Alzheimer’s disease (AD), representing the major cause of dementia worldwide, is a progressive age-related neurodegenerative disorder clinically characterised by behavioural, cognitive, and memory impairment. The main neuropathological hallmarks are the accumulation of extracellular senile plaques principally composed of amyloid-β (Aβ) peptide, the presence of intracellular neurofibrillary tangles of hyperphosphorylated tau, an altered neuronal connectivity, and massive neuronal loss. Although post-mortem AD brains show these pathological features, both cognitive and non-cognitive alterations subtly occur prior to clinical diagnosis.
In this pre-clinical stage of AD, only mild changes in behaviour and cognition occur, even though the disease is in progress. During this phase, rather than neuronal loss and Aβ-plaque deposition, changes in synaptic function and failure of neuronal communication take place and these alterations may be associated with progressive accumulation of Aβ oligomers. Indeed, there is evidence that Aβ accumulates in mitochondria, providing a causative link to mitochondrial dysfunction in AD, pointing to mitochondrial Aβ as a potential critical player in neuronal metabolic dysfunction during pathological processes operating at very early disease phases.
In a recent issue of The Journal of Pathology, Chen et al describe the role of Cisd2 in a validated transgenic mouse model of AD, showing how Cisd2 overexpression may be able to protect mitochondria from Aβ load. Below, we summarise the key results of this paper, together with previous breakthroughs on Cisd2 functions. Undoubtedly, these studies add important new pieces to the complex mosaic of the disease, opening up new avenues of investigation.
Cisd2 and cell homeostasis
The CDGSH iron-sulfur domain proteins are expressed by a small gene family codifying for proteins characterised by a conserved CDGSH (Cys-Asp-Gly-Ser-His) domain. Only three family members are currently known. CISD1, also known as ZCD1 or mitoNEET, codes for a homodimeric protein that is localised to the mitochondrial outer membrane (MOM) and is involved in regulating oxidation.
CISD3, also known as Miner2 or MiNT, differs from the other family members because it contains two CDGSH domains and even though it probably differs in structure and localisation from its counterparts, it coordinates a complementary role in mitochondrial iron and reactive oxygen species (ROS) regulation within the mitochondrial matrix. Similar to CISD1, CISD2 (also called NAF-1, Miner1 or WFS2) codes for a heterodimeric membrane protein whose physiological role has been disentangled only recently. The CISD2 gene plays a pivotal role in mammalian lifespan control and is located in a region of human chromosome 4q recognised as a region containing genes predisposing for longevity.
A CISD2 homozygous mutation in humans has been linked to Wolfram syndrome 2, an autosomal recessive neurodegenerative disorder characterised by diabetes mellitus, hearing loss, and optic atrophy or neuropathy. In earlier work, Chen et al generated Cisd2 knockout mice (Cisd2KO) that stressed the role of Cisd2 in longevity, constituting a stringent animal model of early ageing. Remarkably, this study showed that Cisd2 levels decrease in an age-depending manner in wild-type mice, while Cisd2KO mice show a phenotype that recapitulates most features of the Wolfram syndrome.
In particular, Cisd2KO mice have prominent eyes and protruding ears; ocular abnormalities such as blindness, cornea damage, and opacity; fur depigmentation; and hair follicle atrophy accompanied by reduction in hair density and regrowth. These mice also show severe osteopenia that is detectable also in Cisd2 heterozygous mice, pulmonary abnormalities, and progressive degeneration of muscle fibres. All of these defects determine an early senescence insurgence accompanied by shortened lifespan, highlighting Cisd2’s role in promoting longevity.
To better understand the role of Cisd2 in ageing, the authors analysed the protein’s function at a cellular level. Ultrastructural analysis from Cisd2KO mice showed that several tissues show mitochondrial degeneration, including sciatic nerve axons, neurons, and cardiac and skeletal muscle cells. Particularly, MOMs show altered morphology prior to distortions in the inner cristae.
Cisd2 in Alzheimer’s disease
In their most recent article in this Journal, Chen et al cleverly made another important step forward in disentangling the importance of Cisd2 for proper neuronal function, by crossing their Cisd2-overexpressing mice with the APP/PS1 mouse model of AD. One of the leading theories of degeneration in AD is the Ca2+ hypothesis, which proposes that alterations in intracellular Ca2+ in neurons is a central mechanism connecting amyloid metabolism to neuronal cell death, linking alterations to Ca2+ homeostasis with mitochondrial dysfunction, metabolic and oxidative stress, impaired lysosomal function, and deficits in autophagic mechanisms.
The work by Chen et al focuses on the deleterious effects of Aβ on mitochondria function: many studies showed that deregulated Ca2+ signalling in neurites affected by Aβ accumulation can induce massive Ca2+ entry into neurons, imbalanced communication between ER and mitochondria, mitochondrial Ca2+ overload, functional and structural disintegration, oxidative stress, and finally cell apoptosis. The rationale for Tsai and co-workers’ work is brilliantly simple: given these effects of Aβ on mitochondrial function and integrity, and given that Cisd2 overexpression ameliorates agerelated degeneration and protects mitochondria from age-associated damage, the crossing of an AD mouse with the Cisd2-overexpressing mouse should ameliorate the AD phenotype and protect neurons from cell death.
Indeed, the authors demonstrated that Cisd2 levels in the hippocampus and cortex were nearly double in AD/Cisd2 mice compared with either AD or wild-type mice, and this overexpression significantly increased the lifespan of female AD/Cisd2 mice (while it had no effect in males that anyway showed a normal lifespan). The overexpression of Cisd2 had no apparent effect on amyloid burden. Importantly, with MRI and immunofluorescence the authors showed that overexpression of Cisd2 could restore the hippocampal volume in AD/Cisd2 female mice, reduce microglia-mediated inflammation in the hippocampus, and protect CA3 neurons from degeneration.
To further validate the importance of Cisd2 for longevity, Chen et al also crossed AD mice with their Cisd2KO mice. The AD Cisd2KO mice showed higher loss of hippocampal CA3 neurons compared with either AD or Cisd2KO animals and more pronounced neuroinflammation mediated by microglia and astrocytes, suggesting that Cisd2 deficiency can indeed accelerate AD pathology. Based on this evidence, the authors focused on understanding how Cisd2 overexpression ameliorates AD pathology by analysing the neuronal progenitor cells in the hippocampal subgranular zone of AD/Cisd2 mice.
They observed that although AD mice showed fewer proliferative (Ki67+) and progenitor cells (Dcx+ orTbr2+), the number of these cells was rescued following Cisd2 overexpression, probing the authors to conclude that overexpression of Cisd2 in AD mice aids hippocampal neuronal survival. In line with this, Cisd2 overexpression could rescue the deficits in mitochondrial oxygen consumption rate and the mitochondrial morphological abnormalities detected in the hippocampi of AD mice. In addition to the functional characterisation of Cisd2 overexpression in AD mice, Chen et al also performed a transcriptomic analysis using total hippocampal RNA from wild-type, AD, and AD/Cisd2 mice, and demonstrated that Cisd2 overexpression can reverse or rescue the expression of various genes important for synaptic transmission and mitochondrial function.
Author: Nobili Annalisa, Paraskevi Krashia, Marcello D’Amelio