Novel mitochondrial derived Nuclear Excisosome degrades nuclei during differentiation of prosimian Galago (bush baby) monkey lenses

The unique cellular organization and transparent function of the ocular lens depend on the continuous differentiation of immature epithelial cells on the lens anterior surface into mature elongated fiber cells within the lens core. A ubiquitous event during lens differentiation is the complete elimination of organelles required for mature lens fiber cell structure and transparency. Distinct pathways have been identified to mediate the elimination of non-nuclear organelles and nuclei. Recently, we reported the discovery of a unique structure in developing fiber cells of the chick embryo lens, called the Nuclear Excisosome, that is intractably associated with degrading nuclei during lens fiber cell differentiation. In the chick lens, the Nuclear Excisosome is derived from projections of adjacent cells contacting the nuclear envelope during nuclear elimination. Here, we demonstrate that, in contrast to the avian model, Nuclear Excisosomes in a primate model, Galago (bush baby) monkeys, are derived through the recruitment of mitochondria to form unique linear assemblies that define a novel primate Nuclear Excisosome. Four lenses from three monkeys aged 2–5 years were fixed in formalin, followed by paraformaldehyde, then processed for Airyscan confocal microscopy or transmission electron microscopy. For confocal imaging, fluorescent dyes labelled membranes, carbohydrate in the extracellular space, filamentous actin and nuclei. Fiber cells from Galago lenses typically displayed prominent linear structures within the cytoplasm with a distinctive cross-section of four membranes and lengths up to 30 μm. The outer membranes of these linear structures were observed to attach to the outer nuclear envelope membrane to initiate degradation near the organelle-free zone. The origin of these unique structures was mitochondria in the equatorial epithelium (not from plasma membranes of adjacent cells as in the chick embryo model). Early changes in mitochondria appeared to be the collapse of the cristae and modification of one side of the mitochondrial outer membrane to promote accumulation of protein in a dense cluster. As a mitochondrion surrounded the dense protein cluster, an outer mitochondrial membrane enclosed the protein to form a core and another outer mitochondrial membrane formed the outermost layer. The paired membranes of irregular texture between the inner core membrane and the outer limiting membrane appeared to be derived from modified mitochondrial cristae. Several mitochondria were involved in the formation and maturation of these unique complexes that apparently migrated around the fulcrum into the cytoplasm of nascent fiber cells where they were stabilized until the nuclear degradation was initiated. Thus, unlike in the chick embryo, the Galago lenses degraded nuclear envelopes with a Nuclear Excisosome derived from multiple mitochondria in the epithelium that formed novel linear assemblies in developing fiber cells. These findings suggest that recruitment of distinct structures is required for Nuclear Excisosome formation in different species.

Functional Organization of Visual Cortex in the Prosimian Bush Baby Revealed by Optical Imaging of Intrinsic Signals

Cells in primary visual cortex (V1) of primates and carnivores respond most strongly to a visual stimulus presented to one eye, in a particular visual field location, and at a particular orientation. Each of these stimulus attributes is mapped across the cortical surface, and, in macaque monkeys and cats, strong geometrical relationships exist between these feature maps. In macaque V1 and V2, correlations between feature maps and cytochrome oxidase (CO)-rich modules have also been observed. To see if such relationships reflect a conserved principle of V1 functional architecture among primate species, we examined these maps in the prosimian bush baby, a species that has been proposed to represent the ancestral primate organization. We found that the layout of individual feature maps in bush baby V1 is similar to that of other primates, but we found an entirely different organization of orientation preference in bush baby V2 compared with that reported in simian primates. Another striking distinction between bush baby and simian species is that we observed no strong relationships among maps of orientation, ocular dominance, and CO blobs in V1. Thus our findings suggest that precise relationships between feature maps are not a common element of the functional organization in all primates and that such relationships are not necessary for achieving basic coverage of stimulus feature combinations. In addition, our results suggest that specific relationships between feature maps in V1, and the subdivision of V2 into functional compartments, may have arisen comparatively late in the evolution of primates.