Block 5: homeostasis and neuroimmune system Flashcards
What is meant by core and shell regarding thermoregulation?
Core temperature refers to major organs, CNS, and some muscles. Core temperature varies throughout the day (usually down at 35.5 in the morning and up at 37.7 in the evening. The outer shell includes the skin, subcutaneous fat, limbs, etc and can vary hugely (between 2 and 40 degrees). Additionally, core volume varies- it can extend to occupy over 90% of the body volume (only feet, hands, and skin which are below 37) during warm ambient temperatures, or can redact down to cover just the organs and CNS during cold temperatures. By reducing the size of the core in cold conditions, we are increasing the volume of insulation, separating the internal core from the cold external environment.
Explain the mechanisms underlying thermoregulation.
Thermoreceptors are present in the skin (with around 3 times as many receiving cold than receiving hot, meaning that humans are well adapted to detect when we are losing heat through the skin). There are also thermoreceptors present in the CNS (hypothalamus) and in the major central organs (liver and heart). Afferents to the hypothalamus are relayed through two centres (one for hot, one for cold) – the hypothalamus has a temperature resolution of about 0.01 degrees. This results in a graded response (e.g. sweating doesn’t turn off or on, it becomes more or less to match the intensity of heat).
Short-term response to reduced environmental temperature include more heat produced by skeletal muscle (by increased voluntary activity), increased muscle tone and shivering, adrenergic pathway from hypothalamus to muscle (not sympathetic – it turns up muscle metabolism), increased circulating thyroxine and adrenaline will also increase heat production. Short-term response to increased environmental temperature includes increased blood flow to the skin (up to 2.5L/min, can lose heat via conduction if touching something cool, convection by airflow past the skin, or by evaporation of sweat from the skin). This regional flow is managed by constriction/dilation of skin arterioles. By default, deep blood flow in the skin is always on (within subcutaneous fat layer). This blood flow can extend to the superficial skin via the opening of arteriole loops. Vasomotor control of skin arterioles is via adrenergic nerves from the hypothalamus. However, control of blood pressure involves control of arterioles by the adrenergic system from the CVS centre of the medulla. These two systems can act in conflict (for example, vasodilation for temperature control during exercise in hot conditions may conflict with vasoconstriction for control of blood pressure). In these cases, temperature control “wins”.
A long-term response to chronic cold is the allowance of sustained drops in core temperature (set-point gets turned down a bit). Behavioural adaptations include timing of activities (for example, staying sheltered during mid-day sun in the desert), level of movement, choice of clothing, body posture, food preferences (high calorie food in cold, for instance). Piloerection via sympathetic nervous system is another way of increasing insulation through hair. Pyrogens are molecules released by infectious organisms, and they act on the hypothalamus to change temperature set point (hypothalamus thinks that 40 degrees is where you should be, so when you’re actually 39 degrees you shiver even though you’re hot). Aspirin blocks prostaglandins (type of pyrogen).
Describe the concept of allostasis.
Allostasis proposes that efficient regulation requires anticipating needs and preparing to satisfy them before they arise. The advantages: (i) errors are reduced in magnitude and frequency; (ii) response capacities of different components are; (iii) resources are shared between systems to minimize reserve capacities; (iv) errors are remembered and used to reduce future errors. Allostasis is predictive rather than reactive, as classic homeostatic mechanisms are.
Describe the components of a negative feedback loop with reference to an example.
A stimulus (1) is a change which disrupts a controlled condition (2), bringing it out of the body’s desired parameters (necessary for optimal biochemical/physiological functioning). This change is detected by specialised receptors (3) which monitor the condition and become activated if it deviates beyond its normal range. These receptors project to a control centre (4) which is responsible for processing the information and mediating a response to restore homeostasis. It does this by projecting to (or releasing hormones to) effector organs/tissues (5). These tissues ultimately make the change which alters the controlled condition to bring it back within healthy parameters (6).
