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LKB1 and AMPK regulate synaptic remodeling in old age

Nature Neuroscience volume 17, pages 1190–1197 (2014)Cite this article

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Abstract

Age-related decreases in neural function result in part from alterations in synapses. To identify molecular defects that lead to such changes, we focused on the outer retina, in which synapses are markedly altered in old rodents and humans. We found that the serine/threonine kinase LKB1 and one of its substrates, AMPK, regulate this process. In old mice, synaptic remodeling was accompanied by specific decreases in the levels of total LKB1 and active (phosphorylated) AMPK. In the absence of either kinase, young adult mice developed retinal defects similar to those that occurred in old wild-type animals. LKB1 and AMPK function in rod photoreceptors where their loss leads to aberrant axonal retraction, the extension of postsynaptic dendrites and the formation of ectopic synapses. Conversely, increasing AMPK activity genetically or pharmacologically attenuates and may reverse age-related synaptic alterations. Together, these results identify molecular determinants of age-related synaptic remodeling and suggest strategies for attenuating these changes.

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Figure 1: LKB1 deletion induces age-related changes in young mice.
Figure 2: Rods require LKB1 to maintain synapses in the outer retina.
Figure 3: LKB1-AMPK signaling is disrupted in old age.
Figure 4: AMPK acts downstream of LKB1 to maintain retinal synapses.
Figure 5: Synaptic remodeling is accompanied by rod terminal retraction and postsynaptic sprouting.
Figure 6: Rods drive outer retina synaptic remodeling.
Figure 7: Therapeutic attenuation of age-related synaptic changes.

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Acknowledgements

We thank members of our laboratory for scientific discussions and advice, and A. Thanos for help with the AMPK animals. This work was funded by the US National Institutes of Health (AG32322 to J.R.S. and 5K99AG044444 to M.A. Samuel), the Damon Runyon Cancer Research Foundation (M.A. Samuel), Research to Prevent Blindness (J.R.S. and D.G.V.), the Foundation Fighting Blindness (B.P.), and the Intramural Research Program of the National Institute on Aging (R.d.C.).

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Author notes

  1. Melanie A Samuel and P Emanuela Voinescu: These authors contributed equally to this work.

Authors and Affiliations

  1. Department of Molecular and Cellular Biology and Center for Brain Science, Harvard University, Cambridge, Massachusetts, USA

    Melanie A Samuel, P Emanuela Voinescu, Brendan N Lilley & Joshua R Sanes

  2. Laboratory of Experimental Gerontology, Translational Gerontology Branch, Intramural Research Program, National Institute on Aging, Baltimore, Maryland, USA

    Rafa de Cabo

  3. Inserm, U1016, Institut Cochin, Paris, France

    Marc Foretz & Benoit Viollet

  4. CNRS, UMR8104, Paris, France

    Marc Foretz & Benoit Viollet

  5. Université Paris Descartes, Sorbonne Paris Cité, Paris, France

    Marc Foretz & Benoit Viollet

  6. Department of Ophthalmology, The Berman-Gund Laboratory for the Study of Retinal Degenerations, Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, Massachusetts, USA

    Basil Pawlyk & Michael A Sandberg

  7. Department of Ophthalmology, Retina Service, Angiogenesis Laboratory, Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, Massachusetts, USA

    Demetrios G Vavvas

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Contributions

M.A. Samuel, P.E.V. and J.R.S. designed the experiments, and M.A. Samuel and P.E.V. executed them. M.A. Samuel characterized the LKB1 mutant animals, conducted the aging studies, performed the confocal imaging and the quantification and analysis, and generated all of the figures. P.E.V. provided the first description of the LKB1 mutant animal phenotype and generated the CA-AMPK AAV. B.N.L. helped with initial observations in the LKB mutants and provided the SAD conditional knockouts used in this study. R.d.C. provided the cohorts of aged, treated mice. M.F. and B.V. generated the AMPK conditional animals. B.P. and M.A. Sandberg conducted the ERG experiments. D.G.V. provided guidance on AMPK and AMPK mutants and edited the manuscript. M.A. Samuel and J.R.S wrote the manuscript.

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Correspondence to Joshua R Sanes.

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Integrated supplementary information

Supplementary Figure 1 Overall tissue organization and cellular morphology are normal in LKB1 mutants and old mice, but age-related changes develop in the outer plexiform layer.

a. Overall retinal organization is normal. Retinal sections from old wild-type animals (24M) and adult LKB1 mutants and controls (3M) were labeled with a nuclear marker (green) and an antibody to rod bipolar cells (PKCα, red). Scale bar =50μm. b-c. Age-related changes develop progressively in young LKB1 mutant mice. Retinal sections from LKB1 mutants, control animals and old wild-type mice were labeled with an antibody to rod bipolar cells (PKCα, green) and horizontal cells (calbindin, red). LKB1-deficient retina develop normally for 2 weeks, but by the third and fourth postnatal week extend ectopic bipolar and horizontal cell processes (b) similar to those seen in old wild-type mice. In wild-type mice, ectopic bipolar and horizontal cell processes first occur at ∼12 months. By 24 months, numerous ectopic bipolar and horizontal cell processes are observed (c). Scale bar = 25μm. ONL, outer nuclear layer; IPL, inner plexiform layer; INL, inner nuclear layer; IPL, inner plexiform layer; GCL, ganglion cell layer.

