Dystrophin-Glycoprotein Complexes

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MEMBRANE-ASSOCIATED PROTEIN COMPLEXES
Skeletal Muscle Fibers & Connective Tissue

Extrajunctional muscle membrane: Associated proteins ²

Membrane-associated proteins, Extrajunctional Muscle

General
Basal lamina & Connective tissue
Extrajunctional muscle
Muscle Membrane: Associated Proteins
Neuromuscular junction
Signalling complexes

Also see: Myosin & related proteins

Muscle Membrane: Associated proteins
Acetylcholine receptors: α, β, δ, e, γ
Acetylcholinesterase (A12)
Actin
Agrin (Muscle- & Nerve-derived)
AKAP9
Ankyrin
Biglycan
Calmodulin 1
Caveolin-3
Collagens
α1 (IV); α2 (IV); α3 (IV); α4 (IV); α5 (IV)
α1 (VI); α2 (VI); α3 (VI)
Contactin-1
Decorin
Dok-7
Dysferlin
Dystrobrevin
Dystrobrevin binding protein
α-Dystroglycan
β-Dystroglycan
Dystrophin
ErbB receptor tyrosine kinase
Fibronectin
Filamin-2 (Filamin C)
βGalNAc-terminal conjugates
Grb2
Heparan sulfate proteoglycans
Integrins
Laminins
LRP4
MACF1
MASC
MuSK
NCAM
Neuregulins (ARIA)
Neurofascin
Nidogen
NOS
Perlecan
PHLDB2
Plectin
Rapsyn
Sarcoglycans
Sarcospan
Sodium channels, Voltage gated
S-Laminin
β-Spectrin
Syncoilin
SYNE1
Syntrophin
Utrophin
ZBTB42

Neuromuscular Junction: Associated proteins ¹

Muscle Membrane: Associated Proteins

Protein Complexes, Membrane-associated

Neuromuscular Junction
Dystrophin-Glycoprotein
- Muscle membrane
- Extracellular
- Intracellular
  - Structural
  - Signalling
Dystroglycans
Integrin
Caveolin-Dysferlin

Signalling Pathways

Acetylcholine
- Transmitter: Acetylcholine
- Membrane receptor: Acetylcholine receptors
- Signalling mechanisms, Intracellular: Calcium; Depolarization of muscle membrane
- Effects
  - Muscle fiber contraction
  - Downregulates transcription of AChR genes in non-synaptic muscle nuclei
- Disorders: Myasthenic; Other
Neuregulin-1/ARIA
- Transmitter: Neuregulin-1
- Membrane receptor: ErbB
- Signalling molecules, Intracellular: MAP kinases; GAPBα/β
- Effects
  - Induces transcription of AChR genes in subsynaptic muscle nuclei: Especially e & δ
  - Induces transcription of synapse-specific proteins
Agrin, Neural

Transmitter: Agrin, Neural
Membrane receptor: MASC/MuSK
Signalling molecules, Intracellular: Rapsyn; src-like kinases
Effects: Organizes NMJ

Other
- Integrin
- Caveolin-Dysferlin

Muscle Membrane Proteins

Dystrophin-Associated muscle membrane proteins

β-Dystroglycan
Sarcoglycans
Sarcospan

Sarcoglycans ⁹
- Sarcoglycan proteins: Types
  - α-Sarcoglycan (Adhalin) : LGMD R3 (2D)
  - β-Sarcoglycan : LGMD R4 (2E)
  - γ-Sarcoglycan : LGMD R5 (2C)
  - δ-Sarcoglycan : LGMD R6 (2F)
  - ε-Sarcoglycan : Myoclonus-Dystonia syndrome
  - ζ-Sarcoglycan ⁴ : Structurally related to γ and δ
- General features
  - Family: Homologous transmembrane proteins with single membrane spanning domain
  - All Sarcoglycans (SG) have
    - Glycosylation
    - Extracellular region
      - Contains cysteine cluster
      - Location
        
        N-terminal (Type I): α- & ε-SG
        
        C-terminal (Type II): β-, γ- & δ-SG
      - Homologous to a region in extracellular domain of EGFR
    - Membrane-spanning domain: One
  - Organization: Heterotetramers
    - β & δ subunits
      - Core of complex
      - Sarcoglycan pair most tightly bound to each other
    - α, γ (& e) subunits
      - Bind to complex independently
      - Added to complex in Golgi apparatus
  - Tissue composition of sarcoglycan oligomers
    - Skeletal Muscle: α, β, γ, δ sarcoglycans
    - Smooth Muscle: e, β, z, δ Sarcoglycans
    - Myelinating Schwann cell: β, δ, e
  - Tissue expression of sarcoglycan monomers
    - α, β, γ, δ Sarcoglycans: High in skeletal muscle
    - e-Sarcoglycan: Widely expressed but little in skeletal muscle
    - z-Sarcoglycan: Skeletal & Vascular smooth muscle
  - Molecules interacting with Sarcoglycan complex
    - β-Dystroglycan: May bind to γ-Sarcoglycan
    - α-Dystrobrevin, N-terminal: Binds to Sarcoglycan complex
    - Sarcospan: Stabilized at the membrane by Sarcoglycan complex, especially δ-Sarcoglycan
    - Filamin-2: Binds to γ & δ Sarcoglycans
    - Biglycan : Extracellular leucine-rich repeat (LRR) protein
    - No direct association with dystrophin
  - Phosphorylation of Sarcoglycan complex
    - Tyrosine phosphorylation of α- & γ-Sarcoglycans: Associated with muscle cell adhesion.
    - α- & γ-Sarcoglycans phosphorylated by Src kinase
- Sarcoglycan complex: Possible functions
  - Stabilization of Dystrophin-Glycoprotein Complex
    - Especially Dystrophin�Dystroglycan interaction
  - Regulation of adhesion of Dystrophin-Glycoprotein Complex to Laminin-2 in extracellular matrix
  - ? Role in vascular function associated with blood flow: Especially γ-Sarcoglycan
- Sarcoglycan levels in LGMD
  - α, β, & δ Sarcoglycans: Found in similar, often reduced, levels in sarcoglycanopathy muscle
  - Patterns and amount of sarcoglycan staining
    - Not predictive of specific sarcoglycan gene mutations
    - Sarcoglycanopathies may be associated with reduced dystrophin & β-dystroglycan in muscle
      - Dystrophin & β-Dystroglycan usually in parallel
      - N-terminal dystrophin domain (Dys-3) more reduced than C-terminal domain
  - γ-Sarcoglycan
    - Less closely associated with sarcoglycan complex
    - Most often & most severely reduced in all sarcoglycanopathies
    - May vary independently in some LGMD
    - Extracellular portion may be a critical domain for muscle cell survival
  - ζ-Sarcoglycan: Reduced at membrane in muscular dystrophy
- Sarcoglycan complex: Possible functions
  - Mechanical: Bridging between basal lamina & intracellular actin network
  - Signaling: Cysteine cluster & ATPase activity in α-sarcoglycan
- Other sarcoglycans
  - ε-Sarcoglycan
    - Size: 47 kDa
    - Homologous sarcoglycan: α-sarcoglycan; Replaces &alpha- in SG complex
    - Expression: High in lungs; Moderate in brain, Schwann cells, cardiac & skeletal muscles
    - Gene is maternally imprinted
    - Disease: Myoclonus-Dystonia syndrome
  - ζ-Sarcoglycan
    - Homologies with: γ- & δ-Sarcoglycans
    - Expressed in: Skeletal muscle, Vascular smooth muscle
    - Associates with other sarcoglycans
    - Sarcoglycan complex: Functional homologue of γ-sarcoglycan
- External link
Sarcospan
- Protein family: Tetraspan
- Isoforms: Generated by alternative splicing
  - Isoform 1: Specific for skeletal & cardiac muscle
  - Isoform 2: Widely expressed
- Skeletal muscle location: Sarcolemma; NMJs; Myotendinous junctions
- Binds to: Sarcoglycan complex
- Knock-out mouse: No myopathy
β-Dystroglycan

