Comprehensive University Study Notes on Neurochemistry and Cellular Neuroscience and Molecular Biology

General Introduction to the Central Nervous System and Cellular Diversity

The nervous system, particularly the Central Nervous System (CNS), is characterized by an immense structural and functional complexity. The human brain consists of approximately $10^{11}$ neurons and an equivalent number of glial cells. These neurons establish roughly $10^{15}$ synapses, highlighting the intricate connectivity of the neural network. Glial cells are not merely passive structural components; they provide metabolic support for the neuronal complex, furnish the physiological basis for electrical conduction, and play active roles in cerebral functions. Both neurons and glia share a common developmental lineage, originating from neuroepithelial cells of the embryonic nervous system, which dictates many of shared molecular and structural characteristics.

Neurons exhibit extreme diversity, with over 100 different functional types. They are electrically and chemically excitable cells and represent the most morphologically polarized and compartmentalized cells in the body. This high level of compartmentalization facilitates precise communication but poses challenges for signal transduction, membrane trafficking, cytoskeletal dynamics, and gene regulation. Neural networks are complex and precise but are not "hard-wired"; they exhibit synaptic plasticity, where experience modifies the strength and quantity of connections. Neurons contain specific sets of proteins that control ion flow across the plasma membrane, maintaining cellular excitability.

Cellular Morphology and Neuronal Compartments

Neurons are elongated and asymmetrical, featuring organelles and proteins that are specifically localized. The primary structural components include the soma, dendrites, axons, and terminals. The soma, or cell body, contains the nucleus and is the site of most protein synthesis. Dendrites function as the primary reception areas for information from other neurons. The cytoplasm of the soma extends into the dendritic tree, although the density of organelles decreases with distance. Dendritic spines serve as specialized receptive sites for excitatory synapses and are rich in smooth endoplasmic reticulum (SER) and polyribosomes, enabling local protein synthesis and local gene expression regulation.

The axon functions as a transmission cable, varying significantly in diameter and length (reaching up to $2\,m$ to $3\,m$ in large mammals like giraffes). There is a distinct functional boundary between the soma and the axon hillock, or cone of emergence. Generally, ribosomes, rough endoplasmic reticulum (RER), and the Golgi apparatus are excluded from the axon. Axon guidance is the process by which axons and their nervous connections are formed. Axonal terminals are rich in synaptic vesicles and transmit signals to other neurons or effector cells such as muscles and secretory glands. Some axons are insulated by a myelin sheath, which provides electrical insulation and increases the velocity of signal conduction.

Neuroglial Diversity and the Blood-Brain Barrier (BBB)

Neuroglia are categorized into several distinct types with specialized roles. Astrocytes are connected to almost all components of the CNS; they provide growth factors, receive chemical signals from neurons, and influence ion concentrations ( $Ca^{2+}$ , $K^+$ , $Na^+$ , $Cl^-$ ) in the extracellular space via membrane channels, thereby affecting neuronal membrane potentials. They represent roughly $1/3$ of total brain mass and up to $40\%$ of cells. Astrocytes are interconnected via gap junctions and interact with capillary endothelial cells to form the Blood-Brain Barrier (BBB). They also promote synapse formation and regulate blood flow locally.

Oligodendrocytes are responsible for forming the myelin sheath in the CNS, whereas Schwann cells perform this role in the Peripheral Nervous System (PNS). Microglia, originating from the mesoderm rather than the ectoderm, serve as the immune system of the CNS, acting as phagocytes that remove degenerated cells and participate in synaptic pruning during development. Ependymal cells and choroid plexus epithelial cells line the cerebral ventricles and contribute to the formation of cerebrospinal fluid (CSF).

The BBB is formed by endothelial cells characterized by high mitochondrial density (supporting energy-dependent transport) and complex tight junctions composed of transmembrane proteins like Occludin, Claudin, and Junctional Adhesion Molecules (JAM). Transport across the BBB is highly selective: lipophilic substances like $O_2$ and $CO_2$ diffuse freely, while small water-soluble molecules like glucose and amino acids require facilitated or energy-dependent transport. The brain consumes approximately $20\%$ of the body's total glucose and oxygen. The BBB, the blood-cerebrospinal fluid barrier (BCFSB), and the arachnoid-dura barrier (ADB) comprise the three main interfaces between plasma and brain tissue.

