**Epidemiology**

This book contains valuable knowledge and experiences from experts in various parts of the

I thank the authors for their contributions. I also thank Kristina Jurdana, Author Service

*.*

**Dr Samuel Okware PhD**

Uganda National Health Research Organisation

Director General

Entebbe, Uganda

world. It contains useful reference material.

VIII Preface

Manager for the invaluable support and assistance

**Chapter 1**

**Provisional chapter**

**Brain Aging in HIV-1 Infection**

**Brain Aging in HIV-1 Infection**

DOI: 10.5772/intechopen.77029

It has been shown that patients carrying HIV-1 accumulate damage to cells and tissues that are not directly infected by the virus itself (e.g., neurons). Importantly, these include changes known as HIV-Associated Neurodegenerative Disorder (HAND) leading to the loss of neuronal functions. HAND is an outstanding problem in the clinical management of HIV-1 patients because suppression of infectious virus by c-ART does not completely block neurodegenerative changes. Neuropsychological studies disclose cognitive alteration (such as loss of Spatial and Declarative Memory) in a substantial proportion of HIV-1 infected patients, and analysis of post-mortem brain tissues isolated from HIV-1 patients treated with c-ART show signs of neurodegeneration. In the absence of HIV-1 infection of neurons, several mechanisms have been proposed for HAND, including indirect inflammatory effects in the CNS and direct effects of viral proteins (e.g., gp120) shed from activated HIV-1-infected cells. The fact that these viral proteins enter the neurons through several pathways suggests the presence of many competing mechanisms that can contribute to HAND, each of which has its advocates. Their relative contributions to clinical disease in vivo remain to be sorted out, and this is an outstanding problem in HIV research. This chapter will shed some light on the mechanisms used by HIV-1 leading to

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Physiological brain aging is associated with a decrease in gray matter (GM) volume between adulthood and old age, while the volume of white matter (WM) increase from age 19 to 40 and will regress after that [1]*. The reduction of gray matter is probably the result of neuronal shrinkage* 

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

memory impairments and premature brain aging.

**1.1. Thinning of the cortex: white/gray matter (methods and results)**

**Keywords:** HIV, brain, aging, mitochondria

**1. HIV-1 and structural changes**

Maryline Santerre and Bassel Sawaya

Maryline Santerre and Bassel Sawaya

http://dx.doi.org/10.5772/intechopen.77029

**Abstract**

#### **Chapter 1 Provisional chapter**

#### **Brain Aging in HIV-1 Infection Brain Aging in HIV-1 Infection**

Maryline Santerre and Bassel Sawaya Maryline Santerre and Bassel Sawaya

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77029

#### **Abstract**

It has been shown that patients carrying HIV-1 accumulate damage to cells and tissues that are not directly infected by the virus itself (e.g., neurons). Importantly, these include changes known as HIV-Associated Neurodegenerative Disorder (HAND) leading to the loss of neuronal functions. HAND is an outstanding problem in the clinical management of HIV-1 patients because suppression of infectious virus by c-ART does not completely block neurodegenerative changes. Neuropsychological studies disclose cognitive alteration (such as loss of Spatial and Declarative Memory) in a substantial proportion of HIV-1 infected patients, and analysis of post-mortem brain tissues isolated from HIV-1 patients treated with c-ART show signs of neurodegeneration. In the absence of HIV-1 infection of neurons, several mechanisms have been proposed for HAND, including indirect inflammatory effects in the CNS and direct effects of viral proteins (e.g., gp120) shed from activated HIV-1-infected cells. The fact that these viral proteins enter the neurons through several pathways suggests the presence of many competing mechanisms that can contribute to HAND, each of which has its advocates. Their relative contributions to clinical disease in vivo remain to be sorted out, and this is an outstanding problem in HIV research. This chapter will shed some light on the mechanisms used by HIV-1 leading to memory impairments and premature brain aging.

DOI: 10.5772/intechopen.77029

**Keywords:** HIV, brain, aging, mitochondria

#### **1. HIV-1 and structural changes**

#### **1.1. Thinning of the cortex: white/gray matter (methods and results)**

Physiological brain aging is associated with a decrease in gray matter (GM) volume between adulthood and old age, while the volume of white matter (WM) increase from age 19 to 40 and will regress after that [1]*. The reduction of gray matter is probably the result of neuronal shrinkage* 

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*and the reduction of synaptic spines* [2]. Ventricular enlargement associated with normal aging and the Evans' index is used to distinguish normal and pathological enlargement [3].