For example, the stress response increases sympathetic activity, increasing heart rate and vasoconstriction through the actions of noradrenaline/adrenaline (stimulus). This results in increased blood pressure (controlled condition) due to increased cardiac output and peripheral resistance. Baroreceptors in the bifurcation of the carotid artery (receptors) monitor arterial blood pressure, and become activated upon this disrupted condition, activating and projecting to the ventrolateral medulla (control centre). Neurons of the ventrolateral medulla project back to the heart and vascular endothelium (effector tissues), which decrease heart rate and vasodilate blood vessels to reduce cardiac output and peripheral resistance, respectively, ultimately bringing blood pressure back within healthy parameters (response).
Describe the components of a positive feedback loop with reference to an example.
A stimulus (1) is a change which disrupts a condition, and it detected by specialised receptors (2). These send nerve impulses to a control centre (3) which mediates a response through downstream effectors (4) with the purpose of further exacerbating condition, rather than reducing it. This continually reinforces an event within the system until an outside event shuts it off.
For example, during labour, contractions of the wall of the uterus pushes the head of the baby into the cervix (causes stretching of the cervix- stimulus). Stretch receptors of the cervix are stimulated (receptors) and send nerve impulses to the brain (control centre). The brain releases oxytocin which affects the muscles of the uterine wall (effectors), which respond by increasing contractile force. This further increases the strethc on the cervix by increasing the force applied by the abby’s head. The stimulus is removed when the baby is born.
Briefly explain how waste products are removed from the CNS.
Waste products (systemic factors, cytokines, and other molecules from neurons and glia) accumulate within the CSF). The lymphatic system is responsible for clearing metabolic waste and maintaining fluid homeostasis in most body organs. It is comprised of lymphatic capillaries, lymph vessels, and lymph nodes – it removes waste, transports it, and drains into the circulation. There is an active lymphatic system in the meninges of the brain (however, there is no lymphatic system within the brain tissue). The glymphatic system is responsible for clearing debris within the blood-brain barrier. It is a perivascular network where CSF and interstitial fluid (ISF) interface to exchange waste and facilitate its drainage from brain parenchyma. The glymphatic system is much more visible in older brains, because there are larger spaces, and is also dilated in brains with small vessel disease. AQP4 channels expressed on glial cells may be important for rapid exchange of fluids (this is very controversial just now). The theory is that CSF travels through AQP4 channels (on peri-vascular end-feet of astrocytes) into periarterial space, and this propels the diffusion of waste solutes into perivenous space to be drained from the brain.
CSF production peaks during sleep, and it is believed that lymphatic clearance is predominantly active during NREM. Glymphatic system transport is believed to be decreased in the aged brain, especially in those with neurodegenerative diseases. CSF flow through the perivascular system is therefore believed to be deficient in the above conditions, rendering glymphatic transport and drainage less efficient – sleep disturbances associated with neurodegenerative diseases therefore may exacerbate this decline.
Outline the similarities and differences between CSF and blood plasma composition.
CSF and blood plasma is very similar in ion concentrations, pH, and osmolarity. However, glucose in blood plasma is slightly greater than glucose in CSF (this is because we don’t want to subject CNS to be exposed to fluctuating glucose concentrations). Amino acid levels in the blood plasma is much greater than those in CSF – this is also important due to daily fluctuations in bloodstream concentrations which could alter brain homeostasis if crossing the BBB (for instance, excess glutamic acid in the blood could cause glutamate excitotoxicity if carried into CSF). This is also observed in proteins, which are typically higher in blood plasma than in CSF – most of these may be toxin to CNS tissue.
Ouline the key functions of the blood-brain barrier.
1) Maintains ionic composition optimal for synaptic signaling (actively regulated).
2) Keeps central and peripheral neurotransmitter pools separate (as mentioned).
3) Prevents plasma proteins detrimental to neural functioning from entering the CNS (e.g. albumin, pro-thrombin, plasminogen cause cell activation and apoptosis).
4) Protects the brain from neurotoxic substances circulating in the blood (neurotoxins may be endogenous metabolites of proteins, ingested, or otherwise acquired from the environment).
5) BBB has low passive permeability – there is active transport of essential nutrients.
Describe the structure of the BBB.
