IHH — Indian Hedgehog

IHH — Indian Hedgehog

<div class="infobox infobox-gene">
<div class="infobox-header">IHH</div>
<div class="infobox-row"><span>Full Name:</span> Indian Hedgehog</div>
<div class="infobox-row"><span>Gene Symbol:</span> IHH</div>
<div class="infobox-row"><span>Chromosome:</span> 2q35</div>
<div class="infobox-row"><span>NCBI Gene ID:</span> 3549</div>
<div class="infobox-row"><span>OMIM:</span> 600726</div>
<div class="infobox-row"><span>Ensembl ID:</span> ENSG00000163501</div>
<div class="infobox-row"><span>UniProt ID:</span> Q14607</div>
<div class="infobox-row"><span>Protein Length:</span> 426 amino acids</div>
</div>

Overview

IHH (Indian Hedgehog) is a critical signaling molecule that plays essential roles in embryonic development, particularly in bone formation, chondrocyte differentiation, and neurogenesis. It is one of three mammalian Hedgehog ligands alongside [SHH](/genes/shh) (Sonic Hedgehog) and [DHH](/genes/dhh) (Desert Hedgehog). While IHH is best characterized for its roles in skeletal development, emerging research reveals important functions in the [central nervous system](/brain-regions/overview) with implications for [Alzheimer's Disease](/diseases/alzheimers-disease), [Parkinson's Disease](/diseases/parkinsons-disease), and [Amyotrophic Lateral Sclerosis](/diseases/amyotrophic-lateral-sclerosis)[@ihh2020][@ihh2021].

IHH — Indian Hedgehog

<div class="infobox infobox-gene">
<div class="infobox-header">IHH</div>
<div class="infobox-row"><span>Full Name:</span> Indian Hedgehog</div>
<div class="infobox-row"><span>Gene Symbol:</span> IHH</div>
<div class="infobox-row"><span>Chromosome:</span> 2q35</div>
<div class="infobox-row"><span>NCBI Gene ID:</span> 3549</div>
<div class="infobox-row"><span>OMIM:</span> 600726</div>
<div class="infobox-row"><span>Ensembl ID:</span> ENSG00000163501</div>
<div class="infobox-row"><span>UniProt ID:</span> Q14607</div>
<div class="infobox-row"><span>Protein Length:</span> 426 amino acids</div>
</div>

Overview

IHH (Indian Hedgehog) is a critical signaling molecule that plays essential roles in embryonic development, particularly in bone formation, chondrocyte differentiation, and neurogenesis. It is one of three mammalian Hedgehog ligands alongside [SHH](/genes/shh) (Sonic Hedgehog) and [DHH](/genes/dhh) (Desert Hedgehog). While IHH is best characterized for its roles in skeletal development, emerging research reveals important functions in the [central nervous system](/brain-regions/overview) with implications for [Alzheimer's Disease](/diseases/alzheimers-disease), [Parkinson's Disease](/diseases/parkinsons-disease), and [Amyotrophic Lateral Sclerosis](/diseases/amyotrophic-lateral-sclerosis)[@ihh2020][@ihh2021].

IHH is synthesized as a 45 kDa precursor that undergoes autoproteolytic cleavage to generate an active N-terminal fragment (IHH-N). This secreted form signals through the PTCH1/PTCH2 receptors to activate the GLI transcription factor cascade. IHH plays critical roles in endochondral ossification by promoting chondrocyte proliferation and hypertrophy. In the nervous system, IHH regulates neurogenesis, gliogenesis, and neuronal specification[ihh2006].

Molecular Biology

Gene Structure

The IHH gene spans approximately 5.8 kb on chromosome 2q35 and consists of 3 exons. The gene encodes a 426-amino acid precursor protein with complex processing.

Protein Processing

IHH processing involves[ihh2010]:

The mature signaling molecule is approximately 19 kDa.

Receptor Interaction

IHH signals through:

• PTCH1 (Patched-1): Primary receptor
• PTCH2 (Patched-2): Alternative receptor
• SMO (Smoothened): Signal transducer
• HHIP (Hedgehog Interacting Protein): Feedback regulator

Function

Bone Formation

IHH is essential for endochondral ossification[ihh2002]:

• Chondrocyte proliferation: IHH promotes hypertrophic chondrocyte proliferation
• Bone collar formation: Initiates periosteal bone formation
• Growth plate organization: Critical for growth plate structure
• Osteoblast differentiation: Promotes osteoblast differentiation

Chondrocyte Differentiation

IHH regulates chondrocyte biology[ihh2004]:

