Nerve Growth Factors Case File

Nerve Growth Factors Case File

EUGENE C.TOY, MD, RAHUL JANDIAL, MD, PhD, EVAN YALE SNYDER, MD, PhD, MARTIN T. PAUKERT, MD

CASE 38

A 40-year-old Caucasian male presents from psychiatry exhibiting mood swings, increased irrationality, excessive weight loss, and with a recent history of attempted suicide. Changes in behavior were first noticed by alarmed family members who urged the patient to visit a psychiatrist. No psychological basis has been found for these sudden changes except a somewhat vague family history in the patient’s father of similar midlife crisis before passing away years later of pneumonia at home, and a neurological deficit is suspected. Neurological examination revealed uncontrolled dyskinesias of the arms and legs, and the patient remarked about having difficulties eating that have been getting worse. Imaging showed progressive degeneration of the striatum, with enlarged lateral ventricles and widened intercaudate distance. Genetic analysis revealed abnormality on chromosome 4 consisting of a polyglutamine repeat on a gene coding for a protein of unknown function. A diagnosis of Huntington disease (HD) was decided upon.

• What is the molecular genetics behind the disorder?
• What is the cellular pathology and pathogenesis of this disease?
• What neurotrophins have been targeted for HD therapy?

ANSWERS TO CASE 38: NERVE GROWTH FACTORS

Summary: A 40-year-old Caucasian male has physical and behavioral neurological deficits with an appreciable degeneration of the striatum and a genetic abnormality on chromosome 4.

• Molecular genetics: HD is an autosomally dominant disease inherited by 50% of the children of an affected parent, who will all express the disease and pass on the gene with the same probabilities. The critical gene codes for huntingtin, a protein of unknown function, and is located on chromosome 4. The gene contains a polyglutamine (polyQ) repeat that is normally 5-36 amino acids in length, but is lengthened in individuals with HD.
• Pathology and pathogenesis: How the expanded polyQ repeats results in neuronal degeneration in the striatum is currently unknown. Abnormal inclusion bodies have been found in affected striatal neurons with heavy huntingtin staining, but whether this is a cause or consequence of cell death remains to be explored. The relationship between disturbed mitochondrial function observed in HD and pathology also continues to be delineated.
• Neurotrophins: Brain-derived neurotrophic factor has been chosen because it is required for the correct activity of the corticostriatal synapse and the survival of the GABA-ergic medium-sized spiny striatal neurons that die in HD. Neurotrophin 4/5 has been chosen because it increases the support of interneurons, which is protective of the more vulnerable medium spiny neurons. And FGF-2 has also been targeted because it stimulates the development of new spiny neurons from stem cells already present in the brain.

CLINICAL CORRELATION

HD is marked and progressive degeneration of the striatum, usually identified by enlarged ventricles and widened intercaudate distance on MRI and CAT scans. Cell loss spreads in a dorsal medial to lateral ventral direction, and the caudate nucleus is usually affected earlier and more extensively than the putamen. Medium spiny projection neurons are the most affected along with mild gliosis at the cellular level. As the disease progresses to advanced stages, other brain nuclei, particularly those of the basal ganglia, are affected. Cortical input atrophy and cell loss in the pallidal and substantia nigra pars reticulate outputs of the striatum are indications of advanced HD and translate into clinical signs of severe disability, and later death because of infection, pneumonia, or inanition years after diagnosis. Currently there are no available effective treatments of HD, only drugs to suppress motor and psychiatric symptoms. HD has been heavily investigated for neurotrophic protection in the striatum. Striatal neurons in models of the human disease can be rescued with application of a number of different nerve growth factors including NGF, BDNF, NT-4/5, β-FGF, TGF-α, CNTF, and GDNF. The most important issue in developing a neurotrophinbased therapy for HD is the delivery of trophic molecules through either direct infusion, or through the use of genetically engineered cells. Clinical trials of CNTF administration are currently underway for HD.

APPROACH TO NERVE GROWTH FACTORS

Objectives

1. Describe the role of nerve growth factors in determining neuron survival.
2. Describe the typical brain pathological findings of a patient with HD.
3. Describe the speculated mechanism of brain-derived neurotrophic factor and HD.