Dystrophin-associated protein distributions

Dystrophin-associated protein complex Tissue distribution of components ⁷
Protein	Skeletal	Cardiac	Vascular Smooth
Dystrophin	+	+	+
α--Sarcoglycan	+	+	-
β-Sarcoglycan	+	+	+
δSarcoglycan	+	+	+
γ-Sarcoglycan	+	+	-
ε-Sarcoglycan	+	+	+
ζ-Sarcoglycan	+	+	+
Dystroglycan	+	+	+
Sarcospan	+	+	-
Syntrophins (α and β)	+	+	?
Dystrobrevins (α and β)	+	+	?
Caveolin-3	+	+	-
nNOS	+	±	+

Synaptic membrane proteins

Acetylcholine receptors
β-Dystroglycan
ErbB receptor tyrosine kinase
MASC
MuSK
Neurofascin
Sodium channels, Voltage gated
- Location: Peak of synaptic folds
  - Acetylcholine receptors
    - AChRs at NMJs
      - Located at peak of synaptic folds
      - Concentration: 10,000 per μm²
    - AChR Structure: Pentamers
      - Subunits clustered around central receptor channel
      - Adult muscle NMJs: α1 e α1 β1 δ
      - Fetal muscle NMJs: α1 γ α1 β1 δ
      - Extraocular muscles: Some fibers contain both adult & fetal AChRs
      - AChRs are members of superfamily of ligand-gated ion channels
    - Central pore of AChR mediates ion passage
    - Main immunogenic region of AChR: Extracellular; N-terminal
    - AChR Disorders
      - α subunit
      - β subunit
      - δ subunit
      - e subunit
      - γ subunit
  - MuSK receptor (receptor tyrosine kinase) ^4, ¹⁰
    - Tissue distribution: Muscle
    - Transmembrane protein
    - Enzyme activity: Tyrosine kinase
    - Interactions
      - Abl kinases required for agrin-induced AChR clustering ⁶
      - ColQ: Facilitates AChR clustering
      - Co-localizes with AChRs
      - Low-density lipoprotein receptor-related protein 4 (Lrp4)
        
        Binding of nerve-secreted agrin to Lrp4 activates MuSK by phosphorylation
        
        Phosphorylated MuSK triggers AChR assembly & stabilization: Signaling pathway involves
        
        Dok-7 recruitment
        Activation of nonreceptor tyrosine kinases & GTPases
    - Functions
      - AChR aggregation
        
        Positive regulator: Agrin/LRP4/MuSK/Dok7 pathway
      - Mediates rapsyn-dependent AChR aggregation
        
        via Ectodomain
        
        Acts via interaction with Dishevelled & downstream kinase PAK1
      - Phosphorylates AChRs
        
        via Endodomain
        
        Involves src-like kinases
      - Anchors acetylcholinesterase (AChE) to synaptic basal lamina
    - Diseases
      - Myasthenia Gravis, Anti-AChR antibody negative
      - Congenital MG
  - Low density lipoprotein receptor-related protein 4 (LRP4)
    - Protein
      - Locations
        
        Neuromuscular junctions
        
        Spindles: Intrafusil muscle fibers
        
        Sensory synapses
      - NMJ associations
        
        Binds to: Neuronal agrin
        
        Complexed with: MuSK
      - Roles
        
        AChR clustering
        
        Cell surface endocytic receptor binding
        
        Internalizes extracellular ligands for degradation by lysosomes
        
        Dorsal/Ventral pattern formation
        
        Mediates SOST-dependent inhibition of bone formation & inhibition of Wnt signaling
        
        Spindle development
    - Diseases
      - LRP4 Autoantibodies ¹³
      - Cenani-Lenz syndactyly syndrome (CLSS)
      - Sclerosteosis 2
      - Congenital MG
  - ErbB receptor tyrosine kinase
    - Family: Epidermal growth receptor-related transmembrane receptor tyrosine kinase
    - Common ligand: Neuregulins
    - Components
      - ErbB2
        
        Component of neuregulin binding complex
        
        Disease: Visceral Neuropathy 2
      - ErbB3
        
        Binds, & activated by, neuregulins & NTAK
        
        Cytoplasmic domain: May interact with SH2 or SH3 domains of signal-transducing proteins
        
        Disease: LCCS2
      - ErbB4
        
        Binds, & activated by, neuregulins, NRG-2, NRG-3, HBEGF, βCellulin & NTAK
        
        Forms homodimer or heterodimer with ErbB2 or ErbB3
        Activated on binding of: Neuregulins (NRGs) to extracellular ligand-binding domain of ErbB4
        Expression: Soma of large motor neurons
        Interacts with: WWOX
        Transmembrane tyrosine kinase
        ERBB4 homodimers: Tumor suppressors
        ERBB4-EGFR or ERBB4-ERBB2 heterodimers: Oncogenes
        Disease: ALS 19, Dominant
    - General enzyme activity: ATP + Protein tyrosine = ADP + Protein tyrosine phosphate
    - NMJ actions: Increases synthesis of AChR subunits at NMJs
      - Liganded ErbB receptors activate mitogen activated protein (MAP) kinases
      - MAP kinases phosphorylate GAPBα/β: Acts as a transcriptional activating factor
      - Phosphorylated GAPBα/β
        
