Cumulative labeling of embryonic mouse neural retina with bromodeoxyuridine supplied by an osmotic minipump

In vivo pulse labeling of isochronic cohorts of cells in the central nervous system using FlashTag

The tracing of neuronal cell lineages is critical to our understanding of cellular diversity in the CNS. This protocol describes a fluorescence birth-dating technique to label, track and isolate isochronic cohorts of newborn cells in the CNS in vivo in mouse embryos. Injection of carboxyfluorescein esters (CFSEs) into the cerebral ventricle allows pulse labeling of mitotic (M phase) ventricular zone (VZ) progenitors and their progeny across the CNS, a procedure we termed FlashTag. Specificity for M-phase apical progenitors is a result of the somata of these cells transiently contacting the ventricular wall during this cell-cycle phase, exposing them to CFSE injected into the cerebrospinal fluid. Using this approach, the developmental trajectory of progenitors and their daughter neurons can be tracked. Labeled cells can be imaged ex vivo or in fixed tissue, targeted for electrophysiological experiments or isolated using FACS for cell culture or (single-cell) RNA sequencing. Multiple embryos can be labeled within 30 min. The dye is retained for several weeks, allowing labeled cells to be identified postnatally. This protocol describes the labeling procedure using in utero injection, the isolation of live cells using FACS and the processing of labeled tissue for immunohistochemistry.

Spatial geometry of stem cell proliferation in the adult hippocampus

The modes of stem cell divisions (e.g., symmetric vs. asymmetric) can have a profound impact on the number of progeny and tissue growth, repair, and function. This is particularly relevant for adult neural stem cells, since stem cell-derived neurons affect cognitive and mental states, resistance to stress and disease, and response to therapies. Here we show that although dividing stem cells in the adult hippocampus display a certain bias towards paired distribution (which could imply the prevalence of symmetric divisions), this bias already exists in the distribution of the general population of stem cells and may be responsible for the perceived occurrence of symmetric stem cell divisions. Remarkably, the bias in the distribution of stem cells decreases with age. Our results argue that the preexisting bias in stem cell distribution may affect current assumptions regarding stem cell division and fate as well as conjectures on the prospects of brain repair and rejuvenation.

An integrated pipeline for the multidimensional analysis of branching morphogenesis

Developmental branching morphogenesis establishes organ architecture, and it is driven by iterative interactions between epithelial and mesenchymal progenitor cell populations. We describe an approach for analyzing this interaction and how it contributes to organ development. After initial in vivo cell labeling with the nucleoside analog 5-ethynyl-2'-deoxyuridine (EdU) and tissue-specific antibodies, optical projection tomography (OPT) and confocal microscopy are used to image the developing organ. These imaging data then inform a second analysis phase that quantifies (using Imaris and Tree Surveyor software), models and integrates these events at a cell and tissue level in 3D space and across developmental time. The protocol establishes a benchmark for assessing the impact of genetic change or fetal environment on organogenesis that does not rely on ex vivo organ culture or section-based reconstruction. By using this approach, examination of two developmental stages for an organ such as the kidney can be undertaken by a postdoctoral-level researcher in 6 weeks, with a full developmental analysis in mouse achievable in 5 months.

Induction of the ganglion cell differentiation program in human retinal progenitors before cell cycle exit

BACKGROUND

Despite the disease relevance, understanding of human retinal development lags behind that of other species. We compared the kinetics of gene silencing or induction during ganglion cell development in human and murine retina.

RESULTS

Induction of POU4F2 (BRN3B) marks ganglion cell commitment, and we detected this factor in S-phase progenitors that had already silenced Cyclin D1 and VSX2 (CHX10). This feature was conserved in human and mouse retina, and the fraction of Pou4f2+ murine progenitors labeled with a 30 min pulse of BrdU matched the fraction of ganglion cells predicted to be born in a half-hour period. Additional analysis of 18 markers revealed many with conserved kinetics, such as the POU4F2 pattern above, as well as the surprising maintenance of "cell cycle" proteins KI67, PCNA, and MCM6 well after terminal mitosis. However, four proteins (TUBB3, MTAP1B, UCHL1, and RBFOX3) showed considerably delayed induction in human relative to mouse retina, and two proteins (ISL1, CALB2) showed opposite kinetics, appearing on either side of terminal mitosis depending on the species.

