Inhibition of TrkB- and TrkC-Signaling Pathways Affects Neurogenesis in the Opossum Developing Neocortex

Astrocytes of the Anterior Commissure Regulate the Axon Guidance Pathways of Newly Generated Neocortical Neurons in the Opossum Monodelphis domestica

In marsupials, upper-layer cortical neurons derived from the progenitors of the subventricular zone of the lateral ventricle (SVZ) mature morphologically and send their axons to form interhemispheric connections through the anterior commissure. In contrast, eutherians have evolved a new extra callosal pathway, the corpus callosum, that interconnects both hemispheres. In this study, we aimed to examine neurogenesis during the formation of cortical upper layers, including their morphological maturation in a marsupial species, namely the opossum (Monodelphis domestica). Furthermore, we studied how the axons of upper layers neurons pass through the anterior commissure of the opossum, which connects neocortical areas. We showed that upper-layer II/III neurons were generated within at least seven days in the opossum neocortex. Surprisingly, these neurons expressed special AT-rich sequence binding protein 2 (Satb2) and neuropilin 1 interacting protein (Nrp1), which are proteins known to be essential for the formation of the corpus callosum in eutherians. This indicates that extrinsic, but not intrinsic, cues could be key players in guiding the axons of newly generated cortical neurons in the opossum. Although oligodendrocyte precursor cells were present in the neocortex and anterior commissure, newly generated upper-layer neurons sent unmyelinated axons to the anterior commissure. We also found numerous GFAP-expressing progenitor cells in both brain structures, the neocortex and the anterior commissure. However, at P12-P17 in the opossums, a small population of astrocytes was observed only in the midline area of the anterior commissure. We postulate that in the opossum, midline astrocytes allow neocortical axons to be guided to cross the midline, as this structure resembles the glial wedge required by fibers to cross the midline area of the corpus callosum in the rodent.

Delayed cortical development in mice with a neural specific deletion of β1 integrin

The adhesion systems employed by migrating cortical neurons are not well understood. Genetic deletion studies of focal adhesion kinase (FAK) and paxillin in mice suggested that these classical focal adhesion molecules control the morphology and speed of cortical neuron migration, but whether β1 integrins also regulate migration morphology and speed is not known. We hypothesized that a β1 integrin adhesion complex is required for proper neuronal migration and for proper cortical development. To test this, we have specifically deleted β1 integrin from postmitotic migrating and differentiating neurons by crossing conditional β1 integrin floxed mice into the NEX-Cre transgenic line. Similar to our prior findings with conditional paxillin deficiency, we found that both homozygous and heterozygous deletion of β1 integrin causes transient mispositioning of cortical neurons in the developing cortex when analyzed pre- and perinatally. Paxillin and β1 integrin colocalize in the migrating neurons and deletion of paxillin in the migrating neuron causes an overall reduction of the β1 integrin immunofluorescence signal and reduction in the number of activated β1 integrin puncta in the migrating neurons. These findings suggest that these molecules may form a functional complex in migrating neurons. Similarly, there was an overall reduced number of paxillin+ puncta in the β1 integrin deficient neurons, despite the normal distribution of FAK and Cx26, a connexin required for cortical migration. The double knockout of paxillin and β1 integrin produces a cortical malpositioning phenotype similar to the paxillin or β1 integrin single knockouts, as would be expected if paxillin and β1 integrin function on a common pathway. Importantly, an isolation-induced pup vocalization test showed that β1 integrin mutants produced a significantly smaller number of calls compared to their littermate controls when analyzed at postnatal day 4 (P4) and revealed a several days trend in reduced vocalization development compared to controls. The current study establishes a role for β1 integrin in cortical development and suggests that β1 integrin deficiency leads to migration and neurodevelopmental delays.

Postnatal and Adult Neurogenesis in Mammals, Including Marsupials

In mammals, neurogenesis occurs during both embryonic and postnatal development. In eutherians, most brain structures develop embryonically; conversely, in marsupials, a number of brain structures develop after birth. The exception is the generation of granule cells in the dentate gyrus, olfactory bulb, and cerebellum of eutherian species. The formation of these structures starts during embryogenesis and continues postnatally. In both eutherians and marsupials, neurogenesis continues in the subventricular zone of the lateral ventricle (SVZ) and the dentate gyrus of the hippocampal formation throughout life. The majority of proliferated cells from the SVZ migrate to the olfactory bulb, whereas, in the dentate gyrus, cells reside within this structure after division and differentiation into neurons. A key aim of this review is to evaluate advances in understanding developmental neurogenesis that occurs postnatally in both marsupials and eutherians, with a particular emphasis on the generation of granule cells during the formation of the olfactory bulb, dentate gyrus, and cerebellum. We debate the significance of immature neurons in the piriform cortex of young mammals. We also synthesize the knowledge of adult neurogenesis in the olfactory bulb and the dentate gyrus of marsupials by considering whether adult-born neurons are essential for the functioning of a given area.