Biochemistry of the Neuronal Membrane and Lipid Metabolism

The neural membrane follows the fluid mosaic model, composed of a lipid bilayer with polar heads facing outward and hydrophobic tails inward. The brain is remarkably rich in lipids, which constitute $50\%$ of its dry weight. Brain membranes contain a higher percentage of cholesterol and polyunsaturated fatty acids compared to other tissues. Phospholipids include phosphoglycerides (with a glycerol backbone esterified to fatty acids at $C1$ and $C2$ , and a phosphate group at $C3$ ) such as Phosphatidyl-choline, Phosphatidyl-serine, Phosphatidyl-ethanolamine, and Phosphatidyl-inositol.

Sphingolipids utilize a sphingosine backbone. If acylated, sphingosine becomes ceramide. Further esterification with phosphocholine produces sphingomyelin, while linkage to simple sugars produces cerebrosides (e.g., glucosylcerebroside). Complex polysaccharides linked to ceramide form glycolipids; if these include sialic acid, they are termed gangliosides. Lipids are essential for signal transduction, energy metabolism, and structural integrity. Cholesterol metabolism in the brain is largely independent of the rest of the body because lipoproteins do not cross the BBB. Most brain cholesterol is synthesized de novo in the SER, starting from Acetyl-CoA via the rate-limiting enzyme HMG-CoA reductase. Astrocytes provide neurons with cholesterol via lipoproteins stabilized by Apolipoprotein E (Apo-E).

Structural and Functional Classification of Membrane Proteins

Membrane proteins are categorized as integral, peripheral, or lipid-anchored. Integral proteins often utilize $\alpha$ -helices of approximately 20 hydrophobic amino acids to span the bilayer. Some proteins utilize $\beta$ -sheet structures to form a beta-barrel. Hydrophobicity plots can predict transmembrane segments; for example, the nicotinic acetylcholine receptor $\alpha$ -subunit shows four distinct peaks. Lipid-anchored proteins are covalently bonded to lipids, such as GPI (glycosylphosphatidylinositol) anchors on the extracellular side or acetylation on the cytosolic side.

Membrane phospholipids act as precursors for signaling molecules. Phospholipase C (PLC) acts on phosphatidylinositol 4,5-bisphosphate ( $PIP_2$ ) to produce inositol 1,4,5-trisphosphate ( $IP_3$ ) and diacylglycerol (DAG), both acting as second messengers. Phospholipase $A_2$ ( $PLA_2$ ) releases arachidonic acid, a precursor to eicosanoids (prostaglandins, thromboxanes, leukotrienes). The human genome contains numerous genes for ion channels (9 for $Na^+$ , 10 for $Ca^{2+}$ , over 75 for $K^+$ , etc.), which are multi-pass proteins that form hydrophilic pores to select specifically for certain ions.

The Neuronal Cytoskeleton and Structural Polarity

The cytoskeleton is responsible for cell shape, motility, and organelle distribution. It consists of three primary systems:

Microtubules: These are $50\,nm$ diameter cylinders formed by 13 protofilaments of alternating $\alpha$ and $\beta$ tubulin subunits. They are polar (the "+" end exposes $\beta$ -tubulin, the "-" end exposes $\alpha$ -tubulin) and highly dynamic, undergoing cycles of polymerization and depolymerization (catastrophe and recovery). In axons, microtubules are oriented with the "+" end toward the terminal and are stabilized by the Tau protein. In dendrites, they are associated with MAP2 and can have mixed or inverted polarity.
Neurofilaments: These are $10\,nm$ intermediate filaments, specific to neurons, and are the most abundant cytoskeletal element in axons. They provide mechanical resistance and determine axonal diameter. They are heteropolymers of NFH (high), NFM (medium), and NFL (low) subunits. Subunits possess a conserved coiled-coil "Rod" domain. Disruption or mutation of neurofilaments is linked to pathologies such as Amyotrophic Lateral Sclerosis (ALS), Parkinson's, and Alzheimer's.
Microfilaments: Formed by G-actin monomers polymerizing into F-actin double helices ( $7\,nm$ diameter). They are concentrated in the cortical cytoplasm, dendritic spines, and growth cones. They exhibit treadmilling, where the rate of addition at the "+" end equals the rate of loss at the "-" end. Actin-binding proteins like Profilin and Cofilin regulate their dynamics.