CONCLUSION

With some notable exceptions, human and murine ganglion cell differentiation show similar kinetics, and the data add weight to prior studies supporting the existence of biased ganglion cell progenitors.

Programmed cell death during retinal development of the mouse eye

Similar to other parts of the central nervous system, there are two types of programmed cell death during retinal development. In early development, the neuronal progenitor population is affected. In the mouse eye, this kind of programmed cell death begins at around embryonic day (E) 12.5 and peaks between E14.5 and E16.5. The second phase of programmed cell death occurs during synaptogenesis within the first 2 postnatal weeks. Important signaling mechanisms that induce programmed cell death of retinal progenitors appear to involve nerve growth factor acting on the proapoptotic receptor to p75 neurotrophin receptor (p75(NTR)) and transforming growth factor-β.

TGF-β signaling protects retinal neurons from programmed cell death during the development of the mammalian eye

We investigated the influence of transforming growth factor-β (TGF-β) signaling on developmental programmed cell death in the mouse retina by direct and specific molecular targeting of TGF-β type II receptor (TβRII) and Smad7 in retinal progenitor cells. Mice were generated carrying a conditional deletion of the TβRII in cells that originate from the inner layer of the optic cup. The animals showed a significant decrease of phosphorylated Smad3 in both the central and peripheral retina, which indicates the diminished activity of TGF-β signaling. TβRII deficiency significantly increased the apoptotic death of retinal neurons during embryonic and postnatal development without affecting their proliferation. In contrast, treatment with TGF-β2 inhibited cell death of retinal ganglion cells in dissociated retinal cell cultures, an effect that was blocked by inhibiting the phosphorylation of Smad3. The increase in apoptosis during development resulted in a significant reduction in the number of neurons in adult TβRII-deficient mice. The effect was most pronounced in the inner retina neurons and resulted in functional deficits as determined by electroretinography. In contrast, a conditional deletion of TGF-β-inhibiting Smad7 in retinal neurons significantly enhanced Smad3 phosphorylation and significantly decreased apoptosis of retinal neurons in embryos and pups. Moreover, the number of retinal ganglion cells was significantly higher in Smad7-deficient mice compared with control littermates. TβRII-deficient pups showed a lower level of nerve growth factor (NGF) in its mRNA; however, higher levels were observed in Smad7-deficient pups, which strongly suggests that the protective effects of TGF-β signaling on developmental cell death are mediated through NGF.

Reduced BACE1 activity enhances clearance of myelin debris and regeneration of axons in the injured peripheral nervous system

β-Site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is an aspartyl protease best known for its role in generating the amyloid-β peptides that are present in plaques of Alzheimer's disease. BACE1 has been an attractive target for drug development. In cultured embryonic neurons, BACE1-cleaved N-terminal APP is further processed to generate a fragment that can trigger axonal degeneration, suggesting a vital role for BACE1 in axonal health. In addition, BACE1 cleaves neuregulin 1 type III, a protein critical for myelination of peripheral axons by Schwann cells during development. Here, we asked whether axonal degeneration or axonal regeneration in adult nerves might be affected by inhibition or elimination of BACE1. We report that BACE1 knock-out and wild-type nerves degenerated at a similar rate after axotomy and to a similar extent in the experimental neuropathies produced by administration of paclitaxel and acrylamide. These data indicate N-APP is not the sole culprit in axonal degeneration in adult nerves. Unexpectedly, however, we observed that BACE1 knock-out mice had markedly enhanced clearance of axonal and myelin debris from degenerated fibers, accelerated axonal regeneration, and earlier reinnervation of neuromuscular junctions, compared with littermate controls. These observations were reproduced in part by pharmacological inhibition of BACE1. These data suggest BACE1 inhibition as a therapeutic approach to accelerate regeneration and recovery after peripheral nerve damage.

Neurochemical phenotype and birthdating of specific cell populations in the chick retina

The chick embryo is one of the most traditional models in developing neuroscience and its visual system has been one of the most exhaustively studied. The retina has been used as a model for studying the development of the nervous system. Here, we describe the morphological features that characterize each stage of the retina development and studies of the neurogenesis period of some specific neurochemical subpopulations of retinal cells by using a combination of immunohistochemistry and autoradiography of tritiated-thymidine. It could be concluded that the proliferation period of dopaminergic, GABAergic, cholinoceptive and GABAceptive cells does not follow a common rule of the neurogenesis. In addition, some specific neurochemical cell groups can have a restrict proliferation period when compared to the total cell population.

