a genetic disease in which patients age rapidly & die in their 2nd decade of life from advanced atherosclerosis (typically a disease of the elderly)
• very rare disease, fewer than 300 cases world-wide, fewer than 200 cases in the US
What causes Progeria?
a point mutation in gene that codes for Lamin A → resulting in a nuclear lamina that can’t form properly
an inner & outer membrane separated by a perinuclear space (~20 nm wide) that surrounds the nucleus
How are the inner and outer membranes of the nuclear envelope distinct?
• Outer has ribosomes attached & is continuous with the rough & smooth ER
• Inner is associated with a fibrous lamina called the nuclear lamina* (what’s defunct in progeria)
- • a group of proteins that make up the fibrous nuclear lamina
- • they form a tight mesh network underneath the nuclear envelope & act as source of strength & support for the nucleus
- • they connect inner nuclear membrane to cell’s chromatin (DNA)
What are the 3 functions of the nuclear lamina?
1. structural stability of the nucleus
2. anchoring site for chromatin & nuclear pore complexes
3. regulate disassembly/reassembly of the nuclear envelope during mitosis
What is the only intermediate filament present in ALL cells with a nucleus?
a rare, lethal, AR disorder caused by the loss of a gene responsible cleaving pre-lamin into mature, nuclear lamin
without mature lamin, the skin does not grow with the fetus & the fetus suffocates in its own skin
More Examples of Laminopathies
- Restrictive Dermopathy
- Cardiomyopathy (rare forms)
- Muscular Dystrophy (rare forms)
- Mandibular Acryldysplasia
- Atypical Werner’s Syndrome
Different between Progeria & Restrictive Dermopathy
- • P: every cell in patients body produces defective Lamin A
- • RD: expression of defective Lamin occurs ONLY in the skin
represents how defect expressed generally can have different effect than one expressed locally
What happens when Lamins are de/under phosphorylated?
• when lamins are in their underphosphorylated state they’re very good at connecting chromatin to the nuclear envelope
• exist in dephosphorylated state during Interphase (when a cell isn’t dividing/going through mitosis) + at the end of mitosis when a phosphatase dephosphorylates Lamins → nuclear membrane re-assembly
What happens when Lamins are phosphorylated?
- • means a cell is in very early stages of mitosis/division
- • a particular kinase PHOSPHORYLATES Lamins, causing conformational changes where Lamins remain attached to the nuclear envelope but release chromatin
- • end result: nuclear membrane breaks up into many small vesicles
proteins made up of 8 subunits that form annuli (rings) that connect inner & outer nuclear membranes
• serves as a gateway between cytoplasm & inside of the nucleus
- • made up of intermediate filaments that are the ONLY IFs that aren’t cell type specific
- can stretch very wide
What size molecule can travel through nuclear pores via diffusion?
anything smaller that 5-10 kDa
anything larger that crosses the membrane through nuclear pores is mediated by active transport
What is the number of nuclear pores per nucleus correlated with?
the metabolic activity of the cell
- eg. oocytes only have 1-2 pores per nucleus until they begin to mature, then they develop hundreds of pores
- - hepatocytes also have hundreds of pores per nucleus
What is required for cargo transport into the nucleus?
a Nuclear Localization Signal (NLS)
Nuclear Localization Signal (NLS)
• detected on a protein destined for the nucleus by filaments on the cytoplasmic side of the nuclear pore complex
• NLS binds to chaperone proteins called Importins
• Importin-NLS complex binds to receptors on the nuclear pore that facilitates transport of the molecule into the nucleus
What kind of amino acids do NLSs tend to contain?
BASIC amino acids (Lysine, Argenine)
there isn't a single consensus sequence that specifies protein nuclear localization
proteins & ribonucleo-protein complexes destined for export have Nuclear Export Signals (NESs) recognized by exportins
What is the Nuclear Export Signal (NES) that exists on proteins or ribonucleoprotein complexes (RNPs) destined for export?
- a special methylated 5’-pppG cap
- (detected by the nuclear pore complex)
each exportin has selective set of cargos
eg. exportin-1 specializes in exporting tumor suppressors, apoptosis inducers, & anti-proliferative molecules that exert their normal biological activities by binding to DNA
What molecule do cancer cells appear to greatly over-express?
this results in a lack of of tumor suppressors, apoptosis inducers & anti-proliferative molecules in the nucleus → excessive cell proliferation (tumor growth)
Nuclear Export in the Context of Cancer: a Therapeutic Target
• specific SINEs are used to block the binding region of Exportin-1 that usually binds to tumor suppressors
• this keeps tumor suppressors IN the nucleus & able to fight tumor development (eg. p53, Par-4,PP2A, pRB, p21, IkB, BRCA1, p27, eIF4E)
What is a disease in which defective nuclear protein import has been demonstrated?
• brain cell nuclei of patients with HD show high accumulations of huntingtin that usually exists in the cytoplasm
• accumulation of huntingtin in nuclei disturbs nuclear function & may be the cellular pathogenetic mechanism
• mutant huntingtin has an NLS not present in the WT protein
• the 3D conformation of the mutant protein is likely what allows it to be transported into the nucleus
What is the severity of Huntington’s Disease correlated with?
1. the amount of huntingtin in the nucleus
2. the age of onset of the disease, which itself is correlated with the number of trinucleotide repeats in the huntingtin gene