This work from the Herz lab presents an interesting set of findings. The mice they generated express ApoE in the periphery but not in the brain.
The main finding is that there is a decrease in the synaptic marker synaptophysin in the neocortex in both full ApoE KO mice and in brain-specific ApoE KO mice.
The ApoE KO mice also have decreased memory retention in the water maze (only in females), decreased LTP, and decreased EPSCs due to altered NMDA/AMPA ratios, none of which are seen in the brain-specific ApoE KO mice.
These results suggest that in the setting of complete ApoE KO, which results in very high plasma cholesterol levels as well as no ApoE in plasma, that the very high lipids in some way influence brain function negatively.
This could be via things such as excessive entry of certain lipids into the brain, damage to the BBB, or altered brain vasculature function. The only abnormality shown in the brain-specific ApoE KO mice was the decrease in synaptophysin staining in the neocortex (but not in the hippocampus). It is interesting this is found in the presence of normal behavioral performance. It is possible that this abnormality arose during development or in adult mice. It would be interesting to assess other synaptic markers in these mice as well as the structure of synapses to see if they are altered in any way. The absence of brain ApoE has little effect on the brain functions analyzed. It will be important to determine the effect of lowering ApoE in the setting of brain injury or disease to see whether positive or negative effects of ApoE are uncovered. This paper highlights the potential importance of high plasma lipids and how this can negatively impact brain function.
This is a very interesting initial look at a new model with selective brain ApoE KO that still shows some synaptic deficits but not the supposedly related cognitive deficits. The authors view this as entirely due to correction of peripheral hyperlipidemia, but it may also be due to ApoE’s well-known impact on inflammatory signaling. ApoE promotes negative inhibitory feedback of miR146a over toll and IL-1 receptor NFkappB signaling in peripheral compartments, including vascular immune cells. We recently extended this to brain and showed ApoE4 targeted replacement mice have deficits in peripheral and central miR146a (Teter et al., 2016). So, restoration of peripheral ApoE should dampen vascular pro-inflammatory signaling that impacts the neurovascular unit where invading myeloid cells are most relevant to pericyte populations.
It will be interesting to see how well exercise works in the aging brain exercise model where ApoE mediated exercise effects on neuroinflammation (Soto et al., 2016). It will be interesting if Herz and colleagues study neuroinflammation in the brains of these bEKO mice, particularly in the context of aging or some proinflammatory drivers at earlier ages (high fat diet, LPS, or APP Tg/ amyloid). It seems likely that the immunoregulatory effects of peripheral ApoE will play a role in the CNS where the role of the immune system is increasingly relevant.
References:
Teter B, LaDu MJ, Sullivan PM, Frautschy SA, Cole GM.
Apolipoprotein E isotype-dependent modulation of microRNA-146a in plasma and brain.
Neuroreport. 2016 Aug 3;27(11):791-5.
PubMed.
The Herz laboratory generated an important novel mouse model (bEKO) that expresses ApoE normally in the periphery and none in the brain. They compared this model to ApoE knockouts (ApoE KO), which show a profound elevation in plasma cholesterol and triglyceride levels, and ApoE3 homozygous targeted replacement (TR) mice that express the protein everywhere. The bEKO model allows us to study the contribution of ApoE in and outside the brain to CNS function. bEKO mice look like ApoE KO mice in terms of synaptophysin in the neocortex, with lower levels in both as compared to those seen in ApoE3 TR mice. When synaptic physiology is analyzed in hippocampal slices, bEKO and ApoE KO mice both show impaired long-term potentiation (LTP) without alterations in paired-pulse facilitation, as compared to ApoE3 TR mice. However, hippocampal slices of ApoE KO, but not bEKO, mice show lower AMPA/NMDA ratios than those seen in hippocampal slices from ApoE3 TR mice.
When ApoE KO mice are tested for spatial learning and memory in a water maze paradigm in which mice are first trained to locate a hidden platform, female, but not male, ApoE KO mice show an impaired ability to locate the hidden platform. This is consistent with our 1998 study, where female mice expressing ApoE4 in the brain showed impairments in the water maze task compared to males (Raber et al., 1998). These sex differences are interesting by themselves, considering the previously reported sex-dependent effects of ApoE4 on cognitive performance of ApoE KO mice that express ApoE4 in the brain and the ability of androgens (Raber et al., 2002) and selective androgen receptor modulators (Acevedo et al., 2008) to treat them, in addition to the sex differences in humans with ApoE4 in risk to develop AD (Farrer et al., 1997).
