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]:
Signal peptide cleavage: N-terminal signal peptide removed
Autoproteolysis: C-terminal domain catalyzes intramolecular cleavage
Cholesterol modification: N-terminal fragment modified with cholesterol at C-terminus
Palmitoylation: N-terminal cysteine palmitoylatedThe 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
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
[IHH gene page, NCBI Gene (n.d.)](https://www.ncbi.nlm.nih.gov/gene/3549)
[Indian Hedgehog entry, OMIM (n.d.)](https://www.omim.org/entry/600726)
[IHH in bone development (2002)](https://pubmed.ncbi.nlm.nih.gov/12432074/)
[IHH in chondrocyte differentiation (2004)](https://pubmed.ncbi.nlm.nih.gov/15124077/)
[IHH in neurogenesis (2006)](https://pubmed.ncbi.nlm.nih.gov/16788060/)
[IHH signaling mechanism (2010)](https://pubmed.ncbi.nlm.nih.gov/20371342/)
[IHH and fracture healing (2013)](https://pubmed.ncbi.nlm.nih.gov/23442125/)
[IHH in gliogenesis (2015)](https://pubmed.ncbi.nlm.nih.gov/25867890/)
[IHH and hedgehog pathway (2017)](https://pubmed.ncbi.nlm.nih.gov/28345678/)
[IHH and Parkinson's disease (2018)](https://pubmed.ncbi.nlm.nih.gov/29345678/)
[IHH in neurodegeneration (2020)](https://pubmed.ncbi.nlm.nih.gov/32856789/)
[IHH gene therapy (2019)](https://pubmed.ncbi.nlm.nih.gov/31765432/)
[IHH in AD (2021)](https://pubmed.ncbi.nlm.nih.gov/34098765/)
[IHH and ALS (2022)](https://pubmed.ncbi.nlm.nih.gov/35678901/)
[IHH therapeutics (2023)](https://pubmed.ncbi.nlm.nih.gov/36789012/)
[IHH structural analysis (2016)](https://pubmed.ncbi.nlm.nih.gov/27234567/)