Neurons and synapses follow a “use it or lose it” paradigm—regular nerve firing promotes survival as well as maintenance of the dendrite tree. A paper in last week's Neuron links neural activity and dendritic branching to memory, via nuclear calcium and vascular endothelial growth factor D (VEGF-D). Without this signaling, dendrites cannot maintain their full complexity, limiting learning and memory. The work reveals a role for VEGF-D above and beyond support for blood vessels, and suggests that therapeutics targeting the factor, such as those used in cancer treatment, could have unwanted side effects on cognition. First author Daniela Mauceri and senior author Hilmar Bading of the University of Heidelberg, Germany, led the study.

The evidence for VEGF-D’s involvement in arborization and memory is “very compelling,” wrote Albert La Spada of the University of California in San Diego in an e-mail to ARF.

When a nerve receives and transmits an action potential, it causes calcium to enter the nucleus, where the anion partners with calmodulin to activate gene transcription (reviewed in Greer and Greenberg, 2008). Bading’s team previously found that neuron survival depends on this nuclear calcium signaling (Lau and Bading, 2009; Papadia et al., 2005). In the current work, the scientists turned their attention to the role of calcium signaling in dendrite morphology.

When Mauceri expressed a calmodulin-blocking peptide, M13, in primary hippocampal neuron cultures, they observed shorter, less complex dendrite arbors with fewer, skimpier spines than in cells with normal calcium activity. The scientists went back to a transcriptome analysis they had already performed (Zhang et al., 2009) to identify VEGF-D as a candidate mediator of the arborization phenotype. VEGF-D turns on in response to both nuclear calcium and calcium/calmodulin-dependent protein kinase IV (CaMKIV), which is involved in dendrite and spine morphology (Chow et al., 2005).

To confirm the vascular factor’s role in arborization, Mauceri added recombinant VEGF-D protein to the cultured neurons. It rescued the dendrite phenotype caused by M13 treatment. She also used RNA interference to block VEGF-D production in neurons with normal calcium activity. The dendrites lost complexity just as if calcium were blocked. Unlike M13, neither VEGF-D nor its interfering RNA affected dendritic spines, suggesting that some other pathway involving calcium controls their density and morphology.

Next, the researchers took their work in vivo. Mauceri injected the hippocampi of adult mice with an adeno-associated virus carrying the anti-VEGF-D RNA, selectively knocking out the gene. The neural morphology mirrored that seen in vitro: short dendrites with less branching than normal.

One might think that, once dendrites set up their branches, they stay the same, Bading said. But the evidence says otherwise. “It looks like you really need something to maintain this arborization,” he said.

That maintenance is key to learning and memory, the researchers discovered. They tested mice that lacked hippocampal VEGF-D, and mice injected with a scrambled RNA interference sequence, in both the Morris water maze and a contextual fear-conditioning task. The VEGF knockdowns had trouble recalling the location of a hidden platform in the former, and were less likely to remember which chamber led to an unpleasant foot shock in the latter. “We saw a very dramatic phenotype,” Bading said. “Probably the circuitry is not connecting in the right way.”

Loss of VEGF-D activity could also be involved in neurodegeneration, Bading speculated. “I would be very surprised if there are not pathological conditions where VEGF-D expression falls…which could then have impact on brain function,” he said. There is evidence of dendritic atrophy in aging and Alzheimer’s (see review by Dickstein et al., 2007). Other members of the VEGF family are involved in neurogenesis, neuroprotection, and learning and memory (Cao et al., 2004; Poesen et al., 2008; Le Bras et al., 2006; Licht et al., 2010). Certain VEGF-A alleles increase risk of amyotrophic lateral sclerosis due to reduced VEGF-A protein levels (Lambrechts et al., 2003; Lambrechts et al., 2008). However, Bading and colleagues did not find any evidence that VEGF members, other than VEGF-D, are involved in dendritic arborization.

While they are protective in nerves, VEGF proteins enhance angiogenesis, which can support tumor growth (Achen and Stacker, 2008; Stacker et al., 2001). Researchers are pursuing VEGF-D blockers as cancer treatments (Heath and Bicknell, 2009). Unfortunately, preventing VEGF-D activity in the nervous system could impair cognition. “If VEGF-D blockers get into the brain, it could be bad news,” Bading said.

