The research on Angelman Syndrome which was published last year in Nature NeuroScience last year has been chosen as the scientific highlight of the year by the Federation of European Neuroscience Societies (see the full article below). This is great news and the Nina Foundation congratulates everybody involved in this research. We are convinced that this has a positive effect on the research into the Angelman Syndrome and related disorders.

Rescue of neurological deficits in a mouse model for Angelman syndrome by reduction of alphaCaMKII inhibitory phosphorylation

Van Woerden GM, Harris KD, Hojjati MR, Gustin RM, Qiu S, de Avila Freire R, Jiang YH, Elgersma Y, Weeber EJ
Nat Neurosci 2007, 10:280-282

Summary:

What is Angelman Syndrome?
Angelman Syndrome (AS) is a severe neurological disorder affecting 1:15.000 children and is characterized by mental retardation, motor dysfunction, absence of speech, epilepsy and autism. 10-15% of the AS patients have a mutation in the UBE3A gene, which encodes for the E6 associated protein (E6-AP), which is a member of the E3A family of ubiquitin protein ligases and involved in protein degradation. The paternal copy of the UBE3A gene is imprinted in certain cell types, in particular hippocampal neurons and cerebellar Purkinje cells, so these cells show exclusive expression of the maternally inherited copy. Hence, a de novo mutation in the maternally inherited copy of the UBE3A gene will result in the complete loss of E6-AP expression in these brain areas. Because of these findings, the (maternally inherited) Ube3a mutant is now commonly used as a mouse model of the disease. This mutant shows epilepsy, and cognitive deficits (1).

What was already known?
The highlighted study builds directly on two earlier studies.
First, Edwin Weeber (at that time a postdoc in the lab of David Sweatt) and his colleagues found that the activity of calcium/calmodulin-dependent kinase Type 2 (CaMKII) was decreased by approximately 30% in the Ube3a mutant mice, even though the protein amount of CaMKII was unaffected (2).
In parallel, Ype Elgersma (at that time a postdoc in Alcino Silva's lab) showed that CaMKII can inhibit itself by autophosphorylation at Thr305 and Thr306. Furthermore he showed that expression of the constitutive inhibited CaMKII, results in a severe dominant negative phenotype. Hippocampal dependent learning as well plasticity was abolished in this mutant (3).
Edwin Weeber and Ype Elgersma exchanged these findings and decided to investigate whether the reduced CaMKII activity in the Ube3a mutant was indeed due to increased inhibitory phosphorylation at Thr305/Thr306. This turned out to be indeed the case (2).

The highlighted paper.
Although the increased self-inhibition of CaMKII could well explain the phenotype of the Ube3a mutant, the question was still out, whether this was indeed really the case. There could well be a number of other deregulated pathways in Ube3a mutants, each of them contributing to the phenotype.
To test to which extend the increased self-inhibition of CaMKII was responsible for the cognitive deficits of the Ube3a mice, we used an αCaMKII-TT305/6VA mutant, in which the amino acids Thr305/306 were substituted by two nonphosphorylatable residues (Val and Ala) (3). Homozygous loss of αCaMKII self-inhibition results in several behavioral impairments, but heterozygous αCaMKII-TT305/6VA mice showed no discernible phenotype. Thus, crossing this mutation into mice carrying the Ube3a mutation might significantly decrease the amount of inhibited CaMKII and result in some improvement of the observed deficits. To our surprise this was indeed case (4). Both in our (Elgersma) lab, as well as in the Weeber lab, it was found that the Ube3a/αCaMKII-TT305/6VA double mutants showed a decreased amount of inhibitory phosphorylation, and nearly restored levels of CaMKII activity. More importantly, we showed that the learning and motor deficits were completely rescued, and plasticity as measured in brain slices was back to normal. Another important feature of the disease, epilepsy, was still present in some of the double mutants, but the number of animals showing epilepsy was threefold reduced as compared with the Ube3a single mutants. Finally, it was noted that the moderate (but significant) increase in bodyweight of the Ube3a mice was rescued. Although overweight is not known as a characteristic feature of the disease, overweight is observed in the majority (85%) of AS children carrying the UBE3A mutation. Thus, even in that respect the Ube3a mouse turned out to be a good model, and the absence of increased bodyweight in the double mutants indicated that increased self-inhibition of CaMKII was also responsible for this aspect of the disease.

Implication and future direction.
Knowing that the increased inhibition of CaMKII is responsible for the major neurological hallmarks, is an important step to understand the mechanisms underlying this devastating disorder. Future studies should focus on the link between the absence of Ube3a and the increased self-inhibition of CaMKII. Hopefully this knowledge will lead to the development of therapies.

References
1. Y. H. Jiang et al., Neuron 21, 799 (1998)
2. E. J. Weeber et al., J Neurosci 23, 2634 (2003)
3. Y. Elgersma et al., Neuron 36, 493 (2002)
4. G. M. van Woerden et al., Nat Neurosci 10, 280 (2007)