However, in another previous study, when ApoE KO mice were first trained to locate a visible platform, female ApoE KO mice showed no impairment when they were subsequently trained to locate a hidden platform (Raber et al., 2000). So it is possible that increased measures of anxiety and activation of the hypothalamic-pituitary-adrenal (HPA) axis in the ApoE KO mice (Raber et al., 2000; Robertson et al., 2005) and altered histamine receptor-mediated signaling (Bongers et al., 2003) might contribute to the poor performance of ApoE KO mice in the water maze version in which the mice are trained first to locate a hidden platform. For HPA axis activation, the deficiency of ApoE in the adrenal gland might be critical. Restraint stress-induced plasma corticosterone levels in ApoE KO mice expressing ApoE3 or ApoE4 only in the brain are not significantly different and comparable to those in ApoE KO mice without human ApoE expression (Raber et al., 2000). The bEKO model will allow us to study the role of ApoE in the periphery, including the adrenal gland, in modulating measures of anxiety and HPA axis activation when there is very little ApoE in brain. The ability to compare bEKO mice expressing different ApoE isoforms would be ideal for such studies.
Our previous study reported that ApoE KO mice showed no impairment either in spatial memory retention in the probe trial (platform removed), even when tested at 18 months of age. Age-matched ApoE KO mice expressing ApoE4 only in the brain did show impairment in spatial memory impairment in the water maze probe trial. Based on these results, increasing ApoE4 levels in brain is unlikely beneficial for cognitive performance.
Although the ApoE KO mice show a loss of synaptic markers in the Lane-Donovan study, the cognitive impairments are subtle and sex-dependent, as acknowledged by the authors. However, in previous studies, ApoE KO mice showed profound impairments in cognitive tests other than the water maze that might have been more related to impaired cholinergic function than impaired glutamatergic function. The ApoE KO mice have a reduced number of cortical and hippocampal muscarinic acetylcholine receptors (mAChRs), are more sensitive to effects of scopolamine on delay eyeblink classical conditioning, and are unable to acquire performance on the five-choice serial reaction time tasks (5-SRTT) (Siegel et al., 2011). We should be open to the possibility that ApoE has other effects on cognition that aren’t examined here.
References:
Raber J, Wong D, Buttini M, Orth M, Bellosta S, Pitas RE, Mahley RW, Mucke L.
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.
PubMed.
Raber J, Bongers G, LeFevour A, Buttini M, Mucke L.
Androgens protect against apolipoprotein E4-induced cognitive deficits.
J Neurosci. 2002 Jun 15;22(12):5204-9.
PubMed.
Acevedo S, Gardell L, Bradley SR, Piu F, Raber J.
Selective Androgen Receptor Modulators Antagonize Apolipoprotein E4-Induced Cognitive Impairments.
Lett Drug Des Discov. 2008;5(4):271-6.
Farrer LA, Cupples LA, Haines JL, Hyman B, Kukull WA, Mayeux R, Myers RH, Pericak-Vance MA, Risch N, van Duijn CM.
Effects of age, sex, and ethnicity on the association between apolipoprotein E genotype and Alzheimer disease. A meta-analysis. APOE and Alzheimer Disease Meta Analysis Consortium.
JAMA. 1997 Oct 22-29;278(16):1349-56.
PubMed.
Raber J, Wong D, Yu GQ, Buttini M, Mahley RW, Pitas RE, Mucke L.
Apolipoprotein E and cognitive performance.
Nature. 2000 Mar 23;404(6776):352-4.
PubMed.
Raber J, Akana SF, Bhatnagar S, Dallman MF, Wong D, Mucke L.
Hypothalamic-pituitary-adrenal dysfunction in Apoe(-/-) mice: possible role in behavioral and metabolic alterations.
J Neurosci. 2000 Mar 1;20(5):2064-71.
PubMed.
Robertson J, Curley J, Kaye J, Quinn J, Pfankuch T, Raber J.
apoE isoforms and measures of anxiety in probable AD patients and Apoe-/- mice.
Neurobiol Aging. 2005 May;26(5):637-43.
PubMed.
Bongers G, Leurs R, Robertson J, Raber J.
Role of H3-receptor-mediated signaling in anxiety and cognition in wild-type and Apoe-/- mice.
Neuropsychopharmacology. 2004 Mar;29(3):441-9.
PubMed.
Raber J, Akana SF, Bhatnagar S, Dallman MF, Wong D, Mucke L.
Hypothalamic-pituitary-adrenal dysfunction in Apoe(-/-) mice: possible role in behavioral and metabolic alterations.
J Neurosci. 2000 Mar 1;20(5):2064-71.
PubMed.
Acevedo S, Gardell L, Bradley SR, Piu F, Raber J.
Selective Androgen Receptor Modulators Antagonize Apolipoprotein E4-Induced Cognitive Impairments.
Lett Drug Des Discov. 2008;5(4):271-6.