        Binds to neuregulin response element (-CGGAA-), an Ets-binding site
        
        Binding site location: Promoter regions of δ- & e-subunits of AChRs
  - Myotube-associated specificity component (MASC)
    - Poorly characterized protein
    - Interacts with the most N-terminal of 4 immunoglobulin-like extracellular domains on MuSK
  - β-Dystroglycan
- Location: Base of synaptic folds
  - Voltage gated Na⁺ channels
    - Na_v1.4
    - Tethered to post-synaptic membrane by ankyrin G & β-Spectrin
    - Linked to cytoskeleton by syntrophins
- Other
  - Neurofascin
    - L1 family of cell adhesion molecules (L1 CAMs)
    - Regulates ankyrin G & sodium channel clustering at Nodes of Ranvier
  - Pleckstrin homology-like domain, family B, Member 2 (PHLDB2)
    - Associated with cytoplasmic face of postsynaptic membrane
    - Highest levels border regions of highest AChRs
    - Expression in cultures of mouse myotubes inhibit AChR aggregation
    - CLASP binding partner
Other membrane proteins
- Caveolin : LGMD 1C
- Dysferlin: LGMD 2B; Myoshi distal myopathy
- Integrins
  - Integral membrane proteins
  - Receptors for: Laminins 1, 2, 4
  - Interactions are independent of the dystrophin-dystroglycan complex
  - Integrin α7β1
    - Specificity for skeletal and cardiac muscle
    - Concentrated at myotendinous junctions
    - Also detected at neuromuscular junctions and along the sarcolemmal membrane
    - Integrin-α7 mutations: Congenital muscular dystrophy
    - Integrin α7β1 knockout mice
      - Progressive "dystrophic" myopathy
      - Variabile severity in different muscle types
      - changes suggest abnormal function of myotendinous junctions
  - β1 integrin
    - Required in muscle for innervation by motor axons
    - Role in muscle fiber development of normal adhesive & signaling interactions with motor neurons
  - Integrin α5β1
    - Binds to fibronectin
    - Mouse knockout: Embryonic lethal
    - Mouse chimera: Muscluar dystrophy

Intracellular proteins associated with muscle membranes

Dystrobrevin
Dystrophin
Filamin 2
Syncoilin
Syntrophins

Structural proteins: Intracellular, Skeletal muscle
- Dystrophin: Duchenne & Becker Muscular Dystrophy
- α-Dystrobrevin (DTNA) ⁸
  - α-Dystrobrevin Associations
    - Binds directly to dystrophin
      - Binding occurs at coiled-coil domains in C-terminal region of each protein
      - Complexes contain 2 syntrophin binding sites
    - N-terminal region of α-dystrobrevin associates with the sarcoglycan complex
    - α-Dystrobrevin binds to syncoilin, an intermediate filament protein
    - Dystrobrevin-binding protein 1 (DTNBP1) : Colocalizes with α-Dystrobrevin
  - Location: Subsarcolemmal, Cytoplasmic
  - Isoforms: Generated by alternative splicing
    - Size range: 59 to 78 kDa
    - Number: > 5 different forms of α-dystrobrevin
    - 3 muscle-specific isoforms
  - Dystrobrevin phosphorylation
    - May increase affinity for other dystrophin family proteins via coiled-coil motifs
  - Disease
    - α-Dystrobrevin: Disorders
      - Noncompaction of left ventricular myocardium ± Congenital heart defects
      - Myopathy, Dominant
      - ? Atrial fibrillation
    - α-Dystrobrevin membrane staining is severely reduced along with α1-syntrophin in
      - Normal fetal development
      - DMD
      - Sarcoglycanopathies
      - Denervation
    - α-Dystrobrevin deficient mice: Cardiomyopathy & Mild myopathy
      - α1-syntrophin: Mildly reduced is observed
      - Abnormal formation of NMJs: Abnormally distributed & unstable nAChR clusters
- β-Dystrobrevin (DTNB)
  - Coded by different gene than α-Dystrobrevin
  - Location: Neurons of cortex & hippocampus; Not in brain microvasculature or muscle
  - Coprecipitates with brain dystrophin isoforms Dp71 & Dp140
- Dystrobrevin binding protein (DTNBP1; Dysbindin)
  - Locations
    - Muscle fibers: Colocalizes with alpha-dystrobrevin at sarcolemma
    - Large blood vessels
    - Brain: Neurons
  - Diseases
    - Hermansky-Pudlak syndrome 7 (HPS7)
    - Mouse: sandy
- Filamin C (FLN2; FLNC)
  - Tissue expression
    - Skeletal muscle: Z-disk
    - Heart
  - Structure
    - Domains
      - Actin binding domain: 2 Calponin homology (CH) domains
      - Immunoglobulin (Ig) domains: 24; Divided into ROD1 & ROD2 subdomains
      - C-terminal: Dimerization domain
      - 2 transcripts: ± Exon 31
        
        Long isoform: More during cardiac stress
        
        Short isoform: Normal expression; 3.5x higher in skeletal muscle
    - Dimer: 2 identical filamin proteins
  - Interacts with
    - Actin: via amino terminal CH domains
    - γ- & δ-Sarcoglycans
    - Myotilin
    - Calpain-3
    - Myozenin 1 (Calsarcin 2) & 3
    - Pleckstrin homology-like domain, family B, member 2
    - MAP2K4
    - KCND2
    - Furin
    - FILIP1
    - FLNC
    - Inositol polyphosphate phosphatase like 1
    - Xin actin-binding repeat-containing proteins (XIRP) & Aciculin
  - Functions
    - Involved in actin reorganization and signal transduction
    - Maintains membrane integrity during force application
    - Structural protein: Z-disc; Myotendinous junction; Intercalated discs
  - Diseases
- Syntrophins ⁸
  - General
    - Modular adapter proteins
    - Domain organization
      - Pleckstrin homology domains
      - PDZ protein-interaction domain
      - Carboxyl-terminal syntrophin unique domain
    - Interactions
      - Link ion channels and signaling proteins to cytoskeleton
        
        Bind to: Dystrophin & homologous proteins
      - Complexed with dystrophin-related proteins
        
        Dystrobrevins
        
        Dystrophin
        
        Utrophin
      - Signaling proteins
        
        Phosphatidylinositol 4,5-bisphosphate
        
        Neuronal nitric oxide synthase (nNOS)
        
        Aquaporin-4 (AQP4)
        
        Voltage-activated sodium channels
        
        Several kinases
  - Syntrophin proteins: Several isoforms coded for by different genes
    - α1-Syntrophin
      - Highest levels in skeletal muscle; Also in kidney, cardiac muscle & brain
      - Location in skeletal muscle
        
        Development: Cytoplasmic
        
        Adult: Sarcolemma; Concentrated at NMJ
      - Binds to nNOS in skeletal muscle
      - Reduced or absent in sarcolemmal membrane along with α-dystrobrevin
        
        Normal fetal development
        
        Duchenne muscular dystrophy
        
        Denervation
      - Disease: Long QT syndrome 12
      - α1-Syntrophin knockout mice
        
        nNOS & AQP4 lost from muscle membrane
        
        Aberrant formation of neuromuscular junctions (NMJs): Deficient in utrophin & nAChRs
    - β1-Syntrophin
      - High levels in liver, pancreas, skeletal muscle, lung & kidney
      - Location in skeletal muscle: Along sarcolemma; Concentrated at NMJ
      - Predominant syntrophin isoform in muscle membrane during development
      - Upregulated in muscle fibers with immature phenotype: Associated with
        
        Loss of α1-syntrophin in DMD & denervation
        Expression of fetal nicotinic AChR γ subunit, cardiac actin & neonatal myosin
      - Mature muscle: More in fast fibers
    - β2-Syntrophin
      - Highest in testes, lung & brain; Low levels in skeletal muscle
      - Location in skeletal muscle
        