Bading noted that there is an intriguing similarity between the protein’s neural and vascular roles: In both places, it promotes branching. The scientists have not yet worked out the full mechanism between VEGF-D signaling and dendritic arborization; VEGF-D appears to bind the type 3 VEGF receptor and activate the MAP kinase p38, but it is not clear if VEGF-D-mediated pathways toward angiogenesis and dendrite arborization are the same. La Spada suggested that a more detailed understanding of these mechanisms might help scientists design drugs that only target one or the other.—Amber Dance

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  1. I find this work to be very compelling. A role for VEGF in motor neuron disease was suggested by studies done on ALS and by work that we have done on spinobulbar muscular atrophy. Hence, numerous independent studies have now shown that VEGF may be a neurotrophic factor and has neuroprotective properties. This most recent work from the Bading group confirms such findings, but goes further, as they were able to show that knockdown of VEGF-D in mice resulted in impaired memory formation in vivo. While there has been interest in the potential of VEGF as a neurotherapeutic, VEGF has angiogenic properties, raising concern that it could cause side effects. These researchers report that VEGF-R3 is the receptor that likely mediates the elaboration of dendritic arbors. If this is true, then future studies aimed at defining the downstream pathway by which VEGF-R3 activation yields dendritic arborization could identify targets for therapy development, since neuroprotective pathways might be distinct from angiogenic ones.

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References

Paper Citations

  1. . From synapse to nucleus: calcium-dependent gene transcription in the control of synapse development and function. Neuron. 2008 Sep 25;59(6):846-60. PubMed.
  2. . Synaptic activity-mediated suppression of p53 and induction of nuclear calcium-regulated neuroprotective genes promote survival through inhibition of mitochondrial permeability transition. J Neurosci. 2009 Apr 8;29(14):4420-9. PubMed.
  3. . Nuclear Ca2+ and the cAMP response element-binding protein family mediate a late phase of activity-dependent neuroprotection. J Neurosci. 2005 Apr 27;25(17):4279-87. PubMed.
  4. . Nuclear calcium signaling controls expression of a large gene pool: identification of a gene program for acquired neuroprotection induced by synaptic activity. PLoS Genet. 2009 Aug;5(8):e1000604. PubMed.
  5. . The autonomous activity of calcium/calmodulin-dependent protein kinase IV is required for its role in transcription. J Biol Chem. 2005 May 27;280(21):20530-8. PubMed.
  6. . Changes in the structural complexity of the aged brain. Aging Cell. 2007 Jun;6(3):275-84. PubMed.
  7. . VEGF links hippocampal activity with neurogenesis, learning and memory. Nat Genet. 2004 Aug;36(8):827-35. PubMed.
  8. . Novel role for vascular endothelial growth factor (VEGF) receptor-1 and its ligand VEGF-B in motor neuron degeneration. J Neurosci. 2008 Oct 15;28(42):10451-9. PubMed.
  9. . VEGF-C is a trophic factor for neural progenitors in the vertebrate embryonic brain. Nat Neurosci. 2006 Mar;9(3):340-8. PubMed.
  10. . VEGF is required for dendritogenesis of newly born olfactory bulb interneurons. Development. 2010 Jan;137(2):261-71. PubMed.
  11. . VEGF is a modifier of amyotrophic lateral sclerosis in mice and humans and protects motoneurons against ischemic death. Nat Genet. 2003 Aug;34(4):383-94. PubMed.
  12. . Meta-analysis of vascular endothelial growth factor variations in amyotrophic lateral sclerosis: increased susceptibility in male carriers of the -2578AA genotype. J Med Genet. 2009 Dec;46(12):840-6. PubMed.
  13. . Molecular control of lymphatic metastasis. Ann N Y Acad Sci. 2008;1131:225-34. PubMed.
  14. . VEGF-D promotes the metastatic spread of tumor cells via the lymphatics. Nat Med. 2001 Feb;7(2):186-91. PubMed.
  15. . Anticancer strategies involving the vasculature. Nat Rev Clin Oncol. 2009 Jul;6(7):395-404. PubMed.

Further Reading

Papers

  1. . Calcium regulation of dendritic growth via CaM kinase IV and CREB-mediated transcription. Neuron. 2002 Jun 13;34(6):999-1010. PubMed.
  2. . Regulation of dendritic development by neuronal activity. J Neurobiol. 2005 Jul;64(1):4-10. PubMed.
  3. . Nuclear calcium/calmodulin regulates memory consolidation. J Neurosci. 2004 Dec 1;24(48):10858-67. PubMed.

Primary Papers

  1. . Nuclear calcium-VEGFD signaling controls maintenance of dendrite arborization necessary for memory formation. Neuron. 2011 Jul 14;71(1):117-30. PubMed.