FAK deficiency in cells contributing to the basal lamina results in cortical abnormalities resembling congenital muscular dystrophies

Abstract

Targeted deletion of focal adhesion kinase (fak) in the developing dorsal forebrain resulted in local disruptions of the cortical basement membrane located between the neuroepithelium and pia-meninges. At disruption sites, clusters of neurons invaded the marginal zone. Retraction of radial glial endfeet, midline fusion of brain hemispheres, and gliosis also occurred, similar to type II cobblestone lissencephaly as seen in congenital muscular dystrophy. Interestingly, targeted deletion of fak in neurons alone did not result in cortical ectopias, indicating that fak deletion from glia is required for neuronal mislocalization. Unexpectedly, fak deletion specifically from meningeal fibroblasts elicited similar cortical ectopias in vivo and altered laminin organization in vitro. These observations provide compelling evidence that FAK plays a key signaling role in cortical basement membrane assembly and/or remodeling. In addition, FAK is required within neurons during development because neuron-specific fak deletion alters dendritic morphology in the absence of lamination defects.

Figure 1. Emx1^IREScre-Induced Deletion of fak Results in Cortical Abnormalities

Coronal sections (40 µm) from adult control heterozygote (A, D, and G) and emx1^IREScre/fak mutant (B, C, E, F, H, and I) forebrain stained with Nissl. In the more anterior sections (A and B), the neuronal dysplasia was less severe and was manifested in small ectopic clusters of neurons migrating into the marginal zone (MZ) at the midline, shown in higher magnification in (C). In the more posterior sections (D and E), the neuronal dysplasia became more severe, with prominent streams of neurons invading the marginal zone and disrupting the lamination of the cortex (arrows, [E and F]). Marginal zone material accumulated in relatively cell-sparse nodules adjacent to ectopias (asterisk, [E and F]). Aberrant fiber bundles (arrow, [H and I]) inappropriately coursed through some cortical regions near the midline, and caudal sections showed partial agenesis of the corpus callosum (asterisk, [H]). The dentate gyrus showed similar ectopic outgrowths of granule cells toward the presumptive pial surface (thin arrows, [H]). I-VI, cortical layers; cc, corpus callosum; gcl, granule cell layer; HET, heterozygote; MUT, mutant; MZ, marginal zone; vhc, ventral hippocampal commissure. Scale bar, 250 µm (H) and 25 µm (I).

Figure 2. Cellular Composition of fak^−/− Cortical Ectopias

Coronal sections (40 µm) from P14 heterozygote and fak^−/− mice. (A–D) Panneuronal NeuN mAb immunostaining revealed that the ectopias were composed in part by neurons inappropriately migrating through the marginal zone. (E and F) Deep layer V cortical neurons that are normally localized to a discrete interior stripe of cells were mislocalized in the fak mutant and streamed to the pial/glial surface as demonstrated by anti-ER81 labeling. (G and H) Calbindin-positive interneurons that are normally scattered throughout the cortex as well as prominently localized beneath the marginal zone strikingly encircled areas of dysplastic cortex. (I–L) Numerous GFAP-positive astrocytes characterized areas of fak^−/− ectopia, while the control showed normal glial staining within the cortical basement membrane as well as scattered throughout the cortex (I and J). Thinner paraffin sections (7 µm) of the midline revealed the astrocytic contribution to the pial surface in the control more clearly (arrows, [K]). In the mutant, the pial/meningeal surface had clear interruptions (arrows, [L]), and the astrocytes displayed abnormally beaded processes (arrowheads, [L]). Scale bar, 200 µm (B) and 50 µm (D).

Figure 3. Altered Dendritic Orientation and Branching of fak^−/− Neurons

(A–D) MAP-2-positive dendrites of cortical neurons in control brain had a parallel, striped orientation (arrows, [A and C]), whereas, in the fak^−/− cortex, they were frequently abnormally oriented (arrows, [B]) or were drawn into a tangled nodule in areas of cortical dysplasia (arrows, [D]). (E–H) Golgi impregnation of control (E and G) and fak mutant cortex further revealed lamination defects (asterisk, [F]) but additionally uncovered defects in dendritic branching. Normally, pyramidal neurons extend a single apical dendrite perpendicular to the pial surface. fak^−/− cortical neurons displayed a highly branched dendritic morphology. At higher magnification of the boxed areas, the increased branching of fak^−/− pyramidal neurons was clearly evident (arrows, [H]), even in areas of less-disturbed dysplasia. (I) Dendritic spines from cortical neurons of control and fak^−/− mice were relatively normal. Scale bar, 200 µm (B and F), 50 µm (D), 100 µm (H), and 20 µm (I).

Figure 4. Role of Cajal Retzius Neurons, Reelin, and Basement Membrane Constituents in fak^−/− Mice and Developmental Onset of Cortical Abnormalities

