Pten regulates endocytic trafficking of cell adhesion and Wnt signaling molecules to pattern the retina.

The retina is exquisitely patterned, with neuronal somata positioned at regular intervals to completely sample the visual field. Here, we show that phosphatase and tensin homolog (Pten) controls starburst amacrine cell spacing by modulating vesicular trafficking of cell adhesion molecules and Wnt proteins. Single-cell transcriptomics and double-mutant analyses revealed that Pten and Down syndrome cell adhesion molecule Dscam) are co-expressed and function additively to pattern starburst amacrine cell mosaics. Mechanistically, Pten loss accelerates the endocytic trafficking of DSCAM, FAT3, and MEGF10 off the cell membrane and into endocytic vesicles in amacrine cells. Accordingly, the vesicular proteome, a molecular signature of the cell of origin, is enriched in exocytosis, vesicle-mediated transport, and receptor internalization proteins in Pten conditional knockout (PtencKO) retinas. Wnt signaling molecules are also enriched in PtencKO retinal vesicles, and the genetic or pharmacological disruption of Wnt signaling phenocopies amacrine cell patterning defects. Pten thus controls vesicular trafficking of cell adhesion and signaling molecules to establish retinal amacrine cell mosaics.

Mutations in BCOR, a co-repressor of CRX/OTX2, are associated with early-onset retinal degeneration.

Many transcription factors regulating the production, survival, and function of photoreceptor cells have been identified, but little is known about transcriptional co-regulators in retinal health and disease. Here, we show that BCL6 co-repressor (BCOR), a Polycomb repressive complex 1 factor mutated in various cancers, is involved in photoreceptor degenerative diseases. Using proteomics and transcription assays, we report that BCOR interacts with the transcription factors CRX and OTX2 and reduces their ability to activate the promoters of photoreceptor-specific genes. CUT&RUN sequencing further shows that BCOR shares genome-wide binding profiles with CRX/OTX2, consistent with a general co-repression activity. We also identify missense mutations in human BCOR in five families that have no evidence of cancer but present severe early-onset X-linked retinal degeneration. Last, we show that the human BCOR mutants cause degeneration when expressed in the mouse retina and have enhanced repressive activity on OTX2. These results uncover a role for BCOR in photoreceptors in both health and disease.

Hsc70 chaperone activity underlies Trio GEF function in axon growth and guidance induced by netrin-1.

During development, netrin-1 is both an attractive and repulsive axon guidance cue and mediates its attractive function through the receptor Deleted in Colorectal Cancer (DCC). The activation of Rho guanosine triphosphatases within the extending growth cone facilitates the dynamic reorganization of the cytoskeleton required to drive axon extension. The Rac1 guanine nucleotide exchange factor (GEF) Trio is essential for netrin-1-induced axon outgrowth and guidance. Here, we identify the molecular chaperone heat shock cognate protein 70 (Hsc70) as a novel Trio regulator. Hsc70 dynamically associated with the N-terminal region and Rac1 GEF domain of Trio. Whereas Hsc70 expression supported Trio-dependent Rac1 activation, adenosine triphosphatase-deficient Hsc70 (D10N) abrogated Trio Rac1 GEF activity and netrin-1-induced Rac1 activation. Hsc70 was required for netrin-1-mediated axon growth and attraction in vitro, whereas Hsc70 activity supported callosal projections and radial neuronal migration in the embryonic neocortex. These findings demonstrate that Hsc70 chaperone activity is required for Rac1 activation by Trio and this function underlies netrin-1/DCC-dependent axon outgrowth and guidance.

RAS/ERK signaling controls proneural genetic programs in cortical development and gliomagenesis.