Axon growth is driven by the growth cone, a specialized structure at the tip of the axon containing lamellipodia and filopodia. Growth involves actin polymerization at the leading edge, vesicle-mediated membrane expansion, and microtubule stabilization. Defective cytoskeleton dynamics are characteristic of tauopathies and peripheral neuropathies like Charcot-Marie-Tooth disease.

Intracellular Traffic and Axonal Transport Mechanisms

Protein sorting begins with synthesis on ribosomes. Proteins destined for the secretory pathway are translated into the RER, while others are synthesized on free ribosomes for the cytosol, nucleus, or mitochondria. The secretory pathway involves COPII-coated vesicles for anterograde transport (ER to cis-Golgi) facilitated by the Sar1 GTPase, and COPI-coated vesicles for retrograde transport (Golgi to ER) via the ARF GTPase. Clathrin-coated vesicles, utilizing the dynamin GTPase for scission, manage post-Golgi and endocytic traffic.

Vesicular fusion with target membranes requires specific proteins: Rab GTPases mediate initial tethering, and SNARE proteins (v-SNARES like Synaptobrevin on the vesicle and t-SNARES like Syntaxin and SNAP-25 on the target) form stable four- $\alpha$ -helical coiled-coil complexes to drive fusion. NSF and $\alpha$ -SNAP then dissociate the SNARE complex using ATP hydrolysis.

Axonal transport is divided by speed and direction:

Fast Anterograde Transport: $\left( 100-400\,mm/day \right)$ mediated by Kinesin-1, which "walks" toward the microtubule "+" end using ATP. It carries vesicles, proteins, and lipids.
Fast Retrograde Transport: $\left( \approx 200\,mm/day \right)$ mediated by Dynein (associated with the Dynactin complex), moving toward the "-" end. It carries lysosomes and signaling molecules like neurotrophins.
Slow Transport: $\left( 0.5-10\,mm/day \right)$ carries cytoskeletal proteins (Actin, Tubulin) and cytosolic enzymes.

Cell Adhesion Molecules (CAMs) and Myelin Organization

Synapse formation and tissue integration are regulated by CAMs, categorized into four families: Cadherins ( $Ca^{2+}$ -dependent homophilic adhesion), IgCAMs (immunoglobulin-like, e.g., NCAM, L1), Integrins ( $\alpha\beta$ heterodimers for cell-matrix interaction), and Selectins. NCAM can undergo polysialylation, affecting neural development and plasticity. Caderins are crucial for synaptic stabilization. Integrins, such as $\alpha_6\beta_4$ , are critical for Schwann cell radial sorting and myelination.

Myelin provides electrical insulation and reduces the energy cost of impulse conduction. In the SNP, the primary proteins are P0 ( $50\%$ ), MBP, and PMP22. In the SNC, the major proteins are PLP and MBP. MBP is a peripheral protein on the cytosolic face while P0 is a transmembrane IgCAM. The Node of Ranvier is a gap in the myelin sheath enriched with voltage-gated $Na^+$ channels ( $\approx 100 \times$ higher density than unmyelinated axons) and anchoring proteins like Ankyrin G. Mutations in myelin proteins cause conditions such as Pelizaeus-Merzbacher disease (PLP) and Charcot-Marie-Tooth type 1A (PMP22 duplication).

Energy Metabolism and the Glutamate-Glutamine Cycle

The brain utilizes $20\%$ of basal metabolic energy, primarily to maintain membrane potentials via the $3Na^+/2K^+$ -ATPase pump. Glucose is the nearly exclusive fuel, although ketone bodies (acetoacetate) can be used during prolonged fasting. Glucose enters the CNS via GLUT1 (55 kDa isoform on BBB endothelial cells) and is taken up by neurons via GLUT3. Astrocytes store glycogen and are the primary site of anaplerotic reactions, such as the carboxylation of pyruvate to oxaloacetate by pyruvate carboxylase ( $PC$ ).

Neurotransmitter metabolism is compartmentalized. In the Glutamate-Glutamine cycle:

Glutamate is released by neurons and taken up by astrocytes via EAAT2 transporters.
Astrocytes convert glutamate to glutamine via glutamine synthetase.
Glutamine is released and taken up by neurons, where glutaminase (PAG) converts it back to glutamate. This recycling prevents excitotoxicity and ensures a stable neurotransmitter supply. The malate-aspartate shuttle is crucial for maintaining the low cytosolic $NADH/NAD^+$ ratio required for glycolysis. N-acetyl aspartate (NAA) is synthesized in neurons and used by oligodendrocytes for lipid synthesis.

