Cell Fate Determination Case File

Cell Fate Determination Case File

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

CASE 15

A 66-year-old retired marine sergeant complains of worsening trembling in his hands. He first started noticing the problem a year ago when he was signing checks at work. His wife noticed that he could no longer keep up with her on their daily walks, joking that he now shuffles his feet like a penguin. Besides the history of tobacco use and mild hypertension, he does not have any significant medical issues and has no other complaints. Family history is negative. His only medication is an inhaler that he uses to help with his breathing. Physical examination reveals a tremor in both hands, with a rubbing of the thumb and forefinger—“pill rolling.” His muscles are stiff to passive movement, while causing a small amount of discomfort. Examination of his gait showed a wide-based, shuffling walk, that has a noticeable loss of normal arm swing. He has a bright intellect, and has no difficulties with any cognitive tasks presented to him. He is diagnosed with Parkinson disease (PD) and the physician discusses the diagnosis and disease process with the patient, explaining to him that these symptoms usually worsen and that new symptoms may occur such as loss of normal facial expressions (smiling, blinking), impaired speech (often very soft and monotonous), difficulty in swallowing, and even cognitive decline or dementia.

• What brain structure is affected in this patient?
• What is the underlying mechanism of the movement disorder?
• Are there any preventative measures available?

ANSWERS TO CASE 15: CELL FATE DETERMINATION

Summary: A 66-year-old man is noted to have progressive tremor, shuffling gait, and muscle rigidity. He is diagnosed with PD.

• Brain structure affected: Substantia nigra, a component of the basal ganglia. PD is a neurodegenerative disorder that affects movement. It is one of a group of common neurodegenerative disorders, which includes Alzheimer disease (AD), Huntington disease (HD), and amyotrophic lateral sclerosis (ALS). In PD, the symptoms are caused by the selective death of a specific cell type in the basal ganglia, the pigmented dopaminergic (DA) neurons of the substantia nigra. These diseased neurons often contain Lewy bodies, a specific type of cytoplasmic inclusion.
• Underlying mechanism: The basal ganglia are a group of interconnected nuclei consisting of the putamen, caudate, globus pallidus, subthalamic nucleus, and substantia nigra. These nuclei affect multiple cortical functions, including motor control, cognition, emotions, and learning. The loss of dopaminergic neurons leads to reduced levels of the neurotransmitter dopamine, altering the function of the basal ganglia.
• Prevention: Herbicide and pesticide exposure has been linked to the development of PD; however, no specific chemicals have been identified. As with all neurodegenerative diseases, the risk increases with age. There is also a genetic contribution—first degree relatives confer a 5% risk. However, neither age nor family genetics are modifiable.

CLINICAL CORRELATIONS

PD is recognized as one of the most common neurological disorders, affecting approximately 1% of individuals more than 60 years of age, with an incidence of 5-20 cases per 100,000 people a year. Cardinal features include resting tremor, rigidity, bradykinesia, and postural instability. The major neuropathologic findings in PD are a loss of dopaminergic neurons in the substantia nigra pars compacta and the presence of Lewy bodies. The basal ganglia circuit modulates the cortical signals necessary for normal movement. Impulses from the cerebral cortex are processed through the basal ganglia-thalamus circuit and act as a feedback pathway. Output from the basal ganglia motor circuit is directed toward suppressing the thalamocortical pathway and decreasing movement. The loss of dopaminergic signaling in the basal ganglia increases the inhibition of the thalamocortical pathway, leading to further suppression of movement (see Figure 15-1).

Figure 15-1. Neurochemical pathology of basal ganglia in Parkinson disease.

The molecular mechanisms of PD remain elusive. The protein alphasynuclein recently was discovered to be a major structural component of Lewy bodies. Improper folding of this protein may result in abnormal aggregation and toxicity to neurons. Lewy bodies also contain the protein ubiquitin, a key player in targeting proteins for degradation. A variety of rare familial forms of PD have shed light on several possible molecular mechanisms for the disorder. These mutations have implicated oxidative stress and mitochondrial injury, as well as dysfunction of the ubiquitin system for protein degradation as pathways leading to neuron death in PD. The specific nature of the degenerating system (dopaminergic neurons) and the discrete location (substantia nigra pars compacta) have made PD an attractive target for cell-based therapies. The early successful transplantation of fetal mesencephalic neurons (rich in dopaminergic neurons) into the degenerating Parkinsonian brain demonstrated that transplanted cells could survive and functionally integrate into neuronal circuits. Recent work has been focused on engineering stem cells to assume the phenotype of the degenerating substantia nigra neurons. In order to be successful, the cells must (1) behave like dopaminergic neurons and release dopamine, (2) integrate functionally into the neuronal circuitry, (3) yield sufficient numbers for long-term survival, and (4) reverse the symptoms of PD. Critical to this process is an understanding of the pathway that neurons undergo to determine their cellular fate, so that it can be replicated to entrain new populations of stem cells.