Neurogenesis and cell death in the ganglion cell layer of vertebrate retina

The correct formation of all central nervous system tissues depends on the proper balance of neurogenesis and developmental cell death. A model system for studying these programs is the ganglion cell layer (GCL) of the vertebrate retina because of its simple and well-described structure and amenability to experimental manipulations. The GCL contains approximately equal numbers of ganglion cells and displaced amacrine cells. Ganglion cells are the first or among the first cells born in the retina in all the studied vertebrates. Neurogenesis and cell death have been studied extensively in the GCL of various amniotes (rodents, chicks, and monkeys) and anamniotes (fish and frogs), and the two processes highlight developmental differences between the groups. In amniotes, neurogenesis occurs during a defined period prior to birth/hatch or the opening of the eyes, whereas in anamniotes, neurogenesis extends past hatching into adulthood-sometimes for years. Roughly half of GCL neurons die during development in amniotes, whereas developmental cell death does not occur in the GCL neurons of anamniotes. This review discusses the spatial and temporal patterns of neurogenesis, cell death, and possible explanation of cell death in the GCL. It also examines markers widely used to distinguish between ganglion cells and displaced amacrine cells, and methods employed to birth date neurons.

Nestin-expressing cells divide and adopt a complex electrophysiologic phenotype after transient brain ischemia

The intermediate filament nestin is upregulated in response to cerebral ischemia; the significance of this, however, is incompletely understood. Here, we used transgenic mice that express green fluorescent protein (GFP) under control of the nestin promotor to characterize the fate of nestin-expressing cells up to 8 weeks after 30 mins occlusion of the middle cerebral artery (MCAo) and reperfusion. The population of nestin-GFP+ cells increased in the ischemic lesion rim and core within 4 days, did not become TUNEL-positive, and was detectable up to 8 weeks in the lesion scar. Nestin-GFP+ cells proliferated in situ and underwent approximately one round of cell division. They were not recruited in large numbers from the subventricular zone (SVZ) as indicated by absence of colabeling with intracerebroventricularly injected dye DiI in the majority of nestin-GFP+ cells. Nestin-GFP+ cells expressed the chondroitin sulfate proteoglycan NG2 and nestin protein, but typically lacked mature astrocytic markers, that is, glial fibrillary acid protein (GFAP) or S100beta. Vice versa, the majority of GFAP+ cells lacked nestin-expression and surrounded the ischemic lesion by 4 days. Whole-cell patch-clamp recordings in acute brain slices from controls showed that only about half of nestin-GFP+ cells displayed complex membrane properties. In contrast, 4 days after the insult all nestin-GFP+ cells expressed these properties. We hypothesize that the change in physiologic properties induced by the ischemic insult is directed toward a specific function of nestin-expressing cells.

Cell birth and death in the mouse retinal ganglion cell layer

Here we describe quantitatively the birth and death of the two separate populations of neurons, ganglion cells and displaced amacrine cells, in the mouse retinal ganglion cell layer (GCL). The two cell types, which are roughly equally numerous, were distinguished pre- and postnatally by labeling the ganglion cells retrogradely with fluorescent dye. Embryos were labeled cumulatively with bromodeoxyuridine (BrdU) delivered by an osmotic minipump implanted in the mother; cell birth dates were established as having occurred before or after pump implantation. Early cohorts (GCL cells born before embryonic day [E] 11.8 and E12.8) were 98+/-1.1% and 99+/-0.2% ganglion cells (mean+/-SEM), respectively, and a late cohort (born after E15.8) was 97+/-1.2% displaced amacrines. Thus birth date was a strong predictor of a GCL cell's ultimate identity. Cell death in each cohort was estimated by counting cells at different time points (soon after the cohort was produced and later) and subtracting the later from the earlier number. This method avoids the problem of simultaneous birth and death that has plagued many of the earlier attempts to assess cell death. Negligible numbers died during the first week after a cell's birthday. The amount of cell death differed in the two cohorts; 48.5+/-15% and 29.0+/-12.4% in early and late, respectively, and most of it was postnatal. These findings disagree sharply with an earlier conclusion that ganglion cells die within 5 days of their birthdays or not at all.