Hedgehog Signaling in CBS/PSP

Hedgehog Signaling in CBS/PSP

Overview

Hedgehog (Hh) signaling represents a critical developmental pathway that continues to play important roles in adult brain function. In corticobasal syndrome (CBS) and progressive supranuclear palsy (PSP), both classified as 4-repeat (4R) tauopathies, dysregulation of Hedgehog signaling contributes to neuronal vulnerability, impaired neurogenesis, and progressive neurodegeneration. This section examines the role of Hedgehog pathway dysregulation in CBS/PSP pathogenesis and explores therapeutic implications. [@armstrong2020]

CBS and PSP are distinct clinical syndromes sharing common pathological features of 4R tau accumulation [1](https://doi.org/10.1016/S1474-4422(22)00087-0). While CBS presents with asymmetric cortical dysfunction and basal ganglia degeneration leading to apraxia, alien limb phenomena, and cortical sensory loss, PSP is characterized by vertical gaze palsy, postural instability, and axial rigidity with progressive gait disturbance. Understanding the role of Hedgehog signaling in these conditions may reveal novel therapeutic targets. [@litvan2020]

The Hedgehog pathway's involvement in CBS/PSP extends beyond its developmental functions. In the adult brain, this signaling cascade regulates neural progenitor cell proliferation, neuronal survival, synaptic plasticity, and glial cell function. Dysregulation of these processes contributes to the progressive neurodegeneration observed in both disorders. [@boxer2020]

The Canonical Hedgehog Pathway

Hedgehog Ligands

Hedgehog Signaling in CBS/PSP

Overview

Hedgehog (Hh) signaling represents a critical developmental pathway that continues to play important roles in adult brain function. In corticobasal syndrome (CBS) and progressive supranuclear palsy (PSP), both classified as 4-repeat (4R) tauopathies, dysregulation of Hedgehog signaling contributes to neuronal vulnerability, impaired neurogenesis, and progressive neurodegeneration. This section examines the role of Hedgehog pathway dysregulation in CBS/PSP pathogenesis and explores therapeutic implications. [@armstrong2020]

CBS and PSP are distinct clinical syndromes sharing common pathological features of 4R tau accumulation [1](https://doi.org/10.1016/S1474-4422(22)00087-0). While CBS presents with asymmetric cortical dysfunction and basal ganglia degeneration leading to apraxia, alien limb phenomena, and cortical sensory loss, PSP is characterized by vertical gaze palsy, postural instability, and axial rigidity with progressive gait disturbance. Understanding the role of Hedgehog signaling in these conditions may reveal novel therapeutic targets. [@litvan2020]

The Hedgehog pathway's involvement in CBS/PSP extends beyond its developmental functions. In the adult brain, this signaling cascade regulates neural progenitor cell proliferation, neuronal survival, synaptic plasticity, and glial cell function. Dysregulation of these processes contributes to the progressive neurodegeneration observed in both disorders. [@boxer2020]

The Canonical Hedgehog Pathway

Hedgehog Ligands

Three Hedgehog ligands exist in mammals: Sonic hedgehog (Shh), Indian hedgehog (Ihh), and Desert hedgehog (Dhh) [2](https://doi.org/10.1016/j.tins.2020.09.004). Sonic hedgehog is the most extensively studied in the context of neurobiology and neurodegeneration. [@kovacs2022]

• Sonic hedgehog (Shh): A 19 kDa secreted protein expressed in neurons and astrocytes, involved in neuroprotection and neural repair
• Indian hedgehog (Ihh): Expressed primarily in oligodendrocyte lineage cells, regulates myelination
• Desert hedgehog (Dhh): Expressed in peripheral nervous system and testis

In the adult brain, Shh is expressed by various cell types including neurons, astrocytes, and oligodendrocyte lineage cells. The ligand can act in both paracrine and autocrine fashions, and its expression is dynamically regulated in response to injury and disease states. This adaptive regulation makes the pathway particularly relevant to neurodegenerative disease processes. [@stamelou2021]

Receptors and Signal Transduction

The Hedgehog pathway involves a precisely orchestrated cascade of protein interactions: [@chahine2020]

Patched-1 (PTCH1): Twelve-transmembrane domain receptor that serves as the pathway's negative regulator. In the absence of Hedgehog ligand, PTCH1 inhibits Smoothened through direct protein-protein interaction. [@jellinger2020]

Smoothened (SMO): Seven-transmembrane domain GPCR family member that transduces the Hedgehog signal upon pathway activation. SMO localizes to primary cilia, where it initiates downstream signaling cascades. [@lee2014]

Gli transcription factors: Three Gli proteins (Gli1, Gli2, Gli3) serve as the terminal effectors. In the absence of signaling, Gli proteins are phosphorylated by PKA, CK1, and GSK3beta, leading to proteolytic processing into transcriptional repressors. Upon pathway activation, active Gli proteins translocate to the nucleus. [@mandelkow2010]

Suppressor of Fused (SUFU): Negative regulator that sequesters Gli proteins in the cytoplasm. SMO activation promotes SUFU dissociation, allowing Gli nuclear translocation.