        Concentrated at NMJ
        
        Also in extrajunctional muscle membrane
        
        Cytoplasm during fetal development
        
        Post-natal: More in slow muscle fibers
      - Binds to 2 different microtubule-associated protein (MAP) kinases: Via PDZ domain interactions
    - γ1-Syntrophin
      - Brain specific
      - Binds to C-termini of dystrophin & α-dystrobrevin & β-dystrobrevin
    - γ2-Syntrophin
      - Present at sarcolemma of muscle fibers
      - Binds to C-termini of dystrophin & α-dystrobrevin & β-dystrobrevin
      - Reduced in Duchenne MD
  - Syntrophin interactions
    - Binding proteins: Dystrophin; Utrophin; α-Dystrobrevin
    - Binding via PDZ domains* *,
      - Binds to C-terminus of: NOS
      - Voltage-gated Na⁺ channels (Na_v1.4 & Na_v1.5)
      - SAPK-3
      - Diacylglycerol kinase
      - ICA512: Receptor tyrosine phosphatase-like pancreatic islet protein
      - Aquaporin-4
      - Inward rectifier potassium channels (Kir 2.2 and Kir 4.1
      - ATP-binding cassette transporter A1
    - via Pleckstrin homology (PH) domains
      - α1-syntrophin: First PH domain binds phosphatidylinositol-4,5-bisphosphate
      - β2-syntrophin: Second PH domain regulates interaction with utrophin
    - Mechanisms regulating binding: Phosphorylation of dystrophin; Calmodulin
  - Syntrophin phosphorylation
    - Ca⁺⁺�calmodulin-dependent protein kinase : Regulates syntrophin binding to dystrophin
    - Stress-activated protein kinase-3 (SAPK-3)
  - Mouse models
    - α1-syntrophin deficient: Reduction of AChRs, AChE & Utrophin at NMJs
    - α1/β2-syntrophin deficient: Abnormal postsynaptic structure; Absent utrophin
- Syncoilin
  - Intermediate filament protein
  - Binds to α-Dystrobrevin
- Ankyrin-B (ANK2)
  - Skeletal muscle: Located in costameres
  - Mutations cause LQT4
Structural proteins clustered at NMJs
- Utrophin
- Rapsyn
  - Associated with NMJ aggregation of post-synaptic molecules
    - AChRs
      - 1:1 Stoichiometry with AChRs
      - Crosslinks AChRs
    - ErbB receptors
    - β-dystroglycan: Attached to sarcolemma by binding to cytoplasmic tail
    - Binding to MuSK-based scaffold
  - Diseases
    - Myasthenia Gravis, Congenital
    - Fetal hypokinesia with inborn contractures (FADS2) (Arthrogryposis)
- A-Kinase anchor protein 9 (AKAP9; yotiao)
  - Clustered at NMJ: Depths of synaptic folds
  - Binds to
    - Type II regulatory subunit of cAMP-dependent protein kinase (PKA)
    - Type I protein phosphatase (PP1)
  - Brain: Interacts with NMDA receptors
- Ankyrin_G
  - Links ion channels & cell adhesion molecules to spectrin-based skeleton in specialized membrane domains
  - Clustered at NMJs & axon initial segments
  - Binds to Na_v1.6, β IV spectrin & neurofascin at axon initial segments
  - Note: Ankyrin_B mutations cause LQT4 syndrome
- β-Spectrin
  - Clustered at NMJ
  - Protein binding: Tethers Sodium channels to post-synaptic membrane
- Plectin
- Syntrophins: β1; β2
- SYNE1
Signalling proteins in Skeletal muscle, Intracellular
- Grb2
  - Adapter protein
    - Links Dystrophin-Glycoprotein complex to Ras/MAPK signaling pathway
    - Linkage via other adaptor proteins like SOS
  - Binds to β-dystroglycan
    - via Proline rich region
    - Dystrophin inhibits this binding
  - May link DGC complex to apoptotic pathways
- Calmodulin 1 (CALM1)
  - Binds to & regulates phosphorylation of: Dystrophin & α1-Syntrophin
  - CaM kinase II copurifies with this complex: Phosphorylate dystrophin & α1-Syntrophin
  - Calmodulin binding regulates: Dystrophin-Actin & Utrophin-Actin interactions
  - Calmodulin-regulated kinases participate in many cell survival pathways
    - Examples: Pathways mediated by phosphatidylinositide-38-kinase (PI3K) and Akt
  - Disorders
    - Ventricular tachycardia, catecholaminergic polymorphic, 4 (CPVT4)
    - Long QT syndrome 14 (LQT14)
- Nitric oxide synthase 1 (NOS1; nNOS)
  - Association of nNOS with the Dystrophin-Glycoprotein complex: Binding to Syntrophins
  - Nitric oxide (NO) is synthesized in skeletal muscle by nNOS
  - Production of NO� by nNOS may be cell-protective: Several possible mechanisms
    - Actions as a cytoprotective free radical
    - Induction of cGMP-dependent cell survival pathways
    - Local vasodilatory effect by effects on vascular smooth muscle
      - Decrease in ischemic injury to skeletal muscle cells
  - nNOS deficiency
    - Disorders: Muscle denervation; Myosin-loss myopathy
    - Mouse: No muscle disease
- Phosphoglucomutase-related protein

Extracellular matrix proteins

CONNECTIVE TISSUE COMPONENTS: SKELETAL MUSCLE			Collagen VI (Green) and Collagen IV (Orange) in muscle
Synaptic	Extra-synaptic	Synaptic & Extrasynaptic
Agrin (Nerve-derived) Neuregulins (ARIA) Acetylcholinesterase (A12) Laminins: 4, 9, 11 S-laminin Heparan sulfate proteoglycans βGalNAc-terminal conjugates Collagens α3 (IV); α4 (IV); α5 (IV) NCAM S-nidogen	Collagens α1 (IV); α2 (IV) Laminin-2 Tenascin-XB	α-Dystroglycan Nidogen Fibronectin Integrin-α7 Perlecan Agrin (Muscle-derived) Decorin Collagens α1 (VI); α2 (VI); α3 (VI) Biglycan

Basement membrane: General ⁵
- Composition
  - Protein: Mostly glycosylated
  - Carbohydrate: Nearly all covalently bound to protein
  - No lipid or nucleic acid
- Structure: Two layers
  - Internal: Basal lamina (BL)
    
    Perlecan in skeletal muscle
    Staining of basal lamina & endomysial capillaries
    - Felt-like
    - Directly linked to plasma membrane
    - Composition of core basal lamina
      - Collagen: Non-fibrillar
        
        Collagen IV: Most abundant
      - Noncollagenous proteins
        
        Glycoproteins
        Proteoglycans
        Perlecan: Abundant
        
        Laminin: Abundant
      - Entactin/nidogen
        
        Link laminin & collagen networks
    - Interactions of core BL components
      - Bind to other BL components
        
        Perlecan: Binds to BL via laminins & collagen IV
      - Anchor reticular lamina components to BL
      - Ligands for membrane associated receptors
        