Comments
Washington University
This work from the Herz lab presents an interesting set of findings. The mice they generated express ApoE in the periphery but not in the brain.
The main finding is that there is a decrease in the synaptic marker synaptophysin in the neocortex in both full ApoE KO mice and in brain-specific ApoE KO mice.
The ApoE KO mice also have decreased memory retention in the water maze (only in females), decreased LTP, and decreased EPSCs due to altered NMDA/AMPA ratios, none of which are seen in the brain-specific ApoE KO mice.
These results suggest that in the setting of complete ApoE KO, which results in very high plasma cholesterol levels as well as no ApoE in plasma, that the very high lipids in some way influence brain function negatively.
This could be via things such as excessive entry of certain lipids into the brain, damage to the BBB, or altered brain vasculature function. The only abnormality shown in the brain-specific ApoE KO mice was the decrease in synaptophysin staining in the neocortex (but not in the hippocampus). It is interesting this is found in the presence of normal behavioral performance. It is possible that this abnormality arose during development or in adult mice. It would be interesting to assess other synaptic markers in these mice as well as the structure of synapses to see if they are altered in any way. The absence of brain ApoE has little effect on the brain functions analyzed. It will be important to determine the effect of lowering ApoE in the setting of brain injury or disease to see whether positive or negative effects of ApoE are uncovered. This paper highlights the potential importance of high plasma lipids and how this can negatively impact brain function.
View all comments by David HoltzmanUCLA/VA
This is a very interesting initial look at a new model with selective brain ApoE KO that still shows some synaptic deficits but not the supposedly related cognitive deficits. The authors view this as entirely due to correction of peripheral hyperlipidemia, but it may also be due to ApoE’s well-known impact on inflammatory signaling. ApoE promotes negative inhibitory feedback of miR146a over toll and IL-1 receptor NFkappB signaling in peripheral compartments, including vascular immune cells. We recently extended this to brain and showed ApoE4 targeted replacement mice have deficits in peripheral and central miR146a (Teter et al., 2016). So, restoration of peripheral ApoE should dampen vascular pro-inflammatory signaling that impacts the neurovascular unit where invading myeloid cells are most relevant to pericyte populations.
It will be interesting to see how well exercise works in the aging brain exercise model where ApoE mediated exercise effects on neuroinflammation (Soto et al., 2016). It will be interesting if Herz and colleagues study neuroinflammation in the brains of these bEKO mice, particularly in the context of aging or some proinflammatory drivers at earlier ages (high fat diet, LPS, or APP Tg/ amyloid). It seems likely that the immunoregulatory effects of peripheral ApoE will play a role in the CNS where the role of the immune system is increasingly relevant.
References:
Teter B, LaDu MJ, Sullivan PM, Frautschy SA, Cole GM. Apolipoprotein E isotype-dependent modulation of microRNA-146a in plasma and brain. Neuroreport. 2016 Aug 3;27(11):791-5. PubMed.
View all comments by Gregory ColeOHSU
The Herz laboratory generated an important novel mouse model (bEKO) that expresses ApoE normally in the periphery and none in the brain. They compared this model to ApoE knockouts (ApoE KO), which show a profound elevation in plasma cholesterol and triglyceride levels, and ApoE3 homozygous targeted replacement (TR) mice that express the protein everywhere. The bEKO model allows us to study the contribution of ApoE in and outside the brain to CNS function. bEKO mice look like ApoE KO mice in terms of synaptophysin in the neocortex, with lower levels in both as compared to those seen in ApoE3 TR mice. When synaptic physiology is analyzed in hippocampal slices, bEKO and ApoE KO mice both show impaired long-term potentiation (LTP) without alterations in paired-pulse facilitation, as compared to ApoE3 TR mice. However, hippocampal slices of ApoE KO, but not bEKO, mice show lower AMPA/NMDA ratios than those seen in hippocampal slices from ApoE3 TR mice.
When ApoE KO mice are tested for spatial learning and memory in a water maze paradigm in which mice are first trained to locate a hidden platform, female, but not male, ApoE KO mice show an impaired ability to locate the hidden platform. This is consistent with our 1998 study, where female mice expressing ApoE4 in the brain showed impairments in the water maze task compared to males (Raber et al., 1998). These sex differences are interesting by themselves, considering the previously reported sex-dependent effects of ApoE4 on cognitive performance of ApoE KO mice that express ApoE4 in the brain and the ability of androgens (Raber et al., 2002) and selective androgen receptor modulators (Acevedo et al., 2008) to treat them, in addition to the sex differences in humans with ApoE4 in risk to develop AD (Farrer et al., 1997).
