Projection of the retinal ganglion cells to the tectum differentiated from the prosencephalon

Nanoparticles-mediated CRISPR-Cas9 gene therapy in inherited retinal diseases: applications, challenges, and emerging opportunities

Inherited Retinal Diseases (IRDs) are considered one of the leading causes of blindness worldwide. However, the majority of them still lack a safe and effective treatment due to their complexity and genetic heterogeneity. Recently, gene therapy is gaining importance as an efficient strategy to address IRDs which were previously considered incurable. The development of the clustered regularly-interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9) system has strongly empowered the field of gene therapy. However, successful gene modifications rely on the efficient delivery of CRISPR-Cas9 components into the complex three-dimensional (3D) architecture of the human retinal tissue. Intriguing findings in the field of nanoparticles (NPs) meet all the criteria required for CRISPR-Cas9 delivery and have made a great contribution toward its therapeutic applications. In addition, exploiting induced pluripotent stem cell (iPSC) technology and in vitro 3D retinal organoids paved the way for prospective clinical trials of the CRISPR-Cas9 system in treating IRDs. This review highlights important advances in NP-based gene therapy, the CRISPR-Cas9 system, and iPSC-derived retinal organoids with a focus on IRDs. Collectively, these studies establish a multidisciplinary approach by integrating nanomedicine and stem cell technologies and demonstrate the utility of retina organoids in developing effective therapies for IRDs.

An Update on the Molecular Mechanism of the Vertebrate Isthmic Organizer Development in the Context of the Neuromeric Model

A crucial event during the development of the central nervous system (CNS) is the early subdivision of the neural tube along its anterior-to-posterior axis to form neuromeres, morphogenetic units separated by transversal constrictions and programed for particular genetic cascades. The narrower portions observed in the developing neural tube are responsible for relevant cellular and molecular processes, such as clonal restrictions, expression of specific regulatory genes, and differential fate specification, as well as inductive activities. In this developmental context, the gradual formation of the midbrain-hindbrain (MH) constriction has been an excellent model to study the specification of two major subdivisions of the CNS containing the mesencephalic and isthmo-cerebellar primordia. This MH boundary is coincident with the common Otx2-(midbrain)/Gbx2-(hindbrain) expressing border. The early interactions between these two pre-specified areas confer positional identities and induce the generation of specific diffusible morphogenes at this interface, in particular FGF8 and WNT1. These signaling pathways are responsible for the gradual histogenetic specifications and cellular identity acquisitions with in the MH domain. This review is focused on the cellular and molecular mechanisms involved in the specification of the midbrain/hindbrain territory and the formation of the isthmic organizer. Emphasis will be placed on the chick/quail chimeric experiments leading to the acquisition of the first fate mapping and experimental data to, in this way, better understand pioneering morphological studies and innovative gain/loss-of-function analysis.

Isthmus organizer for midbrain and hindbrain development

Classical transplantation studies showed that the isthmus has an organizing activity upon the tectum and cerebellum. Since Fgf8 is expressed in the isthmus and mimics functionally isthmic grafts, it is accepted that Fgf8 plays pivotal role in the isthmic organizing activity. The fate of brain vesicles is determined by the combinations of transcription factors. The neural tube region where Otx2, Pax2, and En1 are expressed early on acquires midbrain identity. Pax3/7 forces the midbrain to differentiate into tectum. En1/2, Pax2/5, and Fgf8 form a positive feedback loop for their expression, thus misexpression of one of these molecules turns on the loop and forces presumptive diencephalon to differentiate into tectum. The isthmic organizer signal, Fgf8, stabilizes or changes the expression of the transcription factors in mid/hindbrain region. A strong Fgf8 signal activates the Ras-ERK signaling pathway, which in turn activates Irx2 in a rostrodorsal part of the hindbrain, and forces this tissue to differentiate into cerebellum.

Polarity and laminar formation of the optic tectum in relation to retinal projection

The mes-metencephalic boundary (isthmus) works as an organizer for the tectum, and the organizing molecule may be Fgf8. The region where Otx2, En1, and Pax2 are expressed overlappingly may differentiate into the mesencephalon. The di-mesencephalic and mes-metencephalic boundaries are determined by repressive interaction of Pax6 and En1/Pax2 and of Otx2 and Gbx2, respectively. The optic tectum is a visual center in lower vertebrates. The tectum and the retina should be regionalized and be positionally specialized for the proper retinotopic projection. Gradient of En2 plays a crucial role in rostrocaudal polarity formation of the tectum. En2 confers caudal characteristics of the retina by inducing ephrinA2 and A5, which are the repellant molecules for the growth cones of temporal retinal ganglion cells. Grg4 antagonizes the isthmus-related genes, and is involved in the formation of di-mesencephalic boundary and tectal polarity formation at an early phase of development. Then, Grg4 plays a role in tectal laminar formation by controlling the migration pathway. Migration pathway of tectal postmitotic cells changes after E5. The late migratory cells split the early migratory neurons to form laminae h-j of SGFS. Grg4 is expressed in the ventricular layer after E5, and forces postmitotic cells to follow the late migratory pathway, though retinal fibers terminate at laminae a-f of SGFS. Misexpression of Grg4 disrupts the lamina g, and in such tecta retinal arbors invade deep into the tectal layer, indicating that lamina g is a nonpermissive lamina for the retinal arbors.

