Fluctuations in central foveal thickness and association with vision outcomes with anti-VEGF therapy for nAMD: HARBOR post hoc analysis.

Objective: To evaluate correlations between variability in central foveal thickness (CFT) and vision with ranibizumab in a HARBOR post hoc analysis. Methods and analysis: Patients with neovascular age-related macular degeneration (nAMD; N=1097) received monthly or as-needed (PRN) ranibizumab (0.5 or 2.0 mg) for 24 months. Fluctuation scores were used to assess CFT variability; every time CFT increased and then decreased (or vice versa), numeric value of the change was added to the score. Magnitude of change <50 µm was considered clinically insignificant and did not count towards the score. Fluctuation scores were grouped into quartiles. Least squares mean (LSM) changes in best-corrected visual acuity (BCVA) were plotted against fluctuation score quartiles for CFT, subretinal fluid (SRF) height, neurosensory retina and neurosensory retina + subretinal hyper-reflective material. Results: Patients with lower fluctuations scores (quartiles 1-3) had greatest vision gains at month 24, with LSM changes from baseline of 9.0-10.8 and 8.7-10.6 letters in the monthly and PRN arms, respectively. Corresponding changes for quartile 4 were 6.7 and 6.5 letters, respectively. There were no differences between quartiles for association between fluctuations in SRF height and BCVA gains. There were inverse correlations between magnitude of fluctuations in neurosensory and inner retina thickness and BCVA gains for quartile 4 vs quartiles 1-3. Patients in quartiles 1 and 2 showed rapid, robust BCVA gains, whereas those in quartiles 3 and 4 had lesser responses. Conclusions: Fluctuations in retinal thickening with ranibizumab may be associated with treatment response in patients with nAMD. Trial registration number: NCT00891735.

GPR56/ADGRG1 regulates development and maintenance of peripheral myelin.

Myelin is a multilamellar sheath generated by specialized glia called Schwann cells (SCs) in the peripheral nervous system (PNS), which serves to protect and insulate axons for rapid neuronal signaling. In zebrafish and rodent models, we identify GPR56/ADGRG1 as a conserved regulator of PNS development and health. We demonstrate that, during SC development, GPR56-dependent RhoA signaling promotes timely radial sorting of axons. In the mature PNS, GPR56 is localized to distinct SC cytoplasmic domains, is required to establish proper myelin thickness, and facilitates organization of the myelin sheath. Furthermore, we define plectin-a scaffolding protein previously linked to SC domain organization, myelin maintenance, and a series of disorders termed "plectinopathies"-as a novel interacting partner of GPR56. Finally, we show that Gpr56 mutants develop progressive neuropathy-like symptoms, suggesting an underlying mechanism for peripheral defects in some human patients with GPR56 mutations. In sum, we define Gpr56 as a new regulator in the development and maintenance of peripheral myelin.

G Protein-Coupled Receptors in Myelinating Glia.

The G protein-coupled receptor (GPCR) superfamily represents the largest class of functionally selective drug targets for disease modulation and therapy. GPCRs have been studied in great detail in central nervous system (CNS) neurons, but these important molecules have been relatively understudied in glia. In recent years, however, exciting new roles for GPCRs in glial cell biology have emerged. We focus here on the key roles of GPCRs in a specialized subset of glia, myelinating glia. We highlight recent work firmly establishing GPCRs as regulators of myelinating glial cell development and myelin repair. These advances expand our understanding of myelinating glial cell biology and underscore the utility of targeting GPCRs to promote myelin repair in human disease.

Type I bHLH Proteins Daughterless and Tcf4 Restrict Neurite Branching and Synapse Formation by Repressing Neurexin in Postmitotic Neurons.

Proneural proteins of the class I/II basic-helix-loop-helix (bHLH) family are highly conserved transcription factors. Class I bHLH proteins are expressed in a broad number of tissues during development, whereas class II bHLH protein expression is more tissue restricted. Our understanding of the function of class I/II bHLH transcription factors in both invertebrate and vertebrate neurobiology is largely focused on their function as regulators of neurogenesis. Here, we show that the class I bHLH proteins Daughterless and Tcf4 are expressed in postmitotic neurons in Drosophila melanogaster and mice, respectively, where they function to restrict neurite branching and synapse formation. Our data indicate that Daughterless performs this function in part by restricting the expression of the cell adhesion molecule Neurexin. This suggests a role for these proteins outside of their established roles in neurogenesis.