Finally, more developed tools and methods (e.g., brain PAD) were also used to measure the influence of HIV-1 on aging. This integrative tool measures brain-predicted age difference (brain-PAD) scores. It associates structural neuroimaging data with neuropsychological test scores, trying to predict brain age and to assess the correlation of brain age to chronological age [30].

Brain Aging in HIV-1 Infection

5

http://dx.doi.org/10.5772/intechopen.77029

Long-term potentiation (LTP) is a persistent increase in the synaptic activity leading to the signal transmission between two neurons. The canonical mode of LTP induction at CA1 hippocampal synapses relies on the glutamate receptor NMDAR and the following biochemical cascade triggered and maintained by the synaptic protein calcium/calmodulin-dependent protein kinase II (CaMKII). The impairment of this cascade would lead to an acute deficit in learning and memory storage. LTP is involved in learning and memory functions in structures like the hippocampus or the amygdala. It is generated by short repetitive high-frequency

An early study in 1999 demonstrated that some factors secreted by HIV-1-infected monocytesderived macrophages (MDMs) inhibit the induction of LTP in the CA1 region of the rat hippocampus [31]. Later, a study shows that mice with severe combined immunodeficiency (SCIDs) injected by HIV-1-infected human monocyte-derived macrophages (MDMs) into the basal ganglia present a gradual decrease in synaptic function, followed by decreased cognition and later by an impairment of multiple phases of synaptic potentiation [32]. Impairment of synaptic functions, as well as the induction and maintenance of LTP, is described in mice with HIVE [33]. HIV-infected brain mononuclear phagocytes (MP) (macrophages and microglia) are the reservoirs for persistent viral infection. They secrete soluble factors like chemokines, free radicals, proinflammatory cytokines, nitric oxide, and eicosanoids. HIV-infected MDM culture supernatants containing same soluble factors have the capability to inhibit synaptic transmission and block LTP from the CA1 part of the hippocampus of rats. A deeper investigation of the mechanism involved shows that IL-8 severely reduces Ca2+ currents in the septal neurons, triggering the closure of L- and N-type Ca2+ channels [34]. Without an increase of the intracellular Ca2+ flux, the LTP in the CA1 region of the hippocam-

The study of isolated HIV-1 proteins on CA1 long-term potentiation (LTP) gave us more information about the mechanisms involved in the impairment of learning and memory by HIV-1. Mice-expressing HIV-1 gp120 are showing a significant decrease in CA1 hippocampal LTP. Gp120-induced impairment is prevented by a pre-treatment with the NMDA receptor antagonist, suggesting that excessive activation of the NMDA receptor, that can lead to excitotoxic cell death, is responsible for the degenerative process triggered by gp120 [36]. HIV-1 gp120 protein inhibits LTP via the chemokine receptor CXCR4 and binds to it through the V3 loop epitope KRIHI [37]. Gp120-associated reduction of LTP is alleviated by a systemic administration of 4-AP, a Kv, channel antagonist. This result supports the evidence that the neuronal

and that Kv channels are linked learning and memory deficiencies in HAND [38]. With normal, non-pathological aging, dendritic trees experience gradual regression in dendritic arbors

) channels (Kv) are targeted by gp120 during the inhibition of LTP

**1.2. Loss of neural circuits and brain plasticity: implication of long-term** 

**potentiation in learning and memory**

pus is impaired [35].

voltage-gated potassium (K<sup>+</sup>

stimulation (HFS) and may persist for hours or days.

Different techniques are used to evaluate different categories of brain changes: neurophysiology, neurochemistry, brain structure, and brain activation networks. Structural magnetic resonance imaging (MRI), diffusion MRI, and X-ray computed tomography (CT) are the tools for structural neuroimaging. Neurometabolites or neurochemicals can be tracked with positron emission tomography (PET) using radiotracers and by magnetic resonance spectroscopy (MRS). Brain activation networks can be studied by functional magnetic resonance imaging (fMRI) methods based on blood oxygenation level-dependent (BOLD) contrast, and arterial spin labeling (ASL) perfusion contrast shows changes in cerebral blood flow (CBF) and blood oxygenation.

Using imaging techniques, scientists and clinicians determined that the global cerebral volume is smaller in HIV-1 patients than in the seronegative population [4]. The HIV+ subjects also present a higher neuronal loss [5] and the patients with detectable viral loads had the highest rates of gray (GM) and white (WM) matter loss [6].

