We have shown that primary meningeal defects, induced by Foxc1 mutation, cause cortical dysplasia characterized by CBM breakdown, neuronal overmigration, RG detachment, and formation of MZ heterotopias. Extensive meningeal defects are established prior to the onset of dysplasia and meningeal defects are associated with changes in the expression of laminin subunits (namely loss of α1 expression).
The dysplasia in the Foxc1
mutants resembles cobblestone cortex, in which CBM breakdown is thought to initiate a process leading to neuronal overmigration and heterotopia formation14
. Regulation of CBM development and stability is likely to be dynamic and complex. The CBM, like other basement membranes (BMs), consists of thin protein sheets. Major components are laminins, collagen IV, nidogens and perlecan, a heparan sulfate proteoglycan. BMs are formed from two independent networks of laminin and collagen IV, while nidogens and perlecan provide stability. Additional minor components contribute to cross-linking of the networks and interaction with the interstitial matrix. BMs have important roles in providing tissue stability and regulating access to growth factors30, 33
mutants in our study differ significantly from previously reported mutants with cobblestone cortex, which are caused either by mutations in structural CBM components13, 34
, CBM receptors 18, 20, 35
, or downstream signaling molecules19, 21
. Importantly, similar dysplasia is characteristic of dystroglycanopathies, caused by abnormal processing of α-dystroglycan, a molecule needed for interaction of cells with the extracellular matrix. Dystroglycanopathies include Fukuyama congenital muscular dystrophy, muscle-eye-brain disease, and Walker-Warberg syndrome14, 15
, however, is expressed in meningeal fibroblasts and is not a known regulator of CBM structural or signaling molecules, and loss of expression does not affect α-dystroglycan glycosylation. Reduction or loss of Foxc1
likely impairs migration of meningeal cells, which are then not in position to provide a critical function needed for CBM stability and remodeling. In addition to directly impacting CBM development, loss of meningeal cells may remove critical secreted factors, analogous to SDF-1 and retinoic acid, needed to help migrating neurons maintain proper positioning. Interestingly, compensatory mechanisms must exist to partially maintain neuronal positioning for at least some cell types in the face of meningeal loss. We found that Cajal-Retzius cells were properly positioned early in development in the Foxc1
mutants, even though the meninges provide an important chemoattractant for these pioneer neurons4, 7
Extensive meningeal defects are seen in the Foxc1
mutants prior to CBM breakdown and dysplasia formation. Therefore, intact meninges are dispensable for initial formation of apparently intact CBM, and are likely more important in maintenance and stability of the CBM. An attractive idea is that the meninges regulate CBM remodeling, a process which requires controlled degradation and synthesis of extracellular matrix36
, as the cortex grows dramatically during embryonic life. The meninges contribute components to already established CBM37, 38
, suggesting they participate in a dynamic turnover of CBM components, a key feature in the development of epithelial organs39
. The meninges may produce enzymes implicated in BM remodeling, such as membrane-tethered matrix metalloproteinases40
, but little is known about specific proteases expressed by embryonic meninges. The decrease in laminin α1 expression with meningeal loss is intriguing given deficits in BM stability seen with loss of other subunits in experimental models and human disease41
, and may promote CBM instability independent of alterations in remodeling. Both disturbances in CBM remodeling and alteration of CBM composition might contribute to instability and breakdown of the CBM as the underlying brain rapidly expands.
In humans, FOXC1
mutations cause Axenfeld-Rieger syndrome (ARS), a disorder of neural crest migration with characteristic ocular and systemic manifestations42
. Patients with ARS and the 6p25 deletion syndrome, which includes FOXC1
and other loci, can have psychiatric and neurologic symptoms43-47
, although comprehensive brain imaging results are lacking. A recent study found human FOXC1
mutations cause abnormal cerebellar development and meningeal defects48
. This study did not find cobblestone cortex on imaging, but small cortical dysplasia or abnormal organization of the neocortex cannot be excluded.
Most often cortical dysplasia is found sporadically, rather than as a part of a genetic syndrome. Since dysplasia can be acquired in association with prenatal insults, such as cytomegalovirus infection49
, based on our study we can suggest that meningeal injury at a critical point in development might be responsible for some cases of sporadic, localized cortical dysplasia. Future pathologic studies on human brain samples, especially dysplasia resected during epilepsy surgery, will be needed to test this idea.