Supplementary Figure 2 LKB1-deficient and old wild-type retina show parallel remodeling of the outer plexiform layer.

a-b. Ectopic rod bipolar and horizontal cell processes cofasiculate. Retinal sections from old wild-type animals (24M, a) and adult LKB1 mutants (3M, b) were stained for rod bipolar (PKCα, green) and horizontal cells (calbindin, red). In both cases, these cells extend long ectopic processes that cofasiculate (arrows) in the outer nuclear layer (blue). Scale bar = 15μm. c. Young LKB1-deficient and old wild-type retina show parallel changes in the outer plexiform layer. Retinal sections from old wild-type and young control and LKB1 mutant mice were visualized using a nuclear marker. The arrow indicates the OPL, which thins in both old and LKB1 mutants relative to controls. Scale bar = 50μm. d. Low power electron microscopic images of the OPL. OPL thinning in LKB1-deficient and old wild-type mice is accompanied by the presence of misplaced cells (arrows) that appear within the boundaries of the OPL (dotted lines). Scale bar = 5μm. ONL, outer nuclear layer; OPL, outer plexiform layer; INL, inner nuclear layer.

Supplementary Figure 3 Young LKB1-deficient and old wild-type mice express an array of synaptic components.

A panel of synaptic markers (green) was used to label retinal sections from young control (3M), LKB1 mutant (3M) and old wild-type mice (24M). In each case, the synaptic markers labeled puncta located along ectopic rod bipolar or horizontal cell processes (red). PKCα, Protein kinase Cα, rod bipolar cells; calbindin, horizontal cells; RIBEYE, ribbon marker; Piccolo, presynaptic synapse active zone protein; PSD95, postsynaptic density 95, scaffolding protein in photoreceptor terminals; dystrophin, scaffolding protein in photoreceptor terminals; CACNA1, postsynaptic protein. Scale bar = 25μm.

Supplementary Figure 4 Lkb1 heterozygous retina display normal levels of LKB1 and proper outer retina organization.

a-b. LKB1 levels are not reduced in retina from Lkb1F/+ Chx10Cre+ or Lkb1 F/+ Six3Cre+ heterozygote animals as assayed by immunoblot analysis (a, quantified b). c. The outer retina is properly organized in Lkb1 heterozygotes. Retinal sections from Lkb1F/+ Chx10Cre+ and Lkb1 F/+ Six3Cre+ animals were stained for horizontal cells (calbindin, green) and a nuclear marker (TO-PRO-3, blue). Processes appeared normal and were restricted to the outer plexiform layer. Scale bar = 50μm. Full-length blots are presented in Supplementary Figure 11.

Supplementary Figure 5 LKB1-deficient animals display reduced retinal function.

Representative dark-adapted (DA) and light-adapted (LA) ERG waveforms from a control and a LKB1 mutant mouse. The waveforms from the LKB1 mutant demonstrate a markedly reduced rod ERG as well as a diminished cone ERG.

Supplementary Figure 6 AAV2/5 infects photoreceptor neurons.

Retinas of control wildtype mice were infected with AAV2/5-GFP to visualize viral localization. Infection (green) was dominant in the photoreceptor layer (ONL), while other neurons were infected only rarely. ONL, outer nuclear layer; IPL, inner plexiform layer; INL, inner nuclear layer; IPL, inner plexiform layer; GCL, ganglion cell layer. Nuclei, blue; AAV2/5, green. Scale bar = 50μm.

Supplementary Figure 7 Ampk and Lkb1 are broadly expressed in the retina.

Retinal sections from young wild-type mice were examined by in situ using antisense probes specific for Ampka2 (a), Lkb1 (b) or a control sense probe for Lkb1 (c). Lkb1 and Ampk were both expressed throughout the retina, including in photoreceptors. ONL, outer nuclear layer; INL, inner nuclear layer; GCL, ganglion cell layer. Scale bar = 50μm.

Supplementary Figure 8 Retinal deletion of SAD kinases or TAK1 does not induce age-related rewiring.

To define the specificity of the LKB1-AMPK pathway in modulating age-related rewiring, we examined retinas lacking the LKB1 targets SAD-A and -B (a-c) as well those deficient in an alternate AMPK activator, TAK1 (d-f). a, d. Retinas were stained with antibodies to rod bipolar cells (PKCα, green) and horizontal cells (calbindin, red). In all cases, no age-related ectopic sprouting was observed. b-c, e-f. Retinas were stained with Bassoon (green) and with a nuclear marker (blue). Boxed regions are shown as higher magnification images. Mutants failed to develop age-related synaptic changes in the outer retina. Scale bar = 50μm.

Supplementary Figure 9 Rods retract in old wild-type animals and young LKB1 mutants.

Retinas of Lkb1ret and Lkb1rod mutants (3M) and old wild-type mice (24M) were infected with AAV2/5-GFP to visualize rods and their terminals. Low magnification images show that rod terminals frequently retract into the outer nuclear layer (arrows). Nuclei, blue; AAV2/5, green. Scale bar = 50μm.

Supplementary Figure 10 Cones and their synaptic partners are subtly altered in LKB1 mutants or old animals.

a. Retinal sections from young control and LKB1 mutants and old wild-type mice were stained with antibodies to cone bipolar cells [type 3a (HCN4); and type 4 (KChip)] (Wässle et al., 2009). Sprouts were detected only rarely among these cells (arrows), which make few rod connections. Scale bar = 25μm. b-c. Retinal sections from young adult control or Lkb1ret mice and old wild-type animals were labeled with mouse cone arrestin antibody. Cone pedicle endings do not retract (black arrow, b; scale bar = 50μm) beyond the OPL into the ONL. However, subtle alterations in cone terminal structure were observed (white arrows, c; scale bar = 15μm).

Supplementary Figure 11 Immunoblots and molecular weight standards.

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Samuel, M., Voinescu, P., Lilley, B. et al. LKB1 and AMPK regulate synaptic remodeling in old age. Nat Neurosci 17, 1190–1197 (2014). https://doi.org/10.1038/nn.3772

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