However, in another previous study, when ApoE KO mice were first trained to locate a visible platform, female ApoE KO mice showed no impairment when they were subsequently trained to locate a hidden platform (Raber et al., 2000). So it is possible that increased measures of anxiety and activation of the hypothalamic-pituitary-adrenal (HPA) axis in the ApoE KO mice (Raber et al., 2000; Robertson et al., 2005) and altered histamine receptor-mediated signaling (Bongers et al., 2003) might contribute to the poor performance of ApoE KO mice in the water maze version in which the mice are trained first to locate a hidden platform. For HPA axis activation, the deficiency of ApoE in the adrenal gland might be critical. Restraint stress-induced plasma corticosterone levels in ApoE KO mice expressing ApoE3 or ApoE4 only in the brain are not significantly different and comparable to those in ApoE KO mice without human ApoE expression (Raber et al., 2000). The bEKO model will allow us to study the role of ApoE in the periphery, including the adrenal gland, in modulating measures of anxiety and HPA axis activation when there is very little ApoE in brain. The ability to compare bEKO mice expressing different ApoE isoforms would be ideal for such studies.
Our previous study reported that ApoE KO mice showed no impairment either in spatial memory retention in the probe trial (platform removed), even when tested at 18 months of age. Age-matched ApoE KO mice expressing ApoE4 only in the brain did show impairment in spatial memory impairment in the water maze probe trial. Based on these results, increasing ApoE4 levels in brain is unlikely beneficial for cognitive performance.
Although the ApoE KO mice show a loss of synaptic markers in the Lane-Donovan study, the cognitive impairments are subtle and sex-dependent, as acknowledged by the authors. However, in previous studies, ApoE KO mice showed profound impairments in cognitive tests other than the water maze that might have been more related to impaired cholinergic function than impaired glutamatergic function. The ApoE KO mice have a reduced number of cortical and hippocampal muscarinic acetylcholine receptors (mAChRs), are more sensitive to effects of scopolamine on delay eyeblink classical conditioning, and are unable to acquire performance on the five-choice serial reaction time tasks (5-SRTT) (Siegel et al., 2011). We should be open to the possibility that ApoE has other effects on cognition that aren’t examined here.
References:
Raber J, Wong D, Buttini M, Orth M, Bellosta S, Pitas RE, Mahley RW, Mucke L. 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. PubMed.
Raber J, Bongers G, LeFevour A, Buttini M, Mucke L. Androgens protect against apolipoprotein E4-induced cognitive deficits. J Neurosci. 2002 Jun 15;22(12):5204-9. PubMed.
Acevedo S, Gardell L, Bradley SR, Piu F, Raber J. Selective Androgen Receptor Modulators Antagonize Apolipoprotein E4-Induced Cognitive Impairments. Lett Drug Des Discov. 2008;5(4):271-6.
Farrer LA, Cupples LA, Haines JL, Hyman B, Kukull WA, Mayeux R, Myers RH, Pericak-Vance MA, Risch N, van Duijn CM. Effects of age, sex, and ethnicity on the association between apolipoprotein E genotype and Alzheimer disease. A meta-analysis. APOE and Alzheimer Disease Meta Analysis Consortium. JAMA. 1997 Oct 22-29;278(16):1349-56. PubMed.
Raber J, Wong D, Yu GQ, Buttini M, Mahley RW, Pitas RE, Mucke L. Apolipoprotein E and cognitive performance. Nature. 2000 Mar 23;404(6776):352-4. PubMed.
Raber J, Akana SF, Bhatnagar S, Dallman MF, Wong D, Mucke L. Hypothalamic-pituitary-adrenal dysfunction in Apoe(-/-) mice: possible role in behavioral and metabolic alterations. J Neurosci. 2000 Mar 1;20(5):2064-71. PubMed.
Robertson J, Curley J, Kaye J, Quinn J, Pfankuch T, Raber J. apoE isoforms and measures of anxiety in probable AD patients and Apoe-/- mice. Neurobiol Aging. 2005 May;26(5):637-43. PubMed.
Bongers G, Leurs R, Robertson J, Raber J. Role of H3-receptor-mediated signaling in anxiety and cognition in wild-type and Apoe-/- mice. Neuropsychopharmacology. 2004 Mar;29(3):441-9. PubMed.
Raber J, Akana SF, Bhatnagar S, Dallman MF, Wong D, Mucke L. Hypothalamic-pituitary-adrenal dysfunction in Apoe(-/-) mice: possible role in behavioral and metabolic alterations. J Neurosci. 2000 Mar 1;20(5):2064-71. PubMed.
Acevedo S, Gardell L, Bradley SR, Piu F, Raber J. Selective Androgen Receptor Modulators Antagonize Apolipoprotein E4-Induced Cognitive Impairments. Lett Drug Des Discov. 2008;5(4):271-6.
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