Regionalisation and acquisition of polarity in the optic tectum

The optic tectum differentiates from the alar plate of the mesencephalon and receives retinal fibres in a precise retinotopic manner. Here, mechanisms of tectum polarisation and regionalisation are reviewed. Misexpression of Pax2, Pax5 or En can change the fate of the presumptive diencephalon to that of the tectum. Ephrin A2 and A5 are expressed in a gradient in the tectum, caudal high and rostral low, and may play important roles in the formation of a precise retinotectal projection map. Retinal fibres that express receptors for these ligands, and which come from the temporal retina, are repulsed by the ligands and do not invade the caudal tectum. Both En1 and En2 can regulate posterior characteristics in the tectum by inducing ephrin A2 and A5. Transplantation experiments in chick have indicated that the mes/metencephalic boundary works as an organiser for the tectum and the cerebellum. Fgf8 is a candidate signalling molecule in the organiser. Pax2/5, En, and Fgf8 are in a positive feedback loop for their expression such that misexpression of one of these genes in the diencephalon turns on the feedback loop and can result in induction of an optic tectum. Otx2 and Gbx2 appear to repress each other's expression and contribute to defining the posterior border of the tectum. Misexpression of Otx2 in the metencephalon can change the fate of its alar plate to a tectum, and misexpression of Gbx2 in the mesencephalon can cause anterior shifting of the caudal limit of the tectum. The anterior border of the tectum may be determined as a result of repressive interactions between Pax6 and En1/Pax2. Along the dorsoventral axis of the mesencephalon, Shh contributes to ventralize the tissue; that is, Shh can change the fate of the presumptive tectum to that of the tegmentum that is the ventral structure. It is proposed that the brain vesicle that expresses Otx2, Pax2, and En1 may differentiate into the tectum.

Regionalization of the optic tectum: combinations of gene expression that define the tectum

The optic tectum differentiates from the alar plate of the mesencephalon. Here, the molecular mechanisms for differentiation of the tectum are reviewed. Mis-expression of Pax2, Pax5 or En can change the fate of the presumptive diencephalon to become the tectum. En, Fgf8, Pax2 and Pax5, exist in a positive feedback loop for their expression so that mis-expression of any of these genes acts on the feedback loop resulting in induction of the optic tectum in the diencephalon. Otx2 and Gbx2 can repress the expression of each other and contribute to the formation of the posterior border of the tectum. Mis-expression of Otx2 in the metencephalon changed the fate of its alar plate to the tectum. The anterior border of the tectum might be determined as a result of repressive interaction of Pax6 with En1 and Pax2. Along the dorsoventral axis of the mesencephalon, Shh contributes to the ventralization of the tissue, that is, the area affected by Shh differentiates into the tegmentum. It is proposed that the brain vesicle that expresses Otx2, Pax2 and En1 might differentiate into the tectum.

Roles of Pax-2 in initiation of the chick tectal development

Transplantation experiments have shown that the mes-metencephalic boundary (isthmus) acts as an organizer for the development of the optic tectum. We have cloned Pax-2 which is expressed in the isthmus. Previously it was shown that Pax-5, a member of the same Pax subfamily as Pax-2, transformed the diencephalon into a tectum-like structure and induced isthmus- and tectum-related genes both in the mesencephalon and in the diencephalon. In order to define the distinct roles between Pax-2 and Pax-5 in development of the tectum, we expressed Pax-2 ectopically in the mesencephalon and the diencephalon of E2 chick embryos by in ovo electroporation. Histological observation demonstrated that Pax-2 transformed the diencephalon into a tectum-like structure. In Pax-2, transfected embryos the expression of isthmus- and tectum-related genes such as Fgf8 and En-2 was induced in the diencephalon. However, neither Fgf8 nor En-2 expression was induced in the mesencephalon, making a striking contrast with the result of Pax-5 misexpression. In E2 chick embryos, the mesencephalon is committed of its fate to differentiate into the tectum, but the diencephalon has plasticity on its fate. Moreover, Pax-2 expression in the isthmus precedes Pax-5 expression. Taking these results into consideration, it is suggested that Pax-2 plays a crucial role in initiation of the tectal development, and that Pax-5 functions to maintain the state of tectal differentiation.

Position, guidance, and mapping in the developing visual system

Positional identity in the visual system affects the topographic projection of the retina onto its central targets. In this review we discuss gradients and positional information in the retina, when and how they arise, and their functional significance in development. When the axons of retinal ganglion cells leave the eye, they navigate through territory in the central nervous system that is rich in positional information. We review studies that explore the navigational cues that the growth cones of retinal axons use to orient towards their target and organize themselves as they make this journey. Finally, these axons arrive at their central targets and make a precise topographic map of visual space that is crucial for adaptive visual behavior. In the last section of this review, we examine the topographic cues in the tectum, what they are, when, and how they arise, and how retinal axons respond to them. We also touch on the role of neural activity in the refinement of this topography.