Cell Biology of Salivary Protein Secretion

Biology of salivary glands (BMS 513)Nisha D’Silva DDS, PhD

Wednesday, May 16, 20019 - 10 am, Rm. G322

Overview

1. Review anatomy and Histology

2. Secretory pathways

3. Signaling mechanism in regulated secretion.

4. Brief discussion about drugs and saliva.

Major salivary glands

Histology of an acinar unit

Histology of major salivary glands

Macromolecule secretion Polypeptides and proteins are synthesized and secreted

by the salivary acinar cells Sublingual saliva -- very thick and viscous

– produced by mucous acinar cells Parotid saliva -- thin and watery

– produced by serous acinar cells– mainly salivary amylase and proline-rich polypeptides

Submandibular saliva -- intermediate consistency– a mix of serous and mucous acini

Regardless of the type of protein

Too large to cross the cell membrane Must be synthesized and stored within a

membrane-bound vesicle and released by exocytosis

Protein synthesis and secretory pathways

Protein synthesis and secretion Genes transcribed in nucleus to make mRNA Message is transferred to ribosomes in cytoplasm Secretory proteins begin with signal sequence which

targets developing peptide to endoplasmic reticulum At ER, peptide is N-glycosylated and folded into

correct 3D structure Small membrane vesicles carry proteins from ER

through several layers of the golgi apparatus for additional processing and packaging for export

Protein synthesis and secretion (cont’d)

Proteins move by default onwards from the ER Specific retention sequences segregate non-secreted

proteins Secretory proteins are concentrated and stored in

secretory vesicles Mature vesicles are transported to apical membrane Secretory stimuli result in vesicle fusion with plasma

membrane Contents of vesicles are discharged outside of cell

Secretory pathways 1. Constitutive - occurs continuously 2. Regulated 3. Paragranular - small vesicles

break-off from SGs that undergo regulated secretion and are released

Constitutive exocytosis

Differs from regulated exocytosis Proteins not concentrated into secretory vesicles

awaiting exocytotic stimulus Continuous flow of protein in small vesicles to

plasma membrane Regulation occurs at synthesis stage

Regulated secretion in salivary glands

Mechanism of action of G- proteins

Control of protein secretion: second messengers

Each stage of secretion is regulated by phosphorylation of target proteins

Phosphorylation is carried out by a protein kinase such as cyclic adenosine monophosphate (cAMP)-dependent protein kinase (protein kinase A) or PKC

» cAMP stimulates maturation and translocation of secretory vesicles to the apical membrane

» cAMP stimulates exocytosis

Regulated secretion in salivary glands

Four stages of cAMP production

Nordrenaline (NA) binds to -adrenergic receptors G-protein (Gs) associated with the -adrenergic

receptor moves to an active GTP-bound state The Gs-GTP stimulates adenylate cyclase to convert

ATP into cAMP cAMP activates protein kinase A which

phosphorylates target proteins

Regulated secretion in salivary glands

Crosstalk

Fluid and protein secretion occurs by different mechanisms controlled by different nerves

Separation between control of protein and electrolytes breaks down at second messenger level

Interaction between Ca2+ and cAMP-mediated events (cross-talk) allows combination of intracellular signaling pathways into an integrated stimulus-secretion coupling mechanism.

Drugs and secretion

1. Propranolol (Inderal): -blocker

2. Pilocarpine (Salagen): cholinergic agonist

parasympathetic pathway

3. Atropine: (Atropisol, Sal-Tropine)

anti-cholinergic.

Summary