MRI technique also revealed that the gray matter of HIV+ subjects may present cortical atrophy [7, 8] and volumetric loss in the caudate, amygdala, and hippocampus [4, 7, 9–12]. Moreover, the medial and superior frontal gyri can show an atrophy [13], as well as the posterior and inferior temporal lobe, parietal lobe, and cerebellum [14].

If changes of white matter integrity are common with age, the abnormalities are more pronounced in aged HIV+ subjects [15]. The white matter of HIV+ subjects displayed some changes, like a tissue loss in the corpus callosum [9], as well as corpus callosum thinning and ventricular expansion [16]. HIV+ subjects showed increased mean diffusivity in frontal and parietal white matter, putamen, and genu [17]. Lower fractional anisotropy is also found at an older age in HIV+ subjects in white matter of frontal, temporal, and parietal lobes but a higher mean diffusion only in the occipital white matter [18]. Small white matter hyperintensities (WMH) are associated with age in seronegative adults [19, 20] and are attributed to inflammatory, vascular, or blood–brain barrier changes [21, 22]. However, these WMHs can be connected to dementia, multiple sclerosis, and cerebrovascular diseases [23, 24]. The increase of WMH volume is linked to lesser brain integrity in the sagittal stratum and the corpus callosum. HIV+ adults over age 60 showed a higher ratio from abnormal to normal WMH, with a subset of individuals in this age group with a significantly high WMH. This high ratio is associated with cardiovascular and is inversely correlated with global psychomotor and cognitive performance. The examination of the microstructure of the white matter by diffusion tensor imaging (DTI) brings a promising disease-activity marker [25]. A disease more advanced associated with a higher rate of decline of the CD4 count is linked to a greater atrophy of the gray and white matter in the brain [26].

Away from human, this degeneration in gray and white matter was also observed in HIV-1 Tat transgenic mice model. The expression of Tat protein diminishes cortical gray matter density in young Tat transgenic mice [27] and alters the structure of myelin examined by either DTI imaging [28] or electron microscopy [29], with declines of fractional anisotropy and behavioral changes.

Finally, more developed tools and methods (e.g., brain PAD) were also used to measure the influence of HIV-1 on aging. This integrative tool measures brain-predicted age difference (brain-PAD) scores. It associates structural neuroimaging data with neuropsychological test scores, trying to predict brain age and to assess the correlation of brain age to chronological age [30].

#### **1.2. Loss of neural circuits and brain plasticity: implication of long-term potentiation in learning and memory**

*and the reduction of synaptic spines* [2]. Ventricular enlargement associated with normal aging

Different techniques are used to evaluate different categories of brain changes: neurophysiology, neurochemistry, brain structure, and brain activation networks. Structural magnetic resonance imaging (MRI), diffusion MRI, and X-ray computed tomography (CT) are the tools for structural neuroimaging. Neurometabolites or neurochemicals can be tracked with positron emission tomography (PET) using radiotracers and by magnetic resonance spectroscopy (MRS). Brain activation networks can be studied by functional magnetic resonance imaging (fMRI) methods based on blood oxygenation level-dependent (BOLD) contrast, and arterial spin labeling (ASL)

and the Evans' index is used to distinguish normal and pathological enlargement [3].

perfusion contrast shows changes in cerebral blood flow (CBF) and blood oxygenation.

highest rates of gray (GM) and white (WM) matter loss [6].

4 Advances in HIV and AIDS Control

atrophy of the gray and white matter in the brain [26].

and behavioral changes.

terior and inferior temporal lobe, parietal lobe, and cerebellum [14].

Using imaging techniques, scientists and clinicians determined that the global cerebral volume is smaller in HIV-1 patients than in the seronegative population [4]. The HIV+ subjects also present a higher neuronal loss [5] and the patients with detectable viral loads had the

MRI technique also revealed that the gray matter of HIV+ subjects may present cortical atrophy [7, 8] and volumetric loss in the caudate, amygdala, and hippocampus [4, 7, 9–12]. Moreover, the medial and superior frontal gyri can show an atrophy [13], as well as the pos-