(A–F) Paraffin sections (7 µm) from P1 control and FAK mutant animals counterstained with Nissl. (A and B) Calretinin staining labeled singular Cajal Retzius (CR) neurons localized to the marginal zone proximal to the pial/meningeal membrane (A and B). This localization was largely maintained in fak^−/− mutants, except in areas of neuronal ectopia where CR neurons were either present at the peak of ectopias or accumulated within the marginal zone contents to either side. (C and D) Reelin was localized normally to the marginal zone in both control and mutant animals. However, ectopic fak^−/− neurons migrated through deposited reelin and did not halt migration at the border of the marginal zone (D). (E and F) Laminin staining with an anti-EHS laminin antibody showed continuous, uninterrupted staining along the cortical basement membrane in control animals (E). In the fak mutants, laminin was degraded over and within areas of ectopia and showed a punctate, fragmented staining suggestive of active degradation. Developmental onset of fak^−/− cortical abnormalities noted in 40 µm frozen sections from E14 embryos (G and H) or paraffin (7 µm) sections of E15 embryos (K–N). (G and H) The earliest time point when ectopias were found was in E14 embryos, stained with an anti-laminin α1 subunit. At this early stage, arrows mark the initiation of ectopic outgrowths. These areas coincided with thinned regions of the basement membrane that upon higher magnification showed punctate staining characteristic of laminin degradation. (K and L) Nissl staining revealed ectopias invading the marginal zone of fak^−/− cortex at E15 (arrow, [B]). (M and N) The preplate split properly into the cortical plate and marginal zone in the fak^−/− mutant cortex as shown by chondroitin sulfate immunoreactivity (CSPG). (I and J) Electron micrographs of the cortex and pial/meningeal interface in adult control and fak^−/− brains. Arrows indicate the first layer of the cortical basement membrane, and bracket indicates overall thickness of membranes underneath the skull. The fak^−/− basement membrane had allowed cortical material to break through (asterisk, [J]) and was considerably thicker. ch, choroid plexus; cp, cortical plate; iz, intermediate zone; lv, lateral ventricle; mz, marginal zone; p, pial membrane; sk, skull; sp, subplate; vz, ventricular zone. Scale bar, 50 µm (A–H), 2.5 µm (J), and 100 µm (K–N).

Figure 5. Defective Radial Glia in fak^−/− Cortex

Fluorescent micrographs of frozen sections (20 µm) from E16 heterozygous (A, B, and C) and fak mutant (D, E, and F) cortex stained with RC2 antibodies to label radial glia processes and DAPI to visualize individual cell nuclei. The tufted endfeet of radial glial cells were prominently stained at this stage, where they contribute to the cortical basement membrane (A). Cortical ectopias within the marginal zone of the fak mutant are marked by asterisks (D and E) and coincided with interrupted RC2 staining due to retracted and disorganized radial glial processes. Occasionally, radial glial processes were seen extending through the ectopic clusters. mz, marginal zone. Scale bar, 100 µm.

Figure 6. fak Deletion in Neurons Is Not Sufficient to Induce Cortical Dysplasia but Does Induce Altered Dendritic Morphology

Coronal sections (40 µm) from adult control and nex-Cre/fak mutant forebrain stained with Nissl are indistinguishable at the gross anatomical level (A–D). The marginal zone was intact with no ectopic clusters of cells, and the hippocampus exhibited a normal morphology. However, Golgi staining of coronal sections (100 µm) did reveal abnormalities in dendritic branching in nex-Cre/fak mutants. Pyramidal neurons from layer III and V have a prominent apical dendrite extending perpendicular to the brain surface in control cells (arrow, [G]) that is notably perturbed in the fak^−/− mutant (arrow, [H]). Scale bar, 500 µm (B) and 50 µm (D and F).

Figure 7. Adenoviral Recombination of Meningeal Fibroblasts Is Sufficient to Induce Cortical Dysplasia

(A and B) Paraffin sections (7 µm) from E18.5 embryo heterozygous (+/flox, [A]) or homozygous (flox/flox, [B]) cortex following in utero injection of an adenovirus bearing Cre recombinase and GFP at E12.5/13 and stained with DAPI (blue) and anti-GFP (green). Cortical ectopias were present in the marginal zone of the flox/flox animal and coincided with interrupted laminin staining of the cortical basement membrane (inset, [B]). (E–H) Laminin staining of primary meningeal fibroblasts isolated from E15 (flox/flox) and infected with either an adenovirus expressing GFP alone (E and G) or GFP plus Cre recombinase (F and H). fak^−/− fibroblasts were unable to organize laminin into fibrillar structures as in control. Scale bar, 50 µm (B) and 25 µm (A–D).

Figure 8. P130CAS Is Altered, While Src, Pyk2, and Dystroglycan Are Unchanged in fak^−/− Cortical Extracts

Cortical extracts from E16 and P1 wild-type and mutant brains were immunoprecipitated or probed with antibodies to p130CAS (A and C), phospho-Src (B and D), Pyk2 (E), or β-dystroglycan (F) and probed with anti-phosphotyrosine (Ptyr) or their corresponding antibodies. In lanes marked Western, 20 µg of cell extracts were directly fractionated and probed with antibodies to p130 CAS, Src, Pyk2, or β- and α-dystroglycan to compare total cellular expression levels. p130CAS phosphorylation was significantly decreased in the FAK mutant brain at both E16 and P1, whereas its protein expression levels were unchanged (A). The level and phosphorylation of the non-receptor tyrosine kinase Src or of FAK family member Pyk2 was unchanged (B and C). Similarly, the level and phosphorylation of β-dystroglycan was unchanged in the mutant, as was the expression level of α-dystroglycan (D).

Figure 9. Model

FAK is involved in bidirectional signaling from the ECM to the cytoskeleton, regulating laminin polymerization and organization of the cortical basement membrane. Integrin binding to laminin activates FAK and associated molecules, such as p130CAS, that in turn regulate the actin cytoskeleton and feed back on receptor/matrix organization within the basal lamina. Other signaling pathways are involved in basement membrane organization, such as the dystroglycan complex, which is thought to initially bind to laminin and then further recruit integrins into a large macromolecular signaling complex. Modulation of α-dystroglycan glycosylation by glycosyl-transferases (POMGnT1, fukutin) affects its ability to bind laminin. Genetic deletion of fak or other components of this signaling complex in the brain results in local basement membrane disruption and cortical dysplasia as seen in cobblestone lissencephaly.