of the Wnt and Hedgehog (Hh) families are modified by various lipid adducts during their biosynthesis, providing them with high affinity for the membrane bilayer. Recent work performed in model organisms suggests that Wnt and Hh proteins are carried on extracellular vesicles. In this Review, we provide our perspectives on the mechanisms of formation of Wnt- and Hh-containing extracellular vesicles, and discuss their functions during animal development, as well as in various human physiopathologies.

Platelets: The missing link between the blood and brain?

It is becoming increasingly clear that interactions between the peripheral immune system and the central nervous system are important in maintaining healthy brain function. Platelets are small blood cells traditionally known for their role in wound healing. However, platelets have recently been shown to exhibit many alternative functions. In this perspective, we summarize the repertoire of platelet functions, focusing on how these cells contribute to the maintenance of brain homeostasis and propose the mechanisms via which they could communicate with brain cells, including exosome and microparticle release and receptor interactions at local sites. In particular, we highlight the potential role that platelets play in maintaining brain plasticity via the modulation of new neuron generation from neural precursor cells, an interaction which could have important implications in the development of therapeutic interventions to promote cognitive function in aging and disease.

Sonic hedgehog signaling in astrocytes mediates cell type-specific synaptic organization

Astrocytes have emerged as integral partners with neurons in regulating synapse formation and function, but the mechanisms that mediate these interactions are not well understood. Here, we show that Sonic hedgehog (Shh) signaling in mature astrocytes is required for establishing structural organization and remodeling of cortical synapses in a cell type-specific manner. In the postnatal cortex, Shh signaling is active in a subpopulation of mature astrocytes localized primarily in deep cortical layers. Selective disruption of Shh signaling in astrocytes produces a dramatic increase in synapse number specifically on layer V apical dendrites that emerges during adolescence and persists into adulthood. Dynamic turnover of dendritic spines is impaired in mutant mice and is accompanied by an increase in neuronal excitability and a reduction of the glial-specific, inward-rectifying K⁺ channel Kir4.1. These data identify a critical role for Shh signaling in astrocyte-mediated modulation of neuronal activity required for sculpting synapses.

A central system of neurons in the spinal cord and brain coordinate most of our body’s actions, ranging from regulating our heart rate to controlling our movement and thoughts. As the brain develops, neurons form specialized contacts with one another known as synapses. If the number of synapses is not properly regulated this can disrupt communication between the neurons, leading to diseases like schizophrenia and autism.

As the brain develops, it first forms an excess of synapses and later eliminates unnecessary or weak connections. Various factors, such gene expression or a neuron’s level of activity, regulate this turnover process. However, neurons cannot do this alone, and rely on other types of cells to help regulate their behavior. In the central nervous system, for example, a cell called an astrocyte is known to support the formation and activity of synapses. Now, Hill and Blaeser et al. show that astrocytes also exert influence over synaptic turnover during development, leading to long lasting changes in the number of synapses.

Hill, Blaeser et al. revealed that disrupting activity of the signaling pathway known as Sonic hedgehog, or Shh for short, in the astrocytes of mice led to disordered synaptic connections. Notably, neurons produce Shh, suggesting that neurons use this signaling pathway to communicate to specific astrocyte partners. Further experiments showed that reducing astrocyte’s ability to respond to Shh impaired synaptic turnover as the brain developed, leading to an overabundance of synapses. Importantly, these effects were only found to influence neuron populations associated with astrocytes that actively use Shh signaling. This suggests that distinct populations of neurons and astrocytes interact in specialized ways to build the connections within the nervous system.

To address how astrocytes use Shh signaling to regulate synaptic turnover, Hill, Blaeser et al. examined gene expression changes in astrocytes that lack Shh signaling. Astrocytes with a reduced capacity to respond to Shh were found to have lower levels of a protein responsible for transporting potassium ions into and out of the cell. This impairs astrocyte’s ability to regulate neuronal activity, which may lead to a failure in eliminating unnecessary synapses.

Understanding how synapses are controlled and organized by astrocytes could help identify new ways to treat diseases of the developing nervous system. However, further studies would be needed to improve our understanding of how this process works.

Suppressor of fused controls perinatal expansion and quiescence of future dentate adult neural stem cells

Adult hippocampal neurogenesis requires the quiescent neural stem cell (NSC) pool to persist lifelong. However, establishment and maintenance of quiescent NSC pools during development is not understood. Here, we show that Suppressor of Fused (Sufu) controls establishment of the quiescent NSC pool during mouse dentate gyrus (DG) development by regulating Sonic Hedgehog (Shh) signaling activity. Deletion of Sufu in NSCs early in DG development decreases Shh signaling activity leading to reduced proliferation of NSCs, resulting in a small quiescent NSC pool in adult mice. We found that putative adult NSCs proliferate and increase their numbers in the first postnatal week and subsequently enter a quiescent state towards the end of the first postnatal week. In the absence of Sufu, postnatal expansion of NSCs is compromised, and NSCs prematurely become quiescent. Thus, Sufu is required for Shh signaling activity ensuring expansion and proper transition of NSC pools to quiescent states during DG development.

