Effects of ocular injury and administration of brain-derived neurotrophic factor on survival and regrowth of axotomized retinal ganglion cells.

Optic nerve transection in adult rats results inthe death of approximately 50% of the axotomized retinal ganglion cells (RGCs)by 1 week and nearly 90% by 2 weeks after injury. The capacity of brain-derivedneurotrophic factor (BDNF) to prevent this early, severe loss of RGCs wasinvestigated in vivo by intravitreal injections of BDNF [5 micrograms in 5microliters of bovine serum albumin/phosphate-buffered saline (BSA/PBS)] orvehicle (5 microliters of BSA/PBS). Using quantitative anatomical techniques, weshow that (i) all RGCs survived 1 week after a single injection of BDNF at thetime of axotomy. (ii) RGC densities decreased in the BDNF-treated retinas by 2weeks but remained significantly greater than in the untreated controls. (iii)An enhanced RGC survival was obtained with single injections of BDNF from 6 daysbefore to 5 days after axotomy. (iv) Repeated injections resulted in greaternumbers of surviving RGCs, an effect that declined to undetectable levels by 6weeks. (v) There were indications for an endogenous local source of trophicsupport whose expression was triggered by ocular injury, particularly to theanterior part of the eye. (vi) With multiple BDNF injections, there was profuseaxonal sprouting around the optic disc. This remarkable intraretinal growth wasnot, however, reflected in increased RGC innervation of the peripheral nervegrafts, which are known to facilitate regeneration when used as optic nervesubstitutes.     

Genetic modulation of horizontal cell number in the mouse retina

Neuronal populations display conspicuous variability in their size among individuals, but the genetic sources of this variation are largely undefined. We demonstrate a large and highly heritable variation in neuron number within the mouse retina, affecting a critical population of interneurons, the horizontal cells. Variation in the size of this population maps to the distal end of chromosome (Chr) 13, a region homologous to human Chr 5q11.1-11.2. This region contains two genes known to modulate retinal cell number. Using conditional knock-out mice, we demonstrate that one of these genes, the LIM homeodomain gene Islet-1 (Isl1), plays a role in regulating horizontal cell number. Genetic differences in Isl1 expression are high during the period of horizontal cell production, and cis-regulation of Isl1 expression within the retina is demonstrated directly. We identify a single nucleotide polymorphism in the 5' UTR of Isl1 that creates an E-box sequence as a candidate causal variant contributing to this variation in horizontal cell number.     

Transfer of a large gene regulatory apparatus to a new developmental address in echinoid evolution

Of the five echinoderm classes, only the modern sea urchins (euechinoids) generate a precociously specified embryonic micromere lineage that ingresses before gastrulation and then secretes the biomineral embryonic skeleton. The gene regulatory network (GRN) underlying the specification and differentiation of this lineage is now known. Many of the same differentiation genes as are used in the biomineralization of the embryo skeleton are also used to make the similar biomineral of the spines and test plates of the adult body. Here, we determine the components of the regulatory state upstream of these differentiation genes that are shared between embryonic and adult skeletogenesis. An abrupt "break point" in the micromere GRN is thus revealed, on one side of which most of the regulatory genes are used in both, and on the other side of which the regulatory apparatus is entirely micromere-specific. This reveals the specific linkages of the micromere GRN forged in the evolutionary process by which the skeletogenic gene batteries were caused to be activated in the embryonic micromere lineage. We also show, by comparison with adult skeletogenesis in the sea star, a distant echinoderm outgroup, that the regulatory apparatus responsible for driving the skeletogenic differentiation gene batteries is an ancient pleisiomorphic aspect of the echinoderm-specific regulatory heritage.