• Proliferation: IHH stimulates chondrocyte proliferation
• Hypertrophy: Promotes chondrocyte hypertrophy
• Matrix production: Regulates extracellular matrix synthesis
• Apoptosis: Controls hypertrophic chondrocyte apoptosis

Neurogenesis

IHH in the nervous system[ihh2006]:

• Neural progenitor cells: IHH promotes neural progenitor proliferation
• Neuronal specification: Regulates neuronal fate determination
• Axon guidance: Important for axonal pathfinding
• Patterning: Essential for neural tube patterning

Gliogenesis

IHH regulates glial cell development[ihh2015]:

• Oligodendrocyte precursors: IHH promotes oligodendrocyte differentiation
• Astrocyte development: Regulates astrocyte maturation
• Schwann cells: Important for peripheral glia

Fracture Healing

IHH in bone repair[ihh2013]:

• Callus formation: IHH promotes fracture callus development
• Cartilage formation: Essential for cartilaginous callus
• Bone remodeling: Regulates remodeling of callus to bone

Disease Associations

Brachydactyly Type A-1

| OMIM ID | Inheritance | Notes |
|---------|-------------|-------|
| 112500 | Autosomal Dominant | Shortened middle phalanges |

Caused by heterozygous IHH mutations, resulting in shortened digits.

Acrocapitofemoral Dysplasia

| OMIM ID | Inheritance | Notes |
|---------|-------------|-------|
| 607156 | Autosomal Recessive | Severe skeletal dysplasia |

Characterized by short limbs, cone-shaped epiphyses.

Holoprosencephaly

| OMIM ID | Inheritance | Notes |
|---------|-------------|-------|
| 236100 | Autosomal Dominant | forebrain cleavage defects |

IHH mutations can contribute to HPE spectrum.

Alzheimer's Disease

IHH in AD[ihh2021]:

• Altered hedgehog signaling in AD brain
• IHH promotes neurogenesis in hippocampus
• Potential for regenerative therapy

Parkinson's Disease

IHH in PD[ihh2018]:

• Reduced IHH in substantia nigra
• Hedgehog pathway dysregulation
• Dopaminergic neuron survival

Amyotrophic Lateral Sclerosis

IHH in ALS[ihh2022]:

• Altered oligodendrocyte IHH
• Potential for glial-mediated therapy
• Hedgehog pathway modulation

Expression

Tissue Distribution

| Tissue | Expression Level |
|--------|------------------|
| Developing bone | Highest (growth plate) |
| Cartilage | High |
| Brain | Moderate |
| Lung | Low-moderate |
| Intestine | Low |

Brain Expression

In the CNS, IHH is expressed in:

• Floor plate of neural tube (development)
• Certain neuronal populations
• Glial precursors
• [Substantia nigra](/brain-regions/substantia-nigra) (low)
• [Hippocampus](/brain-regions/hippocampus) (low)

Development

IHH expression is developmentally regulated:

• High during embryogenesis
• Decreases in most tissues after birth
• Maintained in growth plate and certain brain regions

Signaling Pathway

IHH activates the Hedgehog signaling cascade:

IHH → PTCH1/PTCH2 → SMO → GLI → Target Genes

Key components:

• [PTCH1](/genes/ptch1) — receptor
• [PTCH2](/genes/ptch2) — alternative receptor
• [SMO](/genes/smo) — signal transducer
• [GLI1](/genes/gli1), [GLI2](/genes/gli2), [GLI3](/genes/gli3) — transcription factors
• [HHIP](/genes/hhip) — feedback inhibitor

Therapeutic Implications

Drug Development

IHH as a therapeutic target[ihh2023]:

• Hedgehog agonists: SMO agonists for bone repair
• Small molecule modulators: Pathway-specific compounds
• Recombinant IHH: Protein-based therapy

Clinical Applications

• Fracture healing
• Bone defect repair
• Neurodegeneration therapy

See Also

• [Hedgehog Signaling Pathway](/mechanisms/hedgehog-signaling-pathway)
• [Hippo Signaling Pathway](/mechanisms/hippo-signaling-pathway)
• [Cohesin Complex](/mechanisms/cohesin-complex)
• [SHH](/genes/shh)
• [DHH](/genes/dhh)
• [GLI1](/genes/gli1)
• [GLI2](/genes/gli2)
• [PTCH1](/genes/ptch1)
• [SMO](/genes/smo)
• [LATS1](/genes/lats1)
• [YAP1](/genes/yap1)
• [STK3](/genes/stk3)

External Links

• [NCBI Gene](https://www.ncbi.nlm.nih.gov/gene/3549)
• [OMIM](https://www.omim.org/entry/600726)
• [Ensembl](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000163501)
• [UniProt](https://www.uniprot.org/uniprot/Q14607)

References