This study by L. Mucke and co-workers deserves credit for analyzing the "ApoE-isoform effect" in AD, a problem that is far from understood at the physiological level. The interesting results are situated on two levels:(i)synaptic deficit is evident in non-plaque bearing APP mice and (ii)ApoE3 but not ApoE4 delays the age- and Aß-dependent synaptic deficits. The early synaptic deficits are indeed independent of plaque formation, since they are evident in the form of decreased LTP and defective cognition (water-maze and object recogition) in APP-mice long before plaques form (Moechars et al, JBC, 1999, 274:6483-6492) and in mice that lack neuronal PS1 (Dewachter et al, J Neurosci., 2002, 22:3445-53). This is likely due to defective calcium-homeostasis (Herms et al, JBC, 2003, 275: 2484-2489). That "AD is a synaptic disease" has thereby been demonstrated, at least in the mouse models that we have generated over the years. A note of caution to the second conclusion is in place: the mice express the human ApoE isoforms in neurons as driven by the neuron-specific enolase gene...  Read more

This study by L. Mucke and co-workers deserves credit for analyzing the "ApoE-isoform effect" in AD, a problem that is far from understood at the physiological level. The interesting results are situated on two levels:(i)synaptic deficit is evident in non-plaque bearing APP mice and (ii)ApoE3 but not ApoE4 delays the age- and Aß-dependent synaptic deficits. The early synaptic deficits are indeed independent of plaque formation, since they are evident in the form of decreased LTP and defective cognition (water-maze and object recogition) in APP-mice long before plaques form (Moechars et al, JBC, 1999, 274:6483-6492) and in mice that lack neuronal PS1 (Dewachter et al, J Neurosci., 2002, 22:3445-53). This is likely due to defective calcium-homeostasis (Herms et al, JBC, 2003, 275: 2484-2489). That "AD is a synaptic disease" has thereby been demonstrated, at least in the mouse models that we have generated over the years. A note of caution to the second conclusion is in place: the mice express the human ApoE isoforms in neurons as driven by the neuron-specific enolase gene promoter. Neuronal expression of ApoE, as originally hypothesised by Alan Roses, does affect neuronal functions and integrity by intefering with tau and micro-tubular transport (Tesseur et al, Am J Pathol.,2000,156:951-964 and 157:1495-1510). It is evident and even likely that this would affect synaptic functioning and confound the observations, which need therefore further analysis to confirm their importance.

View all comments by Fred Van Leuven

I would like to add an important piece to the ApoE4 story, namely ApoE4 domain interaction. Since protein function is directly linked to protein structure and biophysical properties, our studies have focused on determining how ApoE4 differs from ApoE3 and ApoE2. One of the differences is protein stability and the formation of a molten globule state mentioned in the news story. However, a second difference is that ApoE4 exhibits interaction between its two structural domains, referred to as ApoE4 domain interaction. This interaction results from a change in the conformation of arginine at position 61, which both ApoE3 and ApoE4 share. This conformational change is the result of an arginine at 112 in ApoE4 versus a cysteine at this position in ApoE3. X-ray structural analysis and site-directed mutagenesis show that in this interaction, ApoE4’s arginine at position 61 in the amino-terminal domain interacts with glutamic acid in the carboxyl-terminal domain. We hypothesized that this interaction alters the conformation of ApoE4 and may also be a contributing factor, along with molten...  Read more

I would like to add an important piece to the ApoE4 story, namely ApoE4 domain interaction. Since protein function is directly linked to protein structure and biophysical properties, our studies have focused on determining how ApoE4 differs from ApoE3 and ApoE2. One of the differences is protein stability and the formation of a molten globule state mentioned in the news story. However, a second difference is that ApoE4 exhibits interaction between its two structural domains, referred to as ApoE4 domain interaction. This interaction results from a change in the conformation of arginine at position 61, which both ApoE3 and ApoE4 share. This conformational change is the result of an arginine at 112 in ApoE4 versus a cysteine at this position in ApoE3. X-ray structural analysis and site-directed mutagenesis show that in this interaction, ApoE4’s arginine at position 61 in the amino-terminal domain interacts with glutamic acid in the carboxyl-terminal domain. We hypothesized that this interaction alters the conformation of ApoE4 and may also be a contributing factor, along with molten globule formation, to the detrimental effects of ApoE4 in both neurodegeneration and heart disease. See Dong and Weisgraber, 1996 for details.

We have engineered mouse ApoE to reflect this property [PNAS (2001) 98 11587]. Mouse ApoE behaves like human ApoE3 because it lacks the critical arginine 61; it contains the equivalent of arginine 112 and glutamic acid 255. Using gene-targeting we introduced arginine 61, and the mutant mouse ApoE exhibited properties mirroring human ApoE4. Thus, this mouse is a model for this specific property of ApoE4. This mouse mutant does not form a molten globule. A search for small molecule inhibitors that Bob Mahley and I are leading is based on the concept of interfering with ApoE4 domain interaction.

View all comments by Karl Weisgraber

The increasing number of isoform-specific pathological effects of ApoE4, which were so effectively reviewed by Gabrielle Strobel, suggest that ApoE4 may exert its pathological effects by several different mechanisms whose relative importance are context-dependent.

