The Acquisition of Target Dependence by Developing Rat Retinal Ganglion Cells

Abstract

Similar to neurons in the peripheral nervous system, immature CNS-derived RGCs become dependent on target-derived neurotrophic support as their axons reach termination sites in the brain. To study the factors that influence this developmental transition we took advantage of the fact that rat RGCs are born, and target innervation occurs, over a protracted period of time. Early-born RGCs have axons in the SC by birth (P0), whereas axons from late-born RGCs do not innervate the SC until P4-P5. Birth dating RGCs using EdU allowed us to identify RGCs (1) with axons still growing toward targets, (2) transitioning to target dependence, and (3) entirely dependent on target-derived support. Using laser-capture microdissection we isolated ∼34,000 EdU+ RGCs and analyzed transcript expression by custom qPCR array. Statistical analyses revealed a difference in gene expression profiles in actively growing RGCs compared with target-dependent RGCs, as well as in transitional versus target-dependent RGCs. Prior to innervation RGCs expressed high levels of BDNF and CNTFR α but lower levels of neurexin 1 mRNA. Analysis also revealed greater expression of transcripts for signaling molecules such as MAPK, Akt, CREB, and STAT. In a supporting in vitro study, purified birth-dated P1 RGCs were cultured for 24-48 h with or without BDNF; lack of BDNF resulted in significant loss of early-born but not late-born RGCs. In summary, we identified several important changes in RGC signaling that may form the basis for the switch from target independence to dependence.

Developmental retinal ganglion cell death and retinotopicity of the murine retinocollicular projection

“…In the rat it has been shown that axons from early‐born (E13–E15) RGCs are present in the SC at birth (Bunt et al, ; Dallimore et al, ), whereas axons of later‐born (E18–E19) RGCs do not grow into the SC until approximately P4–P5 (Dallimore et al, ). Importantly, RGCs born on different days die at different times, the timing of death in a particular age cohort related to when their axons reach central targets (Dallimore et al, ; Moses et al, ). We analyzed a snapshot of RGC death on specific postnatal days, at a time when, depending on their birthdate, RGC axons have either just grown into the SC or had entered some time before birth.…”

“…The most likely alternative is that most RGC death occurs stochastically, and is due to some form of competition for neurotrophic factors (Oppenheim, 1991). Developing RGCs with axons not yet in central targets such as the SC depend on local neurotrophic support for their survival (Spalding et al, 2004;Moses et al, 2015). However, upon innervation, there is a switch towards target derived neurotrophic dependence, where the parent RGCs begin to rely upon neurotrophins secreted by the SC for their continued survival (Ma et al, 1998;Frost et al, 2001;Pollock et al, 2003;Spalding et al, 2004).…”

“…Within CNS‐derived retina for example, retinal ganglion cells (RGCs) undergo significant apoptosis during development: in rats and mice, at least 50% of the RGCs that are produced die late in gestation and early postnatal period (Potts et al, ; Dreher et al, ; Perry et al, ; Fawcett et al, ; Young, ; Crespo et al, ). It is generally held that developing RGCs, initially able to survive independent of target support (Moses et al, ), begin to compete for a limited amount of trophic support as their axons enter central targets, although the parameters that determine survival versus death remain unclear. One possible trigger may be the accuracy with which individual RGCs map to topographically appropriate sites in retinorecipient areas in the brain.…”

Age Related Response of Neonatal Rat Retinal Ganglion Cells to Reduced TrkB Signaling in vitro and in vivo

“…Developing neurons rely on neurotrophic support for their survival and maturation (Huang and Reichardt, 2001). Perhaps the most well studied neurotrophin in RGC development is BDNF, whose neuroprotective effects on RGCs have been documented in vivo and in vitro (Johnson et al, 1986;Ma et al, 1998;Spalding et al, 2004;Moses et al, 2015). BDNF is synthesized and can be released as pro-BDNF to act on the p75 receptor, or the pro domain can be cleaved to produce mature BDNF that acts on the high affinity tropomyosin receptor kinase B (TrkB) receptor to bring about neuroprotective effects (Nagappan and Lu, 2005;Yang et al, 2009).…”

“…BDNF is synthesized and can be released as pro-BDNF to act on the p75 receptor, or the pro domain can be cleaved to produce mature BDNF that acts on the high affinity tropomyosin receptor kinase B (TrkB) receptor to bring about neuroprotective effects (Nagappan and Lu, 2005;Yang et al, 2009). BDNF and TrkB mRNA and proteins are detected in early embryonic development in RGCs, the levels of expression changing throughout development (Ernfors et al, 1992;Jelsma et al, 1993;Koide et al, 1995;Perez and Caminos, 1995;Vecino et al, 2002;Moses et al, 2015). However, neurotrophins are also produced in the SC and can be retrogradely transported to the retina via RGC axons (Ma et al, 1998;Frost et al, 2001;Spalding et al, 2002Spalding et al, , 2004.…”