If changes of white matter integrity are common with age, the abnormalities are more pronounced in aged HIV+ subjects [15]. The white matter of HIV+ subjects displayed some changes, like a tissue loss in the corpus callosum [9], as well as corpus callosum thinning and ventricular expansion [16]. HIV+ subjects showed increased mean diffusivity in frontal and parietal white matter, putamen, and genu [17]. Lower fractional anisotropy is also found at an older age in HIV+ subjects in white matter of frontal, temporal, and parietal lobes but a higher mean diffusion only in the occipital white matter [18]. Small white matter hyperintensities (WMH) are associated with age in seronegative adults [19, 20] and are attributed to inflammatory, vascular, or blood–brain barrier changes [21, 22]. However, these WMHs can be connected to dementia, multiple sclerosis, and cerebrovascular diseases [23, 24]. The increase of WMH volume is linked to lesser brain integrity in the sagittal stratum and the corpus callosum. HIV+ adults over age 60 showed a higher ratio from abnormal to normal WMH, with a subset of individuals in this age group with a significantly high WMH. This high ratio is associated with cardiovascular and is inversely correlated with global psychomotor and cognitive performance. The examination of the microstructure of the white matter by diffusion tensor imaging (DTI) brings a promising disease-activity marker [25]. A disease more advanced associated with a higher rate of decline of the CD4 count is linked to a greater

Away from human, this degeneration in gray and white matter was also observed in HIV-1 Tat transgenic mice model. The expression of Tat protein diminishes cortical gray matter density in young Tat transgenic mice [27] and alters the structure of myelin examined by either DTI imaging [28] or electron microscopy [29], with declines of fractional anisotropy Long-term potentiation (LTP) is a persistent increase in the synaptic activity leading to the signal transmission between two neurons. The canonical mode of LTP induction at CA1 hippocampal synapses relies on the glutamate receptor NMDAR and the following biochemical cascade triggered and maintained by the synaptic protein calcium/calmodulin-dependent protein kinase II (CaMKII). The impairment of this cascade would lead to an acute deficit in learning and memory storage. LTP is involved in learning and memory functions in structures like the hippocampus or the amygdala. It is generated by short repetitive high-frequency stimulation (HFS) and may persist for hours or days.

An early study in 1999 demonstrated that some factors secreted by HIV-1-infected monocytesderived macrophages (MDMs) inhibit the induction of LTP in the CA1 region of the rat hippocampus [31]. Later, a study shows that mice with severe combined immunodeficiency (SCIDs) injected by HIV-1-infected human monocyte-derived macrophages (MDMs) into the basal ganglia present a gradual decrease in synaptic function, followed by decreased cognition and later by an impairment of multiple phases of synaptic potentiation [32]. Impairment of synaptic functions, as well as the induction and maintenance of LTP, is described in mice with HIVE [33]. HIV-infected brain mononuclear phagocytes (MP) (macrophages and microglia) are the reservoirs for persistent viral infection. They secrete soluble factors like chemokines, free radicals, proinflammatory cytokines, nitric oxide, and eicosanoids. HIV-infected MDM culture supernatants containing same soluble factors have the capability to inhibit synaptic transmission and block LTP from the CA1 part of the hippocampus of rats. A deeper investigation of the mechanism involved shows that IL-8 severely reduces Ca2+ currents in the septal neurons, triggering the closure of L- and N-type Ca2+ channels [34]. Without an increase of the intracellular Ca2+ flux, the LTP in the CA1 region of the hippocampus is impaired [35].

The study of isolated HIV-1 proteins on CA1 long-term potentiation (LTP) gave us more information about the mechanisms involved in the impairment of learning and memory by HIV-1. Mice-expressing HIV-1 gp120 are showing a significant decrease in CA1 hippocampal LTP. Gp120-induced impairment is prevented by a pre-treatment with the NMDA receptor antagonist, suggesting that excessive activation of the NMDA receptor, that can lead to excitotoxic cell death, is responsible for the degenerative process triggered by gp120 [36]. HIV-1 gp120 protein inhibits LTP via the chemokine receptor CXCR4 and binds to it through the V3 loop epitope KRIHI [37]. Gp120-associated reduction of LTP is alleviated by a systemic administration of 4-AP, a Kv, channel antagonist. This result supports the evidence that the neuronal voltage-gated potassium (K<sup>+</sup> ) channels (Kv) are targeted by gp120 during the inhibition of LTP and that Kv channels are linked learning and memory deficiencies in HAND [38]. With normal, non-pathological aging, dendritic trees experience gradual regression in dendritic arbors of pyramidal neurons situated in the superior temporal, precentral, and prefrontal cortices in humans [8]. HIV-1 Tat expression in pyramidal CA1 neurons decrease the number of apical dendritic spines, without the evidence of pyramidal death but with the disruption of the distribution of the synaptic proteins gephyrin and synaptogtagmin2 [39]. The Tat expression induces synapto-dendritic modifications in the hippocampus that will disrupt the LTP in CA1 pyramidal neurons and subsequently bring deficits in learning and memory.