Neural cell fate specification is well understood in the embryonic cerebral cortex, where the proneural genes Neurog2 and Ascl1 are key cell fate determinants. What is less well understood is how cellular diversity is generated in brain tumors. Gliomas and glioneuronal tumors, which are often localized in the cerebrum, are both characterized by a neoplastic glial component, but glioneuronal tumors also have an intermixed neuronal component. A core abnormality in both tumor groups is overactive RAS/ERK signaling, a pro-proliferative signal whose contributions to cell differentiation in oncogenesis are largely unexplored. We found that RAS/ERK activation levels differ in two distinct human tumors associated with constitutively active BRAF. Pilocytic astrocytomas, which contain abnormal glial cells, have higher ERK activation levels than gangliogliomas, which contain abnormal neuronal and glial cells. Using in vivo gain of function and loss of function in the mouse embryonic neocortex, we found that RAS/ERK signals control a proneural genetic switch, inhibiting Neurog2 expression while inducing Ascl1, a competing lineage determinant. Furthermore, we found that RAS/ERK levels control Ascl1's fate specification properties in murine cortical progenitors--at higher RAS/ERK levels, Ascl1(+) progenitors are biased toward proliferative glial programs, initiating astrocytomas, while at moderate RAS/ERK levels, Ascl1 promotes GABAergic neuronal and less glial differentiation, generating glioneuronal tumors. Mechanistically, Ascl1 is phosphorylated by ERK, and ERK phosphoacceptor sites are necessary for Ascl1's GABAergic neuronal and gliogenic potential. RAS/ERK signaling thus acts as a rheostat to influence neural cell fate selection in both normal cortical development and gliomagenesis, controlling Neurog2-Ascl1 expression and Ascl1 function.

Neural stem cell self-renewal requires the Mrj co-chaperone.

The Mrj co-chaperone is expressed throughout the mouse conceptus, yet its requirement for placental development has prohibited a full understanding of its embryonic function. Here, we show that Mrj(-/-) embryos exhibit neural tube defects independent of the placenta phenotype, including exencephaly and thin-walled neural tubes. Molecular analyses revealed fewer proliferating cells and a down-regulation of early neural progenitor (Pax6, Olig2, Hes5) and neuronal (Nscl2, SCG10) cell markers in Mrj(-/-) neuroepithelial cells. Furthermore, Mrj(-/-) neurospheres are significantly smaller and form fewer secondary neurospheres indicating that Mrj is necessary for self-renewal of neural stem cells. However, the molecular function of Mrj in this context remains elusive because Mrj does not colocalize with Bmi-1, a self-renewal protein. Furthermore, unlike in Mrj(-/-) placentas, intermediate filament-containing aggregates do not accumulate in Mrj(-/-) neuroepithelium, ruling out nestin as a substrate for Mrj. Regardless, Mrj plays an important role in neural stem cell self-renewal.

The N-methyl-D-aspartate receptor splice variant NR1-4 C-terminal domain. Deletion analysis and role in subcellular distribution.

The intracellular C-terminal domain of the N-methyl-d-aspartate receptor (NMDAR) subunits 1 (NR1) and 2 (NR2) are important, if not essential, to the process of NMDAR clustering and anchoring at the plasma membrane and the synapse. Eight NR1 splice variants exist, four of which arise from alternative splicing of the C-terminal exon cassettes. Alternative splice variants may display a differential ability to interact with synaptic anchoring proteins, and splicing of C-terminal exon cassettes may alter the mechanism(s) of subcellular localization and targeting. The NR1-4 isoform has a significantly different C-terminal composition than the prototypic NR1-1 isoform. Whereas the NR1-1 C terminus is composed of C0, C1, and C2 exon cassettes, the NR1-4 C terminus is composed of the C0 and C2' cassettes. In the present study, we address the importance of the NR1-4 C-terminal exon cassettes (C0C2') in subcellular localization in differentiated pheochromocytoma (PC12) cells, in organotypic cultures of dorsal root ganglia, and also in heterologous cells. NR1-4-green fluorescent protein chimeras were created with deletion of either C0, C2', or both cassettes to address their importance in subcellular distribution and cell surface expression of the NR1-4 subunit. These experiments demonstrate that the NR1-4 splice variant found predominantly in the spinal cord uses the C0 cassette, to a large degree, to organize the subcellular distribution of this receptor subunit. Although the role of the C2' subunit is less clear, it may be involved in subunit clustering. However, this clustering is not always as efficient as that attributed to C0 alone or to the natural combination of C0C2'. Finally, although an intact C-terminal domain is neither necessary for interaction with the NR2A subunit nor surface expression of the NR1-4 subunit, the C-terminal domain fragment alone blocks surface expression of native NR1-4, in a dominant negative fashion, when the two are coexpressed.