        Integrins
        
        Dystroglycans
    - Neuromuscular junctions
      - Present in synaptic cleft
      - Contributes to intercellular adhesion
        
        Between presynaptic terminal and post-synaptic membrane
      - Involved in enzymatic hydrolysis of acetylcholine by acetylcholinesterase
      - Basal lamina provides target for axons reinnervating NMJs
    - Myotendinous junctions: Basal lamina extends into invaginations
  - External: Reticular lamina
    - Fibrillar
    - Composition
      - Collagenous fibrils
      - Proteoglycan-rich ground substance
        
        Amorphous
        
        Embeds fibrils
    - Neuromuscular junctions: Absent
- Function in muscle: Elasticity & Strength, tensile
Laminins
- General
  - Glycoproteins
  - Components: 3 polypeptide chains (alpha, beta, gamma)
  - Chains are bound to each other by disulfide bonds
  - Cross-shaped molecule
    - Arms: 1 long & 3 short
    - Globules at each end: Attachment sites for integrin receptors & α-dystroglycan
    - Long arm: 80-nm α-helical coiled-coil structure formed by three subunits
    - Short arms: Formed by one of the three subunits
  - Location
    - Basement membranes, extracellular
    - Most abundant noncollagenous proteins of muscle basal lamina
    - Laminins at NMJ: α2β2γ1, α4β2γ1, and α5β2γ1
  - Functions during development
    - Mediate cell attachment & migration
    - Organization of cells into tissues
    - Muscle development: Laminin
      - Enhances proliferation of myoblasts
      - Stimulates myoblast motility
      - Induces bipolar shape characteristic of fusing cells
      - Promotes myotube formation
  - Interact with other extracellular matrix components
  - Bind to: α-Dystroglycan; Integrins
- Laminin chains
  - α1 (LAMA1)
    - Mutations: Cerebellar Dysplasia & Cysts
  - α2 (LAMA2) chain disorders: Congenital muscular dystrophy
  - α4 (LAMA4) : CMD 1JJ
  - α5 (LAMA5) : Myasthenia Gravis, Congenital
  - β1 chain
    - Deficiency: Bethlem myopathy
    - Mutations
      - Lissencephaly 5, Recessive (LIS5)
      - Leukoencephalopathy without lacunae, adult-onset, Dominant (LUCAO)
  - β2
    - Binds to Ca⁺⁺ channels, especially Ca_v2.1 or Ca_v2.2
    - β2 chain deficiency
      - Congenital myasthenic syndrome
      - Pierson syndrome
      - Mouse: Reduced clustering of Ca⁺⁺ channels in presynaptic membrane
- Laminin-2
  - Composed of α2 , β1 , and γ1 chains
  - Location: Muscle basal lamina; Major component of extra-synaptic basal lamina
- Laminin 4
  - Composed of α2 , β2 , and γ1 chains
  - Location: Concentrated at NMJs
- Laminin 9
  - Composed of α4 , β2 , and γ1 chains
  - Location: NMJs, Primary synaptic cleft
- Laminin 11
  - Composed of α5 , β2 , and γ1 chains
  - Location: NMJs, Primary & secondary synaptic clefts
  - Stops motor axon growth at NMJs
  - Prevents Schwann cells from entering synaptic cleft
- S-laminin
Agrin
- Agrin molecule
  - Glycosylation: Heparan sulfate glycosaminoglycan chains
  - Clustering ability: Via Y & Z sites in C-terminal
  - Multiple isotypes: Most secreted
  - Several agrin variants: Produced by alternative mRNA splicing
    - Motor neurons contain 4 different agrin transcripts
      - Most abundant at the NMJ
      - Aggregate AChRs during synaptogenesis
      - Induce autophosphorylation of MuSK
      - Binds to MuSK via MASC
    - Agrin forms from other tissues
      - Muscle: Have little AChR aggregating activity
  - Agrin binding to other molecules
    - Membrane
      - MuSK-accessory specificity component (MASC)
        