Pericytes, microglia, astrocytes, and nerve terminals are closely associated with the endothelium and play supporting roles in the barrier induction, maintenance, and function. Barrier function is not fixed, but can be modulated and regulated by other cells of the neurovascular unit. The physical barrier is formed of a single layer of epithelial cells (specialized vascular endothelium) which contain different integral membrane proteins at the luminal and abluminal surfaces (part of the cell in contact with the lumen and with the basal lamina, respectively). These membrane proteins include receptors, transporters, and enzymes that support the functions of the BBB. Tight junctions link endothelial cells together to block passage of substances via paracellular diffusion – they lock adjacent cells together. Endothelial cells also have low rates of pinocytosis (ingestion of liquid and storage of it). They also lack intracellular fenestrations (transcellular pores).
There are two types of junctions in endothelial cells – adherens junctions and tight junctions:
1) Adherens junctions are closer to the basal aspect of the cell (CNS side), and have more of a role in structural integrity and anchoring, whereas tight junctions are in a more apical region (closer to the bloodstream to prevent paracellular diffusion). They are formed of cadherin proteins which span the intercellular cleft and are linked into the cell cytoplasm by catenins (scaffolding proteins).
2) Tight junctions are formed of 3 proteins – occluding, claudin, and zonula occludens proteins. Claudins and occludins span the intercellular cleft and link together, and are anchored to the cytoplasm by zonula occludens proteins. There are also junctional adhesion molecules (JAMs) which are believed to be adhesion molecules for leukocytes in the blood (since they are positioned at the luminal membrane).
What are the 5 ways in which molecules can cross the cerebrovascular endothelium?
1) Lipid-soluble agents can freely diffuse down their concentration gradients (transcellular lipophilic pathway (for instance, blood gases – oxygen in and CO2 out). Active efflux carriers can be used to transport out any molecules which have passively entered.
2) Transport proteins – glucose enters the brain via GluT transporters (expressed on both the luminal and abluminal membranes), other examples include amino acids and nucleosides. There are four subtypes of transport proteins – bidirectional, those which specifically transport solutes into the cell, those which specifically transport solutes out of the cell, and those which exchange solutes.
3) Receptor-mediated endocytosis – hormones (insulin, transferrin, etc) will bind to receptor protein and cell takes up molecule into a vesicle and transports it to the opposite side where it exists via exocytosis.
4) Adsorptive endocytosis is used to transport positively charged plasma proteins (e.g. albumin).
5) Mononuclear cell migration (typically only occurs during pathological conditions) – this involves immune cells from the systemic circulation entering the CNS via diapedesis.
Define and give examples of circumventricular organs.
CVOs are structures in the brain which do not have a BBB – they are typically of the midline in the brain, next to the ventricles (especially the 3rd ventricle). CVOs include the pituitary gland, the median eminence, the subcommissural organ, the pineal gland, the area postrema, and the subfornical organ. They typically have a role in sensory detection of blood or secretion into the blood (bridge the CNS and peripheral circulation) – having a BBB would prevent their functions. The subfornical organ detects hormone levels in the blood (specifically angiotensin II in order to regulate water balance). The area postrema acts as a chemoreceptor, which induces vomiting by detecting drugs/toxins in the blood. The posterior pituitary gland secretes hormones into the bloodstream. The median eminence of the hypothalamus secretes hypothalamic releasing hormones into the circulation. The subcommissural organ secretes glycoproteins, and the pineal gland secretes melatonin and is involved in circadian rhythms.
Which immune cells are present in which areas of the CNS?
Microglia are found throughout the parenchyma of the brain. Meningeal macrophages are found around the meninges (as are dendritic cells). Choroid-plexus macrophages are found around the choroid plexus, and perivascular macrophages around the blood vessels.
Give an overview of the role of microglia in the neuroimmune response.
Microglia in their resting state are highly dynamic, with active processes which constantly survey for damage associated molecular patterns (DAMPs) of pathogen associated molecular patterns (PAMPs). Pathological events such as altered neuronal function, infection, injury and inflammation rapidly activate microglia. When activated, microglia change from ramified to an amoeboid phenotype, proliferate, and migrate to the site of damage. They then secrete both cytotoxic (IL-1, IL-6, TNF-alpha, reactive oxygen species, reactive nitrogen species, proteolytic enzymes) and neurotrophic factors (IGF1, BDNF, TGF-beta, NGF). If they become chronically active, these molecules can cause huge degrees of collateral tissue damage. They cannot be grown in vitro, since the will become constitutively active out with the brain, and can only be prepared from a brain specimen (data should be interpreted with caution).
