This is a mechanism module, not a specific disease entry. Disorder entries reference individual nodes via conforms_to (for example, "pial_basement_membrane_radial_glial_endfoot_failure#Pial Basement Membrane Breach"). Use this module for cobblestone-like or bilateral frontoparietal polymicrogyria mechanisms when the shared pathograph is pial basement membrane/radial-glial basal endfoot failure followed by neuronal overmigration. Do not use it for primary microtubule-dependent neuronal migration arrest, apical neuroependyma/periventricular heterotopia, Reelin terminal translocation defects, or broad PI3K-AKT-mTOR cortical overgrowth unless a disease-specific branch independently shows this pial-boundary skeleton.
Pial Boundary-Failure Overmigration Model
pial_boundary_overmigration_model
CANONICAL
Evidence: 2
Evidence balance
2 support
Pathogenic disruption of the pial ECM interface, especially through alpha-dystroglycan O-mannosylation or GPR56-COL3A1 signaling, weakens basement membrane integrity and radial-glial basal endfoot anchoring. The causal readout is not primarily slow or failed neuronal locomotion; it is loss of the cortical pial boundary that allows overmigration and ectopic extracortical/cobblestone-like tissue.
Which aspects of pial basement membrane and radial-glial basal endfoot failure in mouse or other models faithfully map to human cobblestone-like cortical malformations, PMG overlap, epilepsy risk, and regional gyral patterning?
HUMAN MODEL MISMATCH
OPEN
gap_pial_boundary_human_model_translatability
Attached to:
Pial Basement Membrane Breach
Radial-Glial Basal Endfoot Detachment
Neuronal Overmigration Across the Pial Boundary
The review and primary papers support a coherent boundary-failure mechanism, but several translatability gaps remain: FKRP mouse knock-in versus knock-down severity differs from patients, human GPR56 regulation is shaped by gyrencephalic/regional enhancers, PMG mechanisms remain incompletely resolved, and seizures are common in human MCDs while often absent from animal models. This is a human/model mismatch knowledge gap rather than a generic missing-mechanism gap.
Proposed experiments:
Human cortical organoid pial-ECM and radial-glial endfoot perturbation panel
Alpha-Dystroglycan Glycosylation and ECM Ligand Binding Failure
trigger
Pathogenic variants in DAG1 or in dystroglycan O-mannosylation pathway genes reduce functional alpha-dystroglycan glycosylation and impair binding to extracellular matrix ligands such as laminin. Representative upstream genes include POMT1, POMT2, POMGNT1, FKTN, FKRP, LARGE1, CRPPA (the current symbol for ISPD), POMK, B4GAT1, and B3GALNT2.
Downstream
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Pial Basement Membrane Assembly Failure
Show evidence (2 references)
"Here we show, in both MEB and FCMD patients, that alpha-dystroglycan is expressed at the muscle membrane, but similar hypoglycosylation in the diseases directly abolishes binding activity of dystroglycan for the ligands laminin, neurexin and agrin."
Human disease tissue evidence supports alpha-dystroglycan hypoglycosylation as a biochemical lesion that abolishes ECM-ligand binding.
"Immunohistochemical analysis of muscle from patients with POMT1 mutations corroborated the O-mannosylation defect, as judged by the absence of glycosylation of alpha-dystroglycan."
Links a dystroglycan O-mannosylation gene to loss of alpha-dystroglycan glycosylation in a human Walker-Warburg syndrome context.
GPR56-COL3A1 Pial ECM Signaling Failure
trigger
Pathogenic ADGRG1/GPR56 or COL3A1 variants impair a pial ECM receptor-ligand axis required for regional cortical development, pial basement membrane integrity, and inhibition of inappropriate neuronal migration beyond the pial surface.
Downstream
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Pial Basement Membrane Breach
Show evidence (3 references)
"Here we identify collagen, type III, alpha-1 (gene symbol Col3a1) as the ligand of GPR56 through an in vitro biotinylation/proteomics approach."