        Links agrin to MuSK
      - αvβ1-Integrin: Via C-terminal region of agrin
      - β-Dystroglycan in Schwann cell membranes
    - Muscle basal lamina
      - α-Dystroglycan: Via C-terminal region of agrin
      - Heparin-binding proteins (HB-GAM)
      - NCAM
      - Heparan sulfate proteoglycans: Via C-terminal region of agrin
      - Laminins: Via N-terminal region of agrin
- Agrin-MASC-MuSK scaffold
  - Induces synapse specific transcription
  - Forms locus for aggregation of synapse-specific molecules
    - By interacting with rapsyn
    - Stabilizes synaptic AChR clusters
  - Neural agrin induces the synthesis and aggregation of MuSK and ErbB receptors
  - Mice with absent agrin
    - Do not develop AChR clusters at neuromuscular junctions
    - Presynaptic regions undifferentiated
    - Die at birth: Respiratory failure
- Diseases
  - Myasthenia gravis, Congenital
  - Myasthenia gravis, Autoimmune: IgG antibodies
  - Fetal akinesia
Heparan sulfate proteoglycans
- Perlecan (HSPG2)
Biglycan (BGN)
- Subcellular location: Extracellular matrix
- Two attached glycosaminoglycan chains: Chondroitin sulfate or Dermatan sulfate
- Family: Small leucine-rich proteoglycans (SLRPS); Contains 12 leucine-rich repeats
- Binds to
  - α & γ sarcoglycans: Via core polypeptide
  - α-dystroglycan: Via chondroitin sulfate side chains to COOH-terminal third of α-dystroglycan
  - Collagen VI
  - Transforming growth factor β1 (TGFβ1)
- Mouse models
  - Knockout mouse: Myopathy; Abnormal bone
  - mdx mouse: Upregulated
Nidogen
- Sulfated glycoprotein
- Present in all basement membranes
- Protein binding: Laminin; Collagen, type IV
Fibronectin
- Cell surface protein of many fibroblast lines
- Protein binding: Integrins (α5β1); CAMs; Others
- Functions: Cell adhesion, Cell motility, Opsonization, Wound healing, Cell shape
α-Dystroglycan
NCAM: Located at depths of postsynaptic folds
Collagen ¹¹
- General
  - All collagens consist of: 3 α-chains
    - Identical or Different
    - Entwined as triple helix
  - Collagen molecules
    - 42 different α-chains
    - 29 different collagens
  - Types
    - Fibrillar: Most abundant
    - Non-fibrillar
    - Fibril-associated collagens with interrupted triple helices (FACIT)
  - Maturation
    - Posttranslational modification: In endoplasmic reticulum
    - Mature collagen: Secreted into extracellular matrix
- Collagen: Domain structure
  - Collagen triple helical domain contains repeats of Gly-X-Y
    - X: Often Proline
      - Proline allows for tight turning of colagen strand helix
    - Y: Often hydroxyProline or hydroxyLysine
    - Glycine: Small side chain clusters toward center of triple helix
    - Hydroxylation facilitates crosslinking of α-helices
  - Lysines undergo extensive translational modifications (Hydroxylation or Glycosylation)
- Collagen α-chains
  - Preprocollagen
  - Procollagen: 3 α-chains (preprocollagen) polymerize in a supercoiled right hand helix
  - Additional crossliking then occurs
  - Chaperone proteins (including HSP47) bind
  - Procollagen then transported through the Golgi apparatus: Some side-to-side association occurs
  - Collagen is then secreted
  - Extracellular procollagen peptidases cleave N and C-terminal propeptides: Tropocollagen
  - Mutations in the N-terminal propeptidase: Ehlers-Danlos type 7
- Tropocollagen
  - Tropocollagen monomers laterally associate with a 67 nm stagger
  - Extensive crosslinking occurs: Gives added strength & maturation
  - Crosslinks: Aldol-histidine & Aldol
- Collagen Disorders
  - Collagen I
    - COL1A1 : Osteogenesis imperfecta I-IV; Ehlers-Danlos I & VIIA; Caffey
    - COL1A2 : Osteogenesis imperfecta II-IV; Ehlers-Danlos VIIB & Cardiac valvular
    - ADAMTS2 : Dermatosparaxis; Ehlers-Danlos, type VIIC
  - Collagen II
    - COL2A1: Bone disorders
  - Collagen III
    - Vascular collagen
    - COL3A1 : Ehlers-Danlos III & IV; Aortic aneurysm
  - Collagen IV
    - Basement membrane collagen: Abundant in perineurium & muscle basal lamina
    - Increased: Schwannomas; Small vessels in diabetes
    - Disorders
      - COL4A1: HANAC
      - COL4A2 : Porencephaly; Brain small vessel disease; Rhabdomyolysis
      - COL4A3 & COL4A4 : Alport syndrome; Hematuria
      - COL4A5 : Alport syndrome
      - COL4A6 : Alport syndrome + Leiomyomatosis, diffuse
  - Collagen V
    - COL5A1 : Ehlers-Danlos I & II; Fibromuscular dysplasia, multifocal (FMDMF)
    - COL5A2 : Ehlers-Danlos I
  - Collagen VI (COL6A1, COL6A2, COL6A3)
    - Myopathies: Bethlem; Ullrich CMD
    - Amyloid-β: Blocks association of oligomers with neurons, Increases aggregation; Reduces neurotoxicity
    - Inhibits apoptosis
  - Collagen VII
    - COL7A1 : Epidermolysis bullosa; Toenail dystrophy
  - Collagen VIII
    - COL8A2 : Corneal dystrophies
  - Collagen IX
    - COL9A1 : Epiphyseal dysplasia 6 (EDM6); Stickler syndrome, type IV
    - COL9A2 : Intervertebral disk disease, Epiphyseal dysplasia, multiple, 2 (EDM2); Stickler syndrome, type V; EDM3
    - COL9A3 : Epiphyseal dysplasia + Myopathy (EDM3)
  - Collagen X
    - COL10A1 : Metaphyseal chondrodysplasia, Schmid; Spondylometaphyseal dysplasia
  - Collagen XI
    - COL11A1 : Fibrochondrogenesis; Marshall; Stickler, type II; Lumbar disc herniation, susceptibility
    - COL11A2 : Fibrochondrogenesis 2; Stickler, type III; Deafness; Otospondylomegaepiphyseal dysplasia;
      Weissenbacher-Zweymuller; OPLL
  - Collagen XII
    - COL12A1 : Bethlem & Ullrich myopathies
  - Collagen 13A1: Congenital Myasthenic Syndrome 19 (CMS19)
  - Collagen XV : Mouse skeletal & cardiac myopathy
  - Collagen XVII
    - COL17A1 : Epidermolysis bullosa
  - Collagen XVIII
    - COL18A1 : Knobloch syndrome; Ataxia + Epilepsy + Eye disorders
  - Collagen tail of AChE: Congenital MG syndrome
  - Collagen related proteins
    - COMP: EDM3
    - P4HA1: Congenital disorder of tendon, bone, muscle & eye
Also see: Connective tissue, Skeletal muscle

Dystroglycans and Other proteins associated with the Dystrophin-Sarcoglycan complex

Glycosylation pathways

α & β Dystroglycans
- Encoded by a single gene
- β Dystroglycan formed by cleavage of α Dystroglycan
- Membrane-spanning complex: Forms linkage between cytoskeleton & extracellular matrix
- Links dystrophin�glycoprotein complex to basal lamina
- Mouse knockouts: Dystroglycan
  - General: Lethal in embryonic development
  - Gross developmental abnormalities
  - Disruption of Reichert membrane, an extra-embryonic basement membrane
  - Abnormal localization of laminin & collagen IV
  - Selective depletion of dystroglycan in muscle: Progressive myopathy; Abnormal neuromuscular junctions

α-Dystroglycan protein

Sub-Cellular Location
- Extracellular matrix
- Binds to
  - β-Dystroglycan: Extracellular domain
  - Biglycan
  - Agrin
  - Laminin α2
  - Perlecan
  - Neurexin
Tissue locations
- Neuromuscular: Muscle, Neuromuscular junction, Nerve
- Brain
  - Neuronal elements: Structural element of synapses in cerebellum & hippocampus
  - Astrocytes: Two basal lamina interfaces formed by astrocyte foot processes
    - Abutting on cerebral blood vessels
    - Glia limitans at the pial surface of the brain and spinal cord
α-Dystroglycan protein features
- Core protein: 74 kD
- Highly glycosylated: Glycosylated MW varies from 120 kD (Nerve & Brain) to 156 kD (Muscle) to 200kD
- Location: Extracellular

α-Dystroglycan: Glycosylation

General: O-Mannosylated glycan
Carbohydrate structures
- Classical: NeuAc-α2,3-Gal-β1,4-GlcNac-β1,2-Man-α1-Ser/Thr
  - Can have branches at C-6 of Man: Type M1 = unbranched; Type M2 = branched
  - High abundance in α-DG: Muscle & Brain
  - Not laminin-binding moiety
- Limited amounts: GalNAc-β1,3-GlcNAc-β1,4-[6-phosphoryl]-Man-α1-Ser/Thr
  - Type M3
  - Essential for laminin & Arenavirus (Lassa virus GP1) binding
  - Binding ligand
    - Matriglycan: (-Xylα1,3-GlcAβ1,3-)_n
    - Includes polysaccharide based on repeat of: Xylose & Glucuronic acid
  - Target of IIH6 antibody

α-Dystroglycan Glycosylation Pathway

Dystroglycan structure

α-Dystroglycan Glycoprotein: Carbohydrate Moiety Synthesis Steps

Dolichol

D-Fructose 6-phosphate
↓ MPI

D-Mannose-6-phosphate
↓ PMM2

α-D-Mannose-1-phosphate
GTP ↓ GMPPB

(Inhibited by GMPPA

) Cytoplasm

DOLK ↓

GDP-α-D-Mannose
Dolichyl-PO₄ ↓ DPM1

(with DPM2

& DPM3

)
Dolichyl β-D-Mannosyl phosphate
↓ POMT1

+ POMT2

UDP-GlcNAc UDP-GlcNAc
3-O-(α-D-Mannosyl)-L-threonyl-protein → GlcNAcβ1- 2Manα1-peptide → (Core M2-Glycans)
POMGNT1