During development, all neural cell types are produced in excess. Many of these then die and the debris is cleared my microglial cells. They are also involved in synaptic pruning – the process of refining synapses important for brain development (dysfunctional pruning has been linked with schizophrenia).
As the main cells of innate immunity in the CNS, microglia constitutively express the most important receptors (MHC-I and -II, chemokine receptors) at low levels. Activated microglia upregulate MHC-II expression, which is required for activation of naïve T-cells, and produce numerous pro-inflammatory cytokines, including cytokines that induce the differentiation of effector T-cells. Prolonged or excessive microglial cell activation may result in pathological forms of inflammation that contribute to the progression of neurodegenerative diseases
They also express pattern recognition receptors (PRRs, including Toll-like receptors) that recognise various PAMPs found on bacteria and viruses. The microglia processes the PAMP/DAMP antigen and presents it via MHC-II to T-cell receptors. When activated, it releases antimicrobial peptides, cytokines, chemokines, and reactive oxygen species (as well as nitric oxide)- they also release IL-12, IL-23, and TGF-beta to further activate the T-cell. These T-cells only enter the CNS in great numbers when there are pathogens, and are responsible for the adaptive immune response in the CNS.
Explain how microglial crosstalk with astrocytes contributes to neuroinflammation and neurodegenerative conditions.
Microglia crosstalk with astrocytes, and this can help amplify inflammatory responses and the production of neurotoxic factors. Lipopolysaccharide (LPS, expressed on gram -ve bacterial walls)-induced secretion of factors such as IL-1-beta and TNF by microglia can result in potent induction of pro-inflammatory gene expression and CSF1 production by astrocytes. Astrocyte-derived pro-inflammatory factors can also feedback on microglia to promote further cell activation and microgliosis, thereby establishing a positive feedback loop (co-cultures of microglia and astrocytes stimulated with LIPs produce significantly more neurotoxic factors than either cell type alone). This functional application is likely a contributor to collateral damage in neurodegenerative disorders.
Neuroinflammation is distinct from regular inflammation, and is used to identify a different set of conditions that are specific to the CNS. The concept of neuroinflammation also describes diseases that do not include conventional inflammatory responses (would typically be describe them as degenerative rather than inflammatory). Toxicity of amyloid-beta in AD can activate microglia through PRRs, and microglia may then contribute to AB-induced neurotoxicity by generating ROSs and peroxynitrite (toxic anion). In the mouse model of AD, microglia cause neuron elimination- this was shown via knockout models of Cx3cr1, which prevented neuron loss.
Describe the complement cascade.
The complement system is the link between the innate and adaptive immune systems. It comprises >30 plasma and cell surface proteins (main serum components are C1-9, mostly made in the liver and exist as inactive proenzymes, cleaved to be activated). There are 3 pathways the complement system can follow- classical, MB-lectin, or alternative. All three pathways result in activation of the complement cascade which causes recruitment of inflammatory cells, opsonisation of pathogens, and killing of pathogens. They all integrate at the C3-protein cleavage step (by C3 convertase):
1) The classical pathway involves a C1 protein complex which either binds to the cell surface or to the Fc region of an antibody bound to a pathogenic cell surface. This binding alters the conformation of C1, leading to cleavage of certain proteins and activation of enzymes in a cascade, leading to insertion of proteins into the bacterial membrane (forming membrane attack complex MAC pore). Uncontrolled influx of water and ions results in cell death of the pathogen (lysis).
2) The MB lectin pathway (mannose-binding) is very similar to the classical pathway but uses MASP protein complex.
3) The alternative pathway is triggered by bacterial cell wall endotoxin binding circulating C3, increasing C3 convertase production.
These pathways are highly regulated by complement inhibitors (expressed on our own cells to prevent lysis of our resident cells, e.g. CD-59, DAF).
Complement also potentiates inflammation- it does this by activating anaphylatoxins which bind to receptors on mast cells causing the release of histamine. The complement system is also responsible for chemotaxis (e.g. C5a increases recruitment of inflammatory cells), It also enhances uptake of damaged cells (C3b, 4b, 5b, opsonise cells for phagocytosis).