Definitions

Ontogenetic cell death: A cycle of massive neuronal cell death that occurs during development that serves a number of purposes for neural maturation. For instance, ontogenetic cell death occurs to eliminate redundant and multiplicative neural connections to the same target.

Trk receptors: Trk receptors belong to the tyrosine kinase class of receptors and are the receptor-binding sites on cells for many important neural growth factors.

Cholinergic neurons: Neurons that synthesize and release acetylcholine which functions as a neurotransmitter in the PNS and CNS during interneuronal communication.

Substantia nigra: Large, pigmented, dopamine-producing cells that are layered in the mesencephalon. These cells have been implicated in Parkinson disease.

Autocrine trophic support effect: A motor neuron following axonal injury is capable of achieving an autocrine trophic support effect of selfstimulation by producing its own trkB and BNDF. The extent to which this effect can delay cell death depends on the cell age and the position at which the axon is damaged.

Acetylcholine esterase: Enzyme that removes acetylcholine from the synaptic cleft to allow repolarization to occur. It does this by converting acetylcholine into inactive choline and acetate through hydrolysis. Choline acetyltransferase synthesizes acetylcholine from choline and acetyl CoA.

DISCUSSION

Nerve growth or trophic factors have a well-characterized role in determining which neurons survived into adulthood, and which axonal processes connected to particular targets. Nerve growth factor or NGF was the first to be identified, and has served as a prototype for all that followed. NGF keeps

sensory and all sympathetic neurons through the period of ontogenetic cell death during development. NGF is mainly secreted by the target tissues innervated by the sympathetic sensory neurons. When developing embryos are deprived of NGF, many of these same neurons die before birth. It has been demonstrated in vitro that for neuronal survival it is essential that axons make successful connections to NGF-secreting tissue, and that NGF applied to any part of the neuron, in addition to the end of an axon, will promote survival. We will see that similar to NGF, trophic factors have many different roles in the adult CNS and PNS, neuronal survival and plasticity, and glial cells, both in normal brain function, as well as regenerative responses to injury.

A few main families among the wide spectrum of molecules with neurotrophic effects account for most of the functional effects on neuroplasticity and regeneration. Factors NGF, BDNF, NT-3, and NT-4/5 constitute the first major family of centrally acting growth factors, and are structurally related to NGF. These factors bind to the trk receptors in addition to a low-affinity NGF receptor, p75, which binds to NGF and BDNF. TrkA is found on some neuronal types, usually cholinergic neurons, while trkB and trkC are widely distributed throughout the nervous system, found on most CNS neurons. CNTF

is another neurotrophic factor with a receptor that is widely found in the CNS. The molecule itself is found in Schwann cells in peripheral nerves and is produced by astrocytes after CNS injury. FGF-2 also has receptors widely distributed on neurons and glial cells, especially after injury. Astrocytes are the main sources of FGFs which promote survival and proliferation of most populations of neurons in the developing CNS. FGF-2 and EGF specifically have been implicated in promoting division and proliferation of CNS progenitor/stem cells. Two other families of trophic factors are the TGF-β, and the GDNF family. These two families have more cursory purposes in the nervous system. For example, GDNF and neurturin are of neuronal and persephin is of glial origin. Their receptors (GFR-α1, GFR-α2, RET) are found on neurons that respond to trophic factors, such as the dopaminergic neurons of the substantia nigra. Interestingly, GDNF appears to be the most potent yet identified for promoting survival and growth of dopamine neurons both in vitro and in vivo.

Peripheral nerve damage affects the axons of motor sensory and sympathetic neurons. These neurons generally survive axonal injury as long as it is some distance from the cell body, and mount a massive regenerative response. Periphery neurons lose contact with their targets following axonal injury, and will consequently no longer receive target-derived trophic factors. However, support cells, such as Schwann cells, continue to supply trophic factors. NGF supports sympathetic axons, and CNTF released at the time of injury can have a protective effect on motor neurons. Trk receptors are found on almost all sensory axons: nociceptive and temperaturesensitive neurons carry trkA and are supported by NGF, large proprioceptive fibers carry trkC and are supported by NT-3, and fine touch and vibrationsensitive fibers carry trkB and are supported by BDNF. TrkB and BDNF production in motor neurons following injury have an autocrine trophic support effect. The extent to which these receptors and complimentary trophic factors can prevent neuronal death can be boiled down to the following points:

• Motor neurons vary in terms of death following axonal injury, depending on the age and position at which the axons are damaged.
• Peripheral nerve damage in developing nervous systems leads to extensive motor neuron death, but nerve damage does not cause motor neuron cell death.
• Motor neurons in adults lose choline acetyltransferase following axonal injury; this effect is absent in developing nervous systems.
• Trophic factors are effective in protecting against neuron death following axonal injury in both developing and fully matured nervous systems. BDNF, GDNF, and CNTF are the most effective in motor neuron death, and IGF, BDNF, and NT-3 are the most effective in peripheral nerve damage.

Actions of trophic factors are not restricted to the CNS, nor are they limited to just promoting neuronal survival. Unspecific delivery of trophic factors causes unrelated side effects because of broad incorporation within the CNS. Thus, for some therapeutic benefit to be realized, factors need to be delivered very precisely to neurons where they are needed for protection. For the time being, manipulation of factor-secreting cells seems to be the best option for such specialized delivery of nerve growth factors, although surgical approaches to the peripheral nerves are still relatively effective.

COMPREHENSION QUESTIONS

[38.1] In the developing nervous system, from which source do immature neurons receive the neurotrophic support that allows them to survive to become mature neurons?

A. The targets they innervate

B. Surrounding glial cells

C. Neighboring neurons

D. Themselves

[38.2] During which developmental period do neurons, which are not receiving trophic support from properly innervated targets, undergo apoptosis and die?

A. Neurulation

B. Neurogenesis

C. Synaptogenesis

D. Ontogenetic cell death

[38.3] A 42-year-old man sustains a crush injury to his left forearm while working on a construction site. On evaluation in the emergency department his hand is well perfused, but he has no sensation in the distribution of his median nerve and severely limited flexion of his wrist and fingers. He is diagnosed with traumatic crush injury to his median nerve, and scheduled for exploration and repair of the nerve. Which type of neurons in the nerve can produce their own trophic factors and thereby prevent cell death?

A. Sympathetic neurons

B. Motor neurons

C. Proprioceptive neurons

D. Nociceptive neurons

Answers

[38.1] A. Immature neurons receive neurotrophic support from the target cells they innervate. In the developing nervous system, there is typically redundancy in early development: multiple neurons will extend axons toward a given target. Once this happens, one of neurons (typically the one with the stronger connection) will be selected for, and the others will degenerate. This selection process involves trophic factors secreted by the target cell that allow the neuron to persist. In the absence of these trophic factors, the nonselected for neuron dies. In peripheral nerve injury of mature neurons, they often receive temporary trophic support from surrounding glial cells or themselves so that the cell bodies do not degenerate.

[38.2] D. The period of ontogenetic cell death is a period in which a huge number of neurons, mostly neurons that are redundant or that have not reached the proper target, undergo apoptosis. During this period, neurons that have reached the appropriate target receive trophic factors, and therefore do not undergo apoptosis. In the case of sensory neurons and neurons of the sympathetic nervous system, the trophic factor that prevents apoptosis is NGF.

[38.3] B. Trophic factors are available from a number of other sources besides target cells, including Schwann cells, and in the case of motor neurons, themselves. Motor neurons can secrete their own BDNF which provides them with trophic support in an autocrine manner.

NEUROSCIENCE PEARLS ❖ When developing embryos are deprived of NGF, many of these same neurons die before birth. ❖ Factors with neurotrophic effects that are structurally related to NGF bind to trk receptors. ❖ Neurons generally survive axonal injury as long as it is some distance from the cell body.

REFERENCES

Bear MF, Connors B, Paradiso M, eds. Neuroscience: Exploring the Brain. 3rd ed. Baltimore, MD: Lippincott Williams & Wilkins; 2006. 

Kandel E, Schwartz J, Jessell T, eds. Principles of Neural Science. 5th ed. York, NY: McGraw-Hill; 2000. 

Zigmond MJ, Squire LR, Bloom FE, Landis SC, Roberts JL, eds. Fundamental Neuroscience. 2nd ed. San Diego, CA: Academic Press; 1999.

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