Synaptic Physiology and Neurotransmitter Systems

Synapses are either electrical (Gap junctions composed of Connexons, each made of 6 Connexin subunits) or chemical. Chemical transmission involves several steps: action potential arrival, $Ca^{2+}$ influx via voltage-gated channels, vesicle fusion, and transmitter release. Release is cooperatively dependent on $Ca^{2+}$ binding to Synaptotagmin (5 $Ca^{2+}$ ions required). Neurotransmitters are classified into small molecules (ACh, Glutamate, GABA, Glycine, Catecholamines, Serotonin, Histamine, ATP) and neuropeptides.

Acetylcholine (ACh): Synthesized from choline and Acetyl-CoA by choline acetyltransferase; degraded by acetylcholinesterase (AChE) in the synaptic cleft. It acts on nicotinic (ionotropic) and muscarinic (metabotropic) receptors.
Catecholamines: Synthesized from Tyrosine via L-DOPA. Dopamine is converted to Norepinephrine and then Epinephrine. Degradation occurs intracellularly via Monoamine Oxidase (MAO) and Catechol-O-methyltransferase (COMT). Dopamine pathways include the nigrostriatal (implicated in Parkinson's) and mesocorticolimbic systems.
GABA & Glycine: The primary inhibitory transmitters. GABA is synthesized from glutamate via glutamate decarboxylase ( $GAD$ ). GABAA receptors are chloride channels modulated by benzodiazepines and barbiturates.
Neuropeptides: Synthesized in the soma as large precursors (e.g., POMC) and processed proteolytically during transport in dense-core vesicles. They lack specific reuptake mechanisms and often act as modulators (co-transmission).

Ionotropic and Metabotropic Receptors and Intracellular Signaling

Ionotropic receptors are ligand-gated ion channels mediating fast responses ( $0.1-5\,ms$ ). These include the pentameric Cys-loop family (nAChR, GABAA, Glycine, $5-HT_3$ ) and tetrameric glutamate receptors (AMPA, NMDA, Kainate). NMDA receptors are "coincidence detectors," requiring both glutamate and depolarization (to remove the $Mg^{2+}$ plug) and are permeable to $Ca^{2+}$ , making them central to Long-Term Potentiation (LTP).

Metabotropic receptors are G-protein coupled receptors (GPCRs) with 7 transmembrane helices, mediating slow, modulatory responses. Common pathways include:

$cAMP$ /PKA: $G_s$ activates adenylate cyclase to produce $cAMP$ , which activates Protein Kinase A (PKA).
PLC/ $IP_3$ /DAG: $G_q$ activates Phospholipase C, releasing $IP_3$ ( $Ca^{2+}$ release) and DAG (PKC activation).
Retrograde Signaling: Endocannabinoids (Anandamide, 2-AG) and gases ( $NO$ , $CO$ ) are produced post-synaptically and diffuse to the pre-synaptic terminal to modulate future release.

Gene Regulation and Protein Homeostasis (Proteostasis)

Long-term synaptic changes (LTP) require de novo transcription and translation. The transcription factor CREB is activated by phosphorylation, recruiting chromatin modifiers like CBP (CBP has HAT activity). Immediate-early genes like c-fos and zif268 are rapidly transcribed, followed by late-response genes. Post-transcriptional regulation includes alternative splicing (e.g., Neurexin-1) and localized mRNA translation in dendrites, often regulated by poly-A tail expansion via CPEB.

Proteostasis involves chaperones (Hsp70, Hsp90) that facilitate folding and the Ubiquitin-Proteasome System (UPS) for degradation. E3 ligases (e.g., Parkin) provide specificity for target proteins marked with K48-linked polyubiquitin. Autophagy removes large aggregates. Neurodegenerative diseases like Alzheimer's (Amyloid- $\beta$ , Tau), Parkinson's ( $\alpha$ -synuclein), and Huntington's (Polyglutamine) are proteinopathies characterized by misfolding and aggregation. Prions represent protein-only infectious agents where a misfolded scrapie form ( $PrP^{Sc}$ ) templates the conversion of the normal form ( $PrP^C$ ). Interestingly, physiological prion-like mechanisms in CPEB may support the persistence of long-term memory.