Past, present and future of preserving and restoring function in the visual system: removing galectin-3 as a promising treatment

Past, present and future of preserving and restoring function in the visual system: removing galectin-3 as a promising treatment

Great advances in retinal ganglion cells survival (RGCs), optic nerve preservation and regeneration have been made in the past 15 years. Nowadays, we know that RGCs are capable of regenerating the full length of the optic nerve, cross the chiasm, enter the brain and reinnervate visual targets. In order to obtain successful regen‐eration, RGCs need to activate signaling pathways related to cell survival and turn on their intrinsic growth capacity. Studies that aimed at blocking cell death and inhibiting apoptosis by B‐cell lym‐phoma 2 (Bcl‐2) overexpression showed an increase in cell survival, but these approaches were not sufficient to promote axon regen‐eration, even when axons were put in an permissive environment, as peripheral nerve gra(Inoue et al., 2002). Neither stimulating axon regeneration by intraocular in fl ammation, nor delaying axon degeneration by overexpression ofWldsprotein, or inhibition of calpain activation (Figure 1C) (de Lima et al., 2016) showed any increase in cell survival. Nevertheless, Park et al. (2008) showed that phosphatase and tensin homolog (PTEN) gene deletion on RGCs stimulates both cell survival and axon regeneration (Benowitz et al., 2016 for review).

The scenario revealed by these studies indicates that different mechanisms regulate RGC survival and axon regeneration. From these evidences, investigators started to combine different treat‐ments, focusing on cell survival, axon regeneration, or both. The rationale behind this approach is that one would be able to stimulate both RGCs survival and axon regeneration at the same time, and possibly get additional e ff ect aer a lesion to the optic nerve. For in‐stance, speci fi c single treatments, such as conditional deletion of the PTEN gene in RGCs resulted in 45% of cell survival aer optic nerve crush (ONC), and also promoted modest axon regeneration (Park et al., 2008). However, when combined with intraocular in fl ammation, a RGC survival rate of 54% was achieved as well as a 10 fold increase in axon regeneration, resulting in brain reinnervation (de Lima et al., 2012) (Figure 1E).erefore, a combination of treatments can be a powerful tool to stimulate recovery of visual pathway.us, research‐ers are focusing their e ff orts on identifying potential candidates that can be more e ff ective in one aspect (cell survival), in the other (axon regeneration), or both, so they can combine those candidates and boost brain target reinnervation.

Successful regeneration of RGCs and brain reinnervation:ere are few treatments that successfully stimulated the regeneration of the full length of the optic nerve, reaching subcortical visual tar‐gets (de Lima et al., 2012; Li et al., 2015; Bei et al., 2016; Lim et al., 2016). Although di ff erent groups have shown some level of brain reinnervation, there are still a lot of controversies if regenerated cells are able to fi nd their targets and recover function (de Lima et al., 2012; Bei et al., 2016). While de Lima et al. (2012) showed evi‐dence that RGCs can extend axons all the way from the eye to sub‐cortical visual targets and become remyelinated, some studies, using di ff erent treatments, claimed that the axons cannot be remyelinated (Bei et al, 2016), get stuck at the chiasm and are unable to fi nd their way to visual targets (Luo et al., 2013). However, other studies have reported a complete regeneration of RGCs, visual targets reinnerva‐tion and partial recovery of visual behaviors, activating mechanistic target of rapamycin (mTOR) signaling pathway combined with en‐hancement of neural activity (Lim et al, 2016). More studies have to be done in order to understand, not only if speci fi c treatments can induce remyelination but, more widely why RGCs behave di ff erent‐ly when subjected to a speci fi c treatment — for instance: 1) why to‐tal number of surviving RGCs and speci fi c RGCs subtypes can vary among studies? 2) Why only some treatments can promote long distance regeneration? 3) How regenerating axons interact with myelinating oligodendrocytes in order to become myelinated? 4) How regenerating axons are guided towards visual targets? 5) Does stimulation of activity improves regeneration? 6) How much axon regeneration is enough to produce signi fi cant functional recovery? 7) Does speci fi c subtypes of RGCs change their pro fi le to compen‐sate for speci fi c subtypes that were lost? Answering these questions might help us to achieve the recovery of the visual function aer a lesion to the optic nerve, or in the case of the neurodegenerative diseases, such as diabetic retinopathy, which are the leading causes of blindness in adults. Nevertheless, these studies have shown that the rate of cell survival decreases overtime, with only 34% of cells surviving at 10–12 weeks aer injury (de Lima et al., 2012). So, dif‐ferent approaches are still required to achieve a satisfactory visual function recovery aer lesions to the optic nerve.

