Cell Biology of Salivary Protein Secretion Biology of salivary glands (BMS 513) Nisha D’Silva DDS, PhD Wednesday, May 16, 2001 9 - 10 am, Rm. G322

1
Cell Biology of Salivary Protein
SecretionBiology of salivary glands (BMS
513)Nisha DSilva DDS, PhDWednesday, May 16,
20019 - 10 am, Rm. G322
2
Overview

• 1. Review anatomy and Histology
• 2. Secretory pathways
• 3. Signaling mechanism in regulated secretion.
• 4. Brief discussion about drugs and saliva.

3
Major salivary glands
4
Histology of an acinar unit
5
Histology of major salivary glands
6
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

7
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

8
Protein synthesis and secretory pathways
9
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

10
Protein synthesis and secretion (contd)

• 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

11
Secretory pathways

• 1. Constitutive - occurs continuously
• 2. Regulated
• 3. Paragranular - small vesicles break-off from
  SGs that undergo regulated secretion and are
  released

12
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

13
Regulated secretion in salivary glands
14
Mechanism of action of G- proteins
15
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

16
Regulated secretion in salivary glands
17
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

18
Regulated secretion in salivary glands
19
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.

20
Drugs and secretion

• 1. Propranolol (Inderal) ?-blocker
• 2. Pilocarpine (Salagen) cholinergic agonist
• parasympathetic pathway
• 3. Atropine (Atropisol, Sal-Tropine)
• anti-cholinergic.

21
Summary