Hedgehog Signaling in Normal Brain Function

Adult Neurogenesis

Hedgehog signaling plays essential roles in adult neurogenesis [4](https://doi.org/10.1016/j.tins.2020.09.004):

• Subventricular zone: Shh promotes neural progenitor cell proliferation in the lateral ventricles
• Hippocampal dentate gyrus: Gli-mediated transcription supports continuous neurogenesis
• Gli proteins: Regulate genes involved in neuronal differentiation and migration

Neuroprotection

Shh signaling exerts neuroprotective effects through multiple mechanisms [5](https://doi.org/10.1016/j.neuroscience.2018.04.012):

These neuroprotective functions are particularly important in the context of tauopathies, where neurons face chronic stress from protein aggregation, mitochondrial dysfunction, and neuroinflammation.

Glial Function

Hedgehog signaling also regulates glial cell function:

• Oligodendrocyte development: Ihh signaling promotes oligodendrocyte progenitor cell proliferation and differentiation
• Astrocyte function: Shh modulates astrocyte reactivity and scar formation
• Microglial surveillance: Pathway activation influences microglial activation states

Hedgehog Dysregulation in CBS/PSP

Altered Pathway Activity

Studies suggest Hedgehog pathway dysregulation in CBS/PSP [6](https://doi.org/10.1007/s00401-020-02171-5):

The motor cortex and basal ganglia, regions prominently affected in CBS, show particular alterations in Hedgehog pathway components. Immunohistochemical studies demonstrate decreased Shh immunoreactivity in neurons with 4R tau inclusions, suggesting a direct relationship between tau pathology and Hedgehog dysfunction.

Relationship to Tau Pathology

Hedgehog signaling interacts with tau pathology in several ways:

• Tau phosphorylation: Gli proteins may influence tau kinase activity through cross-talk with GSK3β
• Neurogenesis impairment: Loss of Hedgehog-mediated neurogenesis limits tau clearance mechanisms
• Neuronal vulnerability: Impaired neuroprotective signaling increases susceptibility to tau-induced death
• Axonal transport: Hedgehog signaling regulates proteins involved in axonal transport

Regional Vulnerability

The pattern of Hedgehog dysregulation in CBS/PSP follows the regional distribution of tau pathology:

• Motor cortex: Severe Hedgehog pathway impairment in CBS, correlating with cortical involvement
• Basal ganglia: Reduced Shh and Gli activation in both CBS and PSP
• Brainstem: Moderate changes in PSP, corresponding to midbrain pathology
• Substantia nigra: Loss of neuroprotective signaling contributes to dopaminergic neuron vulnerability

Therapeutic Implications

Hedgehog Pathway Modulation

Therapeutic strategies targeting Hedgehog signaling include [7](https://doi.org/10.1038/s41573-020-0089-1):

• Shh agonists: Recombinant Shh protein or small molecule agonists
• SMO activators: Smoothened-activating compounds that bypass receptor-level defects
• Gli modulators: Direct Gli transcription factor targeting
• BDNF mimetics: Compounds that replicate Hedgehog-mediated neurotrophic effects

Combination Approaches

Given the complex pathophysiology of CBS/PSP, Hedgehog-targeted therapies may be most effective in combination:

• Hedgehog + neurotrophic factors: Combined delivery of Shh and BDNF
• Hedgehog + anti-tau therapies: Pathway activation with tau-targeted interventions
• Hedgehog + neuroinflammation: Modulation with anti-inflammatory approaches

Clinical Considerations

Challenges in Hedgehog-targeted therapy include [8](https://doi.org/10.1016/j.tcb.2020.06.002):

• Cancer risk: Hedgehog pathway activation linked to medulloblastoma and basal cell carcinoma
• BBB penetration: Delivery of large signaling proteins to the brain
• Selectivity: Avoiding off-target effects on developmental processes
• Dosing: Balancing therapeutic benefit with potential side effects

• AAV-mediated gene delivery of Shh
• Nanoparticle-conjugated pathway activators
• Brain-penetrant small molecule SMO agonists

Cross-Links to Related CBS/PSP Mechanisms

• [Neurogenesis in CBS/PSP](/mechanisms/neurogenesis-neurodegeneration) — Hedgehog signaling in neural progenitor cell function
• [4R Tauopathy Molecular Mechanisms](/mechanisms/4r-tau-cbs) — 4R tau isoform interactions with signaling pathways
• [Cellular Senescence in CBS](/mechanisms/cbs-cellular-senescence) — Hedgehog in senescent neuronal cells
• [Neuroinflammation in CBS](/mechanisms/cbs-neuroinflammation) — Cross-talk between Hedgehog and inflammatory signaling
• [Neural Stem Cells in Neurodegeneration](/mechanisms/neural-stem-cells-neurodegeneration) — Stem cell therapies targeting Hedgehog
• [Substantia Nigra Selective Vulnerability](/mechanisms/substantia-nigra-selective-vulnerability-parkinsons) — Dopaminergic neuron susceptibility
• [Hedgehog Signaling in Neurodegeneration](/mechanisms/hedgehog-neurodegeneration) — General Hedgehog pathway overview

References