Galectin-3 (Gal-3) deletion and visual system preservation: Pre‐vious work from our group showed that Gal‐3 absence increased peripheral nerve regeneration aer crush (Narciso et al., 2009) and improved white matter sparing and motor function after spinal cord lesion (Mostacada et al., 2015). These results, in addition to the knowledge that Gal‐3 is required for microglia/macrophage phagocytic activity during optic nerve Wallerian degeneration (WD), led us to hypothesize that the absence of Gal‐3 could reduce RGCs death and modulate axonal degeneration/regeneration aer ONC.is subject was investigated in a study in Abreu et al. (2016), where we showed that the absence of Gal‐3 promotes neuroprotec‐tion of RGC, since Gal‐3–/–mice presented an increase in the sur‐vival of RGC aer injury and less apoptotic RGC in their retinas. Importantly, the authors found the highest survival level of RGCs ever promoted by a single treatment aer optic nerve crush ‐ that is 52% two weeks aer lesion. Albeit surviving, no GAP‐43 positive axons were observed in neither group, indicating that Gal‐3–/–RGC axons were not able to regenerate (Figure 1D).e course of WD within the optic nerve was also analyzed, and the quantitative anal‐ysis of the ultrastructure of Gal‐3 knockout optic nerves two weeks aer crush showed an increased number of myelinated identi fi able fi bers, although presenting degenerating axoplasm, indicating that the WD process is impaired in Gal‐3 knockout mice 2 weeks af‐ter optic nerve injury.e neuroprotection reported by our study might be related to a decrease in the in fl ammatory response caused by lower number of in fl ammatory cells, such as microglia and mac‐rophage at the site of the lesion, associated with less astrogliosis, which resulted in slow degeneration of optic nerve fi bers and RGC survival. It is of great interest to understand the mechanisms by which the increase in cell survival is taking place. It can be inferred that the e ff ect on cell survival is most likely a tissue extrinsic mech‐anism, since Gal‐3 is not expressed in both retinal ganglion cells or in the optic nerve (unpublished observations). Since there was an important e ff ect on cell survival, it would be interesting to know for how long it can be maintained, in order to have Gal‐3 knockdown in association with other therapeutic approaches. If this e ff ect on RGCs survival is sustained overtime, it would be of great interest to associate Gal‐3 knockdown, knockout or blockade with one or more pro‐regenerative therapies that have proved to be effective, such as intraocular inflammation, PTEN or SOCS3 conditional deletion within RGCs, mTOR overactivation or enhancement of visual activity.

Figure 1 Pro fi le of RGC death and regeneration after crush under di ff erent conditions.

Besides, our results raised the possibility to study the role of Gal‐3 in other important visual incapacitating diseases, such as diabetic retinopathy. Estimated to affect 422 million people around the globe, diabetes is one of the main pandemics of the XXI century. Considered the most common cause of acquired blindness, diabet‐ic retinopathy is responsible for 1.9% of the global cases of severe visual impairment worldwide and 2.6% of total blindness cases. Recently, high serum levels of Gal‐3 has been tagged as a new risk factor associated with pre‐diabetes and diabetes in human subjects (Yilmaz et al., 2014). Indeed, the in fi ltration of in fl ammatory cell and β pancreatic cell death are reduced in Gal‐3 knockout mice, decreasing the susceptibility to diabetes induction (Mensah‐Brown et al., 2009). Moreover, the number of microglia/macrophage are drastically reduced in the lesion site of both spinal cord (Mostacada et al., 2015) and optic nerve (Abreu et al., 2016) of the Gal‐3 knock‐out mice. Also, absence of Gal‐3 has been shown to polarize both macrophage and microglia to an M2 phenotype after both spinal cord injury and diabetes induction in mice (Mensah‐Brown et al., 2009; Mostacada et al., 2015). Since previous research from our group has shown preservation of CNS white matter in the spinal cord (Mostacada et al., 2015) and optic nerve (Abreu et al., 2016), also rescuing RGCs from apoptosis aer lesion, it would be of in‐terest to investigate if Gal‐3 absence could relieve the ophthalmo‐logical symptoms of diabetes, focusing studies in visual function, neurodegeneration, neuroregeneration, neuroplasticity and neu‐roin fl ammation.

Since Gal‐3 inhibitors, such as GR‐MD‐02, are available, even being used in clinical trials (Harrison et al., 2016), the identi fi cation of Gal‐3 as a potential target to treat neurodegenerative diseases, may open new possibilities for translational studies and clinical treatments.