Vertebrate Fidgetin Restrains Axonal Growth by Severing Labile Domains of Microtubules.

Individual microtubules (MTs) in the axon consist of a stable domain that is highly acetylated and a labile domain that is not. Traditional MT-severing proteins preferentially cut the MT in the stable domain. In Drosophila, fidgetin behaves in this fashion, with targeted knockdown resulting in neurons with a higher fraction of acetylated (stable) MT mass in their axons. Conversely, in a fidgetin knockout mouse, the fraction of MT mass that is acetylated is lower than in the control animal. When fidgetin is depleted from cultured rodent neurons, there is a 62% increase in axonal MT mass, all of which is labile. Concomitantly, there are more minor processes and a longer axon. Together with experimental data showing that vertebrate fidgetin targets unacetylated tubulin, these results indicate that vertebrate fidgetin (unlike its fly ortholog) regulates neuronal development by tamping back the expansion of the labile domains of MTs.

Pathogenic mutation of spastin has gain-of-function effects on microtubule dynamics.

Mutations to the SPG4 gene encoding the microtubule-severing protein spastin are the most common cause of hereditary spastic paraplegia. Haploinsufficiency, the prevalent model for the disease, cannot readily explain many of its key aspects, such as its adult onset or its specificity for the corticospinal tracts. Treatment strategies based solely on haploinsufficiency are therefore likely to fail. Toward developing effective therapies, here we investigated potential gain-of-function effects of mutant spastins. The full-length human spastin isoform called M1 or a slightly shorter isoform called M87, both carrying the same pathogenic mutation C448Y, were expressed in three model systems: primary rat cortical neurons, fibroblasts, and transgenic Drosophila. Although both isoforms had ill effects on motor function in transgenic flies and decreased neurite outgrowth from primary cortical neurons, mutant M1 was notably more toxic than mutant M87. The observed phenotypes did not result from dominant-negative effects of mutated spastins. Studies in cultured cells revealed that microtubules can be heavily decorated by mutant M1 but not mutant M87. Microtubule-bound mutant M1 decreased microtubule dynamics, whereas unbound M1 or M87 mutant spastins increased microtubule dynamics. The alterations in microtubule dynamics observed in the presence of mutated spastins are not consistent with haploinsufficiency and are better explained by a gain-of-function mechanism. Our results fortify a model wherein toxicity of mutant spastin proteins, especially mutant M1, contributes to axonal degeneration in the corticospinal tracts. Furthermore, our results provide details on the mechanism of the toxicity that may chart a course toward more effective treatment regimens.

MAP kinase phosphorylation is dispensable for cell division, but required for cell growth in Drosophila.

Proper activation of the Ras/MAPK pathway is broadly required during development, and in many cases, signal transduction downstream of the receptor is linear. Thus, different mechanisms exist to properly regulate the large number of specific developmental outputs that are required by the activation of this pathway. Previously, we have reported a regulated cytoplasmic sequestration of phosphorylated MAPK (pMAPK) in developing Drosophila compound eyes and wings "called MAPK Cytoplasmic Hold". In the developing wing, we have shown that cytoplasmic hold promotes the differentiation of wing vein tissue, while pMAPK nuclear translocation regulates growth and division. We had also suggested that the Ras pathway signals for inducing cell growth and cell division split upstream of the nuclear translocation of MAPK itself. Here, we further refine the role of MAPK in Drosophila. We report evidence that suggests, for the first time, that the phosphorylation of MAPK is itself another step in the regulation of cell growth and division in both Drosophila wing and eye cells. We show that inhibition of MAPK phosphorylation, or pMAPK nuclear translocation, is sufficient to block cell growth, but not cell division. These data suggest that non-phosphorylated MAPK is sufficient to induce cell division, but not cell growth, once inside the nucleus of the cell.