In order to assess this possibility, we reviewed the published information regarding the phenotypic effects of the ApoE genotype on neuronal maintenance and repair in ApoE transgenic mice. The results thus obtained (see table below) are from three different lines of transgenic mice that express ApoE either in both neurons and astrocytes, or in only one of these cell types, and which were exposed to aging,       ^(4-6,8,9) head injury,² excitotoxicity, ⁷ brain inflammation³ and environmental stimulation1 paradigms. The nine different paradigms thus examined yielded three phenotypic categories which are each defined by the phenotypes of the four mice groups used in these studies, namely ApoE3- and ApoE4-transgenic mice, control and ApoE-deficient mice.

Accordingly, the...  Read more

The increasing number of isoform-specific pathological effects of ApoE4, which were so effectively reviewed by Gabrielle Strobel, suggest that ApoE4 may exert its pathological effects by several different mechanisms whose relative importance are context-dependent.

In order to assess this possibility, we reviewed the published information regarding the phenotypic effects of the ApoE genotype on neuronal maintenance and repair in ApoE transgenic mice. The results thus obtained (see table below) are from three different lines of transgenic mice that express ApoE either in both neurons and astrocytes, or in only one of these cell types, and which were exposed to aging,       ^(4-6,8,9) head injury,² excitotoxicity, ⁷ brain inflammation³ and environmental stimulation1 paradigms. The nine different paradigms thus examined yielded three phenotypic categories which are each defined by the phenotypes of the four mice groups used in these studies, namely ApoE3- and ApoE4-transgenic mice, control and ApoE-deficient mice.

Accordingly, the "ApoE gain of toxic function" category corresponds to paradigms in which ApoE4 confers a negative phenotype (e.g., increased mortality) relative to the control, ApoE3 transgenic and ApoE-deficient mice. The second category is "ApoE4 loss of function;" it corresponds to paradigms in which control and ApoE3 transgenic mice have the same phenotype, whereas ApoE deficiency and the ApoE4 genotype are associated with loss of this phenotypic feature. The third and less frequent category is "ApoE3 gain of function" and is associated with a gain of function in the ApoE3 transgenic mice (e.g., enhanced recovery following head trauma) relative to the three other mice groups. Interestingly, the transgenic mice that express ApoE in neurons as well as astrocytes display both the ApoE4 gain-of-toxic-function and ApoE4 loss-of-function phenotypes. In contrast, transgenic mice that express ApoE only in neurons display the ApoE4 loss-of-function phenotype, whereas those that express ApoE only in astrocytes are associated with an ApoE4 gain-of-toxic-function phenotype. This suggests that the pathological effects of ApoE4 on neuronal maintenance and repair are mediated by several mechanisms whose expression and cellular targets may be context- and paradigm-dependent. In addition to Alzheimer’s disease, ApoE4 plays a role in the pathogenesis of multiple sclerosis and several other diseases,¹⁰ and it remains to be determined which of the ApoE4-related mechanisms mediate the phenotypic effects of the ApoE4 genotype in these diseases.—Danny Michaelson, Tel-Aviv University, Israel.

References:

References:
1. Levi, U. and Michaelson, D.M. (2000) Proceedings of the 32nd Meeting of the Society for Neuroscience.

2. Sabo T et al. Susceptibility of transgenic mice expressing human apolipoprotein E to closed head injury: the allele E3 is neuroprotective whereas E4 increases fatalities. Neuroscience 2000;101(4):879-84. Abstract

3. Ophir G et al. Neurobiol. of Diseases. 2003. In press.

4. Veinbergs et al. Differential neurotrophic effects of apolipoprotein E in aged transgenic mice. Neurosci Lett 1999 Apr 23;265(3):218-22. Abstract

5. Buttini M et al. Modulation of Alzheimer-like synaptic and cholinergic deficits in transgenic mice by human apolipoprotein Edepends on isoform, aging, and overexpression of amyloidbetapeptides but not on plaque formation. J Neurosci2002Dec15;22(24):10539-48. Abstract

6. Raber J et al. Isoform-specific effects of human apolipoprotein E on brain function revealed in ApoE knockout mice: increased susceptibility of females. Proc Natl Acad Sci U S A 1998 Sep 1;95(18):10914-9. Abstract

7. Buttini M et al. Expression of human apolipoprotein E3 or E4 in the brains of Apoe-/- mice: isoform-specific effects on neurodegeneration. J Neurosci 1999 Jun 15;19(12):4867-80. Abstract

8. Hartman R et al. Behavioral phenotyping of GFAP-apoE3 and -apoE4 transgenic mice: apoE4 mice show profound working memory impairments in the absence of Alzheimer's-like neuropathology. Exp Neurol 2001 Aug;170(2):326-44. Abstract

9. Holtzman DM et al. Apolipoprotein E isoform-dependent amyloid deposition and neuritic degeneration in a mouse model of Alzheimer's disease. Proc Natl Acad Sci U S A. 2000 Mar 14;97(6):2892-7. Abstract

10. Chapman J et al. The effects of APOE genotype on age at onset and progression of neurodegenerative diseases. Neurology 2001 Oct 23;57(8):1482-5. Abstract

View all comments by Daniel Michaelson