Re-establishing the regenerative potential of central nervous system axons in postnatal mice

Abstract

IntroductionAt a certain point in development, axons in the mammalian central nervous system (CNS) lose their ability to regenerate after injury. The mechanisms of this growth failure are unclear. According to the prevailing view, CNS regenerative failure reflects both the intrinsic inability of adult CNS neurons to survive and reinitiate axonal growth and the lack of a permissive environment for such growth (Bregman, 1998;Chen and Tonegawa, 1998;Goldberg and Barres, 2000;Horner and Gage, 2000). Proposed potential growth obstacles within the CNS include myelin-associated inhibitory molecules, glial scarring after injury, and lack of growth-promoting substrates and neurotrophic factors (Fournier and Strittmatter, 2001;Goldberg and Barres, 2000;Morgenstern et al., 2002). Successful regeneration in the adult CNS may require manipulating both the intrinsic features of injured neurons and the CNS environment. Despite the recent identification of growth-inhibitory molecules and their receptors (McGee and Strittmatter, 2003), two pertinent questions remain unanswered. What are the essential intrinsic and environmental components for promoting CNS regeneration, and how can the axonal growth potential lost during development be restored in the mature CNS?Optic nerve injury is a standard model for studying CNS regeneration. Rodent retinal ganglion cells (RGCs), whose axons form the optic nerve, normally cannot regenerate their axons through an injured optic nerve. When provided with permissive substrate or given a novel treatment, only a small population of severed axons are induced to regenerate; the regeneration is slow, and the axons stop extending before they reach their targets Yin et al., 2003). Rodent RGCs lose their intrinsic ability for axon elongation before birth (Chen et al., 1995;Goldberg et al., 2002). Developing RGCs have two distinct stages of axonal growth -elongation and arborization -distinguished by contrasting rates of axon extension (Goldberg and Barres, 2000;Jhaveri et al., 1991). Embryonic RGC axons elongate about 10 times faster than mature ones (0.5-2.0 mm/d versus 40-60 µm/d). This difference has been attributed primarily to the maturational change in the intrinsic property of neurons and contributes critically to the failure of optic nerve regeneration (Goldberg et al., 2002).RGCs lose their intrinsic capacity for axonal elongation at embryonic day 18 (E18), before the onset of growth inhibition in the CNS environment (Chen and Tonegawa, 1998). Such a developmental scheme offers an opportunity to characterize and uncover independently the intrinsic and environmental elements in optic nerve regenerative failure. Naturally, the first important question is what determines the intrinsic regenerative capacity of RGC axons. Previously we showed that one determining factor is the anti-apoptotic gene Bcl-2 (Chen et al., 1997). In lower vertebrates (e.g. fish and frog), CNS neurons upregulate Bcl-2 after injury and readily regenerate axons and re-establish topographically organized connections (Cristin...

Promoting Optic Nerve Regeneration in Adult Mice with Pharmaceutical Approach

“…In the present study, lithium's effects on supporting optic nerve regrowth in mice is likely through inducing Bcl-2 expression and with it, promoting the intrinsic regenerative capacity of RGC axons. In consistent with our previous observation in Bcl-2 transgenic mice [9,13,14], activating the intrinsic growth mechanisms of RGC axons alone, by inducing Bcl-2 expression, does not suffice optic nerve regeneration in adult mice. Two parallel mechanisms work concurrently that prevent the regeneration of CNS axons in the adult: the lack of Bcl-2-supported intrinsic mechanism and the presence of inhibitory environment in the mature CNS.…”

“…As it has been reported, optic nerve regenerative failure reflects both the lack of Bcl-2-supported intrinsic growth mechanism in RGCs and the presence of glial scarring that presents inhibitory mechanisms for such growth [13]. We proposed that successful regeneration of severed optic nerve in adult could be achieved pharmacologically via concurrent induction of Bcl-2 expression and elimination of mature astrocytes-scar forming cells.…”

“…Not surprising, after the CNS environment becomes mature and develops inhibitory mechanisms that further block axon regeneration, the regeneration failed again even in the presence of Bcl-2 [10][11][12]. Our previous studies have suggested that RGCs overexpressing Bcl-2 retain the intrinsic capacity to grow axon throughout life; however, the mature CNS environment develops growth inhibitory mechanisms around postnatal day 4 in mice that further blocks optic nerve regeneration in older Bcl-2 transgenic mice [9,13]. More-over, we have demonstrated that maturation of astrocytes and with it, their ability to form glial scars after injury, contributes critically to the growth inhibition presented by the CNS environment.…”

Neuroglobin Is an Endogenous Neuroprotectant for Retinal Ganglion Cells against Glaucomatous Damage

“…40 To investigate Ngb mRNA and protein expression, mice (n Ͼ 4 in each experimental group per time point) were sacrificed on day 0, 3, and 7 after microbead injection. Individual retinas were isolated and homogenized using a rotor-stator homogenizer.…”

“…Tuj-1-positive cells were counted as described previously. 38,40 At least three nonoverlapping retinal sections of each eye were studied. Values are reported as means Ϯ SD.…”