HIV-1 clade B Tat is responsible for the up-regulation of IDO and the down-regulation of 5-HT gene and protein expressions. Also, HIV-1 clade B Tat reduces 5-HT with a concomitant

Brain Aging in HIV-1 Infection

7

http://dx.doi.org/10.5772/intechopen.77029

HIV+ subjects present a reduced breakdown of Phe to tyrosine (Tyr) [53, 54] and a faster conversion of trp to kynurenine (Kyn) [55], which is correlated with higher levels of immune activation markers like interferon-γ (IFN-γ) or neopterin in HIV-1 individuals [56]. Accelerated trp breakdown was correlated with neuropsychiatric symptoms in HIV patients [55, 57].

It is interesting to note that serotonin treatment decreases the HIV-I replication in human macrophages. Indeed 5-HT decreases the β-chemokine receptor, CCR5, and increases the CCL5 chemokine, MIP-1α, implying an effect of 5-HT on 5-HT1A receptors on macrophages [58]. Further, some studies show that in HIV+ individuals the blocking of the re-uptake of serotonin (SSRIs) is associated with the up-regulation of NK cells [59, 60]. Serotonergic path-

Apolipoprotein (apo) E isoforms (apoE2, apoE3, and apoE4) play a role in cardiovascular disease and lipoprotein metabolism but are mainly studied for their contribution in neurodegeneration in Alzheimer's disease [62–64]. HIV-associated dementia (HAD) is a neurological

Early research presented in Nature Medicine in 1998 measures the risk of dementia in patients who presented E4 isoform for apolipoprotein E (APOE). Compared to the normal subjects, they presented twice more dementia and peripheral neuropathy, concluding that a long-term infection brings an increased risk of dementia for E4(+) subjects [66], with an even bigger risk with low CD4+ cell count and length of infection. It is today widely accepted that the *APOE* ε4/ε4 genotype is associated with a faster disease course and progression to death compared with the *APOE* ε3/ε3 genotype. However, an association between the ε4/ε4 genotype and

APOEε4 allele(s) may lead to premature aging with neurodegeneration in younger HIV patients preceding the development of HAND, potentially because of greater neuroinflammation or more abundant amyloid deposition in younger HIV subjects with APOEε4 allele(s) [4, 68]. Recent neuroimaging studies present conflicting results. One study on 237 patients shows that the ApoEε4 allele does not affect brain integrity, gray, or white matter, in their cohort of HIV+ individuals [69]. Another study on 76 patients shows brain atrophy, especially in the posterior corpus callosum, thalamus, and brainstem [70]. These individuals were older than 60, which could explain the discrepancy between the studies; the deleterious effects

The APOEε4 genotype is a risk factor for elevated cholesterol in ART-adherent HIV(+) men aged >50 years [72] with a risk for a higher cognitive decline associated and cardiovascular

All these studies taken together, it is now clear that individuals with HIV and the ApoE gene exhibited greater cognitive deficits when tested for attention, executive function, and working

memory than HIV-infected individuals with ApoE4 genotype carriers.

ways are important in the function of natural killer (NK) cells and CD8 + T cells [61].

condition with clinicopathological features similar to Alzheimer's disease [65].

increase in KYN levels as compared to HIV-1 clade C Tat [52].

**2.2. HIV-1 and risks of Alzheimer's disease (AD) pathogenesis**

HIV-associated dementia (HAD) was not identified [67].

could be age dependent [71].

problems.

HIV-1 Tat protein injection into the hippocampus showed that Tat plays on extra-synaptic NMDA receptors but not on synaptic. Additionally, it suppresses long-term potentiation (LTP) followed by a diminution of spatial learning. Tat protein induces the phosphorylation of NMDA receptor subunits NR2A and NR2B in a tyrosine kinase-dependent manner, which triggers Ca2+ flux. Ca2+ entry through synaptic NMDA receptors activates cAMP response element binding protein (CREB) activity, and confers antiapoptotic ability, while Ca2+ entry through extrasynaptic NMDA receptors shuts off CREB pathway [40]. Some recent work shows that CREB protein holds an essential role in memory formation. CREB protein brings changes in global neuronal excitability. CREB overexpression results in more action potential for each pulse and a smaller after-hyperpolarization (AHP) after a chain of action potentials. AHP is usually engendered by K<sup>+</sup> channels, and CREB might be involved in variations in K<sup>+</sup> conductance. By enhancing neuronal excitability, CREB might increase the inclusion of neurons into the memory trace [41].