Silmara de Lima, Henrique Rocha Mendonça, Camila Oliveira Goulart, Ana M. Blanco Martinez＊

Laboratories for Neuroscience Research in Neurosurgery and F.M. Kirby Neurobiology Center, Children’s Hospital Boston; Departments of Surgery and Ophthalmology and Program in Neuroscience, Harvard Medical School, Boston, MA, USA (de Lima S) Laboratório de Neurodegeneração e Reparo ‐ Departamento de Patologia ‐ Faculdade de Medicina — HUCFF — UFRJ — Rio de Janeiro — RJ, Brazil (Mendonça HR, Goulart CO, Martinez AMB) Pólo Universitário Macaé, UFRJ, Macaé, RJ, Brazil (Mendonça HR)

＊Correspondence to: Ana M. Blanco Martinez, Ph.D., anamartinez@huc ff.ufrj.br

Accepted:2017-01-16

Abreu CA, De Lima SV, Mendonça HR, Goulart CO, Martinez AM (2016) Ab‐sence of galectin‐3 promotes neuroprotection in retinal ganglion cells after optic nerve injury. Histol Histopathol 3:11788.

Bei F, Lee HHC, Liu X, Gunner G, Jin H, Ma L, Wang C, Hensch TK, Frank E, Sanes JR (2016) Restoration of visual function by enhancing conduction in re‐generated axons. Cell 164:219‐232.

Benowitz LI, He Z, Goldberg JL (2016) Reaching the brain: Advances in optic nerve regeneration. Exp Neurol 287:365‐737.

de Lima S, Koriyama Y, Kurimoto T, Oliveira JT, Yin Y, Li Y, Gilbert HY, Fagiolini M, Martinez AM, Benowitz L (2012) Full‐length axon regeneration in the adult mouse optic nerve and partial recovery of simple visual behaviors. Proc Natl Acad Sci U S A 109:9149‐9154.

de Lima S, Mietto B, Paula C, Muniz T, Martinez A, Gardino P (2016) Rescuing axons from degeneration does not a ff ect retinal ganglion cell death. Braz J Med Biol Res 49:e5106.

Harrison SA, Marri SR, Chalasani N, Kohli R, Aronstein W, Thompson GA, Irish W, Miles MV, Xanthakos SA, Lawitz E, Noureddin M, Schiano TD, Siddiqui M, Sanyal A, Neuschwander‐Tetri BA, Traber PG (2016) Randomised clinical study: GR‐MD‐02, a galectin‐3 inhibitor, vs. placebo in patients having non‐alcoholic steatohepatitis with advanced fi brosis. Aliment Pharmacoler 44:1183‐1198.

Li S, He Q, Wang H, Tang X, Ho KW, Gao X, Zhang Q, Shen Y, Cheung A, Wong F (2015) Injured adult retinal axons with Pten and Socs3 co‐deletion reform active synapses with suprachiasmatic neurons. Neurobiol Dis 73:366‐376.

Lim JH, Sta ff ord BK, Nguyen PL, Lien BV, Wang C, Zukor K, He Z, Huberman AD (2016) Neural activity promotes long‐distance, target‐speci fi c regeneration of adult retinal axons. Nat Neurosci 19:1073‐1084.

Luo X, Salgueiro Y, Beckerman SR, Lemmon VP, Tsoulfas P, Park KK (2013) Three‐dimensional evaluation of retinal ganglion cell axon regeneration and path fi nding in whole mouse tissue aer injury. Exp Neurol 247:653‐662.

Mensah‐Brown EP, Al Rabesi Z, Shahin A, Al Shamsi M, Arsenijevic N, Hsu DK, Liu FT, Lukic ML (2009) Targeted disruption of the galectin‐3 gene results in decreased susceptibility to multiple low dose streptozotocin‐induced diabetes in mice. Clin Immunol 130:83‐88.

Mostacada K, Oliveira FL, Villa‐Verde DM, Martinez AM (2015) Lack of galectin‐3 improves the functional outcome and tissue sparing by modulating in fl ammatory response aer a compressive spinal cord injury. Exp Neurol 271:390‐400.

Narciso MS, Mietto Bde S, Marques SA, Soares CP, Mermelstein Cdos S, El‐Cheikh MC, Martinez AM (2009) Sciatic nerve regeneration is accelerated in galectin‐3 knockout mice. Exp Neurol 217:7‐15.

Park KK, Liu K, Hu Y, Smith PD, Wang C, Cai B, Xu B, Connolly L, Kramvis I, Sahin M, He Z (2008) Promoting axon regeneration in the adult CNS by modu‐lation of the PTEN/mTOR pathway. Science 322:963‐966.

Yilmaz H, Cakmak M, Inan O, Darcin T, Akcay A (2015) Increased levels of galec‐tin‐3 were associated with prediabetes and diabetes: new risk factor? J Endocri‐nol Invest 38:527‐533.

10.4103/1673-5374.198975

How to cite this article:de Lima S, Mendonça HR, Goulart CO, Martinez AMB (2017) Past, present and future of preserving and restoring function in the visual system: removing galectin-3 as a promising treatment. Neural Regen Res 12(1):58-59.

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