Identifies collagen III/COL3A1 as the ligand for GPR56, grounding this receptor-ligand branch.
"Functional studies suggest that the interaction of collagen III with its receptor GPR56 inhibits neural migration in vitro."
Supports the functional interpretation that the GPR56-COL3A1 interaction constrains migration at the pial boundary.
"Type III procollagen, the chains of which are encoded by COL3A1, is the ligand of the G protein-coupled receptor 56 (GPR56), also known as adhesion G protein-coupled receptor G1."
Human-genetics paper independently anchors COL3A1 as the GPR56 ligand in the clinical malformation context.
Pial Basement Membrane Assembly Failure
central effector
Reduced dystroglycan-laminin binding weakens assembly and maintenance of basement membrane components at the pial surface. This node captures the ECM-assembly branch that feeds into pial basal lamina discontinuity.
Downstream
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Pial Basement Membrane Breach
Show evidence (2 references)
"We find that dystroglycan is required for the formation of a basement membrane in embryoid bodies."
Establishes dystroglycan as required for basement membrane formation in a controlled cell-derived model system.
"Our results further indicate that dystroglycanlaminin interactions are prerequisite for the deposition of other basement membrane proteins."
Supports the specific ligand-binding-to-basement-membrane assembly link used in the dystroglycan branch.
Pial Basement Membrane Breach
central effector
Failure of basement membrane assembly or GPR56-COL3A1 signaling produces discontinuities and breaches in the pial basal lamina/glia limitans. This is the central boundary defect that permits downstream overmigration.
Downstream
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Radial-Glial Basal Endfoot Detachment
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Cajal-Retzius Cell Mislocalization
Show evidence (2 references)
"Dystroglycan-null brain loses its high-affinity binding to the extracellular matrix protein laminin, and shows discontinuities in the pial surface basal lamina (glia limitans) that probably underlie the neuronal migration errors."
Directly links dystroglycan loss to pial basal lamina discontinuities and downstream neuronal positioning errors.
"By detailed time course analysis, we reveal that the leading causal events are likely the breaches in the pial BM."
Places pial basement membrane breach early in the causal order.
Cajal-Retzius Cell Mislocalization
amplifier
Boundary failure disrupts marginal-zone organization and mislocalizes Cajal-Retzius cells, which can amplify cortical disorganization and severity across dystroglycanopathy models.
Downstream
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Neuronal Overmigration Across the Pial Boundary
Show evidence (1 reference)
"Furthermore, the mislocalization of Cajal-Retzius cells is correlated with the gradient of these lesions and the severity of the brain phenotype in these models."
Supports Cajal-Retzius mislocalization as a severity-linked amplifier in dystroglycanopathy pial-boundary lesions.
Neuronal Overmigration Across the Pial Boundary
effector
With the pial boundary breached and radial-glial endfeet detached, neurons can migrate beyond the cortical surface rather than stopping within the cortical plate/marginal zone. This overmigration is the key mechanistic output that distinguishes this module from intrinsic neuronal migration arrest modules.
Downstream
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Cobblestone-Like Cortical Malformation and PMG Overlap
Show evidence (2 references)
"Mutations in the GPR56 gene cause a malformed cerebral cortex in both humans and mice that resembles cobblestone lissencephaly, which is characterized by overmigration of neurons beyond the pial basement membrane."
Directly states the overmigration phenotype and links it to the GPR56-associated cortical malformation.
"The foetal case, which was terminated at 35 weeks of gestation in view of suspicion of Walker Warburg syndrome, showed a cobblestone-like lissencephaly with a succession of normal, polymicrogyric and 'cobblestone-like' cortex with ectopic neuronal overmigration, agenesis of the cerebellar vermis..."
Human fetopathology supports ectopic neuronal overmigration and the overlap between polymicrogyric and cobblestone-like cortex.