Epigenetic clock and methylation studies in marsupials: opossums, Tasmanian devils, kangaroos, and wallabies

The opossum (Monodelphis domestica), with its sequenced genome, ease of laboratory care and experimental manipulation, and unique biology, is the most used laboratory marsupial. Using the mammalian methylation array, we generated DNA methylation data from n = 100 opossum samples from the ear, liver, and tail. We contrasted postnatal development and later aging effects in the opossum methylome with those in mouse (Mus musculus, C57BL/6 J strain) and other marsupial species such as Tasmanian devil, kangaroos, and wallabies. While the opossum methylome is similar to that of mouse during postnatal development, it is distinct from that shared by other mammals when it comes to the age-related gain of methylation at target sites of polycomb repressive complex 2. Our immunohistochemical staining results provide additional support for the hypothesis that PRC2 activity increases with later aging in mouse tissues but remains constant in opossum tissues. We present several epigenetic clocks for opossums that are distinguished by their compatibility with tissue type (pan-tissue and blood clock) and species (opossum and human). Two dual-species human-opossum pan-tissue clocks accurately measure chronological age and relative age, respectively. The human-opossum epigenetic clocks are expected to provide a significant boost to the attractiveness of opossum as a biological model. Additional epigenetic clocks for Tasmanian devil, red kangaroos and other species of the genus Macropus may aid species conservation efforts.

Effects of Brain Size on Adult Neurogenesis in Shrews

Shrews are small animals found in many different habitats. Like other mammals, adult neurogenesis occurs in the subventricular zone of the lateral ventricle (SVZ) and the dentate gyrus (DG) of the hippocampal formation. We asked whether the number of new generated cells in shrews depends on their brain size. We examined Crocidura russula and Neomys fodiens, weighing 10-22 g, and Crocidura olivieri and Suncus murinus that weigh three times more. We found that the density of proliferated cells in the SVZ was approximately at the same level in all species. These cells migrated from the SVZ through the rostral migratory stream to the olfactory bulb (OB). In this pathway, a low level of neurogenesis occurred in C. olivieri compared to three other species of shrews. In the DG, the rate of adult neurogenesis was regulated differently. Specifically, the lowest density of newly generated neurons was observed in C. russula, which had a substantial number of new neurons in the OB compared with C. olivieri. We suggest that the number of newly generated neurons in an adult shrew's brain is independent of the brain size, and molecular mechanisms of neurogenesis appeared to be different in two neurogenic structures.

Downregulation of TrkC Receptors Increases Dendritic Arborization of Purkinje Cells in the Developing Cerebellum of the Opossum, Monodelphis domestica

In therian mammals, the cerebellum is one of the late developing structures in the brain. Specifically, the proliferation of cerebellar granule cells occurs after birth, and even in humans, the generation of these cells continues during the first year of life. The main difference between marsupials and eutherians is that the majority of the brain structures in marsupials develop after birth. Herein, we report that in the newborn laboratory opossum (Monodelphis domestica), the cerebellar primordium is distinguishable in Nissl-stained sections. Additionally, bromodeoxyuridine birthdating experiments revealed that the first neurons form the deep cerebellar nuclei (DCN) and Purkinje cells, and are generated within postnatal days (P) 1 and 5. Three weeks after birth, progenitors of granule cells in the external germinal layer (EGL) proliferate, producing granule cells. These progenitor cells persist for a long time, approximately 5 months. Furthermore, to study the effects of neurotrophic tropomyosin receptor kinase C (TrkC) during cerebellar development, cells were obtained from P3 opossums and cultured for 8 days. We found that TrkC downregulation stimulates dendritic branching of Purkinje neurons, which was surprising. The number of dendritic branches was higher in Purkinje cells transfected with the shRNA TrkC plasmid. However, there was no morphological change in the number of dendritic branches of granule cells transfected with either control or shRNA TrkC plasmids. We suggest that inhibition of TrkC activity enables NT3 binding to the neurotrophic receptor p75^NTR that promotes dendritic arborization of Purkinje cells. This effect of TrkC receptors on dendritic branching is cell type specific, which could be explained by the strong expression of TrkC in Purkinje cells but not in granule cells. The data indicate a new role for TrkC receptors in Monodelphis opossum.