Basic biology of nervesproteins: synthesis and movement

DR SERGE EDDY MBA

Historical backgroundoAugustus Waller (1850)

oNeuron doctrine 19th CE ….His(embryonal studies) Forel (response to injury) Cajal(histological observations)

oElectron microscope (1950) : demonstrate lack of cytoplasmic continuity between neurons

ANATOMY OF A NEURON

Anatomy of a nerve cell

oDentrides: branche projectionsoCell body: Soma, perikaryon, cytonoAxons: Greek axis, oBoutons terminaux

NEURONAL PROTEINS “A nerve-cell would be to its effluent nerve fibers what a fountain is to the rivulet which trickles from it—a centre of nutritive energy.” Waller (1861)

The brain expresses more of the total genetic information encoded in DNA than does any other cell in the body

In mature neurons, the chromosomes no longer duplicate and therefore function only in gene expression

Ribosomal RNA is transcribed in the nucleoli

Precursor RNA is transcribed in the nucleus to generate mRNA

Newly synthesized ribosomes and mRNA are exported from the nucleus through nuclear pores

The synthesis of all protein starts in the CYTOSOL

Initiation

elongation

termination

TRANSLATION

The final destination of the newly synthesized protein is encoded into its AA sequence

Free polysomes

RER

Secretory proteins

cytosolic

Actively imported

Functionality of proteins

Determined by amino acid sequence

Correct folding of polypeptide chain (secondary and tertiary structure)

Folding may occur spontaneously or can be catalyzed by interactions with chaperones (e.g.HSP)

Cotranslational post translational

N-acylation Thioacylation

Isoprenylation

Phosphorylation

Glycosylation

Ubiquination

N - ACYLATIONo Transfer of an acyl group to the N terminus of the growing polypeptide chainoAcylation can occur by a Myristoyl group (carbon saturated fatty acid)oThis allows the modified protein to associate with membrane through the lipid chainoExamples of N-myristoylated proteins include:oThe catalytic subunit of cAMP-dependent protein kinaseoCalcineurin

THIOACYLATIONoAnchors proteins to the cytosolic leaflet membranes

oIt occurs i.e in t-SNARE and SNAP25 (fusion)

oGrowth Associated Protein (GAP-43) also called Neuromodulinis another protein that has been thioacylated

oHumans with a deletion in one allele of the GAP43 gene fail to form telencephalic commissures and are mentally retarded

ISOPRENYLATIONoPost translational modification

oImportant in anchoring proteins to cytosolic membranes

oHappens shortly after synthesis and involves a series of steps

o thioacylation with farnesyl or with geranyl-geranyl

oFarnesylation and geranylation have important roles in vesicle transport reactions

oprogeria

PHOSPHORYLATIONoReversible post translational modification …oMost common mechanism for altering physiological processesoCatalysed by protein phosphatasesoExamples of these reactions includeoPhosphorylation/dephosphorylation that regulate the kinetics of ion channels , activity of

transcription factors, the assembly of cytoskeleton, the activity of enzymes…

GLYCOSYLATIONoOccurs on the amino groups of Asparagine residues (N-linked glycosylation)

oResults in addition of complex polysaccharide chain

oCell to cell interaction that occur during development rely on molecular recognition between glycoproteins

oGlycosylation increases the repertoire configurations a protein can have

oGlycosylation helps anchor several proteins to the outer leaflet of the plasmalemma including a form of Acetylcholinesterase and NCAM (Neuronal Cell Adhesion Molecule)

UBIQUITINATIONoUbiquitin is a highly conserved protein with 76 aaoAttaches to the ɛ-amino group of Lysine residues within the protein moleculeoTags the protein for degradationoATP-ubiquitin-proteasome pathway is present in ALL regions of the neuronoNot only for denatured/old protein degradation but also SYNAPTOGENESIS and LT MEMORY

Synthesis of transmembrane proteins

Some of the proteins synthesized in the endoplasmic reticulum remain in this organelle as resident proteins.

Others are targeted to other compartments of the vacuolar apparatus, to the plasmalemma, or to the extracellular space by secretion.

Proteins within the lumen of the endoplasmic reticulum are extensively modified

Disulfide linkages are crucial to the tertiary structure of proteins within the ER lumen

Synthesis of secretory proteins

THE GOLGI COMPLEXoAppears as stack of flattened cisternae aligned with one another in ribbons

oEach subcompartment is specialized for different types of enzymatic reactions

oSeveral protein modifications occur aiming at increasing the hydrophilicity of the protein ( fine tuning their ability to bind other molecules and delaying their degradation.

PROTEIN TRAFFICKINGoMembranous organelles move in an anterograde or retrograde direction by fast axonal transport (400mm/day)

oThese organelles include: synaptic vesicles precursors, mitochondria, elements of SER

oCytosolic and cytoskeletal proteins move only in the anterograde direction by slow axonal transport

oParticles are transported in a salutatory fashion along linear tracts aligned with the main axis: Microtubules

ANTEROGRADE TRANSPORTo Is dependent on ATPo Is not affected by inhibitors of protein synthesiso Does not depend on the cell body

o depends on microtubules that provide stationary tract on which specific organelles move by means of molecular motors

o salutatory movement is due to periodic dissociation of the organelles from the tract and to collision with other organelles

o motor molecules for anterograde transport are KINESINS and a variety of kinesin related proteins called KIF

o slow axonal transport only occurs in the anterograde direction

RETROGRADE TRANSPORToThe rate of retrograde fast transport is about one-half to two-thirds that of fast transport in the anterograde direction.

o particles move along microtubules.

oThe motor molecule for retrograde transport is a microtubule-associated ATPase called MAP-1C.

oRetrograde transport is used to deliver signals to the cell body e.g activated growth factor

oreceptors are thought to be carried along the axon to their site of action in the nucleus.

o Certain toxins (tetanus toxin) as well as pathogens (herpes simplex, rabies, and polio viruses) are also transported toward the cell body along the axon.

oThe faster component of slow axonal transport is about twice as fast as the slower component.

referencesoYoung SG, Yang SH, Davies BS, Jung HJ, Fong LG (2013). "Targeting Protein Prenylation in Progeria". Sci Transl Med 2 (5): 171. doi:10.1126/scitranslmed.3005229.

oBarr’s, THE HUMAN NERVOUS SYSTEM: AN ANATOMICAL VIEW POINT, 10TH Ed

oEric R.Kandel, PRINCIPLES OF NEURAL SCIENCE, 4th Ed

oBates S; Vousden KH (February 1996). "p53 in signaling checkpoint arrest or apoptosis". Curr. Opin. Genet. Dev. 6 (1): 12–8. doi:10.1016/S0959-437X(96)90004-0. PMID 8791489

ohttp://www.uniprot.org/uniprot/P17677

MERCI BEAUCOUP TATENDA

Dr MBA…