MGAT5B

UDP-GlcNAc ↓ POMGNT2 (GTDC2)

UDP-GalNAc ↓ B3GALNT2

ATP ↓ POMK (SGK196)

GalNAcβ1- 3GlcNAcβ1- 4(P6)Manα1-peptide (phosphocoreM3-peptide) Endoplasmic Reticulum

CDP-Rbo ↓ FKTN

Golgi
RboSP- 3GalNAcβ1- 3GlcNAcβ1-4(P6)Manα1-peptide
CDP-Rbo ↓ FKRP

RboSP- 1 Rbo5P- 3GalNAcβ1- 3GlcNAcβ1-4(P6)Manα1-peptide
UDP-Xyl ↓ RXYLT1

Xylβ1- 4Rbo5P-1Rbo5P-3GalNAcβ1- 3GlcNAcβ1-4(P6)Manα1-peptide
UDP-GlcA ↓ B4GAT1

GlcAβ1-4Xylβ1-4Rbo5P-1Rbo5P-3GalNAcβ1-3GlcNAcβ1-4(P6)Manα1-peptide

UDP-Xyl ↓
UDP-GlcA ↓

LARGE1

-[3GlcAβ1-3Xylα1]_n-3GlcAβ1-4Xylβ1-4Rbo5P-1Rbo5P-3GalNAcβ1-3GlcNAcβ1-4(P6)Manα1-peptide

α-Dystroglycan: Functions
- Receptor functions
  - Muscle
    - Dual receptor for G domain of Laminin 2, Neurexin, & Agrin in extracellular matrix
    - Aberrant α-Dystroglycan glycosylation
      - Unable to bind extracellular ligands
      - Binding may be restored by over-expression of LARGE
  - Schwann cell with α-chain of laminin: Mycobacterium leprae
  - Other tissues
    - Extracellular matrix: Laminins; Perlecan
    - Viruses: Lymphocytic choriomeningitis & Lassa fever virus
- May participate in formation of extracellular matrix
- Associated with clustering of AChRs at NMJs: With binding to agrin
- CNS: Role in long-term potentiation, possibly via post-synaptic mechanism
- Receptor: Viral & Bacterial
α-Dystroglycan: Disease associations ³
- α-Dystroglycan glycoprotein changes in disease
  - General
    - Type: O-glycosylation (at Serine & Threonine)
    - Reduced or altered in muscle in muscular dystrophies & congenital muscular dystrophies (CMD)
    - Mechanism: Abnormal basement membrane formation
  - MEB & Fukuyama CMD
    - Normal levels of α-Dystroglycan core protein
    - α-Dystroglycan hypoglycosylation
    - Reduced α-Dystroglycan binding to laminin, neurexin and agrin
  - Pathology in muscle: Reduced staining around muscle fibers with antibody to α-DG glycosylation moiety
  - OMIM terminology: Muscular Dystrophy-DystroGlycanopathies (MDDG)
    - MDDGA: Congenital muscular dystrophy + Brain & Eye abnormalities
    - MDDGB: Congenital muscular dystrophy, less severe ± Mental retardation
    - MDDGC: Limb-girdle muscular dystrophy phenotype with Dystroglycanopathy
- Congenital muscular dystrophy (CMD) & Muscular dystrophy: Disorders with α-Dystroglycan abnormalities in muscle (MDDG)
  - α-Dystroglycanopathy: Primary
    - DAG1; 3p21: MDDGB9; MDDGC9
  - O-glycan structure: Endoplasmic reticulum
    - M1 & M2 Abnormal
      - POMT1; 9q34: MDDGA1; MDDGB1; MDDGC1 (LGMD 2K)
      - POMT2; 14q24: MDDGA2; MDDGB2; MDDGC2 (LGMD 2N)
      - POMGnT1; 1p32: MDDGA3 (Santavuori); MDDGB3; MDDGC3 (LGMD 2O)
    - M3 O-glycan Abnormal
      - POMGNT2 (GTDC2; GO61); 3p22: MDDGA8; MDDGC8
      - B3GALNT2; 1q42: MDDGA11; MDDGB
      - POMK (SGK196); 8p11: MDDGA12; Walker-Warburg
  - α-Dystroglycan glycosylation: ER & Golgi-related (GalNAc--[O--O]₂-- moiety)
    - Fukutin; 9q31: MDDGA4 (Fukuyama); MDDGB4; MDDGC4 (LGMD 2M); Cardiomyopathy (CMD 1X)
    - FKRP; 19q13: MDDGA5; MDDGB5; MDDGC5 (LGMD 2I)
    - α-Dystroglycan glycosylation: Ribitol pathway (P-Ribitol → CDP-Ribitol)
      - ISPD; 7p21: MDDGA7
  - α-Dystroglycan glycosylation: Golgi-related
    - B3GNT1 (B4GAT1) [Glucuronate moiety]; 11q13: MDDGA13
    - TMEM5 [Xylose moiety]; 12q14: MDDGA10
    - LARGE; 22q12: MDDGA6; MDDGB6
    - GOSR2
    - TRAPPC11
  - Dolichol-P-Mannose pathway
    - GMPPB; 3p21: MDDGA14
    - Congenital disorders of glycosylation
      - DPM1; 20q13: Encephalopathy (CDG1E)
      - DPM2; 9q34: Myopathy with Epilepsy & Microcephaly (CDG1U)
      - DPM3; 1q12: MDDGC15 (CDG1O); MDDCB15
      - DOLK; 9q34: Cardiomyopathy + CNS (CDG1M)
  - General: Acute illness-associated weakness (AIAW) ¹²
    - Frequency: 23% of patients with dystroglycanopathy; 5x other dystrophies
    - Disorders: FKTN; FKRP, POMT1, POMT2, POMGNT1
    - Features
      - Age: Children < 7 years old (80%); Median 2 years; Range 3 mo to 38 years
      - Weakness: Reduced function
      - Preceding illness: Often with respirtatory symptoms
      - Recurrent episodes in 30%
      - May precede diagnosis of dystrophy (50%)
      - Serum CK: 2x to 5x baseline; 600 to 30,000
      - Course: Monophasic; Return to previous level of function over 1 to 3 weeks
- Other disorders with possibly reduced α-Dystroglycan in muscle sarcolemma or other membranes
  - HIBM2 (Nonaka): GNE; 9p12:
  - CMD with respiratory failure & muscle hypertrophy (CMD1B; MDC1B): 1q42
  - Myopathy with Satellite cell loss: POGLUT1; 3q13
  - MDCCAID: INPP5K; 17p13
  - Also see
    - Walker-Warburg congenital MD
    - LGD with Mental Retardation (LGMD 2P): Dystroglycan (DAG1); 3p21
- Glycosylation-related disorders: Myasthenic syndromes
  - ALG14: 1p21
  - GMPPB: 3p21
  - Limb-Girdle MG + Tubular Aggregates
- Other glycosylation-related disorders
  - Arthrogryposis, Mental retardation & Seizures (AMRS): SLC35A3
  - Larsen syndrome, Recessive: B3GAT3
  - GPI anchor synthesis
    - Multisystem disorder: PIGY; 4q22
- Mouse models
  - myd mouse
    - LARGE mutation
    - Loss of α-Dystroglycan glycosylation similar to CMD
  - Brain dystroglycan depletion: Causes CMD-like brain malformations in cortex & cerebellum