APPROACH TO CELL FATE DETERMINATION

Objectives

1. Understand the stages of cell fate determination in the nervous system.
2. Explain the different types of signals that cells respond to during differentiation.
3. Appreciate the role of neurotrophic factors in cell fate determination.

Definitions

Competence: The ability of a cell to respond to development signals.

Fate specification/determination: Both terms refer to the mechanism by which precursor cells begin with many possible fates and are progressively limited in their potential, until they finally differentiate into a mature cell.

Phenotype: The characteristics or traits that a cell exhibits. Different cell types display different phenotypes, which are often because of underlying differences in protein expression, especially transcription factors.

Intrinsic determination: A model that explains progressive cell fate restriction by the influences of the cell’s lineage and internal signals. This would be the cell’s metaphorical version of the nature versus nurture debate in human development.

Extrinsic determination: A model that explains progressive fate restriction by the influences of the environment, as experienced by cell–cell interactions and diffusible signaling molecules.

Apoptosis: The active process of programmed cell death, typified by cell shrinkage, condensation of chromatin, cellular fragmentation, and phagocytosis of cellular debris. A prominent feature of neuronal development is the death of cells by apoptosis.

Trophic factor: A substance that encourages survival and proliferation of a cell or tissue.

DISCUSSION

After a newly born cell exits mitosis and becomes committed to the neuronal lineage, it must continue to respond to signals that will influence the type of neuronal fate it will acquire. There is significant debate as to where neurogenesis ends and neural fate determination begins. As previously discussed, during cortical development, the neural precursor cells that are born later have had their fates restricted—they can only populate the superficial cortical layers. In other words, these precursor cells that were born later are no longer competent to form cells in the deep cortical layers. Therefore, during neurogenesis, some aspects of fate determination have already been decided just by the timing of birth. The developmental plan is best viewed as a continuum of overlapping processes.

Many prospective neurons express the same genes early in development, and at some point, as their fates diverge, they begin to express unique genes and proteins required for their eventual fate. In this manner, the cells acquire progressively specialized neuronal phenotypes. The influences which a maturing neuron encounters can be classified as intrinsic or extrinsic signals.

One example of intrinsic determination occurs during early neurogenesis when the neural progenitor pool divides along the ventricular surface in the ventricular zone. The early cell divisions occur with the plane of division perpendicular to the ventricular surface, generating two identical daughter cells, both capable of further proliferation. The later divisions occur with the plane of division parallel to the ventricular surface and are asymmetrical with one daughter cell retaining the ability to proliferate, while the second daughter is postmitotic and migrates out of the ventricular zone (VZ, ie, area surrounding the ventricles). This intriguing process appears to be mediated by two intracellular proteins, Numb and Prospero. These proteins are asymmetrically inherited by the daughter cells during division, and thus confer different phenotypes to the progeny.

Neuronal target tissues often influence the fate of neurons via extrinsic determinants, a concept called target specification. An example is the specification of neurotransmitter phenotype of the autonomic neurons by sweat glands. Most sympathetic neurons use norepinephrine as their primary neurotransmitter; however, the exocrine sweat glands in the hands and feet use acetylcholine instead. The maturing sympathetic neuron first expresses the norepinephrine phenotype, but when it contacts the sweat glands, the neurons gradually convert their neurotransmitter phenotype to acetylcholine. The signals for this fate specification appear to be a soluble cytokine of the interleukin-6 family secreted by the sweat glands. Target tissues play other critical roles in the fate determination of neurons. Target cells of developing neurons produce a limited amount of a trophic factor that can control the survival of the innervating neurons—the neurotrophic factor hypothesis. Nerve growth factor (NGF) was the first isolated neurotrophic factor. Survival of some neurons requires NGF, a target-derived factor. NGF works by binding to tyrosine kinase receptors on the dependent axon and is transported in a vesicle via retrograde fashion to the cell body, where it can affect transcription and cellular differentiation. Many other families of neurotrophic factors have been isolated: the neurotrophins, IL-6 class, transforming growth factor beta (TGF-β) class, as well as fibroblast growth factor (FGF). It now appears that elimination of neurotrophic factors can control survival by activating programmed cell death, or apoptosis.

An important principle in neuron cell fate is the gradual acquisition of progressively more restrictive fates until the final neuron identity is reached. This process is mirrored phenotypically by the expression of various proteins during development. The protein expression patterns can define different developmental stages, resulting in a hierarchy of gene expression. This is best illustrated in the developing spinal cord by the LIM family of transcription factors. This family of proteins is expressed in different overlapping subpopulations of motor neurons. The protein that all the motor neurons express is Islet1, suggesting that this protein is responsible for characteristics common for all motor neurons. Other LIM proteins are subsequently expressed, which progressively identify specific motor neuron subgroups and even their behaviors like axon projection or target selection.