Exercise-Induced Activated Platelets Increase Adult Hippocampal Precursor Proliferation and Promote Neuronal Differentiation

Physical activity is a strong positive physiological modulator of adult neurogenesis in the hippocampal dentate gyrus. Although the underlying regulatory mechanisms are still unknown, systemic processes must be involved. Here we show that platelets are activated after acute periods of running, and that activated platelets promote neurogenesis, an effect that is likely mediated by platelet factor 4. Ex vivo, the beneficial effects of activated platelets and platelet factor 4 on neural precursor cells were dentate gyrus specific and not observed in the subventricular zone. Moreover, the depletion of circulating platelets in mice abolished the running-induced increase in precursor cell proliferation in the dentate gyrus following exercise. These findings demonstrate that platelets and their released factors can modulate adult neural precursor cells under physiological conditions and provide an intriguing link between running-induced platelet activation and the modulation of neurogenesis after exercise.

Sonic Hedgehog Signaling Rises to the Surface: Emerging Roles in Neocortical Development

The mammalian neocortex is composed of a diverse population of neuronal and glial cells that are crucial for cognition and consciousness. Orchestration of molecular events that lead to the production of distinct cell lineages is now a major research focus. Recent studies in mammalian animal models reveal that Sonic Hedgehog (Shh) signaling plays crucial roles in this process. In this review, we will evaluate these studies and provide insights on how Shh signaling specifically influence cortical development, beyond its established roles in telencephalic patterning, by specifically focusing on its impact on cells derived from the cortical radial glial (RG) cells. We will also assess how these findings further advance our knowledge of neurological diseases and discuss potential roles of targeting Shh signaling in therapies.

The Dorsal Wave of Neocortical Oligodendrogenesis Begins Embryonically and Requires Multiple Sources of Sonic Hedgehog

Neural progenitor cells in the developing dorsal forebrain give rise to excitatory neurons, astrocytes, and oligodendrocytes for the neocortex. While we are starting to gain a better understanding about the mechanisms that direct the formation of neocortical neurons and astrocytes, far less is known about the molecular mechanisms that instruct dorsal forebrain progenitors to make oligodendrocytes. In this study, we show that Sonic hedgehog (Shh) signaling is required in dorsal progenitors for their late embryonic transition to oligodendrogenesis. Using genetic lineage-tracing in mice of both sexes, we demonstrate that most oligodendrocytes in the embryonic neocortex derive from Emx1⁺ dorsal forebrain progenitors. Deletion of the Shh signaling effector Smo specifically in Emx1⁺ progenitors led to significantly decreased oligodendrocyte numbers in the embryonic neocortex. Conversely, knock-out of the Shh antagonist Sufu was sufficient to increase neocortical oligodendrogenesis. Using conditional knock-out strategies, we found that Shh ligand is supplied to dorsal progenitors through multiple sources. Loss of Shh from Dlx5/6⁺ interneurons caused a significant reduction in oligodendrocytes in the embryonic neocortex. This phenotype was identical to that observed upon Shh deletion from the entire CNS using Nestin-Cre, indicating that interneurons migrating into the neocortex from the subpallium are the primary neural source of Shh for dorsal oligodendrogenesis. Additionally, deletion of Shh from migrating interneurons together with the choroid plexus epithelium led to a more severe loss of oligodendrocytes, suggesting that the choroid plexus is an important non-neural source of Shh ligand. Together, our studies demonstrate that the dorsal wave of neocortical oligodendrogenesis occurs earlier than previously appreciated and requires highly regulated Shh signaling from multiple embryonic sources.SIGNIFICANCE STATEMENT Most neocortical oligodendrocytes are made by neural progenitors in the dorsal forebrain, but the mechanisms that specify this fate are poorly understood. This study identifies Sonic hedgehog (Shh) signaling as a critical pathway in the transition from neurogenesis to oligodendrogenesis in dorsal forebrain progenitors during late embryonic development. The timing of this neuron-to-glia "switch" coincides with the arrival of migrating interneurons into the dorsal germinal zone, which we identify as a critical source of Shh ligand, which drives oligodendrogenesis. Our data provide evidence for a new model in which Shh signaling increases in the dorsal forebrain late in embryonic development to provide a temporally regulated mechanism that initiates the third wave of neocortical oligodendrogenesis.