β-Dystroglycan
- Location: Membrane protein; Concentrated with AChR clusters at NMJs
- Binds tightly to WW domain in cysteine-rich region of dystrophin
- Other molecular associations
  - Muscle
    - Grb2
    - Rapsyn at NMJs
    - Caveolin-3: Binding at same domain that interacts with dystrophin
    - ? Indirect association with integrins
  - Other tissues: Utrophin; Dystrophin (Dp116 & Dp71); DRP2
- Target for phosphorylation by protein kinases
  - Tyrosine phosphorylation of the WW binding motifs (PPxY) at C-terminal region of β-dystroglycan
  - Phosphorylation regulates β-Dystroglycan interactions with utrophin
  - May serve regulatory signalling role: Adhesion dependent

HEPARAN SULFATE PROTEOGLYCANS (HSPG)

Integral memebrane

Syndecan-like intercalated proteoglycans (SLIPS)
- Properties
  - Cysteine-free
  - Type I membrane proteins
  - Sequence similarities
    - Ectodomain: Glycanation signal sequences; Junction with transmembrane domain
    - Membrane spanning
    - Cytoplasmic
  - Soluble in Triton X-100
- Proteins
  - Syndecan : Epithelial cells
  - Fibroglycan : Fibroblasts & Endothelial cells
  - N-syndecan (Syndecan-3): : Fetal brain, heart, Schwann cells
  - Ryudocan (Amphiglycan) : Ubiquitous

"Amateur": Exist in glycosylated & non-glycosylated forms

Betaglycan : Auxiliary receptor for TGF-β
CD44E : splice variant of CD44

Membrane linkage by glycosyl phosphatidylinositol (GPI)

Synthesized with carboxy-terminal hydrophobic signal peptide
- Anchors protein to endoplasmic reticulum
Sequence then cleaved on luminal ER surface
New carboxy terminal links to GPI
Proteins
- Glypicans
  - Glypican 1
  - Glypican 2 : Immature neurons
  - Glypican 3 : Lung, liver, Kidney; Simpson-Golabi-Behmel Syndrome
  - Glypican 4
  - Glypican 5 : Brain
- Glypican-related integral membrane HSPG (GRIPS)

Basement membrane

Perlecan (HSPG2) : Schwartz-Jampel Syndrome
Agrin

Disorders

Hereditary
- Proteoglycan-4 : Arthropathy-camptodactyly syndrome
- Xylosylprotein 4-beta-galactosyltransferase, polypeptide 7 (B4GALT7) : Ehlers-Danlos syndrome, Progeroid form
- Nyctalopin : Night blindness, congenital stationary, Type 1
- Carbohydrate sulfotransferase 6 : Macular dystrophy, corneal, 1
- Glypican 3 : Simpson-Golabi-Behmel syndrome, type 1
- Keratocan : Cornea plana 2
Immune
- Heparan sulfate antibodies: Guillain-Barré syndrome

LEUCINE-RICH REPEAT (LRR) PROTEINS

General features

Proteoglycan or glycoprotein in extracellular matrix of cartilage, bone, muscle
Structure: Contain 10 to 38 adjacent LRR regions
- LRR regions: 20 to 29 amino acids with asparagine & leucine in conserved positions
- Flanked by disulphide-bonded domains
- Motifs within the LRR regions for N-linked glycosylation
- Function: Molecular recognition processes
Bind TGF-β, collagens & other matrix molecules
Knockout mice: Abnormal collagen fibrillogenesis

Small leucine rich proteoglycan (SLRP) subfamily

General
- Contain cysteine-rich clusters: Flank LRR; Form disulfide bonds
- Size: Protein core 40 kD
- Most are extracellular: Exception is nyctalopin
- Most are proteoglycans: Exception is asporin
- Location: Often ubiquitous
Class I SLRPs
- General features
  - Secreted with propeptide that may be cleaved by BMP-1
  - Contain 10 to 12 LRRs
  - N-terminal cysteine sequence: CX₃CXCX₆C
  - Encoded by 8 exons
- Decorin ;
  - Contains Chondroitin or Dermatan sulfate chains
  - Location: Skeletal muscle
  - Disorders: Immune Myopathies
    - Myopathy with IgM binding to Decorin
    - Myopathy with deficient Chondroitin sulfate C
- Biglycan ; :
  - Contains Chondroitin or Dermatan sulfate chains
  - Binds to several proteins in skeletal muscle
- Asporin/PLAP
Class II SLRPs
- General features
  - Contain 10 LRRs
  - Polylactosamine or Keratan sulfate chains in LRR region
  - N-terminal cysteine sequence: CX₃CXCX₉C
  - Encoded by 3 exons
- Fibromodulin ; : Contains Keratan sulfate
- Lumican ; :
  - Contains Keratan sulfate (Adult form only glycoprotein)
  - Location: Heart & Skeletal muscle
- Keratocan
- Osteoadherin/Osteomodulin
- Matrix protein PRELP ; : No Polylactosamine or Keratan sulfate chains
Class III SLRPs
- General features
  - Contain 6 LRRs
  - Sulfated tyrosine residues in N-terminal end
  - N-terminal cysteine sequence: CX₂CXCX₆C
  - Encoded by 7 exons
  - Most tissue specific
- Epiphycan
- Mimecan/Osteoglycin
- Opticin/Oculoglycan
Other SLRPs: 11 LRRs; 3 exons
- Chondroadherin (CHAD) ; : Mediates chondrocyte-matrix interactions
- Nyctalopin

Transporters

SLC35A1
- Moves nucleotide sugars ito Golgi
- Transports CMP-sialic acid into Golgi
- Synthesis of functional O-mannose glycan on α-dystroglycan
- disease: Congenital disorder of glycosylation, type IIf
SLC35A3
- Nucleotide sugar transporter
- Transports UDP-N-acetylglucosamine (UDP-GlcNAc) from site of synthesis in cytosol to site of use in Golgi
- Disease: Arthrogryposis, Impaired intellectual development & Seizures (AMRS) :
SLC35D1
- Localization: Endoplasmic reticulum
- Transports both UDP-glucuronic acid (UDP-GlcA) & UDP-N-acetylgalactosamine
- disease: Schneckenbecken dysplasia

Return to Neuromuscular Home Page
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Return to Limb-Girdle Syndromes

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11/27/2025

MEMBRANE-ASSOCIATED PROTEIN COMPLEXES Skeletal Muscle Fibers & Connective Tissue

Extrajunctional muscle membrane: Associated proteins 2

Neuromuscular Junction: Associated proteins 1