Treatment options

Unlike most other neurodegenerative disorders, PD responds well to treatment. The mainstay of treatment is medical treatment with oral levodopa (L-dopa), a precursor to dopamine which can cross from the blood into the brain and be converted by neurons into dopamine. L-dopa is often combined with carbidopa and called Sinemet, which helps to decrease the systemic side effects. Other medications help potentiate the effect of L-dopa treatment by inhibiting the degradation of dopamine: selegiline blocks the enzyme monoamine oxidase, and a class of inhibitors to the enzyme catechol-Omethyltransferase (COMT). Another group of drugs, the anticholinergics, alters the basal ganglia signaling to compensate for the loss of dopamine action. Once commonly practiced, surgical treatment has receded as the first line of therapy, but as symptoms progress and are refractory to medications, surgery has been reevaluated. Selective placement of destructive lesions to the thalamus (thalamotomy) and globus pallidus (pallidotomy) can be helpful in controlling the symptoms, but can carry significant risks. A newer form of surgical treatment called deep brain stimulation (DBS) involves stimulating the subthalamic nucleus with tiny implanted electrodes and a pacemaker-like neurostimulator. Regenerative medical interventions, such as gene therapy and stem cell replacement are areas of considerable current interest and funding.

COMPREHENSION QUESTIONS

Refer to the following case scenario to answer questions 15.1-15.2:

A 25-year-old man falls from the roof of his house and injures his spinal cord. Initially he has flaccid paralysis of his lower extremities, but in the subsequent months his legs become hypertonic, hyperreflexic, and spastic because of destruction of his corticospinal tracts. The axons in these damaged tracts project in large part from pyramidal neurons in cortical layer V of the primary motor cortex.

[15.1] In which location did these neurons originate?

A. Ventricular zone

B. Marginal zone

C. In cortical layer V

D. In the neural plate

[15.2] Which of the following is true of the cells that gave rise to these neurons?

A. They are capable of giving rise to neurons of all cortical layers.

B. They are capable of giving rise to only neurons in deep cortical layers.

C. They are capable of giving rise to only neurons in superficial cortical layers.

D. They are capable of giving rise to neurons in none of the cortical layers.

[15.3] A 22-year old-patient presents to your office complaining of excessive sweating in his axillae, without apparent cause, unresponsive to the application of antiperspirant. You diagnose him with hyperhidrosis, and recommend botulinum toxin A (Botox) injections to the sweat glands to help resolve his symptoms. Which of the following best describes the reason that sympathetic nerves innervating sweat glands release acetylcholine, while all other sympathetic neurons release norepinephrine?

A. Apoptosis

B. Intrinsic determination

C. Extrinsic determination

D. Cell fate restriction

Answers

[15.1] A. Cortical neurons all originate from neural precursor cells located in the ventricular zone. Once generated, these immature neurons migrate (via radial glial migration or tangential migration) to their final location in the cortex. While it is true that the precursor cells that give rise to neurons originally came from the neural plate, the neuron itself originated in the ventricular zone, making that a better answer.

[15.2] A. One of the interesting features of the neural progenitor cells in the ventricular zone is that as development progresses, their fate becomes more determined. The stem cells that give rise to cortical layer V neurons (relatively deep in the cortex) are competent to give rise to all neurons more superficial in the cortex. Conversely, a stem cell that gives rise to a layer I neuron (superficial in the cortex) is not competent to generate a neuron in the deeper layers.

[15.3] C. These sympathetic neurons release acetylcholine instead of norepinephrine because of extrinsic determination. The ultimate neurotransmitter released by the sympathetic neurons innervating the sweat glands is determined by trophic factors released from the sweat glands themselves. These factors induce changes in the neuron that causes it to change from an adrenergic neuron to a cholinergic neuron. This is an example of extrinsic determination: the fate of the neuron is determined by a cell other than itself. While this is a type of cell fate restriction, extrinsic determination is more specific and therefore a better answer.

NEUROSCIENCE PEARLS ❖ Cell fate specification relies on both intrinsic and extrinsic cues. ❖ The expression of certain genes dictates the phenotype and eventual fate of developing cells. ❖ Apoptosis, or programmed cell death, is a critical process in the development of the nervous system.

REFERENCES

Bossy-Wetzel E, Schwarzenbacher R, Lipton SA. Molecular pathways to neurodegeneration. Nat Med. 2004 Jul;10(suppl):S2-S9. 

Kandel E, Schwartz J, Jessell T, eds. Principles of Neural Science. 4th ed. New York: McGraw-Hill; 1991. 

Lindvall O, Kokaia Z, Martinez-Serrano A. Stem cell therapy for human neurodegenerative disorders—how to make it work. Nat Med. 2004 Jul;10(suppl):S42-S50. 

Rao MS, Jacobson M, eds. Developmental Neurobiology. 4th ed. New York: Kluwer Academic/Plenum Publishers; 2005. 

Vila M, Przedborski S. Genetic clues to the pathogenesis of Parkinson’s disease. Nature Med. 2004 Jul;10(suppl):S58-S62.

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